Diagnostic Procedures in
OPHTHALMOLOGY
Diagnostic Procedures in
OPHTHALMOLOGY SECOND EDITION
HV Nema
Former Professor and Head Department of Ophthalmology Institute of Medical Sciences Banaras Hindu University Varanasi, Uttar Pradesh, India
Nitin Nema
MS Dip NB
Assistant Professor Department of Ophthalmology Sri Aurobindo Institute of Medical Sciences Indore, Madhya Pradesh, India
®
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[email protected] Diagnostic Procedures in Ophthalmology © 2009, HV Nema, Nitin Nema All rights reserved. No part of this publication should be reproduced, stored in a retrieval system, or transmitted in any form or by any means: electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the editors and the publisher. This book has been published in good faith that the material provided by contributors is original. Every effort is made to ensure accuracy of material, but the publisher, printer and editors will not be held responsible for any inadvertent error(s). In case of any dispute, all legal matters to be settled under Delhi jurisdiction only. First Edition: 2002 Second Edition: 2009 ISBN 978-81-8448-595-0 Typeset at JPBMP typesetting unit Printed at Replika Press
Contributors Jorge L Alió
MD, PhD
Director, Vissum Institute of Ophthalmology of Alicante Alicante, Spain
Sonal Ambatkar
DNB
Glaucoma Service Aravind Eye Hospital Tirunelveli, Tamil Nadu, India
Francisco Arnalich
Sreedharan Athmanathan
MD, DNB
Virologist LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India MD
Professor Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India
Tinku Bali
MS
Consultant Department of Ophthalmology Sir Ganga Ram Hospital, New Delhi, India
Rituraj Baruah
MS
Senior Registrar Lady Hardinge Medical College New Delhi, India
Jyotirmay Biswas
MS
Ex-Fellow Sankara Nethralaya Chennai, Tamil Nadu, India
Taraprasad Das
MS
Director LV Prasad Eye Institute Bhubaneswar, Orissa, India
MD
Vissum Institute of Ophthalmology of Alicante Alicante, Spain
Mandeep S Bajaj
Surbhit Chaudhary
MS, FAMS
Munish Dhawan
MD
Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India
Lingam Gopal
MS, FRCS
Chairman Medical Research Foundation Sankara Nethralaya, Chennai Tamil Nadu, India
AK Grover
MD, FRCS
Chairman Department of Ophthalmology Sir Ganga Ram Hospital New Delhi, India
Roshmi Gupta
MD
Consultant, Narayana Nethralaya Bengaluru, Karnataka, India
Sanjiv Gupta
MD
Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India
Head, Ocular, Pathology and Uveitis Sankara Nethralaya, Chennai Tamil Nadu, India
Stephen C Hilton
Ambar Chakravarty
Santosh G Honavar
MS, FRCP
Honorary Professor and Head Department of Neurology Vivekananda Institute of Medical Sciences Kolkata, West Bengal, India
OD
West Virginia University Morgantown, USA MD, FACS
Director Department of Ophthalmic Plastic Surgery and Ocular Oncology, LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
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Diagnostic Procedures in Ophthalmology Anjali Hussain
MS
Consultant LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
Subhadra Jalali
MS
Head Smt Kanuri Santhamma Retina-Vitreous Centre LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
Sadao Kanagami
FOPS
Professor Kitasato University School of Medicine Teikyo, Japan
Sangmitra Kanungo
MD, FRCS
Consultant LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
Shahnawaz Kazi
MS
Fellow Sankara Nethralaya Chennai, Tamil Nadu, India
R Kim
DO
Head Retina-Vitreous Service Aravind Eye Hospital and Postgraduate Institute of Ophthalmology Madurai, Tamil Nadu, India
Parmod Kumar
OD
Glaucoma Imaging Centre New Delhi, India
S Manoj
MS
Consultant Retina-Vitreous Service Aravind Eye Hospital and Postgraduate Institute of Ophthalmology, Madurai, Tamil Nadu, India
S Meenakshi
MS
Amit Nagpal
MS
Consultant Pediatric Ophthalmology Sankara Nethralaya Chennai, Tamil Nadu, India Consultant Sankara Nethralaya, Chennai Tamil Nadu, India
A Narayanaswamy
Consultant Sankara Nethralaya Chennai, Tamil Nadu, India
Rajiv Nath
MS
Professor Department of Ophthalmology KG Medical University Lucknow, Uttar Pradesh, India
Tomohiro Otani
MD
Professor Department of Ophthalmology Gunma University School of Medicine Maebashi, Japan
Nikhil Pal
MD
Senior Resident Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India
Rajul Parikh
MS
Consultant, Sankara Nethralaya Chennai, Tamil Nadu, India
David Piñero
OD
Vissum Institute of Ophthalmology of Alicante Alicante, Spain
K Kalyani Prasad
MS
Consultant Krishna Institute of Medical Sciences Hyderabad, Andhra Pradesh, India
Leela V Raju
MD
Monongalia Eye Clinic Morgantown, USA
VK Raju
MD, FRCS, FACS
Clinical Professor Department of Ophthalmology West Virginia University Morgantown, USA
LS Mohan Ram
D Opt, BS
LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
Contributors R Ramakrishnan
MS
Professor and CMO Aravind Eye Hospital Tirunelveli, Tamil Nadu, India
Manotosh Ray MD, FRCS
Associate Consultant National University Hospital Singapore
Pukhraj Rishi
MD
Consultant Sankara Nethralaya Chennai, Tamil Nadu, India
Monica Saha
MBBS
Department of Ophthalmology KG Medical University Lucknow, Uttar Pradesh, India
Chandra Sekhar
MD
Director LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
Harinder Singh Sethi MD, DNB, FRCS
Senior Research Associate Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India
Pradeep Sharma
Yog Raj Sharma
MD
Professor Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India
Deependra Vikram Singh
Devindra Sood
MD
Consultant, Glaucoma Imaging Centre New Delhi, India
MS Sridhar
MD
Consultant LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
S Sudharshan
MS
Fellow Sankara Nethralaya Chennai, Tamil Nadu, India
Kallakuri Sumasri
B Optm
Retina-Vitreous Centre LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
T Surendran
MS, M Phil
Professor Dr RP Centre for Medical Sciences AIIMS, New Delhi, India
Vice Chairman and Director Pediatric Ophthalmology Sankara Nethralaya Chennai, Tamil Nadu, India
Rajani Sharma MD (Ped)
Garima Tyagi B Opt
Savitri Sharma
Vasumathy Vedantham
Senior Resident Department of Pediatrics AIIMS, New Delhi, India MD
Head Jhaveri Microbiological Centre LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India
Tarun Sharma MD, FRCS
Director Retina Service, Sankara Nethralaya Chennai, Tamil Nadu, India
MD
Senior Resident Dr RP Centre for Ophthalmic Sciences, AIIMS New Delhi, India
Retina-Vitreous Centre LV Prasad Eye Institute Hyderabad, Andhra Pradesh, India MS, DNB, FRCS
Consultant, Retina-Vitreous Service Aravind Eye Hospital and Postgraduate Institute of Ophthalmology Madurai, Tamil Nadu, India
L Vijaya
MS
Head Glaucoma, Sankara Nethralaya Chennai, Tamil Nadu, India
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Preface to the Second Edition The goal of this second edition of Diagnostic Procedures in Ophthalmology remains the same as that of the first—to provide the practicing ophthalmologists with a concise and comprehensive text on common diagnostic procedures which help in the correct and speedy diagnosis of eye diseases. Like other disciplines of medicine, the knowledge of ophthalmology continues to expand and a number of newer and sophisticated investigative procedures have been introduced recently. Extensive and detailed information on recent diagnostic approaches is available in resource textbooks or online to ophthalmologists. To search these is time consuming, tiring and at times not practical in a busy clinical practice set-up. Therefore, this ready reckoner has been conceptualized. The book covers most of the basic and well-established diagnostic procedures in ophthalmology. It starts with visual acuity and describes color vision and color blindness, slit-lamp examination, tonometry, gonioscopy, evaluation of optic nerve head in glaucoma, perimetry, ophthalmoscopy and ophthalmic photography. Most of these procedures are considered basic and carried out routinely but to obtain an evidence-based diagnosis, a correct procedure for the examination must be followed. Corneal topography is very useful in detection of corneal pathologies such as early keratoconus, pellucid marginal corneal degeneration, corneal dystrophies, etc. It guides the ophthalmic surgeon to plan appropriate refractive surgery. Recent development in the application of wavefront technology can reduce different types of optical aberrations and may provide supervision and improve results of the LASIK surgery. A new chapter on Confocal Microscopy is included. Confocal microscopy, a noninvasive procedure, allows in vivo observation of normal and pathogenic corneal microstructure at a cellular level. It can identify subclinical corneal abnormalities. Procedures like Fundus Fluorescein Angiography and Indocyanine Green Angiography are invaluable diagnostic tools. They are not only useful in the diagnosis, documentation and followup but also in monitoring the management of the posterior segment eye diseases. With the development of high quality fundus camera and digital imaging, utility of both techniques has significantly increased. Ultrasonography, as a diagnostic procedure, has immense importance in the modern ophthalmology. Both A-scan and B-scan ultrasonography are dynamic procedures wherein diagnosis is made during examination in correlation with clinical features. Three-dimensional ultrasound tomography allows improved visualization and detection of small ophthalmic lesions. Ultrasound biomicroscopy is a method of high frequency ultrasound imaging used for evaluating the structural abnormality and pathology of the anterior segment of the eye both qualitatively and quantitatively. It is very helpful in understanding the pathomechanism of various types of glaucoma.
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Diagnostic Procedures in Ophthalmology Optical Coherence Tomography is a noninvasive, cross-sectional imaging technique which provides objective and quantitative measurements that are reproducible and show very good correlation with clinical picture of retinal pathology especially macula. Presently, OCT is often used in assessment of optic nerve head damage in glaucoma. One must remember that imaging technique alone may not contribute to a correct diagnosis. It is complementary to clinical examination. Therefore, results of imaging should always be interpreted in conjunction with clinical findings and results of other relevant tests. Electrophysiological tests are often ordered to assess the functional integrity of the visual pathway and in evaluating the cause of visual impairment in children. Multifocal ERG and multifocal VEP are newer techniques still under evaluation. It is claimed that multifocal ERG can distinguish between the lesions of the outer retina and the ganglion cells or optic nerve. Results of electrophysiological tests should never be analyzed in isolation but always be correlated with clinical findings to establish a definitive diagnosis. Etiological diagnosis of infectious keratitis and uveitis has been more vexing and often fraught with pitfalls. Collection of samples from eye, their microbiological work-up and interpretation of laboratory results have been described in chapters on keratitis and uveitis. Role of optical coherence tomography in the diagnosis and management of complications of uveitis is also discussed. A number of new chapters such as: Retinopathy of Prematurity, Localization of Intraocular Foreign Body, Comitant Strabismus, Incomitant Strabismus, Dry Eye, Epiphora, Proptosis and Neurological Disorders of Pupil have been added in the second edition of the book. Retinopathy of prematurity is one of the important causes of childhood blindness. Risk factors, documentation, staging, classification, screening procedure and management of the disease are briefly described. Precise localization of intraocular foreign body is a tedious procedure but is critically important for its removal and management. Computerized tomography and magnetic resonance imaging have replaced old cumbersome radiological methods for localization of intraocular foreign bodies, metallic and wooden. Strabismus often has an adverse effect on psychological functioning, personality trait and working capabilities of an individual. Patients with strabismus suffer from low self-esteem and have problem in social interaction. Therefore, early correction of strabismus is necessary for improving the quality of the life of the patient. The chapter on comitant strabismus presents various methods for examination and measurement of deviations. Incomitant strabismus, though less common, is more troublesome. It usually results from cranial nerves (III,IV,VI) paralysis. Restrictive strabismus may be associated with interesting clinical ocular syndromes. Dry eye is one of the most common external ocular diseases seen by ophthalmologists. Prevalence of dry eye is on rise mainly due to an environmental pollution, change in lifestyle and increase in aging population. Should dry eye be considered a disorder of tear film and excessive tear evaporation or a localized immune-mediated inflammatory response of ocular surface? Besides the controversy, what is more important is an early diagnosis of dry eye and its proper management.
Preface to the Second Edition Epiphora is an annoying symptom. It may occur either in infants or adults. An understanding of anatomy and physiology of the lacrimal apparatus is necessary for the evaluation of epiphora. A number of invasive and noninvasive tests are available to investigate patients with epiphora and localize site of obstruction in the lacrimal passage. Proptosis has a varied etiology. It may occur due to ocular, orbital and systemic causes. Generally, proptosis requires interdisciplinary cooperation amongst ophthalmologists, neurologists, oncologists, ENT surgeons, internists and radiologists. Investigation of patients with proptosis should begin with simple standard noninvasive techniques and, if necessary, progress to more elaborate and invasive procedures. Ultrasonography, CT and MRI are of immense value in the diagnosis. Examination of pupil (size, shape and pupillary reactions) is essential in neurological disorders. Typical pupillary signs can help in localizing lesions in the nervous system. Characteristics of Adie tonic pupil and Argyll-Robertson pupil and a detailed evaluation of the third cranial nerve palsy are described in the last chapter. Most of the contributors who have vast experience in their respective fields have written chapters for this book. To make the reader familiar, they have not only described diagnostic procedures but also given characteristic findings of eye disorders with the help of illustrations. The book has expanded greatly as many new chapters with numerous illustrations are added. We hope the book should be of great help to the practicing ophthalmologists and clinical residents providing a practical resource to investigative procedures in ophthalmology. HV Nema Nitin Nema
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Preface to the First Edition The word diagnosis comes from a Greek word meaning to distinguish or discern. Besides history and clinical examination of the patient, diagnostic tests are required to aid in making correct diagnosis of eye diseases. The role of diagnostic technology is not inferior to that of a clinician’s acumen. A correct diagnostic report helps in differentiating functional from organic and idiopathic from non-idiopathic diseases. The number of diagnostic tests available to an ophthalmologist has increased significantly in the last two decades. Both selective and non-selective tests are presently used for the clinical and research purposes. Non-selective approach to testing is costly and does not provide useful information. In order to be useful, diagnostic tests have to be properly performed, accurately read, and correctly interpreted. The ordering oculist should always compare the results of test with the clinical features of the eye disease. The main aim of the book—Diagnostic Procedures in Ophthalmology is to provide useful information on diagnostic tests, which an ophthalmologist intends to perform or order during his clinical practice. Some of the procedures described in the book, assessment of visual acuity, slit lamp examination, tonometry, gonioscopy, perimetry and ophthalmoscopy, are routine examinations. However, the technique of proper examination and interpretation of findings to arrive at a correct diagnosis must be known to the practising ophthalmologist or optometrist. Procedures like ophthalmic photography, evaluation of optic nerve head, fundus fluorescein angiography and indocyanine green angiography are invaluable because they not only help in the diagnosis and documentation but also help in monitoring the management of eye disease. Corneal topography gives useful data about corneal surface and curvature and contributes to the success of Lasik surgery to a great extent. The role of A-scan ultrasonography in the measurement of axial length of the eye and biometry cannot be over emphasised. B-scan ultrasonography is needed to explore the posterior segment of the eye when media are opaque or an orbital mass is suspected. Ultrasound biomicroscopy (UBM) and Optical coherence tomography (OCT) are relatively new non-invasive tools to screen the eye at the microscopic level. UBM helps in understanding the pathogenesis of various forms of glaucoma and their management. OCT obtains a tomograph of the retina showing its microstructure incredibly similar to a histological section. It helps in the diagnosis and management of the macular and retinal diseases. Electrophysiological tests allow objective evaluation of visual system. They are used in determination of visual acuity in infants and in the diagnosis of the macular and optic nerve disorders. What diagnostic tests should be ordered in the evaluation of the patients with infective keratitis or uveitis? Chapters on Diagnostic Procedures in Infective Keratitis and Diagnostic Procedures in Uveitis provide an answer. The experts who have credibility in their fields have contributed chapters to the book. Not only the procedures of diagnostic tests are described but to make the reader conversant, characteristic findings in the normal and the diseased eye are also highlighted with the help of illustrations. The book should be of great help to the practising ophthalmologists, resident ophthalmologists, optometrists and technicians as it provides instant access to the diagnostic procedures in ophthalmology. We are indebted to all contributors for their excellent contributions in short time in spite of their busy schedule. Mr JP Vij deserves our sincere thanks for nice publication of the book. HV Nema Nitin Nema
Acknowledgements The publication of the second edition of Diagnostic Procedures in Ophthalmology is possible with the help and cooperation of many colleagues and friends. We wish to express our gratitude to all the contributing authors for their time and painstaking efforts not only for writing the comprehensive and well illustrative chapters but also updating and revising them to conform the format of the book. We are indebted to Prof JL Alió, Dr Vasumathy Vadantham and Dr Tarun Sharma for contributing chapters on a short notice because the initial contributors failed to submit their chapters. Our grateful thanks go to Dr Mahipal Sachdev for persuading Dr Manotosh Ray to write a chapter on Confocal Microscopy. Mrs Pratibha Nema deserves our deep appreciation; without her patience, tolerance and understanding, this book would not have become reality. Finally, Shri Jitendar P Vij (Chairman and Managing Director), Mr Tarun Duneja (DirectorPublishing) and supporting staff of M/s Jaypee Brothers Medical Publishers (P) Ltd, New Delhi especially deserve our sincere thanks for their cooperation and keen interest in the publication of this book. HV Nema Nitin Nema
Contents 1. Visual Acuity ..................................................................................................................... 1 Stephen C Hilton, Leela V Raju, VK Raju
2. Color Vision and Color Blindness ........................................................................... 12 Harinder Singh Sethi
3. Slit-lamp Examination ................................................................................................... 33 Harinder Singh Sethi, Munish Dhawan
4. Corneal Topography ....................................................................................................... 46 Francisco Arnalich, David Piñero, Jorge L Alió
5. Confocal Microscopy ...................................................................................................... 84 Manotosh Ray
6. Tonometry .......................................................................................................................... 95 R Ramakrishnan, Sonal Ambatkar
7. Gonioscopy ...................................................................................................................... 106 A Narayanaswamy, L Vijaya
8. Optic Disk Assessment in Glaucoma ................................................................... 115 Rajul Parikh, Chandra Sekhar
9. Basic Perimetry .............................................................................................................. 128 Devindra Sood, Parmod Kumar
10. Ophthalmoscopy ............................................................................................................. 151 Pukhraj Rishi, Tarun Sharma
11. Ophthalmic Photography ............................................................................................ 165 Sadao Kanagami
12. Fluorescein Angiography ............................................................................................ 181 R Kim, S Manoj
13. Indocyanine Green Angiography ............................................................................ 200 Vasumathy Vedantham
14. A-scan Ultrasonography .............................................................................................. 216 Rajiv Nath, Tinku Bali, Monica Saha
15. B-scan Ultrasonography .............................................................................................. 239 Taraprasad Das, Vasumathy Vedantham, Anjali Hussain Sangmitra Kanungo, LS Mohan Ram
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Diagnostic Procedures in Ophthalmology 16. Ultrasound Biomicroscopy in Ophthalmology .................................................... 259 Roshmi Gupta, K Kalyani Prasad, LS Mohan Ram, Santosh G Honavar
17. Optical Coherence Tomography .............................................................................. 269 Tomohiro Otani
18. Electrophysiological Tests for Visual Function Assessment ........................ 279 Subhadra Jalali, LS Mohan Ram, Garima Tyagi, Kallakuri Sumasri
19. Diagnostic Procedures in Infectious Keratitis ................................................... 316 Savitri Sharma, Sreedharan Athmanathan
20. Diagnostic Procedures in Uveitis ........................................................................... 333 Jyotirmay Biswas, Surbhit Chaudhary, S Sudharshan, Shahnawaz Kazi
21. Retinopathy of Prematurity: Diagnostic Procedures and Management .... 353 Yog Raj Sharma, Deependra Vikram Singh, Nikhil Pal, Rajani Sharma
22. Localization of Intraocular Foreign Body ............................................................ 362 Amit Nagpal, Lingam Gopal
23. Comitant Strabismus: Diagnostic Methods ......................................................... 369 Harinder Singh Sethi, Pradeep Sharma
24. Incomitant Strabismus ................................................................................................. 395 S Meenakshi, T Surendran
25. Diagnostic Procedures in Dry Eyes Syndrome .................................................. 405 MS Sridhar
26. Evaluation of Epiphora ............................................................................................... 412 AK Grover, Rituraj Baruah
27. Diagnostic Techniques in Proptosis ...................................................................... 426 Mandeep S Bajaj, Sanjiv Gupta
28. Neurological Disorders of Pupil ............................................................................. 441 Ambar Chakravarty
Index .................................................................................................................................................. 461
Visual Acuity
STEPHEN C HILTON, LEELA V RAJU, VK RAJU
1
Visual Acuity
Vision is the most important of all senses. Approximately 80% of the information from the outside world is incorporated through the visual pathway. Loss of vision has a profound effect on the quality of life. The process of vision includes: 1. Central resolution (visual acuity) 2. Minimal light sensitivity 3. Contrast sensitivity 4. Detection of motion 5. Color perception 6. Color contrast 7. Peripheral vision (spatial, temporal and motion detection). In the normal clinical settings, we measure only one of these functions – central resolution at high contrast (visual acuity).1
Definition and Terminology of Visual Acuity The most basic form of visual perception is detection of light. Visual acuity is more than just detecting light. It is the measurement of the ability to discriminate two stimuli separated in space at high contrast compared with the background. The minimal angle of resolution that allows a
human optic system to identify two points as different stimuli is defined as the threshold of resolution. Visual acuity is the reciprocal of the threshold of resolution.2 Clinically, discriminating letters in a chart determine this, but this task also requires recognition of the form and shape of the letters, which are processes that also involve higher centers of visual perception. Discrimination at a retinal level may, therefore, be determined by less complex stimuli, such as contrast sensitivity gratings. Theoretically, the maximum resolving power of the human retina could be derived from an estimate of the angle of approximately 20 seconds of arc because this represents the smallest unit distance between two individually stimulated cones. Thus the resolving power of the eye could be much greater than what is measured by visual acuity charts.3 Cones have the highest discriminatory capacity, but rods can also achieve some resolution. The greater the distance from the fovea the level of visual acuity falls off rapidly. At a 5° distance from the foveal center, visual acuity is only one quarter of foveal acuity.4 Luminance of test object, optical aberrations of the eye and the degree of adaptation of the observer also influence the visual acuity.5
1
2
Diagnostic Procedures in Ophthalmology Visual thresholds can be broadly classified into three groups: 1. Light discrimination (minimum visible, minimum perceptible) 2. Spatial discrimination (minimum separable, minimum discriminable) 3. Temporal discrimination (perception of transient visual phenomena such as flickering stimuli). Many clinical tests can assess many visual functions simultaneously. In a healthy observer in best focus, the resolution limit, or as it is usually called, the minimum angle of resolution (MAR), is between 30 seconds of arc and one minute of arc. Clinically, we use Landolt C and Snellen E to assess visual acuity. The minimum discriminable hyper-acuity or vernier-acuity is another example of spatial discrimination. The eye is capable of subtle discrimination in spatial localization, and can detect misalignment of two line segments in a frontal plane if these segments are separated by as little as three to five seconds of arc, considerably less than the diameter of a single foveal cone. The mechanism subserving hyper-acuity is still being investigated.
Charts and Scales to Record Visual Acuity The function of the eye may be evaluated by a number of tests. The cone function of the fovea centralis is assessed mainly by measurement of the form sense, the ability to distinguish the shape of objects. This is designated as central visual acuity. It is measured for both near and far, with and without the best possible correction of any refractive error present. Because only cones are effective in color vision and because they are concentrated in the fovea, the measurement of the ability to recognize colors is also a measurement of foveal function. The function of the peripheral retina which
contains mainly rods, may be assessed by peripheral visual field.1 Visual acuity is the first test performed after obtaining a careful history. Measurement of the central visual acuity is essentially an assessment of the function of the fovea centralis. An object must be presented so that each portion of it is separated by a definite interval. Customarily, this interval has become one minute of an arc, and the test object is one that subtends an angle of five minutes of an arc. A variety of test objects has been constructed on this principle, so that an angle of five minutes is at distances varying from a few inches to many feet5 (Figs 1.1 and 1.2). The most familiar examination chart is Snellen chart (Fig. 1.3). Conventionally, reading vision is examined at 40 cm (16 inches). The testing distance of a preferred near distance chart
Fig. 1.1: Snellen letters subtend one minute of arc in each section, the entire letter subtends five minutes of arc
Fig. 1.2: Each component of Snellen letters subtend one minute of visual angle the entire letter subtends five minutes of visual angle at stated distance
Visual Acuity
Fig. 1.4: ETDRS chart
Fig. 1.3: Snellen chart
should be observed accurately. The Snellen notation is simply an equivalent reduction for near, maintaining the same visual angle. Most of the Snellen-based distance acuity charts are also commercially available as ‘pocket’ charts to check the near acuity at a preferred distance for every patient or at a defined distance for clinical trial purposes including ETDRS (Fig. 1.4) and Snellen letter “E”. The Jaeger notation is a historic enigma and Jaeger never committed himself to the distance at which the print should be used. The numbers on the Jaeger chart simply refer to the numbers on the boxes in the print shop from which Jaeger
selected his type sizes in 1854. They have no biologic or optical foundation. Clinically, Jaeger’s charts (Fig. 1.5) are widely used. Central visual acuity is designated by two numbers. The numerator indicates the distance between the test object and the patient; the denominator indicates the distance at which the test object subtends an angle of five minutes. In the United States these numbers are given in inches or feet, whereas in the Europe the designation is in meters. The test chart commonly used in the United States has its largest test object one that subtends an angle of five minutes at a distance of 200 feet (6 m). Then there are test objects of 100, 80, 70, 60, 50, 40, 30, 20 and 15 feet. If the individual is unable to recognize the largest test object, then he or she should be brought closer to it, and the distance at which he or she recognizes it should be recorded. Thus, if the individual recognizes the test object that subtends a five minute angle at 200 feet when he or she is at 12 feet, the visual acuity is recorded as 12/200. This is not a fraction but indicates two physical
3
4
Diagnostic Procedures in Ophthalmology
Fig. 1.5A: Jaeger's type near vision chart
Visual Acuity
Fig. 1.5B Fig. 1.5B: Near vision chart: Music type and numericals
5
6
Diagnostic Procedures in Ophthalmology
Fig. 1.6: Broken C, letter E and pictures of familiar objects for testing visual acuity in illiterates and children
measurements, the test distance and the size of the test object. The most familiar test objects are letters or numbers. Such tests have the disadvantage of requiring some literacy on the part of the patient. Additionally, there is a variation in their ability to be recognized. “L” is considered the easiest letter in the alphabet to read and “B” is considered the most difficult. To obviate this difficulty, broken rings (Fig. 1.6) have been devised in which the break in the ring subtends one minute angle, and the ring subtends a five minute angle. Similarly, the letter “E” may be arranged so that it faces in different directions (Fig. 1.6). These test objects are easier to see than letters, eliminate some of the difficulties inherent in reading, and
can be used in the testing of illiterates and persons not familiar with the English alphabet. A variety of pictures (Fig. 1.6) have also been designed for testing the visual acuity of children. When a person is unable to read even a top letter, he or she is asked to move toward the chart or a chart can be brought closer. The maximum distance from which he or she recognizes the top letter is noted as the nominator. When visual acuity is less than 1/60, the patient is asked to count fingers from close at hand (CF at 20 cm). When a patient cannot even count fingers, the patient is asked if he or she can see examiner’s hand movements (HM positive). When hand movements are not seen we have to record whether the perception of light (LP) is present or absent by asking the patient if he or she sees the light. Standard illumination should be used for the acuity chart (10 to 20 foot candles for wall charts). When a patient is examined with the Snellen chart in a dark room, the subject sees a high contrast and glare-free target. But in real circumstances, contrast and glare reduce visual acuity, and even more so in a pathological conditions. The contrast sensitivity function of a subject may be affected even when Snellen acuity is normal. The contrast sensitivity tests are more accurate in quantifying the loss of vision in cases of cataracts, corneal edema, neuroophthalmic diseases, and retinal disorders. A patient with a low contrast threshold has a high degree of sensitivity; therefore, a healthy young subject may have a threshold of 1%, and a contrast sensitivity of 100% (inversely proportional). It is important to have adequate lighting when testing visual acuity so that it does not become a test of contrast sensitivity.
Factors Affecting Visual Acuity Factors affecting visual acuity may be classified as physical, physiological and psychological.
Visual Acuity Uncorrected refractive error is a common cause of poor acuity. Physical factors include illumination and contrast. Increased illumination increases visual acuity from threshold to a point at which no further improvement can be elicited. In the clinical situation this is 5-20 foot candles. When contrast is reduced more illumination is required to resolve an object. Beyond a certain point, illumination can create glare. Therefore, visual acuity is recorded under photopic condition and one wants to evaluate best visual acuity at the fovea. Physiological conditions include pupil size, accommodation, light-dark adaptation and age.2
A
Pupil Size The pupil size has great influence on visual acuity. Visual acuity decreases if pupils are smaller than 2 mm due to diffraction. Pupil diameters larger than 3.5 mm increase aberration. Variation in pupil size changes acuity by altering illumination, increasing depth of focus, and modifying the diameter of the blur circle on the retina.
Accommodation An accommodation creates miosis, which could account for small hyperopic prescriptions being rejected for distance viewing in younger individuals. It is worth while to discuss the role of a pinhole in obtaining the best visual acuity in the clinical setting. The optimum pinhole is 2.5 mm in diameter. A pinhole in an occluder (Fig. 1.7) may be introduced in a trial frame with the opposite eye occluded. Single pinhole device is not adequate. The patient must be able to find a hole, therefore, multiple pinholes are preferred. If the patient is older or infirm, or has tremors, he is asked to read only a single letter from each line as we proceed down the chart to record the vision.
B Figs 1.7A and B: Occluder with multiple holes
Many patients have been referred for neuro-ophthalmologic consultation because of painless loss of vision in one eye only. The best visual acuity may be 20/60 in the affected eye but when properly tested with the pinhole, the acuity may improve to 20/20. This indicates that the macula and optic nerve are functioning normally. When the patient’s vision is improved with pinhole one knows the problem is a refractive one and simply need the change in glasses. If the patient’s vision is less when looking through the pinhole; it indicates that the patient has either an organic lesion at macula, or a central scotoma, or functional amblyopia. A patient with 20/400 vision that improves with pinhole to 20/70 indicates that the improvement is refractive, but some pathology may also be present.
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Diagnostic Procedures in Ophthalmology
Visual Acuity Testing in Young Children Early determination of vision loss and refractive error is an essential component of assessing the infant’s ultimate visual development potential. The visual acuity of a newborn as measured by preferential looking is in the range of 30 minutes of arc (20/600); acuity rapidly improves to six minutes of arc (20/120) by three months. A steady but modest improvement to approximately three minutes of arc (20/60) occurs by 12 months of age. One minute of arc (20/20) is usually obtained at the age of three to five years.6 The examination is generally performed on the parent’s lap. The room should never be totally darkened because this may provoke anxiety. Objective retinoscopy remains the best method of determining a child’s refraction. Other clinical methods involve estimation of fixation and following behavior. A test target should incorporate high contrast edges. For infants younger than six months the best target represents the examiner’s face. For the child of six months and older, an interesting toy can be used. After assessment of the binocular fixation pattern, the examiner should direct attention to differences between the two eyes when tested monocularly. Objection to occlusion of one eye may suggest abnormality with the less preferred eye.7 Three common methods are used for determining resolution acuity: 1. Behavioral technique (preferential looking Fig. 1.8) 2. Detecting optokinetic nystagmus (OKN Fig. 1.9) 3. Recording visual evoked potentials (VEP Fig. 1.10). It is desirable to measure the visual acuity of children sometime during their third year to detect strabismic or sensory amblyopia and to recognize the presence of severe refractive errors.
Fig. 1.8: Preferential looking test chart
Fig. 1.9: OKN drum
In this group of preschool children, visual acuity testing is easier to perform with the use of the following charts: 1. Allen and Osterberg charts (Fig. 1.11) 2. Illiterate E chart 3. Landolt broken ring.
Visual Acuity
Fig. 1.10: VEP testing
Contrast is defined as the ratio of the difference in the luminance of these two adjacent areas to the lower or higher of these luminance values. The amount of contrast a person needs to see a target is called contrast threshold. The contrast sensitivity is assessed by using the contrast sensitivity chart. It has 5-8 different sizes of letters in six or more shades of gray. Some contrast sensitivity charts contain a series of alternating black and white bars; 100 line pairs per mm is equivalent to space of one minute between two black lines. The alternating bar pattern is described as spatial frequency. The contrast sensitivity is measured in units of cycles per degrees (CPD). A cycle is a black bar and white spaces. To convert Snellen units to units of cycles per degree, divide 180 by Snellen denominator. Contrast sensitivity measurements differ from acuity measurements; acuity is a measure of the spatial resolving ability of the visual system under conditions of very high contrast, whereas contrast sensitivity is a measure of the threshold contrast for seeing a target.8
Visual Acuity in Low Vision Patients
Fig. 1.11: Allen and Osterberg chart
Contrast Sensitivity A general definition of spatial contrast is that it is a physical dimension referring to the lightdark transition at a border or an edge of an image that delineates the existence of a pattern or object.
Individual near acuity needs are different among different population groups. For low vision patients these differences are magnified. Two persons with the same severe visual impairment may exhibit marked differences in their ability to cope with the demands of daily living. Visual acuity loss, therefore, is the aspect that must be addressed in individual rehabilitation plans. Colenbrander9 subdivides several components of visual loss into impairment aspects (how the eye functions), visual ability (how the person functions in daily living), and social/economic aspects (how the person functions in society (Table 1.1).
9
20/12.5 20/16 20/20 20/25 20/32 20/40 20/50 20/63 20/80 20/100 20/125 20/160 20/200 20/250 20/320 20/400 20/500 1.6in 20/630 1.2in 20/800 1in 20/1000 20/1250 1cm 20/1600 1cm 20/2000 1cm
NLP
Normal vision
Mild vision loss
Moderate vision loss
Severe vision Loss
Profound vision loss
Near-blindness
Total Blindness
No visual reading must rely on talking books or other
Marginal with aids Uses magnifiers for spot reading, but may prefer talking books for leisure
Slower than normal with reading aids High-power magnifiers (restricted field)
Near-normal with appropriate reading aids Low-power magnifiers and large-print books
Normal reading speed Reduced reading distance No reserve for small
Normal reading speed Normal reading distance Reserve capacity for small print
Vision substitution aids
Vision enhancements aids
None
Visual aids
10 5 0
30 25 20 15
50 45 40 35
70 65 60 55
90 85 80 75
110 105 100 95
VAS
In this range, residual vision tends to become unreliable, though it nonvisual sources may still be used as an adjunct to vision substitution skills.
In the EU, many benefits start at this level. The WHO includes this range in its blindness category.
In the United States, persons in this range are considered legally blind and qualify for tax-break disability benefits.
In the United States, children in this range qualify for special educational assistance
Many functional criteria (whether for a driver’s license or for cataract surgery) fall within the range
Note that normal adult vision is better than 20/20
Comments
Social and economic aspects (how the person functions in society)
(From Colenbrander A. Preservation of vision or prevention of blindness [editorial]? Am J Ophthalmol 2002;133:2. p.264.)
4in 3in 2.5in 2in
10in 8in 6in 5in
25in 20in 16in 12.5in
63in 50in 40in 32in
Newsprint (1 M)
Visual acuity
Ranges (ICD-9-CM)
Statistical estimate of reading ability
Visual ability aspects/functional vision (how the person functions-daily living skills)
Impairment aspects (how the eye function)
TABLE 1.1: RANGES AND ASPECTS OF VISION LOSS
10 Diagnostic Procedures in Ophthalmology
Visual Acuity
Summary Both distance and near visual acuities are recorded for each eye with and without spectacles. Distance visual acuity is recorded at a distance of 20 feet or in a room of at least 10 feet using mirrors and projected charts. Near visual acuity can be recorded using reduced Snellen or equivalent cards at 40 cm. Acuity performance, like any other human performance, is subject to impairment depending on ocular and general health, emotional stress, boredom, and a variety of drugs acting both peripherally and centrally. The examiner must provide encouragement and must have patience. For clinical studies the ETDRS charts are recommended because near vision is often more important in the daily life of older or infirm patients. Reading charts or other near vision testing charts should be used as part of the routine assessment of the visual acuity. Visual acuity measurement is often taken for granted. Many pitfalls make this most important assessment subject to variability.10Ambient illumination, aging bulbs, dirty charts or slides, small pupils, and poorly standardized charts are just
some of the factors that can lead to erroneous results. A little care in ensuring the proper environment for testing can significantly improve accuracy.
References 1. Newell FW. Ophthalmology Principles and Concepts. St Louis, Mosby, 1969. 2. Moses RA (Ed). Adlers Physiology of the Eye. St Louis, Mosby, 1970. 3. Scheie H. Textbook of Ophthalmology. Philadelphia, WB Saunders, 1977. 4. Duane TD. Clinical Ophthalmology. New York, Harper and Row, 1981. 5. Michaels DD. Visual Optics and Refraction. St Louis, Mosby, 1985. 6. Vander J. Ophthalmology Secrets. Hanley and Belfus. 7. Borish I. Clinical Refraction. Professional Publisher, 1970. 8. Owsley C. Contrast Sensitivity. Ophthalmic Clinics of North America 2003;16:173. 9. Colebrander A. Preservation of Vision or Prevention of Blindness? Am J Ophthalmol 2003; 133:263. 10. Kniestedt, Stamper RL. Visual Acuity and its Measurements. Ophthalmic Clinics of North America 2003; 16:155.
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HARINDER SINGH SETHI
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Color Vision and Color Blindness
Color vision examination is an essential part of screening before a person is taken up for a job. A person who is color vision defective may go through life quite unconscious of his color deficiency and without making any incriminating mistakes, differentiating objects by their size, shape and luminosity, using all the time a complete color vocabulary based on his experience which teaches him that color terms are applied with great consistency to certain objects and to certain achromatic shades, until circumstances are arranged to eliminate these accessory aids and then he realizes that his sensations differ in some way from the normal. Various tests have been developed to enable screening of anomalous subjects with color deficiency from a much larger group of normal subjects.
main characteristics of color namely hue, saturation, and brightness. Hue is a function of wavelength. It depends on what the eye and brain perceive to be the predominant wavelength of the incoming light. An object’s “hue” is its “color.” Saturation refers to the richness of a hue as compared to a gray of the same brightness. Saturation is also known as “chroma.” Brightness correlates to the ease with which a color is seen, other factors being equal. Brightness is a subjective term referring to the sensation produced by a given illuminance on the retina. The spectral wavelengths of different colors are as follows: violet 430 nm; blue 460 nm; green 520 nm; yellow 575 nm; orange 600 nm and red 650 nm. The concept of white light is vague, most agreeable definition is, white surface is one which has spectral reflection factors independent of wavelength (in the visible spectrum) and greater than 70%.
Color Vision Color is a sensation and not a physical attribute of an object. Color is what we see and is result of stimulation of retina by radiant energy in a small band of wavelengths of the electromagnetic spectrum usually considered to span about one octave, from 380 nm to 760 nm. There are three
Factors Affecting Color Vision Crystalline Lens The lens absorbs shorter wavelengths; in young, wavelengths of less than 400 nm and in old people up to 550 or 600 nm are absorbed by
Color Vision and Color Blindness the lens resulting in defective color vision on shorter wavelength side.
Retinal Distribution of Color Vision The center of the fovea (1/8 degree) is blue blind. Trichromatic vision extends 20-30° from the point of fixation. Peripheral to this red-green become indistinguishable up to 70-80° and in far peripheral retina all color sense is lost although cones are still found in this region. In the central 5°, macula contains carotenoid pigment, xanthophyll. The molecules of the pigment are arranged in such a way that they absorb blue light polarized in the radial direction. If one looks at a white card through linear polarizer, one will see two blue sectors separated by two yellow sectors the figure is called Haidinger’s brushes. Macular pigment may also be seen as in homogeneity in the field of blue or white light called Maxwell’s spot.
Wavelength Discrimination The normal observer is able to detect a difference between two spectral lights that differ by as little as 1 nm in wavelength in the regions of 490 nm and 585 nm. In the region of violet and red a difference of greater than 4 nm is necessary.
Hue, Saturation and Lightness Hue is the extent to which the object is red, green, blue or yellow. Saturation is the extent to which a color is strong or weak. Lightness is self explanatory attribute, for example, yellow by color is light.
Illumination Illumination affects color vision of low illuminances, the errors increase due to poorer discrimination for most of the hue range while
testing color vision. An illuminance of 400 lux (± 100 lux) would be practical value for most clinical applications.
Bezold-Burcke Effect von Bezold (1873) and Burcke (1878) discovered independently the phenomenon named after them, that variation of the luminance levels modifies hues.
Color Constancy; Aperture Colors and Surface Colors Color constancy is a phenomenon in which color of the objects can be recognized unchanged in spite of possible differences in the illumination. Aperture colors are colors that alter due to change in illumination. Surface colors do not vary with illumination. Extrafoveal vision favors the appearance of aperture colors and foveal vision that of surface colors.
Complementary Wavelengths Complementary wavelengths are those which, when mixed in appropriate proportions, give white.
Simultaneous Color Contrast Color contrast is visually demonstrated by observing the color of a spot in a surround. The general rule is that the color of the spot tends toward the complementary of the color of the surround.
Successive Color Contrast Successive color contrast is more commonly described as colored after images, when one stares at a red spot for several seconds and then looks at a gray card one sees a green spot on
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Diagnostic Procedures in Ophthalmology the card. The after image tends toward the complementary of the primary image (StilesCrawford effect). The light entering near the edge of the pupil is less effective than light entering at the center of the pupil because of the shape of the receptors and the fact that they are embedded in a medium of different refractive index. This effect is wavelength-dependent.
Color Triangle Color triangle can be drawn to describe the trichromacy of color mixtures and is useful for deciding which bands of wavelength are indistinguishable from each other. Three reference wavelengths are chosen, i.e. 450 nm, 520 nm and 650 nm and are placed at vertices of X, Y and Z of a triangle, the position of other wavelengths is determined. A color triangle does not describe the color of a band of wavelengths unless other circumstances are defined.
Theories of Color Vision This is a complex topic as no theory explains the phenomenon of color vision fully. Few important theories are given below:
Young-Helmholtz Theory (Trichromatic Theory) Young’s concept is that there are three types of retinal receptors with different spectral sensitivities. Young’s principal colors are red, green and violet. Young’s hypothesis was not followed up until it was revived by Helmholtz in 1852. The Young’s theory may be summarized as follows: a. At some stage of visual receptor mechanism there are three different types of sensory apparatus G1, G2, G3. These receptors must be same for everyone but they may not be same at the fovea as at the periphery.
b. Each of these receptors is characterized from the spectral point of view by particular function of wavelengths which may be denoted by G and the response G1 of a receptor for radiation with a spectral energy distribution Eλ may be supposed to have the form. G1 = Sgi Eλ dλ. c. Sensation of color is a function of the relative values of the three responses G1. d. Sensation of light is a function of a linear combination of the three responses. Fundamental sensations By determining approximately the coordinate of the confusion points of dichromats Arthur Konig in1893 established a system of fundamental sensations and identified red, green and violet as fundamental colors. Blue was also identified as fundamental color in addition to red and green by Gothelin.
Granit’s Theory of Color Vision Granit divides retina into receptor units, each unit comprising groups of cones and rods which are connected with a single ganglion cell or several ganglion cells which synchronize their discharges. These units are classified as “dominators” or “modulators”. The dominators which are numerous have a spectral sensitivity curve which indicates that they are responsible for the sensations of luminosity. Modulators show a selective sensitivity which makes them responsible for color discrimination. Granit’s theory does not explain the fact of trichromatism.
Hering’s Theory of Color Vision (Opponent Color Theory) Hering assumed six distinct sensations arranged in three opposing pairs: white-black; yellow-blue and red-green; he explains three pairs as being
Color Vision and Color Blindness due to opposing actions of light on three substance of the retina, a catabolism producing warm sensation (white, yellow, red) and an anabolism the cold ones. This theory is clearly a psychological concept and aims at explaining complex percepts than the intermediate effect of the stimuli.
Anatomy of Color Vision The understanding of visual pathway is complex and not evident fully. There are two types of photoreceptors in the retina: rods and cones. Approximately 120 million rods are responsible for night and peripheral vision. Rods contain a photopigment called rhodopsin, a chemical variant of vitamin A and a protein called opsin that serves at very low levels of illumination. Rods have their maximum density about 5 degrees from the fovea and cannot distinguish one color from another. The fovea itself is essentially rod-free containing only cones. Approximately 7 million cones are responsible for central and color vision. Cones have their maximum density within 2 degrees of the center of the fovea. Both types of receptors diminish in number toward the retinal periphery.
Cones In the retina three types of cones responsible for the red, green and blue sensations have been isolated. Three types of cone pigments in the human retina absorb photons with wavelengths between 400 nm and 700 nm. Color vision is mediated by these three cone photoreceptors referred to as long, middle, and short wavelengthsensitive (LWS, MWS, SWS) cones. The long wavelength-sensitive (LWS) cones (sometimes called “red” or “red-catching”) contain a pigment called erythrolabe, which is best stimulated by a wavelength near 566 nm. Medium wavelengthsensitive (MWS) cones (“green” or “green-
catching”) contain the pigment chlorolabe, which has a maximal sensitivity to a wavelength near 543 nm. Short wavelength-sensitive (SWS) cones (“blue” or “blue-catching”) contain cyanolabe, which have maximal sensitivity at 445 nm. The blue cones are absent in the center of the macula. Trichromatic vision perception occurs in central 30º field. It is not uncommon to hear the cones referred to as blue, green, and red cones, but such nomenclature is misleading because the L-cones are more sensitive to blue lights than they are to red lights. The spectral sensitivities of the three cone pigments overlap somewhat. For example, light of 540 nm and 590 nm stimulate both green (MWS) and red (LWS) receptors yet we can easily distinguish between these two wavelengths as “green” and “yellow.” If the human retina contains all three cone pigments in normal concentrations, and has normal retinal function, the subject is a trichromat. Any color the trichromat sees can be matched with a suitable mixture of red, green, and blue light.
Color Coded Cells Two types of color coded cells are found at peripheral levels (ganglion cells and lateral geniculate body) of the visual system and they have been named opponent color cells and double opponent color cells. More complex types are found at more central levels (striate cortex). Opponent color cells: An opponent color cell is one that gives only polarity of response for some wavelengths and opposite polarity of response for other wavelengths. Opponent color cells are concerned with successive color contrast. Double opponent color cells: These are cells opponent for both color and space. The response may be onto red light, off to green light in the center of the receptive field and off to red light, onto green light in the periphery of the receptive
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Diagnostic Procedures in Ophthalmology field. Double opponent cells are concerned with simultaneous color contrast.
Congenital vs Acquired Color Deficiencies
Simple, complex and hypercomplex cells: In rhesus monkey striate cortex there are a variety of cells that are specific for both color and orientation. They have been categorized as color sensitive simple, complex and hypercomplex cells. Simple cells have a bar-flank double opponent arrangement to their receptive fields. Complex color coded cells respond to color boundaries of the appropriate orientation and the response is independent of the part of the receptive field being stimulated. The edge of hypercomplex cells must be short. Opponent color cells are found among ganglion cells of the retina and lateral geniculate body. Double opponent cells with centersurround or flank receptive fields are present in the input layer IV of the striate cortex. Complex and hypercomplex color coded cells are also found in the striate cortex in layers II, III, V and VI. Vaetichin in 1953 recorded a negative slow potential from fish retinae called “S-potential” of two types: L-type (luminosity type) and Ctype (chromaticity type). Mitarai in 1961 regarded horizontal cells as responsible for S-potentials of L-type and Muller’s fibers for those of C-type. The properties of S-potentials support the Herings opponent color theory more than the trichromatic theory of Young.
Congenital color vision deficiencies can be distinguished functionally from acquired deficiencies in a number of ways. Congenital deficiencies typically involve red-green confusions, whereas acquired deficiencies, more often than not, are a blue-yellow (Köllner’s rule). Also, because some of the most common congenital defects are linked to the X-chromosome, they are more prevalent in males than females. Acquired defects, in contrast, are not related to gender except by gender differences to trauma or toxic exposure. Acquired color deficiencies are more likely to be asymmetric between the two eyes than are hereditary defects; they are also less likely to be stable with time. Congenital defects are usually easier to detect with standard clinical color vision tests, but some acquired ones can be more subtle and thus are difficult to diagnose. Finally, those with acquired color deficiencies are also more likely to display color-naming errors because, unlike those with congenital deficiencies, they lack the life-long experience with defective color perception.
Anomalies of Color Vision Deficiency of color vision first was described by Dalton in1794, the founder of the atomic theory, who himself was color blind; hence the term daltonism was coined. The color deficiency is of two types: (1) congenital and (2) acquired. In clinical evaluation of color vision it is important to distinguish between acquired and congenital defects.
Congenital Color Vision Deficiency The color vision anomalies commonly being X-linked are relatively common (8%) in men and rare in women (Fig. 2.1). Nearly all congenital color defects are due to absence or alteration of one of the pigments in photoreceptors. Congenital color deficits may be divided into classes according to whether the patients are red deficient (protans), green deficient (deuterans) or blue deficient (tritans). The term anopia is used for absolute deficiency and anomaly for relative deficiency (Tables 2.1 and 2.2). Anomalous trichromats are people who generally require three wavelengths to match
Color Vision and Color Blindness TABLE 2.1: CLASSIFICATION OF COLOR BLINDNESS Congenital: Males (8%), Females (0.4%) classically X-linked recessive inheritance pattern, always bilateral (a) Achromatopsia Cone monochromats Rod monochromats (b) Dyschromatopsia Dichromats - Deuteranopia - Protanopia - Tritanopia Anomalous trichromats - Protanomaly - Deuteranomaly - Tritanomaly
Acquired Unilateral Bilateral
Disease Glaucoma Hypertensive retinopathy Diabetic retinopathy AMD Lesions of visual pathway Alcohol-nicotine
Red-green defect Blue-Yellow defect Red-green defect Blue-Yellow defect Acquired defect Blue-Yellow Blue-Yellow Blue-Yellow Blue-Yellow Red-Green Red-Green
TABLE 2.2: VARIOUS TYPES OF COLOR DEFICIENCY
Anomalous trichromats Dichromats Monochromats
Red deficient
Green deficient
Blue deficient
Protanomaly Protanopia Rod monochromat
Deuteranomaly Deuteranopia
Tritanomaly Tritanopia Blue monochromat
Fig. 2.1: Inheritance pattern of congenital color vision defects
another wavelength but do not accept the color matches made by normal people, Lord Rayleigh in 1881 discovered trichromacy. Anomalous trichromats have three classes of cones but one is abnormal. Protanomalous people lack the red receptors and instead they have two pigments both peaking in the range of the normal green. Similarly the deuteranomalous people lack green receptors. Dichromats require only two wavelengths to match another wavelength and will accept the color matches made by normal people. The dichromats have two classes of cone receptors with normal spectral sensitivity, the third class being absent. Measurements of their pigments can be made by reflection densitomer and cone processes isolated by colored backgrounds confirm the findings. Protanopes have normal green and blue cones, red cones being absent. Deuteranopes have normal red and blue cones and tritanopes normal red and green cones.
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Diagnostic Procedures in Ophthalmology Protans color deficient subjects are easier to test and classify than deuterans and tritans; because the red cone pigment is quite sensitive to green wavelengths and both red and green cone pigments are quite sensitive to blue wavelength covering the green and blue range, in deuterans and tritans, as the sensitivity of visual pigment does not fall off sharply on the short wavelength side of the peak. Monochromatics can be blue cone monochromatics and rod monochromatics. Blue cone monochromatics have normal blue cone pigment but no red or green cone pigment. In rod monochromatism only 500 nm pigment is present in the retina and all three cones pigments are absent. Genetics of congenital color deficiencies The protans and deuterons are commonly sexlinked recessive. About 1% males are protanopes, 1% protanomalous, 1% deutaranopes and 5% deuternomalous. The incidence of color vision deficiency (red-green) in females is 0.4%. The gene for tritans is autosomal incompletely dominant. Rod monochromatism is very rare; occurs 1 in 30,000, autosomal recessive and thus an increased incidence is seen in consanguineous offsprings.
Acquired Deficiency of Color Vision Koellner formulated that lesions in the outer layers of the retina give rise to a blue-yellow defect, while lesions in the inner layers of the retina and the optic nerve gives rise to red green defect. However, the correlation is not always true. Some patients with lesions in the cerebral cortex may have color deficits. These may involve naming of the colors or perception of colors.
Factors Responsible for Deficiency of Color Vision Ocular Diseases a. Squint amblyopia: Francois by means of clinical tests stated that color vision deficiencies in squint amblyopia do not correspond to the classical type of acquired deficiencies but rather approximate the normal color sense of eccentric retinal positions. b. Glaucoma: Primary glaucoma and ocular hypertension cause tritan-type of defect. c. Diabetic retinopathy: Diabetic retinopathy may cause color deficiency which may vary from a mild loss of hue discrimination to moderate blue-yellow color vision deficiency. In severe cases of diabetic retinopathy the defect may resemble tritanopia. d. Retinal disorders: Blue-yellow deficits are found in senile macular degeneration, myopia, retinitis pigmentosa, siderosis bulbi and chorioretinitis. e. Optic nerve disorders: In one study about 57% of patients with resolved optic neuritis were found to have color vision defects. Red-green defects have been found in cases of multiple sclerosis and optic atrophy. Tobacco amblyopia causes red-green defect. f. Color vision after laser photocoagulation: After argon-laser photocoagulation there may be overall loss of hue discrimination and color deficiency, mostly of blue-yellow.
Drugs Many drugs are known to cause deficiency of color vision. They can cause more than one type of color deficiency (Table 2.3).
Color Vision and Color Blindness TABLE.2.3: DRUGS CAUSING COLOR DEFICIENCY Drugs
Type of color deficiency
Chloroquine, Indomethacin, oral contraceptives, antihistaminics, estrogens, digitalis and butazolidin.
Blue-yellow
Ethyl alcohol, Ethambutol
Red-green
Tri- and bicyclic antidepressants
Mixed type
Systemic Disorders Besides diabetes, a few systemic disorders are known to be associated with defective color vision. Following diseases may cause color deficiency: a. Cardiovascular disease: Patients with heart diseases have been found to have blueyellow deficiency. b. Turner’s syndrome: Red-green color deficiency is usually encountered in the syndrome.
A
Color Vision Testing The main objective for testing the color blindness is to determine the exact nature of the defect and whether the color deficiency is likely to be a source of danger to the community and/or to the individual, if given a particular job.
B
Types of Color Vision Tests Color Confusion Tests Pseudo-isochromatic (PIC) plates are example of color confusion tests (Figs 2.2 and 2.3). PIC Tests are designed on the basis of the color confusions made by persons with color defects. In these a symbol or figure in one color is placed on a background of another color so that the figure and background are isochromatic for the color-defective person. PIC tests are used primarily as screening tests to identify those with an inherited color defect, although, some of the
C Figs 2.2A to C: A Ishihara pseudo-isochromatic plates, B Transformation plate seen as “3” by patients with anomalous red-green color defect, C “Vanishing” or “disappearing” digit type
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Diagnostic Procedures in Ophthalmology
Fig. 2.3: City University test
tests permit a diagnosis of type and severity. Because the inventory of PIC tests is extensive, only the more commonly used tests are described here. The most widely used test, Ishihara pseudoisochromatic plates, is a screening test used to determine the presence of X-linked congenital (red/green) color deficiency. Most screening tests are designed to give a quick, accurate assessment of red/green deficiencies. The Ishihara test is not designed to detect tritan disorders or acquired color defects unless the optic neuropathy is severe.
Arrangement Tests The arrangement tests require the observer to place colored samples in sequential order on the basis of hue, saturation, or lightness or to sort samples on the basis of similarity. One of the earliest tests of this nature that is still available but is rarely used today is the Holmgren Wool test. In this matching test, 46 numerically coded comparison schemes of yarn are selected to match three test colors: yellow-green, pink, and dark red. The comparison schemes differ from the test schemes in being lighter or darker. The test is
not accurate for screening or classification and is not recommended for clinical use. It is of historical significance as an early occupational test. The clinical arrangement tests that are in use today are colored papers mounted in black plastic caps. The caps are placed in order according to specific instructions, and the order is recorded as the sequence of numbers printed on the underside of the caps. Results are plotted on score forms for analysis and interpretation and quantitative scores computed. The tests are standardized for CIE standard illuminant C. The Farnsworth-Munsell Dichotomous-15 (D-15) and the FM-100 test are examples of hue discrimination based on arrangement tests utilizing color chips mounted in a circular cap that subtend exactly 1.5 degrees at a test distance of 50 cm. This ensures that the observations of the subject are made with the central rod free retina. The D-15 contains 15 colored chips and the FM-100 contains 85 chips. The chips have identical brightness and saturation and differ from one another. Farnsworth-Munsell tests reveal the type of defect, but not the severity.
Color Matching Tests The spectral anomaloscope and PickfordNicolson anomaloscope are used for color matching examinations. They can provide the examiner with information on the severity of a particular color vision defect. The Nagel anomaloscope is the most widely used. It consists of a spectroscope in which two halves of a circular field are illuminated respectively by monochromatic yellow (589 nm) and a mixture of monochromatic red and green (670 nm and 546 nm, respectively). The observer is asked to match the two halves of the circle with the three primary colors available. The most widely used color vision tests are the pseudo-isochromatic plates and the D-15
Color Vision and Color Blindness panel due to their ease of use and relative low cost. The Nagel anomaloscope and FM-100 tests are usually only found in academic or research settings. All color vision tests have specific requirements for lighting, viewing distance, and viewing time. It is important for the examiner to be familiar with the test requirements and score sheets before conducting a color vision test, otherwise the results may be inaccurate.
Lantern Tests Lantern tests are used only for occupational purpose. Different types of lantern tests are in use in different countries. The FALANT is used in the United States by marine and aviation authorities; the Holmes Wright Type A is used in the United Kingdom by aviation authorities; and the Holmes Wright Type B is used in Australia, the United Kingdom and other Commonwealth countries by marine authorities. The Edridge-Green Lantern is included in the United States Coast Guard requirements, but it is surpassed by the FALANT. Electroretinography (ERG) and microspectrophotometry may be used in special circumstances.
Test Conditions Lantern testing is performed after dark adaptation but all other tests require artificial daylight conditions. Light adaptation is critical for anomaloscopy and especially for FM-100 hue testing, but a color neutral glare-free background and correct illumination are more important. Reliable results can be obtained with an artificial daylight source (such as a Macbeth Sol source) or fluorescent lighting with a color temperature between 5850 and 6850 degrees Kelvin and good color rendering index (Ra over 90). If appropriate artificial light is not available then skylight is a good source. The illumination should be
between 250 and 350 lux (approximately 1.5 meters below twin fluorescent globe). A failed Ishihara test under incandescent globe is a failure of the examiner to observe basic principles, not a failure of the subject. A pass on the other hand is still a pass and is statistically the more likely outcome. The viewing geometry should be with the light 45 degrees to the surface and the subject viewing the pages at 90 degrees to the surface. Newly printed books sometimes have differential reflectance between pigments so when tilted back and forth in the light by an anomalous observer they may provide luminance clues. Appropriate optical correction for the 65 cm viewing distance must be available if required. Experienced testers know that some people read the small identifying numbers on the bottom of each page and give a memorized response. Cheating can be prevented by covering these identifying numbers with a secret label.
Clinical Significance of the Various Tests Lantern testing is entirely vocational since around 5% of males fail and these include all those with a severe anomaly but a relatively unpredictable group from those with the milder anomalies. Anomaloscopy is the gold standard for clinical testing, while the D-15 and FM-100 tests have both clinical and vocational applications (diamond sorters and croupiers). A common vocational test battery should consist of: • Ishihara plates 2 - 17 from the 38 plate series • D-15 color sorting test (3 or more cross over errors is a failure) • Lantern testing.
Pseudo-isochromatic Color Plates The most common use of plate tests is to identify
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Diagnostic Procedures in Ophthalmology persons with congenital color defects. Pseudoisochromatic plates (for example, AO-HRR, Ishihara, Dvorine,Tokyo Medical College, SPP1) provide efficient screening of congenital redgreen defects (efficiency 90-95%). Other tests have been designed to detect achromatopsia (Sloan Achromatopsia test), to differentiate incomplete achromatopsia from complete achromatopsia (Berson blue cone monochromatism plates), to detect acquired defects (SPP-2), or to detect color confusion (City University test). Plate tests have the advantages of being relatively inexpensive, easily available, simple to use, and appropriate with children and persons who are illiterate. They are only suitable for screening purpose, however, they neither provide a quantitative evaluation of color vision nor distinguish the type and severity of the color vision defect. Plate tests are designed to distinguish congenital color-defective from color-normal observers, but they do not evaluate the wide range of abilities and aptitudes of observers with normal color vision to distinguish colors. Given individual differences in prereceptoral filters and normal photo pigment polymorphisms, no plate test can be 100% effective in screening. When used improperly (nonstandard illuminant, binocular viewing, colored lenses not removed from observer), their efficiency can diminish dramatically. The viewing distance required for pseudoisochromatic plates is 75 cm or approximately 30 inches. Proper refractive correction should be provided to the patient in order for them to see the plates clearly. Viewing time for each plate should be no more than 4 seconds. Undue hesitation can be a sign of a slight color deficiency.
color in red-green color defectives (Fig. 2.2B). There are three editions –- a 16 plate series, 24 plate series and a 38 plate series. The 10th edition of Ishihara has 38 plates. It is best to use the larger series because there are relatively few reliable plates in the smaller series. Both 24 set and 38 plate series set consist of two groups of plates — a group for those who are literate / numerate which starts from plate 1 at the front of the book, and a group for illiterates / innumerate in which the colored pattern is a meandering path of connected dots between two X symbols. The second group is arranged so as to commence with the last page of the book and proceed in reverse order. The group of plates for innumerate are seldom used because they are not as easy or reliable to score, but they are based on the same colorimetric principles as the set for numerates. It is not necessary to use both types in the one subject. From a colorimetric perspective there are four different types of test plate employed in both the 38 and 24 plate series preceded by a demonstration plate that is not for scoring. In the large series plates 1 and 38 are both for demonstration only, while in the smaller series plates 1 and 24 are for demonstration. If the subject fails viewing the demonstration plate do not proceed with the test. The following description applies to the numerate plates in the 38 plate series. The different types of plates in the test are:
Ishihara Pseudo-Isochromatic Plates (Confusion Charts)
Transformation plates (Fig. 2.2B): Anomalous color observers give different responses to color normal observers. In these plates, one number is seen by a normal trichromat and another (different) number is seen by a color deficient person. Those with true total color blindness cannot read any numeral. These are the plates numbered 2 to 9 inclusive.
The Ishihara color vision charts are developed by Shinobu Ishihara in 1917. This test is based on the principle of confusion of the pigment
Disappearing digit (Vanishing) plates (Fig. 2.2C): The normal observer is meant to recognize the colored pattern. On these plates, a number can
Color Vision and Color Blindness be seen by a normal trichromat but nothing can be seen by the color deficient person. These are plates 10 to 17 inclusive in the 38 plate series. Hidden digit plates: The anomalous observer should see the pattern. The number on a hidden digit design cannot be seen by a normal trichromat but can be seen by most people with red/green deficiencies. Those people with total color blindness cannot see any numeral. These are plates 18 to 21 inclusive. Qualitative plates: These are intended to classify protan from deutan and mild from severe anomalous color perception. The plates are numbered 22 to 25.
Procedure of Testing The plates are designed to be appreciated correctly in a room which is lit adequately by daylight. Introduction of direct sunlight or the use of electric light may produce some discrepancy in the results because of an alteration in the color values of the charts. It is suggested that when it is convenient only to use electric light, it should be adjusted as far as possible to resemble the effect of natural daylight. The plates are held 75 cm from the subject and tilted at right angles to the line of vision. A missed/ misread plate must be reread (may be in a random order). The findings should be recorded on the Ishihara color vision test and interpretation marking chart (Table 2.4). A correct response to the Ishihara introductory plate is expected and demonstrates suitable visual acuity to perform the test and rules out malingering. • Plates 1-25 have numerals and each answer should be given without more than 3 seconds of delay. • Plates 26-38 are tracings for use in illiterates, and windings lines between the two Xs are traced with a dry soft brush. Each tracing should take less than 10 seconds.
• Each eye should be tested separately (as should be done for all color vision tests). The recommendations of the test state that of the first 21 plates if 17 or more plates are read correctly by an individual his color sense should be regarded as normal. If 13 or less plates are correctly read then the person has a redgreen color defect. It is rare to have persons who read 14-16 plates correctly.
Hardy, Rand, Rittler (H-R-R) Plates Hardy, Rand, Rittler (H-R-R) plates are another type of pseudo-isochromatic (PIC) plate test. This test is similar to the Ishihara test except that the H-R-R plates classify and quantify the type of color defect whether protan, deutran, or tritan (blue/yellow). H-R-R plates have colored symbols/shapes rather than numbers. This makes H-R-R plates a good choice for children and illiterates. Since it is capable of detecting tritan disorders, this test is especially useful when an acquired color vision defect is suspected. Lighting, viewing distance, and viewing time are the same as that of testing with Ishihara plates. The first four (non-numbered) plates of the H-R-R series are for demonstration only (similar to the Ishihara “12”). The first six (numbered) plates are screening plates. Color vision is deemed “normal” and no further testing needs to be done if the subject gives correct responses to the screening plates. If there is an incorrect response to one or more of the screening plates, the examiner must follow the directions on the scoring sheet and show additional plates to the subject in order to specifically classify the color vision defect.
City University Color Vision Test The City University test (Fig. 2.3) was developed by Fletcher. It consists of 10 black charts each of which has 5 color dots. One of the dots is
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TABLE 2.4: INTERPRETATION AND MARKING OF THE ISHIHARA COLOR VISION TEST Number of plate
Normal person
Person with red-green deficiency
1
12
12
12
2
8
3
x
3
6
5
x
4
29
70
x
5
57
35
x
6
5
2
x
7
3
5
x
8
15
17
x
9
74
21
x
10
2
x
x
11
6
x
x
12
97
x
x
13
45
x
x
14
5
x
x
15
7
x
x
16
16
x
x
17
73
x
x
18
x
5
x
19
x
2
x
20
x
45
x
21
x
73
x
Protan Strong
Mild
Strong
Person with total color blindness and weakness
Deutan Mild
22
26
6
(2)6
2
2(6)
23
42
2
(4)2
4
4(2)
24
35
5
(3)5
3
3(5)
25
96
6
(9)6
9
9(6)
The mark x shows that the plate cannot be read. Blank space denotes that the reading is indefinite. The numerals in parenthesis show that they can be read but they are comparatively unclear
Color Vision and Color Blindness located in the center being encircled with 4 other dots so that a subject has to match the central color dot with one of the 4 other dots.
American Optical Company Plates The American Optical Company (AOC) plates, a screening test for protan and deutan defects, appears to be a composite of other tests. In addition to a demonstration plate, there are 14 test plates that include 6 transformation and 8 vanishing plates. The figures are single- and double-digit Arabic numerals. There are at least two different fonts used on different plates. Five or more errors on the 14 test plates constitute failure of the test. Plates with double-digit numbers are failed if the response to either digit is incorrect.
Dvorine The Dvorine is another widely used screening test for protan and deutan defects. The test booklet contains both PIC plates and a Nomenclature test, which is a unique and valuable feature of this test. The plates are presented in two sections: 15 plates with Arabic numerals and 8 plates with wandering trails, with 1 demonstration plate in each section. Any symbol missed is an error. Three or more errors in the first section constitute a failure. The Dvorine Nomenclature test is used to assess color naming ability. There are eight discs (2.54 cm in diameter) of saturated color and eight discs of unsaturated or pastel colors, which include red, brown, orange, yellow, green, blue, purple, and gray. A rotatable wheel allows the presentation of one disc at a time. Color-naming aptitude adds another dimension to a color vision assessment, and the results are appreciated by patients and employers curious to know the impact of a color defect on the ability to name colors.
Tritan Plate (F-2) The Tritan plate, or F-2, is a single plate that Farnsworth designed to screen for tritan color defects. It is a good test and it can also be used for screening for red-green (protan-deutan) defects. The test is performed by a vanishing plate consisting of outlines of two interlocking squares with different chromaticities on a purple background. One square is purple-blue and vanishes for patients with the red-green defects; the other square is green-yellow and vanishes, or is seen less distinctly compared with the purple-blue square, for the tritan. Persons with normal color vision see both squares, but the green-yellow one is more distinct.
Arrangement Tests Farnsworth-Munsell 100-Hue Test (Pigment Matching Test) Farnsworth-Munsell test (Fig. 2.4A) is a psychotechnical test, which quantifies a person’s ability to discriminate hues of pigment color. This simple and useful test consists of 85 colored chips that are designed to approximate the minimum difference between the hues that a normal observer can distinguish (1-4 nm). Color deficient
Fig. 2.4A: Farnsworth-Munsell 100-hue test
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Diagnostic Procedures in Ophthalmology
Fig. 2.4B: Farnsworth-Munsell 100-hue test results from four subjects: A Normal; B Protan defects; C Deutan defects; D Tritan defects
persons make characteristic errors in arranging the chips. The results are recorded on a circular graph. The greater the error arranging the chips, the farther the score is plotted from the center of the circle (Fig. 2.4B). Automated score for FM 100-hue test is also available. The currently available standard version consists of 85 knobs with pigment-colored paper
on top arranged in 4 horizontal panels. Each panel has 2 knobs fixed at its 2 ends. The subject is required to arrange the knobs in each panel in such a manner that the colors of the knobs appear to be changing gradually from one end of the panel to another. Generally recommended time for arranging each panel is 2 minutes. The time spent on
Color Vision and Color Blindness arranging the each panel is recorded. Scores of a knob/cap is the sum of the differences between the number of that cap and the number of the caps adjacent to it on either side. Sum of the scores of the entire set of knob / caps goes to make the total error score (TES). Then, the scores of each knob are plotted on a circular graph. By plotting the scores in a graph, it is seen that characteristic patterns are obtained in specific defects (Fig. 2. 4 B). The test is capable of detecting all types of color deficiencies. The test results show that: 1. Average discrimination lies between 20 to 100 total error score, 2. Superior discrimination is below 20 total error score, and 3. Low discrimination is more than 100 total error score.
Farnsworth D-15 Test The Farnsworth D-15 test (Fig. 2.5) consists of single box of 15 colored chips. The test can be carried out more rapidly than the 100-hue test. Viewing distance required is 50 cm or approxi-
mately 20 inches. Unlimited testing time is usually allowed but the subject may be told he/ she has two minutes to complete the test in order to prevent dawdling. The object of the test is to arrange the caps in order using the fixed reference cap as a starting point. The subject is instructed to take the cap which most closely resembles the fixed reference cap, and place it next to it; then find the cap that most closely resembles the cap he just placed, and place it next to it. Once the subject has arranged all the caps, the lid is closed and the box flipped over. The examiner then scores the test based on the order in which the subject placed them (the caps are numbered on the bottom). The examiner then connects the numbers on the score sheet in the order in which the patient placed the caps. The score is either “passing” or “failing.” A circular pattern on the score sheet indicates passing, a criss-crossing or lacing pattern indicates failing. The D-15 panel uses only saturated colors, therefore, subtle defects such as those seen with an anomalous trichromat may be missed. The D-15 is useful for detecting dichromacy, in particular, tritan defects which are often associated with eye diseases and drug toxicity. The disadvantage with this test is that minor defects are not detected. Dichromatic subjects will generally form a series of parallel or crisscrossing lines with at least two lines crossing the chart in the same direction. The type of deficiency is indicated by the index line most nearly parallel to the crossover lines.
Lanthony Desaturated D-15 Test
Fig. 2.5: Farnsworth D-15 color test kit
The Lanthony desaturated D-15 test (Fig. 2.6) is similar to the Farnsworth D-15 except that the color on chips is much less saturated. This makes the hue circle smaller and the arrangement task more difficult. It is especially useful for detecting subtle acquired color deficiencies.
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Diagnostic Procedures in Ophthalmology are much more difficult to administer than pseudo-isochromatic plates and arrangement tests. The first anomaloscope was designed by Nagel and is based on the color match known as the Rayleigh equation, that is, R + G =Y. Because of their relatively high price, anomaloscopes are rarely used in private practice.
Nagel Anomaloscope (Spectral Matching Test)
Fig. 2.6: Lanthony desaturated D-15
The Sloan Achromatopsia Test The Sloan Achromatopsia test is a matching test designed for rod monochromats described by Sloan in 1954. The test consists of seven plates, each with a different color: gray, red, yellowred, yellow, green, purple-blue, and red-purple. Each plate includes 17 rectangular strips forming a gray scale from dark to light in 0.5 steps of the Munsell value. In the center of each rectangle is a colored disc that has the same Munsell value from one end of the gray scale to the other. The patient’s task is to identify the rectangle that matches the lightness of the colored disc. This is a difficult task for persons with normal color vision because of the color difference, but it is readily and precisely accomplished by complete achromats, who see the colors as grays of different lightness. There are normative data for both persons with normal color vision and achromats.
Nagel (1970) constructed anomaloscope for studying the color vision defects. It is based on the color match known as the Rayleigh equation, that is Red (R) + Green (G) = Yellow (Y). The Nagel anomaloscope (Fig. 2.7) assesses the observer’s ability to make a specific color match. In anomaloscope, the observer is asked to match a mixture of red and green wavelengths to a yellow. This instrument consists of a source of white light, which is split into spectral colors by a prism. These colors are viewed through a telescope. The field of vision consists of a circle divided into two halves. The lower half projects a spectral Yellow (Sodium line) and this has to be matched by a mixture of Red (Lithium line) and Green (Thallium line) in the other half. The ratio of the two component lights can be controlled by press buttons on the base of the telescope on a scale of 0 – 73, where 0 is pure green, and 73 is pure red. The readings are interpreted as follows: the red/green mix
Anomaloscopes Anomaloscopes are instruments that assess the ability to make metametric matches. The results are used for definitive diagnosis and quantitative assessment of color vision status. Anomaloscopes
Fig. 2.7: Nagel anomaloscope
Color Vision and Color Blindness proportions can be expressed in the form of an Anomaly Quotient (AQ). Normal observers have AQ between 0.7 and 1.4; higher AQs indicate deuteranomaly (AQ usually >1.7), whereas lower AQs indicate protanomaly. A major advantage of the Nagel anomaloscope is that it can distinguish between dichromatic and anomalous trichromatic vision by measuring the balance of red and green wavelengths in the mixture field.
Pickford-Nicolson Anomaloscope The Pickford-Nicolson anomaloscope can be used for three different matches or colorimetric equations: The Rayleigh equation [R + G = Y], The Engelking equation [B + G = CY] and The Pickford - Lakowski equation [B + Y = W]. The matching field is presented on a screen for free viewing at a variety of distances, and there are no intervening optics between the patient and the matching field. The size of the field is changed by selecting different apertures: the largest is 2.54 cm (1 inch) in diameter and the smallest, 0.48 cm (3/16 inch). Different colors are obtained by inserting broadband filters. The Pickford-Lakowski equation is used to assess the consequence of senescent changes in the spectral transmission of the ocular media (yellowing of the lens), it also has value in examining acquired color defects. The Engelking equation is used for diagnosis of the blue - yellow or tritan color defects. Individual variability in density of the macular pigment and lens pigmentation affects both the Engelking and PickfordLakowski equations and, accordingly, confounds the interpretation of an individual result.
navigational aids are extensively used. Lantern tests are performance-based, and they do not diagnose, classify, or grade the level of color vision defect. Rather, they attempt to determine whether the person is capable of performing the color signal recognition tasks with adequate proficiency to maintain safety standards. There are two types of lantern tests, those that use actual signal light filters and those that use simulations of signal lights.
Farnsworth Lantern Test (Falant) In the United States, the Farnsworth Lantern (Falant) is the standard lantern test (Fig. 2.8). It simulates marine signal lights under a variety of atmospheric conditions. Two lights are presented in a vertical display in any of the nine possible combinations of three colors—red, green, and white—in the two positions. A subject must average eight out of nine correct responses to
Lantern Tests In marine, rail, and airline transportation, and in the armed forces, colored signals and
Fig. 2.8: Holmes-Wright Lantern
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Diagnostic Procedures in Ophthalmology pass the test. White lights are particularly problematic, especially for milder color defects. It is reported that the test is not representative of actual field conditions.
Edridge-Green Lantern Test The Edridge-Green Lantern (Fig. 2.9) is an instrument used for testing the ability of a person to recognize color of transmitted light. It was built to simulate the light of railway traffic signals, as they are visible from a distance. The apertures represent the equivalents of five and half-inch railway signals at 600, 800 and 1000 yards, respectively when viewed from 20 feet distance. Usually two apertures 1.3 and 13 mm are used, set of filters showing signal red, yellow, green and blue colors are shown, each color being shown twice for each aperture size.
Other Tests Electroretinography Use of electroretinography (ERG) in the modem era is more useful for detection of color vision deficiencies for two reasons: (i) new methods allow to separate and observe accurately the photopic and scotopic components of ERG with the possibility of better study of cone activity and (ii) with the use of computer averaging, picking up of oscillatory potentials is more easy.
Microspectrophotometry In spectrophotometry, an individual cone of a dissected retina is aligned under a small spot of light and its absorption is measured at various wavelengths. The most direct evidence of Young’s trichromatic theory (3 classes of cones) comes from spectrophotometry. The results of microspectrophotometry confirm three groupings with peak sensitivities at 437-458 nm, 520-542 nm and 562-583 nm.
Color Vision Deficiencies and Everyday Life
Fig. 2.9: Edridge-Green Lantern
The recommendations of the test state that a candidate should be rejected if he calls 1. Red as Green 2. Green as Red 3. White light as Green or Red or vice versa 4. Red-Green or White light as Black. Any candidate who makes any other errors should be tested with other test.
Many tasks depend on our ability to discriminate color. Selecting products at the grocery store, matching paint colors or items of clothing, or connecting color-coded wiring all depend on efficient color vision. Color vision deficiencies can seriously affect an individual’s ability to learn, to work at a chosen occupation and move effectively in the world. Young children are expected to learn color names early in their educational experience and color is frequently used to categorize educational materials. Good color vision is also important for students of art, chemistry, biology, geology and geography. A child with deficient color vision will have disadvantage on such tasks as
Color Vision and Color Blindness color naming, coding, and matching. Color vision testing should be done for all children as early as possible, and certainly prior to starting school. If a color deficiency is present, the child’s school, teacher, and parents should be informed so that methods of instruction can be modified to meet their visual needs. Teachers and parents can help the child in a number of different ways. First, images and utensils such as crayons, pencil and pens can be labeled with words or symbols. Second, discrimination between items of different color can be facilitated by the use of high luminance contrast. For example, it would be better to use white chalk on a black or green chalkboard or a dark marker on a white board than combinations that provide less luminance contrast. The level of luminance contrast in colored materials can be determined quite easily by making a black and white photocopy of them or by converting them to black and white on your computer. Third, children should be taught common objects by their usual color (e.g. ”bananas are normally yellow and the sky is blue”). Occupations vary in their requirement of color identification. For some, good color judgment is desirable but not necessary. For others, knowledge of one’s color vision is critical. Examples where good color judgment can be critical for careers include a painter, safety officer, dermatologist, pharmacist, cartographer, coroner, chemist, buyer of textiles, food inspector, electrician, and marine navigator. Color perception failures in such jobs could be costly, even disastrous.
Enhancing Performance with Filters The color performance of the patients with color deficiency can be sometimes enhanced using colored filters. By absorbing wavelengths selectively, these filters help the observer to differentiate stimuli based on their relative brightness. For example, a red object viewed through a green filter or a green object viewed through a red filter will appear much darker.
For example the X-chrom lens is a red contact lens worn on one eye that absorbs shorter wavelengths and passes longer ones. By comparing the relative brightness in eye with the X-chrom lens to that in the eye without it, a dichromat’s ability to distinguish red from green can be enhanced. While such monocular comparisons may be useful in specific applications, the user remains a dichromat and is unlikely to find the approach practical for everyday use.
Summary Ophthalmic personnel are frequently asked to perform color vision testing. Knowing whether a congenital or acquired defect is suspected can help determine which color vision test should be administered. All color vision tests have specific requirements for lighting, viewing distance, viewing time, and scoring. It is important to be familiar with the various testing and scoring guidelines in order to provide the requesting doctor with accurate and useful information.
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Diagnostic Procedures in Ophthalmology 7. Duke-Elder S. Congenital colour defects. In System of Ophthalmology. Henry Kimpton, London 1964; Vol III (Part 2): 661-68. 8. Duke-Elder S. Colour vision. In System of Ophthalmology. Henry Kimpton, London 1968; 4: 617-51. 9. Edridge-Green. Physiology of Vision, London 1920. 10. Farnsworth D. Protan, deutan and tritan. J Opt Soc Amer 1943;33:568. 11. Farnsworth D, Reed. Small-field Tritanopia. USN Submar Med Res Lab Rep No 19, 1944. 12. Farnsworth D. Manual of the FarnsworthMunsell 100-hue test for the Examination of Color Discrimination. 1949; revised 1957, pp 1-7. 13. Francois J, Verriest G. Acquired diseases producing colour vision defects. Vision Res 1961;1:201. 14. Francois J. La discrimination chromatique dans amblyopie strabique. Documents Ophthal 1967;23:318. 15. Geddes. Prevalence of colour vision deficients. Br J Psychol 1946;37:30. 16. Georgia Antonakon Chrousos. Ocular findings in Turner’s syndrome: A perspective study. Ophthalmology 1984;91:926. 17. Gouras P. Identification of cone mechanism in monkey ganglion cells. J Physiol 1968;199:533. 18. Hardy LH, Rand G, Rittler MC. Comparison of HRR with other tests. Arch Ophthalmol 1954; 51:216. 19. Hart WM Jr. Acquired dyschromatopsias. Surv Ophthalmol 1987;32:10. 20. Hart WM Jr. Colour vision. In Adler’s Physiology of the Eye. Mosby, St Louis 1992;708-27. 21. Hering. Zur Lehre vom Lichtsinne Wien, 1878 cited by Duke-Elder ref 6. 22. Holmgren. Holmgren’s wool test. Ann Rep Smithsonian Inst 1877; 131. 23. Ishihara S. Test for Colour Blindness Manual of Ishihara Plates, 1917, 5th ed. Tokyo 1925. and 14th ed. 1959, Kanehara Shuppan Co Ltd, Tokyo – Kyoto, Japan. 24. John A, Fleishman, Roy W Beck. Defects in visual function after resolution of optic neuritis. Ophthalmology 1987;94:1029. 25. Kinnear PK, Sahraie A. New Farnsworth-Munsell 100-hue test norms of normal observers for each year of age 5-22 and for age decades 30-70. Br J Ophthalmol 2002;86:1408-11. 26. Ladd-Franklin. Tetrachromatic Theory. Z Psychol Physiol Sinnes 1893;4:211.
27. Maxwell C. Fundamental response curve of the cone pigment. Trans Roy Soc Edin 1885; 21(2): 275. 28. Michael CR. Colour vision mechanisms in monkey striate cortex: Simple cells with dual opponent colour receptive fields. J Neurophysiol 1978;41:1233. 29. Michael CR. Colour sensitive complex cells in monkey striate cortex. J Neurophysiol 1978;4: 1250. 30. Michael CR. Colour sensitive hypercouplex cells in monkey striate cortex. J Neurophysiol 1979; 42:726. 31. Miller SJH. Colour blindness or achromatopsia. In Parsons’ Diseases of the Eye. 18th ed. Edinburgh, Churchill Livingstone, 1900, 269-70. 32. Mitarai G. Glia-neuron interactions and Adaptional mechanisms of the retina. ln Jung R, Kormaluber H (Eds). The Visual System: Neuroplysiology and Psychophysics 1961. 33. Nakamura K. New color vision test to evaluate faulty color recognition. Jpn J Ophthalmol 2002; 46: 601-06. 34. Neitz J, Jacobs GH. Polymorphism in normal color vision. Vision Res 1990;30:62. 35. Newton I. Composition of white light. Phil Trans 1672;6:3075. 36. Nigel W Daw: Colour vision: Adler’s Physiology of the Eye, Robert Moses (Ed). St Louis, Mosby, 1981. 37. Pearlman AL, Birch J, Meadows JC: Cerebral colour blindness:An aquired defect in hue discrimination. Amer Neurol 1979;5:253. 38. Rushton WAH. A cone pigment in the protanope. J Physiol 1963;168:345. 39. Swanson WH, Cohen JM. Color vision. Ophthalmol Clin N Am 2003;16:179-203. 40. Taylor WOG. Effects on employment of colour vision defectives. Br J Ophthalmol 1971;155: 753-760. 41. Vola JL, Leprince G. 100-Hue at mesopic level. Mod Probl Ophthal 1978;19:67-70. 42. Wald G. Defective colour vision and its inheritence. Proc Nat Acad Sci USA 1966;55:1347. 43. Wiesel TN, Hubel DH: Spatial and chromatic interactions in the lateral geniculate body of the rhesus monkey. J Neurophysiol 1966;29:1115. 44. Young T. A course of lectures on natural physiology. Phil Trans 91, 43, 92, 12, 387, 1801-07. 45. Yves le Grand. Light Colour and Vision. London: Chapman and Hall, 1957.
Slit-lamp Examination
HARINDER SINGH SETHI, MUNISH DHAWAN
3
Slit-lamp Examination
The slit-lamp is one of the important examining tools of ophthalmologists. Clinical ophthalmologists all over the world routinely use a slit-lamp to examine their patients. A raw slit-lamp was introduced in the early 1900s, but presently, it is a sophisticated instrument (Fig. 3.1). One of the most important advantages of slit-lamp
examination is that one can examine the eye structure in three dimensions (3D). There are three basic requirements for appreciation of depth with a slit-lamp. The first depends upon the clinician possessing a third grade of binocular vision called steriopsis. The second involves the direction of the incoming light source, and is dependent upon the fact that the light beam can be moved so it comes in from one side or the other. The third involves the shape of the slit and is dependent upon the fact that the light source can be moved separately from the oculars.
History
Fig. 3.1: Slit-lamp
One of the first individuals to apply microscopy to the living eye was Purkinje, who studied the iris with an adjustable microscope by illuminating the field of view. The uniocular slit-lamp was born years later when Louis de Wecker combined an eyepiece objective and adjustable condensing lens within a tube. It was improved by Siegfried Czapski, who added binocularity to the microscope. However, none of the units had sufficient and adjustable illumination. Allvar Gullstrand, an ophthalmologist and 1911 Nobel laureate developed a true slit-lamp to
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Fig. 3.2: Allvar Gullstrand
illuminate the eye (Fig. 3.2). Then Henker and Vogt improved upon Gullstrand’s device in 1910s by creating an adjustable slit-lamp and combining Czapski’s microscope with Gullstrand’s slit-lamp illumination. The modern slit-lamp is a tool capable of stereoscopically examining optical sections of the anterior segment of the eye in great detail. Vogt used the slit-lamp biomicroscope to study a vast array of eye diseases and documented his findings in a publication, “Lehrbuch und Atlas der Spaltlampenmikroskopie des Leibenden Auges” in 1930s. Besides examination of the anterior segment of the eye, the slit-lamp, in conjunction with certain contact lenses, is often used to examine the anterior chamber angle and posterior segment of the eye.
Optics of Slit-lamp The slit-lamp is a compound microscope with an objective lens and an eyepiece. The two main components of the modern slit-lamp are the illumination system and observation system (Fig. 3.3).
The illumination system of most slit-lamps consists of two different designs. The first design, the Haag-Streit type illumination, allows de-coupling in the vertical meridian. Such vertical de-coupling is particularly useful when performing gonioscopy to minimize reflections and for indirect funduscopy to gain increased peripheral views. The second design, the Zeiss type illumination system, does not allow decoupling in the vertical meridian. The Zeiss illumination is light and compact and makes the slit-lamp easy to use. In either case, the illumination systems are capable of producing a homogenous and aberration-free beam of white light. Most slit-lamps have halogen bulbs to yield shorter wavelengths of light, which allows better visualization of smaller structures compared with longer wavelengths of light (i.e. tungsten bulbs). A condensing lens first focuses the light onto slit aperture. This light is again focused by another lens onto the eye after being reflected by tilted mirror. Blue and green (redfree) filters are available in slit-lamp to study fluorescein staining pattern and microaneurysm and nerve fiber layer.
Observation System The second main component of slit-lamps is the observation system. Modern slit-lamp microscopes can magnify images between X5 and X25, with some microscopes allowing magnification to X40 and even X100. Magnification is generally achieved by three methods: • Flip-type • Galilean rotating barrel, and • Continuous zoom system. However, magnification of the slit-lamp is less important than its resolution. The resolution of a slit-lamp is dependent on the wavelength of light used, the refractive index between the
Slit-lamp Examination
A
D
B
E
C
F
Figs 3.3A to F: A The binocular eyepieces provide stereoscopic vision and can be adjusted to accommodate the examiner’s interpupillary distance. The focusing ring can be twisted to suit the examiner’s refractive error. B The illumination arm can be swung 180 degrees side to side on its pivoting bases allowing the examiner to direct the light beam anywhere between the nasal and temporal aspect of the eye. The dimension of the light beam can be varied in height and width with the levers. C The patient positioning frame consists of two upright metal rods to which are attached a forehead strap and a chin rest. D The joystick allows for focusing by shifting forward, backward, laterally or diagonally. The joystick can also be rotated to lower or elevate the light beam. The locking screw located at the base secures the slit-lamp from movement when it is not in use. E Knurled knob is slit-beam height adjuster, Flip lever controls filters, from left to right: bright, dim, red-free. F ON/OFF power switch provides high or low options in light intensity
eye and objective, the working distance, and the diameter of the objective lens. In practice, the first three of these factors are not easily modifiable, but the objective lens diameter can be modified to increase resolution. However, a very large diameter lens can introduce optical aberrations. The observation system is also influenced by the proximity of the patient’s eye to the examiner’s eyes. This necessitates a convergence system for binocular viewing, and most modern slit-lamp biomicroscopes are designed with 10 to 15 degrees of convergence to minimize eye strain to the examiner.
Clinical Procedure Before using the slit-lamp, it is important to ensure that the instrument is correctly set up. The following points should be checked: • The eyepieces should be focused for the observer for his/her own refractive error. Often a little more minus correction is required than the observer’s actual refractive error due to proximal accommodation and convergence. • The pupillary distance (pd) is adjusted for the observer (perhaps the pd should be
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• •
• •
slightly less than that usually measured to account for proximal convergence). Check that the slit-lamp is parallel on the runners of the table. Check that the observation and illumination systems are coupled, and the slit-beam is of even illumination and has sharply demarcated edge (otherwise irregularity of the beam may be falsely interpreted as irregularity of tissues). The locations of the controls are known. The observer and patient are comfortable in the mid-travel of the slit-lamp. Mid-travel is the location of the slit-lamp when it is half-way up or down.
The slit-lamp examination is conducted in a semi dark room. Patient is seated in front of slit-lamp on an adjustable stool and his head is steadied by placing chin on chin-rest and his forehead rests on the bar of head-rest. As with any technique, a general routine should be followed, in most cases when examining the eye and adnexa, a large field of view is used initially and then focus in on detail when required with higher magnification. The examination should be commenced using the X10 eyepieces and the lower powered objective. Use the lowest voltage setting on the transformer. Select the longest slit-length by means of the appropriate lever. Adjust the chin-rest so that the patient’s eyes are approximately level with the black marker on the side of the head rest. Adjust the height of the slit-lamp until the slit-beam is centered vertically on the patient’s eye. Focus the slit-beam on the eye by moving the joystick either towards or away from the patient. Coarse positioning can be effected without using the microscope but critical focusing should be carried out whilst viewing through the microscope. The angulation between the observation arm and the illumination arm is adjusted. In addition,
accessories like a fixation light, Hruby lens, an applanation tonometer, camera or CCTV can be attached. Laser system can also be attached to a slit-lamp utilizing its optics for laser delivery.
