Fundamentals and Principles of Ophthalmology
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Basic and Clinical Science Course
Fundamentals and Principles of Ophthalmolog'y Section 2 2011-2012 ILast major revision 2009-2010)
t:l~ AMERICAN ACADEMY ~ OF OPHTHALMOLOGY Tile Eye M .D. A ssac;I1I;OI'
t""0NG EDUCATION ..,.""
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The Basic and Clinical Science Co urse is one component of the Lifelong Education for the Ophthalmologist (LEO) framework, which assists mem bers in planning their continu ing medical education. LEO includes an array of clinical education products that members may select to form individu alized, self-directed learning plans for updating their cl inical knowledge. Active members or fellows who use LEO components may accumulate sufficient CME credits to earn the LEO Award. Contact the Academy's Clinical Education Division for further information on LEO. The American Academy of Ophthalmology is accredited by the Accredi tation Council for Continuing Medical Education to provide continui ng medical education fo r physicians. The American Academy of Ophthalmology designates this enduring material for a maximum of 15 AMA PRA Category I Credits TM. Physicians should claim only credit commensurate with the extent of their participation in the activity.
The Academy provides th is material for educational purposes only. It is not intended to represent the only or best method or procedure in every case, nor to replace a physi cian's own judgment or give specific advice for case management. Including all ind ica tions, contraindications, side effects, and alternative agents for each drug or treatment is beyond the scope of th is materia l. All information and recommendations should be verified, prior to use. with current information included in the manufacturers' package inserts or other independent sources, and considered in light of the patient's condition and history. Reference to certain drugs. instruments, and other products in this course is made for illustrative purposes only and is not intended to constitute an endorsement of such. Some material may include information on applications that are not considered community standard, that reflect indications not included in approved FDA labeli ng, or that are approved for use only in restricted research settings. The FDA has stated that it is the responsibility of the physician to determine the FDA st atus of each drug or d evice he or she wishes to use, and to use them with appropriate. informed patient consent in compli ance with applicable law. The Academy speCifica ll y disclaims any and all liability for injury or other damages of any kind, from negligence or otherwise, for any and all claims that may arise from the use of any recommendat ions or other information contained herein.
Cover image tourtesy of Thomas A. Weingeist, PhD, MD.
Copyright © 20 11 American Academy of Ophthalmology All rights rese rved Printed in Singapo re
Basic and Clinical Science Course Gregory L. Skuta. MD. Oklahoma City. Oklaho ma. Senior Secretary for Clin ical Education Louis B. Cantor. MD. Indianapolis. Indiana. Secre tary for Ophthalmic Knowledge Jayne S. Weiss. MD. Detroit. Michigan. BCSC Course Cha ir
Section 2 Faculty Responsible for This Edition K. v. Chalam. MD. PhD. Chair. Jacksonville. Florida Balamurali K. Ambati. MD. Salt Lake City. Utah Hilary A. Beaver. MD. Iowa City. Iowa Sandeep Grover. MD. Jacksonville. Florida Lawrence M. Levine. MD. Jac ksonville. Florida Tony Wells. MD. Consultant. Wel lington. New Zealand Edward K. Isbey. III. MD. Asheville. North Carolina Practicing Ophthalmologists Advisory Comm ittee for Education
Financial Disclosures The authors state the followin g financial relationships: Dr Beaver: Genzyme. lecture honoraria recipient
Dr Wells: Alco n. consultant; Allergan. consultant The other authors state that they have no significant financial interest or other relation ship with the manufacturer of any commercial product discussed in the chapters that they contributed to this publication or wi th the manufacturer of any com peting commercial product.
Recent Past Faculty Gerha rd W. Cibis. MD Karla Johns. MD Shalesh Kaushal. MD. PhD James c. Tsai. MD [n addition. the Academy gratefully acknowledges the contributions of numerous past faculty and advisory committee members who have played an important role in the development of previous edi tion s of the Basic and Clinical Science Course.
American Academy of Ophthalmology Staff Richard A. Zorab, Vice Presidellt, Ophthalmic Kllowledge
Hal Straus, Director, Publications Departmelll Christine Arturo, Acquisitions Manager
Stephanie Tanaka, Publications Mallager D. Jean Ray, Production Mallager Brian Veen, Medical Editor Steven Huebner, Administrative Coordinator
a~ AMERICAN ACADEMY
\!:.J OF OPHTHALMOLOGV Till Ii>'c M.D. ".soc;IJI;OIl
655 Beach St reet Box 7424 San Francisco, CA 94120-7424
Contents General Introductio n
xv
Objectives . .
.1
PART I
Anatomy
1 Orbit and Ocular Adnexa Orbital Anatomy. . O rbital Vo lume. Bony Orbit . . O rbital Ma rgin . Orbital Roof . . Medi al Orbital Wall Orbital Floor. . . Lateral Orbital Wa ll Orbital Foramina, Ducts. Canals, and Fissures. Periorbital Sinuses. Cranial Nerves. Cilia ry Ganglion . . . Branches of th e Ciliary Gan glion Short C iliar y Nerves. . . Extraoc ular Muscles. Extraocular Muscle Insertions Extraocul ar Muscle Distribution in the Orbit. Extraoc ul ar Muscle O rigins. . . Blood Supply to the Extraocular Muscles. Innervation of the Extraocular Muscles Fine Structure of the Extraocular Muscles Eyelids ...... . Anatomy . . . . . . Vascular Supply of the Eyelids. Lymphatics of the Eyelids. . Accessory Eyelid Stru ctures. Lacr imal Gla nd and Excreto ry System. Lacrimal Gland. . . . . . Accessory Gla nds. . . . Lacrim al Excre tory System . Conjun ctiva . . . Tenon Capsule . . ..... Vasc ul ar Suppl y an d Drainage of th e O rbit . Posterior and Anterior Ciliary Arteries. Vortex Ve ins ......,....
3 .5 .5
.5 .5
.5 .5
.7 7 .9 10 12 13 14 15 16 16 16 18 18 18 21
21 22 22
29 30 31 32 32 33 33
34 35
36 36 38 v
vi • Contents
2 The Eye.
41
Topographic Features of the Globe Precorneal Tear Film .... .
41
Cornea . . . .
43 43 43 43 44
. ..... .
Characteristics of the Central and Peripheral Cornea. Epithelium and Basal Lamina. Nonepithelial Cells Bowman's Layer. . . . Stroma . . . .
Descemet's Membrane. Endothelium . Sclera. . . . .
Limbus . . Anterior Chamber
Trabecular Meshwork. Uveal Trabecular Meshwork Corneoscleral Meshwork. Perica nalicu lar Connective Tissue.
Schlemm Canal. Collector Channels Uveal Tract Iris. . . . . Stroma . .
Vessels and Nerves Posterior Pigmented Layer Dilator Muscle . Sph incter Muscle . . . Ciliar y Body . Ciliary Epithelium and Stroma Ciliary Muscle . . Choroid . . Bruch's Membrane.
Choriocapillaris . Lens . . . Capsule Epithelium. Fibers. . . Zo nu les (S uspensory Ligaments) Ret ina.
Retinal Pigment Epithelium Neurosensory Retina Macu la . . Ora Serrata Vitreous. .
42
45 45 46 47 48 50 53
54 54 54 54 54 55
56 56 58 58 60 61 61 61
63 64 64 66 67 68 70 70 70 71 71 73
79 82 82
Contents . vii
3
Cranial Nerves: Central and Peripheral Connections. . . . . . . Cranial Nerve I (Olfactory) . . Cranial Nerve II (O ptic Nerve) . In traocu lar Reg ion Intraorbital Region Intraca nal ic ular Region Intracranial Region . . Blood Suppl y of the Optic Nerve Chiasm . . . . . . . Optic Tract. Lateral Geniculate Body Optic Radiatio ns . Visual Corlex. . . . . Cranial Ne rve III (Oculomotor) Pathways for the Pupil Reflexes . Cranial Nerve IV (Trochlear). Cranial Ne rve V (Tri geminal) Mesencephalic Nucleus . Main Senso ry Nucle us . Spinal Nucleus and Tract. Moto r Nucleus . . . . . Divisions of Cranial Nerve V . Crani al Ne rve VI (Abd ucens). Cranial Ne rve VII (Facial) . Cavernous Sin us . . . Other Venous Si nuses . Circle of Wi llis .
PART II
4
Embryology .
Ocular Development Introducti on. Growth Factors. Homeobox Genes . Ne ural Crest Cells. Embryogenesis . Organogenesis of th e Eye Neurosensory Reti na Fovea . . . . . . . Retin al Pigmen t Epithelium O ptic Ne rve Lens . . . Vitreous.
87 87 87 89
91 93 93
94 96 96 96 96 97 97 99 99
· 100 100
100 100 102 \03
104 105 107 108 109
111 113 11 3 11 3
11 4 11 5 · 1J7
· 120 124 126 127 128 128 · 130
viii • Contents
Choroid . . . Cornea and Sclera. Anteri or Chamber, Angle, Iris, and Ciliary Body. Vascular System . . . . . . . Periocular Tissues and Eyelids Realignment of the Globe Congenital Anomalies. Genetic In fl uences Nongenetic Teratogens.
PART III
5
Genetics
· · · ·
132 133 135 137 139 140 141
141 141
145
Introduction.
147
Te rmin o logy. Glossa ry . . . . .
147 · 147
Molecular Genetics
165
Gene Structure. . . "Ju nk" DNA . . . . . . Ge ne Transcription. . . Transcription Factors and Regulation Intron Excision. . . . . . . . Alternative Splicing and Iso form s Methylation X-Inactivation . . . Imprinting . . . DNA Da mage and Repair Mutations and Disease . Requirements for Identifying a Disease-producing Mutation Mutations . . . . Cancer Genes. Mitochondrial Disease C hroni c Progressive Exter nal Ophth almoplegia. Leber Heredi ta ry Optic Neuropathy. . . . . Neuropathy, Ataxia, and Retinitis Pigmentosa. Other Mitochondrial Diseases . The Search for Genes in Specific Diseases . . . . Synteny . . . . . . . . . . Cytogenetic Markers (Mo rphologi call y Variant C hromoso mes) Gene Dosage . Association. . . . . . . . ....... Linkage. Ca ndidate Gene Approaches Mutat ion Screening. DNA Libraries . . Single-stranded Conformat ional Pol ymorphism
165 165 166 166 · 168 168 168 168 169 169 170 170 170 · 17 1 173 174 175 175 176 176 176 177 . 177 177 177 180 182 182 182
Contents. ix
Denaturing G radient Gel Electrophoresis Direct Sequencing . . . . . . . Use of Restriction Endonucleases
Allele-specific Oligonucleotides . Gene Therapy . . . . . . . . . . Replacement of Absent Gene Product in X-Linked and Recessive D isease. . . , Strategies for Dominant Diseases
6
Clinical Genetics . . . . . Terminology: Hereditary, Genetic, Fam ili al, Co ngen ital Genes and Chromosomes
Alleles. M itosis . . Meios is . .
Segregation Independent Assor tment Linkage . . . . . . . Chromosomal Analysis Indications. . . . Preparation. . . .
Aneuploidy of Autosomes Mosaicism.
. ....
Eti ology of Chromosomal Aberrations. Mutations . . .
Polymo rph isms . . . . . Ge no me, Ge notype, Phenotype . Single-Gene Disorde rs. Va ri ab ility Penetrance .
Expressivity Pleiotropism Racial and Ethnic Concentration of Genetic Disorders. Patterns of Inheritance . . . . . . Recessivity Versus Dominance . . Autosomal Recessive Inheritance . Autosomal Dominant Inheri tance.
· · · · ·
183 184 184 184 184
187 188
191 191 193 194 195
195 196 196
197 197 198 198 199 .203 .204 .207 .207 · 208 .208 · 208 · 209 · 209 .210 · 210 · 2 11 .211 · 212 · 216
X- Linked Inheritance
· 218
Maternal Inheritance . . . . . .
.220 · 220 · 224
Lyonization . . . . .
Polygenic and Multifactorial Inheritance. Pedigree Analys is. . .... Ge netic Co unseling. . . . . . Iss ues in Genetic Co unseling Prenata l Diagnosis
.225 · 226 · 227 · 228
x • Contents Pharmacogenetics . . . . . . . . . . Clinical Management of Genetic Disease. Accurate Diagnosis . . . . . . . . Complete Explanation of the Disease Treatment of the Disease Process . .
Ge netic Counseling. . . . . . . . Referral to Providers of Support for Disabi lities.
PART IV
Biochemistry and Metabolism .
Introduction.
7
.229 .230 · 230 .230 .230 .232 · 232
233 235
Tear Film . .
237
Lipid Layer . . Aq ueous Layer . Mucin Layer. .
.237 .239 .24 1 · 24 1 .244
Tear Secretion .
Tear Dysfunction.
8 Cornea ...
247
Epithelium . . .
Descemet's Membrane and Endotheliulll .
· 248 .249 .249 .25 1
Iris and Ciliary Body . . . . . .
253
Introd uction . . . . . . Protein Types Expressed in Human Cil iary Body Iris-Ciliary Body Smooth Muscle. Aq ueous Humor Dynamics Eicosanoids . . . . . Types and Actions. Synthesis. . . . . Leukotrienes. . . Prostaglandin Receptors. Neurotransmitters. Receptors. and Signal Transduction Pathways. Miotics . . . . . . . . . . . . . . . Mydriatics. . . . . . . . . . . . . . . . . . . Calcium Channels and Channel Blockers. . . . . . Membrane Receptors and lntracellular Commun ications . Signal Transduction. . . . . . . . . . . . . Receptor-Effector Coupling . . . . . . . . . Cyclic AMP and Polyphosphoinos itide Turnove r Receptors . . . . . . . . . . . . . . . . . . .
· 253 . 253 . 253 . 254 . 255 . 255 . 257 . 258 . 258 . 258 . 259 . 260 . 26 1 .26 1 . 26 1 . 26 1 . 262 . 264
Bowman's Layer Stroma . . . .
9
Contents . xi
10 Aqueous Humor . . Aqueous Dynamics. . . Composition of the Aq ueous Humor Inorganic Ions . Organic Anions.
Carbohydrates Glutathione and Urea Prote ins . . . . . .
Growth -Modulator y Factors Vascula r Endothelial Growth Factors Oxygen and Carbon Dioxide . . . . Clinical Implications of Breakdown of the Blood-Aqueous ' Barrier
11 Lens ..
265 · 265 .265 .267 · 267 .268 · 268 .268 .270 · 27 1 · 27 1 .272
273
Structure of th e Lens Capsule Epithelium. . Cortex and Nucleus. Chemical Composition of the Lens
Membranes . . . Lens Proteins. ....,. Physiologic Aspects . . . . . . . Lens Metabolism and Formation of Sugar Cataracts. Energy Production . Carbohyd rate Cataracts
· 273 · 273 .274 · 274 .275 · 275 · 275 .277 .278 .278 · 279
12 Vitreous .
283
Composition .
· 283 · 283 · 284 .285 · 286 · 286 · 286 .287 .287 · 287 .288 .288 · 288 .289
Collage n . Hyaluronan Soluble and Fibril-Associated Proteins . Zonular Fibers, Lipids, and Low-Molecular-Weight Sol utes. Biochemical Changes Wi th Aging and Disease . . . . . . Vitreous Liquefact ion and Pos terio r Vitreous D etachme nt
Myopia
............ .
Vitreous as an Inhibitor of Angiogenes is . .
Physiologic Changes After Vitrectomy . Injury With Hemorrhage and Infl ammati on Invo lvement of Vitreo us in Macu lar Hole Formation. Ge netic D isease InvolVing the Vit reous.
Enzymatic Vitreolysis
13 Retina ... . Neu ral Retina-T he Photoreceptors. Rod Photo transduction . Cone Phototransdu ction .
291 · 29 1 · 29 1 · 293
xii. Conte nts
Rod-Specific Ge ne Defects. Cone- and Rod-Specific Ge ne Defects Cone-Specific Gene Defects . RPE-Spec ific Gene Defects. . Ubiquitously Expressed Genes Causing Retinal Degenerations In ne r Nuclear Layer . Retinal Electrophysiology . . . .
14 Retinal Pigment Epithelium . Anatomical Description.
Biochemical Composition Protein s
Li pids . . Nucleic Acids. . . . Major Physiologic Roles of the RPE . Visual Pigme nt Regene ration . Phagocytosis of Shed Photoreceptor Outer-Segment Discs Transport . . . Pigmentation. . Retinal Ad hesion The RPE in Disease.
15 Free Radicals and Antioxidants Cell ular Sources of Active Oxygen Species Mechanisms of Lipid Peroxidation . Oxidative Damage to the Lens . . . . . Vulnerability of the Retina to Free Rad icals. Antioxidants in the Reti na and RPE. . . . Selenium, Gl utathione, Glu tathione Peroxidase, and Glutat hione-S-Transferase. Vitamin E . Superoxide Dismutase and Catalase Ascorbate Carotenoids .
PART V
O~ular
Pharmacology .
16 Pharmacologic Principles Introduction. . . . . . Pharmacok in etics.
Pharmacodynamics. Pharmacotherapeutics.
Toxicity . . Pharmacokinetics: The Route of Drug Delivery . Topical Administ rat ion Local Administrat ion Systemic Adm inistra tio n.
Methods of Ocular Drug Design and Delivery. Pharmacodynamics: The Mechanism of Drug Ac tion
. . . . . . .
295 296 296 297 298 298 301
303 · 303 .304 .304 · 305 · 305 · 305 · 305 · 306 · 307 .307 .308 · 308
311 · · · · ·
311 3 12 313 3 15 316
· · · · ·
3 16 317 317 317 318
319 321 · 32 1 · 322 · 322 · 322 · 322 .323 .323 · 327 · 328 · 330 · 33 1
Contents . xiii
17
Ocular Pharmacotherapeutics .
333
Legal Aspects of Medical Therapy. Cholinergic Agents . . Muscarinic Drugs. Nicotinic Drugs. . Adrenergic Agents . . a-Adrenergic Agents p-Adrenergic Agents Ca rbonic Anhydrase Inhibitors. Prostaglandin Analog ues Combined Medications Osmotic Agents Actions and Uses Agents . Anti -Inflammatory Agents. Glucocorti coids. . . . Nonsteroidal Anti-Inflammatory Drugs Mast-Cell Stabilizers and Antihistamines . Antifibrotic Agents . . . . . Antibiotics . . . . . . . Penicillins and Ce phalosporins Other Ant ibacterial Agents. Antifungal Agen ts. An tiviral Agents . Medications for AcarItharnoeba Infections Local Anesthetics. Topical Anest hetics in Anterior Segment Surgery Purified Ne urotoxin Complex Medications for the Dry Eye Hyperosmolar Agents. Ocular Decongestants. Irrigating Solutions. Diagnostic Agents Viscoelastic Age nts Fibrinolyt ic Agents Thrombin . . Antifibrinolytic Agents Corneal Storage Medium Vitam in Supplements and Antioxida nts Interferon Growth Factors.
.333 · 334 · 335 · 342 .344 .345 .348 · 351 .354 · 355 · 355 · 355 · 355 · 356 .357 · 361 .364 · 366 · 368 · 368 · 37 1 · 378 .380 · 385 · 386 · 389 · 390 · 390 · 391 .392 .392 .392 .393 · 393 · 393 · 394 · 394 · 395 .395 .395
Basic Texts. . . Related Academy Materials Credit Reporting Form Study Questions Answers. Index . . . . .
· 399 · 401 .403 .407 .414 · 417
General Introduction The Basic and Clinical Science Course (BCSC) is designed to meet the needs of residen ts and practitioners for a comprehensive yet concise curriculum of the field of ophthalmology. The BCSC has developed from its original brief outline format. which relied heavily on outside readings. to a more convenient and educationally useful self-contained text. The Academy updates and rev ises the course annually. with the goals of integrating the basic science and clinical practice of ophthalmology and of keep ing ophthalmologists current with new developments in the various subspecialties. The BCSC incorporates the effort and expertise of more than 80 ophthalmologists. organ ized into 13 Section facu lties. wo rking wi th Academy editorial staff. [n addition. the course continues to benefit from many lasting contributions made by the facul ties of previous ed itions. Members of the Academy's Practicing Ophthalmologists Advisory Committee for Education serve on each facu lty and, as a group. review every volW11e before and after major rev isio ns.
Organization of the Course The Basic and Cli nica l Science Course comprises 13 volumes. incorporating fu ndamental ophthalmic knowledge. subspecialty areas. and special topics: 2 3 4 5 6 7
Update on General Medicine Fundamentals and PrinCiples of Ophthalmology Clinical Optics Ophthalmic Pathology and [ntraoc ular Tumors Ne uro-Ophthalmology Pediatric Ophthalmology and Strabismus Orbit. Eyelids. and Lacrimal System
8 External Disease and Cornea
9 Intraocular In flammation and Uveitis 10 Glaucoma II Lens and Cataract 12 Retina and Vitreous 13 Refractive Surgery In addi tion. a comprehensive Master Index allows the reader to easily locate subjects throughout the entire series.
References Readers who wis h to explore specific topics in greater detail may consult the references cited within each chapter and listed in the Basic Texts section at the back of the book. T hese references are intended to be selective rather than exh austive. chosen by the BCSC facu lty as being important. current. and readily available to residents and practitioners. xv
xvi . Ge nera l Introductio n
Related Academy educational materials are also listed in the appropriate sections. They include books, o nline and audiovisual materials, self-assessment programs, clinical modules, and interactive programs.
Study Questions and CME Credit Each volume of the BCSC is designed as an independent stud y activity for ophthalmology residents and practit ioners. The learning objectives for this vol ume are given on page 1. The text, illustrations, and references provide the information necessary to achieve the objectives; the study qu estions allow readers to test their unde rstanding of the material and their mastery of the objectives. Physicians who wish to claim CME credit for this educational activity may do so by ma il, by fax, or online. The necessary forms and instruct ions are given at the end of the book.
Conclusion The Basic and Clinical Science Course has expanded greatly over the yea rs, with the addition of much new text and numerous iLlustrations. Recent editions have sought to place a greater emphas is on clinical applicability while maintaining a solid fo undation in basic science. As with any educational program, it reflects the experience of its authors. As its faculties change and as medicine progresses, new viewpoints are always emerging on controversial subjects and tech niques. Not all alternate approaches can be included in this series; as with any educational endeavor, the learn er should seek additional sources, including such carefully balanced opinions as the Academy's Preferred Practice Patterns. The BCSC faculty and staff are continuously stri ving to improve the educational usefulness of the course; you, the reader, can contribute to this ongoing process. If you have any suggestions or questions about the series, please do not hesitate to contact the faculty or the editors. The authors, editors, and reviewers ho pe that yo ur stud y of the BCSC wi ll be oflasting value and that each Section will serve as a practical resource for quality patient care.
Objectives Upon completion of BCSC Secti o n 2, Fundamentals and Prin ciples of Ophthalmology, the reader should be able to identify the bones making up the orbital walls and the orbital foramin a
identify the o rigi n and pathways of cranial nerves 1- VII
• identify the origin and insertions of the extraocu lar muscles and use CT and MRI studies to point out the extraocular muscles, optic nerve, and lacrimal gland in axial and coronal views of the orbit describe the distribution of the ar terial and venous circulat ions of the orbit and optic nerve
summari ze the structural -fun cti onal relationships of the outflow pathways for aqueous humo r of the eye
• deli neate the events of early embryogenesis that are importa nt for the subsequent development of the eye and orbit • identify the roles of growth factors, homeobox ge nes, and neural crest cells in the ge nesis of the eye describe the sequence of events in the differentiatio n of the ocu lar tissues during embryo nic and feta l development of the eye • describe the stages in the development of the eye and the correlat ion between congenital ocular disorders and the timing of an insult to the embryo
describe the organization of the human genome and the role of genetic mutations in health and d isease
explain how DNA can be manipulated in the laboratory to map and to clone genes, to identify genes from surrounding DNA, and to create transgenic and knockout animals
demonstrate how appropriate diagnosis and manage ment of genet ic diseases can lead to better patient care
assess the role of th e ophthal mologist in the provision of genetic co unselin g
identify th e biochemical comp ositi on of the various parts of the eye and the eye's secretions discuss new concepts regarding the in teraction between membrane prote in s and G proteins and the effects of thi s in te ractio n o n ocular functions, such as rhodopsin w ith tran sducin in the convers ion of "light-stimulus" to "electric-
signal" • discuss the biochem ical derangements in diabetes mellitus
and the way in which they lead to the disease's oc ul ar compli cat ions. such as diabetic retinopathy and cataract formation
list th e var ied functions of th e retinal pigment epithelium such as phagocytosis and vitamin A metabolism and their relationship to retinal diseases
summari ze the role of free radicals and antioxidants
• describe the features of th e eye that facilitate or impede drug delivery ci te the basic principles underlying the use of autonomic therapeutic agents in a variety of ocular cond ition s • list the indicat ions. co ntraindications, mechanis ms of acti on, and side effects of var io us drugs in the management of
glaucoma describe th e mechanisms of action of antibiotics, antivirais , and antifungal med icati ons: their indications, dosages, and side
effects discuss the anesthetic agents used in ophthalmology, th eir dosages and adverse effects discuss therapeut ic drugs on the horizon and in the process of bein g in trod uced into clinical practice in the imm ediate future
CHAPTER
1
Orbit and Ocular Adnexa
Orbital Anatom Orbital Volume The eyes lie within 2 bony orbits; the volu me of each adult orbit is slightly less than 30 cm 3 . Each orbit is pear-sha ped. with the optic ne rve representing the stem . The o rbital ent rance averages about 35 mm in height and 45 mm in width . The maximu m width is located abo ut I cm behind th e ante rio r orbital margin. In adu lts. the depth of the o rbit varies from 40 to 45 mm from th e o rbital ent rance to the orbital apex. Both race an d sex affect each of these measurements.
Bony Orbit Seven bones make up the bony orbit (Fig I - I ; see also Fig 1-5): I. 2. 3. 4. 5. 6. 7.
fro ntal zygo matic maxi lla (o r max illary bo ne) ethmoid (or ethmoidal bone) sphenoid lacr imal palatine
Orbital Margin T he orbital margin forms a quadr ilate ral spiral (Fig 1-2) whose superior margin is formed by the fro ntal bo ne. which is in terrupted mediall y by the supraorbita l notch. The med ial margin is formed above by the fro nta l bone and below by the posterior lacrimal crest of the lacrimal bone and the anterior lacrima l crest of the maxillary bone. The inferior margin derives from the maxi llary and zygomati c bones. Laterally. the zygomatic and frontal bones complete the rim.
Orbital Roof The orbital roof is for med from both the orbital plate of the frontal bone and th e lesser wing of the sphenoid bone (Fig 1-3). The fossa for the lacrimal gland. lying anterolaterally
5
6 • Fun dame nta ls and Principles of Opht halmology Frontal
Supraorbital foramen
~
Greater wing of sphenoid
~
/
....
Superior orbital ridge Supraorbital notch Superior orbital fissure Optic foramen
Inferior
orbital fissure
,,::::.--.+=_
Lesser wing of
sphenoid Ethmoid
;:;''l-~.--~~
Lacrimal bone and fossa
Zygomatic Maxilla
Infraorbital foramen
Figure 1-1
Frontal view of bony right orbit
(Reproduced with permission from Doxanas MT, Anderson RL.
Clinica l Orbital Anatomy. Baltimore: Williams & Wilkins; 1984.)
Posterior lacrimal
crest
Anterior lacrimal
crest
Figure 1-2 Right orbital margin. The orbital rim forms a quadri lateral spiral (arrows). Note the relationship between the anterior lacrimal crest of the maxillary bone and t he posterior lacrimal crest of the lacrimal bone. (Reproduced with permission from Doxanas MT. Anderson RL. Clin ical Orbital Anatomy. Baltimore: Williams & Wilkins; 7984.)
CHAPTER 1:
Orbit and Ocu lar Adnexa .
7
Supraort,;!a! foramen
Area of the lacrimal fossa
Cribra orbitalia
Orbital plate of frontal bone Ethmoidal sinuses
W'hlF--
Lesser wing of sphenoid bone Optic foramen
Superior orbital fissure
Figure 1·3
View from below, looking up into the orbital roof (superior orbital wall). (Reproduced with permission from Doxanas MT, Anderson RL. Clinical Orbital Anatomy. Baltimore: Williams & Wilkins; 1984.)
behind the zygo matic process of the frontal bone. resides within the orbital roof. MediaUy. the trochlear fossa is located on the fronta l bone approximately 4 mm from the orbital margi n and is the site of the pu lley of the superior oblique muscle. where the trochlea. a curved plate of hyali ne cartilage. is attached. Helveslon EM, Merriam WW, Ellis FD, Shellhamer RH , Gosli ng CG. The trochlea. A study of the anatomy and physiology. Ophthalmology. 1982;89(2): 124- 133.
Medial Orbital Wall T he medial wall of the o rbit is formed from 4 bones (Fig 1-4): I. 2. 3. 4.
fronta l process of the maxilla lacr imal bone orbital plate of the ethmoid lesser wing of the sphenoid
The ethmoidal bone makes up th e largest portion of the m edial wall. The lacrimal fossa is formed by the fro nta l process of the maxillary and the lacrimal bone. Below. the lacrimal fossa is cont inuous with the bony nasolacrimal cana l, which extends into the inferior meatus (the space beneath the in ferior turbinate) of the nose. The paper-thin structure of the med ial wa ll is reflected in its name, lamina papyracea .
Orbital Floor The floor of the orbit. which is the roof of the maxillary antrum. o r sinus. is composed of 3 bones (Fig 1-5): 1. maxilla
8 • Fundamentals and Principles of Ophthalmology
Orbital plate of frontal bone
Ethmoid
Ethmoidal foramina Lesser wing of sphenoid Optic foramen
Lacrimal bone
lacrimal fossa Fossa for inferior oblique muscle
Pterygopalatine foramen and fossa
Figure 1-4
Rig ht medial orbital wall as viewed from lateral side .
(Reproduced with permission from
Doxanas MT, Anderson RL Cl inica l Orbital Anatomy. Baltimore: Williams & Wilkins; 1984.)
Nasolacrimal canal Orbital aspect of maxillary bone
Infraorbital groove
Ethmoidal ~Y' sinuses
Palatine
Right orbita l floor and inferior orbital fissure. (Reproduced with permission from Doxanas MT; Anderson RL. Clinica l Orbita l Anatomy. Baltimore: Williams & Wilkins; 1984.)
Figure 1-5
CHAPTER 1:
Orbit and Ocular Adnexa.
9
2. palatine 3. orbital plate of the zygo matic The infraorbital groove traverses th e fl oor and descends anteriorly into a canal. It exits as
the infraorbital foramen. below the orbital margin of the maxillary bone. Arising from the floor ofthe orbit just lateral to the opening of the nasolacrimal canal is the inferior oblique muscle. the onl y extraoc ular muscle that does not originate from the orbi tal apex. The floor of the orbit slopes downward approximately 20 0 from posterior to anterior.
Lateral Orbital Wall The thickest and strongest of the orbital walls. the lateral wa ll of the orbit is formed from 2 bones (Fig 1-6): the zygomatic and the greater wing of the sphenoid. The lateral orbital tubercle (the Whilnall tuberc/e). a small elevation of the orbital margin of the zygomatic bone. lies approximately II mm below the frontozygo matic suture. This important landmark is the site of attachment for the following: • check li gament o f th e lateral rectus muscle
suspensory ligament of the eyeball (Lockwood suspensory ligament) lateral palpebral ligament apo neurosis of the levator muscle
Whitnaliliga ment
Orbital plate of frontal
Greater wing of sphenoid Superior orbital fissure
Zygomatic--+--
Inferior orbital fissure
Figure 1-6
Right lateral orbital wa ll as vi ewed from medial sid e. (Reproduced with permission from Doxanas MT. Anderson RL. Clinical Orbital Anatomy. Baltimore: Williams & Wilkins; 1984.)
10 • Fundamentals and Princi ples of Ophthalmology
Orbital Foramina, Ducts, Canals, and Fissures Foramina The optic foramen leads fro m the middle cranial fossa to th e apex ofthe orbit. It is directed fo rward, laterally, and somewhat downward and condu cts the optic nerve, the ophth almic artery, and sympathetic fibers fro m the carotid plexus (Fig 1-7). The optic foramen passes through the lesser wing of the sphenoid bone. The supraorbital foramen (in some people, it is a n otch instead of a foramen) is located at the m edial third of the sup erior margin of the orbit. It transmits blood vessels and th e supraorbital nerve, which is a branch of the ophthalmic division (VI) of cranial nerve V (eN V, trigeminal). The anterior ethmoidal foramen is located at the fron toeth moidal suture and transmits the anterior ethmoidal vessels and nerve. The posterior ethmoidal foramen lies at the junction of the roof and the
medial wall of the orbit and tran smits the posterior ethmoidal vessels and nerve through the fronta l bone. The zygomatic foramen lies in the lateral aspect of the zygomatic bone and contains zygomaticofaciaJ and zygomaticotempo ral branches of th e zygomatic nerve
and the zygomatic artery.
Nasolacrimal duct The nasolacrimal duct travels inferiorly fro m the lacrimal fo ssa into the inferior meatus
of the nose.
Infraorbital canal The infraorbital canal continues anteriorly from th e inf rao rbital groove and exits 4 mm below th e in fer ior orbital margin . where it tran smits the in fraorb ital nerve, which is a
branch of V, (the maxillary division of eN V).
Fissures The superior orbital fissure (Fig 1-8) is located between the greater and the lesser wings of the sphenoid bone an d lies below and lateral to the optic foramen. It is ab out 22 mm long and is spanned by the common tendinous rin g of the rectus muscles (annulus of Zinn). Above the ring, the su perior orbital fissure transm its the lacrimal nerve of eN V I frontal nerve of eN V I eN IV (trochlear) superior ophthalmic vein
Within th e ring or between the 2 heads of th e rectus muscle are the following: superior and inferior divisions of eN III (oc ulomotor) nasociliary branch of eN V I sympathetic roots of the ciliary ganglion eN VI (abducens) Occasionally, the inferior ophthalmic vein is below the ring. The inferior orbital fissure lies just below the superior fissure between the lateral wall and the floor of the orbit, givi ng access to the pterygopalatine and in ferote mporal fossae. Hence, it is close to the fora men rotundum and the pterygo id canal. It transmits the
CHA PTER 1:
Orbit and Ocu lar Adnexa . 11
A
c
E
Figure 1-7 Series of axial computed tomography (CT) scans. Each compares tissue and corresponding bone-window density through the optic canal (DC) and superior orbital fissure (SOF). The SOF passes above and below the plane of the OC and is commonly mistaken for the ~C . The OC lies in the same plane as the anterior clinoid processes (Ac/in) and may be cut obliquely in scans so that the entire canal length does not always appear in 1 section . Four differen t planes of section are shown in this series: A-B, Plane 1 is below the canal; C-D, Plane 2 is iust under the canal; E-F, Plane 3 is at the canal; G-H, Plane 4 is iust at the top of and above t he canal.
12 • Fundamentals and Principles of Ophtha lmology Lacrim al branch of
eN v 1
Superior orbital fissure
Frontal branch of CN V1 Superior ophthalmic ve in
Trochlear nerve (C N IV) Superior rectus Levator palpebrae superioris
:'-_ - Superior oblique
Medial rectus
~~~r;z:-t- Optic nerve Ophlhalmic artery Nasociliary branch of CN V1 Inferior rectus
Inferior ophthalmic vein Abducens nerve
Oculomotor nerve (C N III divisions 1 and 2)
(CNVI ) Figure 1-8
Structures pass ing through the supe rior orbital fissure. (From Bron AJ, Triparhl Re. Tflparhi
BJ. Wolff's Anatomy of the Eye and Orbit. 8th ed. London: Chapman & Hall; 1997.)
infrao rbital a nd zygomatic branches of eN V" an o rbital nerve from the pterygopalatine ganglion, and the inferior ophthalmic vei n. The inferior ophthalmic vein con nects with the pterygoid plexus before the vein drains into th e cavernous sinus.
Periorbital Sinuses The pe ri orbital sinuses have a close anatomical relationship with th e orbits, whi ch a re located o n either side of th e root of the nose. The medial walls of the orbits, whi ch border the nasal cavity ante riorly and the ethm o idal sin us (see Fig 1-3) an d th e sphenoid sinus posteriorly, are almost parallel. In th e adu lt, the lateral wall of each orbit for ms an angle of approximately 45" with the medial plane. The lateral walls border th e middle crani al, temporal, and pterygopalatine fossae. Superior to th e orbit are the ante rior cranial fossa and the fro ntal sin us. The maxi llary sinus and the palatine air cells are located inferio rly. The peri orbital sinuses offer a route for the spread of infection. Mucoceles occasionall y arise fro m the sinuses, extend into the adjacent orbit, and may confuse the clinician
CHAPTER 1:
Orb it and Ocu la r Adnexa. 13
in the d ifferential diagnosis of orbital tumo rs. The locations of the paranasal air sinuses and their relation to anatomical features of the skull are shown in Figures 1-9 and 1-10. Figure 1-9 also shows the distribut io n of pain originating from sinusitis. See BCSC Section 7, Orbit, Eyelids, and Lacrimal System, fo r further discussion. Doxanas M1: Anderson RL. Clinical Orbital Atlatomy. Baltimore: Williams & Wilkins; 1984:232. Z ide BM. Surgical Anatomy Around th e Orbit: The System of Zon es. Philadelphia: Lipp incott W illiams & Wilkins; 2005.
Cranial Nerves Six of the 12 cranial nerves (CN 11- VII ) d irectly innervate the eye and periocular tissues. Because certain tumors affecting CN I (olfactory) can give rise to important ophthalmic signs and symptoms, familiarity with the anatomy of this nerve is also important for the ophthalmologist. (C hapter 3 discusses the central and peripheral connections of CN I -V I!. ) See also BCSC Section 7, Orbit, Eyelids, and Lacrimal System.
Area of pain in frontal
Area of pain sphenoethmoidal
Figure 1-9 A , Bones of the face, showing regions where pa in is experienced in sinusitis . B, Pos itions of th e paranasal sinuses relative to the face. (Reproduced with permission from Snell RS, Lemp MA Clinical
A
Anatomy of the Eye. Boston: Blackwell; 1989.)
/ - Frontal sinus Ethmoidal
Cl::§~~~'Tsinuses Sphenoid sinus Maxi llary sinus
B
14 • Fundamenta ls and Principles of Ophthalmology Crista galli ,><-;z;;::~@:~~~s.?<- Frontal sinus Orbital plate ('/< of frontal bone ...fiL ':::'~~f~==j~Anterior ethmoidal sinus Middle ethmoidal sinus Lesser wing of sphenoid " Posterior ethmoidal Sinus
'(2.... ~~ ": _
A
r
•
,,
'
, \Sphenoid SinUS
~ sella turCica
" Anterior clinOid process
Dorsum sellae
Hiatus S8ml lunans Maxillary sinus
Inferior concha
B Figure 1-10 A, Position of the paranasal sinuses relative to the anterior cranial fossa , in axial view. B, Coronal section through the nasal cavity, showing the ethmoidal and maxillary sinuses . (Reproduced with permission from Snell RS, Lemp MA. Cl inical Anatomy of the Eye. Boston: Blackwell; 1989.)
Ciliary Ganglion The Ciliary ganglion is located approximately I cm in front of the ann ul us of Zinno on the lateral side of the ophthalmic artery between the optic nerve and the lateral rectus muscle (Figs l- ll . I- 12). It rece ives 3 roots: I. A long sensory root arises from the nasoc il ia ry branc h ofCN V" It is 10- 12 mm
long and contains sensory fibers from the cornea, the iris, and the ciliary body. 2. A short motor root ar ises from the infer ior di vision of CN Ill . whic h also supplies the inferior oblique muscle. T he fibers of the motor root synapse in the ga nglion. and the postga nglionic fibers carry parasympathetic axons to supply the iris sphincter. 3. T he sympathetic root comes from the plexus around the internal carotid artery. It enters the orbit through the superior orbital fiss ure within the tendino us rin g. passes through the ciliary ganglion without synapse. and inne rvates ocular blood vessels and possibly th e d ilator muscle.
CHAPTER 1: Orbit
and Ocular Adnexa. 15
Sensory root from nasoci liary (Vd Sympathetics from carotid artery
Parasympathetics from nerve to inferior
oblique (I II)
ciliary nerves
Figure 1·11
Contributions to the ciliary ganglion. (Reproduced with permission from Doxanas MT, Ander· son RL. Clinical Orbital Anatomy. Baltimore: Williams & Wilkins; 7984.)
Nasoci liary nerve
Levator Superior division
SR
\
Cranial nerve III
Short ciliary nerves Iris sphincter Ciliary muscle
~----____
MR
Inferior iii
IR
Motorroc't =::f=::::============::~
10
Figure 1· 12 Cranial nerve III and ciliary ganglion . 10 = inferior oblique, IR = inferior rectus, MR = medial rectus, SR = superior rectus . (lfIusrraflOn by Sylvia Barker}
Branches of the Ciliary Ganglion Only the parasympathetic fibers synapse in the ciliary ganglion. The sympathetic fibers are postganglion ic fro m the superior cervical ganglion and pass through it without synapse. Sensory fibers from cell bodies in the trigeminal ga nglion carry sensation from the
16 • Fundamental s a nd Principles of Ophthalmology eye. o rbit. and face. Together. the nonsynapsing sympathetic fibers. the sensory fibers. and th e myelinated. fas t-conducting postganglionic pa rasympatheti c fibers form the sho rt ci li ary nerves.
Short Ciliary Nerves Two groups. totali ng 6-10 short ciliary ne rves. ar ise from the ciliary gan glion (see Figs I - II . 1- 12). T hey travel on both sides of the optic nerve and. together with the long ciliary nerves, pie rce th e sclera aro und the opt ic ne rve. They pass anterio rl y betwee n th e
cho roid and the sclera into the ciliary m usc;le. where they form a plex us that supp lies the cornea. the ciliary body. and the iris.
Extraocular Muscles There are 7 extraoc ular muscles (Figs 1- 13 through 1- 16) : 1. medial rec tus 2. late ral rectus 3. superior rec tus 4. inferio r rectus
5. superi or oblique 6. inferior oblique 7. levator palpebrae supe rio ri s
Extraocular Muscle Insertions T he 4 rectus m uscles insert anterio rl y on the globe. Starting at the medi al rectus and then proceeding to th e in fe rio r rec tus, late ral rectus. and superio r rectus, the muscle in sertio ns
lie progressively farther from the li mbus. An imag inary curve drawn th rough these in sertio ns creates a spiral. which is called the spiral of Til/aux (Fig 1- 17). The relationship between th e muscle insertions and th e o ra serrata is cli nicall y im portant. A misdirected suture passed through the insertion of th e superio r rec tus muscle could perforate th e retina.
__Trochlea -","~ Supe rior oblique lendOll - --k-'
Inferior oblique muscle
---"_ ----J..:
- Superior oblique muscle ___--.-- Superior rectus muscle Medial rectus muscle d- " "lnu"J. of Zinn
0---"--
rectus
Figure 1· 13 Ext raocular mu scles, lateral composite view. (Reproduced With permiSSion from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. Philadelphia: Saunders, 1994.)
CHAPTER 1:
Orbit and Ocular Adnexa.
17
Levator palpebrae superioris muscle
Superior oblique tendon Trochlea
Superior rectus tendon
Medial rectus tendon
Lateral rectus tendon
Inferior rectus tendon
Figure 1-14
Extraocular muscles, frontal composite view.
(Reproduced with permission from Dutton JJ.
Atlas of Clinical and Surgica l Orbital Anatomy. Philadelphia: Saunders; 1994.)
Levator palpebrae .::===;;;;;;:/,....,""'~~----- superioris muscle
Trochlea - - - .:
Superior oblique ----"~"""" muscle
1~FT'i----- superior rectus muscle
Lateral rectus muscle
Medial rectus muscle
Annulus of Zinn - - - - - /
" - - - - Inferior oblique muscle
:.:.._ _...........ffL-______ lnferior rectus muscle Figure
1-15
Extraocular muscles, frontal view, left eye, with globe removed.
(Reproduced with
permission from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. Philadelphia: Saunders.' 1994.)
'------'It- ,~n,'u IrJS of Zinn
!O----t.f- ""e,',or rectus muscle Medial rectus mL,sele Superior oblique tendon
Figure 1-16
Lateral rectus muscle
,.r.'---- Superior rectus tendon
Extraocul ar muscles, superior composite view. (Reproduced with permission from Dutton
JJ. Atlas of Clinical and Surgical Orbita l Anatomy. Philadelphia: Saunders; 1994.)
The superior oblique muscle, after passing through the trochlea in the superior nasal orbital rim, inse rts onto the sclera superiorly, under the insertion of the superior rectus. The inferior oblique muscle inserts onto the sclera in th e posterior inferior temporal quadrant (see Fig 1- 17; Table 1- I),
18 • Fundame nta ls and Principles of Ophthalmology
Superior reclus----,
Figure 1-17 The media l rectus tendon is closest to th e
limbus, and the superior rec-
Spiral of
tus tendon is fa rthest from it. By conn ecting the insertions of the tendons beginning with the med ia l rectus, then
th e in ferior rectus, then th e
Tillaux Medial
rectus
rectus
lateral rectus, and fin ally the superior rectus, a spiral is obtai ned. This is called the spiral of Til/aux. (Illustration by Christine Gralapp,)
"~~~~~~iiii- 'nferlor rectus Inferior Obll'iqu" _
_ ---./
Extraocular Muscle Distribution in the Orbit Figures 1- 15 and 1- 16 show the arran ge ment of the extraocular muscles within the orbit. No te the relatio nsh ip between th e obli que extraoc ul ar muscles and the superior, med ial, and inferior rectus muscles.
T he location of the extraoc ular muscles within the orbit and their relationship to surro unding nerves and bone are illustrated in coronal, cross-sectio nal views (F igs 1- 18,
1- 19) . Longitud inal, axial views are shown in Figures 1-20 and 1-2 1.
Extraocular Muscle Origins The annul us of Zinn consists o f superio r and infer io r orbital tendons and is the origin of th e four recti muscles. The upper tendon gives rise to all o f the superior rectus muscle, as
well as port ions of the lateral and med ial rectus muscles. The inferior tendon gives r ise to all of the inferior rectus muscle and portions of the medial and lateral rectus muscles, The levator palpebrae superi ori s muscle ar ises from the lesser wing of the sphenOid bone, at th e apex o f th e o rbi t, just superior to the annulus of Zinn o
The superi or oblique muscle originates from the periosteum of the body of the spheno id bone, above an d medial to the optic fo ramen. The inferior oblique muscle originates
anterio rl y, from a shallow depression in the orbital plate of the maxillary bone, at the an tero medial corner of the o rbital floor, near the lac ri ma l fossa . From its o rigin, the inferior
oblique muscle then extends posteri orl y, laterall y, and superio rly to insert into the globe.
Blood Supply to the Extraocular Muscles The in ferio r and superior muscular bran ches of th e ophthalmic artery. lacrima l artery.
and infraorbital artery supp ly the extraoc ular muscles. The lateral rectus m uscle is supplied by a Single vessel derived fro m the lacrimal artery; the other rectus m uscles receive
CHAPTER 1: Orbit and Ocular Adnexa .
19
Tabl e , -, Comparison of Extraocular Muscles Muscle
Origin
Insertion
Blood Supply
Size
Medial rectus
Annulus of Zinn
Inferior rectus
Annu lus of Zinn at orbital apex
Medially. in horizontal meridian 5.5 mm from limbu s Inferiorly, in ve rt ical meridian 6.5 mm from limbu s
40.8 mm long; tendon : 3.7 mm long. 10.3 mm wide 40 mm long; tendon: 5.5 mm long, 9.8 mm wide
Lateral rectus
Annulus of Zinn spanning th e superior orbital fissure Annulus of Zinn at orbital apex
Laterally, in hori zo ntal meridian 6.9 mm from limbu s Superiorly, in ve rtical meridian 7.7 mm from limbus
Inferior muscular bran ch of ophthalmic artery Inferior muscu lar branch of op hthalmic arte ry and infraorbital artery Lacrimal artery
Superio r oblique
Medial to optic foramen , between annu lus of Zinn and peri orbita
Inferior oblique
From a depression on orbita l floor near orbital rim (maxi ll a)
To trochlea , through pulley, at orbita l rim , then hooki ng back under superior rectus, inserting posterior to center of rotation Posterio r inferior temporal quadrant at level of macula; posterior to center of rotation
Superior rectus
Superior muscular branch of ophthalmic artery Superior muscular branch of ophthalmic artery
Inferior branch of ophthalmic artery and infraorbital artery
40.6 mm long; tendon: 8 mm long, 9.2 mm wide 41 .8 mm long; tendon: 5.8 mm long, 10.6 mm wide 40 mm long; tendon: 20 mm long, 10.8 mm w ide
37 mm long; no tendon: 9.6 mm wide at inserti on
Figure ' -'8 Location of the plane of sec tion shown in Figure 1-19. (Reproduced with permission from Outron JJ. Atlas of Clinical and Surgical Orbital Anatomy. Philadelphia: Saunders; 1994.}
20 • Fundamentals and Principles of Ophthalmology
Levator palpebrae superioris muscle --;"'illf.;:;:~t!::t~~-4 Ophthalmic artery
Superior oblique
mlJscle -T" ----..~.
Medial rectus muscle - -m/
Supraorbital nerve Supraorbital artery Superior rectus muscle Superior ophthalmic vein Optic nerve
~;;;'.,JI--
Lateral rectus muscle
Inferior rectus muscle
...r----,:It.l~.-!a-
Infraorbital
canal
Figure 1-19 Coronal section through the ce ntral orbit just post erior to the globe. (Reproduced with permission from DuHon JJ. Atlas of Clinica l and Surgical Orbital Anatomy. Philadelphia: Saunders; 1994.)
Figure '-20 Location of the pla ne of section sh own in Figu re 1-21 . (Reproduced wi th permission from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. Phlfadelphia: Saunders; 1994.)
Anterior arm of medial canthal tendon Posterior arm of medial canthal tendon
Intraocular lens Ciliary body Sclera Lateral canthal tendon
Medial rectus muscle Superior medial vortex vein Optic nerve Ethmoidal sinus Ophthalmic artery
Lateral rectus muscle Posterior lateral ciliary artery
Sphenoid sinus Optic canal
Figure 1-21 A xial se ct ion t hrough the m ido rb it at the level of th e optic nerve. T he t hird portion of the ophthalmic artery c rosses the nerve in th e posterior orb it. (Reproduced with permiSSion from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. Philadelphia: Saunders; 1994.)
CHAPTER 1:
Orbit and Ocular Adnexa . 21
2 anterior cil iary arteries that commun icate with the major arterial circle of the ciliar y body via perforating scleral vessels. Vascu lar supply and ve nous drainage of orbital stru ctures are discussed later in this chapter.
Innervation of the Extraocular Muscles The lateral rectus muscle is innervated by CN VI (abducens); the superior oblique muscle is innervated by CN IV (trochlear); the levator palpebrae superior is. superior rectus. medial rectus. inferior rectus. and inferior oblique muscles are innervated by CN III. Cranial nerve III (oculomotor) has a superior and inferior division: the upper division innervates the levator palpebrae superioris and superior rectus muscles, and the lower division innervates the medial rectus, inferior rectus, and inferior oblique muscles.
Fine Structure of the Extraocular Muscles The ratio of nerve fibers to eye muscle fibers in the extraocular muscles is very high ( I:31:5) compa red to the ratio of nerve axons to muscle fibe rs for skeletal muscle (I :50- 1: 125). thereby allowing precise control of ocu lar movements Crable 1-2). The fibers of the extraocu lar muscles are a mixture of the slow. to nic type (Felderstr uktur) and the fast. twitch type (Fibrillenstruktur). The tonic-type muscle fibers are unique to extraocular muscles. Smaller than twitch-type fibers. they have a slow. smooth contraction and tend to be located more superficially in the muscle. nearer the orbital wall. The tonic-type fibers are innervated by multiple grapeli ke nerve endin gs (e /1 grappe) and are useful for smooth pursuit. The twi tch-type fibers are more similar to skeletal muscle fibers. Larger than tonic fibers and located deeper in the muscle, they have a fast contraction and platelike nerve endings (e /1 plaque). Twitch fibers aid in rapid saccadic movements of the eye. The fibers of the extraocu lar muscles ca n be classified further by contract ile properties. histochemical profile. an d myosin content.
Table 1-2 Extraocular Muscles
Myofibrils Sarcoplasm Sarcomere T-system Z-lin e M -line Nuclei Innervation Neuromuscular junction Synaptic vesic les Acetylcholine
Fast. Twitch Fibers !Fibrillenstruktur)
Slow. Tonic Fibers (Fe lderstruktur)
Well defined Abundant Well developed Regular Straight Wel l marked Located peripherally Thi ck; heavily myelinated En plaque (sing le) Agranu lar Twitch contraction
Poorly defined Sparse Poorly developed Absent, or aberrant Zigzag course Absent Located ce ntrall y or eccentrically Thin En grappe (grapelike) G ranu la r/agran ular Tonic contraction
22 • Fundamentals and Principles of Ophthalmology Porter JD, Baker RS, Ragusa RJ. Brueckner JK. Extraocul:lf muscles: basic and cl in ical aspects of structu re and function. SlIrv Ophtha/mol. 1995;39(6):4 51- 484. Spencer FR. Porter ID. Structural organ ization of the ex traocular muscles. In: Buttner- Ennever
lA . ed. Neuroanatomy of the Oculomotor System. A msterdam : Elsevier; 1988.
E elids The palpebral fissure is the exposed zone between the upper and lower eyelids (Fig 1-22). Normally. the adult fissure is 27- 30 mm long and 8- 11 mm wide. The upper eyelid. more mobile than the lower. can be raised IS mm by the action of the levator muscle alone. If the frontalis muscle of the brow is used. the palpebral fissure can be widened an additional 2 mm. The levator muscle is innervated by eN Ill. See also BeSe Section 7. Orbit. Eyelids.
and Lacrinw/ System.
Anatomy Although small in surface area. the eyelid is complex in its structure and function. When describing the anatomy of the upper eyel id. it is helpful to divide it into distinct segments from the dermal surface inward. These segments are the skin. the eyelid margin. the subcutaneous tissue, the orbicularis muscle. orbital septum , levator muscle. Muller mu scle, tarsus. and conjunctiva (Figs 1-23 th rough 1-26).
Skin The eyelid skin. the thinnest in the body. contains fine hairs. sebaceous glands. and sweat glands. A superior eyelid fold is present near the upper border of the tarsus. where the
27-30 mm
Figure 1·22
Landmarks of the exte rnal eye. (Illustra tion by Christine Gralapp.)
CHAPTER "
Orbital septum
Orbit and Ocular Adnexa .
23
Fat Levator ¥OUer muscle - Palpebral glands (of
Krause) Orbicularis
ocull - - II'-'
Figure '-23
Cross sec-
tion of upper eyelid. Note position of cilia, tarsal gland orifices. Tarsal glands (of Wollring)
Tarsal (meibomian) gland in the tarsal plate
t Mlarginal arcade
and mucocutaneous border. (From Bran AJ. Tnpathi Re, Tflpalhi 8J. Wolff's Anatomy 01the Eye and Orbit. 8th ed. London: Chapman & Hall; tOOl.)
Eyelash, glands of Zeis and Moll al its follicle
levator aponeurosis establishes its first insertional attachments. (In many individuals of Asian descent, there are rew attachments or the levator aponeurosis to the skin near the upper tarsal border, and the superior eyelid fold is minimal or absent.) The aponeurosis forms its firmest attachments on th e anterior aspect of the tarsus aboul 3 111111 superior to the eyelid margin.
Margin T he eyelid margin contains several important landmarks (F ig 1-27). A small opening, the punctum of the canaliculus, presents mediall y at the summit of each lacrimal papUla. The superior punctum, normall y hidden by slight internal rotation, is located more medially. The inferior punctum is usually apposed to the globe and is not normally visible without eversion. Along the entire length or the free margin of the eyelid is the delicate gray line (or intermarginal sulcus), corresponding histologicall y to the most superficial portion of the orbicularis muscle, the muscle or Riolan, and to the avascular plane of the lid. Anterior to this line, the eyelashes (o r cilia) arise, and behind it are the openings of the tarsa l (or meibomian) glands just anterior to the mucocutaneous junction. The eyelashes are arranged in 2 or 3 irregular rows along the anterior dermal edge of the eyelid margin. The)' are usually longer and more numerous on the upper eyeHd than on the lower one. The margins contain the glands oj Zeis, which are modified sebaceous glands associated with the cilia, and the glands oj Moll, apocrine sweat glands of skin (Table 1-3).
Subcutaneous connective tissue T he loose connective tissue of the eyelid contains no fat. Blood or other fluids can accu mulate beneath the skin and result in rapid and dramatic swelling of the lids.
24 • Fund amenta ls and Principles of Ophthalmology
A
L-_~-"'
_
__
B Figure 1-24 A, Photo shows the eye with the periorbital skin removed and orbicularis oculi muscle, which is innervated by eN VII, exposed . This muscle acts as an antagonist to the levator palpebrae superioris muscle, which is innervated by eN 1\1. The orbicularis muscle is
divided into the palpebral and orbital (0) portions. The palpebral portion is further divided into pretarsal (PT) and preseptal (PS) portions. B, Diagram depicts the arrangement of muscle fibers of the orbicularis muscle . (Reproduced with permission from Zide 8M,
Jelks GW Surgical
Anatomy of the
Orbit. New York: Raven; 1985.1
Orbicularis oculi muscle The orbicularis oculi muscle is arranged in severa l concentric bands around th e palpebral fissure and can be subd ivided into orbital and palpebral parts (see Fig 1-24). T he muscle fibers are short and are connected by myomyous junctions. Of all the fac ial muscles, the orbicularis muscle has fibers with th e smallest diameter. Innervation is by the facial nerve (eN Vll) . and end plates are arranged in clusters over th e entire length of th e muscle. This
Levator palpebrae
Whitnai l ligament Levator aponeurosis Medial horn
--~~~~~~~~~~-1 superioris muscle
Fascial slips to orbicularis muscle Lateral horn
Figure '-25 A. The upper and lower tarsal plates and their attachments to the levator aponeurosis and to the Whitnaliligament. B. The 3-dimensional organization of the upper eyelid. For convenience, the upper eyelid may be divided into anterior and posterior lamellae. The anterior lamella consists of the skin and orbicularis muscle and its associated fascial and vascular structures. Note the marginal artery (lower arrow) approximately 3 .0-3 .5 mm above the eyelid marB gin . The posterior lamella cons ists of the levator aponeurosis (LI. tarsus (blue). Muller muscle (M). and conjunctival lining (C). At a variable height above the superior edge of the tarsus, the orbital septum (OS) forms the anterior border of the preaponeurotic fat space. The peripheral arterial arcade is situated (upper arrow) at the level of the superior edge of the tarsus, posterior to the levator aponeuro sis within the so-called pretarsal space. The levator musc le usually becomes aponeurotic at th e equator of the globe in the superior orbit. The aponeurosis courses anteriorly to insert onto the lower two thirds of the anterior tarsa l plate. The levator muscle provides origin to the Muller muscle, the nonstriated, sympathetica lly innervated elevator of the upper eyelid, which inserts into the superior edge of the tarsus and into the conjunctiva of the superior fornix. The superior transverse ligament of Whitnall (W) is noted as a fascial condensation along the upper aspect of the levator muscle. This ligament attaches to the trochlear fascia medially and the fasc ia of the orbital lobe of the lacrimal gland laterally. (Par( A reproduced WI(h permission from Dutton JJ. Atlas of Clinical and Surgical Orbital Ana tomy. Philadelphia: Saunders; 1994; part 8 reproduced wi th permission from Zide 8M, Jelks GW Surgical Anatomy of the Orbit. New York: Raven; 1985.)
26 • Fundamental s and Principles of Ophthalmology
Figure '-26
The lacrimal secretory system : (1) The conjunctival and ta rsal mucin-secreting
goblet cells (green) produce a mucoprotein layer covering the epithelial surface of the cornea and conjunctiva. (2) The accessory lacrimal exocrine glands of Krause and Wolfri ng are present in th e subconjunctival tissues (blue) and contribute to th e aqueous layer of th e precorneal tear film. (3) Oil-producing meibomian glands and palpebral glands of Zeis and Moll (pink). The orbital lobe of the lacrimal gland (Lo) and the palpebral lobe of the lacrimal gland (Lp) are separated by the lateral horn of the levator palpebrae superioris (LA). The tear ducts (arrow) from the orbital portion traverse the palpebral portion. (Reproduced with permission from Z/de 8M, Jelks GW Surgical Anatomy of the Orbit. New York: Raven; 1985.J
Figur. 1-27
Anatomical landmarks of the
lower eyelid margin . The gray line. or intermarginal sulcus, is seen between the bases
of the cilia and the orifices of the meibomian glands. The lower eyelid has been slightly everted in this view to clearly expose the inferior lacri mal puncta . (fllustration by Christine Gralapp.)
arrangement may influence the action of botulinum A toxin , which is used in the treat ment of blepharospasm. lander T, Wi rtschafter JO. Me loon lK. O rbicularis oculi muscle fibers are relativel y short and heterogeneous in length . In vest Ophthnlmol Vis Sci. 1996;37(9): 1732- 1739.
The orbital part inserts in a complex way into the medial canthal tendon and into other portions of t he orbital rim and the corrugator supercilii muscl e. The orbital part acts like a sphincter and functions solely as a volu ntary muscle.
CHAPTER 1:
Orbit and Ocular Adnexa.
27
Table '-3 Glands of the Eye and Adnexa Glands
location
Secretion
Content
Lacrimal
Orbita l gland Palpebral gland Plica, caruncle Eye lid Eyelid Tarsus Follicles of cilia Eyelid, caruncle Eye lid Conjunctiva Plica, caruncle
Exocrine Exocrine Exocrine Exocrine Exocrine Holocrine Holocrine Holocrine Eccrine Holocrine Holocrine
Aqueous Aqueous Aqueous Aqueous Aqueous Oily Oily Oily Sweat Mucus Mucus
Accessory lacrimal Krause Wolfring Meibomian Zeis Mol l Goblet cell
The palpebral part of the orbicularis functions both voluntarily and involuntarily in spontaneous and reflex blinking. The preseptal and pretarsal portions unite along the superior palpebral furrow. The pretarsal orbicularis muscle is firmly adherent to the tarsus, with a portion of it attaching to the anterior lacrimal crest and the posterior lacrimal crest (sometimes called the Horner muscle), and plays a role in the drainage of tears (Fig 1-28). Orbicularis fibers extend to the eyelid margin, where there is a smaLl bundle of striated muscle fibers caLled the muscle of Riolan. Disinsertion of the lower eyelid retractors from the tarsus may resu lt in laxity of the lower eyelid, followed by spastic entropion, an inward turning of the eyelid margin.
Orbital septum A thin sheet of connective tissue called the orbital septum encircles the orbit as an extension of the periosteum of the roof and the floor of the orbit (see Fig 1-23). It also attaches to the anterior surface of the levator muscle. Posterior to the orbital septum is the orbital fat. In both the upper and lower eyelids, the orbital septum attaches to the aponeurosis. The orbital septum thus provides a barrier to anterior or posterior extravasation of blood
Deep head of superior preseptal orbicularis +~ muscle
Lacrimal sac + -1--_ Inferior preseptal orbicularis muscle
Figure 1-28
....11<
+-\-__+-
Superior canaliculus Superior pretarsal orbicularis muscle Superior ampulla
Inferior canaliculus
Lac ri mal drainage system and the orb icu laris muscle. (Reproduced wirh permission from Dutton JJ. Atlas of Cl inical and Surg ical Orbital Anatomy. Philadelphia: Saunders; 1994.)
28 • Fundame nta ls and Principles of Ophtha lmol ogy or the spread of inflammation. T he intermuscular orbital septa can be identified in coronal MRI studies with fat suppression and gadolinium enhancement. Superiorly, the septum is attached fir mly to the periosteum of the superi or half of the orbital margin. It passes mediall y in front of the trochlea and continues along the medial margin of the o rbit, alo ng the margin of the frontal process of the maxillary bone, and on to the inferior margin of the orbit. Here, the septum also delimits the lateral spread of edema, inflammation , o r blood trapped an terio r to it and appears clinically as a dramatic barrier to these processes.
Levator muscle T he levator palpebrae sLiperioris mLiscle originates fro m the lesser wing of the sphenoid bone (see Fig 1-25). The body of the levator muscle overlies the superior rectus as it travels anteriorly toward the eyelid. The Whitnallligament resu lts from a condensati o n of tissue su rro undin g the superi or rectus and levator muscles. Near the Whitnailligamenl, the levator muscle changes directio n from horizontal to more ve rtical , and it divides anteriorly into th e aponeurosis and posteriorly into the superior tarsal (Muller's) muscle. The apo neurosis inserts into the anterior surface of the tarsus and passes by the medial and late ral horns into the canthal tendons. The fibrous elements of the apone urosis pass through the orbicularis muscle and insert subcutaneously to produce the superior eyelid fold. The apo neurosis also inserts into the trochlea of the superior oblique muscle and into the fibrous tissue bridging the suprao rbital notch. Attachments also ex ist with the conjuncti va of the upper fornix and with the orbital se ptum. The levator muscle and tendon are 50-55 mm long. The muscle, which elevates the upper eyelid, is 40 mm long and is innervated by the superior division of eN 11 1.
Miiller muscle The Miiller Illu scle is a smooth (nonstriated), sympatheti call y innervated muscle that originates from the undersurface of the levator muscle in the upper eyelid. A similar smooth muscle arise from the capsulopalpebral head of the inferior rectus in the lower eyel id . The Muller muscle attaches to the upper bo rder of the upper tarsus and to the conjunctiva of the upper fornix. The capsulopalpebral muscle, which is much weaker tha n the Muller muscle, attaches to the lower border of the lower tarsus.
Tarsus The tarsal plates consist of dense connective tissue, not cartilage. They are attached to the orbital margin by the medi al and lateral palpebral ligaments. Although the upper and lower tarsal plates are similar in length (29 mm) and in thickn ess ( I mm ), the upper tarsus is almost 3 times as wide ve rtically (11 mm ) as the lower ta rsus (4 mm ). The ta rsal (meibol1lian) glands are mod ified holoc rine sebaceous glands that are oriented vertically in parallel rows thro ugh the tarsus (Fig 1-29). Their d istrib ution and number wit hin the eyelid can be observed by infrared transillumination (Fig 1-30) of the eyelid. A Single row of30-40 meibom ian oriAces is present in the upper eyelid, but there are o nl y 20-30 o rifices in the lower lid. Oil from these orinces forms a reservoir on the skin of th e lid margi n and is spread onto the lear Aim with each blink.
CHAPTER 1:
Orbit and Ocular Adnexa. 29
Figure , ·29
Posterior view of
the eyelids with the palpebral fissure nearly closed. Note the
ta rsal glands with their short ducts and orifices. The palpebral
Lateral
conjunctiva has been removed to show th e tarsa l glands in situ.
I
of eye
(Reproduced with permission from Snell
Anterior margin of lid Posterior margin of lid Lower lid
RS. Lemp MA. Clinical Ana tomy of the Eye. Boston: Blackwell; 1989.)
Figure '-30 Distribution of the meibomian glands in the lower eye lid, as reveal ed by in frared transillumination of the eyelid. The glands appear as dark gray linea r structures. (Courtesy of William Mathers, MD.}
The hair bulbs of the cilia are located anterior to the tarsus and the meibomian gland orifices. Misdirection in the orientation of the eyelashes (trichiasis) or aberrant growth through the orifices of the meibomian glands (distichiasis) may occur as either a congeni tal or an acquired defect; occasionally these defects are hereditary.
Conjunctiva The palpebral conjuncti va is a transparent vascularized membrane covered by a nonkeratinized epit helium that lines the inner surface of th e eyelids. Continuous with the conjun ctiva l fornices (c ul -de-sacs), it merges with the bulbar conjunctiva before termi-
nating at the limbus (F ig 1-3 1). The conjunctiva is discussed in more detail later in this chapter.
Vascular Supply of the Eyelids The blood suppl y of the eyelids is derived from the facial system, which arises from the external carotid artery, and the orbital system, which originates from the internal carotid
artery along branches of the ophthalmic artery (Fig 1-32). The superficial and deep plexuses of arteries provide a vast blood supply to the upper and lower eyelids. The facial artery becomes the angular artery as it passes upward, forward, and lateral to the nose, where it serves as an important landmark in dacryocystorhinostomy (DCR). The marginal arterial arcade is located 3 mm from the free border of the eyelid, just above the ciliary follicles. It is either between the tarsal plate and the orbicularis or within
30 • Fundamentals and Principles of Ophthalmology
Figure '·31
The conjunctiva con-
sists of bulbar (red), forniceal (black), and palpebral (blue) portions.
Supratrochlear artery
- -- -1
Superior marginal ----l,,~ arterial arcade Medial palpebral --i.~~" artery Dorsal nasal artery Angular artery
Supraorbital artery Superior peripheral arterial arcade Orbital branch of supe rficial temporal artery Superficial temporal artery Lateral palpebral artery
Inferior marginal arterial arcade
Transverse facial artery
Facial artery ~-+"---I--~
Figure '-32
Periorbital and eyelid arteries, frontal view. (Reproduced with permission from Dutton JJ Atlas of Cl inical and Surgical Orbita l Anatomy. Philadelphia: Saunders; 1994.)
the tarsus. A smaller peripheral arcade runs along the upper margin of the tarsal plate within the Muller muscle. The venous drainage of the eyelids can be divided into 2 portions: a superfiCial, or pretarsal, system. which drains into the internal and external jugular veins; and a deep, or posttarsal, system, which flows into the cavernous sinus.
Lymphatics of the Eyelids Lymphatic vessels are found in the eyelids and conjunctiva, but neither lymphatic vessels nor nodes are present in the orbit. Lymphatic drainage from the eyelids parallels the
CHAPTER "
Orbit and Ocular Adnexa.
31
course of the veins (Fig 1-33). Two groups of lymphatics exist: (1) a medial group that drains into the submandibular lymph nodes and (2) a lateral group that drains into the superficial preauricular lymp h nodes. Clinically, swelling of the lymph nodes is a diagnostic sign of several extern al eye infections, including adenoviral conjunctivitis and Parinaud oculoglandular syndrome.
Accessory Eyelid Structures
Caruncle The caruncle is a small , fl eshy, ovoid structure attached to the inferomedial side of the plica semilunaris (see Fig 1-22). As a piece of mod ified skin , it contains sebaceous glands and fine, colorless hairs. The surface is covered by nonkeratinized, stratified squamous epithelium.
Plica semilunar;s The plica sem ilunaris is a na rrow, highl y vasc ular, crescent-shaped fold of the conjunctiva located lateral to and partly under the caruncle (see Fig 1-22). Its lateral border is free and separated from the bulbar conjunctiva, which it resembles histologically. The epithelium of the plica is rich in goblet cells. The plica's stroma contains fat and some nonstriated muscle. The plica is a vest igia l stru cture analogous to the nictitat ing membrane, or third
eyelid, of dogs and other an imals.
Superficial parotid lymph nodes
Submandibular lymph nodes
Figure 1·33
The lymphat ic drainage of the eyelids.
MA. Clinical Anatomy of the Eye. Boston. Blackwell; 1989.)
(Reproduced WIth permiSSion (rom Snell RS. Lemp
32 • Fundamenta ls and Principles of Ophthalmology
lacrimal Gland and Excretory System For further discuss ion of the lacrimal system, see BeSe Section 7, Orbit, Eyelids, and Lacrimal System.
Lacrimal Gland The main lacrimal gland is located in a shallow depression within the orbital part of the frontal bone. The gland is separated from the orbit by fibroadipose tissue and divided into 2 parts by a lateral expansion of the levator aponeurosis (Fig 1-34) . When the upper eyelid is everted, the smaller, palpebral part can be seen in the superolateral conjunctival fornix. An isthmus of glandular tissue occasionall y exists between the palpebral lobe and the main orbital gland. A variable number of thin-walled excretory ducts, blood vessels, lymphatics, and nerves pass from the main orbital gland into the palpebral lacrimal gland. The ducts con tinue downward, and about 12 of them empty into the conjunctival forn ix approximately 5 mm above the superior rnargin of the upper tars us. Because the lacrimal excretory ducts
pass through the palpebral portion of the gland, biopsy of the lacrimal gland is usua lly performed on· the main part to avoid sacrificing the ducts.
The lacrimal glands are exocrine glands that produce a serous secret ion. The body of each gland contains 2 cell types (Fig 1-35): l. acinar cells, which line the lu men of the gland 2. myoepithelial cells, which su rround the parenchyma and are covered by a base-
ment membrane
The lacrimal artery, a branch of the ophthalmic artery, supplies the gla nd. The lacri mal gland receives secretomotor cholinergic, vasoactive intestinal polypeptide (VIP)-
....""',
Common canaliculus
Lacrimal sac
J
Lacrimal gland, orbital lobe Lacrimal gland, palpebral lobe
Lacrimal duct
Figure 1-34
Lacrimal system. (Reproduced with permission from Dutton JJ. Atlas of Clinica l and Surg ical Orbital
Anatomy. Philadelphia: Saunders; 7994)
CHAPTER 1:
Figure 1·35
Orbit and Oc ula r Adne xa . 33
Higher magnification of lacrimal gland lobules. Note that the acinar cells forming
the lobules are surrounded by myoepithelial cells that contain flattened nuclei IH&Ex64) . (Courtesyof Thomas A Wemgeisl. PhD, MD.)
e rg ic. and sympath eti c nerve fibers in add itio n to a senso ry inn e rvation via the lac rimal nerve (CN V I)' Cyclic adenosine mo nophosphate is the second messenge r fo r VIP and ~-a drenergic
sti mulation of the gland; choli nergic stimulati o n acts thro ugh an inositol 1,4,5-triphosphate-activated protein kinase C. T he glan d also conta ins ai-adrenergic recepto rs. Ex tremely complex, the gland's neuroa natomy governs both re flex and psychogen ic sti mulatio n. See BCSC Section 5, Neu/'O-Ophthalmology.
Accessory Glands The accessory lacrimal glands of Krallse and Wolfri ng are located at the proximal lid borders o r in the fo rn ices and are cyto logica ll y id entica l to th e main lacri_mal g land , rece ivi ng a s imilar in ne rva tio n. They account fo r abo ut 10% of the total lacrimal secreto ry mass.
lacrimal Excretory System T he lacri mal drain age system includes the upper and lower puncta, the lac rimal can aliculi, the lacri mal sac, and the naso lacri mal d uct (see Fig 1-28; Fig 1-36). The lacri mal pap il lae are loca ted at the extre me nasal bord e r of th e eyelids at their juncti o n w ith the inne r
canthus. T he pu ncta are directed posterio rl y into the tea r lake at the inner cantbus. Each tiny o peni ng. o r lacrimal pUflcl w n. is abo ut 0 .3 mm in di ameter. The in fe ri or punctul11 is 6.5 111m from the medi al canthu s; the superio r pWlCtum is 6 .0 111m fro m it. These o penings
lead to the lacrimal canaliculi, the lacrim al sac, and fin all y the nasolacrimal dllcl to the
34 • Fundamentals and Principles of Ophthalmology -B mm Canaliculi
···"1
Fundus of sac
10mm
.1Body of sac
Figure 1-36
Lacrimal excretory system.
(llIustrarion by Thomas A. Weingelsr, PhD, MD.)
Nasolacrimal duct - 18 mm
Interosseous part
Meatal part
Inferior meatus -20 mm
nose. In 90% of people, the canaliculi join to form a common canaliculus. In about 30% of fu ll -term neonates, the outlet of the nasolacrimal duct is closed and rnay remain so for up to 6 months. Occasionally, probing may be necessary to achieve patency. The lacrimal puncta and the canaliculi are lined with stratified squamous n o n kerati~ nized epithelium that merges with the epithelium of the eyelid margins. Near the lacrimal sac, the epithelium changes into 2 layers: ( I) a superficial columnar layer and (2) a deep, flattened cell layer. Goblet cells and occasional cil ia are present. In the canalicu li , the substantia propria consists of collagenous connective tissue and elastic fibers. The wall of the lacrimal sac resembles adenoid tissue and has a rich venous plexus and many elastic fibers.
Con· unctiva The conjunctiva can be divided into 3 geographic zo nes: palpebral, fornical, and bulbar. The palpebral conjunctiva begins at the mucocutaneous junctio n of the eyelid and covers the lid's inner surface. This part adheres firmly to the tarsus. The tiss ue becomes redundant and freely movable in the fornices (jomiceal conjunctiva), where it becomes enmeshed with fibrous elements of the levator aponeurosis and Muller muscle in the upper eyelid. In the lower eyelid, fibrous expansions of the in ferior rectus muscle sheath fuse with the inferior tarsal muscle, the equ ivalent of the Medler muscle. The conjunctiva is reflected at the cul-de-sac and attaches to the globe. The delicate bulbar conjunctiva is freely movable but fuses with the Tenon capsule and inserts into the limbus. Anterior ciliary arteries supply blood to the bulbar conjunctiva. The tarsal conjunctiva is supplied by branches of the marginal arcades of the lids. The proximal arcade, running along the upper border of the lid, sends branches proximally to supply the forn ical and then the bulbar conj un cti va as the posterior conjunctival arteries. The Iimbal blood supply derives from the ciliary arteries through the anterior conjunctival arteries. The
CHAPTER "
Orbit and Ocular Adnexa. 35
vasc ular watershed between the anterior and posterior territories lies approximately 3 or 4 mOl from the limbus. T he innervation of the conjunctiva is derived from the ophthalmic division of CN V. The conjunctiva is a mucous membrane cons isting of a non keratinizing squamous epithel ium with numerous goblet cells and a thin. richly vasc ulari zed substantia propria containing lymphatic vessels. plasma cells. macrophages. and mast cells. A lymphoid layer extends from the bulbar conjunctiva to the sub tarsal folds of the lids. In places. specialized aggregations of conjunctiva-associated lymphoid tissue (CALT) correspond to l11ucosaassociated lymphoid tissue (MALT) elsewhere and comprise collections of T and B lymphocytes underlying a modified epithelium. These regions are concerned with antigen processin g. The conjunctival epith elium varies from 2 to 5 cells in thickness. The basal cells are cuboidal and evolve into fl attened polyhedral cells as they reach the surface. The goblet cells (unicellular mucous glands) are concentrated in the inferior and medial portion of the conju nctiva. especiall y in the region of the caruncle and plica semilunaris. They are sparsely distributed throughout the remainder of the conjunctiva and are absent in the limb.1 region. Knop N, Knop E. Conjunctiva-associated lymphoid ti ssue in the human eye. Invest Ophthnlmol Vis Sci 2000,41 (6), 1270- 1279.
Tenon Ca sule The Tenon capsule (the fascia bulbi) is an envelope of elastic connective tissue that fuses posteriorly with the optic nerve sheath and anteriorly with a thin layer of tissue called the intermuscular septum. 3 mm posterior to th e limbus. The Tenon capsule is the cavity within which the globe moves. It is composed of compactly arranged collagen fibers and a few fibroblasts. The Tenon capsule is thickest in the area of the equator of the globe. Connections between the Tenon capsule and the periorbital tissues help suspend the globe in the orbit. The extraocu lar muscles penetrate this connective tissue about 10 mm posterior to their in sertions. The connec tive tissues form sleeves around the penetrating extraocular mus ~ cles. creating pulleys suspended from the periorbita. These pu lleys stabilize the position of the muscles relative to the orbit during eye movements. The pulleys are connected to one another and to the Tenon fascia by connective tissue bands containing collagen. elastin. and smooth muscle (Fig 1-37). Demer JL. Mechanics of the orbita. Dev Ophtlwlmol. 2007;40:132 - 157.
The suspensory li gament of Lockwood (F ig 1-38) is a fusion of the sheath of the in ferior rectus muscle, th e inferior tarsal muscle, and the check liga ments of the medial and lateral rectus muscles. It provides support for the globe and the anterioinferior orbit. The fusion of the sheath of the inferior rectus muscles. the Lochvood ligament. and the inferior tarsal IllUscle is an important consideration in surgery. because an operation on the inferior rectus muscle may be associated with palpebral fissure changes.
36 • Fundamentals and Principles of Ophthalmology Horizontal section
Anterior slings
~ Striated muscle
Figure ' ·37 Diagram of the orbital connective tissues. IR = inferior rectus, LPS = levator palpebrae superioris,
Smooth muscle
LR = lateral rectus, MR = medial rectus, SO = superior oblique, SR = superior rectus.
Elastin
(From Derner JL, Miller JM,
Pouken V, Vin ters HV, Glasgow BJ. Evidence for fibromuscular pulleys of
-
Collagen
the recti extraocular muscles. Invest Ophthalmol Vis Sci. 1995;36(6):1125.
-
© Association for Research in Vision and Oph thalmology,)
Tendon
C artilage
~------41-
Lockwood ligament
Figure '-38
-\t?---~ ~".II!'~~.~~~~=~~ -
Whitnall ligament
Arcuate expansion Capsulopalpebral
fascia
Eyelids, anterior fascial support system . (Reproduced With permiSSion from
Dut/onJJ. Atlas
of Clinical and Surgical Orbital Anatomy. Philadelphia: Saunders; 1994.)
Vascular Supply and Drainage of the Orbit Posterior and Anterior Ciliary Arteries Approx imately 20 short posteri o r ciliary arteries and 10 sho rt posterior ciliary nerves en ter the globe in a rin g around the optic nerve (Figs 1-39 through 1-4 1). Usually. 2 long ciliary arteries and nerves enter the sclera on either side of the optic nerve close to the hori zontal
CHAPTER 1:
Orbit and Ocu lar Adnexa .
37
Supraorbital artery - __...,-" Supratrochlear artery - . - \
Lacrimal artery
Dorsal nasal artery -~~.. Angular artery -
Ophthalmic artery Accessory ophthalmic artery
+-11
Muscular branch to inferior oblique muscle
Lateral palpebral artery -f=.--Ioi Facial artery Infraorbital artery -
Maxillary artery
--i---'
A
r - - -- -
Anterior ethmoidal artery
r - - - - Muscular branch to medial rectus muscle __- - - Muscular branch to supe rior rectus muscle Medial palpebral artery
...
t-..,--:~ ~~~---
Posterior ciliary arteries
~~~~~~~~~<:=-------- Muscular branch to ~
inferior rectus muscle
B Figure ' ·39 Orbital arteries . A, Lateral view with extraocular muscles, compos ite view. B, Central dissection. (Reproduced with permission from Dutron JJ. Atlas of Clinical and Surgical Orbital Anatomy. Philadelphia: Saunders; 1994.J
Ophthalmic artery Posterior ethmoidal arte ry
Accessory ophthalmic
T---rr- artery (uncommon variation) ------,1-1-
Lacrima l artery
Lacrimal posterior ciliary arteries
Medial posterior -F'tt-,ciliary artery Anterior ethmoidal
Lateral palpebral artery
,,,,,--.rt.'
o ..
""""'-..(1"
7'''''--- - -
Supraorbital artery
Figure ' ·40 Orbital arteries, superior composite view. (Reproduced with permission from Outran JJ. Atlas of Clinical and Surgical Orbital Anatomy. Philadelphia: Saunders; 1994)
38 • Fun dame ntals and Princi ples of Op hth a lmology Superior
Superior rectus
Vortex veins
N Short posterior ci liary artery rectus
Med ial rectus
Optic
oblique
Vortex veins
Figure 1-41
Inferior rectus
Poste rior vi ew of th e righ t globe. (Modified by Cyndie Wooley from illustration by Thomas A
Weingeist, PhD, MD.)
me rid ia n. The course of these vessels can usually be followed fo r a short distance in the sup rachoroidal space. The posterior ciliary vessels originate from the ophthalmic artery and supply the whole uveal tract, the cilioreti nal arteries, the sclera, the margin of the cornea, an d the adjacent conjunctiva. Occlusion of the posterio r ciliar y vessels (as in gian t cell arter itis) may have profo und consequences fo r th e eye, such as anterior ischem ic optic
neuropathy. The anterior ciliary arte ries also arise fro m the ophthalmic artery and usuall y su pply (in pairs) the su perior, medial, and infe ri or rectus muscles (Figs 1-42, 1-43) . A single anterior ciliary vessel ente rs the lateral rectus muscle from the lacrimal artery. The ante rio r an d posterior ciliary vessels usually anastomose with the long posterio r ci liary vessels via anastomoses that perforate the sclera anterior to the rectus muscle insertions. Within the eye, the posterior cili ary vessel for ms the intra muscular ci rcle of th e iris, from which
branches supply the major arter ial circle (which is usually discontin uous) . This circle lies within the apex of the ciliary muscle, which it supplies together with the iris. The iris vessels have a radial arrange ment that is visible upon sli t-lamp examinatio n in lightly pigme nted blue irises. This can be d istinguis hed fro m the irregular new iris vessels fo rmed in rubeos is iridis.
Vortex Veins The vortex veins drain the ve nous system of the choroid, ciliary body, and iris (see Fig 1-41 ; Fig 1-44) . Each eye contains 4 to 7 (o r mo re) veins. One or more vei ns are usually located
CHAPTER 1:
Orbit and Ocu lar Adnexa.
39
Figure 1-42 Three-dimensional representation of the multilevel collateral circulation in the primate anterior uvea in both surface and cutaway views. To the left, in cross section, perforating branches of the anterior ciliary artery are shown as they pass through the sclera to supply the intramuscular circle and major arterial circle. ACA = anterior cil iary artery, EC = episcl eral circle, fMC = intramuscular circle, LPCA = long posterior ciliary artery, MAC = major arteri al ci rcle, PACA = posterior perforating anterior ciliary artery, RCA = recurrent ci liary artery. (Reproduced with permission from Morrison JC, Van Buskirk EM. Anterior collateral circulation in the primate eve. Ophthalmology. 1983;90:707.)
Opht~,almic artery -~r==~~~~
' -- - Lacrimal artery
Muscular branch to superior oblique muscle Anterior ettlmoidlal
Muscular branch to medial rectus muscle Muscular branch to inferior rectus muscle - ----'---/
~_ _
Muscular branch to lateral rectus muscle
Muscular branch to ' - - - - - inferior oblique muscle
Figure 1-43
Orb ital arteries, fro nta l view wit h extraocu lar muscles. (Reproduced with permission from Dutton JJ. Atlas o f Clinica l and Surgical Orbital Anatomy. Philadelphia: Saunders; 1994.)
40 • Fundamentals and Principles of Ophthalmology Supraorbital vein -~-f-{( Nasofrontal vein
- " -'I
Superior ophthalmic vein
Nasal vein ---f-d"
Angular vein
A
Anterior facial vein
Cavernous sinus Inferior ophthalmic vein
---'ii----II
Infraorbital vein Pterygoid venous plexus
- ---f-
Supraorbital vein Infratrochlear vein
......
-+-F-~
r - - - - - Muscular branch from
superior rectus muscle
Initial branches from lacrimal gland -TlH;p;;:=,:;~
-,-=== ~_ _ _ _
B
Figure 1-44
Muscular branch from -l.-----" inferior oblique muscle
Central retinal vein
Muscular branch from inferior rectus muscle
Orbital veins, latera l view. A, Composite view. B, Central dissection.
(Reproduced
with permission from Dutton JJ. Atlas of Clinical and Surgica l Orbital Anatomy. Philadelphia: Saunders; 7994.)
in each quadrant and exit l4-25 mm from the limbus between the rectus muscles. The ampullae of the vortex veins are 8-9 mm from the ora serrata and are visible by indirect ophthalmoscopy. A circle connecting these ampullae corresponds rough ly to the equator and divides the central or posterior fundus from the peripheral portion.
CHAPTER
2
The Eye
Topographic Features of the Globe The eyeball, or globe, is not a true sphere. The radius of curvature of the corn ea (8 mm ) is smaller than that of the sclera (I2 m m), making the shape of the globe an oblate spheroid (Fig 2- \ ). The anteroposterior diameter of the adult eye is approximately 23- 25 mm. Myopic eyes tend to be longer, and hyperopic eyes tend to be shorter. The ave rage transverse d iameter of the adult eye is 24 m m. T he eye contai ns 3 compar tments: the ante ri o r chambe r, the poste rio r cham be r, and th e vitreo us c av it y. The ante ri or chamber, th e space betwe e n th e iri s and th e
Cornea Anterior chamber
- - - - - - - - - - -;.:: - -:;;:; - :=::t:;f:.--..
Iris Anterior chamber angle
~~~;:=~ posterior chamber J~~~:~~l Lens /. Ciliary body A
'--""~- Zo n ule s
fr"'-"-"'--"-'\.A."-"oV\.)v-~",",'-''-''-'2l~~-Ora serrata E E
'-- - -.JI+-- Lens capsule
"'~ H\--- - - 24 mm - - -- -+.1 N
Vitreous cavity
Neural retina Choroid Sclera
Optic disc Optic nerve
Figure 2-1 Sagittal section of eye with absent vitreous an d major structures identified. Di mensions are approximate and are the average dimension in the normal adult eye. (Illustration by Chfls rine Gralapp.)
41
42 • Fundamentals and Principles of Ophthalmology cornea, is filled with aqueous fluid. It is abo ut 3 m m deep, wit h an average vo lume of
200 ~L. The posterior cham ber is the anatomical porti on of the eye posterior to the iris and an terior to the lens and vitreous face . It is also fill ed with aqueous fluid and has an average vo lu me of 60 ~L. The largest compar tmen t of the eye is the vitreous cavity, which makes up mo re than two thirds of the vo lu me of the eye (5-6 mL ) and contains the vitreous gel. The total vo lu me of the average adult eye is apprOXi m ately 6.5-7 mL. The eyeball is composed of 3 concentric layers. The outermost layer consists of the clear cornea anteriorly and the opaque white sclera posteriorly. The oute rmost corneosderallayer is composed of tough and protective tissues. The corn ea occupies the center of th e anterior pole of the globe. Because th e sclera and conjuncti va overlap the cornea anteriorly, slightl y more above and below th an medi all y and laterally, the cornea appears elliptical when viewed fro m the front. In the adult, it measures about 12
111111
in the horizonta l meridian and about 11 mm in the vertical.
From behind, when the cornea is viewed at its posterior land mark (the Schwa lbe line-the termination of Descemet's membrane), its circumference appea rs circular. The cornea is about I mm thick at its periphery and is 0.5 mm thick centrally. The li mbus, which borders the cornea and the sclera, is gray and translucent. In cont rast to the transparent co rn ea, the sclera is opaque and white. Thi nnes t just behind the in sertions of the rec tu s muscles (0.3 mm), the scle ra in creases to apprOXimately 1 mm thi ck posteriorly but becomes thin and sievelike at th e lamina crib rosa, w here the
axons of the ganglion cells ex it to form the opt ic nerve. The middle layer of the globe is the uvea, which consists of the choroid, cili ary body, and iris. Highly vasc ular, it serves a nutritive and supporti ve function . T he innermost layer of the globe is the ,·elina. T hi s photosensitive layer contains the photo receptors and neural elements that initiate the process ing of visua l inform at ion. Other important surface features of the globe, such as th e vortex veins, the pos terior ciliary artery and nerves, extraocular muscle in sertions, and the optic nerve and its surrounding meningeal sheaths, are discussed in Chapter 1.
Precorneal Tear Film T he exposed surfaces of the cornea and globe are covered by the precorneal tear film , which is com posed of 3 layers: a superficial Oily layer produced predominantly by the meibomian glands; a middle aqueous layer prod uced by the main and accessory lacrimal glands; and a deep mucin lay,,· derived from the conjunctiva l goblet cells. T he surface cells of the cornea and conjunct iva also express a mucinous glycocalyx. Mainte nance of the precorneal tear film is vital for norma l corneal functi on. In addition to lubri cat ing the surface ofthe cornea and conjunctiva. tears produce a sl11 00th optical su rface. provide oxygen and other nutrients, and contain immunoglobulins, lysozyme.
and lactoferrin . Aberrations in the tear film resu lt from a variety of diseases (eg, dry eye) that profoundly affect the integrity of the su rface.
CHAPTER 2,
The Eye • 43
Cornea Characteristics of the Central and Peripheral Cornea The air- tear interface at the surface of the cornea forms a positi ve lens of approximately
43 di opters (D) in air and constitutes the main refractive element of the eye (Fig 2-2). The central third of the cornea is nearly spherica l and measures about 4 mm in diameter in the normal eye. Because the posterior surface of the cornea is more curved than the ante-
rior surface, the central corn ea is thinner (0 .5 mm) than the peripheral corn ea ( 1.0 mm). The cornea becomes fl atte r in the periphery, but the rate of fl attening is not symmetri c. Flattening is more extensive nasall y and superiorly than temporally and inferiorly. This topography is important in contact lens fitt ing. BeSe Section 8, External Disease and Cornea, di scusses th e cornea in detai l.
Epithelium and Basal Lamina The an te rior surface of the cornea is der ived from su rface ecto derm and is covered by a nonkeratinized, stratified squamous epitheli um whose basal columnar layer is attached to
a basal lamina by hemidesmosomes (Fig 2-3). The basal cells have a width of 12
~m
and
a density of approx imately 6000 cells/111m2. The occasional recurrence of corneal eros ion fo llowing a traumatic corneal abras ion may be due to improper formation ofhem idesmosomes after an epithelial abrasion.
Ove rlying the basal cell laye r are 2 or 3 layers of polygonal "wing" cells. The superficial corneal epithelial cells are extremely thin (30 ~m ) and are attached to one another by occluding zo nules. These zo nules confer the properties of a semipermeable mem brane to the epith elium . Microplicae and microvilli make the apical surfaces of the wing cells high ly irregula r; however, the precorn eal tear film renders the surfaces optically smooth. Although the deeper epithelial cells are fir mly attached to one anot her by des mosomes, they migrate continuously from the basal region toward the tear film, into which they are shed. They also migrate cent ripetall y from their stem cell source at the limbus. Division of the slow-cycling stem cells gives rise to a proge ny of daughter cells (transient ampli fying cells), whose di vision serves to ma intain the cornea l ep ith elium. Diffuse damage to
the lim ba l stem cells (eg, by chem ical burns, trachoma) leads to chronic epithelial surface defects. Fin e BS, Yanoff M. OClilar Histology: A Text alld Atlas. 2nd ed. Hagerstown, MD: Harper & Row; 1979, 163- 168.
Nonepithelial Cells Nonepithelial cells may appea r within the corneal epithelial layer. Wandering histiocytes, macrophages, lymphocytes, and pigmented melanocytes are frequent components of the periphera l cornea. Ant igen -presentin g Langerhans cells are fou nd peripherall y and move ce ntrall y with age or in response to kerat iti s.
44 • Fundamentals and Principl es of Ophthalmology
J-
Figure 2·2
see Figu re 2-3 below for diagram of this portion
Cornea. The empty spaces in the stroma are artifactitious (H&E x32) . (Courtesy of
Thomas A Wemgelsr, PhD, MD.)
Microvilli, microplicae
Surface
cells
Figure 2-3 The corneal epithelium and Bowman 's layer, showing hemides-
Wing cells
mosomes along the basal lamina. (lIIus fratlOn by Thomas A Wemgelsr, PhD, MD.)
Columnar
cells Hemidesmosomes
r----1h- rl- Hemidesmosomes
Bowman's Layer Beneath the basal lamina is Bowman's layer, or Bowman's membrane, a tough laye r consisti ng of rand o ml y dispersed collagen fibril s. It is a mod ified region of the anterior stro ma 8- 14 pm thick. Unlike Descemet's membrane, it is not restored after injury but is replaced by scar tissue.
CHAPTER',
The Eye'
45
Stroma The stroma const itu tes approxim ately 90% of the total corneal thickness in humans (see
Fig 2-5) . It is composed of collagen-producing keratocytes, ground substance, and collagen lamellae. The collagen fibr ils form obliquely oriented lamellae in the anterior third of the stroma (with some interl ac ing) and parallel lamellae in the posterior two thirds. T he corn eal collagen fibrils extend across the ent ire diameter of the cornea, finally winding circumferentially around the limbus. The fibrils are remarkably uniform in size and se paration , and this regularity helps determi ne the transparency of the cornea. Sepa rat ion of the collagen fibr ils by edema fluid leads 10 stromal clouding. The macroperiodicity of the fibrils (640 A) is typical of collagen. The stroma's collagen types are I, III, V, and VI. Type VII forms the anchoring fibril of the epitheli um. The ground substance of the cornea consists of proteoglyca ns that run along and between the collagen fibr ils. Their glycosaminoglycan components (eg, keratan su lfate) are highly cha rged and account for the swelling property of the stro ma. The kerato cytes lie between the corneal lamellae and synthesize both collagen and proteoglycan s. Ultrast ructurall y, they resemble fibrocytes. The cornea has apprOXimately 2.4 million keratocytes, which occupy about 5% of the stromal volume; the denSity is higher anteriorly (l058 cells/mm') than posteriorly (77 I cells/ mm' ). Keratocytes are highly active cells, rich in mitochondria, ro ugh endoplasmic reticula , and Golgi apparatuses. They have attachment structures, communicate
by gap junct ions. and have unusual fenestrations in their plasma membranes. Their flat profile and even distribution in the corona l plane ensure a minirnum disturbance of light transmission. Studies with vital dyes suggest that there may be at least 3 different types of keratocytes. Muller Lj , Pels L, Vrensen GF. Novel aspects of the ultrastructural organization of human cor-
neal keratocytes.lflvest Ophtlwlmol Vis Sci. 1995;36( 13):2557- 2567. Muston en RK, McDonald MB, Srivanllaboon S, Tan AL, Doubrava MW, Ki m CK. Nor mal human co rneal cell populations evaluated by in vivo scanning slil co nfocal m icroscopy. Corlien. 1998; 17(Sj,48S-492.
Descemet's Membrane The basal larnina of the corneal e ndotheli um, Descemet's membrane, is periodic acid-
Schiff (PAS)-posi tive (Fig 2-4). It is a tru e basement membrane, and its thickness increases wit h age. At birth, it is 3-4 ~m thick; thickness increases th roughout life to an adult level of 10- 12 flm. It is composed of an anterior banded zo ne that develops in litera and a
posterior non banded zone that is laid dow n by the corneal endothelium throughout life (Fig 2-5). These zones prOVide a historical record of the synthetic function of the eIldotheli um. Like other basal laminae, Descemet's membrane is rich in type IV coUagen. Peripheral excrescences of Descemet's membrane, known as Hassall-Henle warts, are
common, especially among elderly people. Central excrescences (cornea guttae) also appear with increas in g age.
46 • Fundamentals and Principles of Ophthalmology
"
••
".(""II\~• •
J-
See Figure 2-5 below for diagram of this po rtion
Figure 2-4 Posterior cornea. Note the appearance of Descemet's membrane and the cornea l endothelium (H&E x64) . fCourtesy of Thomas A. Weingeist PhD, MD.)
Stroma
Anterior banded layer
Posterior nonbanded layer
Oescemet's membrane
Endothelium
Anterior chamber
Figure 2-5
Corneal endothe lium and Descemet's membrane. (Illus tration by Thomas A. Weingeist.
PhD, MD.)
Endothelium The corneal endothelium is composed of a single layer of mostly hexagonal cells derived from the neural crest (Fig 2~6). The corneal endothelium is therefore of neuroectodermal origin. Approximately 500,000 cells are present, wi th a density of about 3000 cells/mm'. The size, sha pe, and morphology of the endothelial cells can be observed by specular microscopy at the slit lamp. The apical surfaces of these cells face the anterior chamber; their basal surfaces abut Descemet's membrane. Typically, yo ung endothelial cells have a large nucleus and abundant mitochondria. The active transport of io ns by these cells leads
CHAPTER 2,
The Eye.
47
A
B Specular micrographs of the corneal endothelium. A, Norma l patient B, Patient w ith Fuchs endothelial dystrophy. Both were taken at the same magnification. Bottom micrograph shows larger, more irregular cells (polymegethism); the 3 dark areas toward the bottom are cornea guttae. (Courtesv of David Palay, MD. and Da vid Litof f, MD.)
Figure 2-6
to the transfer of water from the corneal stroma and the maintenance of stromal deturgescence and transparency. Mitosis of the endothelium is rare in humans, and the overall number of endothelial cells decreases with age. Adjacent endothelial cells interdigitate in a complex way and form a variety of adherent junctions, but desmosomes are never seen between normal cells. In cross section, pinocytotic vesicles and a terminal web (a meshwork of fine fibrils that increases the density of the cytoplasm) can be seen toward the apical surface of the cells. Junctional complexes are present at the overlapping apicolateral boundaries of contiguous cells. They form a signifi cant but lesser barrier to ion and water flow than the tight junctions of the epithelium. Endothelial cell dysfunction and loss-through surgical injury, inflammation, or inherited disease (eg, Fuchs endothelial dystrop hy)-may cause endothelial decompensation, stromal edema, and visual failure. In humans, endothelial mitosis is limited, and dest ru ction of cells causes cell density to decrease and residual cells to spread and enlarge. Foster CS, Azar DT, Dahlman CH. Smolin fllld Thofts The Cornea: SCientific Foulldations and Clinical Practice. 4th ed. Philadelphia: Lippincott Williams & Wilkins; 2004.
Sclera The sclera covers the posterior four fifths of the surface of the globe, with an anterior opening for the cornea and a posterior opening for the optic nerve. The tendons of th e rectus muscles insert into the superficial scleral collagen. The Tenon capsule covers the
48 • Fundamentals and Principles of Ophthalmology sclera and rectus muscles anterio rly, and both are overlai n by the bulbar co njunctiva. The
capsule and conjunctiva fuse near the limbus. The sclera is thinnest (0.3 mm) just behind th e insertions of the rectus muscles an d thickest (1.0 mm ) at the posterior pole around the optic nerve head. It is 0.4-0.5 mm thick at the equator and 0.6 mm th ick anterior to the muscle inserti ons. Because of the thinness of the sclera . strab ismus and retin al detachment surgery require careful placement of sutures. Scleral rupt ure following blunt trauma ca n occur at a number of si tes: in a circumferential arc parallel to the corneal li mbus opposite the site of impact, at the insertion of the rectus muscles, or at the equator of the globe. The most com mon site is the superonasal quadrant near the limbu s. The sclera, like the cornea, is essentially avascular except for the superfic ial vessels of
the ep isclera an d the intrascleral vascular plexus located just posterior to the lim bus. A num ber of channels, or emissaria, penetrate the sclera, allowing for th e passage of arteries, veins, and nerves. Extrao cular exte nsio n of mali g nant melanoma of the choroid o fte n occurs by way of the emissaria.
Branches of the ciliary nerves that supply the cornea sometimes leave the sclera to form loops posterior to the nasal and tempo ral limbus. These nerve loops, called Axenfeld
loops, are somet,imes pigme nted and , consequently, have been mi staken for uvea l tissue or malignant melanoma. Ante ri o rly, the episclera consists of a dense vascu lar connective tissue that merges
deeply w ith the sup erficial sclera and sup erficiall y with the Tenon caps ul e and the co njunctiva. The scleral stroma is composed of bundles of collagen, fibroblasts, and a moderate amount of gro und substance. Collagen fibrils of the sclera va ry in size and shape and have been shown to taper at th eir en ds, indicat ing that they are not cont in uous fibers as
in the cornea. In ge neral, the outer scleral collagen fibers have a larger diameter (1600 A) tha n the inner collagen fibe rs have (1000 A) . The inner layer of the sclera (lamina fusca) blends imperceptibly with the suprachoroidal and supraci liary lamellae of th e uveal tract. T he collagen fibers in this portion of th e sclera branch and intermingle with the ollter ciliary body and choroid. The bundles of collagen fibers contain electron-dense bodies, fibroblasts, an d melanocytes. The opaque, porcelain-wh ite appearance of the sclera contrasts markedly wit h the transparency of the co rn ea and is primarily due to 2 thin gs: (I) the greater va ri ation in fibril separation and diameter and (2) the greate r degree of fibril interweave in th e sclera. In addit ion, th e la ck of vascular elements such as the scleral emissa ria contributes to corneal cla rity.
Limbus The trans ition zone between the peripheral co rn ea and the anter io r sclera, known as th e
limbus, is defined differently by anatomists, pathologists, and clinicians. Although not a distinct anatom ical structu re, the limb us is important for 2 reasons: (1) its relationship to the chamber angle and (2) its use as a surgical la ndmark. The followi ng stru ctures are included in the limbus: conjunctiva and limbal palisades • Tenon capsu le
CHAPTER',
Th e Eye .
49
episclera
corneoscleral stro ma aqueous o ut flow apparatus The transitio n from opaque sclera to clear cornea occ u rs grad uall y over 1.0-1.5 m m an d is difficul t to define histologicall y. Th e corneoscleral ju nct io n begins ce ntrally in a pla ne connec tin g the end of Bowman's layer and the Schwalbe line, the te rmin ation of Descemet's memb rane. Internally, its posteri or li mit is the an terior tip of th e scleral spur. Pathologists conside r t he pos teri or li mit of the li mbus to be for m ed by anot her plane p erpendi cul ar to the surface of the eye, app roximately 1. 5 111m p oste rior to the termin at ion of Bowm an's layer in the horizontal m eridi an and 2.0 mm poste ri or in th e vertical merid ian, where th e re is greater scleral ove rl ap (F ig 2-7) . T h e surgical limb us can be di vi d ed co ncep tuall y into 2 eg ual zo n es: (I) an ante rior bluish gray zo ne ove rl ying clear co r nea a nd ex tending from Bowman's layer to the Schwa lbe line a nd (2) a poste rior white zone ove rlyi ng th e trabecu lar meshwork and exte ndi ng fro m th e Schwa lbe li ne to th e scleral sp ur, or iris root. Familiar ity with th ese la n d m arks is essential to t he surgeon performing a cataract ext raction or a glauco m a- filterin g procedu re. Jaffe NS, Jaffe MS, Jaffe GF. Cataract Surgery and Its Complications. 6th ed. Phi ladelphia: Elsevier/Mosby; 1997.
Cornea Termination of Bowman 's layer
' - - - - - - Termi nation of Descemet's membrane (Schwalbe line) Trabecu lar meshwork Schlemm canal _ .;;...,;: Scleral spur Iris
Major circle
Figure 2-7 Anterior chamber angle and limbus, depicting the concept of the limbus. Solid lines represent the li mbus as seen by pathologists; the green dotted line represents the limbus as seen by anatomists. (Illustration by Thomas A Weingeist. PhD, MD.)
50 • Funda mentals and Principles of Ophthalmol ogy
Anterior Chamber The anterior chamber is bordered anteriorl y by the corn ea and posteriorl y by the iris diaphragm and the pupil. The anteri or chamber ang le, which lies at the junction of the cornea and the iris, consists of the following structures (Fig 2-8):
• Schwalbe line • the Schlemm canal and trabecula r meshwork • scleral spur anterior border of the ciliary body (where its longitud inal fib ers insert into the scleral spur) iris
The depth of the anterior chamber va ries. It is deeper in ap hakia, pseudophaki a. and myopia and shallower in hyperopia. In the normal ad ult em metropic eye, the anterior chamber is approximately 3 111m deep at its center and reaches its narrowest point slightly central to the angle recess. The volume of the anteri or chamber is about 200 ~L in the emmetrop ic eye.
The anterior chamber is filled with aqueous humor, which is produced by the Ciliary epithelium in the posterior chamber. The fluid passes through the pupil aperture and drains chiefly by the conventional pathway through the trabecular meshwo rk into the Sch lemm canal and partl y by the nont rabecular uveoscleral drainage pathway, across the ciliary body into the supraciliar y space. The uveoscle ral pathway, thought to be influenced by age, accounts for up to 50% of aqueo us outflow in young people. BCSC Section 10, Glaucoma, discusses th e anterior chamber and aqueo us hu mor in detai l. Hi gh~reso iliti o n ultrasou nd biomicroscopy provides detailed 2-dime nsional views of
the anterior segment of the eye and is perfo rmed in vivo (Fig 2-9), allowing the clin ician to view th e relatio nship of th e stru ctures in the ante ri or segment under different pathologic condit ions.
The internal scleral sulcus accommodates the Schlel11111 canal externally and the trabec ular meshwork internally. The Schwalbe line, the periphery of Descemet's membrane, forms the anteri or margin of the sulcus; the scleral spu r is its poste rior landmark. The scleral spur rece ives the insertion of th e longitudinal ciliar y muscle, and contract ion opens up the trabecular spaces. Contra ctil e cell s are found within the scleral spur, as are struc tures resembling mechanorece ptors, which rece ive a sensory inn ervat io n.
Myofi brob last-like scleral spur cells with cont rac tile properties are d isposed circum ferentially within the scleral spu r. They are connected by elasti c tissue to the trabecular meshwork; in experiments, stim ulation with vasoactive intestinal polypeptide (V IP) or calcitonin gene- related peptide (CG RP) causes an increase in outflow facility. Ind ividual scleral spur cell s are innervated by unmyelinated axons, th e termina ls of which contact th e ce ll membranes of the spur ce ll s without an intervening basal lamina. The nerve fi -
bers in this region are immun oreactive for neuropeptide Y, substance P, CGRP, VIP, and nitrous oxide and the refore are med iated by sympathetic, sensory, and pterygopalatin e nerve pathways. There are no cholinergic fibers in thi s region.
CHAPTER 2, The Eye • 51
2-8 Semidiagrammatic representation of the structures of the angle of the anterior chamber and cil iary body. A, Composite gonioscopic and cross-sectional view of the anterior segment of the eye. B, Enlarged view. Note the superimposed trabecular sheets with intratrabecular spaces through which aqueous humor percolates to reach the Schlemm canal. C = cornea , CB = ciliary body, I = iris, IP = iris process, S = sclera, SC = Schlemm canal, SL = Schwalbe line, SS = scleral spu r, TM = trabecular meshwork, Z = zonular fibers. Figure
A
(Reproduced with permission from Triparhi Re, Triparhi 8J. Functional
anatomy of the anterior chamber angle. In: Jakobiec FA. ed. Ocular Anatomy, Embryology, and Teratology_ Philadelphia: Harper & Row;
1982.1
B 1~1 mll1
ER, Koch TA . Mayer B, Stefani FH , Uitjen- Drecoll E. Innervat ion of myofibroblast -like scleral spur cells in human and monkey eyes. II/ vest Ophthalmol Vis Sci. 1995;36(8): 1633- 1644.
Myelinated nerve fibers extendin g forward from the ciliary region to the inner aspect of the scleral spur give branches to the meshwork and to club-shaped endings in the scleral spur. These endings have the morphologic features of mechanoreceptors found elsewhere in the body, such as in the carotid. The endings are incompletely covered by a Schwa nn cell sheath and make contact with extracellular matrix mate rials such as elastin.
52 • Fundamentals and Principles of Ophthalmology
A
B Figure 2·9 A. Ultrasound biomicroscopic composite image of the anterior segment. IncludIng the anterior chamber (AC). The iris is slightly convex. indicating mild pupillary block. The corneoscleral Junction (CS Jeri. ciliary processes. and posterior chamber (PC) region are clearly Imaged. The angle is narrow but open . Iris-lens contact is small. B, High-resolution ultrasound Image of the antenor segment. Note the location of the ciliary sulcus. CB = ciliary body. CP = ciliary process, 55 = scleral spur. (Parr A courtesy o ( Charles Paviln, MD; part B courresy of K N1schal. MD.)
Vari ous fun cti ons have been proposed for th ese endings, including (I ) propr ioception to th e ciliar y muscl e, which inserts into th e scleral spur, Signaling contract ion of the scl eral spur celis, and (2) baroreception in respo nse to changes in intraocular pressure.
CHAPTER 2,
The Eye .
53
Trabecular Meshwork The relationship of the trabecular meshwork (see Figs 2-7, 2-8) and the Schlemm canal to oth er structures is complex because th e outflow apparatus is composed of tissue derived
fro m the cornea, sclera, iris, and ciliary body (Fig 2-10). The trabecular meshwork is a circular spongework of connective ti ssue lined by tra beculocytes. These cells have contractile properties. which may influ ence outflow resis-
tance. They also have phagocytic properties. The meshwork is roughly triangular in cross section , with the apex at the Schwalbe line and the base formed by the scleral spur and the Ciliary body. Some trabecular tissue passes posterior to the spur. The trabecular meshwork can be divided into 3 layers (see Fig 2-2 in BCSC Section 10, Glaucoma, Chapter 2): l. uveal portion 2. corneosclera l meshwork
3. juxtacanalicular tissue, which is directly adjacent to the Schlemm canal The uveal and corneosclera l meshwork can be di vided by an imagin ary line drawn from th e Schwalbe lin e to th e scleral spur. The uvea l meshwork li es internal and th e co rn eascleral meshwork li es externa l to this lin e.
Figure 2-10 Anterior chamber angle, ciliary body, and peripheral lens. Note the triangular shape of the ciliary body. The muscle fibers appear red in contrast with the connective tissue . The scleral spur is clearly delineated from the ciliary muscle in the reg ion of the trabecular meshwork. The lens is artifactually displaced posteriorly. (Masson tr ichrome x 8). (Courtesy of Thomas A. Wemgeisr. PhD. MD.}
54 • Fundamenta ls and Principles of Ophthalmology
Uveal Trabecular Meshwork The uveal meshwork is composed of cord li ke trabeculae, with fewer elastic fibers than in the corneoscleral meshwork. The trabecu locytes usuall y contain pigment granules, and the trabec ular apertures are less circular and larger than those of the corneoscleral
meshwork.
Corneoscleral Meshwork The corneoscleral meshwork consists of a series of thin, flat, perforated connect ive tissue sheets arranged in a laminar patte rn. Each trab ecu lar beam is covered by a monolayer of thin trabeclJar cells exhibiting multiple pinocytotic ves icles. The basal lamina of these cells forms the outer cortex of the trabecular beam; the inner core is of collagen and elasti c fibers.
Pericanalicular Connective Tissue Pericanalicular connec ti ve tissue in vests the ent ire extent of th e Schlem m canal. On its trabecular aspect, between the outermost layers of the corneoscleral meshwork and the endotheli al,lining of the Schlem m canal, lies the endothelial meshwork, a multilayered collection of cells formi ng a loose network. Spaces exist between these cells, up to 10 ~m in width , th ro ugh which aq ueo us humor can percolate to reach the endothelial lining of the Schlemm canal. T his reg ion of the drainage system makes the g reatest co ntribution to outflow resistance, partly because th e pathwa y is narrow and tortuous and partly because of the resistance offered by extracellular proteoglycans and glycopro tei ns.
Schlemm Canal The Schlemm canal is a circular tube closely resembling a lymphatic vessel. It is fo rm ed by a cont inuous monolayer of non fenestrated endothelium and a thin connective tissue wall. The basement membrane of the endothelium is poorly defined. The lateral walls of th e endothelial cells are joined by ti ght junctions. Micropinocytotic vesicles are present at both the apical and the basal surfa ces of th e cells. Larger ves icles (so-called giant vac uoles) have been observed along the in te rn al canal wall (Figs 2-11, 2-12). These vacuo les are lined by a single membrane, and th eir size and number are in creased by in creas in g intraocular pressure. They are th o ught to contribu te to the pressure- dependent outfl ow of aq ueous.
Collector Channels Approximately 25-30 collector chan nels arise from the Schlem m canal (Fig 2- 13) and drain into the deep and midscleral venous plexuses. Up to 8 of these channels drain directly into the episcleral veno us plexus as aqueous veins (Fig 2-1 4), which are visible in the conjunctiva by biomicroscopy. Aging brings about a twofold to threefold thickening of trabecular sheets; the cortex thickens and the core thins. There is loss of endothelial cellularity, an increase in
CHAPTER',
Th e Eye •
55
Figur. 2-" The walls of the Schlemm canal (SC) and adjacent trabecular meshwork (TM). The endothelial lining of the trabecular wall of the Schlemm canal is very irregular; normally, the cells show luminal bulges corresponding to cell nuclei (N) and macrovacuolar configurations (V). The latter represent cellular invaginations from the basal aspect that eventually open on the apical aspect of the cell to form transcellular channels (arrows) through which aqueous humor flows down a pressure gradient. A diverticulum (D) ~ its endothelial lining continuous with that of the canal-is shown on the inner wall of the Schlemm canal next to macrovacuolar configurations. Such blind, tortuous diverticula course for a variable distance into the trabecular meshwork but remain separated from the open spaces of the meshwork by their continuous endothelial lining. The endothelial lining of the trabecular wall is supported by interrupted, irregular basement membrane and a zone of peri canalicular connective tissue (PTJ of variable thickness. The cellular element predominates in this zone, and the fibrous elements, especially elastic fibers, are irregularly arranged in a netlike fashion. Here, the open spaces are narrower than those of the trabecular meshwork. The corneoscleral trabecular sheets show frequent branching, and the endothelial covering may be shared between adjacent sheets. The corneosclera l wall (CW) of the Sch lemm canal IS more compact than the trabecular wall, with a lamellar arrangement of collagen and elastic tissue predominating . (Reproduced with permis+ sion from Tripathl Re, Tripathi BJ. Functional anatomy of the anreriorchamber angle. In: Jakobiec FA, ed. Ocular Anatomy. Embryology. and Teratology. Philadelphia: Harper & Row; 1982.1
connective ti ssue (eg, in the endothel ial meshwork), and an accumulation of debris i n the meshwork and of glycosaminoglycans in the extracellular space. Such changes are exaggerated in chronic open-a ngle glaucoma.
Uveal Tract The uveal trac t is the main vascular compartment of the eye. It consists of 3 parts: 1. iris 2. Ciliary body (located in the anter ior uvea) 3. choroid (loca ted in th e posterior uvea) T he uveal trac t is firml y attached to th e sclera at only 3 si tes: the scl eral spu r, the exit poin ts of the vortex veins, and the optic nerve. These attachments accou nt for the characteristic anterior balloons formed in choroidal detachmen t.
56 • Fundamentals and Principles of Ophthalmology
Figure 2-12
A, Low-magnifica tion electron
microg raph of the endothelial lining of the Schlemm canal (SC) shows that the majority of the vacuolar conf igurations (V) at this level have direct communicat ion (arrows) with the subendothelia l extrace llular spaces, which contain aqueous humor (x3970). B, Electron micrograph of a vacuolar st ructure that shows bot h basal and apical openings, thus constituting a vacuolar transcellular channel (arrow) . Through this channel, the f luid-containing extracel-
A
lular space on the basal aspect of the cell is temporarily connected with the lumen of
the Schlemm canal, allowing bulk outflow of aqueous humor. N = indented nucleus of the cell (x 23,825). (Reproduced with permission from Tripathi Re, Triparhi 8J. Functional anatomy of the anterior chamber angle. In: Jakobiee FA, ed. Ocular Anatomy, Embryology, and Teratology. Philadelphia: Harper & Row: 1982,)
B
Iris The iris is the most anterior extension of the uveal tra ct (Figs 2-15, 2- 16). It is made up of blood vessels and connective tiss ue, in addition to the melano cytes and pigment cells that are responsible for its distinctive color. The mobility of th e iris allows the pupil to change size. During mydriasis, th e iris is thrown into a number of ridges and folds; during miosis, its anterio r surface appears relatively smooth. The iris diaphragm subdivides the anterior segment into th e anterior and posterior chambers.
Stroma T he iris stroma is composed of pigmented cells (melanocytes) and nonpigmented cells, collagen fibril s, and a matrix containing hyaluroniC acid. The aqueous humor flo ws freely through the loose stroma al ong the anterior border of the iris, which contains multiple crypts and crevices that va ry in size, shape, and depth. This surface is cove red by an interrupted layer of connective tissue cells that merges with th e ciliary body. The overall structure of the iris stroma remains similar in irides of all colors. Differences in color are related to th e amount of pigmentation in the anterior border layer and the deep stroma. The stroma of blue irides is lightly pigmented, and brown irides have a densely pigmented stroma th at absorbs light.
CHAPTER 2, Th e Eye • 57
Figure 2-13 Diagram of the Schlemm canal and relationships of the arteriolar and venous vascular supply. For clarity. the va rious systems have been limited to only parts of the circumference of the canal. Small. tortuous. blind diverticula (so-called Sondermann channels) extend from the canal Into the trabecular meshwork. Externally, the collector channels arising from the Schlemm canal anastomose to form the intrascleral and deep scleral venous plexuses . At irregular intervals around the ci rcumference. aqueous veins arise from the intrascleral plexus and connect directly to the episcleral veins. The arteriolar supply closely approximates the canal, but no direct communication occurs between the two. {Reproduced wIth permission from Tripalhl Re, Tnparhl BJ. Funclionalanafomy of (he amenar chamber 8l1gle. In: Jakobiec FA, ed. Ocular Ana tomy, Embryology, and Teratology. Philadelphia' Harper & Row; 1982:276.)
Figure 2· 14 Aqueous vein (arrow). Collector channels from the Schlemm canal drain into the episcleral venous plexus. With high magnification of the slit-lamp biomicroscope. they are visible near the limbus. Laminar flow and the mixing of aqueous and blood are visible. (Reproduced with permission from Thiel R Atlas of DIseases of the Eye Amsterdam: ElseVier; 1963.)
58 • Fundamentals and Principles of Ophthalmology
Sphincter
Figur.2-15
Posterior pigmented epithelium
Iris. Note the relationship between the sphincter and dilator muscles (H&E x 20) .
(Courtesy of Thomas A Weingeisr, PhD, MD.)
Vessels and Nerves Blood vessels form the bulk of th e iris st roma. Most follo w a radial course, arising from the major arterial circle and passing to the center of the pupil. In the region of th e collaretle (th e thickest portion of the iris), anastomoses occur between the arterial and venous arcades to form the minor vascular circle of the iris, whi ch is often incomplete. The major arterial circle is lo cated at the apex of the ciliary body, not the iris. In humans, the anterior border layer is normally avascular. The diameter of the capillaries is relativel y large. Their endothelium is non fenestrated and is surrounded by a basement membrane, associated pericytes, and a zone of collagenous filaments. The intima has no interna l elastic lamina. Myelinated and non myelinated nerve fibers serve sensory, vasomotor, and muscu lar functions throughout the stroma.
Posterior Pigmented Layer The posterior surface of the iris is densely pigmented and appears velvety smooth and uni form. It is continuous with the nonpigme nted epithelium of the ciliary body and thence with the neurosensory portion of the retina. The polarity of its cell s is maintained from embryogenesis. The basal surface of the pigme nted layer borders the posterior cham ber. The apical surface faces the stroma and adheres to the an terior pigmented layer, which gives rise to the dilator muscle (Fig 2- 17). The posterior pigmented layer of the iris curves around the pupillary margin and extends for a short distance onto the anterior border layer of the iris st roma as th e pigment
CHAPTER 2, Th e Eye. 59
B C
A E
-
~
-~_',~E OJ 0
{g ::J
<Jl
(; ·c
2<Jl
,f?
iJ .$'
~
0
a.
Figur.2·16 Composite drawing of the surfaces and layers of the iris, beginning at the upper left and proceeding clockwise. The iris cross section shows the pupillary (A) and ciliary (B) portions; the surface view shows a brown iris with its dense. matted anterior border layer. Circular contraction furrows are shown (arrows) in the ciliary portion of the iris. Fuchs crypts (e) are seen at either side of the collarette in the pupillary and ciliary portions and peripherally near the iris root. The pigment ruff is seen at the pupillary edge (0). The blue iris surface shows a less dense anterior border layer and more prominent trabeculae. The iris vessels are shown beginning at the major arterial circle in the ciliary body (E). Radial branches of the arteries and veins extend toward the pupillary region . The aneries form the incomplete minor arterial circle (F), from which branches extend toward the pupil. forming capillary arcades. The sector below demonstrates the circular arrangement of the sphincter muscle (G) and the radial processes of the dilator muscle (H). The posterior surface of the iris shows the radial contraction furrows (/) and the structural folds (J) of Schwalbe. Circular contraction folds are also present in the ciliary portion . The pars plicata of the ciliary body is at (K). (Reproduced with permiSSion from Bron AJ. Tflpathl Re, Tnparlll BJ. Wolff's Anatomy of the Eye and Orbit. 8th ed. London: Chapman & Hall; 1997. Originally from Hogan MJ, Alvarado JA, and Weddell JE. Histology of the Human Eye. Phlladelphls: WB Saunders; 1971.)
60 • Fun dame nta ls and Principles of Ophth a lmology
Anterior pigmented layer Iris stroma
Posterior pigmented layer
Posterior chamber
Basal lamina
Figure 2·17
Posterior layer of the iri s.
(I/lust:a t'lJn b )' Thomas A. Weingeist. PhD, M D. !
ruff. In ru beosis iridis, the pigmented layer <xte nds farther onto the an terior surface ofthe iris. a cond ition ca ll ed ectropion. The term ectrori<m uveae is a misnomer, because all of these layers are derived from neuroectoderm.
Dilator Muscle T he dil ator muscle is de rived embryologicall y fro m the outer layer of the o ptic cup, which is neuroec toderm. It li es parall el and anterio r to th e posterior pigmented epi theli um. The smooth muscle cells conta in fin e myofil amen ts and melanosomes. The myofi brils are confi ned m ainl y to the b.,a l portion of th e cells and exte nd ante ri o rl y into th e ir is stroma. The melanosomes 3 11'i the nude us are in the ap ical region of each m yoepithelial cell. There is dual sympathetic and parasympat!1etic inner·:ation. The dilator muscle contracts in response to sympathet ic a i-adrenergic ~timu l ation; cholinergic parasympathetic stim ulation may have an inh ibi to ry role. T he firs t-order neuron of the sympathetic chai n begins in the ipsilate ral posterolateral hypothalam us and passes through the brainstem to synapse in the intermedi olateral gray matter of the spinal cord, chiefly at thoracic level l. The second-order preganglion ic neuron exits th e spinal cord, passes over the pulmom.rv apex and th ro ugh the stellate ganglion withou t synapsing, and synapses in the superior cervical ga nglio n. T he third-order postganglionic neuron originates here, joins the in ternal carotid plexus, ente rs the cavernous sinus, and travels with the ophthalmic division of eN V to the orbit and then to the dilator muscle. Inte rruption of the sympathetic nerve supply results in Horner syndrome, wi th miosis, in add ition to ptosis and anhydros is.
CHAPTER', The Eye. 61
Sphincter Muscle Like the dilator muscle, th e sphincter muscle is de rived frol11 neuroectoderm. It is com-
posed of a circular band of smooth muscle fibers and is located nea r the pupillary margin in the deep stroma, anterior to the pigment epithelium of the iris. Although dual innervation has been demonstrated morphologically, the sphincter muscle receives its primary innervation from parasympathetic nerve fibers that originate in the eN III nucleus, and it responds pharmacologically to muscarin ic sti mulat ion. The reciprocal sympatheti c innervation to the sphincter appears to serve an inhibitory role, helping to relax the sphincter in darkness.
The fibers subserving the sphincter muscle leave the Edinger-Westphal subnucleus and fo llow the in ferior division of eN III after it bifurcates in the cavernous sinus. The fibers continue in the branch supplying the inferior oblique muscle, exit, and synapse with postganglionic fibers in the ciliary ganglion. The postganglionic fibers travel with the short ciliary nerves to the iris sphincter. They are unusual in that they are myelinated, presumably reflecting a need for fast conduction.
The ciliary body, which is triangular in cross section, bridges the anterior an d posterior segments (see Fig 2- 10). The apex of the ciliary body is directed poster iorly toward the ora serrata. The base of the ciliary body gives rise to the iris. The only attachment of the cil iary body to the sclera is at its base, via its longitudinal muscle fibers, where they insert into the scleral spur. The ciliary body has 2 principal fun cti ons: aqueous humor formation and lens accommodation. It also plays a role in the trabecu lar and lIveoscleral outflow of aqueous humor.
Ciliary Epithelium and Stroma The ciliary body is 6- 7 111m wide and consists of 2 parts: the pars plana and the pars plicata. The pars plana is a relatively avasc ular, smooth pigmented zone; it is 4 mm wide and extends fr0111 th e ora serrata to the ciliary processes. The safest posterior surgical approach to the vit reo us cavity is through the pars plana, located 3- 4 mm from the corneal limbus. The pars pUcata is richly vasc ularized and consists of apprOXimately 70 radial folds, or ciliary processes. The zonular fibers of the lens attach primarily in the valleys of the ciliary processes but also along the pars plana. The capillary plexus of each ciliary process is supplied by arterioles as they pass anteriorly and posteriorly from the major arterial circle; each plexus is drained by 1 or 2 large venules located at the crest of each process. Sphi ncter tone within the arte riolar smooth muscle affects the capillary hydrostati c pressure gradient. In addition, it influences whether blood fl ows into the capillary plex us or directly to the draining choroidal vein, bypassing the plexus completely. Neuronal innervation of the vascular smooth muscle and humoral vasoactive substances may be important in determining regional blood flow,
62 • Fundamenta ls and Principles of Ophthalmology capillary surface area available for excha nge of fluid . and hydrostatic capillary pressure. All of these affect the rate of aqueous humo r formation. The ciliary body is lined by a double laye r of epithelial cells. the nonpigmented and the pigmented epithelium (Fig 2-18). The inner. nonpigmented. epithelium is located between the aq ueous humor of th e posterior chamber and the outer pigmented epithelium . The apices of the nonpigmented and pigmented ceUlayers are fused by a complex system of junctions and cellular interdigitations. Along the lateral intercellular spaces. near the apical border of the nonpigmented epithelium . are tight juncti ons (zomllae oCc/lldentes) that maintain the blood-aqueous barrier. The basal surface of the nonpigmented epithelium. which borders the posterior chambe r. is covered by the basal lam ina. which is mul tilaminar in the valleys of the processes. The basal lamina of the pigmented epithelium . which faces the iris stroma. is thick and more homogeneous than that of the nonpigmented epithelium. The pigmented epithelium is relatively un iform throughout the cilia ry body. Its cuboidal cells are characte rized by multiple basal infoldings. a large nucleus. mitocho ndria. extens ive endoplasmic reticulum , and man y me lanosomes. The nonpigmented epitheli um tends to be cuboidal in the pars plana region but columnar in the pars plicata. It also has multiple basal infoldings. abundant mitochondria. and large nuclei. The endoplasmic reticulum and Goigi complex in these cells are important to aqueous hUlllor for mation.
Sometimes melanosomes are present. especially anteriorly, nea r the iris. The uveal porti on of the ciliary body consists of comparatively large fenestrated capillar ies. collagen fibrils. and fibroblasts. The main arterial sup pl y to the ciliary body comes from the anterior and the long posterior ciliary arteries. wh ich join together to for m a multilayered arteria.l plexus consisting of a superfiCial episcleral plexus. a deeper intramuscular plexus, and an incomplete major arterial circle often mistakenly attributed to the iris but
Posterior chamber "'.
Nonpigmented epithelium
Apices Pigmented epithelium
Basal lamina
Stroma Figure 2-18
Ciliary epithelium.
(illustra tion by Thomas A Wemge;sl, PhD, MD.J
CHAPTER',
The Eye • 63
actually located posterior to the anterior chamber angle recess. in the ciliary body. The major veins drain posteriorly through the vortex system. although some drainage also occurs through the intrascleral venous plexus and the episcleral veins into the limbal region.
Ciliary Muscle Three layers of fibers have been described in the ciliary muscle (Fig 2-19): I. longitudinal 2. radial 3. circular
Most of the Ciliary muscle is made up of an outer layer oflongitudinal fibers that attach to the scleral spur. The radial muscle fibers arise in the midportion of the Ciliary body. and the circular fibers are located in the innermost portion. Clinically. the 3 groups of muscle fibers function as a unit. Presbyopia is associated with age-related changes in the lens (discussed in the section Lens. later in the chapter) rather than to changes in the ciliary muscle. Even so. the muscle does change with age. with increasing amounts of connective tissue between the muscle bundles and a loss of elastic recoil after contraction. The ciliary muscles behave like other smooth. nonstriated muscle fibers. Ultrastructural studies reveal that they contain multiple myofibrils with characteristic electron-dense
Canal of Sciliernm
Trabecular mesh'vork'::~~ Anterior ch"ml,er_ Iris --!f-'-- "'~
or radial fibers of Ciliary muscle
.Ciliary processes
Figure 2-19
Diagram showing the arrangement of the smooth muscle fibers in the ciliary
body. Note the relat ionsh ip of the ciliary body to the iris, the anterior chamber, the Schlemm canal, and th e corneosclerallimbus. (Reproduced with permission from Snell RS, Lemp MA. Clinical Anatomy of the Eye. Cambridge, MA: Blackwell Scientific Publications; 1989.)
64 • Fundamentals and Principles of Ophthalmology
attachment bodies, mitochondria, glycogen particles, and a prominent nucleus. The smooth muscle cells are surrounded by a basal lam ina separated from the cell membrane by a 300 A space. Bundles of fibers are surrounded by a thin fibroblastic sheath rather than by collagen. The muscle is rich in type VI collagen, which forms a sheath around the anterior elastic tendons. These tendons insert into the scleral spur and around the tips of the oblique and circular muscle fibers as they insert into the trabecular meshwork. Streeten BW. The ci liar y body. In: Duane TD, Jaeger EA, eds. Biomedical Foundation.s of Oph thalmology. Philadelphia: Lippincott; 1995.
Both myelinated and nonmyelinated nerve fibers are observed throughout the ciliary muscle. Innervation is mainly derived from parasympathetic fibers of CN IJJ via the short ciliary nerves. Approximately 97% of these ciliary fibers are directed to the ciliary muscle, and about 3% to the iris sphincter. Sympathetic fibers have also been observed and may playa role in relaxing the muscle. Cholinergic drugs contract the ciliary muscle. Because some of the muscle fibers form tendinous attachments to the scleral spur, their contraction increases aqueous flow by opening up the spaces of the trabecular meshwork.
Choroid The choroid, the posterior portion of the uveal tract, nourishes the outer portion of the retina (Fig 2-20). It averages 0.25 mm in thickness and consists of 3 layers of vessels: I. the choriocapillaris, the innermost layer 2. a middle layer of small vessels 3. an outer layer of large vessels
Perfusion of the choroid comes from both the long and the short posterior ciliary arteries and from the perforating anterior ciliary arteries. Venous blood drains through the vortex system. Blood flow through the choroid is high compared to that of other tissues. As a result, the oxygen content of choroidal venous blood is only 2%-3% less than that of arterial blood.
Bruch's Membrane Bruch's membrane is a PAS-pOSitive lamina resulting from the fusion of the basal laminae ofthe retinal pigment epithelium (RPE) and the choriocapillaris of the choroid (Fig 2-21). It extends from the margin of the optic disc to the ora serrata, and ultrastructurally it has 5 elements: I. basal lamina of the RPE 2. inner collagenous zone
3. thicker, porous band of elastic fibers 4. outer collagenous zone
5. basal lamina of the choriocapillaris Bruch's membrane, therefore, consists of a series of connective tissue sheets that are highly permeable to small molecules such as fluorescein. Defects in Bruch's membrane develop
CHAPTER 2,
The Eye • 65
Choroid - - - -
Figure 2-20
Choroid . The choriocapiliaris lies ju st below the retinal pig ment epithelium. Be-
neath are a middle and outer vascular layer and multiple dendritic melanocytes (H&E x32). (Courtesy of Thomas A Weingeist. PhD, M[?)
Apex
..• •• ..... • • '
:~. 0S~'~ ~e~ Base
Microvilli
",o,(J
Terminal web
01},
_ .. ..,""'.-.. . . .,. _.,."._,. .~_l...__ . _. r .' .
Basal lamina
Bruch's membrane ___-__ - - - ~~ .- -----~.~ --_-:--~~ Elastic layer ~-e '''''~'-''=-~T'':''··'~ '·-~ Basal lamina
Inner Outercollagenous layer
Choriocapillaris Figure 2-21 PhD, MOl
Retinal pigment epithelium and Bruch's membrane. (Illustration by Thomas A. Weingeisr,
66 • Fundamentals and Principles of Ophthalmology spontaneously in myopia or pseudoxanthoma elasticum. or they result from trauma or inflammation. SubretinaJ neovascular membranes can arise as a result of these defects. and they can lead to disciform macular changes as part of exudative age-related macular degeneration and ocular histoplasmOSis syndrome.