Examination Techniques The various techniques of slit-lamp examination are: 1. Diffuse illumination 2. Direct focal illumination a. Narrow beam (optic section) b. Broad beam (parallelepiped) c. Conical beam 3. Indirect illumination 4. Retroillumination a. Direct b. Indirect 5. Specular reflection 6. Sclerotic scatter 7. Oscillatory illumination 8. Tangential illumination.
Diffuse Illumination Diffuse illumination (Fig. 3.4) is a good method for observing the eye and adnexa in general.
Fig. 3.4: Diffuse illumination
Slit-lamp Examination The beam width is kept at maximum and magnification is kept low and light is thrown at an obtuse angle. It gives an overview of lids, conjunctiva, cornea and lens. Detail examination is not possible with diffuse illumination. Its main purpose is to illuminate as much of the eye at once for general observation. A broad beam of light is directed at the cornea from an angle of approximately 45 degrees. Position the microscope directly in front of the patient’s eye and focus on the anterior surface of the cornea. Low to medium magnification (X7-X16) should be used which allows the observer to view as many of the structures as possible. When viewing the eye with achromatic light one should note on gross inspection, any corneal scar, tear debris, irregularities of Descemet’s membrane or pigmentary changes in the epithelium. These findings are investigated more thoroughly with other types of illumination.The diffuse illumination mode is also used with cobalt blue filter after fluorescein staining. Fluorescein staining is also used to evaluate positioning of contact lenses, tear breakup time (TBUT), and staining of the cornea for corneal ulcer. Diffuse, wide-beam, illumination together with the red free (green) filter is helpful when viewing the bulbar conjunctiva, and episcleral blood vessels. With the aid of the red free filter small hemorrhages, aneurysms and engorged vessels stand out well.
Direct Focal Illumination Direct focal illumination is the most commonly used method of viewing tissues of the anterior segment of the eye. The focused slit is viewed directly by the observer through the microscope (Fig. 3.5A). The magnification can be increased (X10 to X40) to view any areas of interest in greater detail.
Fig. 3.5A: Direct illumination: the light source is positioned off to one side, and a bright slit-beam is shone directly onto the object to be studied. The light is scattered in all directions by the object, and some of this scattered light finds its way back to the oculars, where it can be observed by the examiner
Direct/focal illumination can be used with different types of beams: a. Narrow beam (optic section) b. Conical beam c. Broad beam (parallelepiped).
Narrow Beam Narrow beam optical section is used primarily to determining the depth or elevation of a defect of the cornea, conjunctiva or locating the depth of an opacity within the lens of the eye (Fig. 3.5B). With the optic section, it is possible to detect corneal thickness, site of foreign body, scars and opacities, the depth of anterior chamber and location of cataracts. The biomicroscope should be directly in front of the patient’s eye, the illumination source at about 45 degrees and the illumination mirror in “click” position. The slit-width is almost closed (0.5-1.0 mm wide by 7-9 mm high). Set the magnification on low to medium (X7-X10) and focused on the patient’s closed lid. The
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Fig. 3.5B: Direct illumination: Narrow beam (optic section)
thickness of the eyelid (about 1 mm) means focusing on the cornea is accomplished with only slight movement of the joystick. With eyes open, give the patient a point of fixation such as the fixation light, or the top of the examiner’s opposite ear. Once the cornea is in sharp focus, scan the cornea from temporal limbus to nasal limbus. To maintain a clear, distortion-free view, the illumination source is always moved to the opposite side when crossing the mid-line of the cornea. With a clearly focused optic section slightly temporal to the center of the cornea, magnification is increased to X16, then to X20, and brightness is also increased. Try to note the following: 1. The front surface bright zone is the surface of the tears, 2. The next dark line is the epithelium, 3. The next brighter thin line is Bowman’s membrane, 4. The gray wider granular area is the stromal zone, and 5. The last bright inner zone is the endothelium To attain an optic section of the crystalline lens, the angular separation of the illumination source is reduced until the light beam just grazes the edge of the pupil and the vertical height is reduced to approximate the pupil size. This alignment can easily be accomplished from
outside the biomicroscope. When the beam cuts just across the edge of the pupil, the crystalline lens will appear sectioned. By focusing the biomicroscope with joystick with one hand and controlling the direction or angle of the light source with the other hand, the different layers of the lens can be brought into focus. The anatomical location of lens opacities can be visualized. Furthermore, the degree of nuclear opalescence and color can be evaluated and graded. Medium or high magnification gives the best details of lens. Van Herick’s technique for grading the anterior chamber angle uses an optic section placed near the limbus with the light source always at 60 degrees (Figs 3.6A and B). The biomicroscope is placed directly before the patient’s eye. This technique only allows an estimate of the temporal and nasal angles. The classification of the angle grades and risk of angle closure are summarized in Table 3.1. Split limbal technique: It can be used for an estimation of the superior and inferior angles (Fig. 3.7). The slit-lamp and illumination system
Fig. 3.6A: Van Herick angle estimation method
Fig. 3.6B: Split limbal technique for assessing anterior chamber angle depth
Slit-lamp Examination TABLE 3.1: CLASSIFICATION OF ANTERIOR CHAMBER ANGLE BASED ON VAN HERICK ANGLE OF THE ANTERIOR CHAMBER ESTIMATION METHOD Angle grade
Risk of angle closure
Cornea to angle ratio
4
Wide open angle incapable of closure. Iris to cornea angular separation equals to 35-45°
Anterior chamber depth (shadow) is equal to or greater than corneal thickness
3
Moderately open angle incapable of closure. Iris to corneal angular separation equals to 20-35°
Anterior chamber depth (shadow) is between 1/4 and 1/2 of the corneal thickness
2
Moderately narrow angle closure possible. Iris to corneal angular separation equals to 20°
Anterior chamber depth (shadow) is equal to 1/4 of the corneal thickness
1
Extremely narrow angle, closure chance high. Iris to corneal angular separation equals to 10°
Anterior chamber depth (shadow) is equal to less than 1/4 of the corneal thickness
0
Basically closed angle. Iris to corneal angular separation is 0°
Anterior chamber depth (shadow) is nil or only a very narrow slit
are in click position aligned directly in front of the patient. The beam width is that of an optic section which is focused on the limbalcornea junction thus splitting the cornea and limbus. Then view the arc of light through the cornea and that falling on the iris without the aid of the slit-lamp. The angular separation seen at the limbus-corneal junction is an estimation of the anterior chamber angle depth in degrees.
Conical beam Examination of the anterior chamber for cells or flare must be performed before either dilation or applanation tonometry. High magnification (X16-X20) and high illumination may be needed. High illumination is used to detect floating aqueous cells and flare by the Tyndall effect (particles of dust floating in a sun light beam). The traditional method of locating and grading cells and flare is to reduce the beam to a small circular pattern with the light source 45 to 60 degrees temporally and directed into the pupil. The biomicroscope is positioned directly in front of the patient’s eye with high magnification and with as bright illumination as the patient will permit. The examiner always allows a
period of time to dark adapt. The conical beam is focused on a dark zone lying between the cornea and the anterior lens surface. This zone is normally optically empty and appears totally black. Flare (protein escaping from dilated vessels) makes the normally optically empty zone appear gray or milky when compared to the normal eye. Cells will reflect the light and can be seen as white dots. The techniques used may be either to oscillate the light source with the joystick from left to right while focused in the anterior chamber or to focus from the posterior cornea to the anterior lens while oscillating the light source.
Broad beam (parallelepiped) A parallelepiped is one of most common types of illumination used (Fig. 3.7). It is used in combination with a number of different types of illuminations. The biomicroscope should be placed directly in front of the patient’s eye, the illumination source at about 45 degrees and the illumination mirror in “click,” position. A parallelepiped is essentially an optic section with 2.0-4.0 mm slit-width and variable height. The parallelepiped presents a three dimensional
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Fig. 3.7: Broad beam (parallelepiped)
view of the cornea or the crystalline lens. The three dimensional view permits observation of distinguishable details within the crystalline lens “zones of discontinuity”. As with the optic section, the angle between the illumination source and biomicroscope may be varied to expose more corneal epithelium, stroma and endothelium. The whole cornea should be scanned using a parallelepiped. When scanning the cornea, a clear undistorted view must be maintained by positioning the light source to the opposite side when crossing the mid-line of the cornea. Both normal and abnormal findings can be seen when scanning the cornea with varied levels of magnifications and brightness. Look for the following findings: 1. Tear debris is usually related to allergies or occasionally with infections. 2. Corneal nerves are white thread-like structures that bifurcate and trifurcate and are located anywhere within the cornea. 3. Blood filled vessels extend from the limbus onto or into the cornea, and may be diagnostic of chronic or acute insult or inflammation. 4. Ghost vessels extend from the limbus into the cornea. They are empty of blood and diagnostic of past deep corneal inflammation.
5. Corneal scars are white in color and diagnostic of some past corneal damage, ulcer, abrasion or foreign body. 6. Corneal striae are white usually vertical thread-like twisting lines found in the Descemet’s membrane and posterior stroma. They are diagnostic of poor fitting soft contact lens and diabetes. 7. Endothelial pigmentation, when heavy and located vertically on the endothelium, is known as Krukenberg’s spindle, it may be diagnostic of iris atrophy and pigmentary glaucoma. Transillumination of the iris may reveal transillumination iris defects (TIDs). Scanty and very fine pigment deposits are commonly seen and are not pathological.
Indirect Illumination Indirect illumination means looking at tissue outside the area which is directly illuminated and can be used in conjunction with most of the above techniques. Corneal opacities, corneal nerves and limbal vessels are easily seen under indirect illumination as glare is reduced. Examine always directly as well as indirectly illuminated areas of the structure. To use this type of illumination place the biomicroscope directly in front of the patient’s eye and the illumination light source at about 45 degrees. Make sure the illumination mirror is in “click” position. Use a parallelepiped beam sharply focused on a given structure like the cornea. The light passes through the cornea and falls out of focus on the iris. The dark area just lateral or proximal to the parallelepiped is the indirect or proximal zone of illumination. This is the area of the cornea which one surveys through the biomicroscope. This type of illumination is helpful in detection of microcystic edema, faint corneal infiltrates and irregularities of the corneal epithelium and tears. Because it utilizes
Slit-lamp Examination direct, indirect and retroillumination simultaneously, one should consider it to be as important as any other type of illumination.
Retroillumination Retroillumination is another form of indirect viewing. The light is reflected off the deeper structures, such as the iris or retina, while the microscope is focused to study the more anterior structures in the reflected light (Figs 3.8A to D). It is used to study the cornea in light reflected from the iris, and the lens in light reflected from the retina. Structures that are opaque to
light appear dark against a light background (e.g. corneal scars, pigment, and lens opacity). Portions that scatter light appear lighter than the background (e.g. edema of the epithelium, corneal precipitates). This method is useful for examining the size and density of opacities, but not their location. Retroillumination uses a parallelepiped that bounces unfocused light off one structure while observing the back of another. The alignment and angular separation of the biomicroscope to the illumination source will vary. The light source beam is reflected off another structure like the iris, crystalline lens or retina while the
Figs 3.8A and B: Retroillumination: This technique allows the observer to view a clear structure with light that has been transmitted through, rather than just bounced off it. A Light from the slit-lamp is shone through the pupil, reflected off the fundus, and transmitted through the lens and cornea. B Light is reflected off the iris and transmitted through the cornea
C
Figs 3.8C and D: Retroillumination
D
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Diagnostic Procedures in Ophthalmology biomicroscope is focused on a more anterior structure. For retroillumination or transillumination of the iris or crystalline lens a low to medium magnification (X7-X10) is used. A slitwidth 1-2 mm wide and 4-5 mm high is used with the biomicroscope and light source placed in direct alignment with each other. They are both positioned directly in front of the eye to be examined. Focus the slit just off the edge of the iris and on the front of the lens. If there are defects or atrophy of the iris they will be seen as a retinal “orange” glow coming back through each defect or hole. Patients who have numerous endothelial pigment deposits must have their iris transilluminated. The cornea is probably the most common structure viewed on retroillumination. Keratic precipitates will appear white in direct illumination but dark by retroillumination. This technique is valuable for observation of deposits on the corneal endothelium and invading blood vessels.
Sclerotic Scatter Sclerotic scatter examination uses the principle of total internal reflection (Fig. 3.9). Slit-lamp is set to a low X6-X10 magnification and a narrow vertical-slit (1-1.5 mm in width) is directed in line with the temporal or nasal limbus. A halo of light will be observed around the limbus as light is internally reflected within the cornea, but scattered by the sclera. Presence of corneal opacities, edema or foreign bodies will be made visible by the scattering light, appearing as bright patches against the dark background of the iris and pupil. Even minute nebular opacities can be picked up.
Specular Reflection Specular reflection is achieved by positioning the beam of light and microscope in such a position so that the angle of incidence is equal
Fig. 3.9: Sclerotic scatter: A bright, wide-slit is shone directly at the limbus; most of the light is trapped within the cornea through total internal reflection, and, therefore, the cornea appears dark. When the light hits the opposite limbus or anything abnormal located within the corneal substance, it will scatter; some of the scattered light is directed back to the oculars, the abnormality is visible to the observer
to the angle of reflection. The light can be reflected from either the anterior or posterior corneal surface. Note that the reflected light should pass through only one eyepiece, and, therefore, this method is monocular. Any roughness or irregularity as induced by the presence of corneal guttata is visible due to irregular reflection of light. A parallelepiped is used to view the endothelial cells of the cornea. The cells are seen only by one eye and they appear in the opposite direction of the illumination light source. A parallelepiped is used for specular reflection. The angle between the illumination source and the biomicroscope should be approximately 60 degrees and a high magnification and high illumination must be used. Place the biomicroscope directly in front of the patient’s eye and the illumination light source at 45-60 degrees. Just off the limbus, obtain a sharply focused parallelepiped of the
Slit-lamp Examination cornea. Slowly advance the parallelepiped across the cornea until a dazzling reflection of the filament is seen within the biomicroscope. This reflection is only seen by one eye. Keeping the reflected light within the field of view of biomicroscope, the focus is moved back toward the endothelial cells. There will be a point where two images of the filament are seen, one bright, and the other ghost-like or copper-yellow in color. When the biomicroscope is focused on the ghost-like filament a mosaic of hexagonal cells are seen. It should be noted that even with X40 magnification the endothelial cells do not look as large as most texts show. They resemble the appearance of the dimpled surface of an orange peel or basketball. When the slit-lamp illumination system and the biomicroscope are at equal angles of incidence and reflection, the endothelium of cornea is viewable. Both front and back surfaces of the crystalline lens can also be viewed by using the specular reflection.
Oscillatory Illumination In oscillatory illumination, a beam of light is rocked back and forth by moving the illuminating arm or rotating the prism or mirror. This method may be used to determine occasional aqueous floaters and the extent of opacities in the crystalline lens.
Tangential Illumination In tangential illumination iris is examined under very oblique illumination while the microscope is aligned directly in front of the eye. It is useful for examining tumors of the iris.
Clinical Application Slit-lamp biomicroscopy is very useful in the diagnosis of eye diseases. It should routinely be performed in almost all diseases of the eye.
1. Eyelids and lashes: A low magnification, with a long and fairly narrow beam should be used to scan the eyelashes and lid margins. The examination can reveal the presence of crusted material, lash loss, erythema and flaking suggestive of blepharitis. 2. Conjunctiva: For examination of conjunctiva, pull the lower lid away from the globe with hand and look at the palpebral and bulbar conjunctiva. One may find foreign body, purulent material, injection, conjunctival follicles, pinguecula or pterygium. Try to see the entire cul-de-sac while the patient looking up. The upper lid must be everted to examine the upper palpebral conjunctiva. 3. Cornea: A narrow beam should be directed approximately 45 degrees at the cornea. Scan the entire corneal surface, moving lids and beam appropriately while trying to evaluate the epithelium, stromal thickness and endothelium. Note any defects, opacities or pigment dusting on the endothelium. If defects are seen or suspected, instill a topical anesthetic and fluorescein stain. Make the beam as large as possible and flip the cobalt blue filter on. Examine the epithelium for areas of bright yellow-green staining. The staining represents an epithelial defect. 4. Anterior chamber: The depth of the anterior chamber can be determined by comparing the corneal thickness to the space between the posterior surface of the cornea and the iris surface. The beam should be directed at approximately 45 degrees and just inside the temporal limbus. An anterior chamber depth of less than 1/4 of the corneal thickness is considered a narrow-angle. A search for flare should also be made. 5. Iris: The iris is generally screened with a narrow-beam with full height. It should be fairly flat and free of masses. Small
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Diagnostic Procedures in Ophthalmology pigmented nevi are common, but should be flat. The pupillary margin should be round. A slight extension of the posterior pigment around the margin is common but the presence of vessels on the iris is abnormal (rubeosis iridis). 6. Lens: The anterior capsule, cortex, nucleus, and posterior capsule of the lens are scanned with a narrow and full beam of the slit-lamp. When opacity in the lens is present, localize its depth within the lens. Pupillary dilatation facilitates the localization. If the pupils are dilated, widen the beam slightly, lower the height and direct the beam in a straight line toward the retina between the microscope and the eye near the pupillary border. It results in retroillumination and focus on the lens to find iris defects or lens opacities. 7. Anterior vitreous: Anterior vitreous is seen with a narrow beam. Small proteinaceous strands are normal, but cells, blood or opacities in the vitreous are abnormal and warrant investigations.
Fig. 3.10: Goldmann applanation tonometer
Slit-lamp Attachments Besides routine examination of the eye, the slitlamp with the help of its attachments is used for various investigative procedures. Important slit-lamp attachments with their use are mentioned below: Goldmann tonometer (Fig. 3.10) is used for applanation tonometry. Pachymeter (Fig. 3.11) is used for measurement of corneal thickness. Gonioscope (Figs 3.12A to C) is used for visualization of the angle of the anterior chamber. Hruby lens is used for funduscopy. Digital camera for fundus photography (Fig. 3.13).
Fig. 3.11: Corneal pachymeter mounted on slit-lamp
Slit-lamp Examination
A
B Fig. 3.13: Slit-lamp with digital camera
Bibliography
C Figs 3.12A to C: Goldmann gonioscopes: A Singlemirror, B Double-mirror, C Three-mirror
1. Fingeret M, Casser L, Woodcombe HT. Atlas of Primary Eye Care Procedures. Norwalk, Appleton & Lange, 1990. 2. Waring GO, Laibson PR. A systematic method of drawing corneal pathologic conditions. Arch Ophthalmol 1977:95:1540-42.
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FRANCISCO ARNALICH, DAVID PIÑERO, JORGE L ALIÓ
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Corneal Topography
The cornea is the most important refractive element of the human eye, providing approximately two-thirds of optical power of the eye, accounting for about 43-44 diopters at the corneal apex. Because its surface is irregular and aspherical, it is not radially symmetric, and simple measurement techniques are inadequate. The great upsurge in refractive surgery led to a need for improved methods to analyze corneal surface and shape since refraction and keratometric data alone were insufficient to predict surgical outcomes. Understanding and quantifying corneal contour or shape has become essential in planning modern surgical intervention for refractive surgery, as well as in corneal transplantation. It is also very valuable for assessing optical performance of the eye. The different methods for evaluating the anterior surface of the cornea, developed over several centuries, have, in the present era, led to the modern corneal topographers.
History of Corneal Measurement In 1619 Scheiner analyzed corneal curvature by matching the image of a window frame reflected onto a subject’s cornea with the image produced by one of his calibrated spheres.
Fig. 4.1: Helmholtz ophthalmometer
Keratometer In 1854 Helmholtz described the first true keratometer, which he called an ophthalmometer (Fig. 4.1). With some minor improvements, it is still being used clinically for calculating refraction, intraocular lens power and contact lens fitting. This apparatus is based on the tendency of the anterior corneal surface to behave like a convex mirror and reflect light. The projection of four points, or mires, onto the cornea, creates a reflected image that can be converted into a
Corneal Topography corneal radius, “r”, using a mathematical equation that considers distance from the mire to cornea (75 mm in the keratometer), image size and mire size (64 mm in keratometer). The corneal radius can be transformed into dioptric power using the formula: DP= (index of refraction of the lens - 1)/ r The standard keratometric index represents the combined refractive index of the anterior and posterior surfaces of the cornea, considers the cornea as a single refractive surface, and is 1.3375. Thus, the equation can be simplified to: DP= 337.5/ r Although keratometers are still common in ophthalmology clinics, they do have specific limitations that need to be considered in order to avoid misleading conclusions. 1. Most traditional keratometers measure the central 3 mm of the cornea, which only accounts for 6% of the entire surface. 2. It assumes that the cornea is a perfectly sphero-cylindrical surface, which it is not. The cornea is aspheric in shape, flattening between the center and the periphery. Usually the central corneal curvature is fairly uniform, and this is the reason why it can be used to calculate corneal power in normal patients. However, this is not true in some pathogenic conditions like ectatic disorders or after refractive surgery. 3. The keratometer provides no information as to the shape of the cornea either inside or outside the contour of the mire. Several corneal shapes can all give the same keratometric value so this apparatus is of little use should it become necessary to reconstruct the whole corneal morphology.
reflections of a series of illuminated concentric rings (known as Placido’s rings) first time in 1880 (Fig. 4.2). In 1896 Gullstrand developed a quantitative assessment of photokeratoscopy. The keratoscope, like a keratometer, projects an illuminated series of mires onto the anterior corneal surface, usually consisting of concentric rings. The distance between the concentric rings or mires gives the observer an idea of the corneal shape. A steep cornea will crowd the mires, while a flat cornea will spread them out. Surface irregularity is seen as mire distortion. When a photographic camera is attached to the keratoscope, it becomes a photokeratoscope, which gives semi-quantitative and qualitative information about the paracentral, midperipheral and peripheral cornea. Based on the mathematical equation, it is possible to calculate corneal power from object size. Still, photokeratoscopy gives limited information on the central area, which is not covered by the mires.
Fig. 4.2: Placido’s rings
Keratoscopy and Photokeratoscopy
Videokeratoscopy
Goode presented the first keratoscope in 1847. Placido is credited to photograph the corneal
Modern corneal topographers are based on videokeratoscopy. A video camera is attached
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Diagnostic Procedures in Ophthalmology to the keratoscope, and the information is analyzed by a computer that displays a colorcoded map of power distribution or corneal curvature of the anterior corneal surface (Fig. 4.3). It overcomes some of the limitations of other methods, since it measures larger areas of the cornea, with larger number of points thus increasing resolution. Computer technology makes it possible to create permanent records and conducts multiple data analyses.
Q is asphericity, a parameter that is used to specify the type of conicoid. For a perfect sphere this parameter takes the value of zero (Q=0), for an ellipsoid with the major axis in the X-Y plane (oblate surface) the asphericity is positive (Q>0), for an ellipsoid with the major axis in the Z axis (prolate surface) asphericity is negative (-1
Fig. 4.3: Videokeratography system
Shape of the Normal Cornea The anterior corneal surface is a refractive surface characterized by an almost spherical shape. The human cornea is not a perfect sphere and is usually assumed to have a conic section. This model could be represented in a simple way by means of following equation: X2 + Y2 + (1 + Q)Z2 – 2RZ = 0 The Z axis is the axis of revolution of the conic, R is the radius at the corneal apex, and
Fig. 4.4: Different types of conic section
Several studies have shown that the anterior corneal configuration tends to be prolate, i.e. the cornea progressively flattens out towards periphery by 2-4 diopters (Fig. 4.5).The asphericity of the normal cornea, depending on different studies, ranges from -0.26 to -0.11.
Corneal Topography
Fig. 4.5: Corneal profile in principal meridians
This tendency to flatten towards periphery can be detected in the topographic map. Toward the periphery, dioptric power appears to decline, and the nasal area flattens more than the temporal area (Fig. 4.6). This could be helpful in distinguishing right eye topography from the left eye topography. The topographic patterns of the two corneas of the same individual often show mirrorimage symmetry.
Corneal topographic patterns (Fig. 4.7) have been studied in normal eyes and the following shapes have been found: round (23%), oval (21%), symmetric bow-tie typical for regular astigmatism (18%), asymmetric bow-tie (32%), and irregular astigmatism (7%). In the round and oval shapes there is an area of uniform dioptric power close to 43 diopters (D) in the center of the cornea. The bow-tie configuration
Fig. 4.6: Corneal topography in a normal right eye. There is a flattening towards the periphery, more pronounced at the nasal area
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A
B Figs 4.7A and B: A Oval topographic pattern, B Bow-tie pattern that shows an against-the-rule astigmatism
reflects the existence of corneal astigmatism. Depending on the position of the axes, corneal astigmatism is defined as against-the-rule (the
steepest axis is horizontal), with-the-rule (the steepest axis is vertical), or oblique (the steepest axis is near the meridian angles of 45º or 135º).
Corneal Topography
C
D Figs 4.7C and D: Normal corneal topographic patterns: C With-the-rule astigmatism, D Oblique astigmatism
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Fundamentals and Technological Approaches to Corneal Topography Specular Reflection Techniques Placido Disk System A Placido disk system consists of a series of concentric illuminated rings or mires that are reflected off of the cornea and recorded by videocomputerized systems. Currently, several companies manufacture instruments called videokeratoscopes that picture corneal shape based on the Placido disk method, and, in fact, this approach has been the most clinically and commercially successful. Two types of Placido targets have been used: 1. Large diameter target (disk-shaped), this is less sensitive to misalignment due to a long working distance, but there can be a loss of data due to interference by the patient’s brow and nose. 2. Small diameter target (cone-shaped), this is designed for a short working distance and can be influenced by automatic alignment and focusing or compensation of misalignment for accuracy. It does not present data loss due to shadows. Limitations of placido disk system: Placido-based apparatus creates a three-dimensional system by making geometric assumptions about the cornea since the apparatus does not measure corneal surface directly. These assumptions are not accurate for irregular and aspheric corneas. The reflection technique depends on the integrity and normality of the tear layer.
Interferometric Method-based Systems In essence, a reference surface (or its hologram) is compared to the tested surface, the corneal surface, and interference fringes are produced as a result of differences between the two shapes,
which can be interpreted as a contour map of surface elevations. Interference techniques are used in the optical industry to detect lens and mirror aberrations of subwavelength dimensions. High accuracy is theoretically possible, but clinical use has not been very wide-spread as yet.
Moire Deflectometry-based Systems The deflections of the rays reflected off the corneal surface are analyzed to build up a surface elevation map.
Diffuse Reflection Techniques The following three methods, Moire fringes, Rasterography, and the Fourier transform profilometry method, modify the natural specular condition of the anterior surface of the cornea transforming it into a diffusing surface instilling fluorescein in the eye. A structured light pattern (grid or raster) is projected onto the cornea. Due to the topography of the cornea, if the fringes are observed from a point that is different from that of the projecting point, a distorted fringe pattern is observed. These stereo-triangulation methods locate the cornea in space (x, y and z coordinates) and can reconstruct corneal shape. The only difference between the three methods is the way in which data is processed and analyzed.
Techniques using Scattered Light-slitbased Systems When the slit image is on the cornea, it splits into a specular reflection and a refracted beam that penetrates the corneal surface and is scattered by the tissue of the cornea. An image of this scattered light within the corneal tissue is captured by an imaging system, which uses triangulation to measure the elevation of the
Corneal Topography anterior and posterior corneal surfaces with respect to a reference plane. ORBSCAN II TM (Fig. 4.8) uses placido disk and slit-based systems to obtain 40 slit-images of the cornea. These images are captured over one second and recorded.
Fig. 4.9: Photokeratoscope raw image
Fig. 4.8: Orbscan II system
How to Interpret a Corneal Topography Map? Accurate interpretation of corneal shape using color-coded topographic maps is difficult and confusing for many clinicians, even experienced cornea specialists. In order to obtain the best performance in the analysis of corneal maps, several important points must be taken into consideration. It is critical to check the raw image first. Then it is necessary to focus on the scale and step intervals with which the color-coded topographic map is built up. It is also important to review different topographic displays, especially when evaluating irregular or postsurgery corneas.
Raw Photokeratoscope Image The photokeratoscope image displays the placido’s rings projected onto the cornea (Fig.
4.9). When considering a color-coded map, the clinician must check that the unprocessed data upon which it is based, are reliable. If the videokeratoscope image is irregular, data cannot be processed by the instrument in a meaningful way. Thus, for Placido disk-based computerized videokeratoscopes, the videokeratoscope image should not be ignored. In fact, it is recommended to check this map before referring to any of the other topographic displays, and to go back to it when there are any doubts regarding the accuracy of the displayed data. This image provides important information for assessing tear film quality, mire centering on the cornea, lid opening, or the causes of local irregularities, and other artefacts. If the device used displays computer tracking of the placido mires it is important to rule out tracking errors. Devices that rely only on scanning slittechnology to analyze the anterior corneal surface lack valuable information provided by the raw videokeratoscope image. Whether the resulting map is based on reliable primary data or not is impossible to verify without the raw image. Some instruments identify regions of uncertainty, showing mire distortions that cannot be reliable, by leaving blank areas on the colorcoded map. Other instruments merely extrapolate
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Color-Coded Scales The shape of a cornea can be measured and represented by color-coded maps in which a given color indicates a different curvature or
elevation. The usual color spectrum for corneal powers shows near-normal power as green, lower than-normal power as cool colors (blues) and higher than normal powers as warm colors (reds). Most topographers offer absolute as well as normalized scales to allow the clinician to customize the information for maximal clinical value (Fig. 4.10):
A
B Figs 4.10A and B: Corneal topography map represented using a normalized scale A, an absolute scale B
Corneal Topography i. Normalized scale (variable scale) uses a given color for different curvatures or elevations on each cornea analyzed, depending on the range for that particular cornea, determined by its flattest and steepest values. These maps are difficult to interpret and can lead to an incorrect diagnosis since they may magnify subtle changes in corneal surface if the scale is too narrow, or minimize large distortions if the scale is too wide. In addition, color recognition, one of the primary clues used to interpret on corneal topography, is lost with a variable scale, since it uses different colors for different eyes. ii. Absolute scale (fixed scale) uses the same color for the same curvature or elevation no matter which eye is examined. However, there are many different absolute scales since the examiner can choose different variables such as range or step size (intervals in color changes). For the specified scale, however, each display will use the same colors, steps and range. In order to facilitate comparisons
over time and between patients, it is recommended to stick with a given fixed scale for routine examinations and to change the scale in the particular cases in which this becomes necessary. As an example the popular Klyce/Wilson scale ranges from 28 D to 65 D in equal 1.5 D intervals. Currently, there is no consensus as to the best absolute scale, but in general, dioptric scales with intervals smaller than 0.5 D are not clinically useful and provide details that are not relevant and may complicate map interpretation. For corneas with large dioptric ranges, for instance in advanced keratoconus intervals greater than 0.5 D are recommended. Regarding scales for elevation maps, elevation steps of approximately 5 microns appear to be clinically useful. As mentioned previously, color pattern recognition makes it possible to identify common topographic patterns such as the corneal cylinder, keratoconus (local area of inferonasal steepening) or pellucid marginal degeneration (butterflypattern or inferior arcuate steepening), as well
Fig. 4.11: Corneal topography after myopic LASIK
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Topographic Displays: Corneal Maps Different types of maps are used for displaying curvatures, elevations and irregularities of the cornea.
Axial Map (Sagital Map)
Refractive Map The refractive map displays the refractive power of the cornea, which is calculated based on Snell’s law of refraction, assuming optical infinity (Fig. 4.12C). This map correlates corneal shape to vision, and is useful in understanding the effects of surgery.
Elevation Map
Although this is the original and most commonly used map, its values only provide a good approximation for the paracentral cornea (Fig. 4.12A). The axial map measures the radius of curvature for a comparable sphere (with the same tangent as the point in question) with a center of rotation on the axis of the videokeratoscope. Localized changes in curvature and peripheral data are poorly represented, because of the spherical bias of the reference optical axis. However, newer algorithms in some devices (e.g. arc-step method) have improved the accuracy of curvature measurements in the peripheral region.
The elevation map displays corneal height or elevation relative to a reference plane (Fig. 4.12D), with a presumed assumption of the shape, which may be the best-fit sphere, best-fit asphere, average corneal shape, or even based on preoperative data. Points above the reference surface are positive (hot colors) and points below the reference surface are negative (cool colors). This map shows the three-dimensional (3D) shape of the cornea and is useful in measuring the amount of tissue to be removed by a refractive surgical procedure, assessing postoperative visual problems, or planning and/or monitoring surgical procedures.
Local Tangential Curvature Map (Instantaneous Map)
Difference Map
The tangential map displays the tangential/ instantaneous/local radius of curvature or tangential power, which is calculated by referring to the neighboring points and not to the axis of the videokeratoscope (Fig. 4.12B). Tangential maps reflect local changes and peripheral data better than axial maps. They are very useful in detecting local irregularities, corneal ectactic diseases, or surgically induced changes. For example, in keratoconus corneas with a displaced apex, tangential maps are less influenced by peripheral distortion, and can determine the position and extent of the cone more precisely than axial maps.
The difference map displays the changes in certain values between two maps (Fig. 4.13). It is used to monitor any type of change, such as recovery from contact lens-induced warpage or surgery-induced changes.
Relative Map The relative map displays some values by comparing them to an arbitrary standard (e.g. sphere, asphere, or normal cornea) and a specific mathematical model. This map enhances or magnifies unique features of the cornea being examined.
Corneal Topography
Fig. 4.12A: Axial map
Fig. 4.12B: Instantaneous map
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Fig. 4.12C: Refractive map
Fig. 4.12D: Elevation map Figs 4.12A to D: Different kind of topography maps for the same cornea
Corneal Topography
Fig. 4.13: Difference map
Irregularity Map (Surface Quality Maps) The irregularity map uses the same technique as the elevation map, but takes as a reference surface the best-fit spherocylindrical toric surface. The difference between the actual surface and the theoretical surface represents that part of the cornea that cannot be optically corrected. Like refractive power maps, the irregularity map only has clinical meaning when considering the values over the pupillary area. Numerous other displays, including three dimensional maps (Fig. 4.14) and astigmatic vector analysis are available but less commonly used.
Fig. 4.14: Three-dimensional elevation map
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Fig. 4.15: Bad image for topography analysis due to lack of focus
A Good Topography Examination Corneal topography is a non-invasive imaging technique for mapping the surface curvature of the cornea. The specific method varies depending on the device used, but some aspects are common. The patient is seated facing a bowl containing an illuminated pattern which is focused on the anterior surface of his cornea. The reflected pattern is analyzed by a computer that calculates the shape of the cornea by means of different graphic formulae. Although computer programs are created to be very accurate, they can not recognize, and account for, every problem. Critical points for precise measurement are accurate alignment, centering and focusing (Fig. 4.15). They depend on the ability of the examiner to take a good measurement. Another potential source of error is tear film irregularities, for example focal flattening over a dry patch. These may be most easily identified on the raw image. Tear film breakup causes mistracking of the mires and artefacts in the topography pattern and apparently look like significant irregularities (Fig. 4.16). These corneal irregularities could suggest a corneal pathology, such as keratoconus, and result in wrong diagnosis (Fig. 4.17). To avoid disturbing the tear film, corneal topography should be performed before adminis-
Fig. 4.16: Distortion of the placido rings because of tear film breakup
tering dilating drops and taking intraocular pressures. In addition, one must avoid artefacts induced by the nose or the eyelids which can lead to a loss of information in certain areas (Fig. 4.18). These errors are transformed into black areas or areas without data on the topographic map. Correct positioning of the head, eyes and eyelid opening should be ensured to avoid these problems.
Quantitative Descriptors of Corneal Topography: Corneal Indexes Color-coded maps provide a rapid visual method for clinical diagnosis, but do not supply numerical values that can be used for clinical management. Several corneal indexes describe different features of corneal topography quantitatively and are of great aid in contact lens fitting, for improved assessment of the optical quality of the corneal surface, and can be used in artificial intelligence systems to aid in the diagnosis of corneal shape anomalies. Some of the most useful corneal indexes are described below:
Corneal Topography
Fig. 4.17: Topographic irregularities and patches on the map because of a tear film instability
A
B Figs 4.18A and B: Loss of information of certain areas of the cornea due to eyelids not opened enough A, and due to nose B
Basic Topographic Indexes Simulated Keratometry Reading (SimK values) This is a simple descriptor of corneal topography that provides the power and axes of the steepest
and flattest corneal curvatures just as K1 and K2 are provided by the classic keratometer, to which it correlates well. The cylinder is calculated from the difference between SimK1 and SimK2. Its common uses are: a. Contact lenses fitting
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Minimum Keratometry Reading (MinK) This is the minimum meridional power from rings 7, 8 and 9. The average power and axis are displayed.
Corneal Eccentricity Index (CEI) This index estimates the eccentricity of the central cornea, and is calculated by fitting an ellipse to the corneal elevation data. A positive value is for a prolate surface, negative value for an oblate surface (for example flattened corneas after myopic refractive surgery), and zero value for a perfect sphere. Normal central corneas are prolate, meaning they are steeper in the center than in the periphery, and tend to be around 0.30. This value is used for fitting contact lenses.
Average Corneal Power This is the area-corrected average of corneal power in front of the pupil. It usually corresponds to the spherical equivalent of the classic keratometer, except after decentered refractive surgery. It may be helpful in determining central corneal curvature when calculating the appropriate intraocular lens power.
Surface Regularity Index and Potential Visual Acuity Surface regularity index (SRI) measures the regularity of the corneal surface that correlates with the best spectacle-corrected visual acuity assuming the cornea to be the only limiting factor. This index adds up the meridional mire-to-mire power changes over the apparent pupil entrance. The SRI value increases with increase in the
irregularity of the corneal surface, and its normal value is less than 1.0. It measures optical quality. Potential visual acuity (PVA) is a range of the expected visual acuity that is achievable based on the corneal topography and can be calculated based on SRI.
Surface Asymmetry Index Surface asymmetry index (SAI) is a descriptor of the corneal surface that measures the difference between points located 180º apart in a great number of equally spaced meridians. Therefore, as the cornea becomes less symmetric, the index differs more from 0. Other indexes, some of which will be mentioned below, have been developed, and might be exclusive to one particular topographer. The clinician should evaluate the meaning, utility and validity of each index since some indexes have been tested in peer-reviewed literature while others have not.
Screening Tools and Artificial Intelligence Programs (Neural Networks) for Classification and Auto Diagnosis As mentioned previously, even for an experienced person, interpretation of topography can be difficult, particularly when trying to differentiate the early stages of a disease from a normal cornea (suspected keratoconus), or when trying to differentiate between two similar conditions (contact lens warpage vs. early keratoconus). Several mathematical algorithms have been developed to help solve this problem, with high sensitivity and specificity. Rabinowitz and Mc Donnell developed the first numerical method for detecting keratoconus using only topographic data. They use the I-S value, which measures the differences between the superior and inferior paracentral corneal
Corneal Topography regions, the central corneal power (Max K), and the power difference between both eyes. Their study presented that patients with keratoconus (suspect) had central corneal power > 47.2 D or I-S > 1.4 while those with clinical keratoconus had central corneal power > 47.8 D or I-S > 1.9. However, using only these simple measurements for a diagnosis could create specificity problems. To solve the specificity problem, the new strategy must be able to detect and consider the unique characteristics of keratoconus maps, such as local abnormal elevations. The Keratoconus Prediction Index, developed by Maeda et al, is calculated from the Differential Sector Index (DSI), the Opposite Sector Index (OSI), the Center/Surround Index (CSI), the SAI, the Irregular Astigmatism Index (IAI), and the percent Analyzed Area (AA). This method partially overcomes the specificity limitation. Maeda et al also developed the neural network model, based on artificial intelligence. It is a much more sophisticated method for classifying corneal topography and detecting different corneal topographic abnormalities; it employs indexes that were empirically found to capture specific characteristics of the different corneal pathologies, including keratoconus. Further modifications in neural network approach developed by Smolek and Klyce supposedly produce 100% accuracy, specificity and sensitivity in diagnosing keratoconus.
Corneal Aberrometry: Fundamentals and Clinical Applications Whenever a point object does not form a point image on the retina, as it should be in an ideal optical system, one encounters an optical aberration. Although one may feel that he is measuring the total refractive error, when refracting a patient, one is actually only
considering two components of a whole host of refractive components in the optics of the eye. However, these two components — sphere and cylinder do constitute the main optical aberrations of an eye. Even in a normal eye with no subjective need for refraction, optical aberrations can be detected. Since the cornea has the highest refractive power, more than 70% of the eye’s refraction, it is the principal site of aberrations, although the lens and the tear film may also contribute to aberrations.
Fundamentals Measuring Total Wavefront Aberration It is possible to express ideal image formation by means of waves. An ideal optical system will provide a spherical converging wave centered at the ideal point image. However, in practice, the resulting wavefront, differs from this ideal wavefront. The deviation from this ideal wavefront is called wavefront aberration, and the more it differs from zero, the more the real image differs from the ideal image and the worse the image quality. Ocular wavefront sensing devices use four main technologies to determine the resulting or output wave: 1. The Shack-Hartmann method is the most widely used and is inspired by astronomy technology. It consists of analyzing the wave emerging from the eye after directing a small low energy laser beam. This reflected wave is divided by means of a series of small lenses (lenslet array) which generates focused spots. The position of spots is recorded and compared to the ideal one 2. The Tscherning technique uses typically a grid that is projected onto the retina. The distortion of the pattern is analyzed and used to calculate the wavefront aberration of the eye.
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Measuring Corneal Wavefront Aberration It is known that 80% of all aberrations of the human eye occur in the corneal area and only 20% of aberrations originate from the rest of the ocular structures. The effect of corneal aberrations is especially important after corneal surgery such
as keratorefractive procedures or penetrating keratoplasty, since the anterior corneal surface is the only one modified. The corneal wavefront aberration, which is the component of the total ocular wavefront aberration attributed to the cornea, can be derived from the corneal topographic height data. Specifically, the calculation of wavefront aberrations is performed by expanding the anterior corneal height data into a set of orthogonal Zernike polynomials (Fig. 4.19).
Zernike Polynomials For a quantitative description of the wavefront shape there is a need for a more sophisticated analysis than conventional refraction, as the latter only divides the wavefront in two basic terms:
Fig. 4.19: Corneal wavefront analysis derived from height topography data
Corneal Topography
Fig. 4.20: Zernike polynomial expansion
sphere and cylinder. One can obtain more information by breaking down the wavefront into terms which are clinically meaningful, besides the sphere and the cylinder. For this purpose, a standard equation has been universally accepted by refractive surgeons and vision scientists, known as Zernike polynomials. Zernike polynomials are equations which are used to fit the wavefront data in a three dimensional way. The wavefront function is decomposed into terms that describe specific optical aberrations such as spherical aberration, coma, etc. (Fig. 4.20). Each term in the polynomial has two variables, ρ (rho) and θ (theta), where ρ is the normalized distance of a specific point from the center of the pupil, and θ is the angle formed between the imaginary line joining the pupillary center with the point of interest and the horizontal. According to that, we can imagine that aberrations are strongly influenced by pupil size, and, therefore, aberrometric measurements should be related to the diameter of the patient’s pupil. Zernike terms (Znm) are defined using a double index notation: a radial order (n) and an angular
frequency (m). When talking about first, second, third order aberrations we point to indicate the radial order (n). Each radial order involves n + 1 term. There are an infinite number of Zernike terms that can be used to fit an individual wavefront. However, for clinical practice, terms up to the 4th radial order are usually considered: 1. Zernike terms below third order can be measured and corrected by conventional optical means. The first order term, the prism, is not relevant to the wavefront as it represents tilt and is corrected using a prism. The second order terms represent low order aberrations that include defocus (spherical component of the wavefront) and astigmatism (cylinder component). Wavefront maps that measure only defocus and astigmatism can be perfectly corrected using spectacles and contact lenses. 2. After the second radial order comes high order aberrations. These are not measured by conventional refraction or auto refraction. The aberrometer is the only method available that can quantify these complex kinds of distortions. 3. Third order terms describe coma and trefoil defects. 4. Fourth order terms represent tetrafoil, spherical aberration and secondary astigmatism components. Because spherical and coma aberrations refer to symmetrical systems and the eye is not rotationally symmetrical, the terms spherical-like and coma-like aberrations are normally used (Fig. 4.21).
Wavefront Maps Wavefront map describes the optical path difference between the measured wavefront and the reference wavefront in microns at the pupil entrance. The wavefront error is derived mathematically from the reconstructed wavefront
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Fig. 4.21: Spherical-like and coma-like wavefront aberration maps
by one of the techniques described above. It is plotted as a 2D or 3D map for qualitative analysis in a similar fashion to corneal topography maps. In wavefront error maps, each color represents a specific degree of wavefront error in microns (Fig. 4.22) and like corneal topography maps, it is necessary to consider the range and the interval of the scale.
Optical and Image Quality In order to evaluate the impact of aberrations on visual quality following quantitative parameters have been defined (Fig. 4.23): Peak to valley error (PV error): This is a simple measure of the distance from the lowest point to the highest point on the wavefront and is not
Corneal Topography
Fig. 4.22: Corneal wavefront aberration maps that include all kind of aberrations including low and high order
the best measurement of optical quality since it does not represent the extent of the defect.
results and it is linked to the RMS by the Maréchal formula.
Root mean square error (RMS error): This measure is by far the most widely used. In a simple way, the RMS wavefront error is a statistical measure of the deviation of the ocular or corneal wavefront from the ideal (Table 4.1). In other words, it describes the overall aberration and indicates how bad individual aberrations are.
Point spread function (PSF): This is the spread function observed on the retina when the object is a point in infinity. PSF measures how well one object point is imaged on the output plane (retina) through the optical system. In the eye, small pupils (approximately 1 mm) produce diffraction-limited PSFs, because of the pupil border. In larger pupils, aberrations tend to be the dominant source of degradation.
Strehl ratio: This represents the ratio of the maximum intensity of the actual image to the maximum intensity of the fully diffracted limited image, both being normalized to the same integrated flux. This ratio measures optical excellence in terms of theoretical performance
Modulation transfer function, Phase transfer function and Optical transfer function: Sinusoidal gratings greatly simplify the study of optical systems, because irrespective of the amount of eye aberra-
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Fig. 4.23: Visual quality summary obtained with the CSO topographer. It is possible to visualize the wavefront map (gray scale), Strehl ratio, PSF and MTF function
TABLE 4.1:
REFERENCE VALUES FOR CORNEAL ABERRATIONS IN THE NORMAL POPULATION
Pupil (mm)
Total RMS
Astigmatism RMS
Spherical aberration
Coma RMS
Sphericallike RMS
Comalike RMS
3
0.19 ± 0.07
0.14 ± 0.08
0.04 ± 0.03
0.05 ± 0.03
0.07 ± 0.02
0.09 ± 0.03
5
0.53 ± 0.21
0.43 ± 0.24
0.15 ± 0.05
0.14 ± 0.08
0.18 ± 0.05
0.20 ± 0.08
7
1.26 ± 0.43
0.92 ± 0.53
0.52 ± 0.17
0.42 ± 0.23
0.57 ± 0.16
0.52 ± 0.22
RMS: root mean square, Coma primary coma: terms Z3±1, Spherical aberration primary spherical aberration: term Z40 Spherical-like: terms fourth and sixth order, Coma-like: terms third and fifth order Reference: Vinciguerra P, Camesasca FI, Calossi A. Statistical analysis of physiological aberrations of the cornea. J Refract Surg 2003; 19 (Suppl): S265-9.
Corneal Topography tions, sinsusoidal grating objects always produce sinusoidal grating images. Consequently, there are only two ways that an optical system can affect the image of a grating, by reducing contrast or by shifting the image sideways (phase-shift). The ability of an optical system to faithfully transfer contrast and phase from the object to the image at a specific resolution are called respectively the modulation transfer function (MTF) and the phase transfer function (PTF). The eye’s optical transfer function (OTF) is made up of the MTF and the PTF. A high-quality OTF is, therefore, represented by high MTF and low PTF.
Clinical Applications Aberrometers allow practitioners to gain a better understanding of vision by measurement of high order aberrations. These aberrations reflect a refractive error that is beyond conventional spheres and cylinders. There may be a large group of patients whose best corrected visual acuity (BCVA) may improve significantly on removal of the optical aberrations and this new refractive entity has been called aberropia. Reduced optical quality of the eye produced by light scatter and optical aberrations may actually be the root cause of blurred vision associated with dry eye syndrome and tear film disruption. Measurement of these aberrations can also be helpful in keratoconus, orthokeratology, post graft fitting, irregular astigmatism or when refractive surgery has reduced the patient’s optical quality. Customized ablations are the future step in laser technology that should address not only spherical and cylindrical refractive errors (loworder aberrations), but also high-order aberrations such as trefoil and coma (Fig. 4.24). Thus, vision can be optimized to the limits determined by pupil size (diffraction) and retinal structure and function.
Clinical Uses of Corneal Topography Pathological Cornea Corneal topography is a very important tool in the detection of corneal pathologies, especially ectatic disorders. Screening for these anomalies or their potential development is a critical point in preoperative evaluation for refractive surgery. Keratorefractive procedures are contraindicated in these abnormal corneas.
Keratoconus Keratoconus is characterized by a localized conical protrusion of the cornea associated with an area of corneal stromal thinning, especially at the apex of the cone. The typical associated topographic pattern is the presence of an inferior area of steepening (Fig. 4.25). In advanced cases, the dioptric power at the apex is at or above 55 D. In a small group of patients, the topographic alterations may be centered at the central cornea. In these cases there may be an asymmetric bowtie configuration, and normally the inferior loop is larger than the superior loop (Fig. 4.26). Keratoconic corneas have three common characteristics that are not present in normal corneas: 1. An area of increased corneal power surrounded by concentric areas of decreasing power 2. An inferior-superior power asymmetry 3. A skewing of the steepest radial axes above and below the horizontal meridian. Keratoconus suspects are problematic. They may signal impending development of a clinical keratoconus, but they may also represent a healthy cornea. The lack of ectasia in the fellow cornea does not indicate that the keratoconus suspect will not progress to true keratoconus. In these cases the ideal management is close follow-up of the signs of keratoconus in order to check on their stability, and a thorough analysis of the videokeratographic indexes.
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A
B Figs 4.24A and B: Customized ablation designed according to corneal aberration for the correction of aberrations induced by a decentered ablation. There is a large amount of coma: axial map A and customized ablation designed B with the ORK-CAM software (Schwind)
Pellucid Marginal Degeneration Pellucid marginal degeneration is characterized by an inferior corneal thinning between 4 and 8 O’clock positions above a narrow band of clear thinned corneal stroma. The ectasia is extremely peripheral and it presents a crescent-shaped morphology. This pattern has a classical
“butterfly” appearance that results in a flattening of the vertical meridian and a marked againstthe-rule irregular astigmatism (Fig. 4.27).
Keratoglobus Keratoglobus is a rare bilateral disorder in which the entire cornea is thinned out most markedly
Corneal Topography
Fig. 4.25: Keratoconus topography pattern
near the corneal limbus, in contrast to the localized central or paracentral thinning of keratoconus. It is very difficult to obtain reliable and reproducible measurements in these cases due to the high level of irregularity and the poor quality of the associated tear film. Reliable topographic examinations show an arc of
peripheral increase in corneal power (steepening) and a very asymmetrical bow- tie configuration.
Terrien’s Marginal Degeneration In Terrien´s marginal degeneration there is a flattening over the areas of peripheral thinning.
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Fig. 4.26: Keratoconus with an asymmetric bow-tie configuration
When thinning is restricted to the superior and/ or inferior areas of the peripheral cornea, there is a relative steepening of the corneal surface approximately 90 degrees away from the midpoint of the thinned area. Therefore, high against-the-rule or oblique astigmatism is a
common feature, as this disorder involves more frequently the superior and/or inferior peripheral cornea. If the area of thinning is small or if the disorder extends around the entire circumference of the cornea, central cornea may remain relatively spared with a spherical configuration.
Corneal Topography
Fig. 4.27: Pellucid marginal degeneration topography pattern
Fig. 4.28: Corneal astigmatism induced by a pterygium
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Diagnostic Procedures in Ophthalmology Pterygium Pterygium is a triangular encroachment of the conjunctiva onto the cornea usually near the medial canthus. When the lesion continues to grow out onto the cornea, it could lead to a high degree of astigmatism. When the growth of pterygium is about 2 mm or more, a flattening of the cornea at the axis of the lesion occurs (Fig. 4.28). This produces a marked with-the-rule astigmatism, even of more than 4 D. The evolution of the pathology and the surgical outcome could be monitored by changes in corneal topography.
Postoperative Cornea in Refractive Surgery Keratorefractive procedures attempt to alter the curvature of the central and mid-peripheral cornea, and usually have a minimal effect on the corneal periphery. The area in which the curvature is modified is called the optical zone. This tends to be surrounded by a small zone of altered curvature before normal cornea is
reached at the periphery. The corneal effect of surgery could be determined by analyzing the difference map between the preoperative and postoperative measurements.
Postradial Keratotomy (RK) Radial keratotomy (RK) corrects myopia by placing a series of radial incisions (nearly full corneal thickness) leaving a central clear zone (optical zone). These incisions cause a flattening of the central cornea due to retraction of the most anterior collagen fibers and the outward pressure of the intraocular force. This area of flattening is surrounded at an approximately 7 mm zone by a bulging ring of steepening called the paracentral knee. This increases asphericity and corneal irregularity. A very typical finding in these corneas is a topographic pattern with a polygonal shape. Depending on the number of incisions made, squares, hexagons or octagons can be seen. The angles of the polygons correspond closely to the central ends of the incisions (Fig. 4.29).
Fig. 4.29: Polygonal pattern in a postradial keratotomy cornea
Corneal Topography Postastigmatic Keratotomy (AK) Astigmatic keratotomy is a simple modification of the radial keratotomy that is used to correct astigmatism. Rather than placing incisions radially on the cornea, incisions are strategically placed on the steepest meridian. The incisions induce a flattening in that meridian, but provoke steepening in the perpendicular meridian, in a process called coupling. Coupling results from the presence of intact rings of collagen lamellae that run circumferentially around the base of the cornea. With the surgery, these rings become oval in the operated meridian and transmit forces to the untouched meridian. The stigmatic change achieved is the sum of the flattening in one meridian and the steepening in its perpendicular meridian.
Postphotorefractive Keratotomy Photorefractive keratotomy (PRK) is a procedure which uses a kind of laser (excimer laser, a cool
pulsing beam of ultraviolet light) to reshape the cornea. To correct myopia, the excimer laser flattens the central cornea by removing tissue in that area. However, the optical zone needs to be steepened to correct hyperopia. This is achieved by removing an annulus of tissue from the mid-periphery of the cornea. The topographic pattern in myopic corrections shows a flattening of the central cornea, oblate profile (Fig. 4.30). The treatment zone is usually easily delineated by the close proximity of adjacent contours at its edge. Hyperopic corrections have a pattern of central steepening surrounded by a ring of relative flattening at the edge of the treatment zone (more prolate profile) (Fig. 4.31). In astigmatic treatment, the treatment zone is oval. Inadequate ablations during surgery can be detected postoperatively by analyzing the resulting corneal topography. Decentrations can only be identified by a relatively asymmetric localization of the treatment area (Fig. 4.32). Other
Fig. 4.30: Topographic pattern after a myopic ablation
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Fig. 4.31: Topographic pattern after a hyperopic ablation
Fig. 4.32: Pattern of decentered myopic ablation
Corneal Topography
Fig. 4.33: Central island after myopic photoablation
complicated patterns that may lead to severe vision disturbances are the presence of focal irregularities or central islands (Fig. 4.33) produced by an inhomogeneous laser beam or an irregular process of corneal healing.
Postlaser in situ Keratomileusis Postlaser in situ keratomileusis (LASIK) is an excimer laser procedure like PRK, but in this case tissue is ablated under a superficial corneal flap in order to minimize the influence of the epithelium. The topographic patterns for myopic and hyperopic corrections are the same as in PRK (Figs 4.30 and 4.31). Although the ablation is covered by a flap of corneal tissue, surface irregularities and central islands may still occur. Decentration may also occur in a LASIK ablation, depending on the patient’s ability to maintain eye fixation during surgery (Fig. 4.34). Epithelial in-growth at the periphery of the flap-stromal interface produces an area of steepening surrounded by an area of marked flattening
making the corneal surface more irregular (Fig. 4.35).
Postlaser Thermal Keratoplasty In laser thermal keratoplasty (LTK), a Holmium laser is used to heat corneal stromal collagen in a ring around the outside of the pupil. The heat causes the tissue to shrink, producing a zone of localized flattening centered on the spot, and a surrounding zone of steepening. This bulging effect of the central cornea makes it possible to correct hyperopia. The typical topographic pattern shows the central corneal steepening and a ring of flattening overlying the spots.
Postintrastromal Corneal Rings Implantation Intrastromal rings are small segments or rings, made of a plastic-like substance, that are inserted into the periphery of the cornea to correct small degrees of myopia or hyperopia. They act as spacers and by changing the orientation of the
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A
B Figs 4.34A and B: Topographic patterns of LASIK decentered ablations after myopic treatment A and after hyperopic treatment B
Corneal Topography
A
B Figs 4.35A and B: Topographic analysis in a post-LASIK cornea with an epithelial in-growth at the inferonasal area: placido rings image A, and axial map B
collagen lamellae, depending on their shape and position, flatten or steepen the central cornea. Intrastromal rings could also be used to reduce the corneal steepening and astigmatism associated with keratoconus (Fig. 4.36).
Postkeratoplasty Keratoplasty topographies exhibit a wide variety of patterns, depending on the type of keratoplasty
performed, the quality of the surgical procedure, whether sutures are still in place in the cornea, and the time elapsed after the procedure. Sutures usually induce a central bulge in the corneal graft and its removal results in a decrease of the astigmatic component. The prolate configuration after keratoplasty is the most frequent pattern with a high degree of irregularity (Fig. 4.37). There can be multiple regions of abnormally high or low power, or both simultaneously in
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Fig. 4.36: Management of keratoconus by intrastromal rings
the map. Irregular astigmatism over the entrance pupil may be detrimental to optimum visual acuity in the keratoplasty patient.
Contact Lens-induced Corneal Warpage or Molding Corneal warpage is characterized by topographic changes in the cornea following contact lens wear (most frequently in wearers of hard or RGP lenses) as a result of the mechanical pressure exerted by the lens. There are at least 4 different forms of noticeable topography change that usually
occur mixed with one another: (i) peripheral steepening, (ii) central flattening, (iii) furrow depression, and (iv) central molding or central irregularity (Fig. 4.38). Inferior corneal steepening (pseudokeratoconus) is caused by a superiorly riding contact lens that flattens above the visual axis with an apparent steepening below. The topographic image could appear similar to keratoconus, but both conditions are easily differentiated. In corneal warpage, the shape indexes do not indicate any keratoconic condition, and the flat K is not as steep as in keratoconus.
Corneal Topography Other Uses of Corneal Topography
Fig. 4.37: Topographic pattern after penetrating keratoplasty
Corneal topography is a diagnostic tool, but it is also essential before all refractive procedures, to enable the surgeon to understand the refractive status of an individual eye, and plan the optimum refractive treatment. The corneal topography is also used for the following purposes: 1. To guide removal of tight sutures after corneal surgery (keratoplasty, cataract surgery, etc.) that are causing steepening of the cornea (Fig. 4.39). 2. To help in the designing the astigmatic keratotomy.
Fig. 4.38: Corneal warpage
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Fig. 4.39: Superior corneal steepening caused by a tight suture
3. To guide contact lens fitting: Selection of the probe lens and design of the lens. 4. To calculate the keratometry values for the calculation of the required power of an intraocular lens for implantation. This is an important issue in corneas that have undergone refractive surgery, because it is more difficult to estimate the real keratometric values in order to avoid over or under corrections. 5. To evaluate the effect of a keratorefractive procedure.
Bibliography 1. Ambrosio R Jr, Klyce SD, Wilson SE. Corneal topographic and pachymetric screening of keratorefractive patients. J Refract Surg 2003;19: 24-29.
2. Bogan SJ, Waring GO, Ibrahim O, Drews C, Curtis L. Classification of normal corneal topography based on computer-assisted videokeratography. Arch Ophthalmol 1990;108:945-9. 3. Boyd BF, Agarwal A, Alio JL, Krueger RR, Wilson SE. (Eds). Wavefront analysis, aberrometers and corneal topography. Highlights of Ophthalmology, 2003. 4. Cairns G, McGhee CNJ. Orbscan computerized topography: Attributes, applications, and limitations. J Cataract Refract Surg 2005;31:20520. 5. Corbett M, O’Brart D, Rosen E, Stevenson R. Corneal topography: principles and applications. BMJ Publishing Group, 1999. 6. Corneal Topography. American Academy of Ophthalmology. Ophthalmology 1999;106:162838. 7. Courville CB, Smolek MK, Klyce SD. Contribution of ocular surface to visual optics. Exp Eye Res 2004;78:417-25. 8. Dabezies OH, Holladay JT. Measurement of corneal curvature: keratometer (ophthalmo-
Corneal Topography
9. 10.
11. 12. 13. 14. 15.
16. 17.
meter). In Contact Lenses: the CLAO Guide to Basic Science and Clinical Practice. Kendall/ Hunt Publishing Co, 1995;253-89. Hamam H. A new measure for optical performance. Optom Vis Sci 2003; 80:174-84. Joslin CE, Wu SM, McMahon TT, Shahidi M. Higher-order wavefront aberrations in corneal refractive therapy. Optom Vis Sci 2003;80:80511. Karabatsas CH, Cook SD. Topographic analysis in pellucid marginal corneal degeneration and keratoglobus. Eye 1996;10:451-55. Kaufman H, Barron B, McDonald M, Kaufman S. Companion handbook to the cornea. London, Butterworth Heinemann,1999. Klyce SD. Corneal topography and the new wave. Cornea 2000;19:723-29. Krachmer JH, Mannis MJ, Holland EJ (Ed). Cornea. Surgery of cornea and conjunctiva. St Louis, Elsevier-Mosby, 2005. Maeda N, Klyce SD, Smolek MK. Neural network classification of corneal topography. Preliminary demonstration. Invest Ophthalmol Vis Sci 1995;36:1327-35. Mejía-Barbosa Y, Malacara-Hernández D. A review of methods for measuring corneal topography. Optom Vis Sci 2001;78:240-53. Miller D, Greiner JV. Corneal measurements and tests. In Principles and Practice of Ophthalmology. Philadelphia,WB Saunders,1994.
18. Molebny VV, Panagopoulou SI, Molebny SV, Wakil YS, Pallikaris IG. Principles of ray tracing aberrometry. J Refract Surg 2000;16:S572-75. 19. Rabinowitz YS. Keratoconus. Surv Ophthalmol 1998;42:297-319. 20. Rabinowitz YS, Nesburn AB, McDonnell PJ. Videokeratography of the fellow eye in unilateral keratoconus. Ophthalmology 1993;100: 181-86. 21. Rao SK, Padmanabhan P. Understanding corneal topography. Curr Opin Ophthalmol 2000;11:248-59. 22. Thibos LN, Applegate RA, Schwiergerling JT, Webb R. Standards for reporting the optical aberrations of eyes. J Refract Surg 2002;18:S652-60. 23. Vincigerra P, Camesasca FI, Calossi A. Statistical Análysis of phisiological aberrations of the cornea. J Refract Surg 2003;19(suppl):265-69. 24. Wang L, Koch DD. Corneal Topography and its integration into refractive surgery. Comp Ophthalmol Update 2005;6:73-81. 25. Wilson SE, Ambrosio R. Computerized corneal topography and its importance to wavefront technology. Cornea 2001;20:441-54. 26. Wilson SE, Klyce SD. Advances in the analysis of corneal topography. Surv Ophthalmol 1991;35: 269-77. 27. Wilson SE, Lin DT, Klyce SD, Insler MS. Terrien’s marginal degeneration: corneal topography. Refract Corneal Surg 1990;6:15-20.
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MANOTOSH RAY
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Confocal Microscopy
Confocal microscopy, one of the most advanced imaging technologies, offers several advantages over conventional wide-field optical microscopy. It has the ability to control the depth of field, eliminate or reduce the background information away from the focal plane and the capability to collect serial optical sections from thick specimens. The basic key to the confocal approach is the use of spatial filtering techniques to eliminate out-of-focus light or glare. There has been a tremendous interest in confocal microscopy in recent years, due in part to the relative ease with which extremely high quality images can be obtained. Confocal microscopy has enhanced the ability to image the cornea in vivo. The application of this technology permits the acquisition of images of high spatial resolution and contrast as compare to conventional microscopy. Confocal microscope employs an oscillating slit aperture in an ophthalmic microscope configuration, especially suitable for the analysis of cell layers of cornea. It can focus through the entire range of a normal cornea from epithelium to endothelium. A series of scan shows: (a) epithelium, (b) corneal nerves, (c) keratocytes, (d) endothelium and (e) a computer generated slice of cornea. There are distinct advantages
of confocal microscope over the regular microscope. When focused on a transparent tissue like cornea with regular microscope, the unfocused layers affect the visibility of the focused layer. Confocal microscope, on the other hand, can focus on different layers distinctly without affecting the quality of the image.
Optics A halogen light source passes through movable slits (Nipkow disk). A condenser lens (front lens) projects the light to the cornea. Only a small area inside the cornea is illuminated to minimize the light scattering. The reflected light passes through the front lens again and is directed to another slit of same size via beam-splitter. Finally the image is projected onto a highly sensitive camera and displayed on a computer monitor (Fig. 5.1). The confocal microscope utilizes a transparent viscous sterile gel that is interposed between front lens and cornea to eliminate the optical interface with two different refractive indices. The front lens works on ‘Distance Immersion Principle’. The working distance (distance between front lens and the cornea) is
Confocal Microscopy performed, a graphic shows the depth coordinate on the ‘Z’ axis and the level of reflectivity on the ‘Y’ axis. The graphic also displays the distance between two images along the anteroposterior line. This simultaneous graphic recording is called ‘Z’ scan graphic. The reflectivity on ‘Z’ scan is entirely dependent on the tissue being scanned. A transparent tissue displays low reflectivity whereas a higher reflectivity is obtained from an opaque layer. Therefore, different corneal layers would display different reflectivity on ‘Z’ scan. The corneal endothelium displays the maximum reflectivity while stroma is the lowest. An intermediate reflectivity is obtained from epithelial layers. A typical ‘Z’ scan of entire normal cornea shows high endothelial reflection curves followed by low stromal reflection and then late intermediate reflectivity from superficial corneal epithelium. Thus confocal miscroscopy enables to perform corneal pachymetry or even can measure the distance between two corneal layers.
Fig. 5.1: Optics of confocal microscope
1.92 mm. The back and forth movement of the front lens enables scanning of the entire cornea starting from anterior chamber and corneal endothelium to most superficial corneal epithelium. Use of standard X40 immersion lens gives magnified cellular detail and an image field of 440 × 330 μm. Other lenses (e.g. X20) delivers wide field but less distinct cell morphology. Newer model (Confoscan 2.0) captures 350 images per examination at a rate of 25 frames per second. Thickness of the captured layers varies from 3 to 5 microns depending on scanning slit characteristics. In addition, every recorded image is characterized by its position on the ‘Z’ axis of the cornea. Every time a confocal scan is
Confocal Microscopy of Normal Cornea This is a noninvasive technique of imaging of corneal layers that provides excellent resolution with sufficient contrast. A well-executed scan can visualize the corneal endothelium, stroma, subepithelial nerve plexus and epithelial layers distinctly. The limitations are non-visualization of normal Bowman’s layer and Descemet’s membrane since these structures are transparent to this microscope. However, it is possible to view these structures when they are pathologically involved. Eyes with corneal opacity or edema can also be successfully scanned.¹ The quality of image depends on: (a) centration of the light beam, (b) stability of the eye, and (c) optimum brightness of the illumination.
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Diagnostic Procedures in Ophthalmology Epithelium Corneal epithelium has five to six layers. Three different types of cellular component are recognized in the epithelium. • Superficial (2-3 layers): flat cells • Intermediate (2-3 layers): polygonal cells • Basal cells (single layer): cylindrical cells. The superficial epithelial cells appear as flat polygonal cells with well-defined border, prominent nuclei and uniform density of cytoplasm. The main identifying features of superficial epithelial cells are nuclei, which are brighter than surrounding cytoplasm and usually associated with perinuclear hypodense ring (Fig. 5.2). The intermediate epithelial cells are similar polygonal cells as superficial layers but the nuclei are not evident. Basal cell layers are smaller in size and appear denser than other two layers (Fig. 5.3). The nucleus is not evident in basal layers also.
Fig. 5.3: Basal epithelial cells. High cell density with well demarcated cell borders
Now deep vertical fibers derive from deep corneal plexus to run anteriorly to form subbasal and subepithelial nerve plexus. Small nerve fibers from subbasal plexus terminate at the superficial epithelium. This complex anatomy was not possible to visualize in vivo until the advent of corneal confocal microscope. Generally, the nerve fibers appear bright and well contrasted against a dark background (Fig. 5.4). Confocal microscopy can visualize the orientation, tortuosity, width, branching pattern and any abnormality of the corneal nerves.²
Fig. 5.2: Superficial epithelial cells with prominent nuclei
Subepithelial Nerve Plexus Corneal nerves originate from long ciliary nerve, a branch of ophthalmic division of trigeminal nerve. Nerve fibers from long ciliary nerve form a circular plexus at the limbus. Radial nerve fibers originate from this circular plexus and run deep into the stroma to form deep corneal plexus.
Fig. 5.4: Subepithelial nerve fibers
Confocal Microscopy Stroma Corneal stroma represents 90% of total corneal thickness. It has three components: a. Cellular stroma: Composed of keratocytes and constitutes 5% of entire stroma. b. Acellular stroma: Represents the major component (90-95%) of stroma. The main component has regular collagen tissue (Type-I, III, IV) and interstitial substances. c. Neurosensory stroma: Represented by stromal nerve plexus and nerve fibers originating from it. The keratocyte concentration is much higher in the anterior stroma and progressively decreases towards the deep stroma. Generally, the keratocyte count is approximately 1000 cells/ mm² in anterior stroma while the average value drops to 700 cells/mm² in the posterior stroma. Confocal image of stroma shows multiple irregularly oval, round or bean-shaped bright structures that represent keratocyte nuclei. These nuclei are well contrasted against dark acellular matrix (Fig. 5.5). Anterior stromal keratocyte nuclei assume rounded bean-shaped morphology while the same in rear stroma are more often irregularly oval. A bright highly reflective keratocyte represents a metabolically activated
keratocyte of a healthy cornea. In a normal healthy cornea collagen fibers and interstitial substances appear transparent to confocal microscope and impossible to visualize. It is possible to identify stromal nerve fibers in anterior and mid stroma. These nerve fibers belong to deep corneal plexus and appear as linear bright thick lines. The stromal nerve fiber thickness is greater than epithelial nerves. Occasionally, nerve bifurcations are also clearly visible.
Endothelium Endothelium is a non-innervated single layer of cells at the most posterior part of cornea. Endothelial cell density is maximal at birth and progressively declines with age. Normal endothelial cell count varies from 1600 to 3000 cells/mm² (average 2700 cells/mm²) in a normal healthy adult.2-4 However, cornea can still maintain the integrity till the cell count declines below 300-500 cells/mm².
Fig. 5.6: Hexagonal endothelial cells in a healthy cornea
Fig. 5.5: Stromal keratocytes with bright oval-shaped nuclei
Homogeneous hexagonal cells with uniform size and shape represent healthy endothelial cells. Increasing age and endothelial assault cause pleomorphism and polymegathism. Confocal microscopy easily identifies endothelial cells. These cells appear as bright hexagonal and polygonal cells with unrecognizable nucleus. The
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Confocal Microscopy in Corneal Pathologies Keratoconus Keratoconus is a non-inflammatory ectatic disorder of the cornea characterized by a localized conical protrusion associated with an area of stromal thinning. The thinning is most apparent at the apex of the cornea. The steep conical protrusion of the corneal apex causes high myopia with severe irregular astigmatism. Other features of keratoconus include an iron ring, known as Fleischer’s ring that partially or completely encircles the cone.5 The cone appears as ‘oil drop’ reflex on distant direct ophthalmoscopy due to internal reflection of light. Deep vertical folds oriented parallel to the steeper axis of the cornea at the level of deep stroma and Descemet’s membrane are known as Vogt’s striae. An acute corneal hydrops appears when there is a break in the Descemet’s membrane. The corneal edema usually subsides after few months leaving behind scar and flattening the cornea. The corneal nerves become more readily visible due to thinning of the cornea. High irregular astigmatism precludes adequate spectacle correction. In the early stages, use of contact lenses may improve the visual acuity. However, contact lens fitting can be extremely difficult and in advanced cases it ceases to improve visual acuity optimally forcing the patient to rely on only options left, corneal transplantation. The most effective way to identify early cases of keratoconus is computerized corneal topography that has become a gold standard
for diagnosis and follow-up of the disease in recent years.6,7 Confocal microscopy is a relatively newer investigative modality to assess the keratoconic cornea. Morphological changes in keratoconus are mostly confined to the corneal apex and depend on the severity of the disease. Rest of the cornea may appear normal. The typical polygonal shape of superficial epithelial cells is lost. They appear distorted and elongated in an oblique direction with highly reflective nuclei (Fig. 5.7). Cell borders are not distinguishable. There may be areas of basal epithelial loss as evident by a linear dark non-reflective patch in confocal microscopy. The subepithelial nerve plexus generally appears normal. However, the sub- basal nerve fibers are curved and take the course of stretched overlying epithelium. Corneal stroma is also affected by keratoconus. The confocal images of stroma are highly specific. The characteristic stromal changes are multiple ‘striae’ represented by thin hyporeflective lines oriented vertically, horizontally or obliquely (Fig. 5.8). These are confocal representation of Vogt’s striae.8 In advanced stages of keratoconus, the keratocyte concentration is reduced in anterior stroma. The shape of the keratocytes is also altered. Occasionally, highly reflective bodies
Fig. 5.7: Obliquely elongated superficial epithelium in keratoconus
Confocal Microscopy but with progression of the disease they can involve the posterior stroma as well. Confocal microscopy reveals highly reflective, bright, dense structures in the anterior and midstroma. Keratocytes are not involved. Depth of stromal involvement may be ascertained by using ‘Z’ scan function. This is an added advantage over other contemporary investigations that enables surgeon to plan for surgical modalities. Confocal microscopy is also useful in differential diagnosis and follow-up of the disease.
Posterior Polymorphous Dystrophy Fig. 5.8: Advanced keratoconus: vertical striae in the stroma
with tapering ends are visible in anterior stroma near the apex. The nature of these abnormal bodies is not yet known but it may be due to altered keratocytes. The corneal endothelial changes vary from none to occasional pleomorphism and polymegathism.
Corneal Dystrophies Corneal dystrophies are inherited abnormalities that affect one or more layers of cornea. Usually both eyes are affected but not necessarily symmetrically. They may present at birth but more frequently develop during adolescence and progress gradually throughout life. Some forms are mild, others severe.
Granular Dystrophy This is an autosomal dominant bilateral noninflammatory condition that results from deposition of eosinophilic hyaline deposits in the corneal stroma.9 It specifically affects the central cornea and eventually can cause decreased vision and eye discomfort. Initially, the lesions are confined to superficial stroma
Posterior polymorphous dystrophy (PPD) is a rare inherited disorder of the posterior layer of the cornea. It is a bilateral disorder with early onset, although early stage diagnosis is rare since most of the affected individuals remain asymptomatic. The characteristic endothelial changes are small vesicles or areas of geographic lesions. In fact, endothelial cells lining of the posterior surface of the cornea have epitheliallike features.10,11 These cells can also cover the trabecular meshwork, leading to glaucoma in some patients. Most severe cases may develop corneal edema due to compromised pump function of the endothelial cells. Confocal microscopy shows multiple round vesicles at the level of Descemet’s membrane and endothelium.12 PPD usually distorts the normal flat profile of the endothelial cells and present large dark cystic impressions on confocal scan. The endothelial cells surrounding the lesion appear large and distorted.
Fuchs Endothelial Dystrophy Fuchs endothelial dystrophy is a chronic bilateral hereditary (variable autosomal dominant or sporadic) disorder of corneal endothelium. It typically presents after the age of 50 and more
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Diagnostic Procedures in Ophthalmology common in females. There is a loss of endothelial cells that results in deposition of collagen materials in Descemet’s membrane (guttata). Corneal guttata is the hallmark of this disease. The integrity of corneal endothelium is essential to maintain the metabolic and osmotic function of the entire cornea. Corneal edema in Fuchs dystrophy initially involves the posterior and mid-stroma. As the disease advances, the edema progresses to involve the anterior cornea; resulting in formation of bullous keratopathy. Confocal microscopy is useful to visualize the corneal guttata. This technique has a distinct advantage over conventional specular microscopy that fails to visualize the endothelium when there is significant corneal edema.13 The corneal guttata appears dark with bright central reflex (Fig. 5.9).14 In advanced stage the endothelial morphology altered completely but it is still possible to identify the distorted cell borders.14 In the early stages of bullous keratopathy, intraepithelial edema is seen as distorted cellular morphology with increased reflectivity. It can also identify the bullae in the basal epithelial layer.
Fig. 5.9: Distorted endothelium in Fuchs endothelial dystrophy
Laser in situ Keratomileusis Laser in situ keratomileusis (LASIK) is one of the latest techniques of excimer laser refractive
surgery that is currently being successfully used by refractive surgeons for the correction of various types of refractive errors. LASIK has become the technique of choice to correct myopia and hyperopia with or without astigmatism.15 LASEK is a modification of photorefractive keratectomy (PRK) where excimer laser is used to ablate superficial corneal stroma after the epithelium has been removed. LASIK involves the use of microkeratome to prepare a hinged corneal flap of uniform thickness. The excimer laser is subsequently used to ablate the mid-corneal stromal bed and thereafter the flap is reposited to its original position without applying any suture. After LASIK, the healing of corneal tissue occurs quickly since there is minimal damage to the corneal epithelium and the Bowman’s membrane. Traditionally, the cornea is evaluated with slit-lamp biomicroscopy and computerized corneal topography both pre- and postoperatively. Confocal microscopy adds newer dimensions to the commonly employed investigations. Functional outcome of LASIK depends on many factors including the biomechanics, healing process and the inflammatory response of the flap interface that is created between the epithelial flap and stromal bed. Confocal scan is useful in evaluation of following parameters. • Corneal flap thickness • Interface study a) Healing process b) Inflammatory response c) Abnormal deposits • Corneal nerve fiber regeneration, and • Residual stromal thickness. A well-designed flap is the key to successful outcome of LASIK. Thinner flaps are more at risk from flap complications. A few studies with confocal microscopy had suggested that actual flap thickness after LASIK is consistently lower than predicted thickness.16 The reasons are not
Confocal Microscopy yet known. However, corneal edema that may be caused by microkeratome cut and suction may play an important role. Postoperative scarring and tissue retraction could be other possible factors. Using a ‘Z’ scan, it is possible to identify the interface that corresponds to a very low level of reflectivity. The flap thickness is obtained by measuring the distance between high reflective spike from the front surface of the cornea and the low reflective interface (Fig. 5.10).
white bodies (Fig. 5.11). Microstriae are present at the Bowman’s layer. Excessive interface microstriae and bright particles may lead to astigmatism and eventually poor outcome after LASIK. These microstriae can be imaged with confocal microscope even when the slit-lamp examination is unremarkable.
Fig. 5.11: Bright highly reflective particles at the flap-stroma interface
Fig. 5.10: Measurement of flap thickness in LASIK
The interface usually appears as a hyporeflective space in between relatively hyperreflective cellular stroma. Interface can easily be imaged by confocal microscope. Typically, the keratocyte concentration is lower than normal in the interface. Bright particles and microstriae are consistently visible in the interface. These bright particles most probably originate from microkeratome blade and represented by highly reflective
Diffuse lamellar keratitis (DLK) also known as sands of Sahara syndrome, is a noninfectious inflammation of the interface. The etiology is not known but it is assumed to be toxic or allergic in nature. In confocal scan DLK appears as diffuse and multiple infiltrates in the interface with no anterior or posterior extension. Subepithelial nerve fibers are affected by LASIK. No nerve is visible in immediate postoperative period. However, the regenerating nerve fibers appear as thin irregularly branching line when confocal scan is performed 5-7 days after surgery. The residual stromal thickness can also be measured using ‘Z’ scan technique as described while evaluating the epithelial flap.
Corneal Grafts Confocal microscope is a useful tool to followup the corneal grafts and to diagnose the
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Diagnostic Procedures in Ophthalmology abnormal changes that may occur postoperatively. It provides images at the cellular level to identify any pathological changes even before it becomes clinically evident. It can also be used to assess the donor cornea. Corneal graft survival is entirely dependent on optimum number of healthy endothelial cells. Endothelial cell loss occurs rapidly after corneal transplantation17. Majority of cell loss takes place during the first two postoperative years.18 Several studies had suggested that endothelial cell loss is much higher after corneal grafting when the primary indications are bullous keratopathy or hereditary stromal dystrophy in compare to keratoconus and corneal leukomas.19,20 Another interesting fact is that endothelial cell loss is greater when corneal transplantation is performed on phakic eyes than on aphakics.21 Confocal microscopy scores over conventional specular microscopy while evaluating endothelial cell characteristics especially in eyes with stromal edema. Endothelial morphology in confocal scan has been described earlier. Immediate postoperative period, endothelium looks normal and healthy. However, as time progresses, endothelial cell density decreases as evidenced by pleomorphism and polymegathism. Occasionally, a bright preendothelial deposits appear, the significance of which is not yet known (Fig. 5.12). Reinnervation after grafting is another issue well addressed by confocal microscopy. First sign of innervation that starts few months after keratoplasty is visible at the periphery of the graft stroma. However, complete innervation may take many years to develop. Regenerated nerve fibers look similar to that found in a normal cornea. Occasionally, they may take a tortuous and convoluted course depending on age (e.g. older patients) and primary indications of keratoplasty (e.g. bullous keratopathy, corneal dystrophies).
Fig. 5.12: Pleomorphism, polymegathism and preendothelial deposits in a corneal graft
It is well known that allograft rejection is one of the most common causes of graft failure. Graft rejection can be classified as epithelial, subepithelial and endothelial rejection, of which the endothelial rejection is the worst. Confocal
Fig. 5.13: Co-existence of degenerated and normal endothelial cells in early endothelial allograft rejection
Confocal Microscopy features of epithelial rejection are distorted basal epithelial cells with altered subepithelial reflectivity. Subepithelial rejection is identified by discrete opacities underneath the epithelial layer.22 Endothelial rejection, on the other hand, is characterized by coexistence of normal looking and degenerated endothelial cells, focal endothelial cell lesions and bright highly reflective microprecipitates (Fig. 5.13).23
Intracorneal Deposits Sources of intracorneal deposits can be exogenous or endogenous. They can involve various layers of cornea individually or in combination.
inclusion bodies located at the basal epithelial layer.24 Confocal microscopy adds newer dimensions to the existing knowledge. It demonstrates involvement of entire cornea, although vortex keratopathy is primarily a corneal epithelial pathology. The characteristic features are presence of highly reflective, bright intracellular deposits at the basal epithelial layer (Fig. 5.14). Overlying epithelium is usually normal. In advanced cases these microdeposits may extend to the stroma and eventually to the endothelium.25 Stromal keratocyte density is often reduced.
Exogenous sources: • Long-term use of contact lenses • Refractive surgery • Vitreoretinal surgery using silicone oil • Drugs: Amiodarone, Chloroquine Endogenous sources: • Wilson’s disease • Hyperlipidemia • Fabry’s disease • Hemosiderosis The clinical diagnosis is based on slit-lamp biomicroscopy and systemic features in selected cases. The knowledge of confocal features in these disorders is limited except in drug induced keratopathies.
Fig. 5.14: Intracellular deposits at basal epithelial layer in amiodarone toxicity
Vortex Keratopathy
Conclusion
Vortex keratopathy known as cornea verticillata is characterized by whorl-like corneal epithelial deposits. It can be induced by various drugs, e.g. amiodarone (used for cardiac arrhythmias) and anti-malarials (chloroquine, hydroxychloroquine). Clinically, vortex keratopathy is manifested as golden-brown opacities at the inferior corneal epithelium. On electron microscopy, they appear as intracytoplasmic lysosom-like lamellar
Ophthalmic investigations and instrumentations have come long way over the past decades. Confocal microscope is one of those wonderful innovations in recent time. It is becoming more popular everyday and its indications are expanding. Confocal microscopy is truly an exciting tool that can be useful for the clinical diagnosis, follow-up and analysis of many corneal lesions.
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Diagnostic Procedures in Ophthalmology Acknowledgement I would like to thank Aria Mangunkusumo and Vanathi Ganesh for their help.
References 1. Weigand W, Thaer AA, Kroll P, et al. Optical sectioning of the cornea with a new confocal in vivo slit-scanning videomicroscope. Ophthalmology 1995;102(4):485-92. 2. Oliveira-Soto L, Efron N. Morphology of corneal nerves using confocal microscopy. Cornea 2001;20(4):374-84. 3. Tuft SJ, Coster DJ. The corneal endothelium. Eye 1990;4:389. 4. Nucci P, Brancato R, Mets MB, et al. Normal endothelial cell density range in childhood. Arch Ophthalmol 1990;108:247. 5. Gass JD. The iron lines of the superficial cornea: Hudson-Stahle line, Stocker’s line, and Fleischer’s ring. Arch Ophthalmol 1964;71:348. 6. Maguire LJ, Bourne WM. Corneal topography in early keratoconus. Am J Ophthalmol 1989; 108:107. 7. Maguire LJ, Lowry J. Identifying progression of subclinical keratoconus by serial topography analysis. Am J Ophthalmol 1991;112:41. 8. Somodi S, Hahnel C, Slowik C, et al. Confocal in vivo microscopy and confocal laser-scanning fluorescence microscopy in keratoconus. Ger J Ophthalmol 1996;5(6):518-25. 9. Werner LP, Werner L, Dighiero P. et al. Confocal microscopy in Bowman’s and stromal corneal dystrophies. Ophthalmology 1999;106(9):16971704. 10. Hirst LW, Waring GO. Clinical specular microscopy of posterior polymorphous endothelial dystrophy. Am J Ophthalmol 1983;95(2):143-55. 11. Mashima Y, Hida T, Akiya S, et al. Specular microscopy of posterior polymorphous endothelial dystrophy. Ophthalmic Paediatr Genet 1986; 7(2):101-07. 12. Chiou AG, Kaufman SC, Beuerman RW, et al. Confocal microscopy of posterior polymorphous endothelial dystrophy. Ophthalmologica 1999;213(4):211-13.
13. Chiou AG, Kaufman SC, Beuerman RW, et al. Confocal microscopy in cornea guttata and Fuch’s endothelial dystrophy. Br J Ophthalmol 1999;83(2):185-89. 14. Rosenblum P, Stark WJ, Maumenee IH, et al. Hereditary Fuch’s dystrophy. Am J Ophthalmol 1980;90:455. 15. Reviglio VE, Bossana EL, Luna JD, et al. Laser in situ keratomileusis for the correction of hyperopia from +0.50 to +11.50 diopters with Keracor 117C laser. J Refract Surg 2000;16(6):71623. 16. Durairaj VD, Balentine J, Kouyoumdjian G, et al. The predictability of corneal flap thickness and tissue laser ablation in laser in situ keratomileusis. Ophthalmology 2000;107(12): 2140-43. 17. Harper CL, Boulton ML, Marcyniuk B, et al. Endothelial viability of organ cultured corneas following penetrating Keratoplasty. Eye 1998;12(5):834-38. 18. Vasara K, Setala K, Ruusuvaara P. Follow up study of corneal endothelial cells, photographed in vivo before eneucleation and 20 years later in graft. Acta Ophthalmol Scand 1999;77(3):27376. 19. Obata H, Ishida K, Murao M, et al. Corneal endothelial cell damage in penetrating keratoplasty. Jpn J Ophthalmol 1991;35(4):411-16. 20. Abott RL, Fine M, Guillet E. Long-term changes in corneal endothelium following penetrating keratoplasty. A specular microscopic study. Ophthalmology 1983;90(6):676-85. 21. Ing JJ, Ing HH, Nelson LR, et al. Ten-year postoperative results of penetrating keratoplasty. Ophthalmology 1998;105(10):1855-65. 22. Cohen RA, Chew SJ, Gebhardt BM, et al. Confocal microscopy of corneal graft rejection. Cornea 1995;14(5):467-72. 23. Cho BJ, Gross SJ, Pfister DR, et al. In vivo confocal microscopic analysis of corneal allograft rejection in rabbits. Cornea 1998;17(4):417-22. 24. Ghose M, McCulloch C. Amiodarone induced ultrastructural changes in human eye. Can J Ophthalmol 1984;19:178-86. 25. Ciancaglini M, Carpineto P, Zuppardi E, et al. In vivo confocal microscopy of patients with amiodarone induced keratopathy. Cornea 2001;20(4):368-73.
Tonometry
R RAMAKRISHNAN, SONAL AMBATKAR
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Tonometry
Tonometry in reference to the eye is the measurement of intraocular pressure (IOP). A tonometer is an instrument that exploits the physical properties of the eye to permit measurement of pressure without the need to cannulate the eye. The first practical tonometer was invented by Maklakov in 1885. Fick is credited with inventing a second applanation tonometer employing a fixed area produced by an adjustable force. This instrument was a forerunner of the Goldmann applanation tonometer (1954) which is today considered the most accurate clinical tonometer. From a functional standpoint, a normal IOP is one that does not result in optic nerve damage. All eyes do not respond similarly to a particular IOP, therefore, a normal pressure cannot be represented as a specific measurement. Various studies of IOP distribution have shown a mean IOP of 15.5 ± 2.6 mm Hg and the upper limit has been demonstrated to be 2 standard deviations above the mean IOP that is 20.5 mm Hg.
Types of Tonometers The physical properties of a normal cornea determine the limits of accuracy of tonometry. When the cornea is deformed by a tonometer,
the resulting fluid displacement causes the remainder of the globe to distend. The tendency of the wall of the eye is to resist stretching, and deformation of the cornea raises the IOP. Tonometers in which the IOP is negligibly raised during tonometry (less than 5%) are termed as low-displacement tonometers. The Goldmann tonometer displaces only 0.5 μl of aqueous humor and raises IOP by only 3%. Tonometers that displace a large volume of fluid and consequently raise IOP significantly are termed as highdisplacement tonometers. In a normal eye IOP becomes more during Schiøtz tonometry. Highdisplacement tonometers are mostly less accurate than low-displacement tonometers.
Types of Tonometry Tonometry can be broadly classified into 2 types, direct and indirect.
Direct Method A catheter is inserted into the anterior chamber of the eye and the other end is connected to a manometric device from which the pressure is calculated. Though this is the most accurate
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Indirect Method It is based on eyes response to an applied force.
The shape of corneal deformation is truncated cone. It displaces large intraocular volume so conversion tables based on empirical data is used to estimate IOP. The prototype is Schiøtz tonometer.
Applanation Tonometers Palpation Method Intraocular pressure (IOP) is estimated by response of eye to pressure applied by finger pulp (indents easily/firm to touch). The indirect methods can be broadly divided into contact and non-contact methods. Basic types of contact tonometers differ according to shape and magnitude of deformation.
Contact Tonometers IOP measurement is performed by deforming the globe and correlating the force responsible for deformation to the pressure within the eye. Both indentation and applanation tonometers effect a deformation of globe but the magnitude varies (Fig. 6.1).
Applanation tonometers are used to measure force necessary to flatten a small, standard area of cornea. The shape of corneal deformation is simple flattening. The shape is constant so IOP is derived from a mathematical calculation. They are of 2 types on the basis of variable that is measured. Variable force: Area of cornea on applanation held constant, force varies. Prototype is Goldmann tonometer. Variable area: Force applied to cornea held constant, area varies. Prototype is Maklakov tonometer. The volume displacement is sufficiently large to require a conversion table.
Noncontact Tonometer Noncontact tonometer measures time required to deform a standard area of corneal surface in response to a jet of air.
Schiøtz Tonometer
Fig. 6.1: A Deformation of globe during indentation tonometry, B Deformation of globe during applanation tonometry
Indentation Tonometer Indentation tonometer is used to measure the amount of deformation or indentation of the globe in response to a standard weight applied to the cornea or the area flattened by a standard force.
Schiøtz tonometer (Fig. 6.2) consists of metal plunger that slides through a hole in a concave metal plate. The plunger supports a hammer device connected to needle that crosses a scale. The extent to which cornea is indented by plunger is measured as the distance from the foot plate curve to the plunger base and a lever system moves a needle on calibrated scale. The indicated scale reading and the plunger weight are converted to an IOP measurement. More the plunger indents the cornea, higher the scale reading and lower the IOP
Tonometry generated an empirical formula for linear relationship between the log function of IOP and the ocular distension. This formula has ‘C’ a numerical constant, the coefficient of ocular rigidity which is an expression of distensibility of eye. Its average value is 0.025. Technique: Patient should be in supine position, looking up at a fixation target while examiner separates the lids and lowers the tonometer plate to rest on the anesthetized cornea so that plunger is free to move vertically (Fig. 6.3). A fine movement of needle on scale is in response to ocular pulsations. Scale reading is an average of the extremes of these excursions. The 5.5 gm weight is initially used. If scale reading is 4 or less, additional weight is added to plunger. Conversion table is used to derive IOP in mm Hg from scale reading and plunger weight. The instrument is calibrated before each use to check scale (reading is zero). Fig. 6.2: Schiøtz tonometer
The standard instrument has following characteristics: Foot plate has concavity of 15 mm radius of curvature. The tonometer weighs 11 gm. Plunger has 3 mm diameter, a weight of 5.5 gm including the force of the lever rests on top of the plunger. Additional weights are added to plunger to increase it to 7.5, 10, or 15 gm. The scale reading is zero when plunger extends 0.05 mm beyond foot plate curve. Each scale unit represents 0.05 mm protrusion of the plunger.
Fig. 6.3: Technique of tonometry
Basic concept: The weight of tonometer on the eye increases the actual IOP (Po) to a higher level (Pt). The change in pressure from Po to Pt is an expression of the resistance of the eye (scleral rigidity) to the displacement of fluid. Determination of Po from a scale reading Pt requires conversion which is done according to Friedenwald conversion tables. Friedenwald
Sources of error: Accuracy is limited as ocular rigidity varies from eye to eye. As conversion tables are based on an average coefficient of ocular rigidity; eye that varies significantly from this value gives erroneous IOP. High ocular rigidity is seen in high hyperopia, long-standing glaucoma, age-related macular degeneration, and vasoconstrictor therapy. Low ocular rigidity
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Diagnostic Procedures in Ophthalmology is found in high myopia, advanced age, miotics, use of vasodilators, after RD surgery (vitrectomy, cryopexy, scleral band) and intravitreal injection of compressible gas. The variable expulsion of intraocular blood during Schiøtz tonometry may influence IOP measurement. Repeated measurements lower IOP. Either a steeper or a thicker cornea causes greater displacement of fluid during tonometry and gives a falsely high IOP measurement.
Variable Force Applanation Tonometers
Fig. 6.4: Goldmann applanation tonometry
Goldmann Applanation Tonometer (GAT) Basic concept: Based on Imbert-Fick law, an external force (W) against a sphere equals the pressure in the sphere (P) times the area flattened (applanated) by external force (A) W = P × A Cornea being aspherical, wet, and slightly inflexible fails to follow the law. Moisture creates surface tension (S) or capillary attraction of tear film for tonometry head. Lack of flexibility requires force to bend the cornea (B) which is independent of internal pressure. The central thickness of cornea is about 0.55 mm and the outer area of corneal flattening differs from the inner area of flattening (A1). It is this inner area which is of importance.
Fig. 6.5: Biprism in the Goldmann tonometer
When A1 = 7.35 mm2, S balances B and W =P. This internal area of applanation is achieved when the diameter of the external area of corneal applanation is around 3.06 mm. Grams of force applied to flatten 3.06 diameter of the cornea multiplied by 10 is directly converted to mmHg.
biprism (Fig. 6.5) which is used to applanate cornea. Two beam splitting prisms within applanating unit optically convert circular area of corneal contact in 2 semicircles. Edge of corneal contact is made apparent by instilling fluorescein while viewing in cobalt blue light. By manually rotating a dial calibrated in grams, the force is adjusted by changing the length of a spring within the device. The prisms are calibrated in such a fashion that inner margin of semicircles touch when 3.06 mm of the cornea is applanated. Biprism is attached by a rod to a housing which contains a coil spring and series of levers that are used to adjust the force of the biprism against the cornea.
Instrument: Instrument is mounted on the end of a lever hinged on the slit-lamp (Fig. 6.4). Examiner views through the center of plastic
Technique: Cornea is anesthetized, tear film is stained with sodium fluorescein. Cornea and biprism is illuminated by a cobalt blue light.
Modified Imbert-Fick Law is W + S = PA1 + B
Tonometry Fluorescein facilitates visualization of tear meniscus at margin of contact. Fluorescent semicircles are viewed through the biprism. Force against the cornea is adjusted until the inner edges overlap. Ocular pulsations create excursions of semicircular tear meniscus and IOP is read as the median over which arc glides. This is the end point (Fig. 6.6) at which a reading can be taken from a graduated dial which indicates grams of force applied to tonometer and so this number is multiplied by 10 to obtain IOP in mm Hg.
Figs 6.7A and B: Vertical misalignment. To minimize this, tonometer biprism should be rotated so that axis of least corneal curvature is opposite the red line on the prism holder. Other method is to obtain measurements with mires oriented horizontally and vertically and to average these readings
6. More than 6 D astigmatism produces an elliptical area on applanation that gives erroneous IOP. 4D with-the-rule and against-the-rule astigmatism underestimate and overestimate IOP, respectively. 7. Mires may be distorted on applanating on irregular corneas.
Fig. 6.6: End point recording of IOP
Sources of error in applanation tonometry 1. Inadequate concentration of fluorescein in precorneal tear film gives hypofluorescence. 2. Fluorescein may lose fluorescence in acidic solution (quenching of fluorescence) causing underestimation of IOP. 3. Wider meniscus or improper vertical alignment gives higher IOP readings (Figs 6.7A and B). 4. Thin corneas underestimate and thick corneas overestimate IOP. 5. For every 3D increase in corneal curvature, IOP raises about 1 mm Hg as more fluid is displaced under steeper corneas causing increase in ocular rigidity.
Effect of central corneal thickness (CCT): Variations in corneal thickness change the resistance of the cornea to indentation so that this is no longer balanced entirely by the tear film surface tension thus affecting the accuracy of IOP measurement. A thinner cornea may require less force to applanate it, leading to underestimation of true IOP while a thicker cornea would need more force to applanate it, giving an artificially higher IOP. The Goldmann applanation tonometer was designed to give accurate readings when the CCT was 520 μm. As shown by Ehlers et al, there can be under estimation or overestimation of IOP when the corneal thickness is less or more than 520 micron, respectively. They interpolated that deviation of CCT from 520 μm yields a change in applanation readings of 0.7 mm Hg per 10 μm. IOP measurements are also modified after PRK and LASIK. Thinning of the central cornea is believed to give lower readings on applanation.
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Other Variable Force Applanation Tonometers Hand-Held Goldmann-Type Tonometers Perkins Tonometer Perkins tonometer (Fig. 6.8) uses same prisms as Goldmann but is counterbalanced so that tonometry is performed in any position (Fig. 6.9). The prism is illuminated by battery powered bulbs. The force on the prisms is adjusted manually. Being portable it is practical when measuring IOP in infants / children and for use in operating rooms.
Fig. 6.9: Tonometry with Perkins tonometer
Mackay-Marg Tonometer
Fig. 6.8: Perkins tonometer
Draeger Tonometer Draeger tonometer is similar to Perkins but uses different set of prisms and operates with a motor adjusting the force on these prisms.
Basic concept: Force is required to keep the flat plate of a plunger flush with a surrounding sleeve against the pressure of corneal deformation. Tonometer incorporates a 1.5 mm diameter plunger affixed to a rigid spring that extends 10 μm beyond the plane of surrounding rubber sleeve. Movement of plunger is electronically monitored by a transducer and recorded on a moving paper strip. When the tonometer is placed against cornea, the tracing that represents the force applied to the plunger begins to rise. At 1.5 mm of corneal area applanation, tracing reaches a peak and the force applied = IOP + force required to deform the cornea. At 3 mm flattening, force required to deform cornea is transferred from plunger to surrounding sleeve, creating a dip in tracing corresponding to IOP. Flattening of >3 mm of area gives artificial elevation of IOP. It is accurate in eyes with scarred, edematous and irregular corneas.
Tonometry Other Mackay-Marg-type Tonometers: CAT 100 Applanation and Biotronic Tonometers They have an internal logic program which automatically selects the acceptable measurement and 3 or more good IOP readings are averaged and displayed on screen.
Tonopen Tonopen (Fig. 6.10) is a portable and battery operated tonometer. It has the same principle as that of Mackay-Marg tonometer. The tip has a strain gauge that is activated when in contact with cornea. The built-in microprocessor logic circuit senses a trough force and records until an acceptable measurement is achieved. Four to ten such measurements are averaged to give a final IOP which is displayed.
The probe tip is applied perpendicularly to cornea until it is just indented. An audible click indicates that the measurement is acceptable. The process is repeated 4-10 times until a beep indicates a statistically valid average reading.
Pneumatonometer Pneumatonometer or pneumatic tonometer is like Mackay-Marg tonometer. It has a core sensing mechanism for measuring IOP while force required to bend the cornea is transferred to surrounding structure. The sensor is a air pressure like electronically controlled plunger in Mackay-Marg tonometer. It can also be used for continuous monitoring of IOP. It gives significantly higher IOP estimates.
Constant Force Applanation Tonometry Maklakov Applanation Tonometer With Maklakov applanation tonometer IOP is estimated by measuring the area of cornea flattened by a known weight. It consists of a dumb-bell-shaped metal cylinder with flat end plates of polished glass on either end with a diameter of 10 mm. Tonometers weighing 5, 7.5, 10, and 15 gm are used to measure the IOP. Crossaction wire handle to support instrument on the cornea is used. A thin layer of dye is spread onto the bottom of either end plate and the instrument is brought in contact with anesthetized cornea in supine position for 1 second. A circular white imprint on end plate corresponds to the area of corneal flattening. Area is measured and IOP is read from conversion table in the column corresponding to the weight used.
Noncontact Tonometer Fig. 6.10: Tonometry with tonopen
Noncontact tonometer (NCT) was introduced by Grolman. A puff of room air creates a
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Diagnostic Procedures in Ophthalmology constant force that momentarily flattens the cornea. The time from an internal reference point to the moment of flattening is measured and converted to IOP. The corneal apex is deformed by a jet of air. The force of air jet which is generated by a solenoid activated piston increases linearly over time.
normal, accuracy decreases with increase in IOP and in eyes with abnormal cornea or poor fixation. New NCT, Pulsair is a portable hand held tonometer.
Devices under Investigation Flush fitting silastic gel contact lens instrumented with strain gauges that measures changes in meridional angle of corneoscleral junction caused by variations in IOP. A similar device using a pressure transducer is made in form of a cylindrical guard ring applanation tonometer. A scleral gauge is embedded in an encircling scleral band to measure the distension of globe. An instrument using suction cups for recording IOP up to 1 hour in supine position is under investigation.
Fig. 6.11: Tonometry with noncontact tonometer
Original NCT has 3 subsystems: 1. Alignment system: It aligns patient’s eye in 3 dimensions. 2. Optoelectronic applanation monitoring system: It comprises transmitter, receiver and detector, and timer. a. Transmitter directs a collimated beam of light at corneal apex. b. Receiver and detector accept only parallel coaxial rays of light reflected from cornea. c. Timer measures from an internal reference to the point of peak light intensity. 3. Pneumatic system: It generates a puff of room air directed against cornea. When the reflected light is at peak intensity, the cornea is presumed to be flattened. The time elapsed is directly related to the force of jet necessary to flatten the cornea and correspondingly to IOP. NCT is accurate if IOP is nearly
Comparison, Calibration and Sterilization of Different Tonometers Comparison Goldmann Applanation Tonometer (GAT) In eyes with regular corneas, GAT is generally accepted as the standard against which other tonometers must be compared. Even with GAT, inherent variability must be taken in account.
Schiøtz Tonometer Studies indicate that Schiøtz reads lower than GAT even when the postural influence on IOP is eliminated by performing measurements in supine position. The magnitude of difference between the two tonometers and the influence of ocular rigidity are such that Schiøtz indicates only that the IOP is within a certain range and is of limited value even for screening purposes.
Tonometry Perkins Applanation Tonometer Perkins applanation tonometer compares favorably against GAT. In one study, difference between readings with the two instruments was 1.4 mmHg. It is subject to the same influence of corneal thickness as the GAT. It is useful in infants and children and is accurate in horizontal as well as vertical position.
Draeger Applanation Tonometer Comparative studies of Draeger applanation tonometer with GAT have given inconsistent results because of its more complex design. Draeger tonometer is more difficult to use than the Perkins. Patient’s acceptance to Draeger tonometer is poor.
Mackay-Marg Tonometer (MMT) Highly significant correlation is found between MMT and GAT readings. The average mean MMT values are often higher than GAT.
Mackay-Marg Type Tonometers Tonopen has compared favorably against manometric readings in human autopsy eyes but it may cause a significant increase in IOP during measurements. It has good correlation with GAT readings within normal IOP ranges. But most studies indicate that tonopen under estimates IOP in the higher ranges and over estimates in the lower range.
Pneumatic Tonometer Pneumatic tonometer correlates well with GAT readings. However, it gives significantly higher IOP estimates.
Noncontact Tonometer Noncontact tonometer is reliable within the normal IOP range, although its reliability is
reduced in higher IOP ranges and is limited by abnormal corneas or poor fixation. Corneal thickness has greater influence on NCT than on GAT. The hand-held pulsair NCT has compared favorably with Goldmann applanation readings in normal and glaucomatous eyes. It tends to read lower IOP above the normal range.
Tonometry on Irregular Corneas Accuracy of GAT and Maklakov-type applanation tonometers and NCT is limited in eyes with irregular corneas. MMT is considered to be accurate in scarred or edematous corneas. As MMT applanates a small surface area, the effects of corneal resistance to deformation and surface tension of tears are less than that with GAT. Pneumotonometer has also been shown to be useful in eyes with diseased cornea. Tonopen compared favorably with MMT on irregular corneas in a study.
Tonometry over Soft Contact Lens MMT, pneumtonometer and tonopen can measure the IOP through bandage contact lens with reasonable accuracy although soft contact lenses of different powers create a bias with tonopen. Applanation tonometers are affected by the power of the contact lens with high water content and correction tables are developed to compensate it. The power of soft contact lenses influences the difference in IOP between the paired readings by NCT.
Tonometry over Gas Filled Eyes Intraocular gas significantly influences scleral rigidity rendering indentation tonometry unsatisfactory. Pneumatonometer underestimates GAT readings in gas filled eyes while Tonopen compared favorably with GAT readings.
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Diagnostic Procedures in Ophthalmology Calibration of Goldmann Applanation Tonometer It is essential that Goldmann applanation tonometer (GAT) should be calibrated periodically, at least monthly. Following checks are necessary: • Check position 0: Turn the zero calibration on the measuring drum downwards by the width of one calibration marking, against the index marker. When the feeler arm is in the free movement zone, it should then move itself against the stop piece in the direction of the examiner. • Check position 0.05: Turn the zero calibration on the measuring drum upwards by the width of one calibration marking, against the index marker. When the feeler arm is in the free movement zone, it should then move itself against the stop piece in the direction of the patient. • Check position at drum setting 2: For checking this position, check weight is used. Five circles are engraved on the weight bar. The middle one corresponds to drum position 0, the two immediately to the left and right to position 2 and the outer ones to position 6. One of the marks on the weight corresponding to drum position 2 is set precisely on the index mark of the weight holder. Holder and weight are then fitted over the axis of the tonometer so that the longer part of the weight points towards the examiner. • Check position 1.95: The feeler arm should move towards the examiner. Check position 2.05.The feeler arm should move in the direction of the patient. • Check at measuring drum setting 6: Turn the weight bar to scale calibration 6, the longer part shows in the direction of the examiner. • Check position 5.9/6.1 as performed for drum setting 2.
Sterilization Schiøtz Tonometer The tonometer is disassembled between each use and the barrel is cleaned with 2 pipe cleaners, the first soaked in alcohol and the second dry. The foot plate is cleaned with alcohol swab. All surfaces must be dried before reassembling.
Goldmann Applanation Tonometer A variety of techniques are described for disinfecting the tonometer. Applanation tip should be soaked for 5-15 min in diluted sodium hypochlorite, 3% H2O2 or 70% isopropyl alcohol or by wiping with alcohol, H2O2, povidone iodine or 1: 1000 merthiolate. Other methods of sterilization include: 10 min of rinsing in running tap water, wash with soap and water, cover the tip with a disposable film, and exposure to UV light.
Tonopen Tip is protected by a disposable latex cover.
Pneumatonometer Tip should be cleaned with an alcohol sponge, taking care to dry the surface before use. Alternative is the use of disposable latex cover over the tip.
Bibliography 1. Armaly MF. On the distribution of applanation pressure. I. Statistical features and the effect of age, sex, and family history of glaucoma. Arch Ophthalmol 1965;73:11. 2. Bengtsson B. Comparison of Schiøtz and Goldmann tonometry and tonography in a population. Acta Ophthalmol (Copenh) 1972;50: 455.
Tonometry 3. Craven ER, et al. Applanation tonometer tip sterilization for adenovirus type 8. Ophthalmology 1987;94:1538. 4. Drance SM. The coefficient of scleral rigidity in normal and glaucomatous eyes. Arch Ophthalmol 1960;63:668. 5. Dunn JS, Brubaker RF: Perkins applanation tonometer, clinical and laboratory evaluation. Arch Ophthalmol 1973;89:149. 6. Durhan DG, Bigliano RP, Masino JA: Pneumatic applanation tonometer. Trans Am Acad Ophthalmol Otolaryngol 1965;69:1029. 7. Finlay RD. Experience with the Draeger applanation tonometer. Trans Ophthalmol Soc UK 1970;90:887. 8. Forbes M, Pico GJ, Goldmann B: A noncontact applanation tonometer description and clinical evaluation. Arch Ophthalmol 1974;91:134. 9. Friedenwald JS. Standardization of tonometers decennial report. Trans Am Acad Ophthalmol Otolaryngol 1954;58. 10. Friedenwald JS. Contribution to the theory and practice of tonometry. Am J Ophthalmol 1937; 20:985. 11. Friedman E, et al. Increased scleral rigidity and age-related macular degeneration. Ophthalmology 1989;96:104. 12. Glouster J, Perkins ES. The validity of the ImbertFick law as applied to applanation tonometry. Exp Eye Res 1963;2:274. 13. Grolman B. Non-contact applanation tonometry. Optician 1973;166:4. 14. Hollows FC, Graham PA: Intraocular pressure, glaucoma, and glaucoma suspects in a defined population. Br J Ophthalmol 1966;50:570. 15. Imbert A. Theories ophthalmotonometers: Arch Ophthalmol 1885;5:358. 16. Kaufman HE, Wind CA, Waltman SR. Validity of Mackay-Marg electronic applanation tonometer in patients with scarred irregular corneas. Am J Ophthalmol 1970;69:1003. 17. Khan JA, et al. Comparison of Oculab Tono-Pen readings obtained from various corneal and scleral locations. Arch Ophthalmol 1991; 109: 1444. 18. Krieglstein GK, Waller WK. Goldmann applanation versus hand-applanation and Schiøtz indentation tonometry. Graefes Arch Clin Exp Ophthalmol 1975;194:11. 19. Kronfeld PC. Tonometer calibration empirical validation: the committee on standardization of
20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.
32. 33.
34. 34. 36. 37.
tonometers. Trans Am Acad Ophthalmol Otolaryngol 1957;61:123. Langham ME, McCarthy E. A rapid pneumatic applanation tonometer: comparative findings and evaluation. Arch Ophthalmol 1968;79:389. Macro FJ, Brubakar RF. Methodology of eye pressure measurement. Biorheology 1969;6:37. Markiewitz HH. The so-called Imbert Fick law (Correspondence). Arch Ophthalmol 1960;64:159. McMillan F, Forster RK. Comparison of MackayMarg, Goldmann, and Perkins tonometers in abnormal corneas. Arch Ophthalmol 1975;93:420. Moses RA. Fluorescein in applanation tonometry. Am J Ophthalmol 1960;49:1149. Moses RA. The Goldmann applanation tonometer. Am J Ophthalmol 1958;46:865. Pepose JS, et al. Disinfection of Goldmann tonometers against human immunodeficiency virus type I. Arch Ophthalmol 1989;107:983. Perkins ES. Hand-held applanation tonometer. Br J Ophthalmol 1965;49:591. Petersen WC, Schlegel WA. Mackay-Marg tonometry by technicians. Am J Ophthalmol 1973;76:933. Posner A. Practical problems in the use of the Maklakov tonometer. EENT J 1963;42:82. Posner A. An evaluation of the Maklakov applanation tonometer. EENT J 1962;41:377. Rootman DS, et al. Accuracy and precision of the Tono-Pen in measuring intraocular pressure after keratoplasty and epikeratophakia in scarred corneas. Arch Ophthalmol 1988;106:1697. Schmidt T. The clinical application of the Goldmann applanation tonometer. Am J Ophthalmol 1960;49:967. Schwartz NJ, Mackay RS, Sackman JL. A theoretical and experimental study of the mechanical behavior of the cornea with application to the measurement of intraocular pressure. Bull Math Biol 1966;28:285. Schields MB. The noncontact tonometer: Its value and limitations. Surv Ophthalmol 1980;24:211. Starrels ME. The measurement of intraocular pressure. Int Ophthalmol Clin 1979;19:9. Stepanik J. Tonometry results using a corneal applanation 3.53 mm in diameter. Klin Monatsbl Augenheidkld 1984;184:40. Ventura LM, Dix RD. Viability of herpes simplex type I on the applanation tonometer. Am J Ophthalmol 1987;103:48.
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Diagnostic Procedures in Ophthalmology
A NARAYANASWAMY, L VIJAYA
7
Gonioscopy
Gonioscopy, the visualization and assessment of the anterior chamber angle, is an essential procedure in the diagnosis and management of glaucoma. The term gonioscopy was coined by Trantas in 1907. Subsequently, Goldmann introduced the gonioprism, and Barkan mastered the art of gonioscopy and highlighted its role in the management of glaucoma. All cases of glaucoma should undergo a routine and periodic gonioscopic evaluation. The procedure is fairly easy to perform, but experience is needed in accurate assessment and interpretation.
Optical Principles The anatomy of the eye is such that the angle recess is not visualized by routine instrumentation due to total internal reflection of rays emerging from the angle recess. The gonioscope was evolved to overcome this optical problem of critical angle (Fig. 7.1).
Types of Gonioscopy Direct Gonioscopy Direct gonioscopy is performed with the aid of
Fig. 7.1: Optical principles of gonioscopy: a: Incident light from the angle exceeds critical angle resulting in total internal reflection and preventing visibility of the recess. b and c: The gonio lens optically eliminates the cornea as shown in the schematic diagrams and allows visibility of the angle
concave contact lenses (e.g. Koeppe) placed over an anesthetized cornea with the patient in supine position and the space between the lens and cornea filled with normal saline or methyl cellulose as a coupling agent. Viewing is achieved directly using a hand-held biomicroscope and an illuminator. Alternatively, the operating microscope can be used to evaluate the angle of the anterior chamber by making appropriate adjustments. Koeppe’s lenses are available in diameters of 16 mm and 18 mm allowing easy
Gonioscopy TABLE 7.1: CONTACT LENSES USED FOR GONIOSCOPY Type
Lenses
Features
Direct
1. Koeppe
Diagnostic lens—50 diopter concave lens available in two sizes for infants (16 mm) and adults (18 mm) Surgical lens—available in various sizes and has blunted edges allowing access for goniotomy Surgical and diagnostic lens Surgical lens for goniotomy Diagnostic lens for evaluating neonatal angle
2. Barkan 3. Thorpe 4. Swan-Jacob 5. Layden Indirect
1. Goldmann single mirror and three mirrors 2. Zeiss and Posner four mirrors 3. Sussman four mirrors 4. Ritch trabeculoplasty lens
Diagnostic and therapeutic lenses, provide excellent images with good magnification and globe stability Ideal diagnostic lenses, patient friendly and very valuable in evaluating narrow angles and to perform indentation gonioscopy Hand held four mirrors similar advantages as the Zeiss lenses Four-mirrored lens with pairs inclined at 59 and 62 degrees. One of each set has a convex lens over it providing magnification—both diagnostic and therapeutic
use in pediatric patients. This technique can be practiced both in the outpatient clinic as well as in the operation theatre. A major advantage of this method is that it allows simultaneous comparison of different quadrants of the angle. Apart from the diagnostic value, lenses like the Swan Jacob, Barkan and Thorpe aid in surgical intervention (Figs 7.2 and 7.3).
Fig. 7.2: Koeppe’s lenses
Indirect Gonioscopy Indirect gonioscopy employs reflecting prisms (e.g. Goldmann lens) mounted in a contact lens and angulated at appropriate degrees to evaluate the angle structures using the slit-lamp. The most popular lenses are the Goldmann type, Zeiss, Posner and Sussman four mirrors (Table 7.1). Goldmann lenses (Fig. 7.4) are of two types: (i) Single mirrored—has a mirror angulated at 62°, (ii) Three-mirrored lens—has mirrors at 59° (tongue-shaped, used to evaluate the angle), 67°
Fig. 7.3: Surgical lenses: Barkan and Thorpe
Fig. 7.4: Goldmann lenses three and single mirror
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Diagnostic Procedures in Ophthalmology (midsized, used to view midperipheral fundus) and 73° (long, used to view peripheral fundus and ciliary body). The central wall has a diameter of 12 mm and radius of curvature of 7.38 mm. A newer modified version with 8.4 mm radius of curvature eliminates the need of using a coupling solution. The three mirrors also aid in retinal evaluation and laser therapy. Zeiss lens (has under holder), or Posner (has a screw-in handle) four mirror has mirrors angulated at 64° and are amongst the most popular gonioscopy lenses. The Zeiss four mirror (Fig. 7.5) eliminates the need for rotation to evaluate the angle and it’s radius of curvature is 7.8 mm, closer to the corneal curvature, thereby eliminating the need for a viscous coupling agent. The diameter of the lens is 9.0 mm which aids in dynamic or compression gonioscopy, an important technique in evaluating narrow angles and angle-closure glaucomas.
4. Adequate anesthesia is ensured using either 0.5% topical proparacaine or 4.0% lignocaine. 5. The patient and examiner should be in a comfortable posture with adequate support to examiner’s forearm and elbow to make sure of good control and minimal pressure over the eye throughout the procedure. 6. The lens is held in the examiner’s left hand for evaluating the right eye and vice versa. 7. The three-mirror gonioscope is filled with viscous solution and inserted as shown in Figure 7.6. The four-mirror is applied directly (Fig. 7.7).
Fig. 7.6: The inferior rim of three mirror gonioscope is inserted in the lower fornix with patient in upgaze as shown and swiftly tilted on to the cornea preventing loss of any coupling fluid
Fig. 7.5: Zeiss four mirror lens
Protocol for a Routine Gonioscopy 1. Explain the procedure to the patient. 2. Reassure the patient and ensure cooperation. 3. Corneal surface is examined to rule out any contraindication for gonioscopy (abrasion, infection, significant corneal edema or opacity).
Fig. 7.7: The four mirror gonioscope is applied gently and directly on to the cornea. Fingers rested over cheek to ensure adequate support and prevent inadvertent pressure over the globe
Gonioscopy 8. The patient is asked to maintain a straight gaze once the lens is in situ. 9. Low, but adequate illumination, and small beams are focused on the mirror, with viewing and illumination maintained in the same axis. The illumination arm is moved paraxial when needed to evaluate the nasal and temporal recesses. Magnification and illumination can be increased when needed to evaluate finer details like new vessels and foreign bodies. One quadrant can be evaluated at a time with the three mirror by sequential rotation while with the four mirror gonioscope all four quadrants can be evaluated without rotation and with minimum adjustments of the slit-lamp. Always remember the opposite quadrant (e.g. with mirror at 7 o’clock, the 1 o’clock angle) is being evaluated and the image is reversed but not crossed. Other dynamic maneuvers like compression and over the hill evaluation are subsequently done. Over the hill maneuver involves asking the patient to look in the direction of the mirror; which in turn gives access to viewing angle recess over the convex iris. Compression techniques will be dealt with subsequently. 10. Disinfection of lenses is necessary prior and after every use because of the potential of transmitting infection. Lenses can be swiped dry with bacillocid (2% gluteraldehyde) or alternatively lenses can be rinsed with soap solution and water and allowed to dry.
Gonioscopic Anatomy and Interpretation Repeated and routine normal gonioscopic studies are essential in adding to one’s experience in evaluating a pathological angle. A methodical evaluation of each structure either from iris plane
Fig. 7.8: Gonioscopic landmarks of a normal angle: 1 Iris root, 2 Ciliary body band, 3 Scleral spur, 4 Trabecular meshwork, 5 Schwalbe’s line, 6 Schlemm’s canal, 7 Parallelopiped effect
to Schwalbe’s line (Fig. 7.8) or from iris plane to Schwalbe’s should curtail errors in interpretation. To start with, from the peripheral iris plane one can follow upwards to the insertion of iris root. The contour of iris has several variations. The normal adult eye has a slightly convex contour. The same may be exaggerated in hyperopic eyes, where in the anterior segment it is crowded. A flat iris configuration is commonly associated with myopia and aphakia. A flat iris configuration with a peripheral convex roll or hump of iris that lies in close relation to the trabecular meshwork and can be seen in phakic normal eyes which often mimics a narrowangle and is referred to as plateau iris configuration. Contours could also be concave and are associated with high myopes and pigment dispersion syndrome. The insertion of iris root, may vary from a posterior, anterior or high insertions, thereby determining the visibility of the ciliary body band and the contour and depth of angle recess. The ciliary body band is composed of the anterior end of ciliary muscle
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Diagnostic Procedures in Ophthalmology and is seen as a slate gray or dark brown uniform band when insertion of iris root is posterior, anterior and high insertions preclude its view. An unusually wide ciliary body band may be seen in myopes and aphakes and may be confused with angle recession, but comparative gonioscopy and other signs of trauma help to distinguish between the normal and the pathological. The next anterior transition is the scleral spur, the most prominent and most important landmark, identification of which is vital in terms of orientation of the angle. The scleral spur is the posterior lip of the scleral sulcus and is attached to the ciliary body band posteriorly and to the corneoscleral portion of trabecular meshwork anteriorly. It is visible as a glistening opaque white line between the ciliary body band and trabecular meshwork, however, identification at times may be difficult when the trabeculum is nonpigmented. The scleral spur may be obscured in the presence of dense pigmentation of angle structures like in posttraumatic or postsurgical situations. Iris processes, which are fine uveal strands arising from anterior iris surface and running upto the corneoscleral meshwork may also prevent a good view especially when they are prominent, as seen in congenital glaucomas. The spur is not visible in the presence of peripheral anterior synechiae or appositional angle-closure on routine gonioscopy. The trabecular meshwork has a posterior functional, more pigmented portion and a less functional nonpigmented anterior portion. The corneoscleral part of the meshwork extends from the scleral spur to the Schwalbe’s line. The pigmentation of the meshwork varies with the kind of eyes, age and other pathological conditions. Brown eyes and adult eyes tend to have a deeper pigmentation compared to blue eyes and younger individuals. A nonpigmented trabecular meshwork may often present a tricky situation as far as accurate assessment is concerned, since its color and texture seems to
merge with the scleral spur. However, a careful evaluation reveals it to be a more translucent and less white structure. The parallelopiped effect is a useful adjunct that can be used in situations wherein the landmarks are indistinguishable. This effect causes a narrow-slit beam of light that is reflected from the anterior and posterior corneal surfaces to collapse at the Schwalbe’s line. Once this point is identified the other landmarks can be assessed based on the distance from the line. The Schlemm’s canal is usually not visible, but can be seen through a less pigmented posterior trabeculum when reflux blood fills up either due to raised episcleral venous pressure, or rarely as a normal phenomenon. Excess pressure over the globe especially with a threemirror gonioscope can also cause artifactual filling up of the Schlemm’s canal with blood. Schwalbe’s line as described before represents the peripheral termination of the Descemet’s membrane. Usually optically identified by the parallelopiped method, it also at times appears as a prominent white ridge known as posterior embryotoxon, a misnomer. This ridge is better appreciated when the patient looks in the direction of the mirror and is more prominent in the temporal quadrants. The line may occasionally be pigmented and is referred to as Sampaolesi line as seen in pseudoexfoliation and pigment dispersion syndrome.
Pediatric Eye The pediatric eye has definite but subtle variations in its anatomy. The iris contour in a newborn is usually flat and its insertion is posterior to scleral spur with the anterior extension of ciliary body band visible. This contour does eventually become convex as the angle recess develops in 6-12 months. The trabecular meshwork is nonpigmented and appears thick and translucent. Congenital glaucomas present with
Gonioscopy TABLE 7.2: CLASSIFICATION SYSTEMS FOR GONIOSCOPY System
Scheie (1957)
Shaffer (1960)
System basis
Extent of angle structures visualized
Angular width of recess Insertion of iris root
Spaeth (1971) Angular approach to the recess Configuration of peripheral iris
Angle structures and classification All structures visible Angle recess not seen Ciliary body band not seen Posterior trabeculum obscured Only Schwalbe’s line visible
Wide open Grade I narrow Grade II narrow Grade III narrow Grade IV narrow
Wide open (30°-45°) Moderately narrow (20°) Extremely narrow (10°) Partly or totally closed
Grade Grade Grade Grade
Anterior to Schwalbe’s line Behind (posterior) to Schwalbe’s line At scleral spur Deep into ciliary body band Extremely deep 0-40 degrees
A B C D E
Regular (slightly convex) Quirk (posterior bowing) Steep
r q s
3-4, closure impossible 2, closure possible 1, closure probable 0, closure present
anterior insertions of the iris directly on to the trabeculum and at times the anterior iris stroma sweeps upward in a concave fashion to insert onto the trabecular meshwork.
Grading and Recording of Gonioscopic Findings Though multiple individual variations in assessment and grading gonioscopic details are being followed, it is important to follow a certain protocol of documentation, which aids in follow up of the disease process. Among the systems described (Table 7.2), the Spaeth’s system is thought to be complete as it covers details with regard to angle width, iris insertion and configuration. Any gonioscopic data should contain: (a) width of angle recess, (b) iris contour and insertion of iris root, (c) degree of pigmentation and (d) presence of abnormal structures in each quadrant. Figure 7.9 shows a wide-open angle (Shaffer’s grade IV or Speath’s D40r) with regular iris contour and deep recess.
Fig. 7.9: Gonio-photograph of a grade IV Shaffer’s angle (corresponds to Spaeth—D40r). (a) Iris root, (b) Ciliary body band, (c) Scleral spur, (d) Trabecular meshwork. Iris contour is regular with a deep recess
All the landmarks—iris root, ciliary body band, scleral spur and trabecular meshwork are visible. When insertion of iris occurs at scleral spur, the peripheral iris appears slightly convex, the angle of the anterior chamber still remains open (Shaffer’s grade III or Speath’s C30r, Fig. 7.10).
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Fig. 7.10: Gonio-photograph of a grade III Shaffer’s angle (corresponds to Spaeth—C30r). Landmarks are visible upto scleral spur with a mild iris convexity
Compression Gonioscopy Compression or indentation gonioscopy is a simple and invaluable technique that one needs to know to assess narrow angles (Fig. 7.11) and chronic angle-closure situations. It helps distinguish appositional angle-closure from synechial angle-closure. The technique employs exerting external pressure over the cornea using the Zeiss, Posner or Sussman four mirror lenses; thereby forcing the lens iris diaphragm posteriorly and allowing to visualize the hidden angle recess (Fig. 7.12). The technique involves a routine assessment of all quadrants, following which, if one subsequently decides the angle is narrow, each
Fig. 7.12: The same angle on compression widens to reveal landmarks upto scleral spur
quadrant is re-evaluated using a narrow slitbeam (to prevent miosis causing artifactual opening of the angle recess), pressure is applied directed towards the center of the eye. This results in deepening of the anterior chamber in the area of recess caused by bowing back of peripheral iris along with stretching of the limbal scleral ring and straightening of the angle recess; following this one can see structures that were not visible earlier, or confirm the presence of peripheral anterior synechiae. Corneal folds often distort the view but this can be minimized with appropriate technique in application of pressure. The physiological principles involved in compression gonioscopy have been depicted in Figure 7.13. Compression may not be effective when intraocular pressures are beyond 40 mm Hg as this limits the expansion of the limbal scleral ring.
Common Gonioscopic Findings and their Variations Peripheral Anterior Synechia (PAS)
Fig. 7.11: The photograph shows a narrow angle visible upto the Schwalbe’s line
The peripheral anterior synechia is a pathological term referring to the adhesions of peripheral iris to the anterior angle structures, most often the functional trabecular meshwork, or rarely,
Gonioscopy arising from the peripheral iris surface and branching out in an arborizing and lacy pattern onto the corneoscleral portion of trabecular meshwork. Varying amounts of peripheral anterior synechiae may also be associated depending on the stage of disease process.
Pigmentation Fig. 7.13: Compression gonioscopy: a: The narrow angle appears closed on a routine gonioscopy, b: Compression fails to allow visibility of angle structures due to PAS, c: Compression widens the recess and allows a view of all structures in the absence of PAS
extending to the Schwalbe’s line. Typically seen associated with primary angle-closure glaucoma, uveitic and other secondary angle-closure glaucomas, PAS may often be confused with iris processes—which are normal fine lacy cords of uveal tissue extending from the peripheral iris to the trabecular meshwork. PAS on the other hand are broad adhesions commonly localized to quadrants with areas in between widening with indentation technique of gonioscopy. An angle that is closed 360° may often present a dilemma but one can follow the slit-beam from the posterior surface of the cornea which normally does not meet the beam on the iris directly in an angle that is open but instead lies alongside the other. A direct continuation of the beam without a break is suggestive of a closed-angle. Clinical correlation and experience will often help overcome this hurdle.
The trabecular meshwork has a varying amount of pigmentation varying from 0 to 4, which is a subjective grading that correlates to none (0), faint (1), average (2), heavy (3), and very heavy (4). Pigmentation increases with age under normal physiological conditions. Excessive pigmentation is usually pathological and is associated with pseudoexfoliation syndrome, pigment dispersion syndrome, traumatic and uveitic glaucomas.
Other Abnormal Findings A variety of surprises may be hidden in the angle recess. Blood in Schlemm’s canal appears as a uniform linear reddish hue just anterior to pigmented trabecular meshwork and is associated with raised episcleral venous pressure. It can also be observed under normal conditions and as an artifact when excess external pressure is exerted during gonioscopy. Pseudoexfoliative material, microscopic hyphema and hypopyon can be visualized. Foreign bodies and emulsified silicone oil globules are among the other things that can be picked up by a careful gonioscopy.
Blood Vessels Normally all vessels in the angle are restricted to the ciliary body band and iris root and do not extend to the scleral spur or trabecular meshwork. Anomalous vessels are not rare, they, however, can readily be distinguished from neovascularization which are vessels usually
Conclusion In conclusion, the diagnostic basis of any glaucoma should be in correlation to the gonioscopic findings whenever possible. The management and prognosis of the disease depends on a complete diagnosis that includes
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Bibliography 1. Epstein DL. Chandler and Grant’s Glaucoma (3rd edn). Philadelphia: Lea and Febiger, 1986. 2. Fellman RL, Spaeth GL, Starita RJ. Gonioscopy: Key to successful management of glaucoma. In Focal points Clinical Modules for Ophthalmologists, San Francisco, AAO 1984.
3. Kanski JJ, James AM, John FS. Glaucoma—A Colour Manual of Diagnosis and Treatment (2nd edn). London, Butterworth-Heinemann, 1996. 4. Kolker AE, Hetherington J Jr. Becker-Shaffer’s Diagnosis and Therapy of the Glaucomas (5th edn). St Louis, Mosby, 1985. 5. Neil TC, Diane CL. Atlas of Glaucoma. Martin Dunitz, 1998;39. 6. Palmberg P. Gonioscopy. In Ritch R, Shields MB, Krupin T (Eds). Glaucomas (2nd edn). St Louis, Mosby, 1996. 7. Shields MB. Aqueous humor dynamics. II. Techniques for evaluating. In: Textbook of Glaucoma (3rd edn). Baltimore, Williams and Wilkins, 1992.
Optic Disk Assessment in Glaucoma
RAJUL PARIKH, CHANDRA SEKHAR
8
Optic Disk Assessment in Glaucoma
An estimated 67 million people worldwide have glaucoma in the year 2000. At least 50% do not know that they have the disease since it is usually without symptoms.1,2 Rapid advances in imaging technologies such as confocal scanning laser ophthalmoscopy, scanning laser polarimetry and optical coherence tomography for detection of early glaucomatous damage have only moderate sensitivity and specificity.3-5 New psychophysical procedures such as short wavelength automated perimetry, frequency doubling perimetry and motion automated perimetry which are targeted at specific visual functions have been shown to be more sensitive and specific than standard automated perimetry for identifying early glaucomatous damage. 6-8 However, these techniques may not be available to all clinicians and have the limitations of all subjective tests. Several studies have shown that abnormalities in the appearance of the optic disk may precede visual field defects.9,10 Conventional stereoscopic clinical evaluation and imaging of the optic disk with fundus photographs is still the most frequently used and sensitive means of diagnosing glaucoma. 11 With some training, it is possible to clinically evaluate optic nerve head and retinal nerve fiber layer stereoscopically and detect early glaucomatous damage. The aim
of this communication is to describe the morphological changes of the optic nerve in glaucoma, highlight the techniques of clinical evaluation of the optic disk and discuss the differential diagnosis.
Methods of Optic Disk Examination Traditionally, the direct ophthalmoscope has been used for the evaluation of the optic nerve head. Though it has the advantage of providing a magnified view of the optic nerve head, it, however, lacks stereopsis and can result in missing of subtle changes. Therefore, the use of the direct ophthalmoscope is to be strongly discouraged. A variety of contact and noncontact lenses are available which allow stereoscopic view of the fundus at the slit-lamp. Contact lenses such as Goldmann lenses are relatively uncomfortable for the patient, take longer time and the coupling fluid can cause transient blurring and difficulty in obtaining good quality fundus photographs. Noncontact lenses include +60D, +78D, +90D and Volk superfield lenses (Fig. 8.1). These provide excellent stereoscopic and magnified view of the optic disk.
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Fig. 8.1: Noncontact lenses: +60D, +78D and Volk superfield lenses
It is important to draw the appearance of the optic nerve head based on these methods. Though drawing of the optic disk suffers from the disadvantage of being subjective in nature, this does offer a quick and inexpensive method of evaluation of the optic nerve head in patients with glaucoma during follow-up. In addition, photographs may not be possible in all cases such as patients with rigid miotic pupils and those with significant media opacities. However, wherever possible, photographs are an indispensable adjunct to clinical evaluation.
Fig. 8.2: Disk margin (black arrow) and cup margin (white arrow)
Features of Glaucomatous Disk Damage Cup-Disk Ratio Early studies by Armaly et al have reported that the vertical and horizontal cup-disk diameter ratios are useful for the quantification of glaucomatous optic neuropathy and for early detection of glaucoma.12 The ratio has limited value in the identification of glaucomatous damage, because of the wide variability in the size of the optic cup in the normal population. Disk margin is defined by inner edge of white scleral ring (outer arrows), and the optic cup is the level at which neuroretinal rim (NRR) steeps (inner arrow) (Figs 8.2 and 8.3). A large cupdisk ratio can be normal if the optic disk is large13 and a small cup-disk ratio may be glaucomatous if the optic disk is small14 (Fig. 8.4). The problem with estimating cup-disk ratio as a measure of
A
B Figs 8.3A and B: Vertical disk diameter and horizontal disk diameter
Optic Disk Assessment in Glaucoma
A
Fig. 8.5: Measurement of disk diameter with slitlamp biomicroscopy with use of noncontact lenses
B Figs 8.4A and B: Cup-disk ratio in relation to optic disk size. A Optic disk is small with small cup and still has inferior notch (white arrow) with nerve fiber layer defect (black arrows) B Cup-disk ratio in a large disk
glaucomatous damage is that it is difficult to decide if the cup is physiological in a large disk or pathological in a small or normal-sized disk. In a recent study by Garway-Heath et al, vertical cup-disk diameter ratio corrected for the optic disk size was the best variable to separate between normal subjects and patients of ocular hypertension with retinal nerve fiber layer defect.15 Therefore, in the clinical description of the optic nerve head, it is important to state the vertical cup-disk diameter ratio in combination with the estimated disk size. The disk diameter can be easily measured by adjusting the slit-lamp beam height to the edges of the disk while viewing
the disk with a 60D lens (Fig. 8.5).16 The measurement by this method is roughly equal to the measurement obtained by the planimetry of disk photographs by Litmann’s correction. Measurements can also be made with other lenses by multiplying the measured value with the appropriate magnification factor, Goldmann contact lens X1.26 and Volk superfield lens X1.5.16 It is important to differentiate contour cupping from color cupping. The margin of the cup should be determined by the bend of the small vessels
Fig. 8.6: Asymmetry of cupping in relation to asymmetry of disk size. The left optic disk is larger than right optic disk and has a larger optic cup
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Fig. 8.7: HRT print out of the same optic disks shown in Fig. 8.6 showing asymmetry of optic disk cup in relation to disk area
across the disk rim and not by the central area of disk pallor.
Asymmetry of Optic Disk Cupping Asymmetry of cupping is seldom seen in normal eyes and until proven otherwise, must be taken as an indication of early glaucomatous damage. However, while assessing asymmetry, it is important to rule out asymmetry of the disk size, which may be due to anisometropia. This can result in difference in the cup-disk ratio between two eyes, in the absence of glaucoma (Figs 8.6 and 8.7).
Neuroretinal Rim Evaluation Glaucomatous damage can be diffuse, focal or a combination of both. Diffuse damage results in symmetrical enlargement of the cup. Focal damage usually involves a particular area of the rim. Normally, according to the ISNT rule, the inferior rim is the thickest followed by the superior, the nasal and then the temporal (Fig. 8.8).17 During optic nerve head evaluation, one
Fig. 8.8: Shows ISNT rule, the inferior rim is the thickest followed by the superior, the nasal and then the temporal
must look carefully for any areas of thinning of the neuroretinal rim or for notching or in other words extension of the cup into the rim tissue. If the cup is especially deep in the notch, it is known as a pseudo-pit. Notching and pseudopits are usually seen at the superior or inferior poles. The width of the notch tends to correspond to the extent of the visual field defect (Figs 8.9A and B, and 8.10A and B). Optic rim pallor is another important indicator of glaucomatous disk
Optic Disk Assessment in Glaucoma
A
B Figs 8.9A and B: Relation between neuroretinal rim notch and visual field defect. The optic disk photograph shows inferior notch (black arrow) with corresponding superior arcuate field defect
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A
B Figs 8.10A and B: Relation between inferior notch (here inferior notch is wider than the one seen in Fig. 8.9) and visual field defect. The optic disk photograph shows neuroretinal notch (black arrow) with corresponding superior arcuate field defect
Optic Disk Assessment in Glaucoma damage. In the glaucomatous optic disk, the pale and translucent atrophic tissue may replace the normal pink color of the neuroretinal rim which can result in a field defect in the corresponding opposite hemisphere.
Vascular Changes Splinter hemorrhages on the optic disk are a common finding in glaucoma patients (Fig. 8.11). Various studies have shown that disk hemorrhages in association with localized nerve fiber layer defects and notches of the neuroretinal rim are more common among patients of normal tension glaucoma.18, 19 A possible explanation for the difference in frequency has been suggested by Jonas et al. They stated that the amount of blood leaking out of a vessel into the surrounding tissue depends on the intraocular pressure when the bleeding occurs.19 High transmural pressure gradient in normal pressure glaucoma leads to larger disk hemorrhages. Also, since the absorption rate of disk hemorrhages depends on the size of the disk bleed, the hemorrhages in patients of normal pressure glaucoma may take a longer time to disappear and thus have a higher chance to be detected than the disk
Fig. 8.11: Disk hemorrhage
hemorrhages in patients of high pressure glaucoma.20 Hemorrhages in glaucoma usually appear as splinter-shaped or flame-shaped hemorrhages on the disk surface21 (Fig. 8.11). They usually precede neuroretinal rim changes and visual field defects. The defects corresponding to the location of the hemorrhage may be expected to appear weeks to year later.22 The presence of disk hemorrhages is considered an indication for the enhancement of treatment of glaucoma.
Configuration of Vessels The retinal vessels on the optic nerve head can provide clues about the topography of the disk. Nasalization of the vessels and baring of circumlinear vessels can be seen in glaucoma as well as in other diseases of the optic nerve. Bayoneting of the vessels can be seen if the rim is absent or very thin. This causes the vessels to pass under the overhanging edge of the cup and then make a sharp bend as they cross the disk surface. This convoluted appearance of the vessels is called ‘bayoneting’.
Peripapillary Atrophy The zone closer to the optic nerve head with retinal pigment epithelium (RPE) and choroidal atrophy and baring of sclera is called zone β. The more peripheral zone with only RPE atrophy is called zone α (Fig. 8.12). A highly significant correlation has been reported between the location of peripapillary atrophy and visual field defects.23 Sometimes, these changes may represent a congenital anomaly, especially in myopic eyes. However, appearance of these changes de novo or their presence in small, nonmyopic disks should be viewed with suspicion. Peripapillary atrophy may be focal or circumferential (Figs 8.13 and 8.14).
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Fig. 8.12: Peripapillary atrophy. The diagram shows atrophic zone closer to the optic nerve head called zone β and the more peripheral zone called zone α
Fig. 8.13: Peripapillary atrophy: Localized in the temporal area of the disk
Retinal Nerve Fiber Layer Abnormalities Examination of the nerve fiber layer is often useful in detecting early glaucomatous damage among patients of ocular hypertension with normal disk appearance and normal visual fields. The neuroretinal rim is formed by axons converging from the retina to the scleral canal. Since the axons are spread out in a thin layer in the retina, even minor losses of the axons can be observed in the retinal nerve fiber layer. In healthy eyes, the nerve fiber layer appears opaque with
Fig. 8.14: Peripapillary atrophy: generalized
radially oriented striations. The small retinal blood vessels have a blurred and crosshatched appearance, as they lie buried in the nerve fiber layer. The best way to see the nerve fiber layer defect is through a dilated pupil with a stereoscopic lens, at the slit-lamp, using white or green light and a wide-slit beam. In the presence of nerve fiber layer atrophy, the small retinal blood vessels become more clearly visible and appear unusually sharp, clear and well focused (Fig. 8.15). The fundus in the affected area appears darker and deeper red in contrast to the silvery or opaque hue of the intact nerve fiber layer. Defects may be in the form of a wedge shape arising from the disk margin and widening towards the periphery, are pathological (Fig. 8.16), while slit-like defects narrower than the adjacent blood vessels may be physiological. Diffuse areas of atrophy are less common in early glaucoma and more difficult to identify.
Myopic Changes vs Glaucoma Myopic disks can present difficulty in evaluation for glaucoma due to the tilted disks, peripapillary atrophy and shallow cupping. One needs to
Optic Disk Assessment in Glaucoma
Fig. 8.15: Retinal nerve fiber layer defect: Wedge-shaped RNFL defect can be seen between two black arrows. It is more easily marked in red free photograph
Fig. 8.17: Myopic disk with primary open-angle glaucoma Fig. 8.16: Retinal nerve fiber layer defect. Wedge-shaped RNFL defect reaching up to optic disk margin
carefully examine the disk to look for changes in the contour of the blood vessels, as well delineate the disk margin from the peripapillary changes (Fig. 8.17).
Differential Diagnosis In addition to glaucoma, other abnormalities can cause excavation and or pallor of the optic disk
and it is, therefore, important to rule out these possibilities before making the diagnosis of glaucoma.
Physiological Cupping Assessment of the size of the optic disk, careful examination of the neuroretinal rim and the retinal nerve fiber layer can help distinguish physiological cupping from glaucomatous damage in most cases.
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Diagnostic Procedures in Ophthalmology Optic Nerve Coloboma Optic nerve colobomas typically demonstrate enlargement of the papillary region, partial or complete excavation, blood vessels entering and exiting from the border of the defect and a glistening white surface. The visual field defects can be in the form of generalized constriction, centrocecal scotomas, altitudinal defects, arcuate scotomas, enlargement of the blind spot and ring scotomas that can mimic those found in glaucomatous eyes. Morning glory syndrome is a variant of optic disc coloboma and is characterized by a large excavated disk, central core of white or gray glial tissue surrounded by an elevated annulus of variably pigmented subretinal tissue (Fig. 8.18). The retinal vessels appear to enter and exit from the margins of the disk, are straightened and often sheathed.
Fig. 8.19: Optic disk photograph showing congenital optic disk pits
in about one-third. Involvement is usually unilateral in about 80% cases and the optic disk is larger on the involved side. Approximately 55-60% of the eyes have a field defect in the form of arcuate scotomas, paracentral scotoma, altitudinal defect, generalized constriction and nasal or temporal steps.24 In the absence of other indicators of congenital anomaly (like associated fundus coloboma, the differential diagnosis may be difficult and the absence of progression on follow-up may be the only indicator that the patient has a congenital anomaly and not glaucoma. Fig. 8.18: Optic disk photograph showing characteristic morning glory syndrome
Congenital Optic Disk Pit Congenital optic disk pits appear gray or yellowish-white, round or oval, localized depression within the optic nerve (Fig. 8. 19). They are located within the temporal aspect of the disk in over half of the cases and centrally
Anterior Ischemic Optic Neuropathy A history of acute visual loss, initial swelling of the optic disk, absence of marked cupping, rise in ESR, presence of centrocecal scotoma or altitudinal defects can help differentiate it from glaucoma (Fig. 8.20). In the late stages the cupping in some cases may be exactly the same as is seen in glaucoma.
Optic Disk Assessment in Glaucoma
Fig. 8.20: Anterior ischemic optic neuropathy. The right-sided optic disk photograph is from patients with longstanding AION showing typical glaucomatous cupping
A
B Figs 8.21A and B: A Optic disk photograph showing significant cupping, but with out of proportion pallor. B Visual field defect showing a temporal hemianopia suggestive of pituitary tumor
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Diagnostic Procedures in Ophthalmology Neurological Causes Pallor disproportionate to cupping, normal intraocular pressure or unusual history of onset, progression and age should arouse suspicion of a neurological disorder causing optic disk damage (Fig. 8.21). Presence of visual field defects that respect vertical midline and the pattern of the field defects should be able to suggest the possible site of the intracranial lesion.
Summary In summary, the optic disk evaluation in glaucoma is best done stereoscopically at the slit- lamp with a dilated pupil using one of the 60D, 78D or 90D lenses. Changes in the neuroretinal rim, optic disk hemorrhages, peripapillary atrophy and nerve fiber layer defects are more important features than the cup-disk ratio. The cup-disk ratio is to be documented and interpreted along with the disk size and not in isolation. The diagnosis of glaucoma will depend on the presence of a visual field defect that correlates with the anatomic changes on the optic nerve head and the peripapillary retina.
References 1. Quigley HA. Number of people with glaucoma worldwide. Br J Ophthalmol 1996;80:389-93. 2. Dandona L, Dandona R, Srinivas M, et al. Openangle glaucoma in an urban population in southern India: the Andhra Pradesh Eye Disease Study. Ophthalmology 2000; 107(9): 170209. 3. Zangwill LM, Bowd C, Berry CC, Williams J, Blumenthal EZ, SanchezGoleans CA, Vasilie C, Wainreb RN. Discriminating between normal and glaucomatous eyes using the Heidelberg retina tomograph, GDx nerve fibre analyser and optical coherence tomograph. Arch Ophthalmol 2001;119:985-93.
4. Bowd C, Zangwill LM, Berry CC, Blumenthal EZ, et al. Detecting early glaucoma by assessment of retinal nerve fibre layer thickness and visual functions. Invest Ophthalmol Vis Sci 2001;42:2001-03. 5. Medeiros FA, Zangwill LM, Bowd C, Weinreb RN. Comparison of the GDx VCC scanning laser polarimeter, HRT II confocal scanning laser ophthalmoscope, and stratus OCT optical coherence tomograph for the detection of glaucoma. Arch Ophthalmol 2004;122;827-37. 6. Johnson CA, Adams AJ, Casson EJ, Brandt JD. Blue-on-yellow perimetry can predict the development of glaucomatous field loss. Arch Ophthalmol 1993;111:645-50. 7. Bayer AU, Maag KP, Erb C. Detection of optic neuropathy in glaucomatous eyes with standard visual fields using a battery of short wave-length automated perimetry and pattern electroretinography. Ophthalmology 2002;109: 1009-17. 8. Sample PA, Bosworth CF, Blumenthal EZ, Girkin C, Weinreb RN. Visual function-specific perimetry for indirect comparison of different ganglion cell populations in glaucoma. Invest Ophthalmol Vis Sci 2000;41:1783-90. 9. Quigley HA, Dunkelberger GR, Baginski TA, et al. Chronic human glaucoma causing selectively greater loss of larger optic nerve fibers. Ophthalmology 1988;95:357-63. 10. Sommer A, Pollack I, Maumenne AE. Optic disc parameters and onset of glaucomatous field loss: I Methods and changes in disc morphology. Arch Ophthalmol 1979;97:1444-48. 11. Greaney MJ, Hoffman DC, Garway-Heath DF, et al. Comparison of optic nerve imaging methods to distinguish normal eyes from those with glaucoma. Invest Ophthalmol Vis Sci 2002; 43(1):140-45. 12. Armaly MF, Saydegh RE. The cup/disc ratio. Arch Ophthalmol 1969;82:191-96. 13. Jonas JB, Zach F-M, Gusek GC, Naumann GOH. Pseudoglaucomatous physiologic large cups. Am J Ophthalmol 1989;107:137-44. 14. Jonas JB, Fernandez MC, Naumann GOH. Glaucomatous optic nerve atrophy in small disks with low cup-to-disc ratios. Ophthalmology 1990;97:1211-15. 15. Garway-Heath DF, Ruben ST, Viswanathan A, Hitchings R. Vertical cup/disk ratio in relation to optic disk size: its value in the assessment
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16. 17.
18. 19.
of the glaucoma suspect. Br J Ophthalmol 1999; 82:1118-24. Jonas JB, Dichtl A. Advances in the assessment of the optic disc changes in early glaucoma. Cur Opi Ophthalmol 1995;6:61-66. Jonas JB, Gusek GC, Naumann GOH. Optic disc, cup and neuroretinal rim size, configuration, and correlations in normal eyes. Invest Ophthalmol Vis Sci 1991;29,1151-58, Invest Ophthalmol Vis Sci 1993;32. Kitazawa Y, Shirato S, Yamamoto T. Optic disc hemorrhage in low-tension glaucoma. Ophthalmology 1986;93:853-57. Jonas JB, Budde WM. Optic nerve head appearance in juvenile-onset chronic highpressure glaucoma and normal-pressure glaucoma. Ophthalmology 2000;107:704-11.
20. Jonas JB, Xu L. Optic disc hemorrhages in glaucoma. Am J Ophthalmol 1994;118:1-8. 21. Drance S.M, Fairclough M, Butler DM, Kottler MS. The importance of disc haemorrhage in the prognosis of chronic open-angle glaucoma. Arch Ophthalmol 1977;95:226-28. 22. Heijl A. Frequent disc photography and computerized perimetry in eyes with optic disc haemorrhage. Acta Ophthalmol 1986;64: 274-81. 23. Jonas JB. Naumann GOH. Parapapillary chorioretinal atrophy in normal and glaucoma eyes. II. Correlations. Invest Ophthalmol Vis Sci 1989;30:919-26. 24. Brown GC. Congenital fundus abnormalities. In: Duane TD (Ed). Clinical Ophthalmology 1991, Philadelphia, J.B. Lippincott.
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DEVINDRA SOOD, PARMOD KUMAR
9
Basic Perimetry
Visual field is a part of space, seen at any given moment. Changes in the visual field are produced by a number of disease conditions which can affect the visual system and often manifest through changes in the visual field. Hence, it is essential to determine the extent of the visual field for the diagnosis and management of these conditions. The visual field is usually perceived with both eyes. It is, however, measured separately for each eye. The normal visual field extends up to 50 degrees superiorly, 70 degrees inferiorly, 60 degrees nasally and 90 degrees temporally. After defining the visual field for each eye, the two can be compared with each other for asymmetry or compared to a normal reference test for any abnormality and be examined together to look for patterns suggestive of disease conditions. Perimetry is the science of measuring the peripheral vision (“Peri”= peripheral and “-metry" = measurement). Perimetry involves placing the eye at the center of curvature of a hemispherical or arc-shaped instrument. The test objects have a constant angular size and are at a constant distance from the eye. The visual field has been compared to an island of vision in a sea of blindness by Traquair in 1930. This island of vision is a three dimensional structure. The
x and y co-ordinates represent the location of points on the visual field. At the fovea, the x and y co-ordinates are 0,0. The location of all points on the visual field are described along the x and y axis, with respect to fixation (Fig. 9.1). The blind spot is 15 degrees temporal to fixation. The z axis represents the height of the “hill island of vision” at a given co-ordinate (x,y) and corresponds to the retinal sensitivity at that point. Greater the sensitivity at a given point, greater is the height of the island
Fig. 9.1: A point on the island of vision is marked along the x and y axis
Basic Perimetry of vision. Since sensitivity is maximum at the fovea, the height of “the hill island of vision” (z) is also maximum at the fovea. The retinal sensitivity drops to sea level 15 degrees temporal to fixation (blind spot).
Types of Perimetry Kinetic Perimetry Perimetry aims to draw the map of the island of vision, such that it is a true representation for each eye and also aims to present it in a way which is clinically useful. Earlier methods defined the outer limits of the visual field by moving objects from the non-seeing area to the center. This technique of perimetry, called kinetic perimetry, it utilizes a moving object of a fixed size and intensity (e.g. Tangent screen or Goldmann perimeter) to define the boundary of the island of vision at a fixed height. This line representing the outer boundary for a given size of the test stimulus is called isopter. An isopter is synonymous to a horizontal slice through the hill island of vision. Manual kinetic perimetry allows large areas to be traversed in a fairly short order. One can move quickly over areas of little interest and spend relatively more time in examining critical regions. Equipment is inexpensive and durable. Since the perimetrist is constantly communicating with the patient, the patient is more comfortable. However, reproducible and reliable examinations require technical skill and early or subtle changes are more likely to be overlooked on manual kinetic perimetry. Isopters which are stylized representations of the visual field, making quantification and statistical analysis difficult.
Static Perimetry The outer boundary of the island of vision can also be determined by measuring the retinal
sensitivity (z) at each point (x,y). This technique of perimetry is called static perimetry because the test location is fixed, while the intensity of the test object of known size is varied, e.g. Tubinger, Octopus and Humphrey perimeters. Static perimetry provides a vertical slice through the hill island of vision. Because of the difficulty, inability and a potential for lack of reproducibility with kinetic perimetry, static perimetry is preferred for detecting and following subtle non-geographic defects in the diagnosis and follow-up of glaucoma patients. One can perform effective static perimetry with the tangent screen or the Goldmann perimeter. However, manual static perimetry is tedious, cumbersome and at times boring. Both the patient and the examiner find it difficult to concentrate for 30 to 90 minutes at a stretch. Automated /computerized perimetry presents targets at a random sequence undecipherable by the patient. It can test the same patient with the same methodology year after year and still does not get bored. Kinetic testing is difficult to computerize particularly with regard to the decisions regarding same speed and direction of presentation. A static test, on the other hand is relatively straight forward, since the target does not move, the machine has only to choose a site, target intensity and then record whether the patient responds, yes or no. Computers have revolutionized perimetry by allowing precise repetition and meticulous attention to detail, testing the patient’s response under optimal conditions repeatedly by allowing a binary yes/no answer from the patient. All this makes perimetry tailor made for computerization.
Stimulus Presentation During static visual field measurement the stimulus can be presented by projection or nonprojection. In the projection system a simple
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Diagnostic Procedures in Ophthalmology computer video monitor is used to present dark or light combinations of stimuli against a diffuse background. This system has the advantage of being more flexible and allows kinetic color perimetry. Drawbacks pertain to the mechanical aspects of presenting and moving the test target such as mechanical failures, periodic maintenance and servicing. Also, the combination of mirrors, shutters and the rotational unit produces an unsuitable clinking noise with each projection. This was used to advantage in earlier models, to assess the reliability of a given field (false positives). Newer models elicit the false positive response by omitting the light stimulus and assessing the pace of the patient to the rhythm of the testing. In the non-projection system stimuli are generated by the turning on and off of Light Emitting diodes (LED) which are placed into the surface of the perimetric bowl. Advantages of LEDs include silent operation, no moving parts, multiple stimuli presentation and inexpensive and durable equipment. However, in the LED system stimuli are fixed in the bowl surface at the time of manufacture, inability to vary stimulus size and color, test site location or resolution pattern. Further fixed LED positions cannot be expanded to accommodate new programs. LEDs have a condensed light output. Slight variations in positioning and mounting of the LED result in different directional light intensities. All LEDs need to be calibrated individually. This needs to be done at the factory and on a routine basis. In the non-projection system a high resolution, flat video monitor can also be used to present the stimuli. In this method, the patient fixes on a pseudo-infinite target and stimuli are presented throughout the visual field. With this method of presentation, test site location is infinitely variable, kinetic perimetry is possible, and stimulus presentation is without the audible click. Additionally the video monitor projection does away need for a perimeter bowl and the projection
device allows greater flexibility and durability. They also occupy less space. However, video monitor systems are able to assess only the central 30 degrees. Projected stimuli are usually white and of variable size and intensity. The size of the stimuli in automated perimeters is similar to that used for Goldmann perimeter. There are five different sizes designated by Roman numerals I to V. One very often uses stimulus size III. Failure to recognize target size III necessitates testing with stimulus size V. However, tests using stimulus size V cannot be processed statistically by STATPAC 2 on the Humphrey perimeter. In static perimetry, the patient has to respond to a stimulus of predetermined size, color and location projected for a fixed duration at a given intensity level. The patient responds with the button in two ways: stimulus seen or stimulus not seen. Any such response is only suggestive but not actual proof, that the light was seen or not seen. For a stimulus of a fixed size and location to be seen depends on its intensity. This probability of a stimulus of fixed size and location when plotted against the intensity of the stimulus is called probability of seeing curve. That is to say, the intensity of the stimulus where it is seen 50% of the time and missed 50% is called threshold. Similarly, the intensity at which the stimulus is seen 95% of the projected times, is called suprathreshold. A low intensity stimulus which is seen only 5% of the times when projected is called infrathreshold.
Bracketing Determining the threshold for each point in the field would require thousands of stimulii of varying intensity. However, the number of stimuli for threshold determination has been conveniently reduced by a testing algorithm which is also accurate. At a given point on the visual field, the patient responds to a given stimulus
Basic Perimetry
Fig. 9.2: The probability of seeing curve
Fig. 9.3: Threshold determination at the point P (Staircase technique)
intensity (P1). The intensity of the stimulus is then decreased in steps of 4 dB till the stimulus is not seen (3). The threshold lies between 2 and 3. The intensity of the stimulus is then increased in steps of 2 dB till the patient is able to perceive the stimulus. Herein the threshold for the point is lying between 4 and 5, and is a more accurate assessment of the threshold value at that point. This technique of threshold determination is called 4-2 bracketing (Staircase technique). In the Octopus perimeter, the thresholding strategy continues, until a third reversal, in steps of 1 dB, called 4-2-1 algorithm (Fig. 9.3). Normal threshold values are dependant on the location of the point on the visual field and also the age of the patient. Fovea, the most sensitive point of the visual field corresponds to 0 degree of eccentricity. As the point moves from the fovea, the threshold value (sensitivity) decreases by 0.3 dB for every
Fig. 9.4: Effect of location and age on threshold
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Diagnostic Procedures in Ophthalmology degree of eccentricity outside the macula. Sensitivity drops to zero, 15 degrees temporal to fixation (blind spot). Sensitivity also decreases with the age; 0.6-1dB per decade of life. Since, threshold is age-related, the patients date of birth should be correctly entered as the results are compared to age-matched normals. The intensity of light which reflects off the surface is expressed as apostillbs (unit of luminance). The sensitivity of the human visual system varies from 1 to more than 1,000,000 apostillbs (asb). The maximum stimulus intensity of the Octopus Field Analyzer is 1000 asb and for the Humphrey Field Analysis is 10,000 asb. Hence, large numbers representing the listed sensitivity on the printout would be cumbersome. A convenient way of expressing threshold values is in terms of a relative logarithmic scale where the intensity of the stimulus is varied by powers of 10 1 1dB = ———————— log unit (asb) Increasing dB numbers on the printout imply that dimmer stimuli have been perceived. Thresholds corresponding to a dimmer stimulus mean greater retinal sensitivity. In a report of the measured thresholds, large dB values correspond to better sensitivity and small dB numbers indicate reduction in sensitivity.
Testing Strategy With the inherent ability to vary the intensity of the light stimulus, static perimeters explore the visual field in three ways: 1. Suprathreshold screening. 2. Threshold related screening. 3. Full threshold determination. Suprathreshold screening: Very bright stimuli (suprathreshold) intense enough to be seen easily by most normal people are presented. The patient has simply to respond (yes / no) to the presence
of the target. The role of such examinations is related to quick screening of large populations and also to define gross pathology quickly. However, such examinations can miss early changes suggestive of glaucoma. Threshold related screening: Herein, the intensity of the light presented is 5dB brighter than the actual threshold at the test point in question. This allows the entire field to be screened quickly. Threshold related screening is at best a variant of suprathreshold tests which allow for an approximation of the true sensitivity of the visual field. It can be used as a screening test for detection and follow-up of known pathologies. Threshold determination: A more time consuming way of determining the sensitivity of the visual field is by determining the threshold value at each point by the bracketing technique described earlier. After presenting a light stimulus the machine waits for a yes / no response. If the stimulus is not seen, the intensity of the light seen is increased in steps of 4dB till it is visible (machine records this as suprathreshold level). Subsequently, light stimuli are decreased in steps of 2dB till the stimulus is not seen (infrathreshold). The actual threshold is between the suprathreshold and infrathreshold.
Newer Strategies Threshold determination at each point of the visual field is tedious and time consuming. Because by definition threshold is tested by the staircase algorithm, where every patient can see only half of the stimuli presented, newer techniques aim to make the procedure as short as possible, to ensure that the patient maintains concentration and thus provides better reliability. Swedish Interactive Thresholding Algorithm (SITA) is similarly based on the fact that a response at one location has implications at the point tested
Basic Perimetry and also its neighboring points. Just as one tested point is normal, other points on the visual field are likely to be normal too. Tendency Oriented Perimetry (TOP) is available on the Octopus perimeter and takes advantage of each response of the patient five-fold. It tests and adjusts the location where the stimulus is presented and assesses the threshold of the four neighboring locations by interpolation. Several threshold tests are available on the two commonly available Octopus and Humphrey perimeters. In each test a certain number of points can be tested. The number of points tested in a given test is actually a compromise between the time applied and precision, which depends on the type of damage looked for as well as the diagnostic and therapeutic implications resulting therein. The response at each thresholded point is compared with a group of normal individuals. The likelihood of such a response in this population of normal patients is expressed as a probability symbol for each tested point. These probability symbols increase in significance from a set of 4 dots to a black box, p<5%, <2%, <1% and 0.5%. A black box indicates that few normal subjects will have that score; it does not necessarily correspond to an absolute defect. Many points with p<0.5% are relative defects; their actual threshold is available from the raw data.
Test Programs The standard programs on the Humphrey are the 30-2, 24-2, 10-2 and the macular grid program. In the 30-2 the central 30 degrees of the visual field are tested. It consists of 76 points 6 degrees apart on either side of the vertical and horizontal axes, such that the innermost points are three degrees from fixation. In the 24-2 program 54 points are examined. It is near similar to the 30-2 except the two peripheral nasal points at 30 degrees on either side of the horizontal axis are included while testing the central 24 degrees.
The 10-2 program tests 68 points 2 degrees apart in the central 10 degrees. This program helps to assess and follow-up fixation characteristics in patients with an advanced disease along with the macular test which examines 16 points in the central 5 degrees, each being 2 degrees apart. The efficiency and results of an examination are reflected by the location of the points tested. The two commonly used programs on the Octopus are the G1X and the G2 which test 59 locations in the central 30 degrees. Here the test points are concentrated in the central field, arcuate region and nasal midperiphery to maximize detection of significant changes. Fixation characteristics are assessed with the macular program M2X which tests 45 locations in the central 4 degrees, which are 0.7 degrees apart. Automated perimetry provides a large amount of data which is quantifiable, reproducible and amenable to statistical manipulation. However, the magnitude of the data makes interpretation complex, but a logical, consistent and sequential approach helps to make this less complex. The earliest injury in open-angle glaucoma is localized to the nerve fiber bundle, usually in the paracentral nasal region. The initial defect may be seen as a fluctuation in a cluster of points or as a relative defect with normal surroundings. This small area of increased scatter or threshold instability is often overlooked at the initial examination, since it does not meet the criteria for a valid visual field loss. Based on the other clinical data, a subtle area of unstable sensitivity may be suspected as being glaucomatous. It becomes more manifest when progression occurs and a serial review of fields shows that the area in question has changed with time. Progression of visual field defects occur in several ways – increase in density of scotomas, expansion of areas of depression and the development of new ones. Uncontrolled glaucoma will eventually affect all areas of the field.
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Diagnostic Procedures in Ophthalmology The challenge in automated perimetry is to locate and document areas of subtle glaucomatous damage and carefully follow any progression. Finally a diffuse generalized depression affecting the entire visual field is rarely associated with early glaucoma, and is usually due to other conditions such as cataract or uncorrected refractive errors. Visual fields are usually analyzed by using a printout that contains different elements of data. Although several different visual field analyzers are in current use, there are sufficient similarities in the printout to permit interpretation and comparison of the results. However, difference between instruments does not permit direct correlation of their absolute scores. Gaze monitoring is a high precision gazetracking system on the newer Humphrey models which uses real time image analysis to measure the distance between the center of the pupil and the first corneal reflex. It is unaffected by head motion. A continuous record is available on the printout. An upward deflection is indicative of eye movement during stimulus presentation. Downward deflections imply that the gaze could not be detected. The 750 model of the Humphrey perimeter also offers head-tracking wherein the chin rest is automatically moved in increments of 0.3 mm to bring the head back to the initially gaze tracked position and the Vortex monitor wherein a beep as well as a message is produced on the screen when the patients head moves back by more than 7 mm.
Statistical Analysis The Statpac program introduced first in 1987 and then upgraded to Statpac Plus in 1988, was derived from a group of normal patients and helped answer the question: Are the field in question normal or not? It introduced the Global Indices along with the Single Field Printout,
Change Analysis and the Overview format. In 1989 Statpac-2 was introduced. It was formed from a database of patients known to have visual field loss due to glaucoma which was otherwise stable.To detect early changes of glaucoma, groups of points in the superior and inferior hemispheres were also compared to produce the Glaucoma Hemifield Test. An interpretation of the single visual field performed with the Humphrey visual field analyzer (Humphrey Instruments, Inc, San Leandro, C.A) and the Octopus 1 –2 – 3 visual field analyzer (Interzeag AG , Switzerland) is presented.
Components of Automated Visual Field Humphrey Single Field Printout There are eight parts to the single field printout (Fig. 9.5). Each has to be examined serially before drawing a conclusion. First assess the reproducibility ( Zone-1 ) of the concerned fields (Consistency). At the onset, check the printed information at the top of the page, to ensure listing of the correct patient, the type of test done (30-2, 24-2, 10-2), eye in question and date of birth (the software package statistically compares the patients response with age corrected normal population). The recorded visual acuity, refraction and pupil size are important parameters as they all can affect the data. When pupils are miotic, or smaller than 2.5 mm, dilatation is required so as to prevent generalized depression from occurring. The decision to dilate patients with large pupils rests with the clinician, but consistency for all visual fields must be maintained. Next scans the reliability indices (Zone-2). Fixation losses are noted as the ratio of the number of times the patient responded when he saw a target placed in the blind spot against the total number of times fixation was tested. In automated
Basic Perimetry
A
Fig. 9.5: The Humphrey single field printout is divided into eight zones. Each must be reviewed sequentially
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Diagnostic Procedures in Ophthalmology perimetry fixation is assessed and monitored by i. Sensors, ii. Closed circuit TV monitors iii. Heijl-Krakau method. Sensors are used to detect minute shift in eye position. They are highly sensitive to slight movement in eye position but are expensive, too sensitive, such that insignificant physiological fixation shifts induced by respiration, systole and involuntary head movements get registered as fixation losses. Closed circuit TV monitor displays the image taken by an infrared camera. This allows the examiner to view the patient’s eye and judge and assist in fixation quality. Advantages of this system are continuous monitoring of fixation throughout the test with no extra time spent in monitoring fixation per se (Blind Spot Projection Technique). However, continuous video monitoring is expensive, prone to hardware failure and there exists a potential for the machine to disregard fixation losses in patients with fairly good but not excellent fixation. Heijl-Krakau method: In this method, the machine assumes or plots the blind spot at the beginning of the test and then retests after every eight to twelve stimuli by projecting a suprathreshold stimuli in the blind spot. A positive response indicates fixation loss. This, however, does not work well when significant field loss is adjacent to or involving the blind spot. When fixation losses are more than 20%, it is bracketed (XX) and is indicative of questionable reliability. However, not all fixation losses are due to unsteady gaze. A “pseudo-loss” of fixation is seen when there is an improper location of the blind spot, or when the initial blind spot is present near the edge of a scotoma, so even though it is presented throughout the test, it is occasionally visible. Also, a head tilt or change in head position occurring during the test will lead to a faulty blind spot location. Finally a
patient who is continually responding even when a light is not flashed will have a number of fixation losses. For these reasons the fixation loss score is not considered in isolation, but rather compared to the other reliability scores. False positives (FP) result when the patient responds to the audible click of the perimeter with no stimulus projected (trigger happy). It is also expressed as a ratio of the number of times the patient responds to a pause in the testing sequence without presentation of the target against the total numbers of pauses. It is the single most significant reliability indicator. Bracketing occurs when FP’s are 33% but often 15-20% rate can also destroy the credibility of a field. A high rate can also occur due to a poor understanding of the test requirements by the patient. A high FP ratio , will be accompanied by a high positive mean defect, white areas on the gray scale indication of very high threshold levels (white scotomas), a high number of fixation losses and a message of abnormally high sensitivity on GHT. False negatives (FN) are expressed as a ratio, and occur when the patient does not respond when a point previously thresholded is retested with a brighter stimulus. High FN ratio occurs when the patient tires as in the later part of the examination, when he changes his internal criterion on whether or not he sees a point or when the edge points of a scotoma are tested. A 33% FN ratio is considered excessive and makes the test suspect. However, the presence of a scotoma and a high number of FN, with all other reliability measures being normal, is indicative of a reliable field. Foveal threshold measures over 30 dB for a visual acuity of 6/12 or better. A normal foveal value and a poorly recorded acuity indicates need for a refraction or mild amblyopia. Likewise a good visual acuity and a depressed foveal value suggest early damage.
Basic Perimetry The Gray scale (Zone-3) is a rough indicator of the extent of field damage, but can be misleading. Each point on the gray scale is represented by a symbol of varying darkness which corresponds to the threshold level at that point. These are not indicative of disease. A normal elderly patient will have a darker gray scale than a younger patient because of reduced sensitivity in aging eyes. Additionally, there are a fewer points tested in the periphery, each of which occupies a larger space on the gray scale. For these reasons, the gray scale should not be the sole criterion for assessing the visual field. The Total deviation plot (Zone-4) is created by subtracting the actual raw data from the expected value for age matched controls, at each point. This depending on whether the patient did better or worse than expected is expressed as a positive or negative number. The corresponding probability symbols seen below the data indicate the statistical probability of finding such a point in normal subjects. These probability symbols increase in significance from a set of 4 dots to a black box, p<5%, <2%, <1% and 0.5%. The presence of a black box indicating that a few normal subjects will have that score, it does not necessarily correspond to an absolute defect. Many points with p<0.5% are relative defects their actual threshold is available from the raw data. The Pattern deviation plot (Zone-5) based on further calculations, is derived from the total deviation data and the overall depression of the visual field. It highlights focal changes which are concealed within diffuse changes, after making adjustment for the height of the hill of vision. Whereas the statistical significance, expressed as probability symbols, is measured for each point, the total deviation and pattern deviation probability maps are analyzed by taking the entire field into account and identifying how clusters of affected points occur, the number of points involved, their density and location.
The Pattern and Total Deviation need to be compared and a difference if present should be explained. Corneal opacity, cataract and small pupil are the usual causes. Raw data / numeric data (Zone-6): It is the actual threshold score for each thresholded point. Areas flagged in the Pattern and Total Deviation plot should be inspected carefully for confirmatory signs like double thresholded points of abnormal or foci of high local fluctuation. This should be followed by a geographic survey of the entire numeric data. Global indices (Zone-7) are presented in the lower right hand corner of the printout and include: Mean deviation (MD): It is the weighted score of all the points on the total deviation plot. It takes into account both the severity of loss and amount of field affected. A positive MD indicates that the patient scored better than expected for his age, a negative number indicates that the score was worse than expected. Pattern standard deviation (PSD): It measures the extent to which the damaged points vary from the expected hill of vision (localized loss). Short term fluctuation (SF): Though listed under global indices it is a good indicator of intra test reliability. It measures the variation at each point on repeated thresholding in the same test. A SF from a patient with poor reliability scores is high, further indicating a poor test taker. Corrected pattern standard deviation (CPSD): It is calculated with the help of SF to adjust the PSD. It is a more accurate indicator of the extent of damage. Glaucoma Hemifield test (Zone-8) is a sophisticated analysis of 5 geometric point clusters in the superior and the inferior arcuate regions whose probability maps are compared with one another. It is very sensitive and specific at detecting asymmetry between these regions as well as symmetric deviations from normal data. The GHT can be within normal limits,
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Fig. 9.6: Glaucoma hemifield test
outside normal limits, borderline sensitivity, generalized reduction or abnormally high sensitivity (Fig. 9.6).
Octopus Single Field Printout The commonly used seven in one printout is near identical to the Humphrey single field printout (Fig. 9.7). Again, a systematic and sequential approach helps in interpretation. As before there are eight parts to the single field printout. Each has to be examined serially before drawing a conclusion. Reproducibility (Zone-1): Before looking at other components of the printout, one needs to identify the field in question to the concerned patient. One quickly checks the name and date of birth as printed on the upper part of the printout. Test parameters such as the size of the stimulus and pupil, type of test strategy, and test program used are also looked at along with the eye and date of examination. Reliability factors (Zone-2): To assess the reliability of the concerned examination, one assesses the catch trials just beneath zone I and
also the reliability factor listed below, with the visual indices. False positives (FP) result when the patient responds to the audible click of the perimeter with no stimulus projected. It is also expressed as a ratio of the number of times the patient responds to a pause in the testing sequence without presentation of the target against the total numbers of pauses. False negatives (FN) are expressed as a ratio, and occur when the patient does not respond when a previously thresholded point is retested with a brighter stimuli. Each of these should be less than 10%. The reliability factor (value) is determined by the outcome of the catch trials and ideally it should be less than 15%. The Octopus 1-2-3 takes a video photograph of the pupil and stores this in its memory. If the eye deviates or the lid closes, the machine registers the loss of fixation and disregards the patients response till fixation is restored. Loss of fixation for more than two seconds halts the program. Hence the Octopus printout does not document fixation losses. Gray scale (Zone-3): This is the most colorful part of the printout but like its counterpart in the Humphrey single field, it is the least informative since it is obtained by the interpolation of the actually tested sensitivities. Lighter colors are suggestive of higher sensitivities and darker areas suggest depression. Hence only a cursory look is required. Black depicts an absolute loss of sensitivity. Comparison (Zone-4): It is synonymous with the total deviation plot on the Humphrey single field printout. The lower left part of the printout, one on top of the other, is the comparison display, with a numeric display above a probability map. The comparison values are the difference between the patients test results and age-matched normals. The ‘+’ symbol indicates a normal sensitivity. The probability map is displayed graphically below this. Defects are marked as symbols of different shades. Darker the marking,
Basic Perimetry
Fig. 9.7: Octopus 1-2-3 seven in one printout like the Humphrey single field has eight zones which need to be viewed systematically
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display is best assessed by removing the normal short term fluctuation from the loss variance. This gives us the corrected loss variance (called loss variance on the Humphrey single field printout) which is more sensitive to localized defects.
Corrected comparison (Zone-5): It is synonymous with the pattern deviation plot on the Humphrey single field printout. The corrected comparison represents the localized defect after removing the generalized depression of the visual field from the total depression. Similar to the comparison they are represented by values, ‘+’ for normal sensitivity and the probability map is displayed graphically below this.
Bebie’s curve (Zone-8): In the presence of a local defect, it is often difficult to quantify an additional diffuse defect in a particular visual field. The cumulative curve was introduced by Dr. H. Bebie in the late 1980’s to help assess the overall condition of the visual field at a glance. In the Bebie’s curve test locations are arranged according to the extent of their difference from the normal values. The individual test locations are arranged on the x-axis, and the defects in decibels on the y-axis. The test locations with the least difference are found on the left side of the figure, while those with the greatest are on the right side. With this graphical representation, it is simple to differentiate localized from diffuse damage.
Numeric data/raw data (Zone-6): They represent the actual thresholded points from which the entire statistical calculation is done. Close to fixation, values are in their late twenties or early thirties. In the mid periphery, threshold values are in their mid twenties and in the late teens in the periphery. Visual field indices (Zone-7): They were first introduced on the Octopus perimeters in 1985 and include: Mean sensitivity is the average of retinal sensitivities that are measured at all points. Mean defect (called mean deviation on the Humphrey printout) is the average defect of all thresholded points from the age-matched normals, as shown in the comparison chart. It is indicative of the height of the hill island of vision. Loss variance (called pattern standard deviation on the Humphrey printout) is obtained from individual deviations of all measured locations with the mean defect value. These are indicators of localized damage. Short term fluctuation is a reliability factor suggestive of an intra test variation. A value of more than 2.5 is significant. The difference between individual deviations on the numeric
Analysis of Single Field Printout After ensuring good reproducibility the visual field is analyzed using each of the eight areas alone or in combination. The reliability indices give an indication of the credibility and accuracy of the fields. The gray scale gives a rough over view of the field, but is not used in the actual field interpretation. Any suspected change must be confirmed by inspecting other parts of the printout. The total deviation and pattern deviation (Comparison and Corrected Comparison on the Octopus printout) should be compared in tandem. A difference between them if seen must be explained. The pattern deviation symbols are used in the interpretation of the field with the arrangement and severity of the points or clusters analyzed. The greater the number of points involved and greater the depression the more severe the defect is. After a quick look at the
Basic Perimetry numeric data, the global indices (visual field indices on the Octopus printout) are analyzed next, with the mean deviation (mean defect on the Octopus printout) being an indicator of the overall depression of the field. The pattern standard deviation (loss variance) or corrected pattern standard deviation (corrected loss variance) is considered significant when a score of p < 5% is noted. The short term fluctuation is analyzed as a part of the reliability indices and with the total and pattern deviation symbols. The glaucoma hemifield test is analyzed at the end, a reading outside normal limits is significant. The interpretation should also include allowance for artifacts such as drooping lid, prominent brow, or improper positioning of the patient/trial lens. Other mimics of glaucomatous field loss include retinal and neurological disorders along with disorders affecting the clarity of the ocular media. These need to be ruled out by a detailed ocular examination. The minimum criteria for the diagnosis of glaucoma are listed in Table 9.1. TABLE 9.1: MINIMUM CRITERIA FOR DIAGNOSIS OF GLAUCOMA 1. Three or more non-edge points in the pattern deviation plot with sensitivity reduce to level of p < 5% or worse, with at least one point <1% 2. Glaucoma hemifield test is outside normal limits. 3 Corrected pattern standard deviation p <5% Criteria should be fulfilled on at least two occasions
Non-characteristic visual field defects (Figs 9.8 to 9.11) must be substantiated by clinical examination of the retina and optic nerve head. The first visual field test in an inexperienced patient should be taken with caution. After first test the patient becomes more proficient; the resulting improvement in the visual field is known as learning curve. It is, therefore, desirable to test two or more visual fields before proper interpretation. To be clinically significant, the visual field should be reproducible.
While assessing single field printout, the presence of miotic pupil and media opacities should be taken into consideration because they can cause generalized depression of visual field. The interpretation should also include allowance for artifacts such as position of the patient, correcting lens (Fig. 9.12), drooping of the lid and prominent brow. It is not rare to find that visual field changes in neurological disorders (Fig. 9.13) may mimic the glaucomatous field defects. The visual field examination is a useful tool to study the course of an eye disease as well as to monitor the therapy. Periodic visual field testing is usually recommended for all glaucoma patients especially with a view to evaluate the desired target intraocular pressure. In spite of good control of the pressure, the patient’s visual fields may show deterioration on follow-up (Fig. 9.14) while in some patients the fields remain stationary (Fig. 9.15). Assessment of progression is difficult because of the long-term fluctuations. One needs to repeat the field test when in doubt. In clinical practice the recent fields are compared with the earlier baseline fields to judge the progression.
Conclusion In conclusion automated perimetry is an extremely useful tool which has also become the standard technique for evaluating the visual field in patients with glaucoma or glaucoma suspects. Interpretation of the results is difficult and requires experience in addition to a detailed understanding of the underlying principles of automatic static perimetry and the applied statistical analysis. A word of caution is necessary. Automated perimetry should never be used in isolation. Treatment of patients requires combining the results of automated perimetry with an
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Fig. 9.8: Humphrey single field 24-2 SITA standard test of the left eye of a 53 year old patient. Reliability factors have been expressed as a percentage. The visual field is markedly depressed in the inferior hemisphere on the gray scale and total deviation plot. Anderson’s criteria are fulfilled. The height of the hill island of vision represented by the mean deviation is significantly reduced. Clinical correlation with the amount of optic disk cupping is necessary to determine the cause of such a defect
Basic Perimetry
Fig. 9.9: Humphrey single field 30-2 full threshold test of the left eye of a 64 year old patient. High false positives are bracketed. The gray scale and the total deviation plot show a marked depression of the visual field. However, only a cluster of points on the pattern deviation plot (p<2%) in the central 10 degrees are seen. No probability symbols are seen alongside the CPSD and the Glaucoma Hemifield test is showing a borderline/generalized reduction in sensitivity
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Fig. 9.10: Octopus 1-2-3 seven in one single field printout of the left eye of a 61 year old male patient showing an early inferonasal step. There are a number of adjacent points in the inferonasal quadrant on the corrected probability plot, depressed to 5%, one of which is depressed to less than 1%. The left part of Bebie’s curve shows a localized depression. The corrected loss variance is 8.4. This field needs to be correlated clinically
Basic Perimetry
Fig. 9.11: Octopus 1-2-3 seven in one single field printout of the left eye of a 61 years old male patient to assess fixation characteristics. Here the catch trials are suggestive of poor reliability. The gray scale and comparisons are suggestive of depression of the inferior part of the 10 degrees being tested. Within the central 4 degrees of this program, each point is 0.7 degrees apart. This helps to assess fixation characteristics better. One of the four fixation points is depressed p < 2%. The Bebie’s curve is initially suggestive of normal points corresponding to the superior part of the field. A sudden drop in Bebie’s curve is due to the cluster of depressed points in the inferior part of the field. The CLV is also significant. This field is suggestive of extensive damage in the inferior hemisphere which is threatening fixation and needs to be correlated clinically
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Fig. 9.12: Humphrey single field 24-2 full threshold test of the left eye of a 52 years old patient. A ring scotoma on the gray scale and the pattern deviation plot is evident. Anderson’s criteria are also fulfilled. Such visual field loss could be due to glaucoma or retinitis pigmentosa. However, the fundus findings were normal and on repeating the field test (with proper positioning of the lens) the changes in the pattern deviation plot disappeared (Lens rim artifact)
Basic Perimetry
Fig. 9.13: Humphrey single field 30-2 full threshold test of the right eye of a 59 years old patient. The gray scale and the total deviation plot show a depression of the visual field. Here the gray scale shows a marked temporal depression as is evidenced on the pattern deviation plot. Such defects which respect the vertical meridian are neurological in origin. In this patient the other eye also showed a temporal hemianopia
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Fig. 9.14: Change probability analysis showing deterioration in fields over a period of time
Basic Perimetry
Fig. 9.15: Overview printout showing stable fields
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Bibliography 1. Caprioli J. Automated perimetry in glaucoma. Am J Ophthalmol 1991;111:235. 2. Fankhauser F. Problems related to the design of automatic perimeters. Documenta Ophthalmologica 1979;47(1):89. 3. Flammer J. The concept of visual field indices. Graefes Arch Clin Exp Ophthalmol 1986;224:389.