Choriocapillaris The choriocapillaris is a continuous layer of large capillaries (40-60 fll1l in diameter) lying in a single plane beneath the RPE (Fig 2-22). The vessel walls are extremely thin and contain multiple fenestrations. especially on the surface facing the retina (Fig 2-23). Pericytes are located along the outer wall. The middle and outer choroidal vessels are not fenestrated. The large vessels. typical of small arteries elsewhere, possess an internal elastic lamina and smooth muscle cells in the media. As a result, small molecules such as fluorescein, which diffuse across the endothelium of the choriocapillaris, do not leak through medium and large choroidal vessels. Abundant melanocytes as well as occasional macrophages. lymphocytes, mast cells, and plasma cells appear throughout the choroidal stroma. The intercellular space contains collagen fibers and nerve fibers. The degree of pigmentation observed ophthalmoscopically in' the ocular fundus primarily depends on the number of pigmented melanocytes in the choroid. Melanosomes are absent from the RPE and choroid of albinos. In lightly pigmented eyes. pigmentation in the choroid is sparse compared with that of darkly pigmented eyes. The degree of pigmentation in the choroid must be considered when one is performing photocoagulation. because it influences the absorption of laser energy.
Flgur. 2-22 Lobular pattern of choriocapiliaris. Note that the retinal pigment epithelium is internal to the choriocapillaris. A ;:: choroidal arteriole. V = choroidal venule. (Reproduced with permission from Ha yreh Ss. The choriocapillaris. Albrecht Von Grae fes Arch Ktin Exp Ophthalmol. 1974;1 92(3): 165- 779.)
CHAPTER 2, The Eye • 67 Fenestrations
Lumen
Endothelium
Basal lamina A
Pericyte
Basal lamina
Lumen
Endothelium
Basal lamina
B Figure 2·23
Pericyte
A, Fenestrated choro idal capillary.
a, Nonfenestrated ret inal capillary.
(Illustration by
Thomas A. Weingeist, PhD, MD.)
Lens The lens is a biconvex structure located directly behind the posterior chamber and pupil (Fig 2-24). The lens contributes 20 D of the 60 D of focusing power of the average adult eye. The equatorial diameter is 6.5 mm at birth and increases in the first 2 to 3 decades of life, remaining in the region of 9-\0 mm in diameter in late life. The anteroposterior width of the lens is about 3 mm at birth and increases after the second decade of life to about 6 mm at age 80 years. This growth is accompanied by a shortening of the anterior
68 • Fundamentals and Principles of Ophthalmology
Capsule Epithelium Lens fibers Posterior capsule
Figure 2·24
A, Lens: anterior capsule, epithelium, and lens fibers. B, Equator of the lens.
Note the nuclei within the lens bow and the zonular fibers. C, Posterior lens capsule. Note the absence of lens epithelium (H&E x32). (Courtesy of Thomas A. Weingeist, PhD, MO .)
radius of curvature of the lens, which would increase its optical power if it were not for a compensatory change in the refractive gradient across the lens substance. In youth, accommodation for near vision is achieved by ciliary muscle contraction, which moves the ciliary muscle mass forward and inward. This contraction relaxes zonular tension and allows the lens to assume a globular shape, causing a shortening of its anterior curvature. The increased lens thickness during accommodation is entirely due to
a change in nuclear shape. With age, accommodative power is steadily lost. Adolescents have 12-16 D of accommodation, decreasing to 2 D at'age 50. Causes of this power loss include the increased size of the lens, altered mechanical relationships, and an increased stiffness of the lens nucleus secondary to changes in the crystalline proteins of the fiber cytoplasm. Other factors, such as alterations in the geometry of zonular attachments with age and changes in lens capsule elasticity, may also playa role. The lens has certain unusual features. It lacks innervation and is avascular. After regression of the hyalOid vasculature during embryogenesis, the lens depends totally on the aqueous and vitreous for its nourishment. From embryonic life on, it is entirely enclosed by a basal lamina, the lens capsule. BCSC Section 11, Lens and Cataract, discusses the lens in depth.
Capsule The lens is surrounded by a basal lamina, the lens capsule, which is a product of the lens epithelium (Fig 2-25). It is rich in type IV collagen and other matrix proteins. Synthesis of the anterior lens capsule (which overlies the epithelium) proceeds throughout life, so that its thickness increases, whereas that of the posterior capsule remains relatively constant. Values of 15.5 fUll for the thickness of the anterior capsule and 2.8 flm for the posterior capsule have been cited for the adult lens. Morphologically, the lens capsule consists of fine filaments arranged in lamellae, parallel to the surface. The anterior lens capsule contains a fibrogranular material, identified as laminin, which is absent from the posterior capsule at the ultrastructural level. The thinness of the posterior capsule creates a potential for rupture during cataract surgery.
CHAPTER 2.
Lens
C"[}sIlIA " , X
Dividing
A
Embryonic lens Longitudinal section Suture on posterior of lens fibers
surface of lens
c
B
o
The Eye • 69
E
Organization of the lens. At areas where lens cells converge and meet, sutures are formed. A. Cutaway view of the adult lens showing embryonic lens inside. The embryonal nucleus has a V-shaped suture at both the anterior and posterior poles; in the adult lens cortex, the organization of the sutures is more complex. At the equator, t he lens epithelium can divide, and the cells become highly elongated and ribbonlike, sending processes anteriorly and posteriorly. As new lens cells are formed, older cells come to lie in the deeper parts of the cortex. B, The diagram shows, in cross section and corresponding surface view, the difference in lens fibers at the anterior (A), intermediate (B), and equatorial (e) zones. The lens capsule, or basement membrane of the lens epithelium (dJ, is shown in re lation to the zonular fibers (f) and their attachment to the lens (g). C. The diagram shows a closer view of lens sutures. and E. Optica l sections of a young adult human lens (25-year-old female) demonstrated by Scheimpflug photography. The cornea is to t he right. D. Lens in the non accommodative state. E, Lens during accommodat ion-note tha t the anterior radius of curvature is shortened in the latter case. (Parts A-C reproduced with permission from Kessel RG, Kardon RH. Tissues and Organs: A Text-Atlas of
Figure 2-25
o
Scann ing Electron M icroscopy. San Francisco : WH Freeman; 1979. Parts 0 and E courtesy of Jane Koretz.)
70 • Fundamentals and Principles of Ophthalmology
Epithelium The lens epithelium lies beneath the anterior and equatorial capsule, but it is absent under the posterior capsule. The basal aspects of the cells abut the lens capsule without specialized attachment sites. The apices of the cells face the interior of the lens, and the lateral borders interdigitate, with practically no intercellular space. Each cell contains a prominent nucleus but relatively few cytoplasmic organelles. Regional differences in the lens epithelium are important. The central zone represents a stable population of cells whose numbers slowly decline with age. An intermediate zone of smaller cells shows occasional mitoses. Peripherally, there are meridional rows of cuboidal pre-equatorial cells that form the germinative zone of the lens. Here, cells undergo mitotic division, elongate anteriorly and posteriorly, and form the differentiated fiber cells of the lens. In the human lens, cell division continues throughout life and is responsible for the continued growth of the lens. Germinative cells left behind after phacoemulsification can give rise to posterior capsular opacification as a result of aberrant proliferation and cell migration.
Fibers The lens has an outer cortex and an inner nucleus. The nucleus is the part of the fiber mass that is formed at birth, and the cortex forms as new fibers are added postnatally. In optical section with the slit lamp, lamellar zones of discontinuity are visible, differentiating the adult cortex into deep and superfiCial regions. The fiber cells are hexagonal in cross section, are spindle-shaped, and possess numerous interlocking flngerlike projections (Fig 2-26). Apart from the most superfiCial cortical fibers, the cytoplasm is homogeneous and contains few organelles. The high refractive index of the lens results from the high concentration oflens crystallins (a, p, and y) in the fiber cytoplasm. The lens sutures are formed by the interdigitation of the anterior and posterior tips of the spindle-shaped fibers. In the fetal lens, this forms the anterior Y-shaped suture and the posterior inverted Y-shaped suture: As the lens ages, further branches are added to the sutures, each new set of branch points corresponding to the appearance of a fresh optical zone of discontinuity.
Zonules (Suspensory Ligaments) The lens is held in place by a system of zonular fibers that originate from the basallaminae of the nonpigmented epithelium of the pars plana and pars plicata of the ciliary body. These fibers chiefly attach to the lens capsule anterior and posterior to the equator. Each zonular fiber is made up of multiple filaments of fibrillin that merge with the equatorial lens capsule. In Marfan syndrome, mutations in the fibrillin gene lead to weakening of the zonule and subluxation of the lens. Streeten BW. Anatomy of the zonular apparatus. In: Duane TD, Jaeger EA. eds. Biomedical Foundations of Ophthalmology. Philadelphia: Harper & Row; 1992.
When the eye is focused for distance, the zonule is under tension and the lens form is relatively flattened. During accommodation, contraction of the ciliary muscle moves the proximal attachment of the zonule forward and inward so that the lens becomes more globular and the eye adjusts for near vision (see Fig 2-26).
CHAPTER 2,
The Eye.
71
A Figure 2-26
A and 8, Scanning electron m icrographs of the relationship of lens fiber packing
and interdigitation (arrows in 8). (Reproduced with permission from Kessel RG, Kardon RH. Tissues and Organs: A Text-Alias of Scanning Electron M icroscopv. San Francisco: WH Freeman; 1979)
Retina The fundus oculi is the part of the eye that is visible on ophthalmoscopy, including the retin a and its vessels and the optic nerve head (or optic disc). The macula, 5- 6 mm in di ameter, lies between the temporal vascular arcades. At the macula's center lies the fovea, rich in cones an d responsible for color vision and the highest vis ual acuity. In the far periphery, the ora serrata (the junction between the retina and the pars plana) can be seen by gonioscopy or indirect ophthalmoscopy. The reddish color of the fundus is due to the transmission of light reflected from the posterior sclera through the capillary bed of the cho ro id. The retina is a thin, transparent structure that develops from the inner and o uter layers of the optic cup. In cross section, from outer to inner retina. its layers are • RPE and its basallamina • rod and cone inner and outer segments • external limiting membrane outer nuclear layer (nuclei of th e photoreceptors) • o uter plexiform layer • inner nuclear layer • inner plexiform layer ganglion cell layer • nerve fiber layer (axons of the ganglion cells) • internal limiting membrane The retina is also discussed in BeSe Section 12, Retina and Vitreous.
Retinal Pigment Epithelium The structure of the outer pigmented epithelial laye r is relatively simple compared with th at of the overlying inner, or neurosensory, rctina. The RPE consists of a monolayer of hexagonal cells that extends anteriorly fro m the optic disc to the ora serrata, where it merges with the pigmented epithelium of the ciliary body. Its structure is deceptively simple conSidering its many functions: vitamin A metabolism • maintenance of the outer blood-retina barrier
72 • Fundamentals and Principles of Ophthalmology
• • • • •
phagocytosis of the photoreceptor outer segments absorption of light (reduction of scatter) heat exchange formation of the basal lamina production of the mucopolysaccharide matrix surrounding the outer segments active transport of materials in and out of the RPE
Like other epithelial and endothelial cells, the RPE cells are polarized. The basal aspect is intricately folded and provides a large surface of attachment to the thin basal lamina that forms the inner layer of Bruch's membrane (see Fig 2-21). The apices have multiple villous processes that engage with the photoreceptor outer segments, embedded in a mucopolysaccharide matrix (interphotoreceptor matrix) containing chondroitin-6sulfate, sialic acid, and hyaluronic acid. Separation of the RPE from the neurosensory retina is called retinal detachment. Contiguous RPE cells are firmly attached by a series of lateral, intercellular junctional complexes. The zonulae occludentes and zonulae adherentes not only provide structural stability but also play an important role in maintaining the outer bloodretina barrier. Zonulae occludentes consist of fused plasma membranes forming a cir-
cular band or belt between adjacent cells. A small intercellular space is present between zonulae adherentes.
The retina and RPE show important regional differences (Fig 2-27). The retina is thickest in the papillomacular bundle near the optic nerve (0.23 mm) and thinnest in the foveola (0.10 mm) and ora serrata (0.11 mm). RPE cells vary from 10 to 60 flm in diameter. Compared with RPE cells in the periphery, RPE cells in the fovea are taller and thinner, they contain more melanosomes, and their melanosomes are larger. These characteristics
account in part for the decreased transmission of choroidal fluorescence observed during fundus fluorescein angiography. Cells in the periphery are shorter, broader, and less pigmented. The eye of a fetus or infant contains between 4 and 6 million RPE cells. Although the surface area of the eye increases appreciably with age, the increase in the number of RPE cells is relatively small. No mitotic figures are apparent within the RPE of the normal adult eye. The cytoplasm of the RPE cells contains multiple round and ovoid pigment granules (melanosomes). These organelles develop in situ during formation of the optic cup and first appear as nonmelanized premelanosomes. Their development contrasts sharply with that of the pigment granules in uveal melanocytes, which are derived from the neural crest and later migrate into the uvea. Lipofuscin granules probably arise from the discs of photoreceptor outer segments and represent residual bodies arising from phagosomal activity. This so-called wear-and-tear pigment is less electron-dense than the melanosomes, and its concentration increases
gradually with age. Histologically, it stains with Sudan stain and exhibits a golden yellow autofluorescence. Phagosomes are membrane-enclosed packets of disc outer segments that have been
engulfed by the RPE. Several stages of disintegration are evident at any given time. In some species, shedding and degradation of the membranes of rod and cone outer segments folIowa diurnal rhythm synchronized with daily fluctuations of environmental light.
CHAPTER',
The Eye.
73
Figure 2-27
Regi onal differences in the retina . A, Papil lomacu lar bundle. 8, Macula. C, Periph eral retina . (H&E, all same magnification ). (Courtesy of Thomas A. We ingeist PhD, MD )
The cytoplasm of the RPE also contains numerous mitochondria (involved in aerobic metabolism), rough -surfaced endoplasmic reticulum, a Golgi apparatus, and a large round nucleus. Throughout life, incompletely digested residual bodies, lipofuscin pigment, phagosomes, and other material are excreted beneath the basal lamina of the RPE. These contribute to the formation of drusen, which are accumulations of this extracellular material. They can vary in size and are commonly classified by their funduscopic appearance as either hard or soft drusen. They are typically located between the basement membrane of the RPE cells and the inner collagenous zone of Bruch's membrane. Neurosensory Retina
The neurosensory retina is composed of neuronal, glial, and vascular elements (Fig 2-28). Neuronal elements The photoreceptor layer of the neurosensory retina consists of highly specialized neuroepithelial cells called rods and cones. Each photoreceptor cell consists of an outer and an inner segment. The outer segments, surrounded by a mucopolysaccharide matrix, make contact with the apical processes of the RPE. Tight junctions or other intercellular connections do not exist between the photoreceptor cell outer segments and the RPE. The factors responsible for keeping these layers in apposition are poorly understood but probably involve active transport. The rod photoreceptor consists of an outer segment that contains multiple laminated discs resembling a stack of coins and a central connecting cilium. The microtubules of the cilium have a "9 plus 0" cross-sectional configuration rather than the "9 plus 2" configuration found in motile cilia. The rod inner segment is subdivided into 2 additional elements: an outer ellipsoid containing a large number of mitochondria and an inner myoid containing a large amount of glycogen; the myoid is continuous with the main cell body, where the nucleus is located (Fig 2-29). The inner portion of the cell contains the synaptic
74 • Fundamentals and Principles of Ophthalmology Retinal ~'illari es
,II I vessel
t- C,anqlio,n cell layer plexiform layer
plexiform layer
Blood supply retinal vessels
Blood supply choriocapillaris
Photo receptors
A Figure 2-28
A, Schematic cross section of retina demonstrating layers of ret ina and approxi-
mate location of blood s upply to these layers . (con tinues)
body, or spherule, of the rod, which is formed by a single invagination that accommodates 2 hori zontal cell processes and 1 or more central bipolar dendrites (Fig 2-30). The outer segments of the cones have a different morphology depending on their location in the retina. The extrafoveal cone photoreceptors of the retina have conical ellipsoids and myoids, and their nuclei tend to be closer to the external li miting membrane than are the nuclei of the rods. Although the structure of the outer segments of the rods and cones is similar, at least 1 important difference exists. Rod discs are not attached to the cell membrane; they are d iscrete structures. Cone discs are attac hed to the cell membrane and are thought
to be renewed by membranous replacement. The cone synaptic body, or pedicle, is more complex than the rod spherule. Cone pedicles synapse with other rods and cones as well as with horizontal and bipolar cell processes. Foveal cones have cylindrical inner segments like rods but otherwise are cytologically identical to extrafoveal cones. Horizontal cells make synaptic connections with many rod spherules and cone pedicles; they also extend cell processes horizontally through out the outer plexiform layer. Bipolar cells are oriented vertically. Their dendrites synapse with either rod or cone synaptic bodies, and their axons make synaptic contact with ganglion cells and amacrine cells in the inner plexiform layer.
CHAPTER 2: The Eye • 75
~~~~~~~~~~~~~~~~~~~~ElI- Internal limiting membrane Nerve fiber layer
Ganglion cell layer
Inner plexiform layer
Inner nuclear layer
Outer plexiform layer
Outer nuclear layer
External limiting membrane
Figure 2-28
B, Schematic diagram of cell types and histologic layers in the human ret ina . The
basic relationship between rod (R) and cone (e) photo receptors as well as bipolar (B), horizontal (H), amacrine (A), inner plexiform cell (I), and ganglion (G) neurons is depicted. Note that the Muller cell (M) extends across almost the whole thickness of the retina; the apical processes of Muller cells form the externa l limiting membrane; the foot processes of MOiler cells partially form the internal limiting membrane. (Part A modified with permission from D'Amico OJ. Diseases of the retina. N Engl J Med. 7994;331 :95-106. Illustration B by Christine Grafapp.)
The axons of the ganglion cells bend to become parallel to the inner surface of the retina, where they form the nerve fiber layer and later the axons of the optic nerve. Each optic nerve has more than 1 million optic nerve fibers. The nerve fibers from the temporal retina follow an arcuate course around the macula to enter the superior and inferior poles of the optic disc. The papillomacular fibers travel straight to the optic nerve from the fovea. The nasal axons also pursue a radial course. The visibility of the nerve fibers is enhanced when they are viewed ophthalmoscopically using green (red-free) illumination.
76 • Fundamentals and Principles of Ophthalmology
Cone
Outer plexiform layer
Rod
[
Nucleus
Outer nuclear layer Outer fiber
External limiting - - - - membrane
Myoid Inner se~ment
Ellipsoid Cilium - - - - - - Cilium
Outer segment
Discs
, Retinal pigment epithelium
Figure 2-29
,, , Rod and cone photoreceptor cells.
(Illustration by Sylvia Barker.)
CHAPTER',
FB
FMB
I
I
The Eye'
77
RB RB
FB I
• • • Synaptic ribbon
A Figure 2-30
Cone pedicle
_---''<-=-"
B
Rod spherule
Synaptic bodies of photoreceptors. H = horizonta l cell processes . A, Cone pedicle
with synapses to severa l types of bipolar cells. FB = flat bipolar, FMB = flat midget bipolar, 1MB = invaginating midget bipolar. B, Rod spherule with synapses to bipolar cells. RB = rod bipolar.
(Illustration by Sylvia Barker.)
The neuronal elements and their connections in the retina are highly complex. Many types of bipolar, amacrine, and ganglion cells exist. The neuronal elements of more than 120 million rods and 6 million cones are interconnected, and signal processing within the neurosensory retina is Significant.
Glial elements Muller cells are glial cells that extend vertically from the external limiting membrane inward to the internal limiting membrane. Their nuclei are located in the inner nuclear layer. Muller cells, along with the other glial elements (the fibrous and protoplasmic astrocytes and microglia), provide structural support and nutrition to the retina. Recent studies have provided evidence of the importance of Muller cells in retinal development and metabolism. Immunohistochemistry has shown that these cells contain cellular retinaldehyde-binding proteins, glutamine, taurine, and glutamine synthetase. Muller cells have also been shown to be involved in degradation of the neurotransmitters glutamate and gamma-aminobutyric acid (GABA). The presence of messenger RNA coding for carbonic anhydrase II implies that these cells also are important in buffering carbon dioxide liberated into the extracellular space by neurosensory elements of the retina. The production of insulin and growth factors by these cells may also be important in retinal metabolism. The retina. In: Bron AI, Tripathi Re, Tripathi BJ, eds. Wolff's Anatomy of the Eye and Orbit. 8th ed. London: Chapman & Hall; 1997.
78 • Fundamentals and Principles of Ophthalmology
Vascular elements The inner portion of the retina is perfused by branches of the central retinal artery. In 30% of eyes and 50% of people, a cilioretinal artery, branching from the ciliary circulation, also supplies part of the in ner retina. This cilioretinal artery, when present, contributes to some portion of the macular circulation in approximately 15% of people, but it may supply any portion of the retina. The retinal blood vessels are analogous to the cerebral blood vessels and maintai n the inner blood- retina barrier. This physiologic barrier is due to the single laye r of nonfenestrated endothelial cells, whose tight junctions are impervious to tracer substances such as fluorescein and horseradish peroxidase. A basal lamina covers the outer surface of the endothelium. The basement membrane contains an interrupted laye r of pericytes, or mural cells, surrounded by their own basement membrane material. Muller cells and other glial elements are generall y attached to the basal lamina of retinal blood vessels. Retinal blood vessels lack an internal elastic lamina and the continuous layer of smooth muscle cells fo und in other vessels in the body. Smooth muscle cells are occaSionally present in vessels near the optic nerve head. They become a more discontinuous layer as the retinal arterioles pass farther out to the peripheral retina. The retinal blood vessels do not ordinarily extend deeper than the middle limiting membrane. Where venu les and arterioles cross, th ey share a common basement membrane. Venous occlusive disorders are common at an arterioveno us crossing.
Stratification of the neurosensory retina The neurosensory retina can be subdivided into several layers (see Fig 2-28). The outermost layer, which is located next to the RPE, is the external limiting membrane (ELM). It is not a true membrane and is form ed by the attachment sites of adjacent photoreceptors and Muller cells. It is highly fenestrated. The outer plexiform layer (OPL) is made up of the interconnections between the photoreceptor synaptic bodies and the horizontal and bipolar cells. In the macular region, the OPL is thicker and contains more fibers, because the axons of the rods and cones become longer and more oblique as they deviate from the fovea. In this region, the OPL is known as the Henle fiber layer (Fig 2-31). At the edge of the foveola, it lies almost parallel with the internal limiting membrane. The inner nuclear layer (IN L) contains nuclei of bipolar, Muller, horizontal, and amacrine cells. The next region is formed by a zone of desmosome-like attachments in the region of the synaptic bodies of the photoreceptor cells. The retinal blood vessels ordinarily do not extend beyond this point. The inner plexiform layer (IPL) consists ofaxons of the bipolar and amacrine cells and dendrites of the ganglion cells and their synapses. The ganglion cell layer (GeL) is made up of the cell bodies of the ganglion cells that lie near th e inner surface of the retina .
The nerve fiber layer (NFL) is formed by axo ns of the ganglion cells. Normally, these axons do not become myelinated until after they pass th ro ugh the lamina cribrosa of the optic nerve.
CHAPTE R 2,
The Eye.
79
Fovea
I
...-::::::=;
1500~m
=:::,
NFL
GCl
,--_ _ _ FAZ 25Q-600
~m
OPl
.-
_or
"
#
cc::J Figure 2-31
Schema tic section through the fovea. FAZ
..
~
.
= foveal avascular
, Choroid
zone, GeL
= gan-
glion cell layer. tNL = in ner nuclear layer. tPL = inner plexiform layer. NFL = nerve fiber layer. ONL = outer nuclear layer. OPL = oute r plexiform layer/He nle fiber layer. OS = outer segments of the photoreceptors, RPE = retinal pigment epithelium.
(Illustration by
Sylvia Barker.)
Like the ELM, the internal limiting membrane (lLM) is also not a true membrane. It is form ed by the footplates of the Muller cells and attachments to the basal lamina. The basal lamina of the retina is smooth on the vitreal side but appears undulating on the retinal side, where it follows the contour of the Muller cells. The thickness of the basal lami na varies. Drexler W, Morgner U, Ghanta RK. Kartner FX, Schuman }S, Fujimoto JG. Ultra high-resolution o phthalmic o ptical coherence tomograph y. Nature Med. 2001;7(4):502 - 507.
Overall, cells and their processes in the retina are oriented perpendicular to the plane of the RPE in the middle and outer layers but parallel to the retinal surface in the inner layers. For this reason, deposits of blood or exudates tend to form round blots in the outer layers (where small capillaries are found ) and linear or flame-shaped patterns in the nerve fiber layer. At the fovea, the outer layers also tend to be parallel to the surface (Henle fiber layer). As a resu lt, radial or star-shaped patterns may arise when these extracellular spaces are filled with serum and exudate.
Macula The terms macula, macula lutea, posterior pole, area centralis,jovea, and foveola have created confusion among anatomists and clinicians. Clinical retina speCialists tend to regard the macula as the area within the temporal vascular arcades. Histologically, it is the region with more than 1 layer of ganglion cell nuclei (see Fig 2-28; Figs 2-32, 2-33). See also BCSC Section 12, Retina and Vitreous. Orth DH, Fine BS. Fagma n W. Quirk TC. Clarificat ion offoveomacular nomenclature and grid for quanti tat ion of macular di sorders. Tran s Sect Oph thalmol Am Acad Ophthalmol Otolarytlgol. 1977;83(3 PI 1),OPS06-S 14.
80 • Fundamental s and Principles of Ophthalmology
Figure 2·32
Light micrograph of the macula, Compare with Figure 2-31 .
(Courtes y of Thomas A.
Weingeisr. PhD, MD.)
.~::::::=):===tParafoveal zone ~~, Perifoveal zone
ILM
!
Foveola ~
Fovea externa Figure 2-33 Anatom ical macula; also called area centra/is . The anatomical fovea and foveola are contained within the center of the macula. ELM = external limiting membrane, ILM = internal limiting membrane.
CHAPTER 2,
The Eye.
81
The name macula lutea ("yellow spot") derives from the yellow color of the central retina in dissected cadaver eyes; this color is due to the presence of carotenoid pigments, chiefly located in the Henle fiber layer. Two major pigments have been identified- zeaxanthin and lutein-whose proportions vary with distance from the fovea: the lutein to zeaxanthin ratio is 1:2.4 in the central area (0.25 mm from the fovea) and greater than 2:1 in the periphery (2.2-8.7 mm from the fovea). This variation in pigment ratio corresponds to the rod-to-cone ratio. Lutein is more concentrated in rod-dense areas of the retina; zeaxanthin is more con-
centrated in cone-dense areas. Lipofuscin, the yellow age pigment, has been observed in the cytoplasm of the perifoveal ganglion cells by electron microscopy. The fovea is a concave central retinal depression approximately 1.5 mm in diameter; it is comparable in size to the optic nerve head (see Fig 2-31). Its margins are clinically inexact, but in younger subjects the fovea is evident ophthalmoscopically as an elliptical light reflex that arises from the slope of the thickened [LM of the retina. From this point inward, the basal lamina rapidly decreases in thickness as it dives down the slopes of the fovea toward the depths of the foveola, where it is barely visible, even by electron microscopy.
Around the fovea is the parafovea, 0.5 mm wide, where the GeL, the INL, and the OPL are thickest; surrounding this zone is the most peripheral region of the macula, the perifovea, 1.5 mm wide. The masking of choroidal fluorescence observed in the macula during fundus fluorescein angiography is caused partly by xanthophyll pigment and partly by the higher melanin pigment content of the foveal RPE. The foveola is a central depression within the fovea, located approximately 4.0 mm temporal and 0.8 mm inferior to the center of the optic disc. It is approximately 0.35 mm across and 0.10 mm in thickness at its center. The borders of the foveola merge imperceptibly with the fovea. The nuclei of the photoreceptor cells in the region of the foveola bow forward toward the ILM to form the fovea externa. Usually, only photo receptors, Miiller cells, and other glial cells are present in this area. Occasionally, light microscopy reveals ganglion cell nuclei just below the ILM. The photoreceptor layer of the foveola is made up entirely of cones, whose close packing accounts for the high visual acuity for which this small area is responsible. The foveal cones are shaped like rods but possess all the cytologic characteristics of extramacular cones. The outer segments are oriented parallel to the visual axis and perpendicular to the plane of the RPE. [n contrast, the peripheral photoreceptor cell outer segments are tilted toward the entrance pupil. Thefoveal avascular zone (FAZ), or capillary-free zone (see Fig 2-31; Fig 2-34), is an important clinical landmark in the treatment of subretinal neovascular membranes by laser photocoagulation. Its location is approximately the same as that of the foveola, and its appearance in fundus fluorescein angiograms varies greatly. The diameter of the FAZ varies from 250 to 600 ~m or more; often, a truly avascular, or capillary-free, zone cannot
be identified.
82 • Fundamentals and Principles of Ophth almology
Figure 2·34 Scanning electron micrograph of a retinal vascular cast at the fovea, showing the foveal avascular zone and underlying choriocapillaris.
Ora Serrata The ora serrata is the boundary between the retina and the pars plana. Its distance from the Schwalbe line is between 5.75 mm nasally and 6.50 mm temporally. In myopia. this distance is greater; in hyperopia. it is shorter. Bruch's membrane extends anteriorly, beyond the ora serrata. but is modified because there is no choriocapillaris in the ciliary body. At the ora serrata. the diameter of the eye is 20 mm and the circumference is 63 mm; at the equator. the diameter is 24 mm and the circumference is 75 mm. Topographically. the ora serrata is relatively smooth temporally and serrated nasally. Retinal blood vessels end in loops before reaching the ora serrata. The ora serrata is in a watershed zone between the anterior and posterior vascular
system. which may in part explain why peripheral retinal degeneration is relatively common. The peripheral retina in the region of the ora serrata is markedly attenuated. The photo receptors are malformed. and the overlying retina frequently appears cystic in paraffin sections (Blessig-Iwanoff cysts) (Fig 2-35) .
Vitreous The vitreous cavity occupies four fifths of the volume of the globe. The transparent vitreous humor is important to the metabolism of the intraocular tissues because it provides a route for metabolites used by the lens. Ciliary body. and retina. Its volume is close to 4.0 mL. Although it has a gel-like structure. the vitreous is 99% water. Its viscosity is approximately twice that of water. mainly due to the presence of the mucopolysaccharide hyaluroniC acid (Fig 2-36). At the ultrastructural level. fine collagen fibrils (chiefly type II) and cells have been identified in the vitreous. The origin and function of these cells are unknown. They have been termed hyalocytes and probably represent modified histiocytes. glial cells. or
CHAPTER',
The Eye • 83
Figur.2-35 Ora serrata. Note the malformed appearance of the peripheral retina and the cystic chang es at the junction between the pars plana and th e retina (H&E x32). (Courtesy of Thomas A Wemgelst. PhD, MD.)
Connecting fibril
Figure 2·36 Three-dimensional depiction of the molecular organization of the vitreous, showing the dissociation between hyaluronic acid molecules and collagen fibrils. The fibrils are packed into bundles, and the hyaluronic acid forms molecular "coils" that fill the intervening spaces to provide channels of liquid vitreous. (Reproduced with permiSsion from Sebag J, Balazs EA Morphology and ultrastructure of human vitreous fibers. Invest Ophlhalmol Vis Sci. 1989;30(8): 1867-1871.)
molecular coils
fibro blas ts. T he fibril s at the vitreous base me rge with the basal lamina of the no npigmented epitheliu m of the pars plana and the I LM of the retina. The vitreous ad heres to the retina peripherally at the vitreous base, which extends from 2.0 mm anterior to the ora serrata to approxi_mately 4.0 mm posterior to the ora serrata. Ad dit io nal attachments exist at the disc margin, at the peri mac ul ar region, along th e
84 • Funda menta ls and Principles of Ophthalmol ogy
reti nal vessels, and at th e peri phery of th e posterior lens capsul e. T he vitreolls becomes mo re flui d wit h age an d freq uen tly se parates from the inn er re tin a (pos te rior vi treous detachm ent) (Fig 2-37). The asso ciated pe ri phera l ret in al tractio n is a po tent ial cause of rh egmatoge nous retin a l detachm en t (F igs 2-38 thro ugh 2-40).
Figure 2·37 Typical posterior vitreous detachment. The cortical vitreous initially separates from the retina in the posterior pole and the superior
quadran ts . The detachment may then progress farther anteriorly until reaching the posterior margin of the vitreous base in the inferior quadrants. (Reproduced with permiSSion from Michels RG. Wilkinson CP, Rice TA. eds. Retinal Detachment. St Louis: Mosby; 1990.)
Figure 2·38 Localized posterior extension of the vitreous base with firm underlying area of vitreo· retinal attachment may result in greater traction in that area (large arrow) than along the adjacent vitreous base (smafler arrows), (Reproduced with permission from Michels RG, Wilkinson CR Rice TA, eds. Retina l Detachment. 5t Louis; Mosby; 1990.)
Figure 2-39 A , Traction from the posterior vitreous surface on a site of firm vit reoretinal attachment is the usual mechanism causing a retinal break. B, Persistent traction on the flap of the retinal tear and fluid currents in the vitre· ous cavity contribute to retinal detachment. (Repro-duced with permission from MIchels RG, Wilkinson CP, Rice TA, eds. Retinal Detachment. Sr Louis: Mosby; 1990.)
I
CHAPTER 2:
The Eye. 85
Ll1 nd ~ An d ersen
H, Sa nd er B. The vitreous. In: Kaufma n PL, AIm A, eds. Adler's Physiology oj the Eye. 10th ed. SI LOlli s: Mosby; 2003:293- 31 6.
During embr yonic develo pment. regression of the h)'aloid vasculature results in the for mation o f an S-shaped channel (the Cloquet ca nal ). wh ich passes sinuously fro m a poin t slightl y nasa l to th e posterior pole of th e lens (M ittendorf dot; Fig 2-4 I ) to th e m argin of th e op ti c nerve head. Remnants of this fetal vasc ul ature may be obse rved clinically on th e nerve head in th e adult (vascular loops and Bergmeister papilla).
Figure 2-40 Fundus ph oto of a flap ret inal tear w ith associated retinal detachment. (Courtesy of James Folk, M D.)
Figure 2-41 Mittendorf dot. In some individuals, a remnant of the hyaloid vasculature is visible on the posterior pole of the lens, as a normal variant. (Reproduced with permission from Thiel R. A tlas of Diseases of the Eye. Amsferdam: Elsevier; 1963.)
CHAPTER
Cranial Nerves: Central and Peripheral Connections
Cran ial nerves (CN) I - VI are depicted in Figure 3- 1 in relation to the bony canals and arteries at the base of the skuLl . The reader may fi nd it useful to refer back to this fi gure as each ofthe cranial nerves is discussed. CN VII is discussed later in the chapter. For furth er stud y, BCSC Section 5, Neum -Ophthalmology, describes the cranial nerves and their fun ction and dysfunction in detail.
Cranial Nerve I (Olfactory) Cran ial nerve I originates from small olfactory receptors in the mucous membrane of the nose. Unmyelinated CN I fibers pass from these receptors in the nasal cavity through the cribriform plate of the ethmoidal bone and enter the ventral surface of th e olfactory bulb, where they form the nerve. The olfactory tract runs posteriorl y fro m the bulb, beneath the fro ntal lobe of the bra in in a groove (o r sulcus) and lateral to the gy rus rectus (Fig 3-2). The gyrus rectus forms the ante rol ate ral border of the suprasellar cistern. Meningiomas aris ing fro m the
arachnoid cells in this area can produce important ophthalmic signs and symptoms associated with loss of olfactio n.
Cranial Nerve II (Optic Nerve) The optic nerve consists of more than I million axo ns that o ri ginate in th e ganglio n cell layer of the retina and extend towa rd the occipital cortex. The optic nerve Illay be divided into the foLlowing topographic areas: intraocular region of the optic nerve: optic disc, or nerve head; preiaminar area; and laminar area • intraorbital region (located withi n the muscle co ne) • intracanalicuiar region (located withi n th e optic canal) intracranial reg io n (ending in the o ptic chiasm ) See Table 3- 1 for a summary of regional differences. The organizat ion of the optic nerve is sim ila r to that of the white matter of the brain. Developmentally, the optic ne rve is part of the brain , and its fibers are surround ed by glial (and not Schwa nn cell) sheaths. The optic
87
88 • Fundamentals and Principles of Ophthalmology
ACF
Figure 3~1 View from the right parietal bone looking downward into the skull base, showing the relationship between the bony canals IAI. nerves IB), and arteries IC) at the base of the skull. The orbits are located to the right, out of the picture Ithe roof of the orbits is just visible). The floor of the right middle cranial fossa is in the lower part. A, AC = anterior clinoid, ACF = anterior cranial fossa, CC = carotid canal, FO = foramen ovale, FR = foramen rotundum, M CF = middle cranial fossa, OF = optic foramen, PC = posterior Clinoid, SOF = superior orbital fissure, ST = sella turcica. B, I = olfactory nerve; /I = optic nerve; 11/ = oculomotor nerve; IV = trochlear nerve; V = trigeminal nerve, with ophthalmic V" maxillary V2• and mandibular V3 divisions; VI = abducens nerve; TG = trigeminal ganglion. C, ACoA and arrowhead = anterior communicating artery, BA = basilar artery, ICA = internal carotid artery, MCA = middle cerebral artery, OA = ophthalmic artery, PCA = posterior cerebral artery, PCoA = posterior communicating artery_ (Reproduced
PC ST
A
v
B
With permiSSion (rom Zide 8M, Jelks
Gw, ads Surgical Anatomy of the Orbit. New York: Raven; '985.)
c
OF AC
CHAPTER 3:
Cranial Nerves: Central and Pe ripheral Connections .
89
Longitudinal fissure
,-_ _--'1: Anterior cerebral artery Frontal lobe
\
Anler;or common;cal;ng artery
Optic nerve and chiasm --'--+£'1~I9~~_ lnternal carotid arlery Middle cerebral artery
Lateral olfactory stria - Anterior pertorated substance
~
h - - Posterior communicating artery Optic tract Mammillary body
Figure 3-2
Inferior su rfa ce of the brain, depicting CN I and CN II and surrounding structures.
(Illustration by Thomas A. Weingeis(, PhD, MD.)
Tabl e 3- 1
Regional Differences in the Optic Nerve
Segment
Length
Di ameter
Imml
(mOl)
Intraocular optic disc prelaminar laminar Intraorbital Intracanalicular Intracranial
1.0 1.5 x 1.75
25
3-4
4-10 10
4-7
Blood Supply Retin al arterioles Branches of posterior cilia ry arteries
In trane ural branches of cent ral retinal arte ry; pial branches fro m eRA and choroid Ophthalm ic artery Branches of interna l carotid and ophthalm ic arte ries
nerve va ri es in length from 35 to 55 ml11 and ave rages 40 111111. Part of the intraoc ular portion of the op ti c nerve is vis ible op hthalmoscopicall y as the optic nerve head, o r op ti c disc. The in traorbital por tio n is 25- 30 mm long, which is g reater than the dista nce between the back of th e globe and th e optic ca nal ( 18 mm ). Forthis reason, when the eye is in th e pri mary position, the optic nerve ru ns a sinuous co urse.
Intraocular Region The optic nerve head is th e principal site of many congen ital and acquired ocular d iseases; th erefore, de tailed knowledge of its anatomy is important for the p racticing ophthalmo lo gist. Its anterio r sur face is visible ophtha lmosco picall y as th e optic disc, an oval stru cture l11easu ri ng 1.5 111111 ho ri zo ntall y an d 1.75 111111 ver ticall y, with a cup -shaped depressio n, th e phYSiologic cup, located sl ightl y temporal to its geometric center. The main branches of
90 • Fundamenta ls and Princ ipl es of Ophthalmology the cen tra l retinal arte ry (CRA) and central retinal ve in (C RV) pass through th e cen te r of th e cup. The optic nerve head ca n be described in 4 parts: I. superficial nerve fiber layer 2. prelaminar area 3. laminar area 4. retrolamin ar area
Superficial nerve fiber layer As th e non m yeli nated ganglion cell axons ente r the nerve head, they retain their retinotopi c organization. with fibers from the upper retina above and those from the lower
retina below. Fibers from the temporal retina are lateral; those from the nasal side are medi al. Macular fibe rs, constitutin g about one t hird of th e nerve, are late rally placed. In th e nerve head, foveal fibe rs a re located pe riph era ll y and p eripapillary fib ers, ce ntrall y. Prelaminar area The ganglio n cell axons that enter the ne rve head a re supported by a "wicker basket" of astrocytic glial cells and segregated into bundles, o r fascicles, that pass through the lamina cf ibro sa. These astrocytes in vest the opt ic nerve and form co ntinu o us circular tubes that en cl ose groups of ner ve Abers throughout th eir intraocular and intraorbital course, sepa-
rating th em from connecti ve ti ssue ele me nts at aU sites. No Mu ll e r cell s are prese nt in the nerve head, but the astrocytes form an inter nal lim iting membra ne ( ILM) th at covers th e surface of the nerve head and is cont inu ous with th at of the retina. Astrocytes make up
10% of th e ne rve head vol um e. When the optic nerve is damaged, axo ns a nd supp ortin g glial elements ca n be lost, resultin g in pathologic en largeme nt of th e op ti c cup. This cupping may be th e firs t o bjective s ign o f dam age from glauco ma. The ret inal layers terminate as they approach the edge
of the optic disc. T he MU lie r cells th at ma ke up the ILM a re replaced by astrocytes. The pigment epithel ium may be ex posed at the te mporal marg in of the disc to form a narrow pig mented crescent. When the pigment e pithelium and cho ro id fa il to reach th e temporal margin. crescents of partia l o r absent pigme ntation ca n be seen. The relations hi p of the
choro id to the prelaminar portion of th e o ptic nerve partly accounts for th e staining of the disc no rm all y observed in late phases of fluo rescei n fundus a ngiograp hy. The di sc vessels do not leak, but the choro idal capUiari es are freely permeab le to fluorescein, which ca n th e refore diffuse into the lamina.
Laminar area The lamina cribrosa comp ri ses approxi mately 10 con necti ve ti ssue plates, which are integ rated with th e scl era and whose pores transmit the axon bundles. The openings are wider above th an below. which may impl y less pro tecti on from the mec hanical effects of pressure in
glaucoma. The lamina contains type [ and type III collagens, abu ndant elastin, and lam in in and flb ronectin . Astrocytes surround the axon bundles, and small blood vessels a re prese nt. The lam ina c ribrosa serves the following func ti ons: scaffold for the o ptic ne rve axons, po int
of fLxation fo r the CRA and CRY, and reinforcement of the poste ri or segment of the globe.
CHAPTER 3:
Cranial Nerves: Central and Peripheral Connections. 91
Retrolaminar area Behind the lamina cribrosa, the optic nerve increases to 3 mm in diameter as a result of
myelination of the nerve fibers and the presence of oligodendroglia and the surrounding meningeal sheaths (pia, arachnoid, and dura) (Fig 3-3). The retrolaminar nerve continues proximally (as the intraorbital part of the optic nerve) to the apex of the orbit. The axoplasm of the neurons contains neurofilaments, micro tubules, mitochondria, and smooth endoplasmic reticulum.
Intraorbital Region Annulus of Zinn The intraorbital part of the optic nerve lies within the muscle cone. Before passing into the optic canal, the nerve is surrounded by the annulus of Zinn, which is formed by the origins of the rectus muscles. The superior rectus and the medial rectus partially originate from the sheath of the optic nerve. This connection may partly explain why patients with retrobulbar neuritis complain of pain on eye movement. At the optic canal, the dural sheath of the nerve fuses to the periosteum, completely immobilizing the nerve.
Meningeal sheaths The pia mater is the innermost layer of the optic nerve sheath. It is a vascular connective tissue coat, covered with meningothelial cells, which sends numerous septa into the optic nerve, dividing its axons into bundles. The septa continue throughout the intraorbital and intracanalicular regions of the nerve and end just before the chiasm. They contain collagen, elastic tissue, fibroblasts, nerves, and small arterioles and venules (Fig 3-4). They provide mechanical support for the nerve bundles and nutrition to the axons and glial cells. A mantle of astrocytic glial cells prevents the pia and septa from direct contact with nerve axons. The arachnoid mater, which is composed of collagenous tissue, small amounts of elas-
tic tissue, and meningothelial cells, lines the dura mater and is connected to the pia across the subarachnoid space by vascular trabeculae. The subarachnoid space ends anteriorly at the level of the lamina cribrosa. Posteriorly, it is usually continuous with the subarachnoid space of the brain. Because the central retinal vessels cross this space, a rise in intracranial pressure can compress the retinal vein and raise the venous pressure within the retina above the intraocular pressure. This situation causes the loss of spontaneous venous
pulsation at the nerve head. Such an absence of pulsation may clinically indicate raised intracranial pressure. The thick dura mater encases the brain and makes up the outer layer of the meningeal sheath of the optic nerve. It is 0.3-0.5 mm thick and consists of dense bundles of collagen
and elastic tissue that fuse anteriorly with the outer layers of the sclera. The meninges of the optic nerve are supplied by sensory nerve fibers, which account in part for the pain experienced by patients with retrobulbar neuritis and other inflammatory optic nerve diseases.
92 • Fundamentals and Principles of Ophth almo logy
Figure 3-3
Three-dimensional drawing of the optic nerve head. Where the retina terminates at the optic disc edge, the Muller cells 11 a) are continuous wit h the astrocytes, forming the internallimiting membrane 11 bJ. (2) The optic nerve cup. At the posterior termination of the choroid on the temporal side, the border tissue of Elschnig (3) lies between the astrocytes surrounding the optic nerve cana l (4) and the stroma of the choroid. On the nasa l side, the choroidal stroma is direct ly adjacent to th e astrocytes su rrounding the nerve . This collect ion of astrocytes surroundi ng the cana l is known as the "border ti ssue," which is continuous with a similar gl ial lining (5) at the termination of the retina . The nerve f ibers of the ret ina are segregated into approximately 1000 fascicles by astrocytes (6). On reaching the lamina cribrosa, or cr ibriform plate (upper dotted line), the nerve fascicles (7) and their surrounding astrocytes are separated from each other by connect ive tissue. The cribriform plate is an extension of scleral collagen and elastic fibers through the nerve. The exte rna l choroid also sends some conn ective tissue to the an terior part of the lamina . At the external part of the lamina cribrosa (lower dotted line), the nerve fibers become myelinated, and columns of oligodendrocytes and a few astrocytes are presen t w ithin the nerve fascicles . The bundles continue to be separated by connective tissue septa all the way to the chi asm (Sep). Th e septa are derived from the pia mater. Th is connective t issue is also derived from the pia mater and is known as the "septal tissue." A mantle of astrocytes (GI.M), continuous anteriorly with the border tissue, surroun ds the nerve along its orbital cou r.se. The dura iOu), arachnoid (Ar), and pia mater (Pia) are shown. The nerve f ibers are myelina ted. Within the bundles, the cell bodies of astrocytes and oligodendrocytes form a column of nuclei (GI. C). The central ret inal vessels are surrounded by a perivascular connective tissue th roughou t its course in the nerve. This connective tissue blends wi th the connective tissue of the lamina cribrosa and is called th e "cen tral supporting connective tissue stra nd" here. (Reproduced with permission from Anderson DR, Hovt WF. Ultras truc ture of intraorbital portion of human and monkev optic nerve. Arch Ophthalmol. 1969;82(4) :507.)
CHAPTER 3: Cranial Nerves: Central a nd Periphera l Connections. 93
Dura mater
Arachnoid mater
Pia mater
Figure 3-4 Meningeal sheaths . The dura mater, the outer layer, is composed of collagenou s connective tissue. The arachnoid mater, the middle layer, is made up of fine collagenou s fibers arranged in a loose meshwork lined by endothelial cells . The innermost layer, the pia mater, is
made up of fine collagenous and elastic fibers and is highly vascularized. Elements from both the arachnoid and the pia are continuous with the optic neNe septa (Masson trichrome x64). (Courtesy of Thomas A. Weingeisr, PhD, MD,)
Intracanalicular Region Within the optic canal, the blood supply of the optic nerve is derived from pial vessels originating from the ophthalmic arte ry. The optic nerve and surrounding arachnoid are tethered to the periosteum of the bony canal within the intracanalicuiar region. Blunt trauma, particularl y over the eyebrow, can transmit the force of injury to the intracanalieular region, causing shearing and interruption of the blood supply to the nerve in this area, which is called indirect traumatic optic neuropathy. In addition, optic nerve edema in this area can produce a compartment syndrome. further compromising the function of the optic nerve within the confined space of the optic canal.
Intracranial Region After passi ng through the optic canals, the 2 optic nerves lie above the ophthalmic arteries, above and medial to the internal carotid arteries (leAs). The anterior cerebral arteries cross over the optic nerves and are connected by the anterior communicating artery, which completes the anterior portion of the circle of Willis. The optic nerves then pass posteriorly over the cavernous sinus to join in the optic chiasm. The chiasm then divides into right and left optic tracts, which end in their respective lateral geniculate bodies. From these bodies arise the genieulocalcarine pathways (or optic radiations), which pass to each primary visual cortex. Lesions at different locations along the visual pathway produce charac teristic visual field defects that help to localize the site of damage (Fig 3-5).
94 • Fundamentals and Principles of Ophthalmology Right
Left
) Visual Field Oefects
Figure 3-5 The visual pathway and the circle of Willis. (Illustration by Thomas A. Weingeisr,
PhD, MD.)
Temporal lobe Meyer loop ~--t1~'i'
Lateral geniculate body
~~: ()()c ~~~D
Optic radiation
Occipital lobe
Blood Supply of the Optic Nerve The ophthalmic artery lies below the optic nerve. The CRA and, usually, 2 long posterior ciliary arteries branch off from the ophthalmic artery once it has entered the muscle cone at the annulus of Zinno The blood supply of the optic nerve varies from one segment of the nerve to another. Although the blood supply can vary Widely, a basic pattern has emerged from a multitude of studies. The arterial supply of the optic nerve head is as follows: the retrolaminar nerve is supplied chiefly by pial vessels and short posterior Ciliary vessels, with some help from the CRA and recurrent choroidal arteries. The lamina is supplied by short posterior ciliary arteries or by branches of the arterial circle of Haller and Zinn (circle ofZinn-Haller). This circle arises from the paraoptic branches of the short posterior ciliary arteries and is usuaUy embedded in the sclera around the nerve head. It is often incomplete and may be divided into a superior and an inferior half. There is no supply from the CRA in this region. The prelaminar nerve is supplied by the short posterior ciliary arteries (cilioretinal arteries, if present) and recurrent choroidal arteries, although their relative contribution is debated. The nerve fiber layer is supplied by the CRA (Figs 3-6, 3-7). The posterior ciliary arteries are terminal arteries) and the area where the respective capillary beds from each artery meet has been termed the watershed zone. When perfusion pressure drops, the tissue lyi ng within this area is the most vulnerable to ischemia. Consequences can be significant when the entire optic nerve head or a part of it lies within the watershed zone. The intraorbital region of the optic nerve is supplied proximally by the pial vascular network and by neighboring branches of the ophthalmic artery. Distally, it is also supplied by intraneural branches of the CRA. Most anteriorly, short posterior ciliary arteries and occasional peripapillary choroidal arteries contribute.
CHAPTER 3:
Retinal vein
Cranial Nerves: Central and Periph eral Connections. 95 Ch
Figur.3-6 Diagram of blood supply of the opti c nerve head and intraorbital optic nerve. A = arachnoid, Ch = choroid, ColBr = collateral branches, CRA = central retinal artery, CRV = central retinal vein, 0 = dura, LC = lamina cribrosa, 00 = optic disc, ON = optic nerve, PCilA = posterior ciliary arteries, PLR = prelaminar region, R = retina,S = sclera, SAS = subarachnoid space. (Reproduced with permission from Ha yreh 55. Anatomy and physiology of the optic nerve head. Trans Am Acad OphthalmolOtolaryngol. 1974:78(2):OP240-254.J
Figur.3-7 Vascular supply of the optic nerve head. Retrofaminar: I, pial supply; 2, recu rrent short posterior ciliary arterioles; 3, pial -derived longitudinal arterioles; 4, large pia l vessels; 5, branches of CRA; 6, scleral short posterior arteries. Prelaminar: 7, branch of short poste-
rior ciliary artery enters nerve; 8, occasional choroidal supply; 9, choriocapillary anastomosis and epipapillary and peripapillary branches of the CRA; 10, both epipapillary and peripapillary branches of the eRA anastomose with prelaminar vessels in this area. eRA = central retinal artery, SNFL = superficial nerve fiber layer, SPCA = short posterior ciliary arteries. (Reprinted from Lieberman MF. Maumenee AE, Green WR. Histologic studies of the vasculature of the anterior optic nerve. Am J Ophthalmol . 1976;82(3):405-423, C> 1976, with permission from Elsevier Science.}
The intracanalicular region of the optic nerve is supplied almost exclusively by the ophthalmic artery. The intracranial region of the optic nerve is supplied primarily by branches of both the l eA and the ophthalmic artery.
96 • Fundamentals and Principles of Ophthalmology
Central retinal artery and vein The lumen of the CRA is surrounded by non fenestrated endothelial cells with typical zonulae occludentes like those in retinal vessels. The CRA, however, differs from retinal arterioles in that it contains a fenestrated internal elastic lamina and an outer layer of smooth muscle cells surrounded by a thin basement membrane. The retinal arterioles have no internal elastic lamina, and they lose their smooth muscle cells shortly after entering the retina. The CRY consists of endothelial cells, a thin basal lamina, and a thick collagenous adventitia.
Chiasm The optic chiasm makes up part of the anterior inferior floor of the third ventricle. It is surrounded by pia and arachnoid and is richly vascularized. The chiasm is approximately 12 mm wide, 8 mm long in the anteroposterior direction, and 4 mm thick. The extramacular fibers from the inferonasal retina cross anteriorly in the chiasm at the Wilbrand k.nee before passing into the optic tract. Extramacular supranasal fibers cross directly to the opposite tract. Extramacular temporal fibers remain uncrossed in the chiasm and optic tract. The macular projections are located centrally in the optic nerve and constitute 80%-90%
of the total volume of the optic nerve and the chiasmal fibers. The temporal macular fibers pursue a direct course through the chiasm as a bundle of uncrossed fibers. Nasal macular fibers cross in the posterior part of the chiasm. Approximately 53% of the optic nerve fibers are crossed, and 47% are uncrossed.
Optic Tract Each optic tract contains ipsilateral temporal and contralateral nasal fibers from the optic nerves. Fibers (both crossed and uncrossed) from the upper retinal projections travel medially in the optic tract; lower projections move laterally. The macular fibers adopt a dorsolateral orientation as they course toward the lateral geniculate body.
Lateral Geniculate Body The lateral geniculate body, or nucleus, is the synaptic zone for the higher visual projections. It is an oval, cap like structure that receives approximately 70% of the optic tract fibers within its 6 alternating layers of gray and white matter. Layers 1, 4, and 6 of the lateral geniculate body contain axons from the contralateral optic nerve. Layers 2, 3, and 5 arise from the ipsilateral optic nerve. The 6 layers, numbered consecutively from below upward, give rise to the optic radiations.
Optic Radiations The optic radiations connect the lateral geniculate body with the cortex of the OCCipital lobe. The fibers of the optic radiations leave the lateral geniculate body and wind around the temporal horn of the lateral ventricle, approaching the anterior tip of the temporal lobe (the so-called loop of Meyer). They then sweep backward toward the visual area of the occipital lobe. Damage to the optic radiation in the anterior temporal lobe gives rise to a wedge-shaped, upper homonymous "pie in the sky" visual field defect.
CHAPTER 3: Cra nial Nerves: Central and Peripheral Connections . 97
Visual Cortex The visual cortex. the thinnest area of the human cerebral cortex. has 6 cellular layers and occupies the superior and inferior lips of the calcarine fissure on the posterior and medial surfaces of the occipital lobes. Macular function is extremely well represented in the visual cortex and occupies the most posterior position at the tip of the OCcipital lobe. The most anterior portion of the calcarine fissure is occupied by contralateral nasal retinal fibers only. The posterior cerebral artery. a branch of the basilar artery. supplies the visual cortex almost exclusively. The blood supply to the OCCipital lobe does show anatomical variation. however. with the middle cerebral artery making a contribution in some individuals. Trobe ID. The Neurology of Vision . New York: Oxford University Press; 200 1:1- 42.
Cranial Nerve III (Oculomotor) Although CN III contains only 24.000 fibers. it supplies all the extraocular muscles except the superior oblique and the lateral rectus. It also carries cholinergic innervation to the pupillary sphincter and the ciliary muscle. Cranial nerve III arises from a complex group of cells in the rostral midbrain. or mesencephalon. at the level of the superior colliculus. This nuclear complex lies ventral to the periaqueductal gray matter. is immediately rostral to the CN IV nuclear complex. and is bounded inferolaterally by the medial longitudinal fasciculus. The CN III nucleus consists of several distinct. large motor cell subnuclei. each of which subserves the extraocular muscle it innervates (Fig 3-8). Except for a single central
Cranial
Cranial
Figur.3-8 Diagram of the oculomotor nuclear complex (eN 1111. supplied by different subnuclei. Note tha t a central caudal nucleus supplies both levator muscles and tha t the nucleus for each superior rectus supplies the contralateral muscle. EW = Edinger-Westphal nucleus, 10 = nucleus to the inferior oblique mu scl e, IR = nucleus to the inferior rectu s muscle, LP = nucleus
to the levator palpebrae muscle. MR
= nucleus to the medial rectus muscle, SR = nucle us to
the superior rectus muscle. (Illus tration by Sylvia Barker.}
98 • Fundamentals and Principles of Ophthalmology caudal nucleus that serves both levators, the cell groups are paired. Fibers from the superior rectus cross in the caudal aspect of the nucleus and therefore supply the contralateral superior rectus muscles.
The Edinger-Westphal nucleus is cephalad and dorsomedial in location. It provides the parasympathetic preganglionic efferent innervation to the ciliary muscle and pupillary sphincter. The most ventral subnuclei supply the medial rectus muscles. A subnucleus for ocular convergence has been described but is not found consistently in primates. The fascicular portion of CN III travels ventrally from the nuclear complex, through the red nucleus, between the medial aspects of the cerebral peduncles, and through the corticospinal fibers. It exits in the interpeduncular space. In the subarachnoid space, CN III passes below the posterior cerebral artery and above the superior cerebellar artery, the 2 major branches of the basilar artery (Fig 3-9). The nerve travels forward in the interpeduncular cistern lateral to the posterior communicating artery and penetrates the
arachnoid between the free and attached borders of the tentorium cerebelli. Aneurysms that affect CN III commonly occur at the junction of the posterior communicating artery and the ICA. The nerve pierces the dura on the lateral side of the posterior clinoid process, initially traversing the roof of the cavernous sinus. It runs along the lateral wall of the cavernous sinus and above CN IV and enters the orbit through the superior orbital fissure. Cranial nerve III usually divides into superior and inferior divisions after passing through the annulus of Zinn in the orbit. Alternatively, it may divide within the anterior cavernous sinus. The nerve maintains a topographic organization even in the midbrain, so that lesions almost anywhere along its course may cause a divisional nerve palsy.
The superior division of CN III innervates the superior rectus and levator palpebrae muscles. The larger inferior division splits into 3 branches to supply the medial and inferior rectus muscles and the inferior oblique.
The parasympathetic fibers wind around the periphery of the nerve, enter the inferior division, and course through the branch that supplies the inferior oblique muscle. They join the ciliary ganglion, where they synapse with the postganglionic fibers, which emerge as many short ciliary nerves. These pierce the sclera and travel through the choroid to innervate the pupillary sphincter and the ciliary muscle. The superficial location of these Figure
3~9
Cross
section
through the midbrain at the level of the CN III nucleus. Note the
~-:5u~)eriior colliculus
re lationship be-
tween CN III and the posterior cerebral, superior cerebellar, and posterior communicating arteries . CA = cerebral aqueduct, CC = crus cerebri
(includes corticospinal tracti. IPF ~ interpeduncular fossa. MLF ~ medial longitudinal fasciculus, ON = oculomotor
'SlJDerii,,, cerebellar
nucleus. PAG ~ periaqueductal gray. RN ~ red nucleus.
'Posteriil" communicating artery
SN
= substantia nigra.
artery Basilar artery
CHAPTER 3:
Cranial Nerves: Central and Peripheral Connections.
99
fibers makes them more vulnerable to compression, such as from an aneurysm, than to ischemia. Pupillary dilation is a sensitive (and commonly early) sign of compression. Pathways for the Pupil Reflexes Light reflex
The light reflex consists of a simultaneous and equal constriction of the pupils in response to illumination of one or the other eye. The afferent pupillary pathway coincides with that of the visual pathway and includes a decussation of nasal fibers in the chiasm. At the posterior part of the optic tract, the pupillary fibers leave the visual fibers and pass to the lateral side of the midbrain to reach the pretectal nuclei at the level of the superior colliculus. Here, efferent fibers arise and pass to the Edinger-Westphal nuclei, decussating partially (both ventral to the aqueduct and dorsally, in the posterior commissure). Preganglionic parasympathetic fibers leave each Edinger-Westphal nucleus and run in the oculomotor nerve as it leaves the brains tern. The fibers spiral downward to lie medially in the nerve at the level of the petroclinoid ligament and inferiorly in the inferior division of the third nerve as it enters the orbit. These fibers synapse in the Ciliary ganglion and giverise to postganglionic myelinated short ciliary nerves, about 3%- 5% of which are pupillomotor. The rest are designated for the ciliary muscle and are concerned with the near reflex. Near reflex
The near reflex is a synkinesis that occurs when attention is changed from distance to near. This reflex includes accom,modation, pupil constriction, and convergence. The reflex is initiated in the occipital association cortex, from which impulses descend along corticofugal pathways to relay in pretectal and possibly tegmental areas. From these relays, fibers pass to the Edinger-Westphal nuclei, the motor nuclei of the medial rectus muscles, and the nuclei of CN VI. Fibers for the near reflex approach the pretectal nucleus from the ventral aspect, so that with compressive dorsal lesions of the optic tectum 'there is sparing of the near pupil reflex relative to the light reflex (light- near dissociation). Efferent fibers for accommodation follow the same general pathway as those for the light reflex, but their final distribution (via the short ciliary nerves) is to the ciliary muscle.
Cranial Nerve IV (Trochlear) Cranial nerve IV contains the fewest nerve fibers (approximately 3400) of any cranial nerve, but it has the longest intracranial course (75 mm). The nerve nucleus is located in the caudal mesencephalon at the level of the inferior colliculus near the periaqueductal gray matter, ventral to the aqueduct of Sylvius. It is continuous with the caudal end of the CN III nucleus and differs histologically from the CN III nucleus only in the smaller size of its cells. Like the CN III nucleus, it is bounded ventrolaterally by the medial longitudinal fasciculus. The fascicles of CN IV curve dorsocaudally around the periaqueductal gray matter and decussate completely in the superior medullary velum. The nerves exit the brainstem just beneath the inferior colliculus. Thus, CN IV is the only cranial nerve that is
100 • Fundamentals and Principles of Ophthalmology completely decussated and the only motor nerve to exit dorsally from the nervous system. As it curves around the brain stem in the ambient cistern, CN IV runs from beneath the free edge of the tentorium, passes between the posterior cerebral and superior cerebellar arteries, and then pierces the dura mater to enter the cavernous sinus.
Cranial nerve IV travels beneath CN III and above the ophthalmic division of CN V in the lateral wall of the cavernous sinus. Cranial nerve IV enters the orbit through the superior orbital fissure outside the annulus of Zinn and runs superiorly to innervate the
superior oblique muscle. Because of its location outside the muscle cone, CN IV is usually not affected by injection of retrobulbar anesthetics.
Cranial Nerve V (Trigeminal) Cranial nerve V, the largest cranial nerve, possesses both sensory and motor divisions.
The sensory portion subserves the greater part of the scalp, forehead, face, eyelids, eye, lacrimal gland, extraocular muscles, ear, dura mater, and tongue. The motor portion innervates the muscles of mastication through branches of the mandibular division. The CN V nuclear complex extends from the midbrain to the upper cervical segments, often ~s caudal as C4. It consists of the following 4 nuclei, from above downward: 1. mesencephalic nucleus 2. main sensory nucleus
3. spinal nucleus and tract 4. motor nucleus located in the pons Important interconnections exist between the different subdivisions of the CN V sensory nuclei and the reticular formation (Fig 3-10).
Mesencephalic Nucleus The mesencephalic nucleus mediates proprioception and deep sensation from the masticatory, facial, and extraocular muscles. The nucleus extends inferiorly into the posterior pons as far as the main sensory nucleus. Main Sensory Nucleus The main sensory nucleus lies in the pons, lateral to the motor nucleus. It is continuous with the mesencephalic nucleus (above) and with the spinal nucleus (below). It receives
its input from ascending branches of the sensory root, and it serves light touch from the skin and mucous membranes. The sensory root of CN V, upon entering the pons, divides into an ascending tract and a descending tract. The former terminates in the main sensory
nucleus, and the latter ends in the spinal nucleus.
Spinal Nucleus and Tract The spinal nucleus and tract extend through the medulla to C4. The nucleus receives pain and temperature afferents from the descending spinal tract, which also carries cutaneous
CHAPTER 3:
Cranial Nerves: Central and Peripheral Connections.
101
v, = Ophthalmic division /,,~---V2=
-
Maxillary division
V3 = Mandibular divi sion
Perioral region B
Midfacial region
C
Peripheral facial region
Figure 3wl 0 Cranial nerve V complex (dorsal view of brainstem). M = motor nucleus, MES = mesencephalic nucleus. MSN = main sensory nucleus. SN = spinal nucleus and tract. TG = trigeminal ganglion. A, B, and C are portions of the caudal spinal nucleus that correspond to conw cen tric areas of the face: A, perioral; B, midface, including eyes; C, peripheral face and scalp.
components of CN VII. CN IX, and CN X that serve sensations from the ear and external auditory meatus. The sensory fibers from the ophthalmic division of CN V (V,) terminate in the most ventral portion of the spinal nucleus and tract. Fibers from the maxillary division (V, ) end in the midportion of the spinal nucleus (in a ventral-dorsal plane) . The fibers from the mandibular division (V, ) end in the dorsal parts of the nucleus_ The cutaneous territory of each of the CN V divisions is represented in the spinal nucleus and tract in a rostral-caudal direction. Fibers from the perioral region are thought to terminate most rostrally in the nucleus; fibers from the peripheral face and scalp end in the caudal portion. The zone between them, the midfacial region, is projected onto the central portion of the nucleus_ This "onionskin" pattern of cutaneous sensation has been derived from clinical studies in patients with damage to the spinal nucleus and tract (Fig 3-1\)_ Damage to the trigeminal sensory nucleus at the level of the brainstem causes bilateral sensory loss in concentric areas of the face, with the sensory area surrounding the mouth in the center. If a patient verifies this distribution of sensory loss, then the lesion is in the brainstem_ Conversely, sensory loss that follows the peripheral distribution of the trigeminal sensory divisions (ophthalmic, maxillary, and mandibular) indicates the lesion lies in CN V after it exits the brainstem. Brodal A. Neuro logical Anatomy in Relation to Clitlical Medicine. 3rd ed. New York: Oxford; 198 1: 524- 529_
102 • Fundamentals and Principles of Ophthalmology
... ,
>'
,
v,
/
Figure 3·11 Cran ial nerve V (tr ig em inal): patte rn of fac ial sensation. Le sions of the trigem ina l sensory nucleus in the brainstem resu lt in an onionskin distr ibution of altered sensation (sections A, B. and C) delineated by the dashed lines; lesions of the ophthalmic (V,). maxillary IV, ).
and mandibular IV3 ) ne rves result in the pattern of sensory loss delineated by the solid lines. (Illustration by Thomas A. Weingeisr. PhD. MD.)
Axons from the main sensory. spinal, and portions of the mesencephalic nuclei relay sensory information to higher sensory areas of the brain. The axons cross the midline in the pons and ascend to the thalamus along the ventral and dorsal trigeminothalamic tracts. They terminate in the nerve cells of the ventral posteromedial nucleus of the thalamus. These cells in turn send axons through the internal capsule to the postcentral gyrus of the cerebral cortex.
Motor Nucleus The motor nucleus is located medial to the main sensory nucleus in the pons. It receives fibers from both cerebral hemispheres, the reticular formation, the red nucleus, the tectum , the medial longitudinal fasciculus, and the mesencephalic nucleus. A monosynaptic reflex arc is formed by cells from the mesencephalic nucleus and the motor nucleus. The motor nucleus sends offaxons that form the motor root, which eventually supplies the muscles of mastication (pterygoid, masseter, temporalis), the tensor tympani, the tensor veli palatini, the mylohyoid, and the anterior belly of the digastric. The intracranial fifth nerve emerges from the upper lateral portion of the ventral pons, passes over the petrous apex, forms the trigeminal ganglion, and then divides into 3 branches. The trigeminal ganglion, also called the gasserian or semilunar ganglion, contains the cells of origin of all the eN v sensory axons. The crescent-shaped ganglion
CHAPTER 3:
Cranial Nerves: Central and Peripheral Connections. 103
occupies a recess in the dura mater posterolateral to the cavernous sinus. This recess,
called the Meckel cave, is near the apex of the petro us part of the temporal bone in the middle cranial fossa. Medially, the trigeminal ganglion is close to the ICA and the posterior cavernous sinus.
Divisions of Cranial Nerve V The 3 divisions ofCN V are the ophthalmic (V ,), the maxillary (V 2 ), and the mandibular (V, ).
CNV, The ophthalmic division enters the cavernous sinus lateral to the ICA and courses beneath CN III and CN IV. Within the sinus, it gives off a tentorial-dural branch, which supplies sensation to the cerebral vessels, dura mater of the anterior fossa, cavernous sinus, sphe-
noid wing, petrous apex, Meckel cave, tentorium cerebelli, falx cerebri, and dural venous sinuses. CN V I passes into the orbit through the superior orbital fissure and divides into 3 branches: frontal, lacrimal, and nasociliary. The frontal nerve divides into the supraorbital and the supratrochlear nerves, which provide sensation for the medial portion of the upper eyelid and the conjunctiva, forehead, scalp, frontal sinuses, and side of the nose. The lacrimal nerve innervates the lacrimal gland and the neighboring conjunctiva and skin. It was once suggested that postganglionic parasympathetic lacrimal secretory fibers, arising in the pterygopalatine ganglion, were carried to the lacrimal gland via a zygomaticotemporal connection with the lacrimal nerve. However, it is now thought more
likely that the gland receives its parasympathetic supply directly, from the retro-orbital plexus (discussed later in the chapter). The nasociliary nerve supplies sensation through nasal branches \0 the middle and inferior turbinates, septum, lateral nasal wall, and tip of the nose. The infratrochlear branch serves the lacrimal drainage system, the conjunctiva, and the skin of the medial canthal region. Long ciliary nerves carry sensory fibers from the Ciliary body, the iris, and the cornea and provide the sympathetic innervation to the dilator muscle of the iris. Sensation from the globe is carried by short ciliary nerves. The CN V fibers pass through the ciliary ganglion to join the nasociliary nerve. The ciliary nerves also contain postganglioniC parasympathetic fibers from the ganglion to the pupillary sphincter and the ciliary muscle.
CNV2 The maxillary division leaves the trigeminal ganglion to exit the skull through the fora men rotundum, which lies below the superior orbital fissure. CN V2 courses through the pterygopalatine fossa into the inferior orbital fissure, then runs through the infraorbital canal as the infraorbital nerve. After exiting the infraorbital foramen, CN V2 divides into an inferior palpebral branch supplying the lower eyelid, a nasal branch for the side of the nose, and a superior labial branch for the upper lip. The teeth, maxillary sinus, roof of the mouth, and soft palate are also innervated by branches of the maxillary division.
104 • Fundamentals and Principles of Ophthalmology
CNV3 The mandibular division contains both sensory and motor fibers. It exits the skull through the foramen ovale and provides motor input for the masticatory muscles. Sensation is supplied to the mucosa and skin of the mandible, lower lip, tongue, external ear, and tympanum. Stand ring S, ed. Grays Anatomy: The Anatomical Basis of Clinical Practice. 39th ed. Edinburgh. New York: Elsevier Churchill Livingstone; 2005.
Cranial Nerve VI (Abducens) The nucleus ofCN VI is situated in the fioor of the fourth ventricle, beneath the facial colIiculus, in the caudal pons. Fibers of CN VII pass over or loop around the eN VI nucleus and exit in the cerebellopontine angle. The medial longitudinal fasciculus lies medial to the CN VI nucleus. The fascicular portion of the nerve runs ventrally through the paramedian pontine reticular formation and the pyramidal trac t and leaves the brains tern in the pontomedullary junction (Fig 3-12). Cranial nerve VI takes a vertical course along the ventral face of the pons and is crossed by the anterior inferior cerebellar artery. It continues through the subarachnoid space along the surface of the clivus, surrounded by the Batson venous plexus, to perforate the dura mater below the crest of the petrous portion of the temporal bone, approXimately 2 em below the posterior clinoid process. It then passes intradurally through or around the inferior petrosal sinus and beneath the petroclinoid (Gruber) ligament through the Dorella canal, where it enters the cavernous sinus. In the cavernous sinus, eN VI runs
/ " ' \ - - - - - Vestibular nuclei
:.e-::"""'''------ nucleus Abducens I--'>~""""c-- Facial
nucleus
intermedius
I CNVI
Figur. 3-1 2
Cross section of the pons at the level of the CN VI (abducens) nucleus. CS =
corticospinal tract, MLF = medial longitudinal fascicu lus, PPRF = pontine pa ramedian reticular formation. (//{ustrat;on by Sylvia Barker.)
CHAPTER 3:
Cranial Nerves: Central and Peripheral Connections •
105
below and lateral to the carotid artery and may transiently carry sympathetic fibers from the carotid plexus. It passes through the superior orbital fissure within the annulus of Zinn and innervates the lateral rectus muscle on its ocular surface.
Cranial Nerve VII (Facial) Cranial nerve VII is a complex mixed sensory and motor nerve. The motor root contains special visceral efferent fibers that innervate the muscles of facial expression. The so-called sensory root ofCN VII is the nervus intermedius, which contains special visceral afferent, general somatic afferent, and general visce ral efferent fibers. The special visceral afferent fibers, which convey the sense of taste from the ante rior two thirds of the tongue, terminate centrally in the nucleus of the tractus solitarius. The general somatic afferent fibers convey sensation from the external auditory meatus and the retroauricular skin; centrally, they enter the spinal nucleus of CN V. The general visceral efferent fibers provide preganglioniC parasympathetic innervation by way of the sphenopalatine and submandibular ganglia to the lac rimal, submaxillary, and sublingual glands. The motor nucleus ofCN VII is a cigar-shaped column, 4 mm long, located in the caudal third ofthe pons. It is ventrolateral to the CN VI nucleus, ventromedial to the spinal nucleus of CN V, and dorsal to the superior olive. Four distinct subgroups within the nucleus innervate specific facial muscles; the ventral portion of the intermediate group probably supplies axons to the orbicularis oculi. The part of the nucleus supplying the upper half of the face receives corticobulbar input from both cerebral hemispheres. The lower half of the face is influenced by corticobuloar fibers from the opposite cerebral hemisphere. Fibers from the motor nucleus course dorsomedially to approach the floor of the fourth ventricle and then ascend immediately dorsal to the CN VI nucleus. At the rostral end of the CN VI nucleus, the main facial motor fibers arch over its dorsal surface (forming the internal genu of CN VII) and then pass ventrolaterally between the spinal nucleus of CN V and the CN VII nucleus to exit the brainstem at the pontomedullary junction. The bulge formed by the CN VII genu in the floor of the fourth ventricle is the facial colliculus (Fig 3-13). The sensory nucleus of CN VII is the rostral portion of the tractus soli tar ius, sometimes known as the gustatory nucleus. It lies lateral to the motor and parasympathetic nuclei in the caudal pons. Sensations of taste from the anterior two thirds of the tongue are carried by special visceral afferent fibers to this nucleus. The impulses travel along the lingual nerve and chorda tympani; the cell bodies for these impulses are located in the gen iculate ganglion. They eventually reach the brain through the nervus intermedius. Cranial nerve VII, the nervus intermedius, and CN VIII (acoustic) pass together through the lateral pontine cistern in the cerebellopontine angle and enter the internal auditory meatus in a common meningeal sheath. Cran ial nerve VII and the intermedius nerve then enter the fallopian canal, the longest bony canal traversed by any cranial nerve (30 mm). Cranial nerve VII can be divided into 3 segments in its course through this canal. After passing ante rolaterally for a short distance known as the labyrinthine segment, the nerves bend sharply at the geniculate ganglion and are then directed dorsolaterally past the tympanic cavity. This 90° bend, known as the tympanic segment, is the external genu of
106 • Funda me ntals and Principles of Ophthalmology Mesencephalic nucleus 01V M!lin sensory nucleus 01V
Lac rima l ref lex arc (after Ku rihashil. The affe rent pathway is provided by the first and second divisions of CN V. The efferent path proceeds from the lacrimal nucle us (close to the supe rio r salivary nucleusl via CN VII (nervus intermedius!. through the geniculate gang lion,
Figur.3-13
th e greater superficia l petrosa l nerve, and the nerve of th e pterygoid canal (where it is joined
by sympathetic fibers from the deep petrosa l nervel. The nerve passes to the pterygopalatine ganglion, where it synapses w ith postganglionic fibers . These fibers reach the lacrimal gland directly, via the retro-orbital plexus of nerves. The fibers carry cholinergic and vasoactive intestinal polypeptide (VIP)-ergic fibers to the gland. (From Bron Ai Triparhi RC, Tripathi 8J. Wolff's Anatomy of the Eye and Orbit. 8th ed. London: Chapman & Hall; 1997.)
CN VII. Two parasympathetic branches fro m the superior salivatory and lac rimal nuclei leave the nerve at the tympanic segment: the greater superficial petrosal nerve and a small filament that joins the inferior petrosal nerve, The third segment. the mastoid segment. of the nerve is directed straight down toward the base of the skulL The stapedius nerve leaves. and the chorda tympani joins CN VII in the mastoid segment. The CN VII trunk then exits the skull at the stylomastoid foramen and separates into a large temporofacial and a small cervicofacial division between the superficial and deep lobes of the parotid gland. This area of branching is known as the pes anserinus. The temporofacial division gives rise to the temporal. zygomatic. and buccal branches, The cervicofacial division is the origin of the marginal mandibular and colli branches, However. anastomoses and branching patterns are numerous, Commonly. the temporal branch supplies the upper half of the orbicularis oculi. and the zygomatic branch supplies the lower half. The fro ntalis. corrugator supercilii. and pyramidalis muscles are usually innervated by the temporal branch. The parasympathetic outflow originates in the superior salivatory nucleus and the lacrimal nucleus. both of which lie posterolateral to the motor nucleus and which probably receive afferent fibers from the hypothalamus, The superior salivatory nucleus also receives input from the olfactory system. The hypothalamic fib ers reaching the lacrimal nucleus may mediate emotional tearing. and there is supranuclear input from the cortex
CHAPTER 3,
Cranial Nerves: Central and Peripheral Connections.
107
and the limbic system. Reflex lacrimation is controlled by afferents from the sensory nuclei of eN V. These preganglionic parasympathetic fibers pass peripherally as part of the nervus intermedius and divide into 2 groups near the external genu of eN VII. The lacrimal group of fibers passes to the pterygopalatine ganglion in the greater superficial petrosal nerve. The salivatory group of fibers projects through the chorda tympani nerve to the submandibular ganglion to innervate the submandibular and sublingual salivary glands. The greater superficial petrosal nerve extends forward on the anterior surface of the petrous temporal bone to join the deep petrosal nerve (sympathetic) and form the nerve of the pterygoid canal. This nerve enters the pterygopalatine fossa; joins the pterygopalatine ganglion; and gives rise to unmyelinated postganglionic fibers that innervate the globe, lacrimal gland, glands of the palate, and nose. Those parasympathetic fibers destined for the orbit enter it via the inferior orbital fissure. Here, they are joined by sympathetic fibers from the carotid plexus and form a retro-orbital plexus of nerves, whose rami oculares supply orbital vessels or enter the globe to supply the choroid and anterior segment structures. Some of these fibers enter the globe directly; others enter via connections with the short ciliary nerves. The rami oculares also supply the lacrimal gland.
Cavernous Sinus The cavernous sinus is an interconnected series of venous channels located just posterior to the orbital apex and lateral to the sphenoidal air sinus and pituitary fossa (Fig 3-14). The following structures are located within the venous cavity: • the leA surrounded by the sympathetic carotid plexus • eN III, eN IV, and eN VI • the ophthalmic and maxillary divisions of eN V
:-.---
=~;,,~n~~2{C)PhllhallmiC artery . =~--.....
Sphenoid sinus Sphenoid sinus
A
B
Figur.3-14 Cavernous sinus, corona l sections (A) at the level of the pituitary fossa and (S) at the level of the anterior clinoid process. leA = internal carotid artery, ON = optic nerve. (Repro. duced by permission from Doxanas MT. Anderson RL. Clinical Orbita l Anatomy. Baltimore: W illiams & W ilkins; 1984.)
108 • Fundamentals and Principles of Ophthalmology
Other Venous Sinuses The cavernous sinus is only 1 part of an interconnecting series of venous channels that carry blood away from the brain and drain into the internal jugular veins. Other venous sinuses include the superior sagittal, transverse, straight, sigmoid, and petrosal. The various components of the venous system are depicted in Figure 3- 15. Thrombosis in any
""he,wp.,;,"" sinus
_ Pos'e,;o, intercavernous sinus -::::~:,~::~o:m~:e:~n; ; :n:;g:~.:a,'1 diploic vein \" vein ~~.ave"n<)us
sinus
petrosal sinus pelrosal sinus
'0.,"_"," sinus
'V'.,',.h,., artery Superior anastom ic vein
Superior sagittal sinus
Inferior sagittal sinus Internal cerebral vein
1:I!iit'''\--''''ia-Great cerebral vein Anterior cerebral
vein:~~~~~~~~~~~r;Basa, vein
Superficial middle cerebral vein
Deep middle cerebral vein
Straight sinus Inferior anastomotic vein
Superior petrosal sinus Inferior petrosal sinus
Transverse sinus Occipital sinus Sigmoid sinus
Figure 3-15 Three-dimensional drawi ngs of the venous sinuses of the brain, their interconnections, and the rela tionship to the dura. (Reproduced with permission from Williams PL, Waf\lVick R. Gray's Ana tomy. 38th ed. Edinburgh: Churchill Livingstone; 1995.)
CHAPTER 3:
Cranial Nerves: Central and Peripheral Connections. 109
portion of the venous sinuses can lead to increased venous pressure and may cause intra-
cranial hypertension and papilledema.
Circle of Willis The major arteries supplying the brain are the right and left ICAs (which distribute blood primarily to the rostral portion of the brain) and the right and left vertebral arteries (which join to form the basilar artery). The basilar artery primarily distributes blood to the brainstem and posterior portion of the brain. These arteries interconnect at the base of the brain at the circle of Willis (Fig 3-16). These interconnections help to distribute blood to all regions of the brain, even when a portion of the system becomes occluded.
A Figure 3-16 A, Magnetic resonance angiogram showing the circle of Wil lis in an anteroposterior view. B, Same patient shown in an obl ique view. ACA = anterior cerebra l artery, BA = basilar artery, MCA = middle cerebral artery, PCA = posterior cerebral artery, PCoA = posterior communicating artery. (Courtesy of r Ta/fi, MD. and W Yuh, MD)
CHAPTER
4
Ocular Development
Introduction Experimental studies conducted in recent decades have revolutionized our understanding of ocular development. Consequently, the original treatises on the growth and differentiation of the eye have been modified. The classic germ layer theory depicted the epithelium of the cornea, the retina, and the neural components of the uveal tract as derived from ectoderm and the remainder of the ocular structures as derived from mesoderm. Although this general schema is still used, it is currently recognized that the embryonic and fetal development of the human eye involves a series of sequential events, including inductive interactions and morphogenetic movement of cells from distant regions of the embryo. The primary tissues involved in these processes are the head epidermis, neuroectoderm, and mesenchyme. Three elements have been identified as making important contributions to the genesis of the eye: 1. growth factors 2. homeobox genes 3. neural crest cells
Each is described separately in the following sections, but interaction among these elements is also crucial. Part III of this volume, Genetics, also discusses some of the concepts reviewed in this chapter.
Growth Factors The process of induction is mediated by tissue communication through macromolecules that act as chemical Signals. Growth factors are now known to be active in the earliest stages of embryoniC development. They are a class of trophic substances that participate in the control of normal development by modulating the migration, proliferation, and differentiation of cells. These molecules act at nanomolar concentrations by binding with high affinity to specific recepto r sites localized in the plasma membrane of the target cell. The embryoniC genome is not transcribed until the stage of midblastula transition, which takes place several hours after fertilization. The messenger RNAs (mRNA) for the growth factors involved in the earliest aspects of the growth and differentiation of the fertilized egg are endogenous and are supplied from maternal sources until the embryonic tissues become able to syntheSize them de novo.
11 3
114 • Fundamentals and Principles of Ophthalmology These growth factors include fibroblast growth factor (FGF) transforming growth factor ps (TGF-p, and TGF-P,) • insulin-like growth factor I (IGF-I) Experimental studies have revealed that when cells of the animal cap are exposed to FGF, they are induced to differentiate into posterior mesoderm that is destined to form tissues of the caudal region. However, TGF-ps induce animal cap cells to differentiate into mesoderm that forms structures in the head region, including the eye. Growth factors also regulate the levels of expression of hom eo box genes, which function as a mechanism for controlling the establishment of the overall arrangement of the eye as an organ. Visual acuity requires a precise spatial arrangement of tissues of the eye;
thus, it is critical that homeobox genes be expressed at the appropriate level and time. Some growth factors are crucial in directing the migration and developmental patterns of cranial neural crest cells by influencing the synthesis and degradation of the extracellular matrix. Various components of the extracellular matrix act as morphogenetic factors that facilitate a complex series of integrated tissue interactions, movements, and shape changes, especially during the earliest stages of morphogenesis of the optic vesicle and lens. Differentiation of the various ocular tissues appears to be at least partly controlled by a variety of growth factors. For example, the FGFs induce the neuroectodermal cells that line the inner wall of the optic cup to develop as neural retina; FGF is also responsible for certain aspects of differentiation of the lens epithelial cells into lens fibers. However, the differentiation of lens epithelial cells immediately anterior to the equator- as well as their mitotic activity-is promoted by the IGFs. The synergistic action of multiple trophiC factors appears to be a significant regulatory tool for initiating cellular activities and for limiting abnormal development. Tripathi BJ, Tripathi Re , Livingston AM, Borisuth NS. The role of growth factors in the embryogenesis and differentiation of the eye. Am J Anat. 1991;192(4}:442- 471.
Homeobox Genes Homeobox genes contain a distinctive segment of DNA, approximately 180 base pairs in length, that shows similarity in the sequence of the nucleotides. This region is termed the homeobox (from the Greek homoios ["like, resembling"] and box [the extent of the conserved sequenceD. The homeobox encodes an almost identical sequence of approximately 60 amino acids, the homeodomain, in the protein products of these genes. Because they control the activity of many subordinate genes, homeobox genes are considered "master"
genes. Conserved evolutionarily, these genes are present throughout the plant and animal kingdoms. The function of hom eobox genes is mediated by the homeodomain, which recognizes and binds to specific DNA sequences in the subordinate genes, thereby activating or repressing their expression. Thus, these genes act as selector genes, or "master switches;' encoding transcription factors. Transcription factors regulate mRNA production by other genes. On the basis of the pattern of homeobox gene expression, which is restricted both spatially and temporally during the earliest stages of development, the vertebrate embryo
CHAPTER 4:
Ocular Development.
115
can be subdivided anteriorly to posteriorly into fields of cells that have different developmental capacities. This organizational plan precedes the formation of any specific organ or structures. The fact that homeobox genes are located on the chromosomes in the same order in which they are expressed along the anteroposterior axis of the embryo indicates that they are activated sequentially. The same homeobox genes are expressed again later in embryogenesis, apparently to specify the identity of a particular cell. Experimental evidence suggests that homeobox genes are activated not only by growth factors-especially FGFs and TGF-ps-but also by retinoic acid, a direct derivative of vitamin A. Too much or too little vitamin A is teratogenic. Investigations in several vertebrate species have revealed the involvement of specific homeobox genes in the development of the eye. For example, expression of the PAX6 gene marks the location of the lens-competent region in the head ectoderm before the optic vesicle can be recognized. During the early stages of eye development, 2 HOX genes are expressed with a distinct spatial and temporal relationship. The HOXB.l gene is expressed in the surface ectoderm, in a region destined to form the corneal epithelium, and in the optic vesicle, where the retina will differentiate before invagination occurs. HOX7.1 , which is expressed after the formation of the optic cup, marks the region of the future ciliary body. Subsequently, the PAX6 gene has a role in the expression of tissue-specific genes in the eye. For example, it induces differentiation of progenitor ceUs into neurons in the retina, as well as the expression of zeta (1;)-crystaUins in lens epithelial cells. HSiung F. Moses K. Retinal development in Drosophila: specifying the first neuron. Hum Mol Gell.2002;1l(1O);1207- 1214.
Nguyen M, Arnheiter H. Signaling and transcriptional regulation in early mammalian eye development: a link between FGF and MITF. Development. 2000; 127(16):3581 - 359 1.
Neural Crest Cells Neural crest cells arise from neuroectoderm located at the crest of the neural folds at approximately the same time that the folds fuse to form the neural tube. They are a transient population of cells; after they migrate to different regions of the embryo, differentiation occurs. Most mesenchymal cells of the facial primordia are derived from the neural crest. Crest cells do not arise from the region of the forebrain. However, neural crest cells from the diencephalic, mesencephalic, and rhombencephalic regions migrate anteriorly along the dorsum of the embryo. Crest cells that originate from the posterior midbrain form the maxillary primordia, and those from the hindbrain form the mandibular primordia. The crest cells from the diencephalon contribute to the tissue of the frontonasal mass; later, they are joined by cells from the anterior midbrain that migrate to and settle around the optic vesicles. The anterior flexure of the embryo aids the migration of neural crest cells ventrad and cephalad. The extracellular matrix has a significant role in directing the migration of neural crest cells. Molecules such as fibronectin promote migration, whereas others, such as proteoglycans, are inhibitory. The positive and negative cues that the crest cells encounter during migration appear to gUide the cells along the correct pathways to the appropriate destination. Because the synthesis and secretion of extracellular matrix molecules such as
116 • Fundamentals and Principles of Ophthalmology collagen, fibronectin, and proteoglycans can be influenced by growth factors, especially cytokines also have a role in regulating the migration of crest cells. Early in development, neural crest cells are pluripotent, and their final differentiation is considerably influenced by local factors. Crest cells from the hindbrain normally form the connective tissue of the visceral arch and contribute to the formation of the cranial sensory ganglia. However, if these hindbrain neural crest cells are grafted in place of the posterior diencephalic and mesencephalic crest population, they differentiate appropriately into ocular, orbital, and facial tissues. Neural crest cells make a major contribution to the connective tissue components of the eye and orbit. Notable exceptions include the striated fibers of the extraocular muscles and the endothelial cells that line all blood vessels of the eye and orbit. Both of these exceptions arise from mesoderm (Table 4- 1). TGF-~s,
Table 4-1 Derivatives of Embryonic TIssues ECTODERM Neuroectoderm Neurosensory retina Retinal pigment epithelium Pigmented ciliary epithelium Nonpigmented ciliary epithelium Pigmented iris epithelium Sphincter and dilator muscles of iris Optic nerve, axons, and glia Vitreous Cranial Neural Crest Cells Corneal stroma and endothelium Sclera (see also mesoderm) Trabecular meshwork Sheaths and tendons of extraocular muscles Connective tissues of iris Ciliary muscles Choroidal stroma Melanocytes (uveal and epithelial) Meningeal sheaths of the optic nerve Schwann cells of ciliary nerves Ciliary ganglion All midline and inferior orbital bones, as well as parts of orbital roof and lateral rim Cartilage Connective tissue of orbit Muscular layer and connective tissue sheaths of all ocular and orbital vessels Surface Ectoderm Epithelium , glands, cilia of skin of eyelids and caruncle Conjunctival epithelium Lens Lacrimal gland Lacrimal drainage system Vitreous
- - - - - - - - - - - - - MESODERM - - - - - - - - - - - - Fibers of extraocular muscles
Endothelial lining of all orbital and ocular blood vessels Temporal portion of sclera Vitreous
CHAPTER 4: Ocular Development. 117
Ito Y, Yeo JY, Chytil A, et aL Conditional inactivation of Tgfbr2 in cranial neural crest causes cleft palate and calvaria defects. Development. 2003; 130(2 1):5269- 5280.
Neurocristopathy Congenital and developmental anomalies that involve cells derived from the neural crest have been grouped together under the term neurocristopathies. Most of these abnormalities result from defects in either the migration of neural crest cells or their terminal differentiation. Several conditions are common in combination with ocular defects, especially those of the anterior segment: craniofacial and dental malformations • middle-ear deafness malformations of the skull, shoulder girdle, and upper spine Primary cleft palate can be produced by extirpation of the neural folds prior to crest cell migration. The median face malformation (severe orbital hypertelorism) is thought to result fro m an impaired midline coalescence of the frontonasal process. Honkanen RA, Nishimura DY. Swiderski RE. et aL A family with Axenfeld-Rieger syndrome and Peters anomaly caused by a point mutation (Phe 112Ser) in the FOXCI gene. Am J of Ophthalmol.2003;135(3):368-375. Tripathi BI, Tripathi Re. Wisdom IE. Embryology of the anterior segment of the human eye. In: Ritch R. Sh ields MB, Krupin T, eds. The Glaucomas. 2nd ed. St Louis: Mosby; 1996:1.
Embryogenesis In the 2 weeks after fertilization, the impregnated ovum undergoes a series of repeated cell divisions and, through repositioning and reorientation of the cells, becomes sequentially morula, blastula, and gastrula (Fig 4-1). Only the inner cell mass, a small number of cells derived from the fertilized ovum, differentiates subsequently into the embryo. The outer cell mass, or trophoblast, forms the placenta and support tissues. The formation of the epiblast and hypoblast from the inner cell mass precedes gastrulation, a process that results in the establishment of the 3 primary germ layers: ectoderm, mesoderm. and endoderm (Fig 4-2). Cells of the epiblast in the medial region of the embryonic disc begin to proliferate at the caudal end. which causes the development of a thickening known as the primitive streak. The cells of the primitive streak migrate both laterally and cephalad beneath the epiblast. where they give rise to the mesenchymal cells of the intraembryonic mesoderm. The cells that remain in the epiblast are now recognized as the embryonic ectoderm. Some cells of the primitive streak invade the hypoblast and laterally displace most of these cells to give rise to the embryonic endoderm. The primitive streak elongates by the addition of cells at the caudal end and thus establishes the axial orientation of the embryo. The cranial end of the primitive streak enlarges as the primitive node. Mesenchymal cells that migrate cranially from this site form the medial notochordal process, which develops into the primitive mesenchymal axial skeleton (or notochord) of the embryo. The development of the notochord induces the overlying ectoderm to differentiate into neuroectoderm that becomes identified as the neural plate. The brain and the eye develop from the most anterior region of the neural plate.
118 • Fundamentals and Principles of Ophthalmology
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Early stages of human embryonic development. A-C, Fertilization and earliest cell
divisions to moru la stage. 0, Sectioned blastocyst. A fluid-filled cavity has formed , and cells that will form the embryo (the darke r area indicates the inner cell mass! are distinct from cells that will deve lop into support tissues (eg, the placenta!. E, Embryo-forming cells have now separated into 2 layers. the epiblast (e) and hypoblast (hy). F, Dorsal view of an embryo that is slightly more advanced than the sectioned embryo illustrated in E. Gastrulation movements (arrows) bring cells from the upper laver through the primitive streak (ps) into the potentia l space between the 2 lavers to form the middle germ layer (mesoderm!. Mesodermal cells fail to penetrate between the ectoderm and endoderm at the oral plate (b = buccopharyngeal membrane). wh ich later forms the embryonic partition between the oral and pharyngeal cavi-
ties. At this stage, the heart primordium (h) lies anterior to the oral plate. The notochord (n) is formed from the antenor (cepha lic! end of the primitive streak. The prochordal mesoderm (pm) is subjacent to the neura l plate on the reg ion between nand b. G, Earlv stages of neural tube fo lding and closure, and folding of the lateral body walls (solid arrows). The anterior neural plate has begun to "overgrow" (open arrow) the heart primordium and future oral reg ion, including the buccopharyngeal membrane. H, Embryo fold ing is nearing completion. Migration of cranial neural crest cells (nc) in the hindbrain region has been initiated. In contrast to the trunk crest celis, most of t hose form ing in the head region migrate laterally-under the surface
ectoderm but superficial to the somites (5) and the lateral plate
(ip)
of the mesoderm .
(Repro~
duced with permission from Serafin 0 , Georgiade NG. Pediatric Plastic Surgery. Sr Louis: Mosby; 1984.)
Growth of the lateral part of the neural plate results in folds that develop upward and outward, parallel to the neural groove from the head to the caudal region~ At this stage, neuroectoderm lines the inner folds, and surface ectoderm covers the outer surface of the folds. The neuroectodermal cells at the apex of the folds proliferate and produce a population of neural crest cells, which contribute extensively to the tissues of the eye (Fig 4-3).
Figure 4·2 Normal craniofacial development. A. A para sagittal section through the cranial as· pect of a gastrulation-stage mouse embryo. The cells of the 3 germ layers--€ctoderm (Ec), mesoderm (M), and endoderm (EnHave distinct morphologies. B, The developing neural plate (N) is apparent in a dorsal view of this presomite mouse embryo. C, Neural folds (arrowhead) can be seen in the developing spinal cord region . The lateral aspects of the brain (B) region have not yet begun to elevate in this mouse embryo in the head-fold stage. D, Three reg ions of the brain can be disting uished at this 6-somite stage: prosencephalon (P), mesencepha lon (M), and rhombencephalon (R, curved arrow). Optic su lci (arrowhead) are seen as evaginations from the prosencephalon. E, The neural tube has not yet fused in this 12-somite embryo. The stomodeum, or primitive oral cavity, is bordered by the frontonasal prominence (F), the first visceral arch (mandibular arch, MI, and the developing heart (H). F, Medial and lateral nasal prominences (MNP, LNP) surround olfactory pits in this 36-somite mouse embryo. The Rathke pouch (arrowhead) can be distinguished in the roof of the stomodeum . G. In this lateral view of a 36-som ite mouse embryo, the first and second (hyoid, H) visceral arches are apparent. The reg ion of the first arch consists of maxillary (Mx) and mandibular (M) components. Note the presence 01 the eye w ith its invaginating lens (arrowhead). Atrial (A) and ventricu lar (V) heart chambers can be distinguished . (Reproduced with permIsSIon from Sulik KK, Johnston MC Embryonic origin of holoprosencephafy: in terrelarionshlp of the developing bram and face. Scan Electron M lcrosc. 1982;(Pl 1):311.}
'20 • Fundamentals and Principles of Ophthalmology
A
Neural tube
B Figur.4-3 Cross sections through embryos before IAI an d after (B) the onset of migrat ion of crest ce lis (diamond pattern). The ectoderm has been peeled back in B to show the underlyi ng neural crest ce lls. (Reproduced w irh permission from Johnston Me. Sulik KK. Development of face and oral cavity. In: Bhaskar SN, ed. Orban 's Oral Histology and Embryology, 11th ed, 5 1 Louis: M osby; 1991 .)
The cephalic neural fo lds grow and expand markedly. At the end of the third week after conception, the neural folds begin to close to form the neural tube. This process starts in the mid region of the embryo and proceeds anteriorly and posteriorly at the same time. As the neural tube closes, 3 events important to the development of the eye and orbit occur Simultaneously (Fig 4-4): I. Optic pits develop from the optic sulci, small depressions present in the cephalic
neuroectoderm. 2. Neural crest cells begin to migrate. 3. As the anterior neural tube closes, it flexes ve ntrally.
Organogenesis of the Eye The chronology of ocular development is given in Table 4-2. The optic sulci are first recognizable as slight, curved indentations in the widest part of each neural fold just internal to the peak of the ridge. The long axis of the depression is roughly parallel to that of the
CHAPTER 4:
Ocular Development . 121
Figure 4-4 M igration of cra nial neural crest cells from dorsal diencephalic and mesencephalic regions. Left. Cells begin migration ante riorly as tube closes. Center. Crest cells move in waves around the optic vesicle and lose continuity with the surface cells. Right. The neural tube flexes ventrally, carrying th e optic cup and crest cells ventrally. (Redrawn from M Johnston. 1966.)
neural groove. The optic pits, formed of a single layer of neuroectoderm, develop from the continued evagination of the sulci. As the neural tube closes, the pits deepen and become optic vesicles, which appear as symmetric, hollow, hemispheric outgrowths on the lateral sides of what is now the forebrain vesicle. The optic vesicles remain attached to, and continuous with, the neural tube by optic stalks composed of neuroectodermal cells (Fig 4-5). The expansion and ballooning that take place in the hollow optic vesicle do not occur in the stalk, which remains as a tubular link from the cavity of the vesicle to that of the diencephalon. As the optic vesicle approaches the outer wall of the embryo, a focal thickening of the cells, the lens placode, develops in the surface ectoderm, which has been primed by lens-bias signals during earlier embryogenesis. In the fourth week, invagination of the lens placode leads to formation of the lens vesicle, which initially remains attached to the surface ectoderm by the lens stalk. Simultaneously, differential growth and movement of the cells of the optic vesicle result in the invagination of its temporal and lower walls and the formation of the optic cup. The outer layer of the optic cup evolves as a monolayer of cells, the retinal pigment epithelium (RPE). The inner, invaginated layer will differentiate into the neurosensory retina. Initially, the cup is incomplete in its inferior portion (Fig 4-6). The indentation, or fissure , between the folds or margins of the cup is called the embryonic fissure (previously the choroidal, or fetal, fissure). Invagination pushes the neuroectodermal cells originally near the surface deep into the cup near the outer layer. Invagination produces a fold in
122 • Fundamentals and Principles of Ophthalmology Table 4-2 Chronology of Embryonic and Fetal Development of the Eye 22 days
Optic primordium appears in neural folds (1.5-3.0 mm).
25 days
Optic vesicle evaginates. Neural crest cells migrate to surround vesicle.
28 days
Vesicle induces lens placode.
Second month
Invagination of optic and lens vesicles. Hyaloid artery fills embryonic fissure . Closure of embryonic fissure begins. Pigment granules appear in retinal pigment epithelium. Primordia of lateral rectus and superior oblique muscles grow anteriorly.
Eyelid folds appear. Retinal differentiation begins with nuclear and marginal zones. Migration of retinal cells begin s. Neural crest cells of corneal endothelium migrate centrally. Corneal stroma follows. Cavity of lens vesicle is obliterated. Secondary vitreous surrounds hyaloid system. Choroida l vascu lature develops. Axons from ganglion cells migrate to optic nerve. Gliallaminal cribrosa forms. Bruch's membrane appears. Third month
Precurso rs of rods and cones differentiate. Anterior rim of optic vesicle grows forward, and ci liary body starts to develop. Sclera condenses. Vortex veins pierce sclera. Eyelid folds meet and fuse .
Fourth month
Retinal vessels grow into nerve fiber layer near optic disc. Folds of ciliary processes appear. Iris sphincter develops. Descemet's membrane forms . Schlemm's cana l appears. Hyaloid system starts to regress . Glands and cilia develop.
Fifth month
Photo receptors develop inner segments. Choroidal vessels form layers. Iris stroma is vascu larized. Eyelids begin to separate.
Sixth month
Ganglion cells thicken in macu la. Recurrent arterial branches join the choroidal vessels. Di lator muscle of iris forms.
Seventh month
Outer segments of photo receptors differentiate. Central fovea starts to thin . Fibrous lamina crib rosa forms. Choroidal melanocytes produce pigment. Circular muscle forms in ciliary body.
Eighth month
Chamber angle completes formation. Hyaloid system disappears.
Ninth month
Retinal vesse ls reach th e periphery. Myelination of fibers of optic nerve is complete to lami na cribrosa. Pupillary membrane disappears.
CHAPTER 4: Ocular Development. 123 Optic vesicle
Pigment layer) A t' Nervous layer e Ina
Lens
A ectoderm
Optic stalk
Wall of prosencephalon
B
c
Diagram of the development of the human optic cup. The optic vesicle and cup have been partly cut away in A and C, and the lens vesicle is sectioned fo r clarity. A, 4.5-mm embryo (27 days) . B, 5.5-mm embryo. C, 7.5-mm embryo (28 days). (Repmduced from Tdparh; Re. Figur. 4-5
TriTJa fhi BJ. Comparative ph ysiology and anaromy of rhe aqueous ourf/ow TJathway. In: Davson H. ed. The Eye. 3rd ed. Orlando: Academic Press; 1984.)
the neuroectoderm anteriorly, adjacent to the lens, called the rim of the optic cup. At first, the 2 layers of the developing cup have a small space between them, the optic ventricle; however, as invagination proceeds and the inner and outer layers become juxtaposed, the cavity of the optic ventricle progressively narrows. Basement membrane lines both the outer and the inner layers of the cup. The apices of the cells in both layers meet end to end as the ventricle narrows. The ventricle cavity remains throughout life as a potential space, the subretinal space. The embryonic fissure extends from the rim of the cup near the lens to the distal optic stalk. This fissure allows vessels of the hyaloid system to be incorporated within the eye (Fig 4-7). To complete the entire wall of the globe, the 2 lips of the embryoniC fissure meet and fuse. Closure begins in the midregion of the cup near the equator of the globe and proceeds anteriorly to the rim and posteriorly down the stalk, enclosing the hyalOid artery. This process occasionally gives rise to a bridge coloboma with a posterior and peripheral component separated by a band of normal tissue. The inner and outer layers of the cup meet end to end. Because the primitive cells are still labile, they seal the fissure without evidence of a seam or scar. Incomplete or inadequate closure produces a coloboma of the iris, ciliary body, choroid, or optic disc, depending on the extent of the failed closure and secondary attempts to close the defect (Fig 4-8).
124 • Fu ndamentals and Principles of Ophtha lmology
7mm
Figure 4·6
Ocular and somatic development. A, Flexion of the neural tube and ballooning of
the optic vesicle . B. Upper-limb buds appear as the optic cup and embryonic fissure emerge . C. Completio n of the optic cup with closure of the fi ssure .. Convolutions appear in the brain. and leg buds appear. The size of the fetus is given . Lower sequence: optic vesicle; optic cup with open embryon ic fissure; cup with fissure closing .
Figure 4-7
Optic cup and stal k with open embryonic fissure
below. The hyaloid artery from the dorsa l ophthalmic artery e nters the cavity through the posterior aspect of the embryonic fissure. The rim of th e optic cup is above. The lens is not shown.
Neurosensory Retina The neurosensory, or neural, retina arises from the inner layer of neuroectodermal cells of the optic cup (Fig 4-9). Differentiation of this cell layer begins early, and within I month of fertilization, mitotic activity has produced 3-4 compact rows of cells that rapidly increase in number. The nuclei segregate at the outer two thirds of the primordial retina toward the outer layer of the optic cup. This region is recognized as the primitive zo ne. The ciliated apices of the cells are directed outward into the rapidly shrinking cavity of the optic ventricle. The inner third of the developing retina is initially devoid of nuclei and is termed the inner marginal zone; it eventually diffe rentiates into the nerve fiber layer. The primitive and marginal zo nes are recognizable only until the seventh week of gestation. Little
CHAPTER 4:
Ocular Development.
125
1,~
'gl,~
Figure 4-8 Closure of lips of the embryonic fissure. 1, Norma l closure: Inner layers (neurosensory retina) and outer layers (dotted area, RPE) meet and merge. Basement membrane forms on both surfaces. 2, Coloboma formation: Ectropion of the inner retina at the lips of the fissure results in imperfect fusion; pigment epithelium is displaced laterally by cells of the neurosensory retina . a, A simple coloboma results in a defective retina and RPE . The uvea and sclera (not shown) are th in and dysgenic. b, In a cyst ic co loboma, the primary vesicular cavity enlarges adjacent to the point of defective closu re.
Figure 4-9
. ........
A
B
Development of the retina and optic nerve. A. Right, The fetal neurosensory retina develops from neuroectoderm as gangl ion cells migrate from the outer primitive zone of closely packed nuclei to the inner ma rginal zone of fibrils . A few axons from gangl ion cells grow toward the optic nerve. RPE begins melanization in t he posterior pole. Left, A cross section shows the feta l optic nerve with a center of vacuo lating primitive cells throug h wh ich axons from t he ganglion cells w ill grow towa rd the brain. Neural crest cells as mesenchyme loosely ring the nerve. The hyaloid artery enters the vitreous (fifth weeki. B. Right. The migration of nuclei results in 3 nuclear and plexiform layers . Left, A cross section of the optic nerve shows axons of ganglion cells (black dots) migrating through vacuolating ce ll s, first in the periphery of the nerve . Neural crest cells condense to meningeal sheaths of optic nerve (seventh week) .
126 • Fundamentals and Principles of Ophthalmology is known about the stimuli that initiate and direct the complex migration and subsequent differentiation of the primitive neuroepithelial cells into the retina. Differentiation of the retina begins in the center of the optic cup and gradually extends peripherally toward its rim. Neural and glial cells develop simultaneously. By the fifth week of gestation, the putative ganglion and Muller cells have migrated from the outer neuroepithelial layers towa rd the vitreous cavity. As a result, the nuclei of the neuroblastic cells become segregated as 2 distinct layers, the inner and outer neuroblastic layers. These 2 layers are separated by a region of tangled cell processes known as the transient nerve fiber layer of Chievitz, which becomes the definitive inner plexiform layer between weeks 9 and 12 of gestation (except in the macula, where it persists until birth). At 9-12 weeks, the 4 major horizontal layers of the retina become distinguishable. The ganglion cells are the first cells of the retina to become clearly differentiated. Their axonal processes and dendritic trees begin to develop at about the sixth week of gestation. Axons from ganglion cells nearest the posterior pole are the first to enter the optic stalk and induce formation of the optic nerve. The number of ganglion cells increases rapidly between weeks 15 and 17 of gestation, then decreases between weeks 18 and 30 because of apoptosis. The ganglion cell somas grow larger with advancing gestational age. The proce~ses of the Muller cells extend from the inner basal lamina of the optic vesicle toward the optic ventricle. As soon as the photo receptors enlarge and become morphologically distinct as cones, the development of junctional complexes on adjacent lateral surfaces of these cells and of the Muller cell processes gives rise to the external limiting membrane. Photoreceptors arise from the outermost layer of neuroblastic cells. Mitotic activity, abundant in the outer neuroblastic layers in weeks 4-12, ceases in the central retina by week 15 of gestation, and differentiation of the cones begins in the region of the putative fovea. The cilia on the apices of the cells that had invaginated the adjacent RPE disappear, and precursors of outer segments gradually develop. Initially, cylindrical cyloplasmic processes extend toward the apical region of the RPE cells. Differentiation of cone outer segments begins at 5 months, when multiple infoldings develop in the plasma membrane of the processes. The folds separate from the plasma membrane, and their orientation as flattened, lamellar discs parallels the development of the horizontal cells. The cell bodies of the rods are dispersed among the cones and are first recognizable by their dark nuclei with condensed peripheral chromatin. The rod outer segments develop during the seventh month of gestation. Amacrine cells are identified by their large, round, pale-staining nuclei. They are first seen scattered at the inner border of the outer neuroblastic layer by week 14 of gestation. Bipolar cells do not differentiate until week 23. The bipolar dendrites extend to the outer pleXiform layer by week 25, at which time the horizontal cells probably differentiate.
Fovea Differentiation of the neurons, photoreceptors, and glial cells in the fovea occurs early because this region is the focal point for the centraperipheral development of the retina.
CHAPTER 4,
Ocular Development.
127
The different cell types, as well as many synapses and intercellular junctions, are already established by 15 weeks of gestation. Thinning of the ganglion cell and inner nuclear layers begins at 24- 26 weeks of gestation and gives rise to the earliest recognizable depression in the area of the macula. The foveal pit becomes more prominent by the seventh month as a result of the marked thinning of the inner nuclear layer. An acellular fibrous zone is now present on both the nasal and temporal sides of the fovea. By this time, major changes have occurred in the cones: the width of the inner segments has decreased, whereas their length has increased, as has the length of the fibers of the Henle fiber layer. Only 2 layers of ganglion cells remain at 8 months, and the inner nuclear layer at the foveola is reduced to 3 rows or fewer because oftateral displacement. At birth, axons of bipolar cells that pass to the inner plexiform layers constitute the prominent transient layer of Chievitz. Relocation of alllayers to the periphery of the foveal slope, which leaves the nuclei of the cones uncovered in the foveola, occurs by 4 months after birth. However, remodeling of the elements of the fovea continues until nearly 4 years of age, at which time the transient layer of Chievitz is lost completely. Tripathi BJ, Tripathi RC. Development of the human eye. In: Bron A}, Tripathi Re, Tripathi BJ, eds. Wolffs Anatomy of the Eye and Orbit. 8th ed. London: Chapman & Hall; 1997.
Retinal Pigment Epithelium Mitotic activity continues in the pseudostratified, columnar epithelial cells that constitute the outer wall of the optic cup up to the sixth week of gestation. The apical borders of adjacent cells are already joined by zonulae occludentes and zonulae adherentes junctional complexes. At week 6, melanogenesis begins; concurrently, the cilia that had been present on the inner cell surface (ie, adjacent to the developing neurosensory retina) disappear. The RPE cells are the first in the body to produce melanin. Whether i~ the retina or in the choroid, the stages of melanin production are the same: premelanosomes gradually become melanosomes. Differentiation of the RPE begins at the posterior pole and proceeds anteriorly, so that by 8 weeks of gestation the RPE is organized as a Single layer of hexagonal columnar cells located posteriorly. The cells become tall and cuboidal during the third and fourth months, and the terminal web becomes well established at the lateral apical borders. The RPE is thought to be fully functional at this stage. The increase in surface area of the RPE (which takes place after birth to accommodate the subsequent growth of the globe) is achieved by enlargement and expansion of individual cells. The basement membrane of the RPE becomes the inner portion of Bruch's membrane; the outer layer of Bruch's membrane, also basement membrane, is laid down by the choriocapillaris layer. The embryonic pigment epithelial cells have a profound inductive influence on the development of the choroid, sclera, and neurosensory retina. In areas where pigment epithelium does not form, as sometimes happens along the line of closure of the embryonic fissure, the underlying choroid, sclera, and retina are hypoplastic (see Fig 4-8). The nature of the inductive stimulus is not known.
128 • Fundamentals and Principles of Ophthalmology
Optic Nerve The optic nerve develops from the optic stalk, the original connection between the optic vesicle and the forebrain, Initially, the stalk is composed of an inner zone of closely packed neuroectodermal cells surrounded by a less compact layer of undifferentiated neural crest cells. Late in the sixth week of gestation, some cells of the inner region vacuolate and degenerate, and nerve fibers from the ganglion cells migrate through the spaces thus created. Other cells of the inner zone differentiate as glial cells. By the seventh week, the optic disc contains the hyaloid artery, which is surrounded by axons and covered by a mantle of glial cells, many of which disappear by the seventh month. The glial cells also give rise to the glial elements of the lamina cribrosa during the eighth week of gestation. Differentiation of the neural crest cells into the pia, arachnoid, and dura mater of the optic nerve begins in
the seventh week, but the sheaths become well defined only after the fourth month. The number ofaxons increases rapidly: by 10-12 weeks of gestation, some 1.9 million axons are present in the optic nerve, rising to 3.7 million by 16 weeks. Later, attrition of axons causes the number of fibers to drop to approximately l.l million, which establishes the adult condition by 33 weeks. The loss ofaxons parallels the degeneration of ganglion cells in the fetal retina and may be related to the segregation of terminals as discrete laminae in the dorsal lateral geniculate body. As the axons grow toward the lateral geniculate body, partial crossover occurs at the optic chiasm. Cells located at the chiasm midline (probably radial glial cells) express certain repulsive or inhibitory molecules that provide a guidance cue by acting specifically on ipsilateral projecting axons. Myelination starts in the chiasm at the seventh month of gestation, proceeds toward the eye, and ceases at the lamina crib rosa by about 1 month after birth. OccaSionally, medullated fibers develop in the retina. They appear on ophthalmoscopic examination as a flat, serrated white patch on the inner surface of the retina. The medullation is usually interrupted at the lamina, but occaSionally it is continuous across the lamina from nerve to retina.
Some fetuses demonstrate a response to light as early as the eighth week of gestation, which indicates that at least some central nervous system pathways are established. By 5 months, 50% of the growth of the optic nerve and disc has occurred; by birth, 75%; and before 1 year of age, 95%.
Lens One of the earliest events in embryogenesis is determination of lens development. The interaction that takes place between the surface ectoderm and the underlying chordamesoderm during midgastrulation imparts a lens-forming bias on an extensive region of head ectoderm. Next, the anlage of the eye conveys an inductive signal to the ectoderm, which determines the region of the presumptive lens. The mesoderm beneath the putative lens ectoderm transmits another signal late in gastrulation (but when the neural plate is still open) that potentiates the fate of the tissue that will become the lens. Finally, by invoking the final phase of determination and enhancing differentiation during neurulation, the optic vesicle deSignates the specific region of the head ectoderm that will become the
CHAPTER 4: Ocular Development. 129
lens. The surface ectoderm can respond to the influence of the optic vesicle only during a precise period of development. The lens is first apparent at about 27 days' gestation as a disc-shaped thickening of surface epithelial cells over the optic vesicle. This lens placode and its thin basallamina are separated from the basal lamina of the optic vesicle by a narrow space containing fine filaments that have a role in the gradual invagination of the lens placode to form the lens vesicle. Initially, the vesicle, consisting of a single layer of cells with apices directed inward, is covered by a basal lamina that seals anteriorly to complete the formation of the lens capsule (Fig 4- 10). Ultimately, the lens vesicle separates from the surface epithelium at about 33 days' gestation. The area of lens separation from the surface ectoderm heals without
Surface ectoderm Anterior
Anterior capsule lens eDithetiurn ....
Posterior
:iiiiii;_';;~z:onule
~==,,"I11111'=::r zonule D
Diagram of stages in the developm ent of the lens and its ca psule . A, Formation of th e lens vesicle from invagination of surface ectoderm together with its basal lamina in an
Figure 4-10
embryo correspondi ng to 32 days' gestation. 8, Separation of the vesicle from the surface ectoderm and its surrounding basal lamina . C, Obliteration of the lens vesieie cavity by elongation of posterior cells at about 35 days' gestation. 0 , Equatorial reg ion of the fully fo rmed lens. Attachment of zonular fibers to t he anterior, posterior, and equatorial regions of the lens periphery becomes apparen t at approximately 5!-2 wee ks' gestation. Note the change in polarity of ce lls from anterior to posterior regions of the len s. (Modified from Tripathi RC, Tripathi BJ. Anatomy of the human eye. In: Oavson H, ed. The Eye. 3rd ed. Orlando: Academic Press; 1984.)
130 • Fundamentals and Principles of Ophthalmology residuum. The epithelial cells deposit additional basal lamina material, which forms the lens capsule. Initially, the posterior capsule is more prominent than the anterior capsule. The lens capsule isolates the lens constituents immunologically within the globe. During closure of the lens vesicle, DNA synthesis decreases in the cells that form the posterior half of the lens; simultaneously, specific lens proteins (crystallins) are synthesized. By 45 days' gestation, the posterior cells, or primary lens fibers, have lengthened to fill the cavity of the vesicle from posterior to anterior. The posterior cells of the vesicle account for most ofthe growth of the lens during the first 2 months of embryogenesis. The primary fibers form the compact core of the lens, known as the embryonic nucleus. The pre-equatorial epithelial cells retain their mitotic activity throughout life, producing secondary lens fibers. These fibers are displaced inward between the capsule and the embryonic nucleus and meet on the vertical planes, the lens sutures. The first suture marking the fetal nucleus is shaped like a Y anteriorly and an inverted Y posteriorly. The basic anatomy of the lens is established after the first layer of secondary fibers has been laid down at the seventh week of gestation. At first, the lens is spherical, but it becomes ellipsoid with the addition of secondary fibers. As secondary fibers are added, the sutures become more complex and dendriform. In the third mO)1th, the innermost fibers mature; cytoplasmic fibrillar material increases and cellular organelles decrease. The nuclei of the deeper cells, at first homogeneous and dense, are lost; the chromatin and ribosomes disintegrate. The equatorial diameter of the unfixed human lens measures 2 mm at 12 weeks and 6 mm at 35 weeks. Both the growth and the maturation of lenticular fibers continue throughout life. BCSC Section 11, Lens and Cataract, discusses the development of the lens in d~tail. The zonular apparatus begins to develop after the tertiary vitreous has formed. The ciliary epithelial cells then synthesize collagen fibrils of the zonular fibers. By the fifth month of gestation, as they increase in number, strength, and coarseness, the fibers reach the lens and merge with the anterior and posterior capsule.
Vitreous Between the fourth and fifth weeks of gestation, the space between the lens vesicle and the inner layer of the optic cup becomes filled with fibrils, mesenchymal cells, and vascular channels of the hyaloid system. Together, these elements constitute the primary vitreous (Fig 4-11). Initially, the fibrillar content is of ectodermal origin, being derived from the fibrils already in place between the invaginating lens placode and the inner layer of the optic cup. The mesenchymal cells are mostly mesodermal in origin, having invaded the cavity of the optic cup with the hyaloid vessel through the patent optic fissure. However, some mesenchymal cells are derived from neural crest cells that migrated over the rim of
the cup. The vascular primary vitreous attains its maximum development by 2 months' gestation. The development of the secondary vitreous begins soon after the primary vitreous is established. The secondary vitreous is avascular and consists of type II collagen fibrils and hyalocytes, which are presumed to be derived from mesenchymal cells of the primary vitreous that differentiated into monocytes. The content of hyaluronic acid in the vitreous
CHAPTER 4:
35 days
epithelium
A 2 months
~~~~:s::-l:S'AnnUlar vessel Eyelid
Primary vitreous
Secondary
vitreous Vo rtex vein Choroidal
B
Long posterior ci liary artery
Ocu lar Development. 131
vessel nerve artery
3-4 months
Tertiary vitreous
Lens Primary
vitreous Secondary vitreous
Muscle
Figure 4-1' Main features in vi treous developm en t and the regression of the hya loid system shown in drawings of sagittal sections. A, At 35 days, hyaloid vessels and their bran ches, the vasa hyaloidea propria, occupy the space between th e lens and th e nsural ectoderm . A capillary net joins the capsula perilenticularis fibrosa, which is composed of ectodermal fibrils associated wi th vasoformative mesenchyme from the periphery. The ground substance of the primary vitreous is finely fibrilla r. B, By the second month, the vascular primary vitreous reaches its greatest extent. Arborization of the vasa hyaloidea propria (curved arrow) f ills the retrolenta l area and is embedded in collagen fibrils. An avascular secondary vitreous of more fine ly fibrillar com position forms a narrow zone between the peripheral (ou ter) branches of the vasa hyaloidea propria and the retina. The bent arrow (a t top) points to the vessel of the pupillary membrane, The drawing is a composite of embryos at 15-30 mm . C, During the fourth month, hyaloid vessels and the vasa hyaloidea propria, together with the tunica vascu losa lentis, atrophy progressive ly, w ith the smaller peripheral channels regressing first. The curved arrow points to remnants of involuted vesse ls of the superficial portion of the vasa hyaloidea propria in the secondary vitreous. The black arrowhead (upper left! Indicates the pupillary membrane (not sketche d). The straight arrow points to the remnants of the atroph ied capsulopupillary vesse ls. Zonular fibers (tertiary vitreous) begin to stretch fro m the growing ci liary reg ion toward the lens capsule. Vessels through th e center of the optic nerve connect with the hyaloid artery and vein and send small loops into the retina (open arrowhead). The drawing is a composite of fetuses at 75-1 10 mm, (Reproduced with permission from Cook CS, Ozanics V, Ja kobiec FA. Prenatal
Hyaloid artery
c
i artery and vein
development of the e ye and its adnexa. In: Tasman
\IV, Jaeger f A. eds, Duane's Foundations of Clinical Ophthalmology. Philadelphia: Lippincott; 1991 .)
132 • Fundamentals and Principles of Ophthalmology is very low during the prenatal period but increases after birth. Initially, the secondary vitreous occupies only a narrow space between the retina and the posterior limit of the
primary vitreous. The continued development of the secondary vitreous, until the end of the third month, is related to the regression of the hyaloid system and the simultaneous retraction of the primary vitreous. Remnants of the atrophied hyaloid system and primary vitreous remain throughout life as the Cloquet canal. Between the third and fourth months of gestation, collagen fibrils of the secondary vitreous condense and become attached to the internal limiting membrane at the rim of the optic cup. The condensation of fibrils extends to the lens equator and constitutes the tertiary vitreous. The zonular apparatus of the lens ultimately develops anterior to these collagen fibrils.
Choroid The development of the choroid begins at the anterior region of the optic cup and proceeds posteriorly toward the optic stalk. Choroidal development is associated with the condensation of neural crest cells around the cup that differentiate into cells of the choroidal stroma. Endothelium-lined blood spaces appear in this mesenchymal tissue and first coalesce as the embryonic annular vessel at the rim of the optic cup. During the fourth and fifth weeks of gestation, the choriocapillaris begins to differentiate. The choriocapillary network is formed by mesodermal cells that come in contact with the RPE, which is differentiating simultaneously. The embryonic eye is completely invested with a primitive layer of capillaries at the beginning of the sixth week of gestation. Adjacent endothelial cells are joined by punctate junctional complexes and zonulae occludentes. Characteristic diaphragmed fenestrations develop in the endothelium between the seventh and ninth weeks. At the same time, the basal lamina becomes defined as a continuous layer of extracellular material surrounding the capillaries. Toward the RPE, this basal lamina constitutes the fifth layer of Bruch's membrane. The network of vascular channels is supplied by vessels from the internal carotid artery and, later, by the primitive ophthalmic arteries. The channels drain into 2 main blood spaces, the superior orbital and inferior orbital venous plexuses, and from there into what will become the cavernous sinuses. By the end of the second month of gestation, short ciliary arteries enter the capillary coat. Arteries can be distinguished by narrow lumina and walls 2 or more cells thick; veins are larger and lined only by endothelium. Definite layering of the choroidal vasculature begins in the third month, when the outer layer of large vessels develops. Mainly venous, this layer receives small efferent branches of the choriocapillaris and connects with the vortex veins, which eventually perforate the neighboring sclera. During the fourth month of gestation, the anterior ciliary and long posterior ciliary arteries form the major arterial circle of the iris. Recurrent branches extend from this vessel into the ciliary body by the end of the fifth month. (However, the final anastomosis with the arterial circulation of the choroid is not established until the eighth month.) During the fifth month of gestation, the third layer of medium-sized arterioles develops between the choriocapillaris and the outer layer oflarge vessels. This layer
CHAPTER 4:
Ocular Development.
133
is initially confined to the level of the equator and reaches the developing ciliary body only at the sixth month. The choroidal stroma is demarcated by the sclera at the end of the third month of gestation. Initially, the stroma consists of a loosely organized framework of collagen fibrils and abundant fibroblasts. Elastic tissue is laid down during the fourth month. Melanosomes appear between weeks 24 and 27 of gestation, most notably in the melanocy1es of the outer choroid and suprachoroid. Melanocy1es differentiate from neural crest cells. Melanogenesis proceeds anteriorly from the optic disc to the ora serrata. A few immature melanosomes can be found in the choroidal melanocy1es at birth.
Cornea and Sclera The separation of the lens vesicle from the surface ectoderm initiates the development of the cornea (Fig 4-12). By the end of the fifth week of gestation, the ectoderm consists of 2 layers of epithelial cells that rest on a thin basal lamina (Fig 4-13). Detachment ofthe lens vesicle induces the basal layer of epithelial cells to secrete collagen fibrils and glycosaminoglycans, which occupy the space between the lens and the corneal epithelium and constitute the primary stroma. Mesenchymal cells migrate from the margins of the rim of the optic cup along the posterior surface of the primary stroma. The first of 3 successive waves of ingrowth, these neural crest-derived cells form the corneal endothelium. At 5-6 weeks of gestation, the cornea consists of the following: • a superfiCial squamous an? a basal cuboidal layer of epithelial cells a primary stroma
a double layer of endothelial cells posteriorly
Epithelium
III
II
Figure 4~ 12
Three successive waves of ingrowth of neural crest cells associated with dif-
fere ntiation of th e an terior cham bers . I, Fi rst wave forms the cornea l endoth elium. II, Second
wave forms the iris and part of the pupillary membrane . III, Third wave forms ke ratocytes. (Reproduced with permission from Tripathi BJ, Tripathi Re, W isdom
J. Embryology of the anterior segment. In: Ritch R,
Shields MB, Krupin T, eds. The Glaucomas. 2nd ed. Sf Louis: M osby; 1996.)
134 • Fundamenta ls and Principles of Ophthalm ology
A
39 days
C
7% weeks
3 months
D
o
Figure 4-13 Development of the cornea in the central region . A, At day 39, 2-layered epithelium rests on the basal lamina and is separated from the endothelium 12- 3 layersl by a narrow acellular space. B, At week 7, mesenchymal cells from the periphery migrate into the space between the epithelium and endothelium. C, Mesenchymal cells Ifuture keratocytes) are arranged in 4-5 incomplete layers by 71;2 weeks: a few collagen fibrils are present among
the cells. D, By 3 months, the epithelium has 2-3 layers of cells, and the stroma has abou t 25- 30 layers of keratocytes that are arra nged more regu larly in the posterior half. Thin, uneven Descemet's membrane lies between the most posterior keratocytes and the now~single layer of endothelium . (Reproduced with permission from Cook CS, OZ8nics V. Jakobiee FA. Prenatal development of the eye and irs adnexa. In: Tasman W. Jaeger EA, ed. Duane's Foundations of Clinica l Ophthalmology. Philadelphia : Lippin-
cott; 1997.)
Further development of the stroma is preceded by the ingrowth of another wave of mesenchymal cells fro m the rim of the optic cup, which proceeds in 2 di rections, The cells of the posterior extension grow between the lens epithelium and the corneal endothelium and are destined to form the primary pupillary membrane. Concurrently, hydration of the hyaluronic acid component of the primary stroma causes swelling that seems to make space available for the next migratory wave of cells. At approximately 7 weeks' gestation. the anterior extension of mesenchymal cells m igrates into the corneal stroma. These cells differentiate into ke ratocytes that secrete type I collagen fib rils and form the matrix of the mature (or secondary) corneal stroma. When the stroma attains its maximum width. it is approxi mately double the normal postembryonic width. Dehydration (espeCially of hyaluroniC acid) and compression of the connective tissue cause the later reduction in thickness, Morphogenesis ofkeratocytes begins in the posterior stroma and proceeds anteriorly, The cells syntheSize proteoglycans and collage n fib rils. which are organized as lamellae. Each lamella continues to grow by the fo rmation of additional fibrils (interstitial growth); Simultaneously. successive layers
CHAPTER 4:
Ocular Development. 135
oflamellae are added (appositional growth). As the lamellae increase in length and width, the diameter and thickness of the cornea enlarge. The endothelium in the central region of the cornea becomes a single layer of flattened cells by the third month of gestation. The cells rest on an interrupted basal lamina, which is the future Descemet's membrane. At this stage in development, Descemet's mem brane consists of 2 zo nes: the lam ina densa, toward the stroma, and the lamina lucida, adjacent to the endothelium. Subsequent growth of Descemet's membrane forms a unique organ ization that is recognized as the fetal banded zone, which attains a maximum thickness of about 3 ~m at birth. [n postnatal life, the posterior non banded zone of Descemet's membrane is composed of a homogeneous, fibrillogranular material. This region continues to thicken with age. By the middle of the fourth month of gestation, the apices of adjacent endothelial cells are joined by zonulae occludentes. This development corresponds with the onset of aqueous humor production by the ciliary processes. Late in the fourth month, the acellular Bowman's zone of the anterior stroma is formed. It is thought that the most superficial keratocytes synthesize and lay down the collagen fibrils and ground substance as they migrate somewhat posteriorly in the stroma. The diameter of the unfixed cornea measures 2 mm at 12 weeks' gestation, 4.5 mm at 17 weeks, and 9.3 mm at 3S weeks: The sclera is formed by mesenchymal cells that condense around the optic cup. Most of these cells are derived from the neural crest. However, those in the caudal region of the sclera are probably de rived from paraxial mesoderm that lies juxtaposed to the caudomedial surface of the optic cup ·throughout the period of crest cell migration. The sclera develops anteriorly before the seventh week of gestation and gradually extends posteriorly. The alignment of cells into parallel layers and the deposition of collagen fibrils are evidence of differentiation. Deposits of elastin and glycosaminoglycans are added to the extracellular matrix at a later stage. By the third month of gestation, some undifferentiated mesenchymal cells have migrated between the nerve fibers in the optic nerve. These cells become oriented transversely and synthesize extracellular matrix materials to form the lamina crib rosa.
Anterior Chamber, Angle, Iris, and Ciliary Body The anterior chamber is fi rst recognizable as the slitlike space that results after the ingrowth of the first wave of mesenchymal cells and the posterior extension of the second wave. By approximately 7 weeks' gestation, the angle of the anterior chamber is occupied by a nest ofloosely organ ized mesenchymal cells of neural crest origin. These cells will develop into the trabecular meshwork. At the posterior aspect of the angle, mesodermal cells are developing into the vascular channels of the pupillary membrane. Loosely organized mesenchymal cells and the pigment epithelium of the forward-growing optic cup are also present in this region.
Anteriorly, cells that resemble the corneal endothelium form a layer that extends to the angle recess; these cells meet the anterior surface of the developing iris, thus demarcating the angle of the anterior chamber by week 1S of gestation. Beginning at the third
136 • Fundamentals and Principles of Ophthalmology month of gestation and continuing for a considerable time after birth (up to the age of 4 years), the angle recess progressively deepens (Fig 4-14). It also appears to be reposi-
tioned posteriorly because of the differential growth rate of adjacent tissues. Initially, no demarcation exists between the mesenchymal cells that will form the trabecular meshwork and those that will differentiate into the ciliary muscle. The extracellular matrix of the trabecular beams is synthesized and deposited by the differentiating trabecular cells beginning at week 15 and continuing up to the eighth month of gestation. Even as early as 12- 14 weeks' gestation, the cellular layer that lines the trabecular meshwork on its anterior chamber aspect is perforated by gaps of 2- 8 ~m in diameter. As development proceeds, these gaps become larger, and eventually the open spaces of the meshwork directly communicate with the anterior chamber. The Schlemm canal develops from a small plexus of venous canaliculi by the end of the third month of gestation. Derived from mesodermal mesenchyme, these channels function initially as blood vessels. Other mesenchymal cells surround the canal during the fourth month of gestation. These cells and their secreted extracellular matrix materials will form juxtacanalicular tissue. Characteristic vacuolar configurations begin to appear in the endothelial cells that line the Schlemm canal at approximately the beginning of the fifth month. Th~ir development corresponds to the onset of aqueous humor circulation. The canal begins to function as an aqueous sinus rather than as a blood vessel.
Figure 4-14 Light micrograph of the eye of an 11 -wee k fet us in meri dional sect ion. The angular region is poorly defined at th is stage and is occu pied by loosely arran ged, spindle-shaped
cells . The Schlemm canal is unrecognizable, and ciliary muscles and ciliary processes are not yet formed; the latte r are derived f ro m neural ectodermal fold (asterisk). Corneal e ndot heli um appea rs cont inuous wi th the ce ll ular covering of t he primit ive iris . AC = an te rior cha mber,
=
L lens. (Orig inal magn ification x230.) (Reproduced with permission from Triparhi Re, Triparh i Be. Functional anatomy of the anterior chamber angle. In: Tasman \.IV, Jaeger EA. eds. Duane's Foundations of Clin ical Ophthalmology. Philadelphia: Lippincott; 1991.)
CHAPTER 4,
Ocular Development.
137
Differentiation of the ciliary epithelium occurs in the 2 layers of neuroectoderm just behind the advancing optic cup. Late in the third month, longitudinal indentations appear in the outer pigmented layer. Between the third and fourth months, the inner, nonpigmented, layer starts to foll ow the contour and adhere to the pigmented layer. These radial folds, approximately 75 in number, are the beginning of the ciliary processes. At week lOaf gestation, precursor ciliary muscle cells are identified as an accumulation of mesenchymal ceUs between the primitive Ciliary epithelium and the anterior sclera condensation at the margin of the optic cup. Differentiation, which begins in the outermost (sclerad) cells during week 12, is evident from the myofilaments that surround plaques of dense bodies along the plasmalemma. The meridional part of the muscle becomes organized during the fifth month, followed by the circular and radial parts. The circular muscle continues to develop for at least 1 year after birth. The development of the iris is associated with the formation of the anterior portion of the tunica vasculosa len tis. At approximately the sixth week of gestation, vascular channels of this embryonic structure are present as blind outgrowths from the annular vessel that encircles the rim of the optic cup. The developing vessels extend into the mesenchymal cells that cover the anterior lens surface and will ultimately be incorporated into the iris stroma. The most anterior region of the tunica vasculosa lentis is replaced subsequently by the pupillary membrane. At the end of the third month of gestation, after the future ciliary processes have formed, both walls of the optic cup (at its margin) grow forward beneath the pupillary membrane and mesenchymal cells. The mesenchymal tissue of the iris differentiates earlier than does the neuroectoderm. Cells in the developing stroma become fibroblast-like and secrete collagen fibrils and other components of the extracellular matrix. The earliest differentiation of the sphincter muscle from the anterior layer of epithelium (the forward extension of the RPE) occurs at 3 months' gestation. However, myofibrils are not synthesized until the fifth month, and the muscle does not come to lie free in the stroma until the eighth month of gestation. The dilator muscle is not apparent until the sixth month, and differentiation of the myoepi thelial cells continues after birth. Pigmentation of the posterior epitbeliallayer of the iris, which is a continuation of the nonpigmented layer of the Ciliary body and hence of the neurosensory retina, begins at the pupillary margin at midterm and proceeds toward the periphery. It ceases at the iris root by the end of the seventh month. The pupillary portion of the tunica vasc ulosa lentis is resorbed during the sixth month of gestation. The remains of an incomplete arteriovenous anastomosis at the ciliary end of the sphincter muscle demarcate the collarette. The pupillary membrane atrophies near term. The iris is still immature at birth. Much of the extracellular mat rix is yet to be laid down in the stroma. The collarette is closer to the pupil in the newborn eye than it is in the adult eye.
Vascular System The development of the vascular system of the eye and orbit is complex. Many vessels are transitory, arising and regressing in response to the changing needs of the embryonic eye.
138 • Fundamentals and Principles of Ophthalmology Vascular channels from the internal carotid artery develop in the mesenchyme around the optic vesicle late in the fourth week. Primitive dorsal and ventral ophthalmic arteries bud inward from the carotid and join a loose reticulum of capillaries around the optic vesicle. The system is drained into the future cavernous sinuses by way of plexuses. The early vessels are primarily ocular. A transient vessel, the stapedial artery, arises from the carotid to supply the expanding orbit. Later, the distal part of the stapedial artery is annexed to the ophthalmic artery. The hyalOid artery is a branch of the primitive dorsal ophthalmic artery that arises at the juncture of the optic stalk and the optic cup at the time of closure of the embryonic fissure. The annular vessel that develops at the rim of the optic cup is supplied by the dor,sal and ventral arteries. When incorporated in the optic cup, the hyalOid system extends toward and around the lens to join the annular vessel. Together with the tunica vasculosa lentis, the hyalOid system nourishes the interior of the developing eye. The primitive dorsal ophthalmic artery becomes the definitive ophthalmic artery of the orbit at the sixth week of gestation. It supplies the temporal long posterior ciliary artery, the short posterior ciliary arteries, and the central retinal artery. The primitive ventral ophthalmic artery almost disappears; only a portion remains as the long posterior nasal Ciliary art~ry. The major arterial circle of the iris develops in the mesenchyme that surrounds the optic cup. It is located slightly lateral and anterior to the annular vessel and is formed by a coalescence of branches from the long ciliary arteries. Vascular twigs with little connective tissue grow from both the annular vessel and the major arterial circle to form the pupillary membrane, a system of radial vascular loops over the surface of the iris and lens. The pupillary arcades disappear centrally but remain peripherally as the minor circle of the iris. They provide vessels of the mature iris. Because the tissues that demarcate the anterior chamber angle are repositioned during development, the major arterial circle of the iris is ultimately located in the ciliary body. At the fourth month of gestation, spindle-shaped mesenchymal cells arise from the hyaloid artery at the optic disc. These cells infiltrate the inner layers of the retina as solid cords of undifferentiated cells. Lumina develop, initially as slitlike openings behind the advancing edge of the invading mesenchymal cells. Retinal vascularization proceeds centripetally, and a boundary zone consisting of undifferentiated cells distinguishes the avascular and vascular retina. Endothelial cells differentiate first; adjacent cells are joined by zonulae occludentes and gap junctions. Vascularization of the nasal retina is complete before that of the temporal retina because of the shorter distance from the optic disc to the nasal ora serrata. By the fifth month, patent vessels have extended superiorly and inferiorly on the temporal aspect of the retina, sparing the region of the putative macula. Small blood vessels begin to develop in the ganglion cell laye r of the foveal slope at the sixth month. The adult pattern of arterioles, veins, and capillaries is established through a process of remodeling and retraction of the primitive capillary network. Although capillaries reach the ora serrata by the eighth month, the mature pattern of vascularization is not achieved until 3 months after birth. The hyalOid system and the tunica vasculosa lentis atrophy in the third trimester. OccaSionally, either system may persist after birth (Fig 4- 15).
CHAPTER 4:
Figu re 4- 15
Ocular Development . 139
Persistent pupillary membrane.
Penfold PL, Provis JM, Madigan .Me. van Oriel D. Billson FA. Angioge nesis in normal human retinal development: the involvement of astrocytes and macrophages. Graefes Arch CUll Exp
Ophthall1lol. 1990;228(3 ),255- 263.
Periocular Tissues and Eyelids The frontonasal and maxil lary processes of neu ral crest cells occupy the space that surrounds the optic cups by the fourth week of gestation. The bones. cartilage. fa t. and connective tiss ues of the orbit develop from these cells. All bones of the orbit are membran o us except the sphenoid. which is in itia ll y ca rtilagino us. Ossification begi ns during the third month of gestation, and fusio n occurs between th e sixth and seventh months. The extraocular muscles arise from myotomic cell s of the preo tic mesodermal somites that have shifted craniall y. These ceUs become located withi n the neural crest mesenchyme. which is situated on the dorsal and cauda l aspects of the developing eye. Although the ext raocular muscles were once thought to begi n developing at the primit ive muscle cone that surrounds the optic nerve in the fift h week of gestation , recent evidence suggests that the muscles arise in situ. Myoblas ts with myofibrils and immature Z bands are dist inguishable by the fifth week of gestation. At apprOXimately 7 weeks. the dorsomedial aspec t of the superior rectus muscle gives rise to the levator muscle, which grows laterall y and over the superior rec tus toward th e eyelid. The tendons of th e extraocular muscles fuse with the sclera in the vicinity of the equator late in the third month. The upper eyelid fi rst develops as a proliferation of surface ectoderm in the region of the future outer ca nthus at 4-5 weeks' gestation. During the second month. both the
140 • Fundamentals and Principles of Ophthalmology
upper and the lower eyelids are discernible as undifferentiated skin folds that surround mesenchyme of neural crest origin (Fig 4-16). Later, mesodermal mesenchyme infiltrates the eyelids and differentiates into the palpebral musculature. The eyelid folds grow toward each other as well as laterally. Starting near the inner canthus, the margins of the folds fuse at approximately 10 weeks' gestation. As the folds adhere to each other, evolution of cilia and glands continues. The orbicularis muscle condenses in the fold in week 12. The eyelid adhesions gradually break down late in the fifth month, coincident with the secretion of sebum from the sebaceous glands and cornification of the surface epithelium. The lac rimal gland begins to develop between the sixth and seventh weeks of gestation. Solid cords of epithelial cells proliferate from the basal cell laye r of the conjunctiva in the temporal region of the fornix . Neural crest -derived mesenchymal cells aggregate at the tips of the cords and differentiate into acini. Ducts of the gland are formed at approximately 3 months by vacuolation of the cord cells and the development of lumina. Lacrimal gland (reflex) tear production does not begin until 20 or more days after birth. Hence, newborn infants cry without tears. Between the third and sixth months of gestation, eyelid appendages and pilosebaceous units develop from invaginations of epithelial cells into the underlying mesenchyme.
Realignment of the Globe Initially, the axes of the 2 optic cups and the optic stalks form an angle of 180°. At 3 months' gestation, this angle has decreased to 105°. With continued enlargement; remodeling; and repositioning of the head, face, and brai n throughout gestation, the eyes become oriented in their anterior position. At birth, the axes form an angle of 710 . However, the adult orientation of 68° is not achieved until the age of 3 years.
Development of the eyelids. A. Seventh week: upper and lower eyelid folds grow over the eye. B, Eyelids fu se dUring the e igh th wee k; fusion starts along the nasa l margin. C, From the fifth to the seventh month, as cilia and glandular structures develop, eyelids gradually open .
Figur. 4-16
CHAPTER 4:
Ocular Development. 141
Con enital Anomalies Congenital anomalies are defects present at birth. They result both from genetic influences and from a variety of local and systemic environmental effects. A teratogen is an agent that produces or increases the incidence of congenital malformation. Because exposure to a teratogen can occur at any stage of embryonic development, its effects differ according to the time, duration, and intensity of exposure. A teratogen acting in the first trimester on primordial cells produces severe damage to the ocular primordium and its derivative tissues. Because cellular development continues after organogenesis, genetic influences or environmental effects at a later time may lead to dysfunction without gross structural abnormalities. For more discussion of congenital anomalies, see BCSC Section 8, External Disease and Cornea.
Genetic Influences Mutations in homeobox genes are known to produce congenital ocular abnormalities. Often, the mutation results from the deletion or insertion of a single nucleotide that causes a frameshift in the coding region. Several congenital ocular anomalies have been matched to specific mutations-for example, aniridia, posterior embryotoxon, Peters anomaly, Axenfeld anomaly, and congenital cataract are produced by mutations in PAX6 (see the Human PAX6 Allelic Variant Database Web Site, http:// pax6.hgu.mrc.ac.uk, for more information); oculorenal syndrome and coloboma of the optic nerve, by PAX2 mutations; and cyclopia and holoprosencephaly, by Sonic hedgehog mutations. A mutation in different homeobox genes may produce the same clinical manifestations (eg, a mutation in RlEGJ also results in Peters anomaly), or a different mutation in the same gene may produce a similar but distinct phenotype (eg, a mutation in RlEGI causes Jl.ieger anomaly). Future investigations will identify additional roles for homeobox genes in normal development and their aberrant expression in ab normal ocular development. Borges AS, Susanna R Jr. Carani IC, et al. Genetic analysis of PITX2 and FOXC 1 in Rieger syndrome patients from BraziL] Glaucoma . 2002; 11 (1):51 - 56. Kumar JP, Moses K. ExpreSSio n of evolutionarily conserved eye specification genes during Drosophila embryogenesis. Dev Genes Evo!. 2001 ;211 (8·9) :406-414.
Nongenetic Teratogens Nongenetic teratogens include toxins
maternal infections nutritional deficiencies • radiation
• drugs • developmental failures traumatic insult
142 • Fundamentals and Principles of Ophthalmology Early studies showed that an excess or deficiency of vitamin A during development causes eye abnormalities such as coloboma and lens defects. However, it is now apparent that the acid form of this vitamin-retinoic acid- is critical not only to early ocular development but also to the induction of congenital ano malies. Exposure of the developing human embryo to excess amounts of retinoic acid causes malformation of the retina and optic nerve by affecting the expression of homeobox genes such as PAX2, MSH-C, and Sonic hedgehog. Hyatt GA, Dowling JE. Retinoic acid. A key molecul e for eye and photorecep tor development. Invest Ophthalmol Vis Sci, 1997;38(8); 1471 - 1475.
Prenatal exposure to alcohol results in a disti nct pattern of delayed growth, mental retardati on, and abnormal behavior patterns, as well as multiple congen ital malformations,
that is recogn ized as fetal alcohol syndrome (FAS) (Fig 4-17). Various ocular abnormalities include anomalies of the ad nexa (strabismus, blepharoptosis, epicanthus) and intraocular defects (cataract, glaucoma, coloboma of uvea, persistent fetal vasculature, dysmorphogenesis of the retina, and optic nerve hypoplasia). The majo rity of children with FAS have optic nerve hypoplasia, which, in experimen tal models, results from reduced densities of ganglion cells and their axons as well as from damage to glial cells and myelin sheaths in the optic nerve.
8 Figure 4-17
A, A fetus removed from a mouse, to which
alcohol (ethanol) had been adm inistered ea rly in pregnancy (gastrulation stage), shows numerous fac ial characteristics
similar to those of a child with fetal alcohol syndrome (BI. C, A control mouse fetus.
(Parts A and Creproduced with permission from Kathleen Sulik, PhD. In: Serafin 0, Georgiade NG. Pediatric Plastic Surgery. St Louis. Mosby; 1984. Part B courtesy of Marilyn Miller, MD.)
CHAPTER 4: Ocular Development. 143
The mouse model of FAS shows that the teratogenic effect of alcohol starts with the insult to the optic primordia during the gastrula stage. A small optic vesicle results in a deficient lens vesicle, wh ich manifests as microphakia. Delay in lens detachment from surface ectoderm leads to myriad anterior segment anomalies that resemble Peters and Axenfeld anomalies. Such conditions are explained by impairment of the migration of the neural crest cells that should have formed the corneal stroma, endothelium, and iris. Together, they cause microphthalmos with a secondary persistence of primary vitreous. Persistence of the embr yo nic fissure, which leads to a failure in maintaining intraocular pressure, is one explanation for microphthalmos and coloboma formation . In the mouse model , these malformations appear to be primary events caused by faulty induction produced by the early insult of alcohol on the gastrula forebrain , which gives rise to the evaginating optic primordia. Hug TE, Fitzgerald KM, Cibis GW Clinical and electroretinographic findings in fetal alcohol
syndrome. J AA POS. 2000;4(4):200-204.
Introduction
Genetics is the study of human variability. Although genetics is a relatively new science compared to such disCiplines as anatomy and physiology. its significance in the overall understanding of human life cannot be overstated. Genetic knowledge can enhance our understanding of the processes of cellular function. embryology. and development. as well as our concepts of what is and is not a genetic disease. Many researchers think that as much as 90% of medical disease either has a major genetic component or involves genetic factors that may Significantly influence the disease. The discovery of preViously unknown genes has opened new areas of understanding of physiology at the cellular or tissue level. one important example of which is the discovery of homeotic selector genes (eg. the HOX and PAX gene families) that regulate. guide. and coordinate early embr yologic development and differentiation. (These genes. also called homeobox genes. are discussed in Part II . Embryology. as well.) Another example is the identification of the genes that appear to be transcribed as initiating events in the process of apoptosis. or programmed cell deat h. which itself appears critical for normal embryogenesis. Genetic disorders affect about 5% of the liveborn infants in the United States. Approximately 50% of ch ildhood blindness has a genetic cause. At the beginning of this millennium. more than 10.000 human gene loci were know n by mendelian phenotypes andl orcellular and molecular genetic methods. In about 10%-15% of known genetic diseases. clinical findings are limited to the eye; a similar percentage includes systemic disorders with ocu lar manifestations.
Terminolo Not knOW ing the vocabulary of genetics and molecular biology is one of the greatest impediments to understanding these fields. Readers are strongly encouraged to read the glossary before proceeding through the chapters. paying particular attention to those terms with which they are unfamilia r.
Glossary Acceptor splice site The junction between the 3'. or downstream. end of an intron and
the 5'. or upstream. end of the next exon. The consensus sequence is !N.s:AG/G for the intron- exon boundary. where N is a purine (G or A) . See donor splice 9ite Ind splice junction site.
Acrocentric Type of chromosome in which the centromere is located near one end-for example. chromosomes 13. 14. 15.21. and 22. 147
148 • Fundamentals and Principles of Ophthalmology
Allele Alternative form of a gene or DNA sequence that may occupy a given locus on a pair of chromosomes. Clinical traits, gene products, and disorders are said to be allelic if they are determined to be at the same locus and nonallelic if they are determined to reside
at different loci. Allele-specific oligonucleotide (ASO) A synthetic segment of DNA approximately 20 nucleotides in length. When hybridized to an unknown DNA sample, the ASO will bind to and thus identify the complementary sequence or specific string of base pairs. Used in detecting disease mutation. Allelic association See linkage disequilibrium. Allelic heterogeneity When different alleles at the same locus are capable of producing an
abnormal phenotype. Alu repeat sequence A common short interspersed element (SINE), 300 base pairs long,
that occurs 500,000 times scattered throughout the genome. Often involved in errors of duplication or in mutational events. Unique to primates. Amber codon The primitive stop codon TAG, which is thought to become the consensus sequence for the exon-intron and intron- exon boundaries (or splice junction sites) with loss of the thymine. See stop codon. Aneuploidy An abnormal number of chromosomes. Anticipation The occurrence of a dominantly inherited disease at an earlier age (often with greater severity) in subsequent generations. Now known to occur with expansion
of a trinucleotide repeat sequence. Seen, for example, in fragile -X syndrome, myotonic dystrophy, and Huntington disease. Antioncogene See tumor-suppressor genes. Antisense strand of DNA That strand of double-stranded DNA that serves as template for RNA transcription. Also called the noncoding, or transcribed, strand. See sense strand of DNA. Apoptosis The process by which internal or external messages trigger expression of spe-
cific genes and their products, resulting in the initiation of a series of cellular events that involve fragmentation of the cell nucleus, dissolution of cellular structure, and orderly cell death. Unlike traumatic cell death, apoptosis results in the death of individual cells rather than clusters of cells and does not lead to the release of inflammatory intracellular products. Also called programmed cell death (peDJ. Ascertainment The method of selecting families for inclusion in a genetic study. Assortative mating Mating between individuals with preference for or against a specific
genotype; that is, nonrandom mating. Autosome Any chromosome other than the sex (X or Y) chromosomes. The normal
human has 22 pairs of autosomes.
Introd uction. 149
BAC A bacterial a rtificial chromosome used as a cloning vector. BACs can be used to clone up to 150 kilobases (kb) of DNA . Bacteriophage A virus containjng DNA or RNA whose host is bacteria. Bacteriophage can be used for the transduction or insertion of fragments of DNA into bacteria for cloning purposes. Barr body Inactive X chromosome seen in the nucleus of some female somatic cells.
Base pair (bp) Two complementary nitrogen bases that are paired in double-stranded DNA. Used as a unit of physical distance or length of a sequence of nucleotides. Carrier An individual who has a pair of genes consisting of I normal and I abnormal, or mutant, gene. Usually, such individuals are by definition phenotypically "normal;' although in certain disorders biochemical evidence of a deficient or defective gene product may be present. Occasionally, carriers of an X-linked disorder may show partial expression of a genetic trait.
CCAAT box Approximately 75- 80 bp upstream from the transcription initiation site of many genes is this sequence of nucleotides that is thought to playa role in promoter function .
cDNA clone A host cell that has a vector contain ing a fragment of complementary DNA (cDNA) from another organism. Centimorgan (cM) A measure of the crossover frequency between linked ge nes. One centimorga n equals I % recombination and represents a physical distance of approximately I million bp. Centromere The constricted region of the chromosome. It is associated with spindle fibers during mitosis and meiosis and is important in the movement of chromosomes to the poles of the dividing cell. Chorionic villus sampling (CVS) Transcervical procedure in which chorionic vill i are retrieved with a fleJdble suction catheter and used in studies to establish a prenatal diagnosis. Chromatid One of the duplicate arms (also called sister chromatids) of chromosomes that are created after DNA replication during mitosis or the first d ivision of meiosis. Chromatin The complex of DNA and proteins that is present in chromosomes. Clinical heterogeneity Different mutations at the same locus producing different phenotypes. Examples include macular dystrophy and retinitis pigmentosa from differing m utations of peripherinl RDS and Crouzon, Pfeiffer, and Apert syndromes from mutations of FGFR2. Cloning vector Any DNA molecule capable of autonomous replication within a host cell into which DNA can be inserted fo r amplification. Cloning vectors can be derived from plasm ids, bacteriophages, viruses, and yeast. Exam ples of cloning vectors include YAC
150 • Fundamentals and Principles of Ophthalmology (yeast artificial chromosome), BAC (bacterial artificial chromosome), and PAC (P I artificial chromosome).
Cod om ina nee Simultaneous expression of both alleles of a heterozygous locus (eg, ABO blood groups). Codon The basic unit of the genetic code. The DNA molecule is a chain of nucleotide
bases that is "read" in units of 3 bases (triplets), which will translate (through messenger RNA) to an amino acid. Thus, each triplet codon specifies a single amino acid. Complementary DNA (eDNA) DNA created by the action of reverse transcriptase from messenger RNA. cDNA does not have introns, as does genomic DNA. Compound heterozygote Gene locus having 2 different, abnormal alleles. Congenital Present at birth. The term has no implications about the origin of the congeni-
tal feature. Consanguinity Mating between blood relatives, or a genetic relationship by descent from a common ancestor. Consensus sequence The most com mon or idealized sequence of base pairs (or encoded
amino acids) for a given region of a gene. See acceptor splice site for an example. Conservation A genetic sequence or nucleotide position is said to be conserved or show conservat ion if a similar sequence is present among different species at one gene o r related genes of similar sequence.
Contig A set of overlapping clones, each containing a fragment of a specific region or
DNA sequence, that collectively covers the region without an interruption. Cosmid A self-replicating vector (hybrid bacteriophage) used for cloning of DNA frag-
ments into bacteria. Cosmids accommodate a DNA sequence of approximately 40 kb and are useful for creating gene libraries. Crossing over A process in which homologous chromosomes (chromatids) exchange seg-
ments by breakage and by the physical exchange of segments, followed by repair of the breaks. Crossing over is a regular event in meiosis but occurs only rarely in mitosis. Also termed recombination.
Degeneracy of the code The genetic code is termed degenerate because most of the 20
amino acids are encoded by more than I of the 64 possible triplet codons. Digenic inheritance Simultaneous inheritance of2 nonallelic mutant genes, giving rise to a
genetic disorder wherein inheritance of only I of the 2 is insufficient to cause disease. An example is retinitis pigmentosa caused by simultaneous inheritance in the heterozygous state of otherwise tolerable mutations of both the ROM1 and peripherinl RDS genes. The Simplest form of polygenic inheritance.
Introduction. 151 Diploid The number of chromosomes in most somatic cells, which in humans is 46. The diploid number is twice the haploid number, which is the number of chromosomes in
gametes.
DNA Deoxyribonucleic acid, the nucleic acid of chromosomes. Dominant An allele that is expressed in the phenotype when inherited along with a normal allele. See recessive. Dominant medical disorder A distinctive disease state that occurs in a (dominant) heterozygous genotype. Classically, normal dominant traits give the same phenotype in both the heterozygous and the homozygous states. Homozygotes for dominant disease-producing alleles are rare and are usually more severely affected than heterozygotes. Dominant negative An autosomal dominant mutation that disrupts the function of the
normal or wild-type allele in the heterozygous state, giving a phenotype approaching that of the homozygous mutant. Donor splice site The junction between the 3' end of an exon and the 5' end of the next
s:.
intron. The consensus sequence is AG/GT ~ AGT for the exon-intron boundary. See acceptor splice site and splice junction'\ite. G Endonuclease A phospho diester-cleaving enzyme, usually derived from bacteria, that cuts nucleic acids at internal positions. Restriction endonucleases cut at specific recognition
sites determined by the occurrence of a specific sequence of 4, 5, or 6 bp. Endonuclease specificity may also be confined to substrate conformation, nucleic acid species (DNA, RNA) , and the presence of modified nucleotides. Enhancer Any sequence of DNA upstream or downstream of the coding region that acts in cis (ie, on the same chromosome) to increase (or, as a negative enhancer, decrease) the
rate of transcription of a nearby gene. Enhancers may display tissue specificity and act over considerable distances. Eukaryote Organisms with their DNA located within a nucleus (includes all multicellular and higher unicellular organisms). See prokaryote. Exon Any segment of a gene that is represented in the mature mRNA product. See intron. Expressed-sequence tag (EST) A partial sequence of a gene that uniquely identifies the gene's message. These tags are useful, through reverse transcriptase polymerase chain re-
action (RT-PCR), for determining the expression of genes. Expressivity The variation in clinical manifestation among individuals with a particular
genotype, usually a dominant medical disorder. The variability may be a difference in either age of onset (manifestation) or severity. See penetrance. Fragile sites Reproducible sites of secondary constrictions, gaps, or breaks in chromatids. Fragile sites are transmitted as mendelian codominant traits and are usually not associated
152 • Fundamentals and Principles of Ophthalmology with abnormal phenotype. The most notable exceptions are the association of fragile X chromosomes with X-linked mental retardation and postpubertal macro-orchidism (fragile-X syndrome). See trinucleotide repeat expansion. Frameshilt mutation (framing error. frameshilt) Any mutation, usually a deletion or insertion of a nucleotide or a number of nucleotides not divisible by 3, that results in a loss of
the normal sequences of triplets, causing the new sequence to code for entirely different amino acids from the original. The mutation usually leads to the eventual chance forma tion of a stop codon. Gene The segment of DNA and its associated regulatory elements coding for a single trait,
usually a Single polypeptide or mRNA . The definition was expanded to include any expressed sequence of nucleotides that has functional significance, including DNA sequences that govern the punctuation (promoter) or regulation (enhancer) of transcription. Genetic Related to or produced by a gene. Genocopy Different nonallelic genotypes that result in a similar phenotype (often a medi-
cal disorder). Genome The sum total of the genetic material of a cell or of an organism. Genomic clone A host cell that has a vector containing a fragment of genomic DNA from another organism. Genotype The genetic constitution of an organism. Also used to denote the specific set of
2 alleles inherited at a locus. Germinal mosaicism The occurrence of 2 populations of gametes in an individual, one
population with a normal allele and the other with a disease-producing mutant gene. Of "new" cases of some autosomal dominant diseases (eg, osteogenesis imperfecta), 5%-10% are thought to result from germinal mosaicism; offspring of the affected parent are at significant risk for the same disease. Haploid Half the number of chromosomes in most somatic cells, equal to the number of
chromosomes in gametes. In humans, the haplOid number is 23. Also used to denote the state in which only 1 of a pair or set of chromosomes is present. See diplOid. Haploid insuHiciency (haploinsuHiciency) The condition of dominant genetic disease
caused by reduction in gene product to levels that are insufficient to produce the desired function of the protein. For example, aniridia and Waardenburg syndrome result from insuffiCiency of the Single functional copy of the PAX6 and PAX3 genes, respectively, to activate transcription of the genes that they normally control. Haplotype The combination oflinked polymorph isms or marker alleles for a given region of DNA on a Single chromosome. Hemizygous (hemizygote) Having only 1 allele at a locus; usually refers to X-linked loci in
males, who normally have only 1 set of X-linked genes. An individual who is missing an
Introduction . 153 entire chromosome or a segment of Olle chromosome is considered hemizygous for the genes on the homologous chromosome. Hereditary Genetically transmitted or capable of being genetically transmitted from parent to offspring. Not quite synonymous with heritable, which implies the ability to be transmitted to the next generation but does not intrinsically connote inheritance from the last generation. See genetic. Heterogeneity (genetic heterogeneity) The production of a phenotype (or appa rently simi-
lar phenotypes) by differe nt genetic entities. Refe rs to genetic d isorders that are found to be 2 or more fundamentally distinct entities. See genocopy. Heteronuclear RNA (hnRNA) The mRNA from the initiator codon to the stop codon. Ap-
proximately 25% of these represent immature RNAs prior to splicing out of the introns. The function of the other 75% is unknown. Also called heterogeneous nuclear RNA. Heteroplasmy The presence of 2 or more different populations of mitochondria within a cell, each population carrying a different allele (or the presence or absence of a mutation) at a given locus. Heterozygous (heterozygote) Having 2 unlike alleles at a particular locus. See hemizygous,
homozygous. Homeobox A conserved 180 bp sequence of DNA, first detected within homeotic selector genes, that helps determine the cell's fate. Homeotic selector genes Genes that appear to regulate the activity or expression of other genes, eventually gUiding the embryonic developme nt of cells into body segments, body parts, and speCialized organ systems. Examples are the HOX and PAX families of developmental genes. The HOX family represents 38 homeobox genes that are linearly arranged in 4 independent complexes termed HOXl , HOX2, H OX3, and H OX4. These gene clusters reside on chromosomes 7, 17, 12, and 2, respectively. Whereas HOX genes are involved in early body plan organization, PAX genes are involved in somewhat later organ and body part development. See the discussion of homeobox genes in Chapter 4, Ocular Development. Homologous chromosomes The 2 members of a matched pair of (sister) chromosomes, I
derived from each parent, that have the same gene loci, but not necessarily the same alleles, in the same order. Homoplasmy The presence of a Single population of mitochondria within a cell, each car-
rying the same allele (or the same presence or absence of a mutation) at a given locus. Homozygous (homozygote) Having 2 like or ide ntical alleles at a particular locus in diploid genome. The term is sometimes misused to refer to compound heterozygote (see above). Host cell In the context of recombinant genetics, the organism (usually a bacterium such
as Escherichia coli ) into which is inserted the vec tor (us ually a plasmid or bacteriophage)
154 • Fundamentals and Principles of Ophthalmology
containing the foreign DNA. Hosts are used to propagate the vector and, hence, the cloned DNA segment. Hybridization The bonding (by Watson -Crick base-pairing) of Single-stranded DNA or
RNA into double-stranded DNA or RNA. The ability of stretches of DNA or RNA to hybridize with each other is highly dependent on the similarity or identity of the base-pair sequence.
Imprinting The reversible marking or inactivation of an allele by inheritance (through ei-
ther the maternal or the paternal lineage), which may Significantly alter gene expression. The imprinting is reversed if the gene is passed through subsequent generations through the opposite parental line. This phenomenon occurs in Prader-Willi and Angelman syndromes; it may also occur with mutations of the Wilms tumor gene. One mechanism of imprinting is thought to involve methylation of 5' elements of the gene. Initiator codon The triplet code that, when coded into mRNA, initiates translation of the mRNA by causing binding of a speCial type of transfer RNA called initiator tRNA. In
prokaryotes (bacteria) , either AUG or GUG can act as an initiator codon. In eukaryotes, AUG is the only initiator codon, and it codes for methionine. Intervening sequence Intron. Intron A segment of DNA that is transcribed into RNA but is ultimately removed from the
transcript by splicing together the sequences on either side of it (exons). Isochromosome An abnormal chromosome created by deletion of 1 arm and duplication
of the other arm, such that the chromosome has 2 equal-length arms of the same loci sequence extending in opposite directions from the centromere. Karyotype A photographic record or a computer printout of an individual's chromosome
set arranged in a standard pattern in pairs by size, shape, band pattern, and other identifiable physical features. Kilobase (kb) 1000 bp of DNA or 1000 bases of Single-stranded RNA. Liability With reference to polygenic or multifactorial inheritance, the graded continuum of increasing susceptibility to a disease or trait. Library A complete set of clones presumably including all genetic material of interest from an organism, tissue, or speCific cell type at a speCified stage of development. A genomic library contains cloned DNA fragments from the entire genome; a eDNA library contains fragments of cloned DNAs generated by reverse transcription from mRNA. Genomic libraries are useful sources to search for genes, whereas eDNA libraries give information about expression within the source cell or tissue.
Linkage A concept that refers to loci rather than to the alleles that reside on those loci. Ex-
ists when the loci of 2 genes or DNA sequences are phYSically close enough to each other
Introduction . 155 on the same chromosome that alleles at the 2 loci do not assort independently at meiosis but tend to be inhe ri ted together. Linkage disequilibrium The state in which alleles that reside at loci close together in the genome remain inherited togethe r thro ugh many generations because the close physical distance makes crossover between the loci extremely unlikely. Thus, alleles that are in linkage disequilibrium are present in subpopulations of individuals (eg, those with a given disease) in greater-than-expected frequencies . Also called allelic association. Locus The physical site on a chromosome occupied by a particular gene; term is often colloquially used interchangeably with gene. Locus heterogeneity Term applies when a similar phenotype is produced by mutations at different loci, for example, X-linked retini tis pigmentosa resulting from RP2 at Xp ll and RP3 at Xp2 1.
LOD score (logarithm of odds, or log of the likelihood ratio) A statistical method that tests whethe r a set ofl inkage data indicates that 2 loci are linked or unlinked. The LOD score is the logarithm to the base 10 of the odds favoring linkage. By convention, an LOD score of 3 (1000: I odds in favor of lin kage) is generall y accepted as proof of linkage. Lyonization Inactivation of genes on either the maternally or the paternally derived X chromosome in somatic cells, occurring at about the time of implantation. First proposed by Mary Lyon. Meiosis The speCial form of cell division that occurs in germ cells by which gametes of haplOid chromosomal number are created. Each of the chromatids, which are clearly visible by prophase, contains a long double helix of DNA associated with histones and other chromosomal proteins. At anaphase, the chromatids separate at the centromere and mi grate to each half of the dividing cell; thus, each daughter cell receives an identical set of chromatids (which become the chromosomes fo r that cell). During the first , or reduction, division of meiosis, the chromatids of homologous chromosomes undergo crossover (during the diplotene phase), and the number of chromosomes is reduced to the haplOid number by the separation of homologous chromosomes (with duplicate chromatids) to each daughter cell. During the second division of meiosis, the sister chromatids separate to form the haplOid set of chromosomes of each gamete. Mendelian disorder (single-gene disorder) A trait or medical disorder that follows patterns of inheritance suggesting the state is determ ined by a gene at a Single locus. Microsatellite (eg, dinucleotide or trinucleotide repeats) Tandemly repeated segments scattered throughout the genome of varying numbers of 2 to 4 nucleotides in a row. For example, a stretch of consecutive CA combinations of bases (NNNCACACACACACA CACACACANNN or [CA) IO' where N is any base) in a DNA strand. The highly va riable nature of the number of repeats provides information useful as markers for establishing linkage to disease loci. See satellite DNA and short tandem repeats.
156 • Fundamentals and Principles of Ophthalmology Minisatellite Array of repeated. nested segments of the same sequence of multiple triplet codons. each segment (consensus repeat unit) varying between 14 and 100 bp. Minisatellites are extraordinarily polymorphic and extremely useful as markers for establishing linkage. because they are often situated upstream or downstream from genes. The repeats are inherently unstable and can undergo mutation at a rate of up to 10%. Defects of some minisatellites are associated with cancer and insulin-dependent diabetes mellitus. Other terms used are variable number of tandem repeats (VNTR) and variable tandem repeats (VTR). See satellite DNA. Missense mutation A mutation. often the change of a single nucleotide. that results in the substitution of 1 amino acid for another in the final gene product. Mitosis The ordinary form of cell division. which results in daughter cells identical in chromosomal number to the parent cell. Mosaic An individual or tissue with at least 2 cell lines of different genotype or distinctive chromosomal constitution that develop after the formation of the zygote. Multifactorial inheritance The combined operation of several unspeCified genetic and environmental factors in the inheritance of a particular trait or disease. See polygenic
inheritance. Mutation Any alteration of a gene or genetic material from its "natural" state. regardless of whether the change has a positive. neutral. or negative effect. Nitrogen bases Nitrogen-containing compounds, either the purines guanine and adenine
or the pyrimidines cytosine. thymine. and uracil. These bases are abbreviated as G. A. C. T. and V. respectively. Nondisjunction Failure of 2 chromosomes to separate during meiosis or mitosis.
Nonsense mutation Any mutation that either results directly in formation of a stop codon or creates a stop codon in the downstream sequence after a frameshift mutation through creation of a frameshift. Northern blot Imprint of an electrophoretic gel that separates fragments of mRNA according to their size and mobility. The fragments are identified by hybridization to cDNA probes. Nucleoside The combination of a nitrogen-containing base and a S-carbon sugar. The S nucleosides are adenosine (A). guanosine (G). cytidine (C). uridine (U). and thymidine (T). Note that the abbreviations are the same as those for the nitrogen bases that characterize the nucleoside. Nucleosome The primary unit of chromatin. consisting of a 146-bp sequence of DNA wrapped twice around a core composed of 8 histone molecules. Nucleotide The combination of a nitrogen-containing base. a S-carbon sugar. and 1 or more phosphate groups. The nucleotides are designated by 3 capital letters as follows:
Introduction. 157
adenosine monophosphate (AMP), deoxyadenosine monophosphate (dAMP), uridine diphosphate (UDP), adenosine triphosphate (ATP), etc. Although nucleotides are linked together by phosphodiester linkage into long sequences known as nucleic acids, they also perform other important functions, such as carrying chemical energy (ATP), combining with other groups to form coenzymes (coenzyme A, or CoAl, and acting as intracellular signaling molecules (cyclic AMP, or cAMP). Oncogene A defective gene that is capable of transforming cells to a neoplastic phenotype
characterized by loss of growth control and/or tumorigenesis in a suitable host or site. In many cases, cancer is caused by the growth-stimulating effects of increased expression, protein activation, or aberrant regulation of transcription factors required for nor-
mal growth. Certain oncogenes are produced by chromosomal translocations of normal transcription factor genes to other regions adjacent to more abundantly expressed genes, causing inappropriate excessive expression. See tumor-suppressor genes. Open reading frame (ORF) Any part of the genome that could be translated into a protein sequence because of the absence of stop co dons. An exon is an example of an ORE See exon.
Origin of replication The site(s) on a chromosome where replication is initiated and pro-
ceeds bidirectionally. The site of binding of the origin replication complex (ORC). Also called the replication origin. Origin replication complex (ORC) A series of proteins involved in DNA synthesis and
replication that bind to the origin of replication as one of the initiating events of DNA replication. p arm The short arm of a chromosome in relation to the centromere. From petit. Penetrance The proportion of individuals of a given genotype who show any evidence of
an associated phenotype. Usually refers to the proportion of individuals heterozygous for a dominant disease who show any evidence of the disease. Nonpenetrance is the lack of phenotypic evidence of the genotype. See expressivity. Pharmacogenetics The area of biochemical genetics concerned with genetically controlled variations in drug responses. Phenocopy The occurrence of a particular clinical phenotype (often a medical disorder) as a result of nonmutagenic environmental factors (eg, exposure to a drug or virus), when
the more usual basis for the phenotype is an altered genotype. Phenotype The total observable nature of an individual, resulting from interaction of the genotype with the environment (in medicine, often a disease phenotype). Plasmid Circular extrachromosomal DNA molecules in bacteria that can indepen -
dently reproduce in a host. Plasm ids were detected because of their ability to transfer antibiotic resistance genes to bacteria. They can be used as vectors in recombinant DNA research.
158 • Fundamentals and Principles of Ophthalmology Pleiotropism Multiple end effects (in different organ systems) arising from a single (mutant) gene or ge ne pair. Polygenic inheritance Determined by the operation of an unspecified number of genes with additive effects. See multifactorial inherital1ce. Polymerase chain reaction (peR) A procedure whereby segments of DNA or RNA can be amplified without resorting to the conventional techniques of molecular cloning by use of flanking oligonucleotides called primers and repeated cycles of amplification with DNA polymerase. The steps involve heating to separate the molecules into single-stranded DNA • repeated annealing to the complementary target DNA sequences or primers specifically designed to delimit the beginning and ending of the target segment extension of the primer sequences with the enzyme DNA polymerase. creating double-stranded DNA • separation of the products into single-stranded DNA In effect. the amount of DNA is doubled wit h each cycle. Often. 30 or more cycles are used to obtain sufficjent amplification for further testing. Polymorphism Two or more alleles with a frequency greater than 1% in a given population. Posttranslational modification Changes or modifications of gene products after translation. including removal of ami no acids from the end of the peptide. addition or removal of sugars. and addition of lipid side chains or phosphate groups to speci fic sites in the protein. Often. such changes are essential for proper protein localizat ion or function . Proband The affected person whose disorder. or concern about a disorder. brings a family or pedigree to be genetically evaluated. Also called the propositus (male). proposita (female). or index case. Prokaryote Single-cellular organisms. such as bacte ria. that lack a nucleus and have their DNA located within the cytoplasm. See eukaryote. Promoter That sequence of nucleotides upstream (5') from the cod ing sequence of a gene that determines the site of binding of RNA polymerase and. hence. initiation oftranscriptio n. Different promoters for the same ge ne may exist and can result in alternately spliced gene products and tissue-specific expression. The promoter may contain the consensus DNA sequence TATA A A A (the so-called TATA box) approximately 25-30 bp (5') up-
T T
stream from the transcription start site.
Proposita. propositus Same as proband. above. Proto-oncogene A normal gene that is involved in cell division or proliferation. Abnormalities in expression or regulation can cause the gene to become activated to an on cogene. which can lead to cancer. Several proto-oncogenes are involved in intracellular Signal transduction-the process by which external messages influence the machinery that governs growth and differentiation.
Introduction. 159 Pseudodominance The appearance of vertical transmission of a recessive genetic disorder
from one generation to the next, usually through the mating of an affected homozygote with a heterozygote, which produces affected offspring. Pseudogene A defective copy of a gene. It often lacks introns and is rarely, if ever, expressed. Some pseudogenes are thought to have arisen by reverse transcription of mRNA that has had the introns spliced out. Others, such as globin pseudo genes, have arisen from silencing of a tandem duplicate. Since they are released from conservation (the maintenance of essential DNA sequences necessary for function) through selection, pseudogenes (compared to the original functional gene) often contain numerous base-pair changes and other mutational events.
Purine Nitrogen-containing base: adenine (A) and guanine (G) in DNA or RNA. Pyrimidine Nitrogen-containing base: thymine (T) and cytosine (C) in DNA or uracil (U)
in RNA. q arm The long arm of a chromosome. See p arm. Recessive Classically, a gene that results in a phenotype only in the homozygous state. See dominant. Recessive medical disorder A disease state whose occurrence requires a homozygous (or compound heterozygous) genotype-that is, a double dose of the mutant allele. Heterozygotes are essentially normal.
Recombinant An individual who has a combination of genes on a single chromosome unlike that in either parent. Usually applied to linkage analysis, wherein recombinant refers
to a haplotype (a set of alleles on a specific chromosome) that is not present in either parent because of a recombination crossover.
Recombinant DNA DNA that has been cut out of a single organism, reinserted into the
DNA of a vector (plasmid or phage), and then reimplanted into a host cell. Also, any act of altering DNA for further use. Recombination The formation of a new set of alleles on a Single chromosome unlike that in either parent; due to crossover during meiosis. Relatives,lirst-degree Individuals who share on average half of their genetic material with the proband: parents, siblings, offspring. Relatives. second-degree Individuals who share on average one fourth of their genetic mate-
rial with the proband: grandparents, aunts and uncles, nieces and nephews, grandchildren. Replication Creation of a new linear DNA copy by the enzyme DNA polymerase, pro-
ceeding from the 5' side of bound primer to the 3' end of the DNA sequence. Replication of DNA occurs during chromosomal duplication.
160 • Funda me nta ls and Princi ples of Ophthalmology Replication slippage An error of DNA replication or copying. Because of the similarity of repeated base-pair sequences, 1 or more repeats are skipped over and not represented in
the copied DNA sequence. Replicative segregation The process by which, through partitioning of copies of mtDNA
to each daughter cell during division, some cells receive a preponderance of normal or mutant copies. Replicative segregation tends to result in conversion of heteroplasmy to homoplasmy with associated development of disease within the affected tissue, if the tissue becomes homoplas mic for the mutan t mtDNA. This phenomenon explains the development of new organ system involvement in multisystem mitochondrial diseases. Restriction fragment length polymorph isms (RFLPs, RFLPs represent the variation in the length of genomic DNA fragments created by the loss or gain of an endonuclease restriction site. They can be used to map genes or link specific phYSical or genetic traits. Retrotransposition The insertion of a retroposon (a segment of DNA created by reverse
transcription from an RNA template) into the genome. Because of the staggered cut made in the target DNA by the endonuclease involved in the recombination event, a short duplication (3- 12 bp) of the target site sequence is created at the ends of the transposed element. Becatise the transposable element may contain transcriptional initiation and/o r termination Signals, this process is one mechanism by which fusion genes, such as those causing certain forms of leukemia or cancer, can arise. Reverse transcription The process, performed by the enzyme reve rse transcriptase.
whereby mRNA is converted back to DNA. If the introns have already been spliced out of the precursor mRNA, the product of this process is cDNA. Satellite DNA Nuclear DNA that migrates at separate positions or bands from the bulk of
DNA during CsCI gradient centrifugation. Satellite DNAs are long segments of DNA that consist of short DNA sequences repeated hundreds or thousands of times at a stretch in the genome. Satellite DNAs form the ends and centers of chromosomes. Telomeric DNA is a form of satellite DNA. Segregation The separation of pairs of alleles at meiosis. Sense strand of DNA The strand of double-stranded DNA that corresponds in its 5' to 3' sequence to the expressed mRNA. Also called the coding, or nontrans/ated, strand. Sequence-tagged sites (STSs, Short unique sequences of DNA. usually 200-500 bp, scat-
tered throughout the genome that serve as landmarks for the physical mapping of genes. The presence of a specific STS in any sample can be determined by the polymerase chain reaction. If the STS is detected. the sample has genomic material fro m the known region of that STS. Currently. the average d istance between STSs is 100 kb. Sex linked Genes on the X or Y (sex) chromosomes. The term is often used improperly
to mean X linked.
Introduction. 161 Short tandem repeats (STRs) Sequences of repeated copies of2-5 bp that occur every 10 kb
in the human genome. The variation in number of copies within a given STR is highly polymorphic and thus useful for gene mapping. See microsatellite and minisatellite. Simplex A term used to denote that only a single individual is affected within a given fam -
ily. Thus, a single male or female with a genetic disease would be called a simplex case. This term implies no inheritance type. The term isolated is also sometimes used. Smallest region 01 overlap (SRO) The minimum chromosomal or nucleotide sequence that
is deleted among all individuals who have a phenotype thought to be the result of a particular chromosomal deletion. This deleted region is presumed to contain the gene or genes that cause the phenotype. Southern blot Imprint of an electrophoretic gel that separates fragments of DNA accord-
ing to their size. Splice junction site The DNA region that demarcates the boundaries between exons and introns. The specific sequence determines whether the site acts as a 51 donor or a 3' accep-
tor site during splicing. Single base-pair changes or mutations that involve splice junction sites may resuIt in skipping of the following exon or incorporation of part of the adjacent intron into the mature mRNA. See acceptor splice site and donor splice site for the consensus sequences. Spliceosome Multicomponent ribosomal ribonuclear protein complex (40S to 60S) that is involved in the removal of introns from heteronuclear, or precursor messenger. RNAs. Splicing That process by which the introns are removed from the precursor mRNA and
the exons are joined together as mature mRNA prior to translation. Takes place within spliceosomes. Sporadic A trait that occurs in a Single member of a kindred with no other family
members affected. The term has been used by some geneticists to imply that the trait is nongenetic. Stop codon (termination codon) The DNA triplet that causes translation to end when the
translation is coded into mRNA. The DNA stop codons are TAG, TAA, and TGA. Expressed as mRNA, these are UAG, UAA, and UGA. Synteny The presence of genes on the same chromosome, even if linkage cannot be dem onstrated. Also used to denote homologous chromosomal locations between species. TATA box A promoter element approximately 25- 30 bp (5') upstream from the transcrip-
tion start site that contains the consensus sequence
TATA~A~. The TATA box is recogT T
ni zed by transcription factors that bind to the region, and it is critical in the initiation of transcription.
162 • Fundamentals and Principles of Ophthalmology Telomeric DNA A type of highly repetitive satellite DNA that forms the tips of chromosomes and prevents them from fraying or joining. It decreases in size as a concomitant of
aging. Defects in the maintenance of telomeres may playa role in cancer formation. Threshold In polygenic or multifactorial inheritance, a relatively sharp qualitative difference
beyond which individuals are considered to be affected. The threshold is presumed to have been reached by the cumulative effects of the polygenic and multifactorial influences. Transcription The synthesis as catalyzed by a DNA-dependent RNA polymerase of a single-
stranded RNA molecule from the antisense strand of a double-stranded DNA template in the cell nucleus. Translation The process by which a polypeptide is synthesized from a sequence of specific
mRNA. Translocation The transfer of a part of I chromosome to a nonhomologous chromosome. Trinucleotide repeat expansion (contraction) The process by which long sequences of multiple triplet codons (see minisatellite) are lengthened or shortened in the process of gene replication. The process of expansion of trinucleotide repeats over consecutive generations results in ihe genetic phenomenon of anticipation. The underlying mechanisms for
expansion (or contraction) appear to be replication slippage and unequal crossing over in the region of the repeats. Most disorders involving trinucleotide repeats are dominant in inheritance (eg, fragile-X syndrome, myotonic dystrophy, Huntington disease, Kennedy disease), but one is autosomal recessive (Friedreich ataxia). Tumor-suppressor genes Genes that must be present in one fully functional copy in order
to keep cells from uncontrolled proliferation. Two "hits" (inactivations) of the gene, one for each allele, must occur in a given cell for tumor formation to occur. Examples include the genes for retinoblastoma, Wilms tumor, tuberous sclerosis, p53, ataxia-telangiectasia, and von Hippel-Lindau disease. Also called antioncogenes. See oncogene. Unequal crossing over An error in the events of chromosomal duplication and cell divi-
sion occurring during meiosis and, rarely, during mitosis. Probably because of similar sequences or repeated segments, chromosomal exchange occurs between nonhomologous regions of the chromosome, resulting in duplication and deletion of genetic material in the daughter cells. Uniparental disomy The conveyance to a child of 2 copies of an abnormal gene or chromo-
some by only I parent (the other parent makes no contribution). The child can be affected with autosomal recessive disease even if only I of the parents is a carrier for the abnormal gene. This occurrence has been reported in cystic fibrosis and in Prader-Willi and Angelman syndromes. Untranslated region (UTR) The regions upstream (5' UTR) and downstream (3' UTR) of
the open reading frame of a gene. The 5' UTR contains the promoter and part or all of the
Introduction. 163
regulatory regions of the gene. The 3' UTR presumably also serves important functions in regulation and mRNA stability. Vector A viral. bacteriophage. or plasmid DNA molecule into which a stretch of genomic
DNA or cDNA or a specific gene can be inserted. The A-bacteriophage can accept segments of DNA up to 25 kb long. Cosmid vectors can accommodate a segment 40 kb long. BAC (bacterial artificial chromosome) and YAC (yeast artificial chromosome) vec tors can accept much larger frag ments of DNA. Western blot Imprint of an electrophoretic gel that separates proteins according to their
size and mobility. The proteins are usually identified by immunologic methods. Wild type A normal phenotype of an organism. Also. a normal allele as compared to a
mutant allele. X linked Term that rere rs to genes on the X chromosome. Y linked Term that rerers to genes on the Y chromosome. Yeast artificial chromosome (YAe) Used as a cloning vector. YACs can be used to clone ve ry
large segments of DNA (up to 1000 kb).
CHAPTER
5
Molecular Genetics
This chapter provides a review of molecular genetics (with emphasis on clinical applications in ophthalmology), an overview of the techniques for manipulating deoxyribonucleic acid (DNA) in the laboratory, and an appreciation of the power and implications of molecular investigations for the study of inherited diseases.
Gene Structure A gene is the coding sequence for a protein, ribosomal RNA (rRNA), or other gene product, and associated regulatory sequences. Following the initiation codon (start sequence) is the structural open reading frame (ORF), which is composed of exom (sequences that code for amino acids that will be present in the final protein) and introns (sequences that are spliced out during the processing of mRNA). Following the last exon is the 3' untransfated region (3' UTR). The function of this region is partly regulatory. Thus, for example, a mutation within the 3' UTR of the gene for the enzyme myotonin kinase is thought to cause myotonic dystrophy. The development of introns in higher organisms may have had evolutionary benefits. Introns have allowed eukaryotes to evolve beyond the limits of genes seer: in single-celled organisms, and they may have other roles as well. The compartmentalization of coding segments into exons may have allowed for more rapid evolution of proteins by allowing for alternative processing of precursor RNA (alternative splicing) and for rearrangements of exons during gene duplication (exon shuffling). Some introns contain complete separate genes) and some of these may cause disease
or influence the expression of other genes. Expansion of unstable repeats within introns can cause abnormal splicing and result in genetic disease.
"Junk" DNA Approximately 97% of the base sequences in human DNA have been considered "biologically meaningless;' in that they do not encode proteins or RNAs or have any other known function. It is possible, however, that important roles for this so-called junk DNA will emerge when the structure and function of the genome, chromosomes, nucleus, and nuclear proteins are fully understood. It has been suggested, for example, that RNA transcribed from junk DNA may directly influence the transcription of other sequences and
165
166 • Fundamentals and Principles of Ophthalmology participate in normal genome repair and regulation. When defective, junk DNA may lead to cancer. Some of the repetitive sequences of nontranscribed DNA form telomeric DNA, which is essential for the correct formation and maintenance of chromosomes. Indeed,
loss of telomeric DNA correlates with cell senescence and carcinogenesis. Therefore, se~ quences within junk DNA may influence the transcription or otherwise regulate the ex ~ pression of numerous other genes.
Much of junk DNA is composed of highly repetitive sequences, some of which in~ clude sateilites, minisatellites, microsatellites, short interspersed elements (SINEs), and long interspersed elements (LINEs). The most frequently appearing ofthe repetitive DNAs is the 300~base~pair (bp) Alu sequence, named for the restriction enzyme used to identify it. The Alu sequence is a SINE that occurs 500,000 times in the human genome. Alu sequences are distributed through retroposition (Alu ~ Alu RNA ~ Alu eDNA ~ insertion) and may cause disease if one inserts within and disrupts a gene. This process accounts for one cause of neurofibromatosis 1 (von Recklinghausen disease). An important LINE is the L1 repeat sequence, composed of about 10,000 copies, each 1- 6 kilobases (kb) in length. The Ll repeat sequence has also been implicated as a cause of mutations.
Gene Transcription Genes control cellular activity through 2 processes: 1. transcription (expression), in which DNA molecules give rise to RNA molecules,
followed by translation in most cases 2. translation, in which RNA directs the synthesis of proteins. Translation occurs at ribosomes, where mRNA induces tRNA-mediated recruitment of amino acids to
"build" a protein. A fuller discussion of translation is beyond the scope of this chapter.
Transcription Factors and Regulation Transcription factors contain DNA~binding domains that typically include a helical unit (a~helix) within or near positively charged amino acids. Four classes of structural protein motifs characterize 80% of transcription factors (Fig 5~ I): 1. helix~turn~helix (HTH) 2. zinc finger 3. leucine zipper 4. helix~loop~helix (HLH)
Many ophthalmic diseases result from transcription~factor mutations. PAX2 muta ~ tions cause colobomas of the optic nerve and renal hypoplasia. PAX3 mutations cause Waardenburg syndrome with dystopia canthorum (types WSI and WS3). PAX6 muta ~ tions are the basis of virtually all cases of aniridia, occasional cases of Peters anomaly. and
several other rarer phenotypes, specifically autosomal dominant keratitis and dominant foveal hypoplasia.
CHAPTER 5,
Molecular Genetics.
167
Tltll'lsaetiY&tion
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The general protein structures of the 4 major classes of transcription factors are shown on the left. The structures include a transactivation domain linked to a DNA-binding domain and, in certain cases, a dimerization domain. The types of transcription factors take their names from the characteristic motifs involved in DNA binding and protein dimerization and are show n on the right, interacting with the DNA. The cylinders represent a-helical regions, and the areas that contact DNA directly are green. C = cystei ne, HLH = helix-loop-helix, HTH = helix-turn-helix, L = leucine, Zn = zinc. The plus sign indicates a positive charge. (Reproduced with Figure 5-1
permission from Papavassiliou AG. Molecular medicine: transcription factors. N Engl J Mad. 1995;332(1):46.)
Hanson 1M, Fletcher JM, Jordan T, et al. Mutations at the PAX610cus are found in heterogeneous anterior segment malformations including Peters' anomaly. Nat Genet. 1994;6(2): 168-173. Latchman DS. Transcription -factor mutations and disease. N EnglJ Med. 1996;334(1):28-33. Mirzayans F. Pearce WG. MacDonald 1M. Walter MA. Mutation of the PAX6 gene in patients with autosomal dominant keratitis. Am J Hum Genet. 1995;57(3):539-548.
168 • Fundamentals and Principles of Ophthalmology Papavassiliou AG. Molecular medicine: transcription fa ctors. N Engl JMed. 1995;332( 1}:45- 47. Tassabehji M , Read AP, Newton VE, et aL Waardenburg's syndrome patients have mutations in
the human homologue of the Pax-3 paired box gene. Nature. 1992;355(6361):635-636.
Intron Excision The modified mRNA undergoes excision of the introns by a highly organized process called splicing, which leaves the mRNA composed of only exons, or coding segments. The exons can then undergo translation in the ribosomes. Splicing takes place in specialized
structures composed of RNA and proteins called spliceosomes. The exact process of splicing is complex but involves intermediate steps that look like a lariat. Splicing must recognize precisely the beginning and end of each coding sequence, and errors of splicing can lead to genetic disease. Approximately 15% of point mutations that cause human disease do so by the generation of splicing errors that result in aberrations such as exon skipping, intron retention, or use of a cryptic splice site. Mutations in proteins important in splicing can cause retinitis pigmentosa.
Alternative Spl,icing and Isoforms Alternative splicing is the creation of multiple pre-mRNA sequences from the same gene
by the action of different promoters. These promoters cause the transcription of the gene to skip certain exons. The protein products of alternative splicing are often called isoforms. The promoters are usually tissue-specific, so different ti~sues express different isoforms.
The gene for dystrophin is an example of alternative splicing: full-length dystrophin is the major isoform expressed in muscle; shorter isoforms predominate in the retina, periph eral nerve, and central nervous system. Another example of alternative splicing's relevance
underlies the basis of the cornea's avascularity. Vascular endothelial growth factor (VEG F) receptor-I is a key blood vessel receptor that binds and transduces a Signal from the primary mediator of angiogenesis, VEGF. In the cornea, high levels of an alternatively spliced isoform, soluble VEGFR- I are expressed. As this is the soluble form, it is present in the extracellular matrix and serves as an endogenous VEGF trap or decoy receptor. Without it, the cornea becomes vulnerable to vascular invasion. Ambati BK, Nozaki M, Singh N, et aL Corneal avascularity is due to soluble VEGF receptor- I.
Nature. 2006;443(7114):993-997.
Methylation Regions of DNA that are undergOing transcription lack 5-methyl cytidine residues, which normally account for 1%-5% of total DNA. Evidence suggests a close correlation between methylation and gene inactivation. Regulation of DNA methylation may be responsible for imprinting control.
X-Inactivation A major occurrence in early development of the human embryo is the random inactiva-
tion of 1 of the 2 X chromosomes in the female, resulting in the lack of expression of the
CHAPTER 5:
Molecular Genetics.
169
majority of genes on that chromosome. The time of X -inactivation is not precisely known but is thought to vary over a period of several cell divisions during the blastocyst -gastrula transition. X-inactivation is also known as iyonization, after its discoverer, Mary Lyon. Lyonization affects the severity of the phenotype of several X-linked retinal conditions, such as retinitis pigmentosa and incontinentia pigmenti. Lee JT, Jaenisch R. The (epi)genetic control of mammalian X-chromosome inactivation. Curr Opin Genet Dev. 1997;7(2):274- 280. Lyon ME The William Allan memorial award address: X-chromosome inactivation and the location and expression of X-linked genes. Am J Hum Genet. 1988;42(1):8-16.
Imprinting Genetic imprinting, also called allele-specific marking, is a heritable yet reversible process by which a gene is modified, depending on which parent provides it. The mechanism is unclear but appears to operate at the chromatin organization level and involves hetero-
chromatization and methylation of epG sites. Examples of genes that can be imprinted include the Wilms tumor-suppressor gene and the human SNRPN (small nuclear ribonucleoprotein polypeptide N) gene. Prader-Willi and Angelman syndromes are examples of diseases resulting from abnormalities of imprinting. Approximately 70%-80% of patients with Prader-Willi syndrome harbor a deletion of the paternally derived chromosome 15qll -q 13, resulting in the loss of this region's normal contribution from the paternal line. About 70%-80% of patients with Angelman syndrome also have a deletion of 15qll -ql3 but from the maternally derived chromosome, resulting in loss of the maternal contribution. Uniparen-
tal disomy, wherein both 15 chromosomes are inherited from the same parent, can also cause each syndrome. Again, the 2 chromosome 15s in uniparental disomy are maternal
in Prader-Willi syndrome and paternal in Angelman syndrome. The SNRPN gene maps to 15q ll -q 13 but appears to be expressed only from the paternally inherited allele.
DNA Damage and Repair DNA is constantly sustaining damage from mutagens such as ultraviolet light, chemicals, and spontaneous deamination. Each cell loses 10,000 bases per day from spontaneous DNA breakdown related to normal body temperature alone. This process may involve hydrolytic loss of purine bases or deamination of cytosine to uracil and, less frequently, adenine to hypoxanthine. Oxidation, alkylation, generation of free radicals, and other common metabolic reactions can also injure DNA. In the absence of repair, these mutations would accumulate and result in tumor formation. Damaged DNA is estimated to cause approximately 80% - 90% of cancers in humans. Damaged DNA sites are repaired chiefly by 2 mechanisms: excision repair and mismatch repair. The processes of replication, transcription, mismatch repair, excision repair.
and gene expression are closely coordinated by cross-acting systems. Enzymes that cut or patch segments of DNA during crossing over at meiosis are also involved in DNA repair. Molecules that unwind double-stranded DNA (called helicases) are involved in replication, transcription, and DNA excision repair.
170 • Fundamentals and Principles of Ophthalmology The antioncogene p53 appears to play an extremely important role as the "guardian of the genome" by preventing cells from proliferating if their DNA is irreparably damaged. Levels of p53 increase after ultraviolet or ionizing radiation. p53 inhibits DNA replication directly and binds with I of the RNA polymerase transcription factors, TFJIH. If the degree of damage is slight, increased production of p53 induces reversible cell arrest until DNA repair can take place. If DNA damage is too great or irreversible, p53 production is massively increased and apoptosis occurs, probably through stimulation of the expression of the BAX gene, whose product promotes apoptosis. Loss of p53 causes cells to fail to arrest in response to DNA damage, and these cells do not enter apoptosis. Thus, mutations of p53 predispose to tumorigenesis. The gene mutated in ataxia-telangiectasia (Louis-Bar syndrome), a protein kinase called ATM, also appears to be integrally involved in DNA repair, possibly by informing the cell of radiation damage. The ATM gene product associates with synaptonemal complexes, promotes chromosomal synapsis. and is required for meiosis. People with ataxia-
telangiectasia have a threefold greater risk of cancer. Xeroderma pigmentosa is a severe condition in which DNA repair enzyme functions
are crippled. Patients with this condition typically have diffuse pigmented anomalies on their sun-expo~ed skin surfaces and are at high risk for squamous cell carcinoma of the ocular surface. Latchman DS. Transc ription-factor mutations and disease. N Engl , Med. 1996;334( 1}:28- 33. Levine AJ. p53, the cell ular gatekeeper for growth and division. Cell. 1997;88(3):32 3- 33 1. Yu CE, Oshima J. Fu YH . et at. Positional cloning of the Werner's syndrome gene. Science. I 996;272( 5259):258- 262.
Mutations and Disease Requirements for Identifying a Disease-producing Mutation The major requirements for a given DNA mutation to be verified as disease-producing are the following: The mutation does not occur in the normal population (the variation cannot be more frequent than the disease) . It produces a DNA sequence that alters protein function or expression . • The presence of the variation cosegregates with disease in family members according to the inheritance type (the Significance of cosegregation in a given family depends on the number of possible chances for noncosegregation).
Mutations Mutations can involve a change in a Single base pair; simple deletion or insertion of DNA material; or more complex rearrangements such as inversions, duplications, or transloca-
tions. Deletion, insertion, or duplication of any number of base pairs in other than groups of3 creates frameshifts of the entire DNA sequence downstream, resulting in the eventual formation of a stop codon and truncation of the message.
CHAPTER 5: Molecular Genetics. 171
Mutations that result in no active gene product being produced are called null mutations. Null mutations include missense or nonsense mutations that (1) produce either a stop mutation directly or a frameshift with creation of a premature stop codon downstream or (2) cause the loss or gain of a donor or acceptance splice junction site, resulting in the loss of exons or inappropriate incorporation of introns into the spliced mRNA. Mutations can also lead to a gain of function that may be beneficial (leading to evolution) or detrimental (leading to disease). An example of a beneficial gain in function is the emergence, among bacteria, of antibiotic resistance. An example of a detrimental gain of
function is a receptor protein that binds too tightly with its target protein, creating loss of normal physiologic function. Most autosomal dominant disorders are of this type. Single base-pair mutations may code for the same amino acid or a tolerable change in the amino acid sequence, leading to harmless polymorph isms or DNA variations that are in turn inherited. These are called conserved base-pair mutations.
Polymorphisms A polymorphism is any variation in DNA sequence that occurs, by convention, at a fre quency of I % or greater in the normal population. Key polymorphisms that have been associated with disease include the TlGR/myocilin gene with glaucoma, and the HTRAI gene with neovascular macular degeneration.
Cancer Genes
a
Cancer can result from any of number of genetic mechanisms, including the activation of oncogenes and the loss of tumor-suppressor genes. The product of proto-oncogenes is often involved in signal transduction of external messages to the intracellular machinery that governs normal cell growth and differentiation (Fig 5-2). As such, the DNA sequences of proto-oncogenes are highly conserved in nature between such different organisms as humans and yeast. Proto-oncogenes can be activated to oncogenes by loss or disruption of normal regulation.
Oncogenes Oncogenes were first detected in retroviruses, which had acquired them from their host in order to take control of cell growth. Such oncogenes are often identified by names that refer to the viral source, one example being ras (rat sarcoma virus). They are found to be activated not only in virus -induced malignancies but in common nonviral cancers in hu-
mans. Oncogenes behave the same way that autosomal dominant traits behave, and only I mutant allele is needed for tumor formation, presumably by a dominant negative effect on regulation of Signal transduction.
Tumor-suppressor genes Tumor-suppressor genes, also called antioncogenes, are genes that must be present in one
functional copy to prevent uncontrolled cell proliferation. Although some may represent genes whose products participate in checkpoints for the cell cycle, one characteristic of tumor-suppressor genes is the diversity of their normal functions. Some examples of
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Figur.5-2 In the cell cycle, the progression from DNA synthesis (5) to mitosis (M) includes phases before (GI) and after (G2) the replication of DNA. On receiving signals to differentiate, cells leave the cycle and enter the pathway of terminal differentiation. Under certain circumstances, cells may enter the pathway to programmed cell death (apoptosis). Signal transduction begins with the binding of a growth factor to its transmembrane receptor (upper left!. Usually, the next step is the dimerization of the receptor. The receptor subunits then phosphorylate one another on tyrosine residues (Y). The phosphotyrosines (P) create docking sites on the receptor for many proteins, some of w hich undergo phosphorylation ; others recruit multicomponent complexes to the plasma membrane. One such interaction, shown here, is the activation of the Ras GTPase. In the cascade of phosphorylation initiated by the activation of Ras, the Raf kinase phosphorylates another kinase (mitogen-activated protein kinase kinase, or MAPKKI. which in turn phosphorylates a third kinase, the mitogen-activated protein kinase, or MAPK. MAPK directly activates transcription factors and ribosomal S6 protein kinase (RSK), which also phosphorylates transcription factors. MAPK probably represents at least 2 related proteins. Two transcription proteins, Fos and Jun, are shown. They join to form a fully active transcription factor. The phosphorylation of Fos by MAPK and of Jun by RSK causes them to bind to specific DNA sequences near the MYC gene, thereby init iating transcription of the gene . The Myc protein itself is a transcription factor with several binding partners (not shown). The binding of Myc to its specific recogn ition sites on DNA activates another set of genes. Cyelin 01 initiates the progression cells through G1 to the S-phase. GDP= guanosine diphosphate, GTP = guanosine triphosphate. (Reproduced with permission from Krontiris TG . Oncogenes. N Engl J Med. 1995;333(5):304.}
CHAPTER 5: Molecular Genetics. 173
tumor-suppressor genes include the genes for retinoblastoma, Wilms tumor, neurofibromatosis types 1 and 2, tuberous sclerosis, ataxia-telangiectasia, and von Hippel-Lindau disease. All of these examples (except ataxia-telangiectasia) behave as autosomal dominant traits, but the mechanism of tumor formation is very different for tumor-suppressor genes than it is for oncogenes. If 1 allele is already defective because of a hereditary mutation, the other allele must also be lost for tumor formation to occur (also known as the "2-hit hypothesis"). This loss ofthe second allele is termed loss Of heterozygosity, and it can occur from a second mutation, gene deletion, chromosomal loss, or mitotic recombination. Krontiris TG. Oncogenes. N Engl JMed. 1995;333(5):303 - 306.
Mitochondrial Disease A significant number of disorders associated with the eye or visual system involve mito-
chondrial deletions and mutations. Mitochondrial diseases should be considered whenever the inheritance pattern of a trait suggests maternal transmission. Although the inheritance pattern might superficially resemble that of an X-linked trait, maternal transmission differs in that all of the offspring of affected females - both daughters and sons-can inherit the trait, but only the daughters can pass it on. The phenotype and severity of mitochondrial disease appear to depend on the nature of the mutation, the presence or degree of heteroplasmy (coexistence of more than 1 species of mitochondrial DNA [mtDNAJ - ie, wild type and mutant), and the oxidative needs of the tissues involved. Spontaneous deletions and mutations of mtDNA accumulate with age, and the effect of this accumulation is to decrease the efficiency and function of the electron transport system, reducing the availability of ATP. When energy production becomes insufficient to maintain the function of cells or tissue, disease occurs. There appears
to be an important interaction between age and tissue threshold of oxidati,;,e phosphorylation need and the expression of inherited mutations of mtDNA. With each cell division, the number of mutant mtDNA copies that are partitioned to a given daughter cell is random, unlike with mendelian inheritance characteristics. After a number of cell divisions, some cells, purely by chance, receive more normal or more mutant copies of mtDNA, resulting in a drift toward homoplasmy in subsequent cell lines. This process is called replicative segregation. With mtDNA deletions, preferential replication of the smaller deleted molecules causes an increase of deleted copy over time. The trend toward homoplasmy helps explain why disease worsens with age and why organ systems not previously involved in multisystem mitochondrial disease become involved.
Mitochondrial diseases can be subdivided into these categories (Fig 5-3): disorders resulting from large rearrangements of mtDNA (deletions and insertions), such as chronic progressive external ophthalmoplegia (CPEO), Kearns-Sayre syndrome, and Pearson marrow-pancreas syndrome
mutations of mtDNA-encoded rRNA, such as maternally inherited sensorineural deafness and aminoglycoside-induced deafness • mutations of mtDNA-encoded tRNA, such as the syndromes of MELAS, myoclonic epilepsy with ragged red fibers (MERRF), adult-onset diabetes and deafness, and (in about 30% of cases) CPEO
174 • Fundamentals and Principles of Ophtha lmology
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Figure 5-3 Diagram of human mitochondrial DNA and the most common associated pa thogenetic mutations. Point mu tations in structural and protein-coding genes are shown inside the circle, with the clinical phenotype indicated and the nucleotide positi on of the mutation shown in parentheses. The position of the most common single deletion , w hich is 5 kb long, and the mu lti ple delet ions are indicated by the arcs outside th e circle. A6, A8 = ATPase subunits; CO I, CO II, CO III = cytochrome-c oxidase subunits; CYT'b = apocytochrome-b subunit; Leigh = maternally inherited Leigh disease; LHON = Leber hereditary optic neu ropathy; MELAS = syndrome of mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes; MERRF = myoclonic epilepsy with ragged red fibers; NARP = neuropathy, ata xia, and retinitis pigmentosa; NO-I, NO-2, NO-3, NO-4L, NO-4, NO-5, NO-6 = NADH dehydrogenase subunits;
OH = origin of heavy-stranded DN A re pl ication; 0, = origin of light-stranded DNA rep lication; 12S and 16S = ribosomal RNA subunits. The large open space al th e top, which includes OH' is the noncoding D (displacemen t) loop.
(Reproduced with permission from Johns DR. Seminars of medicine
of the Beth Israel Hospital, Boston. Mitochondrial DNA imd disease. N Engl J Med . 1995;333(10):641 .)
• missense and nonsense mutations such as Leber hereditary optic neuropathy (LHON) and neuropathy, ataxia, and retinitis pigmentosa (NARP)
Chronic Progressive External Ophthalmoplegia CPEO is a disorder involving progressive ptosis and paralysis of eye muscles associated with a ragged red myopathy, usually as a res ult of deleti on of a portion of the mitochondrial genome. Patients with C PEO commo nly have pigmentary retinopathy that does not create significant visual disability. Infrequently, th ey may have mo re marked retinal or other system involvement, the so-called CPEO-plus syndro mes. In Kearns-Sayre syndrome, CPEO is associated with heart block and severe retinitis pigmentosa (RP) with
CHAPTER 5: Molecular Genetics. 175
marked visual impairment. Pearson marrow-pancreas syndrome results from a large deletion of mtDNA and presents in younger patients with an entirely different phenotype involving sideroblastic anemia and pancreatic exocrine dysfunction. However. in patients afflicted during their later years. Pearson marrow-pancreas syndrome can present with a
phenotype resembling Kearns-Sayre syndrome. Although roughly 50% of patients with CPEO have demonstrable mtDNA deletions, virtually all patients with Kearns-Sayre syndrome have large deletions. As many as 30% of patients with CPEO who do not harbor demonstrable mtDNA deletions may have a point mutation at nucleotide position 3243, the same mutation in the tRNA for leucine that in other people is associated with MELAS syndrome. For all of the syndromes associated with deletions, such as Kearns-Sayre and CPEO, detection of the deletion usually requires study of the muscle tissue.
Leber Hereditary Optic Neuropathy The most important ophthalmic disease of mitochondria is Leber hereditary optic neuropathy (LHON), which is more prevalent in males than in females but does not fit a classic X-linked pattern of transmission. The trait is not transmitted to the offspring of affected males, but Virtually every daughter and son of a female patient with LHON inherits the trait. In approximately 50% of cases, LHON development is correlated with a Single base change (G to A at nucleotide position 11778 in the ND-4 gene) in human mtDNA involved in the synthesis ofNADH dehydrogenase. In addition to optic atrophy, patients can exhibit peripapillary microangiopathy and cardiac abnormalities, especially WolffParkinson- White syndrome. LHON can also occur from other so-called primary mutations at nucleotide positions 3460 of ND-l , 14484 of ND-6, 14459 of ND-6, and (more controversially) 15257 of cytochrome-b. At least 12 secondary mutations have been associated with LHON, often when multiple mutations are present in an individual's mitochondria. Some authors think that these secondary mutations cause disease by additive detrimental effects on the electron transport system of oxidative phosphorylation. Most of these secondary mutations appear in the general population. Debate persists on whether each mutation alone is truly pathogenic. The likelihood of improvement wi th time in the recovery of visual acuity appears to differ among the separate mutations associated with LHON. Mutation at nucleotide position 11778 is associated with the least, and mutation at nucleotide position 14484 is associated with the greatest, likelihood of recovery. The mutation at 14459 of ND-6 appears to be associated with 2 very different clinical phenotypes, one LHON and the other a severe, early-onset progressive dystonia with pseudobulbar syndrome, short stature, and reduced intelligence. The 2 phenotypes may reflect different proportions or distribution of mutant mtDNA.
Neuropathy, Ataxia, and Retinitis Pigmentosa NARP is associated with a Single base-pair mutation at nucleotide position 8993 in the ATPase-6 gene. The NARP phenotype occurs when the percentage of mutant mtDNA is less than 80%, whereas the same mutation present at much higher proportions (greater
176 • Fundamentals and Principles of Ophthalmology than 95%) can cause Leigh syndrome, a severe neurodegenerative disease of infancy and early childhood. The 8993 mutation is demonstrable in fibroblasts and lymphoblasts.
Other Mitochondrial Diseases Aminoglycoside-induced deafness and streptomycin ototoxicity are instances wherein antibiotic administration, often only a modest dose, is associated with severe hearing loss. This susceptibility to ototoxicity is a maternally inherited trait. Aminoglycosides (kanamycin, gentamicin, tobramycin, and neomycin) "target" the evolutionarily related bacterial ribosome. The mechanism of action is thought to include interference with the production of ATP in the mitochondria of hair cells in the cochlea. Brown MD, Wallace DC. Molecular basis of mitochondrial DNA disease. JBioenerg Biomembr. 1994;26(3}B3-289. Johns DR. Seminars in medicine of the Beth Israel Hospital, Boston. Mitochondrial DNA and disease. N Eng! f Med. 1995;333(10}:638- 644. Nikoskelainen EK, Savontaus ML, Wanne OP, Katila Mj, Nummeiin KU. Leber's hereditary optic neuroretinopathy, a maternally inherited disease. A genealogic study in four pedigrees.
Arch Ophtha!mo!. 1987;105(5}:665-671. Phillips Cl, Gosden eM. Leber's hereditary optic neuropathy and Kearns-Sayre syndrome: mitochondrial DNA mutations. Surv Ophthalmol. 1991;35(6}:463 - 472.
The Search for Genes in Specific Diseases A variety of methods have been used to assign individual genes to specific chromosomes, to link individual genes to one another, and to link diseases to specific genes.
Synteny The presence of genes on the same chromosome, even if the genes are too far apart to demonstrate linkage, is called synteny. Genes that are X-linked, such as red-green color blindness, choroideremia, and hemophilia, are by definition syntenic. The term is also used to denote homologous chromosomal regions between species. For example, the mouse gene for opsin is localized to the distal half of mouse chromosome 1, which is syntenic or homologous to human chromosome 3q. Both the value and the limitations of the study of genetic disease in other animals can be seen in the following example: the mouse's role in the discovery of the gene for a form of Usher syndrome type I (USH1B) (profound congenital deafness, vestibular dysfunction, and RP). A mouse mutant for deafness, shaker-l (shi), had been mapped to a conserved linkage region on mouse chromosome 7, and USH1B was linked to human chromosome llq13, suggesting that the 2 might result from similar or homologous genes. The shaker-l gene was isolated in 1995 by positional cloning and the mutated gene identified as myosin VIla. The human counterpart to this gene, MY07A, was quickly identified as the gene mutated also in USH1B. The subsequent twist to the story is that, because of differences in tissue-specific expression, the mouse mutant has deafness and vestibular dysfunction but not RP. Thus, animal models may help to find genes that cause human
CHAPTER 5: Molecular Genetics. 177
disease, but the expression of mutations in the homologous genes may have important species differences. el -Amraoui A, Sahly J, Picaud S, Sahel J, Abitbol M, Petit C. Human Usher IB/mouse shaker- I: the retinal phenotype discrepancy exp lain ed by the presence/absence of myosin VilA in the photoreceptor cell s. Hum Mol Gellet. 1996;5(8): 1171 - 11 78. Gibso n F, Walsh J, Mburu p, e t al. A type VII myosin encoded by the mouse deafness gene shaker-I. Nature. 1995;374(6517) :62- 64 . Meisler MH. The role of the laboratory mouse in the human genome project. Am JHum Genet. 1996;59(4)]64-771. Wei! D, Blanchard 5, Kaplan J, et al. Defective myosin VIlA gene responsible for Usher syn drome type lB. Nature . 1995;374(6517):60- 61.
Cytogenetic Markers (Morphologically Variant Chromosomes) If a specific chromosomal structure is abnormal or even normally variant, its transmission
through a fam ily with a hereditary disease, as mapped by a pedigree, may allow the assumption that the mutant gene and the va riant chromosome are comigrating. Thus, the mutanl gene is physically located on the variant chromosome- that is, a cytogenetic marker.
Gene Dosage If a portion of a chromosome containing a specific gene is physically deleted, the amount of the ge ne product will be determined only by the remaining homologue. For example, 50% of normal levels of esterase D may be found in the serum of people with an interstitial deletion of part of the long arm of chromosome 13. When several such persons were also found to have retinoblastoma, it was suggested that both the esterase and the retinoblastoma genes are located in the missing segment. By contrast, duplication mapping requires finding 150% of normal activity of a given gene product, together with either a chromosomal trisomy or triplication of a specific chromosomal segment.
Association Certain combinations of traits may occur for reasons other than the physical relationship of genes. For example, blood group 0 and peptic ulcer are found together in the same person more often than would be expected from their individual frequencies in the population. This finding occurs not because the ABO gene and another gene for peptic ulcer are located on the same chromosome but because people with type 0 blood have a physiologic peculiarity that predisposes them to peptic ulcerations. In anothe r example, retinal detachment occurs more frequently in patients with Marfan syndrome and homocystinuria than in the general population. Rather than resulting from the concurrent action of 2 linked genes, this association is the result of pleiotropism, the multiple effects of a Single gene.
Linkage Even if no information is known about the nature or function of a gene for a disease, link-
age studies may be able to localize the gene to a given chromosome or specific marker.
178 • Fundamentals and Principles of Ophthalmology In 1937. Bell and Haldane recognized the first linkage between 2 diseases on a human chromosome: congenital color deficiency and hemophilia on the X chromosome. Subsequent investigations have led to the chromosomal mapping of a large number of different human ocular diseases.
Gene assignments As of March 15.2004. OMIM (Online Mendelian Inheritance in Man: www.ncbi.nlm.nih .govlomim) listed over 15.000 established human gene loci. Every chromosome has numerous defined genes. Human gene mapping has 2 major applications. The first is identification of the gene for a specific genetic disease by its linkage to a known marker. For example. suppose gene A causes a hereditary disease and gene B is a known enzyme or polymorphic marker closely linked to A. Even though no biochemical test exists for A. a tight linkage to B would allow a reasonable probability of identifying the disease for prenatal diagnosis and sometimes for carrier detection. The second impact of linkage is understanding the cause of the phenotypic malformations in specific chromosomal diseases. For example. the phenotype of Down syndrome may result from triplication of only the distal long arm of chromosome 21 through a chromosome rearrangement rather than trisomy of the entire chromosome. Mets MB. Maumenee IH . The eye and the chromosome. Surv Ophthalmol. 1983;28(1):20-32.
Markers: restriction fragment length polymorphisms Polymorph isms detectable by the presence or absence of a specific restriction endonuclease cleavage site are called restriction fragment length polymorph isms (RFLPs). The differences between 2 chromosomes in DNA genetic material fragment size between restriction endonuclease cleavage sites are inherited and become useful markers for following various genetically determined disorders. Identification of RFLPs begins when DNA is isolated from peripheral blood lymphocytes. The DNA fragments are then produced whe n the DNA is cut with restriction en donucleases. Each restriction endonuclease recognizes a highly specific sequence of 4- 9 bases and cuts double-stranded DNA wherever this sequence occurs. Change of a single base within the recognition sequence results in the loss of that cleavage site and a change in the corresponding DNA fragment length. A single base-pair change elsewhere may create a new recognition cleavage site where none had existed. A variat ion in DNA sequence involving a Single base pair occurs with a frequency of approximately I per 200-500 bp.
The variable-length fragments. determined by the spacing of the restriction enzyme recognition sites. are then separated by agarose gel electrophoresis. The gel will contain millions of DNA fragments. To identify a certain fragment that might be linked to a specific genetic locus (ie. a defective gene). a radioactively labeled DNA probe is hybridized with the DNA fragments generated by the restriction endonucleases. The various probes represent cloned DNA sequences. which are complementary (homologous in base-pair sequence) to a part of the DNA fragment containing the RFLPs. Any fragments containing part or all of the radioactively labeled sequence can be identified by radioactive or nonradioactive detection methods (Fig 5-4).
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M olecul ar Genetics.
179
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A'--_ _ _ _ __ _ _ __ --' B Figur.5-4 A. Ana lysis of DNA by gel electroph oresis and Sou thern blotting. In Sou thern blot· ting, genomic DNA is cut w it h restriction enzymes into fragments before being separated according to size by gel elect rophores is. The 4 lanes on the gel represent the digestion of th e DN A w ith 4 differe nt restrict ion enzym es . After electrophoresis, the nucleic acids in the gel are transferred directly onto a charged nylon fi lter to which they are tightly bound . Thus, th e filte r conta ins a prec ise replica of the nucleic acid distribution in the gel. 'The f ilter is t hen hyb ridized in a rota ting sea led chamber with a DNA or RN A probe specific for t he target of interest (in this case, sequences in a microbial pathogen). Probes have traditionally been radioactive ly labeled w ith nucleotides containing phosphorus-32; however, use of nonradiolabeled probes is becoming more common. After the probe has hybridized to its target sequence, t he nonhybridized probe is washed away and the fi lter is exposed to x-ray film . A DNA sequence complementary to the probe is seen as a dark band on th e developed f ilm. The position of th e hybridized target sequence in each lane is unique to the restriction enzyme used to digest the DNA. B, Au toradiograph of a Southern blot with radiolabeled probe L 1.28 after the DNA w as cut with enzyme Taq1 , separated by size on an agarose gel. and then transferred to a nylon filter. Four female carriers of X-linked retinitis pigmentosa are depicte d. Note that 2 females, numbers 1 and 2, have only a 12-kb band, whereas carriers 3 and 4 have both 12-kb and 9-kb bands. (Part A reproduced with permission from Naber SP Molecular pathology-diagnosis of infectious disease . N Engl J Med. 1994;331(18): 1212.)
RFLPs have been used to map the gene locus implicated in numerous diseases. In many cases, the gene in question has been identified by positional cloning or candidate screening; the latter is discussed later in this chapter. It is possible to detect linkage by observing the frequency with which a polymor. phic marker is in herited with a disease trait, prOVided that the disease loc us is withi n 20- 30 centimorgans (cM) of the marker site. The physical d istance represented by I cM
180 • Fundamentals and Principles of Ophthalmology (0.01 recombination fraction) is approximately 1 million bp (1000 kb) and corresponds to a 1% chance that recombination will result from a single meiosis. When a genetic probe is sufficiently close to a disease gene, both are rarely separated by meiotic recombination. The frequency of separation by chromosomal exchange at meiosis is their recombination
frequency. Linked markers should be no more than 20 cM apart. For perspective, the average chromosome contains about 150 eM; there are about 3300 eM in the entire human genome, which corresponds to 3 x 10' bp. Botstein D, White RL, Skolnick M, Davis RW. Construction of a geneti c linkage map in man using restriction fragment length polymorph isms. Am J Hum Genet. 1980;32(3 ):314-331.
When determining linkage between a diseased gene and a marker, geneticists compare different models by calculating likelihood ratios. When the likelihood ratio is 1000: 1 that the odds of 1 model are greater than those of another, the first is accepted over the second. The log of the likelihood ratio (logarithm of odds score, or LOD score) is usually reported. An LOD score of 1- 2 is of potential interest in terms of linkage; 2- 3 is suggestive; and greater than 3 is generally considered proof of linkage. Although an LOD score of 3 gives a probability ratio of 1000: 1 in favor oflinkage versus independent assortment, this score does not ipdicate a type I error as low as 0.001 , but, in fact, it indicates an error that is close to 0.05, the standard Significance level used in statistics. (BCSC Section 1, Update on General Medicine, explains these concepts in depth in Chapter 16, Using Statistics in
Practice and Work.)
Markers: microsatellites, minisatellites, and satellites Within the genome exist variable lengths of repetitive DNA composed of multiple units that may each be 1- 5 bp in length (microsatellites), 14- 100 bp in length (minisatellites), or over 100 bp in length (satellites). One class of such repeats is also called short tan dem repeats (STRs); these are tandemly repeated blocks of 2-5 nucleotides. The variabil ity in the number of repeats produces polymorph isms that are useful for linkage studies (Fig 5-5). STRs have moderate to high mutation rates. The instability of certain minisatellites can lead to cancer and acquired diseases such as insulin-dependent (type 1) diabetes mellitus. Housman D. Human DNA polymorphism. N Engl JMed. 1995;3 32(5):318 - 320. Litt M, Luty JA. A hypervariable microsatellite revealed by in vitro amplification of a dinucleotide repeat within the cardiac muscle actin gene. Am JHum Genet. 1989;44(3):397-401.
Candidate Gene Approaches Candidate gene screening The process of candidate gene screening involves screening for mutations of genes that are abundantly expressed within a tissue and are either important for function or specifically expressed only in that tissue. Sometimes. the candidate gene is one that recapitulates the human disease in transgenic animals. Examples of candidate gene screening discoveries include the findings of mutations of peripherinl RDS in autosomal dominant RP and
CHAPTER 5:
Short tandem-repeat sequences are the basis of highly polymorphic DNA sites in human DNA
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Figure 5-5 A. Variable-length sequences in human DNA can be created by variations in the number of cop ies of a tandem-repeat DNA sequence . Each line in the figure represents a copy of a human DNA sequence. The copies are ident ical in sequence except for the tandemly repeated DNA sequence ind icated by the boxes. The number of caples of the tandemly repeated DNA sequence is indicated by the number of boxes. The size of the DNA fragment that includes the tandem-repeat sequence is measured between 2 fixed points. In Southern blotting, the sites of restriction-enzyme digestion are the fixed points that determine the ends of the DNA fragment. B. A family In wh ich a highly polymorphic marker is used for genetic ana lysis. The 2 copies of the DNA fragment from the offspring can be distinguished from the 2 copies of the fragment from the father, making the inheritance pattern from each parent clear for this chromosoma l site. Detection may be ca rried out by Southern blotting or polymerase chain reaction (peR), depending on the size of the tandem-repeat sequence. (Reproduced with permission from Housman D. Human DNA polymorphism. N Engl J Med_ 1995:332(S):319.}
macular dystrophies and the finding of mutations of the rod cyclic guanosine monophosphate (cGMP) p-subunit of rod phosphodiesterase and the cGMP-gated cation channel in autosomal recessive RP. Dryja TP, Finn JT, Peng YVV, McGee TL, Berson EL, Yan KW. Mutations in the gene encoding the alpha subunit of the rod cGMP-gated channel in autosomal recessive retinitis pigmentosa. Proe Noli Acad Sci USA. 1995;92(22),10177-10181. Kajiwara K, Sandberg MA, Berson EL, Dryja TP. A null mutation in the human peripherinl RDS gene in a family with autosomal dominant retinitis punctata albescens. Nat Genet. 1993;3(3),208-212. McLaughlin ME, Sandberg MA, Berson EL, Dryja TP. Recessive mutations in the gene encod ing the beta-subunit of rod phosphodiesterase in patients with retinitis pigmentosa. Nat Genet. 1993;4(2),130-134. Nichols BE, Sheffield ve, Vandenburgh K, Drack AV, Kimura AE, Stone EM. Butterfly-shaped pigment dystrophy of the fovea caused by a point mutation in codon 167 of the RDS gene. Nat Genet. 1993;3(3),202-207.
182 • Fundamentals and Principles of Ophthalmology
Positional candidate gene screening Whenever linkage studies localize a gene to a given chromosomal region, genes already known to reside in the same region become candidate genes for that disease. Following are
some examples of disease localization that resulted from linkage to a given region, which in turn led to finding the disease-causing gene by screening for mutations of genes in the region: autosomal dominant RP from rhodopsin mutations (3q); Sorsby fundus dystrophy from TIMP3 mutations (22q); and Oguchi disease from point deletions within the arrestin gene (2q). Dryja TP, McG ee TL, Reichel E, et al. A point mutation of the rhodopsin gene in one form o f retinitis pigmentosa. Nature. 1990;343 (6256):364-366. Fuchs S, Nakazawa M, Maw M, Tarnai M, Oguchi Y, Gal A. A homozygous I-base pair deletion in the arrestin gene is a frequent cause of Oguchi disease in Japanese. Nat Genet.
1995;!O( 3}:360- 362.
Weber BH, Vagt G, Pruett RC, Stohr H, Felbor U. Mutations in the tissue inhibitor of metal loproteinases-3 (TIMP3 ) in patients with Sorsby's fundus dystrophy. Nat Gen et. 1994;8(4}: 352-356.
Mutation Screening DNA Libraries DNA libraries exist as a means of collecting and organizing genes of interest for future study. Libraries can be made from either genomic DNA or complementary DNA (cDNA). Genomic DNA libraries are created by cleaving whole DNA from an organism, tissue, or cell type with restriction enzymes that produce fragments of DNA. These fragments can be cloned into vectors and plated onto media. The speCific clones are identified with probes derived from the original sequence or gene of interest and then isolated and grown as needed (Fig 5-6). eDNA libraries are created by using reverse transcriptase to generate complementary DNA from mRNA expressed by the cell or tissue to be studied. Recent technology allows many cDNA sequences or expressed sequence tags (ESTs) to be placed on a microscopic glass slide. Literally thousands of cDNAs can be screened using this microarray technique.
Single-stranded Conformational Polymorphism With the single-stranded conformational polymorphism (SSGP) technique for mutation detection, single-stranded DNA is electrophoresed under nondenaturing conditions, allowing the molecules to fold on themselves according to their inherent similarity of sequences. Molecules of differing sizes and sequences fold differently and migrate at different rates of speed on the gel. Mutations that alter amino acid residues often change the way in which single-stranded DNA folds upon itself, creating different tertiary configurations that can be separated from the normal sequence by differences in mobility on gel electrophoresis.
CHAPTER 5:
Molecular Genetics.
183
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The fi rst step in making a library of DNA sequences is to cut DNA into fragments with restriction enzymes. These DNA fragments, when inserted into vectors, form recomb inant molecules with the DNA of the vector (a plasmid vector is shown, but viral vectors are also used). Bacteria carrying the vectors can replicate on an agar-coated Petri dish, where they grow to form colonies. Each colony originates from a single bacteria l cell and thus contains a sing le type of recomb inant DNA fragment. A nylon filter put on the surface of the Petri dish picks up a portion of each colony. Chemical treatment of the filter lyses the bacterial cells, denaturing the DNA and fixing it in place. A radioactive probe for a known sequence of nucleotides can revea l the desired fragment on the f ilter. The filter, with the replicate library of the colonies on its surface, is incubated in a plastic bag (or glass dish) with a solution containing th e radioactive DNA probe; after the unbound probe is washed away, an x-ray film can locate the radioactive colonies (black ovals!. The position of the signals on the fi lm serves as a map with which to locate the corresponding colonies on the original master plate. Once identified, these colonies can then be ampl ified in culture to produce large quantities of the desired recombinant DNA molecule. (Reproduced with permission from Rosenthal N. Stalking the gene-DNA libraries . N Engl J Med . 1994;331(9):599.)
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Denaturing Gradient Gel Electrophoresis With denaturing gradient gel electrophoresis (DGGE), double-stranded DNA samples are electrophoresed against a gradient of denaturing agent such as urea. Molecules of differing size and composition reach differing points on the gel before they become denatured. Mutations affect the point in the gel for which a given DNA molecule will denature and hence
184 • Fundamentals and Principles of Ophthalmology
alter the migration patterns. The sensitivity of DGGE in detecting mutations is improved if a 40-bp sequence rich in guanine and cytosine (a GC clamp) is added to I primer before polymerase chain reaction (PCR). Identification of a polymorphism detected by DGGE can be determined by direct DNA sequencing of the PCR-amplified exonic product.
Direct Sequencing One of the most important advances in molecular genetics has been the development of techniques for rapid sequencing of DNA. Currently, it is far cheaper to sequence a stretch of DNA than to sequence and characterize the amino acid peptide that it produces. Although other mutation screening techniques exist, sequencing of DNA is the surest and most direct. Sequencing of cDNA derived from mRNA provides a quick look at the reading frames (exons) of the gene, whereas sequencing of genomic DNA is more time-consuming because of the presence of introns between the exons. The intron-exon boundaries must be known and multiple PCR assays set up in order to screen not only the exons and their splice-site junctions but also upstream and downstream regions that may be important for gene activation and regulation. The 2 DNA sequencing techniques used today are the enzymatic (or Sanger) method, which can be implemented manually or semiautomatically, and automated sequencing, which (for high-volume laboratories) is faster and less prone to errors of reading. Figure 5-7 illustrates these procedures. Rosenthal N. Molecular medicine: fine structure of a gene-DNA sequencing. N Engl ] Med. 1995;332(9):589- 591. .
Use of Restriction Endonucleases If a point mutation destroys or creates a restriction site, screening for this mutation can be accomplished quickly through the use of the particular enzyme that recognizes the changed restriction site. Figure 5-8 illustrates the detection of mutations using restriction enzymes, oligonucleotide hybridization, PCR, and Southern blot analysis.
Allele-specific Oligonucleotides An allele-specific oligonucleotide (ASO) is a synthetic probe made of a sequence of nucleotides. It is constructed so that it uses hybridization to recognize a specific DNA sequence in order to detect a specific point mutation. Often, diagnostic testing is done with 2 separate ASOs, 1 that recognizes the specific base-pair change of a given genetic mutation and another that recognizes the normal allelic sequence. ASOs are commonly used for diagnosing point mutations that occur frequently or for testing multiple members of a large family with a previously identified genetic disorder.
Gene Therapy Gene therapy holds much promise, but the field remains in its infancy. The potential for cure is not matched by either technology or understanding. No clinical ophthalmic applications yet exist. Key challenges remain in characterizing linkages of genes to major
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186 • Fundamentals and Principles of Ophthalmology
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Figure 5-8 A, The detection of a point mutation by digestion of DNA w ith a restr iction enzyme. The mutation creates a new recognition site. The region surrounding the mutation is amplified by the polymerase chain reaction (peR), and the resulting PCR product is incubated w ith the restriction en2Yme and then analY2ed by agarose gel electrophoresis. Lane 1 shows DNA from a person without the mutation; only 1 band appears because the enzyme does not cut the DNA. Lane 2 shows DNA fro m a person homozygous for the mutation; 2 bands represen t the 2 fragments obtained after enzyme digestion . Lane 3 shows DNA f rom a heterozygote, with 1 uncut fragment and 2 cut fragments. B, A mutat ion detected by oligonucleotide hybridization. The segment of DNA is amplified by PCR, divided into aliquots, and spotted onto separate filter membranes, which are hybridized w ith a labeled oligonucleotide corresponding to the wild-type or mutant sequence. The amplified segment of DNA from a person with the wi ld-type sequence (WT) hybridizes only with the wi ld-type oligonucleotide, whereas the DNA from a person homozygous for the mutant sequence (MUT) hybridizes only w ith the mutant oligonucleotide. DNA from a hetero2Ygote (HET) hybridizes w ith both oligonucleotides. C, The detection of a mutation by PCR. The mutant sequence ditfers from the wi ld-type sequence by the substitution of an A for a C. To search for t he 2 kinds of sequences by PCR, 2 prim ers are necessary. A separate reaction is carried out with each, together with a common down stream primer. W ith a w ild-type gene, the primer correspondi ng to the wi ld-type sequence yields a PCR product. Sim ilarly, the mutant primer produces a product wi th the mutant sequence. However, with the w ild-type primer and the mutant sequence, or the mutant primer and the w ild-type sequenc e, there is no PCR product. The agarose gel pattern shows t hat DNA from a person homozygous for the wild-type allele reacts only w ith the wi ld-type primer; DNA from a person homozygous for the muta nt sequence reacts only with the mutant primer; and DNA from a heterozygote yields PCR products w it h both primers. (continued)
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Figur.5-8 D, Detection of a triplet-repeat mutation by Southern blot ana lysis or PCR. In the fragila-X syndrome, a eGG repeat occurs near the 5' end of the gene. The number of repeats ranges frorn 5 to 50 in the general population and from approximately 50 to 200 in those with the fragi le-X syndrome . The abnormality is detected as follows : DNA is treated with a restriction enzyme that cuts at recogn ition sites flanking the eGG repeat. Hybridization on a Southern blot (left! with labeled DNA from the reg ion of the gene reveals a single band in a normal male subject (wild typel . An asymptomatic male carrier will have a band of highe r molecular we ight, and a subject with a full mutation will have a very la rge, diffuse band because of the instability of the full-mutation allele. The normal an d asymptomatic-carrier alleles can also be detected by peR (right) . The full-mutation allele cannot be amplified by peR because it is too large. (Reproduced with permission from Karf 8. M o/eculardiagnosis (2). N Engl J Med . 1995;332(22):1500-1501 .)
d iseases (especially chronic diseases that, although common, are likely multifactorial), understanding the pathogenic relevance of identified linked genes, and developing proper delive ry systems fo r curative gene constructs (viruses, the mai n long-term gene therapy vehicle, are plagued by inflam mation and the risk of oncogenesis).
Replacement of Absent Gene Product in X-Linked and Recessive Disease For genetic diseases wherein the mutant allele produces either no message o r an ineffective ge ne product (a so-called null allele), correction of the disorder may be possible by simple replacement of the gene in the deficient cells o r tissues. It is theoretically possible to transfer normal genes into human cells that harbor either null or mutant genes not producing a stable, translated product. Vectors used to carry the genetic material into the cells include adenoviruses, retrovi ruses (especially adeno-associated viruses [AAV]), and plasmidIiposome complexes. AAV vecto r gene therapy has been successful in curing many d isorders in animal models, such as the RPE65 mutation that causes RP in the Briard dog. Plasmid- liposome complexes m ay have advantages as vec to rs because they can be used fo r no ndividing cells and may be less likely to incite inflammation or immune respo nses.
188 • Fundamentals and Principles of Ophthalmology However, as vectors. these complexes are inefficient and may not produce sufficient ex-
pression of the wanted product. Strategies are being developed to direct the new genetic material into the nucleus. with the possibility of subsequent incorporation into the genome to perpetuate expression of the new gene. Blau HM . Springer ML. Gene therapy-a novel form of drug delivery. N Engl } Med. 1995; 333( 18): 1204- 1207. Crystal RG. Transfer of genes to humans: early lessons and obstacles to success. Seierlce. 1995;270(5235) :404-4 1O. Hangai M, Kaneda Y, Tanihara H, Honda Y. In vivo gene transfer into the retina mediated by a novelliposome system. Invest Ophthafmol Vis Sci. 1996;37(13}:2678-2685.
Strategies for Dominant Diseases Dominant diseases are caused by production of a gene product that is either insufficient (haploid insufficiency) or conducive to disease (dominant-negative effect). Theoretically. haploid insufficiency should be treatable by gene replacement as outlined previously for X-linked or recessive disease. (For dominant disorders produced by defective developmental genes. this correction would have to occur in early uterine development.) Disorders resulting from a dominant-negative effect require a different approach. Thus. strategies for treatment of dominant disease differ. depending on whether a functional gene product is produced. Some genes code for RNAs that can bind to mRNA from another gene and block their ability to be translated. Greater understanding of these genes may allow for creation of either drugs or new gene-encoded RNAs that can block the translation of mRNA for defective alleles. thus allowing only the normal allele to be expressed.
Another approach is the use of oligonucleotides that are designed to bind with mRNA from mutant alleles. stopping the mRNA from being translated by ribosomes (Fig 5-9) or ribozymes. molecules that specifically degrade the mRNA. Although many problems need to be worked out for such therapy to be effective. this approach holds promise for autosomal dominant disorders wherein disease is caused by expression of the mutant gene product. One approach to the treatment of autosomal dominant diseases is to target the translated strand of the mutant allele by antisense DNA. a sequence of DNA designed to anneal to and block the processing or translation of the abnormal mRNA. Another approach involves ribozymes. RNA molecules that have the ability to cleave certain RNAs. A third approach utilizes small interference RNAs (siRNAs). also known as short interfering RNAs. to bind to mRNAs and lead to the eventual degradation of specific mRNAs. The use of siRNAs as potential therapeutic agents has become increasingly popular in the last few years. and siRNAs have proven to be a powerful means by which to study the function of novel gene products. However. siRNAs suffer from difficulty in achieving intracellular delivery and from cell-surface TLR3 receptor stimulation. which can induce immune or antiangiogenic processes as a generic class property. Askari FK. McDonnell WM. Antisense-oligonucleotide therapy. N Engf , Merl. 1996;334(5): 316- 318.
CHAPTER 5:
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Della NG. Molecular biology in ophthalmology. A review of principles and recent advances. Arch Ophthalmol, 1996;114(4);457- 463, Kleinman M, Yamada K, Takeda A, et aL Sequence- and target-independent angiogenesis suppression by siRNA via TLR3. Nature. 2008;452(7l87):591-597.
CHAPTER
6
Clinical Genetics
It is important for a clinician not only to diagnose a disease state accurately and minimize
its effects in the patient but to ensure that the Siblings and parents are evaluated for milder or earlier forms of the disease. Any family with a genetic disorder should receive genetic counseling as a primary care responsibility. However, only the ophthalmologist will be sensitive to the wide variability of traits affecting the visual system and to the subtleties of carrier-state detection (both by direct evaluation and with indirect diagnostic technology). The ophthalmologist is an important member of the team that can appropriately counsel the patient and family about the ocular effects of a given genetic disorder and its attendant risks and burdens.
Terminology: Hereditary. Genetic. Familial. Congenital Hereditary indicates that a disease or trait under consideration results directly from an individual's particular genetic composition (or genome) and that it can be passed from one generation to another. Genetic denotes that the disorder is caused by a defect of genes, whether acquired or inherited. In some instances (eg, with large deletions of mitochondrial DNA [mtDNA] associated with chronic progressive ophthalmoplegia), the disease is clearly genetic, but it is not passed to subsequent generations and is therefore not hereditary. These deletions associated with ocular myopathies presumably arise in the oocyte or during early embryologic development. Thus, the terms hereditary and genetic are not exactly synonymous but are sometimes used to convey similar concepts. A hereditary or a genetic disorder mayor may not be congenital. A condition is familial if it occurs in more than I member of a family. It may, of course, be hereditary but need not be. A familial disorder can be caused by common exposure to infectious agents (eg, tuberculosis), traumatizing materials (eg, radiation), deficient or excess food intake (eg, vitamin deficiencies or obesity), or environmental agents such as asbestos or coal dust. The term congenital refers to characteristics that are present at birth. These may be hereditary or familial, or they may occur as an isolated event, often as the result of an infection (eg, rubella, toxoplasmosis, or cytomegalic inclusion disease). The presence of such characteristics at birth is the defining factor. Findings suggesting but not establishing that a congenital anomaly may be genetic include a phenotype similar to that from known genetic disorders (eg, aniridia) or a tendency toward bilaterality (eg, bilateral colobomas) and symmetry. However, some nongenetic congenital disorders. such as cataracts from 191
192 • Fundamentals and Principles of Ophthalmology rubella, can be bilateral. Moreover, not all hereditary disorders are bilateral or symmetric- for example, optic nerve coloboma in only 1 eye has been observed in multiple generations and presumably in this case is an autosomal dominant trait.
Although numerous hereditary disorders are expressed at the time of birth, the complex interrelationships between genomic expression and factors such as the environment may alter the time of onset and the extent to which a disorder is manifested. For example, the overt clinical onset of signs for diabetes mellitus- a condition with a heritable ten -
dency mediated by multiple genes- in individual monozygotic twins can differ appreCiably with each twin's level of carbohydrate intake or other factors. In addition, although the enzyme deficiency responsible for galactosemia is clearly inherited, the expression of systemic disease as well as cataract formation can be avoided by the removal of galactose
from the diet. A condition known to be genetic and hereditary may appear in only 1 individual of a family (eg, retinitis pigmentosa [RPJ). Such an individual is said to have a simplex, or isolated, form of a genetic disease. A genetically determined trait may be isolated in the pedigree for several reasons: • The pedigree is small. The full ' expression of the disease has not been sought or has not manifested in other relatives. The disorder represents a new genetic mutation. The disorder is recessive, and the investigation to determine whether the parents are carriers has been inadequate .
• The disorder is caused by chromosomal changes. Clinically similar disorders may be inherited in several different ways-for example, RP can occur as an autosomal dominant, autosomal recessive, or X-linked trait or result from a mitochondrial mutation. These various genetic forms represent distinct gene de-
fects with different alterations in gene structure and different biochemical pathogeneses, each of which has similar clinical phenotypic expressions. Clarification of genetic heterogeneity is important, because only with the proper diagnosis and inheritance pattern identification can appropriate genetic counseling and prognosis be offered. Some genetic disorders originally thought to be a single and unique entity are found, on close scrutiny, to be 2 or more fundamentally distinct entities, Further clarification of the inheritance pattern or biochemical analysis permits separation of initially similar disorders. Such has been the case for Marfan syndrome and homocystinuria, Although both disorders cause unusual body habitus and ectopia lentis, the presence of dominant inheritance, aortic aneurysms, and valvular heart disease in Marfan syndrome distinguishes it from the recessive pattern and thromboembolic disease of homocystinuria. Genetic heterogeneity is a general term that applies to the phenotypic similarity that may be produced by 2 or more fundamentally distinct genetic entities; this term implies that the genes are nonallelic. The term locus heterogeneity has been used when linkage studies have shown that different families with similar phenotypes map at different loci; hence, the phenotype can be caused by mutations of different genes. When different alleles at the same locus are capable of producing an abnormal phenotype, the term allelic
CHAPTER 6:
Clinical Genetics. 193
heterogeneity can be applied. With clinical heterogeneity, different mutations at the same locus can produce different phenotypes. Once the location on a chromosome is determined for a particular disease gene and once the gene's molecular structure is identified, most examples of genetic heterogeneity cease to be a problem for diagnosis or classification. However, clinical, allelic, and locus heterogeneity can remain perplexing issues. For example, mutations of the Norrie disease gene, NDp, usually result in the typical phenotype of pseudoglioma from exudative retinal detachments, but some mutations of NDP have been associated with X-linked exudative vitreoretinopathy. Mutations of the proto-oncogene RET can give rise to medullary thyroid carcinoma, multiple endocrine neoplaSia 2A and 2B, and Hirschsprung disease. Such examples give added meaning to the term heterogeneity. Ultimately, greater understanding of these and other disorders at the molecular and cellular levels results in more reliable patient management and improved classification of genetic disease. Mulvihill 11. Craniofacial syndromes: no such thing as a single gene disease. Nat Genet. 1995;9(2): 101 - 103. van Heyningen V. Genetics. One gene-four syndromes. Nature. 1994;367(6461):319- 320.
Genes and Chromosomes In 1909, the Danish biologist Wilhelm Johannsen coined the word genes, from the Greek for "giving birth to;' as a name for segments of the DNA molecule containing individual units of hereditary information. Genes are the basic units of inheritance, and they include the length of nucleotides that codes for a single trait or a single polypeptide chain and its associated regulatory regions. Human genes vary greatly in size, from approximately 500 base pairs (bp) to more than 2 million bp. However, more than 98% range from less than 10 kilo base pairs (kb; 1 kb = 1000 bp) to 500 kb in size. Many are considerably larger than SO kb. Whereas a single human cell contains enough DNA for 6 million genes, about 50,000- 100,000 genes are found among the 23 pairs of known chromosomes. The func tion of the remaining 95% of the genetic material is unknown. The relative sequence of the genes, which are arranged linearly along the chromosome, is called the genetic map. The physical position or region on the chromosome occupied by a single gene is known as a locus. The physical contiguity of various gene loci becomes the vehicle for close association of genes with one another (linkage) and their clustering in groups that characteristically move together or separately (segregation) from one generation to the next. Each normal human somatic cell has 46 chromosomes composed of 23 homologous pairs. Each member of a homologous pair carries matched, although not necessarily identical, genes in the same sequence. One member of each chromosome pair is inherited from the father, the other from the mother. Each normal sperm or ovum contains 23 chromosomes, one representative from each pair; thus, each parent transmits half of his or her genetic information to each child. Of the 46 chromosomes, 44 are called autosomes because they provide information on somatic characteristics. (The X and Y chromosomes provide such information as well; eg, genes on the Y chromosome influence teeth and height.)
194 • Fundamentals and Principles of Ophthalmology The 2 sex chromosomes differ in males and females. Whereas males have an X chromosome and a Y ch romosome. females have 2 X chromosomes. Unique to phenotypically normal males. the Y chromosome determines development of the testes and other male secondary sexual characteristics. Bishop IE, Waldholz M. Genome: the story of the most astonishing scientific adventure of our time-the attempt to map all the genes in the human body. New York: Simon & Schuster; 1990.
Hartl DL, Jones EW, eds. Genetics: Analysis afGenes and Genomes. 7th ed. Sudbury, MA: Jones & Bartlett; 2009. Rimoin DL, Connor JM , Pyeritz RL, Korf BR, eds. Emery and Rimoin's Principles and Practice of Medical Genetics. 5th ed. Philadelphia: Churchill Livingstone; 2007.
Alleles Alternative forms of a particular gene at the same locus on each of an identical pair of chromosomes are called alle/es (Greek for "reciprocals"). If both members of a pair of alleles for a given autosomal locus are identical (ie. the DNA sequence is the same). the individual is homozygous (a homozygote); if the allelic genes are distinct from each other (ie. the DNA sequence differs). the individual is heterozygous (a heterozygote). Different gene defects can cause dramatically different phenotypes and still be allelic. For example. sickle cell disease (SS hemoglobinopathy) caused by homozygosity of I mutant gene is significantly different from the phenotypic expression of SC hemoglobinopathy. yet the Hb S gene and the Hb C gene are allelic. The term po/yallelism refers to the many possible variants or mutations of a single gene. Mutant proteins that correspond to mutant alleles frequently have been shown to possess slightly different biochemical properties. Among the mucopolysaccharidoses. for example. the enzyme alpha-L-iduronidase is defective in both Hurler disease and Scheie syndrome. Because these are mutations of the same gene. they are abno rmalities of the same enzyme and thus allelic. However. the clinical severity of these 2 disorders (age of onset; age of detection; and severity of affliction of skeleton. liver. spleen. and cornea) is entirely different. presumably because the function of the mutant enzyme is less altered by the Scheie syndrome mutation. Because the enzyme is a protein composed of hundreds of amino acids. a mutation resulting in a base substitution within a certain codon might cause a change in I or more amino acids in a portion of the
enzyme remote from its active site. thus reducing its effect on the enzyme's function. However, the substitution of I amino acid at a critical location in the enzyme's active site might abolish most or all of its enzymatic activity. Several examples of allelic disorders appear among the mucopolysaccharidoses. The phenotype of the usual heterozygote is determined by I mutant allele and 1 "normal" allele. However. the genotype of a compound heterozygote comprises 2 different mutant alleles. each at the same locus. The genelic Hurler-Scheie compound heterozygote is biochemically proven and clinically manifests features intermediate between the homozygotes of the 2 alleles. Whenever detailed biochemical analysis is possible. the products of the 2 alleles manifest slightly di fferent properties (such as rates of enzyme activity or electrophoretic migration). Among other autosomal recessive diseases. a spectrum of
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phenotypes can be caused by a diversity of mutant alleles occurring in various paired combinations. Most recessive diseases (as well as dominant disorders) do not permit more
than speculation on this alternative in the clinical setting. In contrast, and as noted earlier, some genetic disorders originally thought to be single and unique may, on close scrutiny, reveal 2 or more fundamentally distinct entities. Occasionally, this genetic heterogeneity is seen with diseases that are inherited in the same manner, such as tyrosinase-negative and tyrosinase-positive oculocutaneous albinism. Because these 2 conditions are phenotypically similar and each is inherited as an autosomal recessive trait, it was assumed for some time that they were allelic. When a tyroSinase-negative person bears children with a tyrosinase-positive person, the offspring appear clinically normal. This observation excludes the possibility that these 2 conditions are allelic: each condition occurs only when an offspring is heterozygous for the gene causing the condition. Separate gene loci (the tyrosinase gene and the P gene) are now known to cause oculocutaneous albinism. The offspring of such matings of individuals with phenotypically similar but genotypically different disorders are called double heterozygotes because they are heterozygous for each of the 2 loci. Because a female has 2 X chromosomes, she may be either homozygous or heterozygous with respect to X-linked genes. A male is said to be hemizygous for X-linked genes because he has only a single X chromosome, and the Y chromosome has little comparable material. A person is also termed hemizygous for a given genetic locus when the second allele is missing, either through loss of an entire chromosome or through rearrangements resulting in deletion of any segment of 1 of a pair of chromosomes.
Mitosis A cell may undergo 2 types of cell division-mitosis and meiosis. Mitosis gives rise to the multiple generations of genetically identical cells needed for the growth and maintenance of the organism. When mitosis is abo ut to occur, the cell accurately duplicates all of its chromosomes. The replicated chromosomes then separate into 2 identical groups that migrate apart and eventually reach opposite sides of the cell. The cell and its contents then divide, forming 2 genetically identical daughter cells, each with the same diplOid chromosome number and genetic information as the parent cell.
Meiosis In contrast to mitosis, meiosis leads to the production of cells that have only 1 member of each chromosome pair (Fig 6-1). The specialized cells that arise from meiosis and participate in sexual reproduction are called gametes. The male gamete is a sperm, the female gamete, an ovum. During meiosis, a modified sequence of divisions systematically reduces the number of chromosomes in each cell by one half to the haploid number. Consequently, each gamete contains 23 chromosomes, 1 representative of each pair. This assortment occurs randomly, except that 1 representative of each pair of chromosomes is incorporated into each sperm or egg.
196 • Fundamentals and Principles of Ophthalmology NORMAL MEIOSIS
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At conception, a sperm and an ovum unite, forming a zygote, a single cell that contains 46 chromosomes. Because both parents contribute equally to the genetic makeup of their offspring, new and often advantageous gene combinations may emerge.
Segregation Two allelic genes, which occupy the same gene locus on 2 homologous chromosomes, separate with the division of the 2 chromosomes during meiosis, and each goes to a different gamete. Thus, the genes are said to segregate, a property limited to allelic genes, which cannot occur together in a single offspring of the bearer. For example, if a parent is a compound heterozygote for both hemoglobin S and hemoglobin C, which occupy the same genetic locus on homologous chromosomes, none of the offspring will inherit both hemoglobins from that parent; each will inherit either 1 or the other.
Independent Assortment Genes on different (nonhomologous) chromosomes mayor may not separate together during meiotic cell division. This random process, called independent assortment, states that
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nonallelic genes assort independently of one another. Because crossing over (exchange of chromosomal material between the members of a pair of homologous chromosomes) can occur in meiosis, 2 nonallelic genes originally on opposite members of the chromosomal pair may end up together on either of the 2 or remain separated, depending on their original positions and on the sites of genetic interchange. Thus, the gametes of an individual with 2 nonallelic dominant traits, or syntenic traits, located on the same chromosome could produce 4 possible offspring. A child may inherit both traits if the separate alleles remain on the same chromosome and the child inherits this chromosome • neither trait if the genes remain on 1 chromosome but the child inherits the opposite chromosome with neither allele only 1 of the 2 alleles if crossing over occurred between the loci, and the child received the chromosome with that particular allele This scheme for nonallelic traits depends on the independent assortment of chromosomes in the first division of meiosis. Approximately 50 crossovers (1 - 3 per chromosome) occur during an average meiotic division.
Linkage Linkage is the major exception or modification to the law of independent assortment. Nonallelic genes located reasonably close together on the same chromosome tend to be transmitted together, from generation to generation, more frequently than chance alone would allow for; thus, they are said to be linked. The closer together the 2 loci, the less likely they are to be affected by crossovers. Linear physical proximity along a chromosome cannot be considered an automatic guarantor of linkage, however. In fact, certain sites on each chromosome may be more vulnerable to homologous crossing over 't han others. Jorde LB, Carey Je, Bamshad MI. White RL. Medical Genetics. Updated ed. for 2006-2007. 3rd ed. Philadelphia: Elsevier Mosby; 2006.
Chromosomal Analysis Cytogenetics is a branch of genetics concerned with the study of chromosomes and their properties. Chromosomal defects are changes in the chromosome number or structure that damage sensitive genetic functions and lead to developmental or reproductive disorders. These defects usually result from (1) a disruption of the mechanisms controlling chromosome movement during cell division or (2) alterations of chromosome structure that lead to changes in the number or arrangement of genes or to abnormal chromosomal behavior. Chromosomal abnormalities occur in approximately 1 of 200 term pregnancies and in 1%- 2% of all pregnancies involving parents over the age of 35 years. About 7% of perinatal deaths and some 40%-50% of retrievable spontaneous abortuses have Significant chromosomal aberrations. Virtually any change in chromosome number during early development profoundly affects the formation of tissues and organs and the viability of the
198 • Fundamentals and Principles of Ophthalmology
entire organism. Most major chromosomal disorders are characterized by both developmental and mental retardation, as well as a variety of somatic abnormalities.
Indications The usual indications for chromosome analysis are listed in Table 6-1. Ophthalmologists should be aware of the value of constitutional and tumor karyotypes in infants with retinoblastoma, especially if the tumor represents a new genetic mutation. Chromosome analysis is also suggested in patients with isolated (nonfamilial) aniridia (which is often associated with Wilms tumor) and other systemic malformations and in patients who survive a neoplastic syndrome and experience a second neoplasm. A chromosomally abnormal state in a previous child warrants consideration of amniocentesis or chorionic villus sampling for prenatal diagnosis in subsequent pregnancies to avoid the risk of recurrence. Amniocentesis can be undertaken at approximately the 16th week of pregnancy.
Preparation Karyotype The systematic display of chromosomes from a single somatic cell is called a karyotype. Chromosome preparations can be made from any tissue whose cells will divide in culture. The tissue most commonly used is peripheral venous blood, although bone marrow, skin fibroblasts, and cells from amniotic fluid or chorionic villi are useful under speCific circumstances. In special situations, chromosome analyses can be obtained directly from rapidly dividing neoplastic tissues, as has been done with fresh cells from retinoblastoma and Wilms tumor. The blood of healthy individuals without leukemia contains no dividing cells. Therefore, T Iymphocyles in a small sample (often less than 5 mL) of fresh heparinized blood are
Table 6-1 Some Indications for Chromosome Analysis Clinical diagnosis in newborns: multiple malformations, especially involving more than 1 organ system , with or without intrauterine growth retardation; perinatal death Clinical diagnosis at any age: mental retardation with or without congenital malformations, in the absence of unequivocal identifiable cause; gonadal ambiguity; infertility, amenorrhea, or reproductive dysfunction; ocular malformations associated with any malformations of other organ systems with no known cause Multiple miscarriages: spontaneous abortions or stillbirths without apparent cause, even in clinically normal parents Studies of malignancy: constitutional and tumor karyotypes, especially leukemia, retinoblastoma, aniridia-Wilms tumor, and other embryonal malignancies; specific syndromes with high risks of malignancy-ataxia-telangiectasia, Bloom syndrome, Fanconi anemia; tumor karyotypes on all second tumors in neoplasia syndromes Prenatal diagnosis: in advanced maternal age, known translocation carrier state, X-linked carrier state for sexing; previous child with chromosomal abnormalities; and as part of either amniocentesis or c horionic villus sampling
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stimulated to divide by adding phytohemagglutinin to a culture medium. After approxi mately 72 hours, as the dividing cells approach metaphase, a drug that has colchicine-like effects is added to prevent formation of the mitotic spindle apparatus.
Fluorescence in situ hybridization and chromosome arm painting With the fluorescence in situ hybridization (FISH) technique, DNA fragments from genes of interest are first tagged with a fluorescent compound and then annealed or hybridized to chromosomes. The regions of interest are stained to determine whether duplication, deletion, or rearrangement has occurred. Such fluorescent molecular probes can detect and often quantify the presence of specific DNA sequences on a chromosome and can find microscopic abnormalities that would be indiscernible by conventional cytogenetic methods. Probes have been developed from microdissections of chromosomal regions and FISH that label entire arms of chromosomes and each of the individual chromosomes (multicolor spectral karyotyping and combinatorial multifluor FISH) . With 2-color FISH, both arms of each chromosome can be Simultaneously labeled (Fig 6-2). These probes are valuable for detecting and understanding the mechanisms of complex chromosomal rearrangement (Fig 6-3), such as can occur in cancer. Chromosome arm painting (CAP) is also being used to generate information on the 3-dimensional organization of chromatin domains within the nucleus in interphase. Guan X-Y, Zhang H. Biltner M. Hang Y. Meltzer P. Treat J. Chromosome arm painting probes. Nat Genet. 1996;12(11) ;10-11.
Schrock E, du Manoir S,
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somes. Science. 1996;273(5274):494-497.
Speicher MR, Gwyn Ballard S, Ward DC. Karyotyping human chromosomes by combinatorial multi · fluor FISH. Nat Genet. 1996;12(4P68- 375.
Aneuploidy of Autosomes Aneuploidy denotes an abnormal number of chromosomes in nongametic cells. The presence of 3 homologous chromosomes in a cell rather than the normal pair is termed trisomy. Monosomy is the presence of only I member of any pair of auto somes or only I sex chromosome. The absence of a Single autosome is almost always lethal to the embryo; an extra autosome is often catastrophic to surviving embryos. Aneuploidy of sex chromosomes (such as X, XXX, XXV, and XYY) is less disastrous. Monosomies and trisomies are generally caused by mechanical accidents that increase or decrease the number of chromosomes in the gametes. The most common type of accident, meiotic nondisjunction, results from a disruption of chromosome movement during meiosis (see Fig 6-1). Polyploidy describes a cell that contains an exact multiple of the normal diplOid number, such as 3n(69) or 4n(92). de Grouchy J. Turleau C. Clinical Atlas of Human Chromosomes. 2nd ed. New York: Wiley; 1984. Jorde LB, Carey JC, Bamshad MJ, White RL. Medical Genetics. Updated ed. for 2006- 2007. 3rd ed. Philadelphia: Elsevier Mosby; 2006.
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Figure 6·2 Composite karyotype of all human chromosomes hybridized with chromosome arm painting. Metaphase chromosomes were hybridized with corresponding short arm (red) and long arm (green) painting probes simultaneous ly, and a compos ite karyot ype was generated . Short-arm probes were not generated for the acrocentric chromosomes 13, 14, 15, 21, and 22. M inimal regions of overlap (yellow) between long-arm and short-arm probes we re identified for chromosomes 2, 3, 4, 7, 9, 11, 17, 18, 20, X, and Y The size distribution of PCRamplified microdissected DNA fragments of each individual arm were analyzed by running peR products on 1 % agarose gels. All peR produc ts showed a smear ranging from 200 to 600 bp with no appa rent dominant bands, After PCR amplification, microdlssected DNA fragmen ts from all 19 short arms were labe led wi th SpectrumO range fluorescent label (Abbott Laboratories, Des Plaines, III for DNA probes . The short arms of the acrocentrics we re not dissected so that cross-hybridizat ion between repetitive sequences localized on t he short arms of the acrocentric chromosomes would be avoided. Microdissected DNA fragments from all long arms were labeled w ith biotin and detected with fluorescein-conjugated avidin (Vector Labs, Burlingame, CAl. (Reproduced with permission from Guan X-Y, Zhang H, Bittner M, Jiang Y, Meltzer p, Treat J. Chromosome arm painting probes. Nat Genet. 1996;1 2(11): 10.)
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-.. A ....._ _ _ _ _ _ __ B Fi gure 6·3 Application of chromosome arm painting (CAP) to detect complex chromosome rearrangements. A cell line with complex, defined chromosome rearrangements was identified for hybridization (human lymphoma cell line SU-DHL-41. A, G-banded metaphase from SU-DHL-4. B, The identical metaphase hybridized with CAPs 2p {red! and 2q (green). A normal 2 and a rearranged chromosome 2[t(2;2)(q37;p13)1 (arrow) were observed. (Reproduced with permission from Guan X- Y. Zhang H, Bittner M. Jiang Y. Meltzer p. Treat J. Chromosome arm paiming probes. Nat Genet. 1996;12(1/):17.}
Trisomy 21 syndrome Trisomy 21 syndrome. o r DOlVn syndrome, is the most common chromosoma l sy nd ro me in humans, with an overa ll incidence of 1:800 live births. It was also the first ch ro moso mal disease defined in humans. Clin ical features of this syndrome have been wel l known since th e British physician John Langdon Down originally described th em in 1866.
The frequ ency of Down sy ndrome dearly increases with age of the mother, from about 1:1400 live births (to mothers aged 20-24 years) to appro:
mothers between 15 and 19 yea rs of age than it is in the next- higher age range. Above age 50, the frequency is 1: 1 I li ve births. The epo nym Down syndrome summarizes a clinical description o r certain distinctive if variable phenotypic features, whereas th e karyotype desc ribes the chromosom al cons titution of the cells and tiss ue studied. The chromosoma l basis of this diso rd er was first demo nstrated by French ge neti cist Jerome Lejeune and coworkers in 1959. ApprOXi mately 95% of child ren with thi s di sorder have an extra chromosome 21 as a result of meiotic nondisjunction. Either parent may co ntribute the third chrom oso me 2 1. but the most important risk factor in haVing a child with Down syndrome is ma ternal age. Whe n it has been pOSSible to dete rmine where the meioti c error took place, more th an 80% occurred in the first meiosis, and more th an 95% occurred with maternal rather th an paternal meiosis. App roximately 5% of patien ts with Down syndrome have a translocation resu lting from the attachment of the long ar m of chromoso me 2 1 with the long arm of o ne of th e other acrocentri c chromosomes, usually 14 o r 22. These t ransloca ti ons cause pairing problems during meiosis, an d th e translocated fragme nt of chromosome 2 1 appears in one of th e daughte r cells along with a normal 21. As in nondisjunction , the fragment becomes trisomic on fe rtil ization. Trisomy of on ly th e distal third of chromosome 21 q is
202 • Fundamentals and Principles of Ophthalmology sufficient to cause the disorder. Genes that lie within the q22 band of chromosome 21 appear to be specifically responsible for the pathogenesis of Down syndrome. The increased incidence of this disorder with maternal age is principally the result or a greater likelihood of nondisjunction , because translocation errors are not related to maternal age. The extra chromosome is of paternal origin in fewer than 5% of affected individuals. A major positive correlation between advanced paternal age and an increased incidence of Down syndrome has not been established. The empirica l recurrence risks to parents who have had I child with trisomy 2) arc approximately 1%, although this number is higher among older women. Patients with Down syndrome exhibit the following features: mental retardation
short stature poor muscular control (hypotonia) brachycephaly with a broad, nat occiput hypoplasia of the middle phalanx of the fifth finger wide space between the first and second toes small ears va rious fo rms of co ngenital hea rt disease, including ventricular and atrial septa l defects (40%) and, occasionally, du odenal atresia and tracheoesophageal fistulas infertility • dental hypoplasia characteristic dermatoglyphic findings
A single palmar crease occurs in about 50% ofthose with Down synd rome but in only I % of the general population. Approx imately half of the infants and young children with this disorder have unspeCified hearing loss. The most common ocu lar findings of Down syndrome are presented in Table 6-2. Additional medical complicat ions in patients with Down syndrome include an increased susceptibility to infection and a 20-fold to 50-fold increase in the risk of leukemia. The shortened life span in patients affected by Down syndrome is partly secondary to
Tabte 6-2 Ocular Findings in Down Syndrome (Trisomy 21 ) Almond-shaped palpebral fissures Upslanting (mongoloid) palpebral fissures Prominent epicanthal folds Blepharitis, usually chronic, with cicatricial ectropion Strabismus, usually esotropic Nystagmus (typica ll y horizontal) Aberrant retina l vessels (at disc) Iris stroma l hypopla sia Brushfield spots Keratoconus Cataract Myopia Optic atrophy
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these related medical problems. Studies of autopsy material from affected patients show that virtually all patients with Down syndrome over the age of 35 years develop abnormal microscopic senile plaques and neurofibrillary tangles in the brain, similar to those in Alzheimer disease. Down syndrome patients also appear to be at Significantly increased risk for the cognitive sympto ms of Alzheimer disease. It has been shown that the amyloid - ~ precursor protein (a major component of the neurofibrillary plaques that acc umulate in the brain of people with Alzheimer disease) is identical to the protein that accumulates in apparently identical lesions in people with Down syndrome who are older than 35 years. The relationship of the gene for amyloid precursor protein and the form of Alzheimer disease in Down patients is unknown. Catalano RA. Down syndrome. Surv Ophthalmol. 1990;34(5 ): 385- 398. de Grouchy J. Turleau C. Clinical Atlas of Human Chromosomes. 2nd ed. New York: Wiley; 1984: 338- 349. Patterson D. The causes of Down syndrome. Sci Am. 1987; 257(2):52-60.
Mosaicism Occasionally, an individual or a tissue contains 2 or more cell lines with distinctly different chromosomal constitutions. Such individuals o r tissues are termed mosaics. Sometimes the peripheral blood, which is the usual source for chromosomal analysis, contains populations of cells wi th completely different chromosomal constitutions. One population of cells may be so infrequent that a second tissue, such as skin fibroblasts, must be analyzed to demonstrate the mosaicism. It is not known if mosaicism occurs in all human tissues. Cytogenetic defects arise because of ab normal chromosomal distribution during the early stages of embryonic development. These embryos possess 2 or more chromosomally different cell populations. Mosaicism usually results either from mitotic nondisjunction, in which I replicated ch romosome fails to separate in the dividing cell; or from anaphase lag, in which normal separation occurs, but I member of the replicated pair fails to migrate and is lost. The clinical effects of mosaicism are difficult to predict because the distribution of abnormal cells in the embryo is determined by the timing of the error and other variables. If mitotic nondisjunction immediately follows conception, the zygote divides into 2 abnormal cells, 1 trisomic and 1 monosomic. The mo nosomic cells rarely survive and may decrease in number o r even disappear entirely over time. Mitotic nondisjunction may occur when the embryo is composed of a small population of cells. Thus, 3 populations of cells are established, I normal and 2 abnormal, although some abnormal cell lines may be "discarded" or lost during development. If mitotic nondisjunction occurs at a more advanced stage of development, resulting abnormal populations constitute a minority of the embryo's cells, and mosaicism may have little or no measurable effect on development. A small population of aneuploid mosaic cells may not have a direct effect on development. However, when cells of this type occur in the reproductive tissues of otherwise normal people, some of the gametes may carry extra chromosomes or be missing some entirely. Consequently, mosaic pare nts tend to be at high risk for chromosomally abnormal children.
204 • Fu ndamenta ls and Principles of Ophthalmology The most common example of autosomal mosaicism is trisomy 21 mosaicism. Some
patients with trisomy 21 mosaicism have the typical features of Down syndrome; others show no abnormalities in appearance or intelligence. The critical variable seems to be the frequency and the embryologic distribution of the trisomic cells during early development, which does not necessarily correlate with the percentage of trisomic cells in anyone tissue, such as peripheral blood. Several types of sex chromosome mosaicism may occur. Again, the physical effects tend to vary, probably reflecting the quantity and distribution of the abnormal cells during development. For example, the cell population that lacks 1 of the X chromosomes can arise in a female embryo, leading to 4S,X/46,XX mosaicism. In some cases, these patients develop normally; in other cases, some or all of the features of Turner syndrome appear. Similarly, the Y chromosome may be lost in some cells of a developing male embryo. This produces 4S,X/46,XY mosaicism. X/XY mosaics may develop as normal males, as females with the features of Turner syndrome, or as individuals with physical characteristics intermediate between the sexes (intersexes, or pseudohermaphrodites).
Etiology of Chromosomal Aberrations Long arm 13 deletion (13q14) syndrome: retinoblastoma Retinoblastoma is one of several heritable childhood malignancies. Ocular tumors, which are usually noted before the age of 4 yea rs, affect between I in 15,000 and 1 in 34,000 live births in the United States. The disease exhibits both hereditary occurrence (approximately 30%-40%), in which tumors tend to be bilateral and multicentric; and sporadic occurrence, in which unilateral and solitary tumors are the rule. Only about 10% of patients with hereditary reti noblastoma have a family history of the disease; the remaining 90% have a new mutation in their germ ce lls.