4. Heijl A, Lindgren G, Olsson J. A package for the statistical analysis of visual fields. Doc Ophthalmol Proc Ser 1987;49:153. 5. Humphrey Field Analyzer User’s guide. Humphrey Instruments, Inc. San Leandro, 1994. 6. Octopus Visual Field Digest. 4th ed. Switzerland, Interzeag AG. 7. Johnson CA, Keltner J. Automated suprathreshold static threshold perimetry. Am J Ophthalmol 1980;89:731. 8. Kaiser HJ, Flammer J. Visual Field Atlas – A guide and atlas for the interpretation of visual fields. University Eye Clinic, Basel, 1992.
Ophthalmoscopy
PUKHRAJ RISHI, TARUN SHARMA
10
Ophthalmoscopy
A comprehensive eye examination is a must for a complete assessment of the anterior and posterior segments of the eye—be it a diagnostic or preoperative evaluation. Although there are several methods of eye examination viz slit-lamp biomicroscopy, gonioscopy, perimetry, tonometry, ultrasonography, ophthalmoscopy remains an important tool for a complete evaluation of the posterior segment of the eye. In December 1850, Helmholtz announced the invention of an “eyemirror”, which was the original ophthalmoscope. It was mounted with a holder for one lens, and lenses had to be changed constantly for eyes of different refraction. Rekoss introduced a revolving disk carrying a series of lenses.
Fig. 10.1: Optics of image formation in an emmetropic eye
overlap. In the emmetropic eye this can happen only if the light source and the observer’s pupil are optically aligned. Under normal conditions this does not happen, hence the pupil normally appears dark (Fig. 10.2).
Principles of Ophthalmoscopy The basic principle of ophthalmoscopy is shown in Figure 10.1. If the patient’s eye is emmetropic, light rays emanating from a point in the fundus emerge as a parallel beam. If this beam enters the pupil of an emmetropic observer the rays are focused on the retina and an image is formed. This is called direct ophthalmoscopy. The fundus can be seen only when the observed and the illuminated areas of the fundus
Fig. 10.2: The light source and the observer’s pupil are not optically aligned
The illuminating and the observing beams are aligned using a semi-reflecting mirror or a prism allowing fundal view (Fig. 10. 3).
Indirect Ophthalmoscopy Ruete introduced indirect ophthalmoscopy in 1852. There are several types of indirect
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Diagnostic Procedures in Ophthalmology scopy. Indirect ophthalmoscopy is carried out in a dark room with fully dilated pupils. The equipments required for slit-lamp indirect ophthalmoscopy includes slit-lamp and condensing lens. The condensing lens may be either noncontact or contact lens. Fig. 10.3: The light source and the observer’s pupil are optically aligned
ophthalmoscopes are available. One must understand optical principles of indirect ophthalmoscopy to carry an ocular examination (including fundus angioscopy). The indirect ophthalmoscope can be used in the treatment of disorders of the posterior segment.
Noncontact lenses: They are plus powered with two convex aspheric surfaces. The +60D version has the greatest magnification and is best used for the disk and macula. The +78D version is a commonly used diagnostic lens and the +90D is good for small pupils. They are available in clear or blue-free, ‘yellow retina protector glass’. They are comfortable to the patient and minimize the risk of phototoxic retinal damage due to prolonged exposure to the focused beam. Contact lenses: Goldman, Mainster, SuperQuad, Equator Plus, Area centralis, Super Macula lenses are often used. Field of view and image magnification obtained by these lenses are listed in Table 10.1.
Method of Examination
Fig. 10.4: Optics of indirect ophthalmoscope
There are five indirect ophthalmoscopy techniques. These are, slit-lamp indirect, head mounted indirect, monocular indirect, modified monocular indirect and penlight ophthalmo-
For examination, minimal slit-lamp intensity can be used in a dark room. Always focus the oculars to accommodate any examiner refractive error, then set the pupillary distance, remove all filters
TABLE 10.1: FIELD OF VIEW AND IMAGE MAGNIFICATION OBTAINED BY DIFFERENT CONTACT LENSES Lens
Field of view
Image mag.
Laser spot
Working distance
Super Quad 160®
160°/165°
.5x
2.0x
contact
Equator Plus®
114°/137°
.44x
2.27x
contact
Quad Pediatric
100°/120°
.55x
1.82x
contact
QuadrAspheric®
120°/144°
.51x
1.97x
contact
PDT Laser
115°/137°
.67x
1.5x
contact
Trans Equator®
110°/132°
.7x
1.44x
contact
Area Centralis®
70°/84°
1.06x
.94x
contact
Super Macula® 2.2
60°/78°
1.49x
.67x
contact
mag: magnification
Ophthalmoscopy and keep the magnification to the lowest setting, usually X6-X10. The illumination of the slit-lamp should be adjusted for an intermediate slit height and a 2 mm width, and then placed in the straight ahead position between the oculars (zero degrees or co-axial). Before examination, ensure that the condensing lens surfaces are clean. Hold the lens vertically between the thumb and index finger of the left hand to examine the patient’s right eye and vice versa.
Examination Procedure Instruct the patient to fixate straight ahead, to stare wide and to blink normally. Center the beam in the patient’s pupil and focus on the cornea. Now the lens is placed in front of the patient’s eye, directly in front of the cornea so the back surface just clears the lashes. Examiner’s fingers may be placed on either the brow bar or the patient’s forehead. Using the joystick, focus on the fundus image by slowly moving away from the cornea, keeping the beam centered in the pupil. Once the retinal image is focused, the magnification may be increased. Scan across the entire lens keeping it steady. In order to view the peripheral retina, ask the patient to change fixation into the nine cardinal positions of gaze. The lens is realigned and refocused the slit-lamp as necessary. To reduce interfering reflections, tilt the lens or move the illumination arm upto 10 degrees on either side, once the fundus has been focused. For fine tuning of the fundus view, lateral and longitudinal adjustments of the lens may be made to optimize the field of view. When viewing finer fundus details, intensity and magnification of slit-lamp should be increased.
Head Mounted Binocular Indirect Ophthalmoscopy Binocular indirect ophthalmoscopy (BIO) is a technique used to evaluate the entire ocular
Fig. 10.5: Optics of binocular indirect ophthalmoscopy
fundus. It provides for stereoscopic, wide-angled, high-resolution views of the entire fundus and overlying vitreous. Its optical principles and illumination options allow for visualization of the fundus regardless of high ametropia or hazy ocular media. Light beams directed into the patient’s eye produce reflected observation beams from the retina. These beams are focused to a viewable, aerial image following placement of a high pluspowered condensing lens at its focal distance in front of the patient’s eye. The resultant image is real, inverted, magnified, laterally reversed, and located between the examiner and the condensing lens. The observer views this image through the oculars of the head-borne indirect ophthalmoscope. An indirect ophthalmoscope (Fig. 10.6) consists of a head band for comfortable placement, light source with variable illumination and an adjustable mirrored surface in the main housing and knobs to align the low plus powered eyepieces (+2.00 to +2.50 D) with the examiner’s interpupillary distance. A 20 D condensing lens (Fig. 10.7A), a pair of scleral depressors (Fig.
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Fig. 10.6: Indirect ophthalmoscope
10.7B) and fundus drawing sheet (Fig. 10.7C) are needed for a proper indirect ophthalmoscopy and documentation.
Examination Procedure Proper placement and adjustment of the binocular indirect ophthalmoscope (BIO) is an important step in the examination. Place the loose BIO onto the head and position the bottom of the front headband one index finger width above the eyebrows. Tighten the crown strap until this headband position begins to stabilize then
position the back head strap on or below the occipital notch and tighten until secured. Now the knobs that control the instruments main housing (oculars and light tower) should be loosened and fixate straight ahead and level in vertical position the oculars and aligned tangential to or slightly angled downward from the ocular surface; this should maximize observer’s visual field and minimize horizontal diplopia. Horizontally align each ocular by closing one eye and fixating a centrally positioned thumb of an outstretched hand. Turn on the light source and fixate straight ahead on a wall at 40 to 50 cm looking at the projected light source. Use the mirror knob to vertically place the light source at the upper one-half to one-third of the field. The headset is adjusted and the voltage set to mid-range (occasionally the sneeze reflex may start from the periphery first). The choice of condensing lens depends upon the need for a panoramic view or detail; a 30 D provides panoramic view while fundus details can be obtained with 14 D. Stereopsis is important and depends on the choice of lens. A full stereopsis is obtained with 14 D, three-quarter with 20 D and one-half stereopsis with 30 D. A 30 D lens can be used to get a view of fundus in patients with small pupil. The condensing lens should be held between the tip of the flexed index finger and the ball of the extended thumb of the non-
Figs 10.7A to C: A 20 D condensing lens, B A pair of scleral depressors and C Fundus drawing sheet
Ophthalmoscopy dominant hand and the scleral depressor with the dominant hand. The extended third finger acts as the pivot. The more convex surface should be toward the observer and the white-ringed edge closest to the patient so as to avoid bothersome light reflexes. These reflexes can be made to move in opposite direction from each other by slightly tilting the lens. Condensing lenses have their surfaces coated to reduce such reflexes. The lens must be smudge free. The patient should have atleast some idea of what to expect in the examination. Although the patient may be examined in either sitting or supine position, it is best to recline the patient on a couch with the face directed towards the ceiling to avoid stooping. The couch or table should be just high enough to reach the examiner’s hips. The examiner stands opposite to the clock hour position to be examined. The patient is instructed to keep both the eyes open and fixate towards his outstretched hand which points to the meridian of interest. From a working distance of 18 to 20 inches, direct the light beam into the pupil, producing a complete red pupillary reflex. Pull backward on the lens, maintaining the central position of the pupil reflex, until the entire lens fills with the fundus image. Fine adjustments are made in the lens tilt and vertex distance to produce a distortion-free full lens view. The patient must be repeatedly urged to open the fellow eye. Good cycloplegia is the most important single factor in getting co-operation in this regard. The eye with inadequate cycloplegia is very photophobic. All the vital elements involved in the visualization of the fundus, namely observer’s macula, the eyepiece of the ophthalmoscope, center of the condensing lens, patient’s pupil and the object observed in the fundus must be kept on an axis to maintain the fundal view. In order to develop and achieve a continuous sweeping picture of the fundus, a major retinal blood vessel
must be picked out from the posterior pole and followed as anteriorly as possible by the observer’s movements alone. This vessel should be then followed back to the optic disk. This maneuver needs constant practice to master it. The problem of orientation in the fundus may be solved by learning to accurately draw the image exactly as we see in the condensing lens. The drawing chart may be placed inverted over the patient’s chest. Positioning 180 degrees away from the area of interest, the observer must think in terms of anterior in the fundus or posterior in the fundus (or central and peripheral). Draw the image seen in the lens on that part of the fundus chart that is closest to the observer. Since 30% of the retina lies anterior to the equator, failure to study this region will result in overlooking serious pathology in many cases. Scleral depression not only allows for an easy and complete view of the ora serrata and the pars plana but also allows a better evaluation of the retinal topography making lesions such as horseshoe tears or vitreo-retinal traction more visible. It is of particular value in differentiating a retinal hemorrhage from a retinal break, in recognizing a raised from a depressed lesion and in detecting whether a foreign body lies on or anterior to the retina. The absence of an overhanging orbital margin superonasally makes initial attempts at scleral depression easier. The depressor is applied to the superior lid, without pressure, at the tarsal margin. The patient looks up and the depressor slides posteriorly parallel to the surface of the globe, as the lid retracts. The depressor is gently pressed against the globe at the equatorial region and a grayish mound is seen to come up in view from the inferior part of the fundus. In viewing the ora, it is sometimes necessary to tilt the condensing lens somewhat forward, into a plane more nearly parallel to the iris. It must be remembered that scleral depression is a dynamic technique.
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Diagnostic Procedures in Ophthalmology Fundus Drawing: Color Code (Peter Morse) Color Code Red Solid • Retinal arterioles • Neovascularization • Vascular abnormalities or anomalies • Vortex vein • Attached retina • Hemorrhages (Pre-intra-and sub-retinal) • Open interior portion of retinal break (Tears, holes) • Normal foveola (Drawn as red dot). Cross lines • Open portion of giant tears or large dialysis • Inner portion of chorioretinal atrophy • Open portion of retinal holes in inner layer of retinoschisis • Inner portion of the areas of retina. Color Code Blue Solid • Detached retina (Fig. 10.8) • Retinal veins • Outlines of retinal breaks (Tears, holes) • Outline of ora serrata (Dentate processes, ora bays) • Meridional, radial, fixed star-shaped and circumferential folds • Vitreoretinal traction tufts • Retinal granular tags and tufts (Cystic, noncystic) • Outline of flat neovascularization • Outline of lattice degeneration (Inner chevrons or Xs) • Outline of thin areas of retina • Intra-retinal cysts (with overlying curvilinear stripes to show configuration). Cross lines • Inner layer of retinoschisis
• White with or without pressure • Detached pars plana epithelium anterior to separation of ora • Outer surface of retina seen in rolled edge of retinal tears, inverted flap of giant retinal tear. Stippled or circles • Cystoid degeneration. Interruped lines • Outline of change in area or folds of detached retina because of shifting fluid. Color Code Green Solid • Opacities in the media (Cornea, anterior chamber, lens, vitreous) • Vitreous hemorrhage • Vitreous membranes • Hyaloid ring • Intraocular foreign bodies • Retinal opercula • Cotton wool patches • Ora serrata pearls • Outline of elevated neovascularization. Stippled or dotted • Asteroid hyalosis • Frosting or snowflakes on cystoid, retinoschisis, and lattice degeneration. Color Code Brown Solid • Uveal tissues • Pars plana cysts • Ciliary processes (Pars plicata) • Striae ciliaris • Pigment beneath detached retina • Subretinal fibrosis demarcation lines • Choroidal nevi • Malignant choroidal melanomas • Metastatic and other choroidal tumors • Choroidal detachment.
Ophthalmoscopy Outline • Chorioretinal atrophy beneath detached retina • Posterior staphyloma • Edge of buckle beneath detached retina. Color Code Yellow Solid • Intraretinal edema • Intraretinal or subretinal hard yellow exudates • Deposits in retinal pigment epithelium • Detached macula in some retinal separations • Retinal edema as a result of photocoagulation, cryothreapy or diathermy • Long and short posterior ciliary nerves • Retinoblastoma. Stippled or dotted • Drusen Color Code Black Solid • Pigment within the detached retina (lattice, flap of horse-shoe tear, paravascular pigmentation) • Pigment in choroid or pigmented epithelial hyperpigmentation in areas of attached retina • Pigmented demarcation lines at the attached margin of detached retina or within detached retina • Hyperpigmentation as a result of previous treatment with cryothreapy, photocoagulation or diathermy • Completely sheathed retinal vessels. Outline • Partially sheathed vessels (lattices, retinoschisis) • Edge of buckle beneath attached retina • Long posterior ciliary nerves and vessels (Pigmented) • Short posterior ciliary nerves and vessels • Chorioretinal atrophy.
Fig. 10.8: Showing a long-standing, partial, rhegmatogenous retinal detachment with demarcation lines and intraretinal macrocyst. A horse-shoe tear, lattice degeneration and a retinal dialysis are also seen. An improperly placed scleral buckle effect is made out. Pars plana is detached nasally. Retinoschisis with inner layer hole is seen in inferotemporal periphery. Pars plana cysts are seen inferiorly
Indirect Ophthalmoscopy in Operating Room Many problems may be encountered whilst operating and performing an indirect ophthalmoscopy. The fundus to be examined is usually a difficult one, with a retinal detachment and/ or PVR. The cornea may become edematous or abraded during the course of surgery. Particular care must be taken in patients having undergone LASIK surgery to prevent dislocation of corneal flap. The fundus picture may change with each step in surgery. The advantages of indirect ophthalmoscopy in the operation room stem from its safe working distance from the sterile operating field, in accurate localization of all retinal breaks and other fundus landmarks by scleral depression. It helps in obtaining a fine
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Diagnostic Procedures in Ophthalmology needle aspiration biopsy and treatment of choroidal or retinal tumors. Indirect ophthalmoscopy is a valuable tool in the examination of children and uncooperative adults: Since the field of view is much larger with an indirect ophthalmoscope, fundus examination is possible even in moving eye. A quick comparison with the other eye is also possible. Children would generally react more favorably to the more impersonal distance of indirect examination. It is also useful equipment in examining the anterior segment for rubeosis and tumor seedings in children with advanced retinoblastoma. Fundus angioscopy, and transillumination with the help of a probe (Fig. 10.9) can be performed using indirect ophthalmoscopy; which helps in differentiating various types of fundus mass lesions. Nystagmus, aniridia, albinotic fundus, partial vitreous hemorrhage, fundus coloboma, microphthalmos and persistent hyperplastic primary vitreous can be diagnosed with the help of indirect ophthalmoscope.
Fig. 10.9: Transillumination probe
Monocular Indirect Ophthalmoscopy Monocular indirect ophthalmoscopy combines the advantages of increased field of view (indirect ophthalmoscopy) with erect real imaging (direct
Fig. 10.10: Monocular indirect ophthalmoscope
ophthalmoscopy). By collecting and redirecting peripheral fundus-reflected illumination rays, which cannot be accomplished with the direct ophthalmoscope. The indirect ophthalmoscope (Fig. 10.10) extends the observer’s field of view approximately four to five times. An internal lens system then reinverts the initially inverted image to a real erect one (Fig. 10.11), which is then magnified. This image is focusable using the focusing lever/eyepiece lever. It gives a field of view of approximately 30 degrees, yet it is important that the patient looks in 6 to 8 different directions to see as much of the fundus as possible. The optical system of the monocular indirect ophthalmoscope (MIO) has a lens which erects the image and allows seeing things as they actually appear anatomically. It also gives a greater working distance from the patient of 5 to 6 inches. The MIO has a yellow filter that allows one to see deeper details of the retina at about the level of the choroid. The cost of the MIO is nearly equal to that of a good binocular indirect ophthalmoscope and of course it does not allow a stereoscopic view of the retina.
Ophthalmoscopy
Fig. 10.11: Optics of monocular indirect ophthalmoscopy
Examination Procedure To examine the right eye, remove the patient’s spectacle correction, stand to the patient’s right side, and ask him to fixate straight ahead and level with the left eye. The observer should wear his refractive correction. The iris diaphragm lever is pushed fully to the left to maximally increase the aperture size. Center the red dot on the filter dial to open the aperture for normal viewing. The observer's head should be against the forehead rest and align the eye through the instrument with the patient’s right eye. Then position several inches in front of the patient and focus through the pupil onto the fundus using the thumb and focusing lever. Adjust the focus and iris diaphragm to produce a clear maximally illuminated fundus view. Continue to approach the patient until the observer’s knuckle lightly touches the patient’s cheek, as the working distance decreases, fundus magnification increases. Angle the light slightly nasally to illuminate and visualize the optic disk.
Modified Monocular Indirect Ophthalmoscopy A thorough fundus examination is important and required in all young patients with strabismus or amblyopia in order to rule out organic causes of amblyopia prior to the initiation of treatment. The patient co-operation obtained with head mounted binocular indirect ophthalmoscope (using a 20 D lens), and slit-lamp biomicroscope (using a 90 D) is usually difficult or impossible on younger children. Also the magnification of the fundus may be inadequate to allow accurate evaluation of posterior pole details. The direct ophthalmoscope is often the best available instrument for detailed retinal examination in young patients. However, children often become frightened as the examiner approaches closely, as is necessary with the direct ophthalmoscope and co-operation is lost. Additionally children often fix the ophthalmoscope light and track it as the examiner moves it, allowing examination of the
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Diagnostic Procedures in Ophthalmology macula but not of the disk. The field of view is small and the magnification is more than is usually necessary. This will prevent the examiner from seeing the large area of fundus. To avoid these difficulties the direct ophthalmoscope can be used in conjunction with a 20 D condensing lens. This combination provides a moderately magnified and wider angle view of the posterior pole. This avoids the close proximity between the patient and examiner required when using a direct ophthalmoscope alone. This technique is called modified monocular indirect ophthalmoscopy and has been noted for its ability to provide a good view of the retina through a small pupil.
Examination Procedure To begin the examination a red reflex is visualized through the direct ophthalmoscope held approximately 18 cm from the patient’s eye. A 20 D lens is then placed 3 to 5 cm in front of the patient’s eye in the path of the ophthalmoscope light beam, the examiner then needs to move slightly toward or away from the patient until a clear image of the retina is observed. An inverted, aerial image of the retina is produced, located between the observer and the lens. The apparent magnification will gradually increase as the examiner moves closer to this image (i.e. closer to the patient), allowing more detailed examination. Moving closer to the image obtains a magnification of X4 to X5. As the examiner moves closer additional lenses in the ophthalmoscope are needed, to keep the image clear depending on the accommodative needs of the examiner. A viewing distance of approximately 18 cm from the patient is optimal, providing suitable magnification and a wide field of view A disadvantage of the technique, as with conventional direct ophthalmoscopy is the lack of a true stereoscopic view, however, lateral movement and rotation of the direct ophthalmo-
scope during the examination gives good parallax clues to depth.
Penlight Ophthalmoscopy This is a very old, basically a bedside technique that originally utilized a penlight and a high plus lens. The patient must be dilated to get as much binocularity as possible and large field of view. The ophthalmoscope is held just below the eyes and its light directed into the patient’s eye. The patient’s eye is viewed from over the top of the ophthalmoscope while a 20 D lens is placed approximately 3-4 cm from the patient’s eye. The light leaving the condensing lens must come to focus within the pupil allowing the fullest field of view of the retina, approximately 30 degrees. The image is inverted and laterally reversed and located between the ophthalmoscope and the condensing lens. The degree of stereopsis depends on how fully the pupil is dilated and one’s ability to converge and accommodate on the image. It gives a larger field of view than a MIO though less magnification. This is an alternative method to examine small infants. Should the bulb burn out in a BIO one has an alternative means to get a good view of the peripheral fundus? Do not put hands on the patient’s shoulder or head. Instead, use the back of the chair to steady yourself.
Direct Ophthalmoscopy Direct ophthalmoscope (Fig. 10.12) is most commonly used instrument in ophthalmic practice. The ophthalmologist must familiarize oneself with the use of the direct ophthalmoscope in an appropriate manner. Before being able to recognize the abnormalities in fundus, one must know what normal looks like. It is advisable to examine as many of your colleagues as possible both inside and outside clinic hours. Good observational and recording skills can be developed with practice.
Ophthalmoscopy
Fig. 10.12: Direct ophthalmoscope
Examination Procedure Direct ophthalmoscopy is best carried out in a dark room with fully dilated pupils. One must be familiar with the color coding of the lens wheel and the various apertures and filters. Instruct the patient to look at a distant target (the white spot light on the vision chart) and to ‘pretend’ to still see it even if obscured with your head. The patient may blink as required. Your left eye and left hand should be used to examine the patient’s left eye. The field of view of the fundus is increased when examiner goes closer to the patient’s eye. When patients with low myopes or low hyperopes are to be examined, it is better to remove their glasses. However, for myopes and hyperopes above ± 3.00 DSph and for astigmats above 2.50 DCyl, it is advisable to keep the glasses on in order to overcome problems associated with magnification, minification and distortion. The extra reflexes produced by the spectacle lenses will at first prove distracting but can be overcome with practice.
Using a large diameter aperture, examine the external features of the eye including pupils. With a +1 or +2 D lens in the ophthalmoscope, view the pupils at a distance of 40 to 66 cms from the patient. Look for media opacities. To find the location of the opacity, note movement of the opacity with relation to the movement of the ophthalmoscope, using the pupillary plane as a reference point. If the opacity moves in the same direction as the ophthalmoscope, the opacity is located behind the iris. If the opacity moves in the opposite direction to the ophthalmoscope, the opacity is located in front of the iris. Using the ophthalmoscope as a light source, which is held tangential to the iris one looks for any shadow that appears on the nasal side. If the nasal irido-corneal angle has no shadow, it denotes a wide-open angle. However, as this shadow increases in width relative to the overall cornea size, the angle seems narrow. Dial up +10 DSph lens in the lens wheel and observe the eye from a distance of 10 cm. Study the red reflex to detect any media opacity. The position of opacity can be inferred from its parallax with respect to the pupil. When the patient looks up and the opacity appears to move in the same direction within the red-reflex then it is located anterior to the pupil plane (i.e. in the cornea or in the anterior chamber). Opacity that remains stationary lies in the plane of the pupil but when it moves in the opposite direction to that of the patient’s gaze it lies posterior to the pupil plane (i.e. the posterior lens or vitreous). It may be easier to move yourself slightly from side to side rather than ask the patient to move his eye to achieve the same effect. During ophthalmoscopy it is advisable to keep both eyes open and suppress the image from the other eye. It may take some practice to accomplish this. It is better to move closer to the patient and gradually reduce the power of the lens in the wheel and focus on the crystalline lens, the vitreous and finally the fundus. The power of
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Diagnostic Procedures in Ophthalmology lens necessary to focus on the fundus will depend on patient’s and observer’s uncompensated refractive error and accommodation. Once a blood vessel on the fundus is located, move along it and locate the point at which it branches. Then move your field of view in the direction in which the apex of the branch is pointing till you reach the optic disk. If one controls his accommodation it allows for an estimation of the patient’s refractive error by focusing the optic disk. Retinal blood vessels should be examined in each quadrant after locating the disk. Artery to vein ratio (A/V), arteriolar light reflex (ALR), branching of vessels to all four quadrants and crossing phenomenon must be assessed. Once again focus the disk and move nasally to view the macula. In this position you may obscure the fixation target, cause the pupil to constrict, dazzle the patient and notice some troublesome corneal reflections. These factors make the macula a difficult area to visualize. It may be useful to use a smaller aperture beam. The patient should not be asked to look into the light when viewing the macula through an undilated pupil. The patient will accommodate and this together with the bright light from the ophthalmoscope will make the pupil even smaller reducing the ability to view the whole macular area. Finally ask the patient to look in the eight cardinal directions to view the peripheral fundus. You will need to adjust the lens in the wheel slightly as the periphery is closer to you than the optic disk requiring more focusing power (plus lens). The red-free filter makes small macroaneurysms and small hemorrhages standout more clearly. It can also be helpful in estimating the C/D ratio. It is also used to differentiate between retinal nevus and choroidal nevus. The retinal blood supply and its retinal pigment epithelium (RPE) act like a red filter.
Therefore, a nevus that lies behind the retina and located in the choroid will not be seen when viewed with the red-free filter. On the other hand a nevus located on or in the retina will still be seen with the red-free filter in place. A cobaltblue filter is useful in detection of nerve fibers drop out. The direct ophthalmoscope gives a magnification of approximately X15 and a field of view of 6.5 to 10 degrees. The formula M= 60 D/4 holds well for up to + or –10 Ds of refractive error.
Hruby Lens Direct Ophthalmoscopy The use of the slit-lamp biomicroscope allows a stereoscopic view of the retina. The auxiliary lenses provide high magnification with excellent resolution. The Hruby lens (-55 D) produces an upright virtual image that is not laterally reversed.
Examination Procedure Patient co-operation can be enhanced by attention to his comfort and with the use of a fixation device. Once the illuminated slit is imaged in the patient’s pupil, the Hruby lens is introduced in front of the patient’s eye as close as possible without contacting the cornea or lashes. This mode of direct ophthalmoscopy can provide a very high level of magnification, even greater than that of the monocular hand held direct ophthalmoscope. The actual level of magnification depends on that available through the slit-lamp. Stereopsis is provided to a greater degree than all other examination techniques. The main disadvantage of this technique is the field of view. It is smaller than all other examination methods with the exception of direct monocular ophthalmoscopy (less than two disk diameters for an emmetropic patient). More dilation is required than in other binocular
Ophthalmoscopy techniques. The quality of the image is easily degraded by media opacities; however, increasing the slit-lamp illumination can reduce this problem. As the magnification is so high, small movements of the observer, lens, or patient have an immediately noticeable effect on image quality.
Wide-Angle Viewing System Retcam The Retcam (Fig. 10.13) has a 3 CCD chip video camera. It is lightweight, easy to position and has a long cable for easy patient access. It has five changeable lenses: 130°, 120°, 80°, 30° and Potrait. It has a large LCD display with 20 seconds of real time video per clip and a frameby-frame or real time video review. It has a lighted control panel, a dual DVD-RAM for easy backup, multi-image data recall and display, side by side image comparison, high resolution 24 bit color image, instant-digital image capture and
Fig. 10.14: Wide-angle fundus photograph (Retcam) of a premature infant showing retinopathy of prematurity with a demarcation ridge clearly made out
Fig. 10.15: Fundus photograph (Retcam) of a premature infant showing retinopathy of prematurity with laser photocoagulation marks. Preretinal hemorrhage is seen beyond the superotemporal vascular arcade
is US FDA approved. It provides a 130° view for easy screening for retinopathy of prematurity (Figs 10.14 and 10.15), integrated image and patient management capabilities, comprehensive photodocumentation, fluorescein angiography and built-in software for reporting, storage and archiving.
Panoret Fig. 10.13: Retcam viewing system
Panoret (Fig. 10.16) is a high resolution, wideangle retinal camera based on an innovative
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Bibliography
Fig. 10.16: Panoret
transscleral illumination concept using a fiber optic bundle, where no pupillary dilatation is necessary. Coverage angles are 50° and 100° with interchangeable front lens assembly. It is computer assisted in auto-light, auto-brightness and contrast control along with auto-disk storage.
1. Yanoff M, Duker JS, Augsburger JJ, et al (Eds). Ophthalmology (2nd edn). St. Louis, Mosby, 2004. 2. Benson WE, Regillo CD: Retinal detachment— Diagnosis and Management (3rd edn). Lippincott-Raven, Philadelphia, 1998;75-99. 3. Regillo CD. Brown GC, Flynn Jr HW. Vitreoretinal Disease—The Essentials. Thieme, New York, 1999;41-49. 4. Schepens CL, Hartnett ME, Hirose T: Schepens’ Retinal Detachment and Allied Diseases (2nd edn). Butterworth-Heinemann, Boston, 2000;99129. 5. Rosenthal ML, Fradin S: The technique of binocular indirect ophthalmoscopy. Highlights of Ophthalmology 1967; 9:179-257. 6. Michels RG, Rice TA, Wilkinson CP. Retinal Detachment (2nd edn). Mosby, St. Louis, 1997; 347-70. 7. Havener WH, Gloekner S. Atlas of Diagnostic techniques and Treatment of Retinal Detachment. Mosby, St. Louis, 1997;1-51.
Ophthalmic Photography
SADAO KANAGAMI
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Ophthalmic Photography
Among all types of medical photography, the speciality of ophthalmic photography is perhaps the most difficult to master as it requires in-depth knowledge of not only the ocular structures, and the disease process of the eye, but it also requires special photographic skills in regard to the equipment needed to record ocular pathology on silver base media or electronic medium. Captured ophthalmic images often have a direct influence not only on the diagnosis but also on the treatment choice as in the case of fundus fluorescein angiography (FFA) or indocyanine green angiography (ICGA). The responsibility of accurately capturing this information needed by the treating ophthalmologist becomes critical and weighs heavily on the shoulder of the ophthalmic photographer—especially with the advent of teleophthalmology where images may be captured hundreds of miles away from the treating ophthalmologist. The ophthalmic photography differs greatly from biological photography in general as the images captured by the ophthalmic photographer are part of the treatment decision process or utilized in the management of ophthalmic patients. Recent trends in ophthalmic photographic equipment include computerized equipment that further adds to the long list of specialized technique and changes in ophthalmic imaging.
Some of the ophthalmic photography and imaging equipments include the following in the long list of tools used in our field.
35-mm Camera A 35-mm camera with a motorized drive to automatically advance the film should be fitted with a long macro lens (135 mm to 150 mm or a medical lens such as the Nikon Medikor lens) in order to keep facial distortion to a minimum. This is very important especially when taking photographs in the speciality area of oculoplasty. A macro lens should be selected to include fields of one eye to full face; a second macro lens could include head/shoulder to full body (Fig. 11.1). The selected 35-mm camera should also be fitted
Fig. 11.1: Macro lens for closeup photography
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Diagnostic Procedures in Ophthalmology with either a double-sided macro flash or a builtin ring flash. These macro flashes are typically meant for short range (less than one meter) photography for optimum illumination. When photographing oculoplasty patients for full body photography, a studio flash set-up is still the recommended approach.
Fundus Camera Mydriatic Fundus Camera Conventional non-corneal contact mydriatic fundus camera (Fig. 11.2) can range between 20 and 60 degrees view of the ocular fundus. The ophthalmic photographer can choose the angle of view that will best reflect the needs of the photodocumentation, for example, in imaging the optic nerve for glaucoma one would use a view of 20 degrees, while in the case of a large melanotic choroidal tumor one would select a
wider 60 degree field of view. Mostly, these retinal cameras capture full color images of the retina as well as having capabilities of capturing monochromatic and angiographic images (fluorescein and ICG). Determining the exposure level of electronic flash is completely different from the regular 35-mm camera used in external photography. Usually, these values are predetermined (factory setting) by the angle of view selected on the retinal camera as well as the film sensitivity used. Other determining factors for flash intensity can be the use of a plus diopter setting and angiographic or monochromatic selections (such as cobalt-blue and red-free). Typically, retinal cameras have two or more camera backs; a 35-mm camera for color or monochromatic black and white film, a polaroid camera and in some cases certain retinal camera manufacturers offer optional video camera (analog or digital) to show the images on the monitor and store them in imaging software program on the computer system.
Non-mydriatic Fundus Camera
Fig. 11.2: Fundus camera
As the name suggests, the non-mydriatic fundus camera does not require the use of mydriatic agents to dilate the patient’s pupil. The nonmydriatic fundus camera usually requires a natural dilation of 4 mm; this can be a limiting factor on patients over the age of 60 years old that typically do not naturally dilate well. These fundus cameras are usually very easy to operate as they have no viewfinder but instead they use a large 4-inch monochromatic TV monitor (or in some more modern non-mydriatic cameras, an LCD screen) where the patient’s fundus can be seen by way of an infrared video alignment camera. Since the viewing lamp utilizes infrared wavelength, the patient is not aware of the examination process. The flash illumination, when using a low LUX video charged couple device (CCD) camera, is usually very low as these
Ophthalmic Photography
Fig. 11.4: ICG angiogram Fig. 11.3: Non-mydriatic fundus camera
cameras have a very high sensitivity. The lower the LUX level of the color CCD camera, the faster the pupillary recovery time and thus, the faster the photographic procedure. There are many manufacturers of non-mydriatic fundus cameras some have the ability to capture angiographic images. When one uses mydriatic cameras in the mode of non-mydriatic, these cameras are usually confined for mid-phase only as a waiting period of at least one minute must be allowed to permit full pupillary recovery time. Nonmydriatic cameras can download their captured images to a computerized filing system. Often, non-mydriatic cameras (Fig. 11.3) are used to photograph diseases of the posterior segment of the eye. The camera is very small and light weighted, it can be easily taken outside of the clinic. Fundus images have been stored on a personal computer directly from the camera using USB cable and an exclusive software.
Indocyanine Green Angiography Indocyanine green angiography (ICGA) can be performed with near-infrared illumination using
a retinal camera. ICGA examines the dynamic flow circulation of the choroidal vessels and adjunct structures (Fig. 11.4). Typically, a retinal camera that has been designed with special filters uses a black and white near-infrared CCD video camera (analog or digital) and records static megapixel images stored in a computer bank or dynamic images on videotapes as in the case of the scanning laser ophthalmoscope (SLO).
Digital Hand-held Fundus Camera Digital hand-held fundus camera is recently introduced. This camera is designed for digital images, therefore, it becomes light and easy to operate compared to the previous model. The fundus images are displayed on a small LCD monitor and can be checked. These images can be stored on a memory card. There is an adapter for indirect images. The adapter is very useful while taking the fundus photographs of the premature babies (Fig. 11.5).
Photo Slit-lamp (Kowa Attachment) The hand-held Kowa Genesis camera has a special attachment (Fig. 11.6) that allows for anterior segment photodocumentation using a
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Fig. 11.7: Gonio photography
Fig. 11.5: Digital hand-held fundus camera
gonioscopy, topical anesthetic agent and a transparent gel such as Gogniosol should be used. A lens that has anti-reflection coating should be preferred. Use of gonioscopic lenses need special techniques, however, combined with the use of a video camera it makes it easier to preview the captured field as opposed to capturing on conventional 35-mm film and waiting for the film to be processed to evaluate the photographic technique. However, using video-captured image does not equal the quality of 35-mm film (resolution, hue, color, contrast) but most surgeons agree that the trade-off of immediacy in seeing the images is well worth than the quality of the 35-mm film. For publication a conventional 35-mm film can also be used in conjunction with the video images.
Portable Slit-lamp with Video Camera Portable slit-lamp is very useful when taking pictures of bed-ridden patients and/or small children. This slit-lamp can adapt a very small video camera and can take patients’ anterior segment photographs or video images. Fig. 11.6: Kowa genesis with slit-lamp attachment
slit-adapter. It allows the user to take images on either 35-mm film, video or fully digital backs. Once the adapter is connected, it is possible to capture conventional anterior segment images including of gonioscopy (Fig. 11.7). For
Photography in Operating Theatre There are two main ways of capturing images in the operating theatre, the first consists of positioning the camera next to the operator using a bedside approach, while the other technique
Ophthalmic Photography is to attach a camera directly to the operating microscope and have the operator take all images using one of the optical pathways of the microscope (right or left). Using this technique means that the photography port will be taken through a 70/30° type of prism and that the operator will have to look through only the optical pathway that is occupied by the camera. Using this technique will ensure the operator that what he/she sees is actually captured. Additionally, using this technique will give a good preview of the non-stereo image that is captured by the recording device since only one optical pathway is equipped with a recording device (usually the right optical pathway is best). It is critical that the microscope should be set for focusing the recording device and not the operator’s actual diopteric correction. If this is not done, captured images may not be sharp. The operator will also notice that the field viewed and the field photographed is not exactly the same area (usually the photographed field is smaller) but with practice and years of experience, very good results may be achieved. It is critical as in any other type of photography that the primary lens (lens close to the patient’s cornea) should be free of artifacts such as: dust, fingerprints, water stains, fluorescein stains. Attentive care should be given to the lens cleaning techniques to avoid possible damage to the costly lens. If this is not done, the quality and color of the captured images will be very low with color shift and low contrast images as well as poor optical resolution.
Specular Microscopy Photography of the corneal endothelial cells can be easily performed using a slit-lamp photomicroscope and resulting images can be analyzed using a computer program. Typically, these images can show the borders of the cells that reflect the light towards the high magnification microscope lens when used in conjunction with
specular illumination methods. This illumination can be achieved by using the illumination tower set at 45 degrees (incident light) from the apex of the cornea while observing the return light (reflected light) through the objective when the observation tower is set at 45 degrees from the opposite side of the illumination tower. Recent trends in specular microscopy are the use of noncontact specular microscope that causes little trauma to the patient and risk of cross contamination is less because no corneal applanation is required with the system. In Figure 11.8 one can easily compare the size and/or the arrangement of the endothelial cells. With innovative imaging technology the use of non-contact specular microscopy can be easily observed on large monitor obviating the use of prints or photography on silver highlight film base.
Fig. 11.8: Specular photography: endothelial cells
In the past, the role of the ophthalmic photographer was limited to the capturing of the endothelial cells of the cornea. Today, however, the role of the ophthalmic photographer has evolved to include the analysis of the corneal cells using a computer program (Fig. 11.9).
Imaging System In 1990, the field of ophthalmic photography was introduced to electronic imaging technology. At first, only two companies in the United States
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Diagnostic Procedures in Ophthalmology accepted daily routine tool of major universities and HMO type practices. The electronic imaging has mostly replaced all film based angiography (especially true for ICG) avoiding the long darkroom delays. Although this technology is not comparable to film based technology, yet as far as resolution and gray scale, it does offer certain advantages, such as, instant results viewable on large CRT screens, image processing or enhancement, transfer of images through the internet for teaching, screening or second opinion (teleophthalmology).
Advantages
Fig. 11.9: Noncon robo
were in the forefront of this newly introduced technology—KOWA VK-2 system (Fig. 11.10), Topcon ImageNet and Ophthalmic Imaging Systems (OIS). Soon thereafter, a flurry of imaging systems appeared mostly in PC base and mostly disappearing in a year or two. In the past ten years, this new technology has grown to be an
Imaging system has following advantages: 1. Captured images are displayed on a monitor immediately, 2. Displayed images are large, so the patients who are dilated or have low vision can appreciate them, 3. Images may be reviewed by the treating ophthalmologist as they are being captured, 4. Prints can be produced immediately on thin paper so it is easy to put on a patient’s chart, and 5. Images may be stored in the computer data base system for easy review and follow-up.
Disadvantages
Fig. 11.10: Digital imaging system: KOWA VK-2 system
Imaging system has following disadvantages: 1. The computer systems are quite expensive and technology changes rapidly making systems obsolete in one year, 2. Computer, large CRT screen and printer require additional space, 3. Operation of the computer and system software requires training and maintenance, and 4. Quality of image is not yet comparable with 35-mm film.
Ophthalmic Photography Imaging systems in ophthalmology typically means that the conventional ophthalmic camera recording device such as the 35-mm or polaroid type back is replaced with a charged couple device (CCD) that may be either analog (video signal) or digital (higher resolution than video signal). These CCDs usually can add significantly to the cost of the fundus or slit-lamp camera especially if they are digital in nature. Digital CCD can be either a single chipped red, green and blue chipped or could be 3 chipped, one for each of the RGB wave lengths. The latter is far more expensive than the single chip but the color separation with the three-chip-CCD is superior. The area of sensitization of the CCD chip (usually varying from inch-to-inch) being much smaller than of the 35-mm surface (24 mm × 36 mm) or of the polaroid sheet, the light (flash intensity) required to expose the light sensitive CCD is significantly less than that of traditional film base emulsion to expose the same area of the eye. Much like the film base emulsion, CCD comes in a variety of sensitivity calculated in LUX values. The lower the value in front of the LUX, the more sensitive (and usually more expensive) the CCD is. However, it can also be said that the more sensitive the CCD is, the more electronic “noise” (comparable to large grain when referring to film) can be produced (comparable to higher sensitivity film such as 1,600 or
3,200 ISO). More recently, ophthalmic manufacturers: have introduced non-mydriatic retinal cameras with purely digital recording devices. Non-mydriatic cameras are usually equipped with two CCD, one is a black and white infrared low resolution used for alignment of the patient’s retina (image is viewable on a small CRT screen located on the base of the non-mydriatic retinal camera), while the second is used to actually capture the color image of the retina through the naturally dilated pupil in a dimly lit room. One of the main advantages of the low light CCD chip used in the non-mydriatic camera is that retinal images can be captured sequentially without having to wait 4 to 5 minutes as with instant type photography (polaroid). The captured retinal images typically do not affect seriously the natural dilation of the pupil. Pupillary recovery is usually very fast as opposed to when using instant type film. Additionally, some non-mydriatic retinal cameras can capture ICG angiography since in some cases the infrared cameras used higher resolution.
Photograph of Both Eyes To take the photographs of ocular movements especially in case of strabismus, amblyopia, and ocular muscle disorder, eye gaze position in 9 directions should be captured (Fig. 11.11). To
Fig. 11.11: Eye gaze position in nine directions
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Diagnostic Procedures in Ophthalmology achieve this type of photography, simply place the patient’s head in a straight forward position referred to as primary gaze. Having selected a long lens such as 135-mm macro or 150-mm macro combined with a ring flash, and patient is asked to fixate at a gaze of 30 degrees in each o’clock position such as 12 o’clock, 1:30, 3, 4:30, 6, 7:30, 9 and 10:30 and take photographs in each of these positions. Make certain that the patient maintains his or her head in the primary straight forward position and avoid side-to-side head shifts or frontal and backward tilts. When taking photographs in downward gazes (4:30, 6, 7:30 o'clock), an assistant should help in lifting the eyelids in order to expose those gaze positions. For an overall even illumination, the use of a ring flash should be used, as the ring flash will create a ring pattern on the patient’s corneas, will be equidistant and could be considered as a Heirshberg ring. The long macro lens (135150-mm macro) will avoid facial distortion and give accurate facial renderings.
Photography of Face and Skin For full-face photography of patients (Fig. 11.12), the practice of using a long 135 to 150-mm macro lens still applies in order to maintain correct facial proportions and avoid the distortion created by wider-angle non-macro lenses. It is important that the patient wipes the facial sweat or heavy make-up used by some women as well as any ocular ointment used onto the eyes prior to taking photographs. This practice will avoid getting any unwanted or irregular flash reflexes. Typically, it is a good idea to use an electronic set of flashes mounted as in a photo studio. This type of illumination helps to accentuate areas of interest by creating shadows. If no flash is available, it is possible to use natural outdoor sunlight illumination but caution should be used not to over-expose the area of interest and use a standard blue or gray background. To document
Fig. 11.12: Face and skin photograph
proptosis, the best position is to capture the image from above the patient’s head using two macrotype electronic flashes set at 90 degrees from the patient. This technique will create the appropriate shadows that will help define areas of interest to the oculoplastic surgeon.
Photography of Pupil In some cases of neuro-ophthalmology, it is important to document the pupillary changes of patients and to differences between the right and the left pupil (as both may dilate differently from each other under similar Lux conditions). The best way to record these differences is to use a black and white camera that is mounted on a tripod (for added steadiness) and have the patient place his chin in a chin-rest (also for added steadiness). The room is then darkened and about 5 minutes is needed to allow for each pupil to either dilate or constrict depending on the particular condition of the patient (at times a flash light, white light, may be used to provoke
Ophthalmic Photography a specific pupillary reaction that is recorded on video). Analog iris recorders are available that use infrared CCD cameras in combination with an infrared illumination system that is not perceivable to the patient and where the patient’s pupil does not react. Images are then recorded as either a series of still images or as a string of segments (continuous video images) that are then transferred to a computer for numeric processing. Typically when performing these studies, no mydriatic agents are used unless otherwise indicated by the examiner.
External Photography When taking photographs of the cornea and the lens, the choice instrument is a photo slit-lamp since it has the correct optical magnification and the appropriate flash to accomplish the task at hand. However, when a photo slit-lamp is not available, a 35-mm SLR camera with macro lens and electronic flash or even a fundus camera (using a plus diopter) may be used. External close-up photography of one eye for the purpose of documentation of ocular trauma or tumors can be taken with a macro type lens (usually a long lens) and a side macro flash (usually mounted on either side of the front of the macro lens) to avoid disturbing flash reflexes often found when using a ring flash type systems. Careful evaluation of where the flash reflex will fall is critical in obtaining useful photo-documentation. Many macro type electronic flashes have what is called a modeling light that is mounted directly next to the flash tube. These modeling lights will illuminate the field of interest and give a good idea of where the flash reflexes will show-up when the photograph is captured. Since the cornea and sclera are highly reflective surfaces, special attention needs to be given to the illumination technique. It is possible to limit these reflections by using polarizing filters on the flash
and lens, however, the reflexes will only partially disappear and the iris detail is made very dark.
Conventional 35-mm SLR Camera When using a 35-mm macro lens for ophthalmic photo-documentation, it is critical to select a lens that will keep the true perspective of the area of interest. Nikon Corporation introduced a special macro lens with intergraded ring type macro flash tube. This special macro lens called Nikor Medikor lens, it comes in two focal lengths. This lens works somewhat differently in that the photographer selects the required magnification on the lens and then simply focuses by physically moving towards or away from the patient. Other possible choices for macro lenses are: One eye 135 to 150 mm macro lens Two eyes 105 to 135 mm macro lens Full face 105 to 135 mm macro lens Torso 50 to 105 mm macro lens Full body 50 to 105 mm macro lens
Use of Fundus Camera in External Photography The hand-held fundus camera (Kowa Genesis) may be especially useful in close-up external photography as the system uses a powerful distortion-free macro lens along with a co-axial illumination in the fundus camera that produces a small reflex on the cornea or sclera. This camera is particularly well suited in the pediatric population (Fig. 11.13). Some table-top retinal cameras are also well suited for external (single eye) photo-documentation of the eye; these retinal cameras are usually fitted with a frontal concave lens. To capture the images, simply position the patient in the chinrest as you would for conventional retinal photography and select a plus diopter setting
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Fig. 11.13: Photograph of external eye with handheld fundus camera
Fig. 11.15: Fluorescein stain photography
(as well as in some cases selecting a higher magnification lens) by focusing the retinal camera until the images becomes clear. Film type and flash exposure is the same as for regular fundus photography (Fig. 11.14). For taking fluorescein stain photography of the cornea or sclera, the retinal camera may be the most useful instrument since it already has both the exciter and barrier filter in place (Fig. 11.15). When performing iris angiography, again the retinal camera is best suited for this purpose not only due to the filters but also because these cameras are equipped Fig. 11.16: Anterior segment fluorescein angiogram
with an internal timer that is critical for fluorescein studies requiring dynamic flow analysis (Fig. 11.16). Black and white films ISO 400 or instant type (polaroid or Fuji) film can be used and processed in a similar way as for retinal angiography.
Optical System of Fundus Camera
Fig. 11.14: Photograph of external eye with table-top fundus camera
Fundus camera’s optical system can be compared to the Galilean type telescope and is characteristic by incorporating an internal co-axial type illumination and electronic flash. The light
Ophthalmic Photography emitted through the objective of the camera lens is a ring-shaped image. The distance from this ring to the surface of objective lens is referred to as the working distance and is of great importance in taking good artifact-free fundus photographs. The actual position of this ringshaped light can be best observed by looking from the side of the fundus camera. To keep this relative position constant is one of the most important and basic points in fundus photography to insure good color saturation and artifact-free photography (Fig. 11.17).
agent to achieve best possible pupillary dilation (optimally a pupillary dilation of over 8 mm is desirable). The objective lens should be clean and free from dust and smear. Any dust particles must be carefully removed with a manual blower while smear should be removed with lens cleaning paper. Check that the film is correctly loaded and flash intensity control is properly set according to the film sensitivity as well as the retinal pigmentation. Also adjust the eyepiece diopter scale to match the operator’s diopteric correction (Fig. 11.18). Adjust the height of the motorized camera table as well as the operator’s and patient’s stool so both may be as comfortable as possible in front of the fundus camera (Fig. 11.19).
Fig. 11.17: Working distance
Fundus Photography Preparatory Operations Prior to starting the photographic session, the patient’s eye must be dilated with a mydriatic
Fig. 11.18: Diopteric correction
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Fig. 11.19: Comfortable position
Operational Procedures The patient rests his/her chin on the chin rest and presses his/her forehead lightly against the forehead bar. Adjust the patient’s lateral canthus with the head rest of the fundus camera and align the patient’s eye with the illumination beam and optical pathway of the fundus camera. If necessary, adjust the optical table for optimal patient comfort. Looking through the viewfinder of the fundus camera, focus the camera until you obtain a sharp image of the posterior segment of the eye. Slightly adjust the joystick (left-right-forward and backward) to set the camera to a position in which the subject’s eye is evenly illuminated. It should be free from flares and reflections. One should try to achieve maximum color saturation. Ask the patient to gaze at the fixation target until you have the desired area of the fundus in your viewfinder. It is important for operator to ask the patient to keep both eyes open throughout the entire photographic session. Also make certain that the eyelids as well as eyelashes should not obstruct the light passage. The light
Fig. 11.20: Beam pathway
beam should be projected entirely into the pupil to avoid artifacts to be recorded on the film (Fig. 11.20). If pictures are taken before the above conditions are fully satisfied, reflections and/ or artifacts will be produced and it will result in a lower picture quality and poor contrast. Once all these conditions have been fully satisfied, capture the image with a minimum delay, otherwise the patient may be tired and lose fixation and concentration. When the patient is asked to keep his eye open for over 30 seconds, the tear film starts breaking and cornea gets dry causing a low contrast photograph. It is important to always keep in mind that the patient’s comfort and well-being is critical in order to achieve good photo-documentation. Speak slowly and clearly explain the photographic procedure to the patient in order to lessen his or her anxiety.
Ophthalmic Photography Fluorescein Angiography Ophthalmic photography is unique because the medical photographers also perform dynamic flow studies of the iris, retina or choroid using dyes such as sodium fluorescein or indocyanine green. These studies provide a vital piece of information needed by the treating ophthalmologist in order to understand the vision problems of a patient. Fluorescein angiography (FA) is often more complex than conventional color retinal photography. This, however, is not the case, the main differences between color retinal photography and angiography are a set of filters (usually a set of exciter and barrier filter) and remembering the correct sequence of the flow study (area to be photographed in the early, mid or late phase that are usually recorded with a timer).
Principle of Sodium Fluorescein Angiography Sodium fluorescein is mainly used to perform dynamic flow studies of the integrity of retinal vessels (in some cases, sodium fluorescein may also be used in the study of the vascular integrity of the anterior segment). Once the pupils are sufficiently dilated, a solution with a concentration of 10% (2.5 cc of volume) or 25% (1 cc of volume) of sodium fluorescein is injected in the patient’s vein. Injection volume should be carefully controlled in children or patients weighing less than 100 pounds. When using a concentration of 10% of sodium fluorescein, a recommended dose of 0.066 cc per kg should be used. It not only avoids adverse reactions but gives a good fluorescence standard in the dynamic flow study. The dye travels throughout the body’s circulatory system (first throughout the veins) including the retinal vessels. When observing the retina with a cobalt blue light (referred to as the exciter light set at about
Fig. 11.21: Fluorescein absorption and emission
490 nm), sodium fluorescein reflects a green fluorescence towards the film plane of the retinal camera. Before arriving to the film plane, that green fluorescence passes through a yellow barrier filter (referred as the barrier filter) that removes all unwanted blue light that may interfere with the true appearance of the fluorescence found at about 520 nm. These exciter filters (cobalt blue set at 490 nm) and the barrier filter (sharp cut-off filter set at 520 nm) must be matched perfectly in order to render true fluorescence images of the retinal vessels (Fig. 11.21).
Film Type and Development The amount of fluorescence perceived by the film when properly excited by cobalt blue illumination is somewhat low; therefore, a highly sensitive black and white film such as ISO 400 film should be used. When processing this black and white film (in total darkness), use a medium to high contrast fresh developer in conjunction with an extended processing time in a solution set at 20°C/68°F. Push process is a technique that is used in angiography to see more detail on the film produced by the fluorescence; this technique consists of processing the exposed sensitive film for an extended period of time (50 to 100% longer) or to process the film in a warmer solution say 2 to 4 degrees centigrade higher.
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Diagnostic Procedures in Ophthalmology Photographic Procedures Fluorescein angiographic study consists of several phases based on time sequence. Depending on the particular ocular disease, dynamic flow studies vary between 3 and 15 minutes. Fluorescein angiography has following phases: 1. Preinjection or control photograph: It is a photograph in which both the exciter and barrier filters are in place and a photograph is taken without the presence of sodium fluorescein. This is usually done to determine the presence of pseudo-fluorescence or autofluorescence such as in the case of drusens.
Fig. 11.22: Arterial/venous phase of FA
2. Arterial and venous phase: This is the early phase of the angiogram study usually within 14 to 30 seconds after injection of sodium fluorescein (Fig. 11.22). 3. Mid-phase: When all retinal vessels have been filled (stained) with sodium fluorescein (from 30 seconds to 120 seconds). 4. Late phase: This is the last phase and varies in duration depending on the disease of the patient. In diabetic retinopathy, this phase may vary from 3 to 5 minutes, whereas in some ocular tumors, it may last as long as 15 to 20 minutes (Fig. 11.23).
Fig. 11.23: Late phase of FA
The fluorescein angiography helps in understanding various retinal diseases and abnormalities. One needs to study carefully the retinal drawing of the patient’s chart and look for notes or direction from the retina specialist to understand the areas of interest and the main phase of the study (early, mid or late). It is critical to follow precisely the retina specialist’s notes to understand the diseased eye to be first studied (right or left eye). How soon the retina specialist needs to evaluate the results of the angiogram? Does the retina specialist need to treat the patient with laser immediately after the angiographic study? This is referred to as a STAT angiogram. A good practice is to carefully study the diseased retinal areas when performing color photography, usually done prior to an angiography. Once you understand the ocular disease, you can start the angiographic procedure with a good plan. Number of images in each phase, early, mid and late phases as well as area of interest, are dependant on a particular study. It is, however, important to get different results from what were initially anticipated. In fact, at times, angiographic pattern may be completely different from what was anticipated, a retinal vessel that was thought to be leaking may be intact and
Ophthalmic Photography
Fig. 11.24: Nerve fiber layer with blue filter
Fig. 11.26: Choroidal pigment with red filter
a normal one may be found leaking. Anticipating the unexpected findings comes with years of angiographic experience and a good set of standardized angiographic protocol.
may be very useful while documenting a patient with glaucoma to demonstrate nerve fiber dropout. A green filter (referred to as red-free) will cut out all red-light making those areas black (red is seen as black) creating a nice high contrast image of the posterior pole. Red filters will allow the longer wavelengths of the visible spectrum to penetrate deep into the ocular structures to reveal the choroidal vascular pattern (choroidal vessels appear as white while retinal vessels will appear as black Fig. 11.25) and a choroidal nevus or melanoma (Fig. 11.26). These photographs, in particular those taken with red-free light, are very suitable for printing use.
Monochromatic Fundus Photography Various monochromatic wavelengths penetrate at different layers of the eye revealing specific structures as well as foreign bodies in those layers. With the appropriate monochromatic wavelength filter (cobalt blue filter), it is possible to isolate the first layer of the retina where you can find the nerve fiber layers (Fig. 11.24). This
Anterior Segment Photography with Photo Slit-lamp
Fig. 11.25: Red-free photography
The anterior segment is usually photographed with a photo slit-lamp biomicroscope (Fig. 11.27). It is similar to the clinical slit-lamp biomicroscope that is used in our daily work; with the exception that it incorporates a camera (static or motion such as video) and an electronic flash light. Needless to say, photographers need a good understanding of the clinical instrument before they can become skillful in capturing clinically useful images of the anterior segment (Fig. 11.28).
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Fig. 11.27: Photo slit-lamp
Different from fundus photography, photo slitlamp biomicroscopy is perhaps the most challenging type of photography in the field of ophthalmology. It requires a good understanding of the ocular structures; disease process as well as illumination techniques to illustrate the area of interest to the clinician. The illumination is of key importance. Since pathology varies greatly and may appear differently for each case, simple changes of slit-width, height angle of the illumination tower or even the use of diffuser, the same pathology may show itself quite differently in the final picture. It becomes essential to select most suitable lighting technique for each situation. This challenge is perhaps what gives the photographer greatest pleasure in taking pictures of best area of interest. In observing through the slit-lamp the reflections from the cornea and lens are not so offensive. However, same reflections may become disturbing and even harmful in hiding areas of interest when taking photographs. Adjust the illumination tower angle to avoid unwanted reflections. When using auxiliary light (often
Fig. 11.28: Slit-lamp photograph of lens with various nuclei
referred to as fill light), it is necessary to pay attention to avoid the reflection that light may produce on the cornea. Carefully place the area of interest in the field to be photographed while making certain that you are using the best possible form of illumination. Use appropriate magnification to ensure that not only the area of interest is captured but you leave enough room to have a point of reference for follow-up photographic sessions (for example, in photographing an iris melanoma; use of medium magnification would allow for a portion of the iris to be seen for identification that the mass is located at 12, 3, 6 or 9 o’clock and provides an idea about the size of the mass.
Bibliography 1. Fogla Rajesh, Rao KS. Ophthalmic photography using a digital camera. Indian J Ophthalmol 2003;51:69-72. 2. Kwan A. A simple slit-lamp digital photographic system. Eye News 2000;6:18-21. 3. Prasad S. Digital video in a surgical setting. J Cataract Refract Surg 2004;30:2302-03.
Fluorescein Angiography
R KIM, S MANOJ
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Fluorescein Angiography
The study and diagnosis of retinal, macular and choroidal pathologic lesions have been greatly revolutionized with the advent of fundus fluorescein angiography (FFA). From an initial laboratory tool, it has now become a useful diagnostic tool that has aided the diagnosis and monitoring of the treatment of retinal vascular and macular diseases. Although the retina can be readily examined by direct and indirect ophthalmoscopy and slit-lamp biomicroscopy, the fluorescein angiography provides a valuable addition to these techniques. Over the last 40 years, it has been successfully utilized in many research studies, controlled clinical trials and national collaborative studies and its usefulness and popularity have increased. With the development of high quality retinal fundus cameras, digital imaging and photographic filters, high resolution angiography of the retina and choroid is now possible.
History The technique of using intravenous fluorescein to evaluate the ocular circulation was probably introduced 40 years ago by Mac Lean and Maumenee, who described the direct observation
of the dye and its characteristics by slit-lamp biomicroscopy and ophthalmoscopy. Chao and Flocks provided the earliest description of fluorescein angiography in 1958. Finally, it was introduced into clinical use in 1961 by Novotny and Alvis, who demonstrated the photographic documentation of the fluorescein dynamics. Over the last 3 decades advances have occurred in this sphere, with the development of high quality photography equipment, photographic filters, newer printing techniques, stereophotography and digital imaging which has made possible the generation of high resolution angiography of the retina and choroid.
Basic Principles The basic principle of FFA is based on the understanding of luminescence and fluorescence. Luminescence is the emission of light from any source other than high temperature. When light energy is absorbed into a luminescent material, a few electrons are elevated into a higher energy state. Spontaneous decay then occurs of these electrons into their lower energy states. When this decay occurs in the visible spectrum, it is called luminescence. Fluorescence is
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Diagnostic Procedures in Ophthalmology luminescence that is maintained only by continuous excitation. In fluorescence, excitation at one wavelength occurs and is emitted immediately through a longer wavelength.1, 2
Properties of Sodium Fluorescein Sodium fluorescein (C20H12O5Na) is an orange red crystalline hydrocarbon with a low molecular weight (376.27 daltons) and readily diffuses through most of the body fluids and through the choriocapillaris, but it does not diffuse through the retinal vascular endothelium or the pigment epithelium. Fluorescein is eliminated by the liver and kidney within 24-36 hours, though traces may be found even for one week. Retention may be increased if renal function is impaired.1 The dye absorbs light in the blue range of the visible spectrum with absorption peaking at 465 to 490 nm. It emits light from 500 to 600 nm with a maximum intensity at 520 to 530 nm (green-yellow). Even though the excitation and emission spectra are quite close, as long as suitably matched excitation and barrier filters are used, only substances capable of fluorescence are detected. When fluorescein is injected intravenously 80% becomes bound to protein while 20% remains free in the blood stream and is available for fluorescence. The blue flash excites the unbound fluorescein within the blood vessels or the leaked out fluorescein. The blue filter shields out all other light and allows through only the blue excitation light. Structures containing fluorescein within the eye emit green-yellow light. The blue light is reflected off of the fundus structures that do not have fluorescein. The blue reflected light and green-yellow fluorescent light are directed back toward the film of the fundus camera. Just in front of the film a filter is placed that allows the green-yellow fluorescent light through but keeps out the blue reflected light.2
Technique and Equipment The materials needed for fluorescein angiography are as follows: 1. Fundus camera and auxiliary equipment 2. 23 gauge scalp vein needle 3. 5 ml syringe 4. Fluorescein solution 5. 20 gauge 1 ½ inch needle to draw the dye 6. Armrest for fluorescein injection 7. Tourniquet 8. Alcohol 9. Bandage 10. Standard emergency equipment (Fig. 12. 1)
Fig. 12.1: Emergency set
Equipment The traditional fluorescein angiography unit (Fig. 12.2) has two 35 mm cameras, one for color fundus photography while the other (black & white) for fluorescein angiography. Most fundus cameras take 30° photographs (magnification of X2.5 on a 35 mm film), which are adequate for a detailed study of posterior pole lesions especially macular diseases. Many camera units provide variable magnification at 20, 30 and 50 degrees. The 50° view is most useful for lesions involving a large area of the fundus. The flash unit and powerpack recharges rapidly enough
Fluorescein Angiography to allow angiophotographs to be taken at 2 second intervals. The motor drive in most equipments advance the film automatically and the timer records the interval between the various phases of angiography and is vital especially in conditions when the arterial perfusion pressure is low. The equipment has 2 filters. The exciter filter transmits blue light at 465 – 490 nm, the absorption peak of fluorescein excitation. The barrier filter transmits light at 525 to 530 nm the emitted peak of fluorescein.
Fig. 12.2: Fundus camera
The most frequently used film for FFA is Kodak 400 ASA (black and white). Various developing solutions are available but the best developing time for a particular camera and power pack combination is variable. After the film is developed, the negatives can be counter printed into either film (transparency) or paper (print). On the negative, areas of flourescence
appear black and on positive film or paper it is white. Usually a roll of 35 mm negative film used for FFA has 36 frames.1
Digital Angiography Commercial digital angiography imaging systems have been available for over 15 years and continue to improve in quality each year. Although photographic film is still capable of capturing greater detail than current digital systems, digital imaging offers some distinct advantages over the more traditional film-based angiogram. Instant access to the electronic images increases efficiency and promotes better patient education by reviewing images on a monitor with the patient. Image enhancement and manipulation is easily achieved with imaging software. Lesions can be measured, or digital overlays used to identify changes in lesion size in serial photographs. Images can be stored on magnetic media like CD-ROMs and transmitted electronically to remote sites equipped with a computer for viewing. Digital systems also offer the additional advantage of shortening the learning curve for novice angiographers. Having instant feedback allows the angiographer to adjust exposure settings and camera alignment to correct any flaws in technique. Cutting edge microelectronics and optical designs of unmatched performance enable the present day digital cameras to take retinal images of exceptional resolution with stunning speed and simplicity. Digital imaging system like the IMAGE-net digital imaging system achieves faster, more efficient acquisition, storage, retrieval and analysis of images. These imaging systems also incorporate a full range of image enhancement programs (sharpness, color, contrast) that can be of great help in precisely evaluating difficult pathologies. For easy and precise photography, digital cameras are now provided
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Diagnostic Procedures in Ophthalmology with alignment dots which provide easy confirmation of working distance and are extremely helpful for pre-injection positioning. Also synchronized accessory detection capability and operating sensors guarantee a perfect image every time during fast photography. Precise and delicate control of flash intensity is vital to obtain maximum detail in digital imaging. Multi-step adjustment of flash intensity is provided with most of the modern cameras, which enables optimized results during angiography. The refractive error of the patient can influence the quality of the images obtained especially the large refractive errors. To adjust for variations in the refractive errors, diopter compensation knobs are provided in the modern camera units for accurate photography. For example 0 setting for –10 to +6 diopter, – setting for –9 to –23 diopter, + setting for + 5 to + 23 diopter and a setting for +22 to +41 diopter (ocular anterior photography). Maximal dilatation is critical for optimal images. Modern angiography units now have a threestep illumination diaphragm changing system for patient’s pupil size. Despite these advantages, the high initial cost of digital systems has prevented them from being employed universally.
Fluorescein Solution Solutions containing 500 mg of fluorescein are available in vials of 10 ml of 5% fluorescein or 5 ml of 10% fluorescein, 3 ml of 25% fluorescein solution (750 mg) is also available. With a greater volume the injection time increases, with a smaller volume, more fluorescein remains in the dead space between the arm and the heart. Therefore, 5 ml of 10% solution (500 mg) fluorescein is generally preferred. The venous dead space between the hand or the antecubital vein and the heart may be as much as 5 to 10 ml, leading to sluggish or reduced flow of fluorescein into the central circulation. The fluorescein can be flushed with
5 to 10 ml of normal saline. An alternative is to elevate the patient’s arm above the level of the heart using an adjustable armrest, which reduces the fluorescein transit time to the heart.2
Procedure for Fundus Fluorescein Angiography After informed consent and explaining the procedure to the patient, the patient’s eyes are dilated. The FFA-set namely fluorescein solution in the required concentration, scalp vein needle, 5 ml syringe and the emergency tray (Fig. 12.1) is prepared. The fundus camera (Fig. 12.2) is kept ready after cleaning the lenses, loading the film and test focusing. Patient identification photographs are then taken. Modern digital cameras with imageNet software maintain a data sheet of patients. The patient is positioned and the camera aligned (Fig. 12. 3). Color photography of both eyes is first done and then switched over to black and white photography for FFA. Redfree photograph of the posterior pole is taken. Insert the scalp-vein needle and inject the fluorescein dye and the timer is started as soon as the dye is injected. Take pre-injection photographs and start fluorescein photography from the first appearance of the dye.
Fig.12.3: Digital fluorescein angiography in progress
Fluorescein Angiography Follow an angiography plan depending on the case. No standard and comprehensive plan is possible to evaluat e all the possible retinal vascular and macular diseases. However, the photographer should use his own judgment to follow a particular order in shooting the various quadrants during flourescein angiography. For example, in central serous retinopathy or choroidal neovascular membrane, it is important to take early films and posterior pole photography is sufficient. In macular disorders, concentrating on the posterior pole during angiography is often adequate. Diabetic and other vascular diseases, however, require a detailed fundus study where the first few photographs are taken of the posterior pole and then each peripheral quadrant is specially taken in a clock-wise fashion from the superior quadrant onwards. Photography of the peripheral retina demands patience, precision and skill due to problems in patient’s compliance, light reflexes and awkward camera placements. At the end, reassure the patient and explain the side effects namely discolored skin and urine. If the patient develops nausea or vomiting or signs of allergic response the procedure is stopped and necessary steps taken.1
Stereophotography Stereophotography facilitates interpretation by allowing the images of both eyes to be viewed simultaneously in depth. It helps us in interpreting the condition under study with respect to its relationship to the various layers of the eye. Adequate stereophotographs can be achieved with a pupillary dilation of 4 mm although dilation of 6 mm or more is preferred. The first photograph is taken with the camera positioned as far to the photographer’s right of the pupil’s center. The second photograph of the pair is taken with the camera held as far to the photographer’s left of the pupil’s center. This order is extremely
important because the photographs are taken and positioned on the film so that the angiogram is read from right to left. Most of the modern cameras have a stereo lock, which can be activated to take stereophotographs. Specially made stereo viewers are available to read the stereo images.
Side Effects and Complications Adverse reactions to intravenous fluorescein angiography have ranged from mild to severe.3,4 Mild reactions (1 in 20) are classified as those with transient effects that resolve completely without requiring any specific treatment. The most common side effects are nausea and vomiting. Moderate adverse reactions (1 in 63) require medical intervention and majority have a good recovery. They include pruritis, urticaria, syncope, thrombophlebitis, pyrexia and local tissue necrosis. Severe reactions (1 in 1900) are those requiring intensive intervention and have a variable recovery and at times fatal. They include laryngeal edema, bronchospasm, anaphylaxis, shock, myocardial infarction, cardiac arrest and convulsion.
Nausea Nausea occurs in about 3-15% of patients and is the most frequent side effect. It is most likely to occur in patients under 50 years of age or when fluorescein is injected rapidly. It begins about 30 seconds after injection and lasts for 2 to 3 minutes and then disappears slowly.
Vomiting Vomiting occurs in about 0-7% of patients nearly 40 to 50 seconds after injection. When patients experience nausea or vomiting, they should be reassured that the unpleasant and uncomfortable feeling will subside rapidly.
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Diagnostic Procedures in Ophthalmology Hyperventilation is often known to relieve these symptoms. Restriction of food and water for 4 hours prior to fluorescein angiography may reduce the incidence of vomiting. Promethacine hydrochloride 25 or 50 mg by month may be given about one hour before injection especially in predisposed individuals.
Pruritus Pruritus or itching is one of the most frequent allergic reaction (1 in 82), usually occurring 2 to 15 minutes after the fluorescein injection. Oral or intravenous antihistamics are often beneficial.
Extravasation of the Dye and Local Tissue Necrosis Extravasation of dye is extremely painful and serious. Toxic neuritis caused by infiltration of the extravasated fluorescein along the nerve in the antecubital area can result in considerable pain. An ice pack or injection of local anesthesia is very effective. If extravasation occurs immediately it is best to place the needle in another vein and reinject a full dose of fluorescein.
Vasovagal Attacks Vasovagal attack is caused more by patient anxiety than by the actual injection of fluorescein.
Shock and Syncope Some patients may experience bradycardia, hypotension, reduced cardiovascular perfusion, sweating and the sense of feeling cold.
Anaphylaxis Anaphylaxis to fluorescein may range from hives to laryngeal edema, bronchospasm or cardiovascular collapse. Hives may occur 2 to 15
minutes after the fluorescein injection. Although hives usually disappear within few hours, an antihistamine such as diphenylhydramine hydrochloride may be administered intravenously for an immediate response. Severe reactions involving the respiratory (1: 3800), cardiac (1: 5300) system and seizures (1: 13,900) can occur and may be fatal (1: 221,781). There are no known contraindications to fluorescein injections including patients with a history of heart disease, cardiac arrhythmias or cardiac pacemakers. However, the dye is to be used with caution or avoided in patients with advanced renal failure or in patients with history of drug allergy. Intradermal testing of diluted sodium fluorescein may be required in such patients with history of drug allergy/cross reaction. Although there has been no report of fetal complications from fluorescein injections during pregnancy, it is the current practice to avoid angiography in women who are pregnant, especially in the first trimester.
Basic Anatomic Considerations The inner retina contains the retinal blood vessels, the larger vessels in the nerve fiber layer and the retinal capillaries in the inner nuclear layer. The normal retinal vessels both the large and capillaries with their tight endothelial junctions (inner blood retinal barrier) are impermeable to fluorescein leakage. The outer retina is the primary interstitial space of the retina, where edematous fluid, deep hemorrhages and hard exudate accumulate, is nourished by the underlying choroidal circulation. Normally this layer does not have fluorescein because the retinal pigment epithelium (RPE) tight junctions (outer blood retinal barrier) prevent the leaking fluorescein from the choroid to reach the retina. The larger choroidal vessels do not leak fluorescein but the choriocapillaris show
Fluorescein Angiography fluorescein leakage. Fluorescein freely permeates through the Bruch’s membrane up to the RPE. The RPE blocks to a great degree the visualization of the choroidal fluorescence. The watershed zone refers to the vertical zone of slightly delayed filling choriocapillaris passing through the papillomacular region and/or the disk, which represents the border area between the two main posterior ciliary arteries. The choriocapillaris by virtue of its lobular arrangement has a patchy filling, gradually filling in a transverse fashion with one lobule spilling over into another. The foveal avascular zone (FAZ) represents the area of the macula devoid of any retinal capillaries and measures about 400-500 microns in diameter. Because most of the optic disk is fed by the ciliary system, fluorescein appears simultaneously at the optic nerve head and the choroid before it is apparent in the retinal arteries.
Normal Fundus Fluorescein Angiography Fluorescein angiography is basically a serial study of the vascular pattern of the retina and the choroid at specific time intervals. Prior to dye injection, one or two photographs should be taken of each eye to test the technical quality. Any evidence of fluorescence that appears on the film at this stage in a normal eye is due to suboptimal matching of filters (pseudofluorescence). The first appearance of fluorescein in the eye depends on the arm to retina circulation time, which is approximately 10 to 12 seconds in young patients and 12 to 15 seconds in older patients. The circulation time is greater in the presence of any disease that affects the myocardium and large vessels, causing congestion in the pulmonary and systemic circulation or obstruction in the vascular system.
Phases Fluorescein angiogram consists of five phases according to the appearance of dye in the retinal circulation. 1. The prearterial phase: The choroidal larger vessels and choriocapillaris begin to fill with dye. Fluorescein usually appears approximately one second before in the choroidal circulation as compared to the retinal circulation. Early choroidal fluorescence is faint, patchy and irregularly scattered throughout the posterior fundus. It is interspersed with scattered islands of delayed fluorescein filling. This early phase is referred to as the choroidal flush. When adjacent areas of choroidal filling and non-filling are quite distinct, the pattern is designated as patchy choroidal filling (Fig. 12.4). Within the next 10 seconds due to extreme choroidal fluorescence, the angiogram becomes very bright. The macula does not show choroidal fluorescence because of the taller, more pigmented pigment epithelium present in the fovea and, therefore, remains dark throughout the angiogram. If a cilioretinal artery is present, it fills at the same time as choroidal circulation and even
Fig. 12.4: Prearterial phase of angiogram showing presence of cilioretinal artery (arrow)
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Diagnostic Procedures in Ophthalmology before the central retinal artery fills up with fluorescein (Fig. 12. 4). The central retinal artery begins to fill about 1 to 3 seconds after choroidal fluorescence or approximately 10 to 15 seconds after injection. The less dense the concentration of pigment in the pigment epithelium, the greater the time will be between the visibility of choroidal fluorescence and the filling of the retinal vessels. 2. The arteriovenous phase: The arterial phase of the angiogram occurs at 12 to 15 seconds after injection of the dye (Fig. 12.5). This is followed by the arteriovenous phase a few seconds later and is characterized by complete filling of the arteries and capillaries and the first evidence of laminar flow in the veins (Fig. 12.6). The vascular flow/ blood stream is faster in the center of the lumen than on the sides and so the unbound fluorescein appears to stick to the side creating the laminar pattern of venous flow. The dark central lamina is non-fluorescent blood that comes from the periphery, which takes longer to fluoresce because of its more distant origin. 3. The venous phase: This begins as the arteries are emptying and the veins are filled with dye. In the next 5 to 10 seconds fluorescence of the
Fig.12.6: Early venous phase of the angiogram showing laminar flow of the dye
two parallel laminae along the wall of the retinal veins becomes thicker. At the junction of two veins, the inner lamina of each vein may merge. This creates three laminae, one in the center and one on either side of the veins. As fluorescein filling increases in the veins, the laminae eventually enlarge and meet, resulting in complete fluorescence of the retinal veins (Fig. 12.7)
Fig. 12.7: Venous phase of the angiogram showing both veins and arteries filled with dye Fig. 12.5: Arterial phase showing the dye filling the arteries, background choroidal fluorescence is also seen
Fluorescence of the disk emanates from the posterior ciliary vascular system, both from the
Fluorescein Angiography edge of the disk and from the tissue between the center and circumference of the disk. Filling also comes from the capillaries of the central retinal artery on the surface of the disk. Because healthy disk contains many capillaries, the disk becomes fairly hyperfluorescent on the angiogram. The perifoveal capillary net cannot always be seen on the fluorescein angiogram. It can be best seen in young patients with clear ocular media about 20 to 25 seconds after a rapid fluorescein injection (Fig. 12.8). This is called the ‘peak’ phase of the fluorescein angiogram. Loss of portions of the perifoveal capillary net is believed to be responsible for the decrease in visual acuity in patients with macular disease, diabetic maculopathy and other conditions. The perifoveal net is an important landmark when considering laser therapy.
Fig. 12.8: Peak phase of the angiogram showing the foveal avascular zone and the perifoveal vascular net in the patients with diabetic retinopathy and choroidal neovascularization in the macular area
4. Transit phase: The aggregate of the arterial, arteriovenous and venous phases is commonly referred to as the transit phase of the angiogram. The transit phase represents the first complete passage of fluorescein in blood through the retina and choroid. At the end of the transit phase fluorescein remains in the choroid and sclera due to leakage from the choroidal vessels and choriocapillaris. The transit time is shortest in
the region of the macula and longest in the more peripheral portions of the retina. Approximately 30 seconds after injection, the first high concentration flush of fluorescein begins to empty from the choroidal and retinal circulations. 5. Recirculation phase: During this phase fluorescein at a lower concentration continues to pass through the circulation of the fundus (Fig. 12.9). About 3 to 5 minutes after injection, the choroidal and retinal vasculature slowly empties the fluorescein and the vessels become gray. Vessels of most normal patients almost completely empty fluorescein in approximately 10 minutes.
Fig. 12.9: Recirculation phase of the angiogram showing decreased fluorescence in the retinal vessels
The large choroidal vessels and retinal vessels do not leak fluorescein. The extravasated fluorescein from the choriocapillaris diffuses through the choroidal tissue, Bruch’s membrane and sclera. Leakage of fluorescein with retention of the dye in tissues is designated as staining. In the later phase of the angiogram, staining of Bruch’s membrane, choroid and especially sclera may be visible if the pigment epithelium is lightly pigmented. Fluorescein also leaks from the vessels of the ciliary body, so that in the venous and
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Abnormal Fluorescence Angiography1,5-7 The abnormal fluorescence is primarily of two types: hypofluorescence and hyperfluorescence. Hypofluorescence is a reduction or absence of normal fluorescence, whereas hyperfluorescence is abnormally excessive fluorescence.
Hypofluorescence Hypofluorescence1,5 is an abnormally dark area on the positive print of an angiogram. There are two causes of hypofluorescence namely blocked fluorescence and vascular filling defect.
Hypofluorescence • Blocked fluorescence • Vascular filling defect
Blocked Fluorescence Blocked fluorescence1,5 is also called as masked, obscured or negative fluorescence or transmis-
sion decrease. It indicates a reduction or absence of normal retinal or choroidal fluorescence because of a tissue or fluid barrier located anterior to the respective retinal or choroidal circulation. To differentiate blocked fluorescence from a vascular filling defect, the hypofluorescence on the angiogram must be correlated with the ophthalmoscopic view. If material is seen ophthalmoscopically that corresponds in size, shape and location to the hypofluorescent area on the angiogram then blocked fluorescence is present. If there is no corresponding material, then it is probably due to a vascular filling defect and fluorescein has not perfused the vessels. Moreover, vascular filling defects have a pattern that follows the anatomical distribution of the vessels involved. Blocked retinal fluorescence Any opacification in front of the retinal vessel involving the cornea, anterior chamber, iris, lens, vitreous or the most anterior portion of the retina or disk will reduce fluorescence. The vitreous opacification is often caused by vitreous hemorrhage. Other causes like asteroid hyalosis, inflammatory debris, vitreous membranes or opacification secondary to amyloidosis may prevent visualization of fundus details. The precapillary arterioles and large retinal vessels are located in the nerve fiber layer and the capillaries are located deeper in the inner nuclear layer. When material lies in front of the nerve fiber layer it will block both planes of retinal vessels. When material lies beneath the nerve fiber layer or within or in front of the inner nuclear layer it will block only the retinal capillaries leaving the large retinal vessels unobstructed. If a blocking material lies deeper than the retinal vascular structures, deep to the inner nuclear layer, it will not block the vessels but will block the choroidal vascular fluorescence. The most common cause of blocked retinal vascular fluorescence is retinal hemorrhage
Fluorescein Angiography
Figs 12.10A to C: A Fundus photograph shows a subhyaloid hemorrhage (black arrow), B Mid AV phase shows blocked retinal and choroidal fluorescence corresponding to the hemorrhage and an area of capillary non-perfusion (white arrow), C Late venous phase shows the persistence of capillary non-perfusion (white arrow) Note the disc hyperfluorescence has increased denoting a NVD
(Fig. 12.10). Nerve fiber layer hemorrhage, which is usually flame-shaped, blocks the smaller retinal vessels lying deeper in the retina but only partially blocks the larger retinal vessels in the nerve fiber layer.5 Blocked retinal fluorescence • Anterior segment material • Vitreous material • Inner retinal material Blocked choroidal fluorescence Blocked choroidal fluorescence occurs when fluid, exudate, scar or hemorrhage lie deep to the retina and in front of the choroidal vasculature.
i. Deep retinal material Fluid, hard exudate, hemorrhage and pigment can block the choroidal fluorescence. Deposition of edema fluid usually occurs in the outer plexiform layer. When it reaches a certain volume it tends to form spaces between compressed nerve fibers and Müller’s fibers causing cystoid retinal edema. Retinal edema blocks choroidal fluorescence in the early phase of the angiogram but later it fluoresces.1 ii. Subretinal material Blood under the retina will cause complete blockage of choroidal fluorescence with the retinal fluorescence showing normally. Subretinal hemorrhage has irregular margins
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Figs 12.11A and B: Geographical helicoid pigment epitheliopathy (GHPC) resolved lesion with pigmentation. A Shows the scar tissue with pigmentation (black arrow), B Shows the late phase of the angiogram with hypofluorescence corresponding to the pigmentation and hyperfluorescence (staining) of the scar (black arrow)
(between photoreceptors and pigment epithelium) whereas sub-pigment epithelial hemorrhage is often round and well demarcated. Accumulated pigments (Fig. 12.11), like melanin from diseased retinal pigment epithelium can also cause blocked choroidal fluorescence.1 Blocked choroidal fluorescence • Deep retinal material • Subretinal material • Sub-RPE material • Choroidal material
Vascular Filling Defect Vascular filling defect results from vascular obstruction, atrophy or absence of vessels. The retinal disk or choroidal vessels may be involved. A vascular filling defect of the disk can be easily made out. The absence of retinal vessels is also readily apparent. If the retinal vessels are visible, the hypofluorescence must be choroidal in origin. Stereoscopic photography can help in differentiating the plane of involvement.4 Retinal vascular filling defects Retinal vascular filling defects are most commonly associated with diabetes and atherosclerosis (Fig. 12.12). In the fluorescein
angiogram the retinal arteries fill first, then the retinal capillary bed followed by the retinal veins, and, therefore, it is easy to differentiate arterial and venous occlusion. Also the blocked vessel can usually be traced in the angiogram.1 Vascular filling defects of the disk The capillaries on the disk may not fill due to congenital absence of disk tissue, atrophy of disk tissue and its vasculature, or because of vascular occlusion (Fig. 12.13). All these conditions show early hypofluorescence with late hyperfluorescence resulting from staining of the involved tissue. Choroidal vascular filling defect This is usually caused by obstruction of tissue and has the following characteristics: 1. Normal retinal vascular flow 2. Depigmentation of the pigment epithelium 3. Reduction of choroidal blood flow, and 4. Hypofluorescence in the early phases caused by loss of the normal ground glass choriocapillaris fluorescence. The most common form of choroidal vascular filling defect has been termed patchy choroidal filling. Areas adjacent to the foci that are filling show early hypofluorescence but eventually fill normally usually 2 to 5 seconds later.
Fluorescein Angiography
Figs 12.12A to C: A Fundus photograph of inferotemporal BRVO showing superficial hemorrhages (white arrow) and blocked vascular segment (black arrow), B,C Early and Mid AV phase of angiogram showing blocked fluorescence corresponding to the hemorrhage (white arrow) and area of capillary non-perfusion (black arrow) corresponding to the blocked vascular segment
Figs 12.13A and B: Anterior ischemic optic neuropathy. A Fundus photograph showing disk edema, B Late AV phase of the angiogram showing hypoperfused segment of the disk (black arrow)
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PCA—Posterior ciliary artery
Hyperfluorescence They are abnormally white areas on the positive print of an angiogram. The common possible causes are: 1. Pre-injection fluorescence 2. Transmitted fluorescence 3. Abnormal vessels 4. Leakage
Each angiogram should have one fundus photo with the filters on and before fluorescein is injected. This is called the control or pre-injection fluorescein photograph. Normally it is completely dark. Autofluorescence: It is the emission of fluorescent light from ocular structures in the absence of sodium fluorescein. It occurs with optic disk drusen (Fig. 12.14) and astrocytic hamartoma. Pseudofluorescence: It occurs when the blue exciter and green barrier filters overlap. The green filter usually allows the passage of green light and the blue filter allows the passage of blue
Figs 12.14A to C: A Optic nerve head drusen, B Autofluorescence of the drusen is seen in the pre-injection phase of the angiogram, C FFA shows normal optic nerve head
Fluorescein Angiography
Figs 12.15.A and B: Pigment epithelium defect (PED). A Fundus photograph showing PED (white arrow) and a foci of RPE atrophy (black arrow), B Late phase of the angiogram showing the corresponding well-defined hyperfluorescent lesions
light only. This light reflected off highly reflective surfaces passes through these mismatched filters and stimulates the film. Any light colored or white fundus structure like sclera, exudate, scar tissue, myelinated nerve fibers, foreign body can thus cause pseudofluorescence.
2. 3. 4. 5. 6.
Transmitted Fluorescence (Pigment epithelial window defect)
Abnormal choroidal vessels It can occur with subretinal neovascularization and vessels within a choroidal tumor. In subretinal neovascularization early phases show a lacy, irregular and nodular hyperfluorescence. With a choroidal tumor it is also early vascular type fluorescence although it may increase in the later phases.4
It occurs because of the absence of pigment in the pigment epithelium leading to accentuation of the visibility of the normal choroidal fluorescence. (Fig. 12.15). It has the following characteristics:1 1. Appears early along with choroidal filling 2. Increases in intensity as dye concentration increases in the choroid 3. Does not increase during the later phases of angiography 4. Tends to fade as the choroid empties the dye at the end of angiography.
Abnormal Vessels Abnormal retinal and disk vessels They can be divided into following categories: 1. Tortuosity and dilatation
Anastomosis Neovascularization (Fig. 12.16) Aneurysms Telangiectatic vessels Tumor vessels. All these changes can be viewed in the early phases and usually appear as hyperfluorescence.