Retinoblastoma does not develop in apprOXimately 10% of all obligate carriers of a germline mutation. In addition, a karyotypically visible delet ion of part of the long arm of chromosome 13 occurs in 3%- 7% of all cases of retinoblastoma. The larger this deletion, the more severe the phenotypic syndrome, which includes mental and developmental retardation, microcephaly, hand and foot anomalies, and ambiguous genitalia (Table 6-3). Although the hereditary pattern in fam ilial retinoblastoma is that of an autosomal dominant mutation, the defect is recessive at the cellular level. The predisposition to retinoblastoma is caused by hemizygosity of the Rb locus within human chromosome band 13q 14. The Rb locus is a member of a class of genes called recessive tumor-suppressor genes. The alleles normally present at these loci help to prevent tumor formation. At least 1 active normal allele is needed to prevent the cell from losing control of proliferation . Patients who inherit a defective allele from 1 parent are at greater risk for lOSing the other allele through a numbe r of mechanisms. Thus, tumor formation in retinoblasto ma is caused by the loss offunction of both normal alleles. Homozygous deletions within the 13q 14 region have been noted in retinoblastomas derived from enucleated eyes. The first step in tumorigenesis is a recessive mutation of 1 of the homologous alleles at the retinoblastoma locus
by inheritance, germinal mutation, or somatic mutation. He-
reditary retinoblastomas arise from a Single additional somatic event in a cell that carries an inh erited mutation, whereas sporadic cases require 2 somatic events. In approximately
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Table 6-3 Systemic Findings in the Long Arm (13q14) Deletion Syndrome "Failure to thrive" growth retardation Mental retardation Microcephaly Trigonencephaly; scalp defect Micrognathia Large, malformed, low-set ears Cleft of highly arched palate Facial asymmetry Congenital heart disease Pelvic girdle anomalies Anal atresia Cryptorchidism, bifid scrotum Hypospadias or epispadias, underdeveloped labia Hypoplastic thumbs Short incurved fourth and/or fifth finger Foot anomalies (clubfoot, short great toe, syndactyly of the fourth and fifth toes) Associated esterase D deficiency
50% of tumors, homozygosity for such a recessive mutation results from the mitotic loss of a portion of chromosome 13, including the 13q 14 band. The resulting homozygosity for recessive mutant alleles at this locus allows the genesis of the tumor. Retinoblastoma, therefore, seemingly represents a malignancy caused by defective gene regulation rather than the presence of a dominant mutant oncogene. Those who inherit a mutant allele at this locus have a high incidence 9f nonocular second tumors thought to be caused by the same mutation. Almost half of these tumors are osteosarcomas. Cavenee WK, Dryja TP, Phillips RA, et al. Expression of recessive alleles by chromosomal mechanisms in retinoblastoma. Nature. 1983;305(5937):779-784. Dryja TP, Rapaport JM, Joyce JM, Petersen RA. Molecular detection of deletions in,volving band q14 of chromosome 13 in retinoblastomas. Proc Natl Acad Sci USA. 1986;83(19):7391-7394. Friend SH, Bernards R, Rogelj S, et al. A human DNA segment with properties of the gene that predisposes to retinoblastoma and osteosarcoma. Nature. 1986;323(6089}:643- 646. Friend SH, Dryja TP, Weinberg RA. Oncogenes and tumor-suppressing genes. N Engl J Med. 1988;318(10),618-622.
Godbout R, Dryja TP, Squire J, Gallie BL, Phillips RA. Somatic inactivation of genes on chro mosome 13 is a common event in retinoblastoma. Nature. 1983;304(5925):451-453. Lohmann DR, Brandt B, OehlschHiger U, et a1. Molecular analysis and predictive testing in retinoblastoma. Ophthalmic Genet. 1995;16(4}:135- 142. Wiggs J, Nordenskjold M, Yandell D, et al. Prediction of the risk of hereditary retinoblastoma, using DNA polymorphisms within the retinoblastoma gene. N Engl J Med. 1988;318(3): 151 - 157.
Short arm 11 deletion (l1p13) syndrome: aniridia Aniridia (AN2) occurs from a defect of a gene that encodes a transcription factor needed for development of the eye. This developmental gene, PAX6, is located at Ilpl3. Aniridia is a panophthalmic disorder characterized by the following:
• subnormal visual acuity • congenital nystagmus
206 • Fundamentals and Principles of Ophthalmology strabismus corneal pannus • cataracts • ectopia lent is
glaucoma • optic nerve hypoplasia • foveal or macular hypoplasia • iris absence or severe hypoplasia
Although almost all cases of aniridia result from PAX6 mutations, a rare autosomal recessive disorder called Gillespie syndrome (MIM 206700) also produces partial aniridia, cerebellar ataxia, mental deficiency, and congen ital cataracts. Aniridia (often with cataract and glaucoma) can also occur sporadically in association with Wilms tumor, other genitourinary anomalies, and mental retardation, the so-called WAGR syndrome. This complex of findings is called a contiguous gene-deletion syndrome because it results from a deletion involVing nearby genes. Most of the affected patients have a karyotypically visible interstitial deletion of a segment of chromosome 11 p 13. This region also includes the gene for the enzyme catalase, and an adjacent locus (l1pI2) contains the gene tor lactic dehydrogenase, LDH-A. Patients with aniridia that is not clearly part of an autosomal dominant trait and those with coincident systemic malformatio ns should undergo chromosomal analysis and observation for possible Wilms tumor. When working with a new patient with aniridia, the ophthalmologist should conduct a careful biomicroscopic examination of the patient's parents for the variable expression of autosomal dominant aniridia. For femal e infants with isolated aniridia, a high-resolution banded chromosomal analysis is essential, as the genital variation caused by 11 P deletion can be extraordinarily subtle. If a male infant with isolated ani ridia has no genital aberrations, a chromosomal analysis is desirable although probably not mandatory (because of the more severe expression of 11 p deletions in males). In older children without other anomalies or developmental delay, a baseline intravenous pyelogram and periodiC urinalYSis (for microscopic hematuria) are recommended. Intravenous pyelography is probably a more sensitive procedure than either echography or computed tomography for the embryonal malignancy associated with this chromosomal deletion. The PAX6 gene product is a transcription factor that is required for the normal development of the eye. Mutations of PAX6 have also been reported in Peters anomaly, autosomal dominant keratitis, and dominant foveal hypoplasia. The mechanism for disruption of normal embryology and the degenerative disease in aniridia and other PAX6 disorders appears to be haploinsufficiency, the inability of a Single active allele to activate transduction of the developmental genes that are regulated by the PAX6 gene product. In this way, aniridia is different from retinoblastoma and Wilms tumor, which result from an absence of both functional alleles at each of the homologous gene loci. de Grouchy J, Turleau C. Clin ical Atlas ofHumarl Chro mosomes. 2nd ed. New York: Wiley; 1984: 208-209. Fearon ER , Vogelstein B, Feinberg AP. Somatic deletion and duplicatio n of genes o n chromo· some 11 in Wilms' tumours. Nature. 1984;309(5964): 176- 178.
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Littlefield JW. Genes, chromosomes, and cancer. JPediatr. 1984;104(4):489- 494. Solomon E. Recess ive mutation in aetiology of Wilms' tumour. Nature. 1984;309(5964): 111 - 112.
Mutations Change in the structure or sequence of a gene is called a mutation. A mutation can occur more or less randomly anywhere along the DNA sequence of a gene and may result when one nucleotide is substituted for another (sometimes called a point mutation). A mutation that occurs in a noncoding portion of the gene mayor may not be of clinical consequence. Similarly, a mutation may structurally alter a protein but in a manner that does not notably compromise its function. A new mutation that compromises function appears in a given gene as the gene is tra nsmitted from parent to offspring at a frequency of approximately I in a million. Mutations are more likely within certain genes than in others. Aniridia has a mutation rate (mutations!locus!generation) of 2.5-5.0 x 10- 6 ; retinoblastoma's rate is 5.0- 12.0 x 10-" Two examples of disorders with even higher mutation rates are von Recklinghausen neurofibromatosis 1 and Duchenne muscular dystrophy, each with an estimated mutation rate of approximately 0.4-1.0 x 10- 4 . A classic example of a simple point mutation is sickle cell anemia, which affects approximately 1 in 600 African Americans. This disorder results from a mutant gene that defines the sequence of ami no acids in the p-polypeptide chain of adult hemoglobin. In sickle cell hemoglobin , the valine is substituted for glutamic acid at the sixth position in the p-polypeptide chain. This substitution is caused by an abnormal specific base, where adenine is substituted for thymine. This seemingly small alteration causes a profound reduction of solubility when hemoglobin is deoxygenated: red blood cells tend to become deformed into a characteristic sickle shape when the partial pressure of oxygen is low. More gross mutations may involve deletion, translocation, insertion, OT internal duplication of a portion of the DNA. Some mutations cause either destruction of the offspring or sterility. Others are less harmful or are potentially beneficial and become established in subsequent generations. Mutations can occur spontaneously for reasons that are not understood. They may also be produced by a variety of environmental agents called mutagens, such as radiation, viruses, and certain chemicals. Mutations may arise in somatic as well as germinal cells, but these are not transmitted to subsequent generations. Somatic mutations in humans are difficult to identify, but some account for the inception of certain forms of neoplasia (eg, retinoblastoma). Jord e LB, Carey Je, Bamshad MI. Wh ite RL. Medical Genetics. Updated ed. for 2006- 2007. 3rd ed. Philadelphia: El sevier Mosby; 2006.
Polymorph isms Many mutations have either little or no deleterious effect on the organism. A polymorphism is defined as the occurrence of 2 or more alleles at a specific locus with a frequency greater than I % each. At least one third of all structural genes may exist in polymorphiC forms. For example, at least 400 variants of hemoglobin are known, many with essentially
208 • Fundamentals and Principl es of Ophthalmology no detectable phenotypic abnormalities. Similarly, several dozen functional and electrophoretic variants of glucose-6-phosphate dehydrogenase exist; again, many have no significant effect on the biochemical function of the affected individual. Finding additional polymorphisms will be important for the completion of gene mapping and for linkage to human diseases.
Genome. Genotype. Phenotype The genome is the sum total of the genetic material within a cell or of an organ ism-thus, the total genetic endowment. By contrast, the genotype defines the genetic constitution, and thus biological capacity, with regard to a specific locus (eg, individual blood groups or a specific single enzyme). Phenotype indicates the total observable or manifest physical, physiologic, biochemical, or molecular characteristics of an individual, which are determined by the genotype but can be modified by the environment. A clinical picture produced entirely by environmental factors that nevertheless closely resembles, or is even identical with, a phenotype is known as a phenocopy. Thus, for example, the pigmentary retinopathy of congenital rubella has occasionally been confused with a hereditary dystrophic disorder of the retinal pigment epithelium (RPE) . Similarly, chloroquine-induced changes in the corneal epithelium resemble those seen as cornea verticillata in the X-linked dystrophic disorder Fabry disease.
Single-Gene Disorders Approximately 4500 different diseases are known to be caused by a defect in a single gene. As a group, these disorders are called monogenic, or mendelian, diseases. They most often show 1 of3 patterns of inheritance; autosomal dominant, autosomal recessive, or X-li nked. Disorders of mtDNA are inherited in a four th manner, termed maternal inheritance. These mtDNA disorders obey galtonian rather than mendelian inheritance characteristics.
Variability Variability is an intrinsic property of human genetic disease that reflects the quantitative and qualitative differences in phenotype among individuals with the "same" mutant allele. Even within the homogeneous population of a single family with a genetic disease, each affected individual may manifest the disease to a different degree, with different features, or at a different age. Steinert myotonic dystrophy, for example, presents its features of motor myotonia, characteristic cataracts, gonadal atrophy, and presenile baldness with a wide variation in severity and age of detectio n. Even within a single family, the cataracts may begin to affect vision any time from the second to the seventh decade of life. Such variability of clinical manifestation led to the concept of anticipation. the phenomenon of apparently earlier and more severe onset of a disease in successive genera-
tions wit hin a family. Before 1990, most geneticists thought that anticipation was not a biological phenomenon but an artifact of ascertainment. With the relatively recent discovery of tripl et or trinucleotide tandem -repeat expansion diseases, anticipation has been
shown to reflect the increased length of trinucleotide tandem repeats from 1 generation to the next. Myotonic dystrophy, fragile-X syndrome, Huntington disease, and a form of
CHAPTER 6, Clinical Genetics. 209 spinobulbar muscular atrophy called Kennedy disease are some of the diseases whose discovery contributed to the rejuvenation of the concept of anticipation. Some human variability may result from the intrinsic differences in genetic background of every human being. Other recognizable or presumptive influences on the variable intra- or interfamilial phenotype of the same gene include the following: sex influences or limitations maternal factors such as intrauterine environment and even cytoplasmic (eg, mito-
chondrial) inheritance factors • modifying loci genetic heterogeneity, including both isoalleles and genocopies gene alterations induced either by position effects with other genes or by somatic mutations
Obviously, nongenetic factors extrinsic to a cell, tissue, or organism such as diet, temperature, and drugs may effect major changes in gene expression, either as phenocopies or through ecologiC parameters.
Penetrance The presence or absence of any effect of a gene is called penetrance. If a gene generates any evidence of phenotypic features, no matter how minimal, it is termed penetrant; if it is not expressed at any level of detection, it is termed nonpenetrant. Thus, penetrance is an all-or-nothing concept, statistically representing the fraction of individuals carrying a given gene that manifests any evidence of the specific trait. In families with an autosomal dominant mutant gene that has 100% penetrance of the phenotype, an average of 50% of the offspring will inherit the gene and show evidence of the disease. Even though penetrance has an exact statistical definition, its clinical ascertainment is affected by diagnostic awareness and the methods of physical examination. For example, many mild cases of Marfan syndrome would be missed without careful biomicroscopy of the fully dilated pupil and echocardiography of the heart valves and great vessels. Simi larly, if the criteria for identification of the retinoblastoma gene include indirect ophthalmoscopy and scleral depression, some "nonpenetrant" parents or Siblings in families with "dominantly inherited" retinoblastoma may be found to have a spontaneously involuted tumor, which clearly identifies them as bearers of the gene. In another example, some family members who have a gene for Best macular dystrophy will be identified not by clinical ophthalmoscopic exami nation but only by electro-oculographic testing. Therefore. in examin ing a potential bearer of a gene. the examiner must carefu lly search for any
manifestations of the gene's effects in all susceptible tissues before dismissing someone as a "skipped generation."
Expressivity The presence of a defective gene does not necessarily imply a complete expression of every potential manifestation. The variety of ways and levels of severity in which a particular genetic trait manifests its presence among different affected individuals is called expressivity.
210 • Fundamentals and Principles of Ophthalmology In von Recklinghausen disease, for example, an affected child may have only cafe-au-Iait spots. The affected parent may have Lisch nodules of the iris, extensive punctiform and pedunculated neurofibromas of the skin, a huge plexiform neurofibroma of 1 lower extremity, and a glioma of the anterior visual pathway. It is extremely rare that all affected members in the same family have uniform textbook presentations of the disorder. Differences in the age of onset of manifestation are one way that expressivity commonly varies in dominant disorders. In von Recklinghausen disease, for example, the affected child may have only cafe-au-lait spots at birth, develop iris Lisch nodules that gradually increase in number and size at about age 5- 10 years, develop punctiform neurofibromas of the skin in early adolescence, experience subareolar neurofibromas postpuberty (females), and experience visual impairment from the effect of an optic glioma in the late teens. Although all of these features are phenotypic components of the mutant gene, each feature has a characteristic age of onset and a natural history of growth and effect within the umbrella of the total disease.
Pleiotropism Alteration within a Single mutant gene may have consequences in various tissues in a given individual. The presentation of multiple phenotypic abnormalities produced by a Single mutant gene is termed pleiotropism. For example, in Marfan syndrome, ectopia lentis is coupled with arachnodactyly, aortic aneurysms, and long extremities. Optic atrophy is found in association with juvenile diabetes mellitus, diabetes insipidus, and moderate perceptive hearing impairment in an autosomal recessive syndrome known as
the DIDMOAD (diabetes insipidus, diabetes mellitus, optic atrophy, and neural deafness) syndrome. Neurosensory hearing loss can also be associated with hereditary hematuric nephritis, lenticular changes (anterior lenticonus, spherophakia, cataracts), arcus juvenilis, and whitish yellow retinal lesions in the dominantly inherited Alport syndrome. Similarly, the Bardet-Biedl syndrome comprises pigmentary retinopathy, obeSity, genital hypoplasia, mental debility, and polydactyly. In each of these disorders, a Single mutant gene is responsible for dysfunction in multiple systems. Frequently, however, a d isease is mistakenly termed "pleiotropic" when several different disorders with the same inheritance pattern and similar clinical manifestations are
actually present. Thus, Leber congenital amaurosis has been attributed to a single pleiOtropic gene. Based on the symmetry of the phenotype of affected siblings in individual families, however, the more likely conclusion is that the clinical disease is heterogeneous and is indeed caused by several genes (not necessarily allelic), each of which is autosomal recessive.
Racial and Ethnic Concentration of Genetic Disorders Most genetic diseases occur without regard to the affected individual's racial or ethnic background. Some, however, are concentrated in certain population groups.
Tay-Sachs disease, with its characteristic macular cherry-red spot, occurs predomi nantly in persons of Eastern European Jewish (Ashkenazi) ancestry, especially those
CHAPTER 6: Clinical
Genetics.
211
whose ancestors lived in northeastern Poland and southern Lithuania. An estimated rate of 1 in 30 for carriers of this disorder in the Jewish population of New York City compares with an estimated carrier rate of 1 in 300 in non-Jewish Americans. Although the reported incidence of this d isorder among Ashkenazi Jewish newborns is 1 in 6000. the actual incidence among this population may be closer to 1 in 3600 births. Approximately 50 new cases occur each yea r in the United States. In addition. familial dysautonomia (Riley-Day syndrome) with hypolacrima. corneal hypoesthesia. exodeviation. and methacholineinduced miosis also occurs more frequently in persons of Ashkenazi ancestry. as do Gaucher disease and Niemann -Pick disease. A variety of achromatopsia
others. For example. polydactyly is approximately IO times more frequent in blacks than in whites. and preauricular sinus may be equally more frequent in blacks.
Patterns of Inheritance Recessivity Versus Dominance The terms dominant and recessive were first used by Gregor Mendel. In classical genetics. a dominant gene is one that is always expressed with similar phenotype. whether the mutant gene is present in a homozygous or heterozygous state. Stated Simply. a dominant gene is one that is expressed when present in only a Single copy. A gene is called recessive when its expression is masked by a normal allele or. more preCisely. when it is expressed only in the homozygote
sion of the gene at a clinical level. rather than the gene itself that is dominant or recessive. A trait is recessive if its expression is suppressed by the presence of a normal gene
212 • Fundame ntals and Principles of Ophthalmology sickle cell hemoglobinopathy is recessive if the clinical disease is considered, dominant if the sickle preparation test is positive, and codominant if hemoglobin electrophoresis is used to look for the specific product of each allele. Although, classically, a dominant gene is one that has the same phenotype when the mutant allele is present in either the heterozygous or the homozygous state, most dominant medical diseases stray from this strict definition. For many dominant disorders, individuals who are homozygous for a mutant allele or who harbor 2 mutant alleles (1 on each homologous chro mosome) will have more severe expression. In experiments, the biochemical mechanisms of "dom inant" hereditary diseases appear different from those of "recessive" disorders. Recessive traits usually result from enzyme deficiency caused by structural mutations of the gene specifying the affec ted enzyme. The altered enzyme often can be shown to be structurally abno rmal or unstable. Heterozygotes usually have approximately 50% of normal enzyme activity but are clinically unaffected, implying that half of the normal enzyme activity is compatible with near-normal function. If adequate biochemical testing can be performed and the specific enzyme isolated, the reduced enzyme activity can be quantified and the heterozygous genetic state inferred. Thus, clinically unaffected heterozygotes can be detected for such disorders as homo,ystinuria (decrease in cystathionine p-synthase), galactokinase deficiency (low blood galactokinase activity), classic galactosemia (galactose-I-phosphate uridyl transferase deficiency), gyrate atrophy of the choroid and retina (decreased ornithine-oaminotra nsferase), and Tay-Sachs disease (decreased hexosaminidase A). Table 6-4 outlines several disorders wit h ocular manifestations for which an enzyme defect is known.
Autosomal Recessive Inheritance An autosomal recessive disease is expressed fully only in the presence of a mutant gene at the same locus on both homologous chromosomes (ie, homozygOSity for a mutant gene) or 2 different mutant alleles at the same locus (compound heterozygosity) . A single mutant allele is sufficient to cause a recessive disorder if the nor mal aUele on the homologous chromosome is deleted. A recessive trait can remain latent through several generations until the chance mating of2 heterozygotes for a mutant allele gives rise to an affected individual. The frequency of heterozygotes for a given disorder will always be conSiderably greater than that of homo zygotes. It is estimated that all hu man beings inherit about 6 or 7 mutations for different recessive disorders for which they are heterozygotes. Enzymatic defects Autosomal recessive diseases often result from defects in enzymatic proteins. Most of the so-called inborn errors of metabolism that result from enzymatic defects are autosomal recessive traits, although a few are X-linked recessive disorders (eg, Lesch-Nyhan syndrome). The defect in alkaptonuria involves homogentisic acid oxidase, an enzyme involved in the metabolism of homogentisic acid. Large amounts of homogentiSic acid are excreted in the urine, which turns black when mixed with alkali or exposed to light or air. The black urine causes diaper stai ns, calling attention to the cond ition. In addition, aggregates of homogentisic acid accumulate in the body, becoming attached to the collagen of cartilage and other connective tissues. The cartilage of the ears and nose and the collagenous sclera are stained black or brownish blue. These manifestations are called
Table 6-4 Known Enzyme Disorders and Corresponding Ocular Signs Defective Enzvme
Ocular Sign
Fabry disease
Ceramide trihexosidase (a-galactosidase)
Krabbe leukodystrophy
Cerebroside a-ga lactosidase
Mannosidosis Metachromatic leukodystrophy Hurl er (mucopolysaccharidosis
a-mannosidase Arylsulfatase A
Corneal epithelial verticil late changes; aneurysmal dilation and tortuosity of retinal and conjunctival vessels Macular cherry-red spot; optic atrophy Lenticular opacities Retinal discoloration, degeneration Corneal opacity; pigmentary retina l degeneration Corneal opacity (mi ld type); older-age patients Corneal opacity; pigmentary retinal degeneration Pigmentary retin al degeneration; optic atrophy Macular cherry-red spot; optic atrophy Macular cherry-red spot
Disorder Storage diseases
u-l -iduronidase
I H)
Hunter (mucopo lysaccharidosis
Sulfoiduronate sulfatase
II I Scheie (mucopolysaccharidosis
u-l -iduronidase
15)
Sanfilippo (mucopolysaccharidosis III) Tay- Sachs disease (GM 2 gangliosidosis, type I) Sandhoff disease (GM 2 gangliosidosis, type II) GM 1 gangliosidosis, type I (generalized gangliosidosis)
Heparan sulfate sulfatase Hexosaminid ase A Hexosidase A and B p-galactosidase
Macular cherry-red spot; optic atrophy; corneal clouding (mild)
Alkaptonuri a Albinism
Hom ogen tisic acid oxidase Tyrosinase
Intermittent ataxia Crigler-Najjar syndrome Ehl ers-Danlos syndrome VI
Pyruvate dicarboxylase Glucuronide transferase Lysyl hydroxylase
Familial dysautonomia
Dopamine-p-hydroxylase
Galactokinase deficiency Galactosemia
Galactokinase Galactose-1-phosphate uridyl transferase Ornithine aminotransferase
Dark scle ra Foveal hypoplasia; nystagmus; iris transillumination Nystagmus Extraocular movement Microcornea;' retinal detachment; ectopia lentis; blue scleras Alacrima; cornea l hypoesthesia; exodeviation; methacholine-induced miosis Cataracts Cataracts
Metabolic disorders
Gyrate atrophy of the choroid and retina Hom ocystinuria Hyperglycinemia Leigh necrotizing encephalopathy Maple syrup urine disease Niemann-Pick disease Refsum syndrome Tyrosinosis Sulfite oxidase deficiency Tyrosinemia
Cystathionine synthase Glycine cell transport Pyruvate carboxy lase
Degeneration of the choroid and retina; cataracts; myopia Dislocated lens Optic atrophy Optic atrophy
Branch chain decarboxylase Sphingomyelinase Phytanic acid oxidase Tyrosine aminotransferase Sulfite oxidase Tyrosine aminotransferase
Ophthalmoplegia; nystagmus Macular cherry-red spot Retinal degeneration Corneal dystrophy Ectopia lentis Lens opacity
214 • Fundamentals and Principles of Ophtha lmology
ochronosis. The coloration in the sclera assumes a more or less triangular form, with a limbic base in the region of the palpebral tissue. In the joints, such as those of the spine, the accumulations lead to arthritis. Alkaptonuria is an example of a genetic enzyme block in which the phenotypic features are caused by the accumulation of excess substances just proximal to the block. In some other disorders with genetic blocks in metabolism, the phenotypic consequences are related to the lack of a normal product distal to the block. An example is albinism, in which the metabolic block involves a step between the amino acid tyrosine and the formation of melanin. In still other inborn errors of metabolism, the phenotypic expression results from excessive production of a product through a normally alternative and minor metabolic pathway. Phenylketonuria, like alkaptonuria and albinism, is a genetic defect in aromatic amino acid metabolism. The defect is in the enzyme involved in the conversion of phenylalanine to tyrosine. In an affected person, hair and skin pigmentation is reduced. Severe mental retardation is one of the most prominent symptoms. Alternative metabolites of phenylalanine, especially phenylpyruvic acid, are excreted in the urine, providing one basis for diagnosis of the disorder. The difference in phenotype of these 3 d iseases- alkaptonuria, albinism, and phenylketonuria-is noteworthy, although the diseases involve closely related metabolic pathways.
Carrier heterozygotes The heterozygous carrier of a mutant gene may show minimal evidence of the gene defect, particularly at a biochemical level. Thus, carrier hetero·zygotes have been detected by a variety of methods: • identification of abnormal metabolites by electrophoresis (eg, galactokinase defiCiency) liver biopsy (eg, phenylketonuria) hair bulb assay (eg, oculocutaneous albinism and Fabry disease) monitoring of enzyme activity in leukocytes (eg, galactose-I -phosphate uridyl transferase in galactosemia), fibroblasts from skin culture (eg, ornithine-oaminotransferase defiCiency in gyrate atrophy of the retina and choroid), serum, and tears (eg, hexosaminidase A in Tay-Sachs disease) In contrast to the transmission of dominant traits. most matings resulting in recessive
disorders involve phenotypically normal heterozygous parents. Out of 4 offspring produced by carrier parents with the same gene for an autosomal recessive disease, usually I will be affected (homozygote), 2 will be carriers (heterozygotes), and I will be genetically and phenotypically normal. Thus, clinically normal heterozygous parents will produce offspring with a ratio of I clinically affected to 3 clinically normal. There is no predilection for either sex. In 2-child families, the patient with a recessive disease is frequently the only affected family member. For instance, approXimately 40%- 50% of patients with RP have no family history of the disorder. However, their age of onset, rate of progression, and other phenotypic characteristics are similar to those with defined recessive inheritance
patterns.
CHAPTER 6: Clinical Genetics. 215
Once 1 child is born with a recessive disorder, the genetic risk for each subsequent child of the same parents is 25%. This concept has specific implications for genetic counseling. All offspring of an affected individual will be carriers; they are unlikely to be affected with the disorder unless their clinically unaffected parent is also by chance a carrier of the gene. However, because a specific method for identifying a carrier is lacking with most recessive diseases, the normal-appearing sibling of a child with a recessive disorder has a statistical risk of 2 chances in 3 of being a genetic carrier. This liability must be accounted for in any equation to predict the small risk that a normal-appearing Sibling will have an affected child.
Consanguinity The mating of close relatives can increase the probability that their children will inherit a homozygous genotype for recessive traits, particularly for relatively rare ones. For exam-
ple, the probability that the same allele is present in first cousins is 1 in 8. In the offspring of a first-cousin marriage, 1 of every 16 of the genes is commonly present in a homozygous state. It follows that each offspring from a first-cousin marriage has a 1 in 16 chance of manifesting an autosomal recessive trait within a given family. Approximately 1% of all marriages may be consanguineous. A vigorous search for consanguinity between the parents should be made in any case of a rare recessive disease. Although its occurrence is
not rare, incest is a form of consanguinity that is often not acknowledged. The expression of common recessive genes, by contrast, is less influenced by inbreed-
ing, because most homozygous offspring are the progeny of unrelated parents. This is usually the case with such frequent disorders as sickle cell disease and cystic fibrosis. The characteristics of autosomal recessive inheritance are summarized in Table 6-5. Fran~ois
J. Heredity in Ophthalmology. 5t Louis: Mosby; 1961:86-92.
Pseudodominance Occasionally, an affected homozygote mates with a heterozygote. Of their offspring, 50% will be carriers and 50% will be affected homozygotes. Because this segregation pattern mimics that of dominant inheritance, it is called pseudodominance. Fortunately, such matings are usually rare and are unlikely to affect more than 2 vertical generations.
Table 6-5 Characteristics of Autosomal Recessive Inheritance The mutant gene usually does not cause clinical disease (recessive) in the heterozygote. Individuals inheriting both the genes (homozygote) of the defective type express the disorder. Typically, the trait appears only in siblings, not in their parents or offspring or in other relatives. The ratio of normal to affected in a sibship is 3:1. The larger the sibship, the more often will more than one child be affected. The sexes are affected in equal proportions. Parents of the affected person may be genetically related (consanguinity); the rarer the trait, the more likely. Affected individuals have children who, although phenotypically normal, are carriers (heterozygotes) of the gene.
216 • Fundamentals and Principles of Ophthalmology
Familial penetrance Penetrance of recessive disorders within families is rarely if ever incomplete. Expressivity of recessive disorders is characteristically more uniform among affected siblings within
families, as each affected individual apparently has a double dose of the same gene. However, age of onset, severity, and rate of progression may vary appreciably among families with the same apparent genetic disease. These variations may reflect intrinsic constitu-
tional differences among families or the modifying effects of unrelated, unknown genes in different families. Alternative (even nonallelic) genes, which cause distantly similar phenotypic diseases, may cause dissimilar expression, as might environmental modifiers.
If the homozygote is defined as a specific base (pair) substitution in a codon, many "autosomal recessive" diseases result from genetically compound heterozygotes- that is,
individuals who have 2 different (but both "defective") alleles at a given locus. Whenever detailed biochemical or molecular testing becomes possible, the products of different alleles will show slightly different properties or behaviors. Hemoglobin sickle cell disease and Hurler-Scheie syndrome are well-established compound heterozygote disorders.
Autosomal Dominant Inheritance When an autosomal allele leads to a regular, clearly definable abnormality in the heterozygote, the trait is termed dominant. The first pedigree to be interpreted in terms of mendelian dominant inheritance was a family with brachydactyly (short fingers) reported by Farabee in 1903. Autosomal dominant traits often repre~ent defects in structural nonen-
zymatic proteins, such as in fibrillin in Marfan syndrome or collagen in Stickler syndrome. In addition, a dominant mode of inheritance has been observed for some malignant neo-
plastic syndromes, such as retinoblastoma, von Hippel-Lindau disease, tuberous sclerosis, and Gardner syndrome. Although the neoplasias in these diseases are inherited as autosomal dominant traits, the tumors themselves result from loss offunction of both alleles of autosomal recessive tumor-suppressor genes.
Almost all bearers of dominant disorders in the human population are heterozygotes. In dominant inheritance, the heterozygote is clinically affected, and a single dose of the mutant gene interferes with normal function. Occasionally, depending on the frequency of the abnormal gene in the population and the phenotype, 2 bearers of the same abnormality marry and produce children. Any offspring of 2 heterozygous parents has a 25% risk of being an affected homozygote. This circumstance has been recorded in achondroplastic dwarfism. The homozygous achondroplastic dwarf has severe cranial and thoracic skeletal disorders and dies at an early age. Because the heterozygote has 1 normal allele and the homozygote has none, it is not surprising that the phenotype of the homozygote is more severely abnormal. Homozygotes (or double heterozygotes) for autosomal dominant RP also appear to have a much more severe form of retinal degeneration. It has been suggested that dominant diseases are caused by mutations affecting struc-
tural proteins, such as cell receptor grow1h factors (eg, FGFR2 in erouzon disease), or by functional deficits generated by abnormal polypeptide subunits (eg, unstable hemoglobins). The dominant disorders aniridia and Waardenburg syndrome result from loss of 1 of the 2 alleles for the developmental transcription factors PAX6 and PAX3, respectively.
CHAPTER 6:
Clinical Genetics.
217
However, it is not at all clear exactly how a single gene abnormality can produce the pleiotropic manifestations of such dominant diseases as von Recklinghausen neurofibromatosis or tuberous sclerosis.
In some instances, dominantly inherited traits are not clinically expressed. In other instances- such as with some famili es with autosomal dominant RP-pedigree analysis infrequently shows a defective gene in individuals who do not manifest any discernible clinical or functional impairment. This situat ion is called incompLete penetrance, or
skipped generation. Conclusive evidence of autosomal dominant inheritance requires demonstration of
the disease in at least 3 successive generations. Transmission of the disorder from male to male, with both sexes showing the typical disease, must also occur. The characteristics of autosomal do minant inheritance with complete (lOO%) penetrance are summarized in Table 6-6. In the usual clinical situation, any offspring of an affected heterozygote with a dominant disorder has I chance in 2 of inheriting the mutant gene and thereby demonstrating some effect, regardless of sex. The degree of variability in the expression of certain traits is usually more pronounced in autosomal dominantly inherited disorders than in other types of genetic disorders. Moreover, when a clinical disorder is inherited in more
than 1 mendelian pattern, the dominantly inherited disorder is, in general, clinically less severe than the recessively inherited one.
Counseling for recu rrence risk of autosomal dominant traits must involve thorough examination of not onl y the affected person (who may have the full syndrome) but also the parents. If I parent is even mildly affected, the risk of additional genetically affected siblings rises to 50%. It is unacceptable to miss va riable expressivity when parents and other family members can be examined. In some ocular disorders, family members can inherit a gene for a dominant trait and not show clinically apparent manifestations; electrophysiologic testing must be used to detect the impairment. An example is Best vitelliform macular dystrophy, in which clinically normal family members can be diagnosed as haVing the gene for this disorder only by the presence of an abnormal electro-oculographic light:dark (peak:trough) ratio. Table 6-6 Characteristics of Autosomal Dominant Inheritance With Complete
Penetrance Th e trait appears in multiple generations (vertical transmission). Affected males and females are equally likely to tra nsmit the trait to male and fema le offsp ring. Thus. male-to-male tran smission occu rs. Each affected individual has an affected parent, unless the co ndition arose by new m utati on in the given individual. Males and fem ales are affected in equal proportion s. Unaffected person s do not transmit the trait to their children . Th e trait is expressed in the heterozygote but is more seve re in the hom ozygote. The age of fathers of isolated (new mutation ) cases is usua lly advanced. The more seve rely the tra it interferes with surviva l and reproduction. the greater the proportion of isolated (new mutation) cases. Variability in expression of the trait from generation to generation and between individu als in the same generation is ex pected . Affected persons transmit the tra it to 50% of their offspring on average.
218 • Fundamentals and Principles of Ophthalmology
X-Linked Inheritance A trait determined by genes on either of the sex chromosomes is properly termed sexlinked. This genetic pattern became widely known with the occurrence of hemophilia in European and Russian royal families. The rules governing all modes of sex-linked inheritance can be derived logically by considering the chromosomal basis. Females have 2 X chromosomes. 1 of which will go to each ovum. Males have both an X and a Y chromosome. The male parent contributes his only X chromosome to all his daughters and his only Y chromosome to all his sons. Traits determined by genes carried on the Y chromosome are called holandric and are transmitted from a father to 100% of his sons. Among these Y chromosomal genes is the testis-determiningjactor (TDF-also called sex-determining region Y, or SR¥). Genes controlling tooth size. stature. and spermatogenesis are also on the Y chromosome. Finally. a gene determining hairy pinnae (ie. hai r on the outer rim of the ear) may also be located on the Y chromosome. All other sex-linked traits or diseases are thought to result from genes on the X chromosome and are properly termed X-linked. Some X-linked conditions have considerable frequencies in human populations; the various protan and deutan color vision defects were also among the first human traits assigned to a specific chromosome.
The distinctive feature of X-linked inheritance. both dominant and recessive. is the absence of father-to -son transmission. Because the male X chromosome passes only to daughters. all daughters of an affected male will inherit the mutant gene.
X-linked recessive inheritance A male has only I representative of any X-linked gene and therefore is said to be hemizygo us for the gene. rather than homozygous or heterozygous. Because there is no normal gene to balance a mutant X-linked gene in the male. its resulting phenotype. whether dominant or recessive. will always be expressed. A female may be heterozygous or homozygous for a mutant X-linked gene. X-linked traits are commonly called recessive if they are caused by genes located on the X chromosome. as these genes express themselves fully only in the absence of the normal allele. Thus. males (with their single X chromosome) are predominantly affected . All their phenotypically healthy but heterozygous daughters are carriers. By contrast. each son of a heterozygous woman has an equal chance of being normal or hemizygously affected. A female will be affected with an X-linked recessive trait under a limited number of circumstances: She is homozygous for the mutant gene by inheritance (ie. from an affected father and a heterozygous mother). Her mother is heterozygous and her father contributes a new mutation. She has Turner syndrome. with only 1 X chromosome. and therefore is effectively hemizygous. She has a partial deletion of 1 X chromosome. either by rearrangement or by formation of an isochromosome. and is thereby effectively hemizygous . • She has a highly unusual skewing of inactivation of her normal X chromosome. as explained by the Lyon hypothesis (discussed later in the chapter in Lyonization) . • Her disorder is actually an autosomal genocopy of the X-linked condition.
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Table 6-7 summarizes the characteristics of X-linked recessive inheritance, which should be considered if all affected individuals in a family are males, especially if they are related through historically unaffected women (eg, uncle and nephew, or multiple affected half brothers with different fathers). X-linked dominant inheritance X-linked dominant traits are caused by mutant genes expressed in a single dose and carried on the X chromosome. Thus, both heterozygous women and hemizygous men are clinically affected. Females are affected nearly twice as frequently as males. All daughters of males with the disease are affected. However, all sons of affected males are free of the trait unless their mothers are also affected. Because only children of affected males provide information in discriminating X -linked dominant from autosomal dominant disease.
it may be impossible to distinguish these modes on genetic grounds when the pedigree is small or the available data are scarce. Some X-linked dominant disorders, such as incontinentia pigmenti (Bloch-Sulzberger syndrome), may prove lethal to the hemizygous male. X-linked hypophosphatemic rickets (vitamin D-resistant rickets) is an example of an X-linked dominant disease. The characteristics of X-linked dominant inheritance are summarized in Table 6-8. Jorde LB. Carey JC, Bamshad MJ, White RL. Medical Genetics. Updated ed. for 2006- 2007. 3rd ed. Philadelphia: Elsevier Mosby; 2006.
X-linked disorders Females with X-linked diseases. have milder symptoms than males. Occasionally, males may be affected severely enough that they die before the reproductive period, thus preventing transmission of the gene. Such is the case with Duchenne muscular dystrophy, in which most affected males die before their midteens. In other disorders, males are so
Table 6-7 Characteristics of X-linked Recessive Inheritance Usually only males are affected. An affected male transmits the gene to all of his daughters (obligate carriers) and none of his sons. All daughters of affected males, even those phenotypically normal, are carriers. Affected males in a family either are brothers or are related to one another through carrier females (eg, maternal uncles). If an affected male has children with a carrier female, 50% of their daughters will be homozygous and affected and 50% will be heterozygous and carriers. Heterozygous females may rarely be affected (manifesting heterozygotes) because of Iyonization. Female carriers transmit the gene on average to 50% of their sons, who are affected, and to 50% of their daughters, who will in turn be carriers.
Table 6-8 Characteristics of X-Linked Dominant Inheritance
-------------------
Both males and females are affected, but the incidence of the trait is approximately twice as great in females as in males (unless the trait is lethal in the male). An affected male transmits the trait to all of his daughters and to none of his sons. Heterozygous affected females transmit the trait to both sexes with equal frequency. The heterozygous female tends to be less severely affected than the hemizygous male.
220 • Funda mentals and Principles of Ophthal mology severely affected that they die before birth, and only fe males survive. Fam ilies with such disorders would include onl y affected daughters, unaffected daugh ters, and normal sons at a ratio of I: I: 1. Incontinentia pigmenti is one such lethal genet ic disorder. Perinatally, affected females develop an eryt hematous, vesicular skin eruption, which progresses to marbled, cur vilinear pigmentation. The syndrome incl udes dental abnormalities, congenital or secondary cataracts, proliferative retinopathy and pseudogliomas, and tractional retinal detachment. Among the most severe X- linked dominant disorde rs with lethality fo r the hemizygous males is Aicardi syndrome. No verified birth of males with this entity has ever been reported, although several XXY pseudomales have been reported. Females have profound mental and developmental retardation; muscular hypotonia; blindness associated with a characteristic lacunar juxtapapillary chorioretinal dys plasia and optic disc anomalies; and central nervous system abnormalities. the most common charac teristic of which is agenesis of the corpus callosum . No recur rences have been reported among siblings, and parents can be reassu red that the risk in subsequent childre n is mi nimal. All instances of the disease appear to arise fro m a new X-dom inant lethal mutation, and females do not sur vive long enough to reproduce. The critical area appea rs to be on the distal end of the short arm of the X chromosome, because some patients with a deletion in this region have also been shown to have feat ures of Aicardi syndrome.
Maternal Inheritance W hen nearl y all offspring of an affected woman appear to be at risk for inheriting and express ing a trait, and the daughters are at risk for passing the tra it on to the next generation, the pattern of inheritance is called maternal inheritance. The d isease stops with all-male offs pring, whether affected or not. This form of inheritance is highly suggestive of a mitochondrial d isorder. The structure and molecular aspects of the mitochond rial genome and a ge neral discussion of mitochondrial disease are covered in Chapter 5, Molecular Genetics.
L onization In classical human genetics, females with a gene fo r a recessive disease or trait on only I X ch ro mosome should have no manifestations of the defect. However, ophthalmic examples of structu ral and functional abnormalities in females heterozygous for sup posedly recessive X-linked tra its abound. Such carrier states, usuall y mild but occasionally severe, have been described in carriers of choroideremia, X-linked Nettieship-Falls ocular albinism, X-linked RP, X-linked sutural cataracts, Lowe syndrome, Fabry d isease, and color vision defects of the protan and deutan types, among others (Fig 6-4; Table 6-9). Detection of these carrier states of the X-linked traits has become cl inically relevant, especiall y fo r sisters and matern al aunts of affec ted males. In 196 1, Mary Lyo n (a British geneticist) advanced an explan ation fo r the unanticipated or partial expression of a trait by a heterozygous female. Briefly, Iyonization (X-chromosome inacti vation) stated that in every somatic cell of a female, only I X chromosome is actively functioning. The second
CHAPTE R 6:
Cli nica l Genetics .
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A
Figure 6-4 A, Yellow, "gold-dust" tapetal-like reflex in the left retina of a carrier for X-linked reti nitis pig-
mentosa . B, Nasal mid peripheral retina in the left eye of a carrier for X-linked retinitis pigmentosa,
showing patchy bone spicule-like pigment clumping. C, Peripheral retina from the left eye of a car-
rier of choroideremia, showing a "moth-eaten" fundus appearance from areas of hypopigmentati on
and hyperpigmentation .
B
(continued)
c X chromosome is inact ive and form s a densely stain Lng marginal nuclear structure dem onstrated as a Barr body in a buccal smear or in "dr umsticks;' pedun culated lobu les of th e nucleus ident ified in about 5% of the leukocytes of th e normal female. Wa rburg reasoned that X-chromosome inactivation occu rs betwee n approximately 6 and 11 days after fertili zat ion, before the process of em bryon ic latera lization at 11- 16 days of embryogenesis. The "decisio n" to inac ti vate 1 X chromosome is random, but once it is made, the sam e X chromosome will be irreversibly inactive in every daughter o f each of these "committed"
222 • Fund amenta ls and Principles of Ophtha lmology
Figure 6·4 0, Characteristic iris transillumination from a ca rrier of X-linked ocular albinism. E, Midperipheral retina from the left eye of a carrier f or ocular albinism, showing a chocolate brown pigmentation from areas of appa rently enhanced pigmentation and clusters of hypoplgmen tation.
o
E
Table 6-9 Ocular Findings in Carriers of X-Linked Disorders Disorder
Ocular Findings
X-linked retinit is pigmentosa Choroideremia
Regional fundus pigmentary changes , "gold -dust" tapetal -like reflex; ERG amplitud e and impl icit time abnormalities "Moth -eaten" fundus pigmentary changes, with areas of hypopigmentation, mottling, and pigment clumping in a striated pattern near the equ ator Chocolate brown c lusters of pigment prominent i n the midperipheral retina; mottling of macular pigment; i ris transil lumination Reductions in ERG oscillatory potentials
Ocular albinism
Congenital stationary night blindness with myopia Blue-cone monochromatism Red-green color vision deficiencies (protan and deutan )
Lowe syndrom e Fabry disease
Abn orma lities in cone function on ERG, psychophysical thresholds, and color vis ion testing Abnormally wide or displaced color match on a Nagel anomaloscope; decrease in sensitiv ity to red light in protan carriers (Schm idt sign) Scatte red punctate lens opacities on sliHamp examination Fing erprint or whorl -like (verticillate ) changes within the corneal epithelium
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primordial cells. With only I X chromosome "functioning:' the active gene is dominant at a cellular level. Thus, a heterozygous female for an X-linked disease will have 2 clonal cell populations (mosaic phenotype), I with normal activity for the gene in question and the other with mutant activity. The proportion of mutant to normal X chromosomes inactivated usually follows a normal distribution, because presumably the inactivations in va rious cells are random events. Thus, an average of 50% of the paternal X chromosomes and 50% of maternal X chromosomes are inactivated. It is conceivable, however, that in some cases the mutant X is active in almost all cells; in other cases, the mutant X is inactivated in nearly all cells. By this mechanism, a female may express an X-linked disorder, and rare cases are known of women who have a classic color defiCiency or X-linked ocular albinism, X-linked RP, or choroideremia. Some possible clinical implications of X inactivation are the following: The abnormalities in carrier females from different families and even within the same family may vary greatly in degree because of random inactivation and the resultant tissue derivatives containing di ffering proportions of the active X cells. • A large population sample should have as many severely affected as mildly affected heterozygous carriers . • In some tissues, both normal and abnormal areas could be found if a known biochemical defect could be mapped in a carrier, especially if the gene product is nondiffusible. For example, each hair bulb is ultimately derived from a single primordial cell. Therefore, biochemical analysis of hair roots may demonstrate the mutant phenotype directly: In Fabry disease, X-linked Nettleship-Falls ocular albinism, and Lesch -Nyhan syndrome, the scalp is a mosaic of hairs that have either normal or defective enzyme activity but not an intermediate activity. In certain diseases, interactions betwee n cells with the normal X active alter the ability of cells with the mutant X active to survive. The enzyme hypoxanthine phosphoribosyltransferase travels from "normal" skin fibroblasts through gap junctions to mutant celis, allOWing them to survive. In other situations, normal cells survive by favorable growth characteristics. In bone marrow and white blood cells, there is progressive elimination of the abnormal X-active cells in the heterozygote and protective survival of normal X-active ceUs. Carriers of the X-linked va riety of Nettleship-Falls ocular albinism may have a mottled mosaic fundus: in the pigmented retinal epithelial cells, the normal X chromosome is active; in the nonpigmented cells, the mutant X is active. However, these distinguishing features of the carrier state are not always present. The possibility that the patient is a carrier cannot be entirely eliminated if a given sign is not present, because a female might have undergone chance inactivation of the mutant X chromosome in most of her primordial cells, which evolved into the speCific tissue observed and may appear phenotypically normal. This subtlety is even more important in evaluating family members with X-linked disease if the phenotypic carrier state is age-dependent; thus, even in obligate carrier females for Lowe syndrome, lenticular cortical opacities are not necessarily seen before the third decade of life.
224 • Fundamentals and Principles of Ophthalmology Krill AE. X-chromosomal -linked diseases affecting the eye: status of the heterozygote female. Trans Am Ophtha/mol Soc. 1969;67:535 - 608.
Warburg M. Random inactivities of the X chromosome in intermediate X-linked retinitis pigmentosa. Two hypotheses. Trans Ophthalmol Soc UK. 197 1;91: 553 -560.
Polygenic and Multifactorial Inheritance In chromosomal and mendelian (single-gene) disorders, genetic analysis of phenotypic, biochemical, or molecular parameters is imperative. However, a simple mode of inheritance cannot be assigned and a recurrence risk cannot be predicted for many common normal characteristics or disorders for which genetic variability clearly exists. Such traits as stature, facial features, refractive error, lOP, iris color, and intelligence are usually distributed as a continuous variation over a wide range without sharp distinction between
normal and abnormal phenotypes. This distribution contrasts with the bimodal curve noted in conditions transmitted by a single gene. Common diseases are often superim-
posed on this substrate of normal variation, perhaps with a threshold level beyond which individuals may be regarded as abnormal. Consequently, the level of blood sugar in diabetes mellitus, the level ofIOP for glaucoma, or the intermedial canthal distance for telecanthus is somewhat arbitrary. Such conditions are often termed polygenic, implying that they result from the operation of multiple collaborating genes, each with rather minor additive but individually indeterminate effects. The term multifactorial denotes a combination of g.e nehc and environmental factors in the etiology of disease without specifying the nature of the genetic influence. Examples of these factors in humans include intelligence, stature, blood pressure. atherosclerosis, and refractive index of the eye. The distinction between polygenic and multifactorial inheritance is one not of exclusion but rather of emphasis: most diseases can be thought of as constituting a spectrum of varying degrees of relative importance of the genetic and nongenetic factors in their causation.
Counseling for recurrence may be difficult in this type of inheritance. Ideally, empirical data are summarized from exhaustive analyses of similarly affected families in the population. Regrettably, such empirical data are rarely available for ophthalmic disorders. However, several general gUidelines can be offered. If 1 offspring has the defect (such as cleft lip/palate) and the parents are normal, the chance that a subsequent child will inherit a similar set of genes and thus manifest the same type of malformation is considerably higher than the frequency of the defect in the general population but much lower than the risk of a mendelian defect. The usual estimate for such recurrence is 5% or less for
common polygenic diseases. In addition , the more severe the abnormality in the index case, the higher the risk of recurrence of the trait in relatives, presumably because either a greater number of deleterious genes are at work or a fixed population of more harmful
genes exists. The risk that an affected individual will have an affected offspring is also approximately 5%, similar to the recurrence risk in siblings. The risk of recurrence in future
children is increased when more than 1 member of a family is affected, which is not true for mendelian disorders. Such observations have been offered for various forms of strabismus, glaucoma, and significant refractive errors.
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Polygenic traits with a threshold (either present or absent) may be much more fre ~ quent in I sex if the threshold is sex~influenced . For example, isolated cleft lip is more common in males and isolated cleft palate is more common in females. The risk of recurrence should be higher among relatives of index cases of the less susceptible sex, who are genetically more highly predisposed or who carry more deleterious genes. For example, perinatal pyloric stenosis is much more common in males than females. Thus, if a female infant is affected, she presumably has either more deleterious genes or a higher personal liability to manifest the trait and, accordingly, a greater likelihood of having an affected sibling or relative. Finally, if the malformation or disorder has occurred in both paternal and maternal relatives, the recurrence risk is distinctly higher because of the consanguineous sharing of multiple unspecifiable but potentially harmful genes in their offspring. Such empirical risks clearly increase the likelihood of diabetes mellitus in the offspring of 2 affected par~ ents, even if neither parent has an antecedent family history of the disease. Jorde LB, Carey IC, Bamshad MI . White RL. Medical Genetics. Updated ed. for 2006-200 7. 3rd ed. Philadelphia: Elsevier Mosby; 2006.
Pedigree Analysis Recording a family history for general medical and eye disease is an essential part of an ophthalmologic consultation. Family data can be summarized in a pedigree chart, a short ~ hand method for recording data· for visual reference. The word pedigree is derived from the French expression pied de grue, or "crane's foot;' from the branching pattern of the diagram. The affected individual who brings a family to the attention of the phYSician is the proband (propositus or propos ita). The person seeking counseling is most frequently identified as the consultand. The most commonly used symbols for drawing a pedigree are shown in Figure 6~5. In human pedigree charts, the usual practice is to place the male symbol first on the left; breeding records of other species generally list the female symbol first. Accurate completion of the pedigree drawing is essential to its interpretation. The health history of family members may be as important as the ocular history. The inter~ viewer should inquire specifically about abortions, stillbirths, and deceased family mem ~ bers. Often, information on these individuals is erroneously omitted, and prenatal or postnatal lethal disorders or relevant medical and genetic causes of death are overlooked. Ages at death may be useful in specific situations and can be recorded directly near the appropriate symbols. For example, a clinician evaluating a child with ectopia lent is and no family history of similar ocular disease can find the identification of a relative deceased from a dissecting thoracic aortic aneurysm in his fourth decade of life very informative, leading to a tentative consideration of Marfan syndrome in the differential diagnosis. The casual observation in a young adult of multiple patches of congenital hypertrophy of the RPE in each eye may stimulate the recognition of a parent deceased at age 50 from meta~ static adenocarcinoma of the colon, and a sibling deceased from a brain tumor at 10 years
226 • Fundamentals and Principles of Ophthalmology
PEDIGREE SYMBOLS
0=
0= D= '..0= .= .= ~=
0=
male two males proband male deceased male affected male affected by hisl0ry (male) examined male
00= 60= (0) =
fraternallwins
identical twins (male) adopted male
0= 0= '0= .= @= 0= 0=
two females proband female
0= 0 = $ =
sex unknown
lived one day stillbirth
deceased female
Q
affected female
~'~> =
miscarriage
',-
affected by history (female) examined temale carner
6'0 00 (0)
Figure 6-5
temale
=
fralernallwins
pregnancy
0--0=
marriage
[}=O=
consanguineous marriage
0:-0 ,
=
extramarital mating
[]flO
=
divorce or separation
~=
=
identical twins (female)
=
adopted fema le
no children
Symbols co mmonly used for pedigree analysis.
of age, thus leading to a diagnosis of Gardner syndrome and referral to a gastroenterologist for further diagnostic evaluation. The interviewer should always clarify whether brothers and sisters are half Siblings or full siblings. This procedure may not only limit the possible patterns of inheritance but also identify other individuals at risk for the disorders under consideration. Occasionally, information about parentage must be pursued aggressively (but always privately and confidentially). In the United States in 1990, 28% ofbabies-3 of every II-were born to parents who were not legally married. The frequency of offspring born to teenage mothers outside of conventional marriages ranges from 30% to 80%. Both incest and nonpaternity are sensitive issues, but clearly neither is rare in our society. The national nonpaternity rate is estimated at 5%, but in some urban settings it may be as high as 15%. In considering rare autosomal recessive diseases, the interviewer must ask specifically about consanguinity in the following ways: searching for common last names in the families of both parents finding identical birthplaces for the parents identifying similar parental backgrounds from known ethnic or religiOUS isolates
Genetic Counseling The ophthalmologist who understands the principles of human genetics has a foundation for counseling patients about their diseases. Genetic counseling imparts knowledge of human disease, including a genetic diagnosis and its ocular and systemic implications,
CHAPTE R 6: Clinical Genetics. 227
and information abo ut the risk of occurrence or recurrence of the disorder within the family. It also encourages an open discussion of the options for reproduction. All genetic counseling is predicated on certain essential requirements.
Accurate diagnosis: The physician must be sufficiently aware of the range of human ocular pathology to derive an accurate and specific diagnosis. It is impossible to counselor refer patients on the basis of "congenital nystagmus" or "color blindness" or "macular degeneration"; these are signs, not diagnoses.
Complete family history: A family history will narrow the choices of possible inheritance patterns, but it may not necessarily exclude new mutational events, isolated occurrences of recessive diseases, and chromosomal rearrangements in individual
circumstances. The ophthalmologist must exam ine (or arrange to have examined) the parents, Siblings, and other family members for mild manifestations of dominant diseases or characteristic carrier states in X-linked disorders. Identification
of 1 young adult with the findings of Usher syndrome-prelingual deafness; night blindness; visua l field constriction; and. ultimately. deterioration of central vision-
obligates the ophthalmologist to evaluate a younger sibling who is congenitally deaf but "historically" has no eye problems. The probability is overwhelming that the Sibling has the same disease. Only an ophthalmologist will be cognizant of and attentive to the atypical findings of hereditary ocular disorders. Understanding the genetic and clinical aspects of the disorder: The ophthalmologist should appreciate, perhaps more intimately than any other physician, how some clinically similar diseases inherited in the same pattern may be the result of different and even nonallelic defects. For example, the visual implications of and prognosis for tyrosinase-positive and tyrosinase-negative oculocutaneous albinism are
considerably different. Some entities that are clinically similar may be inherited differently and thus have a different impact on other family members. In both its autosomal dominant and recessive modes, pseudoxanthoma elasticum is often a
late-onset disease that has serious implications for cardiovascular disease, stroke, and gastrointestinal bleeding. Informed counseling falls short if the ophthalmologist advises only about visual disability associated with angioid streaks without attention to the complete disease and risks to other family members.
Issues in Genetic Counseling The ophthalmologist must remember that an individual affected by a heritable condition may represent a homozygous recessive trait; thus, the ophthalmologist should search for parental consanguinity or ambiguous parentage (nonpaternity. incest, and even occult
adoption) or for a new mutation and should inquire about advanced paternal (or maternal grandpa rental) age. Heterogeneity may confuse the diagnosis. Somatic mutations also occur, as with segmental neurofibromatosis or unilateral unifocal retinoblastoma. Non penetrance or mild expressivity in other family members should be excluded by diligent examination. Chromosomal abnormalities and phenocopies caused by infections or drugs
may account for the isolated affected person. Nonetheless, the ophthalmologist's obligation to explain the disorder begins with accurate diagnosis and establishment of the mode of heritability.
228 • Fundamentals and Principles of Ophthalmology Counseling can be considered successful if the patient or family, having acquired the facts, makes a reproductive decision that is reasonable and appropriate. The counselor is an informer, not an adviser. Properly done, genetic counseling is nondirective. It is inap -
propriate, perhaps even unethical, to tell the patient what to do (for instance, not to have any children). In any circumstance, the counseling ophthalmologist should outline the options for family planning when it is necessary. The ophthalmologic practitioner's responsibilities are to
• suspect and establish the diagnosis of inherited disease inform the patient of the findings and their implications for health provide accurate answers to direct and implied questions about risks of recurrence
and burden of disease Some people may accept a high statistical risk and have children. This decision must be based on how they perceive the social and psychologic burdens of the disorder. Attitude toward reproduction may be considerably different for a female carrier of protanopia than for a female carrier of X-linked RP or choroideremia, even though the statistical risk for an affected son is the same for each carrier.
Some people may elect to delay childbearing in hopes of medical advances in prenatal diagnosis or postnatal treatment of a disorder. Others may choose, for a variety of personal and ethical considerations, not to have natural offspring and may proceed with contraception, termination of pregnancy. sterilization, or adoption.
Artificial insemination by donor is a useful option in family planning if the father has a dominant disease or if both parents are carriers of a biochemically detectable recessive disorder. However, it is clearly not applicable ifthe mother is the carrier of an X-linked disorder or is the individual affected by an autosomal dominant mutation. Finally, although its acceptance and legal implications may lag behind, embryo adoption (transplantation) and surrogate motherhood may soon become useful alternatives for some families.
Prenatal Diagnosis Prenatal diagnosis with amniocentesis or chorionic villus sampling for biochemically identifiable disorders (eg, Tay-Sachs disease, many mucopolysaccharidoses, and about 100 other diseases) is also useful in the proper genetic settings. However, because most genes are expressed in a tissue-specific manner, biochemical diagnostic techniques are limited to diseases for which the gene products are expressed in amniocytes. Other possible indications for amniocentesis include advanced maternal age, with its increased risk of chromosomal abnormalities; elevated maternal serum a -fetoprotein,
suggesting a neural tube defect; and the presence of a familial disease detectable by DNA analysis. Amniocentesis is usually performed at 15-16 weeks of gestation, when enough fluid and cells can be obtained for culture and the maternal risk of abortion is relatively low. The risk of spontaneous abortion or fetal morbidity from the procedure is about 0.5%. Earlier prenatal diagnosis of chromosomal abnormalities, at about 10 weeks of gestation, is available through the use of chorionic villus sampling. In this procedure, tissue from the
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placenta is obtained under ultrasound visualizatio n. It is then cultured and karyotyped in a manner similar to that used fo r amniocentesis. As a first-trimester procedure. chorionic villus sampling allows for an earlier diagnosis and a safer means of pregnancy termination . The rate of spontaneous abortion associated with this procedure is estimated at 1%- 2%. Because the yield of DNA is greater than that from the 20 mL of amniotic fluid withdrawn in amniocentesis, direct DNA analysis of cells can often be done without prior cell culture. Thus, information can be obtained considerably sooner than with amniocentesis.
Pharmaco enetics The study of heritable fac tors that determine how drugs are chemically metabolized in the body is called pharmacogenetics. This field addresses genetic differences among population segments that are responsible for variatio ns in both the the rapeu tic and adverse effects of drugs. Investigations in pharmacoge net ics are important not only because they may lead to more rational approaches to thera py but also because they facilitate a deeper understanding of drug pharmacology. Part V of this section, Ocular Pharmacology, offers more detail. The drug isoniazid proVides an example of how pharmacogenetics works. This antituberculosis drug is normally inac tivated by the liver enzyme acetyl transferase. A large segment of the population, which varies by geographic distribution, has a red uced amount of this enzyme; these individuals are termed slow illactivators. When they take isoniazid, the drug reaches higher-than-normal concentrat ions, causing a greater incidence of adverse effects. Family studies have shown that a reduced level of acetyltransferase is inherited as an autosomal recess ive trait.
Several other well-documented examples demonstrate how pharmacogenetics works. One example involves 10% of the male African-American population, a high percentage of male Sephardic Jews, and males from a number of other ethnic groups. These individuals have an X-linked recessive trait that causes affected males to have glucose-6-phosphate dehydrogenase enzyme deficiency in thei r erythrocytes. As a consequence, a number of drugs (including sulfacetamide, vitamin K, acetylsalicylic acid, quinine, chloroquine, and probenecid) may produce ac ute hemolytic anemia in these individuals. Pharmacogenetic causes have also been ascribed to variations in response to ophthalmic drugs, such as the increased lOP seen in a segment of the population after prolonged use of topical corticosteroids.
Several dr ugs have been shown to cause greater reaction in children with Down syndrome than in children without the syndrome. As a result of supersensitivity, some children with Down syndrome have died after systemic administration of atropine. This supersensitivity is also seen with the topical use of atropine. In these patients, atropine exerts a greater-than-normal effect on pupillary dilation. In several children with Down syndrome being treated for strabismus, hyperactivity has occurred several hours after local instillation of 0. 125% echothiophate iodide. One of the earliest examples of an inherited deficit in drug metabolism involved sucCinylcholine, a strong muscle relaxant that interferes with acetylcholi nesterase, the enzyme that catabolizes acetylcholine at neuromuscular junctions. Normall y, SUCCinylcholine is
230 • Fundamentals and Principles of Ophthalmology rapidly destroyed by plasma cholinesterase (sometimes called pseudocholinesterase) so that its effect is short-lived- usually no more than a few minutes. Some people are homozygous for a recessive gene that codes for a form of cholinesterase with a considerably lower substrate affinity. Consequently, at therapeutic doses of sut cinylcholine, almost no destruction occurs, and the drug continues to exert its inhibitory effect on acetylcholinesterase, resulting in prolonged periods of apnea. Jorde LB, Carey ]C, Bamshad MJ, White RL. Medical Gen.etics. Updated ed. for 2006-200 7. 3rd ed. Philadelphia: Elsevier Mosby; 2006.
Clinical Management of Genetic Disease Genetic disease may not be curable, but in most cases the patient benefits considerably from the physician's appropriate medical management. Such care should include all of the following steps.
Accurate Diagnosis Unfortunately, because health care providers may not be as knowledgeable about genetic diagnoses as they are about other areas of medicine, many cases are not precisely diagnosed or, worse yet, are diagnosed incorrectly. A case of deafness and pigmentary retinopathy may be called rubella syndrome when the patient really has Usher syndrome. Such a syndrome associated with RP may not be recognized in patients with RP. For exampIe, patients with RP and congenital polydactyly (surgically corrected in infancy) may not be recognized as haVing Bardet -Biedl syndrome. Besides the obvious reasons, the correct diagnosis in such cases is important to ensure that the patient's educational and lifetime support needs are truly met.
Complete Explanation of the Disease Patients are often very disturbed when they do not understand the nature of their disease. A careful explanation of the disorder, as currently understood, will often dispel myths that patients may have about their disease and their symptoms. Virtually all genetic disorders confer burdens that may interfere with certain activities later in life. The appropriate time to discuss these burdens with patients and family members is often when they first ask about the consequences of a disease. Such explanations need to be tempered with compassion and with an understanding of the possible emotional and psychological effects of this information.
Treatment of the Disease Process Although definitive cures-that is, reversing or correcting of underlying genetic defectsare yet to emerge for various heritable disorders, some conditions in which metabolic defects have been identified can often be managed through 5 fundamental approaches: l. dietary control 2. chelation of excessive metabolites
CHAPTER 6:
Clinical Genetics. 231
3. enzyme or gene product replacements 4. vitamin and cofactor therapy 5. drug therapy to reduce accumulation of harmful products Some genetic disorders affecting the eye that arise from an inborn error of metabolism can effectively be managed by dietary therapy. These include familial hyperlipoproteinemia, tyrosinosis, homocystinuria, Refsum disease, phenylketonuria. fructose or lactose intolerance. galactokinase deficiency. and galactosemia. Implementing a galactose-free diet can reverse some of the main clinical signs of galactosemia. such as hepatosplenomegaly. jaundice. and weight loss. Progression of cortical cataracts can be avoided. and less extensive lens opacities may even regress with a galactose-free diet. With time. galactosemic patients are able to metabolize galactose through alternative pathways. obviating the need for lifelong dietary restriction. In phenylketonuria. mental retardation can be prevented by early phenylalanine restriction. Disorders that result from enzyme or transport protein deficiencies may lead to the accumulation of a metabolite or metal that harms various tissues. For example. in Wilson disease. decreased levels of serum ceruloplasmin result in poor transport of free copper (Cu") ions and in storage of copper in tissues such as the brain. liver. and cornea. Resultant clinical signs can be reversed. at least partially. after the administration of D-penicillamine. a chelator of Cu" ions. Other copper chelators such as British antilewisite (BAL) can be used. along with copper-deficient diets. to reverse the clinical signs of Wilson disease (hepatolenticular degeneration). Although no cure yet exists for most of the metabolic diseases. enzyme replacement therapy for types I and VI have been useful in reducing non neurologic symptoms and pain in some of the affected indivduals. Also. plasma infusions in patients with Fabry disease have succeeded in temporarily decreasing plasma levels of the accu mulated substrate ceramide trihexoside. although clinical improvement was not detected. Further in-depth investigations are necessary before intervention with this type of therapy becomes meaningful. because circulating enzymes from infusions are rapidly degraded or excreted by the kidneys. Nevertheless. a slow-release depot preparation administered intramuscularly is at least feasible in providing short-term improvement in some metabolic disorders. Organ transplantation can be considered as a form of regionalized enzyme replacement. [n patients with cystinosis. cystine crystals accumulate in the kidney. If a normal kidney. with its rich source of enzymes. is transplanted into a patient with cystinosis. cystine does not accumulate in the cells of the renal tubules. and renal function tends to remain normal. In a complementary approach. stem cell transplantation is being investi gated to treat various diseases. including those of the eye. In addition to enzyme replacement. synthetic or recombinant gene product replacement can effectively manage a gene defect. Hemophilia. for example. can be treated through the administration of a missing clotting factor (VIII). The value of gene product replacement is demonstrated in the use of thyroid hormone for hypothyroidism. insulin for diabetes. erythropoietin for anemia. and growth hormone for pituitary dwarfism. However. caution is needed in light of the possible spread of AIDS caused by HIV if the gene product is extracted from pooled human tissues.
232 • Fundamentals and Principles of Ophtha lm ology
Vitamin therapy appears to be of benefit in 2 autosomal recessive disorders. In at least some patients with homocystinuria. vitamin B6 (pyridoxine) administration has been shown to decrease homocystine accumulation in plasma and to reduce the severity of the disorder. Vitamin A and vitamin E therapy have been noted to benefit some patients with abetalipoproteinemia with regard to neurologic impairment; such therapy is also likely to slow or lessen the development and progression of retinal degeneration. More long-term therapeutic trials are necessary to better define the efficacy of vitamin therapy for these and perhaps other metabolic disorders. Various genetically determined disorders can be managed by use of an appropriate drug. For example, excess accumulation of uric acid in primary gout can be prevented or reduced by (I) blocking the activity of the enzyme xanthine oxidase with the drug allopurinol or (2) increasing excretion of uric acid by the kidneys with the use of probenecid. In addition, a reduction in serum cholesterol found with familial hypercholesterolemia can often be achieved with the use of various cholesterol-lowering drugs or substances that bind bile acids in the gastrointestinal tract. Appropriate management of sequelae and complications Some of the sequelae of genetic diseases, such as glaucoma in Rieger syndrome or cataracts in patients with RP, can be successfully managed to preserve or partially restore vision. However, patients need to understand how treatment of the sequelae or complications may differ in their situation, especially if treatment affects the expected outcome.
Genetic Counseling As discussed earlier in the chapter, the physician has an important obligation to either provide genetic counseling or arrange for the service by referral to a geneticist.
Referral to Providers of Support for Disabilities Individuals and families often receive considerable benefit from referral to local, regional, or national agencies, support groups, or foundations that provide services for those with a particular disease. These organizations include local and state agencies for the blind or visually impaired, speCial school education programs, and appropriate consumer groups. Particularly when a disability is chronic and progressive, these agencies or support groups can greatly aid the individual or family in adjusting to changing visual disabilities. Internet: http://ncbi.f1lm.nih.govIOmim through the Online Mendelian Inheritan ce home page website (under OMIM Alli ed Resources).
j"
Man
Introduction
Considerable progress has been made in the biochemistry of vision over the past 15 to 20 years, as witnessed by the numerous reviews, research art icles, and books that have been published during this time. Part IV, Biochemistry and Metabolism, was written for both practitioners and residents in ophthalmology, as well as for students and researchers seeking a concise picture of the current state of knowledge in the biochemistry of the eye. With the recent growth in new information about vision biochemistry has come increasing specialization among ophthalm ic researchers. These chapters cover most areas
of research in ocular biochemistry, including tear film, cornea, iris and Ciliary body, aqueous humor, lens, vitreous, retina, retinal pigment epithelium, and free radicals and an tioxidants. The text attempts to relate basic sc ience to clinical problems that may be faced during reSidency training and in subsequent practice.
235
CHAPTER
7
Tear Film
The primary functions of the tear film are to provide a smooth optical surface at the aireye interface; to serve as a medium for removal of debris; to protect the ocular surface;
and to supply oxygen, growth factors, and other compounds to the corneal epithelium. The tear film carries tear constituents and debris to the puncta. In addition, it contains a vast number of antimicrobial agents, lubricates the cornea- eyelid interface, and prevents desiccation of the ocular surface. Human tears are distributed among
the marginal tear strip (or tear meniscus) the preocular film covering the exposed bulbar conjunctiva and cornea (precorneal tear film) the conjunctival sac (between the lids and bulbar conjunctiva) The precorneal tear film is a trilaminar structure consisting of an anterior lipid layer. a middle aqueous layer, and a posterior mucin layer. Measurements of tear-film thickness
have differed widely. Original measurements of the precorneal tear film gave an average thickness of approximately 8-9 ~m, with the aqueous layer constituting nearly all the thickness (Fig 7-1). More recent, and presumably more accurate, studies using optical coherence tomography (OCT) and reflectometry have found the tear film to be only about 3.4 ~m thick. The separation between the mucin and aqueous layers may not be distinct because mucins absorb electrolytes and water. The steady-state volume of tears is 7.4 ~L for the unanesthetized eye and 2.6 ~L for the anesthetized eye; this volume decreases with age. Some properties of the normal human tear film are given in Table 7-1.
Lipid Layer The anterior layer of the tear film (approximately 100 molecules thick) contains polar and nonpolar lipids secreted primarily by the meibomian (tarsal) glands (Fig 7-2). These glands are located in the tarsal plate of the upper and lower eyelids and are supplied by parasympathetic nerves that are cholinesterase-positive and contain vasoactive intestinal
polypeptide (VIP). Sympathetic and sensory nerves are present but sparsely distributed. Neuropeptide Y (NPY) -positive nerves are also abundant. There are approximately 30-40 meibomian glands in the upper eyelid and 20-30 smaller glands in the lower eyelid. Each gland orifice opens onto the skin of the eyelid margin, between the tarsal gray line and the mucocutaneous junction. The sebaceous glands of Zeis, located at the eyelid margin close to the eyelash roots, also secrete lipid, which is incorporated into the tear film. 237
238 • Fundamentals and Principles of Ophtha lmology
",.. Superficial lipid laye r -0.1- 0.2
,
• Co
•
r
8:,
(
~I m
thick
Aqueo us layer -7- 8 ~lm thick
q
c
Q
Ad sorbed mucin layer
up to 1 ~lm thick Microvilli o f epithel ium extend into and stabilize mucin layer
Figure 7-' Schematic drawing of the structure of the tear film showing the Quter lipid layer, middle aqueou s layer, inner m ucus layer, and microvilli on the apica l cells of the ocular surface
epithe lium.
(From Marshall D. Tear layer mechanics, In: Bennett ES. Weissman BA. eds. Clinical Contact tice, Philadelphia: J8 Lippincott: 1997 :2.)
lens Prac-
Table 7-1 Properties of Human Tear Film Composition Thickness Volume Secreto ry rate
Water Solid Total Lipid layer Unanesthetized Anesthetized Unanesthetized
Schirmer
Turnover rate Evaporation rate Osmolarity pH Electro lytes (mmoll LJ
Fluorophotometry A nesthet ized Schirmer Fluorophotometry Normal Stimulated
No·
K· Ca 2• Mg 2+ CI-
HCOi
98.2% 1.8%
3.4 11m 0.1- 0.2 7.4 ~ L 2.6 ~L 3. 8
~m
~ Um i n
0 .9 )lUmin 1.8 )lUmin 0.3 )lUm in 12%-16%/min 300%/min 0.06 j.lUcm 2/min
296-308 mOsm/L 6.5-7.6 134-170 26-42 0.5 0.3-0.6 120-135 26
CHAPTER 7:
Tear Film.
239
Figure 7-2 Schema tic drawing of the major tear glands and ocular sur-
face epithelia tha t contribute to the tear film. Shown are the me ibomian
glands Isecrete oily layer!. main lacrimal gland, accessory lacrimal gland, co njunctival epithelium, corneal epithelium (secretes aqueous
layer), and conjunctival goblet cells (secrete mucus layer). (From Dartt DA.. Sullivan OA. Werting of the ocular surface In: Albert DM. Jakobiec FA.. eds. Principles and Practice of Ophthalmology. Philadelphia:
Saunders; 1994:967.)
The lipid layer has the following fun ctions:
• retard evaporation contribute to the optical properties of the tear film because of its position at the air-tear film interface
maintain a hydrophobic barrier (lipid strip) that prevents tear overflow by increasing surface tension
prevent damage to lid margin skin by tears Because the polar lipids are charged compounds (phospholipids), they are located at the aqueous-lipid interface, The fatty acids of the phospholipids interact with the other hyd rophobic lipids (cholesterol and wax esters, which make up the bulk of the lipid layer) through noncovalent, noncharged bonds, Tear lipids are not susceptible to lipid peroxidation because they contain extremely low levels of polyunsaturated fatty acids.
Aqueous Layer The middle aqueous layer is secreted by the main and accessory lacrimal glands (see Fig 7-2) . It consists of electrolytes, water, and proteins. The main lacrimal gland is divided into 2 anatomical parts, the orbital and the palpebral portions, by the levator aponeurosis, The glands oj Krause, which constitute two thirds of the accessory lacrimal glands, are
240 • Funda me ntals and Principles of Ophthalmology located in the lateral part of the upper forni x. A number of Krause glands are also present in the lower fornix. The glands oj WolJring are variably located along the proximal margin of each tarsus. The accessory lac ri mal glands are structurally like the mai n lac rimal gland. The main lacrimal gland is richly innervated by parasympathetic nerves containing the neuro transmitters acetylcholine and VI P. The sympathetic innervation is less dense than the parasympathetic and contai ns no repinephrine and NPY as neuro transmitters. The sensory nerves are sparsely supplied with the neurotransmi tters substance P and calcitonin ge ne- related peptide (CG RP ). The accesso ry lacrimal glands are densely innervated, but the majority o f nerves are unidentified. So me of this innervation consists
of nerves containing VIP, substance P, and CG RP. Corneal innervation is predominantly sensory, but there is also sympathetic and (to a lesser extent) parasympathetic innervation. The conjunctival epithelium is innervated by parasympathetic, sympathetic, and sensory nerves.
The aqueous layer of tears consists of electrolytes, water, protein , and a variety of other solutes secreted by the main and accesso ry lacrimal glands, as well as by the corneal and conjunctival epithelia. In addition, with conjuncti val inflammation and in response to drugs such a~ histamine, the blood vessels of the conjunctiva can leak a plas malike fluid into the aqueous layer of tears. Electroly tes and small molecules regulate the osmotic flow of fluids betwee n the corneal epithelial cells and the tear mm, buffe r tea r pH, and serve as enzyme cofactors in controlling membrane permeability. The Na+ concentration of tears parallels that of serum; the concentration ofK+ is 5- 7 times greater than that in serum. Na+, K+, and Cl- regulate the osmotic flow of fluids from the cornea to the tear film . Bicarbonate regulates tear pH . O ther tear electrolytes (Fe' +, Cu'+, Mg'+, Ca'+, P01-) are enzyme cofactors. Tear-Jilm solutes include urea, glucose, lactate, citrate, ascorbate, and ami no acids. All enter the tear film via the systemic circulation, and their concentrations parallel those of serum levels. Fasting tear glucose levels are 3.6- 4.1 mg/cc in those with and without diabetes. However, after a IOO-mg oral glucose load, tear glucose levels exceed II mg/cc in 96% of diabetic persons tested. Proteins in the tear mm include im munoglobulin A (IgA) and secretory IgA (sIgA). IgA is formed by plasma cells in interstitial tissues of the main and accessory lac rimal glands and by the substantia propria of the conjunctiva. The secretory component is produced within lacrimal gland acini, and slgA is secreted into the lumen of the main and accessory lacrimal glands. IgA plays a role in local host-defense mechanisms of the external eye, as shown by increased levels ofigA and IgG in hu ma n tears associated with ocular in flamm ation. Other immunoglobulins in tears are IgM, IgD, and IgE. Vernal conjunctivitis causes elevated tear and serum levels of IgE, increased IgE-producing plas ma cells in the giant papillae of the superior tarsal conjunctiva, and elevated histamine. Increased levels of tear histamine support the concept of conjunctival mast-cell degranulation trigge red by IgE- antige n interaction. Lysozyme, lactoferrin , group" phospholipase A" lipocalins, and defe nsins are important tear antimic robial constituents. Also present in tears is interferon , which inhibits viral
replication and may be efficacious in limiting the severity of ulcerative herpetic keratitis.
CHAPTER 7:
Tear Film. 241
Tears also contain a wide array of cytokines and growth factors, including transforming growth factor ~s, epidermal growth factor, ~ fibroblast growth factor, interleukin 1a and l~, and tumor necrosis factor u. These may playa role in the proliferation, migration, and differentiation of corneal and conjunctival epithelial cells. They may also regulate wound healing of the ocular surface. The aqueous layer has the following functions: supply oxygen to the avascular corneal epithelium maintain a constant electrolyte composition over the ocular surface epithelium provide an antibacterial and antiviral defense • smooth minute irregularities of the anterior corneal surface
wash away debris • modulate corneal and conjunctival epithelial cell function
Mucin layer The mucin layer of the tear film coats the microplicae of the superficial corneal epithelial cells and forms a fine network over the conjunctival surface. It contains mucins, proteins, electrolytes, and water. Functions of the mucin layer include the following: convert the corneal epithelium from a hydrophobic to a hydrophilic layer, which is essential for the even and spontaneous distribution of the tear film interact with the tear lipid layer to lower surface tension, thereby stabilizing the tear film • trap exfoliated surface cells, foreign particles, and bacteria (by the loose mucin network covering the bulbar conjunctiva) • lubricate the eyelids as they pass over the globe Tear mucins are secreted principally by the conjunctival goblet cells and the stratified squamous cells of the conjunctival and corneal epithelia and minimally by lacrimal glands of Henle and Manz (see Fig 7-2). Goblet-cell mucin production is 2- 3 ).tL/day, which contrasts with the 2- 3 mL/day of aqueous tear production. Both conjunctival and tear mucins are negatively charged, high-molecular-weight glycoproteins. Tear dysfunction may result when tear mucins are deficient in number (avitaminosis A, conjunctival destruction), excessive in number (hyperthyroidism; foreign-body stimulation; allergiC, vernal, and giant papillary conjunctivitis), or biochemically altered (keratoconjunctivitis).
Tear Secretion The lacrimal secretory system was once thought to have 2 components: basic secretars and reflex secretars. Basic secretion was ascribed to the accessory lacrimal glands of Krause
and Wolfring; and reflex secretion, to the main lacrimal gland. However, it is now thought that all lacrimal glands respond as a unit. In addition, the cornea and conjunctiva can also respond by secreting electrolytes, water, and mucins. Although the meibomian glands are innervated, it is not known whether nerves mediate lipid secretion from these glands.
242 • Fundame ntals and Principles of Ophthalmology Reflex tear secretion is neurally mediated and induced in response to physical irritation (superficial corneal and conjunctival sensory stimulation by mechanical. thermal. or chemical means). psychogenic factors. and bright light via the optic nerve. Induction of sensory nerves by a local neural reflex activates the parasympathetic and sympathetic nerves that innervate the tear glands and epithelia. causing secretion. Parasympathetic and sympathetic ne rves release their neurotransmitters. wh ich interact with specific G- protein-linked receptors in the lacrimal glands. cornea. and conjunctiva; these receptors then activate their respective signaling pathways. There are 2 main signaling pathways: Ca"'protein kinase C-dependent and cyclic adenosine monophosphate (cAMP)-dependent (Figs 7-3. 7-4). In most tissues. the Ca"'protein kinase C- dependent pathway is activated by acetylcholine and. except in the main lacrimal gland. by norepinephrine. Acetylcholine. released from parasympathetic nerves. activates muscarin ic receptors; norepinephrine. released from sympathet ic nerves, activates aI-adrenergic receptors. Stimulation of muscarinic and a I-adrenergic receptors activates
a guanine nucleotide- binding protein (G protein) of the GOqll \ subtype. which then
Blood
II
Endoplasmic reticulum
ExocytOSiS
Lumen
'if
Ion transport
Figure 7-3 Epithelial cell. Schematic of Ca2+/protein kinase C-dependent signal transduction pathway activated by cholinerg ic and a,-adrenergic agonists in epithelial cells to stimulate mucin, protein, or electrolyte and water secretion . ACh = acetylcholine, GaqJll = q/ 1 1 subtype of guani ne nucieotide-binding protein, Protein-P = phosphorylated (activated) protei n. IFmm Dam OA. Regulation of rear secretion. Adv Exp Med 8101. 1994;350:4.1
CHAPTER 7:
Exocytosis
Lumen
'if
Tear Film.
243
Ion transport
Figur. 7-4 Epithelial ce ll. Schematic of 3',5'-cyclic adenosine monophosphate (cAMP)dependent signa ling pathway activated by vasoactive intestinal polypeptide (V/P) or norepinephrine to stimulate mucin, protein, or electrolyte and wa ter secretion in epithelial cells. 5' AMP = adenosine 5'-monophosphate, ATP = adenosine 5'-tnphosphate, Gp y = ~- and y-su bunits of guani ne nucleotide-binding protein, Gsa = stimu latory a-subu nit of guan ine nucleotide-binding protein, GOP = guanosine 5'-diphosphate, Protein-P = phosphorylated (activated) protein. (Mod· ified from Daft( DA. Regulation of tear secretion. Adv Exp Med Bioi. 1994;350:5.}
turns o n phospholipase C. Phospholipase C breaks down a membrane lipid- phosphatidyLinositol 4,S,-bisphosphate-into inositol IA,S- trisphosphate (IP, ) and diacyLglyceroL IP, releases intracellular Ca h The depletion of Ca2• from intracellular stores causes the influx of extracellular Ca" to refill these stores. Ca" (either by itself or by ac tivating Ca 2 . calmodulin-depe ndent protein kinases) stimulates protein and/or electrolyte and water secretion, The increase in diacylglycerol activates protein kinase C, a family of 11 isozymes that stimulate protein and/or electrolyte and water secreti on. The CAMP-dependent pathway is activated by VIP and norepinephrine, VIP, released from parasympathetic nerves, interacts with VIP receptors; norepinephrine. released from sympathetic nerves, activates ~- adrenergic receptors, Stimulation of VIP or p-adrenergic receptors activates G protein G,,, subtypes, which in turn stimulate adenylyl cyclase. Activati on of adenylyl cyclase produces cAMP from ATP. cAMP ac tivates cAMP-dependent protein kinases to stimu late protein and/or electrolyte and water secretion. The action of cAMP is terminated when it is broken down by CAMP-dependent phosphodiesterases.
244 • Fundamentals and Principles of Ophthalmology
Another mechanism for stimulating tear secretion (in addition to nerves) is peptide and steroid hormones. Peptide hormones, including a-melanocyte-stimulating hormone and adrenocorticotropic hormone (ACTH), stimulate protein secretion from the main lacrimal gland. These hormones activate the cAMP-dependent pathway described for VIP and p-adrenergic receptors. The steroid hormones, specifically the androgens, stimulate secretion of sIgA from the main lacrimal gland and lipid secretion from the meibomian glands. Androgens diffuse into the nucleus and bind to receptors, which are members ofthe steroid/thyroid hormone/retinoic acid family of transcription factors. The monomeric-activated androgen-receptor complex then associates with the response ele-
ments in the regulating region of the target gene (eg, for sIgA secretion, the target would be the secretory component gene). This association promotes dimerization of2 androgenreceptor complexes, a process that then activates gene transcription and eventually protein syntheSiS. Eyelid movement is important in tear-film renewal, distribution, turnover, and drainage. As the eyelids close in a complete blink, the superior and inferior fornices are compressed by the force of the preseptal muscles, and the eyelids move toward each other, with the upper eyelid moving over the longer distance and exerting force on the globe. This force clears the anterior surface of debris and any insoluble mucin and expresses secretions from meibomian glands. The lower eyelid moves horizontally in a nasal direction and pushes tear fluid and debris toward the superior and inferior puncta. When the eyelids are opened, the tear film is redistributed. The upper eyelid pulls the aqueous phase of the tear film by capillary action. The lipid layer spreads as fast as the eyelids move, so that no area of the tear film is left uncovered by lipid. The lipid layer increases tear-film thickness and stabilizes the tear film. Polar lipids, present in the meibomian secretions, concentrate at the lipid- water interface and enhance the stability of the lipid layer.
Tear Dysfunction A qualitative or quantitative abnormality of the tear film may occur as a result of • change in the amount of tear-film constituents
• change in the composition of tear film • uneven dispersion of the tear film because of corneal surface irregularities
ineffective distribution of the tear film caused by eyelid- globe incongruity The amount or composition of the tear film can change because of aqueous defiCiency, mucin defiCiency or excess (with or without associated aqueous defiCiency), and/or lipid abnormality (meibomian gland dysfunction). For example, increases in tear-fIlm osmolarity have been observed in patients with keratoconjunctivitis sicca (KCS, or dry eyes) or blepharitis and in those who use contact lenses. The preocular tear fIlm is dispersed unevenly with an irregular corneal or limbal surface (inflammation, scarring, dystrophic changes) or poor contact lens fit. Eyelid- globe incongruity results from congenital, traumatic, or neurogenic eyelid dysfunction or absent or dysfunctional blink mechanism.
CHAPTER 7:
Tear Film.
245
Diagnostic tests for tear dysfunction include the tear breakup time,lissamine green staining, rose bengal staining, osmolarity tests, and Schirmer tests. The tear-constituent imbalance can be corrected by decreasing the evaporation of tears (through reduced room temperature or increased humidity) and changing contact lenses for glasses. Tear-film instability (secondary to aqueous andlor mucin deficiency) can be reduced by the use of topical tear substitutes. Reduction of tear drainage by punctal occlusion prolongs the effect of artificial tears and preserves the natural tears. Thus, tear dysfunction is managed by creating a more regular corneal or conjunctival contour or by facilitating eyelid- globe congruity. Unfortunately, all these treatments are palliative (and not curative) in nature. There is increasing evidence that KCS is associated with ocular surface inflammation. In various studies, adhesion molecule expression by conjunctiva epithelial cells, T-cell infiltration of the conjunctiva, and increases in soluble mediators (cytokines and proteases) in the tear film have been found in patients with KCS (Fig 7 -5). Preliminary clinical studies have shown that using hypotonic tear substitutes to treat patients with KCS may reduce tear osmolarity and improve ocular symptoms. Moreover, therapy with a multitude of antiinflammatory agents (including corticosteroids, cyclosporine, matrix metalloproteinases, and doxycycline) has been observed to improve the clinical symptoms of patients with KCS (Fig 7-6). Recently, topical cyclosporine A emulsion (Restasis) was approved by the FDA for treating the inflammatory component of dry eye. This fungal -derived peptide emulsion has been shown effective in stimulating aqueous tear production and redUCing symptoms of blurred vision in patients with KCS. No Significant adverse systemic or ocular events (except for burning symptoms) were observed. BCSC Section 7, Orbit, Eyelids, and Lacrimal System, discusses the lacrimal system in depth, with numerous illustrations.
Rheumatoid arthritis Sjogren syndrome
j
'
~Pithelial
.
JOcular Surface Diseasel Secretory Dysfunctionl-----L (Keratoconjunctivitis Sicca) I Lacri mal gland Hyperosmolar
I
Meibomian gland
~ ,-::-,--::--,---'--:-."._ _,-----,
I
Ocular Surface Inflammation
I
An~~~:~ ~:f;~:~CY ~d~~~ inf~;t:"i'on M~PS APOPI~Si S -.
1
1
1
Cyloki nes Chemokines
Figure 7~5 Inflammatory mediators in keratoconju ncti vitis sicca. MMPs := matrix m eta llo pro ~ tein ases. (Reproduced with permission from Pflugfelder Sc. Antiinflammatory therapy for dry eye. Am J Ophthalmol 2004:1 3 7(2):338.)
246 • Fundamentals and Principles of Ophthalmology Rheumatoid arthritis ________ _________ ______ _
SiOgren rndrome
~s::-e-c-re-'t"'o-ry--:D-Y-Sf:-u-n-ct:-:io-n-'
Ocular Surface Epithelial Disease _____
(Keratoconjunctivitis Sicca)
Lacrimal gland Meibomian gland
I
Female gender Androgen deficiency
c Adhesion molecules
1
C T-eell _ -)Eo infiltration
t
MMPs S, T
Apoptosis
t
Cytokines Chemokines
Targets of anti-inflammatory therapies for keratoconjunctivitis sieca . C = esA (cyclosporin A), MMPs = matrix meta ll oproteinases, S = corticostero ids, T = tet racycl ine. (Repro-
Figure 7-6
duced with permission from Pflugfelder Sc. Antiinflammatory th erapy for dry eye. Am J Ophthalmol, 2004; 137(2):340.)
lester M, Orsoni G}, Gamba G, et a1.lmprovement of the ocular surface using hypotonic 0.4% hyaluronic acid drops in keratoconjunctivitis sicca. Eye. 2000;14(Pt 6):892-898. Pflugfelder Sc. Antiinflammatory therapy for dry eye. Am J Ophthalmol. 2004;137(2):337-342. Tiffany JM . Tears and conjunctiva. In: Harding JJ, ed. Biochemistry of the Eye. London: Chapman & Hall Medical; 1997:1 - 15. Tsubota K. Tear dynamics and dry eye. Prog Retin Eye Res. 1~98;1 7(4) :56 5 - 596.
CHAPTER
8
Cornea
The cornea is a remarkable structure, with a high degree of transparency and excellent self protective and reparative properties. The cornea is made up of the following histologic layers (Fig 8- I): • epithelium with basement membrane
• Bowman's layer stroma (or substantia propria) Descemet's membrane
endothelium The human cornea has a rich afferent innervation. The long posterior ciliary nerves
(branches of V" the ophthalmic division of eN V) penetrate the cornea in 3 planes: scleral, episcleral, and conjunctival. Peripherally, approximately 70-80 branches of the long posterior ciliary nerves enter the cornea and lose their myelin sheath 1-2 mm from the limbus. A plexus posterior to Bowman's layer sends branches anteriorly into the epithelium.
<\r-- S,url,,,. cells cells I\~!!t'-B.,;al
cells
Figure 8-1
Diagram of different layers of the
cornea. (Reproduced with permission from Kanski JJ. Clinica l Ophthalmology: A Systematic Approach. 3rd ed. Oxford: Butterworth-Heinemann; 1994: J00)
layer Stroma
,.;
Descemel's membrane
-~!I~.... ~!;;i;~-~-~;~~2i!~~.
EndOlhelium/
247
248 • Fundamentats and Principles of Ophthalmology Oxygen to the cornea is provided by the preocular tear film , lid vasculature, and aqueous humor. The primary metabolic substrate for the epithelial cells, stromal keratocytes, and endothelium is glucose. The stroma receives glucose primarily from the aqueous humor by carrier-mediated transport through the endothelium; the epithelium receives glucose by passive diffusion through the stroma. The preocular tear film and limbal vessels supply approximately 10% of the glucose used by the cornea. Glucose is metabolized in the cornea by all 3 metabolic pathways: 1. tricarboxylic acid (TCA) cycle 2. anaerobic glycolysis 3. hexose monophosphate (HMP) shunt In the epithelium and endothelium, the HMP pathway breaks down 35%-65% of the glucose, but the keratocytes of the stroma metabolize very little glucose via this pathway. The keratocytes appear to lack 6-phosphogluconate dehydrogenase, an important enzyme in the HMP pathway. The TCA cycle is much more active in the endothelium than in the epithelium. Pyruvic acid, the end product of glycolysis, is converted either to CO, and H,O (via the TCA cycle under aerobic conditions) or to lactic acid (u nder anaerobic conditions). Production of lactic acid increases in conditions of oxygen deprivation, as in the case of tight-fitting contact lenses of low oxygen permeability. Accumulation of lactic acid in the cornea has detrimental visual consequences, such as edema (due to an increase
in an osmotic solute load) or stromal acidosis, which can change endothelial morphology and function . Human corneas possess a remarkably high level of aldehyde dehydrogenase and transketolase. Together, these 2 proteins constitute 40%-50% of the soluble protei ns in corneal stroma. Like enzyme crystallins of the lens, both aldehyde dehydrogenase and transketolase are thought to contribute to the optical properties of the cornea. Both proteins are also thought to protect corneal cells against free radicals and oxidative damage by absorb ing UVB irradiation.
E ithelium The epithelium is typically about 65 I'm thick and constitutes 5%- 10% of total corneal thickness. It is composed of 5- 6 layers, which include 1-2 layers of superficial squamous cells, 2- 3 layers of broad wing cells, and the innermost layer of the columnar basal cells. Surface projections (microvilli and microplicae) are present on the apical surface of the most superficial cell layer of epithelium. These projections are coated with filamentous material known as glycocalyx. Mucin glycoproteins, the major constituents of glycocalyx, are thought to promote both stability of the tear film and wettability ofthe corneal surface. Plasma membrane proteins and the lipids of corneal epithelial cells, like those of other cell types, are heavily glycosylated and play an important role in cell- cell ad hesion as well as in the adhesion of the basal cells of corneal epithelium to the unde rlying basement membrane. The sugar residues of the plasma membrane glycoproteins and the glycolipids of corneal epithelium also playa role in wound-healing mechanisms, by mediating corneal
CHAPTER 8:
Cornea •
249
epithelial sheet migration over the wound surface following ocular injury; and in pathogenesis of corneal infection, by serving as attachment sites for microbes. Hydrophilic molecules penetrate the epithelium poorly, but they may pass through intercellular tight junctions if the polar molecule is less than 500 daltons (D) in apparent molecular mass. Knowing the ionic dissociation constant of a molecule is important for determining its permeability across the cornea. To diffuse across the epithelium, organic molecules should exist in an uncharged state. However, a charged molecule more readily penetrates the stroma. Therefore, to penetrate the cornea and enter the anterior chamber, an organic molecule should be able to dissociate at physiologic pH and temperature (ie, within the stroma).
Bowman's Layer Bowmans layer is immediately posterior to the epithelial basal lamina. This layer is 8-12 Ilm thick and is composed of randomly packed type I and type V collagen fibers that are 30 nm in diameter. The fibers are enmeshed in a matrix consisting of proteoglycans and glycoproteins. Bowman's layer is secreted during embryogenesis by the anterior stromal keratocytes and epithelium. It is acellular, and it does not regenerate when damaged. It is thought that this layer, by virtue of its acellularity and packing distribution, serves to prevent exposure of stromal corneal keratocytes to growth factors secreted by epithelial cells, such as transforming growth factor ~s (TGF- ~s). This is notable because, during excimer laser photorefractive keratectomy (PRK) for correction of myopia, Bowman's layer (along with anterior corneal stromal tissue) is removed from the center of the cornea and thus the anterior dome of the cornea becomes flatter. Similarly, Bowman's layer is lost during laser subepithelial keratomileusis (LASEK) for myopia correction. In this procedure, corneal haze is a common and significant postoperative complication (especially if steroids are not used for months), presumably because stromal keratocytes are exposed to regenerating epithelial growth factors and metaplase into fibroblasts. However, during laser in situ keratomileusis (LASIK), Bowman's layer is transected but still retained, with ablation occurring intrastromally on the stromal bed surface; and thus central corneal haze is extremely rare in uncomplicated LASIK.
Stroma The stroma makes up 90% of the corneal thickness. Stromal cells are known as keratocytes. Depending on a person's age, keratocytes constitute 10%- 40% of corneal volume. Usually, these cells reside between the collagen lamellae. The stroma is made up of roughly 200 layers of lamellae, which are 1.5-2.5 Ilm in thickness and are composed of collagen fibrils enmeshed in a matrix consisting of proteoglycans, proteins, and glycoproteins. The stromal fibrils within each lamella are narrow and uniform in diameter. In humans, the average fibril diameter is 30 nm. Collagen fibrils within each lamella run parallel to each other from limbus to limbus. Adjacent lamellae are positioned at roughly right angles to each other: less than 90" in the
250 • Fundamentals and Principles of Ophthalmology 0
anterior stroma but almost 90 in the posterior stroma. That is. alternate arrays of fibrils are nearly perpendicular. and they are observed tangentially in electron micrographs of cross sections of corneal stroma. Also. collagen fibrils in each lamella are regularly spaced. with a center-to-center distance of 55-60 nm. The narrow and uniform diameter of collagen fibrils and their regular arrangement are characteristic of collagen of the corneal stroma and are necessary for the transparency of this tissue. Type I is the major collagen component of the corneal stroma. constituting approximately 70% of the total stromal dry weight. Immunohistochemical and biochemical studies have demonstrated that normal adult corneal stroma also contains collagen types V. VI. VII. XII. and XIV. Type 1lI collagen production is associated uniquely with stromal wound healing. After collagen. proteoglycans are the second most abundant biological constituents of the cornea. constituting approximately 10% of the dry weight of the cornea. It is the proteoglycans that confer hydrophilic properties to the stroma. Proteoglycans are glycosylated proteins with at least I glycosaminoglycan (GAG) chain covalently bound to the protein core. GAGs are composed of repeating disaccharides. The GAGs found in corneal stroma include • keratan sulfate
chondroitin sulfate derma tan sulfate Two major proteoglycan populations have been identified in corneal stroma. I con taining keratan sulfate chains. the other containing both dermatan sulfate and chondroitin sulfate chains. Regulation of spacing between the stromal collagen fibrils is thought to result from highly specific interactions between the proteoglycans and the collagen fibrils; when these interactions are disturbed. the ability of the cornea to remai n transparent is profoundly affected. Matrix metalloproteinases (MMPs) are a family of Zn" -dependent enzymes responsible for degradation of the components of the extracellular matrix (including proteoglycans and various types of collagens) during normal development as well as in disease processes. Of more than a dozen known metalloproteinases. only MMP-2 proenzyme has been found in the normal healthy cornea. However. after corneal injury. additional MMPs (including MMP-l. MMP-3. and MMP-9) are syntheSized. The proteinase inhibitors of the cornea playa key role in corneal protection by restricting damage during corneal inflammation. ulceration. and wound healing. The following proteinase inhibitors have thus far been identified in the cornea: • ai-proteinase inhibitor • fi1-antichymotrypsin
u, -macroglobulin • plasminogen activator inhibitors I and 2 • tissue inhibitors of metalloproteinases
Many of these inhibitors are synthesized by resident cells of the cornea; some are derived from tears. aqueous humor. and limbal blood vessels.
CHAPTER 8:
Cornea.
251
Descemet's Membrane and Endothelium Descemets membrane is a 10-l1m-thick specialized basement membrane present between the endothelium and the posterior stroma. It is secreted by endothelium and comprises an anterior banded portion and a posterior nonbanded portion. Type IV is the most abundant collagen in Descemet's membrane. The corneal endothelium is a Single layer posterior to Descemet's membrane and is composed of polygonal cells 20 11m in diameter. In young adults, the normal endothelial cell count is approximately 3000/ mm'- The number of endothelial cells decreases with aging, with a concomitant spreading and thinning of the remaining cells. A group of tight junctions forms the apical junctional complex between cells that occludes the lateral extracellular spaces from the aqueous humor. Approximately 20- 30 short microvilli per cell extend from the apical plasma membrane into the aqueous humor. The endothelium functions as a permeability barrier between the aqueous humor and the corneal stroma and as a pump to maintain the cornea in a dehydrated state by generati ng the negative hydrostatic pressure that also serves to hold free corneal flaps (eg, LASIK flaps) in place. In vivo, the endothelium derives sufficient oxygen from the aqueous humor to maintain normal pump function.
If the endothelium is injured, healing occurs mainly via cell migration, rearrangement, and enlargement of the residual cells. Substantial cell loss or damage results in irreversible edema because human corneal endothelial cells have a limited ability to divide after birth. Infiltration of polymorphonuclear leukocytes in response to severe corneal in jury induces endothelial cells to become fibroblastic and to synthesize retrocorneal fibrous membrane (RCFM). This RCFM forms between Descemet's membrane and the corneal endothelium and causes a significant loss of visual acuity. Unlike normal corneal endothelial cells, which accumulate little type I collagen protein, the fibroblastic cells isolated from the RCFM predominantly express type [ collagen. Pa nj wan i N. Cornea and sclera. In : Harding Jl. ed. Biochemistry o/ the Eye. london: Chapman & Hall Medical; 1997; 16- 51.
CHAPTER
9
Iris and Ciliary Body
Introduction The iris and ciliary body are the anterior parts of the uveal tract. which is cont inuous with the choroid posteriorly. The iris is a highly pigmented tissue that functions as a movable diaphragm between the anterior and posterior chambers of the eye. regulating the amount of light that reaches the retina. It is a delicate. dynamic structure. capable of precise and rapid changes in pupillary diameter in response to both light and specific pharmacologic stimuli. The ciliary body produces and regulates the composition of aqueous humor and thus directly influences the ionic environment and metabolism of the lens. cornea. and trabecular meshwork.
Protein Types Expressed in Human Ciliary Body Using expressed sequence tags (ESTs) to identify active molecules from a human ciliary body library. researchers found that the largest group (37%) contains unidentified sequences with unknown functions. Of the known compounds. the functions of major groups of expressed genes include protein synthesis. folding. secretion. and degradation (20%) energy supply and biosynthesis (12%) • contractility and cytoskeleton structure (6%) • cellular signaling and cell cycle regulation (7%) nerve cell- related tasks (2%). including neuropeptide processing and putative nonvisual phototransduction and circadian rhythm control Escribano J, Coca-Prados M. Bioinformatics and reanalys is of subtracted expressed sequence tags from the human ciliary body: identification of novel biological functions. Review. Mol Vis. 2002;8:315-332.
Iris-Ciliary Body Smooth Muscle Unlike smooth muscle elsewhere in the body. which is derived from mesoderm. the smooth muscle in the iris and ciliary body is derived from neuroectoderm. The biochemistry of the smooth muscles of the iris includes the following: • contraction- relaxation • receptor characterization
253
254 • Fundame ntals and Princi ples of Ophthalmology second-messe nger formation and regulation protein phosphorylation phospholipid metabolism arachidonic acid (AA) release and eicosanoid biosynthesis The ciliary body is the main pharmacologic target in the treatment of glaucoma. Many treatments employed to lower intraocular pressure (lOP) in glaucoma. such as adrenergic and cholinergic drugs and prostaglandins (PGs), work th rough receptors and their respective Signal transduction pathways. The iris- ciliary body is rich in many types of receptors that bind to various agonists and antagonists. including adrenergic, muscarinic cholinergic, and peptidergic; PG; serotonin; platelet-activating factor; and growth factor receptors. The following discussion concerns biochemical aspects of the iris- ciliary body such as aqueous humor formation, eicosanoids, and membrane signal transduction and secondmessenger systems. Chapter 2 of this volume discusses and illustrates all of the va rious structures mentioned in this chapter.
Aqueous Humor Dynamics Aqueous humor. the transparent fluid that fill s the anterior and posterior chambers, is secreted by the nonpigmented ciliary epithelium from a substrate of blood plasma. Aqueous is the major nutrient source for the avascular lens an.d cornea and provides a route for the removal of waste products. Aqueous humor is essentially protein -free, which allows for optical clarity. The total protein level in hu man aqueous hu mor, about half of which is albumin, is very low-only about 1/500 of that in plasma. Other components include growth factors; several enzymes, such as carbonic anhydrase. lysozyme, d iamine oxidase, plasminogen activator, dopam ine ~ -hyd roxyl ase. phospholi pase A, and PGs, cycliC adenosine monophosphate (cAMP). catecholamines. steroid hormones, and hyaluroniC acid. Ocular fluids are separated from blood by barriers fo rmed by tight junctions between epithelial and endothelial cells. These barriers are called either blood-aqueous or bloodretina, depending on their location in the eye. Because of these barriers, the composition and amounts of all mate rials entering and leaving the eye can be carefully cont rolled, except fo r materials that exit through the Schlemm canal. Perturbations of these bloodocular barriers result in the mixing of blood constituents with ocular fluids, which may be the cause of plasmoid aqueous, retinal exudates, or retinal edema. Aqueous hu mor enters the posterior chamber from the Ciliary processes by means of active and passive physiologic mechanisms: • passive: diffusion and ultrafiltration • active: energy-dependent secretion, includi ng carbonic anhyd rase II (CA II)
activity Diffusion involves the movement of ions like sodium across a membrane toward the side with the most negative potential. Ultrafiltration is the none nzymatic component of
CHAPTER 9:
Iris and Ciliary Body.
255
aqueous formation that depends on lOP, blood pressure, and the blood osmotic pressure in the ciliary body. CA II in humans is present in pigmented and nonpigmented epithelium. Its inhibitors cause a reduction in the rate of entry of sodium and bicarbonate into the aqueous, leading to a reduction in aqueous flow. The formation of aqueous humor is largely a product of active secretion by the inner, nonpigmented ciliary epithelium and involves membrane-associated Na+,K+ -ATPase. The following observations support the involvement of active-transport mechanisms in secretion:
Aqueous concentrations of Na+, K+, Cl- , myo-inositol, certain amino acids, and glucose are maintained by specific active-transport systems located in the Ciliary epithelium. The high level of ascorbic acid in the aqueous humor suggests that an active pump mechanism secretes it into the aqueous. • Both the iris and the ciliary body accumulate p-aminohippuric acid from the aqueous against a concentration gradient (the accumulation shows saturation kinetics;
p-aminohippuric acid is inhibited at O°C and depressed by cyanide, dinitrophenol, ouabain, iodopyracet, and probenecid). The low protein concentration of aqueous humor relative to serum results from the exclusion of large molecules by the blood- aqueous barrier. The presence of such a barrier requires active-transport systems either for moving substances across the cellular layer into the eye or for removing those substances from the aqueous humor. • The rate of aqueous humor formation differs among species, being approximately 2 jlL/min in humans. lOP is maintained by continuous aqueous formation and drainage, which allows the surrounding tissues to remove the waste products of
metabolism. Inhibitors of enzymatic processes decrease aqueous humor inflow by different amounts and thus provide additional evidence for active secretory processes. Carbonic anhydrase inhibitors and P-blockers (discussed later in this chapter) are used systemically and topically in the treatment of glaucoma to reduce the rate of aqueous humor formation. See Chapter 17 of this volume and BCSC Section 10, Glaucoma, for more detail.
Eicosanoids Types and Actions Eicosanoids, which include PGs, prostacyclin (PGI,), thromboxanes (TXA,), and leukotrienes, are an important family of compounds with hormonal activity. They are synthesized as a result of phospholipase A, (PLA, ) stimulation, which causes the release of arachidonic acid (AA) from membrane glycerolipids (Fig 9-1). These agents affect both the male and the female reproductive systems, the gastrointestinal system, the cardiovascular and renal systems, the nervous system, and the eye. PGI, and TXA, are natural biological antagonists. PGI" which is synthesized mainly in the endothelial cells of vascular
256 • Fundamentals and Principles of Ophthalmology
I
Memb"ne phospholipid
Corticosteroids
8
r cox-, (constitutive)
eOSPhonp"e A, 1 /
8
NDGA
leukotrienes,lIpoxlns
G> 5-Upoxygenase
y1'-------'------+L 0 I ____ Cytochrome P·450
Free arachidonic add (M)
(±)
'po _ '_ 'd_eS_ _ _-"
e COX-2 e e
(inducible)
e
I
I
NSAIDs
Endoperoxide intermediates (PGG z. PGH z)
COX-2 inhibitors
I
PGEz (raises lOP) Isomerization
I
Primary prostaglandins
I I
PGF Za (raises lOP) Vascular endothelium
PGI2 ____ 6-Keto-PGF 1a
Thromboxane synthetase
Figur.9-' An outl ine of the synthesis of prostaglandins (PGs) and leukotrienes from arachidonic acid . In response to stimulation of a target cell w ith a relevant stimulus (eg, a cytokine, neurotransmitter, various pharmacologic agents), phospholipase Az is activated, and arachidonic acid is released from the sn-2 posi tion of membrane phospholipids. Arachidonic acid IS
then converted by cyclooxygenase , (COX-1) or cyclooxygenase 2 (COX-2) to PGH" and
then PG H2 is isomerized to biologically active prostanoid products. Arachidonic acid can
also be metabolized through the 5-lipoxyge nase and cytochrome P-450 pathways to generate leukotrienes and epoxides, respective ly. Phospholipase A2 can be inhibited by corticosteroids such as dexa methasone; COX-', by nonsteroidal anti-inflammatory drugs (NSAIDs) such as indomethacin and aspirin; COX-2, by DUP697, SC58125, L-745-337, and NS398; and the 5-l ipoxygenase pathway, by nordihydroguaiaretic acid (NDGA). TXA, = thromboxane A2 , TXB z = thromboxane 8 2 , (Courtesy of Ata Abdel-Latif, PhD.)
tissues. is a potent vasodilator. a potent platelet-antiaggregating agent. and a stimulator of adenylate cyclase. In contrast. TXA,. which is synthesized mainly by platelets. is a potent vasoconstrictor and a platelet -aggregating agent. Prostaglandins have profound effects on inflammation in the eye. aqueous humor dynamics. and blood-ocular barrier functions. PGs of the E and F subtypes and AA. when administered intracamerally or topically at high concentrations. cause miosis. an elevation oflOP. an increase in aqueous protein content. and the entry of white cells into the aqueous and tear fluid. Evidence indicates that some antiglaucoma drugs. such as epinephrine. may affect lOP by influencing the production of PGs. More recently. PGs and their derivatives have been found to be useful as antiglaucoma agents. For example. several PGF, receptor agonists (latanoprost. bimatoprost. travoprost) have been shown to decrease lOP
CHAPTER g,
Iris and Ciliary Body.
257
by 27%-35%. Unlike ~-blockers. CA inhibitors. and CI, -agonists. PG analogues act on enhancement of outflow rather than formation of aqueous humor.
Synthesis Arachidonic acid can be released from plasma membrane phospholipids through a wide variety of stimuli: inflam_rnatory. immunologic. neural. chemical. or simple mechanical agitation. Free AA reacts either with cydooxygenase {also known as PC synthetase}. the first enzyme of the PG biosynthetic sequence. or with lipoxygenase to generate hydroperoxy fatty acids {see Fig 9-1} . Note that these are 2 isoforms of cydooxygenase {COX-l and COX-2}. In the cydooxygenase reaction. the released arachidonate is converted into endoperoxides {PGG, and PGH,} by the membrane-bound cydooxygenase. The endoperoxides are then converted to TXA, by thromboxane synthetase or to various PGs by isomerase or reductase enzymes. Most nonsteroidal anti-inflammatory drugs {NSAIDs}. such as indomethacin and aspirin. can block PG biosynthesis from AA via the COX-I reaction. The NSAIDs bind irreversibly to the cydooxygenase enzyme. Topical NSAIDs have been used in the treatment of anterior segment inflammation. aphakic and pseudophakic cystoid macular edema. allergic conjunctivitis. and other pain after refractive surgery. Flurbiprofen 0.03% {Ocufen} and suprofen 1% {Profenal} drops are used preoperatively for the prevention of PG-mediated pupillary miosis during ocular surgery. Didofenac 0.1% {Voltaren Ophthalmic} has been approved for the treatment of postoperative inflammation following cataract extraction. Ketorolac t,omethamine 0.5% {Acular} is indicated for the relief of itching from allergic conjunctivitis. Several studies support the use of either didofenac 0.1 % or ketorolac 0.5% to reduce the severity and duration of pseudophakic macular edema thought to be due to ciliary body-derived cytokines and in relieving corneal pain after radial keratotomy. . Prostaglandin biosynthesis. via the COX-2 reaction. can also be blocked by recently developed COX-2 inhibitors. Previously developed NSAIDs {ego ibuprofen. naproxen} inhibit both COX-l and COX-2 and compete with arachidonate in binding to the cydooxygenase-active site. These compounds are effective anti -inflammatory agents. but they are also all quite ulcerogenic when given systemically. In response. pharmaceutical firms have developed new cyclooxygenase inhibitors that selectively inhibit COX-2 {while sparing inhibition of COX- I} . These efforts were initially driven by 2 notions. which subsequently proved to be correct: I. COX-2 is the releva nt enzyme in inflammation {it is expressed at low levels under
normal physiologic conditions and regulated only in response to pro-inflammatory Signals}. 2. Constitutively expressed COX-I {but not COX-2} is present in a variety of tissues {including the inner lining of the stomach}. Indeed. COX-2 inhibitors have been reported to be anti-inflammatory and analgesic and to lack gastrointestinal toxicity. Moreover. they provide time-dependent. reversible inhibition of the COX-2 enzyme. However. oral COX-2 inhibitors. including rofecoxib
258 • Fundamenta ls and Principles of Ophthalmology (Vioxx), celecoxib (Celebrex), and valdecoxib (Bextra ), have been reported to increase the risks of cardiovascular toxicity and complications (eg, myocardial infarction). Because each of these different COX-2 inhibitors has been found to be associated with cardiovascular events, the complication appears to be a class effect.
Leukotrienes Leukotrienes are also formed from AA through the lipoxygenase pathway. Their actions in intraocular tissues and fluids has not yet been thoroughly investigated. Leukotrienes C4 and D4 have been identified as the major active constituents of the slow-reacting substance of anaphylaxis (SRS-A) and are powerful smooth-muscle contractors that alter muscle permeability. They are much more active than histamine. NSAlDs do not inhibit lipoxygenase, but nordihydroguaiaretic acid (NDGA) does.
Prostaglandin Receptors There is a large fam ily of G-protein-coupled, 7-transmembrane PG receptors. Complex specificity and regulatory functions arise because individual receptors have differing and overlapping sp~cificities for individual PGs, they are specifically distributed among different cells and tissues, and there are different coupling mechanisms in different cells. The variety of PGs and the variability in receptors, which was described earUer, leads to very complicated and incompletely understood pathophysiologic functions for most PGs. In general, PGs play key roles in regu lation of smooth-muscle contrac tility; in mediation of pain and fever; in regulation of blood pressure and platelet aggregation; and in other physiologic defense mechanisms, including immune and inflammatory responses. Inh ibition of cyclooxygenases, and therefo re prostaglandins, is the mechanism for much of the analgesic, anti-inflammatory, antipyretic, and anti thrombotic effects of NSAlDs. Abdel -Latif AA. Release and effects of prostaglandins in ocu lar tissues. Prostaglandins Leukot
Essent Fatty Acids. 1991 ;44(2);71 - 82. Colin J. The role of NSAIDs in the management of postoperative ophthalmic innammation. Drugs. 2007;67(9); 1291- 1308. Oraze n JM . COX-2 inhibitors-a lesson in unexpec ted problems. N ElIgl/ Med. 2005;352(1 I): 1131 - 1132. Rho OS. Treatment of acute pseudophakic cystoid macular edema: diclofenac versus ketorolac.
J Cataract Refract Surg. 2003;29(12);2378 - 2384.
Neurotransmitters. Receptors. and Signal Transduction Pathways In the iris- ciliary body, the sphincter and Ciliary muscles have parasympathetic innervation by the third cranial (oc ulomotor) nerve, and cholinergic impulses are transmitted to the muscle by acetylcholine (ACh). In the Ciliary processes, non medullated nerve fibers, many of them adrenergic, surround the blood vessels. The dilator muscle fibers of the iris are innervated by the sympathetic nerves from the superior cervical ganglion, and the adrenergic nerve impulses are transmitted to the muscle cells by norepinephrine (NE).
CHAPTER 9,
Iris and Ciliary Body . 259
The neurons that synthesize, store, and release ACh are called cholinergic neurons; those that synthesize, store, and release NE are called adrenergic neurons. There are also 2 major types of auto nomic receptors: cholinergic receptors receive input from cholinergic neurons; adrenergic receptors, from adrenergic neurons. These re-
ceptors are further divided as shown in Table 9-1. The receptors of both the iris sphincter and the ciliary muscle are of the cholinergic muscarinic type, and the iris dilator is mainly of the a-adrenergic receptor type. In addition, the iris muscles contain sensory nerves. The sensory neurotransmitters include substance P and calcitonin gene-related peptide (CGRP), which may be involved in trophic effects, inflammatory reactions, and direct or indirect regulation of iris muscle tone in many species, including humans.
Miotic. Miotic agents, which constrict the pupil, act either by stimulating the sphincter (cholinergic agonists) or by blocking the dilator (adrenergic blockers) .
Cholinergic agonist miotics Sphincter stimulators produce responses similar to ACh. They contract the sphincter (resulting in an increase in pupillary contraction) and the Ciliary muscles (resulting in accommodation). Sphincter activity can be affected by 2 mechanisms: 1. direct action on the muscles through ACh, carbachol, and pilocarpine 2. inhibition of acetylcholinesterase (AChE), either reversibly with physostigmine (Eserine) or neostigmine (Prostigmin), or irreverSibly with diisopropyl fluorophosphate (DFP) or echothiophate iodide (Phospholine Iodide). The AChE inhibitors allow ACh to accumulate at the parasympathetic third nerve endings and produce accommodative spasm.
Table 9-1 Cholinergic and Adrenergic Receptors Receptors
Agonists
Bloc ki ng Agents
Cholinerg ic (sphincter)
Acetylc holine Muscarine Nicotine
Atropine d-Tubocurari ne
Muscarinic Nicotinic Adrenergic (dilator) Alpha *
<x, <x, Beta
p, p,
Norepinep hrine Phenylephrine Phenylephrine Apraclonidine Isoprote ren ol
Tazalot Albuterol
Phentolamine and phenoxybenzamine Prazosin . thymoxamine. dapiprazole Yohimbine Propranolol and tim olol Betaxolol Butoxamine
The cholinergic agonists and the adrenerg ic blockers listed cause miosis; the adrenergic agonists and the cholinergic blockers listed cause dilation . • The prefixes <X l and <X2 have been proposed for post- and presynaptic <x-adrenoceptors, re spectively. According to the present view, the classification into <Xl and <X2 subtypes is based exclusively on the relative potencies and affinities of agonists and antagonists, regardless of their functi on and loca lization .
260 • Fu ndame ntals a nd Principles of Ophthalmology
Adrenergic antagonist miotics Two mechanisms for the action of dilator blockers are available: 1. inhi bition of the release of NE at the myoneural junction by guanethidine (Ismeli n). which acts by depleting NE stores at the nerve term inals (once these stores are depleted. miosis follows) 2. blockage of the a-adrenergic receptors of the dilator muscle by thymoxamine. dapiprazole. phenoxybenzam ine. di benamine. or phentolami ne. which prevent contraction and produce miosis (miosis is due to the unopposed action of the iris sphincter. which is tonically innervated) The pupillary dilator muscle has predominantly a-adrenergic receptors. whereas the ciliary muscle is under parasympathetic control. Thymoxamine and dapiprazole. which are selective o,-adrenergic blocking agents. can cause miosis through d ilator-muscle paralysis without affecting the ciliary muscle-controlled fac ility of outflow. lO P. or amplitude o f accommodat ion. Thymoxamine has been advocated for many uses but is not commercially available in the United States. Until recently. dapiprazole was commercially available fo r reversal of pupillary dilation after pharmacologic mydriasis. With thymoxamine or dapiprazole. the pupil returns to its predilated state in 30 minutes (versus 3-4 hours without these agents). These agents will not prevent angle-closure glaucoma. but they are able to move the pupil thro ugh the more dangerous mid-dilated state more quickly than when they are not used.
Mydriatics Mydriatic age nts. wh ich dilate the pupil. act by stimulating the dilator (adrenergic agonists) or by blocking the sphincter (cholinergic blockers).
Adrenergic agonist mydriatics Dilator stimulators increase dilator ac tivity in 3 ways: 1. by increasing NE release. as with hydroxya mphetamine. which causes NE to be released rapidly. thus resulting in mydriasis 2. by interfe ring with NE reuptake. as with cocai ne. which. in addi tion to acting as a local anesthetic. preve nts NE reuptake (inactivation) and thus prolongs or potentiates the action of released NE 3. by directly stimulating the a i-receptors ofth e dilator. as wit h phenylephrine (NeoSynephrine)
Cholinergic antagonist mydriatics An ad renergic agent is only mydriatic. but an anticholinergic is mydriatic and cycloplegic. An effective anticholinergic (sphincter-blocking) age nt that produces both mydriasis and cycloplegia is atropine. which blocks the action of ACh and pilocarpine at the muscle. Its effects are long lasting (a single drop may last fo r days) . A drug that produces rapid cycloplegia but has a relati vely short action is tropicamide. Cyclopentolate is a blocker of intermediate action and duration. Other agents include homatropine and scopolamine (see Chapter 17. Table 17-2).
CHAPTER 9:
Iris and Ciliary Body .
261
Calcium Channels and Channel Blockers Calcium channels are membrane-bound protein receptors that contain multiple subunits, one of which is the active site for binding of calcium channel blockers. Calcium plays a majo r role in influencing cellular function, and the most common pathway for entry into cells is through the calcium channels. Six subclasses of calcium channe l blockers have been identified: L, T, N, P, Q, and R. The L-type blocker is predominant in skeletal, cardiac, and vascular smooth muscle. Calcium channel blockers bind to membrane-bound calcium channels and inhibit the influx of extracellular calcium in vascular smooth muscle, thereby causing direct arteriola r vasodilation and depression of myocardial contractility. They are widely used for the treatment of hypertension an d, in therape utic doses, do not affect glucose tolerance, lipoproteins, uric acid, or serum electrolytes. Topically applied calcium channel blockers lower lOP. Because normal-tension glaucoma may be associated with vasospastic disease, calcium channel blockers may playa role in the treatment of this poorly understood condition. See also BCSC Section 10, Glaucoma. Netland PA, Erickson KA. Calcium channel blockers in glaucoma management. Ophthalmol Clin North Am. 1995;8:327-334.
Membrane Receptors and Intracellular Communications Signal Transduction Receptors for neurotransmitters and peptide hormo nes are located on the surface of the cell, whereas receptors for steroid hormones are intracellular, so steroid hormones must be able to enter the cell to have an effect. Both the iris and the ciliary body contain adren ergic, muscarinic cholinergiC, peptidergic, PG, serotonin, and purinergic receptors. Mem brane receptors have several characteristics: T hey recogni ze and bind to first messengers, or ligands-molecules such as ho rmones, peptides, drugs, and neurotransmitters. T hey are ligand -specific, but many can cause positive or negative responses depending on their location (ie, different cell types may have different responses to the same messenger) . The number of receptors occupied by ligands determines the magnitude of the cell response.
Receptor-Effector Coupling The response of the cell to th e ligand depends on Signal transducti on across the plasma membrane to the cell interior, which occurs by several basic mechanisms, including • A ligand (eg, neurotransmitters such as glyc ine, glutamate, y-aminobutyric acid) binds to a receptor that functions as an ion channel (a ligand-gated ion channel) and causes it to open, allowing cations to pass into the cells. Receptors that have in tegral enzyme (eg, tyrosine kinase) activity are activated by ligand binding.
262 • Fundamentals and Principles of Ophthalmology
• G-protein-coupled receptors activate effector proteins, which include ion channels and enzymes.
The G-protein -coupled receptor mechanism involves a cascade of 3 steps: A ligand excites a receptor protein to activate a G protein, which in turn activates an effector protein
(E). The effector proteins are usually enzymes such as adenylate cyclase, phospholipase C (PLC), phospholipase A" or phosphodiesterase (PDE). These produce second messengers such as cyclic nucleotides (eg, cAMP, cGMP) and lipid-derived molecules (eg, PGs, discussed earlier in this chapter, and IP" discussed next). Rhodopsin, one of the most studied G-protein-coupled receptors, is discussed in detail in Chapter 13.
Cyclic AMP and Polyphosphoinositide Turnover Activated p-adrenergic and CGRP receptors stimulate the adenylate cyclase system via the stimulatory G proteins (G,u); others, such as M,-muscarinic and " , -adrenergic receptors, inhibit this effector enzyme via an inhibitory G protein (G,u) (Fig 9-2). The primary control for many intracellular events is the concentration of free Ca" in the cytosol. Many Ca'+ -mobiliZing hormones and neurotransmitters, such as ACh, NE, and PGF,a, act by changing the intracellular Ca2+ concentration. Activation of Ca'+mobilizing receptors. such as M3 -muscarinic and a t-adrenergic receptors, causes the
Agonist
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~
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t
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t
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t
Physiologic response
Figure 9-2 Scheme showing the activa tion and inhibition of the adenylate cyclase secondmessenger system. Adenos ine diphosphate (AOP) ribosylation of G ~ by cholera tox in maintains GS!I in the active state , wherea s ADP ribosylation of Gio by pertussis toxin resu lts in
inactivation of GIll" 5 ' -AMP = adenosine 5'·monophosphate; ATP = adenosine triphosphate; cAMP = 3'.5'-cyclic adenosine monophosphate; GOP = guanosine diphosphate; Gs and Gi = stimulatory and inhibitory G proteins. re spectively; GTP = guanosine triphosphate; POE = phosphodiesterase; PGE2 = prostaglandin E2 : Rs * and Ri* = activated receptor complexes; VIP = vasoactive intestinal polypeptide . (Courtesy of Ara Abde/-Lard, PhD.)
CHAPTER 9: Iris and Ciliary Body. 263
release of Ca" from the sarcoplasmic reticulum (SR) via the polyphosphoinositide second messenger IP, . Phosphatidylinositol 4,5-bisphosphate (PIP, ) turnover also has a role in the regulation of ICa" l1 (Fig 9-3). In the iris sphincter, as weU as in other types of smooth muscle. activation of muscarinic receptors leads to [P3 production. Ca 2+ mobilization. and muscle contraction. In contrast, activation of p-adrenergic receptors leads to cAM P for mation, reduction in in tracell ular Ca 2+ concentration, and muscle relaxation. Contraction- relaxation responses are regulated by increases in the level of cAMP. In the iris sphincter, PGs can trigger either IP, production and muscle contrac tion or cAMP formation and muscle relaxation, depending on the type of PG, species, cell type, and tissue. Such interactions between the 2 signaling systems could constitute the biochemical basis for the multiple functions observed for PGs in a wide variety of systems. The sympathetic ne rvo us system , via alterations in cAMP concentrations, can modulate the cholinergic stimulation of IP, production and muscle contraction. Agonist + R
t
R' G,"" . GTP ~Gq
~®
GDP GTP
I Phospholipase Cop, I
t P1P2
t Ca2• -SR - I P + DAG
t
' t
Ca-CaM kinase
Protein kinase C
"x
/
Protein phosphorylation
t
Physiologic response
Figure 9-3 Scheme showing agonist-stimulated breakdown of phosphatidylinosltol 4,5bisphosphate (PIP,) into inositol I A,S-triphosphate liP,) and diacylglycerol (OAG). The agonists bind to the Ca 2+-mobilizing receptor to activate the G protein Gq to G~q and stimula te phospholipase C-PI to hydrolyze PIP, into IP 3 and DAG. IP, diffuses through the cytosol and binds to the IP3 receptor on the sarcoplas mic reticu lu m (SA) to release Ca 2+ from a specif ic Ca 2+ pool. Ca 2+ binds to ca lmodul in (CaM); Ca 2+-calmodulin then activates a protein kinase to ph osphorylate a protein substrate and provoke a cellular response. IP 3 is then rapid ly metabolize d by a series of dephosphorylation and phosphorylation pathways. DAG acts by binding to protein kinase C. Once activated, protein kinase C trans locates to the plasma membrane and there phosphorylates a number of proteins on serine and threon ine residues. Phorbol esters mim ic the action of DAG on protein kinase C; in fact , the enzyme is presumed to be the recepto r fo r phorbol esters. GOP = guanosine diphosphate, GTP = guanosine triphosphate, W = activated receptor comp lex. (Courtesy of Ata Abdef-Lalif, PhD.)
264 • Fundamentals and Principles of Ophthalmology
Receptors Examples of ocular receptors. their subtypes. and their signal transduction mechanisms are given in Table 9-2. In recent years. a number of reports have appeared on the biochemical and pharmacologic characterization of these receptors. Understanding ocular receptors and their transduction mechanisms will help us design better therapeutic agents for the treatment of glaucoma. This can be appreciated from the summary on the mode of action of the various antiglaucoma agents provided in Table 9-3. The therapeutic actions of these drugs are mediated through specific receptors. Abdel-Latif AA. Iri s-ciliary bod y, aqueous humor and trabecular meshwork. In: Harding H, ed. Biochemistry oj the Eye. London: Chapman and Hall Medical; 1997:52-93.
Table 9-2 Ocular Receptor Subtypes and Their Signal Transduction Mechanisms Receptor p-Ad renoceptor a 1-Adrenoceptor
° 2-Ad renocepto r Muscarinic cho linergic Ca lcitonin gene-related peptide Endothelin
Receptor Subtypes and Their Signal Transduction Mechanisms ~,
~,
~,
cAMP ;
cAMP ;
cAMP ;
a,A IP, /Ca" /DAG a,A cAMP t
a,B IP, /Ca" /DAG a ,B cAMP t
a ,D IP, /Ca" /DAG a ,C cAMP t
a,D cAMPt
M, IP,/Ca" /DAG
M, cAMP t
M, IP, /Ca" /DAG
M, cAMP t
CGRP, cAMP;
ETA IP,/Ca" /DAG cAMP t
Prostaglandin
CGRP, cAMP; ET, IP, /Ca" /DAG DP, EP (EP " EP,. EP" EP,1. FP, Ip, TP
Increase in cAMP or 1P3 dependin g on the receptor subtype
Table 9-3 Mode of Action of Antiglaucoma Agents That Act Through Receptors Primary Mechanism of Action
Drug Class
Examples
1. Decrease aqueous humor production
a. p-Adrenerg ic antagonists
TImolol, betaxolol. carteolol , levobuno lo l Apraclonidine , brimonidine
2. Increase trabecular outflow
a. Miotics b . Adrenergic agonists
3. Increase uveoscleral outflow
a. Prostaglandins
b. a 2-Adrenergic agonists
b. a -Adrenergic agonists
Pilocarpine Epinephrine, dipivalyl epinephrine Latanoprost, bimatoprost, travoprost Apraclonidine, brimonidine
CHAPTER
10
Aqueous Humor
The aqueous humor is important in the physiology of the mammalian eye. It provides nutrients (eg, glucose and amino acids) to support the function of tissues of the anterior segment, such as the avascular lens, cornea, and trabecular meshwork; it also removes the metabolic wastes fro m these tissues (eg, lactic acid, pyruvic acid). Aqueous humor also helps to maintain appropriate lOP. These functiona l properties are essential to the eye's structural integrity. In addition, because the aqueous humor is devoid of blood cells and of more than 99% of the plasma proteins, it provides an optically clear medium for the transmission of light along the visual path.
Aqueous Dynamics The aqueous humor is secreted by the ciliary epithelium at a flow rate of2-3 ~L1min. The ciliary epithelium is a bilayer of polarized epithelial cells lining the surface of the ciliary body; the 2 cell layers are nonpigmented ciliary epithelium (NPE), which faces the aqueous humor through the cells' basal plasma membrane; and pigmented ciliary epithelium (PE), which faces the stroma, also through the cells' basal plasma membrane. Therefore, the apical plasma membranes of both NPE and PE cells appose each other, est~blishing cell-tocell communication through numerous gap junctions. Of the 2 cell layers that constitute the Ciliary epithelium, the NPE cells establish a blood-aqueous barrier by the presence of tight junctions proximal to the apical plasma membrane, thus preventing the free passage of plasma proteins and other macromolecules from the stroma to the posterior chamber. In contrast, the PE cell layer is considered a leaky epithelium, as it allows the movement of solutes through the intercellular space between the PE cells.
Composition of the Aqueous Humor Table 10-\ summarizes the composition of aqueous humor. It is clear from the values given in the table that the fluid and electrolyte composition is similar to that of plasma. However, the aqueous secretion is not an uhrafiltrate of plasma (as was once speculated), because it is produced by energy-dependent processes in the epithelial layer of the ciliary body; this mode of production allows precise control to be maintained over the composition of fluid bathing structures essential for normal vision. Macknight AD, McLaughlin CW, Peart D, Purves RD, Carre DA, Civan MM. Formation of the aqueous hum or. Ciin Exp Pharmacal Physiol. 2000;27(1 - 2}:100-1O6.
265
266 • Fundamenta ls and Principles of Ophthalmology Tablel0-l Composit ion of Aqueous Humor Components
(mmol/kg H2O) Rabb it Na K
el HeO, Ascorbate Lactate Glucose
Human Na el HeO, Ascorbate Glucose
Aqueous
Pl asma
143 4.6 108 25 0.04 10.3 6 146 109 28 0.04 6
Posteri or
Anterior
159 4.7 97 34 1.4 9.9 6
138 4.3 101 30 1.1 9.3 6 163 134 20 1.06 3
Vitreous
134 4.6 105 26 0.46 12 4.6 144 114 20- 30 2.21 3.4
From Macknight AD, MacLaughlin CW, Peart D, Purves RD, Carre OA, Civan MM . Formation of the aqueous hu mor. Clio Exp Pharmacal Physio/' 2000;27(1-2) :100- 106.
The ionic composition of aqueous humor is determined by selective active-transport systems (eg, Na+,K+-2Cl- symport, Cl-- HCO,- and Na"H+ antiports, cation channels, water channels, Na+,K+-ATPase, K+ channels, CI- channels, H+- ATPase) that participate in secretion of aqueous humor by the ciliary epithelium. The systems' activity and cellular distribution along the cell membranes of PE and NPE cells determine unidirectional net secretion from the stroma to the posterior chamber, a process that involves 3 steps: 1. uptake of solute and water at the stromal surface by PE cells 2. transfer from PE to NPE cells through gap junctions 3. transfer of solute and water by NPE cells into the posterior chamber
Likewise, it is thought that there is a mechanism for transporting solute and water from the posterior chamber back into the stroma. In this unidirectional reabsorption, another set of transporters may be involved in extruding Na"K+ and Cl- back into the stroma. Krupin T, Civan MM . Physiologic basis of aqueous humor formation. In: Ritch R, Shields MS, Krupin T. eds. The Glaucomas. 2nd ed. Vol 1. SI Louis: Mosby; 1996:25 1-280.
Molecular studies have shown that the secretory properties of the Ciliary epithelium are not limited to ions and electrolytes but extend to a wide range of molecules of different molecular mass. A common feature for many of these molecules is their syntheSis locally in the Ciliary epithelium and then their secretion by the NPE cells through the regulatory pathway into the aqueous humor. Among the proteins identified whose mRNA expression has been demonstrated are • plasma proteins (eg, complement component C4, u,-macroglobulin, selenoprotein P, apolipoprotein D, plasma glutathione peroxidases, angiotensinogen)
CHAPTER 10:
Aqueous Humor.
267
proteinases (eg, cathepsin D, cathepsin 0) • a component of the visual cycle (eg, cellular retinaldehyde-binding protein, or CRALBP)
a neurotrophic factor (eg, pigment epithelium- derived factor) neuropeptide-processing enzymes (eg, carboxypeptidase E, peptidyl-glycine-aamidating monoxygenase) neuroendocrine peptides (eg, secretogranin II, neurotensin, galanin) bioactive peptides and hormones (eg, atrial natriuretic peptide, brain natriuretic peptide) These findings support the view that the ciliary epithelium exhibits neuroendocrine properties that are directly related to the makeup of the aqueous humor and its regulation. The aqueous humor composition is in dynamiC equilibrium, determined both by its rate of production and outflow and by continuous exchanges with the tissues of the anterior segment. It contains the following: • inorganic ions and organic anions
• carbohydrates • glutathione and urea proteins growth-modulatory factors oxygen and carbon dioxide
Inorganic Ions The concentrations of sodium, potassium, and magnesium in the aqueous are similar to
those in plasma, but the level of calcium is only half that of plasma. The 2 major anions are chloride and bicarbonate. Phosphate is also present in the aqueous (aqueous:plasma ratio: ~0.5 or less), but its concentration is too low to have any significant buffering capacity. Iron, copper, and zinc are all found in the aqueous humor at essentially the same levels as in plasma, approximately I mg/mL.
Organic Anions Lactate is the most abundant of organic anions in the aqueous, and its concentration is
always higher than that in plasma. Plasma and aqueous levels oflactate are directly related, and the contribution resulting from the glycolytiC metabolism of intraocular tissues is significant. Ascorbic acid (vitamin C) is perhaps the most unique constituent of the aqueous humor. In most mammalian species, its concentration ranges from 0.6 to 1.5 mmollL, levels that are some 10- 50 times higher than that in plasma. Furthermore, levels of ascorbic acid in the aqueous humor of diurnal mammals may be 20 times higher than ascorbic acid levels in nocturnal mammals. Ascorbic acid is an important antioxidant, both in the aqueous humor and in tissues of the anterior segment. Ascorbic acid is actively trans-
ported by the ciliary epithelial cells from the stroma side into the posterior chamber by a Na+-dependent L-ascorbic acid transporter.
268 • Fundamentals and Principles of Ophthalmology
Carbohydrates Glucose concentration in the aqueous is roughly 70% of that in plasma. The rate of entry of glucose into the posterior chamber is much more rapid than would be expected from its molecular size and lipid solubility, suggesting that its passage across the ciliary epithelium occurs by facilitated diffusion. Saturation studies have demonstrated that a specific carrier is involved, but no evidence of active transport has been detected, nor has insulin been found to affect the entry of glucose into the aqueous. Aqueous glucose levels are increased in people with diabetes, leading to higher concentrations in the lens, with short-term refractive and longer-term cataract implications. Inositol, important for phospholipid synthesis in the anterior segment, occu rs at a concentration approximately 10 times that in plasma.
Glutathione and Urea Glutathione, an important tripeptide with a reactive sulfhydryl group, is found in the aqueous humor. Its concentration in primates ranges from 1 to 10 I'mollL. Blood contains a high concentration of glutathione, but vi rtually all glutathione resides within the erythrocytes, and plasma has only a low concentration of 5 I'mollL or less. Although aqueous glutathione may be derived by diffusion from the blood or by an ac tive-transport system in the Ciliary epithelium analogous to that of the lens, it probably also arises by loss from the lens and cornea. Glutathione acts as a stabilizer of the redox state of the aqueous by reconverting ascorbate to its functional form after oxidation, as well as by removing excess hydrogen peroxide. Glutathione also serves as a substrate in the enzymatic conjugation by a group of cytosolic enzymes involved in the cellular detoxification of electrophilic compounds. These enzymes (glutathione S-transferases) are important in protecting tissues from oxidative damage and oxidative stress and are highly expressed in the ocular ciliary epithelium. The concentration of urea in the aqueous is between 80% and 90% of that in plasma. This compound is distributed passively across nearly all biological membrane systems, and its high aqueous:plasma ratio indicates that this small molecule (molecular weight of 60 kD) crosses the epithelial barrier quite readily. Urea is effective in the hyperosmotic infusion treatment for glaucoma. However, mannitol (with a molecular weight of 182 kD) is preferred to urea because it crosses the barrier more easily.
Proteins The nonpigmented ciliary epithelial cell laye r establishes a blood-aqueous barrier that prevents the diffusion of plasma proteins from the stroma into the posterior chamber; nevertheless, plasma proteins do enter the aqueous humor, and one possible route for this is through the root and anterior surface of the iris. No rmal aq ueous contains approximately 0.02 g of protein per 100 mL, as compared with the typical plasma level of7 gl100 mL. The most abundant plasma proteins identified in aqueous humor are albumin and transferrin, which together may account for 50% of all the protein content. However, there is compelling evidence that proteins that make up the aqueous humor might actually have been syntheSized within the ciliary body and secreted directly into
CHAPTER 10:
Aqueous Humor. 269
the aqueous humor. Molecular techniques (such as the screening of eDNA libraries constructed from the intact human and bovine ciliary bodies) have resulted in the isolation and identification of numerous protein-encoding genes. These studies, therefore, challenge the long-held view that plasma proteins in the aqueous humor are transported into the aqueous from outside the eye. Among cDNAs isolated from the Ciliary body-encoding plasma proteins are • complement component C4, which participates in immune-mediated inflammation responses oy macroglobulin. which serves as a carrier protein and is involved in proteinase inhibition, clearance, and targeting; and processing of foreign peptides • apolipoprotein D, which binds and transports hydrophobic substances, including cholesterol, cholesteryl esters, and arachidonic acid selenoprotein P, which has antioxidant properties
Proteinases and inhibitors A number of proteinases and proteinase inhibitors have also been identified in the aqueous humor. These proteinases include cathepsin D and cathepsin 0, which are synthesized and secreted by the ciliary epithelial cells. Cathepsin D is involved in the degradation of neuropeptides and peptide hormones and has been found in high levels in the cerebrospinal fluid of patients with Alzheimer disease. Less is known about cathepsin 0; it may be involved in normal cellular protein degradation and turnover. Of the proteinase inhibitors, a,-macroglobulin and ai-antitrypsin are perhaps the most extensively studied. An imbalance in equilibrium between proteinases and proteinase inhibitors could lead to an alteration in aqueous humor composition, which may result in disease (eg, glaucoma). Enzymes Activators, proenzymes, and fibrinolytic enzymes are present in the aqueous, and these enzymes could playa role in the regulation of outflow resistance. Both plasminogen and plasminogen activator are found in human and monkey aqueous, but only traces of plasmin have been reported. In addition to the proteins already mentioned, other enzymes have been reported in the aqueous humor. Several of these are of interest chiefly because of their increased levels in certain pathologic conditions. such as retinoblastoma. wherein tissue damage results in the release of intracellular enzymes. The frequent absence of coen zymes or the substrates of such enzymes from the aqueous (as in the case of nicotinamide adenine dinucleotide for lactic dehydrogenase or of oxaloacetate for glutamic-oxaloacetic transaminase) leads to the conclusion that these enzymes have no Significant catalytic role in the normal aqueous. However, 3 enzymes in the aqueous appear to be exceptions to this no nreac tive role: 1. Hyaluron idase may be important in the normal regulation of the resistance to
outflow through the trabecular meshwork because its injection into the anterior chamber has been shown to increase outflow facility in some species. 2. Carbonic anhydrase is present in trace amounts. Because it is an enzyme with an extremely high turnover, even a low concentration may be Significant in the catalysis of the equilibrium between bicarbonate and CO, plus water.
270 • Fundamentals and Principles of Ophthalmology
3. Lysozyme provides significant antibacterial protection. Most likely, lysozyme originates only in part from the blood; in cases of ocular inflammation, the intraocular level may be raised tenfold or more, reaching concentrations well above those in plasma. Neurotrophic and neuroendocrine proteins The Ciliary epithelia, which are derived from neuroectoderm, are functionally similar to neuroendocrine glands elsewhere in the body. Bioactive neuroendocrine markers identified from human ciliary body eDNA subtraction studies, such as neurotensin, angiotensin, endotheli ns, and natriuretic peptides, have systemic vascular hemodynamic effects and, by implication, may have similar roles in lOP regulation or aqueous secretion. Some authors have linked circadian lOP rhythms to these markers, but this has not been clearly established because corneal biomechanical properties also fluctuate. The neuroendocrine properties of the ciliary epithelium could also determine the composition of the aqueous humo r. Neuropeptide-processing enzymes include prohormone convertases, carboxypeptidase E, and peptidylglycine a -amidating monooxygenase; among neuropeptides and peptide hormones are secretogranin II, galanin, neurotensin, and natriuretic peptides. These compounds, synthesized as large precursors, are cleaved to bioactive peptides and secreted by the ciliary epithelial cells into the aqueous in a regulated, calcium-dependent manner. The function of these bioactive peptides in the aqueous humor secretion is not clearly understood. Coca-Prados M, Escribano
J. Ortego j.
Different ial gene express ion in the human ciliary epi-
thelium. Prog Retin Eye Res. 1999,18(3)A03- 429. Cowdrey G. Firth M. Moss R. Karim A. Thompson G. Firth G. The analysis of aqueous humor constituents using capillary zo ne elec trophoresis. Exp Eye Res. 1998;67 (4):449- 455.
Growth-Modulatory Factors The physical and chemical properties of the aqueous humor playa substantial role in modulating the proliferation, differentiation, functional Viability, and wound healing of ocular tissues. These properties are largely influenced by a number of growth-promoting and differentiation factors that have been identified or quantified in aqueous humor. They include transforming growth factor ~s 1 and 2 (TGF- ~, and -~2) acidic and basic fibroblast growth factor (aFGF and bFGF) insulin-like growth factor I (IGF-I) • insulin-like growth factor binding proteins (IGFBPs) • vascular endothelial growth factor (VEGF) • transfe rrin
The growth factors in the aqueous humor perform diverse, synergistic, and sometimes opposite biological activities. Normally, the lack of significant mitosis of the corneal endothelium and trabecular meshwork in vivo is probably controlled by the complex coordination of effects and interactions among the different growth-modulatory substances present in the aqueous humor (see Part V, Ocular Pharmacology). Disruption in the balance among various growth factors that occurs with the production of plasmoid aqueous
CHAPTER 10:
Aqueous Humor. 271
humor may explain the abnormal hyperplastic response of the lens epithelium and corneal endothelium observed in chronic inflammatory conditions and traumatic insults to the eye. Ultimately, however, the effect of a given growth factor in the aqueous humor is determined primarily by the growth factor's bioavailability. Bioavailability depends on many factors, including the expression of receptors on target tissues, interactive effects with components of the extracellular matrix, and the levels of circulating and matrixbound proteases. The role of several growth factors has been studied in patients with diabetes mellitus. IGFBPs are elevated fivefold in human patients with diabetes (without retinopathy), and IGF-I levels are elevated in patients with diabetic retinopathy. These elevations suggest that the increase in vitreal IGFBPs is not the result of a preexisting end-stage retinopathy but is rather an early ocular event in the diabetic process.
Vascular Endothelial Growth Factors The VEGF family of glycoproteins includes VEGF-A, -B, -C, -D and placental growth factor (PlGF). VEGF-A, which has 9 isoforms, is currently the most studied; it is the only one induced by hypoxia and is a critical regulator of angiogenesis and a potent inducer of vascular permeability. Three VEGF receptors have been identified: 1. VEGFRI has both positive and negative angiogenic effects. 2. VEGFR2 is the primary mediator of the mitogenic, angiogenic, and vascular permeability effects ofVEGF-A. 3. VEGFR3 mediates the angiogenic effects on lymphatic vessels.
VEGF-A and its receptors are also present in tissues and organ systems in normal adults, underlining its physiologic role. VEGF may also have roles in retinal leukostasis and neuroprotection. VEGF-A levels are increased not only in patients with active ocular neovascularization from proliferative diabetic retinopathy but also after occlusion of the central retinal vein and with iris neovascularization. The expression ofVEGF-A is increased by hypoxia in retinal endothelial cells, retinal pericytes, Muller cells, and RPE cells. In addition, a soluble VEGF-dependent mechanism has been shown to mediate RPE barrier dysfunction in cocultures ofRPE with endothelial cells (ECs). This EC-RPE contact-induced disruption of barrier properties takes place with ocular conditions such as choroidal neovascularization, wherein ECs pass through Bruch's membrane and contact the RPE. Also, aqueous VEGF-A levels increase in response to anterior segment ischemia in animal models, in addition to the well-described response to retinal hypoxia. Bhisitkul RB. Vascular endothelial growth factor biology: clinical implications for ocular treat-
ments. Br J Ophthalmol. 2006:90(12P542-1547.
Oxygen and Carbon Dioxide Oxygen is present in the aqueous humor at a partial pressure of about 55 mm Hg, roughly one third of its concentration in the atmosphere. It is derived from the blood supply to the Ciliary body and iris, for there is no net flux of oxygen from the atmosphere across the cornea. Indeed, the corneal endothelium is critically dependent on the aqueous oxygen
272 • Fundamentals and Principles of Ophthalmology supply for the active fluid -transport mechanism that maintains corneal transparency. The lens and the endothelial lining of the trabecular meshwork also derive their oxygen supply from the aqueous humor. The carbon dioxide content of the aqueous humor is in the range of 40-60 mm Hg, which contributes approximately 3% of the total bicarbonate. The relative proportions of CO, and HC0 3 - determine the pH of the aqueous, which in most species has been found to be in the range of7.5- 7.6. CO, is continuously lost from the aqueous by diffusion across the cornea into the tear film and atmosphere. The Na+,K+-Cl- cotransporter is also important both in the trabecular meshwork and for control of aqueous outflow. Gong H, Tripathi Re, Tripathi BJ. Morphology of the aqueous outflow pathway. Microsc Res Tech. 1996;33(4):336- 367.
Spector A. Ma W, Wang RR. The aqueous humor is capable of generating and degrading H10 Z' Invest Ophthalmol Vis Sci. 1998;39(7): 1188- 1197.
Clinical Implications of Breakdown of the Blood-Aqueous Barrier With compromise of the blood-aqueous barrier in conditions such as ocular insult (trauma or intraocular surgery), as well as uveitis and other inflammatory disorders, the protein content of aqueous humor may increase 10- 100 times, especially in the highmolecular-weight polypeptides. The levels of inflammatory mediators, immunoglobulins, fibrin, and proteases rise, and the balance among the various growth factors is disrupted (see Chapter 17). The clinical sequelae include fibrinous exudate and clot (with or without a macrophage reaction and formation of cyclitic membranes) and synechiae formation (peripheral and posterior), as well as an abnormal neovascular response, which further exacerbates breakdown of the barrier. Chronic disruption of the blood-aqueous barrier is implicated in the abnormal hyperplastic response of the lens epithelium, corneal endothelium, trabecular meshwork, and iris, and in the formation of complicated cataracts. Degenerative and proliferative changes may occur in various ocular structures as well. The
use of anti-inflammatory steroidal and nonsteroidal drugs, cycloplegics, protease activators or inhibitors, growth and antigrowth factor agents, and even surgical intervention may be necessary to combat these events.
CHAPTER
11
Lens
The lens is a transparent, avascular body that , in concert with th e cornea, focu ses incident
light onto the sensory elements of the retina. To perform this function, the lens must be transparent and must have an index of refraction higher than that of the surround ing flu ids. Maintenance of transparency depends on precise organization of the cellular structure of the lens and a high degree of short-range order of the protein matrix of the lens-fiber cytoplasm. Transparency must be maintained as the lens changes shape during accom modation. The high refractive index is due to the presence of a very high concentration of protein in the lens cells, especially of the soluble proteins called crystallins. Further, because there is little if any turnover of protein in the central region of the lens (where the oldest, denucleated cells are found), the proteins of the human lens must be ext remely stable to remain functi onall y viable for a lifetime. When we consider the lens's mode of growth and the stresses to which the lens is chronically exposed, it is remarkable that in most people, lenses retain good transparency. Nearly all humans develop opacities by the seventh or eighth decade of life. This chapter discusses the structure and composition of the lens as well as aspects of membra ne function, metabolism, and regulatory processes within the lens. Additional information on the lens and on cataractogenesis is proVided in BeSe Section II , Lens and Ca taract .
Structure of the Lens Capsule The lens is enclosed in an elastic basement membrane called the lens capsule (Fig II-I). The capsule is noncellular and is composed primarily of type IV collagen, with smaller amounts of other collagens and extracellular matrix components (including glycosaminoglycans, lam inin, fibronectin, and heparan sul fate proteoglycan). The capsule is a ve ry thick basement membrane, particularly on the anterior side of the lens, where the epithelial cells continue to secrete capsular material throughout life. On the posterior side of the lens, where there is no epithelium, the posterior fiber cells have limited capacity to secrete such material. The zonular fibers, from which the lens is suspended, insert into the capsule near the equator on both the anterior and posterior sides. The capsule is not a barrier to diffusion of water, ions, other small molecules, or proteins up to the size of serum albumin (molecular weight of 68,000 kD).
273
274 • Fundamentals and Principles of Ophthalmology Anterior Pole
Germinative zone
Epithelial cells
Bow
reol ion, -~
Cortical fih,,,o r
Posterior Pole
Figure 11 · 1 Schema tic representation of the mammalian lens in cross sec tion . Small arrow-
heads indicate direction of cell migration from the epithelium to the cortex. (Reproduced with permISsion (rom Anderson RE. ed. Biochemistry of the Eye . San Francisco: American Academy of Ophthalmology; '983: t12.)
Epithelium A single layer of epithelial cells covers the anterior surface of the lens. These cells have full metabolic capacity and play the primary role in regulating the water and ion balance of the entire lens. Although the cells of the central epithelium are not mitotically active. a germinative zone exists as a ring anterior to the equator. where the epithelial cells divide. The new cells migrate toward the equator and begin to differentiate into lens fibers.
Cortex and Nucleus Aside from the si ngle layer of epithelial cells on its anterior surface. the lens is composed of lens fibers. very long ribbo nlike cells. All fibe rs are formed fro m epithelial cells at the lens equator; hence. younger fibers are always exterior to older ones. The lens structure can be equated with the growth rings of a tree in that the oldest cells are in the center. with progressively yo unger layers. or shells. of fiber cells toward the periphery. Unlike the case wi th many tissues. no cells are sloughed from the lens. and cells produced before birth remain at the center of the lens throughout life. As new fiber cells are elongating and differentiating into mature fibers. their cell nuclei form the bow zone. or bow region. at the lens equator. Elongati ng fibers greatly increase their volume and thei r surface area and express large amounts of both lens crystallins and a lens-fiber-specific membrane protein called the major intrinsic protein (MIP). As the fibers become fully elongated and make sutures at each end with fibers that have elongated from the opposite side of the lens. they become mature. terminally differentiated fiber cells. The cell nuclei disintegrate, as do mitochondria and other organelles. This process happens quite ab ruptly by mechanisms that remain obscure. What is understood is that the elimination of cellular organelles is necessary in the central portion of the lens because such bodies are suffiCiently large to scatter light and thereby degrade visual acuity. It should also be noted that with the loss of cell nuclei. the mature fibers lose the machinery required for the syntheSis of proteins. The fiber mass of the adult lens can be divided into
CHAPTER 11 ,
Lens.
275
the cortex (ie, the outer fibers, laid down after the age of about 20 years) and the internal nucleus (ie, the cells produced from embryogenesis through adolescence).
Chemical Composition of the Lens Membranes The chemical composition oflens-fiber plasma membranes suggests that they are both very stable and very rigid. A high content of saturated fatty acids, a high cholesterol:phospholipid ratio, and a high concentration of sphingomyelin all contribute to the tight packing and low fluidity of the membrane. Although lipids make up only about 1% of the total lens mass, they constitute about 55% of the plasma membrane's dry weight, with cholesterol being the major neutral lipid. As the lens ages, the protein:lipid and cholesterol:phospholipid ratios increase as a result of phospholipid loss, especially in the nucleus.
Lens Proteins The lens probably has the highest protein content of any tissue. In some species, more than 50% of lens wet weight is protein. Lens crystallins, a diverse group of proteins expressed in high abundance in the lens- fiber cells and thought to play critical roles in providing the transparency and refractile properties essential to lens function, constitute 90%-95% of total lens protein. In addition to the crystallins, the lens also has a full complement of enzymes and regulatory proteins that are present primarily in the epithelium and in immature fiber cells, where most metabolic activity occurs. Crystal/ins
Crystallins are water-soluble proteins so-named for their high abundance in the "crystalline lens:' Until the 1980s, crystallins were considered lens-specific proteins, lacking biological activity, which were highly evolved structural elements forming the transparent protein matrix of the lens. It is now clear that most (if not all) crystallins are expressed in other tissues as well and have specific biological functions distinct from their roles in the lens as refractile elements. All crystallins now appear to be "borrowed" proteins, recruited by the lens for a function completely distinct from their biological functions in other tissues. Although the specific criteria that make a protein suitable to function as a crystallin are not well understood, crystallins must have 2 obvious attribu tes: (1) Crystallins must be very stable structures because the proteins of the lens are probably the longest-lived proteins in the body. (2) Crystallins must remain soluble under conditions of high protein concentration without forming large aggregates, which would be light-scattering centers within the lens. Crystallins can be divided into 2 groups. One group includes a-crystallin and the p,r,-crystallin family, both of which appear to be present in all vertebrate lenses. The second group is the taxon -specific crystallins, which are each present at crystallin levels only in phylogenetically restricted groups of species. Andley UP. Crystallins in the eye: function and pathology. Prog Retin Eye Res. 2007;26(1 ): 78-98.
276 • Fundamentals and Principles of Ophthalmology a-Crystallin a-Crystallin is the largest of the crystallins, having a native molecular mass in the 600-800 kD range. It is composed of 2 subunits, aA and aB, which are approximately 20 kD in mass and which are nearly 55% identical in sequence. Native a -crystallin has a wide range of molecular masses, apparently because it is a dynamic structure wherein the number of subunits varies somewhat and subunit exchange occurs among the native
multimers. Probably because of this molecular mass variability, a-crystallin has thus far resisted attempts at crystallization, so a definitive 3-dimensional structure for the molecule has not been determined. a -Crystallin is a member of the small "heat -shock protein" family; as a result, the expression of aB is inducible by heat and other stresses. Both aA and aB have a chaperonelike activity whereby they bind proteins that are beginning to denature and prevent further denaturation and aggregation. Zinc ions enhance the chaperone function and stability of a -crystallin. Because protein aggregates in the lens will scatter light and cause loss of transparency, the antiaggregative function of a -crystallin is crucial to the long-term maintenance of transparency in the fibers of the lens nucleus, where syntheSiS of new protein is impossible and where protein molecules must exist for decades.
p,y-Crystallins Until the primary sequences of the various ~- and y-crystallins were determined, they were thought to be 2 unrelated families of proteins. However, it is now known that the 2 groups are related members of the same protein superfamily. The ~ - crystallins, a complex group of oligomers composed of polypeptides, have molecular masses ranging from 23 to 32 kD. The y-crystallins are monomeric proteins with molecular masses near 20 kD. Most
expression of y-crystallins occurs early in development; thus, they tend to be most con centrated in the nuclear region of the lens. Given its compact and symmetric structure
(which can pack very densely), y-crystallin tends to be highly concentrated in aged, hard lenses, which have little to no accommodative ability. Although no specific biological functions have been identified for the ~ - and y-crystall ins, at least some of them are expressed outside the lens, suggesting that such functions do exist. Members of the superfamily have also been identified in microorganisms, where they are expressed during spore or cyst formation (suggesting a possible role in stress response). Taxon-specific crystallins
In addition to the a- and
~,y-crystallins
found in all vertebrate
lenses, other proteins are expressed in large quantities in various phylogenetic groups.
Most taxon-specific crystallins are oxidoreductases, which bind pyridine nucleotides, and their presence in the lens tremendously increases the concentration of the bound nucleotide. Reduced nucleotides absorb UV light and could serve to protect the retina from UV-induced oxidation; or, these strongly reducing compounds could be part of the antioxidant defenses of the lens. It is noteworthy that taxon-specific crystallins generally occur only in strongly diurnal species that are chronically exposed to increased oxidative stress. Taxon-specific proteins actually function as enzymes and are expressed at increased levels in the lens. For example, E-crystallin was found to be actually lactate dehydrogenase and
to be catalytically active.
CHAPTER 11: Lens • 277
Cytoskeletal and membrane proteins Although the great majority of proteins in the normal lens are water-soluble, a number of important structural proteins can be solubilized only in the presence of chao tropic agents or detergents. These water-soluble proteins include the cytoskeletal elements actin (actin filaments), vim en tin (intermediated filaments), and tubulin (microtubules), as well as 2 additional proteins called filensin and phakinin, which have been found only in lens-fiber cells and which compose a cytoskeletal structure unique to the lens called the beaded filament. The filamentous structures of the cytoskeleton provide structural support to the cells and also play critical roles in such processes as differentiation , motility and shape change, and organization of the cytoplasm. Lens-fiber membranes have one quantitatively dominant protein, which has received a great deal of attention, called MIP (as discussed earlier in the chapter). MIP is expressed only in lens-fiber cells and was once thought to be a gap junction protein; in fact, it is not a connexin but rather an aquaporin, a member of a large, diverse family of proteins involved in regulating water transport. Current data suggest that MIP functions as a water channel.
Posttranslational modifications to lens proteins The proteins of the lens are probably the longest-lived in the body, with the oldest ones (in the center of the lens nucleus) haVing been synthesized before birth. As would be expected, these proteins become structurally modified in a variety of ways: oxidation of sulfur and aromatic residue side chains, inter- and intrapolypeptide cross-links, glycation, racemization, phosphorylation, deamidation, and carbamylation. Many of these modifications occur quite early in life and are probably part of a programmed modification of the crystallins that is required for their long-term stability and functionality. There is evidence that certain of these processes (phosphorylation, thiol oxidation) are reversible and may serve a regulatory function, although this remains to be proven. What is known is that with increasing age (particularly in some cataracts), certain oxidative modifications do accumulate, contributing to the cross-linking of crystallin polypeptides, alterations in fluorescent properties, and an increase in protein-associated pigmentation. In particular, the formation of disulfide cross-links in the proteins of the lens nuclear region is associated with formation of protein aggregates, light scattering, and cataract.
Physiologic Aspects Because of its avascularity and its mode of growth, the lens faces some unusual problems. All nutrients must be obtained from the surrounding fluids; likewise, all waste products must be released into those fluids. Most of the cells of the adult lens have reduced metabolic activity and lack the membrane machinery to independently regulate ionic homeostasis. ElUCidating how the lens maintains ionic balance and how solutes move from cell to cell throughout the lens is critical to understanding the normal biology of the organ as well as the process of cataractogenesis.
278 • Fundamentals and Principles of Ophthalmology In the normal lens, sodium is low (~10 mmollL) and potassium is high (~ 120 mmoI/ L); in the aqueous humor, sodium is about ISO mmollL and potassium, about 5 mmollL. When normal regulatory mechanisms are abrogated, potassium leaks out of the lens and sodium floods in, followed by chloride. Water then enters in response to the osmotic gradient, causing loss of transparency by disrupting the normaUy smooth gradient of refractive index. The ionic balance in the lens is maintained primarily by the Na"K+-ATPase pump, an intrinsic membrane protein that hydrolyzes adenosine triphosphate (ATP) to transport sodium out of and potassium into the lens. Functional Na+,K' -ATPase pumps are found primarily at the anterior surface of the lens, in the epithelium and the outer, immature fibers. Studies using ouabain, a speCific inhibitor of the pump, have established the pump's role as the primary determinant of the normal ionic state of the lens. Lens cells also contain membrane channels that pass ions; in particular, K' -selective channels have been studied by patch-clamp techniques and found to be present primarily in the epithelial cells. Communication between lens cells is proVided by gap junctions, which are thought to account for most ion and small-molecule movement between cells. True gap junctions occur in the lens and are composed of members ofthe connexin family. Junctions between epithelial cells are composed of connexin 43; fiber-fiber gap junctions contain connexins 46 and 50. MIP also forms junctionlike structures, although these apparenlly do not provide direct communication from cell to cell. Within the fiber mass, gap-junctional coupling is greatest in the outer layers of the lens, where the junctions can be uncoupled by lowering the pH. In the lens nuclear region, such uncoupling does not occur. This supports the idea that the lens nucleus may be a syncytium, resulting from membrane fusion between adjacent fibers . Some years ago, the vibrating probe technique was used to demonstrate that currents ofions flow around and through the normal lens. As depicted in Figure ll- 2, the currents flow in at the anterior and posterior poles and out at the equatorial region of the lens. The source of these currents is not completely understood, but it is apparent that the various ion pumps and channels near the lens surface and the communicating pathways between cells play important roles in establishing and maintaining the currents. It has been suggested that fluid flow would follow the flow of ions and that this would, in effect, proVide the lens with a circulatory system able to do the work done by blood vessels in other tissues. That is, nutrients could be carried deep into the lens and waste products moved toward the surface. This model implies that the homeostasis of the cells throughout the lens depends on these ionic currents; if that is true, the maintenance and regulation of the currents would be crucial to the functional integrity of the lens.
Lens Metabolism and Formation of Sugar Cataracts Energy Production Energy, in the form of ATP, is produced in the lens primarily through anaerobic glycolysis in metabolically active cells in the anterior lens. This is necessitated by the fact that
CHAPTER 11: Lens •
279
Anterior Pole
Schematic re presentation of ionic current flow around and through t he len s. Th e f low is inwa rd at t he anterior and posterior pol es and outwa rd at t he eq uatorial area. (From M athias RT. Rae JL. Baldo GJ. Ph ysi-
Figure 11-2
,-+-~~--
Equator
,---.-~~
ological properties of th e normal lens. Physio l Rev. 1997; 77(1) :2 1- 50.)
Posterior Pole
the oxygen tension in the lens is much lower than that in other tissues, because oxygen reaches the avascular lens only via diffusion from the aqueous humor. Most of the glucose entering the lens is phosphorylated to glucose-6-phosphate by hexokinase, the rate-limiting enzyme of the glycolytic pathway. Under normal conditions, most glucose-6-phosphate passes through glycolysis, wherein 2 molecules of ATP are formed per original molecule of glucose. A small proportion of glucose-6-phosphate is metabolized through the pentose phosphate pathway (hexose monophosphate shunt). This pathway is activated under conditions of oxidative stress because it is responsible for replenishing the supply of nicotinamide adenine dinucleotide phosphate (NADPH) that becomes oxidized through the increased activity of glutathione reductase under such conditions.
Carbohydrate Cataracts Much of the research activity on lens carbohydrate metabolism has been stimulated by interest in "sugar cataracts:' which are associated with diabetes and galactosemia. True diabetic cataract is a rapidly developing bilateral "snowflake" cataract (see BCSC Section 11, Lens and Cataract, Chapter 5, Fig 5-16) occurring in the lens cortex of patients with poorly controlled type 1 diabetes mellitus. People with adult -onset diabetes do not develop this type of cataract but do have a higher prevalence of age-related cataract with a slightly earlier onset. It is likely that for such persons, diabetes is simply an additional factor contributing to the development of age-related cataracts.
280 • Fundame ntals and Principles of Ophthalmology Defects in galactose metabo lism also cause sugar cataracts. Classic galactosemia is caused by a deficiency of galactose-I-phosphate uridyltransferase. Infants with this inborn error of metabolism develop bilateral catarac ts within a few weeks of birth unless milk (lactose) is removed from the diet. Catarac ts are also associated with a deficiency of galac tokinase. Under certain conditions wherein sugar levels are elevated significantly, some glucose (or galactose) is metabolized through the polyol pathway (Fig 11 -3). Aldose reductase is the key enzyme for the pathway, and it converts the sugars into the corresponding suga r alco hols. Because aldose red uctase has a very high K", (apparent affin ity constant) for glucose (o r galactose), under normal conditions little or no activity occurs through this pathway; however, under conditions of hyperglycemia, aldose reductase competes with hexokinase for glucose (or galactose). Studies using animal models have established the importance of the polyol pathway in experimental suga r cataracts. Animals with diabetes mellitus (either natural or induced) develop cataracts that are associated with the presence of sorbitol in the lens and with influx of water. The osmotic hypothesis was proposed to account for these facts. It invokes the activity of aldose reductase as central to the pathology by markedl y increasing the sorbitol content of the lens. Sorbitol is largely unable to penetrate cell membranes and is thus trapped inside the cells. Because its further conversion to fructo se by polyol dehydrogenase is slow, sorbitol builds up in lens cells under conditions of hyperglycemia such that it creates an osmotic pressure that draws water into the lens, swelling the cells, damaging membranes. and causing cataract. Con firm ation of the theory has come from a va riety of sources. First, ease of induction of cataract by hyperglycemia varies from species to species, depending on the level of aldose reductase activity in the lens. Rats, the most commonly used animal in studies,
GLUCOSE
Figur.II-3 Sorbitol pathway for glucose and galactose metabolism . The reduction of glucose and
GALACTOSE
NADPH
NADPH
NADP'
NADP+
galactose to sorbitol and dulcitol, respect ively, is
catalyzed by aldose reductase us ing the reduced form of nicotinamide adenine dinucleotide phos-
phate (NADPHI as cofactor. The second s tep is the oxidation of sorbitol (du lcitol is not a substrate) to fructose, catalyzed by polyol dehydrogenase using NAO+ as cofactor. Th is step is reversible in
SORBITOL
'- .~__ DULCITOL
NADH
No reaction
the human lens, as indica ted by the arrows.
FRUCTOSE
CHAPTER 11:
Lens .
281
have a high level of aldose reductase activity and develop cataracts readily; mice, with almost no aldose reductase activity in the lens, do not form sugar cataracts. Second, rats fed diets high in galactose develop cataract more rapidly and severely than do diabetic rats. This correlates with the fact that dulcitol levels in their lenses reach higher levels than does sorbitol in diabetic rats because galactose is a better substrate than glucose for aldose reductase. In addition, dulcitol is not further metabolized because it is not a substrate for polyol dehydrogenase. Most convincingly, a number of potent inhibitors of aldose reductase have been developed that can completely prevent cataract in diabetic or galactose-fed animals. Unfortunately, although it is certain that aldose reductase activity is the critical factor in the cascade leading to sugar cataract in these animal models, the situation is not at all clear with respect to human diabetic cataracts. The levels of aldose reductase are much lower in the human lens than in the lenses of the animals used in sugar cataract models, but the levels of polyol dehydrogenase are much higher. There is controversy as to whether sorbitol can accumulate in the lenses of people with diabetes mellitus to levels capable of causing a significant osmotic influx of water. Oxidative stress andlor glycation of proteins in the lens may also be involved in human cataracts associated with diabetes. Garland DL, Duglas- Tabor y, Jimenez-Ase nsio J, Datiles MS, Magno B. The nucleus o f the human lens: demonstration of a highly characteristic pattern by two -dimensional e lec trophoresis and introduction of a new method of lens di ssec tion. Exp Eye Res. 1996;62 (3 ):28S- 291. Mathias RT, Rae IL, Baldo GJ. Physiological propert ies of the normal lens. Physiol Rev. 1997;77(1 ):21 - 50. Piatigorsky J. Gene sharing in lens and cornea: facts and implications. Prog Retin Eye Res. 1998; 17(2): 145- 174. Piatigorsky J, Hejtmancik JE In: Albert DM , Miller JW, Azar DT, Blod i BA, eds . Albert 6- lakobiec's Principles and Practice of Ophthalmology. 3rd ed. Philadelphia: Elsevier Saunders;
2008;chap 105. Quinlan RA , Sandilands A, Procter IE, et aJ. The eye lens cytoskeleton. Eye. 1999;13(Pt 3b): 409- 41 6. Slingsby C. Clo ul NJ. Structure of the crystallin s. Eye. 1999; 13(Pt 3b):395-402. Wistow G. Molecular Biology and Evolution of Crystallins: Gene Recruitment and Multifunctional Proteills in the Eye Lens. Austin , TX: RG Landes; 1995.
CHAPTER
12
Vitreous
The vitreous body is a specialized connective tissue whose functions include
serving as a transparent gel occupying the major volume of the globe • acting as a conduit for nutrients and other solutes to and from the lens The basic physical structure of the vitreous is a gel composed of a collagen framework interspersed with hydrated hyaluronan, also known as hyaluronic acid, molecules. The hyaluronan contributes to the viscosity of the vitreous humor and is thought to help stabilize the collagen network, although most of the hyaluronan can be removed enzymatically without collapse of the gel. The relative amounts of collagen apparently determine whether the vitreous is a liquid or gel, with the rigidity of the gel being greatest in regions of highest collagen concentration. The collagen fibrils supply a resistance to tensile forces and give plasticity to the vitreous; the hyaluronan resists compression and confers viscoelastic properties. Regen -
eration of the vitreous after a vitrectomy occurs very slowly, if at all.
Composition The vitreous contains approximately 98% water and 0.15% macromolecules, including collagen, hyaluronan, and soluble proteins. The remainder of the solid matter consists of ions and low-molecular-weight solutes. The 2 major structural components are collagen and hyaluronan; however, several non collagenous structural proteins and glycoproteins have been identified in the vitreous, including versican, link protein, fibulin-I, nidogen-I, fibronectin, and 2 novel glycoproteins- opticin and VITI. These last 2 proteins were initially identified after extraction of a pellet of collagen fibrils obtained from the bovine vitreous after centrifugation. In addition , the human vitreous contains hyaluronidase and
at least 1 matrix metalloproteinase (MMP-2, or gelatinase) , suggesting that turnover of vitreous structural macromolecules can occur.
Collagen Vitreous collagen fibrils are composed of 3 different collagen types: I. Type II, which forms the major component of the fibrils 2. Type IX, which is located on the surface of the fibril 3. Type V/ XI, which may be located such that its amino terminus projects from the surface of the fibril (Fig 12-1)
283
284 • Fundamentals and Principles of Ophthalmology
Type V/XI collagen
Type II collagen
Fig ur. 12·1 Model for the s tructure of a collagen fi bril from the vitreous. Note that 3 diffe re nt collagen types III. IX. and XI I are assembled to form the fibril. Type II collagen forms the major
structure of the vitreous w ith stagger to form overlap and gap reg ions. Type IX is located on
the s urface of the fibril such that part of the molecule projects fro m the surface. It is in an antiparal le l dire ction compared with the type II collagen molecules . Type IX also has a single chondroitin sulfate chain (eS) that may project from the surface of the fibril. Type V(X I collage n is also located close to the s urface of the fi bril with a part of the molecule projecting fro m the gap reg ion. However, the location of this molecule is controversial; other models suggest it is located in the center of the fibril, whe re it may form a microfibril. C = carboxyl term inus, N = amino term inus. (Adapted from Olsen BR. New insIghts In tO (he funCtion of collagens from genetIc analysIs. Curr Opln Cell 8!01. 1995;7(5,:720-727.)
At present, 19 types of collagen are known, and the ge nes fo r several more have been identified. Type V IXI collagen is unique to the vitreous in that native triple-helical mol · ecules can be isolated that contain th e u, (XI) and u, (V) chains. The vitreous collagens are closely related to the collagens of hyaline cartilage. They differ from the types I, III , XII, and XIV collagens commo nl y found in scar tissue and in tissues such as dermis, cornea. and sclera. The precise mechanism by which the dia meter of the vitreous collagen fibrils is controlled remains poorly understood, although type V I Xl collagen is thought to playa critical role. In most connective tissues, the collagen fibril s (as observed in the electron microscope) show an ax ial periodicity of 67 nm, but vitreous collage n fibrils rarel y show this periodicity with direct staining, although the periodicity can be observed with negative staining. The collagen fibrils of the vitreous are onl y loosely attached to the internal limiting membrane (ILM ) of the retina; however, at th e vitreo us base, the fibrils are fi rm ly anchored to the peripheral retina and pars plana, as well as to th e margins of the optic disc. The origin of vitreous collagen in mammals is not well established. In vit ro, human retinal Muller cell lines synthesize collagens of the vitreo us and vitreo retinal interface. It has been shown in the chicken that during earl y development, the cells of the neural retina synthesize and probably secrete type II collage n. However, other cells (hyalocytes?) present within the vitreous cavity may also cont ribute. In the developing chicken eye, in situ hybridization of th e retina initially yields positive results for type II collagen, whereas transcripts for type IX coll age n are present only at the region of the ciliary body. Late r in develo pment, type II collagen mRNA also becomes localized on ly to the presumptive cil iary region. The origin of type V/Xl collagen within the vitreous is unkn own.
Hyaluronan Hyaluronan isa polysaccharide (glycosa minoglycan) that has a repeating unit of glucuro nic acid and N-acetylglucosamine linked with a ~-1,3 glYCOSidic bond. T he repeating units are
CHAPTER 12:
Vitreous. 285
further linked with ~-1 ,4 glycosidic bonds to form a long, linear, unbranched molecular chain. At physiologic pH, hyaluronan is a weak polyan ion because of the ionization of the carboxyl gro ups present in each glucuron ic ac id residue. In free solution, hyaluronan occupies an extremely large volume relative to its weight and probably occupies all of the space in th e vitreous except for th e space occupied by the collagen fibrils. Hyaluronan molec ules of th e vitreous may undergo lateral inte ractions with each other, and such in teractions may be stabi lized by non collagenous proteins. Link protein, which binds to hyaluronan in cartilage, is known to be present in the vit reous in small amounts. Hyaluronan is present in nearly all vertebrate connective tissues and is nontoxic, noninflammatory, and no nimmunogenic. Both the concentration and the molecular weight of hyaluronan in the vitreous va ry, depending o n the species, location in th e vitreous body, and type of analysis. Hyaluronan in human vitreous can achieve a molecular weight of greater than I x 10'The source of hyaluronan is also poorly understood. In primate eyes, hyaluronan syn thesis has been ident ified in the posterior pars plana, the neural retina, and the hyalocytes (macrop hage- like cells) of the vitreous. Three forms ofhyaluronan synthetase are known, but which isoform is responsible for synthesizing th e hyaluronan of the vitreous has not been determined. In all animal species that have been analyzed, the hyaluronan concentration is highest in the posterior cortical laye r near the retina and lowest in the anterior portion behind the lens. The Chemistry, Biology and Medical Applications of Hyaluronan and Its Derivatives. Miami: Portland Press; 1998.
Lau rent T C, ed.
Soluble and Fibril-Associated Proteins Many proteins remain in solution after the collagen fibri ls and other insoluble elements present in the vit reous gel are rem oved by filtration or centrifugation. Serum albumin is the major soluble vitreous protein, foll owed by transferrin. Poorly defined glycoproteins make up 20% of the total vitreo us protein and are thought to originate from surrou nding tissues and not from blood. Other proteins include neutrophil elastase inhibitor (wh ich may play a role in resisting neovascu larization ) and tissue plasminogen activator (which may have a fibrinolytic role in the event of vitreous hem orrhage). Serum albumin is thought to o riginate from the plas ma, whereas transferrin at least partially o riginates from the regio n of the ciliary body. The concentration of soluble proteins estimated from a number of species is approximatel y 1.0 mg/ mL. However, the concentratio n of serum proteins in the vitreous gel depends on the integrity of the retinal vasculature and the degree of any intraoc ular inflammation that may be present. Consequently, the concentrat ion of soluble proteins within the vitreous cavity can rise dramatically if the blood- retina barrier is compromised. Some st ru ctural proteins are specificall y associated with the collagen fibrils and are isolated by extraction of collagen fibril s after centrifugation of the vitreous. These include a novelleucine-rich-repeat gl ycoprotein called opticin, produced in the posterior nonpigmented ciliary epithelium, and another novel glycoprotein called VITi. T he latter con tains 2 von Willebrand A domains and is closely related to a protein of the cochlear gene,
286 • Fundamentals and Principles of Ophthalmology caCH, called cochlin. Both opticin and VITi are thought to play key roles in the structure of the collagen fibril, and VITI may also interact with hyaluronan.
Zonular Fibers, Lipids. and Low-Molecular-Weight Solutes Some zonular fibers are present in the anterior vitreous and can be observed by electron microscopy. However, most of these fibers form the zonular apparatus, which is the structural connection between the lens and the ciliary body. The major structural protein of these fibers is a large linear protein called fibrillill, which possesses an unusually high cysteine content. Lipids account for about 7% wet weight of the pellet obtained after centrifugation of the vitreous. The major fatty acids in human vitreous include palmitate (25%), stearate (18%), oleate (23%), and arachidonate (17%). Little variation occurs with age. Ions and organic solutes originate from adjacent ocular tissues and blood plasma. The barriers that control entry into the vitreous include the vascular endothelium of retinal vessels, the RPE, and the inner layer of the ciliary epithelium. The concentrations of Na+ and Cl- are similar to those in plasma, but the concentration of K+ is somewhat higher than that in plasma. Bishop PN. Structural macromolecules and supramolecu lar organisation of the vitreous geL Prog Reti" Eye Res. 2000;19(3P23-344. Mayne R, Brewton RG, Ren Z-X. Vitreous body and zonular apparatus. In: Harding JJ, ed. Biochemistry of the Eye. London: Chapman & Hall Medical; 1997: 135 - 143.
Biochemical Changes With Aging and Disease Vitreous Liquefaction and Posterior Vitreous Detachment The human vitreous gel undergoes progressive liquefaction with age, so that typically by the age of 80- 90 years, more than half of the vitreous is liquid. Myopia is associated with faster vitreous liquefaction, which leads to early posterior vitreous detachment (PVD). The process of vitreous liquefaction has, as a crucial component, the breakdown of the thin (12- 15 nm) collagen fibrils into smaller fragments; implicated in this process is less "shielding" of type II collagen due to the age- related exponential loss of type IX collagen. Some proteolytic enzymes, such as plasminogen, may have elevated vitreous concentrations with increasing age, but others, such as MMP-2, do not. The fragments aggregate into thick fibers, or fibrillar opacities, which are visible by low-powered slit-lamp microscopy. As liquefaction proceeds, the collagen fibrils become condensed into the residual gel phase and are absent from (or in low concentration in) the liquid phase. In terms ofhyaluronan concentration or molecular weight, there are no differences between the gel and liquid phases. With increasing age, there is a weakening of adhesion at the vitreoretinal interface between the cortical vitreous gel and the inner limiting lamina. These combined processes eventually result in PVD in approximately 50% of the population. PVD is a separation of the cortical vitreous gel from the ILM as far anteriorly as the posterior border of the vitreous base; the separation does not extend into the vit reous base
CHAPTER 12:
Vit reous • 287
owing to the unbreakable adhesion between the vitreous and retina in that zone. PVD is often a sudden event. during which liquefied vi treous from the center of the vitreous body passes through a hole in the posterior vitreous cortex and then dissects the residual cortical gel away from the inner limiting lamina. As the residual vitreous gel collapses ante riorly within the vitreous cavity. retinal tears sometimes occur at areas where the ret-
ina is more strongly attached to the vitreous than the surrounding retina can withstand. which subsequently can result in rhegmatogenous retinal detachment. A PVD can protect against proliferative diabetic retinopathy by denying a scaffold for fibrovascular proliferation emanating from the disc and the retina. A PVD can be achieved surgically during macular hole surgery. However. it is now clinically recognized that in many eyes thought to have a PVD. collagen fibrils are still extenSively attached to the ILM; and even after production of an acute PVD during vitreous surgery. some collagen fibrils typically remain adherent to the ILM. Removal of the ILM itself is now more frequently the goal in limiting the extent of traction maculopathy.
Myopia When the axial length of the globe is greater than 26 mm. both collagen and hyaluronan concentrations are approximately 20%-30% lower than their concentrations in emme-
tropic eyes. In a model of negative power lens- induced myopic response in the tree shrew. hyaluronan was found to rapidly decrease when the lens was applied and equally rapidly recover when the myopia-inducing lens was removed. Associated with these changes were biomechanical alterations in the sclera and increased axial length .
Vitreous as an Inhibitor of Angiogenesis Numerous studies have shown that the normal vitreous is an inhibitor of angiogenesis. This inhibitory activity is decreased during diabetic vitreoretinopathy. However. the molecular basis of the phenomenon remains poorly understood. Known inhibitors of angiogenesis. such as thrombospondin I and pigment epithelium- derived factor. are present within the mammalian vitreous and may inhibit angiogenesis in normal eyes. In contrast,
VEGF. a promoter of angiogenesis. is markedly elevated in the vitreous of patients suffering from proliferative d iabetic vitreoretinopathy.
Physiologic Changes After Vitrectomy Both the normal vitreous and the vitreous cavity after vitrectomy are 99% water. Most of the changes in ocular phYSiology after vitrectomy result from altered viscosity in the vitreous cavity. which decreases between 300- and 2000-fold when the vitreous is removed. Not only do growth factors and other compounds such as antibiotics transfer between the posterior and anterior segments more easily. but they are also cleared more quickly from the eye. This effect is proportional to the change in diffusion coefficient. which is of the same magnitude as the viscosity change. In addition. fluid currents may be present that may move solutes even more rapidly. In particular. oxygen movement is more rapid. and the normal oxygen gradient between the well-oxygenated anterior segment and the posterior segment flattens
288 • Fundamentals and Principles of Ophthalmology significantly, with greatly increased oxygen tension at the retina. This process has been proposed as a mechanism by which vit rectomy may improve the outcome of retinal ischem ic diseases such as diabetic macular edema. It has also been proposed that in creased oxygen tension at the posterior pole of the lens may be part of postvitrectomy cataractogen esis.
Injury With Hemorrhage and Inflammation If blood penetrates the vitreous cortex, platelets come in contact with vitreous collagen, aggregate, and initiate clot formation. The clot in turn stimulates a phagocytic inflammatory reaction , and the vitreous becomes liquefied in the area of a hemorrhage. In severe cases, hemoglobin-laden macrophages may cause secondary glaucoma by blocking the trabecular outflow channels. Rigid, degenerated blood cells, called ghost cells, can also cause secondary glaucoma (see BCSC Section 10, Glaucoma ). If the vitreous is largely liquefied (as in myopic, apha kic, or senile eyes), the clot that is formed is loosely aggregated , and early resolution is more likely. Hemorrhage into vitreous gel is less freely dispersed, and a more compact clot is formed. The subsequent inflammatory reaction var ies in degree for unknown reasons and rnay result in proliferative
vitreoretinopathy. Streeten BAW, Wilson DJ. Disorders of the vitreo us. In: Garner A, Klintworth GK, eds. PatllObiology afOcular Disease. 2nd ed. 2 va ls. New York: M. Dekker; 1994:70 1- 742.
Involvement of Vitreous in Macular Hole Formation With the development of optical coherence tomography (OCT) and, more recently, ultra high resolution OCT (UHR OCT), it is possible to directly visualize the vitreous cortex and the retina in the region of the macula. The results of studies using OCT suggest that macular holes sometimes originate from traction generated by attachment of the vitreous
specifi cally to the fovea, with the subsequent generation of add itional tangential tractional fo rce along the ILM , causing hole enlargement. Lamellar macular holes may have a similar pathogenesis. Madrepe rla SA , McCuen BW. Macular Hole: Pathogenesis, Diagnosis, mId Treatment. Boston: Butterwort h-Heinemann; 1998.
Witkin AI, Ko TH, Fuji moto }G, et a1. Redefi nin g lamellar holes and the vitreomacular interface: an ultrahigh-resolution optical cohere nce tomography study. Ophthalmology. 2006;113(3): 388- 397.
Genetic Disease Involving the Vitreous In Stickler syndrome or Marshall syndrome, the vitreous collapse is premature, which may induce retinal detachment (see BCSC Section 12, Retina and Vitreous) . Mutatio ns in both the u, (II) and u, (XI) collagen chains have been shown to be responsible for this condition. Howeve r, other fam ilies with these ocular conditions have also been identified in which the genetic basis is still not understood. Snead MP, Yates JR. Clinical and molecu lar genetics of Stickler syndrome. } Med Genet. 1999; 36(SPS3- 3S9.
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Vitreous • 289
Enzymatic Vitreolysis Considerable interest exists in enzyme preparations that may aid in the clearing of blood from the vitreous and in potentially performing noninvasive vitrectomy or producing a PVD in young eyes (see SCSC Section 12, Retina and Vitreous). Enzymes that have been proposed for injection into the vitreous cavity include hyaluronidase, plasmin, dispase, and chondroitinase. Sebag J. Pharmacologic vitreolysis. Retina. 1998;18( I): 1-3.
CHAPTER
13
Retina
The retina is composed of 2 laminar structu res, an outer RPE and an inner neural retina.
(This chapter discusses the neurosensory retina; the RPE is discussed in Chapter 14.) These laminar structures arise from an invagination of the embryonic optic cup that folds an ectodermal laye r into apex-to-apex contac t with itself. The 2 layers form a hemispheric shell on which the visual image is foc used by the anterior segment of the eye. The neural retinal cell types are as follows: • photo receptors-rods and 3 types of cones • bipolar cells- rod on-bipolars and cone on- and off-bip olars interneurons-hori zontal and amacrine cells
ganglion cells and their axons, forming the optic nerve astroglia. oligodend roglia. Schwann cells, microglia, and vascular endothelium and pericytes
Neural Retina-The Photoreceptors Rod Phototransduction Catching light and converting its mi nute amount of ene rgy into a neural response distinguishes the reti na from all other neural structures, which it otherwise resembles. This combined process occurs within a specialized organelle of the photoreceptor cell. the outer segment. Most of our knowledge of phototransduction comes from information known about rods, which are sensitive nocturnal light detectors. Much more biochemical material can be obtained from rods than from cones because rods are much more numerous in most retinas. In addition. they contain much mo re membrane than do cones. which
contribu tes to the rods' higher sensitivity. The outer segment of a rod is composed primarily of plas ma-membrane material o rgan ized in an unusual way. Most of the membrane is in the form o f membrane sacs
fl attened along the long axis of the outer segment. There are about 1000 sacs within a rod outer segment and about a million rhodopsin molecules in each sac. The sacs fl oat within the cytoplasm of the outer segment like a stack of coins d isconnected from the plasma membrane. The sacs contain the protein machinery to capture and amplify light energy. This abundance of outer-segment membrane increases the number of rhodopsin molecules, which can absorb light. Some deep-sea fi sh. which need great sensitivity to detect the small amount of light available, have much longer rods than do humans. 291
292 • Fundame nta ls and Principles of Ophthalm ology Light is absorbed by rhodopsin concentrated in the outer-segment mem brane of rods. Rh odopsin is a freely diffusible membrane protein similar to (L - and ~ - adre nergic receptors. It has 7 helical loops embedded in the lipid membrane (Fig 13- i) . Phosphorylation sites exist on the cytoplas mic side ofthe protein, where rhodopsin is inactivated and suga r is attached on the intradiscal side. At amino acid 296 on the seventh membrane loop, the ll -cis- retinal chro mophore is bound to a lysi ne by a protonated Schiff base linkage. Each molecule responds to a single quantum of light. Rhodopsin absorbs green light best at wavele ngths of approximately 510 nm. It absorbs blue and yellow lights less well and is insensitive to longer wavelengths (red light ). The tuning of rhodopsin to this part of the electromag netic spectrum is due to th e amino-acid sequence of the protein and th e bind-
ing of the II -cis isomer of retinaldehyde, which creates a molecular antenna. Once rhodopsin absorbs a quantum of light, the II -cis doub le bond of retinal is broken and the opsin molecule undergoes a series of rapid config urational changes that lead to an ac tivated state, meta rhodops in II. Activated rhodo psin starts a reaction that cont rols the inflow of cations into the rod o uter segment. The target of this reac tion is a cyclic guanosine monophosphate (cGMP)-gated cationic channel located on the outer membrane of the outer segment. This channel cont rols the fl ow of Na' and Ca" ions into the rod . In the da rk, Na' and Ca" ions fl ow in th rough this channel, which is kept open by cGM P. Ionic balance is maintained by a Na', K'-AT Pase pump in the inner segment and a Na' ,K' -Ca exchanger in the outer- segment membrane, both of which require metabolic energy. Depolarization of the rod causes the trans mitter glutamate to be released from its synaptic terminal, starting the neural signals for vision.
Rod
Rhodopsin
Cytoplasm
lipid membrane of disc Il1lra -
disc space
Figure 13-' The rhodopsin molecu le is embedded in th e lipid membrane of the outer segment w ith 7 helical loops. Each circl e is an amino acid, with the highly conserved ones in black. An arrow shows the lysine to which the vi tamin A chromophore is linked . Phosphorylation sites occur on the cytoplasmic and sugar attachm ent sites on the intra discal (extracellular) ends of the rhodopsin molecule. (Courresyof Peter Gouras, MD)
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Retina. 293
Light-acti vated rhodopsin drives a second molecule, transducin, by causing an exchange of guanosine diphosphate (GOP) for guanosine triphosphate (GTP). One rhodopsin molecule can acti vate a hundred transducin molecules, amplifying the reaction. Activated transducins excite a third protein, rod phosphodiesterase (rod POE), which hydrol yzes cGM P to S'-noncycl ic GMP. The decrease in cGMP closes the gated channels, which stops Na' and Ca" entry and hype rpolari zes the rod. Hype rpolarization stops glu tamate's release from the synaptic terminal. When the light goes off, the rod returns to its dark state as the reaction cascade turns off. Rhodopsin is inact iva ted by phosphorylation at its C-terminal by rhodopsin kinase, assisted by the binding of arrestin. Transducin is inactivated by the hydrolysis of GTP to GO P by transducin's intrinsic GTPase activity, which inactivates POE. Gua nylate cyclase, the enzyme that synthesizes cGM P from GTP, is activated by the decrease in intracellular Ca" caused by the channel closure; the enzyme's action is assisted by guanylate cyclase- assisting proteins. As cGMP levels increase, the gated channels close and the rod is re-depolarized. The corresponding rise in intracellular Ca" restores guanylate cyclase ac tivity to its dark level. Calcium feedback may also regulate rhodopsin phosphorylation by recove rin as well as the sensit ivity of the gated channel.
"Rim" proteins Rod sacs differ from those of cones in that they are disconnected fro m the outer plas ma membrane. The rim of each rod sac has a unique collection of proteins. Two are peripherin and ROM! , which playa role in the development and maintenance of the sac's curvature. Peripherin and ROM ! are also found in cone outer segments. A third protein is a member of a superfamily of ATP-binding cassette (A BC) transporters. These include the cystic fibrosis transmembrane regulator (CFTR); P-glycop rotein, which is involved in multidrug resistance; TAP ! and TAP2, which transport peptides in lymphocytes; prokaryo tic permeases; and others. The ABC protein is unique to rod sacs and is not found in cones. It functions as a transporter of all · trans retinal. Biswas- Fiss EE. Functional analysis of genetic Illutations in nucleoti de binding domain 20fthe human retina speci fic ABC transporter. Biochemistry. 2003;42(36): 10683- 10696.
Outer-segment energy metabolism ATP is necessary to drive the reactions that control the ion ic current generators as well as the transporters in the outer segment. Because o nly the inner, and not the outer. segment contains mitochondria, oxidative metabolism is confined to the former. The outer segtnent is responsible for glycolysis, including the hexose monophosphate path way and the phosphocreatine shuttle, which produces ATP and GTP and modulates NAOPH. NAOPH reduces retinal to retinol before it is re turned to the RPE for isomerization and reduces glutat hione, which protects agai nst oxidative stress.
Cone Phototransduction Qualitatively, cone phototransdllction resembles that of rods. Light -activated cone opsins start an enzymatic cascade that hydrolyzes cGM P and closes cone-specific cGM Pgated cation channels o n the outer-segment membrane. Cone phototransduction is
294 • Fundamentals and Principles of Ophthalmology comparatively insensitive but fast and capable of adapting enormously to the ambient levels of illumination. The greater the ambient light levels, the faster and more temporally accurate is the response of a cone. Speed and temporal fidelity are important for all aspects of cone vision. This is one reason acuity improves progressively with increased illumina-
tion. Because of their ability to adapt, cones are indispensable to good vision. Without cones, one loses the ability to read and see colors and can be legally blind. In comparison, lost rod function is less of a visual handicap. Several factors contribute to light adaptation. For example, higher levels of illumination bleach away photopigments, making the outer segment less sensitive to light. As light levels increase, so does the noise level, which reduces sensitivity. Biochemical and neural feedback speed up the cone response. This feedback must be increased as light intensity increases and the cone absorbs more and more light. The biochemistry responsible for this speedup has not yet been deciphered. All the processes that turn the rod response off are probably stronger in cones. The life span of activated cone opsin must be shorter, its turn-off and the turn -off of cone transducin (a G protein) faster than they are in rods. In addition, increasing light must enhance the turn-off mechanisms in cones. Cones also show neurally mediated negative feedback. Horizontal cells of the inner nuclear layer synapse antagonistically back onto cones, releasing y-aminobutyric acid (GABA), an inhibitory transmitter. When light hyperpolarizes a cone, the cone hyperpolarizes neighboring horizontal cells. This inhibits the horizontal cells, stopping the release of GABA, which depolarizes (disinhibits) the cone by a recurrent synapse. This depolarization antagonizes the hyperpolarization produced by light and tries to put the cone back in the dark. Depolarization occurs with a synaptic delay so that its main effect is on the later response of the cone. Horizontal cell feedback occurs with strong stimuli, undoubtedly preventing the cone from being overloaded. The feedback also turns the cone response off more quickly, enabling the cone to respond more rapidly to a new stimulus. This process thus increases the flicker fusion frequency, which is much higher in cones (about 100 Hz) than in rods (about 30 Hz).
Trivariant color vision To see colors, mammals must have at least 2 different spectral classes of cones. Most normal humans have 3 types of cones and consequently a 3-variable color vision (3 cone opsins) system. Most mammals have divariant color vision with middle-wavelength -sensitive (M) cones detecting high -resolution achromatic (black and white) contrast and short-wavelength-sensitive (S) cones detecting only color, by comparing their Signals with those of the M cones. This mechanism creates blue/yellow color vision. Because the S cones contribute only to color vision, they are much less numerous than M cones. In primates, a third cone mechanism evolved to enhance color vision by splitting the high -resolution M cones into long (L)- and middle (M)-wavelength cones. This creates red/green color vision. Both Land M cones contribute to achromatic and chromatic contrast. Therefore. both are more numerous than S cones in the human retina.
Most color vision defects involve red/green discrimination and the genes coding for the L- and M-cone opsins. These genes are in tandem on the X chromosome. There is I copy of the L-cone opsin gene at the centromeric end of the X chromosome and 1-6
CHAPTER 13:
Retina.
295
copies of the M-cone gene arranged in a head-to-tail tandem array. Normally, only the most proximal of these 2 genes is expressed. Most color vision abnormalities are caused
by unequal crossing over between the L- and M-cone opsin genes. This creates hybrid opsins that have different spectral absorption functions, usually less ideal. Some males have a serine-to-alanine substitution at amino acid 108 on the cone opsin gene, which allows more sensitivity to red light. Therefore, female subjects with both the serine-containing and the alanine-containing opsins could have tetravariant color vision.
Rod-Specific Gene Defects
Rhodopsin More than 100 different mutations cause autosomal dominant retinitis pigmentosa (ADRP). Mutations occur in different ways; they can alter transduction, protein folding, or localization of the protein. The most common mutation is P23H (responsible for 10% of RP cases in the United States), in which the protein does not fold properly and instead accumulates in the rough endoplasm ic reticulum. Generally, mutations affecting the intradiscal area and amino terminal of rhodopSin are less severe than those in the cytoplasmic region and the carboxyl tail. Alterations in the middle of the gene, coding for the transmembrane regions, result in moderately severe defects. Relatively uncommon mutations have been reported in the rhodopSin gene, causing autosoma l recessive retinitis
pigmentosa (ARRP) and a stationary form of nyctalopia.
Rod transducin A dominant G38D mutation produces Nougaret disease, the oldest known form of autosomal dominant stationary nyctalopia. With this mutation, transducin becomes continuously activated, an example of constitutively active rods that do not degenerate.
Rod cGMP phosphodiesterase Defects in either the a - (PDEA) subunit or ~- (PDEB) subunit ofcGMP phosphodiesterase (rod PDE) cause ARRP. These are nonsense mutations that truncate the catalytic domain of the protein. An H258D mutation in PDEB also causes dominant stationary nyctalopia. This mutation is near the binding site of the y-subunit of PDE and may lead to constitu tively active PDE.
Rod cGMP-gated channel Null mutations of the rod cGMP-gated channel from the a- or y-subunits has been reported.
~ -subunit
cause ARRP. No degeneration
Arrestin A homozygous defect in codon 309 causes Oguchi disease, a form of stationary nyctalopia. It produces a frameshift in and truncation of arrestin. There is genetic heterogeneity because rhodopSin kinase gene defects also cause Oguchi disease.
Rhodopsin kinase Null mutations of rhodopSin kinase cause Oguchi disease. These mutations also retard activated rhodopSin's turnoff.
296 • Fundamentals and Principles of Ophthalmology
Guanylate cyclase Nu ll mutations of guanylate cyclase cause Leber congenital amaurosis (LeA), a childhood au tosomal recessive form of RP. LeA shows genetic heterogeneity.
Rod ABC transporter Recessive defects of ABe transporter cause Stargardt disease. There is allelic heterogeneity, which reflec ts the severity of the gene defect. Mild defects cause macular degeneration, intermediate ones cause cone- rod dystrophy, and severe ones cause RP. Heterozygous defects are also found in 4% of age-related macular degeneration.
L-type calcium channel The L-type calcium channel gene codes for an a -subunit, and defects cause X-linked stationary nyctalopia. The protein seems to determine transmitter release from the rod synaptic terminal and may also affect cones.
Cone- and Rod-Specific Gene Defects
Peripherin (RDS) There is great allelic heterogeneity in the peripherin/RDS gene. Defects cause several dom inantly inherited retinal degenerations that ra nge from ADRP to macular dege neration, pattern macular dystrophy, viteUiform macular dystrophy, butterfly macular dyst rophy, and fundus flavimaculatus. A null mutation of the homologous murine gene causes a semidomi nant form of degeneration, with fa ilure of rod outer-segment development and slow degeneration.
ROMI Double-heterozygotic mutations in both the ROM1 and the peripherin gene cause "digenic" RP. A ROM 1 gene defect alone has been reported in a patient wit h a vitelliform macular dystrophy, but this gene is not responsible fo r Best macular dystrophy (d iscussed later in the chapter).
Myosin VilA A heterozygous null mutation in a form of myosin VIlA, causes Usher syndrome type I. Affected subjects have deafness and vestibular ataxia at birth and develop ARRP.
Oxygen-regulated protein Homozygous defects in oxygen -regulated protein cause autosomal dominant RP. Expression of this unusual gene is modulated by oxygen.
Cone-Specific Gene Defects
Cone cGMP-gated channel A homozygous defect in the cone cGMP-gated channel a -subunit causes ach romatopsia, loss of all cone function.
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Retina. 297
L- and M-cone opsins Two genetic steps involving L- and M -cone opsins lead to "blue-cone achromatopsia:' One reduces the tandem array of these genes to 1 gene, and the second eliminates the residual gene. Another defect in a sequence upstream from this tandem array can also cause blue-cone achromatopsia. These defects occur in males because of the gene's location on
the X chromosome. L- or M-cone opsins Defects in one or the other of the X-linked L- or M-cone opsin genes cause red/green color deficiencies, again almost exclusively in males.
RPE-Specific Gene Defects
RPE65 Homozygous defects of RPE 65 cause Leber congenital amaurosis, a generalized loss of photoreceptor function. Mice with this defect retain some cone function. The protein influences the formation of ll -cis-retinoL Cones may have access to another pool of this isomer.
Bestrophin Heterozygous missense mutations of the bestrophin gene produce Best disease, a dominantly inherited form of macular degeneration, which involves the entire RPE layer but causes damage only in the macula. The protein is membrane-bound with 4 transmembrane regions.
TIMP3 Heterozygous point mutations ofTIMP3 produce Sorsby macular dystrophy. This protein is an inhibitor of a metalloproteinase that regulates the extracellular matrix.
CRALBP Homozygous defects of CRALBP (cytoplasmic retinal-binding protein) cause retinitis punctata albescens. This protein facilitates ll -cis-retinal formation and transport; it is also found in Muller cells. 11-cis-retinol dehydrogenase Homozygous defects of ll-cis-retinol dehydrogenase cause fundus albipunctus, a form of stationary nyctalopia. This enzyme forms ll -cis-retinal from ll -cis-retinol and may also exist in Muller cells. EFEMP1 A single heterozygous nonconservative mutation of EFEMPI (EGF-containing fibrillin like extracellular matrix protein) causes malattia leventinese (Doyne honeycomb retinal dystrophy), a dominant form of macular degeneration. It is uncertain whether the protein is unique to RPE.
298 • Fundamentals and Principles of Ophthalmology
Ubiquitously Expressed Genes Causing Retinal Degenerations REP-1 REP-I (Rab escort protein- I) is an X-linked gene that causes choroideremia. The protein is involved in prenylating Rab proteins. a process that facilitates their binding to cytoplasmic membranes and promoting vesicle fusion. Photoreceptors. RPE. and/or the choroid must be uniquely vulnerable for this process to occur.
OAT Homozygous defects of OAT (ornithine amino transferase) cause gyrate atrophy. The enzyme breaks down ornithine. which. in high concentrations. seems toxic to the RPE.
MTP Homozygous defects in MTP (microsomal triglyceride transfer protein) cause abetalipoproteinemia. or Bassen-Kornzweig syndrome. a condition characterized by ARRP and the patient's inability to absorb fat. The condition is treatable with fat-soluble vitamins.
PEX1 Homozygous defects in PEX 1 cause infantile Refsum disease. with RP. retardation. and hearing deficits. Infantile Refsum disease represents the least severe disease in a spectrum of familial disorders involVing mutations in the PEX genes. The PEX genes encode for peroxins, proteins needed for peroxisome biogenesis.
PAHX Homozygous defects of PAHX cause Refsum disease. with RP. cerebellar ataxia. and peripheral polyneuropathy. The enzyme degrades phytanic acid and is located in peroxisomes. Patients with Refsum disease may be treated with a phytanic acid- restricted diet.
Inner Nuclear Layer The inner nuclear layer has 3 classes of neurons (bipolar. horizontal. and amacrine cells) and a glial cell (the Muller cell). There are separate bipolar cells for cones and rods and at least 2 distinctly different types of cone bipolars. on-bipolars and off-bipolars (Fig 13-2). The former are inhibited. the latter excited by the glutamate transmitter released by cones. Thus. whe n light hyperpolarizes the cones. the on-bipolar is excited (turned on) and the offbipolar inhibited (turned off). When a shadow depolarizes the cones. the reverse occurs. Some cone bipolars synapse only with L cones and others only with M cones. a differentiation that is necessary for color vision. In the fovea. some cone bipolars synapse with a single L or M cone (Fig 13-3). which provides the highest spatial acu ity. This cone selectivity is preserved throughout the ganglion cell layer. This selectivity for L- or M-cone inputs is transmitted by a tonic responding system of small ganglion cells. Separate L- and M-cone on-bipolars and off-bipolars transmit a faster. phaSiC Signal to a parallel system of larger ganglion cells. Rods and probably S cones have only on-bipolar cells. Neither rods nor S cones are involved in high spatial resolution. The S cones are involved in color vision and the rods in twilight vision.
CHAPTER 13:
Cones
Retina.
299
Figure 13-2 Basic ci rcui try of the cones. A separate on- and off-bipolar cell contacts each cone.
In the fovea, a cone has "midget" bipolar cells
Horizontal cell
Off
con tacting on ly a si ngle cone for high spatial acu ity. Horizontal cells are antagonistic neurons between cones; when the cone absorbs light, it is hyperpolarized; this, in turn, hyperpolarizes the horizontal cell, which resembles an off-bipola r cell. (Courtesy of Peter Gouras. MD.)
Bipolar cells
On
Tonic System
&~
11 t ~ ,
"
Figure 13-3
The tonic system transmits signa ls fro m th e cones that are relatively maintained for the duration of the light or dark stimulus. This system provides the brain wi th information about a separate cone system, necessary for color vision. It preserves the polarity of the signal for each cone type. This is best shown here in th e S-cone channe l, wh ich receives a signal of opposite polarity (off) from the Land M cones. (Courtesy of Peter Gouras. MD.)
The horizontal cells are antagon istic interneurons that inhibit photoreceptors (see Fig 13-2) by releasing GABA when depolarized. The dendrites of horizontal cells go to cones. One class of horizontal cells goes to Land M cones. Another class goes mainly to S cones. A thin axon terminal that emanates from the cell body of horizontal cells sends dendrites to rods. The de ndrites of horizo ntal cells receive glutamate from cones and rods and release GABA back onto cones and rods. This provides negative feedback. When light
300 • Fundamentals and Principles of Ophthalmology causes the cone to hyperpolarize and stop its transmitter release, the horizontal cell is also hyperpolarized (turned off). This stops the release of GAB A from the horizontal cell onto the cone, consequently depolarizing the cone. Cone amacrine cells mediate antagonistic interactions among on -bipolars, offbipolars, and ganglion cells. The rods have an unusual amacrine cell that receives the input of rod bipolars and delivers Signals to on- and off-bipolar ganglion cells. Thus, rod Signals undergo additional synaptic delays before they reach the ganglion cell output. The retinal ganglion cells can be classified into 2 main types: on (center cells excited) and off (center cells inhibited by light in the center of their receptive field). A shadow initiates the opposite reaction in these 2 cell types. There are 3 main subgroups of retinal ganglion cells: tonic cells driven by L or M cones, tonic cells driven by S cones, and phasic cells. Tonic cells driven by either L or M cones include small cells concentrated in the fovea (responsible for high acuity) and others located extrafoveally (see Fig 13-4). They project to the parvocellular layers of the lateral geniculate nucleus (the main relay station to the visual cortex) and mediate both high spatial resolution and color vision. Tonic cells driven by S cones have a unique phYSiology deSigned to detect successive color contrast, blue/yellow or gray/brown borders. These ganglion cells are excited by short waves entering and long waves leaving their receptive fields (see Fig 13 -3). The phasic cells are larger, less concentrated in the fovea, and faster-conducting than the ganglion cells (Fig 13-4). They project to the magnocellular layers of the lateral geniculate nucleus and may be more important in movement detection. Muller cells are the least understood of all the retinal cells. Nonneural, they playa supportive role to the neural tissue extending from the inner segments of the photoreceptors to the ILM, which is formed by their end-feet. They buffer the ionic concentrations
Phasic System
&~
The phasic system t ransmits signals at th e beginning or end of a light sti mulus . Th is produces a brief or transi ent response. L- and M cone signals of the same pol arity mix in drivi ng t he phasic system. (Courresv of Pe ter Gouras, M D.J
Figure 13-4
11
CHAPTER 13:
Retina. 301
in the extracellular space, seal off the subretinal space by forming the external limiting membrane (ELM), and may playa role in the vitamin A metabolism of cones. The other non neural, or neuroglial, cells of the retina are macroglia (astrocytes, oligodendroglia, and Schwann cells) and microglia. These cells provide physical support, respond to retinal cell injury, regulate the ionic and chemical composition of the extracellular milieu, participate in the blood- retina barrier, form the myelination of the optic nerve, guide neuronal migration during development, and exchange metabolites with neurons. Neuroglia have high-affinity transmitter-uptake systems and voltage-dependent and transmitter-gated ion channels; they can release transmitters, but their role in signaling, as in many other functions, is unclear. In addition, the neural retina contains blood vessels with endothelial cells and pericytes. Pericytes playa role in the autoregulation of retinal blood vessels and are an early target in diabetes. The pathogenesis of diabetic retinopathy seems to be due to defects in the polyol pathway. Aldose reductase, the rate-limiting first enzyme in the conversion of many sugars to their alcohols, has a known role in the formation of diabetic cataracts. Accumulation of sorbitol and its aldose reductase-mediated metabolite, fructose, causes repeated osmotic insults and cataractogenesis. Similar conditions may lead to thickening of the basement membrane and pericyte loss in the blood vessels of the retina.
Retinal Electrophysiology Changes in the light flux on the retina produce electrical changes in all of the retinal cells, including the RPE and Muller cells, as well as neurons. These electrical changes result from ionic currents that flow when ion -specific channels are opened or closed. These currents reach the vitreous and the cornea, where they can be detected noninvasively by an electroretinogram (ERG). The initiating process of the currents is the ionic response started in the rods and cones that influences the ionic current both diredly by changes in Na+ and K+ fluxes and indirectly by synaptically modifying second-order retinal neurons. The changes in the electrical potentials of the rods and cones are shown in Figure 13-5, which depicts the responses of rods and cones to a pulse of light or a pulse of darkness. Na
+
Cone
o Rod Rod
K
Figure 13-5 The response of a rod and cone to a pulse of light and a pu lse of darkness. The
light pulse hyperpolarizes both photoreceptors. The rod responses are prolonged. The cone responses turn off quick ly even wh ile the pulse of light is on. Darkness depolarizes the cone rap idly but has onl y a sma ll effect on the slower rod response .
302 • Fundamentals and Principles of Ophthalmology Light hyperpolarizes cones and rods. The cone response is rapid; it turns off while the light is still on and overshoots the dark potential. The rod response is more prolonged and turns off very slowly. Dark depolarizes the cone and has little influence on the rod, which is saturated at high light levels and too slow to respond to the "shadow." The ionic changes are due to the shift in the photo receptors' conductivity to Na' and K' ions. The concentra ~ tion gradients for these ions are reversed across the membrane of the photoreceptors so that changes in the conductivity to these ions move currents in the opposite di rections. Liou GI, Fei Y, Peachey NS, et al. Early onset photoreceptor abnormalities induced by targeted di sruption of the inter photoreceptor retinoid · binding protein gene. f Neurosci. 1998; IS( J 2): 4511 - 4520. Molday RS. Photo receptor membrane prote ins, phototransduc ti on. and retinal degene rative diseases. The Friede nwald Lecture. In vest Ophthalmol Vis Sci. I 998;39{ 13):2491 - 25 13.
CHAPTER
14
Retinal Pigment Epithelium
The retinal pigment epithelium (RPE) is a single layer of cuboidal epithelial cells that constitutes the outerm ost layer of the retina. The RPE is located between the highly vascu lar choriocapillaris and the outer segments of photoreceptor cells (Fig 14-1). In humans, there are approximately 4- 6 million RPE cells per eye. The ratio of photoreceptor cells to RPE cells is roughl y 45: I. The RPE is derived embryologicall y from the same neural anlage as the sensory reti na, but it differentiates into a secretory epithelium . Although it has no photoreceptive or neural function, the RPE is essential to the support and viability of photoreceptor cells.
Anatomical Descri tion The RPE cells are polarized epithelial cells. They have long microvillous processes on their apical surfaces that interdigitate with outer segments of photoreceptor cells. Their basal surface, which is adjacent to Bruch's membra ne (an extracellular matrix between the RPE
Figure 14-1
A spectral domain optical coherence tomography (OCT) section of retina showi ng
the relationship of RPE to the reti na and the choriocapillaris.
(Courtesy of Sandeep Grover, MD.)
303
304 • Fundamentals and Principles of Ophthalmology and choriocapillaris), has many infoldings. RPE cells are joined near their apical side by tight junctions that block the passage of water and ions. As such, the RPE contributes to the blood-retina barrier. In addition to the organelles found in most cells (eg, the nucleus, Golgi apparatus, smooth and rough endoplasmic reticulum , and mitochondria), the RPE has melanin granules and phagosomes that reflect 2 of its important roles (discussed shortly). The RPE is particularly rich in microperoxisomes, suggesting that it is quite active in detoxifying the large number of free radicals and oxidized lipids that are generated in this highly oxidative and light-rich environment.
Biochemical Composition Biochemically, the RPE is a dynamic and complex cell. It must meet demands for its own active metabolism, its extraordinary phagocytic fun ction, and its role as a biological filter for th e neurosensory retina. These· processes impose a very high energy requirement on
the RPE; not surprisingly, the cells contain all the enzymes of the 3 major biochemical pathways: glycolysis, Krebs cycle, and the pentose phosphate pathway. Glucose is the primary carbon source used for energy metabolism and for conversion to protein. Although
the RPE does make a minor contribution to the glycosaminoglycan- and proteoglycancontain in g interphotoreceptor matrix, glucose is not converted to glycogen in the RPE. Gl ucosamine, fucose, galactose, and mannose are all metaboli zed to some extent in the
RPE, although man nose seems to be passed on almost directly to the photoreceptors. Regarding the chem ical composition of the RPE, over 80% of wet weight is contributed
by water. Proteins, lipids, and nucleic acids contribute most of the remaining we ight. Proteins Nea rl y 850 proteins have been identified in the RPE. Up to 200 acidic proteins are present, and approximately 180 plasma-membrane proteins have been identified. Many proteins that are found in other ceLIs are present also in the RPE. For example, hydrolytic enzymes such as glutathione, peroxidase, catalase, and superoxid e dismutase, which are impor-
tant fo r detoxification, are present in the RPE. The cytoskeletal proteins actin , myosin , u-actinin, fodrin, and vinculin are also present. Other proteins are present in the RPE but are locali zed differently from other cells. A well -known example of this is the sodium pump, which uses energy derived from adenosine triphosphate (ATP) hydrolysis to transport Na+ and K+ against their electrochemical gradients. Na+,K+-ATPase has a unique location in RPE cell s. Whereas most polarized epithelial cells localize this protein to their basolateral surface, the RPE places it on its apical surface. It is thought that Na+,K+-ATPase is apically located to maintain the balance ofNa+ and K+ in the subretinal space. Additional proteins have been shown to have a reversed polarity in RPE cells compared with other polarized epithelial cells, including NCAM- 140 and folate receptor u. In addition , some proteins are expressed only in the RPE. One such protein, RPE65, is an obligate component of the isomerization of vitamin A, which is required for regeneration of visual pigment (described later in the chapter) .
CHAPTER 14:
Retinal Pigment Epithelium. 305
Lipids Lipids accoun t for approximately 3% of the wet weight of the RPE; about half are phospholipids. Phosphatidylcholine and phosphatidylethanolamine make up more tha n 80% of the total phospholi pid content. In ge neral, levels of saturated fatty acids in the RPE are higher than in the adjacent outer segments. The saturated fatty acids palmitic acid and stearic acid are used for retinol esterification and for energy metabolism by the RPE mitochondria. The level of polyu nsaturated fatty acids, such as docosahexaenoic acid (22:6-3), is much lower in the RPE than in the outer segments, although the level of arachidonic acid is relatively high . A number of studies have suggested that the retina may be spared the effects of essential fatty acid deficiency because the RPE effiCientl y sequesters fatty acids from the blood. The RPE actively conserves and effiCientl y reuses the fatty acids, thus preventin g their loss as waste products.
Nucleic Acids Approximately I % of the wet weight of the RPE is contributed by RNA . RNA is synth esized contin ually by the RPE due to production of the ma ny enzymes needed for cell metabolism, phagocytosis of shed discs, and maintenance of the retinoid pathway and transport fun ctions.
Major Physiologic Roles of the RPE The RPE has a num ber of physiologic roles. Critical among these are visua l pigmen t regeneration
phagocytosis of shed photoreceptor outer-segment discs transport of necessary nutrients and io ns to photoreceptor ceLls and removal of
waste products from photoreceptors absorption of scattered and out-of-focus ligh t via pigmentation adhes ion of the retina These functions are discussed briefly in the followin g sections. Other important functions
subserved by the RPE incl ude its role in syntheSiS and remodeli ng of the interphotoreceptor mat rix, for mation of the blood-retina barrier, and elaboration of humoral and growth facto rs.
Visual Pigment Regeneration Regeneration of th e visual pigment rhodopsin, a process that has been studied extenSively,
involves both photo receptors and the RPE. The RPE plays a major role in the uptake, storage, and mobilization of vitami n A for use in the visual cycle. Indeed, the RPE is second only to the liver in its concentratio n of vitam in A.
The basic function of the RPE cell in the visual process is to ge nerate ll -cisreti naldehyde (used in the forma tion of rhodopsin). As desc ribed in detail in Chapte r 13, the photoreceptor cell syntheSizes opsin, which uses II-cis-retinaldehyde in the
306 • Fundam entals and Principles of Ophthalmology rege neration of rhodopsin. In the photoreceptor cell, rh odopsin is photolyzed and undergoes a cis- ta-tra ns isomerization. All -trans- retinaldehyde is released and converted to all-trans- retinol by retinoldehydrogenase. The retinol is returned to the RPE in the presence of interphotoreceptor retinoid-binding protein (!RBP). In the RPE, retinol is converted to retillyl ester in the presence of th e enzyme lecithin retinol acylt ransferase. When needed for regeneration of rhodopsin, the ret in yl ester is converted by an isomerohydrolase (iso merase) to ll -cis- retinol and is subsequently converted to II -cis- retinal by a dehydrogenase. The II -cis- retinal is returned to the photoreceptor cell alo ng with I RBP. The protein RPE65 is thought to playa role in the isomeri zation step because RPE65 knockout mice cannot regenerate rhodopsin. The RPE acq uires vitamin A in 3 ways: I. through release during bleaching of rhodopsin and return via the regeneration process of the visual cycle 2. from circulation, presumably through a receptor-mediated mechanism 3. via phagocytosis of shed photoreceptor outer-segment discs The aldehyde and alcohol forms of vitamin A are membranolytic; hence, several retinoid-binding proteins mediate both vitamin A metabolism within the RPE and vitamin A's exchange w ith adjacent outer segments. A number of retinoid -binding proteins have been isolated and characterized in the RPE, in the subretinal space, and in photoreceptors. The RPE esterifies retinol with available fatty acids (predominantly palmitic acid and, to a lesser extent. stearic and oleic acids) and stores retinol as a reti nyl ester, a form no longer lytic to cell membranes. In conditions of hypervitaminosis A, toxicity to the RPE is minimal because of vitamin A storage as the ester.
Phagocytosis of Shed Photoreceptor Oute r-Segment Discs The RPE plays a crucial role in turnover of the photosens iti ve membrane of rod and cone photoreceptors. In the mid-1960s, au toradiography was used to establish that proteillS were synthesized in the inner segments of the photoreceptor cells and were transported to the outer segment and incorpora ted into new discs fanning at the base of the outer segment. The band of radioactive protein was displaced toward the apex of the cell ove r a period of approximately 9- 11 days. The vital ro le of the RPE in phagocytosis ofthese discs was demonstrated when the radioactive distal tip of the outer segment arrived in the RPE cell and was subsequently phagocytosed. The shed outer-segment discs are encapsulated in phagosomes, \vhich in turn fuse with Iysosomes and are digested . During degradation of the discs, building blocks are recycled into photoreceptors fo r use in the synthesis and assembly of new discs. The li pofuscin characteristic of the RPE is deri ved from photosensiti ve membranes. Each photoreceptor cell sheds approximatel y 100 outer-segment discs per day. Because ma ny photoreceptors interdigitate with a single RPE cell , each RPE cell ingests/digests more than 4000 discs daily! The shedding event fo llows a circadian rhythm: in rods, sheddin g is most vigorous within 2 hours of light onset; in cones, shedd ing occurs more Vigorously at onset of darkness. Recent evidence suggests that the neurotransm_itter dopami ne acts
CHAPTER 14,
Retinal Pigment Epitheli um .
307
within the photoreceptor-pigment epithelial complex to control disc shedding. Defects in the phagocytic fu nction of th e RPE are seen in Royal College of Surgeons rats; these defects lead to degeneration of the photoreceptor cells.
Transport The hea lth and integrity of retinal neurons depend on a well-regulated extraceliular environment. A critical fun ction of the RPE that contributes to this regulation is control of the vo lume and compositi on of fluid in th e su bretinal space through transport of ions, fluid,
and metaboli tes. The distribution of transport proteins res iding in the apical and basolateral membrane domains of the ce ll is clearly asymmetric, and lhis difference is what allows the epithelium to carr y out vectorial transport. The membrane proteins remain in their proper local ion because of tight junction proteins. Inte rcellularl y, asymmetry or
polarity of the cell is ma intained because of the intracellular molecu lar machinery that synthesizes new proteins and de li ve rs them preferentially to the apica l or basolateral cell membranes. Cytoskeletal proteins are fundamental in determining cell polarity and regulating transport.
The aqueous environ ment of the subretinal space is acti vely I1'laintained by the
iOI1 -
transport systems of the RPE. The active transport of a variety of ions (K" Ca'" Na" CI-, and HCO,- ) across the RPE has been well documented. This transport is vectorial in most
cases; for example. Na+ is actively transported from the choriocapillaris toward the subretinal space, whereas K+ is transported in the opposite direction. The apical membrane of the RPE appears to be the major locus of this transport. The ouabain-sensitive Na+,K+-ATPase is present at the apical, but not the basal, side. Similarl y, an active bicarbonate-transport sys tem appears to be located in this portion of th e RPE memb ran e. High ca rboni c anhy-
drase act ivity seems to be associated with both the apical and basal sides of the cell. Net ionic fluxes in the RPE are responsible for the transepitheUal electrical potential that can be measured across the RPE apical membrane, a potential rapid ly modified in the presence of a variety of metabolic in hibi tors (eg, ouabain an d dinitrophenol). In addition, the RPE apical membrane must be responsive to the changing conditions of phototrans-
duction. For example, light evokes a decrease in K+ ion concentration in the subretinal space, thus hyperpolarizing the RPE. Because th e activity of Na' ,K'-ATPase is controlled in part by K' ion co ncentration, light can affect the ionic composition of the subrelinal space and the transport func tions of the RPE. Active vectorial transport systems for other retinal metabolites (eg, taurine, methi onin e, and folate) have also been demonst rated. The
RPE, therefore, appears to be important fo r maintai ning the ionic environment of the subretinal space, which in turn is responsible for maintain ing the integrity of the RPEphotoreceptor interface. The trans-RPE potential is the basis for the e1ectro-oculogram (EOG), wh ich is the most co mmon electrophysiologic test for eval uatin g the RPE.
Pigmentation A characteristic feature of the RPE is the presence of melanin pigment. Pigment granules are ab undant in the cyto plas m of adult RPE cells, predominantly in the apica l and midportions of the cel l. During development, activa ti on of the tyrosinase promoter triggers
308 • Fundamentals and Princ iples of Op htha lmology the onset of melanogenesis in this cell and marks the commitment of the neuroectoderm
to become RPE. Although most m elanogenesis occurs before birth, melanin production in the RPE does occur thro ughout life, albeit at a slow rate. As humans age, the melanin granules fuse with Iysoso mes; thus, the fund us of an olde r person is less pigm ented than that of a young person. The exact role of melanin inside ce ll s remains speculative. One universally recogn ized function of melanin is to act as a neutral-density ftlter in scatteri ng light. In so doing. melanin may have a protective role. In spite of the mi nim ization of light scatter, visual acuity in the minimally pigmented fundus ca n be normal. Visual proble ms in individuals with
albinism are attributable to foveal aplasia, not optical scatter. Genetic ocular disorders associated w ith melanin incl ude va rying forms of albinism . OCA refers to ocuJoclitaneOlIS alb inism, of which there are 10 types. The OCA J and OCA2 types are due to defects in the tyros inase gene and the pink-eyed dil ution gene, respectively. When melanin levels are below a critical level, there is aberrant neuronal migration in the visual pathway, lack of foveal development. low vision, nystagmus, and strabismus (ocu lar albin ism is characterized by a lack of pigment in the eye but relatively normal pigmentation of skin and hair). Melanin is thought to playa role in reti nal development because albino mammals have underdeveloped cent ral retinas, more contralateral projections of ganglion cells, and fa ilure of foveal development. Melanin is a free-radica l stabili ze r an d ca n bind m any toxins. Some regard th is feature as protective; others thin k that it cont rib utes to tissue toxic ity.
Retinal Adhesion No ne of the aforeme ntioned functions would be possible without another RPE functionnamely. maintenance of re tinal adhesion. The subretinal space is never bridged by tissue, and yet the neural ret ina remai ns rat her fi rm ly attached to the RPE throughout li fe. This adhesion is vital to the retina because detachment of the photoreceptors from the RPE can lead to permanent morp hologic change in the tissue.
Multiple systems keep the retina in place. These factors include passive hydrostatic forces, interdigitation of outer segments and RPE microvi ll i, active tra nsport of subretinal fl uid, and the complex structure and bi ndi ng properties of the interphotoreceptor matrix.
In situations of pathology, retinal adhesion can dimin ish, and detac hment of the ret ina occurs. Detachme nt does not occur sim ply because there is a hole in th e ret ina or a leak in the RPE; there must be e ither positive traction pull ing the neura l reti na or pos itive forces
pushing fluid into the subretinal space.
The RPE in Disease Clearl y, the RPE is vita l for normal visua l function. Three ret ina l degenerations in humans
appea r to be caused by defects un ique to the RPE: Sorsby fun dus dystrophy an d 2 fo rms of autosomal recessive retinitis pigmentosa (RP). There are 2 generalized ret inal degenerations: Usher syndrome (type I B) and sex-li nked RP, in which the defective gene is expressed strongly in the RPE and weakly in neural retina. It has been suggested that some forms of macular degeneration, such as vitelli form macular degeneration (Best disease)
CHAPTER '4:
Retinal Pigment Epithelium . 309
and malattia leventinese (dominant drusen) , may be due to a primary defect in the RPE. Age-related macular degeneration and Stargardt disease appear to affect the RPE early in their course, although the genetic defect is in the rods. Finally, choroideremia and gyrate atrophy produce blindness by their early impact on the RPE. [n certain pathologic situations (including proliferative vitreoret in opathy and subretinal neovascu larization). RPE
cells detach from the basement membrane and become migratory. Efforts are now under way to determine effec tive methods of RPE tran splantation that may ameliorate the functional deficits in these diseases. Gallemo re RP, Hughes BA. Miller SS . Retinal pigment epithelial transport mechanisms and their cont ributions to the electroretinogram. Prog Retill Eye Res. 1997; 16:509 - 566.
Marmor MF. Wolfensberger TJ, eds. The Retinal Pigment Epithelium: FlIIlction lind Disease. New York: Oxford: 1998: 103- 134.
CHAPTER
15
Free Radicals and Antioxidants
The adverse effects of reactive forms of oxygen have been repeatedly proposed as causal factors in many types of tissue pathology, including cataract and age-related macular degeneration (AM D). Lipid peroxides are formed when free rad icals or singlet oxygen molecules react with unsaturated fatty acids, which are present in cells largely as glycerylesters in phospholipids or triglycerides. It has been hypothesized that the oxidation of membrane phospholipids increases the permeability of cell membranes and/or inhibits membrane ion pumps. This loss of barrier fun ction is thought to lead to edema, disturbances in electrolyte balance, and elevat ion of intracellular calcium, all of which contribute to cell ma lfunctio n.
Cellular Sources of Active Ox
eCles
The term reactive oxygell intermediates (ROls) is used collectively to describe free radi cals, hydrogen peroxide (H,O, ), and singlet oxygen. Free radicals are molecules or atoms th at possess an unpaired electron. This property makes them highly reactive toward other molec ular species. Free radicals consist of superoxide anion (0,-), the hyd roxyl radical (O H ·), and the lipid peroxyl radicals. Some free-rad ical reactions are involved in normal cell functions; others are thought to be important mediators of tissue damage. Oxygenderived free radicals and their metabolites are generated within aerobic organisms in several ways.
Oxygen necessary for normal metabolism usually undergoes tetravalent (4-e1ectron) reduction by intracellular systems, such as cytochrome oxidase in mitochond ria (Fig J 5- 1), and is fina ll y disca rd ed as water without leakage of RO ls. However, a small percentage of the metabolized oxygen undergoes univalent reduction in four I-electron steps. Oxygen accepts an electron from a reduCing agent in each of these steps, and several ROIs are formed that are highly reactive. Some of the reactive species leak out of their enzyme-binding sites and may damage other components of tissues, such as proteins, membrane lipids, and DNA, if they are not captu red by detOXifying enzymes. Superoxide is not only produced in mitochondria l electron-transport syste ms but also formed in some enzymati c reac tion s. such as the xan-
thine and xanthi ne oxidase system. Hydrogen peroxide is produced directly in peroxisomes, as well as by enzyme-catalyzed dismutation of superoxide (see Fig 15-1). Any free iron (Fe'+) present may catalyze formation of the hydroxyl radical from superOJode and hydroge n peroxide. Iron and other catalytic metals such as copper may also be involved 311
312 • Fundamentals and Principles of Ophthalmology Cytochrome oxidase complex Catalases
Peroxidase
,
--"'H 0 Superoxide di smutases
Figure 15·'
Enzymes involved in the metabolism of oxygen and in the detoxification of oxy-
gen radicals generated by the univalent reduction of molecular oxygen. The univalent pathway involves a series of single electron transfers. producing the superoxide free radical (0 2- )' hy-
drogen peroxide (H, O, ). water. and the hydroxyl radical (OH -). Superoxide dismutase catalyzes
the conversion of superoxide to hydrogen peroxide without oxidizing other molecules. Catalase and peroxidase catalyze the reduction of hydrogen peroxide to water without formation
of the toxic hydroxyl radical. These enzyme systems are ca pable of preventing the buildup of toxic species produced from the univalent reduction of oxygen. The cytochrome oxidase complex appears to catalyze the tetravalent reduction of oxygen to water without leakage of reactive intermediates. (Courresy of F. J. G. M. van Kuijk, MD, PhD.)
in generating these species by accelerat in g nonenzymatic ox.idation of seve ral mol ecules, including glutathione. O th er sources of activated oxygen species include products from the enzymat ic synthesis of prostaglan dins, leukotrienes, and throm boxanes (see C hapter 9, Fig 9- 1). The NADP H oxidase system of phagocytes yields activated oxygen species, especiall y durin g inflammatio n reactions. Oxygen -radical production is also associated with ioni zing radiatio n and th e metabolism of many chemica ls and drugs. including carcinogenic compo unds. Fo rmati o n of reactive oxygen speCies-such as Singlet oxygen by a light-mediated mechanism - is considered in the following section. Superoxide and hydrogen peroxide are relat ively stable in biological systems, whereas th e hydroxyl radica l is extremely reactive an d capable of prod ucing broad , nonspecific oxidative damage. However, free radicals and other active oxygen species also are important in many biological reactions that maintain normal cell functions, such as mitochondrial and microsomal electron-transport systems.
Mechanisms of Li id Peroxidation The mechanism by which random oxidation of lipids takes place is called aLito-oxidation. This oxidation is a free-radical chain reacti on usually described as a series of 3 processes: initiation , propagation, and termination. Durin g the initiation step, the fatty acid is converted to an intermediate radical after remova l of an allylic hydrogen. The propagation step fo llows immediately, and th e fatty -aci d radi cal intermediate reacts with oxygen at either end to prod uce fatty-acid peroxy radi cals. Thus, a new fatty-aCid radi cal is fo rm ed, whi ch again ca n react with oxyge n. As long as oxyge n is avai lable, a Single free radica l can lead to oxidation of thousands of fatty ac ids. A termination reaction, in which 2 rad icals form a no n radi cal product, can interrupt the chain reaction. Auto -ox idation is also inhib ited by free-radical scavengers such as vitamin E, which cause termination reactions.
CHAPTER 15:
Free Radical s and Antiox idants. 313
Polyunsaturated fatty acids are susceptible to auto-oxidation because their allylic hydrogens are easil y removed by several types of initiating radicals. The primary products of auto-oxidation form ed dur ing the propagation step are hydroperoxides (ROOH), which may decompose, especially in the presence of trace amounts of transition metal ions (eg, free iron or copper), to create peroxy radicals (ROO·), hydroxy radicals (HO ·), and oxy radicals (RO·). Photo-oxidation is a process by which oxygen is activated electronically by light to form Singlet oxygen, which in turn reacts at a diffusio n-controlled rate w ith unsatu rated fatty acids or other cellular constitue nts. The mechanism of Singlet-oxygen generation most Widely accepted involves exposure of a photosensitizer to light in the presence of normal triplet oxygen (,0,). A photosensitizer is excited by absorption of light energy to an excited Singlet state, which rapidly relaxes to an excited triplet state. In this state, the sensiti zer may react with triplet oxygen (,0,) to form Singlet oxygen (,0,). Photooxidation can be inhibited by Singlet-oxygen quenchers such as carotenoids, which are discussed later in this chapter. Lipid peroxidation not only causes direct damage to the cell membrane but also causes secondary damage in ceLIs through its aldehydic breakd own products. Lipid hydroperoxides are unstable, and they break dow n to for m many aldehydes, such as malondialdehyde and 4-hyd roxyalkenals. These aldehydes can react quickly with proteins, inhibiting the proteins' normal functions. Both the lens and th e retina are susceptible to such oxidative damage.
Oxidative Damage to the lens The lens is susceptible to challenge by var ious active species of oxygen because it contains low levels of molecular oxygen and trace amounts of transition metals such as copper and iron . It is thought that metal -catalyzed auto-oxidat ion reactions of various reducing agents in the lens can lead to the production of potentially damaging oxidants, such as oxidized glutathione (GSSG) and dehydroascorbiC acid, as weLl as hydrogen peroxide, which can go on to produce hydroxyl rad icals. In addi tion, ultraviolet radiation entering the lens can ge nerate ROls. Although most UVB radiation «320 nm wavelength) striking the human eye is absorbed either by the corn ea or by the high level of ascorbic acid in the aqueous humor, a certain proportion is able to reach the lens epithelium, where it can cause damage. UVA ligh t (320-400 nm wavelength) is able to reach more deeply into the lens, where it can react with various chromophores to generate hydrogen peroxide, superoxide anion, and Singlet oxygen. Altho ugh repair or regeneration mechanisms are active in the lens epithelium and superficial cortex, th ere are no such mechanisms in the deep cortex and the nucleus, where any damage to lens proteins and membrane lipids is irreversible. One resu lt of this damage can be cross-linking and insolubilization of proteins, leading to loss of transparency (see Chapter II ). Certain types of human cataracts appear to initiate at the site of the fiber cell plasma membrane, possibly because oxygen is 5-7 times more soluble in membrane lipids than in cytoplasm. To defend against oxidative stress, the young, healthy lens possesses a variety of effective ant ioxidant systems. These include the enzymes glutathione peroxidase, catalase, and
314 • Fundamentals and Principles of Ophthalmology superoxide dismutase (SOD) (Fig 15-2) . By means of the glu tathione redox cycle. GSSG is reconverted to glutathione (GSH) by glutathione reductase via the pyridine nucleotide NAOPH. which is provided by the hexose monophosphate shun t as the reducing pathway. Th us, GSH acts as a major scavenger of act ive oxygen species in the lens. For reasons that are not well understood, the mammalian lens contains unusua ll y high levels of protein sulfh ydryl groups; however. it is clear that the groups must exist nearly completely in the reduced state for the tissue to remain transparent. The young hurnan lens contains a high level of GSH . which is first sy nthesized in th e epitheliu m and th en migrates to the lens cortex and nucleus. With age. levels of GSH decline significantly in the human lens. particularly in the nucleus. Studies have indicated that a cortical- nuclear barrier may exist in the mature human lens. wh ich inh ibits the free flow of GSH to the nucle us. The result is that with age. the human lens nucleus becomes more susceptible to ox idative da mage and cataract. Nuclear cataracts show high levels of oxidized cysteine and methionine in the lens proteins. The free-radical scavengers ascorbic acid and vitamin E are also present in the lens. These scavengers work in conjunction with GSH and the glu tathione redox cycle to protect against oxidative damage. Carotenoids that can quench singlet oxygen also exist in the lens. Epidemiologic (observational ) studi es have shown that people with higher levels of plasma antioxidants. particularly vitamin E, have a reduced risk of cataract. particularly nuclear cataract. However, 2 prospective. randomized placebo-controlled clinical trialsthe Age-Related Eye Disease Study (AREDS) and the Vitamin E. Cataract and AgePolyunsaturated fatty acids (PuFA)
~t a"
Sulfur
-1 Vitamin E I
Light ------ - 1 02 - - ---- - ---- ~ -- Carotenoids
Free - ----- - Lipid peroXideS radicals (LOOH) (L', La·. LOa·) etc.
t
acids ,
1
GSH (reduced) GSH-Px
NADP'l
GSHRd
Glucose 6-PO, G-6-PDH
(Se-dependeot)
•
mutations enzyme damage cross-linking lipofuscin
etc.
l
Amino
I h
I
Lipid alcohols (LOH)
:
GSSG (oxidized)
NADPH
etc.
Normal
poxidation Figure 15-2 Mechanisms by which several antioxidan ts protect against oxidative damage. Upper fe ft, Free radicals lead to the fo rmation of lipid peroxides. Vitamin E in hibits this autooxidation process by scavenging free-radica l in termediates . Carotenoids inhibit photo-oxidation by que nching singlet oxygen (' 0 ,). Center. If lipid hydroperoxides are fo rmed, they can be reduced by GSH-Px, which requires selenium as a cofactor. If these protective enzymes are not fully active, more free radicals are formed by breakdown of lipid peroxides, wh ich in turn leads to additional oxidation of polyunsa turated fatty acids. G-6-PDH = glucose-6-PO, dehydrogenase. GSH = glutathione, GSH-Px = glutathione peroxidase, GSHRd = glutathione reductase, GSSG = oxidized glutathione. (Courtes y of F. J. G. M. van Kuiik, MD, PhD.}
CHAPTER 15:
Free Rad icals and Antioxidants. 3 15
Related Maculopathy Trial (VECAT) - fou nd that high -dose formulations of vitamin C, vitamin E, and beta carotene (AREDS), as well as vitamin E alone, neither prevented the development nor slowed the progressio n of age-related cataracts. Age-Related Eye Disease Study Research Group. A randomized, placebo-controll ed clin ical trial of high -dose supplem entation wi th vitamin s C and E and beta carotene for age- related cataract and vision loss: AREDS report no. 9. Arch Ophthalmol. 200 1;11 9{l O):1439 - 1452. Lyle BJ, Mares- Pe rlman JA, Klein BE, et al. Serum carotenoids and tocopherols and in cidence of age- related nuclear cataract. Am J Cli n Nut r. 1999;69(2}:272 - 277 . McNeil
JJ, Robman L, Tikellis G, Sinclai r MI, McCarty CA, Taylor HR.
Vi tamin E suppleme n-
tation an d cataract: randomized co ntrolled trial. Ophthalmology. 2004; 111 (1 ):75- 84. Padgaonkar VA, Lin LR, Leve renz VR, Rinke A, Reddy VN, Gblin F). Hyperbaric oxyge n in
vivo accelerates the loss of cytoskeletal proteins and MIP26 in guinea pig lens nucleus. Exp Eye Res. 1999;68(4};493- 504. Sweeney MH, Tru scott RJ. An impediment to glutathione diffusion in older normal human lenses: a possible precondition for nuclear cataract. Exp Eye Res. 1998;67(5):587- 595.
Vulnerability of the Retina to Free Radicals Expe ri mental data have shown that retinal photo receptors degenerate when they are exposed to oxidative challenges such as hyperbaric oxygen, iron overload, or injection of lip id hydroperoxides into the vitreous humor. The retina also degenerates when antioxi dative defen ses are reduced, which presumab ly elevates levels of lipid peroxidation in the absence of unusual oxidative stress. The retina is made vulnerable to damage from lipid peroxidation by several distinctive characteristics, 4 of which are cons idered here: 1. Vertebrate retinal rod outer segments are susceptible to damage by oxygen because of thei r high content of polyunsaturated fatty acids. Their phospholi pids typically contain about 50 mol% docosahexae noic acid, the most highly polyun saturated fatty acid that occurs in nat ure. It is well established that polyunsaturated fatty acids are sensitive to peroxidation in proportion to their number of double bonds. 2. The rod in ner segment is very rich in mitochondria, which may leak activated oxygen species. 3. The excellent oxygen supply thro ugh the choroid and the retinal vessels elevates the risk of oxidative damage. Vertebrate retinas maintained in vitro showed at least a sevenfold higher rate of oxygen consumption per milligram of protein compared with all other tissues tested (except the adrenal gland). The oxygen tension is high est at the choroid and drops toward the inner segments because of the high metabolic demand of their mitochondria. Oxygen consumption has been reported to decrease when the retina is illuminated. 4. Light exposure may tri gger photo-oxidative processes mediated by Singlet oxygen, and the RPE may playa key role. The RPE is tightly packed with endoplasmic reticulum and appears to be rich in antioxidant enzymes in most species tested. The RPE of pigmented animals contains melanin
316 • Fundame ntal s a nd Principles of Ophtha lm ology granules, which funct ion as a light trap. Although melanin is commonly assumed to be photoprotective, its role in the prevention of light damage to ocu lar tissues is not clearly und erstood. Evidence suggests that the RPE is qu ite sensitive to dietary antioxidant deficie ncy, in which light-activated melani n may contribute to phototoxicity. If an RPE cell dies, then the numerous photo receptors supported by that RPE cell may suffer severe damage or death. Intense light at levels that may be encountered in daily li fe is phototoxic to the reti na. Even though the cornea absorbs some UV radiation, the ret inas of young people are exposed to a substantial amount of light in the range of 350-400 nm (you ng lenses transmit these wavelengths). The lens yellows with age, an d the cutoff wavelength in older people moves up to approxi mately 430 nm. Because the ad ult lens absorbs nearly 100% of light below 400 nm, little o r no UV light reaches the retina in older people. In addition to UV light, blue light (400-500 nm) can be harmfu l to the reti na (bluelight hazard). The photoreceptors in the reti na are particu lar ly susceptible to damage by bl ue light, a process that can lead to cell death and retinal disease. This happens due to the formation of the phototoxic compound A2E. A2E (ie, A2E epoxide) specifically targets cytochrome oxidase and induces irreversible DNA damage (apoptosis) ofRPE cells. This molecular process is believed to be a precursor to the pathogenesis of AMD. Carotenoids (eg, lutein and zeaxanthin ) present in the retina act as a blue-light fi lter, sh ielding the photoreceptors in the retin a from th is radiat ion, and in vitro studies have suggested th at vitamin E and other antioxidants inhibit A2E-epoxi de formation.
Shaban H, Richter C. A2E and blue light in th e retina: the paradigm of age-related macular degeneration. Bioi Chem. 2002;383(3- 4):537-545. Sparrow JR, Vollmer-Sna rr HR, Zhou J, et al. A2E-epoxides damage DNA in retinal pigment epithelial cells. Vitam in E and other antioxidants inhibit A2E -epoxide formation. J Bioi Chem. 2003;278(20),18207- 1821 3.
Antioxidants in the Retina and RPE Several antioxida nt mechanisms have been established in biological systems, incl uding free-radical scavenging, quenching of Singlet oxygen, and enzymatic reduction of hyd roperoxides. Antioxidants found in vertebrates include selenium, GS H, selenium dependent glutathione peroxidase, non-selenium -dependent glu tathione peroxidase (g lutathione-S-transferase), vitamin E, and carotenoids. Antioxida nts in vertebrates also include SOD and cata lase, and antioxidant ro les for ascorbate and melanin have also been reported. The relation between some of these an tioxidants and the protective mechanisms is shown in Figure 15-2. Yu BP. Cellular defenses against damage from reactive oxygen species. Physiol Rev. 1994;74{ 1): 139- 162.
Selenium, Glutathione. Glutathione Peroxidase, and Glutathione-S-Transferase A number of enzymes have been identified that can provide antioxidant protection by a peroxide-decomposing mechanism. For example, selenium-dependent glutathione
CHAPTER 15: Free Radicals and Antioxidants. 3 17
peroxidase (GSH-Px) and several enzymes of th e glutathione-S-transferase (GSH -S-Ts) group can reduce organic hydro peroxides. GSH -Px is also active with H,O, as a subst rate, although the GSH-S-Ts gro up cannot act on H,O,. All of these enzymes require GSH, which is conve rted to GSSG during the enzymatic reaction. The hexose monophosphate shunt enzymes produce NADPH, which is needed for reduction of GSSG by GSH reductase. Both GSH- Px and GSH-S-Ts activities have been measured in human and in animal retinas. The highest concentration of seleniu m in th e human eye is present in the
RPE: 100-400 ng in the RPE cells of a Single human eye, up to 10 times more than in the retina (40 ng). In add ition, the amount of selenium appears to be similar in both eyes of the same indi vidual. The selenium level in the human reti na is constant with age; in the human RPE, however, the level in creases with age.
Vitamin E Vitam in E acts by scavengi ng free radica ls, thus terminating the propagat ion steps and leading to interruption of th e au to -oxidation react ion. Reports on the vitamin E content of th e retina of the adult rat raised on normal chow diets show values ranging from 215 to 325 ng. A detailed study on the vitami n E con tent of microdissected parts of verte brate
eyes showed that the RPE is rich in vitami n E relati ve to the photo receptors and that photo receptors are rich in vitam in E relative to most other cells in the eye. Studies on vitamin E in postmortem human eyes also found that the level of vitamin E is higher in the RPE than in the retina. Fu rthermore, vitamin E levels in human retinal tissues increase
wi th age until the sixth decade of life and then decrease. Th is decrease coincides with the age at which the incidence of AMD in creases in the population.
'
Friedrichson T, Kalbach HL, Buck P, van Kuijk FJ . Vitamin E in macular and peripheral tissues of the human eye. Curr Eye Res. 1995; 14(8):693-701.
Superoxide Dismutase and Catalase Superoxide dis mutase catalyzes the dismutation of superoxide to hydrogen peroxide, which is furt her reduced to water by catalase or peroxidase. Two types of SOD are usuall y isolated from mammalian tissues: Cu -Zn SOD, the cytop lasmic enzyme, which is inhibited by cyanide; and Mn SOD, the mitochondrial enzyme, which is not inhibited by cyanide. Catalase catalyzes the reduction of hydrogen peroxide to water. Information on catalase activity in the retina is currently rather limited. Total retinal catalase activity was
found to be ve ry low but detectable in the rabbit. A protective role for catalase has been reported in rats wi th experim ental allergic uveit is.
Ascorbate Ascorbate (vitamin C) is thought to fu nction synergistically with vitamin E to terminate free-radical reactions. It has been proposed that vitamin C can react with the vitamin E
radicals formed when vitamin E scavenges free radicals. Vitam in E radicals are then regenerated to native vitamin E. The vitam in C radicals resu lting from this regeneration can
be reduced by NADH reductase, with NADH as the electron accepto r. Ascorbic acid is
31 8 • Funda me nta ls a nd Principles o f Ophthalmology fo und th roughout the eye of many species in concentrati ons th at are hi gh relative to those in other tissues. Delam ere NA. Asco rbic acid and the eye. In: Harris JR, ed. Subcellular Biochemistry, Vol 25. Ascorbic Acid: Biochemistry and Biomedica l Cel l Biology. New York: Pl enum Press; (996: 3 13-329.
Carotenoids Va rious roles have bee n p roposed fo r ca roteno ids (xanthophylls) in biological systems, in cluding limiting chro mati c aberrati on at the fovea of th e retina and th e qu enching of sin glet oxygen. ~ -Ca ro ten e is the prec urso r of vitam i ll A and can ac t as a free-radical trap at low oxygen tensio n. In postmortem huma n retinas. caroteno ids have been shown to make up th e yellow pigment in the macul a. A mix ture of the 2 caroteno ids lutein and zeaxmll hin is present in the mac ula and located in th e Henle fiber layer. It has bee n demonstrated th at in human s, zeaxanthin is concentrated primaril y in the fovea, whereas lute in is dispersed in the retin a. Inte restin gly, little ~-carote n e is present in th e human eye. Furthermo re, ca rotenoids are prese nt o nly in th e retin a and not at aU in th e RP E. In the periph eral retina, lutein and zeaxanthin are also con centrated in th e photoreceptor outer segments and may act as anti oxidants to protec t the macula against short-wavelength visible light. Figure l S-3A shows th e localization of antioxidants in the human mac ula and peripheral retina; Figure IS-3 B shows their locali zatio n in a cross sectio n of th e peripheral retina. Khachik F, Bernstein PS, Garland DL. ldentification of lutein and zeaxant hin oxidation prod uc ts in humaf\and monkey retinas. 1,lvest Ophtha/mol Vis Sci. 1997;38(9): 1802- 181 1. Mayne ST. Beta-carote ne, carotenoids, and d isease prevention in hu mans. FASEB 1. 1996; 10(7): 690-70 1.
~'~"'~~""
. '';
-;;;-
A (P'l,(ol-- Ganglion Figure 15-3 A, The localization of antioxidants in the human macula and peripheral retina. Yellow represents carotenoids, blue represents vitamin E, and red represents selenium. Vitamin E and selenium are primarily concentrated in the RPE. In the macula, carotenoids
ce ll Amacrine
_
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cell ~~P6~, Horizontal
cell
are present in the fibers of Henle; in the peripheral retina, they are also present in the rods. B. The localization in a cross section of the peripheral retina. Vitamin E and selenium remain primarily concentrated in the RPE but are also enriched in the rod outer segments. Carotenoids have been found in rod outer segments in the peripheral retina. (illus tratIOns by J. Woodward, MD: counesy of F. J.
Cone Rod
G. M. van Kuijk, MD, PhD.) RPE cell
B
CHAPTER
16
Pharmacologic Principles
Introduction Ophthalmic medications and distant humoral and local neural transmitters both manipulate the same receptors. The drugs differ from the natural transmitters in not only the mechanism and location of delivery but also the quantity of agent released and the feedback control of the active agent. For example, mydriatic and cycloplegic dilating drops are applied in a location remote from their intended sites of action in enormous quantit ies that swamp homeostatic mechanisms. In contrast, the intrin sic neurotransmitters
controLling the pupil are released in precise locations in minute quantities as part of a delicatel y balanced system. Phar macothera py depends on a frequently unreliable mechanism- the patient-to deliver agents. These medication s are given infrequently in volum es greater than the tear
film capacity; the type of medication and the dosing regimen are adjusted based on the clinical response. The more reliable physiologic processes in healthy patients, in contrast, continuously deliver the appropriate quantities of transmitter via precisely controlled feedback mechanisms. Further, pharmacologic agents often produce unwanted side ef-
fects, known as toxicity, by acting at sites other than the intended location. Drug therapy may also induce counteract in g mechanisms, such as up-regu lating drug-metabo li zing en -
zymes or down -regulating receptors that alter the therapeutic effect. Occasionally, medications may cause an allergic reaction. These effects all differ from the highly specific physiologic neural and humoral agents, which work in concert with feedback mechanisms and have immune privilege.
In the future, we will be able to model most, if not all, of our pharmacologic interventions on natural physiologic processes. Delivering agents directly to the desired site of action or continuously from a sustained -release device or deposit will improve efficacy and compliance and reduce side effects. Additional areas of future research include
coupling drug release to feedback-control mechanisms (eg, glaucoma therapy regulated by a pressure-sensing strain ga uge)
achieving higher specificity of action by designing drugs to complement the geometry of receptor sites designing medication with structural modifications that eliminate allergenic moi eties or min imize side-effect profiles
recognizing genetic traits that predispose individuals to disease, and devising drugs that repair or modify the defective gene product manipulating the genome to effect repa ir or regeneration 321
322 • Fundamentals and Principles of Ophthalmology The study of ocular pharmacology begins with a review of some general principles of phannacology, with particular attention to special features of the eye that facilitate or impede ocular therapy.
Pharmacokinetics Pharmacokinetics deals with the cycle of a drug through the body, including the absorption, distribution, metabolism, and excretion of that drug. To achieve a therapeutic effect, a drug must reach its site of action in sufficient concentratio n. The concentration at the site of action is a function of the route of administration, the amount administered, the
extent and rate of absorption at the adm inistration site, the distrib utio n and binding in tissues, the movement by bulk flow in circulating fluids, the transport between compartments. biotransformation, and excretion. Pharmacokinetics and dose together determine
bioavailability. or concentration of the active drug at the therapeutic site.
Pharmacodynamics Pharmacodynamics refers to the biological activity and clinical effect of drugs. It is the drug action after pharmacokinetics has distributed the act ive agent to th e therapeutic site.
Included within the area of pharmacodynamics are the tissue receptor for the drug and the intracellular changes initiated by the binding of the active drug with the receptor. The pharmacodynamic action of a drug is often described using the receptor for that drug, for example an a-adrenergic agonist or ~-adrenergic antagonist.
Pharmacotherapeutics
Pharmacotherapeutics is the application of a drug in order to reach a given cl inical endpoint, such as the prevention or treatment of disease. The therapeutic dose may vary for any patient, based on the patient's age, gender, race, other currently prescribed medications, and preexisting medical conditions. Phannacotherapeut.ics are covered in Chapter 17. Toxicity Toxicity refers to the adverse effects of e ither medication or environmental chemica ls, including poison ing. Toxicity may be influenced by pharmacokinetics and/or pharmacodynamics. For example, topicaJJ y applied ophthalmic medications are readil y absorbed through the mucous membranes of the eye and nasophar ynx, as well as by absorption through the iris and ciliary body. Topical absorption avoids the first-pass metabolism of the liver and increases systemic bioavailability. The system ic toxicity of these medications may therefore be more than expected relative to the total topical dose. Local toxicity of topical agents is more common than systemic toxicity. howeve r, Local toxicity may be a type I IgE-mediated hypersensitivity reactio n or may represent a delayed reaction to either the medication or the preservatives. The older preservatives thimerosal and benzalkonium chloride are frequently implicated in ocular toxicity. New preservatives developed in response to this problem dissipate upon exposure to light or to the ions in the tear fUm. Two examples of these are oxychloro complex (Pur ite), which breaks down to sodium
CHAPTER 16:
Pharmaco logic Principles.
323
chloride and water, and sodium perborate, which breaks down to hydrogen peroxide before becom ing oxygen and hydrogen. These "disappearing preservatives" theoreticall y should have no toxicity to the corneal surface. These pharmacologic principles apply differently to the elderly. Compared with yo un ger patients, older patients have less lean body mass due to a decrease in muscle bulk, less body water and albumin, and an increase in the relative percentage of adipose tissue. These physiologic differences alter tissue binding and drug distribution. Human renal function decreases 50% with age; both hepatic perfusion and enzymatic activity are va riably affected as well. The elderly tend to be on more chronic medications, many of which are processed simultaneously by the same, already compromised, metabolic systems. The pharmacokinetic processing of dru gs in the elderly is therefore significantly altered, extending the effective half- life of most medications. The pharmacodynamic action of a drug is often independently potentiated in the elderly. The increase in both drug effect and side effects occurs even if the dose is decreased in consideration of these pharmacokinetic changes. Thus, the pharmacotherapeutic effects and the toxicity of medication may be altered simply by the aging process, independent of drug dosage. Accordingly, the selection of a specific therapeutic agent should be guided by the general health, age, and concomitant medication taken by a patient.
Pharmacokinetics: The Route of Drug Delivery Topical Administration Eyedrops Most ocular medications are administered topically as drops. This rou te of administration maximizes the anterior segment concentrat ions while minimizing systemic toxicity. The drug gradient from the concentrated tear reservoir to the relatively barren corneal and conjunct iva l epithelium forces a passive route of absorption. Some features of topical ocular therapy limit its effectiveness. Very little of an administered drop is retained by the eye. When a 50-~lL drop is delivered from the usual commercial dispenser, the volume of the tear lake rises from 7 ~ L to only 10 ~L in the blinking eye of an upright patient. Thus, at most, 20% of the ad ministered drug is retained (l0 ~ L/50 ~L). A rapid turnover of fluid in the tear lake also occurs, 16% per minute in the undisturbed eye. with even faster turnover if the drop elicits reflex tearing. Consequently, for slowly absorbed drugs, only 50% of the drug that was initially retained in the tear reservo ir (50% of the 20% of the delivered medication, or 10%) remains 4 minutes after instillation (0.84' X 0.50), and only 17% remains after 10 minutes, or 3.4% of the original dose. The amount of time that a drug remains in the tear reservoir and tear film is called the "residence time" of a medication. It is affected not onl y by drug formulation but also by the timing of subsequent medication, tear production, and drainage. Some simple measures have been shown to improve ocular absorption of materials that do not traverse the cornea rapidly. Patients using more than one topical ocular
Amount administered
324 • Fundamentals a nd Principles of Ophthalmology medication should be instructed to allow 5 minutes between drops; otherwise, the second drop may simply was h o ut the first. Blinking also dimi nishes a drug's effect by activating the nasal lacrimal pump mechan ism, forcing fluid from the lacrimal sac into the nasop harynx, and creating a negative sac pressure that empties the tear lake (see BeSe Secti on 7, Orbit, Eye/ids, and Lacrimal System). Patients can circumvent this loss of drug reservoir
by e ith er com pressing th e nasolacrimal duct with digita l pressure at the medial ca nthus o r closing the eyelids for 5 minutes after instillation of each drop. These 2 measures will prevent emptying of the tear lake and wi ll reduce systemic toxicity by decreasing absorption through the nasal mucosa. Nasolacrimal occl usion wi ll increase the absorption of topically applied materials (Fig 16-1) and decrease the systemic absorpti on and potential toxicity (Fig 16-2). Tear rese rvoir retentio n and dr ug contact time ca n also be extended either by increasing the viscosity of the vehicle or by using drug delivery objects such as contact lenses, collagen sh ields, and inserts. Topical medications that are absorbed by the nasal mu cosa can attai n Signi ficant levels in the blood. One to 2 drops of a top ical medication may provide a Signi ficant systemic dose of th at drug. For exa mple, a 1% solution of atropine has I gllOO mL, or 10 mgll mL. A Simpler way of remembering this is to add a 0 to the dr ug percentage to change to
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Figure 16-1
Fluorescein concentration in the anterior chamber at various times after applica-
tion: with nasolacrimal occlusion (NLOl. with 5 minutes of eyelid closure, or with no interven-
1ion (no NLOI.
CHAPTER 16: Pharmacologic Principles . 325
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Systemic absorption of timolol at various times after application: with nasolacri-
mal obstruction (NLOI. with 5 minutes of eyelid closure. or with no intervention (no NLO).
mg/ mL As there are 20 drops per m illiliter (up to 40 in some newer, small-tip dispensers), there is Yo-I> mg per drop. If this drop is given bilaterally, there is up to I mg of active agent available fo r systemic absorption. Because the contact time of topical medication is short, the rate of transfer from the tear fluid into the cornea is critical. The fenestrated ba rriers in the vascular endothelium an d th e mucosa of the stomach allow rapid d iffus ion of all but large molecules through extracellular passages. In contrast, the corneal epithelium and endothelium have tight in te rcell ular junct ions that limit passage of mo lec ules in the extracell ular space. Topically applied medicatio n must fi rst pass through hyd rophobic/li pophilic cell membranes in the epithelium, t hen thro ugh the hydroph ilic/li pophobic corneal stroma, and fi nally through the hydrophob ic/lipophilic cell memb ranes in the endothelium in order to enter the anterio r segment. Topical opht halmic drug for mulatio ns must therefore be both lipoph ilic and hydrophi lic. As non ionic particles are more lipophilic than ionic particles. they pass through the cellular phospholipid membra nes more readily. The pH of the medication can be manipulated to increase the percentage of the drug in an uncharged or non ionized form. Mechanical disruption of the epitheli al barrier in corneal abrasion or infection also increases the rate of intraocular drug penetration. Sim il ar considerations apply to the conjunctiva. The permeability of the conjunctiva to small water-soluble molecules is thought to be 20 times that of the cornea. Perilimba l conjunctiva offers an effective transscleral route for deli very of drugs to anterior segment strllctures. The factors determini ng the amolln t of medication that can penetrate the cornea are co ncentration and sol ubility in the deli ve ry vehi cle, viscosity, li pid sol ubility, and the drug's pH, ionic and steric form, molecular size, chemical structure and configuration, veh icle, and surfactants. In add itio n, reflex tearing and the bind ing of the active medication to proteins in tears and tissue affect drug bioavailability.
326 • Fundamentals and Princi ples of Ophthalmology Drug concentration and solubilitv [t may be necessary to load the tear reservoir with concentrated solutions in order to get a sufficie nt amount of a drug throu gh th e corn eal barriers (eg, selecting pilocarpine 4% instead of pilocarpine 1%). A practical limit to exploiting these high concentrations is reached when the high tonicity of the resulting solutions elicits reflex tearing or when drugs that are poorly water-soluble reach their solubility limits and precipitate. A drug with adequate solubility in an aqueous solution can be formulated as a solution, whereas a drug with poor solubility may need to be provided in a suspension. A suspens io n requires agitat ion so that the active medicat ion is redistributed prior to administrat ion. Suspensions may be more irritat ing to the ocular surface than solutions are, a facto r that may affect th e choice of drug formulation. Viscosity The additio n of high-viscosity substances such as methylcellulose and polyvin yl alcohol to the drug increases drug retention in the inferior cul-de-sac, aiding drug penetration. There is little correlation between increasing solution viscosity and in creasing efficacy, however, thus indicating that these substances may act by altering the barrier function of the corn eal epithelium as well as by increasing drug contact tim e with th e cornea.
Lipid solubility To traverse the cornea, a drug must pass sequentiall y through the lipidrich environment of the epith elial cell m embranes) through th e wate r-rich environmen t of the stroma, and finally through the lipid barrier at the endothelilun. Studies of the permeab ility of isolated corneas to families of chemical compounds show that lipid solubility is more important than water solubili ty in promoting penetration. To determine th e solubility of a drug or grou p of drugs, researchers ascertain the ratio of lipid solubility to water solubility for each compound in the series by (I) measurin g the phase separation of a drug between 2 solvents-l lipid-soluble and 1 water-soluble (eg, octanol and water); and (2) calculating the ratio of the drug concentration in the 2 compartments (partition coefficient). Drugs with greater relative lipid solubili ty have a higher partition coefficient. For su bstituted ethoxzolam ides, the permeability coefficient is 70 times higher for compounds of high lipid solubility than for those oflow lipid solubility. Drugs with higher lipid solubility and a higher partition coefficient have increased penetration of cell membranes. However) compounds with excess ively high partition coefficients are often poorly soluble in tears. Experimenta l studies of su bstituted compounds need to account for the effects of the substituents on potency, on solubility, and on the permeability coefficient. pH and ionic charge Many eye medications are alkaloids, or weak bases. Such drugs as tropicamide, cyclopentolate, atropine, and epinephrine exist in both charged and un charged form s at the slightly alkaline pH of tears (p H 7.4). The partition coefficients, and therefore the drug penetration, can be increased by raising the pH of the water phase, thereby increasing the proportion of drug molecules in the more lipid-soluble, uncharged form. Surfactants Many preservative agents used in topical drops to prevent bacterial contamination are surface-active agents that alter ceU membranes in the cornea as well as in bacteria. They reduce the barrier effect of the corneal epithelium and increase drug
CHAPTER 16:
Pharmacologic Principles .
327
permeability. For example. a 0.1 % carbachol solution containing 0.03% benzalkonium ch loride can elicit the same m.iotic response as a 2% solutio n without it.
Reflex tearing
Ocular irritation and secondary tearing wash out the drug reservoir in the tear lake and reduce the contact time of the drug with the cornea. Reflex tearing occurs when topical med icat io ns are not isoto nic and when they have a nonphysiologic pH or contain irritants. Tissue binding of medication Tear and surface proteins, as weLl as ocular melanin, may bind topical or system ic medication. making the drug unavai lable or creating a slow release reservo ir. This bindi ng may alter the lag tim e, or onset of action, of a medication, as well as th e peak effect and duration of action. and can cause a delayed local toxicity despite discontinuation of the medication. The retinal toxicity that progresses even after discon tinuation of the 3minoquinoli ne an timalarial agents chloroquine and hydroxychloroquine is one example of this effecl.
Ointments Another strategy for increasing the contact time of ocular medications is the use of ointments. Com mercial oil-based ointments usually consist" of petrolatum and mineral oil. The mineral oil allows the ointment to melt at body temperature. Both ingredients are also effective lipid solvents. However. most water-soluble medications are insoluble in the ointment and are present as microcrystals. Only those microcrystals on the surface of the ointment dissolve in the tears; the rest are trapped until the ointment melts. Such protracted. slow release may prevent the drug from reaching a therapeutic level in the tears. Only if the d ru g has hi gh lipid solubility (wh ich aUows it to diffuse thro ugh the oin tment ) and some wa ter solu bility wi ll it escape fro m the ointment into both the corn eal epithelium and the tears. Fluorometholone. chloramphenicol. and tetracycline are examples of drugs that ac hieve higher aqueo us levels when admi nistered as ointment rather than as drops.
local Administration Periocular injections Injection of medication beneath the conjunctiva or the Tenon capsule allows drugs to bypass the conjunctival and corn eal epithelial barriers and absorb paSSively down a concentrat ion gradient into the sclera and intraocular tissues. Subconjunctival, sub-Tenon, and retrobulbar injections all allow medications to reach therapeutic levels behind the lens-iris diaphragm. This approach is espeCiall y usefu l for drugs with low lipid solubility (such as penicillin). which do not penetrate the eye adequately if they are given topically. Injections can also be helpful in delivering medication closer to the local site of actionfor example. posterior sub -Tenon injections of steroids for cystoid macular edema (CME) or subconjunctiva l injection of 5-f1uorouracil after trabecu lectomy. Retrobulbar and peribulbar anesthesia techniques for ocular surgery are covered in BCSC Section 11 . Lens and Cataract. Other examples oflocal. injectable medications are botulinum toxin. which is used in the treatment of ben ign essential blepharospasm and hemifacial spasm and for cosmesis; and retrobulbar alcohol as therapy for chronic pain in blind eyes.