Leak The fluorescence of the retinal and choroidal vessels diminishes about 40 to 60 seconds after injection and empties almost completely about 15 minutes after injection. Any fluorescence that remains after the retinal and choroidal vessels have emptied is leakage. Certain forms of leakage occur in the normal eye. They are: 1. Fluorescence of the disk margins from the surrounding choriocapillaris
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Figs 12.16A to C: Proliferative diabetic retinopathy (PDR). A Fundus photograph showing NVD, B Late AV phase of the angiogram showing hyperfluorescence of the disk (black arrow), C Late venous phase showing increased disk hyperfluorescence (Leak–black arrow)
2. Fluorescence of the lamina cribrosa 3. Fluorescence of the sclera at the disk margin if the retinal pigment epithelium terminates away from the disk 4. Fluorescence of the sclera when the pigment epithelium is lightly pigmented.1 Vitreous leak Vitreous leak is caused by: 1. Neovascularization growing from the retinal vessels onto the surface of the retina or disk or vitreous cavity 2. Intraocular inflammation 3. Intraocular tumors. The vitreous leak due to neovascularization is usually localized and appears as a cotton ball type of fluorescence, and following inflammation,
the leak is usually generalized. If secondary to tumors it is most often localized over the tumor.1 Disk edema In the early phases, dilation of the capillaries on the optic nerve head may be seen and in the late phases, the dilated vessel leak resulting in fuzzy fluorescence of the disk margin. Retinal leak When the leakage is severe, the extracellular fluid may flow into cystic pockets and the angiogram shows fluorescence of the cystic spaces. Cystoid retinal edema is apparent as the fluorescein pools in small loculated pockets (Fig. 12.17). In the fovea it takes on a stellate appearance, elsewhere it has a honeycombed appearance. Fluorescent staining of non-cystoid
Fluorescein Angiography
Figs 12.17A and B: A A case of ruptured macroaneurysm with a ring of hard exudates and edema (white arrow), B FFA- late AV phase showing macroaneurysm (black arrow) and edema as a diffuse hyperfluorescence (white arrow)
Figs 12.18A and B: Central serous retinopathy. A Fundus photograph shows the serous collection involving the macula, B Late phase of the angiogram shows the site of leak, smoke stack appearance and pooling of the dye (white arrow)
edema is diffuse, irregular and not confined to well demarcate spaces. Sometimes the large retinal vessels can also leak. This is called perivascular staining and is seen in inflammation, traction and occlusion.1 Choroidal leak It can appear as pooling or staining. Pooling is leakage of fluorescein into a distinct anatomic space, staining is leakage of fluorescein diffused into tissue. There are specific differences between the fluorescent pooling patterns of sensory and pigment epithelial detachment. In a sensory
retinal detachment the pooling tends to fade away gradually toward the site where the sensory retina is attached (Fig. 12.18). In contrast in a pigment epithelial detachment the pooling extends to the edges making the entire detachment and its margins hyperfluorescent.3 Staining refers to leakage of fluorescein into a tissue or material. The most common form of staining occurs with drusen. Drusens hyperfluoresce early in the angiogram since choroidal fluorescence is transmitted through defects in the pigment epithelium overlying them. However,
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Figs 12.19A and B: Disciform scar of Age-related macular degeneration (AMD). A Fundus photograph showing the macular scar with pigmentation (white arrow), B Late phase of the angiogram showing staining of the scar tissue (white arrow)
some of the drusens remain hyperfluorent even in the late phases of angiogram due to staining. Scars also demonstrate staining- hyperfluorescence (Fig. 12.19). Sclera usually exhibits late hyperfluorescent staining1.
collarette. In abnormal conditions such as rubeosis, leakage of fluorescein dye from the abnormal vessels is extensive. This leakage occurs early in the angiogram.2
Iris Neovascularization
Iris Fluorescein Angiography The vasculature of the iris can be examined by focusing a retinal fundus camera directly on the iris. This technique is useful in patients with suspected neovascularization (rubeosis iridis), iris ischemia or iris and ciliary body tumors. Normal iris blood vessels leak fluorescein slightly and are characteristically straight in configuration, with anastomotic connections between the vessels near the iris root and those at the
In rubeosis iridis an abnormal growth of new blood vessels occurs on the surface of the iris. Only vessels on the anterior surface are clearly detected. However, if leakage of fluorescein dye from behind the iris is considerable, posterior surface vessels should be suspected. Abnormal new vessels have an irregular distribution across the iris surface, with a tendency to concentrate at the pupillary border and at the chamber angle. Normal iris vessels follow a fairly straight pattern from the iris root to the pupillary border. Some anastomotic connections exist between the vessels at the iris root and the vessels at the collarette. Leakage of fluorescein occurs from the abnormal vessels in the early phase of the angiogram.2
References 1. Ryan SJ, Schachat AP (Eds). Retina. St Louis, Mosby-Year Book Inc, 2001;875-942.
Fluorescein Angiography 2. Joseph WB, Robert WF, David HO, James SK. Fluorescein and indocyanine green angiography— Technique and interpretation. American Academy of Ophthalmology, San Francisco, 1997. 3. Stein MR, Parker CW. Reactions following intravenous fluorescein. Am J Ophthalmol 1971; 72: 861-68. 4. Yannuzzi LA, Rohrer MA, Tindel LJ, et al. Fluorescein angiography complication survey. Ophthalmology 1986;93:611-17.
5. Rabb MF, Burton TC, Schatz H, Yannuzzi LA. Fluorescein angiography of the fundus: a systematic approach to interpretation. Surv Ophthalmol 1978;22:387-403. 6. Schatz H. Flow chart for the interpretation of fluorescein angiograms. Arch Ophthalmol 1978;10:625. 7. Schatz H. Essential fluorescein angiography – A compendium of 100 classic cases. San Ansalmo, Pacific Medical Press, 1985.
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Indocyanine Green Angiography
Indocyanine green (ICG) angiography (ICGA) is fast emerging as a popular and useful adjunct to the traditional fundus fluorescein angiography (FFA) in the diagnosis of macular, choroidal and outer retinal disorders. This technique was introduced in ophthalmology in 1973 by Flower and Hochheimer.1 FDA approved the ophthalmic use of ICG dye in 1975. Yet, for the next twenty years the ICGA remained largely unpopular owing mainly to technical difficulties. With the advent of videoangiogram recordings and the recognition of its potential in delineating occult choroidal neovascular membranes, the clinical use of ICGA has increased tremendously.
Indocyanine Green Angiography vs Fluorescein Angiography The visualization of the choroidal circulation is better with ICGA due to two reasons. Firstly, the ICG molecules are not as rapidly extravasated from the choroidal circulation as those of fluorescein. Secondly, the near-infrared wavelengths of light that excite and are emitted by ICG dye penetrate the pigmented ocular structures, hazy media and small pupils much more readily than the light of visible wavelengths associated with
fluorescein dye. The excitation and fluorescence of the blue-green wavelengths of FFA are absorbed and scattered by the pigments in the fundus including macular xanthophyll. These factors result in a much better visualization of the choroidal circulation and its dynamics with ICGA than FFA. However, adequate mydriasis is essential as, its fluorescence efficiency is quite poor in comparison to fluorescein which requires a larger quantity of light to be transmitted for adequate resolution.
Indocyanine Green The indocyanine green (ICG) is a tricarbocyanine dye that comes packaged as a sterile lyophilized powder and is supplied with an aqueous solvent. It was first used in 1957 to measure cardiac output. It is an anhydrous 3,3,3’,3’-tetramethyl1,1’-di-(4-sulfobutyl)-4,5,4’,5’-dibenzoindotricarbocyanine hydroxide sodium salt. Its empirical formula is C43H47N2O6S2Na. It contains less than 5% sodium iodide (in order to increase its solubility). It has a pH of 5.5 to 6.5 in the dissolved state, and also has limited stability, and hence must be used within 10 hours after reconstitution. Ninety eight percent of the injected dye is bound to plasma proteins, with
Indocyanine Green Angiography 80% being bound to globulins, especially alpha1 lipoproteins.2 The dye is secreted unchanged by the liver into the bile.3 There is no renal excretion of the dye and it does not cross the placenta. The dye also has a high affinity for vascular endothelium, and hence persists in the large choroidal veins, long after injection. ICG absorbs as well as emits (fluoresces) light in the near-infrared region of the spectrum. The peak absorption after injection is at around 805 nm and peak emission is around 835 nm. It also exhibits a phenomenon referred to as concentration quenching. After a period of increasing fluorescence with increasing serum concentration, that results in peak fluorescence, further increase in concentration, paradoxically leads to decreased fluorescence. This is referred to as quenching and is thought to result from dimer formation.
Adverse Reactions The rate of mild, moderate and severe reactions to ICG dye is 0.15%, 0.2% and 0.05%, respectively.4 The reported death rate following ICGA is 1 in 333,333 (in contrast to 1 in 222,000 following FFA).5 Owing to its iodine content, it has to be used cautiously in patients with known allergy to iodine containing substances such as shell fish. ICGA is contraindicated in patients with history of severe allergies, uremia and liver disease. In fact, persistence of the ICG dye in the retino-choroidal circulation of the eye for more than 30 minutes in the late phase of the angiogram should prompt the search for hepatic dysfunction. The ICG should also be avoided in pregnancy due to lack of human toxicity data in this area. No more than 5 mg per Kg of body weight of ICG dye should be used for safety purposes. Extravasation of the dye causes a painless greenish-blue stain that migrates from the injection-site to the elbow, which often disappears in a week. Removal of the injection needle before
all the dye has been injected will produce a transient dye-stained needle track.
Low- and High-speed Angiography Two types of fundus camera systems are used for ICGA. The first type is based on the imaging optics of the Zeiss fundus camera, but utilizes a digital charge coupled device (CCD) or a vidicon video camera for recording of images. The high resolution CCD camera contains light sensitive elements (pixels) that are analog to the silver base of the photographic film of a traditional camera. Opening of a built-in electromechanical shutter exposes these pixels to light. The camera then converts this analogous signal into a digital one and sends it to the computer for storage or immediate viewing. The camera employs filtered light from either a xenon flash lamp or quartz halogen lamps. As is true for most of the fundus cameras, the optics are optimized for a field of 50 degrees, which allows maximum field, maximum light entry with minimum of noise which in turn increases the image contrast. Despite a very good spatial resolution, the temporal resolution is only several images per second (low-speed angiography). Hence the late phases are captured in good detail. The early choroidal arterial filing, especially in younger subjects with fast blood flow, can be completely missed using such systems. Another disadvantage associated with CCD systems is the image “blooming” or “blow-out” which is a CCD camera photographic artifact. It results when the amount of imaging light exceeds a system’s capacity resulting in saturation of the CCD’s pixels, which in turn overflows to the surrounding pixels. This appears as overexposure of the hyperreflective surfaces such as optic nerve head, drusen, and exudates. Large overlapping vessels, large scars, melanomas, etc. may be associated with blooming. It can be corrected by reducing the flash intensity.
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Procedure of Indocyanine Angiography The patient is seated comfortably in front of the fundus camera, with an extended forearm for dye injection. Red-free and near-infrared reflected light images (with ICG excitation light on and barrier filter removed) are obtained prior to injection. The former image demonstrates the landmark retinal vessels well. The latter image demonstrates the light-transmission irregularities in the retinal and choroidal tissues and
helps to differentiate between hypo- and hyperfluorescent areas due to irregularities in blood flow. This technique of Red (green-free) photography is referred to as “poor man’s ICG” and is tried when there is a contraindication to ICG. This is achieved using a band pass filter centered at 640 nm. Pseudofluorescence is also checked for prior to injection, with the excitator and barrier filters in place. Areas such as dehemoglobinized blood that reflect the incident light are associated with pseudofluorescence. In the late stages of ICGA study, where the camera gain and flash are at the maximum, such areas would appear highly hyperfluorescent. As the dye is diluted 600 times in the systemic circulation before entering the choroidal circulation, a dye concentration of 0.03 mg/ml is required for maximal fluorescence. The total amount of dye injected varies from 25 to 50 mg in 2 to 4 ml of aqueous solvent, with smallervolume higher concentrations (1 ml bolus of 15 to 20 mg/ml injection, followed by a 5-ml saline flush) being used for high-speed angiography.5 For patients with poor dilatation or dark fundi, 50 mg in 3 ml of aqueous solvent would be optimal. The timing of photography is determined by arm to retina time (approximately 10 in young and 12-18 seconds in older patients) since the fundus cannot be observed. 6 The flash and gain are set at the maximum, at 300 watt seconds and +24dB respectively. To capture the earliest phases, photographs should be taken even before fluorescence is evident on the alignment monitor. When the first images of the choroidal or retinal filling phases are seen clearly, the gain should be progressively lowered with each subsequent photograph. Images are obtained at intervals of several seconds in low-speed angiography systems until maximum hyperfluorescence of the retinal and choroidal circulations are reached after which images are taken at 30 to 60 second intervals for the first few minutes, for the early
Indocyanine Green Angiography phase. Later images are taken between 8 and 12 minutes for the middle phase and between 18 and 25 minutes for the late phase. Occasionally, images obtained 30 to 40 minutes into the study are helpful.
Advantages of ICGA over FFA 1. ICGA can be used even when the ocular media are too hazy for FFA. This is due to the phenomenon of Rayleigh scatter that occurs when the scattering particles are small with respect to incident-light wavelengths, with the scatter intensity being greater for shorter wavelengths and hence more troublesome during FFA than ICGA. 2. ICG fluorescence can be imaged even in the presence of considerable blood, due to the phenomenon of Mie or forward scatter. It occurs when the dimension of the scattering particles is nearly similar to that of the wavelength of the incident light. Due to this effect, the 800 nm light used in ICGA allows the visualization of large blood vessels hidden behind hemorrhages. 3. The peak absorption of ICG coincides with the emission spectrum of diode laser, which allows the selective ablation of chorioretinal lesions using ICG dye-enhanced laser photocoagulation wherein a target tissue containing ICG is exposed to the diode laser beam. 4. Infrared light appears as barely visible red light to the patients, and, therefore, photophobic patients tolerate ICGA better than FFA. 5. ICGA accurately measures the size of an occult choroidal neovascular membrane (CNVM) when compared to FFA that might over or underestimate it. On FFA, the occult CNVM may appear larger due to leakage into pigment epithelial or neurosensory detachments or artificially smaller due to blocked fluorescence from adjacent exudates or blood.
Limitations of ICGA 1. The choriocapillaris cannot be imaged separately with ICGA since their average cross-sectional diameter (21 μm) is much smaller than that of their feeding and draining vessels, and hence the fluorescence of the former cannot be differentiated from that arising from the latter. The edge of one capillary vessel too, cannot be distinguished from that of an adjacent one since the intercapillary spaces are on an average only 5 to 7 μm, which is below the limit of resolution of ICGA. 2. The phenomenon of Mie scatter also masks the unfilled retinal vessels that cannot be visualized well in low speed angiography systems. 3. Bright areas do not necessarily signify dye leakage due to the phenomenon of additive fluorescence which the fluorescence increases linearly with increase in vascular thickness until an aggregate thickness of 50 μm is reached, when a plateau is reached and no further increase in brightness occurs. Mie scatter contributes to this additive fluorescence by making the bright area fuzzy and apparently larger. 4. ICGA is poorer than FFA in the imaging of classic CNVM since the early hyperfluorescence of the CNVM is overwhelmed by the intense background choroidal filling. Moreover, since the affinity of the ICG dye to the serum proteins is considerably greater than fluorescein, the leakage of the former from the classic CNVM is lesser than that of the latter even in the late phases. 5. Although superior to FFA in the imaging of occult CNVM, ICGA may underestimate the size of the CNVM, when there is little dye leakage. It is, therefore, imperative to view the films as late as 30 to 40 minutes after injection.
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Diagnostic Procedures in Ophthalmology Normal Phases of ICGA Depending on time the ICG filling of the posterior pole choroid follows the following phases:7
During First 2 Seconds (Prearterial and Arterial Phases) 1. Choroidal arteries fill rapidly, usually in the peripapillary area (nasal to the fovea, since this region is the area of highest blood perfusion pressure in the eye) first and then radiate to the periphery (prearterial phase). The entire posterior pole appears as a uniform network of arterial vessels. Distinct areas of delayed filling can be seen at times corresponding to the watershed zones described by Hayreh.8 The period beginning with the dye injection to the first appearance of the dye in the choroidal arteries is referred to as the prearterial phase. Interarterial anastomoses, although common cannot be imaged by ICGA. 2. Rapid filling of the choriocapillaris occurs and is complete within 2 seconds after entry of the ICG dye into the eye. The choriocapillaris filling pattern produces faint and diffuse fluorescence that prevents the visualization of the deeper choroidal layers. 3. Concomitant with choriocapillaris filling, the choroidal veins begin to fill. The arterial phase refers to the period from early filling of the choroidal arteries to the first appearance of the dye in the choroidal veins. The mean time taken for this is 1.8 seconds. 4. In SLO angiograms, the major retinal vessels remain dark, blocking the underlying choroidal fluorescence.
Between 2 and 5 Seconds (Arteriovenous phases) 1. The fluorescence from the choroidal arteries begins to fade, while that of the choroidal
veins increases, making them more prominent. Arteriovenous phase refers to the period from the late arterial phase to the point when the veins are beginning to fill. 2. The areas of delayed filling get filled up. 3. The retinal arteries begin to fill up.
Between 5 Seconds and Several Minutes There is diminishing fluorescence from the choroidal veins and overall the choroidal vascular features become less distinct. The period from the early filling of the choroidal veins to their emptying is also referred to as venous phase of ICG. The choroidal veins run parallel to the periphery and eventually form the vortex veins. Venous anastomoses occur between large vessels. Laminar flow (the layered blood-flow pattern in veins caused by the slower, nonturbulent movement of blood along the vessel wall), is sometimes seen in large choroidal veins of myopic eyes in ICGA.
Beyond Several Minutes 1. The optik disk becomes dark. 2. There is a uniform, faint dimly fluorescent background against which the major choroidal and retinal vessels are seen as dark structures. The period after the venous phase when there is leakage or retention of dye in the choroidal or retinal tissue is referred to as the late phase of ICGA. This phase demonstrates choroidal neovascularization best. Images using low speed angiography with standard 1024-line digital systems (non-SLO systems) do not allow imaging of the earliest phases and hence for these systems, three phases have been described: 1. Early phase (0-3 minutes): This encompasses the period from the first appearance of the dye in the choroidal arterial circulation till
Indocyanine Green Angiography maximal ICG choroidal fluorescence is achieved (normally within one minute after dye injection). The medium sized choroidal arteries and veins are well imaged along with hyperfluorescent retinal vessels (Fig. 13.1A). In this phase, choroidal fluorescence predominates over retinal circulation since these vessels are larger, more numerous and layered in three dimensions. 2. Middle phase (5-15 minutes): This is seen 6 to 15 minutes after injection of the dye, wherein the hyperfluorescence of the choroidal veins and retinal vessels diminishes (gradual dye washout) to be replaced by a homogenous diffuse background choroidal fluorescence, due to perfusion of the choriocapillaris (Fig. 13.1B). 3. Late phase (30 to 40 minutes): This is seen beyond 18 to 22 minutes after injection, wherein the choroidal vessels stand out in relief (silhouettes) as relatively hypofluorescent structures against the hyperfluorescent background, with a dark optic nerve head. The leakage of the dye from the choriocapillaris into the choroidal stroma accounts for the background fluorescence that persists for hours or days after a single injection of ICG dye (Fig. 13. 1C). Since there is maximal contrast at this stage with hyperfluorescent lesions, this stage identifies CNVMs best by their late staining and fuzzy margins.
Applications of Indocyanine Green Angiography Age-related Macular Degeneration (AMD) The occult choroidal neovascular membrane (CNVM) that occurs in AMD is better imaged by ICGA than FFA. In fact, ICGA can convert occult CNVM (as per FFA) into well-defined classic CNVM eligible for ICG-guided laser treatment in 30% of cases. This is because the
Figs 13.1A to C: Show the various phases of a normal ICG angiogram (ICGA). A Early phase of ICGA of the left eye of a patient (1 minute after injection) showing well-delineated choroidal and retinal vessels. Note that the hyperfluorescence of the choroidal vessels is superior to that of the retinal vessels in this phase. B Mid phase of ICGA of the left eye of a patient (7 minutes after injection) showing decreased hyperfluorescence of the choroidal and retinal vessels (dye washout) with homogenous background fluorescence. C Late phase of ICGA of the left eye of a patient (30 minutes after injection) showing a dark optic disk and ill-defined late background choroidal hyperfluorescence. Note that the choroidal vessels stand out in relief (silhouettes) as relatively hypofluorescent structures against the hyperfluorescent background
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Yannuzzi et al have shown that the finding of a hyperfluorescent spot on the late ICG angiogram (hot spot) can separate the neovascularized portion from the serous portion of a pigment epithelial detachment (PED).10 The same authors have recognized three morphologic types of CNVM, namely, focal spots (comprising 29% of cases), plaques (well and ill defined, comprising 27% and 34% of cases respectively), and combination lesions (with both focal spots and plaques, comprising 8% of cases).11 Focal spots are less than 1 disk diameter in size (Figs 13.2A to F) and are located outside the foveal vascular zone (hence they are amenable to ICG-guided laser photocoagulation); while plaques are larger lesions (usually more than one disk diameter in size). The plaque-like lesions are the
Figs 13.2A to F: Hot spot. A Color fundus photograph of the left eye of a patient showing a hemorrhagic detachment of the posterior pole. B Arteriovenous (AV) phase of fundus fluorescein angiogram (FFA) of the left eye showing an area of blocked fluorescence corresponding to the hemorrhagic detachment. Also seen are multiple hyperfluorescent areas suggestive of pigment epithelial detachments (PEDs). There is no hyperfluorescence that could point towards the underlying choroidal neovascular membrane (CNVM). C Early phase of ICGA of the left eye showing a small spot of intense hyperfluorescence suggestive of a hot spot (white arrowhead). D Early phase of ICGA of the left eye showing the increasing hyperfluorescence of the hot spot (white arrowhead). E Mid phase of ICGA of the left eye showing the increasing hyperfluorescence of the hot spot suggestive of leakage (white arrowhead). F Mid phase of ICGA of the left eye showing the progressively increasing hyperfluorescence of the hot spot (white arrowhead). The area of blocked fluorescence corresponding to the hemorrhagic detachment is also obvious
Indocyanine Green Angiography
Figs 13.3A to D: Plaque. A Color fundus photograph of the left eye of a patient showing a hemorrhagic PED with a notch (black arrowhead). Also seen are hard exudates with retinal pigment epithelial (RPE) degeneration at the fovea. B Venous phase of the FFA of the left eye showing blocked fluorescence corresponding to the hemorrhagic PED (black arrowhead). There is an ill-defined hyperfluorescence in the area of the notch (white arrow). C Mid phase of ICGA of the left eye showing blocked fluorescence corresponding to the hemorrhagic PED (black arrowhead). D Late phase of ICGA of the left eye showing a well defined plaque of hyperfluorescence suggestive of CNVM (white arrowhead) along with an adjacent area of blocked fluorescence of the hemorrhagic PED (black arrowhead)
commonest type of occult CNVMs and they correspond to the thick subretinal pigment epithelial membranes (Figs 13. 3A to D). They are usually subfoveal in locations and hence ICG-guided laser photocoagulation is not advisable and either transpupillary thermotherapy (TTT) or photodynamic therapy (PDT) may be tried. Combination lesions can further be divided into marginal spots, focal spots at the edge of a plaque in 3% of cases (Figs 13.4A to D),
overlying spots, hot spots overlying plaques in 4% of cases (Figs 13.4A to D) or remote spots (focal spots remote from a plaque of neovascularization seen in 1% of cases). Interestingly, the patients are often found to develop the same morphologic type of CNVM in the other eye as well.12 ICGA also reveals the retinochoroidal anastomosis (RCA) in eyes with occult CNVM along with a vascularized pigment epithelial
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Figs 13.4A to D: Marginal and overlying hot spots. A Color fundus photograph of the right eye of a patient showing an occult CNVM with subretinal blood seen superiorly and sub-RPE blood inferiorly and overlying the fovea. B Mid phase ICGA of the right eye showing the blocked fluorescence due to subretinal and sub-RPE blood with a vague central hyperfluorescence. C Late phase ICGA of the right eye showing blocked fluorescence (black arrow) and central hyperfluorescence (white arrowhead). D Late phase ICGA of the right eye showing a well-defined plaque (white arrowhead) along with two hyperfluorescent hot spots (white arrows). The vertical arrow denotes the overlying hot spot while the horizontal arrow denotes the marginal hot spot
detachment (PED). This is a variant of CNVM that is fed by both a choroidal and retinal vascular component.13 RCA is the stage III of a retinal angiomatous proliferation (RAP) which originates in the inner retinal layers, progresses into the subretinal space and becomes eventually associated with new vessel growth from the choroid. Associated features in this type of CNVM include pre- or intraretinal hemorrhages at the lesion site, dilated tortuous retinal vessels, sudden termination of a retinal vessel and cystoid
macular edema. Of these, intraretinal hemorrhage is considered pathognomonic of RAP. This entity has to be distinguished from small branch retinal vein occlusions. RAP responds poorly to treatment. These lesions are difficult to be detected on early phase of ICGA and are better imaged on the mid-late phases when there is progressive intraretinal dye leakage (Figs 13.5A to H). They are best identified when they overlie a serous PED that produces a homogenous background of relative hypofluorescence. FFA is poorer to
Indocyanine Green Angiography
Figs 13.5A to H: Retinochoroidal anastomosis (RCA). A Color fundus photograph of the right eye of a patient showing an occult CNVM with an inverted “C”-shaped subretinal hemorrhage (SRH). B Arterial phase of the FFA of the right eye showing blocked fluorescence corresponding to the SRH. The white arrow points to the two hyper fluorescent spots (choroidal in origin) connected to the vasculature arising from the inferior temporal artery. C Arteriovenous phase of the FFA of the right eye showing the spots to progressively increase in hyperfluorescence (white arrow). Also seen are a few hyperfluorescent spots representing RPE window defects in the papillomacular bundle. D Venous phase of the FFA of the right eye showing progressive increase in hyperfluorescence of the spots (white arrow). E Late venous phase of the FFA of the right eye showing increased hyperfluorescence of the spots suggestive of leakage (white arrow). F Early phase of the ICGA of the right eye showing a small area of hyperfluorescence at the choroidal level suggestive of a new vessel (white arrow). G Mid phase of the ICGA of the right eye showing the communication of the choroidal vessel to the retinal vasculature (arising from the inferior temporal artery) (white arrow). H Mid phase of the ICGA of the right eye showing the communication of the choroidal vessel to the retinal vasculature (arising from the inferior temporal artery) with progressively increasing hyperfluorescence (white arrow)
ICGA in the detection of RAP lesions due to obscuration of the lesion due to progressive dye leakage both intra and subretinally. In contrast, the RAP lesions in ICGA remain localizable to a small spot of hyperfluorescence due to lesser dye leakage till late into the study. Polypoidal choroidal vasculopathy (PCV) that is considered to be a variant of AMD in recent years has a characteristic appearance on ICGA.
The characteristic lesion is a vascular bulge from the surface layer of the choroidal vessels, visible as a spheroidal orange-red polyp-like structure. These lesions have a predilection to the peripapillary areas but isolated lesions in the macula or the periphery can also occur and are associated with serosanguineous detachments of the neurosensory retina and the retinal pigmentary epithelium. When the leakage is
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Figs 13.6A to D: Polypoidal choroidal vasculopathy (PCV). A Color fundus photograph of the right eye of a patient showing the characteristic orange lesions of PCV (white arrowheads). Also seen is an area of subretinal hemorrhage (SRH) superior to the disk. B Venous phase of FFA of the right eye showing the blocked fluorescence corresponding to SRH superior to disk. Also seen are mottled hyperfluorescent areas over the macula. C Mid phase of ICGA of the right eye (10 minutes) showing multiple polyps in relation to large choroidal vessels (white arrowheads). D Mid phase of ICGA of the right eye (20 minutes) shows that the polyps (white arrowheads) are not leaking
predominantly serous from the polpys the entity might be mistaken for central serous chorioretinopathy (CSCR). In the early frames, larger choroidal vessels of the PCV network are easily identifiable, with the area around and within the network remaining relatively hypofluorescent. Shortly thereafter, small hyperfluorescent polyps, corresponding to the reddish-orange choroidal excrescences seen clinically, become visible (Figs 13.6A to D).14 The late phases of ICGA first show a reversal of the
fluorescence pattern (hypofluorescent core and a hyperfluorescent surrounding casement of the polpys); and later show usually a uniform disappearance of the dye (“washout”) from the polyps (except when they are actively leaking), with no late staining characteristic of classic or occult CNVM. The ring of ICG staining due to reversal of fluorescence has also been noted in retinal arterial macroaneurysms and serous pigment epithelial detachments (PEDs). The central core of polyps less than 0.5 disk diameters
Indocyanine Green Angiography in size appear to have uniform intense fluorescence while the internal details are generally visible in larger polyps, suggestive of the presence of an internal architecture. Recently, both PCV and retinal angiomatous proliferation (RAP) are classified as types of hot spots by Yannuzzi. The CNVM that is imaged as a hot spot which does not fall into either category is referred to as focal occult CNVM. In geographic atrophy (GA), in the early phase, there is hyperfluorescence due to transmitted fluorescence from the large choroidal vessels that are imaged better due to the absence
of the attenuating influence of the RPE. In the mid phases, this hyperfluorescence decreases as the ICG dye washes out of the choroidal circulation. In the late phases of the angiogram, hypofluorescence is observed due to absence of the choriocapillaris (which is the source of late background fluorescence) in the area of GA.
Central Serous Chorioretinopathy (CSCR) In the early to the mid phases of ICGA in CSCR, diffuse or multifocal areas of choroidal hyperpermeability,15 not associated with abnormalities
Figs 13.7A to D: Central serous chorioretinopathy (CSCR). A Color fundus photograph of the left eye of a patient showing a central blister of subretinal fluid with subretinal fibrin. B Early phase of ICGA of the left eye showing widespread choroidal hyperpermeability with no clear cut vasculature. C Mid phase of ICGA of the left eye is similar to the early phase, showing multiple islands of choroidal hyperpermeability. D Late phase of ICGA of the left eye showing multiple hyperfluorescent spots representing PEDs (black arrowheads). The central hyperfluorescent streak represents a leaking PED
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Diabetic Retinopathy (DR) FFA remains the gold standard in the management of DR. ICGA, however, has demonstrated irregular and delayed choroidal filling in most of the proliferative DR cases and in up to 50% of background DR cases.6
Choroidal Inflammatory Conditions Digital ICGA is invaluable in the diagnosis and follow-up of patients with choroidal inflammatory disorders. In eyes with choroiditis, areas with active inflammation block the choroidal fluorescence and are imaged as hypofluorescent areas that might resolve with treatment, as in serpiginous choroiditis.16 Late hyperfluorescence is seen at sites if a CNVM has evolved. In fact, ICGA is superior to FFA in bird shot choroiditis, in defining the typical patches. Multiple hypofluorescent lesions radiating to the periphery, corresponding to the areas of choriocapillaris
drop-out, are observed between the choroidal veins in this condition, by ICGA. Eyes with enlarged blind spots on visual field testing can show confluent hypofluorescence around the optic nerve. Acute posterior multifocal placoid pigment epitheliopathy (APMPPE) lesions remain hypofluorescent both in the initial and late phases of ICGA, and these recover with resolution, in contrast to the initial hypofluorescence and late hyperfluorescence in FFA. This hypofluorescence might be due to a partial choroidal vascular occlusion secondary to occlusive vasculitis. 17 The hypofluorescent lesions are also seen beyond the areas of clinically observed yellow lesions, implying that they are not due to masking of the choriocapillaris by abnormal RPE (Figs 13. 8A to D). Additionally, the larger choroidal vessels are visualized within the hypofluorescent areas pointing towards the non-perfusion of the overlying choriocapillaris. The ICGA findings have thus lent credence to the theory of primary choriocapillaris rather than RPE involvement in APMPPE. In resolved APMPPE, the hypofluorescent areas are seen to be decreased in size or completely resolved representing areas of RPE hyperpigmentation (the hyperpigmented RPE remains transparent to the infrared wavelengths of ICGA). However, a few persistent hypofluorescent areas might remain corresponding to RPE hypopigmentation, suggestive of persistent hypoperfusion due to choriocapillaris damage. In eyes with multiple evanescent white dot syndrome (MEWDS), ICGA shows a pattern of hypofluorescent spots seen in the mid-phase, approximately 10 minutes after injection, throughout the posterior pole and the peripheral retina. These spots persist throughout the remainder of the study. These spots appear larger than the white spots seen clinically and more in number on ICGA.18A ring of peripapillary hypofluorescence corresponding to the enlarged blind spot is also seen. The hypofluorescent
Indocyanine Green Angiography
Figs 13.8A to D: Acute posterior multifocal placoid pigment epitheliopathy (APMPPE). A Color fundus photograph of the left eye of a patient showing yellowish white plaque like peripapillary lesions. Also seen are peripapillary concentric lines suggestive of subretinal fluid. B Venous phase of FFA of the left eye showing hyperfluorescent and hypofluorescent spots. The former are seen in the peripapillary area. C Mid phase of ICGA of the left eye (10 minutes) showing peripapillary hypofluorescent spots (white arrowheads). D Mid phase of ICGA of the left eye (20 minutes) is similar showing persistence of the hypofluorescence of the peripapillary spots (white arrowheads)
lesions and the peripapillary hypofluorescence disappear with clinical resolution.
Choroidal Tumors Heavily pigmented tumors such as choroidal melanomas absorb the near-infrared light and block ICG fluorescence. However, the tumor borders are better delineated by ICGA than FFA, which is essential in the assessment of tumor size in response to treatment as well as in follow-
up.6 Choroidal hemangiomas, due to their vascular channels demonstrate progressively increasing hyperfluorescence on ICGA, with very intense late hyperfluorescence.19 Choroidal metastasis show variable characteristics on ICGA depending on their vascularity and pigmentation. For instance, while metastasis of thyroid carcinoma and metastatic bronchial carcinoid tumors are hyperfluorescent, metastasis of breast carcinoma blocks the choroidal fluorescence of ICGA.19
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Recent Advances in Indocyanine Green Angiography 1. Wide-angle angiography: This is carried out by performing ICGA with the aid of wideangle contact lenses, such as Volk SuperQuad and a traditional Topcon fundus camera. The diopter compensation knob of the camera should be set to the (+) setting to compensate for the image plane of the contact lens. This allows real-time imaging of a wide field of the choroidal circulation up to 160 degrees of field of view. 2. Overlay technique: This technique allows lesion on one image to be traced on to another color or red-free image. This would allow precise localization of the lesion for thermal laser treatment and assessment of the adequacy of treatment.
3. Digital stereo imaging: As with FFA images, ICGA images too can be viewed by stereo imaging. Elevated lesions such as PEDs can be better imaged in this way. 4. ICG as a photo sensitizer: It is considered to be a cheaper alternative to vertoporfin in photodynamic therapy of neovascular AMD and other disorders. The advantage of ICG is that it can be imaged and hence proper timing of dye activation (theoretically the ideal timing is when the ratio of ICG fluorescence of the CNVM to that of the normal tissue is greatest) and precise localization for treatment are possible. The need for a separate angiogram prior to treatment is also eliminated. However, this technique is still in its infancy and several investigators are working on it. 5. Digital subtraction ICGA: It uses digital subtraction of sequentially acquired ICG images along with pseudocolor imaging. It shows occult CNVM in greater detail and within a shorter time than conventional ICGA.
The Future Applications of Indocyanine Green Angiography In the future, ICGA is expected to play a more important and wider role especially in the management of macular disorders. 1. Identifying subclinical neovascular lesions in the other eye of patients with AMD. There are several reports that mention that 10% of such eyes with no clinical or fluorescein angiographic evidence of an exudative process harbor plaques of neovascularization evident on ICGA. 2. ICG-guided feeder vessel photocoagulation: SLO high-speed ICGA can adequately image the feeding vessels of the CNVM which are 0.5 to 3 mm in length and are believed to
Indocyanine Green Angiography lie in the Sattler’s layer of the choroid. Either conventional thermal laser or micropulsed (810 nm diode laser) with ICG-dye enhancement are treatment modalities gaining popularity. The drawbacks of this technique is the high cost of the SLO imaging systems, frequent occurrence of multiple feeder vessels and a high rate of reperfusion after a single treatment, both of which necessitate multiple treatment sessions.
References 1. Flower RW, Hochheimer BF.A clinical technique and apparatus for simultaneous angiography of the separate retinal and choroidal circulations. Invest Ophthalmol 1973;12: 258-61. 2. Cherrick GR, Stein SW, Leevy CM, Davidson CS. Indocyanine green: observation of its physical properties, plasma decay and hepatic excretion. J Clin Invest 1960;39:502-600. 3. Sutoh N, Murakoka K, Takahashi K, et al. Remodeling of choroidal circulation in carotid cavernous sinus fistula. Retina 1996;16: 497-504. 4. Hope-Ross M, Yannuzzi LA, Gragoudas ES, et al. Adverse reactions to indocyanine green. Ophthalmology 1994;101:529-33. 5. Lutty G. The acute intravenous toxicity of biological stains, dyes and other fluorescent substances. Toxicol App Pharmacol 1978;44: 22549. 6. Bischoff PM, Flower RW. Ten years’ experience with choroidal angiography using indocyanine green dye: A new routine examination or an epilogue? Doc Ophthalmol 1985;60:235. 7. Yannuzzi LA, Flower RW, Slakter JS (Eds). Indocyanine Green Angiography. St. Louis, Mosby, 1997. 8. Hayreh SS. In vivo choroidal circulation and its watershed zones. Eye 1990;4:273-89. 9. Hayashi K, Hasegawa Y, Tazawa Y, et al. Clinical
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application of indocyanine angiography to choroidal neovascularization. Jpn J Ophthalmol 1989;33:57. Yannuzzi LA, Slakter JS, Sorenson J, et al. Digital indocyanine green videoangiography and choroidal neovascularization. Retina 1992;12: 191. Guyer DR, Yannuzzi LA, Slakter JS, et al. Classification of choroidal neovascularization by digital indocyanine green videoangiography. Ophthalmology 1996;103:2054. Chang B, Yannuzzi LA, Ladas ID, et al. Choroidal neovascularization in second eyes of patients with unilateral exudative age-related macular degeneration. Ophthalmology 1995;102:1380. Kuhn D, Meunier I, Soubrane G, Coca G. Imaging of chorioretinal anastomoses in vascularized retinal pigment epithelial detachments. Arch Ophthlamol 1995; 113:1392. Yannuzzi LA, Ciardella AP, Spaide RF, et al. The expanding clinical spectrum of idiopathic polypoidal choroidal vasculopathy. Arch Ophthalmol 1997;115:478. Guyer DR, Yannuzzi LA, Slakter JS, et al. Digital indocyanine green videoangiography of central serous chorioretinopathy. Arch Ophthalmol 1994; 112:1057. Krupsky S, Friedman E, Foster CS, et al. Indocyanine green angiography in choroidal diseases. Invest Ophthalmol Vis Sci 1992;33:723. Howe LJ, Woon H, Graham EM, et al. Choroidal hypoperfusion in acute multifocal posterior placoid pigment epitheliopathy. An indocyanine green angiography study. Ophthalmology 1995;102:790. Ie D, Glaser BM, Murphy RP, et al. Indocyanine green angiography in multiple evanescent white dot syndrome. Am J Ophthalmol 1994;117:7. Mones J, Guyer DR, Krupsky S, Freidman E, Gragoudas ES, Ciardella AP. Indocyanine green Videoangiography (2nd edn). In Principles and Practice of Ophthalmology. Albert DM, Jakobiec FA, Azar DT, Gragoudas ES (Eds). Philadelphia, WB Saunders, 2000.
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RAJIV NATH, TINKU BALI, MONICA SAHA
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A-scan Ultrasonography
Ophthalmic ultrasonography is a non-invasive, efficient and inexpensive diagnostic tool to detect and differentiate various ocular and orbital pathologies. It is an indispensable tool for the calculation of intraocular lens (IOL) power, the evaluation of the posterior segment behind dense cataract or vitreous hemorrhage, the diagnosis of complex vitreoretinal conditions and the differentiation of ocular masses. Ultrasound, unlike other imaging modalities, is examinerdependent and needs a high level of skill and expertise. It is a dynamic test where diagnosis is best reached during examination and not from still pictures. However, a correlation with clinical findings is essential to make a precise diagnosis.
History Ultrasound was first used in ocular diagnosis in 1956 by Mundt and Hughes who employed the A-scan technique. Oksala and Lehtinen of Finland further refined this technique in the early 1960s. Baum and Greenwood developed the Bscan using the immersion method in late 1950s. The quality of these B-mode images was quite poor and, therefore, almost all the ultrasono-
graphy of the eye was initially performed using the A-mode. Later on the biometric precision of A-scan was increased by increasing transducer frequencies and using more advanced time measurement techniques to replace the ruler measurement of photographed A-mode displays. In 1967, Giglo and Ludlam developed the system, using 20 mHz focused transducer with a multitrace oscilloscope display. In the 1970s the interpretation of A-mode patterns became more precise and standardized due to the efforts of Ossoinig of Vienna. However, its acceptance was limited because the multiple peaks of an A-scan were bewildering for the uninitiated. Standardized echography is a widely used ultrasonic method in ophthalmology conceptualized by Ossoinig, which combines diagnostic A-scan, diagnostic B-scan, biometric A-scan and at times Doppler evaluation. Ultrasonograhy has thus become a reliable and simple procedure with increasing indications. The advent of high resolution, high frequency probes has improved B-mode studies for intraocular and orbital imaging thus pushing A-scan into the backdrop. However, A-scan still remains the best modality for biometry.
A-scan Ultrasonography
Physics of Ultrasound Ultrasonography is based on the propagation, reflection and attenuation of sound waves. Ultrasound consists of high frequency sound waves of greater than 20 kilohertz (20 kHz). Those used for diagnostic ophthalmic ultrasound have a frequency of 7.5 to 12 megahertz (1 MHz = 106 Hz). These high frequency waves have a small penetration (approximately 6 cm at 7.5 MHz) but provide good resolution of minute structures in the eye and orbit. The speed of the ultrasound depends on the medium through which it passes. As the ultrasound passes through tissues, part of the wave may be reflected back towards the probe; this reflected wave is referred to as an echo. Echoes are produced by acoustic interfaces that are created at the junction of media with different sound velocities. The greater the difference in sound velocities of the media at the interface, the stronger is the echo. For example, the lens (velocity = 1641 m/s) produces a stronger echo when adjacent to aqueous (velocity = 1532 m/s) as opposed to blood (velocity = 1550 m/s), such as in hyphema. The returning echoes are affected by many factors, including the size and shape of acoustic interfaces, the angle of incidence of sound beam, absorption, scattering and refraction. The detected echo is highest when the beam is incident perpendicular to the interface.
Instrumentation An ultrasound unit is composed of four basic elements : pulser, receiver, and display screen, all contained within the same unit and connected to the transducer located at the tip of the probe, which acts as sending and the receiving device (Figs 14.1A and B).
Fig. 14.1: (A) A-scan biometer, (B) Basic components of A-scan
The pulser produces electric pulses that excite the piezo-electric quartz of the transducer probe generating sound waves. The returning echoes are received by the transducer and transformed into electric signals, which are processed in the receiver and then displayed on the screen as echograms. The examiner can adjust the amplitude of the echo signal displayed by changing the gain or sensitivity of the instrument. The display may be in one of the two modes: A-scan or B-scan.
A-mode (Amplitude Modulation) A-scan (A stands for amplitude) is a onedimensional display in which echoes are represented as vertical spikes from a baseline.
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B-Mode (Brightness Modulation)
Fig. 14.2: Schematic representation of A-scan in orbita mode of examination
These spikes represent reflectivity, location and size of the anatomic structure (Fig. 14.2). The A-mode display is a time-amplitude display. The X-axis represents time elapsed, which is a function of tissue depth. Knowing the speed of ultrasound in soft tissues the distance between two spikes can be derived. The horizontal expansion can be modified according to type of examination for which three modes orbita, bulbus and varia have been provided. The orbita mode is used for orbital examination, each microsecond measures approximately 1 mm on screen. In the bulbus mode examination (in intraocular examination) each microsecond measures approximately 2 mm of horizontal expansion on the screen. The varia mode is used for axial length measurement. The reflectivity is measured in decibels on the Y-axis and is directly related to the height of the spike above the baseline. When on highest gain, the sound beam is widest, the penetration highest and the spike amplitude maximum, enabling visualization of the weak signals. When gain is lowered, the sound beam
B-scan (B stands for brightness) differs from Ascan in that it produces a two dimensional acoustic section. An echo is represented as a dot on the screen rather than a spike. The strength of the echo is depicted by the brightness of the dot and coalescence of multiple dots on the screen forms a two dimensional picture of the reflecting tissue. A focused beam is used, as the examination takes place in a focal zone (Figs 14.3B and 14.4).
Figs 14.3A and B: A Nonfocused beam of A-scan, B Focused beam of B-scan
A-scan Ultrasonography
Fig. 14.4: Schematic representation of B-scan
patient reclining or sitting, after anesthetic drops are instilled in both eyes. No other coupling agent is needed. The echographer sits on an adjustable examining stool on one side of patient. The ultrasound probe is first applied at 6 o’clock limbus (Fig. 14.6), aiming at the center of the globe. It examines the opposite chorioretinal layers at the 12 o’clock meridian. The patient is instructed to look away from probe to avoid scanning through the lens. The probe is shifted from limbus to fornix (Fig. 14.7) still aiming it towards the center of the globe, thus screening a particular meridian from the posterior pole to the ora serrata. The ultrasound beam is always kept perpendicular to the opposite retina (Fig. 14.8). The same procedure is repeated in eight o'clock meridians, moving the probe temporally around the globe (Fig. 14.9).
Fig. 14.5: Simultaneous display of A and B modes (Note: White arrow is pointing at A-scan spikes corresponding to the B-scan display above it)
Vector A-scan Display A-scan display is provided with some B-scan units, which allows a simultaneous display of both modes (Fig. 14.5). The A-scan pattern corresponds to the vector’s direction.
Fig. 14.6: Ultrasound probe applied at 6 o’clock limbus
Procedure To perform a successful ultrasound examination, two key components need to be mastered viz. the acquisition of images, and the interpretation of images.
Basic Screening Examination The screening examination is used to detect a lesion. The examination is performed with the
Fig. 14.7: Ultrasound probe is placed at fornix
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Diagnostic Procedures in Ophthalmology Further examinations are performed at higher system sensitivity (gain) allowing the detection of any vitreous opacity missed during the examination at tissue sensitivity. However, examination at lower system sensitivity allows detection of flat fundus lesions.
Anterior Segment Evaluation — Immersion Technique Fig. 14.8: Ultrasound probe is kept perpendicular to the opposite retina
Indications for anterior segment evaluation are limited. However, A-scan may be performed by using a simple immersion technique. A scleral shell filled with methylcellulose is inserted between the lids and the probe placed on it. Using this technique the cornea, anterior chamber, iris, lens and retrolental space can be evaluated and axial length of the eye can be measured.
Special Examination Techniques If a lesion is detected on screening examination then special techniques are employed to differentiate lesions through the analysis of specific acoustic characteristics. The special examination techniques include: 1. Topographic echography 2. Quantitative echography, and 3. Kinetic echography.
Topographic Echography Fig. 14.9: Screening-probe position for scanning in eight meridians
The printout is labeled according to the meridian that has been screened, and the segment of the meridian that has been examined, using P for posterior, E for equator and A for anterior. For example, when the probe is placed at 6 o’clock limbus for examining the posterior pole at the 12 o’clock meridian, the picture is labeled as 12P.
It entails the assessment of shape, location and elevation of lesions. The following maneuvers are used: a. The probe is placed at the limbus of the meridian opposite to the center of the lesion and then moved from limbus to fornix to assess the lesion anteroposteriorly (radially). b. The probe is shifted from side-to-side (parallel to limbus) to evaluate the lesion laterally.
A-scan Ultrasonography TABLE 14.1: TOPOGRAPHIC DIFFERENTIATION OF LESIONS ON A-SCAN Category
Point-like
Membrane-like
Space-occupying
Echogram Differential diagnosis
Single spike Foreign body Vitreous opacities
Single spike or chain of spikes Retinal detachment Choroidal detachment Vitreous membranes Tumor surfaces
Chain of spikes Melanoma Retinoblastoma Hemangioma Vitreous hemorrhage
c. The probe is placed in positions that are 90° apart, to examine the lesion from different beam directions. The pathological findings are classified into one of the three categories point-like, membranelike and space occupying (Table 14.1).
Quantitative Echography Once the topographic findings have been ascertained, quantitative echography is performed with A-scan to determine the reflectivity (i.e. spike amplitude) of a lesion, after directing the sound beam perpendicular to it. The resultant spike height is expressed as a percentage of the maximum height that can be displayed on the screen and the lesion can be categorized (Table 14.2). The determination of reflectivity is necessary for evaluation of the internal structure and sound attenuation of a mass lesion. Internal structure refers to the histological configuration (size and arrangement of interfaces) of mass lesions. An internal acoustic structure of a lesion is classified as regular when the echo spikes are uniform. The spikes are uniformly low in melanoma and uniformly high in hemangioma.
The acoustic structure is irregular (heterogenous) if the echo spikes show marked variation in amplitude as seen in a metastatic carcinoma. Sound attenuation occurs when incident sound energy is scattered, reflected or absorbed by a given medium. It is indicated by decreasing spike height within, or posterior to a lesion (occurring from left to right). This spike decrease called angle kappa is determined by drawing an imaginary line through the peaks of the lesion spikes and estimating the angle then formed with the vitreous base line (Fig. 14.10). The steeper the angle, the greater is the
Fig. 14.10: Sound attenuation showing large angle kappa ‘K’
TABLE 14.2: CATEGORIZATION OF LESIONS ON SPIKE HEIGHT PERCENTAGE
1. 2. 3. 4. 5.
Spike height
Lesions
Low (2-20%) Low medium (10-60%) Medium (20-80%) High (80-100%) Very high (100%)
Senile vitreous floaters Choroidal melanoma Vitreous membrane Asteroid hyalosis, metastatic carcinoma Retinal detachment, organized vitreous hemorrhage or foreign body
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Diagnostic Procedures in Ophthalmology sound attenuation. Bone, calcium and most foreign bodies produce strong sound attenuation. This results in a marked decrease of scleral or orbital spikes.
Kinetic Echography The purpose of this examination is to detect spontaneous movements and after movements. It is done at a low gain. Spontaneous movements indicate a vascular lesion as evidenced by multiple, very quick, small amplitude, vertical oscillations in the echo spike pattern. This is assessed with the probe stationary and the eye fixing steadily on a target. After movements indicate mobility and are seen as a vertical motion of the echo spikes following cessation of eye movements. Non-solid lesions like PVD or retinal detachment display after movements, whereas solid lesions like tumors do not.
Indications of A-scan A-scan ultrasonography is indicated for evaluation of the posterior segment of the eye in the presence of complete or partial opacification of the anterior or posterior segments. It is also used to localize and measure and differentiate tumors and evaluate growth during follow-up of patients as well as to detect intraocular foreign bodies and assess extent of intraocular damage in case of trauma. Biometry is another important indication of A-scan for accurate axial length measurements required in IOL power calculation. Measurement of the axial length of globe, is also important in evaluating congenital glaucoma, microphthalmos, nanophthalmos, myopia, PHPV and phthisis bulbi. Morphological characteristics of the eyeball and its contents, like corneal thickness, lens
thickness, anterior chamber depth and relative lens position in the anterior segment, have been extensively studied in various conditions such as narrow-angle glaucoma and refractive errors, with the help of A-scan ultrasonography (Table 14.3). Ultrasonic pachometry which uses the principle of A-scan is now the standard for measurement of corneal thickness. TABLE 14.3: INDICATIONS OF A-SCAN ULTRASONOGRAPHY Anterior Segment • Corneal opacification • Anterior chamber hyphema or hypopyon • Miosis • Pupillary membrane • Cataract Posterior Segment • Vitreous hemorrhage • Endophthalmitis Clear Ocular Media • Tumors and masses – detection, differentiation and follow-up • Vitreous pathologies • Choroidal detachment • Retinal detachment – rhegmatogenous and exudative Biometry • Axial length of eyeball • Anterior chamber depth • Lens thickness • Tumor measurements Ultrasonic pachometry • Corneal thickness
Interpretation of Normal A-scan Examination of a normal globe displays the following echo spikes from left to right (Fig. 14.11). 1. The initial spike (I) represents reverberations at the probe tip and has no clinical significance. 2. The baseline (B) represents the vitreous cavity which is characterized by absence of echo spikes in normal conditions. The presence of any blip on the horizontal line needs evaluation to rule out a pathological condition.
A-scan Ultrasonography
Fig. 14.11: Normal A-scan with sound beam bypassing lens; I: Initial spike, B: baseline representing echo-free vitreous, R: retina, S: sclera, O: orbital soft tissues, E: electronic scale
3. The retinal spike (R) is a straight, high rising echo spike perpendicular to the baseline. A jagged echo spike means that the probe is not perpendicularly placed. 4. The choroidal spikes are multiple high reflective spikes, which are seen between the retinal spike (R) and the scleral spikes (S). 5. The scleral spike (S) is difficult to differentiate from choroidal spikes. 6. The orbital spikes (O) are multiple spikes behind the scleral spike. The initial spikes are high reflective and the reflectivity decreases rapidly because of sound attenuation in the orbit. 7. An electronic scale (E) is displayed on the lower part of the screen. Examination at low system sensitivity (low gain) clearly identifies the retinal and scleral spikes.
Fig. 14.12: A-scan pattern of a vitreous floater (arrows)
A-scan in Common Ocular Pathologies Vitreous Vitreous Floaters They are found due to condensation of vitreous sheets in an aging eye. Very low reflective (2-20%) spikes are displayed as small blips along the baseline, which are better displayed at a higher gain (Fig. 14.12).
Asteroid Hyalosis Multiple echo spikes with medium to high reflectivity (50–100%) are displayed along the baseline. The high reflectivity results due to presence of calcium within the asteroid bodies.
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Fig. 14.13: A-scan of organized vitreous hemorrhage
Vitreous Hemorrhage
Posterior Vitreous Detachment
In fresh, mild vitreous hemorrhage with dispersed red blood cells, a chain of low amplitude spikes is found on A-scan. These are often limited posteriorly by a higher reflective spike representing a posterior vitreous detachment. Denser the hemorrhage, the higher is the reflectivity of the echo spikes. If the blood organizes larger interfaces are found, which may present even 60-100% reflectivity (Fig. 14.13).
A single lesion spike is present along the baseline. The reflectivity is low (5–10%) if the posterior vitreous layer is thin. The reflectivity is high (8090%) if the posterior vitreous layer is thick or lined by red blood cells (Fig. 14.15).
Endophthalmitis In endophthalmitis diffuse inflammatory cells are present in the vitreous, which are displayed as multiple echo spikes with low to medium reflectivity (10–60%). With organization and membrane formation, the reflectivity increases (Fig. 14.14). Daily follow-up examinations are required.
Retina Retinal Detachment Retinal detachment is characterized by a single, steeply rising, extremely high (100%) and moderately thick retinal spike when the sound beam is perpendicular to the retinal surfaces (Fig. 14.16). Other directions cause a change in pattern – an oblique beam gives lower and wider spikes with two or more peaks and tangential beams show a long chain of low to medium high spikes.
Fig. 14.14: A-scan of endophthalmitis, a week after its occurrence. Spike due to organized inflammatory membrane in vitreous (arrow)
A-scan Ultrasonography
Fig. 14.15: A-scan at high gain: A:medium reflective spike of PVD, B: low reflective spike from subvitreal blood
Fig. 14.16: A-scan of retinal detachment showing 100% tall single peak spike (R)
The distance between the retinal spikes and the ocular wall spikes in a given beam direction is equal to the degree of elevation. The presence of signals between the retinal and scleral spikes is indicative of an exudative or hemorrhagic retinal detachment. Sometimes it is difficult to differentiate between a thick vitreous membrane due to
inflammation or trauma, and a retinal detachment, as both may show a highly reflective (100%) spike. However, they have different reflectivities in the periphery. A retinal detachment is highly reflective both posteriorly and in the periphery. Vitreous membranes tend to be highly reflective posteriorly but less in the periphery (Fig. 14.17).
Fig. 14.17: A-scan technique for differentiating a dense PVD or thick vitreous membrane from RD in the superior portion of eyeball. On scanning the membrane posteriorly 100% high spike is first seen (1). The probe is then shifted so as to follow the membrane to its insertion in the periphery. A PVD shows low reflective spikes in the periphery, while the retina remains highly reflective as seen in (4)
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Diagnostic Procedures in Ophthalmology Retinoschisis A 100% high spike is produced in retinoschisis which may demonstrate slight vertical after movements. Retinoschisis differs from a retinal detachment by its more focal, smooth and thin character.
Intraocular Tumors A-scan helps in the detection, differentiation and measurement of intraocular tumors.
Extrascleral extension of the melanoma is seen as jagged scleral echospike with low reflective spikes immediately behind the sclera. A-scan can also be used for follow-up of tumor growth and to assess effectiveness of therapy. Typically, a treated melanoma becomes highly irregular and more highly reflective due to tumor necrosis, with decreased elevation and loss of vascularity. A-scan can help in differential diagnosis of choroidal tumors (Table 14.4 and Fig. 14.18).
Metastatic Carcinoma Choroidal Melanoma Most malignant melanomas can be diagnosed or suspected from their characteristic ophthalmoscopic appearance. Ultrasound provides confirmation of the diagnosis especially in eyes with opaque ocular media; provided the lesion is elevated by at least 0.75 mm from the inner scleral wall. The key acoustic criteria of a choroidal melanoma are regular acoustic structure and a low to medium internal reflectivity due to a homogeneous cellular architecture. Vascularity is present, with fast spontaneous vertical spike movements seen during examination; and a solid consistency, with no after movements of spikes following ocular movements. Larger tumors may have a more irregular internal structure due to tumor necrosis and large blood vessels. They also show a moderately steep angle kappa due to strong sound attenuation within the tumor.
The acoustic structure of metastatic carcinoma is irregular with a high internal reflectivity (6080%) and absence of vascularity. Measurements of tumor height on follow-up show slow growth or no growth.
Choroidal Hemangioma The acoustic structure of choroidal hemangioma is regular with a very high internal reflectivity due to multiple blood filled channels. Vascularity is present and follow-up shows no growth.
Choroidal Hemorrhage Choroidal hemorrhage may show a reflectivity similar to that of melanoma but profoundly differs from it by displaying after movements during kinetic echography if it is sufficiently elevated.
TABLE 14.4: A-SCAN ULTRASONIC DIFFERENTIATION OF CHOROIDAL TUMORS Criteria
Choroidal melanoma
Metastatic carcinoma
Choroidal hemorrhage
Regular Low to medium (10-60%)
Irregular High (80-100%)
Regular High (100%)
Spontaneous movements (vascularity)
Fast, vertical
Minimal or no movements
Fast, vertical
Growth during follow-up
Significant
Slow
None
Internal structure Reflectivity
A-scan Ultrasonography
Fig. 14.19: A-scan of retinoblastoma. Very high reflective tumor spikes (T) are seen with decreased reflectivity behind them due to shadowing of sclera and orbital tissues. Vitreous seeding is seen as a low reflective echo spike (V)
Figs 14.18A to C: Differential diagnosis of choroidal tumors: A Choroidal melanoma, low internal reflective, B Choroidal hemangioma, uniform high reflectivity, C Metastatic carcinoma, variable reflectivity, Arrow – internal tumor spikes, T: tumor surface, S: sclera
Retinoblastoma Retinoblastoma is best diagnosed by indirect ophthalmoscopy. A-scan offers additional diagnostic information through the quantitation of sound attenuation by the lesion. The measurement of axial length helps in differentiating it from other causes of leukocoria. The status of
the lesion and the effect of treatment can also be assessed by A-scan. On A-scan, a retinoblastoma shows an irregular acoustic structure with high internal reflectivity (70-100%) (Fig. 14.19). Tumor cell arrangement, large vessels and particularly calcifications are responsible for the high reflectivity. Vascularity is present as evidenced by spontaneous movements of the lesion spikes. The axial length may be normal or increased. The A-scan pattern may vary depending upon size of tumor and degree of tumor calcification and necrosis. A small retinoblastoma without calcification will not produce a high reflectivity. Other conditions that can cause leukocoria but can be differentiated from a retinoblastoma by ultrasonography include, persistent hyperplastic primary vitreous (PHPV), retinopathy of prematurity, and Coats’ disease (Flow Charts 14.1A to D). In PHPV, an A-scan examination confirms the absence of a retinal pathology, as normal retinal echo spikes are seen. The axial length of the affected eye is shorter than the fellow eye. In retinopathy of prematurity, the A-scan shows absence of a mass lesion and the presence of a large echo spike representing the detached retina with normal axial length. Coats’ disease consists of retinal detachment with subretinal exudates. A-scan shows a high reflective echo
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Diagnostic Procedures in Ophthalmology Flow chart 14.1A: Differential diagnosis of intraocular lesions on A-scan. Point-like lesions on topographic echography
Flow Chart 14.1B: Membrane-like lesions on topographic echography
A-scan Ultrasonography Flow chart 14.1C: Diffuse mass lesions on topographic echography
Flow chart 14.1D: Well-defined mass lesions on topographic echography
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Diagnostic Procedures in Ophthalmology spike of detached retina followed by low reflective echo spikes representing red blood cells and cholesterol in the subretinal exudates.
Choroid Choroidal Thickening A low gain should be used to detect choroidal peaks as it improves the resolution of the closely spaced posterior layers. The choroidal thickening may be diffuse or focal, and its reflectivity may be high or low depending on its etiology. High reflective thickening is seen in macular edema, endophthalmitis and uveitis while low reflective thickening in VKH syndrome and sympathetic ophthalmitis. Certain tumors may also appear as thickened choroid.
Choroidal Detachment A thick steeply rising 100% high spike is produced by choroidal detachment on A-scan. On lowering the gain the spike is observed to be double peaked. If choroidal hemorrhage is present, low to medium spikes are seen in the subchoroidal space. If choroidal effusion is present the space is echo-free.
Ocular Trauma In a traumatized eye, the fundus visualization may be obscured by a hyphema, a cataract or a vitreous hemorrhage. A-scan examination of the eye in such cases is used to detect any intraocular damage and the presence of an intraocular foreign body (IOFB). It is advisable to repair an open wound before ultrasonic examination. However, if intraocular assessment is imperative before closure, the Ascan probe should be placed on the conjunctiva in an area away from the wound. Marked lid swelling or severe pain may prevent placement
of probe directly on the globe in such case it may be placed on the closed lids with methylcellulose applied for better sound penetration. It is important to use a very high gain when examining through the closed lids.
Foreign Body Detection and Localization A foreign body in the eye can be easily recognized by the characteristic steeply rising overloaded echo spike with extremely high reflectivity (100%). It may show a great width at lower gain. Because of the small size of the IOFB, the high reflective single echo spike is seen whenever the foreign body is centered in the path of the sound beam, irrespective of the beam’s direction. A foreign body produces strong sound attenuation and the ocular wall spikes behind it are significantly lower due to shadowing. A-scan complements radiologic evaluation and can detect radiolucent foreign bodies missed on X-ray. A small spherical foreign body, like a pellet, shows a high echo spike followed by a long chain of echo spikes with decreasing height. These multiple echoes are caused by multiple reverberations of sound between the anterior and posterior surfaces of the spherical foreign body. A larger foreign body has anterior and posterior echo spike and its thickness may be measured between these surface spikes.
Preretinal Foreign Bodies When a foreign body is lying within 2 mm of the sclera or in the coats of the eye, it becomes difficult to decide on A-scan whether the foreign body is intraocular or extraocular. In such situation, it is important to know that the sclera yields a high reflective echo spike only when the sound beam is perpendicular to it, while a high reflective foreign body signal can be
A-scan Ultrasonography
Fig. 14.20: A-scan of traumatic retinal detachment: R: 100% tall spike from retinal detachment, H: low reflective echo spike from subretinal hemorrhage
displayed if it is centered within the sound beam at any angle. Therefore, the sound beam is aimed towards the foreign body at an angle oblique to the sclera, thus decreasing its reflectivity. High reflective foreign body spikes are then displayed in front of the lower reflective ocular wall spikes if the foreign body is intraocular.
Traumatic Retinal Detachment The high reflective echo spike of retinal detachment is followed by low reflective echo spikes between the retinal spike and ocular wall spikes. These represent a subretinal hemorrhage (Fig. 14.20).
Dislocated Lens in Vitreous A dislocated lens in the vitreous shows two smooth and high reflective surface spikes, displayed in front of the ocular spikes. There may be lower
Fig. 14.21: A-scan of dislocated lens in vitreous: A: anterior lens spike, P: posterior lens spike seen in vitreous cavity
reflective spikes representing the lens nucleus, separating the surface spikes (Fig. 14.21).
Phthisis Bulbi In phthisis bulbi the globe is atrophic and shrunken, the intraocular contents are disorganized and intraocular calcification may be present. The A-scan represents these changes as an irregular pattern of high and low reflective echo spikes which fill the globe. High reflective echo spikes may be present due to ossification and the normally high orbital echo spikes are absent. The axial length of the eyeball is shorter than normal.
Biometry The most commonly used function of the A-scan is for measurements in the eye, i.e. biometry. This
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Diagnostic Procedures in Ophthalmology includes measurement of axial length of the eye for IOL calculations, for monitoring eyes with congenital glaucoma, myopia and nanophthalmos and measuring intraocular parameters like anterior chamber depth and lens thickness.
Method The A-scan biometer probe is a 10 MHz solid probe with an inbuilt fixation light. The probe has to be aligned with the optical axis of the eye for accurate axial length measurement. This can be done by the immersion or the contact technique.
Immersion Technique The patient is placed in a supine position or in a reclining examination chair and local anesthesia is instilled. A scleral shell is applied to the eye, the most commonly used being Hansen or Prager shell, which is available in different diameter sizes. The scleral shell is filled with 1% or 2% methylcellulose, which should be free of air bubbles; the presence of air bubbles causes
Fig. 14.22: A: scan display of phakic eye measured with Immersion technique. IS: Initial spike produced at the tip of the probe. C: The corneal spike C is double peaked representing the anterior C1 and posterior surfaces C2 of the cornea. L1: The anterior lens spike generated from anterior surface of lens. L2: The posterior lens spike generated from posterior surface of lens and is usually smaller than L1. R: The retinal spike, from anterior surface of retina. It is straight, highly reflective and tall whenever the ultrasound beam is perpendicular to the retina. S: Scleral spike. O: The orbital spikes are low reflective behind the scleral spike
variations in the speed of sound and is responsible for noise formation, within the ultrasound pattern. The ultrasound probe is immersed in the solution keeping it 5-10 mm away from cornea. Since the probe does not touch the cornea, corneal compression is avoided. The patient is asked to look with the other eye, at a fixation point placed at the ceiling. The probe is gently moved until it is properly aligned with the optical axis of eye and the echo spikes displayed as shown in Figure 14.22.
Contact Technique The contact technique for axial length measurement is an alternative to immersion biometry. It does not use scleral shell. Instead the probe comes in contact with the cornea, which can be done in two ways: either hand held by examiner or attaching the probe to slitlamp biomicroscope or applanation tonometer holder (Fig. 14.23). The patient is examined in the seated position after instilling local anesthetic drops. The patient is asked to fixate a target straight ahead with the non-testing eye or to look directly at the probe’s fixation light with the tested eye. The probe is brought forward to touch the cornea
Fig. 14.23: A-scan probe fit into the applanation tonometer holder
A-scan Ultrasonography
Fig. 14.24: A-scan display of a phakic eye measure with contact A-scan biometry. Since the probe is in contact with the eye, the initial spike and the anterior corneal spike become one: C: cornea/probe, A: anterior lens surface, P: posterior lens surface, R: retina, S: sclera
without indenting it. It is properly aligned along the visual axis to optimize the five high amplitude spikes on the screen. The five spikes in a phakic patient represent from left to right: (1) anterior surface of cornea, (2) anterior surface of lens, (3) posterior surface of lens, (4) anterior surface of retina, (5) sclera (Fig. 14.24). An aphakic eye will not show the lens spikes (Fig. 14.25) though sometimes a spike of intact posterior capsule, if present, may be seen. The leading edge of each echo spike should be perpendicular to the horizontal baseline. The gain is kept at the minimum level that allows proper resolution of these spikes. The density of the cataract determines the need for changing the gain setting due to absorption of sound. Dense cataract requires higher gain to achieve good resolution. The anterior chamber depth which appears on the screen should also be monitored to detect corneal compression during contact biometry. The biometer has an automatic as well as manual mode. Use of the automatic mode
Fig. 14.25: A-scan of an aphakic eye with probe on cornea. Note the striking absence of the lens spikes: C: cornea/probe: R: retina, S: sclera
increases the risk of error as the biometer may capture poor quality scans. Biometers are programed to capture any scans with spikes that are of high amplitude within their given appropriate area. However, they cannot determine if the spike arose steeply from the baseline or if a step or hump was present in the spike origin. Manual mode is preferable, in which the examiner presses a foot switch to capture the scan when it is seen to be of high quality. The axial length of the eyeball is measured from corneal surface to retinal surface and an electronic readout is obtained. A comparison between contact and immersion techniques of biometry is given in Table 14.5.
Biometry in Ocular Pathologies Congenital Glaucoma Axial length measurement in congenital glaucoma allows confirmation of the clinical diagnosis, and differentiates congenital
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Diagnostic Procedures in Ophthalmology TABLE 14.5: COMPARISON BETWEEN CONTACT TECHNIQUE AND IMMERSION TECHNIQUE OF BIOMETRY Contact technique
Immersion technique
Patient is in a more comfortable position, sitting
Patient is in a supine or reclining position
Variability from one test to next is present due to inconsistent corneal compression
No variability since probe does not come in contact with cornea
Axial length measured is shorter by an average of 0.24 mm
Axial length measured is closer to the true value
glaucoma from megalocornea in which axial length remains normal. It can also monitors efficacy of glaucoma therapy. The immersion technique is preferable as it can detect minute changes in axial length in small eyes of children.
Nanophthalmos
Myopia
Limitations and Pitfalls of A-scan
Biometry helps differentiating the axial myopia from the lenticular myopia. A posterior staphyloma in highly myopic eyes causes an increase in axial length. A comparison with previous axial length measurement or with that of the other eye may reveal a difference of more than 1 mm.
Artifacts
The diagnosis of nanophthalmos is made when the globe of an adult is smaller than 17 mm with thickening of retinochoroid and sclera.
Acoustic artifacts result from multiple reflections, attenuation and variations in propagation speed in tissues. One must be aware of these artifacts in order to avoid misdiagnosis.
Multiple Reflection Artifacts Tumor Height Tumor height can be obtained by measuring the distance between tumor spike and scleral spike. Follow-up measurements are performed to monitor the height of the lesion. An increase of 0.5 mm suggests tumor growth.
Multiple reflections (reverberations) may occur between the probe tip and a highly reflective surface, or between two highly reflective ocular interfaces. Calcified lens, intraocular implants (Fig. 14.26), foreign bodies, scleral buckles and air bubbles are common producers of multiple
Fig. 14.26: A-scan of intraocular lens implant producing multiple signals: L: highly reflective spike from IOL, M: multiple signals (reverberations). PMMA lens has a longer chain of reverberations than a silicone lens
A-scan Ultrasonography signals and may cause error in axial length measurements. These artifacts can be distinguished from true echoes by their position in the echograms as well as by their more pronounced movements.
Attenuation Artifacts Silicone oil disperses the ultrasound beam, and the examination is, therefore, very difficult to perform. The sound attenuation prevents resolution of posterior ocular wall and orbital contents (Fig. 14.27). The velocity of sound in silicone oil is much less than in vitreous. This causes the echograms to appear larger than normal.
Low Reflective Spike Low reflective spikes can occasionally be seen in front of the retinal spike when examination is performed at high gain. This occurs due to the lateral portion of the ultrasound beam reaches the concave retina earlier than the central portion.
Tumors A tumor mass less than 0.75 mm will be missed on A-scan. To detect the acoustic structure the thickness should at least be 2 mm. A false negative result may occur in case of a small retinoblastoma with no calcification, as it will show low reflective spikes. A diagnosis of retinoblastoma may be made if a mass shows
high reflectivity. Reflectivity may be due to some other causes such as intraocular calcification or bone formation in phthisis bulbi. Axial length measurement and clinical history should be helpful in making the correct diagnosis.
Vitreoretinal Diseases Dispersed vitreous cells or hemorrhage may be missed initially due to low reflectivity. The gain should be increased to improve resolution. It is sometimes difficult to differentiate between a thick vitreous membrane and retinal detachment as both show high reflectivity.
Intraocular Foreign Bodies A foreign body may be missed on A-scan if its surface is less than 1 mm2 or if it is embedded in the sclera. A wooden foreign body may initially be highly reflective but its reflectivity may decrease making its localization extremely difficult. Small air bubbles which enter the eye as a result of penetrating injury may mimic an intraocular foreign body, but they usually disappear within a day or two.
Errors in the Axial Length Measurement by Biometry In the measurement of axial length by biometry, a few errors can creep in. They are mentioned here along with the corrections required.
Fig. 14.27: A-scan of globe filled with silicone oil: A: spikes from anterior surface of silicone drop, B: spikes from posterior surface of silicone drop, C: markedly attenuated spikes from ocular wall and orbit
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Posterior staphyloma: Problem should be suspected if a difference of more than 0.3 mm is present between the two eyes or a difference of more than 0.2 mm is found on consecutive measurements in the same eye. In these instances, the patient’s history should be taken to find reasons for the difference, or probe for a macular lesion like posterior staphyloma.
Misalignment: Erroneous axial length measurements occur when the ultrasound beam is not aligned with the visual axis of the eye or is not perpendicular to the macula. The retinal and scleral spikes should be of high amplitude, with the retinal spike arising steeply from the baseline. No sloping of the retinal spike should be present and there should be no jags, humps or steps on its ascending edge. If the posterior or the anterior lens spikes are not of high amplitude, the sound beam is misaligned and is not in the visual axis. The posterior lens spike may be slightly shorter than anterior lens spike because of its greater curvature. However, if the posterior lens spike is more than 10% shorter than the anterior one, the sound beam is misaligned (Fig. 14.28).
Cataract: Extremely dense cataracts can be a challenge due to absorption of the sound beam as it passes through the lens. A higher gain setting may be needed to achieve high amplitude retinal and scleral spikes. In an extremely dense, calcified lens, the entire sound beam may be absorbed with no echoes at all from the posterior segment. In such case, measurements from the fellow eye must be used in calculation.
Methylcellulose or thick tear film: Longer axial length may appear due to presence of a fluid meniscus between the probe and the cornea, caused by use of ointment, methylcellulose from previous eye examination or abnormally thick tear film. If these are suspected, the eye should be washed with saline prior to biometry.
Fig. 14.28: Misalignment demonstrated by the decreased amplitude of the posterior lens spike (arrow): A: anterior lens surface, P: posterior lens surface, R: retina
Difference between the average sound velocity and the specific sound velocities: The human eye is mostly composed of aqueous and vitreous humors, both of which have an ultrasound velocity of 1532 m/s. Only the cornea and crystalline lens have different ultrasound velocities. If the eye is measured at an ultrasound velocity of 1532 m/s then the true axial length (TAL) = MAL + 0.32, where MAL is the measured axial length and 0.32 mm is the addition, which stands for the correction for underestimation due to corneal thickness and lens thickness. Refractive errors: The formula, distance = velocity X time is programed into the biometer to calculate
A-scan Ultrasonography the distance between the corneal and retinal spikes, with the average velocity in a phakic eye taken as 1550 m/s. However, the velocity of sound in various ocular media in the same eye and in same ocular media of different eyes is not the same, but the machine does not differentiate it. For example, in a myopic person who is likely to have a fluid vitreous, sound waves should be able to travel faster in the vitreous cavity than in a hyperopic person. Since the biometer is not capable of recognizing the difference in velocities it may underestimate the length of vitreous cavity in myopia. It will be the reverse in hyperopia, where axial length may get overestimated. An axial myopia of 29 mm is best measured at an average velocity of 1550 m/s while an axial hyperope of 20 mm is best measured at average velocity of 1560 m/s. The type of eye (phakic, aphakic or pseudophakic) should also be carefully fed in before biometry as the average velocities programed for them are different. The errors which creep into the estimation of axial length prevent accurate IOL power calculation. An error of 1 mm in measuring axial length affects the postoperative refraction by at least 2.5 diopters. This causes a large residual postoperative error of refraction (spherical) in eyes with high ametropia. Modification in IOL calculation formulae have been suggested (Holladay modification), but these are complex, time consuming and require additional software.
Importance of Clinical Correlation in Making a Diagnosis Before beginning the examination, the echographer should be informed of the pertinent history and clinical findings. Some clinically disparate conditions may have similar acoustic properties; for example, it is difficult to differentiate vitreous hemorrhage from endophthalmitis
on A-scan as both show low to medium reflective echo spikes. A clinical history assists in their differentiation. In case of a dense cataract, an ultrasound examination is warranted when other clinical features raise the suspicion of a posterior segment abnormality. Such indications include a rapidly developing cataract, history of trauma or a possible IOFB, heterochromia, afferent pupillary defect, posterior synechiae, acute red eye, acute rise of IOP and diabetes mellitus.
A-scan versus B-scan The B-scan covers a considerably larger area than a single A-mode pattern. By providing a twodimensional topographic documentation, the Bscan is used to define a lesion’s shape and position. However, the information obtained from amplitude, shape and motion of A-scan spikes is missing. The quantitative reflectivity of ocular lesions, the most important ultrasonic differential criterion can be evaluated only on A-scan. Thus, A- and B-scan displays provide specific acoustic information which in either of one display mode is absent or poorly differentiated. The A-scan has certain unique advantages. The probe used is smaller and can be angled to detect peripheral lesions and small vitreous opacities (which may be missed on B-scan). Because of the small probe size it is the method of choice in posttraumatic cases with open globe wounds. However, orbital lesions are better delineated by B-scan. The two modes are, therefore, complimentary to each other and an optimal echographic examination results from a combination of both modalities.
Bibliography 1. Atta HR. Ophthalmic Ultrasound: A Practical Guide. London,Churchill Livingstone, 1996.
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Diagnostic Procedures in Ophthalmology 2. Baum G, Greenwood I. The application of ultrasonics locating techniques to ophthalmology. Theoretic considerations and acoustic properties of ocular media. I: Refractive properties. Am J Ophthalmol 1958, 46:319. 3. Binkhorst RD. Biometric A-scan ultrasonography and intraocular lens power calculation. In: Emery JM (Ed). Current Concepts In Cataract Surgery: Selected Proceedings of the Fifth Biennial Cataract Surgical Congress. St. Louis, Mosby: 1987. 4. Byrne SF. Standardized echography, Part I: Ascan examination procedures. Int Ophthalmol Clin 1979;19:267-81. 5. Baum G. Ultrasonic characteristics of malignant melanoma. Arch Ophthalmol 1967;78:72-75. 6. Blumenkranz MS, Byrne SF. Standardized echography for the detection and characterization of retinal detachment. Ophthalmology 1982;89: 821-31. 7. Coleman DJ, Carlin B. A new system for visual axis measurements in the human eye using ultrasound. Arch Ophthalmol 1967;77:124-27.
8. Coleman DJ. Ophthalmic biometry using ultrasound. Int Ophthalmol Clin 1969;9:667-83. 9. Coleman DJ, Lizzi FL, Jack RL. Ultrasonography of the Eye and Orbit. Philadelphia: Lea and Febger, 1977. 10. Coleman DJ, Lizzi FL, Silverman RH. A model for acoustic characterization of intraocular tumors. Invest Ophthal Vis Sci 1985;26:545-50. 11. Dallow RL. Ultrasonography in ocular and orbital trauma. Int Ophthalmol Clin 1994;14:23-56. 12. Holladay JT. Standardizing constants for ultrasonic biometry, keratometry, and intraocular lens power calculations. J Cataract Refract Surg 1997;23(9):1356-70. 13. Nhindatz GH Jr, Hughes WB. Ultrasonics in ocular diseases. Am J Ophthalmol 1956;41: 488-89. 14. Ossoinig KC. Standardized echography: Basic principles clinical application and results. Int Ophthalmology Clin 1979;19:127-210. 15. Price PR, Jones TB, Goddard J. Basic concepts of ultrasonic tissue characteristics. Radiologic Clinics of North America 1980;18:21-30. 16. Shammas HJ. Atlas of Ophthalmic Ultrasonography and Biometry, St Louis, Mosby, 1984.
B-scan Ultrasonography
TARAPRASAD DAS, VASUMATHY VEDANTHAM, ANJALI HUSSAIN, SANGMITRA KANUNGO, LS MOHAN RAM
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B-scan Ultrasonography
Since the first application in ophthalmology by Mundt and Huges,1 ultrasonography, in little over four decades, has emerged as an indispensable tool in the diagnosis and management of various ocular and orbital abnormalities. The value of ultrasonography in the diagnosis of vitreoretinal diseases, and particularly in preoperative evaluation of the posterior segment of the eye need not be over emphasized.2 Ultrasonography is mostly indicated in hazy media when the traditional optical evaluation is not possible. It is also of immense diagnostic and therapeutic value in selected situations despite media clarity such as intraocular space occupying lesions. This chapter briefly describes the technique, and evaluation of the posterior segment eye diseases using B-scan contact ultrasonography. Care is taken to describe the ultrasonic features of commonly seen vitreoretinal diseases with representative illustrations. An acquaintance with the technique and interpretation is imperative to appreciate the technical potential of ocular ultrasound.
kilohertz (kHz). The tissue ultrasound interaction consists of reflection (and refraction), scattering, and absorption of the sound energy.
Physics and Basic Technology
Scattering
Ultrasound consists of high frequency sound waves over 20,000 cycles per second or 20
Scattering of ultrasonic energy occurs both at rough interfaces between different tissues and
Reflection and Refraction When the pulse of ultrasound energy meets a large smooth boundary between two tissues that differ in physical properties, some of the incident pulse energy may be reflected between the two media. This part of the pulse energy is redirected in a specific direction back into the original tissue with the same speed with which it approached the boundary; some energy, however, continues to be transmitted forward into the tissue beyond the boundary, with the speed of propagation determined by the medium. If the incident pulse strikes the boundary perpendicularly, the reflected energy will be maximal and the transmitted pulse will propagate forward with none or minimal change in direction. If the boundary is approached obliquely by the incident pulse, then the reflected pulse will be reduced and the transmitted pulse will be refracted.
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Diagnostic Procedures in Ophthalmology different homogeneity of density or elasticity within a tissue. It can be considered to be the redirection of the incident ultrasound energy into many directions. There is no particular direction ascribed to the reflected energy, but a continuum of directions. In true scattering no energy is lost from the pulse, but the energy is redirected.
Absorption In the ocular tissues, an ultrasound pulse loses energy due to conversion of the vibrational energy of the pulse to other energy forms such as heat. The mechanisms of absorption in media are not properly understood; different tissues exhibit different frequency dependent absorptions. Ophthalmic ultrasonography utilizes 8-10 MHz sound waves. As it travels through the eye, it is reflected by the intraocular structures, and the echoes or the signals are returned to the screen.
Ultrasound Unit An ultrasound unit is composed of four basic elements: the pulser, the receiver, and the display unit are all contained within the same chassis and connected to the transducer, located at the tip of the probe by an electrically shielded cable. The pulser produces electric pulse at a rate of 1000 pulses per second. Each pulse excites the electrodes of the piezo-electric crystal of the transducer, generating sound waves. The returning echoes are received by the transducer and transformed into electric signals. These signals are processed in the receiver and demodulator, and then displayed on the screen of the display unit. Ophthalmic ultrasonography commonly uses two modes of display—the A-scan, and B-scan. A-scan or amplitude modulation scan provides one dimensional image of vertical deflections from a base line. The A-scan provides information
regarding structure (size, distribution), reflectivity (height of spikes), sound attenuation (absorption), and vascularity. B-scan or brightness modulation scan provides two dimensional images of a series of dots and lines. B-scan provides the topographic information of shape, location, extension, mobility, and gross estimation of thickness of the tissue. While independently each mode of ultrasonography do provide a wealth of information, the combination of both A- and Bscan (Vector A-scan) is invaluable in a variety of occasions where diagnostic dilemma exists. In vector display the A-scan pattern corresponds to the vector’s direction. The vector B-scan uses a focused 10 MHz transducer in contrast to 8 MHz unfocused transducer used in standardized A-scan. Three-dimensional ultrasound tomography of the eye is a new advanced ultrasound technique and digital computer technology where ocular pathology can now be viewed in “3D”. The sonographer scans the eye using a regular B-scan ultrasound probe which is inserted into the motorized scanner assembly. This in turn rotates the probe allowing the computer to acquire over 200 B-scan images in 5 to 10 seconds. Using digital technology, the 2D images and their global positions are recorded, reconstructed and displayed in real time as part of a 3D volume. Three-dimensional ultrasound tomography provides the following benefits over the current one- and two-dimensional evaluations: 1. Improved visualization: Information is presented in a format that reflects the 3D nature of the pathology under examination. The multiple acquired scans in 3D imaging also reduce the risk of missing a small pathology that can be overlooked if the probe is not properly aimed toward it. 2. Volume measurements: The 3D examination provides volume measurement capabilities
B-scan Ultrasonography that surpass any of the volume estimation methods available with conventional 2D ultrasound techniques. Accurate volume measurements of intraocular tumors allow the physician to monitor changes over a certain period of time, i.e. growth of a small choroidal tumor, decrease in size of a disciform macular degeneration, or the response of a melanoma to radiation, laser or drug therapy. 3. Profile A-scan analysis: Using an S-shaped amplifier that allows an evaluation of the internal echo-spikes an accurate linear measurement in any chosen direction can be made. 4. Analysis of the volume-of-interest: With multidirectional slicing can show a tomographic display of intraocular pathology. 5. Surface rendering with a three-dimensional view: The surfaces and boundaries of the ocular pathology can be made under examination.
Screening Techniques It is best to begin with a maximum gain (80 decibels) setting on the B-scan, with the patient lying on his back. The eye is anesthetized with topical paracaine when the transducer can be placed on the sclera; alternately, the probe can be placed on the closed eyelid and in such a situation the eye need not be anesthetized. The probe is placed on the globe opposite the area to be examined. The marker on the probe acts as the orientation point and corresponds to the upper portion of the echogram. To evaluate the superior and inferior fundus the marker is directed towards the nose (horizontal transverse), and to evaluate the nasal and temporal fundus, the marker is directed at 12 o’clock meridian (vertical transverse). The best detail of pathology is obtained in the central portion of the echogram;
if the pathology is not located in one of the major meridians (3, 6, 9, 12 o’clock) an oblique transverse scan can be used to evaluate the pathology. In order to completely scan the eye it is prudent to first direct the probe face at the limbus, and then slowly shift to the fornix. Thus one could evaluate from the posterior pole to the periphery in each quadrant. Once the crosssectional evaluation is completed, the area of interest is scanned by longitudinal scan. Longitudinal scans allow for evaluation of a single meridian from its most posterior aspect to the far periphery. This is accomplished by directing the marker at the corneal limbus opposite the area to be examined. Axial scan provides a pleasing, generally understandable picture; however, it requires placement of the probe directly on the cornea and thus the risk of corneal abrasion increases. A-scan produces a series of deflections from the base line. The amplitude of the spike is directly related to the density of the interface, and the space between the spikes indicates the time it takes for the sound to encounter an interface and return as a signal.
Screening Technique with a 3D Unit To begin scanning with 3D-unit, the operator either presses a foot switch or may press the on-screen scan button. The scanner assembly rotates for several seconds (5, 7.5, or 15 according to the chosen scan type), and then return to its starting position. The system emits a beep tone at the start and end of actual scanning period, during which fixation must be maintained. After scanning, the images become static, the recorded initial scan plane images replace the line B-scan display. Several buttons appear on the right side of screen, which allow the operator to review the recorded images and send to the 3D reviewing mode.
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Evaluation of the Vitreous
Activating the 3D viewing function produces a screen, where central part of the screen shows the polyhedron (cube), with its sides positioned at the extreme borders of the scanned volume. The view is initially oriented as seen by the ultrasound probe, i.e. the near field is in front and the far field is behind. The 3D viewing software includes many functions.
A maximum high gain should be used for evaluation of the vitreous humor. The normal vitreous cavity is devoid of any acoustic signals and appears black or sonolucent on the B-scan. On the A-scan the baseline remains flat throughout the scan. During normal aging the vitreous begins to degenerate, and varying amounts of opacities are seen. Also there may be significant contracture of the vitreous gel leading to complete separation of the posterior hyaloid.
The Normal Eye Examination of a normal globe at high system sensitivity reveals two echographic areas, separated by an echo free area. The echographic area at the beginning of the scan represents reverberations at the tip of the probe and has no clinical significance. When the scan resolution is good, one could see the posterior convex structure of the crystalline lens. The large echo free area represents the vitreous cavity. The echogenic area after the vitreous represents the retina, choroid, sclera, and the orbital tissue behind it. The retina is seen as a concave surface proximally. The optic nerve shadow is seen as a triangular shadow within the orbital fat (Fig. 15.1).
Asteroid Hyalosis Asteroid hyalosis, a unilateral condition characterized by formation of calcium soaps within the vitreous cavity, appears as bright round signals on B-scan, and medium amplitude spikes in A-scan, with an echo free space just in front of the retina that represents the echofree vitreous gel (Fig. 15.2). This is in contrast to an eye with emulsified silicone oil, where there is no echo-free space. Generally, these opacities exhibit distinct movement on movement of the eye.
Fig. 15.1: Normal globe: Ultrasonogram shows an echolucent vitreous cavity, concave retinochoroidal layer and the triangular shadow of the optic nerve
B-scan Ultrasonography
Fig. 15.2: Asteroid hyalosis: Bright round signals seen on B-scan with echo free space separating them from the retina
Posterior Vitreous Detachment Posterior vitreous detachment (PVD) appears as an undulating membrane in front of the retinochoroidal layer that moves with movement of the eye. It may separate completely from the posterior pole or may remain attached to the optic disk. On A-scan it appears as a tall spike, but not as tall as the spike of a retinal detachment (RD) or a retained intraocular foreign body. The height of the A-scan spike and the brightness
of B-scan of PVD reduce as the gain is reduced; in contrast the RD maintains its 100% reflectivity all the time. Kinetic scanning is also useful where a PVD shows wafting after-movements (Fig. 15.3). PVD may be complete or incomplete. It is incomplete in most of the vascular retinopathies associated with vitreous hemorrhage, particularly proliferative diabetic retinopathy (PDR). One could also image vitreoschisis that usually occurs in PDR.
Fig. 15.3: Posterior vitreous detachment (PVD): B-scan shows an undulating membrane in front of the retinochoroidal layer attached to the optic disk. The configuration of the detached vitreous is changed with the movement of the eye (right)
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Diagnostic Procedures in Ophthalmology Vitreous Hemorrhage The ultrasonic pattern of vitreous hemorrhage depends on the density, location, extent, and associated fibrous changes. The density of hemorrhage is best estimated from the A-scan amplitude and the area of vitreous hemorrhage from B-scan. Hemorrhage in the vitreous appears as small white echoes on B-scan and low amplitude spikes on A-scan. With greater density of vitreous hemorrhage, usually greater opacities are seen on the B-scan. A fresh diffuse and unclotted hemorrhage produces very little or no echo response so that many time the vitreous might appear sonolucent. Membranes are easily differentiated from blood clots by their patterns, and the height of the echoes. One can also image the location of vitreous hemorrhage such as confined within the PVD, preand post-hyaloid location, or diffusely dispersed (Fig. 15.4). One can also differentiate old clotted blood from fresh hemorrhage. We have earlier reported that the overall accuracy of ultrasonic diagnosis of vitreous hemorrhage and retinal detachment in opaque media vis-a-vis the intraand postoperative findings were nearly 92%.3 Subhyaloid hemorrhage typically does not clot. On echography, high gain settings are often
required to detect mild subhyaloid hemorrhage. However, dense subhyaloid hemorrhage shows high echo reflectivity (Fig. 15.5).
Endophthalmitis Ultrasonography of the eye with endophthalmitis depends on the degree and severity of infection and the extent of vitreous involvement. Generally opacities are noted, and membrane formation becomes apparent in severe cases. Choroidal thickening, choroidal detachment, retinal detachment and retained IOFB are possible associated findings (Fig. 15.6).
Evaluation of the Retina The retina appears as a dense membrane on B-scan, and in normal circumstances one can not differentiate retina from the choroid. On A-scan it typically gives a 100% tall spike.
Retinal Detachment Retinal detachment appears as tall (100% amplitude) spike separated from the choroidoscleral layer; it is attached, however, to the optic nerve and the ora serrata. By serial scanning
Fig. 15.4: Vitreous hemorrhage: Intragel and subhyaloid in location and the posterior vitreous is partially detached
B-scan Ultrasonography
Figs 15.5A to D: B-scan of posterior hyaloid detachment. A shows a high echoreflectivity due to thickening of posterior hyaloid with medium echo reflectivity due to less dense subhyaloid hemorrhage. Corresponding A-scan, B shows initial high reflective spike with low to medium echospikes. In contrast, dense subhyaloid hemorrhage, C shows high echo reflectivity and corresponding A scan, D shows medium to high echoreflectivity
Fig. 15.6: Endophthalmitis: Ultrasonogram shows low to medium echoreflective vitreous opacities with choroidal thickening
the extent of retinal detachment can be determined. Recent retinal detachments are characterized by a mobile retina and translucent subretinal space (Fig. 15.7). With time when the proliferative vitreoretinopathy (PVR)4 sets in, the vitreous space becomes
limited, there is decreased mobility of the retina in kinetic scanning, and membranes form and adhere to the retina from all sides. This causes a variety of configurations in the B-scan and the most prominent one is the funnel configuration of the detached retina (Fig. 15.8 ). Two
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Fig. 15.7: Retinal detachment (fresh): B-scan shows detached retina as a thin, attached to the optic disc and fanning peripherally. Vector A-scan showing a tall, highly echoreflective spike signifying a retinal detachment. The subretinal space in fresh retinal detachment is usually sonolucent
Fig. 15.8: Closed funnel retinal detachment: Ultrasound shows a detached thick retina in a triangular configuration, with apposition of the sides of the triangle in front of the optic disc
configurations—open, and closed funnel are described in PVR. In triangular retinal detachment the sides of the triangle represent the highly detached stiff retina, and the base of the triangle is the proliferating vitreous membrane. An attempt was made to ultrasonically differentiate advanced grades of PVR.5 In PVR C1 the detached retinal leaves are thickened, and the subretinal space is sonolucent in contrast to PVR C2 and C3 where the subretinal space is not sonolucent. In PVR D1 and D2 the retinal leaves are thickened and shortened and subretinal space is no longer sonolucent. In PVR D3 three configurations are observed—triangular, morning glory, and T-shape.
Long-standing retinal detachments may also develop retinal cysts (Fig. 15.9) and become partially calcified, and cholesterol debris may accumulate in the subretinal space. It is important to remember that an axial B-scan view may not always demonstrate the insertion of a retinal detachment into the optic nerve. Therefore, a longitudinal approach should be used to properly assess the relationship of a membrane to the optic nerve.
Retinal Tear Large retinal tears can be visualized easily, but the smaller ones require a meticulous examina-
B-scan Ultrasonography
Fig. 15.10: Retinal tear: B-scan showing a breach of retinal tissue. Vitreous is attached to this breach of tissue suggesting the element of traction in causing retinal tear
Traction Retinal Detachment
Fig. 15.9: Longitudinal B-scan shows formation of intraretinal cysts (white arrow) and retinal detachment with high reflective surface spikes on corresponding A-scan. Often intraretinal cysts may mimic a tractional retinal detachment
tion. It appears as a breach of tissue on B-scan, and on A-scan it appears as a highly reflective tissue separate from the other fundus spikes (Fig. 15.10). Giant retinal break with detachment appears as a rolled out tissue on B-scan with clear breach of tissue. In general, however, detecting retinal tears on ultrasonography is not easy and it is never as specific or sensitive as on optical evaluation. It is useful in situations when fresh vitreous hemorrhage due to retinal tear obscures the fundus view; in these situations the retinal tears are mostly located in the upper half of the retina.
Traction detachment is a common finding in vascular retinopathies, chiefly diabetic retinopathy. It is caused by the strong adhesion of vitreous membranes, bands, or the posterior hyaloid face to the retina and subsequent traction. The vitreoretinal adhesion could be focal causing a tent-like, or broad, causing a table-top traction of the retina (Fig. 15.11); the detached retina appears to have a concave configuration, in contrast to the convex configuration of the rhegmatogenous retinal detachment.
Exudative Retinal Detachment The configuration of the detachment is convex and bullous. It is usually secondary to tumors, inflammatory conditions, e.g. Vogt-KoyanagiHarada disease (VKH), or vascular disorders such as hypertensive choroidopathy, and toxemia of pregnancy. In VKH syndrome there is a diffuse choroidal thickening with low to medium echo reflectivity (Fig. 15.12).
Retinoschisis This condition most often involves the inferotemporal peripheral fundus. It may be
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Fig. 15.11: Tractional retinal detachment: B-scan shows a concave configuration of the retina with a broad area of vitreoretinal adhesion signifying a table-top traction of the retina. The corresponding vector A-scan showing a highly reflective spike, signifying RD
Fig. 15.12: Exudative retinal detachment and choroidal thickening in VKH syndrome: B-scan shows diffuse choroidal thickening (better appreciated at the low gain of 77.0 dB), with overlying exudative RD. Corresponding vector A-scan shows a highly reflective spike signifying retinal detachment and low to medium reflective spikes behind it signifying choroidal thickening
B-scan Ultrasonography demonstrate slight vertical after movement. It differs from retinal detachment by its more focal, smooth and thin character. A choroidal detachment is thicker than retinoschisis and may have a double peaked spike.
Cysticercosis There is a characteristic echographic appearance with a sharply outlined, oval cyst within the vitreous cavity and/or in the subretinal space (Fig. 15.14). The scolex of the parasite is seen as a very highly reflective, echo-dense nodule that is located adjacent to the inner wall of the cyst.
Evaluation of the Choroid
Fig. 15.13: Retinoschisis: Transverse B-scan shows a moderately elevated thin smooth dome-shaped membrane echo (arrow) located in the inferotemporal periphery. Very thin 100% spike is also seen on A scan
unilateral or bilateral. On B-scan it appears as smooth, thin, dome-shaped membrane that does not insert in the optic disc (Fig. 15.13). On Ascan, 100% high spike is produced, which may
The retinochoroidal layer has a smooth concave configuration on B-scan and gives a tall spike on A-scan.
Choroidal Thickening Thickening of choroid can be localized or diffuse, and is seen in a number of conditions. They include posterior uveitis, sympathetic ophthalmia, Vogt-Koyanagi-Harada disease, late stage of endophthalmitis and uveal effusion syndrome.
Fig. 15.14: Subretinal cysticercosis: B-scan shows a sharply outlined cyst in the subretinal space, with a bright spot adjacent to the inner wall corresponding to the scolex. The vector A-scan through the scolex shows a tall and highly reflective spike
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Fig. 15.15: Choroidal detachment: B-scan shows smooth, dome-shaped, thick membranous structure. The corresponding vector A-scan, shows a series of medium to high reflective spikes behind the retinal spike with a sonolucent suprachoroidal space
Choroidal Detachment On B-scan a choroidal detachment appears as a smooth, dome-shaped, thick membranous structure that does not insert to the optic nerve (Fig. 15.15). The choroidal detachment can be localized, or involve the entire fundus (kissing choroidal detachment). The B-scan also can demonstrate the nature of suprachoroidal fluid; in serous detachment, the suprachoroidal space is echo-lucent, and in hemorrhagic detachment, the suprachoroidal space is echo-dense. On A-scan the thickened choroid appears as a series of high reflective spikes just behind the retinal spike. The detached choroid produces a 100% reflective, double peaked spike (retina and choroid together). This spike exhibits little or no after movement on kinetic scanning. The suprachoroidal space appears sonolucent or with low to medium height spikes depending on the nature of suprachoroidal fluid.
patient cooperation. Ultrasonography permits evaluation of the intraocular structures, locating a retained intraocular foreign body, and identifying any posterior wall disruption.
Vitreous Hemorrhage The ultrasonic character of vitreous hemorrhage is not different than vitreous hemorrhage in nontraumatic conditions. However, a large retinal dialysis can be easily detected. Occasionally the trail of hemorrhage in the solid vitreous can be traced to the site of bleeding such as avulsion of major vessel or scleral rupture (Fig. 15.16).
Evaluation of Traumatized Eye Ultrasonography adequately supplements the careful and meticulous evaluation a traumatized eye needs.6 Very often indirect ophthalmoscopy is not useful because of media opacity, or poor
Fig. 15.16: Traumatic vitreous hemorrhage: Intravitreous gel trail of traumatic vitreous hemorrhage. B-scan shows linearly placed bright spots in the vitreous cavity, leading to site of retinal vessel avulsion causing vitreous hemorrhage
B-scan Ultrasonography strands of vitreous might be attached to the dislocated lens (Fig. 15.17).
Intraocular Foreign Body
Fig. 15.17: Dislocated lens: B-scan shows a globular structure in the posterior vitreous signifying a dislocated lens. Acoustic shadowing is seen, implying that the lens could be cataractous or calcified
Dislocated Lens Dislocated lens appears as a round or oval globular structure in the posterior vitreous, and
Ultrasonography can detect both metallic and non-metallic foreign bodies. Metallic foreign bodies produce very bright signals on B-scan that persist on lowering the gain (Fig. 15.18). When the sound beam is focused on the metallic foreign body, much of the sound waves are absorbed by the foreign body, thus creating a shadowing artifact on the adjacent orbit. Round metallic foreign bodies classically produce reverberation artifact just behind the foreign body, and the sound signals gradually reduce as it progresses to the orbit. On A-scan metallic foreign bodies produce high (100%) reflective echoes, and reduplication echoes are seen as progressively decreasing amplitude spikes behind the
Fig. 15.18: Intraocular foreign body: B-scan shows a bright signal in front of the optic disk in the posterior vitreous, with a high 100% reflectivity on vector A-scan, that persists on lowering of the gain. Orbital shadowing is also seen at low gain
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Fig. 15.19: Posterior globe rupture: B-scan shows breach of scleral tissue with echolucent space in the subTenon’s space signifying fluid
round metallic foreign body. Glass and vegetative matter (radiolucent) are more challenging, but they also produce bright signals on B-scan, and tall reflective echo on A-scan.
Posterior Globe Rupture Traumatic posterior globe rupture (Fig. 15.19) is seen as a breach of scleral and choroidal tissue with associated choroidal thickening. Associated findings may be vitreous hemorrhage, retained intraocular or orbital foreign body, and retinal detachment.
Optic Nerve Avulsion
Fig. 15.20: Optic nerve avulsion: B-scan shows a scleral break near the optic disk signifying optic nerve avulsion
This is seen secondary to trauma. In acute injury, vitreous hemorrhage may be present, and an actual peripapillary scleral break may be seen in B-scan (Fig. 15.20). In long-standing cases, there may be proliferative tissue at the optic disk.
features such as shape, location, and extension. A-scan provides information on structure, reflectivity, vascularity, and height. Serial ultrasonography is useful in measuring the height and growth of the tumor over a period of time.
Evaluation of Intraocular Tumors
Melanoma
The intraocular tumors display different acoustic characteristics on ultrasonography because of their vast difference in histologic composition. B-scan provides information on topographic
Ultrasonically melanomas appear as solid, regularly structured, vascular lesions of low to medium reflectivity. Vascularity of the tumor is well appreciated as distinct spontaneous
B-scan Ultrasonography irregular contour, with a central elevation. They have medium to high reflectivity, with minimal to none internal vascularity. The large interface between the choroidal tissue and the carcinoma mass is responsible for the high reflectivity. On A-scan, irregular spikes of medium to high amplitudes are seen.
Choroidal Hemangioma
Fig. 15.21: Choroidal melanoma: B-scan shows a collarbutton-shaped mass from the choroid into the vitreous cavity
movements of the lesion spikes during examination in A-scan. While the most common shapes are a dome or collar-button, they can also be diffuse. A collar-button shape signifies rupture of Bruch’s membrane, and it is usually associated with retinal detachment (Fig. 15.21). There can be other signs such as acoustic hollowing (decreased reflectivity at the tumor base due to uniform echotexture of the tumor), choroidal excavation at the tumor base and posterior scleral bowing (noted in younger individuals).
Metastatic Choroidal Carcinoma On B-scan metastatic choroidal carcinomas appear diffuse; they have a typical bumpy,
These tumors appear as a flat, echogenic, solid, subretinal mass, often located at the posterior pole, with minimal sound attenuation, with or without concomitant exudative retinal detachment (Fig. 15.22). On A-scan, it has a regular acoustic structure with very high (95-100%) internal reflectivity, that results from the large interfaces formed by the vessel surfaces. By reducing the gain, the vascularity of the tumor can be better appreciated.
Retinoblastoma On B-scan retinoblastoma, if large, is seen as an irregular echogenic mass involving the vitreous, retina, and/or the subretinal space. Area of calcification is seen as area of high echogenicity. This causes strong sound attenuation, and is seen as an area of echolucency behind the calcification (Fig. 15.23). This is because the
Fig. 15.22: Choroidal hemangioma: Left—On B-scan, a flat echogenic solid subretinal mass is seen with concomitant exudative retinal detachment of 4.16 mm thickness. Right—A decrease in thickness is seen after photocoagulation
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Fig. 15.23: Retinoblastoma: B-scan shows an irregular, large echogenic mass involving the vitreous from the retina. Corresponding vector A-scan shows high internal reflectivity (70 to 90%), due to spots of calcification
sound is almost totally reflected by calcification, thus preventing its further propagation beyond. On A-scan, the characteristic features are solid consistency (absence of after movements following a sudden ocular movement), high internal reflectivity (70-90%), and presence of vascularity. High internal reflectivity is due to calcification and the large interface between area of necrosis and viable tumor cells.7 The axial length of the eye may be normal or increased in case the tumor invades the ocular wall. The increased axial length is thus an important point in differentiating retinoblastoma from other conditions causing leukocoria.
Disciform Macular Scar (Secondary to Age-related Macular Degeneration) Disciform macular scar is often confused with choroidal melanoma due to its subretinal
location, and solid consistency, it can be differentiated by its irregular acoustic structure, medium to high reflectivity, absence of vascularity, and rarity of associated retinal detachment (Fig. 15.24).
Structural Anomalies Structural anomalies of globe include phthisis bulbi, atrophic bulbi, posterior staphyloma, choroidal coloboma, optic nerve head drusen and anophthalmos.
Phthisis Bulbi In phthisis bulbi the globe is smaller than normal with multiple echogenic vitreous opacities, choroidal thickening, and calcification of ocular coats, with resultant absence of high reflective orbital echospikes due to shadowing (Fig. 15.25).
B-scan Ultrasonography
Fig. 15.24: Disciform macular scar: B-scan shows a solid subretinal lesion. In contrast to a melanoma, it has an irregular acoustic structure, and medium to high reflectivity in the corresponding vector A-scan
Fig. 15.25: Phthisis bulbi: B-scan shows a smaller than normal globe, with multiple echogenic vitreous opacities and calcification of ocular coats. The corresponding vector A-scan shows the resultant orbital shadowing
Atrophic Bulbi
Choroidal Coloboma
Atrophic bulbi is characterized by a normal globe contour with calcification of ocular coats (Fig. 15.26). It has normal axial length.
Choroidal coloboma is seen as an excavation, usually involving the posterior pole; but in contrast to posterior staphyloma, its edges are sharp. Associated findings include microphthalmos, and retinal detachment.
Posterior Staphyloma Posterior staphyloma is seen as a shallow excavation of the posterior pole with smooth edges on sonographic evaluation of highly myopic eyes.
Optic Nerve Drusen Optic nerve drusen are calcified nodules seen echographically to produce an echo of extremely
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Fig. 15.26: Atrophic bulbi: Ultrasonogram shows a normal globe contour with calcification of the ocular coats
Fig. 15.27: Optic nerve head drusen: B-scan showing bright echogenic spot over the optic disk. Corresponding vector A-scan showing a highly reflective spike that persists on lowering the gain
B-scan Ultrasonography high reflectivity at or within the optic nerve head. They are best seen with transverse and longitudinal B-scan approaches, which bypass the lens, and demonstrate the calcified nodules better than the axial approach (Fig. 15.27).
Optic Nerve Head Coloboma Coloboma involving the optic disk is easily imaged by B-scan. These can be small and shallow. The sharp edge of the defect margin differentiates a coloboma from a staphyloma on ultrasonography (Fig. 15.28).
Immersion B-scan Immersion B-scan is used to study the anterior segment structures (Fig. 15.29). A water bath is used to incorporate the delay zone. Ophthalmic ultrasonography is an invaluable tool in diagnosis and evaluation of the posterior segment of the eye. Knowledge of various features and appropriate clinical correlation is essential to gain maximum information from this technology.
Fig. 15.28: Optic nerve head coloboma: Horizontal Bscan showing sharp defect over the optic disk area suggestive of coloboma of the optic disk
Fig.15.29: Left: Immersion Bscan shows a total cataract with intact posterior capsule. Right: Immersion B-scan showing partially absorbed cataractous lens. Note the thickness of the lens and increased reflectivity of the posterior capsule
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References 1. Mundt GH, Huges WF. Ultrasonic in ocular diagnosis. Am J Ophthalmol 1956;41:488-98. 2. Das T, Namperumalsamy P: Ocular ultrasound in preoperative evaluation of posterior segment of the eye. Indian J Ophthalmol 1983;31:1022-24. 3. Das T, Namperumalsamy P. Ultrasonographic characterisation of vitreous hemorrhage and retinal detachment. Afro-Asian J Ophthalmol 1985;4:10-16. 4. The Retina Society Terminology Committee. The classification of retinal detachment with proliferative vitreoretinopathy. Ophthalmology 1983;90:121-25. 5. Das T, Namperumalsamy P. Ultrasonic characterisation of proliferative vitreoretinopathy. Afro-Asian J Ophthalmol 1987;5:180-85.
6. Das T, Namperumalsamy P. Ultrasonography in ocular trauma. Indian J Ophthalmol 1987;35: 121-25. 7. Das T, Namperumalsamy P. Ultrasonic evaluation of retinoblastoma. Afro-Asian J Ophthalmol 1986;5:4-10.
Bibliography 1. Coleman DJ, Lizzi FL, Jack RL (Eds). Ultrasonography of the eye and orbit. Philadelphia, Lea and Febiger, 1977. 2. Shammas HJ. Atlas of Ophthalmic Ultrasonography and Biometry. St Louis: CV Mosby Co, 1984.
Ultrasound Biomicroscopy in Ophthalmology
ROSHMI GUPTA, K KALYANI PRASAD, MOHAN RAM, SANTOSH G HONAVAR
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Ultrasound Biomicroscopy in Ophthalmology
Ultrasound biomicroscopy (UBM) uses high frequency sound waves to provide noninvasive in vivo imaging of the anterior segment with microscopic resolution. It acts on a principle similar to that of the B-scan; sound waves in the ultrasonic range are reflected off the structure of interest, and the reflected waves form images. However, frequency of the waves used in the UBM range between 35 and 50 MHz, while the ophthalmic B-scan probes generate sound waves of 10 MHz frequency. The B-scan, with a lower frequency, has better penetration, and can image structures of the posterior segment well. Anterior segment structures can be visualized only by the immersion technique, the resolution being poor compared to the UBM. The UBM, with higher frequency sound waves, can penetrate only about 5 mm into the eye; however, it can form images of the anterior segment with a much better resolution than the B-scan. UBM can be used to image and assess the morphology of structures easily seen on conventional examination (with slit-lamp) such as cornea, iris and sclera, as well as structures hidden from clinical observation, including the ciliary body and zonule. The normal anatomical relations and pathophysiologic changes in the anterior segment structures can be examined both
qualitatively and quantitatively with the help of UBM.
Basic Physics and Instrumentation Signal processing for ultrasound biomicroscope is similar to that in conventional B-mode ultrasound. A monocycle high voltage (200V peak to peak) 40 to 100 MHz pulse is used to excite the transducer. The resulting 40-100 MHz ultrasound pulse is transmitted into the tissue while the transducer is moved linearly over the imaging field (typically 4-8 mm). The commercially available machines use 50 or 35 MHz transducers. The back-scattered ultrasound is detected by the same transducer, the data being collected at each of 512 equally spaced lines. The radio-frequency signal is received and amplified in proportion to the depth from which it originated using time-gain compensation (TGC). That is the signals from deeper structures are amplified more than those from more superficial structures, thus compensating for the attenuation of the ultrasound beam in the tissue. After the radio-frequency signals are processed non-linearly to enhance the low level signals, its envelope is detected to produce an A-scan
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Fig.16.1: Schematic diagram of the ultrasound biomicroscope. TGC, time gain compensation
signal. This signal is displayed as B-scan data on a video monitor as real time images, the whole process being controlled and synchronized by a computer. B-mode imaging is currently performed at 8 frames/second (Fig. 16.1). By increasing the frequency in an ultrasound biomicroscope, microscopic resolution is attained over a limited depth. The units operating at 50 MHz provide a lateral and an axial resolution of 50 μm and 25 μm, respectively. In contrast, the axial resolution of a typical 10 MHz system is 190 μm. Tissue penetration of the UBM is approximately 4-5 mm.
the ultrasound beam strikes the targeted surface perpendicularly. In the UBM manufactured by Paradigm Instruments, the probe is suspended from a gantry arm to minimize motion artifacts, and lateral distortion is minimized by a linear scan format. In the OTI instrument, the probe is small eliminating the need for a suspension system, and a sector scanning method is used (Fig. 16.2).
Procedure Scanning is performed with the patient in supine position. A flared plastic eyecup of the appropriate size is inserted between the lids, holding methylcellulose or normal saline, which acts as a coupling medium. The reflected signal is best detected when the transducer is oriented so that
Fig.16.2: Ultrasound biomicroscope
Ultrasound Biomicroscopy in Ophthalmology The entire ciliary body can be defined from the ciliary processes to the pars plana. The zonule is imaged as a medium reflective line extending from the ciliary process to the lens surface.
Quantitative Ultrasound Biomicroscopy Fig.16.3: UBM photograph showing normal ocular structures, cornea, corneoscleral junction, sclera, anterior chamber angle, iris, ciliary body and anterior surface of lens
Ultrasound Biomicroscopic Anatomy of the Normal Eye and Adnexa The appearance of ocular anatomy on ultrasound biomicroscopy is similar to a low power microscopic section (Fig. 16.3). The superficial cornea appears as two parallel highly reflective lines; the first indicating the epithelial surface and the second the Bowman’s membrane. The corneal stroma shows a lower internal reflectivity than the sclera. The posterior corneal surface is depicted by a high reflective line corresponding to the endothelium and the Descemet’s membrane. The anterior chamber can be outlined and its depth to the lens or iris at any point can be determined. The angle structures are well outlined by ultrasound biomicroscopy. The corneoscleral junction can be defined well. The sclera has a high internal reflectivity due to the irregular arrangement of the collagen bundles. The high reflectivity of the sclera differentiates it from the less reflective episcleral tissue, ciliary body and peripheral choroid. The scleral spur (thickest region) is located where the trabecular meshwork meets the interface line between the sclera and ciliary body. It is utilized as a landmark for interpreting UBM images of the anterior chamber angle and analyzing angle pathology.
The ultrasound biomicroscopy provides precise and reliable measurements and relationships of the anterior segment structures. The UBM measurement software measures distance by counting the number of pixels along the measured line, and multiply it by the theoretical size of the pixel. The theoretical precision of measurement of the lateral and axial distances in the commercially available machines is 6 and 12 μm, respectively, while the resolutions of the two are 50 and 25 μm. The UBM cannot distinguish between two points along an axial line which are less than 25 μm apart, but if the points are more than 25 μm apart, the distance between them can be measured with 6 μm precision. The axial and lateral measurements of the UBM are accurate and reliable, as compared to histologic sections and ultrasound pachymetry. The reproducibility is good in intra-observer UBM measurements, but not in the inter-observer values. Both image acquisition differences and measurement process contribute to the variability. A semiautomated software that calculates the parameters after a single user input of a reference location has improved the reproducibility. Multiple parameters have been proposed to facilitate the quantitative study of UBM measurements (Table 16.1).
Clinical Applications of UBM Keratoplasty The UBM may be useful in imaging underlying structures and defining the state of the angle
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Abbreviation
Description
Angle opening distance
AOD
Distance between the trabecular meshwork and the iris at 500 μm anterior to the scleral spur
Trabecular-iris angle
TIA θ 1
Angle of AC angle recess
Trabecular-ciliary process distance
TCPD
Distance between the trabecular meshwork and the ciliary process at 500 μm anterior to the scleral spur
Iris thickness
ID1
Iris thickness at 500 μm anterior to the scleral spur
Iris thickness
ID2
Iris thickness at 2 mm from the iris root
Iris thickness
ID3
Maximum iris thickness near the pupillary edge
Iris-ciliary process distance
ICPD
Distance between the iris and the ciliary process along the line of TCPD
Iris-zonule distance
IZD
Distance between the iris and the zonule along the line of TCPD
Iris-lens contact distance
ILCD
Contact distance between the iris and the lens
Iris-lens angle
ILA θ 2
Angle between the iris and the lens near the pupillary edge
of the anterior chamber prior to keratoplasty (Fig. 16. 4). In corneal transplant cases, the graft-host junction can be defined. Posterior wound gape and the Descemet’s stripping can also be imaged.
Fig.16.4: UBM of an eye with adherent leukoma: visualization of anterior chamber and other structure helps planning of management
Limbal Dermoid Limbal dermoid can be well-delineated with the help of UBM. The extent of the dermoid into the cornea or intraocularly may be demonstrated, and the surgical approach for removal can be planned (Fig. 16.5). The UBM is capable of
Fig.16.5: UBM of a limbal dermoid showing extension into the layers of the cornea, but no intraocular extension
Ultrasound Biomicroscopy in Ophthalmology detecting cystic as well as solid lesions of the conjunctiva. The margins of the lesion and their intraocular extension can be defined. However, it is not yet possible to differentiate the different types of solid and cystic tumors of the conjunctiva based on the UBM image alone.
Refractive Surgery Excimer laser keratoablation results in a loss of Bowman’s membrane and double lines of the normal corneal surface are converted into a single line.
crystalline lens, and the changes in the anatomic relationships pre- and post-implantation of different types of lenses.
Glaucoma The ability of ultrasound biomicroscopy, to image various angle structures and the ciliary body, has helped to define mechanisms in various types of glaucoma (Fig. 16.7).
Intraocular Lenses The location of optic and haptic of an intraocular lens can be assessed accurately by looking for a strong echo at their interface plane (Fig. 16.6). The technique is used to study different types of intraocular lenses including accommodating intraocular lenses. Studies have been conducted on angle-fixated, iris-fixated (Artisan) and posterior chamber phakic intraocular lenses using the UBM. It is possible to assess distance of phakic intraocular lenses from the corneal endothelium, iris, and the surface of the
Fig.16.7: Iris bombé in uveitic glaucoma: Accumulation of aqueous behind the iris balloons the iris forward
Relative Pupil Block Glaucoma In primary angle-closure glaucoma (PACG), the angle closure results from relative pupillary block. The iris is always convex, with variable degrees of angle-closure (Figs 16.8A to C). It is possible to quantify the area of iris-lens touch present in this condition. The iris-lens touch is generally smaller than that seen in normal patients.
Plateau Iris Syndrome
Fig.16.6: UBM showing pupillary capture of intraocular lens optic
Peripheral angle-closure persists in plateau iris syndrome even in the presence of patent iridectomy. It has been shown by the ultrasound biomicroscopy that the peripheral iris is supported by an anterior positioning of the ciliary
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Fig.16.8A: UBM of an eye with a narrow-angle glaucoma
Fig.16.8B: UBM of the same eye after laser iridotomy, demonstrating opening of the angle
Fig.16.9A: UBM of an eye with plateau iris configuration showing narrow-angle due to anterior location of ciliary processes
Fig. 16.9B: UBM showing plateau configuration of iris with a closed-angle
Fig. 16.9C: UBM of the eye (shown in Fig. 16.9B) showing open-angle after laser iridoplasty Fig.16.8C: Eye with angle-closure, UBM showing peripheral anterior synechiae, iridotomy is unlikely to be effective in opening the angle
processes. The ciliary processes provide structural support behind the peripheral iris that prevents it from falling away from the trabecular meshwork following iridectomy. Peripheral iridoplasty can produce thinning of the iris in this region improving angle opening (Figs 16.9A to C).
Supraciliary effusion can produce angleclosure by anterior rotation of the ciliary processes producing direct angle-closure and pupil block secondary to the anterior position of the lens. Supraciliary fluid that is undetectable by other means can be detected by the UBM.
Ciliary Block Glaucoma Ciliary block glaucoma has been studied by the ultrasound biomicroscopy. Swelling or anterior
Ultrasound Biomicroscopy in Ophthalmology rotation of the ciliary body with forward movement of the lens-iris diaphragm and relaxation of the zonular apparatus cause direct angle-closure by morphologically pushing the iris against the trabecular meshwork. UBM often reveals a shallow supraciliary detachment not evident on routine B-scan examination.
Pigment Dispersion and Pigmentary Glaucoma Pigment dispersion occurs most likely due to mechanical contact between the posterior iris and zonular pockets. The UBM demonstrates a wide open-angle in pigmentary glaucoma. The iris configuration is typically concave, with a variable amount of irido-zonular contact (Fig. 16.10). The iris which is in apposition to the anterior lens capsule acts as a flap valve that does not permit the flow of aqueous from the anterior chamber to the posterior chamber, thereby increasing the pressure in the anterior chamber compared with the posterior chamber, a condition termed reverse pupillary block. It has been shown both by clinical observation and ultrasound biomicroscopic studies that miotics
and iridectomy can produce a straightening of a bowed iris in this condition. It has also been demonstrated by the ultrasound biomicroscopy that accommodation can produce iris bowing in pigmentary dispersion. With accommodation, the anterior surface of the lens moves forward (which increases anterior chamber aqueous pressure) resulting in the pressure reversal that produces posterior iris movement.
Failure of Filtering Surgery Since it is possible to image the internal ostium with ultrasound biomicroscopy it can be helpful in determining the causes of filtering surgery failure. The filtering bleb shows a spongy appearance on the ultrasound biomicroscopy with an occasional clear fluid space. The UBM can accurately localize the site of obstruction to aqueous flow (Figs 16.11A and B). Sites of potential blockage include internal ostium (within the eye), beneath the scleral flap (within the surgical drainage tunnel), at the episclera
Fig. 16.11A: UBM of eye after trabeculectomy showing a filtering bleb (white arrow)
Fig.16.10: UBM showing irido-zonular contact due to posterior bowing of the iris in an eye with pigmentary glaucoma
Fig. 16.11B: UBM showing scarred bleb (black arrow) in failed trabeculectomy
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Ocular Trauma Ocular trauma may result in hyphema, cyclodialysis, angle-recession and iridodialysis. UBM can be used in the detection of these conditions, especially in the presence of hazy media. It can be used to locate a foreign body in the angle or the iris (Fig. 16.12). Persistent hypotony after ocular trauma may be due to cyclodialyisis (Fig. 16.13), or cyclitic membrane (Fig. 16.14). The diagnosis may be elucidated by the UBM.
Tumors of Uvea Tumors of the iris, ciliary body and peripheral choroid lie within the penetration limit of the
Fig.16.14: UBM demonstrating a cyclitic membrane in an eye with persistent hypotony
ultrasound biomicroscope. This imaging method is valuable in measuring tumor thickness, defining tumor extent and differential diagnosis.
Iris Nevi Iris nevi are benign tumors which do not require any intervention. UBM is useful in measuring the thickness and extent of nevus.
Leukemic Infiltration of Iris Fig.16.12: UBM showing a high-reflective foreign body in ciliary body
UBM is valuable in measuring the stromal thickness in leukemic infiltration (Fig. 16.15A). It can also be used to assess the effect of radiotherapy (Fig. 16.15B), and follow-up.
Iris Melanomas
Fig.16.13: UBM showing cyclodialysis cleft in an eye after blunt trauma (white arrow)
Iris melanomas have varied clinical presentations, and differentiation between melanomas and nevi can be difficult, requiring serial observations. UBM is useful in defining the tumor boundaries and also detecting a change in its characteristics.
Ultrasound Biomicroscopy in Ophthalmology
Fig.16.16: UBM showing ciliary body cyst
Fig.16.15A: UBM showing leukemic infiltration of iris in a patient of leukemia in remission
Fig.16.15B: UBM of the same eye after external beam radiotherapy
Fig.16.17: UBM of ciliary body tumor extending through the angle into the anterior chamber and iris
Iris Cyst The iridociliary junction is a common location for iris cysts. UBM is useful in differentiating cysts from solid masses. The ultrasound appearance consists of a thin-walled cyst with no internal reflectivity (Fig. 16.16).
Ciliary Body Tumor The UBM is very helpful in differentiating purely iris tumors from ciliary body tumors. Ciliary body tumors can be defined and identified at a stage when they cannot be detected by conventional ultrasound (Fig. 16.17).
Peripheral Choroidal Tumors The UBM cannot define the full extent of the
peripheral choroidal tumors. The anterior borders can be frequently detected and this information can be helpful if radioactive plaque therapy is contemplated.
Scleral Diseases UBM can differentiate between the diseases of sclera proper and diseases of episclera.
Nodular Scleritis The involvement of the sclera can be detected by a change in the reflectivity of the scleral tissue. The edematous scleral tissue becomes weakly reflective.
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Scleral Staphyloma The UBM can detect the thinning that occurs in a scleral staphyloma and also the changes in the underlying ciliary body.
Episcleritis Episcleritis appears as a thickening of the episcleral layer without involvement of the sclera itself.
Conclusion The strength of UBM lies in its ability to produce cross-sections of the living eye at microscopic resolution without affecting the relationships of the structures imaged. It is a tool for qualitative and quantitative assessment of the anterior segment. It has already contributed considerably to our understanding of ocular pathophysiology. The scope of applications of UBM is increasing in the diagnosis and management of various eye diseases.
Bibliography 1. Buchwald HJ, Muller A, Spraul CW, Lang GK. Ultrasound biomicroscopy of conjunctival lesions. Klin Monatsbl Augenheilkd 2003;220(1-2): 29-34. 2. Hoops JP, Ludwig K, Boergen KP, Kampik A. Preoperative evaluation of limbal dermoids using high-resolution biomicroscopy. Graefes Arch Clin Exp Ophthalmol 2001;239(6):459-61. 3. Iishikawa H, Schuman JS. Anterior segment imaging: ultrasound biomicroscopy. Ophthal-
mology Clinics of North America 2004;17:7-20. 4. Jimenez-Alfaro I, Benitez del Castillo JM, GarciaFeijoo J, Gil de Bernabe JG, Serrano de La Iglesia JM: Safety of posterior chamber phakic intraocular lenses for the correction of high myopia: anterior segment changes after posterior chamber phakic intraocular lens implantation. Ophthalmology 2001;108(1):90-9. 5. Jimenez-Alfaro I, Garcia-Feijoo J, Perez-Santonja JJ, Cuina R. Ultrasound biomicroscopy of ZSAL4 anterior chamber phakic intraocular lens for high myopia. J Cataract Refract Surg 2001;27(10): 1567-73. 6. Kawana K, Okamoto F, Nose H, Oshika T. Ultrasound biomicroscopic findings of ciliary body malignant melanoma. Jpn J Ophthalmol 2004;48(4):412-4. 7. Kunimatsu S, Araie M, Ohara K, Hamada C. Ultrasound biomicroscopy of ciliary body cysts. Am J Ophthalmol 1999;127(1):48-55. 9. Liebmenn JM, Ritch R, Esaki K. Ultrasound biomicroscopy. Ophthalmology Clinics of North America 1998;11:421-33. 8. Lanzl IM, Augsburger JJ, Hertle RW, Rapuano C, Correa-Melling Z, Santa Cruz C. The role of ultrasound biomicroscopy in surgical planning for limbal dermoids. Cornea 1998;17(6):604-6. 10. Lin HC, Shen SC, Huang SF, Tsai RJ. Ultrasound biomicroscopy in pigmented conjunctival cystic nevi. Cornea 2004;23(1):97-9. 11. Marchini G, Pedrotti E, Sartori P, Tosi R. Ultrasound biomicroscopic changes during accommodation in eyes with accommodating intraocular lenses: pilot study and hypothesis for the mechanism of accommodation. J Cataract Refract Surg 2004;30(12):2476-82. 12. Pavlin CJ, Foster FS. High frequency ultrasound biomicroscopy, imaging the eye at microscopic resolution. Ophthalmology Clinics of North America 1994;7:509-22. 13. Pavlin CJ, Foster FS. Ultrasound Biomicroscopy of the Eye. New York: Springer-Verlag, 1995. 14. Pavlin CJ, Foster FS. Ultrasound biomicroscopy in glaucoma. In Ritch R, Shields MB, Krupin T (Eds). The Glaucomas, Basic Sciences 2nd ed. St Louis: Mosby, 1996;1:471-90. 15. Pop M, Payette Y, Mansour M. Ultrasound biomicroscopy of the Artisan phakic intraocular lens in hyperopic eyes. J Cataract Refract Surg 2002;28(10):1799-803.
Optical Coherence Tomography
TOMOHIRO OTANI
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Optical Coherence Tomography
Optical coherence tomography (OCT) is a diagnostic technology, which provides a crosssectional image of the anterior eye and the retina in vivo with a high resolution similar to a histological section by light microscopy.1-3 OCT has demonstrated the intraretinal structure of fundus diseases including macular hole, 4 macular edema,5, 6 highly myopic eye.7 It also enables us to evaluate the surgical outcome of macular diseases on the histopathologic level. A third-generation OCT (OCT3), with less than 10-μm axial resolution, provides more detailed imaging of the retinal structures than the former one. In this chapter, the principle of OCT, procedures, limitations and the cross-sectional images of various macular diseases using OCT conducted in our institution are being described.
Instruments and Principle of OCT3 System The OCT3 system hardware consists of the patient module; the computer unit; the flat screen video monitor; the keyboard, mouse and color inkjet printer (Fig. 17.1). OCT uses low-coherence interferometry to produce cross-sectional images of optical
Fig. 17.1: OCT3 system
scattering from intraretinal microstructures (Fig. 17.2). These images are similar to those provided by B-mode ultrasound. Low coherence light from a super luminescent diode source connects with a Michelson interferometer. Infrared light from the source is divided at an optical beam-splitter into reference beam and measurement beam. The measurement beam is directed onto the patient’s eye and is reflected from intraocular structures at different distances. The reflected light (reflected measurement beam) is composed of multiple echoes which include information about the range or distance and thickness of different intraocular structures. The reference beam is
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Difficulties and Limitations The use of OCT is limited by intraocular media opacities such as vitreous hemorrhage, dense cataract and corneal edema, which attenuate measurement beam and reflected light.
Pattern of OCT in Macular Diseases Normal Macula
Fig. 17.2: The optical interferometer (Courtesy from: Schuman JS, Puliafito CA, Fujimoto JG. Principle of Optical Coherence Tomography. In Optical Coherence Tomography of Ocular Diseases (2nd edn). New Jersey, SLACK Incorporated, 2004)
reflected from a reference mirror. The reflected reference beam returns to the beam-splitter where it combines with the reflected measurement beam. Time delay information between the two light pathways is then determined by a photo diode, which detects back-scattered light along a reference optical delay path. Once the light is detected, a signal is sent which is processed electronically and used within the OCT internal computer data acquisition bank for analysis and storage. Measuring the interferometric signal creates A-mode type scans. Cross sectional images are constructed from a sequence of single longitudinal A-mode type scan. There is no contact between the OCT scanner and the eye. Slit-lamp biomicroscopy of the retina may be performed simultaneously with the image acquisition. The obtained OCT images are displayed in a false color representation. The intensity of the reflected optical signal is represented on a logarithmic scale with varying degrees of brightness. The maximum optical reflection and back-scattering are represented by red-white colors, while the minimum signals are represented by blue-black colors.
Cross-sectional images of the normal macula showed a physiological foveal depression with an intraretinal layered structure (Fig. 17.3). A high reflectivity was obtained from the retinal nerve fiber layer, plexiform layers and the retinal pigment epithelium (RPE). The boundary between the photoreceptor inner segments and outer segments (Fig. 17.3, bottom, arrows) was also seen as a highly reflective band on OCT3.8, 9 The ganglion cell and nuclear layers produced low reflectivity. The thickness of the fovea (center of the macula) averaged 144 μm and was independent of either age or state of refraction.10
Macular Hole Kishi and Takahashi evaluated the threedimensional structure of idiopathic macular hole (Fig. 17.4) in 89 affected eyes using OCT and scanning laser ophthalmoscopy (SLO).4 In stage 1 hole, OCT revealed retinal split or cystic changes at the fovea in 11 of 15 eyes (73%) and foveal retinal detachment in 4 eyes (27%). Intraretinal splitting involving the perifoveal area was present in 16 eyes with stage 2 hole. A break was present in the anterior cyst wall. The outer retina could not be identified at the fovea by OCT. A full thickness hole surrounded by intraretinal split or cystoid edema was present in all of 50 eyes with stage 3 hole. Opercula were seen in 32 of the 50 eyes. A detached vitreous
Optical Coherence Tomography
Fig. 17.3: Normal macula. Fundus photograph (top) and OCT3 (bottom). OCT3 shows a physiologic foveal depression with an intraretinal layered structure. The boundary between the photoreceptor inner segments and outer segments is also seen as a highly reflective band (arrows).
cortex could be observed in 24 of the 32 eyes. Intraretinal split seen by OCT appeared as radiating striae of elevated Henle’s fiber layer by SLO. The findings show that idiopathic
macular hole initiates as intraretinal split or cysts at the fovea and that a full-thickness macular hole forms when the anterior cyst wall is operculated.
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A
B
C Figs 17.4A to C: Macular hole. A Stage 1 macular hole. OCT3 demonstrates a foveal cyst. B Stage 2 macular hole. OCT3 shows a flap consists of retinal tissue extending from the perifoveal retina. The perifoveal retina has cystic changes. C Stage 3 macular hole. The perifoveal retina is elevated and has cystic changes. An operculum is seen above the hole
Preretinal Macular Fibrosis
High Myopic Eyes
Maruyama and coworkers examined 19 eyes with preretinal macular fibrosis (Figs 17.5 and 17.6) using OCT.11 The foveal thickness ranged from 300-650 μm. The macula was swollen in 15 eyes lacking the physiological foveal depression. In 4 eyes with pseudomacular hole, the foveal structure showed sharp columnar depression surrounded by thickened perifoveal retina. Swelling of the retina was more pronounced in the outer retinal layers showing a low reflective zone. The findings show that preretinal macular fibrosis is not a mere retinal surface disorder but may also be associated with fluid accumulation in the outer retina.
Takano and Kishi evaluated the OCT features of the retina in patients with severe myopia and posterior staphyloma7 (Fig. 17.7). The study included 26 phakic and 6 pseudophakic eyes. The refractive errors of the 26 phakic eyes ranged from –8 to –31 diopters (average – 16.7 diopters). Although refractive errors were within – 8 diopters in the 6 pseudophakic eyes, the eyes had apparent posterior staphyloma. The axial lengths measured by A-mode ultrasonography ranged from 25.7 to 32.7 mm (average, 29.2 mm). Slit-lamp examination with a contact lens showed that none of the eyes had a macular hole. In 9 eyes with shallow retinal elevation
Optical Coherence Tomography
Fig. 17.6: Preretinal macular fibrosis with pseudohole. Fundus photograph (top) and OCT3 (bottom). The foveal structure showed sharp columnar depression (arrows) surrounded by thickened perifoveal retina Fig. 17.5: Preretinal macular fibrosis. Fundus photograph (top) and OCT3 (bottom). Contraction of preretinal membrane (arrows) caused retinal thickening with fluid accumulation in the outer layer of the retina
on slit-lamp examination, optical coherence tomography disclosed a foveal retinal detachment with retinoschisis in 8 eyes and a foveal retinal detachment in 1 eye. Two of the remaining 23 eyes had retinoschisis. Foveal retinal detachment and retinoschisis are common features in severely myopic eyes with posterior staphyloma. Retinal detachment may precede the formation of a macular hole in severely myopic eyes.
Diabetic Macular Edema Otani and Kishi reported cross-sectional images of diabetic macular edema by OCT.6 OCT showed three patterns of structural changes in diabetic macular edema (Figs 17.8 to 17.12): sponge-like
Fig. 17.7: Highly myopic eye. Fundus photograph (top) and OCT3 (bottom). The fundus has retinochoroidal atrophy within the staphyloma. OCT3 shows a localized retinal detachment (*) and perifoveal retinoschisis (Δ)
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Fig. 17.10: Diabetic macular edema with hard exudates. Fundus photograph (top) and OCT3 (bottom). In OCT3 image, hard exudates are seen as highly reflective areas located in the outer retinal layers (arrows)
Fig. 17.8: Diabetic retinopathy with cystoid macular edema. Fluorescein angiography (top) and OCT3 (bottom). In the late phase of angiogram, hyperfluorescent cystoid spaces occupy most of the macula. OCT3 shows round cysts mainly in the outer retina that caused the fovea to protrude
Fig.17.11: Diabetic macular edema with subretinal hard exudates. Fundus photograph (top) and OCT3 (bottom). Fundus photograph shows hard exudates at the fovea. In OCT3 image, subretinal hard exudates (arrows) are observed as highly reflective plaques, which are slightly elevated from the retinal pigment epithelium
Fig. 17.9: Diabetic retinopathy with serous retinal detachment. Fundus photograph (top) and OCT3 (bottom). OCT3 reveals a serous retinal detachment at the fovea (arrows)
retinal swelling (88%), cystoid macular edema (47%), and serous retinal detachment (15%). Some eyes had more than one pathologic change. Retinal swelling was more pronounced in the outer than in the inner retinal layers. Cystoid macular edema was located mainly in the outer retinal layers. In eyes with long-standing cystoid macular edema, cystoid spaces had fused, resulting in a large cystoid cavity involving almost the entire retinal layer. Hard exudates are seen as highly reflective areas located in the
Optical Coherence Tomography
A
B
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D Figs 17.12A to D: Diabetic macular edema. A Before vitrectomy, the retina is thickened with an area of low intraretinal reflectivity (yellow arrows) and cystoid cavities are seen in the retina. The fovea protrudes. A serous retinal detachment is seen at the fovea (white arrows); the foveal thickness is 780 μm. The visual acuity is 20/500. B Two months after vitrectomy, a serous retinal detachment (white arrows) is enlarged in diameter. The visual acuity is 20/300. C Four months after vitrectomy, an intraretinal area of low reflectivity is diminished and the serous retinal detachment has resolved; the foveal thickness decreased to 400 μm. The visual acuity is 20/200. D Ten months after vitrectomy, the foveal pit is restored. The visual acuity is 20/70
outer retinal layers. In eyes with a serous retinal detachment, hard exudates tend to deposit not only in the retina but also in the subretinal space.12 Otani and Kishi also evaluated the retinal structure before and after vitrectomy for diabetic macular edema13 (Figs 17. 8 and 17. 9). The foveal thickness (the distance between the inner retinal surface and the retinal pigment epithelium) and the retinal thickness (thickness of the neurosensory retina) were measured by OCT preoperatively and postoperatively. All 13 eyes had retinal swelling with a low intraretinal reflectivity. In addition to retinal swelling, there were cystoid spaces in 5 (38%) of 13 eyes, a serous retinal detachment in 3 (23%), and both cystoid spaces and serous detachment in 3 (23%). Six months postoperatively, the mean foveal thickness significantly decreased from 630 to 350 μm (P <.01, paired t-test) and the mean thickness of neurosensory retina decreased from 540 to
320 μm (P <.01, paired t-test). A serous retinal detachment occurred transiently in 3 eyes. Compared with the preoperative level, the postoperative visual acuity level improved by more than 2 lines in 5 of the 13 eyes (38%), remained the same in 7 eyes (54%), and decreased in 1 eye (8%). Vitrectomy was generally effective in treatment of diabetic macular edema. OCT demonstrated the intraretinal changes of macular edema and the process of edema absorption.
Central Serous Chorioretinopathy Iida et al evaluated central serous chorioretinopathy with OCT during the acute phase and after resolution of the phase14 (Fig. 17.13). In a prospective study, 23 consecutive eyes of 23 patients with central serous chorioretinopathy were examined. In the acute phase, neurosensory retina was thickened within the area of serous
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Rhegmatogenous Retinal Detachment
Fig. 17.13: Central serous chorioretinopathy. Fundus photograph (top) and OCT (bottom). A fundus photograph shows a serous retinal detachment. In OCT image through the fovea, the detached retina is swollen, with intraretinal areas of low reflectivity
retinal detachment in all 23 eyes. The detached retina was thicker than the reattached retina after resolution of the retinal detachment in all eyes. The retinal thickness at the center of the fovea during the acute phase (range 157 to 236 μm; mean ± SD 196.9 ± 22.6 μm) was significantly thicker compared with that after resolution (range, 105 to 152 μm; mean ± SD, 124.8 ± 10.7 μm; P<.0001, Wilcoxon test). In the acute phase, areas of low reflectivity localized within the detached retina were observed in 18 of the 23 eyes. In the area of a grayish-white lesion, OCT showed a moderately reflective mass bridging the detached neurosensory retina and retinal pigment epithelium in all 4 eyes; the outer layer of the detached retina was more highly reflective in these eyes. The retinal pigment epithelium was focally detached beneath the subretinal reflective mass in 3 of the 4 studied eyes. In all eyes studied, neurosensory retina was thickened within the area of serous retinal detachment in
Hagimura et al reported the pathologic changes of the detached neurosensory retina in rhegmatogenous retinal detachment15 (Fig. 17.14). Retinal images were prospectively examined by OCT in 25 eyes with rhegmatogenous retinal detachment. OCT of the detached neurosensory retina, adjacent to the center of the fovea, demonstrated normal retinal structure in 10 eyes (40%), intraretinal separation in 7 eyes (28%), and an
Fig. 17.14: Rhegmatogenous retinal detachment. Fundus photograph (top) and OCT (bottom) show the detached retina with intraretinal separation (arrows)
Optical Coherence Tomography undulated separated outer retina in 8 eyes (32%). Three statistically significant factors affected bestcorrected visual acuity: intraretinal separation (P = <.001), intraretinal separation with undulated outer retina (P = <.001), and height of retinal detachment at the central fovea (P<.001). Visual acuity was significantly worse in the 15 eyes with intraretinal separation with or without an undulated outer retina than in the 10 eyes with retinal thickening but no intraretinal separation (P = <.036). The 8 eyes with undulated separated outer retina showed significantly higher retinal detachment at the central fovea than the 7 eyes with intraretinal separation but no undulated outer retina (P = <.009) and the 10 eyes without intraretinal separation (P = >.016). The duration from onset of subjective symptoms to OCT was not related to the occurrence of intraretinal separation of the detached retina. Intraretinal separation of the detached retina occurred frequently and shortly after retinal detachment in this condition and was one of the factors associated with poor vision in rhegmatogenous retinal detachment. Visual acuity significantly decreased in the highly detached retina.
Juvenile Retinoschisis Ikeda et al reported a cross-sectional image of juvenile retinoschisis16(Fig. 17.15). The retina was split into two layers in the central fovea which extended into the perifoveal area. The inner retina contained two highly reflective zones corresponding to the nerve fiber and inner plexiform
Fig. 17.15: Juvenile retinoschisis. OCT3 shows columnar-shaped structures bridging the separated two layers
Fig. 17.16: Vitelliform macular dystrophy (Vitelliform stage). Fundus photograph (top) and OCT (bottom): OCT shows a highly reflective fusiform thickening of the layer (white arrows) at the level of retinal pigment epithelium and choriocapillaris
layers. Columnar-shaped structures, presumably Müeller cells, bridged the separated two layers. Scanning laser ophthalmoscope showed elevation of the Henle’s fiber layer. These findings seemed to show that the retinal splitting occurs at the outer plexiform layer.
Vitelliform Macular Dystrophy Honma et al reported a cross-sectional image of vitelliform macular dystrophy17(Fig. 17.16). Eyes at the vitelliform stage showed a highly reflective fusiform thickening of the layer at the level of the retinal pigment epithelium (RPE) and choriocapillaris. The vitelliform lesion lacked background fluorescence due to blocking when seen by fluorescein angiography. In eyes with scrambled egg lesion, OCT showed two highly reflective zones posterior to the sensory retina. A flat dome-shaped space was present between
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Conclusion Examination of ocular fundus is a routine examination in the clinical practice of ophthalmology. Ophthalmologists can observe ocular fundus at 10 μm of resolution using a direct ophthalmoscope or a biomicroscope. Because biopsy of the retina is impossible, histopathologic information of retinal disorders has not been well known. OCT allows us to investigate the clinicopathologic correlation of fundus diseases in vivo. As described in this review, OCT has made a great contribution to our understanding of chorioretinal diseases.
References 1. Haung D, Swanson EA, Lin CP, et al. Optical coherence tomography. Science 1991;254:1178-81. 2. Puliafito CA, Hee MR, Schuman JS, Fujimoto JG. Macular diseases. In Optical Coherence Tomography of Ocular Diseases. New Jersey, SLACK Incorporated, 1996. 3. Hee MR, Izatt JA, Swanson EA, et al. Optical coherence tomography of the human retina. Arch Ophthalmol 1995;113:325-32. 4. Kishi S, Takahashi H. Three-dimensional observation of developing macular holes. Am J Ophthalmol 2000;130:65-75. 5. Hee MR, Puliafito CA, Wong C, et al. Quantitative assessment of macular edema with optical coherence tomography. Arch Ophthalmol 1995;113:1019-29.
6. Otani T, Kishi S. Patterns of diabetic macular edema with optical coherence tomography. Am J Ophthalmol 1999;127:688-93. 7. Takano M, Kishi S. Foveal retinoschisis and retinal detachment in severely myopic eyes with posterior staphyloma. Am J Ophthalmol 1999;128: 472-76. 8. Drexler W, Sattmann H, Hermann B, et al. Enhanced visualization of macular pathology with the use of ultrahigh resolution optical coherence tomography. Arch Ophthalmol 2003;121:695-706. 9. Schuman JS, Puliafito CA, Fujimoto JG. Interpretation of the Optical Coherence Tomography Image. In. Optical Coherence Tomography of Ocular Diseases, 2nd ed. New Jersey, SLACK, 2004. 10. Hagimura N. Optical coherence tomographic features of normal ocular fundus. Jpn J Clin Ophthalmol 1998;52:1459-62. 11. Maruyama Y, Otani T, Kishi S. Optical coherence tomographic features of preretinal macular fibrosis. Jpn J Clin Ophthalmol 1999;52:1468-70. 12. Otani T, Kishi S. Tomographic findings of foveal hard exudates in diabetic macular edema. Am J Ophthalmol 2001;131:50–54. 13. Otani T, Kishi S. Tomographic assessment of vitreous surgery for diabetic macular edema. Am J Ophthalmol 2000;129:186-90. 14. Iida T, Hagimura N, Sato T, Kishi S. Evaluation of central serous chorioretinopathy with optical coherence tomography. Am J Ophthalmol 2000;129:519-20. 15. Hagimura N, Suto K, Iida T, Kishi S. Optical coherence tomography of the neurosensory retina in rhegmatogenous retinal detachment. Am J Ophthalmol 2000;129:16-20. 16. Ikeda F, Takahashi K, Kishi S. Optical coherence tomographic features of juvenile retinoschisis. Jpn J Clin Ophthalmol 1998;52:1479-82. 17. Honma R, Utsugi N, Maruyama Y, Kishi S. Optical coherence tomographic features of vitelliform macular dystrophy. Jpn J Clin Ophthalmol 1998;52:1515-18.
Electrophysiological Tests for Visual Function Assessment
SUBHADRA JALALI, LS MOHAN RAM, GARIMA TYAGI, KALLAKURI SUMASRI
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Electrophysiological Tests for Visual Function Assessment
Visual Electrophysiology Tests Visual electrophysiology is an extremely powerful tool to assess functional integrity of the visual pathway. Visual pathway starts from the photoreceptor and retinal pigment epithelial layer, proceeds through inner retinal layers, ganglion cell layer and then via optic nerve through the chiasma to the optic radiations in the brain, finally ending at the occipital cortex. This chapter aims to introduce some basic concepts of visual electrophysiological tests (VET) with the help of some representative clinical cases. Visual electrophysiological tests include the various types of electroretinogram (ERG), electrooculogram (EOG) and visual evoked potential (VEP). A patient may need some tests to ascertain the abnormality. Before ordering the tests a clear understanding of the nature of each of these is absolutely essential to derive a valid interpretation. A thorough clinical evaluation is a prerequisite before ordering any visual electrophysiological test.
History To understand how visual electrophysiological tests reached its present status, some of the milestones are described here. DuBois-Reymond
of Berlin discovered standing potential of 6 millivolts in excised fish eyes and found that cornea was positive with respect to posterior pole of the eye in 1849. He thought that these signals originate in optic nerve. Holmgren showed electrical responses to light in excised frog and demonstrated that these to originate in the retina. Dewar and McKendrick showed that electrical potentials could be recorded from intact animal eyes on illumination of the retina. In 1877, Dewar succeeded in recording ERG from the human eye but the resulting curves were not published. The first human ERG was published by Kahn and Lowenstein. Between 1933 and 1947 Ragnar Granit in Oxford did extensive studies with various chemical agents to analyze the origins of various phases of the ERG. American psychologist Lorrin Riggs, and Gosta Karpe at the Karolinska Institute designed a contact lens electrode independently. The credit for taking the science of ERG from the laboratory to the clinic goes to Gosta Karpe who invited ophthalmologists to visit his clinic where routine diagnostic ERG was introduced. Before ordering and interpreting these tests, a thorough understanding of the nature and limitations of each of test is a essential to arrive at a valid interpretation and diagnosis.
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Diagnostic Procedures in Ophthalmology The visual electrophysiology tests follow a hierarchal pathway along the various cell layers of the visual system. The EOG examines the function of the retinal pigment epithelium (RPE). Following stimulation by light, the electrical responses from retinal photoreceptors and the inner retinal cells are assessed by the a- and b-wave components of the Flash ERG, respectively. The macular photoreceptor function and the ganglion cells function is revealed and separated by the technique of Pattern ERG recording. The integrity of the visual pathway from optic nerve via optic chiasma to the occipital cortex is assessed by various techniques of VEP recording. For each of these recordings in the clinic, certain minimum standards have been laid down by the International Society for Clinical Electrophysiology of Vision (ISCEV pronounced as eyesev). These are available on the website www.iscev.org.
Importance of Electrophysiological Tests Sometimes, the clinical examination of the eye cannot explain the exact cause of decrease in vision. These tests help to detect and categorize the site of lesion in the visual pathway. In other cases, especially in retinal degenerations, these tests help to know the type and extent of disease and its prognosis. In vascular pathology, these tests can assess the extent of ischemia of the inner retinal layers. Other indications of the tests include detection of drug or metal toxicity, pediatric visual assessment and cause of poor vision in infants. In a given situation, these tests prove invaluable in the proper management plan.
side effect. ERG testing very rarely leads to irritation and watering of the eyes for a few hours after the test and can be easily treated with lubricating eyedrops. Rarely, patient can get infectious keratitis. The total test can take up to 3 hours. Alcohol or sedatives should not be taken for 24 hours before the tests as these can interfere with the results. Other medicines such as for diabetes, asthma and hypertension can be continued. For VEP testing, the hair should be preferably washed and dried a night before so as to be free of oil and greasiness. The patient should be electrically isolated according to current standards for safety of clinical biologic recording systems in the user’s country.
Electrooculogram Electrooculogram (EOG) examines the function of the retinal pigment epithelium (RPE) and the interaction between the RPE and the rod photoreceptors.1All vertebrate eyes are like a dipole, with a resting potential in which the cornea is positive with respect to the back of the eye. This creates a standing or resting potential of about 6 millivolts. This standing potential rises when the retina is illuminated to a steady light. EOG measures changes in the standing potential to light and dark conditions. Clinically, EOG measures the standing potential indirectly using the fact that the spatial orientation of a polarized eye is detected by skin electrodes placed nasal and temporal to the eye. Saccadic eye movements result in flow of current around orbit proportional to the magnitude of standing potential of each eye. Skin electrodes record these voltage changes.
Side Effects and Precautions Electrophysiology testing of the eye is very safe and there are no major side effects. VEP and EOG recording is done from skin and has no
Clinical Measurement Geoffrey Arden and colleagues2,3described the indirect method of recording of clinical EOG.
Electrophysiological Tests for Visual Function Assessment Skin electrodes are placed at the medial and lateral canthi to detect the amplitude of the signal between these two points. A ground electrode is fixed to the forehead. Pupils are dilated. A Ganzfeld is used to illuminate whole retina uniformly. Eye should not be exposed to too bright or too dim lights before EOG. After an initial 6 minutes of light adaptation, test is started. The patient makes fixed 30-degree lateral eye movements (using diode fixation lights) during a period of 20 minutes of dark adaptation, and then during a 12-15 minute period of light adaptation. The eye movements are made every 1-2 seconds for approximately 15 seconds and a pause of 45 seconds, every minute. The dipole generated by the resting potential induces current flow in the skin electrodes upon shift of the eye position (Fig. 18.1).The changes in voltage are
amplified and displayed on a computer data acquisition system (Fig. 18.2). The changes in this indirectly measured potential, from darkness to light is the light-induced rise of the resting potential.3 In the dark, the resting potential decreases while it slowly rises to a peak (Slow oscillation of EOG) after the lights are switched on (Fig. 18.2). The amplitude of the signal is recorded at its minimum during dark adaptation (the dark trough) and at its maximum during light adaptation (the light peak). The ISCEV has laid down standards for EOG testing.4The normal light peak occurs in conditions of normally functioning photoreceptors in contact with a normally functioning RPE, and is caused by progressive depolarization of the RPE basal membrane. The EOG is quantified by calculating the amplitude of the light peak in relation to
Figs 18.1A to C: EOG recording procedure. A Sites of skin electrode placement. B Ganzfeld fixating lights (LED) 15 degrees apart, with 30° excursion from right to left. C 16 to 20 sweeps per minute following a baseline recording of 6 minutes in white light. Recording is for 15 minutes in dark and 15 minutes in light
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Fig. 18.2: Showing raw waveforms of the saccades (left) and the final EOG graph (right). Note the light rise and normal Arden ratio of >200% in each eye
the dark trough as a percentage, the Arden index.3A normal index would be > 185% (Fig. 18.2).
Clinical Uses A normal ERG and abnormal EOG are classically seen in Bests’ vitelliform macular dystrophy5 (Fig. 18.3) even in very early stages of the disease with minimal fundus changes and in asymptomatic carriers. EOG abnormality is also seen in a variety of RPE and rod-photoreceptor disorders such as retinitis pigmentosa, choroideremia and age-related macular degeneration. EOG is also abnormal in choroidal melanomas and could be an adjunct tool to differentiate melanoma from nevi.6 EOG is normal in isolated inner retinal cell dysfunction such as in congenital stationary night blindness (CSNB) where RPE and photoreceptors are normal. EOG can be used to study drug toxicity against RPE. One must remember that because light is used to provoke the voltage change in EOG, this test cannot separate the photoreceptor and RPE dysfunction. In recent
years, Arden et al.7 have shown that after intake of low doses of ethanol an EOG peak similar to the light-induced EOG peak can be recorded. This could test RPE layer function independent of its interaction with the photoreceptors.7
Limitations of EOG Recording Patient cooperation and central fixation limit the clinical recording of EOG. Patients with poor central fixation or variable eccentric fixation, children, infants and uncooperative adults cannot be tested satisfactorily. Many testing variables such as media opacities and illumination levels can influence the voltages. Therefore, borderline EOG abnormalities need to be interpreted with caution and test may have to be repeated for confirmation.1
Fast Oscillations of EOG It was reported by Kolder and colleagues8 that the EOG responses could be slow or fast, if the frequency of the light and dark periods for
Electrophysiological Tests for Visual Function Assessment
Fig. 18.3: Shows poor light rise on EOG in a patient with subnormal vision and bilateral macular lesions. ERG recordings including macular photoreceptors (PERG) are normal as shown in ERG results
stimulation were altered. They found the ‘slow’ oscillations were greatest with repeated light and dark periods of 12.5 minutes each, whereas, the greatest amplitude of ‘fast’ oscillations of EOG were seen when the light and dark cycles were of 1.1 minute each. The amplitude of ‘fast’ oscillations increased in dark phase and reduced in light phase. The clinical value of these fast oscillations needs further study.
Electroretinogram (ERG) Due to selective transport of ions, the inside of the photoreceptor cells is more negative than the outside resulting in a standing membrane potential in the dark. Once light falls on the retina, it induces a change in the transmembrane
movement of especially sodium and potassium ions, making the cells hyperpolarized, that is, they become more negative to the extra cellular space than in the dark. These voltage changes are reflected in various ERG components. Various techniques are in clinical use to assess the electrical response of retinal cells to light. The most common of these is the Full-field Flash ERG. Others are Pattern ERG, Focal ERG and Multifocal ERG (Table 18.1). The Flash ERG is the mass response of the neural and nonneural retinal cells to a full field luminance stimulation. The test reflects the function of the photoreceptors and inner nuclear layers of the retina in response to light stimulation. It is recorded by using stimuli delivered by an integrating sphere, called Ganzfeld, which provides a uniform whole field illumination to
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Diagnostic Procedures in Ophthalmology TABLE 18.1: SPECIALIZED TYPES OF ERG (NOT COVERED BY ISCEV STANDARD)10 1. 2. 3. 4. 5. 6. 7. 8. 9.
Macular or focal ERG Multifocal ERG Early receptor potential (ERP) Scotopic threshold response (STR) Direct-current ERG Long-duration flash ERG (on-off responses) Bright-flash ERG Double-flash ERG Chromatic stimulus ERG (including S-cone response) 10. Dark and light adaptation of the ERG 11. Stimulus intensity-response amplitude analysis (Naka-Rushton) 12. Saturated a-wave slope analysis
the retinal spherical surface.9 The Ganzfeld provides both flash stimulation and a diffuse background for photopic adaptation besides fixation lights (Fig. 18.4) By varying the background illumination, the light or dark-adapted state of the eye and the intensity of the stimulus flash, one can elicit and isolate response from different retinal cells. The ISCEV standard describes simple technical
procedures that allow reproducible ERGs to be recorded under a few defined conditions, from patients of all ages including infants.9,10 Details of the equipment standardization is beyond the scope of this chapter but is available in literature.11
Recording Electrode The ERG is recorded using corneal or non-corneal electrodes (Fig. 18.5). The closer the electrode is to the cornea, the higher the amplitude one gets, though latency will not change. Prototypes of corneal electrode are Burian-Allen and Jet electrodes. The corneal electrodes can be unipolar like the Jet-electrode or bipolar like the BurianAllen electrode. The Burian-Allen electrode is centrally transparent with a large optical opening and incorporates a device to hold the lids apart. Topical anesthesia and a nonviscous solution like 0.5% methylcellulose are needed. More viscous solutions can attenuate signal amplitude. Corneal electrodes may be difficult to maintain due to a silver coating that needs resurfacing periodically, and are expensive and cause some
Fig. 18.4: An integrated sphere called Ganzfeld provides a uniform, whole field illumination to the retinal spherical surface. It provides both flash stimulation and a diffuse background light for photopic adaptation. The inside surface has three light emitting diodes as fixation targets for the eye and also for excursion of the eyes during EOG recordings. A chin rest allows proper positioning of the subject. Two prototypes are shown
Electrophysiological Tests for Visual Function Assessment
Fig. 18.5: Electrodes used in visual electrophysiology. Gold-foil and H-K loop electrodes (Courtesy: Dr. G. Holder, London)
discomfort besides rare possibility of corneal abrasion. The advantage, however, is that higher amplitudes are recordable due to proximity to the cornea. All reusable electrodes should be cleaned and sterilized after each use to prevent disease transmission. The non-corneal electrodes include gold foil, the DTL-fiber (Dawson-Trick-Litzkow) and our own devised LVP-Zari electrode.12-14 The LVPZari electrode is disposable, inexpensive, rigid and reliable and made from locally available Zari-embroidery thread. It has a core of nylon (traditionally had cotton thread core) covered with layers of silver, copper and gold, making it a good conductor of electric currents. Due to its nylon core, and multiple metallic coatings, the movement of the fiber across the limbus is minimal, making the recordings very reliable.
The recording electrodes, bipolar or nonbipolar are placed on the cornea. Topical anesthesia is necessary for contact lens electrodes but may not be required for other types of corneal and conjunctival electrodes. It is important to learn the technical requirements of a chosen electrode, to ensure good ocular contact, to ensure proper electrode impedance, to ensure that waveforms are comparable to standard responses, and to define both normal values and variability (which may be different with different electrodes) for their own laboratory.9,10 Skin electrodes are in general not recommended as active ERG recording electrodes. Reference electrodes: Reference electrodes may be incorporated into the contact lens-speculum assembly as in Burian-Allen (Fig. 18.5) or can be placed near each ipsilateral outer canthus
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Diagnostic Procedures in Ophthalmology as a reference for the corresponding eye. The forehead as a reference has a theoretical risk of signal contamination by ocular crossover or from cortical evoked potentials. Ground electrode: A separate skin electrode, such as an ear-clip (Fig. 18.5) should be attached to an indifferent point and connected to ground.
Electrode Placement After topical anesthesia, corneal electrodes are filled with a mild viscous coupling solution such as 0.5% methylcellulose and inserted gently like a contact lens in the center of the cornea, with the lid speculum holding the lids apart simultaneously. The non-corneal electrode is placed in the lower fornix as close to the inferior limbus as possible. It should be stable, non-mobile and not injure the cornea. The reference electrode is placed at the outer canthus. An ear-clip electrode serves as a ground electrode. For all skinelectrodes, good contact is essential with low impedance. To achieve this grease and dead cells on the skin are removed by rubbing with an abrasive and an alcohol pad. Figure 18.6 (left) shows a subject with the LVP Zari electrode in place, held across the fornix with a crocodile clip (red color) and the reference electrode (blue color) at the outer canthus. The ear-clip ground electrode is also seen. All the electrodes are then connected to a junction box (middle) which sends the signals through an interface box into the
computerized amplifiers and recorders. Care should be taken to connect the electrodes to the correct site on the junction box. The outer canthal electrodes go to the positive and recording electrodes to the negative poles of the junction box.
Flash Stimulus Characteristics The light stimulus should consist of flashes having a maximum of about 5 ms duration so that duration of each flash is considerably shorter than the integration time of any photoreceptor. These short white flashes obtained by stroboscopes and gas discharge tubes have a color temperature of 7000 degrees K. A standard flash (SF) strength is defined as one that produces a stimulus strength (in luminous energy per square meter) at the surface of the Ganzfeld bowl of 1.5-4.5 photopic cd.s.m-2 (candela-seconds per meter squared).10 In addition to producing flashes, the stimulator must be capable of producing a steady and uniformly even, white (colored in rare special situations) background luminance of 17- 34 cd.m-2 across the full field. Prolongedflash ERGs and chromatic lights are currently used for studying slow potentials and for separating on-and off-responses.
Technical Requirements The system should be capable of attenuating the flash strength from standard flash over a range of at least 3 log units, either continuously or
Fig. 18.6: LVP electrode placement (left) and connections (middle) to junction box (arrow). ERG in progress (right)
Electrophysiological Tests for Visual Function Assessment in steps of no more than 0.3 log unit. This attenuation should not change the wavelength. It is essential to periodically calibrate the stimulus and background illumination by integrated and nonintegrated photometers to achieve standard test conditions.11 The bandpass of the amplifier and preamplifier should include at least the range of 0.3 to 300 Hertz and should be adjustable for oscillatory potential recordings and other specialized requirements. Amplifiers are generally AC (alternating current) coupled. The recording equipment should be able to represent the full amplifier bandpass without attenuation. The computer digitizers should sample responses at the rate of 1000 Hertz or higher. The observer should be able to watch the displays so as to monitor and make adjustments to get clean and less noisy recording. The computerized digitizers are usually capable of averaging multiple responses so as to remove some of the artifacts.
Clinical Protocol9,10 ERG is recorded after full pupillary dilatation so that all parts of retina get illuminated. Avoid any extra illumination (as in fluorescein angiography or fundus photography) but if these examinations have been performed, a period of dark adaptation of at least one hour is needed before scotopic recording. The subject is placed in a completely dark room for 30 minutes. Next, the subject with electrodes in place is seated comfortably with the chin on the chin rest and eyes open with the face inside the Ganzfeld bowl (Fig. 18.6). The height of patient should be adjusted so that the neck and back muscles are not in a tensed-up position as this can induce muscle-generated artifacts. The cable from junction box is fixed to the shoulder at the subject’s end and plugged
into the interface box at the other end, sending the retinal signals into the amplifiers and computer analyzers. Patients are encouraged to fixate at the central target to reduce eye movement and artifacts. The ISCEV standard describes9,10 a minimum of 5 basic ERG response recordings, three in dark adapted or scotopic conditions and two in lightadapted or photopic state. These basic ERG waveforms are a mass response of the photoreceptors and inner retinal cells. The retinal ganglion cells do not contribute to the flash ERG. Various ERG responses (Fig. 18.7) are described below.
Isolated Rod Response To isolate the signals from the rod system of photoreceptors, a dim white flash of strength 2.5 log units below the white SF is used. Serial responses are recorded with a minimum of 2 second interval between the flashes to allow the rods to return to dark-adapted state in between the flashes. A blue stimulus is equally appropriate if equated to the white standard. At this low intensity level, the cones are insensitive to the stimulus. The isolated rod response has almost no a-wave and a slowly rising, broad-peaked, b-wave. The b-wave in the isolated rod-response waveform is a post-receptor phenomenon, i.e. inner retinal cell response that is driven by only the rod photoreceptors. At this low luminance a-wave is not recordable due to poor photoactivation. There is progressive appearance and increasing amplitude of the awave as stimulus intensity is increased from low level to the higher level of the standard flash (intensity response curve). As the a-wave starts appearing with increasing intensity of stimulus, it represents activity of the rod photoreceptors but with maximum flash intensity, the cones also start contributing to the a-wave as is seen in the maximal combined response.
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Fig. 18.7: Normal Flash ERG waveforms from a normal fundus. Under scotopic conditions, we can record the isolated rod response (IRR), the maximal retinal response (MCR), and the scotopic oscillatory potentials (OP’s). The photopic responses include the single flash for cones (PSF) and the 30-Hertz flicker responses (30 Hz)
Maximal Combined Response The maximal response is dominated by rod responses but also has a small component of cone activity. The initial negative a-wave is generated by the photoreceptors, i.e. both rods and cones. The positive b-wave is generated postreceptoraly in relation to depolarization of the ON-bipolar cells.15 Under scotopic conditions, flash ERG is obtained using the Standard white flash which is 0 decibel attenuated. A sharp awave and a much larger, rapidly rising peaked b-wave which comes to baseline very slowly, are characteristic of this response. Duration between two flashes should be at least 10 seconds to remove effect of bleach of photoreceptors by the bright flash of light.
Oscillatory Potentials The oscillatory potentials (Ops pronounced as opees) are small but high frequency oscillations
on the ascending limb of the b-wave of the maximal combined response. These are extracted and amplified to present the oscillatory potentials as seen in Figure 18.7. They are generated in relation to amacrine cell activity in the middle and inner retinal cell layers. Under scotopic condition and using standard flash intensity as a stimulus, other wavelets are removed by resetting of the filters. The high-pass filter must be reset from the usual 0.3 Hz to 75 Hz, so that an overall bandpass of 75 at high end and 300 Hz at low end is achieved. The response varies with stimulus repetition rate and changes after the first stimulus. Flashes should be given 15 seconds apart to the dark-adapted eyes (1.5 seconds apart to light-adapted eyes), and only the second or subsequent responses should be retained or averaged.9,10 Normal response is characterized by 3 major peaks followed by 1-2 smaller peaks. Comparison with normal individual laboratory values is often adequate to assess any abnormality.
Electrophysiological Tests for Visual Function Assessment Single-Flash Cone Response To record the photopic responses, the retina is exposed to 10 minutes of light adaptation by using the background light in the dome of 17-34 candelas per meter square of luminance (that saturates rods and makes them unresponsive). After this the retina is exposed to a standard flash (SF) to obtain the photopic single flash (PSF) cone response. Inter-stimulus intervals should not be less than 0.5 seconds. This cone response is characterized by a small a-wave and a very sharply rising b-wave that rapidly returns to the baseline. Better localization of cone functions is seen with the single flash cone response than with the flicker response. The photopic cone awave has contribution from the hyperpolarizing (OFF) bipolar cells and also cone photoreceptors.16 The cone b-wave probably reflects postphototransduction activity. Separation of the cone ON (depolarizing bipolar cells) and OFF (hyperpolarizing bipolar cells) pathways is done by using a long duration stimulus with a photopic background.17
30 Hz Flicker Cone Response Under the photopic condition repetitive standard flashes are presented at a frequency of 30 stimuli per second. Rods are suppressed by the photopic condition and are incapable of responding to the highly repetitive stimuli. The amplitude is measured from trough to peak of each response. The latency is measured as the distance between stimulus onset and time-to-peak. A vertical line in the trace should indicate the time of onset of the stimulus. The 30 Hz response is a sensitive measure of cone dysfunction, but is generated at an inner retinal level.18 The response is affected in inner retinal ischemic states.
amplitude of the initial cornea negative a-wave is measured from baseline to the trough, while b-wave amplitude is from the trough of a-wave to peak of cornea positive b-wave. Latency of each wave is measured from stimulus onset, marked by a vertical line across the baseline, to peak of the response (Fig. 18.8). Both amplitude and implicit time should be measured for each component of the waveform. For practical purposes, the variables most often measured are the b-wave amplitudes of the isolated rod response, maximal combined response and of single flash cone response. The time-to peak of the single flash 30 Hz flicker response and bwave latency of maximal combined response is measured. Amplitudes and appearance of the oscillatory potentials9,10 are highly dependent upon stimulus conditions, adaptation and amplifier filter characteristics, but most authors describe three major peaks often followed by a fourth smaller one. Comparison of the response to the laboratory normative wavelets may be adequate for many clinical purposes at our present state of knowledge. An overall index of oscillatory potential amplitude can be obtained by adding up measurements of the three major peaks, preferably from lines spanning the bases
ERG Measurements and Recording A typical flash ERG record is a double peak waveform. According to current convention the
Fig. 18.8: Methodology of ERG amplitudes and latency measurements (see text for details)
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waveforms and choose the best and largest of these reproducible responses.
Normal Values
ERG has following limitations: 1. Flash ERG is affected only if the retinal dysfunction is widespread. In localized conditions, even if they involve high-cell density area say of the macula, the flash ERG can be normal. This is seen in conditions like Stargardts’ heredomacular degeneration and early stages of cone dystrophy or localized RP. Ganglion cell function is not reflected in flash ERG. Flash ERG does not correlate with visual acuity. 2. Diurnal variation exists in rod-ERG b-wave amplitudes, therefore, it should be accounted in serial measurements or research protocols. 3. A number of artifacts such as a blink reflex, muscular tension artifacts (photomyoclonic response) or improper electrode placement and contact can lead to erroneous results. The electrophysiologist should be aware of these and know how to get valid recordings. 4. ERG recordings require a certain level of cooperation from the patient. Fixation is not critical in ERG recording but photophobia, claustrophobia and excessive blinking and anxiety are known to alter the response. 5. Hazy media and miotic pupils can cause erroneous results, as sufficient light does not reach the photoreceptors. Appropriate adjustments would be required to get meaningful data. 6. Adjustments are also needed for age and high refractive error as these affect the ERG responses.
Due to multiple variables that can affect the ERG waveforms, it is recommend that each laboratory should confirm normal values for its own equipment and patient population taking an appropriate sample size. All ERG reporting should include normal values and the limits of normal.
Reporting of the ERG The reports or communications of ERG data should include two representative waveforms of each of the standard responses displayed with amplitude and time calibrations and labeled with respect to stimulus variables and the state of light or dark adaptation. Details of the standardized reporting conditions are available in the literature.9,10
Pediatric ERG Recording The ERG can be recorded from infants and young children9,10but one needs to account for immature eyes and limited cooperation. Special care is required to monitor electrode position and compliance in order to avoid artifactual recordings. Pediatric subjects can be studied without sedation or general anesthesia. Non-cooperative children are given oral sedation and rarely general anesthesia. The latter can modify the ERG responses. Repeat measurements may be needed to confirm the findings, especially in cases of poor recordable waveforms. Pediatric ERG responses should ideally be compared to those from normal subjects of the same age, even though there may be little normative data available. Several examples of each response should be recorded in order to recognize reproducible
Limitations of ERG
Pattern Electroretinogram Some of the limitations of flash ERG can be overcome by more recent techniques of pattern
Electrophysiological Tests for Visual Function Assessment electroretinogram (PERG) and multifocal ERG. Pattern ERG is a contrast response driven by macular photoreceptors but originates in the ganglion cell layer of retina. It allows both a measure of central retinal function, and retinal ganglion cell function. It is the only electrophysiological test that can provide direct assessment of the ganglion cells. PERG helps in improved interpretation of VEP abnormalities and helps to differentiate optic nerve pathology from the macular pathology.19
Recording Parameters and Measurement The PERG is recorded (Fig. 18.9) with refractive correction in place, without mydriasis, using noncontact lens electrodes.12,13,19 Reference electrodes are placed on ipsilateral outer canthus, and not on forehead or ear, to avoid the contamination
from the cortically generated VEP. Binocular stimulation and recording is usually preferred, except in cases of squint, so the better eye can maintain fixation and accommodation. It is a small response and may be difficult to record without stringent controls. Diurnal variation and test-retest variability may be important in longitudinal studies. PERG is measured as the electrical response to a pattern reversal checkerboard stimulus where the overall luminance is unchanged during pattern reversal. A high contrast (near 100%) black and white checkerboard pattern of 15 and 30 minute check size with pattern reversal method is recommended. Field size of stimulus should be between 10 and 16 degrees, and the frame rate of the Cathode rate tube should be a minimum of 75 Hz or above. Stimulus strength of the white checks should be 80 cd m-2. Steady
Fig. 18.9: PERG measurements (Top). Bottom left shows PERG stimulus and bottom right shows actual recording of PERG from two eyes
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Diagnostic Procedures in Ophthalmology fixation is very important because eye movement and blinking will cause severe artifacts. As the amplitudes of PERG signals are small, more averaging is needed. Often more than 150 responses are averaged to get each response. Sweep time of recording is about 150 milliseconds. Computerized artifact rejection is essential and this should be set at no higher than 100 microvolts peak to peak. Background illumination should not be very bright or dim. Ordinary background room light suffices and should be kept constant for all recordings. At a stimulus reversal rate of 16 reversals per second a sinusoidal waveform called Steadystate PERG is obtained. This needs Fourier analysis to measure the amplitude and phase shift and is not often used clinically. At a slow rate of 1-3 Hz (2 to 6 reversals per second) pattern reversal, a transient PERG is obtained. Three components are seen in PERG. There is an initial negative wave (N35) at 35 milliseconds, a positive peak (P50) at 50 milliseconds and a final negative N95 wave at 95 milliseconds from stimulus onset. Clinically, the transient PERG has two main components (Fig. 18.9). P50 is an inner retinal component driven by the macular photoreceptors. N95 is the second component which is contrast related and is generated by the ganglion cells.19 PERG P50 amplitudes can vary from 0.5 to 8 microvolts depending on stimulus characteristics such as the temporal frequency of the stimulus.20 Bandpass filters of the AC coupled amplifiers are set from 1-100 Hz and notch filters should be switched off. P50 amplitude is measured from trough of N35 to the peak of P50, the N95 is measured from peak of P50 to the trough of N95 (Fig. 18.9). P50 latency is a more consistent measurement and used clinically while peak of N 95 is often broad and this precludes accurate latency measurement for N95. If N35 is poorly defined then N35 is replaced by the average time
between time zero and onset of P 50. Age-matched control data should be generated in each laboratory.
Clinical Uses Pattern ERG is most useful in assessing the visual loss of unknown etiology. It helps in differentiating visual loss due to macular photoreceptors/ macular inner retinal cells from diseases of ganglion cell and optic nerves. PERG also helps to monitor early drug toxicity.21 Primary evaluation of macular function: In macular disorders, the P50 component of the PERG is abnormal, often with preservation of the N95:P50 ratio. P50 amplitude is usually affected, with latency changes only, occasionally being seen, particularly in association with macular edema or serous detachment at the macula.19 Primary macular dysfunction such as Stargardt-fundus flavimaculatus, will usually have a normal (rarely subnormal) flash ERG and an abnormal PERG. In generalized retinal dysfunction with macular involvement (cone-rod dystrophy) both ERG and PERG are abnormal. In patients with rod-cone dystrophy, but normal central retinal function, the PERG may be normal even when the Flash ERG is almost extinguished. Ganglion cell dysfunction: Primary ganglion cell dysfunction is associated with marked N95 component loss, particularly in Lebers hereditary optic atrophy and advanced dominant optic atrophy.19 Very severe optic nerve disease will also reduce P50 amplitude, and P50 latency. Complete extinction of the PERG in relation to optic nerve disease rarely if ever occurs, providing at least one eye has enough vision to maintain fixation for binocular PERG recording. The PERG may still readily be detectable in an eye with no light perception.19 It must be remembered that though pattern VEP is primarily used to detect
Electrophysiological Tests for Visual Function Assessment optic nerve dysfunction, macular diseases can cause delayed VEP latency. PERG P50 defects associated with or without VEP abnormalities, point to macular dysfunction. Normal or a defect of only N95 component of PERG with an abnormal VEP suggests optic nerve/ ganglion cell dysfunction.19
Limitations of PERG 1. The PERG amplitudes are very small and due to technical demands, not all laboratories record PERG as a routine. Stringent controls are required to avoid artifacts. The ISCEV standards are available for PERG recordings.20 2. Patient cooperation is essential in recording the PERG. 3. All equipments for ocular electrophysiology do not have the capability to perform PERG. 4. In eyes with hazy media where the pattern stimulus cannot be projected on the macula, results can be erroneous.
Visual Evoked Potential Visual evoked potential (VEP) is a sensitive indicator of optic nerve function. It is an evoked electrophysiological signal that is recorded at the scalp in response to visual stimuli. The responses are much smaller than the full-field flash ERG responses, typically measuring only 5-10 microvolts in amplitude, which lie buried in the electroencephalographic (EEG) noise of 50 microvolts or greater. Averaging of the recorded signals over a given time period after repeated stimulation can help in extraction of VEP from the background EEG activity.
Recording and Measurement The visual stimuli used to elicit VEP are of three types: flash, pattern-reversal and pattern-onset.22
The standard flash used in ERG recording can be used for Flash VEP also. The pattern stimulus consists of an isoluminant checkerboard or grating of various spatial frequencies. Skin electrodes used are silver-silver chloride or golddisk electrode (Fig. 18.5). Good contact of the electrodes using conducting paste and thorough cleaning of skin, help in obtaining clean and reliable recordings. The electrodes are placed on the scalp relative to bony landmarks in relation to the head size as per the international 10/20 system23 (Fig. 18.10). The anteroposterior midline measurements are based on the distance between the nasion, inion and vertex. The active electrode is placed on the midline over the visual occipital cortex (OZ) while reference electrode at the frontal pole (FZ). The ground electrode is at the forehead or earlobe. Recordings are done with refractive correction without mydriasis using monocular stimulation. Prechiasmal lesions are reliably detected by pattern-reversal stimulation while flash stimulus is used in difficult and uncooperative patients or those with dense media opacities and very poor vision. Pattern-onset/offset stimulus is especially useful in malingerers and patients with nystagmus, due to short stimulus duration and inability of the subject to consciously defocus this stimulus. For chiasmal and postchiasmal lesions, multichannel recordings are required as a single midline channel with active electrode only over the occipital cortex can miss lesions. The VEP traces (two reproducible records of each) can be presented as positive upwards (Fig. 18.11) or negative upwards. The polarity convention and stimulus parameters used should be indicated in the report besides the amplitude and latency. Latency is measured from the stimulus onset to peak of the component measured. It must be remembered that interocular difference in the pattern-reversal VEP indicates dysfunction of the entire prechiasmal pathway
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Fig. 18.10: The international 10/20 system of electrode placement for midline single channel VEP. Inset shows Pattern VEP recording in progress
Fig. 18.11: Normal pattern-reversal to three different check sizes (top-15, 30 and 60 minutes), Pattern-onset (bottom left) and Flash (bottom right) VEP
Electrophysiological Tests for Visual Function Assessment and includes ocular, retinal and optic nerve causes.
Normal Waveforms22 1. Flash VEP: It consists of a series of positive and negative peaks that are designated in numerical sequence. Commonest components recorded are N2 and P2 at 90 and 120 msec, respectively (Fig. 18.11). 2. Pattern-reversal VEP: The peaks are named as negative or positive followed by the latency. Commonest wave used for clinical cases is the P100 component, (positive peak at 100 msec) since it is a very robust measure with minimal interocular and inter-subject measurement variation (Fig. 18.11). 3. Pattern-onset/offset VEP: Three components described are C1 (positive at 75 msec), C2 (negative at 125 msec) and C3 (positive at 150 msec). With a stimulated hemifield, the response will appear contralateral to the hemifield stimulated.
Limitations of VEP VEP has following limitations: 1. Age, refractive error, inattention and conscious defocusing of the pattern affect the VEP latency. 2. Stimulus parameters such as contrast, luminance, check size and field size are important determinants of the waveform (Fig. 18.11) and it is essential for each laboratory to establish their own normal controls. 3. Since the amplitudes of VEP are very small, surrounding noise can easily contaminate them and, therefore, strict vigil has to be kept on the recording equipment, recording technique and the stimulus parameters used. 4. Numerous specialized types of VEP22 are being assessed and these are still used as
TABLE 18.2: SPECIALIZED TYPES OF VEP NOT COVERED BY THE ISCEV STANDARD22 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Steady state VEP Sweep VEP Motion VEP Chromatic (color) VEP Binocular VEP Stereo-electro VEP Multichannel VEP Hemifield VEP Multifocal VEP Multifrequency VEP LED goggles VEP
investigational tools (Table 18.2). Knowledge in these areas is still evolving.
Clinical Uses of Visual Electrophysiological Tests A number of ocular disorders may require visual electrophysiology testing for proper diagnosis (Tables 18.3 and 18.4). It must be remembered that ERG needs to be interpreted in the context of other clinical features and investigative reports to arrive at the correct diagnosis. One can be way off the true diagnosis if it is based on ERG recording alone.