328 • Fundamentals and Principles of Ophthalmol ogy
Intraocular medica/ions The intraocular injection of drugs instantly delivers effective concent rations at the target site. There are 2 types of intraocular injections: intracameral injections into the anterior chamber and intravitreaJ injections into the vi treolls cavity. Great care must be taken to avoid using preserved medication and to control the concentration of such agents so that the delicate internal structures of the eye are protected fro m toxicity. Lntraocular injections have been used to treat diabetic di ffuse macular edema, persistent CME, and central retinal vei n occlusion; th ey have also been lIsed after vitrectamy for severe proliferative diabeti c retin opathy as well as for some ocular neoplasms. Int raoperative use of tissue plasminogen acti vato r (tPA) assists fibrino lys is and subretinal he morrhage displacement and drainage; silicon o il, intraoc ular gases, and perfluorooctane faci li tate vitreo retinal surgery, decrease postoperati ve complicati ons, and enha nce postoperative o utcomes. Intraocular injections of expa nsile gases are rout inely used in pneumatic retinopexy to treat superior retinal detachments, to fl atten the posterio r pole after vitrectomy for macular holes, and to disp lace submacul ar blood. Intraocular implants allow th e slow release of medication over months to years. Ganciclovir implants may be placed into th e vit reo us cavity for treatment of cytomegalovirus (CMV) retinitis, allowing immediate and sustained therapeu tic levels with a prolonged disease remission ofB to 13 months. Intravit real ant ivirals and ga nciclovi r implants have also been used with Limited success in progressive outer ret inal necrosis in AIDS patients. Fluocinolone acetonide intravitreal implants (Retisert) are used for th e control of chro ni c posterior uveitis. Chen SN, Yang TC, Ho CL, Kuo YH, Yip y, Ch~o AN. Retinal tox icit y of in trav it real ti ssue plasminogen activator: case report and literatu re review. Oph thalmology. 2003; 110(4) :704-708. de Smet MD, Vanes VS, Kohler 0 , Solom on 0, Chan Cc. Intravitreal chemotherapy for the treat ment of recurrent intraocular lymphoma. Br J Opllt/will/ol. 1999;83(4):448-45 1. Jonas IB, Hayle r JK, Sofker A, Panda-Jonas S. [ntravi treal inject ion of crystalli ne cortisone as adj unctive treatment of proliferative diabeti c retinopat hy. Am } Ophthalmol. 2001;131(4): 468-47l. Rahhal FM. Treatment adva nces for CMV retinitis. A IDS Read. 1999;9( 1):28-34 . Roig-Melo EA , Macky TA, Heredia-Eli zondo ML, Alfaro DV III. Progress ive outer retinal necrosis synd rome: successful treatment with a new combination of antiviral d rugs. Eur J Opil,/w/lllo/. 200 1; It (2),200-202. Velez G, Bo ldt He, Whitcup SM, Nussenblatt RB, Robinson MR. Loca l methotrexate and dexa methasone phosphate for the treatment of rec urrent primary int raocu lar lymphoma. Ophthalmic SlIrg Lasers. 2002;33(4):329-333.
Systemic Administration Just as th e intercellular ti gh t junctions of the corn ea l epith elium and endothelium li mi t anterio r access to the interior of the eye, simi lar barr iers li mit access thro ugh vascular channels. The vasc ul ar endothelium of th e retina, like that of th e brain, is non fenestrated an d kni tted together by tigh t junctions. Altho ugh both th e choroid and th e Ciliary body have fenestrated vascu lar endothelia, the choro id is effectively sequestered by the RPE; and the Ciliary body, by its nonpigmented epithelium .
CHAPTER 16:
Pharmacologic Principles.
329
Drugs with higher lipid solubilities more readil y penetrate the blood- ocular barrier. Thus, chloramphenicol, which is highl y lipid -soluble, penetrates 20 times better than does penicillin, which has poor li pid solubili ty. The ability of systemicall y administered dru gs to gai n access to the eye is also infl uenced by the degree to which the y are bound to plasma proteins. Only the unbound form can cross the blood-ocular barrier. SuLfonamides are lipid-soluble but penetrate poorly, because at therapeutiC levels, more than 90% of the medication is bound to plasma proteins. Similarly, the greater binding of plasma protein to oxacillin reduces its penetration, compared with that of methicillin. Because bolus administration of a drug exceeds the binding capacity of plasma proteins and leads to higher intraocular drug levels than ca n be achieved by a slow int ravenous drip, this approach is used for the administration of antibiotics in order to attain high peak intraoc ular levels.
Sustained-release oral preparations The practical value of susta in ed- release preparations is significant. For example, a sin-
gle dose of acetazolamide will reduce lOP for up to 10 hours, whereas a single dose of sustained -release acetazolam ide will produce a comparable effect lasting 20 hours. Sustai ned-release med ications offer a more steady blood leve l of th e d rug, avoid ma rked
pea ks and valleys, and reduce the frequen cy of administration.
Intravenous injections Intravenous medicat ion can be administe red for either diagnostic or th erapeutiC effect. Two diagnostic medications, sodium fluoresce in and indocyanjne green, are used for reti -
nal angiography to diagnose retinal and choroidal disease. Edrophonium chloride is used intravenously in the diagnosis of myasthenia gravis.
Intrave nous medications are also used therapeutically in ophthalmology. Although intravitrea l injections have replaced intravenolls therapy for postoperat ive endophthalmitis, con tinu ous intrave nous administration of an antibiotic is an effective way of maintaining intraocular levels in endogenous infection (see BCSC Section 12, Retina and Vitreous).
The barriers and reservoir effects of the eye change the ocular pharmacodynamics of antibiotics such as ampicillin, chloramphenicol, and erythromYCin. These agents pen etrate the eye with higher initia l intraocular le ve ls and maintain comparable bioavail -
ability for 4 hours when give n as a single intravenous bolus rather than by continuous infusion. Medication may have better intraocu lar penetration in the inflamed eye than
in the healthy eye du e to the disruption of the blood- aqueous and blood-retina barriers. This disruption is demonstrated by the leakage of fluorescein from inflamed retinal vessels into the vitreous during angiography. Studies of the rabbit eye have found that the bioavailability of intravenous ampicillin , tetra cycline, and dexamethasone is different in va rio us structures of the rabbit eye, with
the highest levels of these medications found in the sclera and conjunctiva, followed by the iris and ciliary body, and finally the corn ea, aqueous humor, choroid, and retina. Very low level.s appeared in the lens and vitreous. The dru gs showed no marked differences in their vascular distribution, however. The tissue bioavaiiabUity is determined by the vascula rity of the tissue and the barriers that exist between the blood and that tissue.
330 • Fundame ntals and Principles of Ophthalmology
Intramuscular agents In ophthalmology, intramuscular agents are used less frequentl y than to pical, oral, or intravenous age nts. Notable exceptio ns include the use of prostigmine in the di agnos is of myasth enia gravis and the local th erapeut ic li se of botulin um toxin in fa cia l dystonias.
Methods of Ocular Drug Design and Delivery New ocular drugs are deSigned with a foc us o n specificity and safety, wit h deli ve ry systems aimed at imp roving conve nience and patie nt compliance. Each of th e follow ing new
approaches respo nds to a specific pro blem in ocular pharmacokin etics.
Prodrugs Prodrugs are inert compo unds that are ac ti vated by o ne of the enzymatic systems within the eye. Dipivefrin HCI (Propine) is a prod ru g of epinephri ne; it has better li pid solubility than epinephrine and increases corn eal penetrati on 17-fold. Thus, a 0. 1% solution ca n be ll sed in place of epi nephrine I %-2%. Corn ea l esterases convert dipivefrill to epinephrine.
and, as dipivefrin has low intrinsic activity, it is virtuall y free of the system ic side effects of epinephrine. Another topical prodr ug, latanoprost (Xalatan), is converted to prostaglandi n F2u by corn eal esterases for the treatment of glaucoma. Va lacyclov ir HCI (Va ltrex) is an anti viral pro dru g th at is easil y absorbed th rough the gastro intestinal system and q ui ckl y converted to the active form of acyclOVir. Likewise, famciclovi r is a prod rug of the ac ti ve antiviral penciclovir.
Sustained-release devices and gels Drop therapy involves periodiCdelivery of relatively large q uantities of a drug. A suffi cient amo unt of the drug must bind to receptors to ac hieve a therapeutiC effect. T he surro unding ti ssues ofte n retain additional medica ti on, acti ng as a local reservo ir fo r sli stained
release betwee n applications. The high peak drug levels attained in bolus dosing can cause loca l and syste m ic side effects. as fo r example. the miosis and ind uced accomm odation
resulting fro m th e use of pilocarpine. Delivery devices Devices have been developed that deli ve r an adequate supp ly of medicatio n at a steady-state level, thus achieving benefi cial effects wi th fewer adverse effects. The Ocusert del ive ry system, deS igned to delive r pilocarpine at a steady rate of 40 ~lgl hr, was the therapeutiC equivalent of 2% pilocarpine used 4 times a day. However, because the total daily dose of pilocar pine was onl y 960 ~ g (24 hours x 40 ~ g/h r) when deli vered with the device as compared to 4000 ~l g (4 doses x 2000 mg/l OO mL x 0.05 m L/dose) with drops, miosis was less marked, and the ind uced accommodation was red uced. The OCllsert dev ice was disco ntinued as the use of pilocarpine decreased, but it re main s an in teresting example of steady-state d rug delive ry. This device may be resurrected in th e future fo r li se with other med ications. The gel fo rm of timolol m aleate (Timoptic-XE) contains a heteropolysaccharide that thickens on co ntact w ith th e tea r film, ma intainin g the rapeut ic levels w hi le decreaS in g th e
dosing to o nce daily. T he ga ncicJovir sustained-release in traoc ular device (Vitrase rt) is surgically implanted and deli vers a steady source of gancicJovir for 5- 8 mo nths. An ethylene vinyl acetate disc
CHAPTER 16:
Pharmacologic Principles . 33 1
with polyv in yl alco hol coating serves as a drug reservoir. The thickness of the polyvinyl alcohol lid regu lates the delive ry of gancidovir to target tissue. Fluocinolone acetonide vitreal implants (Retisert) are li kewise used for a slow, targeted delivery of corticosteroids to the vitreous cav ity for posterior uveitis. Collagen cornea shields Porcine sderal tissue is extracted and molded in to contact lens-like sh ields that are useful as a delivery system to prolong the contact between a drug and the cornea. Drugs can be incorporated into the co llagen matrix during th e manufacturing process, absorbed into the shield duri ng rehyd ration, or applied topica lly whi le the shield is in the eye. Because the shield dissolves in 12,24, o r 72 hours, depen di ng o n the manufac turing process for collage n cross-lin kin g, the drug is released gradu all y into the tear fUm, and high concen trat io ns are mainta ined on the corneal surface and in the conjunct ival cul-de-sac. The shields have been used in the earl y management of bacterial keratitis, as well as for antibiotic prophylaxis. They have also been used to promote epithelial healing afte r ocular surgery, tra uma, or spontaneous erosion. Despite these therapeutic benefits, collagen shields are poorly tolerated because they are very uncomfortable. New technology in drug delivery Liposomes are synthetic lipid m icrospheres that serve as multipurpose vehides for the topical delivery of drugs, genetic material, and cosmetics. They are produced when phospholipid molecules interact to form a bilayer lipid membrane in an aqueous environment. The interior of the bil ayer consists of the hyd rophobic fatty-acid ta ils of the phospholipid molecule, whereas the o uter layer is composed of hydrophi lic polar-head groups of the molecu le. A water-solu ble drug can be dissolved in the aqueous phase of the interior compartment, whereas a hyd rophobiCdr ug can be intercalated into the lipid bilayer itself. Biodegradable nanopa rticles, such as nanospheres, nanocapsules, and micelles, are also used to transport hydrophobiC d rugs and ge nes and are modeled after the molecular structure of viruses. The physical process of moving charged molec ules by an electrical current is called iontophoresiS. This procedure places a relatively high concentration of the drug locally, where it can achieve maximum benefit with little waste or systemic absorption. Ani_mal studies have demonstrated that iontophoreSiS increases penetration of various antibiotics and antiviral drugs across ocular surfaces into the cornea and the interior of the eye. However, patient discomfort, ocular tissue damage, and necrosis restrict the popularity of th is mode of drug delivery.
Pharmacodynamics: The Mechanism of Drug Action Most d rugs act by bi nd ing to and altering the functio n of regu latory m acromolecules, usually neurotransm itter receptors, hormone receptors, or enzymes. Binding may be a reversible association mediated by electrostatic and/or van der Waals forces, or it may invo lve for mation of a covalent intermediate. If the drug-receptor interaction stimulates the receptor's natural function, the d rug is termed an agonist. Stimulation of an opposing
332 • Fundam enta ls and Principles of Ophthalmology
effect characterizes an antagonist. Corresponding effectors of enzymes are termed activators and inhibitors. This terminology is cr ucial to und erstanding the next chapter. The relati o nship between the initial drug- receptor interaction and the drug's cl inical dose- response curve may be simple or complex. In some cases, the drug's clinical effect closely reflects the degree of receptor occupancy on a moment-to-moment basis. Such is usually the case for drugs that affect neural transm ission o r for drugs that are enzyme inhibitors. In contrast, some drug effects lag hours behind receptor occupancy or persist
long after the drug is gone. Such is the case with many drugs acting on hormone receptors, because th eir effects are often mediated through a series of biochemical events. In addition to differences in timing of receptor occupancy and drug effects, the degree of receptor occupancy can differ considerabl y fro m the corres ponding dru g effect. For example. because the amount of carbonic anhydrase prese nt Ln the ciliary processes is 100 times that required to support aqueous secreti on, more than 99% of th e enzyme must
be inhibited before secretion is reduced. On the other hand. some maximal hormone re sponses occur at concentrati ons well below that requ ired for receptor saturation. ind icating the presence of "inbou nd receptors:' Bochot A, Couvreur P, Fattal E. Intravitreal ad minist ration of antise nse oligonucleotides: potential ofliposomal delivery. Prog Refill Eye Res. 2000;19(2): 131- 147. Eller MG. Schoenwald RD, Dixson JA. Segarra T. Ba rfknecht CF. Topical carbon ic anhydrase in hibitors. III: optimization model for co rneal pene tration of et hoxzolam ide analogues.
J Pilarlll Sci. 1985;74(2): 155- 160. Hsiu e CH, Chang RW, Wang CH. Lee SH. Development of in situ thermose nsi tive drug vehicles for glaucoma therapy. Biomaterials. 2003;24( 13):2423-2430. Wi lliams PB, Croll ch ER Jr, Sheppard JD Jr, Lattanz io FA Jr, Parke r TA, Mitrev pv. The birth of ocular phar macology in the 20th centuf}'. J Clill Phnrll1t1col. 2000;40(9):990- 1006. Zimmerman TJ, Kooner KS, Kandarakis AS, Ziegler LP. Improvi ng the th erapeutic index of top ica lly applied ocu lar drugs. Arclz Ophthalmol. 1984; I02(4):55 1-553.
CHAPTER
17
Ocular Pharmacotherapeutics *
legal Aspects of Medical Therapy The US Food and Drug Adm inistration (FDA) has statutory authority both to approve the marketing of prescription dru gs and to specify the uses of these dru gs. The FDA has created a 3-step process regulating human testing of new dru gs before they are approved for marketing. After animal and in vi tro studies, phase 1 testing begins; this involves testing 10-80 people for toxicology and pharmacokinetic data concerning dosage range, absorptiOIl . metabol ism, and toxicity. Phase 2 testing in volves randomi zed, controlled clinical trials on a minimu m of 50- 100 affected people to determine safety and effectiveness. Phase 3 testing uses controlled and uncontrolled trials to evaluate the overall risk- benefit relationship and to provide an adequate basis for physicia n labeling. The data gathered from these tests are then submitted as part of a new drug application for marketing. The FDA's approval of each dru g and its specific uses is based on documentation sub mitted by manufacturers that supports the safety and efficacy of specific drug applications. Once approved for any use, a drug may be prescribed by individ ual physicians for any indication in all age gro ups without violatin g federal law. Howeve r, phys icians remain liable to malprac tice actions. In particular, a nonapproved lise that does not adhere to an applicable standard of care places a practitioner in a difficu lt legal position. If a respectable minority of Similarly situated physicians prescribes in the same manner, a standard of cafe cou ld be met in most jurisdictions. Informed consent in equivocal cases is helpful. Many common dru gs have off-label application in ophthalmology. A limited listing of these includes, but is not li mited to, the following drugs: bevacizumab (Avastin ), an an tia ngiogenic drug used off-label in intravitreal injection for multiple neovascular ocular diseases acetyleysteine (Mucomyst 10% and 20%), used as a mucolytiC in fil amentary keratopathy and as an an ti collage nase agent in severe alkali injuries
*Th is chapter may include information 0 11 pharmaceutical appl ications that are not conside red COI11 munity standard. that are approved for use on ly in restricted research settings. or that reflect indications not included in approved FDA labeling ("off- label"). For exampl e, many ophthalmiC uses of system ic med ications are off-label, including 1110St sys temic an tibiotics and ant ifungal agents compou nded for treatment of ocular infections sllch as keratit is or endopb thalmitis. Many antifu ngal agents are lIsed with an off-label appl ication based o n in vitro and anim al data. because hu man data for unusual infectious agents are often limited. The FDA has stated that it is the responsibi lity of the physician to determine the FDA status of each drug or device he or she wishes to use and to use them with appropriate, in formed patient consent in compl iance with applicable law.
333
334 • Fundamentals and Principles of Ophthalmology tissue plasminogen activator (Activase). used as an intravitreal injection for th rombolysis and fibrinolysis flu orouracil (S-FU). used to improve the outcomes of glaucoma filtration su rgery mitomyci n (Mutamycin). used to improve the outcomes of glaucoma filtration surger y cyclosporine A (Sa nd immune}. used off- label as a 2% compounded solution in high-risk corneal transplants and in severe vernal. li gneous. and autoim mune
keratopathies doxycycline. used for ocular rosacea edetate disodium (EDTA). used for band keratopathy One of the most com monly used medications. topical prednisolone. has not been approved specifically for postoperative care. When used postoperatively for cataract surgery. it is an off-label usage. The FDA has established clear gUidelines on investigational drugs. and their use must meet specified com mercia l an d investigative requirements.
A number of commonly used ophthalm ic medications affect the activity of acetylcholine receptors in synapses of the somatic and autonomic nervous systems (Fig 17-1 ). Such recepto rs are found in
• the motor end plates of the extraocular muscles and levator palpebrae superioris (supplied by somatic motor nerves) the cells of the superior cervical ganglion (sympathetic) and th e ciliary and sphenopalatine (parasym pathet ic) ganglia (s upplied by preganglionic auto nomic nerves) parasympathetic effector sites in the iris sphincter and ciliary body and in the lacrimal. accessory lac rimal. and meibomian glands (supplied by postga nglionic parasympathetic nerves) Although all cholinergic receptors are by definition responsive to acetylc holine. they are not homogeneous in their response to other age nts. which fall into 2 categories: Muscarinic agents are supplied by postganglionic parasympathetic nerves and are responsive to muscarine. Nicotr,lic agents are supplied by somatic motor and preganglionic autonomic nerves and are responsive to nicotine.
Cholinergic age nts are further di vided into the fo llowing gro ups (Fig 17-2) : direct-acting agonists. which act on the receptor to elicit an excitatory postsynaptic potential indirect-acting agonists. which in hibit the acetylcholinesterase of the synaptic cleft. preventing deactivation of endogenous acetylcholine antago nists. which block the action of acetylcholine on the receptor
CHAPTER 17:
Ocul ar Pharm acoth erapeutics . 335
AUTONOMIC
SOMATIC
Sympathetic innervation o adrenal medulla
Sympathetic
Parasympathetic
Acetylcholine
Acetylcho line
Acetylcho line
.u.
.u.
1 1 1
Preganglionic neuron
Ganglionic transmitter
.u.
•
cE"'O')
(Nicotinic
•
•
(no ganglia)
(Nlcollnic recept or)
Adrenal medulla
,[j, Neuroetfec tor transmitt er
Epinephrine released into Ihe blood
Norepinephrine
Acel ylch oline
Ac ety tchollne
.u. ....
.u. ....
.u.
.u.
(Adrenerg ic receptor)
(Adrenergic receplor )
(Muscarinic receptor)
Effector organs
•
•
~ yo recep lor) .......
Striated muscle
Figure 17-1 Summary of the neurotransmitters relea sed and the types of receptors found within the autonomic and somatic nervous systems. (Reproduced with permission from Mycek MJ, Harvey RA. Champe Pc. ads. Pharmacology. 2nd ed. Lippincott's Illustrated Reviews. Philadelphia: Lippincott-Raven; 1997:32.)
Muscarinic Drugs
Direct-acting agonists Topicall y app lied direc t-acti ng ago nists have 3 actions. Fi rst, th ey cause co ntraction of the iris sphi ncte r, which not o nly constricts the pupil (miosis) bu t also changes the anatomical rel ations hi p of the iris to both th e lens an d the cham ber angle. Second , t hey cause contraction of the circular fibe rs of the ciliary muscle, relaxing the zonular tension on the lens equ ato r and allowing the lens to shi ft fo rward and ass u me a more spher ical shape (accommodation). Th ird, t hey cause co ntracti on of the longitud inal fi bers of th e ciliary muscle, prod UCing tension on th e scleral sp ur (o pening the trabec ul ar meshwork) and faci litatin g aq ueous outfl ow. Co ntracti on of th e ciliary musc ul atu re also produces tension on th e periph eral retin a, occaSionally resultin g in a reti na l tea r or even rh eg matogenous detac h me nt. Ace tylc ho line does not penetrate the co rneal epithelium well, and it is rap idly d egraded by acetylcholinesterase (Fig 17-3). T hus, it is not used topically. Acetylcholine 1% (M iochol) and carbachol 0.0 1% (M iostat) are ava ilable for int raca meral use in ante rior
336 • Fundam entals and Principl es of Ophthalmology
CHOLINERGIC AGONISTS
NEURON
• ••
Neurotransmitter
o
•• •
•
SYNAPSE
=r-~
Acetylcholine
f-
8ethanechol
f-
Carbachol
L
Pilocarpine
INDIRECT ACTING (reversible)
,
E~" II ~
f-
I---
~= ~lfm' receptor
DIRECT ACTING
-
POSTSYNAPTIC
f-
Physostigmine
f-
Neostigmine
L
Edrophonium
TARGET CELL MEMBRANE
INDIRECT ACTING (irreversible)
I--Neurotransmitter bound to receptor L
~~ Receptor activated by neurotransmitter
A
INTRACELLULAR RESPONSE
REACTIVATION OF ACETYLCHOLINESTERASE
'-----
B
Isoflurophate
L
Pralldoxlme
Figure 17-2 A, Neurotransmitter binding triggers an int racel lular response . 8, Summary of cholinergic agonists. (Part A reproduced with permission from Harvey RA, Champe PC. eds. Pharmacology. Lippincott's Illustrated Reviews. Philadelphia: Lippincott; 1992:30. Part B reproduced with permission from Mycek MJ, Harvey RA, Champe PC, eds. Pharmacology. 2nd ed. Lippincott's Illustrated Reviews. Philadelphia: Lippincott-Raven; 7997:35.)
CHAPTER 17:
Ocular Pharmacot herapeutics .
337
OF [J SYNTHESIS ACETYLCHOLINE •
Transport Of chohne Inhibited by hern.cholin>um
iii ..
C~>ne
N,'JrA""'A
•
UPTAKE INTO
~ STORAGE VESICLES •
Acetylchohne protected trom degrad;ll>on 10 ves>cle
Acetylchohne
I
~ RECYCLING
L!.I
•
OF CHOLINE Chahne'5 taken up by neuron
~
. , . RELEASE OF ~ NEUROTRANSMITIER •
Release block ed by botuhnum tox,n Spider venom tauses release 01 atetylcholme
II
SynaptK: ' \ ) yeslCle
PostsynaptK: receptor acc,yaled by bmd>ng 01 neurol ransmltter
~
E!.II •
DEGRADATION OF ACETYLCHOLINE Acetylcholine >s rapidly hydrolyzed by chohn. esterase In IIIe synaptic clelt
INTRACELLULAR RESPONSE
Figure 17-3 Synthesis and release of acetylcholine from the cholinergic neuron. AcCoA acetyl coenzyme A. (Reproduced with permission from Mycek MJ, Harvey RA, Champe PC, eds. Pharmacology. 2nd ed. L,ppmcotl's //Ius/rated Reviews. PhIladelphIa: Llppmcott-Raven; 1997:37.)
segment surgery. These dr ugs produce prompt and marked miosis. which helps avoid iris capture by the optic of posterior chamber lenses and may prevent iris incarceration in surgical wo unds. Intracameral acetylcholine 1% has a more rapid onset than intracameral carbachol, acting within seconds after instillation. but the effect is short-lived. Acetylcholine is not stable in aqueous form and is rapidly broken down by acetylcholinesterase in the anterior chamber. Intracameral carbachol 0.0 1% is 100 times more effective and longer lasting than acetylcholine administered sim ilarl y. Maximal miosis is achieved within 5 minutes and lasts for 24 hours. In addition. carbachol 0.0 1% is an effective hypotensive agent and lowers intraocula r pressure (lOP) during the critical 24-hour period afte r surgery.
338 • Fundam enta ls and Principles of Ophtha lmolog y Pilocarpine 0. 12% is used diagnostically to confirm an Adie tonic pupil, a condition in which the parasympathetic innervation of the iris sphincter and ciliary muscle is defective because of the loss of postganglionic fibers. Denervated muscarinic 511100th muscle
fibe rs in the affected segments of the iris exhibit supersensitivity and respond well to this weak miotic, whereas the normal iris does not.
Pilocarpine 0.25, 0.5, 1,2,3,4,6% (q id) and carbachol 1.5,3% (tid) are used in the treatment of primary open-angle gla ucoma (POAG) because they lower lOP by increasing outflow facility (Table 17-1). Use of 4% pilocarpine is contraindicated in acute attacks of angle-closure glaucoma, as this strong miotic may induce intense anterior movement
of the lens-iris diaphragm, closing the angle completely. Miotic therapy is not an adequate substitute for laser iridotomy and should not be used for chronic control or prophylaxis of pupillary-b lock ang le-closure glaucoma. (See also BCSC Section 10, Glaucoma, Chapter 7.) Mio sis, cataractogenesis, and indu ced myopia are generally unwelcome side effects of
muscarinic therapy. Although the broad range of retinal dark adaptation usually compensates sufficiently for the effect of miosis on vision during daylight hours, patients may be visually incapacitated in dim illumination. In addition, miosis often compoun ds the effect
of axial lenticular opacities; many pat ients with cataract are unable to tolerate miotics. O lder patients w ith ea rly cata racts have visua l d ifficul ty in sco topic co ndit io nsj the miosis
induced by cholinergic agents may increase the risk of falls. Younger patients may have diffi culty with miotics as well. Ind uced myopia and induced accommodation occur because of th e drug- in duced contraction of the cili ary
body, which both increases the convexity of the lens and shifts the lens forward. Patients younger than 50 years may manifest disabling myop ia and induced acco m modation from this side effect. Miot ic iris cysts and an increased incidence of retin al detachment due to ci li ary body cont ractio n and tract ion on the pars plana are other co mplications seen with higher concentrations. Systemic side effects afte r ocular use of pilocarpine are rare. They include salivation.
diarrhea, vomiting, bronchial spasm, and diaphoresis (Fig 17-4). A slowly dissolvi ng gel (Pilopine HS gel) taken at bedtime mi nimizes the unwanted side effects of pilocarpine and is useful in younge r patients. in patients bothered by variable myopia or intense miOSis.
in older patients with lens opacities, and in patients who have diffi culty complying wi th mo re frequent dos in g regimens.
Table 17-1 Miotics Generic Name
Trade Name
Strengths
Pilocarpine HCI ointment
Isopto Carbacho l Isopto Carpine Available generically Pilopine HS gel
1.5,3% 0.25, 1, 2, 4% 0.5,1,2,3,4,6% 4%
Cholinesterase inhibitors Physostigmine Echothiophate iodide
Available generically Ph ospholine Iodide
1 mg/mL ampule 0.125%
Cholinergic agents Ca rbac hol Pilocarpine HCI
CHAPTER 17:
Figure 17-4
Ocular Pharmacotherapeutics. 339
Some adverse effects observed w ith chol in-
ergic drugs.
(Reproduced with permission from Mycek MJ. Harvey RA, Champe Pc. eds. Pharmacology. 2nd ed. Lippincott's Illustrated Reviews . Philadelphia: Lippincott-Raven; 1997:40.)
Ciliary muscle stimulation can be desirable in managing accommodative esotropia. The near response is a synkinesis of accommodation, miosis, and convergence. As discussed previously, muscarinic agonists contract the ciliary body and induce accom modation as a side effect. The patient does not therefore need to accommodate at near, decreasing not only the synkineti c convergence response but also the degree of accom modative esotropia.
Indirect-acting agonists Topically applied indirect-acting muscarini c agonists (choli nesterase inhibitors) have the sam e actions as direct-acting muscarinic agonists, althou gh they have a longer duration of action and are frequently more potent. TWice-daily treatment is sufficient. These agents react with the active serine hydroxyl site of cholinesterases, forming a slowly hydrolyzed intermediate. Thus, they render the enzyme unavailable for hydrolyzing spontaneously released acetylcholine. There are 2 classes of cholinesterase inhibitors: I. reversible inhibitors, such as physostigmine (preViously marketed as Eserine oint-
ment; available as powder for compounding and as solution for injection) and carbamylate acetylcholinesterase 2. irreversible inhibitors, such as echothiophate (Phospholine Iodide) and diisopropyl phosphorofluoridate (DF P; no longer available for ophthalmic use in the United
340 • Fundamentals and Principles of Ophthalmology States), which phos phorylate both the acetylc holinesterase of the synaptic cleft and the butyrylcholinesterase (pseudocholinesterase) of plasma One carbamylati ng age nt, de meca rium bromide (previously marke ted as Humorsol; no longer ava ilable for op hthaLmic use in the United States), is also irreve rsible; it contains 2 carbamyl groups and cross-links units of the enzyme. Carbamylen zyme is regenerated by hydrolys is of the ca rbamyl-ester li nkage over 3-4 hours. Regeneration of dial kyl phosphorylated enzyme is so slow, however, that recovery of activity depends primarily o n new enzyme synthesis. T he act ion of phosphorylating cholinesterase inhibito rs can be reve rsed acutely by treatment with oxime-contai ning compounds that remove the dialkylphosphate moiety from the enzyme. This treatment must take place rapidly, before the spontaneous elimination of I of the alkyl residues (Till = 20 minutes for butyrylcholi nesterase, 270 minutes for acetylcholinesterase), which makes the monoalkylphosphate intermediate no lo nge r susceptible to regeneration by oxime. Thus, the oxime pralidoxime (2-PAM)-although useful in the treatment of acute organophosphate poisoning (eg. insecticide exposure)- is of little va lue in reversing the marked reduction of plasma butyrylcholinesterase activity that occurs w ith chronic irreversible cholinesterase-i nhibitor therapy.
Patients on chronic, irreversible choli nesterase- inhibitor therapy such as echothiophate may experience toxic reactions from systemic absorption of local anesthetics containing ester groups (eg, procaine) that are normally inactivated by plasma cholineste rase. Administration of the muscle relaxant succin ylcholine during induction of general anesthesia is also hazardous in such patients, because the drug would not be metaboli zed and would result in prolonged respiratory paralysis. Phosphorylating cholinesterase inhibitors also have local ocular tox icities. Children may devel op cystlike proli ferations of th e iris pigment epithelium at the pupil margin that can block the pupil. For unknown reasons, cyst development can be minimized by concomitant use of phenylephri ne (2.5%) drops. In adults, cataracts may develop or preexisting opacities may progress. Interestingly, children rarely if ever develop slIch cataracts, and adults rarel y if ever develop Significant epithelial cysts. Therapy with choli nesterase inhibito rs should not be combined with direct-acting cholinergic agonists, because the combinat ion is less effecti ve than ei ther drug g ive n alone. Because cholinesterase inhibitors are potent insecticides, they were used in the past as treatment for lice infestations of th e eyelashes. The adult louse ma y be smothered with bland ophthalmic ointment or removed mechanically. Nits, however, must be mechanically removed. because they are resistant to suffocation or any medication. Physostigmine. diisopropyl phosphoroflu oridate, an d demecarium bromide are no longer commercia ll y available for ophthalmic use in the Uni ted States. Antagonists Topically applied muscarinic antagonists. such as atropine, react with postsynaptic muscarinic receptors and block the action of acetylcholine. Paralysis of the iris sphincter, coupled with the unopposed action of the dil ator muscle, causes pupillary dilation, or mydriasis (Table 17 -2) . Mydriasis facilitates exam ination of th e periph eral lens, ciliary bod y,
CHAPTER 17: Ocular Pharmacotherapeutics . 34 1
and retina and is approved for use therapeutically in the treatment of iritis in adults, as it reduces contact between the posterior iris surface and the anterior lens capsule, thereby preventing the formation of iris-lens adhesions, or posterior synechiae. Atropine and cyclopentolate are FDA approved for use in pediatric patients but not for all indications. Muscarin ic antagon ists also paralyze the ci li ary muscles, which helps to relieve pain associated with iridocyclit is, inhibit accommodation for accurate refraction in child ren (cyclopentolate, atropine) , and treat ciliary block (malignant) glaucoma. However, use of cycloplegic agents to dilate the pupils of patients with POAG may elevate the lOP, especially in patients requiring miotics for pressure control. It is advisable, therefore, to use short-acting agents and to consider monitoring the pressure in patients with severe optic nerve damage.
Table 17-2 Mydriatics and Cycloplegics Generic Name
Trade Name
Strengths
Pheny lep hrine HCI*
AK-Dilate A ltafrin Mydfrin Neofrin Available generically
Soln, Soln, Soln, Soln, Soln,
Hydroxyamphetamine hydrobromide 1% t Atropine sulfate
Cyclopentolate HCI *
Atropine-C are Isopto Atropine Ava ilable generically AK-Pentolate
Cyclogyl
Homatropine hydrobromide Scopo lamine hydrobromide Trop icamide
Cyclopento late HCII pheny lep hrine HCI
Cylate Available generically Iso pta Homat ropine Homatropa i re Isopto Hyoscine Mydral Mydriacyl Tropicalyl Available generically Cyclomydril
Onset
Duration of Action
30- 60 min
3-5 h
45-120 min
7-14 days
30-60 min
2 days
Soln, 2%, 5%
30-60 min
3 days
So l n,5% So l n, 0.25%
30-60 min
4-7 days
20-40 min
4-6 h
2.5%, 10% 2.5%, 10% 2.5% 2.5% 2.5%, 10%
Available as powder for compounding So ln,1% So l n,1 % So l n,1% Ointment, 1% Soln, 1% Soln, 0.5%-2% Soln, 1% Soln, 1%,2%
Soln, Soln, Soln, Soln,
0.5%, 1% 1% 0.5%, 1% 0.5%, 1%
Soln, 0.2%,1 %
* A dilute com bination agent, Cyclomydril (cyclopentolate HCI O.2%/phenylephrine HCI 1%). is available for in fant exams. t Hydroxyamphetamine hydrobrom ide, in combination with tropicamide , is available commercially as Paremyd 1%; however, it is used for dilating the pupil and can not be used to test for Horner syndrome.
342 • Fundamenta ls a nd Principles of Ophthalmology In situations requiring complete cycloplegia, such as the treatment of iridocyclitis (ad ults: scopolamine. homatropine. atropine) or the full refractive correction of accol11 modative esotropia, more potent agents are preferred. Although a single drop of atropine has some cycloplegic effect lasting for da ys, 2 or 3 instil lations a day ma y be required to ma intain full cycloplegia to relieve pain in iritis. It may become necessary to change medications if atropi ne elicits a characteristic local irritation with swelling and maceration of the eyelids and conju nctival hyperemia. When mydriasis alone is necessary to faci litate examination or refraction, agents with shorter residual effect are preferred. because they allow quicker ret urn of pupil res po nse and read ing ability. Systemic absorption of topically adm inistered muscarinic antagonists can produce dose-related toxicity, espec iall y in chi ldren, whose dose is distributed in a sm all er body mass. Flushing, fever. tachycardia. and even delirium can result from a combin ati on of central and peripheral effects (Fig 17-5). Mild cases may req uire o nl y discontinuation of the drug, but seve re cases can be trea ted with intravenous physost igm ine (a pproved in adul ts and ch ildren), slowly titrated, unt il the symptoms subside. Physostigmine is used because it is a tertiary am ine (un charged), and it is able to cross the blood - brain barrier. Systemic admini strat io n of atropi ne blocks the oculocard iac reflex, a reflex bradycard ia sometimes eli cited during ocular surgery by manipulation of the conjunctiva, the globe, or the extraocular muscles. T he re!lex can also be prevented at the affere nt end by retrobulbar anesthesia, although it can occur during admi nistratio n of the retrobulbar block.
Nicotinic Drugs
Indirect-acting agonists The only cholinesterase inhi bitor that ophthalmologists administer in a dose sufficient to allow it to work as an indirect-acting nicotinic agonist is edrophoniu m (Enlo n; previously marketed as Tensilon). This is a short-acting competitive inhibitor of acetylcholinesterase that binds to the enzyme's active site but does not form a covalent link with it. Edrophonium (En Ion) is used in the diagnosis of myas th enia gravis, a neuromuscular disease caused by an au toi mmunity to acetylcholine receptors in the neuromuscu lar junc· tion and characterized by muscle weakn ess and marked fat iga bility of skeletal muscles. This disease may manifest primarily as ptosis an d diplopia. T he diagnosis is confirmed by firs t administering a 2-mg test dose of rapidly injected intravenous edrophonium, followed 60 seconds later by an ad dit ional 8 mg if the first dose has no effect. T he test dose is given to confirm the pat ient is not prone to side effects of the med ication, including nau sea, vom iting, and bradycardia. The patient is then exami ned for improvement in muscle funct ion. sese Section 5, Ne uro-Ophtha{lIIo{ogy, d iscusses myas thenia g rav is and the use of edrophonium in detail. Neostigmine methylsu lfate (P rostigm in) is a lo nge r-act ing in tramuscular agent that is also used in the diagnosis of myasthenia gravis. The longer dura· tion of activity allows the examiner time to specifically assess a complex endpoint, such as orthoptic measurements.
CHAPTER 17: O c ular Pharmacotherapeu t ics . 343
CHOLINERGIC ANTAGONISTS
ANTIMUSCARINIC AGENTS
r--
)
Atropine Scopolamine
GANGLIONIC BLOCKERS
f--
r r L
B
A, Adverse effects commonly observed with cholinergic antagonists. B, Summary of chol inergic antagonists. (Reproduced with permission from Mycek figure 17-5
MJ, Harvey RA, Champe PC, eds. Pharmacology. 2nd ed. Lippincott's
Nicotine
Illustrated Reviews. Philadelphia: Llppincott·Raven; 199 7:45, 49.)
Trimethaph an Mecamylamine
NEUROMUSCULAR BLOCKERS
'----
)
-
Tubocurarine
-
Pancuronium
)
Gallamine Succln y Icholine
In patients wi th myasthenia gravis, the in hi bition of acetylcholinesterase by edrophoni um allows acetylcholine released into the synapti c cleft to accum ulate to levels adequate to act through the reduced number of acetylcholine receptors. Because edrophonium also augments muscarinic transmission, muscarinic side effects (vo miting, diarrhea, urination , and bradycardia ) may occur un less atropine, 0.4- 0.6 mg. is co adm inistered intravenously.
344 • Fundamenta ls and Principles of Ophthalmology
Antagonists Nicotin ic antagonists are administered as neuromuscular blocking agents to facilitate intubation for general anesthesia. T hey are of 2 types: 1. nondepolarizing agents, including curare-l ike drugs slich as gallamine and pan( urani um, which bind competitively to nicotinic receptors on striated muscle but do not cause contraction 2 . depolarizing
agents. sli ch as SUCCinylcholine and decamethonium , which bind
competiti vely to nicotinic receptors and cause an initial receptor depolarization and muscle cont ract ion
In singly innerva ted (en plaque) muscle fibers, this depolarization-contraction is followed by a prolonged unresponsiveness and fl accidity. However. these drugs produce sustained contractions of multiply innervated fibers. which make up one fifth of the muscle fibers of extraocular muscles. Such contractions of extraocular muscles (a nicotin ic agonist acti on) exert force on the globe. an undesirable effect in cases in which the lOP is to be measured. T he use of these agents in the induction of general anesthesia should be avoided in operations on lace rated eyes. because the force of the muscles on the globe could expel intraocular co ntents.
Several ophthalmic medications affect the ac tivity of adre nergic recepto rs in synapses of the peripheral nervous system. Such receptors are found in the following locations:
the cell membranes of the iris dilator muscle, the superior palpebral smooth muscle of Mulle r. the Cilia ry epithelium and processes. the trabecular meshwo rk. and the smooth muscle of ocular blood vessels (s up plied by postganglionic autonomic fibers from the superior cervical ganglion) the presynaptic terminals of some sympathetiC and parasympathetic nerves. where they have feedback inhi bitory action s Alth o ugh adrene rgic receptors were originally defined by their response to epi nephrine (adrenaline). the tra nsmitter of most sympathetiC postganglionic fibers is ac tually norepinephrine. Adrenergic receptors are subclassified into 4 categories-a), a 2 , P) I and ~2 - based o n their profile of responses to nat ural and synthetic catecholam ines (Fig 17-6). The a )-receptors generally mediate smooth-muscle contraction , whereas u 2-receptors mediate feedback inhi bitio n of presynaptic sympathetiC (a nd sometimes parasympathetic) nerve terminals. The PI- receptors are found predomina ntly in the heart, where they med iate stimulatory effects; P2-receptors med iate relaxation of smooth muscle in most blood vessels and in the bronchi. Systemic absorption of ocula r adrenergic agents is frequ ently sufficient to cause system ic effects, which are manifested in the cardiovascular system, the bronchial air ways, and the brain. Adrenergic agents may be direct-acting agonists, indi rect-acting agonists. or antagonists at 1 or more of the 4 types of receptors.
CHAPTER 17:
I ex,
I
I
VasoconstriClion
Increased blood pressure
I
~,
I
345
I
ADRENOCEPTORS
":z
Increased peripheral resistance
Ocular Pharmacotherapeutics.
~2
I
Inhibition 01 norepinephrine release
Tachycardia
Vasodilation
Increased lipolysis
InhIbition 01 norepinephrine
Increased myocardial contractility
Slightly decreased peripheral resistance
Mydriasis
Bronchodilation Increased muscle and liver glycogenolysis
Increased closure 01 inlernal sphincter 01 the bladder
Increased release 01 glucagon Relaxed uterine smooth muscle
A Acebutolol Atenolol Metoprolol
Propranolol
nR", \ j Force
~., ..
rtf
Ell Decreased IiII cardiac output
Figure 17-6
and
~1
""
:'
. ,' :_,'', : _r_,
'
I ,
z~i\
~.,. bJJ
B
"
r r' ,
...... / ,'
-"
~:::
Ii'I
Peripheral . . vasoconstriction
'''-
"
-,
~Broncho
IiaIII constriction
Increased sodium retention
A, Major effects mediated by a- and ~-a drenoceptors. B, Actions of propranolol
blockers.
(Reproduced with permission from Mycek MJ, Harvey RA Champe PC, eds. Pharmacology. 2nd ed.
LIppincott's Illustrated Reviews. Philadelphia: Lippincott-Raven; 7997."60, 75.)
a-Adrenergic Agents Direct-acting of-adrenergic agonists The primary clinical use of direct-acting a)-adrenergic agonists such as phenylephrine is stimulation of the iris dilator muscle to produce mydriasis. Because the parasympathetically innervated iris sph incter muscle is much stronger than the dilator muscle. the dilation achieved with phenylephrine alone is largely overcome by the pupillary light reflex during ophthalmoscopy. Coadministration of a cycloplegic agent allows sustained dilation. Systemic absorption of phenylephrine may elevate systemic blood pressure. This effect is of clinical significance if the patient is an infant or has an abnormally increased sensitivity to a -agonists, which occurs with orthostatic hypotension and in association with the use of drugs that accentuate adrenergic effects (eg. reserpine. tricyclic antidepressants. cocaine, monoamine oxidase [MAO] inhibitors~di scussed later). Even with lower doses of phenylephrine (2.5%), infants may exhibit a transient rise in blood press ure, because the dose received in all eyedrop is large for them 011 a per-weight basis. Phenylephrine 10% should be used cautiously, particularly in pledget application and in patients with vasculopathic risk factors. A 10% sol ution contains 5 mg of drug per drop, and ocular
346 • Fundamen tals and Principles of Op hth almo logy medications passing through the canalicular system are available for systemic absorption through the vascu lar nasal mucosa (see Chapter 16). In contrast. the typica l systemic dose of phenylep h rine for hypotension is 50- 100 ~I g given at a time. T he oph th almic use of 10% phenylephrine has been associated with stroke. myocard ial infarctio n. and cardiac arrest. Vascular baroreceptors are particularly sensitive to phenylephrine. An increase in blood pressure after topical application may therefore cause a significant drop in pulse rate that can be particularly dangerous in a vasculop.thic individual already on a ~-blocking systemic medication.
Indirect-acting adrenergic agonists Indirec t-acting adre nergic agonists are used to test fo r and localize defects in sym pathe tiC in nervation to the iris dil ator muscle.
Normally. pupil response fibers originating in the
hypothalamus pass down the spinal co rd to synapse wit h cells in th e in termedi olateral columns. In turn. preganglionic fi bers ex it the co rd th rough the an teri o r spinal roots in th e upper thorax to synapse in the superi or cervical ganglio n in th e neck. Fi nally. postga ngli-
onic adrenergic fibers terminate in a neuroeffector junction with the iris dilator muscle. The norepinephrine released is inactivated primarily by reuptake into secretory granules in the nerve terminal (Fig 17-7). Approximately 70% of released norepinephrine is recaptured. T he testing to confirm and localize a sympathetic lesion is twofold. To confirm a lesion, 4% cocaine is given and the pupil size is measured at I hOllr. Cocaine blocks reuptake of norep in ephr ine in to the presynaptic vesicles in an intact neuron, causing synapt ic accu mu lation and pupillary d ilation. T he injured side wi ll have less accumu lation and show less dilation. Through the use of hydroxyamphetamine. currently ava iJab le onl y th rough compounding pharm ac ies. the site of the lesion can then be de term ined to be ei th er prega ngli oni c or pos tga nglio ni c. Hydroxya mphetamin e releases stored norepinephrine fro m an intact ne uro n, res ul ting in pupillary d ilati on. (See also BCSC Secti o n 5. NeuroOphtlwlmology). Apraclonidine hydrochloride (para-a mi noclonid ine; lopidine) is an u,-adrenergic agonist and a c10nidine derivative that prevents release of norepi nephrine at nerve terminals (Table 17-3) . It decreases aq ueous production as well as episcle ral ve nous pressu re and
improves trabecu lar outtlow. However, its true ocular hypotenSive mechanism is not fu ll y unde rstood. When administered p re- and postoperatively, the drug is effective in dim inishing the acute lOP rise that follows argon laser iridectomy, argo n laser trabeculoplasty, Nd:YAG laser capsulotomy. and cataract extraction. Apraclonidine hydrochloride may be effective for the short-term lowering of lOP. but the deve lopment of topical sensitivity and tachyphylax iS ofte n lim its lo ng- term use.
Bri monidi ne tart rate causes less tachyphylaxiS than apraclonidine in long-term use; the rate of allergic reac ti ons, such as follicul ar co njun cti vitis and contact blepharodermat itis, is also lower (less than 15% fo r brimon idine but up to 40% for apraclo nid ine).
Cross-sensitivity to brimonidi ne in patients with known hypersensitivity to apracloni dine is minimal. Brimonid ine's rnechani sm of lowering lOP is thought to in volve both decreased aqueous production and increased uveoscleral outfl ow. Similar to the case with P-blockers, a central mechanism may account for part of the lOP reduction frol11 brimonidine 0.2%: a I-week trial of treatment for a Single eye caused a statistically sign ificant
CHAPTER 17:
Ocular Pharma cothe rapeutics. 347
n
SYNTHESIS OF . . NOREPINEPHRINE •
iii
Tyrosine
• Dopamine enlers veSicle and Is converted to norepinephrine • Norepinephrine Is protected I rom degradation in vesicle
Tyro~ne
l
• Transport Into vesicle Inhibited by reserpine
DOPA
Inacllve metabolites
MAO
UPTAKE INTO
~ STORAGE VESICLES
HydroltylaUon 01 tyrosine Is Ihe rate-llmlUng step
~ E;'I RELEASE OF
CI NEUROTRANSMITTER
r=I CI
REMOVAL OF NOREPINEPHRINE
•
Released norepinephrine Is repldly taken Into neuron
•
Uptake Is Inhibited by cocaine and Imipramine
C",,,hol-O- ) methyltranslerase
<~;;;;;;;;.
(COMT)
•
METABOLISM Norepinephrine Is
~ne~'!i~r~iz~b;~~~o_
amine o)(ldase
inllUII: of calcium causes luslon 01 vesicle wIth cell membrane
•
Release blocked by guanethidine and bretylium
BINDING TO RECEPTOR
"'J ~----/- ~orePlnePhrlne
Inactive metaboUtes
~
•
•
Postsynaptic receptor actIva ted by bIndIng 01 neurotrllnsmtner
__-
:"r ~~~~I~!~rif
lWlWJ~ INTRACELLULAR RESPONSE
Figure 17-7 Synthesis and re lease of norepinephrine f rom the adrenergic neuron. MAO = monoamIne oxidase inhibitor. (Reproduced with permission from Mycek MJ, Harvey RA, Champe PC, eds. Pharmacology_2nd ed Lippincott's Illustrated Reviews. Philadelphia: Lippincott-Raven; 1997:57.)
Table 17-3 Adrenergic Agonists Generic Name
Trade Name
Strengths
Propine Availab le generical ly Not available in US
0.1% 0.1% 0.5%,1%,2%
lopid in e Alphagan P Ava i lable generica l ly Combigan
0.5%, 1% (single-use container) 0.1%,0. 15% 0.2% 0.2%,0.5%
P2-Adrenergic agonists Dipivefrin HCI Epinephrine HCI
u2-Selective agonists Aprac lonidine HCI Brimon idin e tartrate Brimonidine tartrate and timo lol maleate
348 • Fundamentals and Principles of Ophthalmology
reduction of 1. 2 mm Hg in the fellow eye. Brimonirline is now ava ilable in a generic 0.2% solution and in a 0. 1% and 0.1 5% solution with the new preservative Purite as Alphagan P. Studies have show n brimonidine tartrate 0.15% (Al phaga n P) to be co mpa rabl e to AIphagan 0.2% (now discontinued) when give n 3 times dail y. Brimonidine's peak lOP reducti on is approximately 26% (2 hours postdose). At peak, it is comparable to a nonselective ~-bl ocker a nd superio r to th e selective ~-b l ocker betaxo lol, a lthough at trough (12 ho urs postdose), the reduction is o nl y 14%-15%, whi ch makes brimonidine at trough less effective than the nonselective ~ - bl ocke rs but compa-
rable to betaxolol. Brimonidine may also have potential neuroprotecti ve properties, as shown in ani mal mod els of optic nerve and retinal injury that are independent aflOP reduction. The proposed mechanism of neuroprotec ti on is up-regulation of a neurotrophin,
basic fibro blast g rowth factor, and cellular regu lato ry genes. Caution is recommended when using apraclonidine or brimonirline in patients on an
MAO inhibitor o r tr icyclic antidepressa nl therapy and in pat ients wit h severe cardiovascular disease. Use of these drugs co ncomit antl y with ~-blockers, a ntih yperten sives, and cardiac glycosides (oph thalmi c and systemic) also requires prudence. Although effective in acutely lowe rin g the lOP in angle-closure glaucoma, these drugs may also induce vasoconstriction that can prolong iri s-sphincter ischemia and reduce th e efficacy of concurrent miotics. Apraclonidine has much greater affinity for a I-rece pto rs than does brimonidine and is th erefore more likely to produce vasoconst rict ion in th e eye. Brimon idine has been shown to not induce vasocons tri ction in th e posterior segment o r optic nerve. Ligand binding to uz-receptors in othe r systems has been shown to med iate inhibiti on of th e enzyme ade nylate cyclase. Adenylate cyclase is present in the Cilia ry epithel ium and is thou ght to have a role in aqueous production.
Antagonists Thymoxamine hydroc hloride (mox isylyte), an ai-adrene rgic blocking agent, acts by competitive inhibi tion of norepinephrine at the receptor site. Thymoxamine inhibits a-adrenergic recep to rs of the dilator muscle of th e iris and results in pupil constr ict ion but does not have any Signi fi cant effect o n cil iary muscle contrac ti o n and therefore does not induce a significant change in anterior cham ber depth . facility of outflow, lOP. or accommodation. It is usefu l for differentiating angle-closu re glaucoma from POAG with closed angles an d to reverse th e pupil dilation caused by phe nylephri ne. Thymoxam ine is not avaiJable co mmerciall y in the United States even th ough it has been Widely used in Europe for yea rs. Dapiprazo le hydrochlorid e (previously ma rketed as Rev-Eyes 0.5%; no longer ava il able in th e United States) is an a-adrenergic blocking agent that reverses, in 30 minutes, the mydriasis produced by phenylephrine a nd tropicamide but not cycloplegics. It affects th e dil ato r muscle but not Ciliary muscle contrac ti o n (ante rior cha mber depth, facility of outflow. or accommodation) .
p-Adrenergic Agents
PrAdrenergic agonists ~2-Adren e rgi c ago nists lower lOP by increasing uveoscle ral ou tnow and perhaps
also by increasing out n ow through the trabecular m eshwork. The beneficial effect o n
CHAPTER 17:
Ocular Pharmacotherapeutics . 349
outn ow more than compensates for a small increase in aqueous innow as detected by fluorophotometry. ~,- Receptors linked to adenylate cyclase are present in the ciliary epithelium and processes as well as in the trabecu lar meshwork. Treatment wi th L-epinephri ne, an 0.- and ~ -agoni st. increases int race llular levels of cycl ic adenosine monophosphate (cAMP) in these tissues and in the aqueous humor. In other tissues, ~-receptor-mediated generation of cAMP in turn act ivates cAMP-dependen t enzymes, which results in responses such as glycoge nolys is and gluconeogenesis in the li ver and lipolysis in ad ipose tissue. However. the biochemical mechanisms responsible for loweri ng 101' remain to be determined. L-Epinephrine is no longer commerciall y available in the Uni ted States (see Table 17-3). Local allergic an d irritative manifestations as well as systemic adverse effects (headache. palp itations, and card iac arrhythm ias) are cOlllmon causes of into lerance of long-term epinephrine therapy. Oxidation prod ucts of epinephr ine may prod uce black deposits in the conjun ctiva. Although such deposits are harmless. they have been mistaken for foreign bodies or even melanomas. Epin ephrine therapy has also been associated with a reversible cystoid maculopathy that occurs in app roximately 25% of chronically treated aphakic eyes that lack a posterior lens capsule. Alt hough epinephrine maculopathy does not occur in phakic eyes. it has not yet been established whet her an intact posterior lens capsule. hyalOid membrane. and/or the presence of an intraocular lens retards the development of the condition. T he prod rug dipi vefrin HCI (Propi ne) 0. 1% is therapeutically equi valent to epinephrine compou nds of I %-2%. because the presence of 2 pivalyl residues (see Chapter 16 for more information on prod rugs) increases its lipid solubility and corneal penetration by a factor of 17. The dru g has little adrenergic activity until th e pivalyl gro ups are cleaved by cornea l esterases. The reductions in bo th in tr insic activ ity an d concentration employed virt uall y eliminate the systemic side effects of epin ephrine. Extraocu lar irri tation is aJ so reduced because of e ith er the lower concentratio n or the red uct ion in auto-oxidation of the phenolic hydroxy ls. However. because epinephrine is liberated from the prod rug inside the eye. the risk of epinephri ne maculopat hy should be unchanged. Chronic therapy with epineph rine has been shown in an animal model to result in a down- reg ulatio n of the number of ~-receptors. This phenomenon may underlie the loss of some of its therapeutic eFFectiveness over time (tachyphylaxis).
p-Adrenergic antagonists ~ -Adrenergic
antagonists. also known as ~ - blockers. lower lOP by redUC ing aqueous humor production as much as 50% (Table 17-4). Six agents are approved for use in the treatment of glaucoma: betaxolol, carteo lal , levobunolo l, metipranolal. timal ol mal eate, and timolol hem ih yd rate. Although it is li kely that the site of ac ti o n resides in the Ciliary body, it is no t known which is primari ly affected: th e vasculature of th e d liary processes or the pumpin g mechan ism of the ciliary epith elium. O ne possible mechanism may be an effect on the ~ - adrenergic recepto r-coupled adenylate cyclase of the Ciliary epitheli um. Although systemic admin istratio n of ~ - blockers has been reported to elevate blood lipid . sllch elevation has not been demonstrated with topical ~-blockers such as timolol. All ~-b locke rs can inhibit the increase in pulse and blood pressure that is exhibited in response to exertion. For this reason. they may be poorly tolerated in elderly patients
350 • Fundam e ntals and Prin cipl es of Ophthalmo logy Table 17-4 p-Adrenergic Antagonists Generic Name Betaxolol
Her
Carteo lol
Her Her
Levobunol ol
Trade Name
Strengths
Betoptic S Available generically Ocupress Available generically
0.25%
8eta gan
0.25%, 0.5% 0.25%, 0.5%
Timolol maleate (prese rvative
Available generical ly OptiPranolol Ava il ab le generica lly Istalol Timopti c Oc umeter Tim optic Ocumeter Plus Available generica ll y Tim opt ic-XE Ocumeter (ge l) Tim opti c-XE Oc um eter Plus (ge l) Available generical ly (gel) Betima l Timopt ic in OcuDose
free) Timolol maleate and
Cosopt Ocumeter Plus
Metipranolol
Her
Timo lol ma leate
Timolol hemihydrate
dorzolamide
Her
Ttmolol maleate and
brimonidine tartrate
Combigan
0.5% 1% 1%
0.3% 0.3% 0.5%
0.25%,0 .5% 0.25%,0.5% 0.25%,0.5% 0.25%, 0.5% 0.5% 0.25%, 0.5% 0.25%,0.5% 0.25%, 0.5%
TImolo l 0.5%; Dorzolam ide 2% Timolol mal eate 0.5%; brimonidine tartrate 0.2%
during routin e activit ies, as well as in young, physica ll y act ive individuals. Nonselec ti ve ~ - bl ocke rs
inhibit the pulmonary ~,- receptors that dilate the respiratory tree. The induced
bronchospas m may be significant in patien ts wit h asthma and chronic obstructive lun g
disease. In patients wi th heart failure, bradyca rdia, and second-degree or third-degree atrioventricular block, the underl ying card iac condition may be exacerbated with use of
these agents. T imolol maleate 0.25% or 0.5% (Timoptic) and levobun olol (Be tagan) 0.25% or 0.5% are mixed ~1 / ~ 2- antago nists. Tests of more specific ~ - blocke rs suggest that ~,-antago ni sts have a greater effect o n aqueous secretion than PI -an tagoni sts. For example, comparative studies have shown that the specific ~ l-a ntagonist betaxolol 0.5% is approximately 85% as
effective in lowering lOP as timolol. Metipranolol hydlOch loride (O ptiPranolol) is a nonselective ~l - and ~,- adrenergi c receptor- blocking agent. As a 0.3% topical solution, it is similar in effect to other topica l nonselective beta-blockers and is efficacious in reduci ng lOP. Carteolol hydrochloride (Oc upress) demonstrates intrinsic sympat homimetic activity, which means that, wh il e act ing as a competiti ve antago nist, it also causes a sli ght to
moderate activation of receptors. Thus, even though carteolol produces ~ - bl ocking effects, these may be tempered, redUCing the effect on card iovascular and respiratory systems. Carteolol may also be less li kely to adversely affect the systemic lipid profile compared with other ~ - blockers.
CHAPTER 17:
Ocular Pharmacotherapeutics. 351
Betaxolol is a selective ~ l- antagoni s t that is sign ificantly safer than the nonselective ~ - blockers whe n pulmonary. cardiac. central nervous system , or other systemic conditions are considered. Betaxolol may be useful in patients with a history of bronchospastic disorders, although other thera pies should be tried in lieu ofbetaxolol (~sel ectivity is o nly relative and not absolute, and some ~2 effect can therefore remain) . In general, the IOPlowering effect of betaxolol is less than that of the nonselecti ve ~ - adre ne rgic antagonists. Betaxolol is avai lable as a ge neric 0.5% solution, as well as a 0.25% solution (Betoptic S). Betoptic S causes less irritat ion on insti llation yet maintains its clinical efficacy when compared with Betoptic 0.5% (now discontin ued), a finding that is generally extrapolated to the generic 0.5% solution cu rrently avai lab le. Prod rugs of nonselecti ve ~ - blockers are being de veloped, and they may offer the benefit of the higher potency of ~ l/ ~, - blockin g age nts while reducing their potential systemic side effects. It is curious that both ~ - agonist and ~-antagonist dru gs can lower lOP. T his paradox is compounded by the observation that ~-agonist and ~ -a ntagonist drugs have slightly add itive effects in lowering lOP.
Carbonic Anh drase Inhibitors Aqueous humor is secreted into the posterior chamber by the nonpigmented epithelillln of the ciliary processes. Although the phYSiologic mechanisms of secretion are not fully understood, secretion is known to depend largely on active transport of sodium by Na+, K+-ATPase on the surfa ce of nonpigmented epithelial cells. Inhibition of that enzyme by ouabain inj ected into th e vitreous cav ity of experimental animals reduces secretion markedly. Unfortunatel y, o uaba in and other cardiac glycosides cannot be used clinically to treat glaucoma because effecti ve doses for the eye would require systemic doses toxic to the heart. However. Na + transport and coupled aqueous secretion can be inhibited indirec tl y. Na+ transport and fluid flow seem to be partially linked to HC0 3- formation in th e ciliary epithelium, and HC0 3- formation can be su bstantiall y reduced by inhibition of the enzyme carbonic anhydrase (Table 17-5). The linkage between HC0 3- and Na+ transport is demonstrated by the fact that the effect on aqueous flow is no greater when ouabain and a carbonic anhydrase inhibitor (CAl) are given together than when each drug is given alone. Carbonic anhydrase catal yzes the hydration of dissolved CO, to H, C0 3 , which ionizes into HC0 3- and H+. The HC0 3- is then available to accompany secreted Na +. When the enzyme is inh ibited, the transport of radiolabeled HC0 3- in to the posterior chamber is redu ced by up to 60%. That portion (60%) ofNa+ transport accompanied by Cl- is un affected by inhibition of carbonic anhydrase and, curiously, is unaffected by coadmini stra tion of ouabain . The mechan ism(s) of th e residual secretion and th e means of inhibiting it are yet to be determined. CA ls such as acetazolamide and methazolam ide are approved for the treatment of glaucoma in ad ults. and some preparations of acetazolamide are ap proved fo r this use in children. These CA ls may also be effective in treating cystoid macular edema (CME) and pseud otumor cerebri.
352 • Fun damen tals and Principles of Ophthalmology Table 17-5 Carbonic Anhydrase Inhibitors Generic Name
Duration of Acti on
Trade Name
Stre ngths
Diamox Seque ls
500 mg (time-release)
1-1.5 h; 2 h
Available generically Availab le generically
125,250 mg 500 mg, 5- 10 mgl kg'
1-1.5 h 2 min
8-12 h; 18-24 h 8-12 h 4-5 h
Availab le generically
25,50 mg
2-4 h
10-18 h
Trusopt Ocumeter Plus Azopt
2% so lution 1% suspension
5 min 2h
8h 8-12 h
Casapt Oc umeter Plus
Do rzo lam ide Hel 2% and timo lol 0.5%
2h
1h
Onset
System ic Acetazolamide
Acetazolamide sodium Methazolamide Topica l Dorzolamide He! Brinzo lamide Combin at io n Agent Dorzo lam ide Hel and timolol maleate
The concentration of carbonic anhydrase in the ci li ary epithelium of rabbits is one tenth the concentration of that found in the kidney an d the choroid plexus of the brain. Carbonic anhydrase is present in considerable excess of what is needed to supply the amount of HCO,- transported. Calcu lations based o n the K", (catalysis constant) and Km (apparent affinity constant) of the enzyme and the concentrations of substrates and product indicate that lOO times as much enzyme is present as is needed in the cil iary bod y. Correspondingly, in clinical use, the enzyme must be mo re than 99% inhibited to significantly red uce aqueous flow. The enzyme in the kidney, present in I OOO-foid excess, must be more tha n 99.9% inhibited to affect the usual pathway for HCO,- reabsorpt ion. Wi th the inhibitor methazolam ide, the difference in concentration of carbonic an hyd rase in the ciliary body and the kidney can be exploited to lowe r lOP without inc urri ng renal HCO,- loss, and an unpleasant me tabolic acidosis can be avoided. Eve n though renal stone formation has been reported with use of met hazo lam ide, it is significantly less than w it h other agents because of this specific property. In contrast, acetazo lamide is actively secreted in to the rena l tubul es, and rena l effects are unavoidable. The topical forms ofCAls-dorzolamide (Trusopt Ocumeter Plus) and brinzolam ide (Azopt)-are agents that are also available for chron ic treatme nt of glaucoma. They pen etrate the corn ea easil y, and they are water-solub le and speciall y formu lated for top ical ophtha lm ic use. When administered as solution 3 times a day, they effect ively in hibit carbonic anhydrase II and avoid the system ic side effects of oral ad mi nistration. Both agents are equa lly effective and reduce lOP by 14%- 17%. The concomitant administration of do rzo lamide and oral CAls is not recommended because of potent ially add iti ve side effec ts. Adverse effects of topical CAls include burning on instillation, punctate keratitis, local allergy, and bitter taste. Dorzolam ide hydrochloride and timolol maleate are available as a combination agent. Cosopt. which provides superior antih ypertens ive action compared with either drug admin istered alone, but it has slightly less efficacy than both drugs administered concomitantly in clinical trials. However. the lise of a Single combi ned
CHAPTER 17:
Ocul ar Ph armacoth erapeutics .
353
medicati on may imp rove patie nt compliance an d th erefore lead to an eq ui va lent efficacy in clin ica l practice. The systemic CAls are administered orall y and /o r parenterally. The longer half- li fe of me thazo lam id e all ows it to be used twice da il y; acetazolamide is also available in a sustained-release, 500-mg for m (Diamox Sequels) used twice a day. No ne of the compo un ds has the idea l combination of high potency (low Ki ), good ocul ar penetra tio n (hig h percentage in the no n io ni zed form and high lipid solubili ty to facilitate passage th ro ugh the blood-ocula r barri er), high proportion of the dr ug present in the blood in the unbo un d form, and long plas ma half-li fe. In add ition to lowering lO P by inhib iting cili ary body carboni c anhydrase, each of the agents at hi gh doses fur ther lowe rs lO P by causing renal metabolic ac idosis. The mechani sm by which acidos is lowe rs secretio n is uncerta in, but it plaus ibl y involves reduction in HCO]- formation. At the onset of acidosis, th e renal effects cause alka li ne diures is, with loss of Na+, K+, and HCO,- . In patients receiving CA l therapy concurrently with diuretics, steroids, or ad renocorticotropic hormone (ACTH), severe hypo kalemi a ca n resul t. This situatio n may be dangero us fo r pati ents usin g digitalis, in who m hypo kal emia may eli cit arrh ythmias. Such patie nts on chronic CA l th erapy sho ul d have potassium levels checked at reg ul ar interva ls, preferably by th ei r primary care physic ian, The ac idosis promp ts a renal mechanism for HCO]- reabsorpt ion unrelated to ca rbo ni c an hyd rase; this mechan ism li mits the degree of aci dosis and ha lts both the d iu resis and K+ loss after the fi rst few days of treat ment. However, dichlo rphenamide (Da ranide) also acts as a chloruretic agent and may cause contin ued K+ loss; dich lo rphenam ide is no longe r ava ilable in th e United States. CAl therapy may interac t un favora bl y with certain systemic cond itions. T he alkalin ization of the urine present during init ial CA l treatment prevents excretion of N H4+, a factor to consider in patients with cirrhosis of the li ver, Metabolic acidosis may exacerbate diabet ic ketoacidosis. In patie nts with severe chronic obs tr uctive pulmonary disease, respira to ry acidosis may be call sed by impai rment of CO 2 transfer from th e pulmo nary vasculature to the alveoli. Elderl y pat ients have a ph YS iologicall y redu ced renal fun ctio n, whic h pred isposes th em to severe metabolic ac idos is wi th the use of systemic CAls. The use of acetazo lamide has been linked to the for mati on of stones in th e urina ry tract. In a retrospective case-contro l series, the incidence of stones was II times higher in pati ents us in g this d rug. T he in creased risk occurred primarily during the first year of the rapy. Continued use after occ urrence of a stone was associated with a high risk of recurrent stone formation. However, a history of spontaneolls stone fo rmation more than 5 years prior to acetazolamide therapy d id no t appear to be associated wi th a spec ial risk. The mechanisms responsible for such stone formation may be related to metabolic acidosis an d associated pH changes, as well as to decreased excretion of citrate. Nearly 50% of patients are intolerant of systemic CAls because of d ist ressing centra l nervous system and gastro intestinal side effects, which include numbness an d tingling of the han ds, feet, and lips; malaise; meta llic taste when drin king carbonated beverages; ano rex ia and we ight loss; nausea; somn olence; impotence and loss ofl ibid o; an d depress ion . W henever the cl inical situation allows, it is wise to begin th erapy at low d oses (eg, 125 mg acetazo lamide g id or 25-50 mg methazolam ide bid), because side effects wi ll be less severe
354 • Fundamentals and Principles of Ophthalmology and weaning may actually reduce their incidence. Patients should be informed of the potential side effects of these agents; otherwise, many pat ients may fail to associate their
systemic symptoms with the use of a medication prescribed by the ophthalmologist. Rare adverse effects from this class of drugs, including the topical age nts, comprise those common to other members of the su lfonamide family, sllch as tran sient myopia.
hypersensitive nephropathy, skin rash, and thrombocytopen ia. One potential side effect, aplastic anemia, is idiosyncratic. White blood cell counts do not detect susce ptible patients. CA ls have been assoc iated with teratogenic effects (forelim b deformity) in rodents, and their lise is not advised during pregnancy_
Prosta land in Analo ues Prostaglarldin (PC) analogues are the newest c1assofocular hypote nsive agents (Table 17-6). Currentl y, 4 PG ana logues have been approved for clinical use. Latanoprost (Xalatan), bim atoprost (Lumigan), and travoprost (Travatan, Travatan Z) are used once daily, with nighttime dOSing; unop rostone (Resc ula), used twi ce daily, has been discont inued in the United States. Latanoprost is a prodrug of prostaglandin F,u (PGF,u); it penelrates the corn ea and becomes biologically active after being hydrolyzed by corn eal tissue esterase. It appears to lower lOP by enhancing uveoscleral outflow and may reduce the pressure by 6-9 mm Hg (25%-35%). O ne potential advantage of this agent is once-daily dOSing. Other adva ntages include the lack of cardiopulmonary side effects and the additivity to other antiglaucoma medications.
An ocular side effect unusual to this class of drugs is the da rkening of the iris and periocular skin as a resul t of increased numbers of mclanosomes ( in c reased melanin con-
tent, or melanogenesis) within the melanocytes. The risk of iris pigmentation correlates with baselin e iris pigmentation. Light-co lored ir ides may exper ien ce in creased pigmen-
tation in 10%-20% of eyes in the initial 18-24 months of therapy, IVhereas nearly 60% of eyes that are light brown or 2-toned may experience in creased pigmentation over the
same time period. The long-term sequelae of this side effect are un known. Other side effects reported in association with the use of a topical PG analogue include conjunctival hyperemia, hypertrichosis of the eyelashes, CME, and lIveitis. The 2 latter side effects are more common in eyes with preexisting risk factors for either macular edema or uveitis. Reported systemic reactions include flulike symptoms, sk in rash, and poss ible uterine
Table 17·6 Prostaglandin Analogues Generic Name
Trade Name
Strength s
Bimatoprost Latanoprost Travoprost
Lumigan Xalatan Travatan Travatan Z Rescula
0.03% 0.005% 0.004% 0.004% Discontinued in US
Unoprostone isopropyl
CHAPTER 17: Ocular Pharm acotherapeuti cs . 355
bleeding in postmenopausal wome n. Reactivation of herpetic keratitis has been reported with use of lata noprost.
Combined Medications Med icati ons that are combined and placed in a single bottle have the potential benefits of jmproved efficacy, conven ience. and compliance, as well as reduced cost. FDA guidelines require the fixed combi nation to be more efficacio us than either agent give n alone. The adva ntage of combined therapy is the convenience and lessened confusion of 1 bottle rather than 2, which may in crease th e potential for greater compli ance and thus improved efficacy in clinica l practice. The ocular sid e effects are the same, whether the drugs are administered in th e combined form or individually. It is important to prove that each component has an effect o n lOP before using the combined medicati on. Cosopt. the combination of a P-blocker (timolol maleate 0.5%) and topical CAl (dorzolamide 2%), is more efficacious than either agent administered alone, altho ugh it has slightly less efficacy than both agents given concomitantl y. If Cosopt is used as mo notherapy, a monocular trial of timolol sho uld be tried first. If timolol is effective in Significantl y lowering th e lO P, dorzolam ide should be used with timolol in a monocular trial. An alternative trial could involve Cos opt in I eye tw ice daily and timolol in the oppos ite eye. A formulation ofbrimo nidine 0.2% and timolol 0.5% (Co mbiga n) is approved for the reduction of elevated lOP in patients with glaucoma or ocular hypertension who requ ire adj unctive or replacement therapy due to inadequately controlled lOP. Each agent shou ld be Similarl y proven effective in a given patient before the combined agent is prescribed.
Osmotic A ents Actions and Uses Increased serum osmolari ty reduces lOP and vitreous volume by drawing fluid out of the eye across vasc ular barriers. The osmotic ac tivity of an agent depends on the number of particles in solutio n and the maintenance of an osmotic gradient between the plasma and the intraocu lar fluids. It is independent of the molecular weight. Low-molecular-weight age nts such as urea th at penetrate the blood- ocu lar barriers produce a small rebound in lOP after an initial lowering because of a reversal of the osmotic gradien t when the kidneys clear the blood of excess urea. Osmoti c age nts are approved fo r the sho rt-term management of acute glaucoma in adu lts and may be used in the reduction of vitreous vo lu me prior to cataract surgery.
Agents Osmot ic agents should be used with care in patients in whom card iovascular overl oad might occur with moderate vasc ular vol ume expansion , such as patients with a histo ry of congestive heart failure. angina, syste mic hypertension, or recent myoca rdi al infarct. The osmotic agents glycerin, mannitol, and urea are currently ava ilab le for ophthalmic lise in the United States (Table 17-7).
356 • Fundamentals and Pri nciples of Ophtha lmology
Table 17·7
Hyperosmotic Agents
Generic Name
Trade Name
Strengths
Glycerin Mannitol
Available generically Osmitrol Available generically Available generica lly
5%-20% 5%-25% Powder
Urea
Onset
Duration of Action
Dose
Route
1- 1.5 g/kg 0.5-2 g/kg
Oral IV
10- 30 min 5h 30-60 min 4-8h
0.5-2 g/kg
IV
30-45 min 5-6 h
Intravenous agents Ma nn itol (Os mi trol) m ust be given intravenous ly because it is not abso rbed from the gast roin testina l t ract. Urea is u npalatab le and thus is llsed int rave no usly. Use of u rea is out of favor beca use of rebound (see the p revious sect ion Actio ns and Uses) and beca use of its tendency to cause tissue necrosis if it extravasates during administration. Intravenous (IV) administration produces a rapid onset of action, which is usually desirable, but both mannitol and urea have bee n associated with subarachn o id hemorrhage att rib uted to rap id volume overload of the blood vessels and/or rap id shrinkage of the brain with traction of the subarachnoid vessels. This shrinkage is of particular concern in the elderly, who already have brain shrinkage from microischemic disease and are therefore at increased risk of bleeding. These agents are cleared by the kidneys and produce a marked osmotic d iuresis that may be troublesome in the operating room. The conscious patient should void shortly befo re surgery, and a u rinal should be ava ilable. If general anesthesia is em ployed, an indwelling ureth ra l catheter may b e req uired to preve nt bl adder d istension.
Oral agents Glycerin 50%, discontinued in 2004, can be co mpounded by diluting the 100% solution. This frequent ly llsed oral osmotic agent is given over cracked ice to minimize the nauseating sweet taste. T he no n metabolized sugar isoso rb ide (ls mo tic) was p referred in dia betic patients but has been discon tinued in t he United States. Cunliffe I. New drugs in glaucoma therapy. Hosp Med (Lol/doll). 2003;64(3): I 56-160. Ingram el, Brubaker RF. Effect of brinzolamide and dorzolamide on aqueous humor now in human eyes. Am / Ophtlwlmol. 1999;128(3):292 - 296. Lichter PRoGlaucoma clinical trials and what th ey mean for our patients. Am J Ophtlwlmol. 2003; 136( I); 136- 145. Moroi SE, Gottfredsdottir MS, Schte ingart M 'I~ et al. Cystoid macular edema associated wi th latanoprost the rapy in a case series o f patie nts with gl
Anti-Inflammator A ents Ocular inflammation can be treated with drugs administered topically, by local injection, by ocular implantation, or systemically. These drugs may be classified as
CHAPTER 17:
Ocular Pharmacotherapeutics.
357
glucocorticoids, no nsteroidal anti-inflammatory age nts, mast-cell stabi li zers, antihis tamines, or antifibrotics.
Glucocorticoids Corticosteroids, or steroids, are applied topically to prevent or suppress ocul ar inflammation in trauma and uveitis, as well as after 1110st ocular surgical procedures (Table 17-8). Subconjunctival and retrobulbar injections of steroids are used to treat more severe cases of ocular inflammation. Systemic steroid therapy is used to treat systemic immune diseases such as giant cell arteritis, vision-threatening capi llar y hemangiomas in childhood, and severe oc ul ar inflammati ons that are resistant to topical therapy. Intravenous meth ylpred nisolone is an opt ion in the treatment of demyelinating optic neuritis; sese Section 5, NeLiro-Ophthalmology, discusses this issue in depth. Glucocorticoids induce cell -specific effects on lymphocytes, macrophages, polymorphonuclea r leukocytes, vascu lar endothelial cells, fibroblasts, and other cells. In each of these types of cells, glucocorticoids must penetrate th e cell membrane bind to soluble receptors in the cy tosol
Table 17-8 Topical Anti-Inflammatory Agents Generic Name
Trade Name
Strengths
Maxidex Available genericall y FML S.O.P. FML Liquifilm FML Forte Liquifilm Available generically Flarex Alrex Lotemax Econopred Plu s Pred Forte Available generical ly Pred Mild Inflamase Forte Prednisol Ava il able genericall y Vexo l
Ophthalmic susp, 0.1 % Ophthalmic soln, 0.1 % Ophthalmic ointment, 0.1 % Ophthalmic susp, 0.1% Ophthalmic susp, 0.25% Ophthalmic susp, 0.1% Ophthalmic susp, 0.1% Ophthalmic susp, 0.2% Ophthalmic susp. 0.5% Ophthalmic susp, 1% Ophthalmic susp, 1% Ophthalmic susp, 1% Ophthalmic susp, 0.12% Ophthalmic so ln, 1% Ophthalmic soln, 1% Ophthalmic soln, 1%, 0.125% Ophthalmic susp, 1%
Vo ltaren Ocufen Available generi ca ll y Acu lar Acular PF Acular LS Nevanac Xibrom
Ophthalmic so ln, 0.1% Ophthalmic so ln *, 0.03% Ophthalmic so ln, 0.03% Ophthalmic so ln, 0.5% Ophthalmic soln, 0.5% Ophthalmic soln, 0.4% Ophthalmic susp, 0.1% Ophthalmic so ln, 0.09%
Corticosteroids
Dexamethasone Dexamethasone sod ium phosphate Fluorometholone
Fluorom etho lone acetate Loteprednol eta bonate Prednisolone acetate
Prednisolone sodium phosp hate
Rimexolone Nonsteroidal Anti -Inflammatory Drugs Dicl ofe nac sod ium Flurbi profe n sodium Ketorolac tromethamine
Nepafenac Bromfenac sodium * Indicated for intraoperative miosis only.
358 • Fundamentals and Principles of Ophthalmology allow the translocation of the glucocorticoid receptor complex to nuclear binding sites for gene transcription induce or suppress the transcription of specific mRNAs
The proteins produced in the eye und er the control of these mRNAs are not known, and only resultant effects have been described. At the tissue level, glucocorticoids prevent or suppress the local hyperthermia, vascular congest ion , edema. and pain of initial inflammatory responses, whether the cause is traumatic (radiant, mechanical, or chemical). infectiolls, or immunologic. They also sup-
press the late inflammatory responses of capillary proliferation, fibroblast prol iferation, co llagen deposition, and scarring. At the biochemical level, the most important effect of anti-i nflammatories may be
the inhibition of arachidonic acid release from phospholipids (see Part IV, Biochemistry). Liberated arachidonic acid is otherwise converted into PGs, PG endoperoxides, leukotrienes, and thromboxanes, which are potent mediators of inflammation. Gtucocorticoids also suppress the liberation of lytic enzymes from Iysozymes. The effects of glucocor ticoids on immune-mediated inflam mation are complicated.
Glucocorticoids do not affect the titers of ei ther IgE, which mediates allergic mechanisms, or IgG. which mediates autoimmune mechanisms. Nor do glucocorticoids appear to interfere with the normal processes in th e afferent limb of ce ll -mediated immunity. as in graft rejection. They interfere instead with the efferent limb of the immune response. For example. glucocorticoids prevent macrophages from being attracted to sites of inflammation by
interfering with the macrophages' response to lymphocyte-released migration-inhibiting factor. Systemically administered glucocorticoids cause seq uestration of lymphocytes, especially the T lymphocytes that mediate cellular immunity. However, the posttranscriptional molecular mechanisms of these responses are as yet unknown. BCSC Section 9. Intraocular Inflammation and Uveitis, discllsses imm une res ponses in detai l.
Adverse effects Glucocorticoids may calise a nmnber of adverse effects in the eye and elsewhere in the body. Complications in the eye include the follOWing: glaucoma posterior subcapsular cataracts
exacerbation of bacte rial and viral (es pecially herpetiC) infections through suppression of protective immune mechanisms
ptosis mydriasis
scleral melting eyelid skin atrophy In the body, oral doses can cause the follOWing: suppression of the pitUitary-adrenal axis • gluconeogenesis resulting from hype rglycemia, muscle wasting. osteoporosis
CHAPTER 17:
Ocular Pharmacotherapeutics.
359
redistribution of fat from the periphery to the trunk • centra l nervous system effects. such as euphoria insomnia
aseptic necrosis of the hip peptic ulcer diabetes occasionally. psychosis Elderly patients have particular difficulty taking long-term systemic steroids. For example. one side effect. proximal muscle wasting. may make it difficult for these patients to climb stai rs. Another adverse effect of glucocort icoids, osteoporosis. exacerbates the risk
of falls and fractures for the elderly. who are generall y at an increased risk of both. Elderly patients with inflammatory disease may require a steroid-sparing regimen with metho-
trexate due to steroid-induced complications. The system ic side effects of steroids. as well as the benefits and limitations of altern ate-day therapy. are discussed in BCSC Section I.
Update 011 Gelleral Medicine. Steroid -induced elevation in lOP ma y occur with topical. periocular. nasal. and systemic glu cocorticoid th erapy. Individua ls differ in their responsiveness: approximately
4% develop pressures higher than 3 1 mm Hg after 6 weeks of therapy with topica l dexamethasone. High levels of response are genera ll y reproducible. The mechanism by which steroids decrease the faCility of aqueous outflow through the trabecular meshwork remains unknown. lOP response to topical prednisolone in a normotensive cat model is
comparable to that with topical dexamethasone. IndividuaJ response to steroids is highly dependent on the duration. strength. and frequency of therapy and the potency of the agent used. Steroid-induced lOP elevation alm ost never occurs in less than 5 days and rarely in less than 2 weeks. It is not genera ll y
apprecia ted that late responses to therapy are com mon and that failure of the lOP to rise after 6 weeks of therapy does not ensure that the patient will maintain normal lOP after several months of therapy. For this reason. lOP monitoring is required at periodic interva ls during the entire course of chronic stero id therapy to prevent iatrogenic glaucoma to us nerve damage. Steroid-indu ced lOP rises are usually reversible by discontinuance of
thera py if the dru g has not been used for more than 1 yea r. but permanent elevations of pressure are common if therapy has cont inu ed for 18 months or more.
The anti -inflammatory and pressure-elevating potencies of 6 steroids used in ophthalmic therapy are given in Table 17-9. The an ti-inflammatory potency was determined by an in vitro assay of inhibition of lymphocyte transformation . and the lOP effects were determined by testing in individuals already known to be highly responsive to topical dexamethasone. However, until all these agents are compared in a model of ocular in flammation relevant to human disease, no conclusion ca n be reached about the observed
dissociation of effects. The lower-than-expected effect on pressure of some of these agents may be explai ned by more rapid metabolism of f1uorometholone in the eye compa red with dexamethasone and by the relatively poor penetration of med rysone. The efficacy of th ese agents for intraocular inflammation may be Similarly reduced.
360 • Fundamenta ls and Principles of Ophthalmology Table 17-9
Comparison of Anti-Inflammatory' and IOP-Elevatingt Potencies
Glucocorti coid
Relative Potency
24 21 2.3
Dexamethasone 0.1 % Fluorometholone 0.1 % * Prednisolone 1% Med ryso ne 1% § Tetra hydrotriamci no lone 0.25% Hydrocortisone 0.5%
1.7
1.4 1.0
Rise in lOP (mm Hg )
22 6
10 1 2 3
• Anti-inflammatory potency determined by in vit ro assay of inhibition of lymphocyte transformation t lOP effects determined in topical dexamethaso ne responders :I: Rapid metabolism of fluorometholone in the eye compared with dexamethasone § Relatively poor ocular penetration of medrysone
When a steroid-induced pressure rise is suspected but continued steroid therapy is warranted, the physician faces the following choices:
Continue the same treatment and cl osely moni tor th e status of the optic nerve. Attempt to offset the pressure rise with other age nts. Reduce the potency, concentration, or frequ ency of the steroid used, while moni toring both pressure and inflammation. When alternative classes of anti -in fla mmatory age nts can be employed, a change may be
advisa bl e.
Agents and regimens Choice of available corticosteroid agents and dosage regimens remains somewhat em-
pirical. Steroids can be used topicall y (iritis), intrave nously (optic neuri tis), intravi treall y (e ndophth almitis), or in a periocular fas hio n (uveitis) (Table 17- 10). All corticosteroids may exacerbate bacterial, viral, mycobac terial, and funga l d iseases of the eye and should be used with caution in these settings. Prolonged use may result in secondary glaucoma. cataract formation. and secondary ocular infec tions following suppression of the hos t re-
sponse andlo r perfo ration of the globe. Table 17-10
Usual Route of Corticosteroid Administration in Ocular Inflammation
Condition
Route
Bleph ari tis Conjunct ivitis Episcle ritis Scleritis Keratitis Anterior uveitis Poste rio r uveitis En do ph thalmitis Macular edema, diabetic Macular edema, cystoid Optic neuritis Temporal arteritis Sym pathetic op hthalmia
Top ica l Top ica l Topical Systemic Topica l Topical and/or periocular, systemic System ic and/or perioc ul ar, intravitreal inject ion or implant Syste m ic/pe riocu lar, intrav itreal Pe riocular, intravitreal Topical. periocular, intravitreal Systemic Systemic Systemic and topical, intravi treal imp lant
CHAPTER 17: Ocula r Pharmacotherapeutics . 36 1
Recent developments in cort icosteroids are aimed at developing agen ts with less lOP effect and age nts that can be used in traocularl y and periocularly. Rimexolone 1% (Vexol) is a synthetic topical steroid designed to minimize lOP elevations, similar to fluorinated steroids. Common side effects still include visual field defects and posterior subcapsu lar cataracts. Elevated lOP has been reported, but it is rare. Systemic side effects, including headache, hypotension, rh initis, pharyngitis, an d taste perversio n, occur in fewe r than 2% of patients. Loteprednol etabonate 0.5% (Lotemax) is structurally sim ilar to other steroids but lacks a ketone group at position 20. In corticosteroid responders, studies show that patients treated wi th lotepred nol demonstrate a low incidence of clinically sign ificant, in creased lOP. Loteprednol etabonate 0.2% (Alre x) is marketed fo r the temporary treatment of allergic conjunctivitis. Loteprednol etabonate 0.5% with tobramyci n 0.3% (Zylet) is approved for superficial bac terial infection of the eye with inflammatio n. The fluocino lone aceton ide implant (Retisert) was approved for intraocular implantatio n in the vitreous cavi ty fo r chronic, noninfec tious uveitis in 2005. Triesence, a 40 mg/mL. preservative- free triam c inolone ace ton ide injectable suspension \vas app roved for intraoc ular use in 2007. FDA-approved indicat ions include visual ization during vitrec to my and treatment of sym pathetic ophthalm ia , temporal arteritis, uveitis. and ocular inflammatory cond itio ns un responsive to top ical corticostero ids. Arma ly ME Effect of corti costero ids on intraocular pressure and fluid dynamics II. The effect of dexamethasone in the glaucomatous eye. Arch Ophthalmol. 1963;70:492 - 499. Arma ly ME Effect of corticos teroids on the intraocu la r pressu re and flu id dynamics. Th e effect of dexamethasone in the normal eye. Arch Ophtl/(l/mol. 1963;70:482- 49l. Becker B, Mills OW. Corti costeroids and intraoc ular pressure. Arch aphtha/mol. 1963;70: 500-507. Burk SE, Oa Mat ,l AP, Snyder ME, Schneider S, Osher RH , Cionni RJ. Visua li zing vitreous using Kena log suspens ion. J Cataract Refract Surg. 2003;29(4):645- 651. Fosler CS, Forstot SL, Wilson LA. t'Vlorl atity rate in rheumatoid arthritis patients developing necrotizing scle riti s or peripheral ulcerative keratit is. Effec ts of syste mic immunosuppression. Ophthallllology. 1984;9 1(10): 1253- 1263 . Leibowitz HM, Bartlett JD, Rich R, McQuirter 1-1 . Stewart R, Assil K. Intraocular pressureraising potential of 1.0% rimexolone in patients responding to corticostero ids. Arch Oph IIJah"ol. 1996; I 14(8):933- 937.
Zhan GL, Miranda OC. Bito LZ. Steroid glaucoma: corticosteroid- indu ced ocular hyperten sion in cats. Exp Eye Ues. 1992;54(2) :2 1 1- 218.
Nonsteroidal Anti -Inflammatory Drugs
Derivatives Derivatives of arachidonic acid, a 20-carbon esse nt iaJ fatty acid , have been shown to be mediators of a wide va riety of biological func tions, including regulation of smooth muscle tone (in blood vessels, bronchi, uterus, and gut ), platelet agg regation , hormone release (growth hormone, ACT H, insulin , renin, and progeste ron e), and inflammation. The synthetic cascade that results in the production of a wide variety of derivatives (depe nding on the stimulus and tissue ) begins with stimulation of phospholipase A" the
362 • Fundamentals a nd Principles of Ophtha lmology enzyme that liberates arach idonic acid from phospholipids of the cell membra ne. (Phospholipase A, is inhi bited by corticosteroids.) Arachido nic acid is then converted either into cyclic endoperoxides by cyclooxygenase (PG synth ase) or into hydrope roxides by Iipoxygenase. Among the subsequent products of the endope roxides are the PGs, which mediate inflammation and other responses; prostacyclin, a vasodilator and platelet antiaggregant; and thromboxane, a vasoconstrictor an d platelet aggregant. The hydro peroxides form a chemotactic agent and the leukotrienes C" D" and E" previously known as the slow-reacti ng substance of anaphylaxis.
Classification The currently available nonsteroidal anti-inflammatory dr ugs (NSAlDs) inhibit the prod uction and, thus, the inflammation-inducing effects of PGs through the cyclooxygenase pathway. On the basis of chemical structures, NSAlDs ca n be classified as • salicylates: acetylsa licylic acid (aspirin ), etofenami c acid, flufenamic acid, meclofenamate. mefenamic acid, tolfenamic acid indoles: indomethacin, sulindac, toimetin phenylalka noic acids: di clofenac, fenoprofen, flurbip rofen, ibuprofen, keto profen, ketorolac tromethamine. naproxen, piroxicam. sutoprofen pyrazolones: oxyphenbutazone, phenylbutazone
Agents Table l7- ll lists a number of NSAID agents, with their starting doses. Aspirin and other NSAlDs inhibit the local signs of inflammation (local heat, vasodilation, edema, swelli ng) as well as pain and fever. They have complex effects on clotting. At low doses (300 mg every other day), aspirin perm anently inhibits the cyclooxygenase in platelets that is essential for the conve rsion of arachidonic acid to PGG 2 and thromb oxa nc. Inhibition of thromboxane
production in turn prevents coagulat ion. Whereas nucleated ce lls can rep lenish their cyc1ooxygenase, anucleate platelets cannot. T he anticoagulant effect of aspirin therefore lasts fo r 7- l0 days, mirro rin g the li fe span of the inhibited platelets, despite the discontinuat ion of aspi rin therapy. Other NSA IDs inh ibit clotting in a reversible fas hion, and their use does not need to be discontinued as far in advance before elective surgery.
Table 17-11
Nonsteroidal Anti-Inflammatory Drugs Drug Aspirin Fen oprofen (Na lfon) Ibuprofen (Motrin, IBU, Advil) Indomethacin (lndocin) Ketoprofen (Orudisl Naproxen (Na prosyn , Naprelan) Piroxicam (Feldene) Sulindac (Clinoril) Tolmetin
Starting Dosage
650 mg qid 600 mg qid 400 mg qid 25 mg tid
75 mg tid 250 mg bid 20 mg qid 150 mg bid
400 mg tid
CHAPTER 17:
Ocular Ph arma coth erap eutics . 363
The relati ve risks and benefits of aspi rin therapy sho uld be assessed specifically for each patient. Asp irin therapy for postop erati ve pain or for pain associated wi th traumatic hyphema may increase the risk of hemorrhage because of the antiaggregant effect on platelets. This same side effect may benefit those patients having platelet emboli , as in some cases of amaurosis fugax. Diversion of arachidonic acid to the lipoxygenase pathway by inhibition of cyclooxygenase may explain why aspirin use can be assoc iated with ast hm a attacks and hypersensitivity reacti ons (mediated by leukotrienes e" D" and E4 ) in susceptible people. High doses of aspirin, such as those employed in the treatment of arthritis, may occasionally have toxic effects stich as headache, dizziness, tinnitus, dimmed vision, mental confusion, drowsin ess, hyperventilation, nausea, and vo mi ting. These effects may be potentiated by th e concom itant use of CAls at doses suffic ient to cause systemic ac id osis. During metabo lic acidosis, a higher proportion of aspirin molecu les is shi fted into the more li pid-so luble un -ioni zed for m, which more readily penetrates the blood-brain barrier. Aspirin and o th er cyclooxygenase inhibitors are less effective than steroids in the treatment of scleritis and uveitis. NSAIDs such as indomethacin can be effective in treating orbital inflammatory diseases. The prophylactic use of indomethacin in patients with catara ct has been reported to reduce the incidence of angiographically detected eME, but its effect o n Visuall y Signi ficant eME has yet to be reported. Topical NSAIDs have been used to treat ocular inflammation and to prevent and treat postoperative CME. Flurbiprofen sodiu m (Ocufen) was the fi rst commerCially ava ilable topical ocular NSAID. When applied preoperatively, it reduces PG-mediated int raope rat ive miosis. Topical diclofenac sodium (Voltaren) (see Table 17-8) is FDA-approved for the postoperati ve prophylaxis and treatment of ocul ar inflammat ion and has also been used successfully to prevent and treat eME. Ketorolac tromethamine (Acular, Acula r PF, Acula r LS) is approved for the treatment of postoperative inflammati on and allerg iC conj unctiv itis. Ketorolac blocks the metabolis m of arachidonic acid by cyclooxygenase. Arachidonic acid metabolites are present in higher quant ities in the tears of ocular allergiC disease patients. Two double- masked studies have revealed tha t patients wit h ocu lar allergies who were treated with keto rolac tromethamine had Significantl y less co njuncti val inflammation, ocular itch in g, and tearing than those give n placebo. Ketorolac does not have a decongestant effect and does not relieve redness. The recommended dose of ketorolac is I drop (0.25 mg) 4 times per day. The most com mon side effects are stinging and burning on instillation (40%). Nepafenac (Neva nac) was approved iJl 2005 for tid dosing fo r pain and inflammatio n 1 day before and 2 weeks after cataract surgery. Brom fenac sodium (Xibrom) was approved in 2006 for twice-daily dosing from 24 hours to 2 weeks post-cataract surgery for pain and inflammation. Topical NSAlDs may be used fo r the ir topical analgeSiC properties after corneal abrasion and after anterior segment surgery and refractive surgical procedures. All agents are associated with corneal compl icati ons, including melting and corneal perforation, seen both in postoperative patients as well as in cases of uvei tis, most often in patients with preexisting diabetes and ocu lar surface disorders. The preponderance of these patients
364 • Funda me nta ls a nd Princ iples of Ophtha lmology was fo und to be o n gene ric diclofenac. and this product was subsequ e nt ly removed fro m
th e United States ma rket. Congdon NG, Schein 00, von Ku lajta P, Lubomski LH, Gilbert 0 , Ka tz
J. Corneal complica -
tions assoc iated with topical oph thalmic use of nonsteroidal antii nnammato ry drugs. J Cat-
(nact Refract Stlrg. 2001 ;27(4):622- 631. Flach AJ. Corneal melts associated with topically applied nonsteroidal anti -inflammatory drugs. Trans Alii Ophtlwflllof Soc. 2001 ;99:205-2 1O. Guidera AC, Luchs JI, Udell IJ. Ke ratitis, ulce ration, and perforat ion assoc iated wi th top ical nonsteroidal an ti- inflammatory drugs. Ophthalmology. 200 I ; I 08(5):936-944. Noble AG, Tr ipalhi RC. Levi ne RA. Indome thacin for the treatment oridiopat hic orbital myo · sit is. Alii J Op"'!,"!IIIO/. 1989;108(3)336- 338.
Mast-Cell Stabilizers and Antihistamines The human eye has approx im ately 50 mill ion mas t cell s. Each cell co nta in s several hun · d red granules that in turn contain preformed chem ical mediators. All ergic co njunct ivi ti s is an immediate hyperse nsitivity reaction in which triggeri ng ant igens couple to reaginic
ant ibodi es (IgE) on th e cell surface of mast cells and basophils, leadi ng to the release of histam ine PG, leukotr ie nes, and chemotactic factors fro m secretory gran ules. The released histamine ca uses capi ll ary di latation and increased permea bility and thus conjunctival inject ion and swelling. It also sti mulates ne rve endings, causin g pai n and itching. Agents th at inte rfe re at di ffe re nt poin ts along this pathway ca n treat ocular all ergy. Corticos ter·
o ids are very effecti ve, but oc ul ar side effects lim it their app lication fo r th is chronic cond iti o n. Mast-ce U stab il ize rs, NSA IDs) an tihistam in es, and decongestants have fewer and less
da ngerous ocul ar side effects and can be used Singly o r in combinat io n. Table 17- 12 lists age nts fo r th e relief of allergic conjun cti vi tis. Short-term reli ef for mild all e rgic sym ptoms may be achieved w ith over-th e-counter preparat ions of topical anti histami nes such as antazo lin e and phe niramine, usuall y com-
bined with the deco nges tant na phazo line (Vasoco n-A and Naphco n-A, res pecti vely). Speci fic H I-a ntagonists have been developed , such as emedas tine (E mad ine), levocabastine
(Livostin), and azelas tine (Optivar). Emedas tin e difum arate 0.05% (E m adin e) is a relatively selective H ,- recepto r an tagonist indi cated for the tem porary reli ef of the signs and symptoms of all e rgic conjunctivit is.
Recomme nded dos ing is 1 drop up to 4 times per day. T he most common side effect reported is headac he (1 1%) . Bad taste, blurred vision, burnin g or stin ging, corn eal infil trates, dr y eye, rhinitis, and sinus iti s are other noted side effec ts. Levocabastine HCI has an onset of ac tio n that occurs w ith in min utes an d lasts for at least 4 hours; it is as effecti ve as cromolyn sodium . The usual dosage of levocabast in e
0.05% is 1 drop 4 tim es per day fo r up to 2 weeks. T his d rug has been disconti nued in the Uni ted States. Azelastine Hel (Optivar) is effective and well tolerated at a dose of 0.05%. Th is d ru g is also avai lable as a nasa l spray fo r th e treatment o f all ergic rhi ni tis.
Ketoro lac trometh amine (Ac ul ar), a top ical NSAID, is used to preve nt itching and provides a rapid onset but does not reli eve conj unc ti val hyperemia.
CHAPTER 17:
Table 17-1 2
Ocular Pharmacothe rapeu ti cs .
365
Agents for Relief of Allergic Conjunctivit is
Generic Name
Trade Name
Cl ass
Azelastine HCI
Optivar
Cromolyn sodium Emedastine difumarate Epinastine HCI
Crolom , available generically Emadine Elestat
Ketorolac tromethamine Ketotifen fumarate
Ac ul ar, Acu lar PF, Acular LS Zaditor IOTC) A laway IOTC) Available generica ll y Discontinued in US Alomide Alrex Ak-Con, Alba lon, available generically Vasocon-A
H, -antagonist/mast-cell inhibitor Mast-cell inhibitor H,-antagonist H ,-a ntagon ist/mast-cell inhibitor NSAIO H ,-a ntagonist/mast-cell inhibitor
Levocabastine HCI Lodoxamide tromethamine Loteprednol eta bonate Naphazoline HCI Naphazoline HCI/antazoline phosphate Naphazoline HCI/pheniramine maleate Nedocromil sodium
H, -antagonist Mast-cell inhibitor Corticosteroid Antihistamine Antihistamine/decongestant
Naphcon-A, Opcon -A
Anti hista m ine/decongesta nt
Alocri l
H ,-a ntagon ist/mast-cell inhibitor H ,-a ntagonist/mast-cel l inhibitor Mast-cell inhibitor
Olopatadine HCI
Patanol, Pataday
Pem irolast potassium
Alamast
Mast-cell stabilizers have previously been viewed as preventing calcium influx across mast-cell membranes. thereby preventing mast-cell degran ulation and mediator release. C romolyn sodium (Crolom) inhibits neut rophil. eosinoph il. and monocyte activation in vitro. Trad itional mast-cell stabilizers such as cromolyn sodium. lodoxamide (Alomide). and pemi rolast (A lamast) prevent mast-cell deg ranu lation but take days to weeks to reach their peak efficacy. They have litt le or no anti histam ine effect and do not provide immediate relief from allergic symptoms. They are used for allergic. vernal. and atopic conj unctivitis. Lodoxa mide has been shown to produce stabilizat ion of the mast-cell membrane 2500 ti mes greater than does cromolyn sodium. In treat ing allergiCconjunctivitis, its onset of action is quicker. with less stinging. than that seen with cromolyn sodium. One recent multicenter. double-masked study showed that lodoxamide was superior to cromolyn sodium in treating vernal keratoconjunctivitis. However. as with all mast-cell stabilizers, lodoxamide does not become clinicall y effective for several weeks. It may therefore be necessary to use topical steroids or H)-antagonists concurrently with mast-cell stabi lizers for the first several weeks. until these agents are fully effect ive. The usual dose of lodoxamide 0.1 % for adults and children older than 2 years is 1- 2 drops in the affected eye 4 times da ily for up to 3 months. The most frequently reported adverse reactions were burning, stinging) and discomfort upo n instillation ( 15%).
366 • Fundamentals and Principles of Ophthalmology Pemirolast potassium 0.1 % (Alamast) is used for the preventi on of itchy eyes due to allergic conjunctivitis. In clinical studies. the most comlnon side effects were headache, rhinitis, and cold and flu symptoms, which were generall y mild. Some agents, incl uding olopatadine (Patanol), ketotifen (Zaditor), nedocromil (A Iocril ), epinastine (Elestat), and azelastine (O ptivar), have a mast-cell stabilizing effect as we U as H I-antagonism. These agents provide immediate relief aga inst released histamine and also prevent the future degranu lation of mast cells. Olopatadine Hel 0.1 % (Patanol) has a rapid onset and at least an 8-hour duration of action. Recommended dosing is 1- 2 drops in the affected eye 2 times a day at an interval of 6-8 hours. T his drug is now also available for once-a-day dosing as olopatadine 0.2% (Pataday). Adverse reactions of ocular burni ng, stinging, d ry eye, foreign-body sensation , hyperemia, keratitis, eyelid edema, pruritus. asthenia, cold syndrome, pharyngitis, rhinitis, sinusitis, and taste perversion were all reported at an incidence ofl ess than 5%. For ketotifen fumarate 0.025% (Zaditor, A1away), recommended dosing is I drop every 8- 12 hours. Th is medication is now avail·
able without a prescription. Side effects of conjunctival injection , headaches, and rhinitis were reported at an incidence of 10%-25%. Nedocromil sod ium 2% (Alocril) is a mastcell stabili zer with a twice-daily dosiJlg regimen. Corticosteroids are very effective at treating ocu lar allergies but are prone to abuse and have a more dangerous side-effect profile. Loteprednol etabonate 0.2% (Alrex) is a steroid designed to cause less lOP elevation that can be used for the temporary treatment of ocular allergies. Recalcitrant cases of severe allergic, vernal, and atopic conjunctivit is may require the short-term use of stronger topical steroids, but these cases should be carefu ll y monitored and patients switched to one of the previously mentioned agents as soon as clinically prudent. Giede-Tuch C, Westhoff M , Za rth A. Azelasti ne eye-drops in seasonal allergiC conjll nctiviLi s or rhinoconjllnctivitis. A double-blind, randomized, placebo-controlled st udy. Allergy.
1998;53(9):857-862. Hingo rani M, Moodaley L, Calder VL, Bll ckley RJ, Lightlllan S. A randomized, placebocon trolled tri al of topi cal cyclospor in A in ste rOid -dependent atopic keratoconjunctivitis.
Ophtiw/IIl%gy. 1998; 105(9): 17 15- 1720. Verin P. Treating severe eye allergy. Clill Exp Allergy. I998;28{suppl 6):44-48.
Antifibrotic Agents Antiproliferative agents, also known as afltimetaboiites, are occasionall y required Ln the treatment of severe ocalar inflammatory di seases, such as Beh<;et syndrome and sympath etic ophthalmia, or for ocular diseases that are part of a systemic vasculit is. Systemic therapy with such agents is best carried out in consultation with a chemotherapist. The uses and side effects of these agents are disc ussed in BeSe Section 9, Intraocular Inflam-
mation and Uveitis. Fluorouracil is a fluorin ated pyrimidine nucleoside analogue that blocks prod uction of thymidylate synthase and interrupts normal ceJl ular DNA and RNA syntheS iS. Its primary action may be to cause cellular thymine defiCiency and resultant cell death. T he effect of fluorouracil is most pronounced on rapidly growing cells. and its use as an antiviral agent is primarily related to destruction of infected cells (eg, warts) by topical app lication.
CHAPTER 17:
Ocular Pharmacotherapeutics. 367
Intravitreal injection of 5- fluorouracil has been reported to be beneficial in preventing rec urrent proliferative vitreo retinopathy after surgery for complex retinal detachments in an experimental model. Fluorouracil is used postop eratively as a subcon junctival injection and intraoperativel y as a topi cal application to the trabec ulectomy site. The drug is thou ght to inhi bit the cellular proli fera t ion th at co uld otherwise occur in response to inflammation. In hi gh -risk patients, including young glauco m a patients (:0;40 years), the ini tial trabec ul ectomy with adjunct ive 5-fluorouracil had a hi ghe r success rate than surgery without the adjun ct. Mitom yc in C is a co mpound isolated fro m the fungu s Streptomyces caespitosus. The parent compound beco n1 es a bifunctional alkylating agent after en zym atic alteration with in the cell; it th en inhibits DNA synthesis and cross-links D NA. M itomycin's immu nosu ppressive proper ti es are fa irl y weak; however, it is a potent inhibitor of fibroblast proliferation. Like fluorouracil, topical mitomycin C has been used in filterin g surgery. However, unlike 5-fluoroura cil, which req uires repeated postoperative inj ecti ons, it has th e ad va ntage of functioning with a Single in traoperative application. Randomi zed co mparative studi es of mitomycin C wit h S-tluorouracil in high -risk patien ts show lowe r average press ures with fewer co rn eal -s ur face and hypoton y- related compli ca tions in th e gro ups treated with mitomyc in C than in th ose treated with 5-fluorouracil. Mitomycin C is used as a Single topi cal app licati on during glaucoma filte ring ope rations to impede sca rring and prevent surgi cal fa ilure. Compli catio ns of th erapy are wo und leakage, hypotony, an d localized scleral melting. Severe toxicity has bee n rep orted in an a nimal model with intraocular instillation of mitomyci n C, resulting in irreve rsible progress ive bullous keratopathy in 3 of 4 rabbits. Both m itomycin an d fluorouracil have bee n used in th e treatm en t of co njunctival int raepithelial neoplas ia. Topical mitomycin C has also been reco mmended both as a Single-dose therapy and as postoperative drop s for use in the preven tion of rec urrence of pte ryg ia after pterygi um excision. Reco mm end ed dosage is 0.02%-0.04% 4 times dail y for 1-2 weeks after su rgery. The recurrence rate with such th erapy h as been repo rted to be as low as 0%- 11 %. Unfortunatel y, seve ral adve rse effects-such as corneal ed em a, co rnea l and scleral perforation, corec topi a, iritis, cataract, and intractable pain-have been reported. A primar y conjunctiva l graft after pterygi um removal may offer similar low recurrence rates with out th ese se rious complications. Both mitomycin and thiotepa have been used to red uce haze in photorefractive keratectomy ( PRK) patients. Allinson RW. Adjuvant 5-FU and heparin preve nt PVR. Ophtlwlll1ology. 2002;109(5):829- 830 . Anderson Penna E, Braun DA, Kamal A, Hamilton WK, Gimbel HV. Topical thiotepa treatment for recurrent corneal haze after photo refract ive keratectomy. ] Cataract Refract Surg. 2003;29(8): 1537- 1542. Asaria RH, Kon CH, Bunce C, et al. Adjuvant 5-fluorouracil and heparin prevents proli ferative vitreoretinopath}': results from a randomized, double-blind, controlled clinical trial. Oph thalmology. 2001; 108(7): 1179-1 183. JoUmaitre P, Malet-Ma rtino M, Mar tino R. Fluorouracil prod rugs for the treat ment of prolirerative vitreoret inopathy: formulation in silicone oil and in vitro release of fluorouracil. Int J Pha"n. 2003;259( 1- 2): 18 1- 192.
368 • Fundamentals and Principle s of Ophtha lm ology
Khaw PT. Advances in glaucoma surgery: evolution of antimetabolite adjunctive therapy. / Glaucoma. 200 I; I 0(5 slIppl I ):S81-S84. Wormald R, Wilkins MR. Bunce C. Post-operative 5- fluoroura ci l for glaucoma surgery. Coch rane Database Syst Rev. 2000;(2):CDOOI132. Review. (Update in Cochrane Database Syst Rev.2001;(3):COOOI132. J Yamamoto N, Ohmura l~ Suzuki H, Shi rasawa H. Successful treatment wilh S-fluoroll racil of conjunctival intra epithelial neoplasia refractive 10 mitom),cin -C. Ophthalmology. 2002; 109(2):249-252.
Antibiotics Penicillins and Cephalosporins The penicillins and cephalosporins are ~ - lactam -contai nin g antibacte rial age nts that react with and inactivate a particular bacterial transpeptidase that is essential for bacterial cell-wall synthesis (Table 17- 13). The amide bond of the ~- lactaJ1l gro up is surround ed Table 17-13 Principal Antibiotic Agents Drug Name
Topic al
Subconjunctival
Intravitreal
Amikacin sulfate
10 mg/mL
25 mg
400
~g
Ampici llin sodium Bacitracin zinc
50 mg/mL 10,000 units/mL 4-6 mg/mL
50-150 mg 5000 units
500
~g
100 mg
250-2000
50 mg/ mL 50 mg/mL
100 mg 200 mg 15-50 mg
2250 2200 1000
10 mg/ mL
15-25 mg
100
50 mg/ mL 8-15 mg/mL
100 mg 10-20 mg
500 ~g 100-200
Carbenicillin disodium Cefazolin sod ium Ceftazidime Clindamycin Colistimethate sodium Eryth romycin Gentamicin sulfate Imipenem/cilastatin sod ium Kanamycin sulfate Methicillin sod ium Neomycin sulfate Penici llin G Polymyxin B sulfate Ticarcillin disodium Tobramycin sulfate Vancomycin Hel
Intravenous (Adult) 15 mg/kg daily in 2-3 doses 4-12 9 daily in 4 doses
~g
~g ~g ~g
~g
8-24 9 daily in 4-6 doses 2-4 9 daily in 3-4 doses 1 9 daily in 2- 3 doses 900-1800 mg daily in 2 doses 2.5-5.0 mg/kg daily in 2-4 doses 3- 5 mg/kg daily in 2- 3 doses 2 9 daily in 3-4 doses
~g
5 mg/ mL 30-50 mg/mL 50 mg/mL 5-8 mg/mL 100,000 units/mL 10,000 units/mL 6 mg/mL
30 mg 50-100 mg 125-250 mg 0.5-1.0 m ill ion units 100,000 units
8-15 mg/mL
10-20 mg
100-200 ~Ig
20-50 mg/mL
25 mg
1000
~g
1000-2000
&-10 9 daily in 4 doses 12-24 million units daily in 4 doses
100 mg
200-300 mg/kg daily
~g
3-5 mg/kg daily in 2- 3 doses 15- 30 mg/kg daily in 1-2 doses
CHAPTER 17:
Ocular Pharmacotherapeutics. 369
by structural features in th e antibiotic molecule that resemble the portion of the natural substrate with which the transpep tidase reac ts. The peptidase reacts with the antibiotic, forming an inactive acyl intermediate. Some bacteria are resistant to the action of penicillins and cephalosporills. The lipopol ysacc haride outer coat of Illany gram-negative bacteria may prevent certain hydrophilic antibiotics from reaching their cytoplasmic Illembrane site of action. Furthermore, so me bacteria produce ~ - lacta mases (penicillinase), enzymes capable of cleaving th e critical amide bond within these antibiotics. The different penicillins and cephalosporins vary in susceptibility to the ~ - Iactamases produced by different bacterial species. The penicillins and cep hal os porins penetrate the blood- ocular and blood-brain barriers poorl y and are actively transported out of the eye by the organic-acid transport system of the ciliary body. However, their penetration into the eye increases with inflammation and with coadnlinistration of probenecid. Serious and occasionally fatal hyperse nsitivity (a naphylactoid ) reactions can occur in association with penicillin and ce phalosporin therapy. Penicillin hypersensitivity affects 0.7%-4% of all treatment courses and has an overall incidence between 0.7% and 10%. Reactions to drugs in the penicillin fami ly may be the most common drug allergy. Although anaphylaxis is more frequent following parenteral administration, it can occur with oral therapy. Such reactions are more likel y to occur in individuals with a history of sensitivity to multip le allergens. A history of immediate allergic response (anaphylaxis or rapid onset of hives) to any pe nicillin is a strong contraindication to the use of any other penicillin. App rOximatel y 10% of people allergiC to a penicillin will have cross- reacti vi ty to cepha losporins. Kelkar PS, Li IT. Cephalospo rin alle rgy. N Engl J Med. 2001;345(11):804 -809 .
Penicillins There are 5 classes of penicillins, which differ in their spectrum of antibiotic activity and in their resistance to penicillinase: l. Peni cillin G, penicillin V, and phenethicillin are highl y effective against most gram-
positive and gram-negative cocci, many anaerobes, and Listeria, Actinomyces, Leptospiral and Treponema organisms. However) most strains of Staphylococcus aureus and many st rains of S epidermidis. anaerobes. and Neisseria gOllorrhoeae are now resistant, often through production of penicillinase. Resistance by enterococci is often from altered penicillin-binding proteins. Penicillin V and phenethi cillin are absorbed well orally, whereas penicillin G is better given intravenously because it is inactivated by stomach acid. These penicillins are excreted rapidly by the kidneys and have short half-lives unless they are given in depot forms (ie, procaine penicillin G) or administered with probenecid, which competitively inhibits excretion by the kidneys. 2. The penicillinase-resistant penidUins include methicillin sodi um, nafcillil1, oxacillin sod ium, cloxacillin sodium, dicloxacillin sodium, and f1oxacillin. They are less potent than pe ni cillin G against susceptible organisms but are the drugs of choice for infections that are caused by penicillinase-producing S aureus and that
370 • Fundam e ntals a nd Principl es of Oph t ha lmo logy
are not methic illin resistan t. Melhicillin and nafci lJ in are acid -labile an d are therefore give n eit her parenterall y or by subconjun ctival injection. The other agents in th is group have reasonable oral absorp tion. W hen they are given system icall y. coadministrat ion of p robenecid reduces re nal excretion and outward t ransport from th e eye.
3. The broad-spectrum penicillins sli ch as ampici ll in, i:lmox iciUin, and baca mpicil lin Hel have ant ibacterial activi ty tha t extends to such gram -negative organisms as Haemophilus inJluenzae, Escherichia coli, Salmonella and Shigella species, an d Proteus l11irabilis. Resistan t strains of H inj1uenz(le are becom ing mo re com mon. T hese dr ugs are stable in acid and may be given o rally. T hey are not res istant to pe ni ci llin ase or to th e broader-spectr um p-lac tamases that are increasingly commo n amo ng gram -negat ive bacte ri a. 4. Carb eni ci llin and ti ca rcilli n have ant imicrobi al acti vity that exte nds to Pseudomonas and Enterobacter species and indole-positive stra ins of Proteus. T hese d rugs are give n pa renterall y or subconjunctivall y, al though t he indanyl ester of carben icillin may be give n orally. T hey are not resistan t to peni ci lli nase and are less act ive against gram-positive bacteria and Listeria species. 5. Piperacillin sodi um , mezlocillin sodi um, an d az locill in are parti cul arl y pote nt against Pseudomonas and Klebsiella spec ies and retain a stro ng gram -positive coverage and act ivity aga inst Listeria species. They are ad min istered pa renterall y or sub co njunctiva ll y. an d they are no t' resistant to peni cillinase. Cepha/osporins
Bacterial susceptibil ity patterns and resista nce to ~-lac tam ases have determi ned the classifica tion of th e cephalos porins as fi rst-, second-, lhird-, or fo urth-generation, altho ugh fifth - and Sixth -generat ion age nts are u nde r development.
First-generation. Cephalo thin, cefazolin, cephalex in , cefadroxil, and cep hradi ne have strong antim icrobial activity aga inst gram-pos itive o rga nisms. especiall y Streptococci species and S aureus. They re tain moderate act ivity aga inst gram-negative orga nisms. Ce phalothin is the mos t resistant of these age nts to staphylococcal ~ - I actamase an d is used in severe staphylococcal in fect ions. Because cep halothi n is pain ful when give n int ramuscu larl y, it is used o nly intravenously. In con trast, cefazo lin is mo re sensiti ve to ~ - I acta m ase but has so mew hat grea ter activity aga inst Klebsiella species and E coli. Cefazolin has a longe r half-life and is to lerated bo th intramusc ul arly and in travenouslYi th us it is used more frequ entl y th an the other fir st-generatio n cephalospo rins. Cephalexin, cefadroxil, and cephrad ine are stab le in gastr ic acid and ava ilable in ora l for ms. Second-generation. T hese agents were developed to expand the ac tivity againsl gramnegative o rganis ms whiJe retain ing much of th eir gram- positi ve spectrum of activity. Cefam and ole. cefoxitin. and cefu roxime display greater ac ti vity agai nst H il1fluenzae, Enterobacter aerogenes, and Neisseria species. Cefamando le has increased act ivity against Enterobacter and indole-positive Proteus species, H influenzae, and Bacteroides species. Cefoxitin is active agains t indo le- positive Proteus and Serratia o rganisms, as well as aga inst Bacteroides Jragilis. Cefuroxime is va luable in th e treat men t of
CHAPTER 17:
Ocular Pharmacotherapeutics. 371
penicillinase-producing N gonorrhoeae and ampicillin-resistant H inJluenzae. and its penetration of the blood-brain barrier is adequate for initial treatment of suspected pneumococcal, meningococcal, or H inJluenzae meningitis. Third-generation. The third-generation cephalosporins have further enhanced activity against gram-negative bacilli, specifically the ~-Iactamase- producing members of the Enterobacteriaceae family, but they are inferior to first -generation cephalosporins with regard to their activity against gram-positive cocci. Commonly used agents include cefotaxime, cefoperazone sodium, ceftriaxone sodium, ceftazidime, and ceftizoxime sodium. These agents have a similar spectrum of activity against gram-positive and gram-negative organisms; anaerobes; Neisseria, Serratia, and Proteus species; and some Pseudomonas isolates. Cefoperazone and ceftazidime are particularly effective against Pseudomonas but lose more coverage of the grampositive cocci. Cefotaxime penetrates the blood-brain barrier better than the other cephalosporins can, and it presumably also penetrates the blood-ocular barrier. Fourth-generation. Cefepime HCI and cefpirome have a spectrum of gram-negative coverage similar to that of the third-generation cephalosporins, but these agents are more resistant to some p-lactamases.
No cephalosporin provides coverage for enterococci, Listeria and Legionella species, or methicillin-resistant S aureus.
Other Antibacterial Agents See Tables 17-14 and 17-15.
Fluoroquinolones Fluoroquinolones are tluorinated derivatives of nalidixic acid and are available in a variety of chemical structures. The most commonly used ophthalmic agents are ofloxacin, levo floxacin, ciprofloxacin, moxifloxacin, and gatifloxacin. These agents are highly effective, broad-spectrum antimicrobials with potent activity against common gram-positive and gram-negative ocular pathogens. Their mechanism of action targets bacterial DNA supercoiling through the inhibition of DNA gyrase and topoisomerase IV, 2 of the enzymes responsible for replication. genetic recombination, and DNA repair. Mutations in the bacterial genes for these enzymes allow the development of resistance to tluoroquinolone agents. There has been an increased incidence of resistance to these agents, as well as evidence of cross-resistance among agents. Fluoroquinolone resistance has been reported in Mycobacterium che/onae, S aureus, coagulase-negative Staphylococcus species, Pseudomonas aeruginosa, Clostridium diffici/e, Salmonella enterica, E coli, and Helicobacter pylori. Studies in vitro have demonstrated that the fluoroquinolones, espeCially ciprofloxacin and temafloxacin, inhibit 90% of common bacterial corneal pathogens and have a lower minimum inhibitory concentration than the aminoglycosides gentamicin and tobramycin and the cephalosporin cefazolin. They are also less toxic to the corneal epithelium than are the aminoglycosides. Five fluoroquinolones currently available are ofloxacin ophthalmic solution 0.3% (Ocuflox), ciprofloxacin 0.3% (Ciloxan), levofloxacin 0.5% (Quixin), gatifloxacin 0.3%
372 • Fundamentals and Principles of Ophthalmology Table 17-14 Ophthalmic Antibacterial Agents Generic Name Individual Agents Azithromycin
Bacitracin zinc Ch loramphenico l
Trade Name
Solution Strength
AzaSite Ak-Traci n; ava ilable generically Powder availab le for
1% Ointment (500 units/g) 0.5%. ointment (1%)
co mpounding Ciprofloxacin Hel
Erythromycin Gatifloxacin Gentamicin sulfate
l evofl oxac in M ox ifloxaci n Hel Ofloxaci n Sulfacetamide sodi um
Tobramycin sulfate
Ciloxan; avai lable ge ne rica ll y Romycin ; avaitab le generical ly Zymar Ga ramycin Genopti c Gentaso l Gentak Ava ilab le generically Ouixin Vigamox Ocuflox Available g enerica lly Bleph- 10 Av ailable generica lly
Ak-Tob Tobrasol Tobrex Av ailabl e generically
Mixtures Polymyxin B sulfate/bacitracin zinc
Poly myxin 8 su lfate/neomycin su lfate/bacitracin zinc Po lymyxin B su lfate/neomycin su Ifate/g ramicid i n Po lymyxin B su lfate/ oxytetracycline Polymyxin B sul fate/ trimeth oprim sulfate
0.3%. ointmen t (0.3% ) Not available; ointment (0.5%) 0.3% 0.3%; ointment (0.3%) 0.3% 0.3% 0.3%, oi ntm ent (0.3%) 0.3%, ointment (0.3%) 0.5% 0.5% 0.3% 0.3% 10% 10%, ointment (10%) 0.3% 0.3% 0.3%. ointment 10.3%) 0.3%
Ak-Poly-Bac
Not available; ointment (10.000 units/g. 500 units/g) Polycin -B Not avai lable; ointme nt (10,000 units/g, 500 units/gl Available generical ly Not ava ilab le; ointment (10,000 unitS/g, 500 unitS/gl Available gene rically Not available ; oi ntment (10,000 units/g, 3.5 mg/ base, 400 units/g) Neosporin; avai lable generica lly 10.000 units/m L, 1.75 mgt base mL, 0.025 mg/ mL Te rak Not available; oint ment (t o,OOO units/g, EQ 5 mg base/g) Polytrim; available generically 10.000 units/ mL. EO 1 mg base/mL
(Zymar), and moxifloxacin 0.5% (Vigamox). They are used in the treatment of corneal ulcers caused by susceptible strains of S aureus, S epidermidis, Streptococcus pneumoniae, P aeruginosa, Se rratia marcescens (efficacy studied in fewer than 10 infections), and Propionibacterium acnes. They are also indicated for bacterial conjunctivitis due to susceptible strains of S aureus, S epidermidis, S pneumoniae, En terobacter cloacae, H influenzae, P m irabilis. and P aeruginosa.
CHAPTER 17:
Ocular Pharmacotherapeutics . 373
Table 17-15 Combination Ocular Anti-Inflammatory and Antibiotic Agents
---
Generic Name
Trade Name
Preparation and Concentration
Dexa methasone/neomycin su lfa te/ polymyxin B su lfate
Ak-Trol, Poly-Dex, Dexacidin, Maxitro l, ava il ab le generica ll y Maxitrol Available gene rically Tobradex Available generica lly Tobradex Cort isp ori n, ava ilable generically Ak-Spore, Cortisporin, available gene ri ca lly
Susp, 0.1%; EO 3.5 mg base/m L; 10,000 units/m L
Dexa methason e!tobramycin
Hydrocortisone/ neomycin sulfate/ po lymyxin B sulfate Hydrocortisone/neomycin sulfate/ po lymyxin B sulfate/bacitracin zinc Loteprednol etabonate/tobramycin Prednisolone acetate/gentamicin su lfate Neo mycin su lfa te/polymyx in B sulfate/predni so lo ne acetate Predniso lone acetate/ sulfacetamide sodium Prednisolone sodium phosphate/ sulfacetamide sodium
lyle! Pred -G
Pred -G S.O.P. Poly-Pred Blephamide, available generically Blephamid e S.O.P. Vasocidin, avai lab le generically Ak-Cide
Ointm ent, 0.1%; EO 3.5 mg base/g; 10,000 units/g Susp, 0.1%, 0.3% 5usp, 0.1 %, 0.3% Ointment, 0 .1%, 0.3% Susp, 1%; EO 3.5 mg base/ mL; 10,000 units/m L Ointment, 1%; EQ 3.5 mg base/g; 5000 units/g; 400 units/g 5usp, 0.5%, 0.3% 5usp; EO 0.3% base; 1% Oin tm ent; EO 0.3% base; 0.6% 5usp; EO 0.35% base; 10,000 units/mL; 0.5% Susp, 0.2%, 10% Ointment, 0.2%, 10% 50 1n, 0.25%, 10% Ointment, 0.5%, 10%
These fluoroquino lones have a high rate of penetration into ocular tissue. Their sustained tear concentration levels exceed the minimum inhibitory concentrations of key ocular pathogens for up to 12 hours or more after 1 dose. They also deliver excellent susceptibility kill rates, with one in vitro study confirming eradication of 87%-100% of indicated pathogen ic bacteria, including P aeruginosa. In addition, ofloxacin provides patient comfort and prevents precipitates from forming. Ofloxacin has a high intrinsic solubility that enables it to be formulated at a near-neutral 6.4 pH. Ciprofloxacin is formulated at a pH of 4.5, gatifloxacin at a pH of 6.0, and moxifloxacin at a pH of 6.8. The combination of high efficacy and safety has made fluoroquinolones the most prescribed an ti-infectives for the treatment of ocular pathogens. The most frequently reported d rug-related adverse reaction is transient ocular burn ing or discomfort. Other reported reactions include stinging, redness, itching, chemical conjunctivitis/keratitis, periocular/facial edema, foreign -body sensation , photophobia, blurred vision, tearing, dry eye, and eye pain. Although rare, reports of dizziness have also been received. Both norfloxacin and ciprofloxacin have been reported to cause white, crystalline corneal deposi ts of medication, which resolve after d iscontinuation of the drug.
374 • Fundamentals and Principles of Ophthalmology
Su/fonamides Sulfonamides are derivatives of para-aminobenzenesulfonamide. They are structural ana-
logues and competitive antago nists of para-am inobenzoic acid for the bacterial synthesis of folic acid. These drugs affect only the bacteria that must synthesize their own foli c acid. Mammalian cells are not affected, because they are unable to synthesize folic acid. Sulfonamides are bacteriostatic only. They are more effective when administered with trimethoprim, a potent inhibitor of bacteri al dihydrofolate reductase; together, they block successive steps in the synthesis of tetrahydrofolic ac id. Systemic pyrimethamine, sulfadiazine, and folinic acid are used in the treatment of toxoplasmosis. Chlamydia requires a
3-week course of systemic sulfonam ide therapy. Sulfacetamide ophthalmic solution ( 10%-30%) and ointment (10%) penetrate the cornea well but may sensitize the patient to sulfonamide medication. Susceptible organ isms include S pneumoniae, Corynebacterium diphth eria e, H influenzae, A ctinomyces species, and Chlamydia trachomatis. Local irritation, itching, periorbital edema, and transient stinging are some of the common adverse effects from topical administration. As with all sulfonamide preparations. severe sensitivity reactions such as toxic epidermal necrolys is
and Stevens- Johnson syndrome have been reported. The incidence of adverse reactions to all sulfonamides is approximately 5%. Tetracyclines The tetracycline family includes agents produced by Streptomyces species (chlortetracycline, oxytetracycline, demeclocycline), as well as the semisynthetically produced agents tetracycline, doxycycline, and minocycline. Tetracyclines enter bacteria by an active transport across the cytoplasmic membrane. They inhibit protein synthesis by binding to the 30S ribosomal subunit, thus preventing access of aminoacyl tRNA to the acceptor site on the mRNA- ribosome complex. Host cells are less affected because they lack an ac tivetransport system. Doxycycline and minocycline are more lipophilic and thus more active by weight. Doxycycline is the most commonly used tetracycline for ophthalmic conditions, as well as the most commonly used parenteral tetracycli ne in the United States. The tetracyclines may depress prothrombin, thus prolonging the bleeding in patients on anticoagulation medication. As bac teriostatic drugs, tetracyclines may inhibit bactericidal medication such as the penicillins and therefore should not be used concurrently. The use of tetracyclines may decrease the efficacy of oral contraceptives. Patients should be instructed to use an additional form of birth control during administration of tetracyclines and for I month after di scontinuat ion of th eir use. Tetracyclines are broad-spectrum bacteriostatic antibiotics, act ive agai nst many gram-positive and gram-negative bacteria and also against Rickettsia species, Mycoplasma pneumoniae, and Chlamydia species. However, many strains of Klebsiella and H influenzae and nearly all strains of Proteus vulgaris and P aeruginosa are resistant. These agents dem-
onstrate cross-resistance. Tetracycline is poorly water-soluble but is soluble in eyedrops containing mineral oil; it readily penetrates the corneal epithelium . Chlortetracycline has also been used previously in ophthalmic preparation, but neither chlortetracycline nor tetracycline is currently available for ophthalmic use in the United States. Oxytetracycline is available in combination with polymyxin as an ophthalmic ointment.
CHAPTER 17:
Ocular Pharmacotherapeutics. 375
Systemic therapy with the tetracyclines is used for chlamydial infections; because these drugs are excreted into oil glands, they are also used for staphylococcal infections of the meibomian glands. They chelate to calcium in milk and antacids and are best taken on an empty stomach. As tetracyclines may cause gastric irritation, they can be taken with nondair y agents to improve compliance. Tetracyclines should not be given to children or pregnant women, because they can be deposited in growing teeth, causing permanent discoloration of the enamel, and may also deposit in bone and inhibit bone growth. Tetracyclines depress plasma prothrombin activity and thereby potentiate warfarin (Coumadin). They have been implicated as a cause of pseudotumor cerebri, a condition discussed in BCSC Section 5, Neuro- Ophthalmology. The use of tetracyclines also causes photosensitivity; consequently, patients taking tetracycline should avoid extended exposure to sunlight. Degraded or expired tetracyclines may cause renal toxicity, also called Fanconi syndrome.
Chloramphenicol Chloramphenicol, a broad-spectrum bacteriostatic agent, inhibits bacterial protein synthesis by binding reverSibly to the 50S ribosomal subunit, preventing aminoacyl tRNA from binding to the ribosome. Chloramphenicol is effective against most H influenzae, Neisseria m eningitidis, and N gonorrhoeae, as well as all anaerobic bacteria. It has some activity against S pneumoniae, S aureus, Klebsiella pneumoniae, Enterobacter and Serratia species, and P mirabilis. P aeruginosa is resistant. Chloramphenicol penetrates the corneal epithelium well during topical therapy and penetrates the blood-ocular barriers readily when given systemically. However, the use of this agent is limited because it has been implicated in an idiosyncratic and potentially lethal aplastic anemia. Although most cases of this anemia have occurred after oral administration, some have been associated with parenteral and even topical ocular therapy. Chloramphenicol is available as a powder for compounding, but it should not be used if an alternative agent with less potential toxicity is available. Aminoglycosides The aminoglycosides consist of amino sugars in glycosidic linkage. They are bactericidal age nts that are transported across the cell membrane into bacteria, where they bind to the 30S and 50S ribosomal subunits, interfering with initiation of protein synthesiS. The antibacterial spectrum of these agents is determined primarily by the efficiency of their transport into bacterial cells. Such transport is energy-dependent and may be reduced in the an aerobic environment of an abscess. Resistance to aminoglycosides may be caused
by failure of transport, low affl nity for the ribosome, or plasmid-transmitted ability to enzymaticall y inactivate the drug. The coadministration of drugs such as penicillin that alter bacterial cell-wall structure can markedly increase aminoglycoside penetration, resulting in a synergism of antibiotic activity against gram-positive cocci, espeCially enterococci. One such aminoglycoside, amikacin, is remarkably resistant to enzymatic inac tivat ion. Gentamicin, tobramycin , kanamycin. and am ikacin have antibacterial activity against aerobic, gram-negative bacilli such as P mirabilis; P aeruginosa; and Klebsiella, Enterobacter,
376 • Fundamentals and Principles of Ophthalmology and Serratia species. Gentamicin and tobramycin are also active against gram -positive S aureus and S epiderm idis. Kanamycin is generally less effective than the others against gram-negative bacilli. Resistance to gentamicin and tobramyci n has graduall y increased as a result of a plasmid-transmitted ability to synthesize inactivating enzymes. Thus, ami kacin, which is ge nerally impervious to these enzymes, is particularly valuable in treating such resistant organisms. It is effective against tu berculosis, as well as atypical mycobacteria, and can be compounded for topical use against mycobacterial infect ion . Aminoglycosides are not absorbed well orally but are given systemically by intramuscular or intravenous routes. They do not readily penetrate the blood- ocular barrier but may be administered as eyedrops, ointments, or periocular inj ections. Gentamicin and
carbenicillin should not be mixed for IV administration because the carbenici llin inactivates the gentamicin over several hours. Sin1ilar incompatibilities exist in vitro between gentamicin and other penicillins and cephalosporins. Use of streptomycin is now limited to Streptococcus viridans bacterial endocard itis, tularemia, plague, and brucellosis. Neomycin is a broad-spectrum antibiotic, effective against Enterobacte r species, K pneumoniae, H influenzae, N meningitidis, C diphtheriae, and S au reus. It is given topically in ophthalmology and orally as a bowel preparation for surgery. Neomycin is too toxic to be used intravenously but can be given orall y because it is not absorbed from the gut. Topical allergy to ocular use of neomycin occurs in approximately 8% of cases. Neomycin can cause punctate epitheliopathy and retard re-epithelialization of abrasions. All aminoglycosides can cause dose-related vestibular and auditory dysfunction and nephrotoxicity when they are give n systemically. Systemic use of aminoglycosides should be limited to serious infections, and the plasma concentration of drug and blood urea nitrogen should be monitored to avoid overdosing.