Photoreceptor Dysfunction In widespread genetic retinal photoreceptor disorders like retinitis pigmentosa (RP) or choroideremia, a profound reduction of ERG is seen even when retina looks apparently normal. The diagnosis of RP is often obvious in patients with history of night blindness, progressive peripheral field constriction and typical retinal changes including equatorial pigment migration, arterial attenuation, RPE atrophy and disk pallor as seen in a 40 years male with visual acuity of 20/50 and residual visual fields of 10 degrees centrally (Fig. 18.12A, top). ERG has a limited role in diagnosis but helps to assess residual
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macular photoreceptor function. In the test ERG may not be recordable with routine testing using standard flash. With extensive filtering and averaging (Fig. 18.12B, bottom), response (arrow) can be elicited identifying residual cone function. PERG is a more reliable method of eliciting residual central macular function (Fig. 18.13). Fields are also important in such cases to define legal blindness and functional disability in the patient. ERG, however, is essential in research studies to demonstrate diffuse, severe photoreceptor dysfunction that characterizes even early stages of RP. A normal ERG recorded beyond 6 years of age practically rules out possibility of developing RP in future. ERG helps to diagnose patients with atypical findings and also in carrier detection. Flash ERG in RP (Figs 18.12 to 18.14) can be either extinguished, or show a rod-cone or rarely a cone-rod or even a negative type of ERG dysfunction. All such types usually point to a progressive disease especially if implicit time abnormalities are present. True sector or localized central (restricted) disease (Fig. 18.15) may give amplitude reduction with no implicit time
change, whereas diffuse or generalized disease is usually associated with abnormal implicit time.24,25
Localized Photoreceptor Loss with Pigmentary Retinal Dystrophy In atypical retinal pigmentary dystrophies, ERG may help to confirm the diagnosis and differentiate various conditions. For example in a 65 years old female patient with BCVA of 20/80 in each eye, there was no history of night blindness or reduced dark adaptation but gradual progressive loss for reading since 5 years. Posterior pole showed RPE atrophy and mild pigment migration while the rest of the retina was normal (Fig. 18.15). Retinal arteries showed attenuation and disk had temporal pallor. ERG was not extinguished or severely affected, excluding the diagnosis of typical RP. However, both scotopic and photopic responses showed 20-40% reduction in amplitudes with only mild increase in latency. The condition can be interpreted as an Inverse RP / Central RP26 or central
Electrophysiological Tests for Visual Function Assessment TABLE 18.4: INDICATIONS OF ELECTROPHYSIOLOGY TESTS IN SPECIFIC DISEASES A. Retinal and Choroidal Disorders 1. Congenital and infantile forms of blindness (a) Leber congenital amourosis (LCA) (b) Stationary congenital retinal dysfunction (1) Congenital achromatopsia (complete blue cone monochromatacy) (2) Congenital stationary night blindness (incomplete and complete CSNB) (3) Fundus albipunctatus (4) Oguchi disease (c) Blindness as part of a pediatric neurologic syndrome (1) Infantile Refsum syndrome, Zelweger syndrome (retinal degeneration associated with generalized peroxisomal disease) (2) Neuronal ceroid lipofuscinoses (infantile, late infantile, juvenile) (3) Mucolipidosis type IV (4) Hallervorden-Spatz syndrome (iron storage in basal ganglia, mental retardation, spasticity, RP) (5) Senior-Loken syndrome (LCA or severe early-onset RP with renal failure) (6) Joubert syndrome (retinal aplasia, cerebellar hypoplasia, neonatal tachypnea) 2. Rod-cone photoreceptor dystrophy/degeneration (hereditary dystrophies) (i) Rod and rod-cone dystrophy/degeneration (retinitis pigmentosa) (1) Autosomal dominant, autosomal recessive, X-linked (2) RP with slightly greater cone loss (3) RP with electronegative ERG (ii) Cone and cone-rod dystrophies 3. Macular Dystrophies (a) Peripherin/Retinal degeneration slow type (RDS) (b) X-linked (juvenile) retinoschisis (c) Stargardts macular dystrophy (d) Bests macular dystrophy (e) Pattern dystrophy B. Choroidal dystrophies (a) Choroidal atrophy (b) Gyrate atrophy of the choroid and retina (c) Choroideremia – patients and carriers (d) Central areolar choroidal atrophy C. Retinal dystrophies associated with other diseases (a) Usher syndrome (RP and congenital deafness) (b) Bardet-Biedl syndrome (RP, hexadactyly, obesity, hypogenitalism, and mental retardation)
(c) Kearns-Sayre syndrome [mitochondrial myopathy, chronic progressive external ophthalmoplegia (CPEO), RP, heart block] (d) Chronic progressive external ophthalmoplegia plus (CPEO+) D. Bruch’s membrane disorders (a) Angioid streaks (PXE) (b) Dominant drusen E. Hereditary vitreoretinal disorders (a) X-linked juvenile retinoschisis (b) Goldmann-Favre syndrome (c) Enhanced S-cone syndrome (ESCS) F. Inflammatory conditions (a) Multiple evanescent white dot syndrome (MEWDS) (b) Birdshot retinochoroidopathy (c) Pars planitis (d) Syphilis (e) Pigmented paravenous retinochoroidal atrophy (PPRCA) (f) Diffuse unilateral subacute neuroretinitis (DUSN) (g) Rubella G. Vascular disorders (a) Sickle-cell retinopathy (b) Ophthalmic artery occlusion (c) Central retinal artery occlusion (d) Central retinal vein occlusion (e) Carotid insufficiency (ocular ischemic syndrome) (f) Diabetic retinopathy H. Toxic disorders (a) Chloroquine and hydroxychloroquine (b) Quinine (c) Digoxin (d) Thioridazine (e) Chloropromazine (f) Indomethacin (g) Methanol I. Miscellaneous (a) Albinism (b) High myopia (c) Acquired retinal dysfunction/degeneration (i) Vitamin A deficiency (malabsorption syndromes) (ii) Autoimmune retinopathy, including cancerassociated retinopathy (CAR) and melanoma-associated retinopathy (MAR) (d) Retinal (cone-rod) dystrophy with supernormal and delayed rod ERG b-waves
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Fig. 18.12: Unrecordable PERG and flash ERG in advanced retinitis pigmentosa depicting macular involvement VA 20/80 OU, Fields central 10 degrees, night blindness present (Top row). Extensive filtering and averaging of the maximal combined response to elicit a microvolt ERG (arrow, outside ISCEV standard) showing residual retinal function in patient of RP with visual acuity of 20/800 and macular atrophy (Bottom row)
Fig. 18.13: Preserved PERG in a patient of RP with extinguished flash ERG responses showing macular sparing. Visual acuity of a 25-year male was 20/25 and visual fields showed central island of 10 degrees
Electrophysiological Tests for Visual Function Assessment
Fig. 18.14: Cone-rod dystrophy: Retinal dystrophy with Bulls’ eye macular lesion, arterial narrowing, peripheral RPE degeneration and disk pallor. ERG showed absent cone functions with subnormal but recordable isolated rod response suggestive of cone-rod dystrophy in a 29 years patient with VA 20/80. Note large blink artifacts towards end of recordings (arrows) that are not uncommon due to photophobia in these subjects
Fig. 18.15: Central RP: Fundus photograph showing central location of pigmentary retinopathy with normal periphery. ERG shows subnormal rod and cone functions and ERG is not extinguished. The disease is likely to remain localized and minimally progressive. Visual acuity of patient 20/80, central scotoma on fields but with no night blindness
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dystrophy is a patient with visual loss of unknown etiology (Fig. 18.17) where ERG gives the correct diagnosis.
Cone Dystrophies
Inner Retinal Dysfunction: Negative ERG
Cone dystrophies (Fig. 18.16) have normal rod responses, but subnormal, though not extinguished, cone responses.27 The 30 Hz flicker response usually shows both amplitude reduction and delayed implicit time. In early stages, the patient may present with normal macula and mild temporal disc pallor and be misdiagnosed as optic nerve dysfunction if abnormal VEP is demonstrated, without recording the ERG. In later years such patients develop typical Bull’s eye lesion. Some patients can have supernormal rod responses. One common presentation of cone
In a negative ERG the a-wave is unaffected but the b-wave in the scotopic maximal retinal response has a selective reduction of amplitude. It usually signifies diseases sparing the photoreceptors and involving the dysfunction of post-photo transduction and probably postreceptor cells in the middle retinal layers. In a majority of cases an etiology can be detected after correlating clinical and ERG findings, but in some cases the clinical entity cannot be labeled as specific.
Fig. 18.16: Cone dystrophy: Male 36 year with VA 20/200, color vision loss and central scotoma. Localized cone dystrophy involving only macular photoreceptors, it shows severely reduced and delayed P50 in PERG. Other flash ERG responses are normal including photopic responses as the peripheral cones (that are more in numbers than macular cones) are uninvolved
Electrophysiological Tests for Visual Function Assessment
Fig. 18.17: One of the commonest indications for ERG testing in a patient with a visual loss of unknown etiology. This 42-year-old female had history of mild visual loss since 4-5 years. The best corrected visual acuity was 20/ 40 in each eye. Clinically, ocular examinations including detailed anterior and posterior segment evaluation were normal. Visual fields showed no abnormality. ERG showed markedly subnormal, but not absent, cone flicker response (arrow) with normal rod response suggestive of an early adult-onset cone dystrophy. The photopic single flash is not depicted
Causes of negative ERG28 include congenital stationary night blindness (CSNB, complete and incomplete), fundus albipunctatus, Oguchi's disease (Figs 18.18 to 18.20),29 X-linked retinoschisis,30 quinine toxicity, melanoma associated retinopathy (MAR),31 Battens disease, and occasionally in cone-rod dystrophy (Table 18.5). Carcinoma associated retinopathy (CAR) does not usually give a “negative” ERG but profound global ERG reduction in keeping with dysfunction at the level of the photoreceptor.31 It occurs due to damage from circulating antibodies. Central retinal artery obstruction (CRAO) also has a negative ERG (vide infra). In patient of Oguchi’s disease an increased amplitudes of responses in PERG and single flash cone ERG (Fig. 18.19) may occur after prolonged dark adaptation that also changes the golden metallic color of retina to a relatively normal color.
Ischemic Vascular Retinal Disorders ERG changes are profoundly helpful to detect inner retinal ischemia since fundus fluorescein
TABLE 18.5: CONDITIONS ASSOCIATED WITH ELECTRONEGATIVE ERG (i) (ii) (iii) (iv) (v) (vi) (vii) (viii)
CSNB/Oguchi Juvenile retinoschisis CRAO, CRVO Familial optic atrophy Siderosis bulbi Quinine Some forms of RP and cone-rod dystrophy Melanoma associated retinopathy, CAR
angiography or fundus appearance may not detect the true extent of retinal ischemia.32,33 ERG is an indispensable and extremely powerful but unfortunately underutilized tool to differentiate ischemic from non-ischemic obstruction of the central retinal vein (Figs 18.21 and 18.22). Reduced b-wave amplitude has 80-90% sensitivity and 70-80% specificity to detect inner retinal ischemia. An absolute increase of more than 37 msec in latency of the flicker ERG responses or a difference of more than 7 msec between affected and normal eye are almost pathognomic of ischemic type of CRVO in a given
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A
B
A1
B1 Figs 18.18A and B: Oguchi's disease. A & A1 Fundus appearance before and B & B1 2 hours after dark adaptation. Corresponding ERG are shown in Figure 18.19
clinical setting. Reliable information from FFA may be available only in 50-60% cases of CRVO due to media haze, extensive hemorrhages, poor quality photographs and inability to visualize peripheral retina. ERG circumvents all these limitations as it is a global response from the whole retina and is not too much affected by media haze. Other conditions like ocular ischemic syndrome34 (Fig. 18.23), central retinal artery occlusion (Fig. 18.24) and ophthalmic artery occlusion (extinguished ERG) 35 are also very well detected on ERG. The “negative” ERG in central retinal artery occlusion (CRAO) 35 occurs due to
the double blood supply of the retina. The RPE/ photoreceptors (a wave) are spared as they are supplied via choroidal circulation, but bipolar cells and amacrine cells (b-wave and oscillatory potentials) are affected as they are supplied via central retinal artery. In ophthalmic artery obstruction where both retinal and choroidal circulation are affected, the ERG is unrecordable as all retinal cell layers are involved.35 In ocular ischemic syndrome ERG is extremely useful since clinical presentation of this under diagnosed clinical entity is variable.36 In diabetic retinopathy37progressive abnormalities in ERG are seen with progression of the
Electrophysiological Tests for Visual Function Assessment
Fig. 18.19: Oguchi's disease: ERG findings after 20 minutes and after 2 hours of dark adaptation in Oguchi's disease. In each case the left graph is before and right graph is after the prolonged dark adaptation. The OP’s and on-off responses were recorded only once before prolonged dark adaptation and show absence of off-response. Baseline findings are similar to those seen in complete form of CSNB with normal fundus with the exception of a much smaller or nearly absent MCR b-wave in classical complete CSNB
Fig. 18.20: Oguchi's disease: Negative ERG with preserved isolated rod responses and photopic flash and flicker ERG suggestive of incomplete CSNB
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Fig. 18.21: Non-ischemic CRVO: A 28-year male had mild blurring of vision since 5 days due to CRVO with mild macular edema. The full field ERG of right eye is very much comparable to the left eye; both of which are within normal limits suggestive of non-ischemic CRVO. Note the PERG is showing reduced P50 amplitude in the right eye compared to left eye. Although the vision is same (20/20) in both eyes but the macular function of the right eye is not same as the left eye possibly due to macular edema leading to symptomatic reduced contrast sensitivity in the patient
retinopathy. The oscillatory potentials show profound reduction in case of disk new vessels (NVD). The ERG can be subnormal even before clinical retinopathy, possibly due to metabolic effects on the retinal cells. ERG, however, cannot predict accurately the presence of PDR and is, therefore, not used clinically for monitoring diabetic retinopathy.
Drug Toxicity and Monitoring Health of Retina 25 ERG helps to differentiate nyctalopia due to vitamin A deficiency from CSNB and RP. ERG is indicated particularly in adults such as those with alcoholic liver disease, chronic pancreatitis, or malabsorption syndromes. The rod responses
are markedly affected and white flecks may be seen in the retina. Visual acuity is unaffected. Similarly, ERG abnormalities can be seen in drug toxicity especially with hydroxychloroquine,38 chloroquine, quinine and thioridazine. VEP is useful to detect ethambutol toxicity (Fig. 18.25). ERG can detect and prognosticate siderotic changes in eyes with retained iron IOFB.31 Initially ERG has a subnormal b-wave on maximal combined scotopic response that can progress to a negative ERG with time and ultimately become extinguished. Removal of IOFB in eyes with recordable ERG, may lead to improvement in ERG changes and a stable outcome. In advanced siderosis, removal of IOFB will not stop progressive visual loss and sometimes phthisis bulbi develops.
Electrophysiological Tests for Visual Function Assessment
Fig. 18.22: Ischemic CRVO in right eye and non-ischemic in left eye. Right eye has reduced b/a wave ratio and increased latency of b-wave in MCR; reduced amplitudes and delayed stimulus-to-peak time of 30 Hz flicker with absence of PERG, isolated rod response and oscillatory potentials. Left eye has no delays in responses but reduced amplitudes of all waveforms
Pediatric Visual Impairment39 ERG is indispensable in evaluating the cause of poor vision in children. Commonly seen conditions include Lebers congenital amaurosis (LCA), rod monochromatism (Fig. 18.26), Stargardt’s macular dystrophy, ocular albinism and delayed visual maturation. Correct identification of the underlying dysfunction helps in proper counseling as regards risks to relatives, long-term prognosis and sometimes identi-
fication of an underlying systemic disease such as abetalipoproteinemia, neuronal ceroid lipofuscinosis, mucopolysccharidoses and cystinosis.
Carrier Stage Detection ERG can be helpful in detection of the carrier stage of certain X-linked conditions such as X-linked RP,40 blue-cone monochromatism,41 and X-linked cone dystrophy.
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Fig. 18.23: Ocular ischemic syndrome: A 68-year female with VA 20/50 and early cataract in each eye. Fundus had features of NPDR, dilated veins and minimal disk pallor. ERG showed reduced amplitude of rod mediated inner retinal responses (IRR), and reduced b/a wave ratio in maximal combined response (MCR).The inner retinal ischemia was depicted by reduced amplitudes and poorly recordable oscillatory potentials, with delayed stimulus-to-peak time of 30-Hz flicker ERG. Carotid artery doppler (not shown) showed moderate atheromatous changes. Patient developed neovascular glaucoma six months later without worsening of retinopathy in the right eye
Optic Nerve and Visual Pathway Optic nerve and visual dysfunction42 include following conditions: 1. Optic nerve demyelination: The pattern VEP (PVEP) latency (P100) is usually delayed in optic nerve demyelination and the delay may be subclinical, i.e. it may occur with no signs or symptoms of optic nerve involvement.43,44 This may significantly affect clinical management in a patient with spinal cord disease and possible multiple sclerosis (MS). The VEP is almost invariably delayed following symptomatic optic nerve involvement in MS, even when vision has returned to normal. 2. Papilledema: In papilledema the VEP is normal unless secondary optic atrophy occurs.
3. Anterior ischemic optic neuropathy: In anterior ischemic optic neuropathy (AION) the PERG shows normal P50 amplitude and latency, elevation of N95, normal flash ERG and reduced amplitude with normal P100 latency in VEP (Fig. 18.27). Using multichannel VEP recordings, the chiasmal lesions, such as pituitary tumors, show a “crossed asymmetry” where there is an abnormal distribution over the two hemispheres which is in an opposite direction for the two eyes.45 Stimulus parameters are crucial for accurate localization. In general, use of a large field, large check stimulus gives paradoxical lateralization 45 whereas a small field, small check stimulus gives anatomical lateralization. Retrochiasmal lesions give an
Electrophysiological Tests for Visual Function Assessment
Fig. 18.24: CRAO: Left eye with CRAO shows preserved a-wave and absent b-wave (negative ERG) in maximal combined response depicting preservation of outer retinal cell layers supplied by choroidal vasculature and ischemia in inner retinal layers supplied by central retinal artery. Right eye responses are normal
“uncrossed” asymmetry where there is an abnormal distribution that is the same for the two eyes. Serial VEP recordings can help detect recurrences or non-responsiveness to medical therapy as VEP abnormalities can occur before visual fields or visual acuity become abnormal. 4. Ocular albinism: In some cases of ocular albinism, the condition may not be apparent in the absence of typical phenotypic expression of skin or iris, but child can have nystagmus and poor vision due to an albino genotype. All albinos, irrespective of genotype or phenotype exhibit misrouting. Heterozygote carriers do not demonstrate misrouting. The diagnosis of the intracranial misrouting
of albinism, where the majority of optic nerve fibers from each eye do not decussate to the contralateral hemisphere, is readily demonstrated by multichannel VEP. Abnormalities may occur in response to either pattern appearance or diffuse flash stimulation, but the flash VEP appears to be more effective in infants and the pattern appearance VEP in adults. 5. Visual acuity assessment: Objective assessment of visual acuity is performed with pattern appearance stimulation using a very brief appearance time in order to minimize the possibility of voluntary closure or defocusing. 6. Other optic nerve diseases: Lebers hereditary optic neuropathy (LHON), toxic and nutri-
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Fig. 18.25: Ethambutol toxicity: This 18-years male has rapidly progressive, bilateral sequential loss of vision from 4 months (20/400). There was bilateral optic disk pallor with ill-sustained pupillary reactions but no RAPD. The pattern VEP was unrecordable. Flash ERG was normal. In PERG, the N95 was absent (arrow) and P50 was preserved confirming the patient to have bilateral optic neuropathy. Visual fields showed central 7 degrees of scotoma in both eyes. History of antitubercular treatment in the past pointed to a diagnosis of possible ethambutol toxicity
tional optic neuropathies and traumatic optic neuropathy show variable changes in amplitude and latency of VEP depending on extent of involvement. 7. Visual loss assessment in infants and children: VEP is a useful tool along with ERG and other clinical assessments to differentiate various conditions such as cortical visual impairment, delayed visual maturation, and amblyopia. 8. Malingering: Along with other tests, VEP helps to differentiate malingering from visual pathway lesions. Pattern onset is a useful
technique as subjects cannot voluntarily blur this stimulus.
Recent Advances in Multifocal ERG and Multifocal VEP Multifocal ERG (mfERG) technique developed initially by Bearse and Sutter46 allows local ERG responses to be recorded simultaneously from many regions of the retina. The response is thought to originate from outer retina with relatively little contribution from the ganglion cells.47
Electrophysiological Tests for Visual Function Assessment
Fig. 18.26: Rod monochromatism: Showing poorly recordable PERG (due to nystagmus), and absent cone-mediated responses (PSF, 30 Hz) with normal scotopic rod-mediated responses (IRR, MCR). This child of 8 years had VA of 20/400, congenital nystagmus that had reduced with time and photophobia with complete achromatopsia
Responses are recorded to a scaled hexagonal pattern-reversal stimulus in photopic conditions (Fig. 18.28) although some laboratories are attempting to record scotopic mfERG also. MfERG helps to distinguish between diseases of the outer retina and ganglion cells or optic nerves. Along with multifocal VEP (mfVEP),48 the mfERG helps to differentiate organic and non-organic causes of visual loss. There are some limitations of these techniques. Since it is an evolving technology the recording parameters and interpretation are still not standardized, though guidelines have been formulated.49 The techniques are still not widely available. Full field ERG helps to evaluate the function of the retina as a whole. However, it cannot detect focal areas of abnormal function. Multifocal ERG is a new technique. It allows analysis of local ERG responses to assess focal retinal function. Basic technology is similar to full-field ERG in some
aspects. In mfERG the recording, ground and reference electrodes and their placement close to or on the cornea, lateral canthus and ear lobe are similar to the routine ERG. Recording is done with dilated pupils with subject placed in ordinary room light for 15 minutes before testing.
Effect of Stimulus on mfERG
46,47
Stimulus can be delivered by a cathode ray tube (CRT), i.e. monitor LCD projectors, LED arrays or scanning laser ophthalmoscope. The commonest frame frequency of the CRT is 75 Hz and should never be 50 or 60 Hz as this is similar to the line current frequency which interferes as noise with the recordings. Stable fixation is essential to get reliable mfERG recordings and various fixation targets and monitoring devices may be used that do not interfere with the recordings.
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Fig. 18.27: Anterior ischemic optic neuropathy: A 48-year-old male had one month reduction of vision in the left eye. VA was 6/6 and 6/60 in the right and left eyes respectively. Left eye showed diffuse field loss (not shown). Right eye color fundus (Top left) and red free photograph (Middle left) showed normal color of the disk with few RPE changes at macula. Color fundus photograph of the left eye (Top right) showed small disk with no cup and diffuse pallor. Red free photograph of left eye (Middle right) showed 3 quadrants disk pallor with sparing of inferotemporal segment. Pattern ERG showed normal P50 and N95 responses in right eye. Left eye showed reduced amplitude and delayed latency of P50, with secondary elevation of N95 component (arrow) (Extreme top right). The pattern VEP had normal amplitude and latency in right eye (Bottom right) but was poorly recordable in the left eye (Bottom left)
The retina is stimulated with a black and white pattern of hexagonal elements each of which has a 50% chance of being illuminated every time the frame changes. The hexagonal pattern was designed to compensate for the local differences in signal density (cone density) across
the posterior retina. Thus the central hexagons are smaller than the peripheral ones (Fig. 18.28). Each hexagon element follows a fixed predetermined sequence called m-sequence that controls the order of flicker of the stimulus elements between light and dark. This sequence
Electrophysiological Tests for Visual Function Assessment
Fig. 18.28: Normal multifocal ERG stimulus and variety of output display
is designed in such a way that the overall luminance of the screen over the time of recording is relatively stable, i.e. equiluminant. The overall stimulus pattern should subtend a visual angle of 20-30 degrees on either side of fixation. The stimulus region can be divided into different numbers of hexagons such as 61, 103 or 241. Duration of recording varies from 4-8 minutes depending on whether 61 or 103 elements are used. Various artifacts in mfERG recordings include electrical noise, movement errors due to fixation losses, eccentric fixation, shadowing errors due to edge of refraction lenses, and errors due to too much averaging.
Multifocal ERG Responses By correlating the continuous ERG signal with the on or off phases of each stimulus element,
the focal ERG signal associated with each element is calculated. The data obtained can be displayed in various ways; commonly as a topographic array, a three-dimensional plot or as group averages (Fig. 18.28). The trace arrays are essential to display as they not only show the topographical variations due to focal pathology but also demonstrate the quality of the records. It is important to remember that the tracings of mfERG are not responses in the sense of direct electrical signals from a local region of the retina. The mfERG waveforms are a mathematical extraction of signals that correlate with the time that one portion of the screen is illuminated. The signals are hence influenced by adaptation effects from previous stimuli and by scattered light from other fundus areas. The typical waveform of the primary mfERG (first order or first order kernel K1) is a biphasic
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Clinical Uses of Multifocal ERG Multifocal ERG is still under evaluation for clinical usage. However, it is used in the study of following conditions: 1. Maculopathies such as cone dystrophy, central areolar atrophy (Fig. 18.29), and Stargardt's macular dystrophy (Fig. 18.30). 2. Retinal vascular disorders 3. Inflammatory conditions of optic nerve
4. Field loss due to ocular and non-ocular pathology 5. Toxic retinal pathology and 6. Visual loss of unknown etiology.
Focal Macular ERG Focal macular ERG50 is another technique to record ERG responses from the macular area alone. There is, however, no consensus on the best technique or standardized technique for focal macular ERG. With advent of PERG and multifocal ERG there is still a need to assess as to which of these techniques is useful in clinical situations. Presently, focal macular ERG is not in widespread use.
Conclusion The objective information provided by electrophysiological examination of the visual
Fig. 18.29: Central areolar atrophy: It shows subnormal PERG, normal full-field ERG and reduced multifocal ERG. Right bottom shows clinical fundus picture
Electrophysiological Tests for Visual Function Assessment
Fig. 18.30: Multifocal ERG in Stargardt's heredomacular degeneration showing reduced central cone function
system is important in the diagnosis and management of diseases of visual pathway. The clinician recording the waveforms and the one interpreting the test results should be thoroughly conversant with the pitfalls and interrelation of various tests ordered. The electrophysiology results must always be
interpreted and correlated to the clinical and other test parameters to avoid misdiagnosis. Newer techniques in this field such as multifocal ERG, multifocal VEP, focal macular ERG and motion VEP are constantly evolving to improve our diagnostic ability and understanding of the visual pathway.
TABLE 18.6: NORMAL VALUES IN THE LVPEI LABORATORY USING THE METROVISION SYSTEM (FIG. 18.7) Response
Isolated rod response Maximal retinal response Photopic cone 30 Hz Flicker
a-wave Amplitude Latency (microvolts) (milliseconds) 105-130
20.0-22.0
b-wave Amplitude Latency (microvolts) (milliseconds) 130-160 350-450 120-180 100-150
90-110 45.00±4 27-31 33-35
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References 1. Welber RG, Eisner A. Retinal function and physiological studies. In: Retinal Dystrophies and Degenerations. Newsome DA (Ed). New York, Raven Press 1988;44-69. 2. Arden GB, Barrada A, Kelsey JH. New clinical test of retinal function based on the standing potential of the eye. Br J Ophthalmol 1962;46: 449-67. 3. Arden GB, Fojas MR. Electrophysiological abnormalities in pigmentary degenerations of the retina. Arch Ophthalmol 1962;68:369-89. 4. Marmor MF, Zrenner E (for the International Society for Clinical Electrophysiology of Vision): Standard for Clinical Electrooculography. Doc Ophthalmol 1993;85:115-24. 5. Krill AE, Morse PA, Potts AM, Klein BA. Hereditary vitelliruptive macular degeneration. Am J Ophthalmol 1966;61:1405-15. 6. Brink HM, Pinckers AJ, Verbeek AM. The electrooculogram in uveal melanoma: A prospective study. Doc Ophthalmologica 1990;75:329-34. 7. Arden GB, Wolf JE. The human electrooculogram: interaction of light and alcohol. Invest Ophth Vis Sci 2000;41:2722-29. 8. Kolder H, Brecher GA. Fast oscillations of the corneoretinal potential in man. Arch Ophthalmol 1966;75:232-37. 9. Marmor MF, Zrenner E (for the International Society for Clinical Electrophysiology of Vision): Standard for Clinical Electroretinography (1994 Update). Doc Ophthalmol 1995;89:199-210.
10. Marmor MF, Zrenner E (for the International Society for Clinical Electrophysiology of Vision): Standard for Clinical Electroretinography (1999 Update). Doc Ophthalmol 1999;97:143-56. 11. Brigell M, Bach M, Barber C, Kawasaki K, Kooijman A. Guidelines for calibration of stimulus and recording parameters used in visual clinical electrophysiology. Doc Ophthalmol 1998;95:1-14. 12. Dawson WW, Trick GL, Litzkow CA. Improved electrode for electroretinography. Invest Ophthalmol Vis Sci 1979;18:988-91. 13. Ram LSM, Jalali S, Reddy PSR, Rao VS, Das T, Nutheti R. Safety and efficacy evaluation of a new Electrode (The LVP Electrode) Part I. Pattern ERG pilot study. Doc Ophthalmol 2003; 107:171-77. 14. Ram LSM, Jalali S, Faheemuddin S, Das T, Nutheti R. Safety and efficacy evaluation of a new ERG electrode (The LVP Electrode) Part II. Flash ERG pilot study. Doc Ophthalmol 2003;107:179-83. 15. Shiells RA, Falk G. Contribution of rod, onbipolar and horizontal cell light responses to the ERG of dogfish retina. Vis Neurosci 1999; 16:503-11. 16. Bush RA, Sieving PA. A proximal retinal component in the primate photopic ERG a-wave. Invest Ophthalmol Vis Sci 1994;35:635-45. 17. Bush RA, Sieving P A. Inner retinal contributions to the primate photopic fast flicker electroretinogram. J Opt Soc Am A 1996;13:557-65. 18. Sieving PA. Photopic ON- and OFF-pathway abnormalities in retinal dystrophies. Trans Am Ophthalmol Soc 1993;91:701-73. 19. Holder GE. The pattern electroretinogram and an integrated approach to visual pathway diagnosis. Prog Retin Eye Res 2001;20:531-61. 20. Bach M, Hawlina M, Holder GE, Marmor MF, Meigen T, Vaegan, Miyake Y. Standard for Pattern Electroretinography. Doc Ophthalmol 2000;101:11-18. 21. Neubauer AS, Steifelmyer S, Berninger T, Arden GB, Rudolph G. The multifocal pattern electroretinography in chloroquine retinopathy. Ophthal Res 2004;36:106-13. 22. Odom JV, Bach M, Barber C, Brigell M, Marmor MF, Tormene AP, Holder G, Vaegan. For the International Society for Clinical Electrophysiology of Vision. Visual evoked Potential standard (2004). Doc Ophthalmol 2004;108.
Electrophysiological Tests for Visual Function Assessment 23. American Encephalographic Society. Guideline thirteen: Guidelines for standard electrode placement nomenclature. J Clin Neurophysiol 1994;11:111-13. 24. Marmor MF. The electroretinogram in Retinitis pigmentosa. Arch Ophthalmol 1979; 97:1300-04. 25. Berson EL. Retinitis pigmentosa and allied diseases: applications of electroretinographic testing. Int Ophthalmol 1981;4:7-22. 26. Marmor MF, Aguirre G, Arden G, et al. Retinitis pigmentosa: Symposium on terminology and methods of exmination. Ophthalmology 1983; 90: 126-31. 27. Krill AE, Deutman AF, Fishman M. The cone degenerations. Doc Ophthalmol 1973;35:1-80. 28. Koh AH, Hogg CR, Holder GE. The incidence of negative ERG in clinical practice. Doc Ophthalmol 2001;102:19-30. 29. Miyake Y, Yagasaki K, Horiguchi M, Kanda T. Congenital stationary night blindness with negative ERG. A new classification. Arch Ophthalmol 1986;104:1013-20. 30. Tantri A, Vrabee TR, Cuunjieng A, Frost A, Annesley WH Jr., Donoso LA. X-linked Retinoschisis. A clinical and molecular genetics review. Surv Ophthalmol 2004;49:214-30. 31. Scholl HP, Zrenner E. Electrophysiology in acquired retinal disorders. Surv Ophthalmol 2000; 45:29-47. 32. Hayreh SS, Klugna MR, Beri M, Kimera AL, Podhajsky P. Differentiation of ischemic from non-ischemic CRVO during the early acute phase. Graefes Arch Clinical and Exp Ophthalmol 1990;228:201-17. 33. Johnson MA, McPhee TJ. Electrophysiologic findings in iris neovascularization due to acute central retinal vein occlusion. Arch Ophthalmol 1993;111:808-14. 34. Brown GC, Magragal LE. The ocular ischemic syndrome: clinical, fluorescein angiographic and carotid angiographic features. Int Ophthalmol 1988;11:243-51. 35. Brown GC, Magragal LE, Sergott R. Acute obstruction of the retinal and choroidal circulation. Ophthalmology 1986;93:1373-82. 36. Hussain N, Jalali S, Kaul S. Carotid artery diseases and ocular disorders. Ind J Ophthalmol 2001;49:514.
37. Tzekov R, Arden GB. The electroretinogram in diabetic retinopathy. Surv Ophthalmol 1999;44:5360. 38. Tzekov RT, Serrato A, Marmor MF. Electroretinogram findings in patients using hydroxychloroquine. Doc Ophthalmol 2004;108:87-97. 39. Kriss A, Jeffrey B, Taylor D. The Electroretinogram in infants and young children. J Clin Neurophysiol 1992;9:373-93. 40. Berson EL, Rosen JB, Simonoff EA. Electroretinographic testing as an aid in detection of carriers of X-chromosome linked Retinitis pigmentosa. Am J Ophthalmol 1979;87:460-68. 41. Berson EL, Sandberg MA, Maguire A, Bromley WC, Roderick TH. Electroretinograms in carriers of blue cone monochromatism. Am J Ophthalmol 1986;105:254-61. 42. Heckenlively JR, Weleber RG, Arden GB. Testing levels of the visual system. In: Heckenlively JR and Arden GB (Eds). Principles and Practice of Clinical Electrophysiology of Vision. St Louis, Mosby Year Book, 1991;485-93. 43. Holliday AM, McDonald WI, Mushin J. Visual evoked response in the diagnosis of multiple sclerosis. Br Med J 1973;4:661-64. 44. Holder GE. Multiple sclerosis. In: Heckenlively JR and Arden GB (Eds). Principles and Practice of Clinical Electrophysiology of Vision. St Louis, Mosby Year Book 1991;797-805. 45. Holder GE. Chiasmal and Retrochiasmal lesions. In: Heckenlively JR and Arden GB (Eds). Principles and Practice of Clinical Electrophysiology of Vision. St Louis, Mosby Year Book, 1991;557-64. 46. Bearse MA Jr., Sutter EE. Imaging localized retinal dysfunction with the multifocal ERG. J Opt Soc Am A 1996;13:634-40. 47. Hood DC, Odel JG, Chen CS, Winn BJ. The multifocal ERG. J Neuroophthalmol 2003; 23: 225-35. 48. Hood DC, Odel JG, Winn BJ. The multifocal VEP. J Neuroophthalmol 2003;23:279-89. 49. Marmor MF, Hood DC, Keating D, Kondo M, Seelinger MW, Miyake Y. For The International Society for Clinical Electrophysiology of Vision. Guidelines for basic multifocal ERG. Doc Ophthalmol 2003;106:105-15. 50. Miyake Y. Focal macular electroretinography. Nagoya L Med Sci 1998;61:79-89.
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SAVITRI SHARMA, SREEDHARAN ATHMANATHAN
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Diagnostic Procedures in Infectious Keratitis
Microbial keratitis may be caused by bacteria, fungi, parasites or viruses and each of these may produce a spectrum of disease which may or may not have distinctive clinical appearance. Many a time it may not be possible to discriminate between infected or non-infected corneas. To minimize morbidity that may occur secondary to delay in diagnosis and to achieve favorable outcome within a reasonable cost and time, laboratory investigations are indicated in patients with suspected microbial keratitis. Two entirely different protocols are required to be followed while investigating viral and nonviral corneal ulcers, so determined on the basis of clinical features. A combination of the two protocols may be called for when a distinction of viral versus non-viral is not obvious clinically. In the interest of clarity, this chapter is divided in two parts to describe microbiologic procedures required for work-up of clinically non-viral and viral corneal ulcers. Familiarity of ophthalmologists to the functions, limitations, and scopes of microbiology laboratory is important for proper and meaningful interpretation of results. A well equipped ocular microbiology laboratory with well trained technical personnel has great advantages over a general microbiology laboratory, in handling
and processing minute quantity of ocular samples, especially corneal samples. Special orientation towards processing and interpretation of results is of paramount importance.1
Protocol for Non-viral Keratitis: Bacterial, Fungal and Acanthamoeba Ideally, samples for the microbiologic investigations of a suspected microbial keratitis must be collected before the start of any antibiotic treatment. Treatment can be initiated based on the result of the smears and, if required, modified in accordance with the culture and sensitivity results. The protocol essentially consists of four steps, viz: collecting, transport, and processing of the clinical samples and interpretation of the results.
Collection of Samples Prior to the collection of sample from the corneal ulcer itself, it is generally recommended to obtain a culture from the lids and conjunctiva of both the infected and the uninfected eye.2 This procedure is supposed to help in two ways: firstly, the organism(s) grown from the uninvolved
Diagnostic Procedures in Infectious Keratitis eye (indicating normal flora) may be used for comparative purposes, secondly, in the absence of growth from the ulcer the organism(s) from the cul-de-sac of the involved eye may well be the causative organism(s).2 Despite recommendation for this procedure in several textbooks, in our experience, samples from lids and conjunctiva have not yielded useful results in the management of corneal ulcers.3 Similar observation has been made in the newer edition of Laboratory diagnosis of ocular infections published by the American Society of Microbiology,4 which is a deviation from the earlier edition recommending collection and processing of samples from the eyelid margins and conjunctiva. Samples collected from the site of lesion, i.e. the infected corneal tissue are the most valuable for microbiological diagnosis of microbial keratitis. If available, any foreign body on the cornea, contact lens, contact lens case, or lens solutions may be collected. Corneal samples can be collected using the slit-lamp or operating microscope after instillation of topical anesthetic (4% Lignocaine hydrochloride or 0.5% Proparacaine hydrochloride). These anesthetic agents may have variable effect on the growth of organisms,5 however, allowing some time interval between instillation of anesthetic agent and collection of sample would help reduce their effect, if any. Cotton swabs are not recommended for collection of corneal samples, however, calcium alginate swabs, if available, may be used in cases of bacterial keratitis. 6 Platinum spatula, disposable blade (#15), bent needle, surgical knife and disposable cautery have all been used for collection of corneal scrapings for microbiological processing. We routinely use blade no. 15 on Bard Parker handle. No difference was found by us in the quantitative yield of organisms from bacterial and mixed (fungal with bacterial) keratitis while comparing the use of calcium
alginate swab with blade no. 15.7 Although the yield of fungi was more with calcium alginate swab than with blade in this study we did not recommend replacing blade with swab. Swabs are likely to get contaminated by normal flora in the tear film and are less efficient in transferring clinical material onto slides and culture media. While collecting samples from the corneal ulcer the eyelids must be held widely apart to reduce inadvertent contamination by the lid margins or eyelashes. Adherent exudate on the surface of the ulcer may be removed using a sterile cotton swab prior to collection of scrapings. The blade or spatula is scraped over the surface of the area of suppuration by a series of short, moderately firm strokes in one direction to sample both the central and peripheral margins of the infiltrated area of the cornea. Each scraping is used to inoculate one medium or to prepare one smear. Viable organisms may be present throughout the inflamed area or localized to the advancing margin or the ulcer crater. In the absence of accessible corneal suppuration, a corneal biopsy can be done with a disposable skin punch, diamond knife or small corneal trephine.8 The tissue specimen is placed in a sterile petri dish for sectioning. Additional corneal scrapings can be obtained from the base of a partial thickness corneal biopsy. Collection of anterior chamber exudates is advised only under exceptional circumstances owing to risk of inoculating organisms into the eye. The possible circumstances are deep stromal suppuration that cannot be sampled by an anterior approach and infections that have extended into the anterior chamber.4
Transport of Corneal Samples to the Microbiology Laboratory Transportation of corneal scrapings in any transport medium is not recommended. The
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Diagnostic Procedures in Ophthalmology TABLE 19.1: SEQUENCE OF SMEAR PREPARATION AND CULTURE MEDIA INOCULATION FOR THE DIAGNOSIS OF NON-VIRAL KERATITIS Smears
Media
Fig. 19.1: Corneal scraping collection tray containing culture media, blades, glass slides marker pen, reagents and coverslips
Optional Smears/ media
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1. 2. 3. 4. 5.
scrapings are plated directly onto culture media or smeared onto clean glass slides by the side of the patient in the clinic or operating room. It would help to maintain a corneal collection kit in the clinic or operating room containing a set of media, sterile slides (wrapped in foil), spatula/blades, glass marking pencil and swabs (Fig. 19.1) in case the microbiology laboratory cannot be reached and requested to provide the materials whenever required. Corneal biopsy tissue can be transported to the microbiology laboratory in a sterile dry Petri dish or in a sterile bottle with sterile saline. Aqueous fluid is usually collected and transported in a tuberculin syringe. Exudates from the anterior chamber may also be directly plated on culture media and smeared on slides.
Processing of Corneal Scrapings A complete microbiological work-up of a nonviral corneal ulcer may require up to 10 corneal scrapings for a number of smears and culture media (Table 19.1). In case of small ulcer, with limited material availability, high priority needs to be given to inoculation of blood agar or chocolate agar and to prepare only one or two
Potassium hydroxide and/or Calcofluor white Gram stain Giemsa stain Blood agar—aerobic Blood agar—anaerobic Chocolate agar Brain heart infusion broth Thioglycollate broth Non-nutrient agar Sabouraud dextrose agar Potato dextrose agar Lowenstein-Jensen medium Brain heart infusion broth with antibiotic Additional non-nutrient agar Extra smear on slide
smears. Preferred media may be selectively included based on clinical impression, for example, non-nutrient agar for a suspected Acanthamoeba keratitis patient. A schematic diagram to guide non-viral corneal ulcer workup is shown in Figure 19.2.
Direct Smear Examination Methods Material is transferred from the blade/spatula to a glass slide over an area of approximately 1 cm in diameter within a wax-pencil marked (on the reverse) area to avoid needless searching under the microscope. While the specimen is thinly spread for dry smears (Gram, Giemsa, GMS) it can be just placed within the circle for wet smears (KOH, CFW, LPCB) under a coverslip. Table 17.2 outlines the various staining procedures in brief.8 At least two smears should be prepared. For several years, a combination of KOH + CFW, Gram, and Giemsa-stained smears has provided a high sensitivity and specificity in our laboratory for the detection of bacteria, fungi, and Acanthamoeba in corneal scrapings. Common laboratory light microscope
Diagnostic Procedures in Infectious Keratitis
Fig. 19.2: Schematic diagram for microbiology processing of non-viral keratitis
suffices in most instances for the examination of the smears except when fluorescent stains (calcofluor white or acridine orange) are used which require a fluorescence microscope.
Culture Methods Inoculation: Agar plates such as blood agar (BA), chocolate agar (CA), are inoculated by lightly streaking both sides of the blade/spatula over a surface in a row of separate C-shaped marks without penetrating the agar. This procedure helps distinguish valid growth from plate contaminants (Fig. 19.3). Slopes of Sabouraud dextrose agar (SDA) or potato dextrose agar (PDA) in bottles are similarly inoculated by making a row of streaks from below upwards. Liquid media such as brain heart infusion broth
Fig. 19.3: Blood agar inoculated with corneal scraping and incubated at 35°C for 48 hours showing confluent gray, moist colonies (Pseudomonas aeruginosa) on the inoculum (‘C’ streaks) and a contaminant colony away from the inoculum
(BHI) is inoculated by agitating the blade/spatula directly in the broth. To facilitate this procedure
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Diagnostic Procedures in Ophthalmology without inviting contamination, the BHI should be available in screw-capped tubes with the top level of the medium not below 1 cm from the brim of the tube. The inoculation of thioglycollate broth (thio) requires transfer of the scraped material onto a cotton or calcium alginate swab and insertion to the bottom of the tube to facilitate growth of anaerobic bacteria. It is a good practice to limit the inoculation of non-nutrient agar (NNA) with 1-2 strokes in the center of the plate with minimal disturbance of the surface of the medium. While inoculating the plates/bottles, care must be taken to minimize exposure of the medium to the atmosphere. Corneal biopsy tissue can be cut into small fragments and inoculated into media or it can be emulsified in sterile saline using tissue homogenizer and then distributed in culture media, preferably under a bio-safety laminar flow hood. Aqueous fluid drops can be placed over agar plate surfaces as such without streaking and dropped directly into liquid media, preferably under a bio-safety laminar flow hood. Incubation The inoculated culture media are placed in appropriate incubators. NNA (after sample inoculation) requires to be overlaid with a few drops of heat killed or live E coli suspension prior to incubation. While BA (aerobic), BHI broth, thio broth, NNA, SDA and PDA are incubated under normal atmospheric conditions, CA is incubated in a candle jar which provides 5% CO2, and another BA (anaerobic) is incubated in anaerobic jar or cabinet, if available. All media are incubated at 35°C (± 1) except SDA and PDA which are kept at 27°C (± 1) in BOD incubator. Petri dishes are incubated with lids facing downwards to prevent condensed moisture from dripping onto the medium. Broth tubes are held upright in racks. Early growth may be detected on culture plates in most instances within 2448 hours of incubation, however, media such
as BA (aerobic), CA, thio and BHI that show no growth, should be incubated until at least 7 days before discarding. In case of no growth BA (anaerobic), SDA, PDA and NNA may be incubated until 2 weeks. Incubation beyond 2 weeks, in our experience, has not resulted in increased positivity. Instead, incubation longer than 2 weeks may lead to drying of media, and growth of contaminants due to repeated opening of plates for observation. Observation: On solid agar plate growth on inoculation marks (C streaks) are regarded important while growth outside the inoculation marks is disregarded as contaminants (Fig. 19.3). All culture media [except BA (anaerobic) in a jar/cabinet] must be examined daily for detection of any growth. BA (anaerobic) may be examined at intervals of 2-3 days for 2 weeks. Size, color, texture, consistency, and number of colonies on the inoculation marks are counted and recorded. An arbitrary semiquantitative growth estimation graded in our laboratory is + (10 colonies), ++ (10-50 colonies), and +++ (50 colonies). While bacterial and fungal colonies are examined with unaided eyes, the observation of Acanthamoeba growth requires use of microscope. NNA plates (with lid on) are placed under X4 or X10 objective lens of the microscope and presence of trophozoites is looked for in the vicinity of the inoculation mark on the surface of the medium. No colonies are formed by Acanthamoeba. Growth in liquid media appears as turbidity which requires to be subcultured and Gramstained for identification. Identification: Microbiological identification details of various organisms that may be isolated from cases of non-viral keratitis are neither the intent nor the scope of this chapter. Bacterial colonies are usually Gram-stained and identified after consideration of colony characteristics, Gram-reaction, morphology, and results of
Diagnostic Procedures in Infectious Keratitis biochemical tests. Conventional procedures may be adopted for biochemical tests or commercial kits from a number of companies (bioMerieux, France, Lachema, Czech Republic; Organon Technika, USA)9 may be obtained. Some of these companies have recently launched their products in India. Identification of fungal species requires observation of rate of growth, color, consistency and texture of the colony and characteristic microscopic features. Though most species are identified easily more than 20% of filamentous fungal isolates may remain unidentified because of the lack of characteristic spores. Biochemical tests for identification are needed only in case of yeast or yeast-like fungal growth. Helpful hints for identification are available.10 Presence of characteristic cysts and trophozoites on the surface of NNA (Fig. 19.4) helps to identify Acanthamoeba genus. Specification of this genus is presently controversial11 and has no place in the realm of a clinical ocular microbiology laboratory.
Fig. 19.4: Acanthamoeba trophozoites (irregular, vacuolated) on the surface of NNA with E. coli (original magnification × 500)
Antimicrobial Susceptibility Testing Antimicrobial susceptibility testing is done in vitro to identify the response of an organism to a panel of selected drugs. Commercially available panels for Gram-positive and Gram-negative
Fig. 19.5: Antibiotic susceptibility test for Pseudomonas aeruginosa isolated from corneal ulcer. The diameter of zone of inhibition around antibiotic discs is measured and reported as sensitive, intermediate, or resistant (Disk diffusion test)
bacteria are used to determine sensitivity by diskdiffusion method. In this method (Kirby-Bauer) the bacteria is cultured on Mueller-Hinton agar, and antibiotic impregnated disks are applied. After incubation, the diameter of the zone of inhibition around each disk gives an approximation of susceptibility or resistance of the organism (Fig. 19.5). Commercially available kits provide a zone size interpretative chart to facilitate interpretation. Slow-growing bacteria and anaerobes cannot be reliably tested with diskdiffusion method. Estimating the minimal inhibitory concentrations (MIC) of antibiotics may provide a more useful information than labeling organisms as sensitive or resistant,4 especially because the results of disk-diffusion tests relate to levels of drug achievable in serum and do not relate directly to concentration of drug produced in the preocular tear film and ocular tissues by standard routes of administration. The MIC of a drug can be tested by broth dilution or agar dilution method. The antibiotic is serially diluted and added to tubes with broth or wells of a microtiter plate or incorporated into agar plates. A standard suspension of the organism is then inoculated. The MIC is recorded as the lowest concentration with no visible
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Diagnostic Procedures in Ophthalmology growth. The tubes or wells with inhibited growth can be subcultured and the lowest concentration with no growth is recorded as minimum bactericidal concentration. The availability of antifungal and antiamoebic susceptibility testing is limited. In vitro test methods are diverse for fungi 12 and Acanthamoeba13 and clinically predictive value of the results obtained is not known.
Immunological and Molecular Methods Immunology and molecular biology based diagnostic tests that are applicable to eye infections including keratitis have been described in the literature.14,15 Such methods are most useful for the identification and characterization of microorganisms for which culture methods are difficult, time consuming or unavailable. Macroscopic latex and co-agglutination methods may be applicable for certain bacterial and fungal eye infections.16 Diagnostic molecular microbiology is an emerging field that applies the principles of nucleic acid hybridization and nucleic acid amplification, notably polymerase chain reaction (PCR), to the detection and characterization of pathogenic microorganisms. There is an explosive growth in the number and variety of applications of PCR in microbiology and ocular microbiology is no exception. PCR based diagnosis of fungal keratitis17 and Acanthamoeba keratitis18,19 have been published recently.
Interpretation of Microbiology Results Smears Commonly used stains for evaluation of smears and the organismal identification are listed in Table 19.2. Results of smear examination form the basis for provisional diagnosis and initial choice of an antimicrobial agent.
Figs 19.6A and B: Corneal scrapings stained with KOH + CFW showing A Septate fungal filaments, and B Acanthamoeba cysts under fluorescence microscope (original magnification × 500)
Though reported to be useful in the detection of bacteria in corneal scrapings,20 we do not have much experience with acridine orange. However, we have used calcofluor white (CFW) for several years and find the stain very useful in the detection of fungi and Acanthamoeba in corneal scrapings (Figs 19.6A and B). The Gram-stain is useful in identifying bacteria, fungi, as well as Acanthamoeba cysts (Figs 19.7A to D). Precipitated stain, carbon, salt crystals, and necrotic debris can lead to troublesome artefacts in Gram-stained smears. It is easier to detect Gram-positive bacteria (especially S. pneumoniae) than Gram-negative bacteria. Gram-variable bacteria may sometimes be seen.21 Fungal hyphae and Acanthamoeba cysts stain variably since their cell walls do not stain well and may often be
Diagnostic Procedures in Infectious Keratitis TABLE 19.2: COMMON STAINING PROCEDURES FOR CORNEAL SCRAPINGS IN THE DIAGNOSIS OF NON-VIRAL KERATITIS Stain
Steps
Gram stain
1. 2. 3. 4. 5. 6. 7. 8. 9.
Fix smear in 95% methanol Flood smear with crystal violet for 1 minute Rinse with tap water Flood smear with Gram’s iodine solution for 1 minute Rinse with tap water Decolorise with acetone-alcohol solution Rinse with tap water Flood with safranin or dilute Carbol Fuchsin for 30 seconds Rinse with tap water and allow to dry
Giemsa stain (quick)
1. 2. 3. 4.
Fix smear in fixative for 5 (Diff Quik)TM seconds Dip in reagent A for 5 seconds Dip in reagent B for 5 seconds Rinse with water and allow to dry
Giemsa stain
1. Flood with Giemsa solution for 45-60 minutes 2. Rinse in 95% ethanol
Potassium hydroxide (KOH) preparation
1. Add one drop of 10% KOH with 10% glycerol 2. Place a coverslip 3. Apply nail polish around the coverslip edges to prevent drying
KOH+ Calcofluor white
1. 2. 3. 4.
Add one drop of 10% KOH Add one drop of 0.1% calcofluor white with 0.1% Evans blue solution Place a coverslip Examine under UV light
Ziehl-Neelsen acid fast
1. 2. 3. 4. 5. 6.
Flood fixed smear with hot (steaming) strong carbol fuchsin and leave for 5 minutes Rinse with water Decolorize with 20% H2SO4 for 1-2 minutes Rinse with water Flood with methylene blue counter stain for 2 minutes Rinse with water and allow to dry
Kinyoun’s modification of Acid fast stain
1. 2. 3. 4. 5. 6.
Flood fixed smear with strong carbol fuchsin for 2 minutes Rinse with water Decolorize with 1% H 2SO4 Rinse with water Flood with methylene blue counter stain for 2 minutes Rinse with water and allow to dry
LactophenolCotton blue
1. Mix specimen colony in a drop of LPCB 2. Apply coverslip 3. Apply nail polish around edges of coverslip to prevent drying
Acridine orange
1. Mix specimen in 0.01% of acridine orange 2. Apply coverslip 3. Examine under UV light
seen as negative outlines (Fig. 19.7). Trophozoites of Acanthamoeba are difficult to recognize owing to their irregular morphology and similarity to macrophages.22 Giemsa-stained smear serves as a supportive smear. Cytological details are seen well and bacteria, fungi as well as Acanthamoeba cysts can be seen.
Arbitrary quantification of bacteria per high power field may help determine the significance as bacteria comprising the indigenous microflora of the conjunctiva and tear film may be detected in small numbers. Smears with more than ten organisms are more determine. However, detection of bacteria in smears often needs to
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Fig. 19.7D: Corneal scrapings stained with Gram stain showing Acanthamoeba cysts (original magnification × 500)
Presence of partially stained or unstained bacilli in Gram or Giemsa-stained smears has often indicated possibility of atypical mycobacteria (Fig. 19.8) and successful diagnosis of the same.23 Thin, branching and beaded filaments in these smears are indicative of Nocardia sp. To confirm the diagnosis, acid fast stains using 20% H2SO4 (Ziehl-Neelsen technique) in the former and 1% H2SO4 (Kinyoun method) in the latter (Fig. 19.9) are very rewarding.
Cultures While smear examination provides preliminary evidence, culture isolation gives diagnostic
Figs 19.7A to C: Corneal scrapings stained with Gram stain showing A Gram-positive cocci in pairs, B Gramnegative bacilli (arrow), C Septate fungal filaments
be correlated with corresponding bacterial growth in culture for determining significance. Failure of an organism, seen in smears, to grow in culture would indicate either non-viable organism or sample variation. Sampling error must always be ruled out in case of discrepant results.
Fig. 19.8: Corneal scraping from a case of Mycobacterium chelonae keratitis (post-LASIK surgery) showing acid fast bacilli by Ziehl-Neelsen staining (20%H2SO4) (original magnification × 500)
Diagnostic Procedures in Infectious Keratitis medium of the same organism identified in smears, confluent growth at the inoculation site on at least one solid medium, or repeat isolation from the same patient. These criteria are more applicable to bacteria and fungus than Acanthamoeba as it is neither a normal commensal nor a laboratory contaminant.
Antibiotic Susceptibility
Figs 17.9A and B: Corneal scraping from a case of Nocardia keratitis showing A Gram-positive, thin, beaded, branching filaments in Gram stained smear, and B Acid fast, thin, beaded, branching filaments in the same smear stained by Kinyoun method (1% H2SO4) after decolorization (original magnification × 500)
Interpretation of agar disk diffusion test (for bacterial susceptibility) that relates to levels of drug in serum is often controversial. However, since higher antibiotic concentrations can be achieved in the cornea by topical administration of antibiotics, an organism labeled as resistant or intermediate in sensitivity by this test may respond to the drug in vivo. The reverse is unlikely to be the case. The quantitative MIC can be compared to the antibiotic concentration expected at the site of infection. However, resistance breakpoints for ocular isolates have not been determined and there are no generally accepted cut off points.
Polymerase Chain Reaction (PCR) confirmation. Culture report should indicate the day the growth appeared and its quantification or significance. Less than 10 colonies on only one solid medium or growth in only one liquid medium is usually equivocal. Growth of organisms such as S. epidermidis, Corynebacterium sp. and Propionibacterium sp. in small numbers or in a single liquid medium is generally of uncertain significance. The same organisms, however, may be significant in the presence of a strong predisposing factor in the patient. All isolates must be considered in the light of clinical relevance and laboratory significance. Laboratory criteria for definitive infection include growth on two or more media, growth on at least one
The results of PCR on corneal scrapings are usually as good as the choice of primers (oligonucleotide sequence for a particular gene of a particular organism) and the stringent performance of the test. It is a highly sensitive test but instances of false positives can be high if PCR test is not handled carefully. Any laboratory that undertakes molecular diagnostics must comply with all requirements to contain contamination, use appropriate controls and provide reliable results. The PCR results are best viewed in conjunction with the clinical impression and, if possible, with another supporting laboratory evidence towards the diagnosis.
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Protocol for Viral Keratitis
Transport of Samples
The advancement made in the field of laboratory techniques for the diagnosis of viral infections in the past decade has been enormous with the introduction of newer techniques and improvisation of earlier techniques. These techniques have been extensively employed for the diagnosis of viral keratitis, especially in developed countries. However, owing to the cost constraints, the techniques are yet to become a routine in most laboratories in India, including large university laboratories. Several factors may need to be considered before a laboratory chooses to adopt techniques for the diagnosis of viral keratitis or in fact any viral disease. These factors include information regarding the prevalence of a particular viral infection, need of screening for the same in a given population, cost of the technique, availability of the infrastructure, and whether a rapid diagnosis can be provided. An ideal technique should be cost-effective, provide a rapid diagnosis in a reasonable frame of time, easy to perform and interpret, and adaptable in routine microbiology laboratories. As pertinent in non-viral keratitis, the collection, transport, and processing of corneal samples for the diagnosis of viral keratitis have a distinct protocol which may be combined with the former in case of clinical uncertainty.
Unlike the banishment for transport of corneal scrapings (in a transport medium to the laboratory) in the protocol for non-viral corneal ulcer diagnosis, the sample for viral diagnosis always needs to be collected in an appropriate transport medium (except the smears) and sent to the laboratory. Methods of transport would vary according to the type of sample collected. Table 19.3 outlines the methods of transportation of samples to the virology laboratory.
Collection of Samples A variety of samples including corneal scrapings, corneal swabs, corneal impression smear, and corneal button may be submitted for viral diagnosis. In addition or instead of corneal samples, conjunctival scrapings/swabs or aqueous fluid may also be helpful in some situations. As is true for most diseases, collection of clinical sample early in the disease prior to administration of antimicrobial agents, is most useful for laboratory diagnosis.
Processing of Samples Samples received in a virology laboratory may be processed using a variety of techniques. The choice of technique would depend on the type of sample and the specific virus that is being looked for. Most of the procedures can be performed in a moderately equipped laboratory. The procedures standardized and adopted by us for the diagnosis of Herpes simplex virus (HSV) keratitis are outlined in Table 19.4. Of all available laboratory techniques for diagnosis of viral infections only a few can be adopted in a particular laboratory. The choice is made based on the advantages, disadvantages and cost effectiveness of the techniques and their overall utility.
Direct Smear Examination (Cytology) A rapid diagnosis of viral keratitis can be established by observing stained smears of corneal scrapings, conjunctival scrapings/ swabs, or centrifuged deposits of aqueous fluid (cytospin).24 These may be accomplished using non-specific staining techniques such as Giemsa, Papanicolaou, and Hematoxylin-Eosin stain. These techniques help visualize multinucleated giant cells, koilocytic changes (Fig. 19.10A), and intranuclear/intracytoplasmic inclusions (Fig. 19.10B), and various inflammatory cells which
Diagnostic Procedures in Infectious Keratitis TABLE 19.3: METHODS OF TRANSPORTATION OF SPECIMEN TO THE VIROLOGY LABORATORY FOR INVESTIGATION OF VIRAL KERATITIS Corneal scrapings 1. Smear on glass slide, air dry and send for staining/immunofluorescence (IF)/immunoperoxidase (IP) 2. Transfer in a vial (0.5 to 1 ml) of viral transport medium (VTM) and send for culture. Can be stored at 4°C. Do not freeze 3. Transfer on a cellulose acetate membrane, air dry, fix in acetone/methanol and send for staining/IF/IP 4. Transfer in 1 ml of phosphate buffered saline/minimum essential medium/Hank’s balanced salt solution and send for PCR Corneal impression smear on glass slide or cellulose acetate membrane Air dry, fix in acetone/methanol/15 minutes and send for staining/IF/IP Corneal/conjunctival swab 1. Use cotton swab to collect material and transfer in VTM and send for culture. Can be stored at 4°C. Do not freeze 2. Dry swab and calcium alginate swabs are unacceptable Corneal button 1. Place in VTM and send for culture 2. Place in 10% buffered formalin and send for histopathology 3. Place in phosphate buffered saline/minimum essential medium/Hank’s balanced salt solution and send for PCR Aqueous humor 1. Place few drops in VTM and send for culture 2. Place in sterile tube/eppendorf and send for PCR or staining/IF/IP
are predominantly lymphocytes. Koilocytic changes are characteristic perinuclear clearing (halo) with increase in density of surrounding rim of cytoplasm, classically described in human papilloma virus infected squamous epithelial cells of the cervix. Intranuclear inclusions are more efficiently seen in Papanicolaou stain than Giemsa-stained smears, however, Giemsa stain is good for enumerating cell types. Though these staining techniques have the advantage of being rapid and inexpensive, they are often non-specific and offer low sensitivity in the diagnosis of viral infection. For example, these stains cannot TABLE 19.4: METHODS FOR LABORATORY DIAGNOSIS OF VIRAL KERATITIS FOLLOWED AT LV PRASAD EYE INSTITUTE 1. Non-specific smear examination (cytology) methods: • Papanicolaou stain • Giemsa stain 2. Cell-associated antigen detection methods • Direct/indirect immunofluorescence assay (IF) • Indirect immunoperoxidase assay (IP) 3. Virus isolation (tissue culture) methods • Conventional tissue culture • Shell-vial technique 4. Molecular virology method • Polymerase chain reaction
Figs 17.10A and B: Corneal scrapings from a case of HSV keratitis showing. A Multinucleated giant cell (arrow) and koilocytic changes (arrowhead), and B Intranuclear inclusion, in an epithelial cell (Papanicolaou stain, original magnification × 500)
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Diagnostic Procedures in Ophthalmology differentiate the intranuclear inclusions of HSV from that of Varicella zoster virus (VZV). Specific cytology techniques used for viral diagnosis are techniques that indirectly suggest the presence of viral antigen in the clinical sample. Detection of cell associated viral antigen in a corneal scraping or conjunctival scraping is very useful in the diagnosis of viral keratitis. We have been routinely using direct and indirect immunofluorescence (Fig. 19.11), and indirect immunoperoxidase (Fig. 19.12) assays in the diagnosis of HSV, VZV keratitis and adenoviral keratoconjunctivitis. Both these tests are rapid,
Fig. 19.11: Corneal scraping from a case of HSV keratitis showing presence of HSV-1 antigen in the epithelial cells (indirect immunofluorescence assay, original magnification × 250)
specific and sensitive when suitable monoclonal or purified polyclonal antibodies are used in the test system. Relatively higher sensitivity and lower specificity is achieved with purified polyclonal antibodies tests while monoclonal antibodies show high specificity but low sensitivity. Indirect immunoperoxidase (IP) assay has distinct advantages over indirect immunofluorescence (IF) assay. The former provides a permanent preparation for records and utilizes an ordinary light microscope while the latter has the inherent problem of quenching (fading) of fluorescence and requires a sophisticated and expensive fluorescence microscope. In addition, the IP technique can be used on paraffin embedded tissue while the IF technique provides better results with frozen tissue sections. Detection of soluble viral antigens in corneal scrapings collected in buffer and body fluids including tears, aqueous, and vitreous, have been described using enzyme-linked immunosorbent assay (ELISA),25latex agglutination,26 and radio immunoassay (RIA). Results obtained by ELISA and RIA are more objective compared to IF and IP assays (which tend to be subjective), however, we do not have experience using these techniques for the diagnosis of viral keratitis. Some of the rapid methods of antigen detection in viral keratitis are described in Table 19.5.
Tissue Culture Methods
Fig. 19.12: Corneal scraping from the same patient of HSV keratitis (Fig. 19.11) showing presence of HSV-1 antigen (stained brown) in epithelial cells (indirect immunoperoxidase assay, original magnification × 500)
Classically described techniques of virus isolation have been embryonated eggs and animal inoculation, which are not favored by most virology laboratories for routine diagnostic purposes. A much favored technique is that of tissue culture, especially cell cultures. Established cell lines such as HeLa, Vero, HEp 2 and MRC5 have been used for isolation of HSV from corneal scrapings.27We have recently succeeded in using immortalized human corneal epithelial cell line28 for isolation of HSV.
Diagnostic Procedures in Infectious Keratitis TABLE 19.5: RAPID DIAGNOSTIC TESTS FOR VIRAL KERATITIS Type of test Less than 1 hour 1. Giemsa stain (Diff Quik) TM 2. Papanicolaou stain 3. HSV test kit (Sure cell herpes)(R) 4. Latex agglutination (Virogen)(R) Between 1-6 hours 1. Immunofluorescence 2. Immunoperoxidase 3. HSV antigen detection (Herp check)(R) 4. ELISA
Time
Viruses detected
5 minutes 45 minutes 20 minutes
HSV 1 & 2, VZV, Adenovirus HSV 1 & 2, VZV, Adenovirus HSV 1 & 2
35 minutes
HSV 1 & 2
2-3 hours 4-5 hours 5 hours 3-4 hours
HSV 1 & 2, VZV, Adenovirus HSV 1 & 2, VZV, Adenovirus HSV-1 HSV 1 & 2
Growth of virus in the cell lines can be determined either by characteristic cellular changes or cytopathic effect (CPE) as shown in Figure 19.13 or by IF or IP technique, which detect viral antigens in the infected cell lines. Appearance of CPE may take several days but antigens can be detected even before CPE occurs, therefore, IF or IP is a more rapid method. Viruses may be cultured in cell lines maintained in tubes (tube culture) or on cover-slips in vials (shell vial) as shown in Figure 19.14. Shell vial technique is a modification of conventional tissue culture technique wherein entry of virus into the monolayer of susceptible cells (on a cover-slip in a vial) is facilitated by centrifugation of the
Fig. 19.13: Monolayer of vero cell line showing cytopathic effect caused by HSV-1 indicating growth of the virus in the cells (tube culture, phase contrast, original magnification × 200)
vial containing cells and the clinical sample (spin amplification).29 The virus growth occurs in a shorter period (18-72 hours) by this method and additionally, both IF and IP techniques can be performed easily on the cover-slips retrieved from the vials for antigen detection. Both these factors are responsible for increased sensitivity of shell vial technique in isolation of viruses.
Molecular Virology Methods In recent times, the PCR technique has been employed extensively for the detection of viral DNA in clinical samples which is one of the best indications for diagnostic use of this technique.30 By virtue of being extremely sensitive and specific, and at the same time simple and rapid, PCR is presently the most sought after technique for viral diagnosis. In our experience, combination of cytology coupled with antigen detection by IF or IP technique and viral DNA detection by PCR may obviate the role of cumbersome procedures of viral isolation by tissue culture, in the diagnosis of viral keratitis. By cost considerations, setting up and running a molecular virology laboratory may be less expensive than tissue culture laboratory. Diagnosis of atypical herpetic epithelial keratitis using primers for 142 base pair segment of the DNA polymerase gene of the HSV genome by
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Diagnostic Procedures in Ophthalmology herpetic keratitis diagnosis has also been described.35 Proper safe guard against false positives in PCR based test is a great challenge to a molecular virology laboratory. Top of the line quality control, appropriate controls and good laboratory practices are mandatory to obtain reliable laboratory reports.
Interpretation of Virology Results Our laboratory routinely performs a variety of techniques for the laboratory diagnosis of viral keratitis which includes cytology by Papanicolaou and Giemsa stain, antigen detection by IF/IP technique, culture by shell vial technique and PCR (Fig. 19.15). In an analysis of 70 clinically suspected cases of HSV keratitis whose corneal scrapings were tested by PCR, shell vial culture, antigen detection by IF/IP, and cytology by Papanicolaou stain, the sensitivity of the tests was 55.8%, 28.3%, 22.7% and 15.6%, respectively (unpublished data). A laboratory diagnosis of HSV keratitis was offered when HSV-1 antigen was detected and/or HSV-1 was isolated and/ or HSV DNA was detected by PCR with or
Figs 19.14 A to C: Virus cultures using cell lines. A Tube culture, B Shell vial culture, C Containing cover-slip
PCR has been reported.31,32 A variety of primers for PCR diagnosis of HSV type 1 and 2 and VZV infections of the eye (other than keratitis) has been described and can be adapted for the diagnosis of keratitis.33,34 A nested PCR for stromal
Fig. 19.15: Detection of HSV 1 and 2 by PCR in a corneal scraping from a case of HSV keratitis. Agarose gel electrophoresis (Ethidium bromide stained) showing positive control (lane 1), negative control (lane 2), test sample (lane 3), and molecular weight marker (lane 4). Note the band of 179 bp size (DNA polymerase gene specific) in lane 3 corresponding to positive control
Diagnostic Procedures in Infectious Keratitis without cellular changes in Papanicolaou stained smear. PCR results were interpreted with caution when this test alone was positive. It was always correlated with clinical findings and with the results of other tests. False positives were avoided by using a different primer set and adopting a reduced sensitivity PCR.36 It is evident from our observations that adopting a single technique alone may result in under diagnosis. Papanicolaou stain, though less sensitive than others, is a valuable test. Presence of multinucleated giant cells, intranuclear inclusions, and koilocytic changes are indicative of HSV/VZV infection. Initiation of antiviral therapy is indicated based on these smear findings coupled with positive antigen detection by IF or IP assays. Both IF and IP assays detect viral proteins in the absence of viable virions when cultures would be negative. We have often been more successful in detecting the viral antigen than isolating the virus. It is, therefore, recommended that antigen detection assays and Papanicolaou staining should be done in the laboratory diagnosis of viral keratitis where facilities for culture and PCR are not available.
References 1. Agarwal V, Biswas J, Madhavan HN, et al. Current perspectives in infectious keratitis. Ind J Ophthalmol 1994;42:171-92. 2. Burd EM. Bacterial keratitis and conjunctivitis. In: Smolin G, Thoft RA (Eds). The Cornea: Scientific Foundations and Clinical Practice. Boston: Little Brown and Co, 1994. 3. Sharma S, Sankaridurg PR, Ramachandran L, et al. Is the conjunctival flora a reflection of the pathogenic bacteria causing corneal ulceration? Invest Ophthalmol Vis Sci 1994;35(Suppl): S1947. 4. Wilhelmus KR, Liesegang TJ, Osato MS, et al. Cumitech 13A, Laboratory Diagnosis of Ocular Infections. Coordinating (Ed). Specter SC, American Society for Microbiology, Washington, DC 15, 1994.
5. Badenoch PR, Coster DJ. Antimicrobial activity of topical anaesthetic preparations. Br J Ophthalmol 1982;66:364-67. 6. Benson WH, Lanier JD. Comparison of techniques for culturing corneal ulcers. Ophthalmology 1992;99:800-04. 7. Jacob P, Gopinathan U, Sharma S et al. Calcium alginate swab versus Bard Parker blade in the diagnosis of microbial keratitis. Cornea 1995;14: 360-64. 8. Sharma S. Diagnostic methods in ocular microbiology. In: Modern Ophthalmology. Datta, LC, (Ed). 2nd ed. New Delhi: Jaypee Brothers Medical Publishers 1999;216-24. 9. Murray PR, Baron EJ, Pfaller MA et al. Manual of Clinical Microbiology. 6th ed. American Society of Microbiology, Washington DC, 1995. 10. Larone DH. Medically Important Fungi: A Guide to Identification (3rd edn). Washington DC: ASM Press, 1995. 11. Gast RJ, Ledee DR, Fuerst PA et al. Subgenus systematics of Acanthamoeba: Four nuclear 18S rDNA sequence types. J Euk Microbiol 1996;43: 498504. 12. Thomas PA. Mycotic keratitis: An underestimated mycosis. J Medical Veter Mycology 1994;32: 235-56. 13. Saunders PPR, Proctor EM, Rollins DF et al. Enhanced killing of Acanthamoeba cysts in vitro using Dimethylsulfoxide. Ophthalmology 1992;99:1197-2000. 14. Rao NA. A laboratory approach to rapid diagnosis of ocular infections and prospects for the future. Am J Ophthalmol 107: 283-91,1989. 15. Gordon YJ. Rapid diagnostic tests for infectious ocular disease. Int Ophthalmol Clin 1993;33:15361. 16. Sobol WM, Gomez JT, Osato MS et al. Rapid streptococcal antigen detection in experimental keratitis. Am J Ophthalmol 1989;107:60-64. 17. Alexandrakis G, Gloor P. Diagnosis of Fusarium keratitis in an animal model using the polymerase chain reaction (abstract). Invest Ophthalmol Vis Sci 1994;35:S1676. 18. Mathers WD, Nelson SE, Lane JL et al. Confirmation of confocal microscopy diagnosis of Acanthamoeba keratitis using polymerase chain reaction analysis. Arch Ophthalmol 2000;118: 17883. 19. Lehmann OJ, Green SM, Morlet N, et al. Polymerase chain reaction analysis of
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20. 21. 22. 23.
24.
25.
26. 27.
28.
Acanthamoeba. Invest Ophthalmol Vis Sci 1998;39: 1261-65. Groden LR, Rodnite J, Brinser JH et al. Acridine orange and Gram stains in infectious keratitis. Cornea 1990;9:122-24. Choudhury K, Sharma S, Garg P et al. Clinical and Microbiological profile of Bacillus keratitis. Cornea 2000;19:301-06. Sharma S, Srinivasan M, George G: Acanthamoeba keratitis in non-contact lens wearers. Arch Ophthalmol 1990;108:676-78. Garg P, Bansal AK, Sharma S et al. Bilateral infectious keratitis following laser in situ keratomileusis: A case report and review of the literature. Ophthalmology, 2000. Jack I, Marmion BP: Direct virus diagnosis. In: Collee JG, Duguid JP, Fraser AG, Marmion BP (Eds). Mackie, McCartney Practical Medical Microbiology (13th ed). Edinburgh, Churchill Livingstone,1989. Kowalski RP, Gordon YJ, Romanowski EG et al. A comparison of enzyme immune assay and polymerase chain reaction with the clinical examination for diagnosing ocular herpetic disease. Ophthalmology 1993;100:530-33. Kowalski RP, Gordon YJ. Evaluation of immunologic tests for the detection of ocular herpes simplex virus. Ophthalmology 1989;96:1583-86. Simon MW, Miller D, Pflugfelder SC et al. Comparison of immunocytology to tissue culture for diagnosis of presumed herpes virus dendritic epithelial keratitis. Ophthalmology 1992;99:1408-13. Sasaki KA, Ohashi Y, Sasabe T et al. An SV40immortalized human corneal epithelial cell line and its characterization. Invest Ophthalmol Vis Sci 1995;36:614-21.
29. Johnson FB, Luker G, Chow C. Comparison of shell vial culture and the suspension-infection method for the rapid detection of herpes simplex viruses. Diagn Microbiol Infect Dis 1993;16:6166. 30. Podzorski RP, Persing DH. Molecular detection and identification of microorganisms. In Murray et al (Eds). Manual of Clinical Microbiology (6th ed). American Society of Microbiology, Washington DC, 1995. 31. Tei M, Nishida K, Kinoshita S. Polymerase chain reaction detection of herpes simplex virus in tear fluid from atypical herpetic epithelial keratitis after penetrating keratoplasty. Am J Ophthalmol 1996;122:732-35. 32. Koizumi N, Nishida K, Adachi W et al. Detection of herpes simplex virus DNA in atypical epithelial keratitis using polymerase chain reaction. Br J Ophthalmol 1999;83:957-60. 33. Yamamoto S, Langston DP, Kinoshita S et al. Detecting herpes virus DNA in uveitis using polymerase chain reaction. Br J Ophthalmol 1996;80:465-68. 34. Cunningham ET, Short GA, Irvine AR et al. Acquired immunodeficiency syndrome— associated herpes simplex virus retinitis. Arch Ophthalmol 1996;114:834-40. 35. Kudo E, Shiota H, Kinouchi Y et al. Detection of herpes simplex virus DNA in tear fluid of stromal herpetic keratitis patients by nested polymerase chain reaction. Jpn J Ophthalmology 1996;40:390-96. 36. Yamamoto S, Shimomura Y, Kinoshita S et al. Detection of herpes simplex virus DNA in human tear film by the polymerase chain reaction. Am J Ophthalmol 1994;118:160-63.
Diagnostic Procedures in Uveitis
JYOTIRMAY BISWAS, SURBHIT CHAUDHARY, S SUDHARSHAN, SHAHNAWAZ KAZI
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Diagnostic Procedures in Uveitis
In recent years, there have been remarkable advances in the diagnosis and management of uveitis/intraocular inflammation. The advances are in part, from the progress noted in the arena of ocular immunology, immunopharmacology, vitreoretinal surgical techniques and laboratory investigations. This chapter on diagnostic procedures in uveitis provides an overview of the advances in the field of clinical and laboratory diagnosis of uveitis, indications and surgical techniques of chorioretinal biopsy.
and erythrocyte sedimentation rate (ESR) can give initial clues to the systemic association of the uveitic disease and also provide a base-line to therapeutic response and drug side effects. ESR may be raised in non-infectious uveitic diseases such as connective tissue disorders and sarcoidosis and also in infectious conditions such as tuberculosis and syphilis and is, therefore, included in routine investigations of uveitis.
Clinical and Laboratory Diagnosis of Uveitis
Rheumatoid Factor
Fifty percent of cases of uveitis are considered idiopathic. Many others are associated with, or form a part of, other systemic immune-mediated disease. Diagnostic laboratory based immunological tests often provide not only the differential diagnosis of uveitis but also aid in the management of these patients. Moreover, in combination with other serologic, laboratory-based investigations, these assays assist in defining a noninfectious entity.
Basic Investigations Total and differential white blood cell counts
Serological Tests Rheumatoid factor per se is not associated with uveitis and can be ordered in cases of sclerouveitis. Rheumatoid factor is negative in cases of juvenile rheumatoid arthritis (JRA) and ankylosing spondylitis (AS).
Antinuclear Antibody Antinuclear antibody (ANA) is elevated in a number of diseases such as: AS, JRA, dermatomyositis, systemic lupus erythematosus (SLE), scleroderma, Sjogren’s syndrome, chronic hepatitis, apical pneumonia and lymphoma. ANA testing, therefore, gathers more relevance when limited to patients with a
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Diagnostic Procedures in Ophthalmology reasonable likelihood of disease, where a positive result is more likely to be true positive. Obtaining diagnostic titres adds to the significance of the test.
Antineutrophil Cytoplasmic Antibody Test Antineutrophil cytoplasmic antibody (ANCA) test is positive in Wegener’s granulomatosis, Churg-Strauss syndrome, microscopic polyangitis and polyarteritis nodosa. Two distinct subtypes are noticed—perinuclear ANCA (pANCA) and cytoplasmic ANCA (cANCA). The sensitivity of cANCA is as high as 85-96% in patients with widespread Wegener’s granulomatosis, but sensitivity falls to 80% in patients with limited disease. There are a number of conditions where a false-positive cANCA is demonstrated. These include: atrial myxomas, human immunodeficiency virus (HIV) infection, bronchial carcinoma, non-Hodgkin’s lymphoma, endocarditis, eosinophilic myalgia syndrome and mycobacteriosis. Scleritis is more common with Wegener’s granulomatosis than anterior uveitis. Therefore, ANCA is not routinely done in uveitis and should be done in selected cases like uveitis with scleritis or peripheral ulcerative keratitis.
Angiotensin Converting Enzyme Many cells in the body including normal capillary endothelial cells and monocytes, particularly macrophages, produce angiotensin converting enzyme (ACE). Normal levels in males are 1255 mole/min/ml and in women 11-29 mole/ min/ml. The levels are elevated in active systemic sarcoidosis and reduced with oral steroid intake. However, serum ACE is not specific for sarcoidosis and is also elevated in leprosy, tuberculosis and histoplasmosis. As patients with ocular sarcoidosis are usually in systemic remission, a normal serum ACE level does not rule out sarcoidosis.1
In combination with gallium scan and a roentgenogram of the chest, serum ACE has acquired more significance in the diagnosis of sarcoid uveitis.2 In one of the studies the aqueous ACE levels were found to be more specific than the serum ACE values.3
Serological Tests for Syphilis Fluorescent Treponemal Antibody Absorption Test Fluorescent treponemal antibody absorption (FTA-AbS) test is positive in all cases of syphilis, a negative test rules out syphilis. Once the patient has had syphilis, he remains positive throughout life.
Venereal Disease Research Laboratory Test Venereal disease research laboratory (VDRL) test measures non-specific reaginic antibody. The test is negative in many cases of tertiary syphilis. As uveitis is often a feature of late stage of syphilis, the test may be negative. It is a titrable parameter and turns negative following adequate treatment. If syphilis is suspected as a cause of uveitis, both FTA-AbS (for high degree of sensitivity and specificity) and VDRL (to determine state of activity) must be ordered.
Human Leucocyte Antigens Human leucocyte antigens (HLA) are located on the sixth chromosome. The position on the chromosome is indicated by A, B, C (class I) or D (class II). HLAs are found to be associated with several specific diseases. Some of the anterior uveitic entities have been found to have a strong HLA association and are listed in Table 20.1.
Diagnostic Procedures in Uveitis TABLE 20.1: HLA ASSOCIATION WITH UVEITIS Disease
HLA association Comments
Acute retinal necrosis
DR 9 HLA DQw7
50% of patients with fulminant disease; 55% of of patients vs 19% of control subjects
Behçet disease
B5101*
80% of patients vs 26% of control subjects
Birdshot retinochoroidopathy
A29.2
80% of patients vs 7% in control subjects
Inflammatory bowel disease
B27
6 of 13 patients
Juvenile onset arthritis with iridocyclitis
DRw5
62% of patients vs 19% in control subjects
Pars planitis
DR2
68% of patients vs 28% in control subjects
Presumed ocular histoplasmosis
DRw2 HLA B7
81% of patients with disciform scarring vs 28% in control subjects
Psoaritic arthritis
B27
6 of 9 B27 positive patients
Spondyloarthritis
B27
56 of 63 patients with either AS or Reiters syndrome
Recurrent anterior uveitis
B27
52% of patients vs 4% in control subjects
Vogt-Koyanagi-Harada(VKH) syndrome
DRB1*0405, DRB4*0101, DRQA1*0301 HLA DR1, DR4
Several class II alleles associated, depending on the racial group DR1 (36% vs 9% of control subjects) and in Hispanics DR1 56%
Systemic lupus erythematosus (SLE)
DR2 and DR3
36% vs 24% of control subjects
Diagnostic Biopsies Diagnosis of isolated intraocular inflammatory process (without accompanying systemic manifestations) is characteristically based on observation of clinical signs, the evolution of affection and the final outcome. Laboratory tests based on findings in the serum are of some value, especially when the intraocular inflammation is associated with disease involving other organs. When, however, the affection involves the eye only, these tests are of little value. Sampling of the ocular tissue may be more revealing. Following improvement in instrumentation and aseptic microsurgical techniques, intraocular material for diagnostic purpose and testing is more commonly being utilized by ophthalmologists. Histopathology is a part of the ophthalmologist’s armamentarium that is useful in the diagnosis and management of intraocular
inflammation/uveitis. Initially, uveitis is broadly classified based on a thorough history and physical examination. Once classified, the ophthalmologist can efficiently use special tests and procedures to aid in the diagnosis and management of uveitis.
Indications for Diagnostic Biopsies A large number of new diagnostic laboratory techniques allows for the identification and characterization of cells, proteins, and histopathologic specimen and even for flow cytometric studies of very small samples obtained by paracentesis. The diagnostic paracentesis of the eye, keratocentesis of the anterior chamber fluid and vitreous biopsy, have definite value in the following situations. 1. Diagnosing the presence of specific microbial pathogens that are the likely cause of infectious disease in the eye.4,5
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Diagnostic Procedures in Ophthalmology 2. Detecting a predominance of a certain cell types (e.g. macrophages, epithelial ingrowth, ghost erythrocytes, phacolytic cells) that may provide a clue as to the etiology of an inflammatory disease, which may be autoimmune or allergic in nature6 (Table 20.2). 3. For the identification of: a. Specific antibody in the aqueous humor or the vitreous aspirate that would be suggestive of infection (toxocara,7 toxoplasma8). b. Serum ACE in the aqueous humor would be indicative of granulomatous inflammation (sarcoidosis).3 c. Miscellaneous conditions include immune complex and antibody associated with Behçet’s disease9 and identification of tumor cell infiltration of the eye such as large cell lymphoma, 10 leukemia, retinoblastoma, malignant melanoma, and metastatic cells11,12 (Fig. 20.1). It is recommended that diagnostic paracentesis be performed in all cases of postoperative endophthalmitis.4,5Anterior chamber tap should also be done to rule out infectious etiology (particularly Propionibacterium acne) in delayed onset postoperative uveitis following intraocular
Fig. 20.1: Anterior chamber tap of a patient showing metastatic cells
lens implantation. Furthermore, any elderly patient who presents with deteriorating vision (usually with vitritis as predominant feature) of undetermined etiology, should undergo vitreous biopsy to rule out reticulum cell sarcoma (Table 20.3).. The biopsy is also indicated in a malignant neoplasm that involves the eye, the central nervous system or the visceral organs. In one series, the diagnosis of ocular reticulum cell sarcoma was made by vitreous biopsy in 56% of the eyes.13 In cases diagnosed by vitreous biopsy, the average interval from onset of symptoms to the diagnosis was 13 months, as
TABLE 20.2: INDICATIONS AND FINDINGS ON DIAGNOSTIC PARACENTESIS Indications
Findings
Endophthalmitis Retinoblastoma, malignant melanoma, reticulum cell sarcoma, leukemia metastatic tumor Toxocara canis Toxoplasma gondii, T. canis, Reticulum cell sarcoma, syphilis, Behçet’s disease Sarcoidosis Retinoblastoma Phacolytic glaucoma Hemorrhagic glaucoma Epithelial ingrowth Persistent hyperplastic primary vitreous Amyloidosis
Bacteria, fungi Tumor cells Eosinophils Antibodies (ELISA) ACE LD isoenzymes Macrophages/lens matter Ghost erythrocytes Epithelial cells Mesenchymal fibrous cells Amyloid
Diagnostic Procedures in Uveitis TABLE 20.3: INDICATIONS FOR AND FINDINGS ON VITREOUS TAP Indications
Findings
Endophthalmitis Retinoblastoma, malignant melanoma, reticulum cell sarcoma, leukemia, Metastatic cancers T. canis, T. gondii Reticulum cell sarcoma, syphilis, Behçet’s disease Sympathetic ophthalmia CMV retinitis Behçet’s disease Asteroid hyalosis Amyloidosis
Bacteria, fungi
Tumor cells Eosinophils Antibodies Macrophages PCR of virus DNA Monoclonal antibodies Calcium soaps Amyloid
opposed to 21 months in patients where the diagnosis was made by histology of any other sites.13 Ocular reticulum cell sarcoma often responds to radiation and chemotherapy. Therefore, an early diagnosis with prompt therapeutic intervention may contribute to the preservation of visual function and prolongation of life. Similarly, any patient suspected of being an intravenous drug abuser presenting with endogenous endophthalmitis-like picture should undergo diagnostic anterior chamber paracentesis and vitreous biopsy.14
anesthetic is instilled. One may also use sterile cotton tipped applicator soaked in antibiotic and applied at the planned site of needle insertion. 2. A tuberculin or 2 ml syringe with a 27 to 30 gauge needle is used (Fig. 20. 2). However, in case of granulomatous uveitis, it is preferable to use a large bore 25-26 gauge needle. Conjunctival toothed forceps may be used to stabilize the globe. 3. The needle entry into the anterior chamber is oblique through the stroma via the lower limbus. This acts as a valvular self-sealing paracentesis wound on withdrawal of the needle. One should avoid touching the corneal endothelium and particularly the lens in phakic patients and should stay over the peripheral iris at all times. The needle should not be aimed towards the center of the pupil and the beveled end should face upwards throughout the procedure. 4. Obtain a 0.1 to 0.3 ml yield of aqueous, and on withdrawal external pressure is applied to the entrance with sterile cotton tipped applicator. A drop of antibiotic is instilled in the conjunctival sac and the eye is patched
Anterior Chamber Paracentesis Paracentesis of anterior chamber is a relatively simple outpatient procedure, which can be performed when the patient is seated at the slitlamp or lying in a supine position. Many techniques have been described for the anterior chamber paracentesis. A simple and safe technique, which can be performed in an OPD setting taking adequate aseptic precautions is described below: 1. Broad-spectrum antibiotic drops should be instilled. After 30 seconds, a drop of local
Fig. 20.2: Shows the technique of anterior chamber paracentesis using limbal approach with a 30 gauge needle, in a patient suspected to have postoperative endophthalmitis
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Diagnostic Procedures in Ophthalmology for half an hour after which the patient is re-examined on the slit-lamp to ensure anterior chamber reformation.
Vitreous Tap or Diagnostic Vitrectomy A vitreous tap has to be considered in cases with intraocular inflammation of suspected infectious etiology, masquerade syndrome, or intraocular inflammation not responding to treatment. Tapped vitreous is often small, but can provide valuable diagnostic information if processed properly. Diagnostic vitrectomy provides a large amount of material, though diluted. In the presence of a hazy vitreous precluding visualization of the fundus, the combination of vitreous sampling for diagnostic purpose with a therapeutic vitrectomy is certainly a sound and logical approach.
Technique of Vitreous Tap 1. The procedure can be carried out in the operation theatre using a surgical microscope after a sub-conjunctival or retrobulbar injection. However, it can also be performed in the OPD. When performed in the OPD, the technique is similar to that of anterior chamber paracentesis. As the perforation of the sclera is more painful than performing a keratocentesis, a sub-conjunctival injection of 0.1 ml of 2% lidocaine can be given at the site of scleral perforation and entry into the vitreous space 3-mm posterior to the surgical limbus. 2. The vitreous sampling is done using 25 to 23 gauge needles. Most eyes with longstanding intraocular inflammation have liquefied vitreous or fluid pockets within the vitreous. In such a situation fine bored 25G needle can be used. When organization of vitreous is seen 23-G needle is preferred. The vitreous sample may be easier to obtain
with the use of a three-way stopcock. One end is attached to the needle and the other two openings are attached to two tuberculin syringes. The globe is immobilized with a conjunctival forceps, and the needle is inserted in the vitreous cavity under direct visualization with slit-lamp. The empty syringe withdraws the vitreous, and manipulating the stopcock, a similar quantity of antibiotic is injected into the vitreous cavity. After the injection the needle is slowly withdrawn from the eye. For analysis of the vitreous, it is essential to obtain an undiluted vitreous specimen under sterile conditions intraoperatively. Newer techniques using vitrectome with attachments15 and pneumovitrector16 have been described. Doft et al17 obtained vitreous samples in eyes with endophthalmitis by directly connecting a syringe to the aspiration tube of the vitreous cutter. Others have used a collecting bottle with openings at both sides integrated into the aspiration system.18 Smiddy et al19 obtained vitreous samples through a three-way stopcock through a manual aspiration. Scholda and co-workers15 have developed a new technique, in which a metal devise is integrated into the aspiration system of the vitrectomy unit which fits on standard laboratory plastic containers with integrated caps. Recently, Peyman has described a full functional vitrectomy instrument (pneumovitrector) composed of an aspiration and a cutting system combined with an infusion line for injecting air or gas into the vitreous cavity.16 By simultaneous injection of air and removal of vitreous, the pneumovitrectomy allows to obtain a large undiluted vitreous biopsy specimen. A more thorough standard three-port vitrectomy can be performed especially when therapeutically indicated in cases of endophthalmitis. The undiluted vitreous can be sampled and aspirated via a side-port.20
Diagnostic Procedures in Uveitis
Tests and Handling of Aqueous and Vitreous Specimen The specimen should be handled in such a way as to allow maximum number of tests to be performed for the diagnosis. The volume of sampled vitreous is relatively large, compared to the aqueous specimen, increasing the yield on various agar plates and the chances of obtaining a positive culture. To maximize the chance of detecting the offending factor, the aqueous humor and vitreous specimen obtained should be divided in two equal volumes. One volume is used for the following tests:
Microbiology Direct smears are prepared for Gram stain, Giemsa stain, Gomori’s methanamine silver and calcofluor white stain.The samples should also be immediately inoculated onto blood agar, chocolate agar, brain-heart-infusion-broth (BHIB), thioglycolate fluid (maintained at body temperature), Sabouraud’s agar, Brucella agar and BHIB with gentamicin (maintained at room temperature for fungal isolation).