Miscellaneous antibiotics Vancomycin is a tricyclic glycopeptide derived from cultures of Nocardia orientalis. It is bactericidal for most gram-positive organisms th ro ugh the inhibition of glycopeptide polyme ri zation in the cell wall. It is useful in the treatment of staphylococcal infections in patients who are allergic to or have not responded to the penicillins and cephalosporins. It can also be used in combination with ami noglycosides to treat S viridans or Streptococcus bovis endocarditis. Oral vancomycin is poorly absorbed but is effective in the treatment of pseudomembranous colitis that is caused by C difficile. Vancomycin resistance has increased in isolates of Enterococcus and Staphylococcus, and antibiotic resistance is
transmitted between pathogens by a conjugative plasmid. Vancomycin may be used topically or intraocularly to treat Sight- threatening infectio ns of the eye, including infectious keratitis and endophthalmitis caused by methicilIinresistant staphylococci or streptococci. It has been used within the irrigating fluid of balanced salt solution during intraocular surgery. There is controversy concerning the
contribution of this prophylactic use of vancomycin to the emergence of resistant bacteria, as well as to an increased risk of postoperative eME. Vancomycin is a preferred substitute for a cephalosporin used in combination wit h an aminoglycoside in the empirical treatment of endophthalmitis. See BCSC Section 8, External Disease and Cornea, and Section 9, Intraocu lar Inflammation and Uveitis, for further discussion.
CHAPTER 17:
Ocular Pharmacotherapeutics.
377
The IV dosage of vancomycin in adults with normal renal function is 500 mg every 6 hours or I g every 12 hours. Dosing must be adjusted in subjects who have renal impairment. Topical vancomycin may be compounded and given in a concentration of 50 mg/ mL in the treatment of infectious keratiti s. Intravitreal vancomycin combined with an aminoglycoside may be used for initial empirical therapy for exogenous bacterial en dophthalmitis. A dose of I mg/O.I ml establishes intraocular levels significantly higher than the minimum inhibitory concentration for most gram-positive organisms. Unlike systemic treatment, topical va ncomycin and intraocular vancomycin have not been associated with ototoxicity or nephro toxicity. Hourly use of 50 mg of vancomycin per milliliter delivers a dose of 36 mg/day. which is well below the recommended systemic dose. In addition to the ototoxicity and nephrotoxicity associated with systemic therapy. chills. rash. fever. and anaphylaxis are other possible complications. Further. rapid IV in fu sion may cause "red -man syndrome" due to flushing. Erythromycin is a macrolide (many-membered lactone ring attached to deoxy sugars) antibiotic that binds to the 50S subunit of bacterial ribosomes and interferes with protein synthesis. It is bacteriostatic against gram-positive cocci such as S pyogenes and S pneumoniae. gram-positive bacilli such as C diphtheriae and Listeria monoeytogenes. and a few gram -negative organisms such as N gonorrhoeae. It may be bactericidal. in sufficient dosing. against susceptible organisms. Drug resistance is rising and is as high as 40% among streptococcus isolates. There are 4 mechanisms of resistance: I. 2. 3. 4.
esterases from Enterobacteriaceae mutations that alter the 50S ribosome enzyme modification of the ribosomal binding site active pumping to extrude the drug
Macrolide antibiotics such as erythromycin are the treatment of choice for Legionella pneumophila. the agent of legionnaires' disease. as well as for M pneumoniae. Erythromycin is administered orally as enteric-coated tablets or in esterified forms to avoid inactivation by stomach acid. It can also be administered parenterally or topically as an ophthalmic ointment. The drug penetrates the blood-ocula r and blood- brain barriers poorl y. Clarithromycin and aZithromycin are semisynthetic macrolides with spectrums of ac tivity similar to that of erythromycin. Clarithromycin is more effective against staphylococci. streptococci. and M leprae and azith ro mycin is more active against H inj7uenzae. N gonorrhoeae. and Chlamydia species. Both agents have enhanced activity against Mycobacterium avium-il1tracellulare, atypica l mycobacteria. and Toxoplasma go ndii. Azithromycin 1% (AzaSite) was recently FDA approved for bacterial conjunctivitis caused by CDC coryneform group G. H influenzae. S aureus. S mitis group. and S pneumoniae. Polymyxin B sulfate is a mixture of basic peptides that function as cationic detergents to dissolve phospholipids of bacterial cell membranes. thus disrupting cells. It is used topicall y or by local injection to treat corneal ulcers. Gram-negative bacteria are susceptible. including Enterobacter and Klebsiella species and P aeruginosa; bacterial sensitivity is related to phospholipid content of the cell membrane. and resistance may occur if a cell wall prevents access to the pathogen cell membrane. Topical hypersensitivity is
378 • Fundamentals and Principles of Ophthalmology an uncommon occurrence. Systemic use of this medication was abandoned due to severe
nephrotoxicity. Bacitracin is a mixture of polypeptides that inhibit bacterial cell -wall synthesis. It is active against Neisseria and Actinomyces species and H infJuenzae and most gram-positive bacilli and cocci. It is available as an ophthalmic ointment either alone or in va rious combinations with polymyxin, neomycin, and hydrocortisone. The primary side effect is local hypersensitivity, although this is not commonly seen. Topical povidone-iodine solution (Betadine 5%) exhibits broad-spectrum antimicrobial activity when used to prepare the surgical fi eld and to rinse the ocular surface and has FDA approval for this purpose. It is the only agent that has been shown to have a Significant effect on postsurgical endophthalmitis. It is contraindicated in patients with hype rsensitivity to iodine or to intravenous contrast dye. Povidone-iodine scrub may be used periocularly, but it is contraindicated in the eye because it is damagi ng to the corneal epithelium. Castillo A, Benitez del Castillo ]M, Toledano N, Diaz -Valle 0, Sayaq ues 0, Garcia-Sanchez J. Deposits of topical no rfloxacin in the treatment of bacte rial keratitis. Cornea. 1997; 16(4): 420-423.
Ciulla TA, Starr MB, Masket S. Bac terial end ophthalmil is prophylaxis for cataract surgery: an eviden ce· based update. Ophthalmology. 2002; 109(1 ): 13-24. Gall C. Balmer p, Schwab F. Ruden H. Eckmanns T. Different trends of MRSA and VRE in a German hospital, 1999 -2005. Infection. 2007;35( 4):245-249. Han DP, Wisniewski SR, Wilson LA, et al. Spectrum and susceptibilities of microbiologic isolates in the Endo phthalmitis Vitrectomy Study. Am J Ophthalmol. 1996; 122(1): 1- 17. (Erratum appea rs in Am JOphtha/mol. 1996; 122(6):920. J Kollef MH . Limitations of vancomycin in th e management of resistant staphylococcal infections. elin Infect Dis. 2007;45(suppI3):S 19 1- S195. Li esegang TJ. Use of antimicrobial s to prevent postoperative infection in patients with cataracts. Curr Opin Oph thalmol. 2001;12(1 ):68 - 74. Werner G, Klare I, Fleige C, Witte W. Increasing rates of vancomycin res istance am ong Enterococcus faecium isolated from Germ an hospitals between 2004 and 2006 are due to wide clonal di sseminati on of van comycin -resistant enterococci and horizontal spread of vanA clusters.lnt J Med Microbial. 2008;298(5-6):5 t 5- 527.
Antifungal Agents See Table 17-16. Po/yenes The polyene antibiotics are named for a component sequence of 4 to 7 conjugated double bonds. That lipophilic region allows them to bind to sterols in the cell membrane of susceptible fungi , an interaction that results in damage to the membrane and leakage of essential nutrients. Other antifungals (such as flucytosine and the imidazoles) and even other antibiotics (such as tetracycline and rifampin) can enter through the damaged membrane, yielding synergistic effects. Natamycin and amphotericin Bare 2 examples of polye ne macrolide antibiotics. Natamycin is available as a 5% suspension for topical ophthalmic use (once an hour) .
CHAPTER 17:
Ocular Pharmacotherapeutics. 379
Table 17-16 Antifungal Agents
Generic (Trade) Name Polyenes Amphotericin B (Fungizone; available generically)
Route
Dosage
Topical
0.1%-0.5% solution; dilute with water for injection or dextrose 5% in water
Subconjunctival Intravitreal IV
0.8-1.0 mg 5 ~g Because of possible side effects and toxicity, dose needs to be carefully adjusted. 5% suspension
Natamycin (Natacyn)
Topical
Imidazoles Ketoconazole (Nizoral; available generically)
Oral
200 mg daily, up to 400 mg for severe or incomplete response
Miconazole nitrate (available as powder for compounding)
Topical Subconjunctival Intravitreal
1% solution 5 mg 10 ~g
Triazoles Fluconazole (Diflucan)
Oral
200 mg daily
Oral IV
200 mg daily
Oral
50- 150 mg/kg daily divided every 6 h 1% solution
Itraconazole (Sporanox)
Fluorinated Pyrimidine Flucytosine (Ancobon)
Topical
Indication! (additional reports of use)
Aspergillus Candida Cryptococcus (Blastomyces) (Coccidioides) (Colletotrichum) (Histoplasma)
Fusarium (Aspergillus) (Candida) (Cephalosporium) (Curvularia) (Penicillium) Blastomyces Candida Coccidioides Histoplasma Candida Cryptococcus Aspergillus Candida Cryptococcus (Acremonium) Aspergillus Blastomyces Histoplasma (Candida) (Curvularia) (nonsevere Fusarium) Candida Cryptococcus (Aspergillus)
Local hypersensitivity reactions of the conjunctiva and eyelid and corneal epithelial toxicity may occur. Amphotericin B may be reconstituted at 0.25%- 0.5% in sterile water (with deoxycholate to improve solubility) for topical use (every 30 minutes). It may also be administered systemically for disseminated disease, although careful monitoring for renal and other toxicities is required. Both of these agents penetrate the cornea poorly. They have been used topically against a variety of filamentous fungi, including species of
380 • Fundame ntals and Princ iples of Ophthalmology Aspergillus, Cephalosporium, Curvularia, Fusarium, and Penicillium, as well as the yeast Candida albicans. Systemic amphotericin B has been reported as useful in the treatment of systemic Aspergillus, Blastomyces, Candida, Coccidioides, Cryptococcus, and Histoplasma infections.
Imidazoles and triazoles The imidazole- and triazole-derived antifungal agents also increase fungal cell-membrane permeability and interrupt membrane-bound enzyme systems. The triazoles have less effect on human sterol synthesis, as well as a longe r half-life, than the imidazoles, and they are being more actively developed. The imidazole miconazole is available in a 1% solution that may be injected subconjunctivally (5 mg/0.5 OIL, once or twice daily) or applied topically. Miconazole penetrates the cornea poorly. Ketoconazole is available in 200-mg tablets for oral therapy (once or twice daily). Ketoconazole normally penetrates the blood- brain barrier and, presumably, the bloodocular barrier poorly, but therapeutic levels can be achieved in inflamed eyes. The triazole itraconazole, with an expanded antifungal spectrum and less systemic toxicity, has largely replaced ketoconazole. However, there is an extensive and growing list of potentially dan gerous drug interactions with itraconazole, and this list should be consulted prior to instituting systemic therapy. Fluconazole, anothe r triazole, may also increase the plasma concentrations of other medications. The imidazole and triazole antifungals acl against various species of Aspergillus, Coccidioides, Cryptococcus, and Candida. Flucytosine F1ucytosine (5-fluorocytosine) is converted by some species offungal cells to 5-fluorouracil by cytosine deaminase, and then to 5- fluorodeoxyuridylate. This last compound inhibits th ymidylate synthase, an important enzyme in DNA synthesis. Host cells lack cytosine deaminase activity and are less affected. Only fungi that have both a permease to facilitate flucytosine penetration and a cytosine deaminase are sensitive to flu cytos ine. Flu cytosine
is taken orally at 50- 150 mg/kg daily, divided every 6 hours. Although the drug is well absorbed and penetrates the blood- ocular barrier well, the majority of Aspergillus and half of Ca ndida isolates are resistant. F1ucytosine is used primarily as an adju nct to systemic amphote ricin B therapy.
Antiviral Agents See Table 17- 17.
Topical antiviral agents Three agents that compete with natural nucleotides for incorporation into viral and mammalian DNA have been used to treat herpes Simplex virus (HSV) keratitis. ldoxuridine (5- iodo-2' -deoxyuridine) and trifluridine (Viroptic) are structural analogues of thymidine and work in a similar man ner; vidarabine is an analogue of adenine. Trifluridine (l % drops, every 2-4 hours) is more soluble than the other agents and can be used in drop form , providing adequate penetration of diseased corneas to treat herpetic iritis. Trifluridine is currently marketed in the United States; idoxuridine and vidarabine powder are available for compounding. Vidarabine can be used if an agent with a different mechanism of action is reqUired. Cross-resistance does not seem to occur among these agents.
Table 17-17 Antiviral Agents Generic Name
Trade Name
Topical Concentration! Ophthalmic Solution
Trifluridine
Viroptic; available
1%
Idoxuridine
Vidarabine monohydrate Acyclovir sodium *
generically Avai lable as powder for compou nding Avai lable as powder for compounding Zovirax; available
0.1%
3% (ophth ointment)
generically
Azidothymidine (AZf)/zidovudine
Retrovir; available generical ly
Cidofovir*t
Vistide
Fa m vi r HZV
Foscarnet sodium
Foscavi r; avai lable generica ll y
Ganciclovir
Vitrasert
Ganciclovir sodium ... t
Cytovene IV
Va lacyc lovir HCI*t
Va ltrex HZV
Systemi c Dosage
Oral: Herpes simplex keratitis (HSV) 200 mg 5 times daily fo r 7- 10 days Oral: Herpes zoster ophtha lmicus (HZV) 600-800 mg 5 times daily for 10 days; IV if patient is immunocompromised Dosage variable per source consulted; dosing per internal medicine consultation recomme nded IV induction: 5 mg/kg constant infusion over 1 h once weekly for 2 consecutive weeks Maintenance: 5 mg!kg constant infusion over 1 h administered every 2 weeks 500 mg 3 times dail y for 7 days IV induction: By controlled infusion on ly, either by central ve in or by pe ripheral vein ind uction: 60 m g/kg (adjusted for rena l function) given over 1 h every 8 h for 14-21 days Maintenance: 90--120 mg/kg given over 2 h once daily Intravitreal: 4.5 mg steri le intravitreal insert designed to release the drug over a 5-8 mo period IV induction: 5 mg!kg every 12 h for 14-2 1 days Maintenance: 5 mg/kg daily for 7 days , 9 3 times daily fo r 7-1 4 days
* Dose adjustment is recommended for geriatric and renal patients or with concomitant nephrotoxic medications. t Because of potentia l side and toxic effects with systemic dosage, the possible dosage adjustments and
warnings should be followed properly.
t- At high doses, valacyclovir has been associated with thrombotic thrombocytopenic purpura/hemolyt ic uremic syndro m e (TIP/HUS) in immunocompromised patients.
382 • Fundamentals and Principles of Ophthalmology
Systemic antiviral agents Acyclovir is a synthetic guanosine analogue that requires phosphorylation to become active. It undergoes monophosphorylation by viral thymidine kinase. Because the viral thymidine kinase in HSV types I and 2 has many times more affinity to acyclovir than does host thymidine kinase, high concentrations of acyclovir monophosphate accumulate in infected cells. Acyclovir monophosphate is then further phosphorylated to the active compound acyclovir triphosphate. The triphosphate cannot cross cell membranes and accumulates further. This increased concentration of acyclovir triphosphate is 50- 100 times greater in infected cells than in uninfected cells. Acyclovir triphosphate inhibits virus growth in 3 ways: I. It can function as a competitive inhibitor of DNA polymerases, with viral DNA polymerases being significantly more susceptible to acyclovir triphosphate than are human DNA polymerases. 2. It can be a DNA chain terminator. 3. It can produce irreversible binding between viral DNA polymerase and the interrupted chain, causing permanent inactivation.
The result is a several-hundred-fold inhibition of HSV growth, with minimal toxicity to uninfected cells. Acyclovir-resistant thymidine kinase HSVs have evolved. They occur primarily in patients receiving multiple courses of therapy or in patients with AIDS. Thymidine kinase mutants are susceptible to vidarabine and foscamet. Changes in viral DNA polymerase
structures can also mediate resistance to acyclovir. Acyclovir can be used topically (this product has been discontinued in the United States), orally, or intravenously. Oral acyclovir is only 15%-30% bioavailable, and food does not affect absorption. For unknown reasons, bioavailability is lower in patients with transplants. Acyclovir is minimally protein-bound (10%- 30%), and drug interactions through binding displacement have not been reported. The drug is well distributed, with cerebrospinal fluid (CSF) and brain concentrations equaling approximately 50% of serum values. Concentrations of acyclovir in zoster vesicle fluid are equivalent to those in plasma. Aqueous humor concentrations are 35% that of plasma; and salivary concentrations, 15%. Vaginal concentrations are equivalent to those of plasma, and breast-milk concentrations exceed them. The percutaneous absorption of topical acyclovir is low and occurs primarily when large areas are treated. Plasma concentrations 0[0.3 Jlg/mL were noted in patients treated topically with this drug for herpes zoster (HZV). Peak serum concentrations after oral ingestion of acyclovir average 0.6 mg/mL and occur 90 minutes after dosing, but peak serum concentrations after IV administration reach approXimately 10 mg/mL. The plasma half-life for normal adults and neonates is 3.3 and 3.8 hours, respectively. It increases to 20 hours in anuric patients. In the urine, 60%-90% of acyclovir is excreted unchanged through both glomerular filtration and tubular secretion. As a result, acyclovir may interfere with the renal excretion of drugs that are eliminated through the renal tubules (eg, methotrexate); probenecid significantly decreases the renal excretion of acyclOVir. A major metabolite of acyclOVir, carboxymethoxymethylguanine, accounts for
CHAPTER 17: Ocular Pharmacotherapeutics. 383
10%- 20% of the total administered dose and is excreted in urine. Acyclovir is effectively removed by hemodialysis (60%) but only minimally removed by peritoneal dialysis. A commonly used intravenous dosage for acycloVir is 1500 mg/m'/day. Acyclovir is off-label for HSV and HZV ophthalmicus but has proven to be effective in preventing the recurrence of HSV epithelial and stromal keratitis in oral doses of 400 mg twice a day. Although this prophylactic dosage was originally studied over a I-year treatment period. clinicians are now using this dosage indefmitely to decrease the likeli hood of disease recurrence. Similar dosing of acyclOVir has proven beneficial in reducing the likelihood of recurrent herpetic eye disease after corneal transplantation. However. oral acyclovir was not found to be of benefit when used with topical steroids and trifluridine in the treatment of active HSV stromal keratitis. The addition of oral acyclovir to a regimen of topical antiviral agents may be considered in patients with HSV iridocyclitis. Although the benefit of this drug did not reach statistical Significance. study enrollment was halted due to inadequate numbers of patients. Although well tolerated in oral form. parenteral acyclovir can cause renal toxicity due to crystalline nephropathy. Neurotoxicity may also occur with intravenous use. Valacyclovir is currently approved for management of HZV infections in immunocompetent persons but not for HSV. It is an amino acid ester prodrug of acyclovir; its bioavailability is much higher than that of acyclOVir (54% vs 20%). The recommended dosage is I g 3 times a day for 7-14 days. Valacyclovir has been associated with nephrotoxicity and thrombocytopenia in immunocompromised patients. Famciclovir is the prod rug of penciclovir and is currently approved for the management of uncomplicated acute HZV. It has demonstrated efficacy in relieving acute zoster signs and symptoms and reducing the duration of postherpetic neuralgia when administered during acute zoster. The recommended dosage for the management of acute HZV is 500 mg 3 times a day for 7 days. Ganciclovir (9-2-hydroxypropoxymethylguanine) is a synthetic guanosine analogue active against many herpesviruses. It is approved for cytomegalovirus (CMV) retinitis and for CMV prophylaxis in advanced HIV and in transplant patients. As with acyclovir. it must be phosphorylated to become active. Infection-induced kinases. viral thymidine kinase, or deoxyguanosine kinase of various herpesviruses can catalyze this reaction. After monophosphorylation. cellular enzymes convert ganciclovir to the triphosphorylated form . and the triphosphate inhibits viral DNA polymerase rather than cellular DNA polymerase. Ganciclovir triphosphate competitively inhibits the incorporation of guanosine triphosphate into DNA. Because of its toxicity and the availability of acyclOVir for treatment of many herpesvirus infections, its use is currently restricted to treatment of CMV retinitis, predominantly as an intraocular implant. SystemiC ganciclovir is used primarily intravenously. because less than 5% of an oral dose is absorbed. The intravenous induction dose is 5 mg/kg every 12 hours for 14-21 days. Once the infection is under control. a daily dose of 6 mg/kg is required for maintenance of the virus-free state. CSF concentrations are approximately 50% those of plasma. with peak plasma concentrations reaching 4- 6 sLg/mL. The plasma half-life is 3- 4 hours in people with normal renal function, increasing to over 24 hours in patients with severe renal insufficiency. Over 90% of systemic ganciclovir is eliminated unchanged in urine,
384 • Fundamentals and Principles of Ophthalmology and dose modifications are necessary for individuals with compromised renal function. Ganciclovir is approximately 50% removed by hemodialysis. Bone marrow suppression is
the primary side effect of systemic therapy. Oral ganciclovir may be used in the suppression of CMV retinitis after initial control is obtained with parenteral therapy. Ganciclovir can be administered intravitreally or as a sustained-release intraocular device (Vitrasert).
Because of the long duration of remission and the low side-effect profile, the ganciclovir implant has become the preferred treatment for CMV retinitis, specifically in isolated ocular infection.
Foscarnet (phosphonoformic acid) inhibits DNA polymerases, RNA polymerases, and reverse transcriptases. In vitro, it is active against herpesviruses, influenza virus,
and HIV Foscarnet is approved for the treatment of AIDS patients with CMV retinitis and for acyclovir-resistant mucocutaneous HSV infections in immunocompromised patients. It acts by blocking the pyrophosphate receptor site of CMV DNA polymerase. Viral resistance is attributable to structural alterations in this enzyme. Foscarnet inhibits herpesviruses and cytomegaloviruses that are resistant to acyclovir and ganciclovir. It is
administered intravenously in doses adjusted for renal function and with hydration to establish sufficient diuresis. Foscarnet bioavailability is approximately 20%. Because it can bind with calcium and other divalent cations, foscarnet becomes deposited in bone and may be detectable for many months. Distribution follows a 3-compartment model and produces peak serum concentrations of approximately 30 pg/mL. It is eliminated by both glomerular filtration and tubular secretion, with 80%-90% of the administered dose appearing unchanged in the urine. Dosage adjustment is required in persons with impaired renal function. Treat-
ment may be limited by nephrotoxicity in up to 50% of patients; other side effects include hypocalcemia and neurotoxicity. Cidofovir (Vistide, formerly known as HPMPC) is a third agent approved for the treatment of CMV retinitis, and it is approved only for that use. Cidofovir is a cytidine nucleoside analogue active against herpesviruses, poxviruses, polyomaviruses, papillomaviruses, and adenoviruses. The mechanism of action is inhibition of DNA synthesis, and
resistance is through mutations in DNA polymerase. The prolonged intracellular half-life of an active metabolite allows once-weekly dosing during induction, with dosing every 2 weeks thereafter. The primary side effect is renal toxicity, which can be decreased by IV prehydration and by both pretreatment and posttreatment with high-dose probenecid. Ocular side effects include uveitis and hypotony. Cidofovir does not have direct crossresistance with acyclovir, ganciclovir, or foscarnet, although some virus isolates may have multiple resistances and even develop triple resistance. Cidofovir was shown to inhibit
CMV replication when administered intravitreally in a small series of patients. Dosage administered was 20 ~g/O.l mL. Long-lasting suppression of CMV retinitis was noted, with an average time to progression of 55 days. Cidofovir is the second-line therapy for complications after smallpox vaccination (vaccinia virus) and has been used in selected studies for varicella-zoster retinitis, as well as adenoviral keratoconjunctivitis.
Zidovudine (aZidothymidine [AZTJ) is a thymidine nucleoside analogue with activity against HIV. Zidovudine becomes phosphorylated to monophosphate, diphosphate, and
CHAPTER 17:
Ocular Pharmacothe ra peutics . 385
triphosphate forms by cellular kinases in infected and un infected cells. It has 2 primary methods of action : I. The triphosphate acts as a competitive inhi bitor of viral reverse transcriptase. 2. The azido group prevents further chain elongation and acts as a DNA chai n terminator.
Zidovudine inhibits HIV reve rse transcriptase at much lower concentrations than those needed to inhibit cellular DNA poly me rases. It is currently indicated as treatment for some stages of HI V infection . Zidovudine is administered orally in doses of 1500 mgtday and is approximatel y 60% bioavai lable, with 40% metabolized by first pass. Peak concentrations occur within 30- 90 minutes to give steady-state peak concentrations of 0.05-\.5 fgt mL. Zidovudine is about 40% protein-bound. eSF concentrations va ry widely and range fro m 25% to 100% of serum values. Zidovud ine enters the brain, phagocytic cells, liver, muscle, and placen ta. Plasma half-life is app roximately \ hour. Intravenous dosage is 1- 2 mgt kg 4 times a day. Fluorouracil is a fluorinated pyrimidine nucleoside analogue that blocks production ofthymidylate and interrupts no rmal cell ular DNA and RNA synthesis. Its primary action may be to cause cellular thymine deficiency and resu ltant cell death. The effect of flu orouracil is most pronounced on rapidl y growing cells, and its use as an antiviral agent is primarily related to destruction of infected cells (warts) by topical application. Acyclovi r for Ihe prevenlio n of recurrent herpes Simplex virus eye disease. Herpetic Eye Di s· ease Study Group. N Ellg! J Med. 1998;339(5): 300- 306. Oral acyclovir for herpes simplex virus eye disease: effect o n prevention of epithelial keratiti s and stromal kerati ti s. Herpetic Eye Disease Study Group. Arch Ophthalmo/, 2000; 11 8(8):
1030- 1036.
Medications for Acanthamoeba Infections Acallthamoeba is a genus of ubiquitous, free-living amebae that inhabit soil, water, and air. Their appearance as corn eal pathoge ns has increased due to a number of factors, including the increased use of contact lenses. The species responsible for corneal infections, which include A polyphaga, A castellan ii, A hatchelli, and A culbertsolli, exist as both trophozoites and double-walled cysts. Because of the va riations among species of Acanthamoeba, no Single drug is effective in treating all Acanthamoeba keratitis. Polyhexamethylene biguanide (0.02% solution) is a non- fDA-approved disinfectant and the firstline agent with lowest minimal amebicidal concentration. Medications that are effect ive include chlorhexidine, neomycin, polymyxin B- neomycin-gramicidin mixtures, nata mycin 5% topical suspens ion, imidazoles such as miconazole (powder compounded to 1% topical solution), systemic imidazoles and tri azoles, propamidine isethionate 0. \ % drops (Brolene; not approved in the United States), and topical dibromopropamidine 0. 15% ointment (not approved in the United States). See BeSe Section 8, External Disease and Cornea, for updated treatment recommendations.
386 • Fundam e ntals and Principles of Ophtha lmology Kumar R, Lloyd D. Recent advances in the treatment of Acanthamoeba keratitis. eli" Infect Dis. 2oo2;35( 4):434 - 44 1. O'Day OM. Head WS. Advances in the management of keratomycosis and Acanthamoeba keratiti s. Com ea. 2000;19(5):681-687.
Seal DV. Acanthamoeba keratitis update- incid ence. molecular epidemiology and new drugs
for treatment. Eye.
2003;17(8 ):893- 905.
Sudesh 5, Laibson PRoThe impact of the herpetic eye di sease studies on the management of herpes simplex virus ocular infections. Curr Op;" Opllthalmol. 1999; 10(4 }:230-233. van Rooij I, Rijn eve1d WI, Remeijer L, et aL Effect of oral acyclovir after penetrating keratoplasty for herpetic keratitis: a placebo-controlled multicente r tria l. Ophthalmology. 2003;11 0(10): 19 16- 1919.
Local Anesthetics Local anesthetic age nts are used extensively in ophthalmology. Topical preparations yield corneal and co njunctival anesthesia for comfortable performance of examination tech niques such as tonometry, gonioscopy, removal of superficial foreign bodies, corneal scraping for bacteriologic studies, and paracentesis, as well as for use of contact lenses associated with fundus examination and laser procedures. Topical and intraca meral anesthesia has gained increasing acceptance in cataract, pterygium , and glaucoma surgery. Local retrobulbar and eyelid blocks yield excellent anesthesia and aki nesia for intraocular and orbital surgery (Tables 17-1S, 17- 19). The local anesthetic agents used in ophthalmology are tertiary amines linked byeither ester o r amide bonds to an aromatic residue. Because the protonated form is far more soluble and these compounds undergo hydrolysiS more slowly in acidic solutions, local anesthetic agents are supplied in the form of their hydrochloride salts. When exposed to tissue fluids at pH 7.4, approximately 5%-20% of the anesthetic agent molecules will be in the unprotonated form, as determined by the pK, (S.0- 9.0) of the individual agent. The more lipid-soluble unprotonated form penetrates the lipid-rich myelin sheath and cell membrane ofaxons. O nce inside, most of the molecules are agai n protonated. The protonated form gains access to and blocks the sodium channels on the inner wall of the cell membrane and increases the threshold for elect rical excitability. As increasing numbers of sodium channels are blocked, nerve conduction is impeded and finally blocked. After administ ration of a local anesthetic, nerve fibers that are small or unmyelinated are blocked most qUickly because their higher discharge rates open sodium channel gates more frequently and because conduction can be prevented by the disruption of a shorter length of axo n. The action potential in unmyelinated fibers spreads continuously along the axo n; in myelinated fibers, the action potential spreads by saltation. Therefore, only a short length of an unmyelinated fiber need be functionally interrupted, whereas I or more nodes musl be blocked in a myelinated fiber. In larger myelinated fibers, the nodes are farther apart. Clinically, local anesthetics first block the poorl y myelinated and narrOW parasympathetic fibers (as evidenced by pupil dilation) and sympathetic fibers (vasodilation), followed by sensory fibers (pain and temperature), and fina lly the larger and more
CHAPTER 17: Ocu la r Pharmacotherapeutics . 387
Table 17-1a Regional Anesthetics Generic (Tradel
Concentration (%l Maximum Dose
Onset of Acti on
Durati on of Action
Bupivacaine* (Sensorcaine, Marcaine)
0.25%-0 .75%
5- 11 min
480-720 min (w ith epinephrine)
Lidocaine* (Xylocaine, Anestacaine) Mepi vaca ine* (Ca rbocaine)
0.5%-2% (500 mgl
4- 6 min
2% (500 mg)
3- 5 min
40- 60 min; 120 min (with epinephrine) 120 min
Proca inet (Novocain)
1%- 2% (500 mg)
7-8 min
It
30-45 min; 60 min (with epinephrine)
Major Advantages/ Disadvantages Long duration of action/increased toxicity to the extraocu lar muscles Spreads readi ly without hyaluronidase Duration of action greater without epinephrine Short duration; poor absorption from mucous membranes
Amide-type compound
t Ester-type co mpound
Table 17-1 9 Topical Anesthet ic Agent s Generic Name Cocaine Fluoresce in sodium/ benoxinate
Fluo resce in sodiu m/proparacaine Lidocain e Propara ca ine
Tetracaine
Trade Name Fluress Flurox Avai lable generically Fluoracaine Flucaine Topi ca l solution Viscous gel Alcaine Parcaine Ophthetic Availabl e generically Alta ca ine Tetravisc Available generically
Strength
1%-4% 0.25%; 0.4%
0.25%; 0.5% 4% 2% 0.5% 0.5% 0.5% 0.5% 0.5% 0.5% 0.5%
myelinated motor fibers (akinesia). The optic nerve. enclosed in a meningeal lining. is often not blocked by retrobulbar injections. Amide local anesthetics are preferred to ester agents for retrobulbar blocks because the amides have a longer duration of action and less systemic toxicity. However. this duration of action is limited by diffusion from the site of injection because amide agents are not metabolized locally but are metabolized and inactivated in the liver. primarily by dealkylation. Ester agents are susceptible to hydrolysis by serum cholinesterases in ocular vessels as well as by metabolism in the liver. Toxicity of ester anesthetics may occur at lower doses
388 • Fundamentals and Principles of Ophthalmology when serum cholinesterase levels are low because of treatment with echothiophate eyedrops or a hereditary serum cholinesterase deficiency. The toxic manifestations of local anesthetics are generall y related to dose. However. patients with severe hepatic insufficiency may have symptoms of toxicity wi th either amide or ester local anesthetics, even at lowe r doses. These 1l1anifestations include restlessness and tremor that may proceed to convulsions. and respiratory and myocardial depression. Central nervous system stimulation can be counteracted by IV diazepam; respiratory depression calls for ventilatory support. Because local anesthetics block sympathetic vascular tone and dilate vessels. a 1:200.000 concentration of epinephrine is frequently added to shorter-acting agents to retard vascular absorption. Such use of epinephrine raises circulating catecholamine levels and may result in systemic hypertension and cardiac arrhythmias. Topically applied anesthetics disrupt intercellular tight junctions. resulting in increased corneal epithelial permeability to subsequently administered agents (ie. dilating drops). They also interfere with corneal epithelial metabolism and repair and thus cannot be used for chronic pain relief. Because topical anesthetics can become drugs of abuse that can eventually lead to chronic pain syndromes and vision loss. they should not be dispensed to patients. Lidocaine (Xylocaine) is an amide local anesthetic used in strengths of 0.5%. 1%. and 2% (with or without epinephrine) for injection and 2%-4% for topical mucosal anesthesia. although it is off-label for topical cataract surgery. It yields a rapid (5-minute) retrobulbar or eyelid block that lasts 1-2 hours. The topical solution. applied to the conjunctiva with a cotton swab for 1-2 minutes. reduces the discomfort of subconjunctival injections. Topical lidocaine is preferable to cocaine or proparacaine as the agent for conjunctival biopsy because it has less effect on epithelial morphology. Lidocaine is also extremely useful for suppressing cough during ocular surgery. The maximum safe dose of the 2% solution for local injection is 15 mL in adults. A common side effect is drowsiness. Mepivacaine (Carbocaine) is an amide agent used in strengths of 1%-3% (with or without a vasoconstrictor). It has a rapid onset and lasts about 2- 3 hours. The maximum safe dose is 25 mL of a 2% solution. Bupivacaine (Marcaine) is an amide agent that has a slower onset of action than does lidocaine. It may yield relatively poor akinesia but has the advantage of long duration of action. up to 8 hours. It is available in 0.25%-0.75% solutions (with or without epinephrine) and is frequently administered in a mi xture with lidocaine or mepivacaine to achieve
a rapid. complete. and long-lasting effect. The maximum safe dose is 25 mL of a 0.75% solution.
The following agents are commonly used for topical anesthesia of the ocular surface. Because of their higher lipid solubilities. these agents have a more rapid onset than other agents; thus the initial discomfort caused by the drops is shortened. Proparacaine (eg. Ophthaine. Ophthetic) is an ester topical anesthetic available as a 0.5% solution. The least irritating of the topical anesthetics. it has a rapid onset of approximately 15 seconds and lasts about 20 minutes. Used without a preservative. proparacaine reportedly does not inhibit the growth of Staphylococcus. Ca ndida. or Pseudomonas and thus might be preferred to other agents for corneal anesthesia prior to obtaining a scraping for culture from a
CHAPTER 17: Ocu lar Pharmacot herapeutics. 389
corneal ulcer. Its structure is different enough from the other local anesthetics that crosssensit ization apparently does not occur. Benoxinate is an ester topical anesthetic available in a 0.4% solution with flu orescein (Fluress) for use in tonomet ry. [t has an onset and duration similar to that of proparacaine. Tetracaine is an ester topical anesthetic available in 0.5% solution (Altacai ne) and is approved for short-duration ocular surface procedures. It has a longer onset of action and duration of action than proparacaine has and causes more extens ive corneal epithelial toxicity.
Topical Anesthetics in Anterior Segment Surgery The first modern use of topical anesthetics was Koller's use of cocaine in 1884. Since then, synthetic dru gs have become ava ilable; cocaine is no longer used because of the potential risk of side effects and drug abuse. Tetracaine 0.5% or I% (amethocaine) and proparacaine 0.5% are short-ac ting (20 minutes) and are the least toxic of the regional and topical anesthetics to the corneal epithelium . Lidocaine 4% for injection (Lignocaine) can be used topically, as can 2% lidocaine jelly. Bupivacai ne 0.5% and 0.75% have a longer duration of ac tion but an increased risk of associated corneal toxicity.
Technique The ai m of topical anest hetics is to block the nerves that suppl y the superficial cornea and conjuncti va- namely, the long and short ciliary, nasociliary, and lacrimal nerves. Patients should be wa rned that they will experience some st inging upon application of the drops onto the surface of the cornea. Because visual perception is not lost, the patient is asked to foc us on the source of the light, the intensity of which is subsequently reduced. Topical anesthetics may be combi ned wit h subconjunctival anesthetics. This combi nation is well tolerated by patients and allows subconjuncti val and scleral manipulations to be carried out. To pical anesthet ics can be augmented with a blunt cannula sub-Tenon infusion of anesthetic as a primary anesthetic or intraoperati vely in pat ients who become intolerant of topical anesthetics. Intraocular lidocaine Recently, intraoc ular lidocaine has been used to provide analgesia during surgery. The solution used is 0.3 mL of 1% isotonic, nonpreserved lidocaine administered intracamerall y. No side effects have been reported, except for possible transient retinal toxicity if lidocaine is injected posteriorly in the absence of a posterior capsule. Lidocaine obviates the need for intravenous and regional anesthetic supplementation in most patients. Adequate anesthesia is obtained in approximately 10 seconds. As with topical techniques, the ability of the patient to cooperate during surge ry is des irable. Contrasting studies have shown no diffe rence with and without intracamerallidocaine as a supplement to topical anesthetics. Because of unreliable patient cooperation, topical and intraca meral anesthetics should be used cautiously, if at all, in patients with deafness, dementia, and severe photophobia.
390 • Fundamentals and Principles of Ophthalmology Crandall AS. Anesthesia modalities for cataract surgery. Curr Opin OphthalnlOl. 200 1; 12( 1): 9-11.
Crandall DC. Pharmacology of ocular anesthetics. In: Tasman W, Jaeger EA, eds. Duane's
Foundations a/Clinical Ophthalmology. Vol 3. Philadelphia: Lippincott; 1999:1-22. Kansal S, Moster MR, Gomes Me, Schmidt e M Jr, Wilson RP. Patient comfort with combined anterior sub -Tenon's, topical, and intracameral anesthesia versus retrobulbar anesthesia in
trabeculectomy, phacotrabeculectomy, and aqueous shunt surgery_ Ophthalmic Surg Lasers. 2002;33(6) :456- 462. Roberts T, Baytell K. A comparison of cataract surgery under topical anaesthesia with and without intracamerallignocaine. Clin Experiment Ophtha/mol. 2002;30( I): 19 - 22 . Varga JH, Rubinfeld RS, Wolf Te , et al. Topical anesthetic abuse ring keratitis: report of four cases. Cornea. 199 7;16(4):424- 429.
Purified Neurotoxin Complex Botulinum toxin type A (Botox, formerly called Oculinum) is produced from cultures of the Hall strain of Clostridium botulinum. It blocks neuromuscular conduction by binding to receptor sites on motor nerve terminals, entering the nerve terminals and inhibiting the release of acetylcholine. Botulinum toxin type A injections provide effective relief of the excessive. abnormal contractions associated with benign essential blepharospasm and hemifacial spasm . Cosmetic use of botulinum toxin, speCifically in the treatment of glabellar folds, has gained popularity as welL Botulinum is approved to treat strabismus, possibly by inducing an atrophic lengthening of the injected muscle and a corresponding shortening of the muscle's antagonist.
n. Fowler AM. Botulinum toxin in ophthalmology. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2007, module 3. Harrison AR. Chemodenervation for facial dystonias and wrinkles. Curr Opin Ophthalmol. 2003; 14(5):241-245. Scott AB. Botulinum toxin treatment of strabismus. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 1989, module 12. Dutton
Medications for the Dry Eye Artificial tear preparations (demulcents) and emollients form an occlusive film over the corneal surface to lubricate and protect the eye from drying. The active ingredients in demulcent preparations are polyvinyl alcohol, cellulose, and methylcellulose and their derivatives: hydroxypropyl cellulose, hydroxyethylcellulose, hydroxypropyl methylcellulose (HPMC), and carboxymethylcellulose. One demulcent is glycerin 0.3% or 1% variously combined with polysorbate 80, HPMC, PEG-400, and dextran 70. Polycarophil is available in combination with dextran 70 and PEG -400. Hydroxypropyl methylcellulose is available alone in 0.3%, 0.2%, and 0.5% solutions, and in formulation with glycerin or dextran 70 or PEG -400, or a combination of these additives. Polyvinyl alcohol may be used alone or with either povidone or a combination of PEG-400 and dextrose. Hydroxypropyl methylcellulose with dextran 70 and propylene glycol with PEG-400 are 2 additionallubricating agents.
CHAPTER 17:
Ocular Pharmacotherapeutics. 391
The viscosity of artificial tears varies in part due to the concentration of the wetting age nt. For example, carboxymethylcellulose is available in 0.25%, 0.5%, and 1% solutions; the increasing viscosity of these solutions is aimed at the increasing severity of dry-eye symptoms. Multidose preparations contain preservatives, including benzalkonium chloride, EDTA (ethylenediaminetetraacetic acid) , methylparaben, polyquad, potassium sorbate, propylparaben, Purite, sodium perborate, and sorbic acid. Although there was significant toxicity with the early preservatives such as thimerosal, the newest generation of ophthalmic preservatives is virtually nontoxic. Un preserved unit-dose preparations eliminate the cytotoxic effects of preservatives. Ocular emollients are ointments prepared with sterile petrolatum, liquid lanolin, mineral oil, methylparaben , and polyparaben. Ophthalmic lubricating ointments are useful for easing the symptoms of severe dry eye and exposure keratopathy, as well as for nighttime use in dry eye and nocturnal lagophthalmos. Kunert KS, Tisdale AS, Gipson IK. Goblet cell numbers and epithelial proli fera tion in the conjunctiva of patients with dry eye syndrome treated with cyclosporine. Arch Ophthafmof. 2002; 120(3),330- 337. [Erratum appears in Arch Ophthalmol. 2002; 120(8),1099.1 Kunert KS, Tisdale AS, Stern ME. Smith JA. Gipson IK. Analysis of topical cyclosporine treatment of patients with dry eye syndrome: effect on conjun ctiva llymphocytes. Arch Ophthal-
mol. 2000; 11 8( 11),1489- 1496. Pflugfelder Sc. Antiinfl ammatory therapy for dry eye. Am J Ophthalmol. 2004;137(2),337 - 342. Small DS, Acheampong A, Re is B. et at. Blood concentratio ns of cyclospori n A during longterm treatment with cyclospori n A ophthalmic emulsio ns in patients with moderate to severe dry eye disease. JOwl Pharmacol Tiler. 2002; 18(5):411 - 418.
Preservative-free topical cyclosporine emulsion 0.05% (Restasis) has been developed to target the inflammatory etiology of dry eye. Because cyclosporine is poorly wate rsoluble, it is prepared in a glycerin, castor oil, polysorbate 80 emulsion. Studies have shown that twice-daily dosing with this agent has negligible systemic absorption and side effects. A measurable repopulation of goblet cells and a decrease in conjunctival epithelial cell turnover and the number of lymphocytes have been demonstrated by biopsy. Patients reported a decrease in their subjective dry-eye complaints and had measu rable increases in Schirmer wetting at 6 months. The oily vector is marketed separately as the tear supplement Refresh Endura. Future development of medications for dry eye should be aimed at providing nourishment for the keratoconjunctival surface as well as reVitalizing the tear-secreting system. (See also BeSe Section 8, External Disease and Cornea.)
Hyperosmolar Agents Hyperosmolar agents are used to decrease corneal and epithelial edema. O ne such agent is sodium chloride (Muro 128), which is available without a prescription in a 2% or 5% solutio n or as an ointment. Products such as these are used for the treatment of corneal edema from Fuchs dystrophy, other causes of endothelial dysfunction, prolonged edema postoperat ively. and recurrent erosion syndrome.
392 • Fundamentals and Principles of Ophthalmology
Ocular Decon estants Common agents such as naphazoline, oxymetazoline, tetrahydrozoline, and phenylephrine hydrochloride are used as topical drops to cause temporary vasoconstriction of conjunctival vessels. Possible side effects include rebound vasodilation and conjunctival hyperemia. These medications can be abused by patients and can cause ocular surface toxicity. Although they are available as over-the-counter preparations, patients should be in structed not to use them on a chronic bas is.
Irrigating Solutions Sterile isotonic solutions are for general ophthalmic use. Depending on the solution, nonprescription ocular irrigating solutions may contain sodium chloride, potassium chloride, calcium chloride. magnes ium chloride. sodium acetate, sodium citrate, bo ric acid , sodium
borate, and sodium phosphate. They are preserved with EDTA, benzalkonium chloride, and sorbic acid. Sterile, phYSiologically balanced, preservative-free salt solutions (BSS and BSS PLUS) are isotonic to eye tissues and are used for intraocular irrigation during surgical procedures. Glucose glutathione bicarbonate solution (BSS PLUS) has been shown to cause less change in the corneal endothelial morphology postoperativel y and to augment the postoperative endothelial pump function. It is not routinely used due to cost concerns, but it may be used in patients who preoperatively are noted to have compromised corneas.
Diagnostic Agents Fluorescein 2%, lissamine green I %, and rose bengal, as impregnated paper strips, are examples of solutions commonl y used in the examination and diagnOSiS o f external ocular
diseases. The first 2 stains outline the defects of the conjunctival and corneal epithelium, whereas the rose bengal indicates abnormal devitalized epithelial cells. For the study of retinal and choroidal circulation as well as abnormal changes in the RPE, sodium flu orescein solution in concentration s of 5%, 10%, or 25% is injected intravenously. Fundus
fluorescein angiography is helpful in diagnOSing various vascular diseases and neoplastic disorders. Fluorescein dye can also be used in anterior segment angiography to demonstrate anterior segment vascular disorders.
Indocyanine green, a tricarbocyanine type of dye, is approved to study choroidal vasculature in a variety of choroidal and retinal disorders. Typically, 25 mg of dye is injected as IV solution. Indocyanine green angiography is particularl y helpful in identifying and delineating poorly defined choroidal neovascular membranes in AMD. Indocyanine green and trypan blue dye are useful in delineating the anterior capsule during phacoemulsification of mature cataracts. Whereas indocya nine green may be constituted off-label for this use, the FDA has approved trypan blue for use as an anterior capsule stain during surgery. Matsuda M, Kinoshita S, Ohashi y, et al. Comparison of th e effects of intraoc ul ar irri gating solutions on the corneal endothelium in intraocular lens implantation. Br J Ophthalmol. 1991 ;75(8);4 76 ~ 479.
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McDermott M, Snyder R, Slack J, Holley G. Edelhauser H. Effects of intraocu lar irrigants on the preserved human corneal endothelium. Cornea. 1991;10(5):402-407. Saini JS. Jain AK, Su khija J, Gupta P, Saroha V. Anterior and posterior capsulorhexis in pediat ric cataract surgery with or without trypan blue dye: randomized prospective clinical study.
/ Cataract Refract Surg. 2003;29(9): 1733- 1737. Werner L, Pandey SK, Escobar-Gomez M, Hoddinott OS, Apple DJ. Dye-enhanced cataract surgery. Part 2: learning critical steps of phacoemulsification. J Cataract Refract Surg. 2000;26(7): 1060- 1065.
Viscoelastic A ents Viscoelastic agents possess certain chemical and physical properties that include the capacity to resist flow and deformation . Viscoelastics for ophthalmic use must also be inert, isosmotic. sterile. nonpyrogenic, nonantigenic. and optically clear. In addition. they must be sufficiently hydrophilic to allow easy dilution and irrigation from the eye. Naturally occurring and synthetic compounds include sodium hyaluronate. chondroitin sulfate. hydroxypropyl methylcellulose. and polyacrylamide and are available in a variety of concentrations. Combined chondroitin sulfate/sodium hyaluronate materials are also available. Viscoelastic agents protect ocular tissues. such as the corneal endothelium and epithelium, from surgical trauma; help to maintain intraocular space; and facilitate tissue manipulation. Thus. they are indispensable tools in cataract or glaucoma surgery. penetrating keratoplasty, anterior segment reconstruction surgery. and retinal su rgery. (See also the discussions of hyaluronic acid and vitreous collagen cross-linking in Chapter 12 of this volume and BCSC Section II , Lens and Cata ract.)
Fibrinolytic Agents Tissue plasminogen activator (tPA). urokinase. and streptokinase are all fibrinolytiC agents. tPA is a naturally occurring serine protease with a molecular mass of 68 kD. Because tPA is normally present at a higher concentration in the aqueous humor of the human eye than it is in blood. it is less toxic to ocular tissues and is specific for dissolution of fibrin clots. tPA has been used successfully in the resolution of fibrin clots after vitrectomy. keratoplasty. and glaucoma filtering procedures. These drugs are not approved by the FDA for ocular use and are therefore used with an off-label application. Tripathi Re, Tripathi 6J, Park JK, et al. Intra cameral tissue plasminogen activator for resolution of fibrin clots after glaucoma filtering procedures. Am J Ophthalmol. 1991 ;III (2):247- 248.
Thrombin Thrombin. a sterile protein substance. is approved to control hemorrhage from accessible capillaries and small venules, as would be seen with standard surface incisions. Its use in maintaining hemostasis during complicated intraocular surgery is off-label, as this requires injection. Intravitreal thrombin has been used to control intraocular hemorrhage
394 • Fundamentals and Principles of Ophthalmology during vitrectomy. The addition of thrombin (l00 units/ mL) to the vitrectomy infusate significantly shortens intraoc ular bleeding time. and thrombin produced by DNA recombinant techniques minimi zes the degree of postoperative inflammation. Thrombin causes
significant ultrastructural corneal endothelial changes when human corneas are perfused with 1000 units/mL.
Antifibrinolytic Agents Antifibrinolytic agents. such as E-aminocaproic acid and tranexa mic acid. inhibit the activation of plasminogen. These agents may be used systemically to treat cases of hemorrhage secondary to excessive fibrinolysis and to prevent recurrent hyphema. which most commonly occurs 2- 6 days after the origi nal hemorrhage. They are contraindicated in the presence of active intravascular clotting. such as diffuse intravascular coagulation (DIC). as they can increase the tendency for th rombosis. They should not be used in pregnancy. in patients with coagulopathies or on platelet inhibition therapy. or in patients with renal or hepatic disease. Patients with larger hyphe mas and those with delayed presentation are at a higher risk for rebleeding. but patients with early presentation and those with smaller hyphemas are at a low risk for rebleeding. The use of E-aminocaproic acid is usually reserved for patients at a higher risk of rebleeding. E-Aminocaproic acid is used in a dosage of 50-100 mg/kg every 4 hours. up to 30 g daily. Possible adverse reac tio ns include nausea, vomiting. muscle cramps. conjuncti val
suffusion. nasal congestion. headache. rash. pruritus. dyspnea. tonic toxic confusional states. cardiac arrhyt hmias. and systemic hypotension. Gastrointestinal side effects are similar with doses of either 50 or 100 mg/kg. The drug should be continued for a full 5- 6 days to achieve maximal clinical effectiveness. Topical E-aminocaproic acid may be an attractive alternative to systemic delivery in the treatment of traumatic hyphema. but the efficacy of topical treatment has been questioned. Optimal topical concentration to maximize aqueous levels and minimize corneal epithelial toxicity is 30% E-aminocaproic acid in 2% carboxypolymethylene. Tranexamic acid (Cyklokapron ) is another antifibrinolytic age nt used off-label to reduce the incidence of rebleeding after traumatic hyphema. It is 10 times more potent in vitro than is E-aminocaproic acid. The usual dosage is 25 mg of tranexamic acid/kg 3 times daily for 3- 5 days. Gastrointestinal side effects are rare. Karkhaneh R, Naeeni M. Chams H, Abdollahi M, Mansouri MR. Topical aminocaproic acid to prevent rebleeding in cases of traumatic hyphema. Eur J Ophtha fmol. 2003;13(1 }:57-6 1. Shiuey Y. Lucarelli MJ. Traumatic hyphema: outcomes of outpatient management. Ophthalmology. 1998; 105(5};85 1- 855.
Corneal Storage Medium The corneal storage medium helps to prolong the viability of donor corneas to be used for transplantation. The main components of the various kinds of media include a bicarbonate-buffered minimum essential medium (MEM) or a hybrid medium of MEM/TC 199.
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chondroitin sulfate, and dextran (to retard proteoglyca n loss during storage and reduce intraoperative and postoperative rebound swelling), as well as gentamicin sulfate or other antibiotics used as prophylactic agents. Corneal tissue storage can be prolonged with the addition of recombinant growth factors such as epidermal growth factor and with antioxidants, insulin , adenosine triphosphate precursors, anticollagenases, and antiproteases.
Vitamin Supplements and Antioxidants Nonprescription vitamin supplements have enjoyed increased popularity because of their antioxidant properties and are used for intermediate-to-severe AM D. The Age- Related Eye Disease Study (A REDS) is discussed in depth in BCSC Section 12, Retina and Vitreous.
Interferon A naturally occurring speCies-specific defense against viruses, interferon is synthesized intracellularly and increases resistance to virus infection. Synthetic analogues such as
polyinosinic aCid- polycytidylic acid have been used to induce patients to form their own interferon .
Topically administered interferon has been found to be ineffective in the treatment of epidemic keratoconjunctivitis caused by adenovirus. In herpes simplex keratitis patients, however, interferon used in conjunction with acyclOVir showed significantly qUicker healing time than treatment with acyclOVir alone (5.8 vs 9.0 days). Interferon has also been found to speed the healing of an epithelial defect when used in combination with trifluridine. The dosage of interferon (30 million IU/mL) was 2 drops per day for the first 3 days of treatment. Interferon alone has little effect on the treatment of herpes Simplex keratitis. In combination, however, it seems to act as a topical adjuvant to traditional antiviral therapy in resistant herpes simplex keratitis. Interferon has also been shown to inhibit vascular endothelial cell proliferation and differentiation. It is particularly effective in the treatment of juvenile pulmonary hemangiomatosis, which used to be a fatal condition before the development of interferon. Intralesionol administration of interferon has been reported to be especially effective in ocular Kaposi sarcoma.
Growth Factors Growth factors are a diverse group of proteins that act at autocrine and paracrine levels to affect various cellular processes, including metabolic regulation, tissue differentiation, cell growth and proliferation, maintenance of Viability, and changes in cell morphology. The growth factors are syntheSized in a variety of cells and have a spectrum of target cells and tissues. Various growth factors have been found in retina, vitreous humor, aqueous humor, and corneal tissues. These include
• epidermal growth factor fibroblast growth factors
396 • Fun da mentals an d Principles of Ophthalmol ogy
• transforming growth factor ~s • vascular endothelial growth factor • insulin-like growth factors These growth factors are capable of diverse. synergistic, and sometimes antagonistic biologic activities.
Under normal physiologic conditions, the complex and delicate coordination of the effects of and the interactions among growth factors maintains the homeostasis of intraocular tissues. The net effect of a growth factor depends on its bioavailability, which is determined by its concentration, its binding to carrier proteins, the level of its receptor in the target tissue, and the presence of other complementary or antago nistic regulatory factors. Pathologically, the breakdown of blood- ocular barriers disrupts the balance among growth fac to rs in th e ocular media and tissues and may resu lt in various ab normalities.
The disruption in the balance among isoforms of transforming growth factor ~s, basic fibroblast growth factor, vascular endothelial growth factor, and insulin -like growth factors is suspected to cause ocular neovasculari zation. Transforming growth fac tor ~s and platelet-derived growth factor are implicated in the pathogenesis of proliferative vitreoretinopathy and in the excessive proliferation of Tenon capsule fibrobl asts, which can result in the scarring of the glaucoma filtration bleb. Increased concentrations of insulin like growth factors in plasmoid aqueous humor may be responsible for the abnormal hyperplas tic response of the lens epithelium and corneal endothelium seen in inflammatory conditions and in traumatic insults to the eye. Identifying growth factors and understanding their mechanisms of action in the eye offer great potential for providing the ophthalmologist with new methods for manipulation of and intervention in ocular disorders. Epidermal growth factor and fibroblast growth factor can accelerate corneal wound repair after surgery, chemical burns, or ulcers and can increase the number of corneal endothelial cells. Fibroblast growth factor has also been shown to delay the process of retinal dystrophy in Royal College of Surgeons rats. Vascular endothelial growth factor (VEGF), also known as vasculotropin, deserves special mention. It is a dimeric, heparin -binding polypeptide mitogen and has 4 isoforms that are generated from alternative splicing of mRNA. The VEGF gene is widely expressed in ac tively proliferating vascular tissue and is implicated in the pathogenesis of va rious neovascular retinopathies such as diabetes mellitus and age-related choroidal neovascu larization (CNV) . Intravitreal injections of VEGF inhibitors are used in the treatment of wet macular degeneration. Patients with CNV who have been treated with anti-VEGF have shown a slower loss of vision, especially moderate (>3 lines of vision lost) to severe vision loss (>6 lines lost) and, in some cases, an improve ment in vision . Pegaptanib (Macugen), the first approved agent, requires intravitreal injections every 6 weeks for up to 2 years and shows a decrease in efficacy in the second year of treatment. Subsequent agents have largely supplanted pegaptanib. Ranibizumab (Lucentis) is approved, and bevacizumab (Avastin) is being used off-label, for the treatment of wet age-related macular degeneration.
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Ranibizumab is given as an intravitreal injection of 0.5 mg (0.05 mL) monthly, with a possible decrease to every 3 months after 4 injections. Bartlett J0, Jaanus SD, eds. Clinical Ocular PhamUlcology. 5th ed. St Loui s: Butterworth Heinemann/Elsevier; 2008. Brunton LL, Lazo JS, Parker KL, eds. Goodman & Gilmmls the Pharmacological Basis of Therapeu/ics: Digital Edition. II th ed. New York: McG raw- Hili; 2006. Eyetech Study Group. Anti-vascular endothelia l growt h factor therapy for subfoveal choroidal neovasc ularization secondary to age- related macular degeneration: phase II study results.
Ophthalmology. 2003; 110(5):979-986. Fraunfelder FT, Fraunfelder FW. Drug-II/duced Ocular Side Effects. 5th ed. Boston: Butterworth-Heinemann; 2001. KrzystolikMG, Afshari MA, Adamis AP, et al. Prevention of experimental choroidal neovascu larization with intravitreal anti-vascular endot helial growth fa ctor antibody fragment. Arch Ophtlwlmol. 2002; 120(3):338-346. Micromedex" Healthcare Seri es. n.d . Thomson Micromedex, Greenwood Village. www.thomsonhc.com. Accessed 24 January 2006.
co.
http://
Murray L. Drug Topics Red Book Up date. Montval e. NJ: Thomson PDR; 2007. Murray L, ed. Physicians' Desk Reference. 58th ed. Montvale, NJ: Thomson PDR; 2004. Physicians' Desk Reference for Ophthalmic Medicines. 35 th ed. Montval e, Nl: Thomson PDR;
2007. U.S. Food and Drug Admini stration. Drugs@FDA: FDA Approved Drug Products. 2007. Available at: http://www.accessdata.fda.gov/scripts/cder/ drugsatfda/. Accessed September 2007.
The authors would like to thank Joan Murhammer, RPh, from the Drug Information Center at the University of Iowa Hospitals and Clinics for her invaluable assistance in updating the information on available medications marketed in the United States.
Basic Texts Anatomy Beard C, Quickert MH. Anatomy of the Orbit: A Dissection Manual. 3rd ed. Birmingham, AL: Aesculapius; 1988. Bran AJ, Tripathi RC, Tripathi BJ, eds. Wolff's Anatomy of the Eye and Orbit. 8th ed. London: Chapman & Hall; 1997. Duke-Elder S, Wybar KC, eds. The Anatomy of the Visual System. St Louis: Mosby; 1976. System of Ophthalmology; vol 2. Dutton J]. Atlas of Clinical and Surgical Orbital Anatomy. Philadelphia: Saunders; 1994. Fine BS, Yanoff M. Ocular Histology: A Text and Atlas. 2nd ed. Hagerstown, MD: Harper & Row; 1979. Hogan MJ, Alvarado JA, Weddell JE. Histology of the Human Eye: An Atlas and Textbook. Philadelphia: Saunders; 1971. Miller NR, Newman NJ, Biousse V, Kerrison JB, eds. Walsh and Hoyt's Clinical Neuro Ophthalmology. 6th ed. Philadelphia: Lippincott Williams & Wilkins; 2004. Snell RS, Lemp MA. Clinical Anatomy of the Eye. 2nd ed. Malden, MA: Wiley-Blackwell; 1998. Tasman W, Jaeger EA, eds. Duane's Ophthalmology. Philadelphia: Lippincott Williams & Wilkins; 2007. Zide BM, ed. Surgical Anatomy Around the Orbit: The System of Zones. Philadelphia: Lippincott Williams & Wilkens; 2006. Zide BM, Jelks GW, eds. Surgical Anatomy of the Orbit. New York: Raven Press; 1985.
Embryology Jakobiec FA, ed. Ocular Anatomy, Embryology, and Teratology. Philadelphia: Harper & Row; 1982. O'Rahilly R, Muller F. Human Embryology and Teratology. 3rd ed. New York: Wiley-Liss; 2001.
Genetics Merin S. Inherited Eye Diseases: Diagnosis and Management. 2nd ed. Boca Raton, FL: Taylor & Francis; 2005. Nussbaum RL, McInnes RR, Huntington FW. Thompson & Thompson Genetics in Medicine. 7th ed. Philadelphia: Elsevier/Saunders; 2007. Traboulsi El, ed. Genetic Diseases of the Eye. New York: Oxford University Press; 1998.
399
400 • Basic Texts
Biochemistry and Metabolism Berman ER. Biochemistry of the Eye. Perspectives in Vision Research. New York: Springer; 1991. Kaufman PL, Aim A, eds. Adlers Physiology of the Eye. lOth ed. Philadelphia: Elsevier/ Mosby; 2003. Tombran-Tink J, Barnstable C). Retinal Degenerations: Biology, Diagnostics, and Thera peutics. Totowa, N): Humana Press; 2007.
Ocular Pharmacologv Brunton LL, ed. Goodman and Gilmans The Pharmacological Basis of Therapeutics. 11th ed. New York: McGraw-Hill; 2006. Bartlett )0, )aanus SO, eds. Clinical Ocular Pharmacology. 5th ed. St Louis: Elsevier/ Butterworth-Heinemann; 2008. Zimmerman T), Karanjit K, Mordechaie S, Fechtner RO, eds. Textbook of Ocular Pharmacology. 3rd ed. Philadelphia: Lippincott Williams & Wilkins; 1997.
Related Academy Materials Focal Points: Clinical Modules for Ophthalmologists For information on Focal Points modules, go to http://one.aao.org/CE/Educational Products/FocaIPoints.aspx. Ahmed M, Foster CS. Steroid therapy for ocular inflammatory disease (Module 7, 2006). Hertle RW, Kowal LM, Yeates KO. The ophthalmologist and learning disabilities (Module 2, 2005). Oester A, Baffi JZ, Balamurali BK. Pharmacotherapy targeting ocular neovascularization (Module 7, 2008). Sheth BP. Drugs and pregnancy (Module 7, 2007). Stead SW, Bell SN. Ocular anesthesia (Module 3, 2001). Wygnanski-Jaffe T, Levin AV. Introductory genetics for the ophthalmologist (Module 5, 2005).
Print Publications Arnold AC, ed. Basic Principles of Ophthalmic Surgery (2006). Jordan DR, Anderson RA. Surgical Anatomy of the Ocular Adnexa: A Clinical Approach. Ophthalmology Monograph 9 (1996). Parke OW II, ed. The Profession of Ophthalmology: Practice Management, Ethics, and Advocacy (2005). Rockwood EJ, ed. ProVision: Preferred Responses in Ophthalmology. Series 4. Self-Assessment Program (2007). Traboulsi EI. A Compendium of Inherited Disorders and the Eye. Ophthalmology Monograph 18. Published by Oxford University Press, in cooperation with the American Academy of Ophthalmology (2005). Wilson FM II, ed. Practical Ophthalmology: A Manual for Beginning Residents. 5th ed. (2005).
Online Materials For Preferred Practice Patterns and Complementary Therapy Assessments, go to http:// one.aao.org/CE/PracticeGuidelines/default.aspx. Basic and Clinical Science Course (Sections 1-13); http://one.aao.org/CE/Educational Products/BCSC.aspx Clinical Education Cases; http://one.aao.org/CE/EducationaIContent/Cases.aspx Clinical Education and Ethics Courses; http://one.aao.org/CE/EducationaIContent/ Courses.aspx
Focal Points modules; http://one.aao.org/CE/EducationaIProducts/FocaIPoints.aspx 401
402 • Related Academy M ateri als
Maintenance of Certification Exam Study Kit, version 2.0 (2007); http://one.aao.org/CE/ MOC/default.aspx Rockwood E), ed. Pro Vision: Preferred Responses in Ophthalmology. Series 4. SelfAssessment Program, 2-vol set (2007); http://o ne.aao.org/CE/EducationaIProducts/ Provision.aspx
Preferred Practice Patterns Preferred Practice Patterns are available at http://one.aao.org/CE/ PracticeGuidelines/ PPP.aspx. Preferred Practice Patterns Committee. Comprehensive Adult Medical Eye Evaluat ion (2005) . Preferred Practice Patterns Committee, Pediatric Ophthalmology/Strabismus Panel. Pediatric Eye Evaluations (2 007). Preferred Practice Patterns Committee. Summary Benchmarks for Preferred Prac tice Patterns (2008).
Complementary Therapy Assessments Complementary Therapy Assessments PracticeGuidelinesITherapy.aspx.
are
available
at
http://one.aao.org/CE/
Complementary Therapy Task Force. Acupuncture for Ocular Conditions and Headaches (2003). Complementary Therapy Task Force. Antioxidant Supplements and Age-Related Macular Degeneration (2002). Complementary Therapy Task Force. Antioxidant Vitamin and Mineral Supplements and Cataract Prevention and Progression (2002). Complementary Therapy Task Force. Ginkgo Bi/oba Extract and Ocular Conditions (2002). Complementary Therapy Task Force. Marijuana in the Treatment of Glaucoma (2003). Complementary Therapy Task Force. Nutritional Supplements: Perioperative Implications for Eye Surgery (2003). Complementary Therapy Task Force. Vision Therapy for Learning Disabilities (2001).
CDs/DVDs Basic and Clin ical Science Course (Sections 1- 13) (CD-ROM, 2009). Guyton DL. Retinoscopy and Subjective Refraction (DVD; reviewed for currency 2007) . Movaghar M, Lawrence MG. Eye Exam: The Essentials. From The Eye Exam and Basic Oph thalmic Instruments (DVD, 2004; reviewed for currency 2007) . Tang R. Ocular Manifestations ofSystemic Disease. From Eye Care Skills: Presentations for Physicians and Oth er Health Care Professionals (CD-ROM; reviewed for currency 2005).
To order any of these m alerials, please o rder online al www.aao.orglslore o r call the Academy's Customer Service toll-free number 866-561-8558 in the U.S. If out side the U.S., call 41 5-561 -8540 between 8:00 A M and 5:00 P M PST.
Credit Reporting Form Basic and Clinical Science Course, 2011-2012 Section 2 The American Academy of Ophthalmology is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.
The American Academy of Ophthalmology designates this enduring material for a maximum of 15 AMA PRA Category 1 Credits™. Physicians should claim only credit commensurate with the extent of their participation in the activity. If you wish to claim continuing medical education credit for your study of this Section, you may claim your credit online or fill in the required forms and mail or fax them to the Academy. To use the forms: 1. Complete the study questions and mark your answers on the Section Completion Form.
2. Complete the Section Evaluation. 3. Fill in and sign the statement below. 4. Return this page and the required forms by mail or fax to the CME Registrar (see below). To claim credit online: L Log on to the Academy website (v..'WW.aao.org/cme). 2. Select Review/Claim CME. 3. Follow the instructions.
Important: These completed forms or the online claim must be received at the Academy by June 2013.
r hereby certify that r have spent _ _ (up to 15) hours of study on the curriculum of this Section and that I have completed the study questions. Signature: Date
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403
404 • Credit Reporting Form
2011- 2012 Section Completion Form Basic and Clinical Science Course Answer Sheet for Section 2 Question
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Section Evaluation Please complete this CME questionnaire. I. To what deg ree will you use knowledge from
BeSe Section 2 in your practice?
o Regularly o Sometimes o Rarely 2. Please review the stated objectives for sese Section 2. How effective was the material at meeti ng those object ives?
o All objectives were met. o Most objectives were met. D Some objec tives were met.
o Few or no objectives were mel. 3. To what degree is BeSe Section 2 likely to have a posit ive impact on health outcomes of your patients?
o Extremely likely o Highly li kely o Somewhat likely o
Not at all li kely
4. After you review the stated objectives for BeSe Section 2, please let us know of any additional knowledge. skills, or information useful to your practice that were acqu ired but were not in cluded in the objectives.
5. Was BeSe Section 2 free of commercial bias?
o Yes o No 6. If you selected "No" in the previous question, please comme nt.
7. Please tell us what might improve the appli cability of sese to your practice.
Study Questions Although a concerted effort has been made to avoid ambiguity and redundancy in these ques tions, the authors recognize that differences of opi nion may occur regarding the "best" answer. The discussions are provided to dem onst rate th e rationale used to derive the answer. They may also be helpful in co nfirming that your approach to the problem was correct or, if necessary, in fixing the principle in your memory. I. Which bone is not part of the orbital floor?
a. maxilla
b. palatine c. greater wing of th e sphenoid d. zygomatic 2. Which structure does not pass through the annulus of Zi nn? a. superi or division of cranial nerve III b. cranial nerve IV c. nasociliary branch of cranial nerve V (V I) d. optic nerve
3. Which extraocular mu scle originates from the annulus ofZinn? a. levator palpebrae superioris b. superior oblique c. lateral rectus d. inferior oblique 4. Which of the rectus muscles inserts closest to the limbus? a. lateral rectus b. medial rectus c. superior rectus d. inferior rectus 5. Regarding the stratifi ed layers of th e neurose nsory retina, which is a true membrane? a. external limiting mem brane b. internal limiting mem brane c. inner plexiform layer d. None of the above are true memb ranes. 6. The air~tear interface at the surface of the co rnea is responsible for approximately what percentage of the refractive power of the human eye?
a. 25% b. 33%
c. 50% d.66% 407
408 • Study Questions 7. Which of the following structures is not part of the uveal tract of the eye' a. iris
b. ciliary body c. choroid
d. neurosensory retina 8. Which of the following cranial nerves exits from the dorsal aspect of the midbrain? a. eN III (oculomotor) b. eN IV (trochlear) c. eN V (trigeminal)
d. eN VI (abducens) 9. The optic nerve averages what total length? a. 10 mm
b. 20 mm c. 40 mm
d. SOmm 10. An expanding lesion of the cavernous sinus is least likely to affect which of the following? a. internal carotid artery b. cranial nerve 111
c. ophthalmic division of cranial nerve V (V I) d. mandibular division of cranial nerve V (V 3) II. Aneurysms that affect the oculomotor nerve (eN III ) commonly occur at the junction of which 2 arte ries? a. internal and external carotid arteries b. anterior cerebral and an terior communicating arteries c. basilar and posterior cerebral arteries d. posterior communicating and internal carotid arteries 12. The human lens is derived from which embryo nic ti ssue?
a. neuroectoderm b. neural crest cells c. surface ectoderm d. mesoder m 13. All of the following structures include neural crest cell - derived tissue except: a. trabec ular meshwork b. iris stroma c. ciliary epithelium d. corneal endothelium
Study Questions' 409 14. Which is not true of ho meobox genes?
a. They are about 180 base pairs long. b. They control the activity of subordinate ge nes. e.
They are found only in mammals.
d. The PAX6 ge ne is a homeobox gene that appea rs to be a master control gene for the development of th e eye. 15. Whi ch of the followin g di sorders is not associated with a defect in a mitochondrial gene?
a. Leber hereditary optic neuropathy (LHO N) b. chron ic progressive external ophthalmoplegia (CPEO) c. neuropathy, atax ia, and retinitis pigmentosa (NARP) d. retin oblastoma 16. Which of th e following is not correct ab out retinoblastoma?
a. It may be associated with chromosome 13 long arm (l 3q14) deletion synd ro me. b. It affec ts between I and 15,000-34,000 live births in the United States. c. Approximately 50% of patients with hereditary retinobl astoma have a family history of the disease. d. The hereditary pattern of familial retinoblastoma is autoso mal dominant, but the defec t is autosomal recess ive at a cellular level. 17. A cert ain ocular disorder is fo und to occur mo re often in males than females. but affected males do no t transmit the disorder. However, virtually every son and daughter of an affec ted female inh erits the trait. Wh at is th e most likely mode of inheritance o f thi s di sorder? a. autosomal domin ant b. autosomal recessive
c. X-linked recessive d. mitochondrial 18. Mutations in the rhodopsin ge ne are associated with whi ch inh erited ocular disease? a. juve nile glaucoma
b. Lebe r optic neuropathy c. retinitis pigmentosa
d . Stargardt disease 19. Mitochondrial inheritance is based on whi ch of the followin g? a. pate rnal mitochondria b. matern al mitochondria
c. acquired mitocho ndria d. de novo mitochondria
410 • St udy Qu estio ns 20. Linkage analysis is a metho d th at determ ines which of th e followin g? a. patern ity
b. type of inherit ance c. how o ften a medicati on should be g iven
d. the proximity of I gene to anoth er 2 1. Ma ny of the ge nes encoded in the mitochond rial genome are important fo r whi ch of the following processes? a. cell motiHty
b. pro teasomal degradatio n
c. energy metabolism
d . autophagy 22. PAX6 mutatio ns are associated with a. aniridia
h. optic nerve hypoplasia c. renal hypoplas ia d. corneal granul ar dystrophy 23. Which of the following pairs accurately desc ribes th e cell type that makes th e tea r-layer co mponent?
a. goblet ceUs-lipid layer b. meibomian glands- mucin layer c. gland of Krause-aqueo us layer
d . gland of Wolfrin g- mu cin laye r 24. Which of the following immunoglobulins is found in the tea r film ? a. IgA b.lgM c. IgE d. lgD 25. Which of th e fo llowing is a true base ment membran e?
a. Bowman's layer b. zonule of Zinn c. Descemet's membrane
d. anterior border layer of iris 26. Descemet's membrane is principall y made fro m
a. type I coUagen b. type II collagen c. type III collagen d . type IV collagen
Study Questions. 411
27. The main corneal mechanism for holding the LASIK flap in place after surgery is a. endothelial- Descemet's membrane interaction
b. endothelial pump c. Bowman's layer- stromal adhesio ns
d. strom al coll age n adhesions
28. Which of th e following lens proteins is not found in the human lens'
a. a -crystallin b. p-c rystallin c. y- crystallin d . E-crystallin 29. Photoreceptor cones have 1 of 3 visual pigments. with absorptive max ima of 570 nm , 540 nm, and 440 nm. Which of these would absorb blue light the most? a. 570 nm
b. 540 nm c. 440 nm
d. none of the above 30. Cone density is greatest in which area of the retina? a. macula
b. peripapillary region c. arcuate regions
d. peripheral retina 31. Whi ch vitamin is most critical for th e photo recepto r response to light?
a. A
b. B c.
C
d. D 32. Pharmacologic princi ples apply differently to the elderly because a. hepatic perfusion and enzymatic activity increase with age b. renal function decreases with age c. the
elderly have more albumin relative to weight
d. the elderly have more body water relative to weight e. the elderl y have less body fat relative to weight 33. Clinical effects of direct-acting muscarinic agents (miotics) include all except which of the following' a. increased myopia h. decreased range of accommodation c. central anterior chamber shallow in g d. reduced night vision
412 • Study Ques t io ns
34. W hich of th e following is a direct-acting cholinergic agent? a. pilocarpine (lsopto Carpine, Pilocar, Pilostat) b. echothiophate iod ide (Phospholine Iodid e) c. physostigmine (Eserin e Salicylate) d. demecarium bromide (Humorsol) 35. Wh ich of the follow ing statements is correct about cocai ne?
a. It directly stimulates adrenergic receptors. b. It directly stimulates muscarinic receptors.
c. It blocks re -uptake of norepinephrine. d. It blocks adrenergic receptors. 36. Systemic side effects of o ral carbonic anhydrase inhibitors incl ude all except which of the following? a. paresthesias b. weight loss c. hyperkalemia d. metabolic alka los is
37. Which of the following is not a mast -cell stabilizer or an tihista mine ? a. lodoxa mide tromethamine (Alo mide) b. cromolyn sodium (C rolo m) c. olopatadine h ydrochloride (Patanol) d. ketorolac tromethamine (Acula r) 38. Antiviral agents include all except which of the following? a. trifluridine (Viro ptic) h. amphotericin B c. vidarabine
d. acyclovir 39. Which of the following locallregional anesthetic age nts has the longest du ration of effect? a. lidocaine
b. procaine c. bupivacaine
d. mepivacaine 40. A retrobulbar anest hetic is least likely to produce anesth esia of cranial nerve
a. II b. III c. IV
d. VI
Study Questions . 413 4 1. Factors that influence drug penet ration of th e cornea include all of the following except:
a. co ncentration b. drop volume e. pH d. vehicle 42. Which of the following statements about drug penetration is correct? a. Drugs with a higher percentage of charged mo lec ules. or ionic for ms, penetrate more rapidly.
b. Drugs with higher lipid solubility have better penetration of cell membranes. c. Preserved medication has a decreased ocular penetration and may require a higher concentration of the active drug.
d. Most drugs obtain a higher ocular penetration in ointment form than in solution. 43. Timolol 0.5% has how many milligrams (mg) per drop, given 20 drops per milliliter (mL)?
a. 1 mg
b. 0.75 mg c. 0.50 mg
d. 0.25 mg 44. Which of the follOWing series correctly depicts the relative duration of drug ac tion? a. atropine> homatropine>scopoiamine>cyclopentolate>tropicamide b. alropine>scopoiamine> homatropine>cydopentolate>tropicamid e c. cyclopentolate>tropicamide>scopolamine>homatro pine>atropine d. homatropin e>cyclopentolate>tropi cami de >scopolamine>atropine e. homatropine>atropine>scopolamine>cyciopentolate>tropicamide
45. Which of the follOWing agents are associated with co rneal complications, including melting and corneal perforations? a. topical alpha agonists b. topical steroids
c. topical carbonic anhydrase inh ibitors d. topical nonsteroidal anti -inflammato ry drugs
46. Which of the following is a potential side effect of tetracyclines? a. depressio n of prothrombin h. inhibiti on of bactericidal antibiotics c. decreased efficacy of oral contraceptives
d. all of th e above 47. Whi ch of the following is the only agent to have a significant effect on postsurgical en dophthalmitis? 3.
preope rative preparation of the eye with topical povidone-iodine
h. intracamera l vanco mycin c. intracameral aminoglycosides d. subconj unctiva l fluoroquinolo nes
Answers 1. c. The greater wing of the sphenoid is part of the lateral wall of the orbit.
2. d . Cra ni al nerve IV passes through the superior orbital fissure but not through the annulus of Zinno 3. c. The lateral rectu s. The superior, inferior. medial, and lateral recti muscl es all arise from the annulus of Zinno 4. b. The 4 recti muscle insertions form the spiral ofTillaux, with the medial rectus inserting closest to the limbus, at 5.5 mm, and the supe rior rectus most distal from the limbus, at 7.7mm. 5. d. None of the above. The inner and outer limiting membranes are not true membranes. 6. d. The adult human eye has a focusing power of approximately 60 diopters (60 D). The air- tear interface of the cornea is responsible for approximately 40 D, and the lens contributes approximately 20 D of power. 7. d. The neurosensory retina. The uveal tract is the main vascular compartment of the eye and consists of the iris. ciliary body, and choroid. 8. h. C ranial nerve IV exits from the dorsal midbrain. It also has the longest intracranial course of any cranial nerve, approximately 75 mm. 9. c. 40 mm. The intraorbital portion averages 25 mm in length; however, the total nerve also co nsists of intraocular, intracanalicular, and intracranial portions. 10. d. The mandibular division of cranial nerve V (V J) does not pass through the cavernous si nus. 11. d . Aneurysms at the junction of the posterior communi cating an d internal carotid arteries may affect CN III (oculomotor). 12. c. Surface ectoderm. One of the earliest events of embryogenesis is the determination of lens development. The underlying mesoderm signals a region of surface ec toder m to become the lens. The lens becomes apparent by 27 days' gestation. 13. c. Ciliary epithelium is derived from th e optic cup, which is an extension of th e forebrain. 14. c. Homeobox genes are found across the entire plant and animal kingdom. 15. d. The hereditary pattern in familial reti noblastoma is autosomal dominant. The other 3 conditions appear to be a result of a defect in mitochondrial genes. 16. c. Only about 10% of patients with hereditary retinoblastoma have a famil y history of the disease. The remaining 90% appear to have a new mutation in th eir germ cells. 17. d. The inheritance pattern of mitochondrial disease might superficially resemble that of an X-linked trait. Maternal transmission, however, differs from X-linked inheritance in that all the offspring of affected femal es (both daughters and sons) can inherit the trait but only daughters can pass it on. L8. c. More than 70 diffe rent mutations are known to cause retinitis pigmentosa. 19. h. A significant number of disorders associated with the eye or visual system involve mitochondri al deletions or mutations. Mitochondrial diseas e should be considered whenever the inheritance pattern of a trait suggests maternal transmission.
414
Answers. 415 20. d. Linkage is the major exception or modification to the law of independent assortment.
Nonallelic genes located reasonably close togeth er on the same chromoso me tend to be transmitted together more frequently th an by chance alone; thus. th ey are said to be linked. 21. c. Mitochondrial genes are important in th e prod ucti on of peptides, which are involved in
the mitochondrial respi ratory chain and ATP synthase. 22. a. A PAX6 mutation is associated with aniridia. 23. c. Goblet cells make the mucin layer and meibomian glands make the lipid laye r. Glands
of Krause and Wolfring produce the aqueous laye r. 24. a. IgA and IgG are found in th e tear film. 25. c. Descemet's membrane is a true basement membrane produced by th e corneal endothe-
lium. 26. d. Descemet's membrane is a IO-Ilm-thick basement membrane between the endothelium and posterior co rneal stroma. Type IV collage n is the most abundant collagen in Descemet's membrane. Type I collagen, however, is th e major collage n component of th e corn eal stroma. 27. h. The endothelial pump is responsible for ge nerating the negative hydrostatic pressure that is necessary for holding the LASIK flap in place after surgery. 28. d. E-Crystallin is a taxon -specific crystallin, which is found only in a phylogenetically restricted group of species. 29. c. Blue light is predominantly absorbed by co nes that absorb the shorter wavelengths (ie, 440 nm ).
30. a. Cone photoreceptor density is greatest in th e macular region of the retina. 31. a. Vitamin A, a metabolic precursor of ll -cis-retinaldehyde, is most necessary for the light-induced photoreceptor response. 32. b. Compared with younger patients, older pat ients have less lea n body mass due to a de-
crease in muscle bulk, less body water and albumin, and an increase in relative adipose tissue. These physiologic differences alter tissue binding and drug distribution. Human renal function decreases with age. HepatiC perfu sion and enzymatic activity are also affected by age. 33. h. Miotic agents co nstrict the pupillary sphincter and the ciliary muscle. Increasing myopia and decreased central anterior chamber are a result of ciliary muscle contraction. Pu pillary constriction causes decreased night vision but increases the range of accommodation (pinhole effect ). 34. a. In this list, pilocarpine (lsopto Carpine, Pilocar. Pilostat) is the onl y direct-acting cholinergic age nt. 35. c. Cocaine blocks re-uptake of norepineph rine at adrenergic receptor terminals. This increases the adrenergic response but does not directly stimulate or block receptor response. 36. d. Oral carbonic anhyd rase inhibitors can have many system ic side effects, one being a system ic metabolic acidosis, not alkalosis. 37. d. Ketorolac tromethamine (Acular) is a nonsteroidal anti-inflammatory age nt. Each of the others is a mast-cell stabilizer and an antihistamine, or they combine the effects of both.
416 • Answers
38. b. Amphotericin B is an antifungal agent. 39. c. Bupivacaine has the longest duration of effect (8-12 hours) of the locallregional agents listed. 40. c. The fourth cranial nerve (eN IV) is located outside the muscle cone in the orbit and is least likely to be affected by injection of retrobulbar anesthetics. 41. b. When a 50·~L drop is delivered from the usual commercial dispenser, the volume of the tear lake rises from 7 ~L to only 10 ~L in the blinking eye of an upright patient. Thus, changing the volume of the drop does not enhance the absorption of medication. 42. b. Studies of the permeability of isolated corneas to families of chemical compounds show that lipid solubility is more important than water solubility in promoting penetration. 43. d. A 0.5% solution has 0.5 gramllOO milliliters (mL), or 5 milligrams (mg)lmilliliter. As there are approximately 20 drops per milliliter, there are 5 mg120 drops. So, 5 mg120 drops ~ 0.25 mgll drop. If this drop is given bilaterally, there is up to 0.50 mg of active agent available for systemic absorption. 44. b. The duration of action of atropine is 7-14 days, scopolamine is 4-7 days, homatropine is 3 days, cyclopentolate is 2 days, and tropicamide is 4-6 hours. 45. d. Both diclofenac and ketorolac have been associated with corneal complications, including melting and perforation. The preponderance of patients were found to be on generic diclofenac, which was subsequently removed from the US market. 46. d. Tetracyclines may depress prothrombin, thus prolonging the bleeding in patients on anticoagulation medication. As bacteriostatic drugs, tetracyclines may inhibit bacterial medications and should not be used concurrently. They also may decrease the efficacy of oral contraceptives. 47. a. Topical povidone-iodine solution (Betadine 5%) exhibits broad-spectrum antimicro-
bial activity when used to prepare the surgical field and rinse the ocular surface. It has been shown to have a significant effect on postsurgical endophthalmitis.
Index (j = (lgurc; t = table) AA. See Arachidonic acid ABC transporters. See ATP binding cassette (ABC)
transporters Abducens nerve. See Cranial nerve VI Ahetalipoprole inem ia
microsomallriglyccride transfer protein defects causing. 298
vitamin supplements in management of, 232 AC(llI tIUlIIlOcba, 385 treat me nt of keratitis/ocular infection caused by.
385- 386 Acceptor splice site, 147
Accessory lac rimal glands, 26f. 27/, 33, 239-240, 239/ of Krause, Bf, 26f, 271, 33, 239-240 of Wolfring. 23/. 26f. 27/, 33, 240 Accommod ation, 67, 69j. 70 aging affecting, 67 muscarinic drugs affecting. 335 in near reflex. 99 Accommodative esotropia, m uscarinic agents for managem ent of, 339
Accbulola!' 345/ Acetazolamide, 351, 352t, 353 susta ined- release ora l preparation fo r, 329, 353 Acetylcholine, 2591 clin ical usc of, 335~33 7, 336/ drugs affec ting receptors for, 334 - 344, 335f. 336f. 337f. 339f. 343f Sec also Cholinergic agents in iris-ciliary body, 258, 2591 sphin cter activity affected by, 260 synthes is/ release/degradat ion of, 335- 337, 337j in tear sec retion, 240, 242, 242/ Acetylch olinesterase. See Cholinesterasel acetylcholinesterase Acetylcholineste rase inhibitors. Sec C holinesterasel acety lcholinesterase inhib itors Acetylcysteine, 333 Acetyltransferasc. in isoniazid pharmacogenet iCS. 229 Ach . Sec Acetylcholine Achromatopsia gene defects causing. 297 with myopia. racial an d e thnic concentration of, 21 1 Achromycin. See Tetracycl ines Acidic fibroblast growth fac to r. Sce aiso Fibroblast growth fac to r in aqueous humor, 270 Acidosis, carbonic anhydrase lOP lowering and. 353 Acinar cells, lacrimal gland, 32. 33/ differentiation of, 140 Acroce ntric chromosome, 147 ACTH (adrenocorticotropic hormone), in tear secretion, 244 Actin. 277 Actin fila ments. 277 Activase. Sce Tissue plasminogen activator Activators, 332
Active transport /secret ion ac ross retinal pigmen t epithelium. 307 aqueous humor composit ion affected by, 266 in aqueous humor d ynam icS, 254. 255 in lens, 278 Acular. See Ke torolac Acyclovir, 38 lf, 382-383 Adaptation, light, 294 Adenylyl/ adenylate cyclase adrenoceptor binding and, 348, 349 recepto r- effector coupling a nd, 262, 262/ tear secretion and, 243, 243/ Adie pupi l (tonic pu pil). pharmacologic test ing for, 338 Adnexa. See Ocular adnexa Ad renaline. See Ep inephrine AdrenergiC agents. 344- 35 1. 345f. 347f. 3471, 350f See
also specific agenr for glaucoma agoni sts, 2641, 346-347, 3471, 348- 349 antago nists /p-blockcrs, 264 !, 349- 35\, 3501 as miotics, 260, 348 modes of action of, 2641 as mrd riatics, 260, 345-346 systemic absorp tion of, 344 AdrenergiC n eurons, 259 AdrenergiC receptors, 259, 259! drugs affecting, 344-35 1, 345/ in iris- ciliary body, 259, 259!, 260, 262~263, 344 locations of, 344 signal transduction and , 262- 263, 262f. 263f, 264! in tear secretion, 242, 242f, 243, 243f Adrenocorticotropic h o rmo ne (ACTH), in tea r sec retion, 244 ADRP. See Retinitis p igmen tosa, autosomal dom inan t Advi1. See Ibu profen Afferent fibers somatic. 105 visceral, 105 Afferent pupillary pathway, 99 nFGF. See Acidic fibrobl ast growth fac tor Age/aging accommodative response/ presbyopia and, 68 ciliary muscle affected by, 63 Descemet's membra ne/ corneal e ndot h elium affected by. 45, 251 lens changes associated wi th . 67-68, 314 len s proteins affected by. 277 mitochondrial DNA diseases and. 173 parental, chromosomal aberrations in Down syndrome and, 20 1, 202 pha rmacologic principles aflected by. 323 vit reous changes associated with, 84, 84f, 286-289 Age- related cataracts, in d iabetes, 279 Age- Related Eye Disease Stud y (A REDS), 314~3 1 5 Age- related macula r d egene ration/ macu!opathy (sen ile macula r degeneration), retinal pigment epitheliu m abno r malities associated wit h , 309 Agonist, 33 1
417
418 • Index Aicardi syndrome, 220
Air-tear film interface. 43, 44/ AK Beta. See levobunolol AK -Chlor. See Chloramphenicol AK-Con. See Naphazoline AK-Dex. See Dexa methasone AK -Dilatc. See Phenylephrine AK-M)'d n. See Eryth romycin AK·PenlOlate. See Cydopentolate AK-Poly- Bac. See Polymyxin B. in combination
preparations AK-Pred. See Prednisolone
AK-Sulf. See Sulfacetamide AK-Tob. See Tobrnmycin AK-Tracin. See Baci tracin Akarpine. See Pilocarpine AKPro. See Dipivefrin Alamast. See Pemirolast Alaway. See Ketotifen Albalon. See Naphazolin e Albinism, 2 13t, 2 14, 308 defective melanin synthesis and, 308 enzyme defect in , 2 131 , 214 oc ular, 308
ocular findings in carriers of. 222J. 222t X-li nked (Nettleship-Falls), ocular findings in carriers of. 222j. 223 oculocutaneous, 195. 308 racial and ethn ic concentration of. 211 tyrosinase-negative/ positive. 195.2131,308 Albumin in aqueous h umor, 268 in vitreous, 285 Alburerol,2591 Akainc. See Proparacaine Alcohol (ethanol), use/abuse of, maternal, malfo rmat io ns associated \vith . 142- 143. 142/ Aldehyde dehydrogenase, in cornea, 248 Aldose reductase in cataract formation , 280-28\ in lens glucoselcarbohydrate metabolism, 280, 280/ Alkaptonuria, 2 12- 2 14, 2UI AJI-trmls-retinol/ all -tralls-retinaldehyde. 293. 306 Allele-specific marking (genetic imprinting). 154, 169 Allele-spedfic o ligon ucleotides. 148 in m utation screening, 184 Alleles, 148. 194 - 195 n uU. gene therapy and. 187- 188 Allelic association. See Linkage disequilibrium Allelic heterogeneity. 148, 192- 193, 193 Allergic conjunctivitis, drugs for. 364- 366, 365t Allergic reaction s. to penicillin. 369 Alocri!. See Nedocromil Alomide. See Lodoxamide Alpha (u )· adrene rgic agents, 345-348, 347f, 347t agonists, 347/ direct-acting, 345-346 for g laucoma. 264/. 346- 347 indirect-acting, 346- 348. 347/ antagonists, 260. 348 Alpha (a)· adrenergic receptors. 344, 345/ blocking, 260 in iris-ciliary body, 259. 2591. 260. 262- 263
signal transduction and. 262 - 263. 262j. 263J, 2641 in tear secretion, 242, 242/ Alpha l (oj)-antit rypsi n, in aqueous humor, 269 Alpha (a)- blockers, 260. 348 Alpha (a)-crystallins. 70. 276 Alpha (u)-galaClosidasc, defective, 2 131 Alpha (a) helix. 166, 167/ Alpha (u)- L - i duronid;l.~e, defective. 2 131 Alphal (ul l-macroglobulin , in aqut.'Ous humor, 269 Alpha (u)-m:mnosidase. defective, 2 13t Alpha «(1) -mehlllocyte-stim ulating hormone (u- MSH ). in tear secretion, 244 Alphagan. See I3rimonidine Alport d isease/syndro me. pleiotropism in , 2 10 Alrex. Sec Loteprednol Altacai ne. See Te tracaine Altafri n. See Phe n),lephrine Alternat ive splicing, 165, 168 Alu repeat sequence, 148, 166 Amacrine cells, 75f, 300 differentiation of, 126 Amau rosis Leber congeni tal (congenital/inrantile/ childhood retinitis pigmentosa) guanylate cyclase mutations causing. 296 RPE65 gene defects causing, 297 Amber codon , 148 Amethocaine. See Te tracaine Amikacin, 3681. 375- 376 Amino acids in aqueous hu mor. 255 140 in tear Aminoca proic acid/ c-aminocaproic acid. 394 Aminoglycos ides, 3721. 375- 376 ototoxicity of, m itochond ria l DNA mutations and, 173. 176 p-Aminohippuric acid , in aqueous h umor dynam ics, 255 Am niocentesis, 228- 229 Amoxicill in .370 Amphotericin H, 379- 380, 379t Ampicillin, 368/, 370 intravenous administration of, 329 Amplifying cells. transient, in corneal epithelium, 43 Anaerobic glycolYSiS. in glucose/carbohydrate me tabolism in cornea. 248 - 249 in lens. 278 Anaphase lag, mosaicism caused by, 203 Anaphylactoid reactions, penicillin causing, 369 Anaphylaxis, penicillin allergy caus ing, 369 Ancobon. Sec Flucytosine Androgens, in tear secretion, 244 Ane mia. aplastic carbonic anhydrase inhibitors caUSing, 354 chloramphenicol causing, 375 Anestacaine. Su Lidocaine Anesthesia (anestheticsl, local (topical/regio nal). 386- 390. 387t for anterior segment surgery, 389- 390 in patients taking cholinesterase inhibitors, 340 AneuplOidy. 148. See IIlso specific disorder of autosomcs, 199-203 of sex chromosomes. 199
mm,
Index . 4 19 Aneurysms, cranial nerve 111 affected by. 99 Angelman syndrome. imprin ting abnormalities causing. 154 , 169 Angiogenesis, vitreous as in hibitor of. 287 Angular artery eyelids supplied by, 29 orbit supplied by, 37/ Angu lar vein. 40j Aniridia. 205- 207 mutation rate of. 207 short arm I I deletion synd romelPAX6 gene mutations and, 141. 166.205.206.2 16 Annulusof Zinn , 10, 16f.1 7J. 18. 191.9 1 Anomalies. congenital. Sec specific lype and Congen ita l anom alies Antagonist, 332 Antazoline.364 Anla1.Oline/napnazoiine, 364, 3651 Anterior banded zone, of Desccmet's membrane, 45. 46[,25 1 Anlerior cerebral artery. 93 Anterior cha mber, 41 -42. 41J. SO- 52, 51/. 52/ depth of, 50 development of. 135- 136, 136/ topography of, 41 - 42, 41/ Anterior chamber angle, 41J. 49f. 50. 5 1f, 52/. 53j development of, 135- 136. 136j Ante rior ci liary arte ries. 2 1,34. 36- 38,38, 38j; 39f. 62. 64 development of. 132 Anterior cli noid process, Ilf Anterior communicating artery. 93 Anterior inferior cerebellar artery. 104 An terior lacrimal crest. 5, 6J. 27 An terior lamella, 25f Anterior pigmented layer, of id s. 58, 60f Anterior .~cg ment , surgery on , topical anesthetics for.
389-390 Ante rior uvei tis, corticosteroid route of administration in . 3601 Antibacterial agents. 368- 378. 3681. 3721. 3731. See elisa specific agenl (lnd Antibiotics Ant ibiotics. 368-378, 368(, 3721. 3731. Sec (lIsa specific
Iype aud specific agelll for ACClllllmmoeba keratitis, 385- 386 p-Iactam, 368- 371 , 3681 in ((l mbination preparat ions with ant i-in namm atory agents, 373t intrave nous administration 0(, 329 ototoxicity of, mitochond rial DNA mutations and. 176 resistance to. 369, 37 1,375.377 Anticholinergic agents. 260. 334. 340-342. 34 11,342 adverse effects of, 342. 343f Anlicipalion (genetic). 148. 162.208- 209 Ani idt·pressants. aprac\onidine/brimonidinc interaclions and, 348 Anlifibrinolytic agents, 394 Alltifibroti c agents, 366-368 Anti fungal agents, 378 - 380. 3791 An tigen -presenting cells. ill cornea, 43 Anliglaucoma agents ad renergic agonists u -adrenergic agonists. 264 (, 346-347 p-adrenergic agonists, 3471. 348-349
p-blockers, 2641, 349-351. 3501 calcium channel blockers. 261 carbonic anhydrase inhibitors. 255, 351-354, 3521 ci liary body as target fo r. 254 combined preparati ons. 355 cycloplegics, 34 1 hypc rosmotic!os rnotic ,lgcn ts, 355-356, 3561 hltanoprost, 354. 3541 miotics. 264(, 338 prostaglandin analogues. 256- 257, 2641. 354-355, 3541 receptors in mode of action of. 264 , 2641 Antihistamines. 364. 365t Anti-infective age nts, 368- 386. See also specific type and AntibiOlics Ant i-in nammatory agellts, 356- 368. 357r, 3601, 3621, 3651. See also specific agenl (HId Corticosteroids: No nsteroidal anti-int1a lllmatory drugs ar'lChidon ic acid release affec ted by, 358 in combination preparations wit h antibiotics, 3731 fo r keratoconjunctivitis sicca, 245 AntimetaboHtes, 366-368 Anlimuscari nic agents. 340-342, 341 1 adverse effects of, 342. 343/ Anliollcogenes. See Tumor-suppressor genes Antioxidants in lens, 313 - 314, 3 14f in retina and retina l pig mcnt epithelium, 315-3 16, 316- 318 supplemental, 395 Antisense DNA, 148 in gene therapy, 188. 189/ AntiSe nse oligonucleotides. in gene (he rap)'. 188, 189/ It ,-A nt itrypsin. in aqueous humo r, 269 Antiviral agents, 380-385, 38 1t fo r herpetic eye diseasc. 380, 38 11. 382-383 intravitreal admin istrati on 0(, 328 resistance to, 382 systemic. 38 1t, 382-385 topical. 380, 38 1f Aplastic anemia carbonic anhydrase inhibitors causing, 354 chloramphenicol causing. 375 Apocri ne glands of eyelid , 23. 271 Apolipoprotein D, in aqueous humor. 269 Aponeurosis, levator. See l.evator aponeurosis Apop\()sis, 147, 148 in DNA repair, 170 Appositional growth , in development of lamellae, 135 Aprac\onid ine, 259 /, 346, 3471 Aquaporin 0 (majo r intrinsic protei n/MIP). 274, 277 in lens transport, 277 Aqueous humor, SO, 254, 265- 272 biochemistry and metabolism of. 254- 255.165-272 carbohydrates in, 266/, 268 carbon dioxide in, 272 ca rbonic anhydrase inhibito rs su ppressing formation of, 35 1, 352 com position of, 265- 272. 266t dynamics of. 254- 255, 265 glutathione in, 268 growth mod ulator)' fac tors in, 270-27 1 inorganic ions in, 2661, 267 intraocular pressure and. 255. 265
420 • Index organic anions in, 2661, 267
ATPasc-6 gene, in neuropathy with ata.'( ia and retinitis
oxygen in. 27 1- 272
pigmentosa. 175 Atrial natriuretic peptide. in aqueous humor, 267 Atrophy, gyrate, 2131 ornithine aminotransferase defec ts causing. 2 131. 298 rel inal pigment epitheliu m in, 309 Atropine. 2591, 260. 340- 342. 34 11 adverse effects of. 3431 in Down syndrome patients. pharmacogenetics and. 229 systemic absorpt ion of. 324 Atropi ne-Care. See Atropine Atropisol. See Atropine Auditory meatus. internal, 105 Aulomated DNA seq uencing, 184, 185f Autonomic pathways. cholinergic drug action and, 334, 335/ Aulo-oxidation, 31 2- 3 13 in lens. 313. 3 14f vitamin E affecting, 3 12. 3 17 Autosomal dominant inheritance. 2 16-21 7, 217t Autosomal recessive inh eritance. 2 12- 216, 213/, 21 51 Autosomes, 148, 193 aneuploidy of, 199- 203. See also specific disorder Axenfc1 d anomaly/synd rome, 141 Axenfeld loop, 48 AzaSite. See Azit hromycin Azelastine, 364, 3651, 366 Azidothymidine. See Zidovud ine Azithromycin. 3721. 377 Azlocillin.370 Azapi. See Brinzolamide AZT (azidOlhymidine). See Z idovud ine
proteins in , 254, 255. 266. 268-270 separat ion of from blood . Sec Blood-aqueous bar rier sodium transport and, 351 urea in, 268 vascular endothelial growth fa cto r (VEGF) in, 270, 27 1 Aqueous layer (component ) of tear 42, 237, 238[ Aqueous tear deficiency, 244- 245 Aqueous veins, 54, 57! Arachidonic add (a rachidonate) in eicosanoid synt hesis. 255, 256f, 257 nonsteroidal allli -inflammatory drug derivation lind,
mm.
36 1-362 release of, 257 anti-inflammatory drugs affecti ng. 358 in re tinal pigment epithelium. 305 in vitreous, 286 Arachnoid mater, optic ne rve. 9 1. 92f. 93/ Area centralis. 79, SOj. See also Macula AREDS (Age-Related Eye Disease Study), 314-315 Arrestin, 293 mutations in, 182. 295 ARRP. See Retinit is pigmentosa. autosomal recessive Arterial circles, 38. 391, 571, 58 development of, 132, 138 Arteria l plexus, cili ary body, 62 Arteritis, giant cell (tempora]), corticosteroid route of administration in, 3601 Arlifkial tears, 245, 390- 39 1 Arylsulfatase A, defective. 2 131 Ascertainment. 148 Ascorbate. See Ascorbic acid Ascorbic acid (vitamin C) antioxidant effect of, 317- 3 18 in lens, 314- 3 15 in retina and retinal pigment epitheli um, 3 17-318 in aqueous humor, 255. 2661 oral supplements and. 3 14-3 15 in tea r film , 240 in vitreous. 2661 ASO. See Allele-specific oligonucleotides Aspirin. 362-363. 3621 prostaglandin syn thesis afrected by, 257 Association (genetic). 177 al lelic. See Linkage d isequi librium Assortalive mati ng. 148 Astrocytes optic ne rve, 90. 9 1, 921 retinal, 77, 30 I Ataxia interm ittent. 2131 with neuropathy and retinitis pigmentosa, 175- 176 mitochondrial DNA mutations and, 174[. 175 - 176 Ataxia-telangiectasia ( Louis- Bar syndrome). ATM mutat ion in, 170 Atenolol. 3451 ATM gene mutation . 170 ATP binding cassette (A BC) transporters, 293 mutations in, 296 ATP production in lens, 278-279 in rod outer segments, 293
SAC (bacte ria l artificial chromosome). 149 Sacampicillin, 370 Bacitracin, 3681. 3721, 378 in combination preparations. 3721, 3731 Bacterial artificial chromosome (BAC), 149 Bacteriophage. 149 HAL ( British anti lew isite), for Wilson disease, 231 Ha rdet-Hiedl syndrome, pleiOl ropism in , 2 10 Baroreceptors. phe nylephrine affec ting. 346 Barr body, 149 Basal lamina (basal cell layer) of choriocapillaris. 64, 651 ciliary body. 62. 621 corneal. 43, 44J. 45. 46J. 248. See also Descemet's membranellayer lens_ See Lens capsule of retinal blood vessels. 78 of retinal pigment e pithelium , 64. 651 Base pair. 149 mutations of, 17 1 conserved,17 1 Basic fibroblast growth factor, 270, 348. See also Fibroblast growth factor in aqueous humor. 270 Hasic secretion test/basic secretDrs. 241 Basilar arlery, 109 Bassen-Kornzweig s)'ndromc. MTP defects causing, 298 Balson venous plexus, 104 BAX gene, in DNA repair, 170
Index . 421 Beaded fila ments. 277 Bcnoxi nutc. with flu orescein , 387 1, 389 Bem.alkonium absorption affected by, 327 in art ificial tears, 39 1 toxic reactions to. 322 Bergmeister papilla. 85 Best disease (vitelli form macular dystrophy/ vitel liruptive macular dege neration ) bestrophin defect causing, 297 identification of carriers fo r, 217 retinal pigment epitheli urn in, 308- 309 Bestrophin, mutations in. 297 Beta (Il)-ad renergic agents, 3471. 348- 35 1, 350r ago nists. 3471, 348- 349 antago nists, 349- 35 1, 3501 for glaucoma. 264 /, 3471. 348- 349 Beta (Il)-adre nergic receptors, 2641. 344. 345J. 3471, 348- 349 in iris-ci liary body, 2591. 262, 263 signal transduction and. 262. 262J. 263. 264 1 in tear secret ion . 243, 243f Beta (P)-blockers, 349- 35 I , 350t for glaucoma, 264 1. 349- 35 I . 350t Beta (Il) carotene. antioxidant effect of. 3 15 oral supplements and . 3 15 Beta H!l-crystallins. 70, 276 Bela (p)-galactosidase, defective. 2131 Beta
Blinking (blink reflex) medication absorption and, 323, 324, 327 lear secretion and. 144 Bloch-Sulzberger syndrome (inconti nent ia pigmcnt i), 219. 220 Blood- aqueous barrier. 254 . 265 clinica l implications of breakdown of. 272 medication ad ministration and. 329 Blood pressure, phenylephrine affecting. 345-346 Blood- retina ba rrier. 72. 78. 254 breakdown of, mcdication adm inistration and, 329 Bl ood vessels. Sec VlIsc ular system Blue-cone achromatopsia, ge ne defects causing, 297 Blue-cone monochromat ism. ocular fin dings in carriers of, 2221 Blue-light phototoxic ity. 3 16 Blunt trau ma. optic neuropathy caused by. 93 Ho ny o rbit. See O rbit Botox. Sec Botulinum toxin. type A Botulinum toxin . type A, 26. 390 periocular injectio n of, 327 Bow zone/ region, 274. 274f Bowman's layer/ mem brane. 44 , 44J, 247J. 249 anatomy of, 44. 44J. 247f biochcmistry and metabolism of, 249 development of. 135 refract ive surgery and, 249 Bowman's zone, development of. 135 bp. See Base pair Brain natriuretic pept ide, in aqueous humor. 267 Branch chain decarboxylase, defective. 2131 Brimonidinc. 346- 348. 347/ in combinalion preparations. 3471. 3501, 355 Brinzolam ide. 352. 3521 British antilewisitc ( BAL), for Wilson disease, 23 1 13romfenac. 3571, 363 Bruch's membrane. 64- 66. 65J. 751 development of, 127 . 132 Bu lbar conjunctiva, 29. 30f, 34 Bupivacaine. 3871. 388. 389 RUloxamine, 2591 C4 complement. in aqueous humor. 269 Cah/protcin kin ase C-dependent signal t ransduction, in tear secretion, 242 - 243, 24 2f C Als. Sec Carbon ic anhydrase inhibi tors Calcitonin gene- related peplide in iris-ciliary bod y. 259 outflow facility affec ted by. SO in sigllaltransduclion. 2641 in tea r secretio n. 240 Calcium in aqueous humor. 267 in signal transd uction in iris- ciliary body, 26 1,262- 263. 263f in tear secreti on, 242- 243, 242J in lear fil m. 2381 Calcium channel blockers, 26 1 ClIlcium channels, 26 1 L-type.26 1 mutations in. 296 CALT. See Conjunct iva-associated lymphoid tissue
422 • Index c AMP. in signal t ran sduction in iris-ciliary body. 262- 263. 262/ in lear secretion, 243. 243/
Canal. See specific lype Cana liculi. lacrima l, 32f, 33, 34 C:mcer DNA damage and. 169 genetic and familial fac tors in, 171 - 173. 172/ junk DNA and, 166 toss of telomeric DNA and. 162. 166 Cancer genes, 171- 173. I nj Candidate gene screening. 180- 182 pos itio nal, 182 CAP. See Chromosome arlll painti ng Capi llary-free zone (foveal avascula r zone), 79f. 81. 82f Capillary plexus. of ciliary process, 6 1 CapsulopalpebraJ m uscle. 28 Ca rbachol, 335- 337, 336f, 338, 3381 Ca rbenicillin, 3681, 370, 376 Carbocaine. See Mepivacainc Carbohydrate ("suga r") cataracts, 279-281 aldose reductase in development of, 280-281 Ca rbohyd rates in aqueous humor, 2661. 268 metabolism of in cornea. 248 in lens, 279. 279-2 81,280/ in re tinal pigment e pit hel iulll , 304 Carbon dioxide, in aqueous hUlllor. 272 Carbonic anhydrase in aqueous humor, 269 dynamics and, 255, 351 in ciliary epithelium , 351. 352 Carbonic a nhyd rase inhibitors. 255. 35 1- 354, 3521 for glaucoma, 255 . 35 1- 354. 3521 in suppression of aqueous formation, 255, 351,3 52 Ca rboxypeptidase E, in aqueous humor, 267, 270 Carcinogenesis, loss of telome ric DNA and, 162, 166 Carotenoids (xanthophylls) in lens. 3 14- 3 15, 3 14/ in retina, 81, 31S, 3 18! Ca rotid arteries, interna l. 107, 107/. 109 optic nerve supplied by, 93, 95 Ca rotid plexus, 105, I06/. 107 Carrier (genetic), 149,2 14 - 2 15, 220 ocu lar findings in, 221 - 222f, 2221 Carteolol, 350, 3501 Caruncle, 22J, 3 1 Catalase. 312! in lens, 313 in retina and retinal pigment epithelium, 3 17 Cataract antioxidants affec ting r isk of, 3 14-3 15 carbohydrate (diabelicl"suga r 279- 28 1 aldose reductase in development of, 280-2S1 conge nital and infanti le, 14 1 muscarinic therapy 'l1ld, 338, 340 Cathepsin D, in aqueous humor, 267, 269 Calhepsin 0, in aqueous humor, 267, 269 Cavernous sinus. 104- 105, 107. 107/ CCAAT box, 149 M
),
e DNA,150 from ciliar}' body-encoding plasma proteins. 269 sequencing, IS4, 185f c DNA clone, 149 eDNA library, 154, 182 Cefadroxil.370 Cefamandole, 370 Cefa7.0lin, 3681. 370 Ce fepime, 37 1 Cefoperazone, 37 1 Cefolaxime, 37 1 Cefoxilin, 370 Cefpirome, 371 Certa7.idime, 368/, 371 Ceftizoxime.371 Ccft ria xone.371 Cefu roxime, 370-37 1 Celebrex. See Celecoxib Ce1ecoxib. 258 Cell death , programmed (I)C Dfapoptosis). 147, 148
in DNA repair. 170 Cel lular retinaldehyde- binding proteinfc),toplaslllic retinal.binding protei n (C RALBP) in aqueous hUlllor. 267 mutations in, 297 Centimorgan , 149 Central ret inal artery, 78, 90, 94. 95j, 96 Central retinal vein. 40j. 90, 96 Cent ral zone, 70 Centromere, 149 Cephalcxin. 370 Cephalosporins. 368- 369. 3681, 370-37 1 allergiC reaclion to, 369 Cephalothin, 370 Ccphradine, 370 Ccram ide trihexosidasc, defective, 2131 Ceramide trihexosidc, in Fabry disease, plasma infusions for, 23 1 Cerebellar arteries, anterior in ferio r, 104 Cerebellopontine angle. 104. 105 Cerebral artery, anterior, 93 Ce rebroside u -galactOSidase, defective, 2131 Cer uloplasmin, in Wilson disease, 23 1 Cervical ganglion. superior. 15, 60 dilator muscle inn ervation and, 258 Ce rvicofacia l tfunk, of Crllllia l ncrve VII , 106 Cetamide. See Sul facetamide eGM P in cone phototransduc tion, 293 m utations in. 181. 295 in rod phototransduction. 292, 293 cGNIP-gated channel cone, 293 mutations in, 296 rod, 292-293 mutation s in , 295 CGR P. See Calciton in ge ne-related peptide Chamber angle_See Anterior chamber a ngle Chelation therapy for band keratopathy, 334 for Wilson disease, 23 1
Index . 423 Chiasm (optic), 89f. 93, 96 ocular developme nt and, 128 Chievitz, tran sient nerve fiber layer of, 126 Chloramphenicol. 372/, 375 in travenous administration of, .329 Chloride in aqueous humor, 255, 266 /, 267 ;n lens. 278 in tear film , 238/, 240 in vitreous, 266t, 286 Chloromycetin. See Chloramphenicol Chloroptic. See Chloramphenicol Chlortetracycline, 374 Cholinergic agents, 334- 344, 335[. 336f. 337f. 339[. 343f See also specijh.: agent am/ Muscarinic agents; Nicotinic agents adverse effects of, 338, 339/ antagonists, 334, 340- 342, 341 / 344, 348 adverse effects of, 342. 343/ as mydriatics. 260, 340- 342. 341 1 direct.acting agonists. 334, 335- 339, 336f. 337f. B8t. 339/ indirect·acting agonists, 334, 336f, 339-340, 342- 343. 346-348,347/ tiS Illiotics, 259, 335- 340, 337/. 338t, 339/ Cholinergic neuron s, 259 Cholinergic receptors. 259, 259r drugs affecting, 334- 344. 335J. 336[' 337J. 339J. 343/ in iris-cilia ry body, 259. 259t locations of, 334 signal transd uction and, 2M! C holi nesterase/acetylcholinesterase in acetylcholine degradation , 335, 337/ de fective, succinylcholi ne effe cts and. 229- 230 Cholinesterase/acetylcholin esterase inhibitors, 259, 334, 336j. 338', 339-340, 342- 343 miotic action of, 259, 338t. 339- 340 Cho ndroitin sulfate in cornea, 250 in corneal storage med ium, 395 as viscoelastic, 393 C hondroitinase. for e nzymatic "itreolysis. 289 Chorda tympa ni . 106 Choriocapillar is, 65f, 66. 66f. 67[' 74/ differe ntiation of. 132 Chorionic vill us sampling. 149,229 Cho roid, 41/, 64 -66 anatomy of, 4 If, 64- 66, 65[, 66f. 67/ development of, 132- 133 gyrate atrophy of, 21 31 vasculature of. 64, 65f, 66. 66f. 67 development of. 132- 133 Choroidal arte ry. opt ic nerve supplied by, 94. 95/ Choroidal fi ssure. Sec Embryonic fi ssure C horoideremia ocular fin dings in carrie rs of. 22 If. 2221 REP I (Rab escort protei n I) gene mutat io ns in , 298 reti nal pigment e pithelium in , 309 Chromatid, 149 Chromatin , 149
Chromosomal aneuploidy, 148. See also specific disorder autosom e, 199- 203 sex chromosome, 199 Chromosome arm ptlinting, 199, 2oof, 20 1/ Chromosomes, 193-194. See a/so Autosomes abnormalities of. See a/50 5pecific Iype congenital anom alies and. 141, 197- 198 et iology of, 204 - 207, 2051 identification of, 197-207, 198!, 200f. 20 If See also Cytogenetics incidence of, 197 acrocentric, 147 analysis of. 197- 207, 198/, 200f, 201/ See also Cytogenetics bacterial artificial (SAC), 14 9 homologous, 153, 193 morphologically variant (cytogenetic markers). 177 sex, 193, 194. See ailo X chromosome; Y chromosome aneuploidy of. 199 mosaicism and, 204 translocation of, 162 Down syndrome caused by, 20 1-202 yeast artificia l (YAC), 163 Chronic progressive external ophthalmoplegia (CPEO). 174- 175 mitochondrial DNA deletions ;lIld, 173, 174- 175, 191
Chronic progressive ex ternal opht halmoplegia (CPEO)·pius synd romes, 174- 175 Cido(ovir, 38lt, 384 Cilia. See Eyelashes Ciliary arteries, 21 , 34 , 36-38. 37f. 38, 38/. 39f. 62 anterior, 21, 34, 36-38, 38. 38f. 39f. 62. 132 choroid supplied by, 64 de\·clopment of. 132, 138 ex traocular muscles su pplied by, 38, 38f optic ne rve supplied by. 94 , 95, 95/ poste rior, 36-38. 37J. 38, 38[' 39J. 62, 132 Ciliary body. 4 If, 5 If, 52f, 53J. 6 1-64, 253-264 anatomy of. 4 If. 5 If. 52f, 6 1- 64 . 62f, 63/ bioche mistry and metabolism of. 253-264 development of. 137 epit helium of. See C iliary epithelium protein types expressed in. 253 Signal t ransduction in , 258-261 smoot h muscle in, 253- 254 st roma of. 61-63 topography of, 41/ uveal portion of. 62- 63 C iliary epithelium. 6 1- 63. 62f. 265 aqueous humor formation and. 254. 265. 266-267. 270 development of, 137 no npigmented. 62, 62/, 265 pigme nted. 62, 62f. 265 C iliary ga nglion. 14-16, ISf branches of. 15-16 Cilia ry muscle. 63-64. 631. 258, 259 in accommodation , 68 development of. 137 miotics affecting, 335. 339 muscarinic d rugs affecting. 335. 339. 341 -342
424 • Index Cilia ry nerves long, 36, 247 posterior, 36, 24 7 short , ISf, 16,36, 99 ciliary m uscle sup p lied by. 64 iris sphin cter supplied by, 61 Ci liary processes, 52/. 6 1, 258 developme nt of, 137 C ilio retinal artery, 78, 94 , 95/ C iliu m, ph otoreceptor, 73 , 76/ Ciloxan. See Ciprofl oxacin C iprofl ox;lcin , 37 1-373, 3721 Circle of Willis. 93, 94j, 109, 109! Circle o f Z inn -Haller (circle of Ha lle r a nd Z inn ), 94 I I -cis retinaldeh yde, 292, 305-306 I I-cis reti nol dehydrogenase, mutations in , 297 C itrate, in tear fil m, 240 CI. See Chlo ride Cla rith romyci n, 377 Clinda mycin, 368 t Cli n ica l helerogeneity, 149, 193 Cli n ica l tria ls (experim enta l/in ter ventio nal studies), for new d r ug, 333 Clinoid process anterior, I I/ posterior, 104 Clinori!' See Sulindac Clivus (umbo), SOf, 104 C lone cDNA, 149 geno m ic. 152 Clonin g veClor, 149~1 50 Cloquet ca na l, 85 Cloxacilli n , 369- 370 eM . See Cent imorgan Cocaine n o repinep h rine up take affec ted by, 260, 346 as topical anesthetic, 387 t, 389 C o chlin , in vitreous, 286 Coding strand o f D NA (sense D NA), 160 Cod o mina nce (co d om inant inherit ance), 150,2 11 Codo n , ISO am ber, 148 initiato r, 154 sto p/ te rmination, 16 1 fra meshift m utation and , 152 Colistimethate, 368t Collage n in choroid , 66 in ciliary m uscle. 64 corneal. 44 . 45, 134,249, 249- 250 in len s capsu le, 68, 273 scle ral. 48 st ro mal , 44, 249- 250 in vitreous, 82- 83, 83f, 130 , 132,283, 283- 284, 284f liquefactio n and, 286 Collagen cornea sh ields, fo r dr ug admin ist ratio n , 33 1 C olla rette (i ris), 58, 59f, 137 C o llecto r ch annels. 54 - 55, 57f Coll icul us, (acial, 104 Colobo m as, 113, 125J, 166 em bryonic fi ssure closu re and, 123, 125f o pti c nerve/optic d isc, 14 1
Color vision, 294- 295 . 298 defects in genetic basis oC 294-295 o cular find ings in ca rr ie rs o f, 222 t trivariant, 294- 295 C o mbigan. See Bri m o nidi ne, in combination p re parat io ns C omm u nicatin g arlery, anterio r, 93 Com plem ent, in aq ueous h umor, 269 Com plementary DNA (e DNA), 150 fro m ciliary body- e ncod ing p lasma proteins, 269 seq uencing, 184 , 18 5f Complementary D NA clo ne (cD NA clone), 14 9 Complementary DNA libra ry (cD NA library). 154, 182 Compound he te rozygo te, 150, 194 -1 95, 21 6 Cone inner seg me nts, 76f See also C ones Cone outer segme nts. 76/. 8 1. See also C o nes different iatio n o f, 126 sh ed, re tinal p igm ent ep ith eli um ph agocytos is o f, 306~307
Co ne respo n se, 294, 30 1[' 302 Cones, 73, 74, 76[, 77f See (l /so C o ne o uler segments amacrine cells fo r, 300 bipo la r cells for, 298, 299/ developmen t o f. 126 electrophysiologic res po nses of, 30 1[' 30 2 extrafovea!' 74 fovea!' 74, 8 1 gen e d efects in , 296- 297 ho rizon tal cells for, 299~ 300 p holotransd ucl ion in . 293-294 C o ngenita l. defin ition of. 150, 191 - 192. See also Genetics C o ngenital anoma lies. 14 1- 143, 142f Sce also specific Iype
chrom osomal ana lysis and , 198 , 1981 genetic in fl uences a nd , 141, 197- 198 nongenet ic teratoge ns causing, 14 1- 143, 142f Congenita l nigh t blindness, stationa ry, with myo p ia, ocular fi ndings in carrie rs of, 2221 C o njunctiva, 29, 30j. 3 4 ~ 35 anato my of, 29, 30j. 34 - 35 b lood vessels of, 34-35 b ul bar, 29, 30/' 34 drug absorpti on and, 32 5 ep ithelium of, 35, 239f tear secretion and , 239/. 240, 24 1 fo rn iceal, 29, 3D/. 34 palpebral, 29, 3D/. 34 C onju nctiva-associated lympho id t issue (CA LTI conjunctiva l MALT ). 35 C onju nctival arteries. poste rio r, 34 Conjunctival sac, 237 C o njunctivitis allergiC, d rugs fo r, 364-366, 365 t con icoste roid ro ute of adm in istration in, 3601 Conn ective tiss ue eyelid,23 pe ricanalic ula r. 54 , 55/ Consangui n ity, 150, 215,227 Consensus sequence. 150 Co nservation (genet ic). 150 C on served base-pair mutations, 17 1
Index . 425 Consuhand .225 Contig, 150 Contiguou s gene d eletion synd rome, 206 Contraceptives, oral. tetracycline use and. 374 Convergence, in near reflex, 99 Copper in aqueous humor. 267 in free radica l for mation. 3 11- 3 12, 3 13 in Wilson disease, 231 Cornea anatomy of, 4 If, 42, 43-4 7, 44f, 46f, 47/ basal lamina of, 43. 44f, 45. 46/ biochemist ry and metabolism of. 247- 251. 247/ Bowman's layer/ membrane of, 44 , 44f, 247f, 249 anatomy of. 44, 44f, 247/ biochem istry and metabolism of, 249 develo pment of. 135 refractive surgery and, 149 central, 43 , 441 Descc rnet's membrane of. See Descemet's membranel layer develo pment of, J33- 135, 133f, 134/ donor. See Do nor cornea drug absorption a nd, 315 e ndothelium of, 46-47, 247f, 25 1 anatomy of. 46-47, 46f, 47j. 247/ biochemi stry nnd metabolism of, 25 I development of, 133, 134f, J 35 glucose metabolism in, 248 epitheli um of, 43, 44f, 239f, 247f, 248-249 a natomy of. 43, 44f, 247/ biochemistry and metabolism of. 248- 249 development of, 133, 133f, 1341 glucose metabolism in, 248 tear secretio n and, 239f, 240, 241 guttata/guttac ce ntral, 45 in Fuchs d ystro phy, 47/ periphe ra l ( Hassall-Henle bodies/warts), 45 in nervat ion of, 247 layers of, 247, 247/ nonepithelial cells in, 43 peripheral, 43, 44/ stroma of, 45. 46f, 247f, 249-250 a natomy of, 45, 46f, 2471 biochemistry and metabolism of, 249- 250 development of. 133-1 35, 134/ topography of, 4 If. 42. 43 Cornea shields. collagen, for drug admini st ration, 331 Corneal dystroph ies, endotheli:tl, Fuchs, 47. 47/ Cor neal nerves,247 Corneal sto rage medium. 394-395 Corneosderal junction, 49, 52/ Corneosderal meshwork, 53, 54, 551 Cortex, lens, 70, 274- 275, 274/ Cortical-n uclear barrier, in lens, 3 14 Corticosteroids (ste roids), 357- 361, 3571. 3601 adverse effects of, 358-359 a nti-i nflamma tory effects/pote ncy of. 359-360, 3601, 364.366 fo r dry eye, 245, 246/ intraoc ular pressure affec ted by, 359- 360, 3601, 361
fo r ocula r allergies/i nflamma tion. 364. 366 route of administration of. 360, 360 t in tear secretio n, 244 Cosmid vector. 150 Cosopt. 350t, 352-353, 3521, 355 Counsel ing. genet ic. See Genetic testing/counseling COX-I/COX-2 . See Cyclooxygenase COX-2 inhibitors, 257-258 CPEO. See Chronic progressive external ophthalmoplegia CPEO · plus syndromes. 174- 175 CRA. See Central retinal arte ry CRAI.BP. See Cell ula r retinaldehyde-binding protein' cytoplasmic retinal-bi nding protein Cranial nerve I (olfac tory nerve), 87, 88f, 89/ C ranial ne rve II . See O ptic nerve C ranial ne rve 111 (oculomotor nerve), S8f, 97-99, 97j. 98! anatomy of, 88f, 97- 99, 97f, 981 aneurysms affecting, 98 extraocular muscles inne rvated by. 21 , 97- 99, 97/ motor root of, 14, IS! Crania l nerve IV (troc hlear nerve). S8f, 99- 100 anato my of. 88f, 99- 100 extraocular muscles inne rvated by, 21 Cranial nerve V (trigem inal nerve). 881. 100- 104, IOIj. 102! anatomy of, 88f, 100- 104, 10 11, 1021 divisions of. 100, 102f, 103- 104 VI (ophthalm ic nerve), 881. 10 1. 101j. 102[, 103 sensory root of, 14, 15/ VI (maxHiary nerve) , 88! 101, 101f, 102f, 103 VJ (mandibular nerve). S8f, 10 1. 10 1f, 102[. 104 Cronial nerve VI (abducens nerve). 88f, 104- 105. 104/ anatomy of, S8j. 104-1 05. 104/ extraocu lar muscles innervated by, 2 1 C ranial nerve VII (facial nerve), 105- 107, 106/ anatomy of. 105- 107, 106/ cervicofacial d ivision of, 106 labyri nthjne segment of, 105 mastoid segm ent of, 106 temporofacial d ivision of, 106 tympanic segment of, 105 Cranial nerves, 13.87- 109. S8f. See "~ Iso specific lIerl'l! a natomy of. 13,87- 109.88/ C raniofacial dysostosis (Crouzon syndrome). 2 16 Crest cells. See Neural crest cells C ribriform plate, 92f See also Lamina cfibrosa C rigle r-Najjar syndrome. 2 131 C rolom. See C romol)'n C romolyn, 365, 3651 C rossing over (genetic), ISO. 197. See a/so Recombination unequal,162 color vision defects and, 295 Crol.lzon syndrome (craniofacial dysostosis), 216 CRY, See Central ret inal vein Crystalline lens. See Lens Crysta llins, 70. 130. 273 U, 70, 276 p. 70. 276 p.y.27. development of, 130
426 • Index ~,2 7 6
y, 70, 276 taxon-specific, 276 zeta ( ~ ), 11 5 Cu -Zn SOD, 317 Cupping of optic disc, 90 CVS. See Chorionic villus sampling Cyclic adenosine monophosphate (cAMP), in signal transduction in iris~ciliary body, 262--263, 2621, 263f in tear secretion, 243, 243f Cyclic guanosine monophosphate (cGMP) in cone phototransduction, 293 mutations in, 181 in rod phototransduction, 292 , 293 Cyclic guanosine monophosphate (cGMP)-gated channel cone, 293 mutations in, 296 rod,292- 293 mutations in, 295 eyclogy!. See Cyclopentolate Cydomydril. See Cyclopcntolate Cyclooxygenase (COX-l/COX-2) aspirin affect ing, 257, 362 in eicosanoid synthesis, 256f, 257 nonsteroidal anti-inflammatory drugs and, 362 in prostaglandin synthesis, 256f, 257 Cyclooxygenase-2 (COX-2) inhibitors, 257- 258 Cyclopentolate, 260, 3411 Cyclopia, sonic hedgehog mutations causing, 141 Cycloplegia/cycloplegics, 260, 34 J t anticholinergic agents/muscarinic antagonists and, 260,341-342,34lt Cyclosporine/cyclosporine A, 334 for dry eye, 245, 246f, 391 topical, 245, 246f, 334 Cyklokapron. See Tranexamic acid Cylate. See Cyclopentolate Cystathionine p-synthase, abnormality of in homocystinuria, 2 131 Cystinosis, renal transplant for, 231 Cystoid macular edema, corticosteroid route of administration in, 3601 Cytochrome-b gene, in Leber hereditary optic neuropathy, 175 Cytogenetics, 197- 207, 1981, 200f, 201f indications for, 198, 198t markers in, 177 preparation for, 198 - 199 Cytokines, in tear film, 241 Cytomegalovirus retinitis cidofovi r for, 38 It, 384 foscarnet for, 381(, 384 ganciclovir for, 38lt, 383-384 Cytoplasmic retinal -binding protein/ cellular retinaldehyde -binding protein (CRALBP) in aqueous humor, 267 mutations in, 297 Cytoskeletal (urea-soluble) lens proteins, 277 Cytovene. See Ganciclovir D-penicillamine, for Wilson disease, 231 DAG. See Diacylglycerol
Dapiprazole, 2591, 260, 348 Daranide. Sec Dichlorphenamide Deafness (hearing loss) gene for, synteny and, 176 mitochondrial DNA mutations and, 173, 174f, 176 Decadron. Sec Dexamethasone Decamethonium, 344 Decongestants, ocular, 392 Deep petrosal nerve, 107 Defensins, in tear film, 240 Defy. See Tobramycin Degeneracy, of genetic code, 150 Degenerations, retinal gene defects causing, 298 retinal pigment epithelium defects and, 308 Deletion mapping, 177 Deletions, mitochondrial DNA, disorders caused by, 173, 191 Demecarium, 340 DemecJocycline, 374 Demulcents, 390-391 Denaturing gradient gel electrophoresis, 183-184 Deoxyribonucleic acid. See DNA Depolarizing neuromuscular blocking agents, 344 Dermatan sulfate, in cornea, 250 Descemet's membrane/layer, 45, 46f, 247f, 251 anatomy of, 45, 46f, 247f biochemistry and metabolism of, 25 I development of, 134f, 135 guttae in, 45 in Fuchs dystrophy, 47f Desmosomes, in corneal epithelium, 43 Deutan defects (deuteranopia), ocular tlndings in carriers of, 2221 Developmental anomalies. See specific type and Congenital anomalies Dexamethasone, 3571, 359 anti-i ntlammatory/pressure-elevating potency of, 3601 in combination preparations, 3731 intravenous administration of, 329 Dextran in corneal storage medium, 395 in demulcents, 390 DFP. See Diisopropyl phosphorotluoridate DGGE. Sec Denaturing gradient gel electrophoresis DHA. Sec Docosahexaenoic acid Diabetes mellitus aqueous humor growth factors and, 271 cataracts associated with {carbohydratel"sugar" cataract),279- 281 aldose reductase in development of, 280-281 type 2 (non- insu lin-dependent/ NIDDM/adultonset), mitochondrial DNA mutations and, l73 Diabetic macular edema, corticosteroid route of adm inistration in, 360t Diacylglycerol (DAG), in tear secretion, 242j; 243 Diagnostic agents, 392 - 393 Diamox. See Acetazolamide Dibenamine, 260 Dichlorphenamide, 353 Dic1ofenac, 257, 357t Dic1oxacillin, 369- 370
Index . 427 DIDMOA D synd ro me. pleiot ro pism in. 2 10 Diet/diet therapy, in inborn erro rs or metabolism , 23 1 Diffusion, in aqueo us h umo r d yna m ics/formation, 254 DiOuca n. See Fluconazole Digenic inheritance , 150 Dii sopropyl phospho roflu o ridate. 259. 339-340 Dil ator m uscle. 58f, 59f, 6Of, 258, 259 (l·adrcnergic agents afrec ting. 345. 346. 348 development of, 60, 137 miotics affecting, 259t. 260 myd riatics affecting. 2591. 260. 345 Di nucleotide repeats, 155 Dipivefri n (d ipiva lyl epineph rine) . 330. 347t Diplo id/diploid num be r. l SI Direct sequenci ng, 184, 185! Discontinuity, zo ne of, 70 Disod ium e thylened iaminete traacetic acid (EDTA) in artificial tears, 39 I for band keratopathy. 334 in irrigating solutions, 392 Disomy. un iparental, 162, 169 Dispase, fo r enzym atic vitreolysis, 289 Distichiasis.29 DNA, 15 1 a mplifica tio n of, in polymcrascchll in relictio n, 158 llIu isense, 148 in gene the rapy, 188. 189/ complem entary (cDNA), 150 fro m ciliary body-encoding plasma proteins, 269 clone. 149 .sequenci ng, 184, 185J damage to. 169- 170 junk. 165- 166 methylation of, 168 mi tocho nd ri al di seases assoc iated with delet ions/ mutations of, 173- 176. 174f. 19 1,208 ga ltonian inheritance of, 208 Leber heredi tary optic neuropathy, 174f, 175 maternal inheritance/ transmission and. 208, 220 in replicative segregation, 160, 173 recombi nant, 159 repair of, 169- 170. See (llso M utation replicatio n of, 159 o rigin . 157 segregatio n an d, 160. 196 slippage, 160 satellite. 160, 166, 180 sense. 160 sequencing of, 184, 185J telomeric (telomeres). 162, 166 DNA libraries, 182, 183! cDNA. 154. 182 genomic. 154. 182. 183/ Docosahexaenoic acid. in retinal pigment epithelium . 305 Do m inan t allele, 15 1,2 11,2 16 Do minant in he ritan ce (d o minant gene/ tnlit ), 15 1, 2 11 -2 12 autosomal. 2 16-217. 217t disorders associated with, 15 1, 212 gene therapy fo r. 188. 189/ X· linked.2 19-220 X·linked , 219, 2191
Dominant negative mutation, 15 1 ge ne therapy fo r disorders caused by, 188, 189J Donor cornea, sto rage of. 394-395 Donor splice site, 15 1 Dopam ine. o ute r-segment disc shedd ing and, 306- 307 Dopami ne-p . hydroxylase, d efective, 2 131 Dorello ca03.l, 104 Dorzolamide, 352. 3521 in com bination preparations. 3501. 352- 353. 3521. 355 Double hete rozygotes, 195 Down syndrome (trisomy 2 1).20 1- 203,2021 mosaicism in, 204 pharmacogenetics and, 229 Doxycycli ne, 334. 374 [Joy ne honeycom bed dystrophy, EFE MPI defects causi ng. 297 DPE. See Dipivefrin Drug resistance. See also specifi( ageflt atld specific
orgatlism antibiotic. 369. 37 1,375, 377 antiviral. 382 Drugs a lle rgic reaction to, pe nicillins, 369 a nt ibiotic. 368-378, 3681, 3721. 3731 genetics affec ting (pharmacoge net ics), 157, 229- 230 ocular. See also specific agetll absorpt ion of, 322 , 323-327. 324f. 325f adrenergic, 344- 35), 345J antibiotic, 368-378. 368 /, 3721, 373 , antifibrinolytic.394 antifungal, 378-380. 3791 a nti · infla mmatory. 356-368, 3571, 3601, 3621, 3651 antioxidant/vita min supplements. 395 antiviral, 380-385, 38 1/ carbonic anhydrase in hibitors, 35 1- 354, 352t cho linergic. 334- 344 , 335f. 336j. 337f. 3391; 343J combined agents. 355 concentration of, absorption a ffected by, 326 fo r corneal storage medium , 394- 395 decongesta nts, 392 diagnostic agents, 392-393 for dry eye. 245. 246f, 390-39 \ in eyedrops, 323 - 327. 324f, 325/ fibrinol ytic,393 growth fa ctors, 395- 397 hyperosmolar, 39 1 inte rfe rons, 395 intraocular injectio n of. 328 int raveno us adm ini strat ion of. 329 investigation alJdinicaltesti ng of, 333 for irrigation. 392 legal aspects of use of. 333-334 local administration of, 327-328 local a nesthetics. 386- 390, 3871 m echanisms of action of. 322. 33 1-3 32 m ethods of desig n a nd de live ry of, 330- 33 I
new future research a reas a nd, 32 1 investigationalJcli nical testing of, 333 off- label usage of, 333- 334 in o intments. 327 oral prepa rations of. susta ined· release. 329 osmot ic agents. 355- 356. 356r
428 • Index partition coefficients of. 326 periocular injection of, 327 pharmacodynamics of, 322, 331-332 pharmacokinetics of, 322. 323- 331 pharmacologic principles and. 32 1-332 phar macotherapeutics of. 322 prostagla ndin analogues. 354-355, 3541 purified neurotoxin complex, 390 receptor inte ractions and, 33 1- 332 solubil ily of. absorption affected by, 326 systemic absorpt io n of. 322. 324- 325. 32sf systemic administrat ion and , 328-330 thrombin . 393-394 tissue binding of, 327 topical. 323-327, 324f. 325/ See also Eyedrops sustained-release devices for, 330-33 1 toxicity of, 321,322-323 aging and, 323 tissue binding and, 327 viscoelasli cs, 393 viscosity of. absorption affected by. 326 vitamin/ antioxidant supplements. 395 Drusen, 73 Dry eye synd ro me surface inflammation and, 245, 245j. 246/ tear film abnormalities and. 244. 246/ treatment of. medica l management, 245, 246/, 390-39 1 Duchenne muscular dystrophy. mutation rate of. 207 Dulcitol (galactitol) in cataract fo rmation , 28 \ in lens glucose/carbohydrate metabolism, 280/ Duplication mapping, 177 Dura mater, optic nerve. 9 1. 92j. 93f Dysautonomia, fami lia l ( Riley- Day syndrome). 2 13t racial and ethn ic concent ration of, 21 1 Echolh iophale. 259. 338/. 339-340 in Down syndrome patients. pharmacogenetics and , 229
Econopred. See Pred nisolone Ectoderm . 117, 119j. 120/ ocular st ructures derived fro m , 116t cornea, 133, 134/ lens, 128-129. 129/ Ectropion. 60 uveae, 60 Edetate disodium, for band keratopathy, 334 Edi nger-Westphal complex/nucleus, 6 1, 971. 98, 99 Edrophonium, 336j. 342- 343 in myasthenia gravis diagnosis. 329. 342- 343
EDTA in artificialtcars, 391 for band keratopathy, 334 in irrigating solutions, 392 EFEMP I gene/EFEMPI protei n (EGF-cont:'Lining fibrillin -like extracel lular matrix gene/protein) mutations, 297 Efferent fibers, visceral. 105 Effe re nt pupillary pathway. 99 Efl o ne. See Fluorometholone EGF-containing fibri ll in-Iike extracellular matrix protein (EFEMP I ) mutations, 297
Eh lers- Danlos syndrome, ll3t Eicosanoids, 255- 258, 256/ Electrolytes. in tear film. 238t, 240 Elec tro -oculogram, trans- RPE pote ntial as basis for, 307 Electrophoresis, denaturing gradient gel. 183- 184 Electrophysiologic testing of retina, 30 1- 302, 30 1/ Electroretinogram , 30 1 Eiesta!. See Epinasti ne Elevated intraocular pressure cort icostero ids causing, 359- 360. 3601. 361 cycloplegics causing, 34 1 dr ugs for. See specific ngelll alld Antiglauco ma agents Il pl3 synd rome (short arm I I del etion syndrome). 205- 207 Ellipsoid of cone. 74, 76/ of rod, 73, 76/ ELM. See External limit ing membrane Emadine. See Emedastinc Embryogenesis, 117- 120, II Bf, 11 9f, 12 1f, 122/ Embryology. See Congeni tal anomalies; Eye, development of Embryonic fissure, 121, 123, 124/ clos ure of, 123, 125/ colobomas and, 123, 125/ persistence of, in fetal alcohol syndrome, )43 Embryonic nucleus, 130 Embryotoxon, posterior, 141. See a/so Neurocristopathy Emedastine. 364. 3651 Emissaria/ emissarial cha nnels. 48 Emollients. oc ular, 390. 39 1 En grappe nerve endings. 2 1 En plaque nerve endings, 21 Encephalopathy, Le igh necrotiz-ing (Leigh syndrome), )76 , 213t mitochondrial DNA m utation and. 174f, 176 Encopred. See Prednisolo ne Endoderm . 117. 119f, 120/ Endo nucleases, 15 1. See a/so Restriction endonucleasE's Endophtha lmitis. corticosteroid route of admi nistration in ,360t Endothelial d ystrophies, Fuchs. 47. 47/ Endothelial mesh .....o rk, 54 Endothelin receptors, in signal transduction, 2641 Endothelium corneal, 46- 47, 24 7j. 251 anatomy of, 46- 47. 46j. 47f, 247/ biochemistry and metabolism of. 25 1 development of, 133, 134f, 135 glucose metabolis m in, 248 retinal blood vessel. 78 Energy production in lens, 278-279 in retinal pigment epithelium, 304 in rods, 293 Enhancer, 151 En Ion . See Edrophonium Enz.ymatic method. for DNA sequencing. 184. 185/ Enz.ymatic vit reolysis, 289 Enzyme defects, disorders associated with, 2 12. 212- 214,2131 management of, 231 - 232 recessive inheritance of, 2 12- 2 14 , 2 131
Index. 429 Enzymes, in aqueolls humor, 269-270 Epiblast, 11 7, 118/ Epidermal grm ... th faClo r, 395- 396 in tear film , 24 1 Epidermal growth faClor- containing fibrillin · like extracell ular matrix protein (EFEMP!) mutations, 297 Epifrin. See Epinephrine Ep in astine, 365t, 366 Epinephrine, 347t adrenergic agentsfreceptor response a nd , 344-35 1. 345/ dipivaly!. See Dipivefr!n with local anesthetic, 388 Episclera, 48 Episcleral plexus, ciliary body, 62 Episcleritis, corticosteroid route of administration in, 360r Epithelium ciliary. 6 1-63, 62f, 265 aqueous humo r formation a nd, 254, 265, 266- 267, 270 development of, 137 nonpigmented, 62, 62!, 265 pigmented, 62, 62[. 265 conjunctival, 35. 239/ tear sec retion a nd, 239j, 240, 24 1 corneal. See Cornea, epithelium of le ns, 68, 68f. 69f, 70, 274, 274/ Epsilon (£)-crysta lli ns, 276 Eq uator (lens), 274, 274/ Erythromycin, 3681. 372 t, 377 intravenous administ ration of, 329 Eseri ne. See Physostigm ine Esotropia, accommodat ive. muscar inic agents for management of, 339 EST. See Expressed sequence tags Esterase 0 , retinoblastoma and, 177 EthanoL See Alcohol Ethmoid/ethmoid al bone (lami na papyracea), 7 Ethmoidal air cells/ ethmoidal sinuses, 7f, 8f, 12. 13f, 14J Ethmoidal artery, 37j, 39/ Ethmoidal foram ina, anterior/ posterior, 8f, 10 Eth nic background, genetic disorde rs and, 210-211 Et hylenediaminetetruacetic acid (EDTA) in artificial tears, 39 1 for band keratopathy. 334 in irrigating solutions, 392 Eukar yotes/eukaryotic cells, 151 Exc ision repair, 169 Excretory lacrima l system, 27j, 33- 34, 34J Exon, 151 , 165 Exon shufflin g, 165 Experimental (interventional ) studies (clinical trials), for new drug, 333 Expressed sequence tags, 15 1, 182,253 Expression, of genes. See Transcription (gene) Exp ressivity (gene tic), 15 1, 209-210 External limi ting mem brane, 75/, 76f, 78, 80/ development of, 126 Extrafoveal cones, 74 Extraocular m uscles, 16- 22. 16j, 17f. 18f, 19f, 191 , 20j, 21t. See a/so specific muscle anatom y of, 2 1- 22 . 2 1t blood supply of, 18- 21, 19t, 38, 38/
development of, 139 fiber types in. 21 , 21t innervation of, 2 1 insertions of, 16- 17, 18f, 191 origins of, 18, 19t within orbit, 18, 19f, 20/ Eye anatomy of, 4 1- 85. See also specific slructllre development of, 113- 143. See also specific structure abnormalities of, 11 7, 141- 143. 142f See also specific type atld Congenital anomalies chronology of, 122t embryogenesis, 11 7- 120, 11 8f, 119f, 121/ growth factors in, 113- 114 homeobox genes and, 114-11 5 neural crest cell s and, 115-117, 116t, 120f, l21/ organogenesis. 120- 140, 1221 gla nds of, 271 vasc ular system of, development of, 137- 138, 139/ Eye movements, control of, extraocular muscles in, 2 1 Eyeba ll. See Globe Eycdrops (topica l medications), 323-327, 324f, 325/ sustained release devices for, 330-33 1 Eyelashes (cilia), 23, 23f, 29 misdirection of. See Trichiasis Eyelid- globe incongruity, in tear deficiency, 244 Eyelids, 22-31,22/ accessory structures of. 3 1 anatomy 0(, 22-29, 22f, 23f, 24/, 2Sf, 26f, 27f, 29/ anterior fa scial support system of. 36/ conjuncti... a of, 29, 30f, 34-35. See also Conjunctiva connective tissue structures of, 23 developme nt of, 139- 140, 140/ lower anatomy of, 22, 26/ developme nt of, 140, 140/ lymphat iCS of, 30- 3 1,31/ margin of, 23, 26/ movement of, in tear film renewal and distribution, 244 orbital fat and, 27 orbital septum and, 2Sf, 27-28 ski n of, 22-23 subcutaneous tissue of, 23 tarsus of, 25/ upper anato my of, 22, 22- 29. 22f, 23f, 24J, 25f, 26/, 27f, 29/ development of, 139- 140, 140/ vascular supply of, 29- 30, 30/ venous drainage of, 30 Fabry disease, 2 131 ocular find ings in carriers of, 2221 plas ma infusions in mar.agement of, 231 Facial a rtery eyelids supplied by, 29, 30/ orbit supplied by, 37/ Facial colliculus, 105 Facial nerve. See C ranial nerve VII Facial vein, 40/ Fallopian canal, 105 Famciclovir, 329, 38 1/, 383
430 • Index Fam ilia l, definition of. 19 1. Sec also Genetics Fami lial dysautonomia (Riley-Day syndrome), 2131 racial and ethnic con centration of, 211 Familial penetrance. 216 Family history/ familial fa ctors in genetic counseling, 225- 226, 226j. 227 pedigree analysis and, 225-226, 226/ Famv ir. See Famcidovir Fanconi syndrome, tetracyclines causing. 375
FAS. See Fetal alcohol syndrome Fascia bulbi. See Tenon capsule Fascicles cranial nerve IV, 99 opt ic nerve, 90, 92/ Fast-twitch fibe rs, 21, 21t Fat, orbital, 27 Fatty acids peroxidat ion of, 3 11 , 312-3 13 in retina, 315 in retinal pigment epithelium, 305 in vit reous, 286 PAZ. See Foveal avascula r woe FDA. See Food and Drug Administration Feldene. See Piroxicam Felderstruktur muscle fibers, 2 1,2 11 Fenoprofen, 3621 Fetal alcohol syndrome, 142 - 143, 142/ Fetal banded lone, 135 Fetal fi ssure. See Embryonic fissure FGF. See Fibroblast growth factor FGFR2 gene, in C ro uzon syndrome, 216 Fiber layer of Hen le, 78, 79, 79/ Fibril-associated proteins, in vitreous, 285-286 Fibrillar opacities, 286 Fibrillenstru ktur muscle fibe rs, 2 1. 21 t Fibrillin .286 Fibrinolytic agents, 393 Fibroblast growth fa ctor (FGF), 11 4,395-396 in aqueous humor, 270 homeobox gene expression affected by, 11 5 in ocular development, 114 in tear film , 241 Fibroblast growth factor receptor-2 gene, in C rouzon syndrome, 2 16 Fibronec lin, in vitreous. 283 Fibrous astrocytes. 77 Fibulin - I. in vitreous, 283 Fifth c rania l nerve. See C ranial nerve V FiJensin, 277 First c ranial nerve. See Cranial nerve I First-degree relatives, 159 First-order neuro n, dilator m uscle innervation and,60 FISH. Sec Fluorescence in situ hybridization Fissures embryonic, 121, 123, 124/ closure of, 123, 125/ colobomas and, 123, 125/ persiste nce of, in fetal alcohol syndrome, 143 orbital , 10- 12. 12/ inferior, 8f, 9f, 10-12 superior,7f, 10, II/. 12/ palpebral, 22, 22f, 29/
Flarex. See Fluorometho lone Flicker fusion freq uency, 294 FloxaciUin, 369- 370 Flucaine. See also Fluo rescein, with proparacaine Fluconazole. 379t, 380 Flucytosine/s -tluorocytosinc, 379/, 380 Fluocinolone implant, 328, 331 , 361 Fluor-Op. See Fluorometholo ne Fluorncai ne. See Fluorescein, with proparacaine Fluorescein, 392 with benoxinate, 3871. 389 intravenous ndministral'ion of, 329. See (/Iso Fluorescein angiography with proparacaine. 3871 Fluorescein angiography, 392 Fluorescence in situ hy bridization (FISH), 199,
200f S-Fluorocytosine. See Flucytosi ne Fluorometholon c, 357/, 359 anti -inflammntory/pressu re-elevating potency of, 3601 Fluoroquinolones, 37 1- 373. 3721 Fluorouracil, 334, 366-367, 385 Flurbip rofen , 257, 3571. 363 Fluress. See Fluorescein . with benoxinate Hurox. See Fluorescein, with benoxinate FML. See Fluoro metholone Folinic acid, 374 Food and Drug Adm ini stration, 333 Foramen (foram ina) e thmoidal, anterior/posterior, Sf, 10 infraorbital,9 opt ic. 7f, Sf, 10. 1 tf orbital, 10, II/ supraorbital. 5, 6f. 7j. \0 zygomatic, 10 Fornices, 29, 3Df, 34 FosC;lrnet, 38 1I . 384 for cytomegalovirus retinitis, 381/, 384 Foscilvir. See Foscarnet Fossae, lacrimn l, 7, 7f, 8/ Fourth cranial nerve. Sce C ranial nerve IV Fovea (foveacen trali s), 71. 74f. 79, 79f. 80/' 81. 82/ development of, 126- 127 Fovea externa, 80/' 81 Fovea l avascular zone, 79f, 8 1, 82[ Foveal cones, 74 ,8 1 Fovea l fibers, 90 Foveal pit, 127 Foveo)a, 79, 80f, 81 Fragile site/ fragilit y, chromosomal, 151-1 52 Fragile X syndrome, 152 Frameshift mut'alion (fram ing error/ frameshift). 141, 152 Free radica ls (oxygen radicals), 3 11 -318 cell ular sources of, 311 - 3 12, 3 12/ in lens, 313-3 15, 314/ lipid peroxidation and, 31 1, 312- 313 in retina. 3 15- 3 16 Front al nerve, 103 Frontal sinuses, 9j, 12, 13f, 14/ 5-FU. Sec Fluorouracil Fuchs d ystrophy. 47, 47/
Index. 431 Fundus albipunctatus, II-cis retinol dehydrogenase defects causing, 297 flavimaculatus (Stargardt disease/juvenile macular degeneration) ABC transporter mutations causing, 296 retinal pigment epithelium in, 309 oculi,71 Fungizone. See Amphotericin B G38D mutation, 295 G-protein-coupled receptors, 262 prostaglandin, 258 in tear secretion, 242, 242f, 243, 2431 G proteins in cone phototransduction, 294 in signal transduction, 262- 263, 2621, 2631 in tear secretion, 242, 2421 G6PD deficiency. See Glucose-6-phosphate dehydrogenase deficiency GABA in cone phototransduction, 294 horizontal cell release of, 299, 300 GAGs. See Glycosaminoglycans Gain of function mutations, 171 Galactitol (dulcitol) in cataract formation, 281 in lens glucose/carbohydrate metabolism, 2801 Galactokinase, defective, 213t Galactokinase deficiency, 213t cataract associated with, 280 Galactose in cataract formation, 281 in lens glucose/carbohydrate metabolism, 2801 Galactose I-phosphate uridyltransferase (Gal-I-PUT), galactosemia caused by defects in, 213t, 280 Galactose-free diet, 231 Galactosemia,213t cataracts in, 280, 281 dietary therapy for, 23 1 a -Galactosidase, defective, 213t [3-Galactosidase, defective, 213t Galanin, in aqueous humor, 267, 270 Gallamine, 344 adverse effects of, 343f Galtonian inheritance, of mitochondrial DNA diseases,
208 Gametes, 195 Gamma (y)-aminobutyric acid (GABA) in cone phototransduction, 294 horizontal cell release of, 299, 300 Gamma (y)-crystallins, 70, 276 Ganciclovir, 330-331,381 t, 383 - 384 for cytomegalovirus retinitis, 38lt, 383 - 384 Ganciclovir sustained-release intraocular device, 328, 330-331, 38 It, 384 Ganglion cells, retinal, 74f, 75, 75I, 78, 79f, 299f, 300 development/difterentiation of, 126 Ganglionic blocking agents, adverse effects of, 3431 Ganglionic neurons, dilator muscle innervation and,60 Gap junctions, in lens, 278 Garamycin. See Gentamicin
Gasserian ganglion (semilunar/tr igeminal ganglion), 101,101-102,lOlf
Gastrula, 117 Gastrulation, 117, 118f, 1I9f Gatifloxacin , 371-373, 372t Gaucher disease, racial and ethnic concentration of, 21 1 GCAPs. See Guanylate cyclase assisting proteins GeL (gangli on cell layer). See Ganglion cells Gel electrophoresis, denaturing gradient, 183- 184 Gelatinase (MMP-2), in vitreous, 283 Gene, 152, 165, 193-194 cancer, 171-173, 172f candidate, 180-182 defective copy of (pseudogene), 159 expression of. See Gene transcription structure of, 165 Gene dosage, 177 Gene duplication. See DNA, replication of Gene mapping, 178, 193 Gene replacement therapy, 187-188,231 Gene therapy, 184-189, 189f Gene transcription, 162, 166-169, 167f reverse, 160 Gene translation, 162, 166 gene product changes/modification after (posttranslational modification), 158 of lens proteins, 277 Generalized gangliosidosis (GM 1 gangliOSidosis type 1),2131 Genetic, definition of, 152, 191 Genetic code, degeneracy of, 150 Genetic heterogeneity, 153, 192, 193 Genetic imprinting, 154, 169 Genetic mapping/genetic map, 178, 193 Genetic testing/counseling, 226-229 autosomal dominant disorders and, 217 family history/pedigree analysis and, 225 - 226, 2261 issues in, 227-228 polygenic and multifactorial inheritance and, 224- 225 prenatal diagnosis and, 228-229 support group referral and, 232 Genetics clinical, 191-232. See also specific disorder alleles and, 194-195 chromosomal analysis and, 197- 207, 198t, 200f, 20 1/ congenital anomalies and, 141 disease management and, 230-232 genes and chromosomes and, 193- 194 genetic counseling and, 226-229 independent assortment and, 196- 197 inheritance patterns and, 211 - 220 linkage and, 193, 197 Lyonization (X chromosome inactivation) and, 220- 224 meiosis and, 195-196, 196f mitosis and, 195 mutations and, 207-210 pedigree analysis in, 225-226, 226/ pharmacogenetics and, 229-230 polygenic and multifactorial inheritance and, 224- 225
432 • Index racial and ethnic concentration of disorders and. 210- 2 11 segregation and , 193, 196 terminology used in , 19 1- 193 molecula r, 165-189 correlation of genes wi th specific diseases and, 176- 182, 179[ 181/ DNA damage/ repai r a nd, 169- 170 gene slructu.re and, 165
genctherap)' and, 184-189. 189/ junk DNA and. 165- 166 L)'onization (X chromosome inactivation) an d, 168- 169 mitochondrial disease and , 173 - 176, 174/
mutations/disease and , 170- 173, 172! sc reening and. 182- 184. IS3/. ISS/, 186- 187/ transcription (expression) and. 166-1 69, 167/ translat ion and, 166 te rms used in, 147- 163 Genic ulate body/nucleus/ganglio n, 105 lateral. 92, 94j. 96 Geniculocalcarine pathways (optic radiations), 93, 94f. 96
Genocopy. 152 Genome, 11 3, 152, 19 1,208 Genomic done, 152 Genomic DNA library, 154 , 182. 183/ Genoptic. See Gentamicin Genotype. 152, 194,208 Gentacidin . See Gentam icin Gentak. See Gentamicin Gentamicin, 368t. 3721, 375- 376 in combination preparations, 3731 ototoxicity of, mitochondrial DNA mutat ions and . 176 Gen tasol. See Gentami cin Germ layer theory, 113 Germinal m osaicism, 152 Germ inative zone, 70 Ghost cells. 288 Giant vac uoles, in Schlem m cana l. 54, 55f. 56/ Gillespie syndrome (M IM 206700). aniridia in, 206 GIOD. See Ga nciclovi r sustained -release int raocula r device Glands of Krause, 23f, 26f 27 /, 33, 239-240 G lands of Moll, 23 . 23f, 26f. 27 1 G la nds of Wolfring, 23f, 26f 27/. 33, 240 G lands ofZeis. 23, 23f, 26f, 271 in tear-film lipids/ tear production, 237 Glaucoma Ciliary bod y as target in management of. 254 corticoste roid-ind uced . 358, 359. 360 drugs for. See Antiglau coma agents ocular receptors and, 264. 2641 vitreous injury and . 288 Glauctabs. See Methazolam ide Glial cells. re tinal, 77, 300- 30 1 differentiatio n/develo pment of. 126 Globe embryologic/pe ri natal realignment of. 140 topographic fea tures of, 4 1- 42. 4 1J
Glucocorticoids/g1ucocorticosleroids, 357-361, 3571. 3601. See also Cort icosteroids Glucose in aqueous hum or, 255, 2661. 268 in cataract fonmlt ion, 279- 281 metabulism of in cornea, 248 in lens. 279. 279- 28 1. 280f in retinal pigment epitheli um . 304 in tear film, 240 in vit reous, 266t Glucose-6-phosphate, in lens glucose/carbohydrate metabolism. 279 Glucose-6-phosphate dehydrogenase defiCiency. pharmacoge netics and, 229 Glucuronide transfe rase, defective. 2131 Glutath ione. 3 14f in aqueous humor, 268 in lens. 313 . 3 14 oxidative changes a nd. 3 14 in retina and retinall)igment epithelium, 3 16-3 17 Glutathione peroxidase, 314J in lens. 313 in retina and retinal pigme nt epithelium . 316- 3 17 G lutath ione redox cycle. 314 G lutathione-$-transferase, in relina and retinal pigment epithelium . 3 16- 3 17 Glycerin , 356, 3561 for dry eye, 390 Glyci ne cell transport, defective, 2 13t Glycocalyx, corneal epithelium. 42. 248 Glycolysis, in glucose/carbohydrate metabolism in cornea, 248-249 in lens, 278 Glycoproteins corneal. 248, 249 in vitreous, 283, 285 Glycosaminoglyca ns. stromal, 45, 250 Glyrol. Sec Glycerin GM L gangliosidosis type J (generali7.ed ), 213t GM ~ gangliosidosis type 1 (lay-Sachs disease), 2131 racial a nd e thn ic concentration of. 210-2 11 GM 1 ga ngliosidosis type 11 (Sand hoff disease). 2 13t Goblet cells, 26f, 271, 34, 239J mucin tear secretion by, 42 . 239f, 241 Gramicidin , with polymyxin B and neomycin. 372 1 Gray line (i nte rmargina l sulcus), 23, 26f, 237 Greater (major) arterial circle, 38. 39J. 58 development of. 138 Greater superficial petrosal nerve. 106. 107 Growth factors, 11 3, 395- 397 in aqueous humor, 270- 27 1 homeobox gene expression affected by. l i S in ocular development, 11 3- 114 in tear film , 241 Gruber (petroclinoid) ligame nt, 104 GSH. See Glutath ione Guane th idine. 260 G uanylate cyclase mutations in. 296 in rod phototransd uct ion. 293 Guanylate cyclase -assisti ng proteins (GCAPs) . 293
Index . 433 Gustatory nucleus, 105 Gyrate atrophy, 2J31 ornithine ami notransferase defects causing, 2 131, 298
retinal pigment epitheli um in, 309 Gyrus, rectus, 87, 89/ Haller and Zinn , circle of, 94 Haploid/haploid number, 152, 195 Haploid insuffic iency (haploinsufficiency), 152,206 in aniridia/ PAX6 gene di sorders, 206 gene therapy for disorders caused by, 188 Haplotype, 152 Hassall·Henle bodies/,va rts (periphe ral corneal guttae). 4S
HCO J . See Bicarbonate Hearing loss (deafness) gene fo r, synteny and , 176 mitochond rial DNA mutat ions and , 173, 174f, 176 Helicases, 169 Hel ix·loop-helix motif. 166, 167/ Helix-tum-helix motif, 166, 167/ Hemidesmosomes, 43. 44/ Hemizygote/ hemizygous alleles, 152- 153. 195.2 18 Hemoglobin, sickle cell, 207 Hemoglobinopathies, sickle cell. See Sickle cell disease Hemorrhages, vitreous, 288 Henle fiber layer/ Henle layer, 78. 79, 79f Heparan sulfate sulfatase, defective. 2 J3t Hepatolelltic ular degeneratio n (Wilson disease). 23 1 Hereditary, d efinition of. 153, 19 1. See also Genetics Hereditary optic neuropathy, Leber. mitochondrial DNA mutations and , 174f Heritable, definition of, 153 Hermansky- Pudlak synd rome. racial ;lOd ethnic concentrat ion of, 211 Herpes sim plex virus acyclovir for infect ion caused by. 38 11 , 382-383 antiviral agents for, 380. 38 1I. 382-384 keratitis caused by, topica l antiviral agents for, 380, 38 11.382- 383 Herpes zoster fa mcid ovir for, 38 1I, 383 valacyclovir fo r, 381 t, 383 Herpex. See Idoxuridine Heterogeneity, 193-194 allelic, 14 1;1, 192-193, 193 clinical, 149, 193 genetic, 153, 192, 193 locus, ISS, 192, 193 Heterolluclear (heterogeneous nuclear) RNA (hnRNA), 153, 161 Heteroplasmy. 153, 173 Heterozygosity, loss of, 173 Heterozygote/heterozygous alleles, 153, 194. See also Carri er (genetic) carrier. 149, 2 14- 21 5 compound, 150, 194.2 16 double. 195 Hexokinase. in lens glucose/carbohydrate mctabolism. 279
Hexosaminidase, defective, 213t
Hexose monophosph ate shunt, in glucose/carbohydrate metabolism in cornea. 248 in lens. 279 Hexosidase. defect ive. 2 13t Histamine, 364 History, family in genetic counseling. 225- 226, 226f, 227 ped igree analysis and , 225- 226, 226/ HLH . See Helix-loop -heli x motif HM P shunt. See Hexose monophosphate sh unt HMS. See Medrysone hnRNA. See Heteronudea r (heterogeneous nuclear)
RNA Hola nd ric inheritance/trait, 2 18 Holes macular, vitreous in formation of, 288 optic (optic pits), 120, 121 Holoprosencephaly, sonic hedgehog mutation causing, 141 Homatropine. 260. 34 1t Homeobox, 114, 153. See also Homeobox genes Homeobox genesJhomeotic selector genes, 114, 14 7. 153 congenital anomalies associated with mutat ions in, 1<1 1, 142
gro\"th fa ctors affec ting expression o f, 115 in ocular development, 114 - 11 5 Homcodomain, J 14 Homeotk genes/homeotic selector genes. See Homeobox genes Homocyslin uria, 2 131 vitamin B6 replacement therapy fo r. 232 Homogentisic acid/homogent isic acid oxidase, alkaptonuria and, 2 12, 21 3t Homologous chromoso mes. 153. 193 Homoplasmy. 153. 173 Homotropaire. See Homatropine Homozygote/homozygous alleles. 153, 194 Horizontal cells, 74, 75f, 299-300, 299f in cone phototransduct ion. 294 Hormones, in aqueous humor, 267 Horner muscle, 27 Horner syndromc, dilator muscle and, 60 Host cell, 153-1 54 HOX genes, 11 5, 14 7, 153 H OX7. 1 gent', 115 HOX8. 1 gene, l iS HPM C, See Hydroxypropyl methylceUulose HPMPC. See Cidofovir HT H. See Helix-tum-helix moti f HTRA I, in age-related macular degeneration, 171 Human gene mapping, 178, 193 Humorsol. See Demecarium Hunter syndrome, 21 31 Hurler syndrome, 2131 Hyalocytes, 83-84 , 130 Hyaloid artery/system development of. 123, 124f, 13 1f, 138 persistenceirem na nts of, 85, 85f, I 31f, 138, 139f Hyaluronan/ hyal uronic acid in corneal development, 134 in vi treous, 83f. 130- 132.283.284- 285
434 • Index Hyaluronate/sodium hyaluronate, as viscoelastic, 393 Hyaluronidase in aqueous humor, 269 fo r enzymatic vitreolys is. 289 in vit reous, 283 Hybridization , 154 fluore scence in situ (FISH) , 199, 200! oligonucl eotide. in mutation screening. 184, 186f Hydrocortisone anti -i nllammatory/pressure-elevating potency of.
3601 in combination preparations. 3731 Hydrogen pe roxide, 311,3 11 - 312. 31Z/' 3 L3, 31 7. See also Free radicals Hydroperoxides. 3 13 Hydroxyamphetamine, 260, 3411 for pupillary testing, 346 Hydroxyl radicals, 3 13. See also Free radicals HydroxypropyJ melhylcellulose (HPMC) fo r dr y eye, 390 as viscoelastic, 393 Hyperglycemia, cataract formati on and . See Diabetes mell itus, cataracts associated with Hyperg lycinemia,2 13t Hyperosmolar agents, 39 1 Hyperosmotic/osmotic agents, 355- 356. 3561 Hypertension. phenylephrine causing, 345-346 Hypoblast. 11 7, 11 8f Hypokalem ia. carbonic anhydrase inh ibitors causing, 353 IBU. See Ibuprofen Ibuprofe n. 3621 leA. See Interna l carotid arler y Idoxuridine. 380, 38lt CL -l-lduronidase. defective, 2131 Ig. See under Immunoglobulin IG F- l. See Insulin-li ke growth fa ctor 1 IGFBPs. See Insulin -like growth factor binding proteins ILM . See internal lim iling membrane lIotycin. See Er)1 h romycin Imidazoles, 379t, 380 Imipenem -cilastin , 368/ Immune response (im munity) , g lucocor ticoids affecting, 358 Immunoglobulin A (lgA)Jsecretory IgA . in tear film , 240 Immunoglobulin 0 (lgO), in tear film, 240 Imm unoglobulin E OgE) in tear film, 240 in type I hypersensitivity/anaphylactic reacti ons, 264 Immunoglobulin G (igG), in tear film, 240 Immunoglobu li n M (lgM ), in tear film , 240 Impr int ing (genetic), 154. 169 In situ hybridization, fluorescen ce, 199. 200f Inborn errors of metabolism dietary therapy in, 231 enzyme defect in, 212, 213t. 214 ocular findings in. 213t Incom plete penctrance (skipped gene ration), 209, 217 [ncontinent ia pig menti (Bloch-Sulzberger syndrome ), 219,220 Independent assort ment, 196- 197
Index case (proband ), 158,225 Indirect traumatic optic neuropathy, 93 Indocin. See Indomethacin Indocya ni ne gree n, 392 Indocyanine g reen angiography. 392 indo les. 362. See also Nonstero idal anti- inflammatory drugs Ind omethacin. 362t, 363 prostaglandin synthesiS affected by, 257 induction. in oc ula r developmen t, 113 Infantile phytanic ac id storage d isease (Refslim disease).
2131 gene defects causing, 2131. 298 Inferi or meatus. 7. 1O,34f Inferior oblique muscles, 9, 16f, 17J, 18, 19(, 38f, 39); 40f insertion relationships of, 17, LSf, 19/ nerves supplying. 21 Inferior o rbital fi ssure, 8f, 9f, 10 - 12 Inferior petrosal nerve, 106 Inferior petrosa l sin us. 108f Inferior punctum . 23. 33 Inferior rectus muscles, 16f, 17J, 191,201, 37f, 38, 38J,
39;; 40J insertion relationships of. 16, 18f, 19t nerves supplying, 21 Inflamase Fortell nflamase Mild. See Prednisolone Inflammation {ocular} treatment of. See Anti -inflammator y agents vitTeal,288 In flammatory mediators. See Mediators Infraorbita l artery extraoc ular m uscles supplied by, 18 orbit supplied by. 37/ Infraorbital canal, JO Infraorbital foramen, 9 Infraorbital groove, 8f, 9 Infraorbital nerve. 14J, 103 Infraorbital vein. 40/ Inheritance. See also Genetics codominant. 150.2 11 digenic, 150 dominant , 151,21 1-2 12 autosomal, 216-2 17. 2 17/ disorders associated with , 15 1.212 gene therapy for, 188, 189f X-linked. 2 19- 220 X-linked, 2 19, 2 191 gaitonian, of mitochond rial DNA diseases, 208 holandric, 2 18 m atern31, J 73.208.220 m ultifactori:d. 156.224- 225 patterns of, 21 1- 220 polygenic, 158, 224-225 recessive, 21 1- 2 12 autosomal, 212-2 16, 213/. 21St disorders associated with, J 59.2 12-216,2131, 2 1St gene therapy (or, 187- 188 X· linked, 218- 2 19, 2 191 sex lin ked , 160. 218 X-linked, 163, 185 disorders associated with. 219- 220 gene therapy for. 187- 188 Y-lin ked. 163
Index . 435 Inh ibitors, 332 Initiator codon . 154 In itiator tR NA, 154 IN L. See Inne r nudear layer Inner cell mass, 117, 118/ Inner marginal zone. 124 Inner neuroblastic layer, 126 Inn er nuclear layer, 2. 74f, 75f. 78. 79f, 298- 301 . 299f. 300f Inner plexiform layer, 74f. 75J. 78, 79/ developme nt of, 126 Inner segments, photoreceptor, 73, 74f, 76/ See also Cone inner segments: Pholo reccptorsi Rod inner segments [nosilo l- I ,4,5-triphos phate (IP J ), in sign al transduct ion in iris-ci liary body, 263, 263/ in lear secretion, 242f, 243 Insertio ns, mitochondrial DNA, disorders caused by. 173 Insulin-like growth fac tor I, 114,396 in aqueous humor. 270 in ocular developm ent. 114 Insulin-like gro\" th fac tor binding proteins. in aqut"ous humor, 270 Inter ferons (IFNs). 395 in tea r mm . 240 Interleukin- Iu , in tear film , 241 Inte rle ukin - IJ3, in tear fil m. 24 1 Intermarginal sulcus (gray line ). 23. 26j. 237 Inte rmediate zone, 70 Intermittent ataxia. 213t Intermuscular sepl'um. 35 [nternal auditor y meatus. lOS Internal carotid artery. 107. !07f, 109 optic nerve supplied by, 93, 95 Internal limiting membrane, 74f, 75f, 79, 80f optic disc. 90. 92f vi treous detachment and, 286- 287 Interna l sclera l sulcus. 50 Interphotorecepto r mat rix (lPM/subretina l space) , 72. 74f development of, 123 retinal pigment epit helium in maintena nce of, 308 lnterphotoreceplor retinoid-binding pro tein. 306 Inte rsex. 204 Inte rstitial g rowth. in developmen t of lamellae, 134 Inte rvening sequence. See Lntran Interventi onal (e xperimental) studies (clinical trials). fo r new drug, 333 Intraca meral injections. 328 acetylcholine. 337 carbachol,337 lidocaine. 389 lntracanalicular region of optic nerve, 87, 891, 93 blood supply of. 93, 95 Intrac ranial region of optic nerve, 87, 89/, 93. 94f blood supply of, 95 Int ramuscular agents. in ocular pharmacology, 330 Intramuscular ci rcle of the iris, 38, 39J Intramuscular plexus. ci liary body. 62 Intraocular medications, 328. See also specific drug lidocai ne. 389
Intraocular pressure aqueous humor dynamics a nd. 255. 265 corticosteroids affecting, 359-360. 3601, 361 cycloplegics affecting. 341 drugs for lowering. See Antiglaucoma agen ts Intraocular region of optic ne rve. 87, 89, 89-9 1, 891 Int raorbital region of o ptic nerve, 87, 89, 89t blood supply of. 94 , 95J Intraveno us drug adm inistration, in ocular pllarmacology. 329 Inlravitreal medications, 328 Intron. 154, 165 excision of, 168. See also Splicing Ionic charge. of oc ular medication. absorption affected by, 326 Io nizing radiation DNA repa ir after, 170 in free radical generation. 3 12 Iontophoresis, for drug delivery. 33 1 lopidine. See Aprac!onjdine [P~ . Sec Inosito l- IA .S-triphosphate IPL. Sce Inner pleX iform laye r IPM. See Inter pho to recepto r mat rix IRBP. See Interpho toreceptor retinOid -binding protein Irides. See Iris IridocyclitiS. muscarinic antagonists fo r, 341. 342 I.;" 4 If. 5 If. 52f, 56-61, 253 absence of (aniridia), 205- 207 m utat io n rate of, 207 sho rt arm II deletion syndromel PAX6 gene mutations and. 141. 166, 205 , 206,2 16 .""omy of, 4 If, 5 If, 52f, 56-61 , 58f, 59f, 6()f ante rior pigmented layer of. 60J bioche mistry and metabolism o f, 253-264 collarette of, 58, 59J cysts of, m uscarinic therapy and. 338, 340 devel opment of. 137 innervation of. 58 intramusc ular ci rcle of. 38, 39/ pigme ntation of, 56 develo pme nt 0(, 137 prostaglandin analoguesllatanoprost affect ing, 354 poste rior pigmented layer of, 58-60. 58! development of, 137 signal transduction in, 258-261 smooth muscle in, 253- 254 stroma of, 56, 60! topography of. 4 If vessels of. 58, 59! development of, 138 Iris-ciliary body smooth muscle. 253-254 Iris diaphragm/iris diaphragm pallern, 56 Iris dilator. See Dilato r muscle Iris pigment epithelium , 58/ development of. 137 Iri s processes, 51f Iris sphincter. See Sphincter muscle Iritis musca rinic antagonists fo r. 341. 342 mydriasis in treatment of. 34 1 Iron in aqueous hum or. 267 in free radica l formation , 3 11 -3 12. 314
436 • Index Ir rigation, solutions for, 392 Is melin. See Guanethidine Ismotic. See lsosorbide isochromosomes, 154 Isofl urophate, 3361 isoforms, 168 Isolated (simplex) case/geneti c disease, 161, 192 Isoniazid, pharmacogenetics of, 229 Isoproterenol. 259t Isoplo Atropine. See Atropine Isoplo Carbachol. See Ca rbachol Isoplo Carpine. Sec Pilocarpine Isoplo Cetamide. See Su lfacetamide IsopIa Eserine. See Physostigmine Isopio Homatropine. See Homatropine [sopto Hyoscine. See Scopolamine Isosorbide, 356 [stalo!' See Timolo1 Itraconazole, 3791 , 380 Junctional com plexes in corneal endothelium, 47 in retinal pigment epithelium, 72 Junk DN A, 165- 166 Juvenile macula r degeneration (Stargard t disease! fundus flavimaculatu s) ABC transporter mutations causing, 296 retinal pigment ep ithelium in, 309 Juxtacanalic ular tissue, 53 development of, 136 K. See Potassium Kanamycin, 3681, 375-376 ototoxicity of, mitochondrial DNA m utations and, 176
Karyotype/karyotyping, 154, 198- 199,200/ kb. See Kilobase Kearns-Sayre syndrome, 174 - 175 mitochond rial DNA deletions and, 173, 174 - \ 75 Ken nedy disease, 209 Ke ratan sulfate, i.n cornea, 250 Keratitis Acanthamoeba, treatment of, 385-386 corticosteroid route of admin istration in , 3601 herpes simplex, topical antiviral agents for, 380, 38 1I, 382-383 Keratocytes, 45 , 249 differentiation of, 134 Ketoconazole, 379(, 380 Ke toprofen, 362t Ke torolac, 257, 3571, 363, 364, 3651 Ke totife n. 365(, 366 Kilobase, 154 Krabbe disease/ leukodystrophy, 2131 Krause, glands of, 23f, 26f, 271, 33, 239-240 L-cone opsin genes, 294 - 295 mutations in, 297 Leones, 294 bipolar cells for, 298 gene fo r on X chromosome, 294-295 horizontal cells fo r. 299 retinal ganglion cells for. 299f, 300
Ll repeat element/seque nce, 166 L-type calcium chan nels. 26 1 mutation s in. 296 Labyrinthin e segment of cranial nerve VII (facia!), 105 Lacrimal artery, 32, 37J, 38, 39/ extraocular muscles supplied by, 18,38,39/ Lacrimal canaliculi. See Canaliculi Lacrimal crests, anterior and posterior, 5, 6f, 27 Lacrimal d rainage system, 27f, 33- 34, 34/ Lacrimal ducts. 32, 32/ development of, 140 Lac rimal fossa, 7. 7f, 8f Lacrimal glands, 26f, 27/, 32- 33, 32f, 33J, 239- 240, 239/ accessory, 26J, 27/, 33, 239- 240, 239f development of, 140 orbital, 32, 32f, 239 palpebral, 32, 32f, 239 in tear secretion, 239- 240, 239/ Lacrimal ner ve. 103 Lacrimal nucle us, 106 Lacrimal papillae, 33 Lacrimal pump, ocular med ication abso rpt io n affected by, 324 Lacrimal puncta. See Pun cta Lacri mal sac (tear sac), 27f, 32f, 33 , 34 Lacrimal system. See also specific structure excretory apparatus of, 27f, 33-34, 34/ secretory apparatus/function of, 26f, 32-33, 32f, 33f, 24 1-244, 242f, 243/ See also Lacri mal glands Lactate in aqueous humor, 266t, 267 in tea r film , 240 in vitreous, 266t Lactoferrin, in tear film, 240 Lamellae corneal, 45, 249 development of, 134 - 135 eyelid,25/ Lamina cribrosa, 42, 90, 92/ blood supply of, 94 Lamina densa, 135 Lamina fusca, 48 Lamina lucida, 135 Lamina papyracea (ethmoid/ethmoidal bone), 7 Laminar area of optic nerve, 90 Laminin,68 Langerhans cells, 43 LASEK. See Laser subepi thelial keratomileusis Lase r in situ keratomileusis ( LASlK), Bowman's layer and,249 Laser subepithelial keratomileusis (LASE K), Bowman's layer a nd, 249 l.ASIK. See Laser in situ keratomi leusis Latanoprost, 330, 354, 3541 Lateral geniculate body/nucleus, 93, 94f, 96 Lateral orbital tubercle ofWhitnall, 9 Lateral pontine cistern, 105 Lateral rectus muscles, 17f, 18, 19t, 20/, 38, 38f, 39/ insertion relationships of, 16, 18f, 19t nerves supplying, 2 1 Leber congenita l amaurosis ( Leber amaurosis congenita; congenital/in fa ntile/childhood retinitis pigmen tosa) guanylate cyclase mutations causing, 296
Index. 437 pleiot ropism and, 210 RPE65 gene defects causing, 297 Leber hereditary optic neuropathy, 175 mitochondrial DNA mutations and, 174f, 175 Leigh syndrome (Leigh necrotizing encephalopathy), 176,2 131
mitochondrial DNA mutations and, 174f, 176 Lens (crystalline), 67-70, 273-281 anatomy of, 4 If, 53f, 67- 70, 68f, 69f, 7 If, 273- 275,
274/ antioxidants in, 313-314, 314/ biochemistry and metabolism of, 273-281 carbohydrate ("sugar") cataracts and, 279-281 capsule of. See Lens capsule carbohydrate metabolism in, 279-281, 280/ changing shape of. See Accommodation chemical composition of, 275 -277 cortex of, 70, 274-275, 274/ development/embryology of, 69f, 128-130, 129/ energy production in, 278- 279 epithelium of, 68, 68f, 69f, 70, 274, 274/ free radicals affecting, 313-315, 314/ ionic current flow around/through, 278, 279/ membranes of, 275, 277 nucleus of, 70, 274-275, 275/ embryonic, 69/ oxidative damage to, 313-315, 314/ physiology of, 277-278, 279/ size of, 67 sutures of, 69f, 70 development of, 130 topography of, 41/ transport functions in, 277- 278 zonular fibers/zonules of, 41f, 5 If, 52f, 61, 69f, 70, 273 development of, 129f, 130 Lens capsule, 4 If, 68, 68f, 69f, 273, 274/ anatomy of, 4 If, 68, 69f, 273, 274/ development of, 129-130, 129/ Lens crystallins. See Crystallins Lens-fiber plasma membranes, 275, 277 Lens fibers, 68f, 69f, 70, 7 If, 274-275, 274/ development of, 129f, 130 zonular (zonules ofZinn), 41f, 51f, 52f, 61, 69f, 70,
273 development of, 129f, 130 Lens placode, 121, 129 Lens proteins, 275-277 crystallins, 70, 130,273,275-276 cytoskeletal and membrane, 277 posttranslational modification of, 277 Lens vesicle, formation of, 121, 129, 129/ Lesser (minor) arterial circle, 58, 59/ development of, 138 Leucine zipper motif, 166, 167/ Leukodystrophy Krabbe, 2131 metachromatic, 213t Leukotrienes, 25 5, 256I, 258 Levator aponeurosis, 23, 25f, 27, 28, 239 Levator muscle (levator palpebrae superioris), 17f, 20f, 25j, 26j, 28 disinsertion of, 27 innervation of, 21
Levobunolol, 350, 350t Levocabastine, 364, 365t Levofloxacin, 37 1-373, 372t LHON. See Leber hereditary optic neuropathy Liability, in inheritance, 154 Library, DNA, 154, 182, 183/ cDNA, 154, 182 genom ic, 154, 182, 183! Lice, ocular infection caused by, cholinesterase inhibitors for, 340 Lid margin. See Eyelids, margin of Lidocaine, 387t, 388 intraocular, 389 Ligand-gated channels, in Signal transduction, 261 Ligands, 261, 262 Light cone opsin changes caused by, 293 eye injury caused by exposure to, 315, 316 photo-oxidative processes triggered by, 313 in retina, 315 pupillary response to, pathways for, 99 retinal changes caused by, 315, 316 retinal pigment epithelial transport systems affected
by, 307 rhodopsin changes caused by, 291 -293, 292/ Light adaptation, 294 Light-near d issoci ation, 99 Light reflex (pupillary response to light), pathways for, 99
Light toxicity/photic damage/phototoxicity free radical damage and, 313, 315-316 retinai, 315, 316 Lignocaine. See Lidocaine Likelihood ratio, 180 logof(LODscore), 155, 180 Limbal stem cells, in corneal epithelium maintenance,
43 Limbus, 48 - 49, 49/ Limiting membrane external, 75I, 76f, 78, 80! development of, 126 internal, 74f, 75f, 79, 80/ optic disc, 90, 92/ vitreous detachment and, 286-287 LINEs. See Long interspersed elements Link protein, in vitreous, 283 Linkage (gene), 154-155, 177-180, 179f, 181f, 193, 197 Linkage disequilibrium (allelic association), 155 Lipid layer of tear film, 42, 237-239, 238f, 238t, 239/ Lipid peroxidation, 311,312-313 retinal vulnerability and, 315 Lipid strip, 239 Lipids in retinal pigment epithelium, 305 solubility of, medication absorption affected by, 326, 329
in tear film. See Lipid layer of tear film in vitreous, 286 Lipocalins, in tear film, 240 LipofUScin granules (wear-and-tear pigment), 72 Liposomes for drug delivery, 331 for gene therapy, 187-188
438 • Index Lipoxygenase in eicosanoid synthesis. 256f, 257 in leukotriene synthesis, 256[. 258 Lissami ne green, 392 Livostin. See Levocabasti ne Local anesthesia. See Anesthesia (anesthetics), local Lockwood, suspensory ligament of (Lockwood ligament), 35, 36/ Locus (gene), 155, 193. See (llso Gene Locus heterogeneity, 155, 192, 193 LOD score, 155, 180 Lodoxamide, 365, 365t Log oflhe likelihood ratio (LOD score), ISS, 180 Long (q) arm, 159 Long arm 13 deletion (1 3q 14) syndrome, 204 -205, 20S t Long ciliary nerves, 36, 247 Long interspersed elements, 166 Longitudinal fasci culus, medial . 104, 104/ Loop of Meyer. 94f. 96 Loss of heterozygosity. J73 Lolemax. See Lotepredno l Loteprednol, 3571, 361. 365/, 366 with tobramycin, 3731 Louis ~ Bar syndrome (ataxia-telangiectasia), ArM mutation in, 170 Lowe syndrome (oculocerebrorena l syndrome) , ocular findings in carriers of, 222t, 223 Lucentis. See Ranibizumab Lurnigan. See Bimatoprost Lutei n. 81 antioxidant effect of. 3 16 Lymphatics. eyelid. 30-3 1, 3 1f Lymphoid tissue, conjunctiva-associated (CALT), 35 Lyonization (X chromosome inactivation/Barr body), 149, 155,168-1 69,220- 224 ocular findings in carrier states and, 220-224, 221-222f, 2221 Lysozyme in aqueous humor, 270 in lear film , 240 Lysyl hydroxylase. defects in gene for. in Ehlers Danlos syndrome, 213t
M-cone opsin genes, 294-295 mutations in, 297 M cones, 294 bipolar cells for, 298 gene for on X chromosome, 294- 295 horizontal cells fo r. 299 relinal ganglion cells for. 299J. 300 Macroglia. retinal. 30 1 a I-Macroglobulin, in aqueous humor, 269 Macrolides.377 polyene, 378-380, 379t Macugen. See Pegaptanib Macu la/macula lutea, 71 , 73J. 79- 8 1 anatomy of, 7 1, 73J. 79-81, 80J. 821 antioxidants in, 3 18/ Macular degeneration juven ile (Stargardt disease/fundu s navimaculatus) ABC transporter mutations causing, 296 retinal pigme nt epithelium in, 309
vitell iform, Best disease/ vitelli ruptive macular degeneration bestrophin defect causing, 297 reti nal pigment epithelium in, 308-309 Macular dystrophy, Sorsby retina l pigment epithelium in, 308 TIMP3 defects causing. 182,297 Macular edema cystoid, corticosteroid roule of administration in, 3601
d iabetic. corticosteroid route of administration in, 3601 Macular fibers/ project ions, 90, 96 Macular holes, vitreous in formation of, 288 Magnesium in aqueous humor, 267 in tear film, 238t Main sensory nucleus, of cranial nerve V (trigeminal), 100, 101/ Major (greater) arterial ci rcle. 38, 39J, 58, 62- 63 development of, 138 Majo r intrinsic protein (M IP/aquaporin 0). 274, 277 in lens transport . 277 Malatt ia leventinese EFEMP I defects causing, 297 retinal pigment epitheli um in, 309 MALT. See Mucosa-associated lymphoid tissue Mandibular nerve. See Cranial nerve V (t rigeminal nerve), V3 Ma nnitol, 356, 356/ « -Mannosidase, defective, 2 131 Mannosidosis, 2131 Maple syrup urine disease, 2UI Marcaine. See Bupivacaine Marfan syndrome. pleiotropism in, 2 iO Margin, lid . See Eyelids, margin of Marginal arterial arcade, 23J. 29- 30, 34 Marginal tear strip (tear meniscus)' 237 Marke r site, i 79 Markers (gene). See also specific type microsatellites/ minisatellites/satellites, 155. 156. 166, 180, 18 1/ restriction fragment length polymorph isms. 160, 178-1 80, 179/ Marshall syndrome, vitreous collapse in, 288 Mast-cell stabilizers, 364-366, 3651 . See also specific agent Mast cells, 364 Mastoid segment of cranial ne rve VII (facial ), lO6 Maternal age, Down syndrome incidence and, 20 1,202 Maternal drUg/alcohol use. malformations associated with, 142- 143. 142/ Maternal inheritance, 173,208. 220. See also Mitochondrial DNA Mating, assorlalive, 148 Matrix metal loproteinases in cornea, 250 in vitreous, 283 Maxidex. See Dexamethasone Maxillary arte ry, 37/ Maxillar y nerve. See C ranial nerve V (trigeminal nerve), VI Maxillary sin uses, 12, 13J, 14f
Index . 439 Mecamylami ne, adverse effects of, 343/ Meckel cave, 103 Mediul longitudil1al fa scic ulus, 104, 104/ Medial notochorda l process, 11 7 Medial rectus muscles. 16j. 17/. 191. 20j. 37f, 38. 38f, 39/ insertio n relatio nships of, 16. 18f, 191 nerves supplyi ng, 2 1 Mediato rs, 364 Medications. See Drugs Medrysone, 359 anti· infla m m;ttory/prcssure-eleval ing pOlency of, 3601 Meibomian glands, 23. 23f, 26/ 27t, 28, 29j. 239/ dysfu nction of, 244 in tear-mm lipids/tear production, 42. 237. 239/ Me ios is, 155, 195- 196, 196/ nond isjunction d uring, 196/ See (/(so Nondisj unction Melanin defective synthesis of, in albinism , 308 photopro tec tive fu nction of. 308, 3 15- 3 16 in retinal pig ment e pithel ium , 304, 307-308, 3 15-3 16 a -Melanocyte-sti m ulating hormo ne (u · MSH), in lear sec retion, 244 Mel3 nocytes choroidal, 65f, 66, 133 in iris stroma, 56 Melanogenesis, 127. 133, 308 latanoprost and, 354 Melanosom es in cho roid, 66, 133 in ciliary body epithelium, 62 in iris d ilator muscle. 60 in re tinal pigment epithelium. 72 MELAS (mitocho ndrial myopathy wilh e ncephalop3thy/ lactic acidosis/strokelike episodes) syndro me, mitochondria l DNA l11utation and. 173. 174/ MEM . See Minimum essential medium Membrane sacs, in rods, 291 Membrane structural (urea -insolu ble) lens prote ins, 277 Me mbranes Descemet. Sce Descemet's membrane retrocorneal flbro us. 25 1 Mendel ia n disorder (Single -gene d isorder), 155, 208 Meninges. optic nerve. 9 1. 92j. 93/ Mepivacaine. 3871 , 388 MERRF. See Myoclonic epilepsy with ragged red flbers Mesencephalic nucleus, of cranial nerve V (trigeminal), 100,101/ Mese ncephalon, cranial nerve III arising from , 97 Mesode rm, 117. 118j. 11 9j. 120/ oc ula r struct ures derived from . 1161 Messenger RNA (mRNA) , precursor. 16 1 Metabolic acidosis, carbon ic an hydrase lOP lowering and . 353 Met3bolic d isorders, enzyme defects/ocular signs in, 2 13/ Metachromatic le ukodystrophy, 2 131 Metalloproteinases, matrix in cornea. 250 in vitreous, 283 Metarhodopsin 11.292 Me thazolamide. 35 1. 3521, 353 Methicill in. 3681. 369- 370
Methylation. DNA , 168 Methylcellulose. hyd roxypropyl (HPM C) fo r d ry eye, 390 as viscoelastic, 393 Metipranolol. 350. 3501 Metoprolol,345/ Meyer, loop 0(, 94j. 96 MezlociJlin, 370 Miconazole. 3791, 380 Microglial cells, 77. 301 Microperoxisomes. in re tinal pigment epithelium, 304 Microphakia, in fet31alcoho l syndrome, 143 Microphth almia (micropht.halmos). ;n fe ta l alcohol syndrome, 14 3 Mic roplicae, in corneal epithelium . 43, 44j. 248 Mic rosatellites. ISS, 166, 180 Microsomal triglyceride transfer pro tein (MTP). m utations in . 298 M icrovilli, in cornea l epithelium . 43. 44j.248 Minimum essential medium , 394 Minisatellites, 156. 166. 180 Minocycl ine.374 Minor (lesse r) arterial ci rcle, 58, 59/ developm ent of. 138 M iochoL See Acetylcholine Miosis/ miotic agents, 259- 260. 3381 adrenergic antagonists, 260, 348 adve rse effects of. 338, 339/ cholinergic/m uscari nic 3gonists. 259. 335- 340, 337f, 338t , 339/
cholinesterase in hibitors. 334, 336f, 3381, 339- 340 for glaucoma. 2641, 338 Mioslat. See Carbacho l M IP. See Major intrinsic protein Mi smatch repair. 169 Mi ssense mutation, 156, 171 Mitochondrial DNA diseases aSSOCiated with deletions/ m utations of, 173- 176, 174f, 191,208 gaitonian inheritance of. 208 Leber hered ita ry o ptic neuropathy. 174f, 175 mate rnal inhe ritance/t ransmission and, 208. 220 in replicative segregation. 160, 173 Miloc hondri31 myopathy, wilh encephalopathy/ lactic acidoSis/ strokelike episodes (M ELAS), mitochondrial DNA mutation and, 173, 174/ Mitomycin/ mitomycin C, 334, 367 Mitosis, 156. 195 no ndisjunction during, 203. See (lisa Nond isjunc tion Mitte ndorf do t. 85, 85/ MM P. See Matrix metallopro teinases MMP·2 (gelatinase). in vitreous. 283 MM P-2 proe nzyme. in cornea. 250 MnSOD,3 17 Molecular genetics. See Genetics, molec ular Moll. glandS of, 23, 23f. 26f, 271 Monoamine oxid3se inhibitors, apraclonidi ne/ brimonidine interactions and, 348 Monochromatism, blue-cone, ocular findings in carriers of. 2221 Monogenic (mendelia n) disorder, 155,208 Monosomy, 199 Morula, 11 7. 11 8/
440 • Index Mosaicism (mosaic), 156,203- 204 germinal, 152 sex chromosome, 204 trisomy 21, 204 Motor nucleus of cranial nerve V (trigeminal), WIJ, 102-103 of cranial nerve VII (facia l), lOS Motor root of cranial nerve III (oculomotor), 14, 151 of cranial nerve V (trigeminal), 102 of cranial ne rve V II (facial), 105 Motrin. See Ibuprofen Moxifloxacin, 371-373. 3721 Moxisylyte. See Thymoxamine mRNA. See Messenger RNA u -MSH (a lpha-melanocyte-stimulating hor mone), in tear secretion, 244 mtDNA. See Mitochondrial DNA MTP (microsomal triglyceride transfer protein), mutations in , 298 M ucins/m ucin gel/ layer, tear film, 42. 238j. 24 1, 248 deficiency of, 241, 244 secretion of, 241 Mucoceles, 12-13 Mucomyst. See Acety1cysteine Mucosa-associated lymphoid tissue (MALT), of conjunctiva (conjunc tiva-associated/CALT), 35 Miiller cells/fibers optic nerve, 92J retina!, 75f. 77, 300- 301 developmentldifferentiat ion of, 126 Muller m uscle (superior tarsal muscle), 23f. 25/, 28 Multifactorial inher itance, 156,224-225 Mural cells, retinal blood vessel, 78 Muro 12B. See Sodium chloride Muscarine, 259t Muscarinic agents, 334, 335-342, 338t, 339f. 341/ adverse effects of, 338, 339/ antagonists, 340- 342 , 34 1t adverse effects of, 342, 343J di rect-acting agonists, 334, 335-339, 337f. 3381, 339/ indirect -acting agon ists. 339- 340 Muscarinic receptors in iris-ciliary body, 259, 2591, 262, 263 signal transduction and, 262, 262f. 263, 263f. 2641 in tear secretion, 242, 242/ Muscle of Rio lan, 23, 23f. 27 Muscles, extraocular. See Extraocular muscles Muscular dystrophy, Duchen ne, mutation rate of, 207 Mutagens, 169 Mutamyci n. See Mitomycin Mutation, 156, 169, 207- 210. See also specific Iype base pair, 17 1 conserved, 171 carrier of, 149 disease-producing, 170 dominant negative, lSI frameshift (framing error/frameshift), 141 , 152 gain of funct ion and, 171 missense, 156, 17 1 nonsense, 156, 17 1 null, 17 1
point, 207 mitochondrial DNA , 174/ screening for, 184, 186- 187/ repair and, 169- 170 screening for, 182- 184, IB3f. IBSf. 186-187J transcription factor, J 66 Myasthenia gravis, 342 diagnosis of, edrophonium in, 329, 342-343 Mydfrin. See Phe nylephrine Mydral. See Tropicamide Mydriacyl. See Tropicamide Mydriasis/mydriatics.260 adrenergic agents, 260, 345- 346 cholinergic agents, 260, 340-342. 341/ MY07A gene, 176 Myoclonic epilepsy with ragged red fibers (MERRF), mitochondria l DNA mutations and, 173, 174/ Myoepithelial cells iris, 60/ differentiation of, 137 lacrimal gland, 32, 33J Myoid of cone, 74, 76J of rod, 73, 76/ Myo-i nosilol, in aqueous humor, 25S Myopathies, m itochondrial, with encephalopathyl lactic acidosis/strokelike episodes (MELAS), mitochondrial DNA mutation and, 173, 174f Myopia with ach romatopsia. racial and ethnic concentration of,2 11 congenilal stationary night blindness with, ocular findings in carriers of, 2221 muscarinic agents causing. 338 vitreous changes associated with, 286, 287 Myosi n VIlA, mutations in, 176,296 MZM. See Methazolamide Na. See Sodium Na' .K'- ATPase (sodium -potassium pump) in aqueous humor secret ion, 351 in lens ionic balance, 278 in retinal pigment epithelium. 304. 307 in rods, 292 Na' , K' -Ca exchanger (sodium-potassium-calcium exchanger), in rods, 292 NADPH in free radi ca l generation, 3 12 in lens glucose metabolism, 279, 280/ glutathione redox cycle and, 3 14 Nafcillin .369-370 Nalfo n_See Fenoprofen Naphazoline. 364. 365l, 392 Naphazoline/antazoline, 364, 365r Naphazoline/pheniramine, 364, 3651 Naphcon-A. See Naphazoline/pheniramine Naprelan . See Naproxen Naprosyn . See Naproxen Naproxen , 3621 NA RP (neuropathy Ine urogenic muscle weakness]1 ataxia/ retinitis pigmentosa) syndrome, 175- 176 mitochondrial DNA m utations a nd, 174f. 175- 176 Nasociliary nerve. 103
Index . 441 Nasofrontal vein, 40/ Nasolacrimal d uct . 10.33, 34 occlusion of, 34 oc ular medicat ion absorption a nd , 324, 324f, 325/ Natacyn. See Natamyci n Nata mycin, 378- 379, 3791 Nat riuretic peptides, in aqueous humo r, 267, 270 ND· / ge ne, in Leber he reditary opt ic neuropathy, 174f,
175 ND·4 gene. in Leber he reditary o pt ic neuropathy, 174f. 175 N D·6 gene. in Leber he reditary o ptic ne uropathy, 174f, 175 NDPgene.193 Near reflex, pathways for, 99 Nec rotizing encephalopathy. Le igh (Leigh syndrome). 176. 2131 mitochondrial DNA mutat ions and, 174f, 176 Nedocromil. 3651, 366 Neofrin. Sec Phenyleph rine Neomycin . 3681, 376 in combinatio n preparations, 3721, 3731 ototoxicity of, mitochondrial DNA mutations and,
176 Neosporin,372t Neostigmine. 259, 336f. 342 Neo-Synephrine. See Phenyleph rine Nepafenac, 3571, 363 Neptazane. Scc Methazolamide Nerve block, local anesthetics for, 386-390, 3871 Nerve endings en grappe, 2 1 en plaque. 21 in scleral spur, 5 1- 52 Nerve fiber layer, 74f, 75, 75f. 78, 79/ o ptic nerve, 90 blood supply of, 94, 95/ transient, of Ch icvitz, 126 Nerve loops, 48 Nerve of pterygoid canal, 107 Nervus inte rmedius, 105 Ncttleship· Fa!1s X-li nked ocular albini sm, ocula r findings in carrie rs of, 222f, 223 Neural crest cells, I l S- 117, 116t, 120f, 121/ ocula r str uctures derived from , 1161 cornea l endothe lium . 46 Neu ral folds, 11 5. 119f, 120 Neural plate. 117, 118. 118f, 120/ Neural tube, 118/. 11 9f, 120, 124/ Neuri tis, opt ic. Sce O ptic neuritis Neuroblastic layers, in ner a nd outer, 126 Neurocristopathy, 11 7 Neuroectoderm . li S. 11 8 ocular st ructures derived from , 115, 11 61 Neuroendocrine pcptides, in aqueous humor. 270 Neuroll bromatosis, von Recklinghausen (type J) express ivity in , 210 mutation rate of, 207 Neurogenic muscle weakness/ ataxia/retinitis pigmentosa (NARP) synd rome. 175- 176 mitochondrial DNA mutations and, 174f, 175- 176 Neuromuscular blocking agents, 344 adverse effects of, 3431
Neuropathy with ataxia and re tin itis pigmentosa (NARP), 175- 176 m itochondrial DNA m utations and, 174j. 175- 176 optic. Sce Optic neuropathy Neuropeptide· processing e nzymes, in aqueous humor,
267,270 Neuropeptide Y, in tear secretion, 237, 240 Neurosensory retina, 41/, 73-79. 29 1- 298. See also Retina ,n'tomy of. 4 1j. 73- 79, 74j. 75j. 76j. 77j. 79/ bioche mistry and metabolism of, 29 1-298 development of, 12 1, 124-126, 1251 glial eleme nts of, 74f, 77 neuronal elemen ts of, 73 -77, 74f, 75/. 76f, 77/ stratifica tion of. 74/. 78- 79, 79/ vascula r elements of, 74f, 78. See also Retinal blood vessels Neurotensin , in aqueous humor, 267, 270 Neurotoxin complex. pu rified. 390 Neurotransmitte rs in iris- ci liary body, 258-26 1 in tear secretion, 240, 242 Neurotrophic prote ins. 267, 270 Neutrophil elastase inhibitor, in vitreous, 285 Nevanac. See Nepafenac N FL. See Nerve fibe r layer Nicotinamide adeni ne dinucleotide phosphate (NADPH ) in free radical generation , 3 12 in lens glucose metabolism, 279, 280/ glutathione redox c ycle and, 313 Nicotine, 259" 334. See also Nicotin ic agen ts ad verse effects of, 343f Nicotinic agents, 334, 342 - 344 Nicotin ic receptors, in iris- cilia ry body. 2591 Nidogell · l , in vitreous, 283 Niemann·Pick disease, 213t racial and e th nic concentration of, 2 11 Night blindness, congen ital stationary. with myopia . ocula r fi nd ings in carriers of. 2221 Nitrogen bases, l 56 Nizoral. See Ketoconazole Noncoding strand of DNA (anti sense DNA), 148 Nondepolarizing neuromuscula r blocking agents, 344 Nondisjunction, 156, 196J. 199,203 aneuploidy and, 199,203 in Down syndrome, 202 in m osaicism, 203 Nonhomologous chromosomes, independent assortment a nd, 196- 197 Nonpenetrant gene. 209 Nonpigmented epithelium, ciliary body. 62, 62f, 265 Nonsense mutation, 156. 171 Nonsteroidal anti-inflamm atory d r ugs (NSA IDs), 3571. 361- 366,362(,365( COX · 1/COX -2 in hibition by, 257, 362 prostagla ndins affec ted by. 257 Nontranslated st rand of DNA (sense DNA) , 160 Norepinephrine. 259t adrenergic agents/receptor response and, 344-35 1, 345j. 347/ d ilator m uscle affected by, 258, 260
442 • Index in iris-ciliary body. 258, 259t, 260 in tear secretion. 240, 242, 242f, 243. 243[ Norrie dise ase, gene for, 193 Northern blot analysis, 156 Notochord, 117, li B/. 1201 Notochordal proceSs, med ial, 117 Nougaret disease, rod transducin m utation causing. 295 Novocain. See Procaine
NPY. See Neuropepticle Y NSA IDs. See Nonsteroidal a ni i-inflammatory drugs Nuclear layer ;n nee, 74f, 7Sf, 78, 79f, 298-301 , 299f, 300f o"tee, 74f, 7Sf, 76f, 79f Nucleic acids, 157 in retinal pigment epithelium, 305 Nucleoside, 156 Nucteosom e, 156 Nucleotides. 156- 157 Nudeus, lens. See Lens (crystalline), nucleus of Null allele, gene the rapy and. 187-188 Null mutat io ns, 17 1 Nycta lopia, rod -specific m utatio ns causing, 295, 296 OAT (ornithin e aminotransferase) m utations, 298
in gyrate atrophy, 2 13t, 298 Oblique muscles, 9, 16f, 17f, 18, 19r, 20f, 38f, 39f, 40/ insertion relationships of, 17, 18f, 191 nerves supplyi ng, 21 origins of, 18, 19t OCA 1 albinism , 308 OeA2 albinism , 308 Occi pita l association cortex, near reflex initiated in, 99 Occipital (prim ary visuallcalcarine/striate) cortex, 97 Occluding zonules, corneal, 43 Och ronosis, 214 Ocu-Chlor. See Chlo ram phenicol Ocufen. See Flurbiprofen ocunox. See O floxaci n Ocular adnexa. See also specific strrlcture anato my of, 5- 13 g lands of, 27t Oc ular albinism , 308 oc ular findin gs in carriers of, 222f, 222t X-linked (Net1leship -Fall s), ocular findings in carriers of, 222f, 223 Ocular development. See Eye, development of Ocular inflamm ation, treatment of. See Antiinflammatory agents Ocular motility, extraoc ular muscles controlling, 21 Ocular pharmacology. See (liso specific agent ami Drugs, oc ular future research areas and, 32 1 legal aspects of. 333- 334 principles of. 32 1- 332 Ocular receptors. See Receptors Oculinum. See Botulinum toxin Oc ulocardiac reflex, atro pine affecting, 342 Oc ul ocerebrorenal syndrome (Lowe syndrome), oc ular findings in carrie rs of, 222 t, 223 Oculocutaneous albinism, 195, 308 racial and ethnic concentration of. 21 1 Oculo mo to r nerve. See C ra nial nerve III Oculorenal syndro mes, 141
Ocupress. See Carleola l Ocusert Delivery System , 330. See also Pi locarpine Off-bipolars, 298, 299/ Off-label application of drugs, 333- 334 Off reti nal ga ng li on cells, 299I. 300 Ofloxacin, 37 1- 373, 372t Oguchi di sease arrestin nmtat ion caus ing, 182, 295 racial and ethnic concent ration of, 2 11 rhodopsin kinase m utations causing, 295 O intments, ocula r, 327 O leate, in vitreous. 286 Olfactory bulb, 87, 89f Olfactory nerve. See Cranial nerve I Olfactory pits, 119/ Olfactory tract, 87,89/ Oligodendrocytes, 92/ Oligodendroglia, retinal, 30 I Oligonucleot ide hybridil.a tion, in mutation screen ing. 184, I 86f
Oligonucleotides allele-specific. 148 in mutation screen ing, 184 ant isense. in gene therapy, 188 , 189/ Olopatadin e, 365t, 366 OM lM. See On line Mendelian Inheritance in Man O n-bipolars, 298, 299/ O n retinal ga ng li on ceils, 299f, 300 O ncogenes/oncogenesis. 157. 17 1. 172f O nline Mende lian Inhe ritance in Man (OM 1M), 178 O pacities, vi treous, fib rillar. 286 O pcon-A. See Naphazoline/ pheniramine O pen-angle glaucoma, cycloplegic use in, 341 Open reading frame . 157, 165 Ophthacet. See Sulfacetamide Ophthaine. See Proparacai ne Ophthalmia, sympatheti c. co rticosteroid route of admin istration in, 360t Ophthalmic a rtery, 37f, 38. 39f development of, 138 extraoc ular m uscles supplied by, 18, 38 eyelids supplied by, 29 optic nerve supplied by, 94, 94f, 95 Ophthalmic irrigants. 392 Ophthalmi c nerve. See Cranial nerve V (trigemi nal ner ve), VI Ophthalmi c vein , 40f Ophthalmoplegia, progressive external/chronic progressive external, 174- 175 mitochondria l DNA deletio ns and, 173, 174 - 175, 191 Ophthetic. See Propa racaine O PL See Outer plexifo rm layer Opsins cone,293,294 genes for, 294- 295 rod. See Rhodops in Opti c canal, III. 93 Opti c chiasm . See C hiasm Optic cup, 89, 92/ choroidal development and, 132 development of, 12 1- 122, In , Inj, 124f enla rgeme nt of. 90 retina l developme nt and, 125-126. 125/
Inde x . 443 Optic disc (optic nerve head). 4 If, 71, 89- 92, 92f anatomy of, 41f, 71, 89-92, 92/ blood supply of, 94 , 95f coloboma of, 14 1 differentiation of, 128 Optic fo ramen, 7j. 8f, 10, Ilf Oplic nerve (cranial nerve II ). See (/150 Optic disc anatomy of, 4 If. 75, 87-97, 88f, fi9/. 94J blood supply of, 93, 94- 96, 95/ coloboma of. 14 1 congenital abnormalities of, in fel::al alcohol synd rome, 142 development of, 12 5f. 128 hypoplasia of, in fetal alcohol syndrome, 142 intracana licular region of, 87, 89t, 93 blood supply of, 93 , 95 intrac ranial region of. 87, 89t, 93, 94f blood suppl)' o f, 95 intraocular region of, 87. 89, 89-91. 891 intraorbital region of, 87, 89. 89t blood supply of, 94 , 95/ lamin ::ar area of, 90 prelaminar area of, 90. See (//so Optic disc blood supply o f, 94. 95/ regional differences in, 87, 89t rctrolami nar area of. 91 blood supply of, 94. 95/ Optic nerve head. See Optic disc Optic neuritis. corticosteroid route of administration in. 360r Optic nt' uropath)' Leber hereditary, 175 mitochondrial DNA mutat ions and , 174f. 175 traumatic, indirect. 93 Optic pits (optic holes) , 120, 121 Optic radiations (genicli loca1carine pathways), 93. 94f. 96 Optic stalks. 128 Optic tracl, 94f, 96 Optic ventricle, 123 Optic vesicle, 121, 121f, 123f, 124f lens development and. 128- 129 Opticin, in vitreous, 283, 285 Opt iPrallolot. See Mctiprano[ol Optiva r. See Azeiaslinc Ora serrata. 4 If. 71, 82, 83/ muscle insertion relationships and, 16 Oral contraceptives. tetracycline use and , 374 Oral medication. sustained -release preparations of, 329 Orbicularis oculi muscle, 23. 23f. 24- 27, 24f. 27/ Orbit anatol11Y of, 5- 13 vascula r arter ial su pply in, 36- 38, 37f, 38f. 39/ venous drain::agc in, 38- 40 , 40f bony,S, 6f, Sf connective tissues of, 35, 35/ developme nt of, 139 fi ssures in, 10- 12, 121 floor of. 7-9, 8/ foram ina in, 10, I I/ lateral \"all of. 9, 9/ margin of, S, 6/ medial wall of. 7. 8f
optic nerve in, 87. 89, 89t blood supply of, 94, 95/ roof of. 5- 7, 7/ septum of. 23f, 25f. 27 -28 vascu lar system of anatomy of, 36- 40. 37f. 38f. 39/ development of. 138 volume of. 5 walls of lateral, 9, 9/ medial, 7. 'Of Orbital fat , 27 Orbital fi ssures, 10-12 , 12/ inferior, 8/, 9f. 10- 12 superior.7f. 10, I If, 12f Orbilallacri mal gland. 32, 32f. 239 ORe. See Origin replicatio n complex ORF. See O pen reading fra me Organogenesis. in eye. 120- 1<10, 122t. See also specific
structure Origin of replication. 157 Origin replication complex. 157 Orn ithine aminot ransferase (DAn mutation/defect, 298 in gyrate atrophy, 213(, 298 O rudis. See Ketoprofen Osmi trol. See Mannitol Osmoglyn. See Glycerin Osmolarity, tear film . 238t dysfunctions and , 244 Osmotic/hype rosmotic agen ts, 355- 356, 356t Osmotic hypothesis, of carbohydrate ("sugar") cataract formation , 280 O totox ici ty. m itochondrial DNA mutations and . 176 O uter cell mass. 117 OUler neuroblasti c layer. 126 O uter nuclear layer, 74f. 7Sf, 76f, 791 Q Uler plexiform la)'er. 74f, 75j. 76f. 78, 79/ QUler segmems. photoreceptor, 73, 74J. 76f. 79/ See also Cone outer seg ments; Photoreceptors: Rod outer segments Ovum, 196 Oxaci llin. 369-370 protein bindi ng of, 329 Oxidored uctases, taxon-speci fi c crystall ins as, 276 Ox), radicals. 313. See also Free rad ica ls Ox)'chloro complex, as ocular medication preservative, 322- 323 Oxygen in aqueous humor, 27 1- 272 singlet, 311, 312, 313, 314. 315 vit rectomy affecling movement of, 287-288 Oxyge n radicals. See Free radicals Oxygen -regulated protein, mutations in, 296 Oxygen tension in lens. ene rgy production and, 279 in retina, free radical reactions and , 315 Oxymetazoline, 392 Oxyradicals. Sec Free radica ls Ox)1etracycl ine, 374 with polymyxin B, 372/, 374 p23 H rhodopsin mutation, 295 p53 gene, in DNA repair, 170
444 • Index parm,157 P gene, defects of in albinism, 195
PAHX gene, Refsum disease caused by mutations in, 298 Palatine air cells, 12 Palmitate/pal mit ic acid in retinal pigment epithelium, 206, 305 in vitreous, 286 Palpebral artery, 37f Palpebral conjunctiva, 29, 30!, 34 Palpebral fissures, 22, 22f, 29f Palpebral lacrimal gland, 32, 32f, 239 2-PAM. See Pralidoxime Pancuronium, 344
adverse effects of, 343f Papillae, Bergmeister, 85 Papillomacula r bundlelfibers, 72, 73f, 75 Para-aminoclonidine. See Apraclonidine Para foveal para foveal zone, SOI, 81 Paramedian pontine reticular format ion, \04, 104/ Paranasa l sinuses, 12-13, 131, 141 Parasympathetic ganglia/nerves/pathway
cholinergic drug action and, 334, 335/ in ciliary ganglion, 14, ISf ciliary muscle innervation and, 64 cranial nerve JlI and, 98-99 cranial nerve VII and, \06, \07 dilator muscle innervation and, 60 sphincter muscle innervation and, 61, 258 in tear secretion, 237, 240, 242, 242I, 243, 243f Parcaine. See Proparacaine Paredrine. See Hydroxyamphetamine Paremyd. See Hydroxyamphetam ine Parental age, chromosomal aberrations in Down syndrome and, 20J, 202 Pars plana, 61 Pars plicata, 59I, 61 Partition coeffi cient, drug lipid solubility and, 326 Passive transport, in aqueous humor dynamics, 254, 254-255 Pataday. See Pemirolast Patanol. See Olopatadine PAX genes, 115, 142, 147, 153,206 in aniridia, 141, 166,205,206,216 PAX2 gene, mutation in, 141 , 166 PAX3 gene, mutation in, 166,216 PAX6 gene, 115, 205, 206 mutation in, 141 , 166,206 in aniridia, 14 1, 166,206,216 peD. See Programmed cell death PCR. See Polymerase chain reaction POE. See Phosphodiesterase Pearson marrow-pancreas syndrome, 175 mitochondrial DNA deletions and, 173, 175 Ped icle, of cone, 74, 77f Pedigree analysis, 225 - 226, 226f PEG -400, in demulcents, 390 Pegaptanib,396 Pem irolast, 365, 365t, 366 Penciclovir, 383 Penetrance/penetrant gene, 157,209 familial, 216 incomplete (skipped generation), 209, 217
Penicillamine, for Wilson disease, 23 J Penicillin G, 368t, 369 Penicillin V (phenoxymethyl penicillin), 369 Penicillinase, 369 Penicillinase-resistant penicillins, 369-370 Penicillins, 368-369, 368(, 369-370. See also specific agent allergic reaction to, 369 broad-spec trum , 370 penicillinase-resistant, 369-370 Pentolair. See Cyclopentolate Pentose phosphate pathway (hexose monophosphate shunt), in gll.lCose/carbohydrate metabolism in cornea, 248 in lens, 279 Peptide hormones in aqueous humor, 267, 270 in tear secretion, 244 Peptidyl-glycine-u -amidating monoxygenase, in aqueous humor, 267, 270 Pericanalicular connective tissue, 54, 55f Pericytes, retinal blood vessel, 78 Peri fovea/peri foveal zone, 80I, 81 Periocular drug administration, 327 Periocular tissues, development of, 139- 140 Periorbital sinuses, 12- 13, 13I, l'if Peripapillary fibers, 90 Peripheral arterial arcade, 23I, 25I, 30, 34 Peripheral corneal guttae (Hassall-Henle bodies/warts),
45 Peripherin,293 mutations in gene for, 296 Peripherin/ RDS gene mutations, 296 Peroxidase, 312I, 317 Peroxidation, lipid, 311, 312-313 Peroxy radicals, 311 , 312,313. See also Free radicals Persistent pupillary membranes, 139f Pes anserinus, 106 Peters anomaly, 141 , 166 Petroclinoid (Gruber) ligament, \04 Petrosal nerve deep, 107 greater superficial, \06, \07 inferior, 106 Petrosal sinuses, inferior/superior, 108, 108f PEX I gene, Refsum disease caused by mutations in, 298 PG analogues. See Prostaglandin analogues PG/PGH synthetase (prostaglandin synthetase/ prostaglandin G/H synthetase). See Cyclooxygenase PGl 1 . See Prostacyclin PGs. See Prostaglandins; Proteoglycans pH, of ocular medication, absorption affected by, 326 Phagocytosis, by retinal pigment epithelium, 306-307 Phagolysosomes/phagosomes, in retinal pigment epithelium, 72, 304, 306 Phakinin,277 Pharmacodynamics, 322, 331-332 Pharmacogenetics, 157,229- 230 Pharmacokinetics, 322, 323-331 age-related changes in, 323 Pharmacology, ocular. See also specific agent and Drugs, ocular future research areas and, 321
Index . 445 legal aspec ts of, 333-334 principles of. 32 1- 332 Pharmaco the rapeutics. 322. Sec also specific agenl a"d Drugs, ocular fu ture research areas and , 321 principles o f, 321 - 332 Phasic cells, 298, 300, 300/ Phenet hicillin, 369 Phenira mine, 364 Pheniramine/ naphazoline, 364 . 3651 Phenocopy, 157,208 Pheno type, 157,208 alleles a nd, 194- 195 Phenoxybenzamine, 2591. 260 Ph entolam ine, 2591. 260 Phe nylalanine in phenylketo n uria, 214 dietary rest riction and, 23 1 Phe nylalkanoic acids, 362 . See also Nonste roida l an ti ~ inllammatory drugs Phenylephrine, 259r, 260, 341 I, 345- 346, 392 d rug interactions and , 345 Phenylketo nu ria, enzyme defect in, 2 14 Phosphate, in aqueous humo r, 267 Phosphatidyl-inositol -4,5-bisphosphate ( PIP!), in signal transduction in iris-ci liary body, 263, 263f Phosphat idylcholine, in retinal pig ment e pithelium. 305 Phosphatidylet hanolam ine, in retinal pigment epithelium, 305 Phosphodiesterase receptor- effecto r coupling a nd, 262, 262f rod (rod POE). 293 Ill utations in , 295 Phospho line. See Echothiophate Phospholipase A l in eicosanoid synthesis, 255, 256/ no nsteroida l an li ~ inl1ammiltory d rug derivatio n and, 36 1- 362 receplOr-effector coupling and, 262 in tear film , 240 Phospho lipase C receptor-effecto r coupling a nd, 262, 263f in tear secretion, H 2f, 243 Ph ospholipids in retinal pigm ent epitheliulll , 305 in lea r film , 239. See also Lipid layer of tear fil m Phosphonoformic acid. See Foscarnct Photo-oxidation, 3 13 in retina, 3 15 Photoreceptor inner segmenls. 73, 74f. 76f See also Cone inner segments; Photoreceptors; Rod inner segments Photoreceptor outer segments, 73, 74f. 76f, 79f See (liso Cone o ule r segments; Photorecepto rs; Rod oute r seg ments shed , retinal pigment epilheJium phagocytosis of. 306-307 Photoreceptors, 73 - 74, 74f, 75f. 76f. 77f, 29 1-298. See also Co nes; Rods biochemistry and metabolism of, 29 1- 298 development of. 126 oxidative damage to, 3 15-3 16
Pho to refractive keratectomy ( PRK ), Bowman's laye r and, 249 Phototransduc tion conc,293- 294 rod ,29 1- 293,292/ Physiologic c up/ physio logic cupping, 89, 90, 92f See also Optic cup Physostigmine, 259, 336/, 3381, 339. 340 for muscarinic a ntagonist side effects. 342 Phytanic acid oxidase, defective. 2 131. See also Refsul11 disease/ syndrome Ph ytanic acid storage disease. See Refsum disease/ synd ro me Pia m ater. optic nerve, 9 1, 92f, 93/ Pi e in the sky defect, 96 Pigme nt epithelium. See C iliary epithelium , pig mented; Iris pigme nt epit helium; Re tina l pig ment epithelium Pigment epithelium- derived factor in aqueous humor. 267 in vitreous, 287 Pigment granules. in retinal pigm ent epithelium, 307- 308 Pigmentations/pigment depOSits choroid,66 iris, 56 development of, 137 prostaglandin a nalogues/lata noprost and . 354 ret inal/retinal pigment epithelium , 307- 308 Pigment s, visual, regeneratio n of, retinal pigment epitheli um in . 305- 306 PHagan . See Pilocarpin e Pilocar. See Pilocarpine Pilocarpine, 260, 336f, 338, 338 r sustained release gel for adm inist ration of, 330, 338, 3381 system ic side e ffects of. 338, 3391 Pilopine. See Pilocarpine Pilo pt ic. See Piloca rpine Pilosta!. See Pilocarpine PIP!. See Pnosphat idyl-inosiw]-4,5-bisphosphate Pipe racil lin, 370 Piroxicam, 362t Pits foveal, 127 optic (optic holes), 120, 121 PITX2/ RI£G I gene, 141 P L A ~. See Phospholipase A l Placode, lens, 121, 129 Plasma proteins. See also Pro tdns in aqueous humor, 254, 255, 265. 266, 268 - 270 dr ug-binding by, system ic administration and, 329 Plasmid- liposome co mplexes, as gene therapy vectors, 187 - 188 Plasm ids, 157 for gene therapy, 187- 188 Plas min , fo r enzymatic vitreolysis, 289 Plasminogen anti fibrinolyti c agents affecti ng. 394 in aqueous humor, 269 Plasminogen activato r in aqueous humor, 269 tissue. See Tissue plasm inogen ac tivator Platelet-derived growth factors, 396
446 • Index Platelets, aspirin affecting. 362 Pleiotropism, 158, 177,210 Plexiform layer inner, 74f, 7Sf, 78, 79/ development of, 126 out". 74f. 75f. 76f. 78. 79/ Plica semilunaris. 22f. 3 1 Point mutations, 207. See a/so Mutation mitochondrial DNA, 174/ screening for, 184. 186- 187f Polyacrylamide, as viscoelastic, 393 Polyallclism. 194 Polycarbophil, for dry cye, 390 Polyci n -B. See Polymyxi n H, in combination preparations Po lyenes, 378-380, 3791 Pol ygenic inhe ritance. 158,214-115 Polyhexamethylene biguanide. for AcantJwmoeun infection, 385 Polyme rase chain reac lion (pe R), 158 in mutat io n screening. 184, 186- 187/ Polymorphisms. 158, 171,207-208 denaturing gradien t gel electrophoresis in ide ntification of, 184- 185 restriction fra gme nt length (RFLP). 160. 178- 180, 179/ single-stranded conformational, 182 Polymyxin S, 368 1, 377-378 in combination preparations, 3721, 3731, 374 Polyol (sorbitol) pathway in catarac t formation . 280, 28 1 diabetic retinopathy and. 30 1 in lens glucose/ca rbohydrate metabolism, 280. 280/ Polyphosphoi nositide turnover, 262-263, 262f, 263/ PolyplOidy, 199 Po lysorbate, in demulce nts, 390, 39 1 Poly trim. See Trim ethoprim -po lymyxin B Polyvin yl alcohol. for dry eye. 390 Pontine cistern, lateral, 105 Pontine reticular formati o n, paramedian. 104. 104/ Pontomeclullary ju nction, 105 Positional candidate gene screening. 182 Posterior chamber, 4 If, 42 , 52/ Posteri or ci liary arteries, 36- 38, 37f, 38, 38j, 39f. 62, 64 development of. 132 Posterior Ciliary nerves, 36, 247 Posterior clinoid process. 104 Posterior co njunctival arteries, 34 Posterio r e mbryotoxon, 141 . See also Neuroc ri stopathy Posterior lacrima l crest,S, 6f, 27 Posterio r lamella, 25/ Posterior nonbanded 7.One, o f Descemet's membra ne, 45.46f. 135.25 1 Poste rior pigme nted layer, of iris, 58-60, 58f, 60/ development of. 137 Posterio r po le, 79. See also Macula Posterio r synechiae, myd riasis in preven tion of, 34 1 Posterior uveitis. corticosteroid route of administration in ,3601 Posterio r vitreous detachme nt, 84, 84f, 85/. 286-287 Postganglio nic nerves, 98, 103, 107 cholinergic drug action a nd, 234. 235/ iris sphincter supplied by. 6 1
Posttranslational modificat ion, 158 of lens proteins, 277 Potassium in aqueous hu mor, 255, 26111. 267 in lens, 278 in tear flhn, 2381, 240 transport of, 307 in vitreous, 2661, 286 Potassium channels, in lens, 278 Povidone· iodine, 378 Prader" Willi syndrome, imprinting abnormalities causing, 154, 169 Pralidoxime, 336f, 340 Prazosin, 259t Precornealte;lr m m, 42, 237. See also Tea r mm (tears) Precursor messenger RNA, 16 1 Pred Forte/ Pred Mild. See Prednisolone Prednisol. See Prednisolo ne Prednisolone, 3571 a nti" inOammatory/pressure-elevati ng po te ncy of. 3601 in combination pre parations. 3731 Preganglio nic autonomic nerves. cholinergic drug ac tio n and, 334, 335/ Pregnancy alcohol use during (fetal alcohol syndrome), 142- 143. 142/ carbonic a nhyd rase use during, 354 Prelaminar nerve/prelaminar area of optic nerve, 90. See also Optic disc blood supply of, 94. 95/ Premcianosomes. in retinal pigment epi thelium. 72. 127 Prenatal diagnosis, 1981. 228-229 Preo(lliar tear film, 237 tear dysfuncti on and, 244 Presbyopia, 63 Preservatives, in ocu lar medications allergic/adverse reactions to, 322 demulcents, 391 irrigating solutions, 392 Pretarsal space, 25/ Pretectum/ pretectal nuclei, 99 Primary lens fibers, development of, 130 Primar}, vitreous, 130, 131f Primers, in polymerase chain reaction, 158 Primitive node, 117 Primitive streak, 117, 118/ Primitive zone, 124 Proband, 158.225 Procaine, 3871 in patients taking cholinl'Sterase inhibito rs, 340 Prodrugs, 330 Profe na!' See Suprofe n Program med cell death ( Pe D/apoptosis), 147, 148 in DNA repai r, 170 Progressive (chronic progressive) exte rnal opht ha lmoplegia, 174- 175 mitocho ndrial DNA deletions a nd, 173, 174- 175, 19 1 Prokaryotes/prokaryotic cells. 158 Promoter, 158 in alternative splicing, 168 Proparacaine, 3871, 388-389. 389 with Ouorescei n, 3871
Index .447 Propine. See Dipivefrin Proposita/propositus. 158.225 Propranolol, 259t. J45/ Propylene glycol, fo r dry eye. 390 Prostacycl in,255-256 Prostaglandin analogues, 256- 257. 2641. 354-355, 354r for glaucoma. 256-257. 264 t. 354-355. 3541 Prostagland ins, 255, 256- 257 modes of action of. 264 t nonsteroidal anti ~ infl a l1lmator )' drugs and . 257 receptors fo r. 258 in Signal transduction. 2641 synthesis of, 256f, 257 anti-inflammatory drugs affecting, 257 Prostig min . See Neostigmin e Prota n defects (protanopia ), oc ular fi ndings in carriers of, 222t Protein kinase C/Ca 1 • - dependent Signa l transduction, in tear secretion , 242- 243. 242/ Proteinase inhibitors in aq ueous humor, 267. 269 in cornea, 250 Proteinases. in aqueous humor, 269 Proteins in :lq ueous humor. 265. 266. 268- 270 bre:lkdown ofbloud - aq ueous b:lrrier and, 272 dynam ics and, 254. 255, 268- 269 cilia ry body expression of. 253 drug-binding by, system ic admi nistration and . 329 in retinal pigment epithelium, 304 in rod outer segments ("rim" proteins). 293 in tear film , 240 vitreous, 283, 285- 286 Proteogl ycans, corneal. 45. 249. 250 Proto·oncogene. 158, 17 1 Protoplas mic astrocytes. 77 Pseudocholinesterase. See (1150 Cholinesterase defective. SUCCinylcholine effect s and. 230 Pseudodominance, 159, 215 Pseudogene.159 Pseudohermaphrodites.204 Pterygoid canal, nerve o f. 107 Pterygoid venous plexus, 40/ Pterygopalatine ganglion/ nerves. 107 Puncta, 23,33. 34 Pupil. See Pupils Pupillary light reflex (pupilla ry response to light), pathways for, 99 Pupillary membrane development of. 133f. 134. 135. 137, 138 persistent, 139J Pupillary near reflex. pathways fo r, 99 Pupils constriction of, in near and Iighl reflexes. 99 tonic (Adie pupillsy ndrume), testing for. 338 Purified neurotoxin complex , 390 Purine bases/purines, 159 Purile. See Oxychloro complex PVD. See I)osterior vitreous detachment Pyrazolones. 362. See also Nonsteroida l antiinflam matory drugs Pyridoxi ne (vitamin Bh ). for homocystin uria,132 P)'rimethamine, 374
Pyrimidi ne bases/pyrimidines, 159 fluorinated, 379 f, 385 Pyruvate carboxylase. defective, 213t Pyruvate dica rboxylase. defective, 213t q arm. 159 Quixin . Sec Levofloxaci n Rab escort protein I ( REP I) gene mutat ion. 298 Race, genetic disorders and. 210- 211 Rad iation DNA repair after. 170 in free radical generation, 312 Ranibizumab, 396-397 mSOllcoge ne, 171 Rathke pouch, 119/ Rb locus. 204 RBI ( retinoblastoma) gene, 204- 205 RCFM . Sec Retrocorneal fibrous membrane RDS/peripherin gene mutations, 296 Reactive oxygen intermediates. 31 1. Sec also Free radicals Receptor agonist, 331 Receptor antagonist . 332 Receptor- effector coupling, 26 1- 262 Receptors, 258-261 , 259t, 264. 264t in tracellular communicat ion and, 26 1- 263, 262/. 263/ in iris-ciliary body. 258-261. 259t ocular d rug interactions and, 33 1- 332 in Signal transduction. 258-261. 261 Recessive inheritance ( recessive gene/t rait), 159, 211 - 212 autosomal. 212-2 16. 213t. 21St disorders associated With. 159,2 12- 216, 2 13t. 21 5t ge ne therapy for, 187- 188 X- linked, 218-219, 2 19 t Recessive tumor-suppressor gene. Rb locus as, 204 Recombinant, definiti on of, 159 Recombinant DNA. 159 Recombination , 150, 159 Recombination frequency. 180 Rectus muscles, 16f, 17J, 19t. 20J, 39f See also specific
type blood supply of, 18- 21. 19/, 37J. 38, 38I. 39I. 40j insertion relationships of. 16, 18/. 19t nerves supplying, 21 origi ns of, 18, 19t Red-green color vision. 294 defects in gene defec ts causing. 297 ocular fin dings in carriers of, 222t "Red -man syndrome," 377 Reduction division. in meiosis. 155 Reflex secretors, 241 Reflex tea r arc, 106/ Refle x tearing, 106- 107, 106/.24 1, 24 2 absorption of ocular medication affec ted by, 323. 327 init iation o f, 140 Refsum disease/syndrome (phytan ic acid storage disease). lUt gene defects causing. 213(.298 infantile, 213t gene defects causing, 213t. 298
448 • Index Regional anesthesia, 386-390, 3871 Relatives, first~degree/second-degree, 159 Renal metabolic acidosis, carbonic anhydrase lOP lowering and, 353 REP 1 (Rab escort protein 1) gene mutation, 298 Repeat element/sequence Aiu, 148, 166 LJ,166 Replication, DNA, 159 origin, 157 segregation and, 160, 196 slippage, 160 Replicative segregation, 160, 173 Rescula. See Unoprostone Residence time (of medication), 323 Resistance (drug). See also specific agent and specific
organism antibiotic, 369, 37 1, 375, 377 antiviral, 382 Restasis. See Cydosporine/cyclosporine A, topical Restriction endonucleases, 151, 178- 180, 179f for mutation screening, 178-180, 1791, 184, 186- 187/ Restriction fragment length polymorphisms (RFLPs), 160,178- 180, 179/ RET oncogene, mutations of, 193 Reticular formation, paramedian pontine, 104, 104/ Retina, 4 If, 42, 71-79, 291 - 302. See also under Retinal anatomy of, 4 If, 42, 71 - 79, 73/, 74f, 75f, 76f, 79/ antioxidants in, 316-318, 318/ biochemistry and metabolism of, 291 - 302 blood supply of. See Retinal blood vessels detachment of. See Retinal detachment development of, 121, 124-126, 125/ melanin in, 308 electrophysiology of, 301 - 302, 301/ free radicals affecting, 315- 316 gyrate atrophy of, 2131 neurosensory, 41f, 73-79, 291-298 anatomy of, 41f, 73-79, 74f, 75f, 76f, 77f, 79/ biochemistry and metabolism of, 291 - 298 development of, 121, 124- 126, 125/ glial elements of, 74f, 77 neuronal elements of, 73 - 77, 74f, 75f, 76f, 77/ stratification of, 74f, 78-79, 79/ vascular elements of, 74f, 78. See also Retinal blood vessels development of, 138 pigment epithelium of. See Retinal pigment epithelium regional differences in, 72, 73/ topography of, 41f, 42 Retinal adhesion, retinal pigment epithelium in maintenance of, 308 Retinal artery, central, 78, 90, 94, 95f, 96 Retinal blood vessels, 74f, 78. See also specific I'Cssel development of, 138 Retinal degenerations gene defects causing, 298 retinal pigment epithelium defects and, 308 Retinal detachment, 72 muscarinic therapy and, 335 posterior vitreous detachment and, 84, 84f, 85/
retinal pigment epithelial maintenance of adhesion and,308 rhegmatogenous, posterior vitreOlls detachment and, 84, 84j; 85/ Retinal pigment epithelium (RPE), 64, 65f, 71-73, 73f, 74f, 75f, 76f, 303-309, 303/ anatomy of, 64, 65f, 71 - 73, 73f, 303 - 304 antioxidants in, 315- 316 biochemistry and metabolism of, 303 - 309 composition of, 304-305 detachment of, 72. See ,lIso Retinal detachment development of, 121, 125f, 127 in disease, 308 gene defects affecting, 297 lipids in, 305 nucleic acids in, 305 phagocytosis by, 306- 307 physiologic roles of, 71 - 72, 305- 308 pigment granules in, 307- 308 proteins in, 304 regional differences in, 72, 73/ retinal adhesion maintained by, 308 sub retinal space maintenance and, 308 transport functions of, 307 visual pigment regeneration in, 305- 306 Retinal tears muscarinic therapy and, 335 in posterior vitreous detachment, 287 Ret inal vein, central, 40f, 90, 96 Retinaldehyde, in retinal pigment epithelium, 305- 306 Retinitis cytomegalovirus cidofovir for, 3811, 384 foscarnet for, 38lt, 384 ganciclovir for, 38lt, 383 - 384 punctata albescens, CRALBP defects causing, 297 Retinitis pigmentosa autosomal dominant, 217 oxygen~related protein mutations causing, 296 rhodopsin mutations causing, 295 autosomal recessive retinal pigment epithelium defects in, 308 rhodopsin mutations causing, 295 rod cGMP gated channel mutations causi ng, 29 5 rod cGMP phosphodiesterase mutations causing,
295 congenital/infantile/childhood (Leber congenital amaurosis) guanylate cyclase mutations causing, 296 RPE65 gene defects causing, 297 with neuropathy and ataxia, 175-1 76 mitochondrial DNA mutations and, 174f, 175-176 oxygen-regulated protein defects causing, 296 retinal pigment epithelium in, 308 ROM I gene mutations in, 296 X-linked ocular findings in carriers of, 221f, 222t retinal pigment epithelium in, 308 Retinoblastoma, 204-205 genetics of, 204-205 long arm 13 deletion syndrome and, 204-205, 205t mutation rate of, 207 Retinoblastoma (RBI) gene, 204-205
Index . 449 Retinoic acid homeobox gene expression affected by. l iS ocular development/congenital anomalies and, 142 Retinoid-binding proteins, interpholoreceptor, 306 Retinol. 306 Retisen . See Fluocinolone implant Retrobulbar anesthesia, agents for. 387 Retrobu lbar drug adm inistration. 327 Retrocorneal fibrous membrane. 25 1 Retrolam inar nerve/retrolaminar area of optic nerve, 9 1 blood supply of. 94, 95f Retro-orbital plexus. 106f, 107 Retroposon, l60 Retrotransposition, 160 Retrovir. See Zidovudine Rev-Eyes. See Dapiprazole Reverse tnmscription, 160 RFLPs (rest riction fragment length polymorph isms) .
Rod transduci n, 293 mutations in. 29S Rods, 73, 73 -74. 76[, 77f See {liso Rod outer segments amacrine cells fo r, 300 bipolar cells for, 298 development of, 126 electrophysiologic responses of, 30 If, 302 gene defects in. 295- 296 pholotranscluction in. 29 1- 293. 292f Rofecox ib, 257-258 ROls. Sec Reactive ox:ygen intermediates ROM 1 gene/ROM I protein, 293 mutations in. 296 Romycin. See Eryth romycin Rose bengal stain , 392 RPE. See Retinal pigment epilhelium RPE65 genefRP E65 protein, 304. 306 mutations in, 297 in Leber congenital amaurosis. 297
160. 178- 180. 179/ Rhodopsin, 291-293. 292f gene mutations in, 182,295 light affec ting, 29 1- 293, 292f phosphorylation of, 292, 292f, 293 regeneralion of, retinal pigment epithelium in,
305-306 Rhodopsin kinase. 293 mutations in, 295 Ribonucleic acid. Sec RNA RIEGIIP1TX2 gene. 141 Rieger ano maly/syndrome. 14 1 Riley- Day synd rome (fa milial dysautonomia), 2 13t racial and ethnic concentnuion of. 211 Ri m of optic cup. 123 "Rim" proteins, 293. See (lIsa specific Iype Rimexolone. 357/, 36 1 Riolan, muscle of. 23, 23f, 27
RNA amplification of, in polymerase chain react ion, 158 heteronuclear/ heterogeneous nuclea r (hnRNA), 153,
161 messenger (mRNA). precursor. 161 in reti nal pigment epithelium. 305 small interference (short interfering) (siRNA). in gene therapy. 188 splicing of. 161. 168 alternative, 165 transfer (tRNA). initiator, 154 Rod cGM P phosphodiesterase (rod POE), 293 mutations in, 295 Rod inner segments. 73, 76f, 292. See also Rods free radical leakage from, 315 Rod outer seg ments. 73, 76f, 291,292. See also Rods development of, 126 energy metabolislll in, 293 ox idative damage to, 3 15 phototransduction in, 29 1- 293. 292/ shed. retinal pigment epithelium phagocytosis of. 306- 307 Rod POE. See Rod phosphodiesterase Rod phosphodiesterase (rod POE). 293 mutat ions in. 295 Rod response. 294, 30 I/. 302
Scones. 294 horizorHal cells fo r. 299 retinal ganglion cells fo r, 299j. 300 slgA. See Secretory ISA SagiHal sinus. superior. J08, 108f Salicylates. 362. See (liso Nonstero idal antiinflammatory drugs Salivary nucleus, 106. 106- 107, 107 Sand hoff disease (G M1 gangliosidosis type II ). 213/ Sand im mune. See Cyclosporine/cyclosporine A Sanfi Bppo syndrome. 2 13t Sanger method . for DNA sequencing. 184. 185/ Satellite DNA , 160. 166, 180 Scheie syndrome. 213t $chlemm canal, 5 If, 54, 55f, 56f, 57f developmen t of. 136 Schwalbe line/ ring, 39. 42, 49J, SO. 5 If, 53 Schwann cells, retinal. 301 Sclera, 4 1f, 42, 47- 48 anatomy of. 4 1J, 42. 47-48 development of, 135 rupture of, 48 stroma of. 48 topography of, 41J, 42 Scleral spur. 50, 5 1f, 52/, 53/ Scleral spur cells. 50 Scleral sulcus. internal, 50 Scleritis. cort icosteroid route of administration in ,
360 < Scopolami ne. 260, 34lt adverse effects of, 343/ Second cranial nerve. Sec Optic nerve Second-degree relatives. 159 Second messengers. in signal transduction, 262-263.
262/263/ Second-order neuron, d ilator muscle in nervation and .
60 Secondary lens fibers, development of, 130 Secondary vitreous. 130- 132, 13lf Sec retogra nin II. in aqueous humor, 267, 270 Sec retory IgA. in lear film , 240 Secretory lacrimal apparatus. 26J, 32- 33, 32f, 33f, 24 1-244 . 242f, 243f See also Lacrima l glands
450 • Index Segregation (genetic), 160, 193, 196 replicative, 160, 173
Selen ium. 316J in retina and retinal pigment epithelium, 3 16-3 17,
318/ Selenoprotein P, in aq ueous humor, 269 Semilunar ga nglion (gasserian/trigemina l gangli on). 101,101-102, 101/ Se nse st rand of DNA, 160 Sensorcaine. See Bupivacaine Se nsory nucleus of c ra nial nerve V (t rigeminal), 100, 10 1/ o f c ranial nerve VII (facia l), 105 Sensory root o f c ranial nerve V (t rigeminal), 100 of c ra ni al nerve VI (ophthalmic ), 14, I Sf of c ranialncrve VII (facial ), 105 Sequence-tagged sites, 160 Sequencing, DNA, 184 , 18sf Sex ch ro mosomes, 193, 194 aneuploidy of, 199 mosa icism and, 204 Sex-determi ning region Y (S RY Itestis-dctcrm ining fa ctor/TDF),218 Sex-linked inhe ritance (sex-linked genes), 160,2 18. See niso X- linked inheritance; Y-Iinked inheritance Slinker-I (511 I) gene, 176 Short (p) arm, 157 Sho rt arm 11 deletion syndrome ( 11p 13 syndrome), 205-207 in an iridia, 205-207 Short ciliary ne rves. lSI, 16, 36, 99 ciliary muscle supplied by, 64 iris sphincter supplied by, 6 1 posterior. 36 Short interfering (small interfere nce) RNA (siRNA). in gene therapy, 188 Short interspersed elements, 166 Alu repeat sequence, 148, 166 Short tandem repeats, 161 , 180, 18If Sickle cell disease (sickJe cell a nemia) point mutation causing, 207 racia l and ethn ic concent ration of, 2 11 s IGA . See Secretory IgA Sigmoid sinus, 108, 108f Signal transduction, 261 in iris-ciliary body, 258 - 261 in tear sec retion, 242- 243, 2421, 243/ Signal transduction pathways. 258-261 Simplex case/genetic disease, 16 1, 192 SINEs. See Short interspersed ele ments Single-gene disorder (me ndelian disorder), 155.208 Single-stranded conformational polymorphism, 182 Singlet oxygen. 311 , 312, 313,3 14,3 15 Sinus thrombosis. 108-109 Sinuses paranasal, 12- 13, 13I, 141 periorbital. 12-13, 13f, 14f Si nusiti s, pain distribution and, 13, 131 siRNA. See Sma ll interference (short interfering) RNA Sister chro matids, 149 Sixth cranial nerve. See Cranial nerve VJ Skin, eyelid, 22-23
Skipped generation (incomplete penetrance), 209, 2 I 7 Slow inact ivators, isoniazid use in, 229 Slow-reacting substance of anaphylaxis, lellkotri cnes in,258 Slow-twitch fibe rs, 21, 2 1t Small interference (short interfering) RNA (siRNA), in gene therapy, 188 Smallest region of overlap, 16 1 Snowflake cataract, 279 SNRPN gene, imprinting of, 169 SO ~. See Superoxide dism utase Sodium in aq ueous humor, 255, 266/, 267 in le ns. 278 in tear film , 2381. 240 transport of. 307 aqueous humor secretion and, 35 1 carbon ic anhydrase inhibitors affecting. 35 1 in vitreous. 266/, 286 Sodi um chloride, for ocular edema, 391 Sodi um hyaluronate, as viscoelast ic. 393 Sodiu m perborate, as ocular medi cat ion preservat ive,
323 Sodi um-potassium -calcium exchanger (Na t, K' -e a exchanger), in rods, 292 Sodi um -potassium pump (Na' ,K' -ATPase) in aqueo us humor secretion, 351 in le ns ionic balance, 278 in retinal pigment epithelium. 304, 307 in rods, 292 Somatic afferent fibers. 105 Somatic motor nerves. cholinergiCdrug action and . 334.
335/ Sondermann channels, 57f Sonic hedgehog mutations. 141 , 142 cyclopia caused by. 141 Sorbitol/sorbitol pathway in cataract for mation, 280, 28 1 diabetic retinopathy and, 301 in lens glucose/carbohydrate metaboli sm, 280, 280f Sorsby macular dystrophy retinal pigment epithelium in, 308 T IMP3 defects causing, 182, 297 Southern blot analysis, 161 , 179f in m utation screening, 179/. 184, 187f Sperm, 196 SphenOid sinuses, 9I, 12, 13/. 14/ Spherule (synaptic body), of rod, 74, 77f Sphincter muscle, 58I, 59I, 61, 258, 259 development of, 6 1, 137 miotks affecting, 259, 259t, 335 muscarinic drugs afiecting, 335, 340 myd riatks affecti ng. 259t, 260, 340 Sphingomyelinase, defective, 213t Spina l nucleus and trac t, of c ranial nerve V (trigemi na l), 100- 102,101/ Spiral ofTillaux , 16, 18f Splice junction site, 148, 161 Spliceosome, 161 , 168 Splicing, 16 1, 168 alternative, 165, 168 Sporadic, defini tion of, 16 1 Sporanox. See Itraconazole
Index . 451 SRO. Sec Smallest regio n of overlap SRY. See Sex-determin ing region Y SSCP. See Single-stranded conformatio nal polym orphism Stapedia l artery. 138 Staped iallstaJ>edi us nerve, 106 Stargard t disease (juvenile macular degeneration/ fund us Ilavimaculams) ABC transpo rter mutatio ns causing, 296 retinal pigme nt epithelium in, 309 Statio na ry night blindness. congenital. with myopia, ocular findings in carriers of. 2221 Stationa ry nyctalopia, rod-specific mutations causing,
295, 296 Stearate/stear ic acid in retinal pigment epitheli um , 305, 306 in vit reous. 286 Stero ids. See Corticostero ids Stickler synd rome. vitreous coll apse in . 288 Stomode um. 119/ Stop codon (ter minatio n codon). 16 1 fra meshi ft m utatio n and, 152 Sto rage diseases. enzyme defects/oc ular signs in, 2 13t Straight sinus. 108. 108/ Streptokinase, 393 Strepto myci n, 376 o to toxicity of. mitochondrial DNA mutations and . 176 Stro ma choroidal, development of. 133 ciliary body. 6 1-63, 62/ corneal. 45. 46j. 247f, 249- 250 a nato my of. 45. 46f, 247/ bioche mistry and membolism of. 249- 250 dewlopmenl of, 133- 135, 134/ iris, 56, 60f sclera l, 48 ST Rs. See Short tande m repeats STSs. See Sequence-tagged sites Su barach noid space, o ptic nerve. 9 1 Subconjunctival d rug admi nistration , 327 Subretina l space (interphOloreceplor m atrixIl PM), 72.
74/ development of, 123 retinal pigme m epithelium in maintena nce o f. 308 Su bstance P in iris- ciliary body, 259 in lea r secretion, 240 Substantia pro pr ia, corneal. See Stroma, corneal Sub-Teno n d rug adm inistratio n. 327 of local anesthetic. 389 Succ inylcholine, 344 adverse effects of, 343f in patie nts t3king cholinesterase in hibitors, 340 phar macogenetics and , 229- 230 "Sugar" cataracts. 279- 28 1 a ldose red uctase in development of, 280-281 Sulamyd. & c Sulfacetamide Sulc us/sulc i. interna l sclera l, 50 Sulf- 10. See Su lfaceta mide Sulface tam ide, 3721, 374 in combination preparations, 37JI
Sulfad iazine, 374 Sul fl te oxidase, de fective, 23 11 Sulfite oxidase deflciency. 2 131 Sulfoiduronate sulfa tase, defective, 2 131 Sulfo na mides, 374 protein binding of. 329 Sulindac. 3621 Superficial petrosal nerve, greater, 106. 107 Superior ce rvical ganglion, 15, 60 di lato r muscle in ner vatio n a nd, 258 Super ior oblique m uscles, 16j. 17f, 18, 19/, 20f, 38j. 39/ insertion relat ionships of, 17, 18j. 191 ne rves supplying, 2 1 Superior o rbital flssure, 7/, 10, 11j. 12f Supe rior petrosal si nus, 108f Superior punctum. 23, 33 Superior rectus m uscles, 16f, 17/, 19/. 20j. 37j. 38, 38f,
39f, 40/ inserlion relatio nships of, 16. I Sj. 191 ne rves supplying, 2 1 Superior sagitta l sinus. 108, 108/ Supe rior tarsal muscle of Muller, 23f, 25j. 28 Superio r transverse ligament ( Whit nailligament l. 25j.
28 Superox ide, 3 11 , 3 12/ Superox ide dism utase. 3 12f, 3 17 in lens. 3 14 in retina and ret inal pigment epithelium. 3 17 Support groups, ge netic d iso rders and , 232 Supraorbital a rtery, 20j. 37/ Supraorbital fora men / no tch, S, 6j. 7f, 10 Supraorbital nerve, 103 Suprao rbital vein , 40/ Supratrochlear a rtery. 37/ Supratrochlear nerve. 103 Suprofen, 257 Surfac ta nts, in eyedrops, absorption and, 326-327 Suspensory ligament of lens. See Zonules Suspensory ligament of Lockwood, 35. 36f Susta ined- release preparations o ral,329 fo r to pical admini stration, 330- 33 1 Sutures (lens). 69f. 70 developmen t of, 130 Sweat glands, of eyelid, 23. 23f, 27 1 Sym pathetic nerves/pathway adrenergiC drug action and. 346 cholinergic drug action and. 334, 335/ in cilia ry ganglion, 14.15- 16, 15/ ci liary muscle innervation and, 64 d il ato r muscle innervation and, 60, 258 sphincter m uscle inne rvation and. 61 in tear secre tion, 237, 240, 242, 242/ Sympathetic ophthalmia, corticosteroid roule of administration in, 3601 Sympathetic root , in ci liary ganglion, 14, 15f Synaptic body of cone (pedicle), 74. 77/ of rod (spherule), 73-74. 77/ Synechiae, poslerior, mydriasis in prevention of, 341 Synkinesis. near re Oex and, 99 Sym cny/syntenic traits, 161, 176- 177 , 197 Systemic drug therapy, for ocular disorders. 328- 330
452 • Index Tandem repeats short, 161, 180, lSI/ variable/variable n um be r of (minis3tellites), 156, 166, 180
Tarsal glands. 23, 23J, 28. 29f, 237, 239/ See (lIsa Glands
of Wolfring; Meibomian glands Tarsal muscles. superior (Muller), 23f, 2S/. 28 Tarsal piates/rarsus, 2Sf, 27. 28, 29/ TATA box, 16 1
Taxon -specific crystallins, 276 Tay- achs disease (GM 1 gangliosidosis type I), 213t racial a nd ethn ic concent ratio n o f. 2 10- 2 11
Tazolol, 259r TeA cycle. See Tricarboxylic acid (TeA) cycle TDF. See Testis-determining fac tor Tear d efiCiency states, 244-246. 24Sj. 246/ aqueous, 244-245 muci n, 24 1, 244 Tear film (tears), 237 - 246. 238f, 238t. 239f
aqueous layer of, 42. 237, 238/ biochemistry and metabolism of. 237- 246 dysfunction/alterations of. 244-246, 245f, 246/ lipid layer o f, 42 , 237- 239, 238f, 2381, 239/ muci n layer o f, 42, 238f pH o f. 2381. 240 precorneal. 42. 237 preocular.237 sec retio n o f, 24 1- 244, 242J. 243/ solu tes in. 240 thickness of. 237. 238f. 238t Tear meniscus (marg inal tear strip), 237 Tear pump, ocu lar medication absorp tio n a ffec tcd by, 324 Tearing/epip ho ra, refl ex, 106- 107, 106f. 241 , 242 absorption o f oc ular m ed ication a ffected by. 323, 327 init iation o f, 140 Tears (artificial ), 245, 390- 39 1 TeJomeres/ telo m eric DNA, 162, 166 Tema floxacin, 37 1 Tem poral arteritis. corticosteroid ro ute o f administration in, 3601 Temporofacial trunk, of cranial nerve VII, 106 Tenon capsu le, 35 Tensilon. See Edro phoni um Terak. See Oxytet racycline, with polymyxin B Teratogens.141 congenital anoma lies caused by. 141 - 143. 142/ Te rm inal web, 46f, 47. 65/ Termination cod on (stop codon). 16 1 frameshift m utation and. 152 Te rt iar y vitreous, 132 Testis-d etermini ng fac to r (TD F/sex-determi n ing regio n
YISRY), 21 8 Tetracaine, 3871, 389 Tetracyclines. 374-375 int ravenou s a dm in istration of. 329 Tc tra hydrotriam cino lo ne, a nti -inflammatory/ pressu reelevating poten cy of, 3601 Tetra hydrozoline.392 Te lravisc_ See Tetracaine TGF- ps. See Transfor ming growth fac to r ps T h imerosal. a llergic/sensitivity reactions and. 322 T hird cranial nerve. See C ranial ne rve III
T hird -order neurons, d ilato r muscle in nervation a nd, 60 13q 14 (long arm 13 deletion) syndrome, 204-205, 205 t T hree cone opsi ns, 294. Sec also Color vision Threshold. genetic, 162 po lygenic traits with. 215 T hrombin , 393- 394 Th ro mbospond in I. in vitreous, 287 T h romboxa nes, 255, 256. 257 aspirin affec ting. 362 Thymoxamine, 2591. 260. 348 Ticarcilli n. 368t. 370 Tight junct io ns (zonulae occlude ntes) in ciliary body epith elium . 62, 265 in relinal blood vessels, 78 in retinal pigment epithelium, 72 TlGRlm yocilill gene, in glaucoma, 17 1 Tillaux , spiral of. 16, 18/ Timolol. 2591. 350, 3501 in combination p repa ratio ns, 347t. 3501, 352- 353, 3521, 3 55 fo r glauco m a. 350. 3501. 3521 sustained-release preparation of, 330 systemic absorption of, 325/ 'J'i moptic. See T imolol TIMP3 gene/T IM P3 protein mutation, 182. 297 Ti ssue plas minogen activato r, 334, 393 intraoc ular administration of. 328 in vitreous. 285 Tobralcon . See Tobramyci n To bramycin. 368(, 3721. 375-376 in combination p repa ratio ns, 3731 ototoxic ity of. m itochond ria l DNA mu tations and , 176 Tob rasol. See Tobramycin To brex. See Tobra myci n Toleclin . See Tolmctin Tolmctin, 3621 Tonic cells. 298. 299f, 300 To nic pu pil (Adie p upil). p harmacologic testing for, 338 To nic-type fibers. 2 1. 2 1t To pical anestheSia. 386- 390. 3871 fo r a nter ior segment surgery. 389-390 Topical medications. 323- 327, 324f, 325f Sec fliso Eyedrops sustained -release devices fo r, 330-3 31 Topography. 4 1- 42. 4 If Tox.icity. drug therapy. 32 1. 322- 323 aging and . 323 tissue binding and. 327 tPA_See Tissue plasminogen ac tivato r Trabecular meshwork. 49f. 5 1/. 53-55. 53f. SSf, 56/ corneoscleral. 53, 54 develo pment of. 135. 136 cnd otheli al. 54 uveal. 53. 54 Trabeculocytes. 53. 54 Trait, 21 1 holandric. 2 18 Tranexam ic acid . 394 Trans-RPE poten tial. electro-oculogram and. 307 T ranscribed strand of ON A (antise nse DNA ), 148 Transcription (gene). 162. 166- 169, 167/ reverse. 160
Index. 453 Transcription fac tors, 166-1 68. 167/ Transd ucin, rod, 293 mutations in, 295 Transfer RNA (tRNA ). initiator, 154 Transferrin in aqueous humor, 268, 270 in vit reous, 285 Transforming growth fac tor ps. 114.396 in aqueous humor, 270 homeobox gene expression affected by, 115 in ocular development, 11 4 in lear mm. 24 1 Transient amplifying celis, in corneal epithelium. 43 Transient nerve fiber layer ofChievitz., 126 Transketolase. in cornea, 248 Translated strand of ON A (antisense DNA), 148 Translation . 162. 166 gene product cha nges/modi fica tion after (posnranslational modi fica tion), 158 ofl ens proteins, 277 Translocation, chromosome, 162 Down syndrome caused by. 20 1- 202 Transplantation , organ. for enzyme deficiency disease, 231 Transport mechanisms in aqueo us hu mor dynamics, 254 - 255 lens, 277-278 retinal pigment epithelium, 307 Transverse ligamen t, superior (W hitnall ). 25f. 28 Transverse sinus, J08, J08f Trauma, optic neuropat hy caused by, 93 Travatan. See Travoprost Travoprost, 354, 354 1 Triazoles. 3791. 380 Tricarboxylic acid (TeA) cycle, in corneal glucose met abolism. 248 Trichiasis, 29 Tricyclic antidepressants, apraclonidine/brimonidine in teractions and, 348 Triesence, 36 1 Trifluridine. 380, 3811 Trigemina l ganglion (gasserian/semilunar ganglion), 10 1, 101-1 02,10 1/ Trigeminal nerve. Sec Cranial nerve V Trimcthaphan, adverse effects of, 343/ Trimethoprim, with sulfonamides, 374 Trimethoprim -polymyxin B, 372t Trinucleotide repeats, 155 expansion/cont raction of, 162 ant icipation and. 148, 162.208-209 Triplet, ISO Trisomy, 199 Trisomy 21 (Down sy ndrome), 20 1- 203, 202t mosaicism in, 204 pharmacogenetics and, 229 Trisomy 21 mosaicism, 204 Triva riant color vision. 294-295. See also Color vision tRNA. See Transfer RNA Trochlea, 7 Trochlear fossa, 7 Trochlear nerve. See Cranial nerve IV Trophoblast, 117 Tropicalyl. Sec Tropicamide
Tropicamide, 260, 341 t Trusopt. Sce Dorzolamide Trypan blue, 392 Tubocurari ne, 2591 adverse effects of. 343/ Tubulin , 277 Tumor necrosis factor u . in tear mm, 24 1 Tumor-suppressor genes, 162. 170, 171- 173. See also
wccific type in DNA repair, 170 recessive, Rb locus as, 204 Wilms tum or, imprinti ng of. 154 Tunica vascuiosa ientis, 137 remnant of, 138 Mittendorf dot, 85, 85/ Twitch-type fi bers. 21,2 1t 2· hit hypothesis, 173 TXA. See Thromboxa nes Tympanic segment of cranial nerve VII (facial), 105- 106 Tyrosinase gene mutations. in albinism, 195. 2 13t, 308 Tyrosinase-negative/posit ive albinism, 195.2 131,308 Tyrosine aminotransferase, d efective, 23 1t Tyrosinemia, 2 J31 Tyrosinosis,2 13t Ul trafiltration, in aqueous humor dyna mics/fo rmation, 254- 255 Ultrasound biomicroscopy. 50, 52/ Ultraviolet blue phototoxicity, 31 6 Ultraviolet light (ult raviolet radiation), eye d isordersl injury assoc iated with , 313. 316 Umbo (clivus) . 80/' 104 Unequal crossing over. 162 color vision defects and . 295 Uniparental disomy. 162. 169 Unoprostone. 354, 354t Untranslated region. 162-163, 165 Upper eyelids. See Eyelids, upper Urea in aqueous humor, 268 as hyperosmotic agent, 355, 356, 3S6t in tear film, 240 Ureaphil. See Urea Urinary tract stones, acetazolamide use and, 353 Urokinase, 393
USHIB gene. 176 Usher syndrome gene for. 176 myosi n VilA mutation in, 176, 296 retinal pigment epitheli um in, 308 synteny and, 176 UTR. See Vntranslated region Uvea (uvea l tract), 42, 55-66, 253. See also specific
structure IOpography of, 42 Uveal meshwork. 53.54 Uvei tis, cort icosteroid route of ad mi nist ration in, 360t Uveosd eral d rainage/outflow, SO Valacyd ovir. 329. 38 It , 383 Valdecoxib.258 Valtrex. See Valacyclovir Vancomycin, 368 t, 376-3 77
454 • Index Variability. in genetic disease, 208- 209 Variable/ variable number of tandem repeats (variable tandem repeats/ min;satcllite), 156, 166, 180 Vascu lar endothelial growth fa ctor (VEGF), 396 alternative splicing and. 168 in aqueous humor, 270, 27 1 in vitreous, 287 Vascular loops, 85 Vasc ular system of choroid, 64, 65f, 66, 66f. 67 development of, 132- 133 of Ciliary body, 62- 63 development of. 137- 138. 139/ of eyelids, 29-30, 30/ of iris, 58, 59/ of orbit a natomy of, 36 - 40, 37f. 38j, 39f development of, 138 Vasculotropin. See Vascula r endothelial growth factor Vasoactive intestinal polypeptide (V] P) o utflow facility affected by. 50 in tear secretion, 237. 240. 243. 2431 Vasocon-A. See Naphazoline/antazoline VECAT (Vitamin E, Cataract and Age-Related Maculopathy Trial), 3 14-3 15 Vector, 163 cloning, 149- 150 cosm id , 150 in gene thera py, 187 VEGF. See Vascular e ndothelial g rowth factor Veno us sinuses, 108- 109, 108f Versican, in vitreous, 283 Ve rtebral artery, 109 Vesicle lens, 12 1, 129, 129/ optic, 121, 121f, 123f, 124/ lens development and, 128- 129 Vexol. See Rimexolone Vidarabine, 380, 38 1t Vigamox. See Moxinoxacin Vim entin, 277 Vioxx . See Rofecoxib VI P. See Vasoactive intestinal po lypeptide Viroptic. See Trifluridine Visceral afferent fi be rs, 105 Visceral efferent fibers, 10 5 Viscoelastic agents, 393 Viscos ity, ocular medication abso rption affected by, 236 Vistide. See Cidofovi r Visual (calcarine/ occipita l) cprlex. 97 Visual field defects, 93, 94f Visual path ways, 9 3. 94/ Visual pigments, regeneratio n of, retinal pigment epithelium in . 305- 306 Visual radiations (geni culocalcarine pathways), 93 VITI , in vitreous, 283, 285- 286 Vilam inA for abetalipoproteinemia, 232 oc ul ar development/congenital anoma lies and, 11 5. 142 in retinal pigment epitheli um . 305, 306 in visual cycle, 305-306 Vitam in B6 (pyridoxine). for hOl1locystinuria, 232
Vi tamin C (ascorbic acid ) antioxidant effec t of. 3 17- 31 8 in lens, 31 4- 3 15 in retina and retinal pigment epithelium, 3 17- 3 18 in aqueous h umor. 255 , 2661 o ral supplements and, 3 14- 3 15 in tear film, 240 in vi treous, 266t Vitamin E abetalipoproteinemia and, 232 a nt ioxida nt e ffec t of, 3 12, 3 14f, 3 17 in lens. 314-3 15 oral su pplemen ts and , 314- 3 15 in retina and retina l pigment epithelium , 3 17. 3 18/ Vitamin E, Cataract a nd Age-Related Maculopalhy Trial (VECAT),3 14- 315 Vitamin suppl ements, 395 in ge netic disorders, 232 Vitrasert. See Ganciclovir Vitrecto my, ocular physiologic changes after. 287- 288 Vitreolysis. e nzymatic, 289 Vitreous, 4 If, 42, 82- 8 5, 283- 289 aging affect ing, 84 , 84[' 286- 289 anatomy of, 41f, 42, 82- 8 5, 83f, 84f, 85/ as angiogenesis inhibitor, 287 biochemistry and metabolism of, 283-289 collagen in, 82- 83, 83f, 130, 132,283,283 - 284,284/ liq uefaction and , 286 composition of. 28 3- 286 detachment of. See Vit reous detach ment development of, 85. 130- 132, 13 If disorders of, biochemical changes associated with, 286-289 genetic disease involving, 288 hyal uronan/hyaluronic acid in, 83f, 130-1 32,283. 284-285 in flammatory processes affecting, 288 injury to. 288 lipids in, 286 liquefaction of, 286 injury/ he mo rrhage/ inOammation and. 288 macular ho le for matio n a nd, 288 m yo pia caused by changes in. 286. 287 primary, J 30, B l/ protein s in . 283, 28 5- 286 secondary, 130- 132, 13 1/ tertiary, 132 topography of, 41f, 42 zonu lar fibers in. 286 Vitreous base. 83 Vitreous detachment , posterio r, 84, 84/. 85f, 286-287 Vitreous hemorrhage, 288 VNTRs. See Variable/variable n umber of tandem repeats Voltaren. See Diclofenac von Recklinghausen di sease. See Neurofibromatosis, von Recklinghausen Vortex veins, 38- 40, 38f, 40f, 64 VTRs. See Variable/variable number of tandem repeats Waarde nburg syndrome, 166, 2 16 WAGR syndrome. 206 Watershed zone , 94
Index .455 Wear-and-tear pigment (lipofusci n granules), 72 Western blot analysis, 163 Whitnaliligament (superior transverse ligament), 25[, 28 Whitnall tubercle, 9 Wilbrand knee. 96 Wild type, 163 Will is, circle of, 93, 94f, 109, 109J Wilms tumor- suppressor gene, imprinting of, 154, 169 Wilson disease (hepatolent icular degeneration), 231 "Wing" cells. 43. 44/ Wolfr ing, glands of, Bf, 26f, 271. 33, 240 X chromosome, 193, 194, 195 genes for color vision on, 294- 295 inactivation of (Lyo nization/ Barr body), 149, ISS, \68- 169, 220- 224 ocular fin dings in carrier stales and , 120-224, 22 1- 22 2[, 2221 X-linked disorders, 219-220. See also specific disorder and X-linked inheritance albin ism (Nettleship-Falls), ocular findings in carriers of, 222[, 223 blue-cone monochromatism, ocular findings in carriers of, 2221 dominant , 219-220 gene therapy for, 187- 188 ocular findings in carriers of, 220, 22 1- 222f, 2221 retinitis pigmentosa, ocular findings in carriers of,
mf. 2221 X-linked inheritance (X-lin ked genes), 163, 185. See also X- linked disorders dominant, 219, 2191 recessive, 218-2 19, 2191 Xalatan . See Latanoprost Xanthophylls (carotenoids) inlens,314- 315, 3 14f in retina, 81, 3 18, 3 18J Xeroderma pigmentosum , 170 Xibrom. See Bromfcnac Xylocaine. See Lidocaine Y chromosome, 193, [94 inheritan ce/traits determined by (holandric inheritance/trait),2 18
Y-linked in he ritance (Y-linked genes), 163 V-sutures, lens, 69[, 70 development 0(, 130 YAC. See Yeast art ificia l chromosome Yeast artificial chromosome (YAe) , 163 Yohimbine, 259/ Zaditor. See Kctotifen Zeaxanthin , 81 antioxidan t effect of. 316 Zeis, glands of. 23, Bf, 26f, 27/ in tear-fil m lipids/tear production, 237 Zeta (s)-crystall ins. 115 Zidovudin e, 381 t, 384-385 Zinc, in aqueous humor, 267 Zinc finger motif, 166, 167f Zinn annulus of, 10, 16[, 17f, 18, 191, 91 zonules of (zo nular fibe rs), 4 If, 5 If, 52f, 61 , 69f, 70 , 273 development of, 130 Zinn- Haller, circle of, 94 Zones of discontin uity, 70 Zonulae adnerentes. in retinal pigment epithelium , 72 Zonulae ocdudentes (tight junct ions) in ciliary body epithelium, 62 , 265 in corneal endothelium, 135 in retinal blood vessels, 78 in retinal pigment epitheli um, 72 Zonular apparatus, 286 Zonular fibe rs lens (zonules ofZinn), 4 If, 5 If, 52[, 61, 69f, 70, 273 development of, 129f, 130 vitreous, 286 Zo nules occluding, corneal, 43 of Zinn (zonular fibe rs/suspensory ligaments), 4 If, 51f. 52/. 6 1, 69f. 70, 273 development o f, 130 Zovirax. Sec Acyclovir Zygomatic foramen, 10 Zygomaticofacia l artery, 39f Zygomaticotemporal artery, 39/ Zygote, 196 Z)'mar. Sce Gat ifloxaci n