Polymerase Chain Reaction A minimum volume of 0.05 ml should be reserved for this test especially when P. acne, fungal endophthalmitis and uveitis of viral etiology is suspected. The details of PCR are mentioned later. The other half of the original sample can be processed for the following tests: 1. Cytology: The entire sample can be spinned down, the supernatant transpipetted and the pellet resuspended in formalin or glutaral– dehyde. These pellets are passed through two or more millipore filters and number of specific staining including immunohistochemistry is carried out to identify the infiltrating cell types.20 The other technique
includes cytocentrifuge by cytospin method (about 1000 revolutions for 5 minutes). 2. Antibodies: The supernatant obtained after spinning down the cellular components within the aqueous humor should be subjected to ELISA. The local production of specific antibodies within the ocular fluids is an important indication for the possible etiology especially when Toxocara or Toxoplasma is suspected.20 3. Flow cytometric analysis: 21 Flow cytometry (FCM) measures the physical and chemical properties of individual particles or cells moving in a single file in a fluid stream. Constituent cells must be dispersed in a fluid medium before a specimen can be analyzed by FCM. Intact tissue specimens (for example, solid tumors) must first undergo disintegration by mechanical, chemical, or enzymatic methods. Some of these methods allow the cytoplasm to remain intact, thereby permitting analysis of cytoplasm or cell surface membrane properties. The most clearly defined application of FCM is in diagnostic surgical pathology. It is an adjunct to histologic examination in the diagnosis of lymphoproliferative and leukemic processes. FCM is applied to the study of uveal melanomas, retinoblastomas and ocular lymphoid proliferations, especially masquerade syndrome. It is now also being used to provide valuable information regarding the ratio between the cytotoxic and helper T-cells, an indication for the immunologic events taking place during the course of the ocular disease.
Biopsy Iris and Ciliary body Biopsy Biopsy of iris and ciliary body are usually performed in suspected tumors in these regions.
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Diagnostic Procedures in Ophthalmology These lesions include glioneuromas, medulloepitheliomas, iridociliary cysts, leiomyomas, malignant melanomas and nematode granulomas.22 Indications of iris and ciliary body biopsy in uveitic conditions are few and include: 1. Metastatic lesions to the iris and ciliary body masquerading as uveitis. 2. Ruptured iris cysts mimicking as anterior uveitis. 3. Iris and ciliary body nodules secondary to granulomatous conditions like tuberculosis and sarcoidosis. In a recent publication, Moorthy and co-workers were able to diagnose 3 patients with coccidioidomycosis iridocyclitis following biopsy of iris nodules.23
Choroidal and Retinochoroidal Biopsy Lesions within the choroid can be difficult to differentiate clinically, although technological advances in non-invasive imaging have helped to monitor the size and the growth. When the inflammatory process is relentless and the anterior chamber tap and/or vitreous tap (or vitrectomy) are unrevealing, the ophthalmologist has to consider the option of performing a choroidal or retinochoroidal biopsy. With advances in instrumentation and microsurgical techniques, endoretinal biopsy and chorioretinal biopsy can be performed more easily. There are several reports of retinochoroidal biopsy establishing etiological diagnosis of uveitis, especially to differentiate subretinal lesions of infective origin from non-infective ones. 24 The diagnosis of ocular reticulum cell sarcoma can usually be made on the basis of vitrectomy alone, sometimes it requires a more aggressive approach with choroidal biopsy, when pars plana vitrectomy and extensive medical examination fail to confirm the diagnosis of reticulum cell sarcoma.25
Chorioretinal Biopsy (External Approach) Trap-door approach of Stallard: After a conjunctival peritomy, a Flieringa ring is sutured to the bare sclera. A lamellar scleral flap is dissected in a trapdoor fashion over the lesion. Penetrating diathermy is applied around the lesion with adequate margins. The lesion is dissected leaving the retina intact. This procedure is not in use any more. Full-thickness eye wall resection of Peyman: Preoperatively, the mass lesion is surrounded by rows of heavy laser photocoagulation burns, which is performed in two sessions 3 to 4 weeks apart.26-31 The resection is performed 4 weeks after the initial session.30 A conjunctival 360° peritomy is carried out and transillumination and diathermy localize the tumor. A Peyman basket is sutured to the globe. A partial thickness flap is dissected around the lesion. Penetrating diathermy is applied around the tumor in the scleral bed. Pars plana sclerotomies are made for later vitrectomy and sealed with scleral plugs. The lesion is excised using curved Vannas scissors. The scleral flap is sutured back in place and pars plana vitrectomy is performed to remove any vitreous blood and incarcerated vitreous from vitrectomy site. Surgery can also be performed under systemic hypotensive anesthesia to reduce the risk of hemorrhage.29 Endoretinal biopsy (Internal approach): Endoretinal biopsy is done in patients with uncertain retinal inflammation. A pars plana vitrectomy is performed before the retina is biopsied. After the vitrectomy, the retinal site is demarcated and surrounded by endodiathermy or a barrage of endolaser. A shallow retinal detachment is induced by injection of a minute amount of balanced salt solution under the retina to slightly elevate it. The internal part of demarcation zone is cut out using fine intraocular scissors. The underlying tissue is sampled which can be removed by gently aspirating it into a draining
Diagnostic Procedures in Uveitis flute. At times, additional material for examination can be collected from the subretinal space using a soft tipped flute needle connected to a tuberculin syringe. Additional endodiathermy and endolaser burns may be added, if necessary, in order to prevent fluid seepage under the cut edges of the non-biopsied retina which can cause a retinal detachment. This is followed by internal tamponade using expansile gases. In some instances, a biopsy of the retina may be necessary to establish the diagnosis, particularly when both eyes are involved or there is a great potential for loss of vision, as in cases of acute retinal necrosis (Herpes and CMV). 32 Retinal biopsy is needed in patients if retinal lesions have atypical presentations. A case was reported, where an HIV-positive patient presented with a clinical picture similar to CMV retinitis. As the retinitis was found to be ganciclovir resistant, retinal biopsy was carried out which showed it to be toxoplasmic chorioretinitis33 highlighting the importance of retinal biopsy in establishing the diagnosis.
Fine-Needle Aspiration Biopsy Fine-needle aspiration biopsy (FNAB) offers a histologic correlation to the clinical diagnosis in cases of atypical presentation of intraocular lesions. It aids in effective planning and management and enables histopathological diagnosis without having to sacrifice the eye or having to resort to open biopsy methods. FNAB has been recommended in following conditions: 1. Cases of suspected infectious subretinal lesions (abscess or tuberculoma) mimicking as choroidal tumors. Gregor and coworkers 34 diagnosed a Nocardia asteroides subretinal abscess following a trans-vitreal FNAB. 2. Cases where diagnosis is difficult, distinction between benign and malignant is not clear, all ancillary tests are inconclusive, and where
therapeutic decisions have to be made on the basis of cytological findings.35 3. Patients with metastatic disease of the choroid but with no primary. 4. Cases where patient refuses recommended therapy until histopathological confirmation is obtained.
Techniques of Fine-Needle Aspiration Biopsy The method of choice of FNAB is contingent on the existing anatomic state of the eye, the location of the lesion, size of the lesion, and the presence or the absence of a retinal detachment.
Approach 22,36,37 Limbal, pars plana, corneolimbal-zonule and subretinal approaches are used for taking FNAB. Limbal approach: This approach is used for iris lesions (Fig. 20.3) or posterior ciliary body lesions in aphakia. Pars plana approach: In this approach, the needle is passed from the pars plana region (3.5 mm from the limbus) in the quadrant opposite the lesion, through the vitreous gel (Fig. 20.4). For some of the eyes with tumors located posteriorly, a vitrectomy has to be performed
Fig. 20.3: Shows the technique of FNAB of an iris mass lesion using a limbal approach
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Complications of FNAB22,36,37
Fig. 20.4: Technique of FNAB of a subretinal mass lesion using a pars plana approach
before aspiration. The purpose of vitrectomy is to maintain the clear visualization of the lesions and the needle path, to remove the vitreous that could potentially adhere to the needle (reducing unnecessary retinal traction), to eliminate adherence of the vitreous to the tumor cells in the needle as the needle is withdrawn (mitigate potential of tumor cells tracking in the wound), and to control bleeding after aspiration. Corneolimbal-zonular approach: It prevents dissemination of the tumor mass through the needle track. This approach is used in patients with retinoblastoma, a highly friable tumor, as the chance of needle track dissemination is extremely high. Through a corneolimbal approach the needle passes through multiple planes, thus wiping out the tumor cells as the needle is removed from the eye. In addition the absence of blood vessels theoretically decreases the chances of dissemination. Subretinal approach: It is adopted in cases of subretinal abscess and tuberculoma, considering the site is approachable. Most surgeons prefer to use a 25-gauge needle with a flexible connector to a 2 ml syringe to minimize the movement and surgical trauma during biopsy. Others prefer a spinal needle with
The most common complication of FNAB is bleeding from the site of the needle track. Virtually all intraocular FNABs are associated with a small degree of hemorrhage, which can be subretinal, retinal or intravitreal. Orbital dissemination of tumor cells and distant metastatic spread caused by tumor implantation along the needle track has been reported. It is reduced with the use of smaller 25-gauge needle. Theoretically, the procedure can also disseminate a subretinal focus of infection.38 Iatrogenic retinal perforations are unavoidable by the indirect needle approach to the choroidal lesions and may cause a retinal detachment after FNAB. The number of cases developing the retinal detachment following FNAB is few, possibly because the blood clot closes the site of perforation.
Test and Handling of Biopsy Material For obtaining maximal information from retinal and chorioretinal biopsy, a close cooperation amongst the clinician, the surgeon, the microbiologist and the pathologist is of utmost value. The differential diagnosis should be communicated to the laboratory personnel and a plan of handling the tiny biopsy specimen should be worked out. As soon as the biopsy specimen is removed, it should be divided into four parts based on the clinical suspicion. One part should be snap frozen for immunostaining, one for light and electron microscopy, one for in situ hybridization and the remaining for microbiological cultures and PCR detection of infective agents.
Diagnostic Procedures in Uveitis Microbiological Cultures A small piece of tissue is seeded onto the agar plate or preferably onto the agar medium. After incubation for 24-48 hours, initial indications for the type of infective microbial agent can be obtained.
Light and Electron Microscopy These two techniques are complimentary and should be used in parallel. The specimen to be used for these tests should be fixed in 10% formaldehyde or preferably glutaraldehyde solution.
Immunohistochemistry The material should be snap frozen. Frozen sections can be studied with appropriate antibody to identify infectious agents like viruses and autoimmune diseases. Immunohistochemistry can also be done from sections of formalin fixed tissues.
In situ Hybridization39 In situ hybridization test can be done in cryo preserved tissue as well as sections from formalin or glutaraldehyde fixed tissues. Radiolabelled probes are used specially for infectious organisms like viruses. Localization of such infectious agents within a cell is possible.40
Polymerase Chain Reaction Polymerase chain reaction (PCR) is a new molecular biological technique, which involves enzymic application of specific sequence of DNA or RNA (Fig. 20.5). This technique was first described by Mullis and coworkers in 1985. PCR is based on the principle of three steps namely: (i) denaturation, (ii) annealing, and (iii) amplification (Fig. 20.6). Since its introduction in 1995, PCR has been widely used in both research and
Fig. 20.5: Photograph of the PCR machine
clinical medicine. Its application in ophthalmology and medical sciences as a whole has increased exponentially over the last few years.41,42 The ocular tissues which can be submitted for PCR include: intraocular fluid (aqueous and vitreous), fresh retinal and choroidal tissues, formalin fixed or paraffin embedded tissues, and DNA material extracted from a stained or unstained cytology slide. PCR is a reliable test for detection of adenoviruses from the conjunctiva and Propionibacterium acne and other bacterial endophthalmitis.43 It is also employed in the diagnosis of tuberculous uveitis,44 presumed ocular tuberculosis45 and also to emphasize the role of tuberculosis in the etiology of Eales’ disease46,47 and toxoplasmosis. Some general precautions are needed to minimize the risk of contamination, which include performing the initial processing in a biologic safety hood not used for any other PCR related procedure. All reagents should be prepared in another biologic safety cabinet using materials dedicated solely to the PCR and should be aliquoted in sterile tubes.
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Fig. 20.6: Diagrammatic representation of the three steps of PCR: Denaturation, Annealing and Amplification
Lastly, PCR can detect agents for which the primers exist. There are limited numbers of primers available. It also does not provide cellular morphology. In contrast, the in situ DNA hybridization can show the hybridized DNA within the infected cell. PCR detects only the amplified DNA and not necessarily reflect the etiological agents.
syphilis and coccidiomycosis.22 Stains used include hematoxylin and eosin supplemented by Gomori’s methamine silver, Warthin-Starry stain and acid-fast stains, as well as immunohistochemical stains using antibodies. Occasionally, a gallium scan will demonstrate lacrimal gland uptake and support the need for lacrimal gland biopsy.
Conjunctival and Lacrimal Gland Biopsy
Mucosal Biopsy
Conjunctival and lacrimal gland biopsy should be reserved for those patients with visible conjunctival masses or lacrimal gland enlargement, as can occur with sarcoidosis, tuberculosis,
The commonest indication of oral mucosal biopsy in a uveitic scenario is for the diagnosis of Behçet’s syndrome, which shows evidence of an occlusive vasculitis. Similarly, characteristic inflammation of one of the minor salivary glands
Diagnostic Procedures in Uveitis on scanning can confirm a clinical suspicion of Sjogren’s syndrome, whereas inflammation of the intestinal mucosa can support the diagnosis of ulcerative colitis, Crohn’s disease or Whipple’s disease.22
Lymph Node Biopsy In patients with uveitis and enlarged lymph nodes, FNAB or excision biopsy of the affected nodes can be performed to rule out tuberculosis or sarcoidosis2.
Ancillary Tests Fundus Fluorescein Angiography Fundus fluorescein angiography (FFA) is a helpful adjunct test in inflammatory diseases involving the fundus of the eye. The main advantage of this technique is its ability to better visualize the retinal vessels and delineate their walls. FFA is most often used to diagnose cystoid macular edema, retinal or choroidal neovascularization, areas of retinal non-perfusion and active retinal vasculitis. FFA is also useful in patients with neurosensory retinal detachment and other outer retinal inflammations, particularly those involving the RPE. The major drawback of FFA is its inability to image the choroid and to detect inflammatory events affecting the choroid and choriocapillaris especially in areas where the lesion is deep.20 Furthermore, one has to remember that although FFA findings are helpful in illustrating the inflammatory processes and anatomic changes within the retina and the vessels, generally the FFA patterns are not diagnostic or pathognomonic for any particular intraocular inflammatory disease. Posterior uveitic entities where FFA is particularly indicated are birdshot retinochoroidopathy, geographic helicoid peripapillary choroiditis, acute posterior multifocal placoid
pigment epitheliopathy, multiple evanescent white dot syndromes, multifocal choroiditis, sympathetic ophthalmia and VKH syndrome (early stage). FFA can also distinguish a macular retinitis or choroiditis from central serous choroidopathy or choroidal neovascular membrane.
Indocyanine Green Angiography48 The choroid is composed of vascular elements and is involved in majority of chorioretinal vascular disorders. FFA provides information regarding the alterations in the blood-aqueous barrier at the level of the retinal vessels and the retinal pigment epithelium in intraocular inflammatory conditions. However, it is unable to describe the choroid, as the absorption and emission of photonic fluorescein energy in the blue-green wavelength range is impaired by the retinal pigment epithelium. Indocyanine green angiography (ICGA) uses the ICG molecule, which absorbs and emits photonic fluorescein energy in the near infrared wavelength range, which penetrates melanin pigment, hemorrhage, macular xanthophyll pigment and other obstacles such as turbid fluids.20 These characteristics allow imaging of the normal and disturbed choroidal and retinal circulations as well as the normal and disturbed fluid distribution in the choroid. ICGA can aid in the diagnosis of Behçet’s disease, sarcoidosis, tuberculosis, birdshot retinochoroidopathy toxoplasmic retinochoroiditis, acute posterior multifocal placoid pigment epitheliopathy, multiple evanescent white dot syndromes, multifocal choroiditis, VKH syndrome and sympathetic ophthalmia.. Different patterns of ICG fluorescence have been identified during examinations of patients with similar disorders. Fardeau and coworkers 49 established a precise ICGA semiology in 52 patients with Birdshot retinochoroidopathy. In another
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Diagnostic Procedures in Ophthalmology landmark article by Oshima et al, 50 ICGA was performed in 20 eyes with VKH syndrome and the findings suggested a transient choroidal circulatory disturbance during the acute stage. A standardized procedure for combining FFA and ICGA has been found to be more useful.
Ultrasound B-scan ultrasonography is used most commonly in patients with uveitis to investigate inflammatory choroidal and scleral thickening that can occur with VKH syndrome (Fig. 20.7), posterior scleritis and sympathetic ophthalmia20and to evaluate the posterior segment in patients with dense cataracts or other media opacities. It is also useful in detecting exudative retinal detachment, detachment of choroid, evaluation of the ONH and thickening of macula (edema). It is useful in detection of panophthalmitis and scleritis where classically “T-sign” is present. Ultrasound has a very important role in the management of endophthalmitis to determine its severity and extent of infection. USG is also useful in diagnosis of granuloma and abscess in tuberculosis, cysts with scolex in cysticercosis and nematode infection.
Fig. 20.7: Ultrasound B-scan with vector A-scan showing gross choroidal thickening in a patient with VKH syndrome
Ultrasound Biomicroscopy51,52 Conventional ultrasound use frequencies in 10 MHz range. The use of ultrasound frequencies in the 50-100 MHz range is a relatively new development in the ultrasound imaging of the eye. Ultrasound biomicroscopy (UBM) is a new tool available for evaluation inflammation in areas, which are not visualized clinically. It can be used to study up to the anterior 4-mm of the globe. In conditions like small undilating pupil due to posterior synechiae with or without complicated cataract this modality is extremely useful in identifying the presence of inflammation in the area of the pars plana (Fig. 20.8). It has also been used in ciliary body metastatic tumors, which can masquerade as uveitic entities,12 (Figs. 20.9 and 20.10) and for the diagnosis and management of pars planitis caused by caterpillar hair.53
Optical Coherence Tomography Optical Coherence Tomography (OCT) is a noncontact, non-invasive imaging technique used to obtain high-resolution cross-sectional
Fig. 20.8: UBM photograph showing a membrane (m) over the pars plana region (arrows denoting the extent of membrane) with vitreous exudates suggestive of pars planitis in a patient with complicated cataract obscuring retinal view
Diagnostic Procedures in Uveitis
Fig. 20.9: Metastatic Lesion: Slit-lamp photograph showing anterior chamber reaction and fluffy exudates on the superior iris
Fig. 20.10: UBM image showing cystic metastatic lesions in the ciliary body region
images of the retina. It is analogous to ultrasound B-scan imaging except that light rather than sound waves are used in order to obtain a much higher longitudinal resolution of approximately 10 μm (axial) and 20 microns (transverse) in the retina.54 Its resolution is 8-25 times greater than
any sonic modality. Newer fourth generation OCT uses a femtosecond laser light source and has achieved an axial resolution of 3 microns. Spectral OCT and the en-face OCT are newer developments. Among the newer modalities of investigations OCT has come to stay and is a very useful supplement to conventional techniques. Macular changes can occur in various forms of uveitis and can be studied clinically by slitlamp biomicroscopy, indirect ophthalmoscopy or by fundus flourescein angiography. Macular edema and its sequelae are among the leading causes of decreased vision in patients with uveitis. Other changes that can occur in the macula due to uveitis include: serous retinal detachment at the macula, epiretinal membrane, macular hole, vitreomacular traction (tractional retinal detachment), and choroidal neovascular membrane. Uveitic conditions are by nature recurrent and hence the patients have to be followed-up at frequent intervals. OCT being a non-invasive technique has the advantage of repeatability. OCT thus is helpful in not only diagnosing but in follow-up of patients at regular intervals with treatment. It is helpful in the management of intraocular inflammation as it is able to define the extent, depth and thickness of the inflammatory lesion. It is also helpful in localizing the layer of retina and choroid harboring lesion. This localization is helpful in not only diagnosing the disease but also in monitoring the response to treatment – for example cystoid macular edema is a classical complication of ocular inflammation. It results from either a rupture of the inner or outer blood ocular barrier. OCT can detect precisely even very minimal amount of fluid in particular layers of retina. OCT can be more helpful in detecting subtle macular edema which may not be detected on FFA.55-57 Macular edema is an important cause
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A A
B B Figs 20.11A and B: A Pretreatment OCT picture showing large cystoid spaces at the macula. B Posttreatment OCT picture of the same patient showing resolution of macular edema with restoration of foveal contour
of defective vision due to various uveitic conditions especially intermediate uveitis. Early detection and treatment is important (Figs 20.11A and B) because it can lead to complications and vision loss. We have found intermediate uveitis to be the commonest cause of macular edema. OCT is extremely sensitive in identifying neurosensory retinal elevation because of the distinct difference in optical reflectivity between photoreceptors and underlying RPE/choriocapillaris. It can detect the presence of shallow subretinal fluid or macula involvement in retinal detachment. OCT is helpful in the early diagnosis as well as resolution of shallow retinal detachment (Figs 20.12A and B). It can also be used to distinguish true retinal detachment from retinoschisis. OCT appearance of CNVM is described as a bump with moderate slope extending upward or as a fusiform thickening with disruption of the reflective band. OCT is useful before planning
Figs 20.12A and B: A OCT picture showing neurosensory detachment at the macula, B OCT picture showing posttreatment resolution
macular surgery for removal of subfoveal CNV especially in patients with presumed ocular histoplasmosis syndrome and multi-focal choroiditis. Following patterns of subfoveal CNVM can be made out with the help of OCT: • Reflective band anterior to and clearly separated from the RPE. • Highly reflective red band anterior to and adherent to the RPE, similar to the bump • Highly reflective band indistinguishable from the RPE. Submacular surgery results have shown that eyes for which OCT reveals the triad of hyperreflective tissue with anterior location, a separation zone and an underlying optically clear zone, surgical removal may represent a reasonable alternative.
Color Vision Testing Color vision testing serves as an objective measure of optic nerve dysfunction. In addition, patients with birdshot choroidoretinopathy can develop color vision loss disproportionate to the visual
Diagnostic Procedures in Uveitis acuity or fundus finding, presumably reflecting the widespread outer retinal dysfunction.
Visual Field Testing Visual field testing (VFT) is an important ancillary investigation in uveitic entities, especially in posterior uveitis like serpiginous choroiditis, multifocal choroiditis, birdshot retinochoroidopathy, Behçet’s disease, sarcoidosis, toxoplasmic retinochoroiditis, acute posterior multifocal placoid pigment epitheliopathy and multiple evanescent white dot syndromes. It can document the progression of a disease process involving the retina which manifests as well-demarcated scotomas. De Courten and co-workers58 discussed the potential role of computerized visual field testing for appraisal and follow-up of patients with birdshot retinochoroidopathy. VFT can also categorize the patients developing field loss secondary to steroid induced glaucoma.
Audiometry Audiometry can record the extent of hearing loss seen in VKH syndrome and syphilis.
Radiological Studies The sacroiliac joint is inflamed in 60% to 90% of patients with HLA-B27 related uveitis. Plain radiographs are quite useful in demonstrating the inflammatory narrowing of the sacroiliac joint. CT-scan and MRI offer increased sensitivity for documenting sacroiliitis, but are more expensive and indicated in selected cases. Chest X-ray is indicated in sarcoidosis and tuberculosis while cases with ankylosing spondylitis need radiograph of sacroiliac joint.
Radionucleotide Studies Intravenously injected gallium-67 localizes to normal liver, spleen and bone, as well as areas
of active inflammation, such as inflamed lymph nodes, parotid and lacrimal glands, and joints. Although any cause of inflammation can produce a positive test, the gallium scan is used frequency to identify pulmonary hilar gland or lacrimal, parotid and submandibular gland inflammation, as in the case of sarcoidosis.
Lumbar Puncture Lumbar puncture (LP) is most often used in patients with suspected intraocular lymphoma. LP should be done after a complete neurological evaluation and imaging procedure like CT and/ or MRI-scan to avoid unexpected shifting of intracranial contents. LP is also used in selected cases to test suspected meningitis due to syphilis, tuberculosis, toxoplasmosis, cryptococcal infection and coccidiomycosis.
Skin Testing Purified protein derivative of tuberculin (Mantoux test): Mantoux test is a non-specific test. Skin testing involves intradermal injection of 0.1 ml of antigen to elicit a delayed type hypersensitivity response indicative of prior exposure. Normally all patients with panuveitis are tested for tuberculosis (TB) with 0.1 ml of 5 units of purified protein derivative (PPD). This includes patient with a prior Bacillus CalmetteGuerin (BCG) vaccination or a distant history of tuberculosis. Although a positive test does not reflect tubercular activity, a negative test often rules out a tubercular focus in the body. Due to the prior exposure to TB, a large number (60 to 90%) of healthy adults have a positive PPD skin test in India. Therefore, there are high possibilities of false positive results. Hence, all patients of suspected ocular tuberculosis should be evaluated by associated findings like X-ray chest showing pulmonary tuberculosis. A positive Mantoux test in a case of granulomatous
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Diagnostic Procedures in Ophthalmology anterior uveitis with poor general health or in a nonresponder to systemic steroids is of greater significance. Aqueous samples should be subjected to PCR study for Mycobacterial tubercular genome in such cases. Patients with active Behçet’s syndrome occasionally show increased dermal sensitivity, termed pathergy, which is manifestated by formation of local pustule in response to intradermal injection. This test is of limited sensitivity even in the active phase of the disease.
References 1. Baarsma GS, La Hey E, Glasius E, et al. The predictive value of serum-angiotensin converting enzyme and lysozyme levels in the diagnosis of ocular sarcoidosis. Am J Ophthalmol 1987;104:211-17. 2. Power WJ, Rodriquez A, Perroze-Seres M, et al. The value of combined serum-angiotensin converting enzyme and gallium scan in diagnosing ocular sarcoidosis. Ophthalmology 1995;101:2007-11. 3. Weinreb RN, Sandman R, Ryder MI, et al. Serum-angiotensin converting enzyme activity in human aqueous humor. Arch Ophthalmol 1985;103:34-36. 4. Allansmith MR, Skaggs C, Kimmuras SJ. Anterior chamber paracentesis: Diagnostic value in postoperative endophthalmitis. Arch Ophthalmol 1970;84:745-48. 5. Foster RK. Etiology and diagnosis of bacterial postoperative endophthalmitis. Ophthalmology 1978;85:320-24. 6. Witmer R. Clinical implications of aqueous humor studies in uveitis. Am J Ophthalmol 1978;86:39-42. 7. Benitez del Castillo JM, Herreros G, Guillen JL et al. Bilateral ocular toxocariasis demonstrated by aqueous humor enzyme linked immunosorbent assay. Am J Ophthalmol 1996;119:51-54. 8. O’Connor GR. Precipitating antibody to toxoplasmosis in blood and aqueous humor. Am J Ophthalmol 1957;75:44-48. 9. Michelson JB, Chisari FV, Kansu J. Antibodies to oral mucosa in patients with ocular Behcet’s disease. Ophthalmology 1985;92:1277-34.
10. Michels RG, Green WR, Engel HM, et al. Intraocular reticular cell sarcoma:diagnosis by pars plana vitrectomy. Arch Ophthalmol 1975;93:1331-35. 11. Takahashi T, Oda Y, Chiba T, et al. Metastatic tumour of iris and ciliary body simulating as iridocyclitis. Br J Ophthalmol 1984;188:266-69. 12. Woog JJ, Chess J, Albert DM, et al. Metastatic tumour of iris simulating as iridocyclitis. Br J Ophthalmol 1984;188:167-69. 13. Freeman LN, Schachat AP, Knox DL, et al. Clinical features, laboratory investigations and survival in ocular reticulum cell sarcoma. Ophthalmology 1997;94:1631-39. 14. Michelson JB, Whitaker JP, Wilson S, et al. Invisible foreign body granuloma of the retina associated with intravenous cocaine addition. Am J Ophthalmol 1979;87:278-79. 15. Scholda CD, Egger SF, Lakits A, et al. A system for obtaining undiluted intraoperative vitreous biopsy specimen. Arch Ophthalmol 1996;114:127172. 16. Peyman GA. A Pneumovitrector for the diagnostic biopsy of the vitreous. Ophthalmol Surg Laser 1996;27:246-47. 17. Doft BK, Donelly K. A single sclerotomy vitreous biopsy in endophthalmitis. Arch Ophthalmol 1991;109:465. 18. Tamai M, Nakazawa M. A collection system of obtaining vitreous humor in clinical cases. Arch Ophthalmol 1991;109:465-66. 19. Smiddy WE, Michels RG, de Bustros, de la Cruz Z, Green WR. Histopathology of tissue removed during vitrectomy for impending macular holes. Am J Ophthalmol 1989;108:360-64. 20. Ben Erza D. Diagnostic intraocular interventions. In: Ben Erza D (Ed). Ocular inflammation: Basic and clinical concepts. Martin Dunitz,1999;83-89. 21. Crofty TB, Campbell RJ. Flow cytometry. Ophthalmol Clinic North Am 1995;8:37-46. 22. Ma PE, Peyman GA. Uveal biopsy. In: William T, Jaeger EA (Eds). Duannes Clinical Ophthalmology. Lippincott Williams and Wilkins, 1998.Vol 4,Ch 36,1-13. 23. Moorthy RA, Rao NA, Sidikaro Y, et al. Coccidioidomycosis iridocyclitis. Ophthalmology 1994;101:1923-28. 24. Kirmani MH, Thomas EL, Rao NA, et al. Intraocular reticular cell sarcoma: diagnosis by choroidal biopsy. Br J Ophthalmol 1987;71:74852.
Diagnostic Procedures in Uveitis 25. Peyman GA, Charles H. Internal eye wall resection in the management of uveal melanoma. Can J Ophthalmol 1988;23:219-24. 26. Peyman GA, Juarez CP, Raichand M. Fullthickness eye wall biopsy: Long-term results in 9 patients. Br J Ophthalmol 1981;65:722-25. 27. Peyman GA, Axelrod AJ, Graham RO. Fullthickness eye wall resection: An experimental approach for treatment of choroidal melanoma: Evaluation of cryotherapy, diathermy and photocoagulation. Arch Ophthalmol 1974;91: 21930. 28. Constable IJ, Chester GH, Horne R, et al. Human chorioretinal biopsy under controlled systemic hypotensive anaesthesia. Br J Ophthalmol 1980;64:559-63. 29. Peyman GA, Diamond JG, Axelrod AJ. Sclerochorio-retinal (SCR) resection in humans. Ann Ophthalmol 1974;6:1347-49. 30. Peyman GA, Nelson PT, Axelrod AJ. Fullthickness eye wall resection: Evaluation of preoperative photocoagulation. Invest Ophthalmol 1973;12:262-66. 31. Peyman GA, Mary DR, Ericson ES, et al. Fullthickness eye wall resection: An experimental approach for the management of choroidal melanoma II. Homo- and heterograft. Invest Ophthalmol 1973;11:668-72. 32. Culbertson WW, Blumenkranz, Pepose JS, et al. Varicella zoster virus is a cause of acute retinal necrosis. Ophthalmology 1986;93:559-69. 33. Cote MA, Rao NA. The role of histopathology in the diagnosis and management of uveitis. Int Ophthalmol 1990;14:309-16. 34. Gregor RJ, Chong CA, Augsburger JJ, et al. Endogenous Nocardia asteroides subretinal abscess diagnosed by transvitreal fine needle aspiration biopsy. Retina 1989;9:118-21. 35. Shanmugam MP, Biswas J. Fine needle aspiration biopsy in the diagnosis of intraocular lesions. Indian J Ophthalmol 1997;45:105-08. 36. Glasgow BJ, Straatsma BR, Kreiger AE. Fineneedle aspiration of the posterior segment intraocular tumors. Ophthalmol Clinic North Am 1995;8:59-72. 37. Glasgow BJ, Brown HH, Zargoza AZ, et al. Quantification of tumor seeding from the needle aspiration of ocular melanomas. Am J Ophthalmol 1988;105:538-56. 38. Freeman WR, Wiley CA. In situ nucleic acid hybridization. Surv Ophthalmol 1989;34:187-92.
39. Henderly DE, Atalla LR, Freeman WR, Rao NA. Demonstration of cytomegalovirus retinitis by in situ DNA hybridization. Retina 1988;8:17781. 40. Della G. Molecular biology in ophthalmology: review of principles and recent advances. Arch Ophthalmol 1996;114:457-63. 41. Adleberg JM, Wittwer C. Use of the polymerase chain reaction in the diagnosis of ocular disease. Current Opinion in Ophthalmology 1995;6:III:8085. 42. Chan CC, Palestine AG, Li Q, Nussenblatt RB. Diagnosis of Ocular toxoplasmosis by the use of immunocytology and the polymerase chain reaction. Am J Ophthalmol 1994;117:803-05. 43. Therese KL, Anand AR, Madhavan HN: Polymerase chain reaction in the diagnosis of bacterial endophthalmitis. Br J Ophthalmol 1998;82:1078-82. 44. Biswas J, Therese L, Madhavan HN. Use of polymerase chain reaction (PCR) in the detection of Mycobacterium tuberculosis complex DNA from aqueous sample of suspected tubercular uveitis. Uveitis Today. Proceedings of the Fourth International Symposium on Uveitis, Yokohama, Japan, 10-14th October 1997, pp. 227-230, 1998. 45. Gupta V, Arora S, Gupta A, Ram J, Bambery P, Shegal S: Management of presumed intraocular tuberculosis: possible role of the polymerase chain reaction. Acta Ophthalmologica Scandinavica 1998;47:679-82. 46. Madhavan HN, Therese KL, Gunisha P, Biswas J. Polymerase chain reaction for the detection of Mycobacterium tuberculosis in epiretinal membrane in Eales' disease. Invest Ophthalmol Vis Sci 2000;41:822-25. 47. Biswas J, Therese L, Madhavan HN: Use of polymerase chain reaction in detection of Mycobacterium tuberculosis complex DNA from vitreous sample of Eales’ disease. Br J Ophthalmol 1999;83:994. 48. Ruiz-Moreno JM, Ben Erza D. Indocyanine green angiography in uveitis. In: Ben Erza D (Ed). Ocular inflammation: Basic and clinical Concepts. Martin Dunitz, 1999;91-102. 49. Fardeau C, Herbert CP, Kullmann N, et al. Indocyanine green angiography in Birdshot choroidoretinopathy. Ophthalmology 1999;106: 1928-34.
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of patients with uveitic macular edema. Ophthalmology 2004;101(12):1181-88. 56. Antcliff RJ, Stanford MR, Chauhan DS, Graham EM, Spalton DJ, Shilling JS, Ffytche TJ, Marshal J. Comparison between optical coherence tomography and fundus fluorescein angiography for the detection of cystoid macular edema in patients with uveitis. Ophthalmology 2000; 107(3):593-99. 57. Markomichelakis NN, Halkiadakis I, Pantelia E, Peponis V, Patelis A, Theodossiadis P, Theodossiadis G. Patterns of macular edema in patients with uveitis: qualitative and quantitative assessment using optical coherence tomography: Ophthalmology 2004;111(5): 94653. 58. De Courten C, Herbort CP. Potential role of computerized visual field testing for the appraisal and followup of Birdshot choroidoretinopathy. Am J Ophthalmol 1998;106:430-36.
Retinopathy of Prematurity: Diagnostic Procedures and Management
YOG RAJ SHARMA, DEEPENDRA VIKRAM SINGH, NIKHIL PAL, RAJANI SHARMA
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Retinopathy of Prematurity: Diagnostic Procedures and Management
The improved survival rate of extremely premature infants has indirectly led to increase in the incidence of retinopathy of prematurity. Ophthalmologists are now being required to examine these premature babies with greater frequency. The understanding of pathogenesis, screening and management of retinopathy of prematurity (ROP) has markedly changed since Terry first described it.1 The Multicenter Trial of Cryotherapy for Retinopathy of Prematurity (Cryo-ROP) 2-5 and lately, Early Treatment Retinopathy of Prematurity (ETROP) study6 have influenced the management of ROP.
Etiology Retinopathy of Prematurity is the result of abnormal development of immature retinal vessels capable of progressing to a vasoproliferative retinal disorder. ROP can result in severe visual impairment and has been reported to attribute to as much as 40% of the perinatal blindness.
reach nasal ora serrata by 36 weeks of gestation and the temporal ora serrata by 39 to 41 weeks of gestation. The interruption of this normal vasculogenesis leads to development of retinal ischemia and ROP. The location of this interruption which is related to time of premature birth determines the development of various stages of ROP.
Risk Factors The multiple factors that are associated with the severity of ROP are: low birth weight, young gestational age, non-black race, multiple birth, prolonged elevation of arterial oxygen levels, hypoxemia, hypercarbia, hypocarbia, respiratory distress syndrome, apnea, erythrocyte transfusions, sepsis, intraventricular hemorrhage (IVH), prolonged parentral nutrition, methylxanthine administration, and treatment with indomethacin.7-12
Classification and Staging
Arrested Vasculogenesis
Zones
During normal retinal development, the vessels start at optic disk at approximately 16 weeks of gestation and migrate towards ora serrata. They
The coordinated developmental sequence permits the retina to be subdivided in to 3 concentric zones.
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of vascularized and avascular retina. The demarcation line can develop in any zone depending upon the level of prematurity and very premature babies can have it nasally only.
Zones II and III are circular extensions to the area encompassed by Zone-I, while the zone-III being the “residual crescent of the retina anterior to zone-II. The periphery of zone-II in the nasal portion of the retina is the ora serrata, but in the temporal portion, the junction of zones II and III cannot be accurately defined clinically. Thus, all 3 retinal zones are derived from a spatial coordinate system centered on the optic disk (Fig. 21.1).
Stage 2: Demarcation ridge The demarcation line progresses to ridge which is pink or white elevation of the thickened tissue. Some neovascular tufts can be seen posterior to this ridge. Stage 3: Extraretinal fibrovascular proliferation Neovascular growth occurs into and above the ridge. The vessels also grow into the vitreous and can lead to vitreous hemorrhage (Fig. 21.2).
Fig. 21.1: Standardized zones of retina used for classification and documentation of ROP
Documentation International Classification of ROP The International Classification of Retinopathy of Prematurity (ICROP) was a consensus statement of an international group of retinopathy of prematurity experts.13 The original classification has facilitated the development of large multicenter clinical treatment trials and furthered our understanding of this potentially blinding disorder. The different stages described by ICROP are as follows:
Stage 1: Demarcation line Earliest feature of ROP in a premature baby is the development of a flat white line at the junction
Fig. 21.2: Stage 3 ROP: 6 clock hours of extraretinal neovascularization with demarcation ridge inferiorly
Stage 4: Partial retinal detachment With progressive growth into the vitreous, contraction of the fibrovascular proliferation exerts traction on the retina, leading to partial retinal detachment (Stage 4 ROP), either without foveal involvement (Stage 4A) (Figs 21.3 and 21.4) or with foveal involvement (Stage 4B). Stage 5: Total retinal detachment These retinal detachments are always funnelshaped and their configuration can further be described as open and closed anteriorly and open or closed posteriorly (Fig. 21.5).
Retinopathy of Prematurity: Diagnostic Procedures and Management
Fig. 21.3: Early Stage 4A ROP: Traction along the ridge with peripheral retinal detachment
Fig. 21.6: Zone-I ROP with plus disease
Plus Disease and Rush Disease
Fig. 21.4: Stage 4A ROP: Tractional retinal detachment not involving macula
Plus disease is defined as dilated and tortuous blood vessels at posterior pole along with pupillary rigidity and media haziness (Fig. 21.6). Rush disease is defined as rapid progression through the three stages of ROP, with plus disease and retinal detachment occurring within a few weeks.14 It often occurs in the zone-I and the smallest babies are frequently affected. Failures of treatment are highest in this group and hence such eyes need prompt and aggressive management. Recently, an international group of pediatric ophthalmologists and retinal specialists has developed a consensus document that revises some aspects of ICROP.15 The aspects that differ from the original classification include introduction of (1) the concept of a more virulent form of retinopathy observed in the tiniest babies (aggressive, posterior ROP), (2) a description of an intermediate level of plus disease (pre-plus) between normal posterior pole vessels and frank plus disease, and (3) a practical clinical tool for estimating the extent of zone-I.
Threshold ROP (Figs 21.7A and B) Fig. 21.5: Stage 5 ROP: Total retinal detachment with white reflex
Threshold ROP is defined as Stage 3 + ROP in zone-I or -II occupying at least 5 contiguous clock
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B
A
Figs 21.7A and B: Threshold ROP. A prelaser and B postlaser ablation
hours or 8 noncontiguous clock hours of retina. The Cryo-ROP trial found that 62% of the untreated eyes as compared to 44% of the treated eyes with threshold ROP progressed to unfavorable outcome5 (Table 21.1).
Prethreshold ROP Prethreshold ROP is defined as any stage of ROP in zone-I with plus disease and ROP stage 3 plus with 3 contiguous or 5 noncontiguous clock
hours of involvement of retina in zone-II, but less than threshold stage. ROP may not progress through all these stages sequentially. ROP in zone-I frequently progresses to stage 3 without an intervening demarcation line or ridge. With advancements in neonatal support and intensive care units and increasing availability of screening modules more and more prematures are being diagnosed ROP in zone-I, which progresses at a faster rate and almost always to threshold stage. Therefore,
TABLE 21.1: CATEGORIES OF STRUCTURAL OUTCOME5 Favorable
Unfavorable
(1) Essentially normal posterior pole (near periphery and zone-I), including angle of vessels (2) Abnormal angle of major temporal vascular arcade in the posterior pole (3) Macular ectopia (4A) Stage 4A partial retinal detachment, also including retinoschisis, or fold in the posterior pole (fovea spared)
(4B) Stage 4B partial retinal detachment, also including retinoschisis, or fold, all with foveal involvement (4C) View of macula (and presumably patient’s central vision) blocked owing to partial cataract, partial retrolental membrane, or partial corneal opacity due to retinopathy of prematurity (ROP) (5) Stage 5 total retinal detachment, or total retinoschisis, or retrolental membrane (blocking all view of fundus) (5A) Entire view of posterior pole and near periphery blocked by total cataract or total corneal opacity from ROP. (6) Enucleation for any reason
Unable to grade (UG) Unable to determine (e.g. view impossible because of corneal opacity unrelated to ROP or because of miotic pupil) None of the above (e.g. extreme vascular attenuation and optic atrophy)
Retinopathy of Prematurity: Diagnostic Procedures and Management zone-I eyes and high risk zone- II eyes should be treated earlier. This issue was addressed in the multicenter study of Early Treatment for Retinopathy of Prematurity (ETROP). 6 Comparison between untreated eyes, high risk pre-threshold eyes treated early, and high-risk eyes treated at threshold demonstrated that retinal ablative therapy is beneficial for preventing unfavorable outcome. The ETROP concluded that the early treatment can be considered for Type 1 ROP defined as; (a) Any eye that has any stage of ROP in zone-I with plus disease, (b) Stage 3 ROP in zone-I with or without plus disease and (c) Stage 2 or 3 ROP in zone-II with plus disease.
Screening for ROP The natural history data from the CRYO-ROP and other studies were combined to answer the question of when to begin and conclude screening for acute ROP (Table 21.2).
Screening Procedure • Screening is best done at Neonatal ICU along with trained neonatology staff to monitor vital parameters during examination. • Mydriasis can be achieved by 2.5% phenylephrine and 0.5% tropicamide instilled thrice at an interval of 15 mins. • Instruments required include: 28 D/20 D lens, pediatric scleral depressor, pediatric lid speculum (Fig. 21.8) and indirect ophthalmoscope. • Since examination with lid speculum and scleral depressor is often distressing to the infant, presence of a pediatrician is extremely useful. • Examination should be carried out with utmost gentleness and minimal possible illumination. A quick examination of the posterior pole gives impression whether the plus disease is present or not. Screening all along the 4 major blood vessels in four quadrants up to the retinal periphery should be carried out.
TABLE 21.2: GUIDELINES FOR SCREENING AND FOLLOW-UP EXAMINATION Screening criterion
• • •
First examination
By 32 weeks postmenstrual age or 4 weeks chronological age whichever is earlier
Follow-up
Final examination
All infants with a birth weight < 1500 gm All infants born at postmenstrual age of 32 weeks or earlier All infants weighing between 1500 and 2000 gm requiring supplemental oxygen, or with an unstable clinical course
48-72 hours
(a) After treating threshold ROP (b) High risk prethreshold ROP (consider treatment if in zone-I)
Weekly
(a) Retinal vessels immaturity with vessels ending in zone-I but no ROP in that zone (b) Low risk prethreshold ROP
Fortnightly
(a) Retinal vessels immaturity with vessels zone-III but no ROP in that zone (b) Less than prethreshold ROP in zone-I
ending
in
zone-II
or
(a) Attainment of 45 weeks post-menstrual age without development of ROP (b) Progression of retinal vascularization into zone-II without previous zone-II ROP (c) Full vascularization within 1 disk diameter of the ora serrata on two occasions
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A C B Figs 21.8A to C: The instruments required for screening: A scleral depressor, B speculum, C condensing lens for indirect ophthalmoscope
Retcam The Retcam (Fig. 21.9) is a real time wide-angle (120-130 degrees) digital imaging system for
viewing pediatric eyes manufactured by Massie Research Laboratories, Dublin CA. Retcam fills the need for wide-field imaging and is fully digital enabling efficient assessment and monitoring. Nearly the entire retina is documented with only five images. Real-time imaging display provides immediate feedback. Inexpensive digital image storage eliminates film. One is able to retrieve and manage patient information with built-in image database and also transmit images to colleagues. Retcam II is the latest addition to the series with newer benefits like flat LCD color display, frame by frame video review and 20 second video capture especially useful for fluorescein angiography.
Intervention for ROP Cryotherapy
Fig. 21.9: Retcam
The most detailed and comprehensive data regarding the safety and efficacy of ROP was made available by the multicenter trial of Cryotherapy for Retinopathy of Prematurity (CRYO-ROP) study.2-5 The study was carried out in 23 centers across USA. CRYO ROP results indicated an unfavorable outcome in 25.7% of the eyes that received cryotherapy compared with 47.4 % of the control eyes. Though the data signify a definite advantage of treatment over no treatment but the rate of 25% blindness is still very high.
Retinopathy of Prematurity: Diagnostic Procedures and Management Laser Ablation Laser treatment applied through laser indirect ophthalmoscope (LIO) has become the method of choice for treatment of threshold and high risk prethreshold ROP.16-19The cryotherapy is also effective in decreasing the incidence of unfavorable outcome, but laser has following advantages: 1. The laser is more precise than cryo in treating the retina, especially for the areas near the ridge and thus reduces the risk of vitreous hemorrhage.20 2. Laser is less painful and allows treatment under a topical anesthesia with sedation and monitoring. 3. Laser leads to lesser dispersion of the retinal pigment epithelial cells and less or no breakdown of blood-retinal barrier.21 4. Laser photoablation seems to be particularly effective for zone-I disease.20 The diode laser (810 nm) has the advantage of being portable so that it can be easily transported to the neonatal intensive care unit.22 The treatment is carried out in presence of a trained neonatologist, who monitors the infant. An intravenous infusion maintains the hydration of the infant. Adequate sedation is achieved by administration of oral sedative one hour prior to the surgery. The treatment is applied in a near confluent pattern with moderate intensity burns placed half burn apart covering the whole avascular area.16 However, new vessels are not to be treated. The combination of 20 D and 28 D lenses with scleral indentation is utilized to approach different areas. Continuous monitoring by a trained neonatologist is essential during laser ablation. Any suspicion of severe apneic episode should lead to curtailment of the treatment, which can be planned at a later date. It is not infrequent to find large skip areas in the eyes treated by beginners, so all attempts should be made to
do maximum laser photocoagulation in the entire avascular area. Reevaluation after 48 hours to assess the disease progression and the adequacy of the photocoagulation is most vital. Presence of plus disease and of skip areas usually guides the advisability of the supplementary treatment. The infants showing definite signs of regression like disappearance of plus disease can safely be followed-up after a week. Other infants should be reexamined after 3 days. It is to be stressed that while more than 90% of cases with ROP will regress, a significant percentage of cases will keep progressing despite laser therapy.23 Complications of laser treatment are few. Besides systemic problems like apneic episodes, ocular side effects include iris atrophy, posterior synechiae and cataract.24
Surgical Intervention Scleral Buckling The buckling is done for stage 4A ROP. After performing 360-degree limbal peritomy, a band is passed under the four recti and tied. Indirect ophthalmoscopic examination is done to ensure adequate retinal and choroidal perfusion and position of the buckle. Care should be taken to avoid pulling the band too tight. Within a year of surgery, the scleral band is divided or removed to permit growth of the globe and orbit. Buckling does reduce the progression of stage 4 to stage 5 ROP.25, 26
Vitrectomy Pars plana vitrectomy is being increasingly utilized to manage advanced ROP cases.27-31 Although recent reports describe encouraging anatomical results, the functional results have been disappointing so far.28, 30 The vitrectomy is usually performed for stage 5 ROP.27 The lens sparing vitrectomy for stage 4a and 4b has also
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Visual Rehabilitation and Parental Counseling Management of ROP involves not only a proper follow-up of neonates with prompt laser ablation at the required stage and/or vitreoretinal surgery, but also a proper refraction including low vision aid assessment. As well, parents need to be educated about the severity of the disease and to cope with psychosocial issues in children disabled due to ROP.
Conclusion ROP is becoming a major cause of blindness among children worldwide because of the introduction of the neonatal intensive care
services for preterm and low-birth-weight babies. The future challenge is to make accessible to these infants laser, cryo or surgical treatment. To conclude, recent surgical advances have made ROP from untreatable to manageable in most cases. The investigators need to focus on improving the surgical techniques for stage 4 and stage 5 ROP, preventing the prematurity and preventing the development of ROP.
References 1. Terry TL. Extreme prematurity and fibroblastic overgrowth of persistent vascular sheath behind each crystalline lens. Am J Ophthalmol 1942;25:20304. 2. Cryotherapy for Retinopathy of Prematurity Cooperative Group. Multicenter Trial of Cryotherapy for Retinopathy of Prematurity: preliminary results. Arch Ophthalmol 1988; 106:471-79. 3. Cryotherapy for Retinopathy of Prematurity Cooperative Group. Multicenter Trial of Cryotherapy for Retinopathy of Prematurity: one-year outcome—structure and function. Arch Ophthalmol 1990;108:1408-16. 4. Cryotherapy for Retinopathy of Prematurity Cooperative Group. Multicenter Trial of Cryotherapy for Retinopathy of Prematurity: Snellen visual acuity and structural outcome at 5½ years after randomization. Arch Ophthalmol 1996;114:417-24. 5. Cryotherapy for Retinopathy of Prematurity Cooperative Group. Ophthalmological outcomes at 10 years. Arch Ophthalmol 2001; 119:1110-18. 6. Early Treatment for Retinopathy of Prematurity Cooperative Group. Revised indications for the treatment of retinopathy of prematurity: results of the Early Treatment for Retinopathy of Prematurity Randomized Trial. Arch Ophthalmol 121:1684-96. 7. Darlow BA, Horwood LJ. Retinopathy of prematurity: risk factors in a prospective population-based study. Paediatr Perinat Epidemiol 1992,6:62-80.
Retinopathy of Prematurity: Diagnostic Procedures and Management 8. Flynn JT, Bancalari E, Snyder ES, et al. A cohort study of transcutaneous oxygen tension and the incidence and severity of retinopathy of prematurity. N Engl J Med 1992;326:1050-54. 9. Hammer ME, Mullen PW. Logistic analysis of risk factors in acute retinopathy of prematurity. Am J Ophthalmol 1986;102:1-6. 10. Palmer EA, Flynn JT, Hardy RJ, et al. Incidence and early course of retinopathy of prematurity. Ophthalmology 1991;98:1628-40. 11. Schaffer DB, Palmer EA, Plotsky DF, et al. Prognostic factors in the natural course of retinopathy of prematurity. Ophthalmology 1993;100:230-37. 12. Shohat M, Reisner SH, Krikler R, et al. Retinopathy of prematurity: incidence and risk factors. Pediatrics 1983;72:159-63. 13. The Committee for the Classification of Retinopathy of Prematurity. An International Classification of Retinopathy of Prematurity. Arch Ophthalmol 1984;102:1130-34. 14. Pierce E, Peterson R, Smith L. Retinopathy of prematurity. In: Principles and Practice of Ophthalmology. Jakobiec A. (Ed). WB Saunders, Philadelphia, PA 2000;4443-59. 15. An International Committee for the Classification of Retinopathy of Prematurity. The International Classification of Retinopathy of Prematurity Revisited. Arch Ophthalmol 2005;123:991-99. 16. Fallaha N, Lynn MJ, Aaberg TM Jr, Lambert SR, Clinical Outcome of Confluent Laser Photoablation for Retinopathy of Prematurity. J AAPOS 2002;6:81-85. 17. Paysse EA, Lindsey JL, Coats DK, Contant CF, Steinkuller PG. Therapeutic outcomes of cryotherapy versus transpupillary diode laser photocoagulation for threshold retinopathy of prematurity. J AAPOS 1999;3:234-40. 18. Connolly BP, McNamara A, Regillo CD, Tasman W, Sharma S. Visual outcomes after laser photocoagulation for threshold retinopathy of prematurity. Ophthalmology 1999;106:1734-8. 19. Connolly BP, McNamara A, Sharma S, Regillo CD, Tasman W. A comparison of laser photocoagulation with trans-scleral cryotherapy in the treatment of threshold retinopathy of prematurity. Ophthalmology 1998;105:1628-31.
20. Hammer ME, Pusateri TJ, Hess JB, Sosa R, Stromquist C. Threshold retinopathy of prematurity.Transition from cryotherapy to laser treatment. Retina 1995;15:486-89. 21. Hunter DG, Repka MX. Diode laser photocoagulation for threshold retinopathy of prematurity. Ophthalmology 1993;100:238-44. 22. McNamara JA. Laser treatment for retinopathy of prematurity. Curr Opin Ophthalmol 1993,4:7680. 23. Hartnett ME, McColm JR. Retinal features predictive of progressive stage 4 retinopathy of prematurity. Retina 2004;24(2):237-41. 24. Ibarra MS, Capone A Jr. Retinopathy of prematurity and anterior segment complications. Ophthalmol Clin North Am 2004;17(4):577-82. 25. Chuang YC, Yang CM. Scleral buckling for stage 4 retinopathy of prematurity. Ophthalmic Surg Lasers 2000;31(5):374-79. 26. Noorily SW, Small K, de Juan E Jr, Machemer R. Scleral buckling surgery for stage 4B retinopathy of prematurity. Ophthalmology 1992; 99(2):263-68. 27. Gopal L, Sharma T, Shanmugam M, Badrinath SS, Sharma A, Agraharam SG, Choudhary A. Surgery for stage 5 retinopathy of prematurity: the learning curve and evolving technique. Indian J Ophthalmol 2000;48(2):101-06. 28. Seaber JH, Machemer R, Eliott D, Buckley EG, deJuan E, Martin DF. Long-term visual results of children after initially successful vitrectomy for stage V retinopathy of prematurity. Ophthalmology 1995;102(2):199-204. 29. Hartnett ME, Maguluri S, Thompson HW, McColm JR. Comparison of retinal outcomes after scleral buckle or lens-sparing vitrectomy for stage 4 retinopathy of prematurity. Retina 2004;24(5):753-57. 30. Prenner JL, Capone A Jr, Trese MT. Visual outcomes after lens-sparing vitrectomy for stage 4A retinopathy of prematurity. Ophthalmology 2004;111(12):2271-73. 31. Hubbard GB, Cherwick DH, Burian G. Lenssparing vitrectomy for stage 4 retinopathy of prematurity. Ophthalmology 2004; 111(12): 227477.
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AMIT NAGPAL, LINGAM GOPAL
22
Localization of Intraocular Foreign Body
Intraocular foreign body (IOFB) is defined as an intraocular retained unintentional projectile. Of all open globe injuries, 18-41% harbor IOFB.1 Most of such injuries occur in the 20-40 years age group. 2 This being the most productive age, the effect on the economy is significant. The most common type of injury associated with IOFB is metal on metal injury exemplified by hammering activity (60-80%). 3-5 Power machine tools contribute to 18-25% of IOFBs and weapon related injuries contribute to about 19%.3-5 Seventy to 90% of IOFBs are metallic and 80% of these are magnetic which has a significant bearing on the management and the ease with which the FB can be extracted from the eye.
Types of Intraocular Foreign Bodies As alluded to, IOFBs can be broadly grouped under metallic and non-metallic. Metallic foreign bodies can be magnetic such as iron and some of its alloys and non- magnetic. Among the nonmagnetic foreign bodies the most important are lead foreign bodies seen in bullet injuries. Brass and other metal pieces can be seen in explosive injuries such as bomb blasts. Glass forms the
most frequently encountered non-metallic foreign body. Other non-metallic foreign bodies include wooden pieces classically seen with broomstick, and caterpillar hair.
History of Injury History can guide the clinician as to the possibility of the IOFB being present in a given eye and as well as the type of IOFB. From the management perspective, it is important to note the type of instrument or tool being used at time of the injury. In cases of metal on metal, the identity of the IOFB is fairly certain. Injuries in a rural set up are likely to be due to thorn and plant twigs and could be associated with high incidence of fungal infection. The findings of the surgeon who examines the patient immediately after the injury would be very important because subsequently visualization of the fundus becomes difficult due to hazy media.
Slit-lamp Examination Thorough slit-lamp examination can provide very useful information.
Localization of Intraocular Foreign Body Cornea
Caveats
Corneal entry wound can often be made out easily. However, fine linear corneal scars can be missed unless one looks for them carefully. The size of the corneal wound is usually smaller than the foreign body since the foreign body traveling at a high velocity is able to stretch the elastic tissues. Presence of localized corneal edema especially inferiorly can be indicative of foreign body lying at 6 o’clock angle.
1. Glass IOFB in anterior chamber can be particularly difficult to see even on slit-lamp examination. 2. Gonioscopy may be the only way to identify a small foreign body in the angle. 3. Foreign body located in the ciliary body area is difficult to identify by both slit-lamp evaluation and fundus examination.
Iris
Fundus Examination
Presence of iris hole is a very important clue to the presence of the IOFB. The relationship between the location of the iris hole and the corneal scar also indicates the direction in which the foreign body was traveling. Iris hole can be hidden under a dense arcus senilis. Presence of siderosis bulbi may be evident on slit-lamp examination of the iris and lens. The rusty brownish hue is striking.
Evaluation of fundus using indirect ophthalmoscopy cannot be over emphasized. Since the injury is likely to produce vitreous hemorrhage and vitritis, the visualization of fundus details may deteriorate very rapidly. Therefore, the initial fundus examination should be as thorough as possible. The documentation of fundus findings made by the first examiner is often valuable for the subsequent ophthalmologist who may be called upon to manage the case. Binocular indirect ophthalmoscopy with scleral indentation may detect IOFB (Fig. 22.1),
Lens Presence of a track of foreign body in the lens can be seen occasionally. However, in most cases lens opacity rapidly becomes total. Intactness of posterior capsule can be assessed sometime clinically and if not possible by slit-lamp examination, then the ultrasound evaluation is recommended. If the vitreous is perceived to be clear and posterior capsule is intact despite lens injury, one can presume that the FB is located in the anterior segment. On occasions, foreign body can traverse across the zonule of the lens without disrupting the lens. Hence corneal wound with presence of intact posterior capsule does not necessarily exclude the possibility of posterior segment foreign body. Intralenticular foreign body can be obvious on slit-lamp examination.
Fig. 22.1: Showing a large metal foreign body on the retina
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Diagnostic Procedures in Ophthalmology if the media are clear enough. In delayed cases, the IOFB may be surrounded by fibrous capsule and could be missed. In case of iron foreign bodies present on the retina for significant period of time, one may find tell tale signs of localized siderosis bulbi. Large glass pieces may evade clinical detection with ophthalmoscopy if they are embedded in the peripheral opaque vitreous following vitreous hemorrhage. Signs in the fundus that may facilitate localization of foreign body: 1. Vitreous track formed by blood may point to the location of foreign body 2. Intra-retinal hemorrhage may indicate the site of impaction of foreign body. On occasion, the foreign body may hit the retina and ricochet to another location or fall down to the inferior periphery. Therefore, inferior periphery should be inspected carefully if foreign body is not located elsewhere. 3. Signs of double perforation of globe indicate the presence of foreign body in the ocular coats posteriorly or even behind the eye in the orbit.
two probes that can be used intraoperatively by covering them with sterile sleeves. One probe has higher penetrance and is used to grossly locate the foreign body to a quadrant. The second probe is more sensitive for precise localization.6
Ultrasonography A combined B- and vector A-scan is the easiest way of evaluating the eye for presence of IOFB7 (Figs 22.2 and 22.3). A 10 Mega hertz (MHz) probe is routinely employed. A 20 MHz probe
Electrical Induction Methods for Localization of IOFB
Fig. 22.2: Ultrasound B-scan showing metallic IOFB near the retinal surface
The Berman and Roper-Hall localizers are purely of historic importance. Induction is a physical phenomenon wherein an alternating current passed through a primary coil will induce current in a secondary circuit. If the voltage in the secondary circuit is equalized, no current flows between them. If such an instrument approaches a metallic foreign body, a difference in potential is created in the secondary circuit resulting in a flow of current. Roper-Hall localizer gives audio signal if foreign body is metallic. A continuous sound is heard for an iron foreign body and a discontinuous sound for a nonferrous metal foreign body. The instrument is provided with
Fig. 22.3: Ultrasound B-Scan showing large piece of metallic IOFB with orbital shadowing
Localization of Intraocular Foreign Body can give higher resolution. For anteriorly located IOFB, one may have to use immersion scan or stand off or use ultrasound biomicroscopy. Since the injured eye could potentially have a wound that can open on pressure, the ultrasonography should be done very gently. For the same reason, ultrasonography is done over closed lids.
Features Foreign bodies are characterized by a high echogenecity. They are seen as dense white spots on gray scale display and persist even at low gain. Depending on the size, reverberating echoes may also be seen. Metal and stone have a high echogenecity, more than any other normal structure except bone. Wood and vegetable matters reflect only intermediate amplitude echoes. Glass gives a high amplitude echoes only when the ultrasound beam strikes the surface of the glass with perpendicular incidence.
Caveats Very large foreign bodies can cause shadowing. Linear glass foreign bodies can sometimes produce misleadingly low amplitude echoes due to the ultrasound beam not being perpendicular to the surface of the foreign body. With regards to foreign bodies in the eye wall, it may be difficult to be certain whether the foreign body is closer to the vitreous cavity or scleral surface. The shadowing caused by the foreign body will make it difficult to assess the integrity of the coats of eyeball. Very anteriorly located foreign bodies and small foreign bodies entrapped in dense vitreous hemorrhage can be missed by routine ultrasonography. Air bubble in the vitreous cavity can mimic IOFB due to the high reflectivity. However, air tends to float in the vitreous cavity and hence is located in the nondependent position irrespective of the position of the head.
Multiple foreign bodies can present bizarre echo patterns. Foreign body with irregular surface can give the impression of multiple foreign bodies. Organic matter degrades with time and the IOFB could be difficult to detect. The injury related damage to the retina, choroid and the vitreous can be detected on ultrasonography. One should look for the presence or absence of choroidal detachment (hemorrhagic or serous), retinal detachment and vitreous detachment. Vitreous incarceration in the posterior coat of the eye indicates possible site of double perforation. In general, echography tends to overestimate the size of an IOFB.
Ultrasound Biomicroscopy Ultrasound biomicroscopy (UBM) is a relatively new investigational modality. Using a 50 MHz probe, the resolution is increased multifold at the expense of penetrance. Foreign bodies located in the anterior segment can be well imaged with this modality. Foreign bodies located beyond the posterior capsule of the lens cannot be reached because of the low penetrance. Since the investigation can only be done in contact with the globe, it is obviously contraindicated in eyes with open globes or precariously approximated wounds. IOFB such as caterpillar hair can only be picked up with UBM.8
Radiological Methods Computerized tomography has replaced most of the other radiological methods in the investigation of injured eye with suspected IOFB. However, from the historical perspective, these methods are reviewed.9
Direct Methods a. Plain X-ray, true lateral view: The affected side is towards the film with infraorbital line at right angles to the film.
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Methods Based on Rotational Movements of the Eye a. Movement of the eye in lateral position: Three exposures are made on the same film arranged for a true lateral view. With the head steadily placed, exposures are made with the eye looking straight, up and down. If the foreign body moves with the eye, three images of the foreign body will be seen. This is indicative of the presence of foreign body in the eye. b. Use of limbal ring: A metallic ring made of either silver or steel of suitable diameter is sutured to the limbus. The same procedure as described above is followed for the lateral view. In addition, a posteroanterior view Xray is also taken. In a perfect true lateral view, the limbal ring is imaged as a straight line corresponding to the limbus. Three such lines will be seen corresponding to the three positions of the eyeball. An outline of the eye can be drawn using the limbal ring as guide. The location of the foreign body can be identified with respect to the outline drawn. Movement of the foreign body with respect to the ring movement also gives a clue as to the location of the foreign body. The posteroanterior view gives the clock meridian location of the foreign body while the lateral view gives the anteroposterior location. Movement of the foreign body with the ring indicates that the FB is within the eyeball while if it does not move it is likely to be extraocular. There are obviously a lot of fallacies
in the interpretation. A foreign body stuck to the eye wall is likely to move to the same extent as the ring while one in the vitreous cavity is likely to move more or less than the limbal ring. Foreign body in the center of rotation of the eye will not move while a foreign body in the extra ocular muscle, although outside the eyeball will move with the contraction of the extra ocular muscle. Suturing a ring to a recently traumatized eye is not a pleasant procedure and could be associated with risk of further damage if the globe has precariously approximated wound. c. Radio opaque markers: Other radio opaque markers that have been used are contact lens with 4 radio opaque dots incorporated in it.
Method Based on Different Angle of Exposure to X-rays a. Sweet’s method: Using two reference markers, one located just in front of the cornea, and the other temporal to it, two exposures of X-ray are taken from two different directions on the same plate. The superimposition of the markers on the plate along with the foreign body permits the localization of the foreign body. However, the calculations are cumbersome. b. Other methods: Similar to Sweet’s method there are others methods such as Mac Kenzie’s method, Dixons’ method, Bromley’s method, and Mc Rigor’s method. None of them are in vogue now.
Use of Contrast Material to Delineate the Globe This method envisages injection of radio opaque dye into the subTenon’s space with a view to delineate the globe surface and use it as a reference to locate the FB. This technique for obvious reasons is obsolete.
Localization of Intraocular Foreign Body Computerized Tomographic Scan Computerized tomographic scan (CT-scan) has replaced all other radiological methods for localization of IOFB. It is noninvasive and does not need placement of any radio opaque marker on or near the injured eye. It images the orbit equally well and hence superior to ultrasonography from that perspective. It enables the localization of the foreign body easily in vitreous (Figs 22.4 and 22.5) and very precisely in the coats of eyeball (Fig. 22.6). Associated damage Fig. 22.6: CT-scan showing a metallic IOFB within the ocular coats with retinal detachment
to the orbital bones and brain can also be evaluated. Multiple foreign bodies can be easily identified. For foreign bodies of more than 0.06 cu mm in size, the sensitivity is 100%.10 However, soft tissue details inside the eye cannot be seen well. For detecting small IOFBs, high-resolution scans with overlapping slices are needed. Wooden foreign bodies are not easily imaged by CT scan.
Magnetic Resonance Imaging (MRI) Fig. 22.4: CT scan showing a piece of metallic wire within the vitreous cavity.
In general, magnetic resonance imaging (MRI) is not indicated in detection of IOFBs. In the presence of magnetic IOFB it can be positively harmful. The application of powerful magnetic field can move the magnetic IOFB and damage the intraocular structures.11 However, wooden IOFBs are best picked up on MRI.
References
Fig. 22.5: CT-scan showing metallic IOFB within mid vitreous cavity
1. Shock JP, Adams D. Long-term visual acuity results after penetrating and perforating injuries. Am J Ophthalmol 1985;100:714-18. 2. Punnonen E, Laatikainen L. Prognosis of perforating eye injuries with foreign bodies. Acta Ophthalmol 1989;66:483-91. 3. Roper-Hall MJ. Review of 555 cases of intraocular foreign bodies with special reference to the prognosis. Br J Ophthalmol 1954;38:65-99.
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Gopal L. Ultrasound biomicroscopy in the diagnosis and management of pars planitis caused by cater pillar hairs. Am J Ophthalmol 2000;130:125-26. 9. Duke-Elder S. System of ophthalmology vol 14 Part I- Mechanical injuries. Intraocular foreign bodies. Henry Kimpton. London 1972;579-611. 10. Chacko JG, Figueroa RE, Johnson MH, Marcus DM, Brooks SE. Detection and localization of steel intraocular foreign bodies using computed tomography. Ophthalmology 1997;104:319-23. 11. Ta CN, Bowman RW. Hyphema caused by a metallic intraocular foreign body during magnetic resonance imaging. Am J Ophthalmol 2000;129:533-34.
Comitant Strabismus: Diagnostic Methods
HARINDER SINGH SETHI, PRADEEP SHARMA
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Comitant Strabismus: Diagnostic Methods
Introduction A strabismus or squint is a misalignment of the two eyes when they do not point together towards the same object. This may take the form of one or other eye turning in (convergent strabismus) or out (divergent strabismus). Occasionally, one eye may be higher than the other (vertical strabismus). The strabismus may be constant (present at all times) or occur only intermittently. In a comitant strabismus there is a full range of movement of each eye.
Incidence It is estimated that a strabismus occurs in about 5% of the population. Most strabismus develops in the first few years of life with the majority appearing either in the first or the third year of life.
Etiology Both eyes are moved by six muscles and the movements of the two eyes are linked by reflexes which are normally fully developed within six months of birth. A strabismus occurs because of the failure of these reflexes to develop fully in early life. In most cases the reason for this failure of the reflexes to develop is unclear. In some cases the development of the strabismus
is related to uncorrected refractive error, trauma and general ill health.
Comitant Strabismus Comitant strabismus is usually congenital. It is not associated with diplopia. Extra-ocular muscles and nerves are often normal. The angle between the longitudinal axes of the eyes remains constant on testing eye movements. Both eyes have full movement if tested separately. There is excess tone in one muscle compared with its antagonist resulting in deviation of the eye.
Types of Comitant Strabismus • Convergent strabismus termed esotropia • Divergent strabismus termed exotropia • A and V syndromes The examination of strabismus requires a few equipments, a list of essential and desirable equipments is given in the Table 23.1.
Examination of a Case of Strabismus The examination of a case of strabismus requires assessment of: 1. The motor status 2. The sensory status
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Diagnostic Procedures in Ophthalmology TABLE 23.1: ESSENTIAL AND DESIRABLE EQUIPMENTS FOR EXAMINATION OF STRABISMUS Essential equipments
Desirable equipments
Prism bars: horizontal and vertical prism bars and loose prisms set, at least 30 and 45 prism diopter prisms Occluder Fixation targets for distance and near to control the accommodation as desired, trial set with prisms of 1 to 8 pd Bagolini’s striated glasses Red and green goggles. Double Maddox rod set Snellen chart with letters and E in rows and a single letter E chart Protractor with a foot ruler Direct ophthalmoscope Randot stereotest
Synoptophore Hess chart or Lees screen Perimeter Indirect ophthalmoscope Teller acuity cards with screen Haidinger brushes and after images attachment for synoptophore Spielmann occluder, translucent or one way reflecting Optico Kinetic Nystagmus (OKN) drum VER Digital camera Electronystagmography and videonystagmography system
Examination of Motor Status The examination of the motor status includes: 1. Head posture 2. Ocular deviation 3. Limitation of movements or the extent of the versions 4. Fusional vergences.
Head Posture Observation of head posture starts at the first glance of the patient, as he enters the clinic. He must not be made conscious of keen observation. Much of information is lost after the patient becomes conscious of being examined. Head posture has three components: (a) Chin elevation or depression (vertical), (b) Face turn to right or left side (horizontal) and (c) Head tilt to right or left shoulder (torsional). These three components at three different joints between head and neck may correct the motility disturbances in the three directions. The patient prefers a head posture at which the ocular deviation is the least, and image can be fused. Rarely, a head posture which causes the maximal deviation is chosen so that the peripheral image can be easily suppressed or ignored.
Common causes of abnormal head posture are tabulated in Table 23.2. TABLE 23.2: CAUSES OF ABNORMAL HEAD POSTURE 1. Incomitant strabismus either paralytic, restrictive, or musculofascial anomalies 2. Comitant strabismus with A and V phenomenon; chin up in a V-exotropia or A-esotropia and chin down in V-esotropia and A-exotropia 3. Nystagmus cases with a null position 4. Under corrected glasses with the peripheral stronger power or a wrong cylinder axis 5. One eyed persons 6. Homonymous hemianopia
Ocular torticollis is a classic example of an abnormal head posture seen in a patient with congenital superior oblique palsy who maintains a binocular vision in spite of the congenital defect. Such cases may present later with their head posture forcibly corrected. An old photograph of the patient helps in diagnosing the condition and rule out a supposedly acute onset. The head posture in a case of left superior oblique (LSO) palsy will present chin depression, face turn to the right, and head tilt to the right shoulder. For comprehension it is told that LSO being a depressor, chin depression occurs, it being an intorter, a head tilt towards the opposite shoulder
Comitant Strabismus: Diagnostic Methods occurs, and a face turn to the right brings the eyes in abduction so that the vertical movements can be executed by the vertical recti. Head posture ensures that the eye is out of the field of action of the paralytic muscle.
An asymmetric face requires an asymmetric adjustment of the optical centers of the spectacle. The IPD should also be similarly measured with the patient looking at the distant target (6 m). There may be slight (2-3 mm) difference between the near and distance measurements.
Measurement of Interpupillary Distance
Using the Pulzone-Hardy rule Pulzone-Hardy rule is a special device for measuring the pupillary distance of each eye. It has a slot for the nose and the central line is aligned with the midline of the patient. With the patient and examiner seated as in the above method, with the left eye occluded the patient fixates with his right eye at the examiner’s left eye. The vertical wire is moved till it bisects the pupil. The reading is taken (half IPD). Similarly the left half IPD is taken. Add the two which gives the interpupillary distance. Difference between the two readings indicates asymmetry.
Interpupillary distance (IPD) is the distance between pupils of the two eyes. The interpupillary distance is a very important parameter that can give information about the craniofacial disorders, a true hypertelorism versus telecanthus and vergence requirement. It is helpful in proper centering of the spectacles. Decentered lenses have prismatic effect and can increase or decrease an existing deviation, or induce a strabismus causing eye strain. A narrow interpupillary distance may predispose, simulates and accentuates esotropia. A wide IPD gives an impression of exotropia. Interpupillary distance can be measured by following methods: By an ordinary millimeter scale: A 15 mm rule is required to measure the IPD. The patient is seated with the examiner positioned about 33 cm in front, both in the same vertical plane. The millimeter rule is placed on the nasal bridge of the patient in the spectacle plane. The examiner closes his right eye and asks the patient to look at his left eye with his right eye (left eye may be closed). The scale reading bisecting the pupil is aligned to one. Next the right eye of the patient is covered and the patient asked to look at the examiner’s right eye with his left eye. Again the scale reading bisecting the pupil of left eye of the patient is taken. The difference between the two readings gives the IPD. It is better to note the scale reading aligned with the midline of the patient. This gives the pupillary distance of each eye from the midline and helps in detecting asymmetry of the face.
Using the synoptophore The IPD can be simply determined on the synoptophore. It is considered a first step before the synoptophore can be used for any measurement. This is done by adjusting the distance between the two eye pieces each of which can be separately adjusted and the distance between the two read on a millimeter scale. The arms of the synoptophore are kept at zero and the patient is asked to look at the center of the slide in the right picture tube with his right eye. The examiner with his right eye closed, aligns the central white line on the top of mirror unit of the tube with the reflection of the light in the center of patient’s pupil. The procedure is repeated with the left eye of the patient similarly and the IPD read from the millimeter scale. Once set, it is locked for the different procedures for the patient.
Ocular Deviation The examination of ocular deviation is the most important aspect of strabismus examination as
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Diagnostic Procedures in Ophthalmology it not only establishes strabismus but also quantifies it. Further it is very important to differentiate the true strabismus from the apparent strabismus, i.e. pseudostrabismus which needs only reassurance to the patient. A cover test is required to establish the existence of a strabismus.
Pseudostrabismus A true strabismus is a misalignment of the two visual axes, so that both do not meet at the point of regard. An apparent strabismus is just an appearance of strabismus in spite of the alignment of the two visual axes. Apparent strabismus or pseudostrabismus can be due to an abnormality of adnexal structures like the lids, canthi or orbits, or due to abnormal relationship between the visual axis and optical axis of the eyes. A telecanthus or a broad nasal bridge covers the nasal bulbar conjunctiva and gives the appearance of a convergent strabismus (pseudoesotropia). This becomes more prominent whenever a lateral gaze is attempted, the adducting eye getting covered by the telecanthal fold. Similarly the epicanthus covers the nasal bulbar conjunctiva to cause a pseudoesotropia. Neonates and young infants are commonly suspected to have such a strabismus. A proper examination can exclude this and reassure the mothers. A greater interorbital separation (hypertelorism) gives the appearance of a divergent strabismus (pseudoexotropia). On the other hand, euryblepharon, a condition with horizontally large palpebral apertures gives a look of pseudoesotropia. Similarly, a ptosis or lid retraction can masquerade as a pseudohypotropia and hypertropia, respectively. A ptosis may mask an existing hypotropia or aggravate hypertropia. And a telecanthus may mask an exotropia and highlight an esotropia. These appearances, therefore, assume importance even in a case of strabismus posted for surgery. The patient should be explained
of the consequence of a surgery in advance to avoid any discontentment later. Angle kappa: Angle kappa is the difference between visual and the optical axis. The visual axis (the line joining the fovea and the target) is not the same as the optical or geometric axis (the line passing through the center of the pupil or cornea). They differ normally by about + 5°, that is the eye would appear to be looking 5° out (exotropic) when it looks at any object. This is the nature’s mechanism to offset some optical aberrations. The angle kappa gives a look of exotropia in spite of perfect alignment, but it is within our limits of acceptance. When it is more than 5°, as in some hyperopes, it causes pseudoexotropia. On the other hand, an angle less than 5° or a negative angle kappa, as in some myopes, causes pseudoesotropia. Occasionally, displacement of the macula (heterotopia) can occur in some conditions like retinopathy of prematurity, leading to displacement of the corneal reflection. If only one eye is affected, the squinting appearance is accentuated. Angle kappa can be measured on the synoptophore with a special slide. Detection of a strabismus: A cover-uncover test is required to confirm the diagnosis of strabismus and to differentiate it from pseudostrabismus. It is necessary to perform this test when the corneal reflections are unequal, or if the history suggests a strabismus. It is an objective test which is a cornerstone of the diagnosis and management of strabismus. It has two components: 1. Observations to be made during covering (Cover test, Fig. 23.1A) and 2. Observations to be made during uncovering (Cover-uncover test, Fig. 23.1B).
Cover Test It is important to have a proper fixation target. It should be a figure or letter of size 6/9 of Snellen chart. This is to control the accommodation. A
Comitant Strabismus: Diagnostic Methods
Figs 23.1A and B: Cover/uncover test: A cover test, B uncover test
fixation achieved by a torch light is not desirable. Lang fixation stick which has very small figures is very useful for young children and reduced Snellen letters or numbers are ideal for the adults and older children. The fixation distance should be 33 cm for near and 6 meters for distance. It is important that target for near should be held slightly below eye level and for distance it should be at eye level to avoid a false impression of strabismus. Thirdly an occluder is required and in case of children it is the hand or a thumb which can be used to avoid scarring him. The subject is asked to fixate on the target at the requisite distance and an observation is made whether both eyes appear to fixate (no apparent strabismus) and one appears to fixate as the other deviates (apparent strabismus). Cover test (Observation made during cover test) The next step is to cover the apparently fixating eye and observe what happens to the other (apparently deviating) eye. If that moves to take up fixation, it confirms the presence of a manifest or true squint (heterotropia). If one had used a Spielmann translucent occluder (Fig. 23.2) one would have observed the eye behind the cover, deviating. However, if both the eyes appear to fixate in the first instance, the examiner attempts to cover either of the eyes to observe the behavior
Fig. 23.2: Spielmann occluder
of the eyes. If it moves to confirm a heterotropia it would imply a true squint masked by appearance. Uncover test (Observations made during uncovering) The second part, uncover test is helpful in unmasking the latent strabismus (heterophoria) which presents with both eyes appearing to fixate the target. One of the eyes is covered, which breaks the fusion, and if there is any heterophoria (tendency for strabismus) the eye behind cover deviates (in/out/up/down). The examiner then observes the behavior of this eye as he removes the cover. If it remains deviated it confirms a latent strabismus with poor fusion (poor recovery) and if it recovers, the examiner observes the speed of recovery. The speed of recovery indicates the
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Alternate Cover Test In the alternate cover test, the eyes are rapidly and alternately occluded—from one eye to the other and then back again (Fig. 23.3). This procedure causes breakdown of the binocular fusion mechanism and reveals re-fixation movements of each eye at the moment of uncovering. The cover/uncover test is less dissociating than alternate cover test. In the absence of a manifest strabismus, such a strabismus in fixation implies a latent strabismus.
Prerequisites of Cover-Uncover Test The cover-uncover test requires the following prerequisites: 1. Ability of both eyes to fixate the target 2. Ability of both eyes to have central fixation and
3. Ability of both eyes to have no gross / severe motility defect. The cover-uncover test may be fallacious if the eye is blind, have markedly subnormal vision or eccentric fixation and limitation of moments. For infants who would not allow an occluder or a hand close to their face, the examiner can use indirect occlusion test or distant cover test. Here the fixation light or target is obstructed for one eye by an occluder at some distance away from the child The cover-uncover test helps to confirm a true manifest or latent strabismus along with its type: exodeviation, esodeviation or vertical deviations. It also indicates the visual dominance or the presence of amblyopia. It also tells about comitance of the strabismus by comparing the primary and secondary deviations. The characteristics of strabismus like unilateral or bilateral, constant or alternating can be noted. The variability in strabismus for near and distance, the effect of accommodation and patient’s refractive error can also be studied. The test can uncover the associated latent nystagmus if any. A cover-uncover test needs to be done in all the nine cardinal positions of gaze, as also for near and distance fixation. With experience the examiner can detect even small angle strabismus except microtropia of less than 5 prism diopter deviation.
Fig. 23.3: Alternate cover test
Comitant Strabismus: Diagnostic Methods Measurement of Ocular Deviation Deviations can be measured by two methods: objective and subjective. Both require the cooperation of the patient. The objective tests depend on the observations by the examiner of the patient’s fixation pattern. This is based on neutralizing the movement of the deviating eye as it takes up the fixation. Methods based on this principle requires patient to have foveal or steady parafoveal fixation. Inherently the subjective tests are more precise and reveal status of the sensory system. Tests cannot be done if the patient lacks binocular vision, or the ability to comprehend the directions or express the response. The subjective measurement is based on dissociation of two eyes to induce the maximal angle of deviation or observe the position of the images on a calibrated scale. Both the tests would have to be used judiciously by the examiner in order to understand the sensorimotor aspects of strabismus.
Prism Bar Cover Test The deviations can be measured whether subjectively or objectively by various methods. The simple and best way to measure deviation is by using prisms or prism bar (Fig. 23.4) along with the cover test known as prism bar cover test (PBCT). In fact it is the cover-uncover with the addition of neutralization of deviation by the prisms (Fig. 23.5). For neutralizing esodeviations, prisms are placed base out and for exodeviations they are placed base in. A simple rule to remember is
Fig. 23.4: Prism bar and loose prisms
Fig. 23.5: Prism bar cover test
that apex of the prism should point towards deviation. Therefore, in a vertical deviation, base up prism is used in front of right eye if there is right hypotropia and base down if there is hypertropia. If there is combination of horizontal and vertical deviations, the prisms are placed horizontally in front of one eye and vertically in front of the other eye. For large deviations, a combination of loose prism of 30 or 45 prism diopter in front of one eye and prism bar in front of the other eye is used. The plastic prisms are placed in the frontal position, that is, parallel to the infraorbital margin. But the glass prisms are placed in the Prentice position, that is, the posterior face of the prism is perpendicular to the line of sight. It may be reiterated that the fixation distance (both for near and distance), fixation targets (Fig. 23.6) and proper dissociation of the two eyes should be ensured. A hurriedly done test can be fallacious. An accommodative or fusional convergence should be relaxed. The latter by making the subject wear occlusion for at least 4 hours (even extended up to 24 hours in cases of intermittent exotropia of simulated divergence excess type). The accommodative convergence should be controlled by making the subject wear his proper refractive correction for the test
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Fig. 23.6: Fixation targets
distance. For near fixation additional reading correction may have to be added in cases of accommodative esotropia of convergence excess type. Following precautions should be taken for prism bar cover test: a. It is essential to prevent fusion by continuous use of alternate cover test. b. It is essential to control accommodation by use of an accommodating target. c. Since a high powered prism reduces the clarity of vision, often impairs fixation if placed in front of an ambylopic eye, therefore, it should preferably be placed in front of better eye. d. Children can cooperate for short time only so it is preferable to start with approximately the correct prism rather than to work up from lower strength. e. In cases with combined vertical and horizontal strabismus, it is preferable to use square prisms as they can be easily held together between thumb and the finger. It is important to understand that the deviation to be measured is to be static deviation and should be free of the aforementioned dynamic factors of accommodation and fusion. It is the static angle which requires surgical correction whereas the dynamic deviation of accommodation should be corrected by glasses.
Prism Bar Under Cover test: Measurement of Dissociated Vertical Deviations In patients with dissociated vertical deviations (DVD) the alternate cover test reveals that each eye turns upward under cover in contrast to the situation in vertical heterophoria. After removal of the cover, the eye makes a slow downward movement to reach the midline, at times even going below it, accompanied by incycloduction. The translucent occluder of Spielmann is especially useful in the diagnosis of this condition as well as demonstrating it to the patient’s parents. A precise measurement of the vertical excursions of each eye during DVD is nearly impossible because of its variable nature. An accurate quantitative assessment of DVD may be obtained provided visual acuity in each eye is sufficient to visualize the fixation target, using a modification of the prism and cover test. As the patient focuses on the fixation target at 6 m distance, the occluder is quickly shifted to the fixating eye, allowing the previously dissociated and elevated eye to take up fixation. The cover is then returned to the nonfixating eye. As the alternate cover test is continued, increasing amounts of base-down prisms are held under the occluder infront of the nonfixating eye until the downward fixation movement of that eye is neutralized. The procedure is then repeated with the fellow eye fixating. Effect of high plus or minus glasses on measuring strabismus deviations Plus lenses always measure less deviation than actual, both in esodeviation and exodeviation (base-out effect in eso and base in effect in exo). Minus lenses always measure more deviation than actual, both in esodeviation and exodeviation (base in effect in eso and base out effect in exo). Measurements To determine the different aspects of strabismus, deviations can be measured in various ways:
Comitant Strabismus: Diagnostic Methods 1. Deviation with distance and near fixation to determine its nature as to whether esotropia is: basic/convergence excess divergence insufficiency, and exotropia is: basic/ convergence insufficiency /divergence excess. 2. Deviation in nine different cardinal positions of gaze to determine any incomitance (paralytic, restrictive or spastic). 3. Deviation in up gaze of 25 degree and down gaze of 35 degree for determining A-V patterns. 4. Deviations with right and left eye fixating alternatively to determine primary and secondary deviation in case of incomitant strabismus. 5. Deviations with subjective method and objective method to determine the type of retinal correspondence (normal or anomalous). 6. Deviations after prolonged cover to differentiate a true divergence excess type from the simulated divergence excess exotropia as also to determine the fully undissociated deviation.
Examination of Eyes in Nine Gaze Positions It is important to measure the ocular deviations in different gaze positions for diagnosing motility defects. Although, except for the down gaze, one does not use 35 degree gaze positions physiologically, these are helpful for diagnosis. These are, therefore, called diagnostic positions. Just like the measurements in the primary position, measurements in these peripheral eight positions are also best done by prism bar cover test, with an accommodative target. Some clinicians prefer to use deviometers, which are devices that can give different fixed/repeatable fixation target positions. For near measurements any Lister’s perimeter (Fig. 23.7) or a simple vertical stand with a vertically rotatable arm around a pivot, with the free end carrying the fixation target,
Fig. 23.7: Lister perimeter
can be used. For distance measurements one may use multiple fixed points on the opposite wall. Alternatively a single fixed target may be used with the head being turned to bring the eyes in the desired positions. The deviation of head can be read on a protactor along with scale. A cephalodeviometer, a calibrated mirror can also be used.
Synoptophore Synoptophore (Fig. 23.8) is a basic orthoptic instrument based on the haploscopic principle. It is also known as amblyoscope (Major, Curpax—Major types), and troposcope. It consists of a chin rest and forehead rest with two tubes carrying the targets seen through an angled eye piece. The tubes are placed horizontally and are movable in the horizontal and vertical planes. The distance between the two tubes can also be adjusted with the subject’s interpupillary distance (IPD). The targets in the tubes are illuminated slides which can be raised up or down and also be tilted to test for vertical and torsional deviations. All these adjustments can be read on the scales in degrees and prism diopters. The tube can be locked individually
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Corneal Reflection Tests
Fig. 23.8: Synoptophore
or both with respect to each other. The illumination of each target can be increased or decreased and flashes can be given if desired. Additional devices like Haidinger brushes can be attached. The targets are placed at a fixed distance from the eye piece which are of + 6.0 D or +6.5 D, so that the targets are at optical infinity. This should theoretically not stimulate accommodation. However, in reality proximal convergence does come into play distorting the deviations. This factor has significantly reduced the applicability of the synoptophore as a reliable instrument to measure deviations, especially horizontal ones. Uses of Synoptophore In cyclovertical strabismus the synoptophore is a useful instrument to measure torsion. It is also useful for studying accommodative convergence and for imparting orthoptic exercises. The synoptiscope of Curpax-Major is a modification which uses semi-transparent mirrors in place of opaque mirrors in front of eyes. This
Hirschberg’s test: The test estimates the deviation of corneal light reflex from the center of the pupil and provides a rough measurement of degree of strabismus. The corneal reflections, even normally are not exactly centered, because of angle kappa, but are symmetrical in the absence of a strabismus. In case of esodeviation, the corneal reflection falls more temporally and in exodeviation, reflection falls nasally. Roughly a 1 mm shift signifies a 5° deviation (earlier thought to be 7°). Thus if the reflex falls on the nasal limbus, the exodeviation is 30° (approximately 60 prism diopters). This test can be used in infants, who are not very co- operative or in cases of eccentric fixation or non-fixation (blind eyes). Krimsky test: In Krimsky test (or prism reflex test), a prism bar is utilized to quantify the deviation using the corneal reflection. It is preferable to place the prism bar on the fixating eye and to neutralize the amount by observing the corneal reflex in the deviating eye (nonfixating). Synoptophore: A foveal sized slide is placed in front of the fixing eye and the position of the corneal reflections is noted while the amblyoscope tubes are both at zero. The tube in front of the fixing eye is moved along the eye in such a way that reflection in the squinting eye moves into the normal position (comparable with that in other eye). The angle can be recorded in degree or prism diopter. Methods using corneal reflections give approximate values but are useful when PBCT cannot be done, for e.g. young children, uncooperative patients, patients with poor
Comitant Strabismus: Diagnostic Methods fixation, etc. Deviations measuring less than 10 prism diopters cannot be measured.
Subjective Tests of Deviation These tests utilize the subject’s perception of the deviation. When there is misalignment, the subject perceives diplopia and the separation between the two images indicates the subjective deviation. This is the diplopia principle. Here the single physical location is perceived by the subject as two perceptual localizations. Diplopia testing with the red green goggles is based on this principle. Measurement of deviation on Maddox tangent scale with the help of Maddox rod is also based on it, Subjective tests can also be done on the haploscopic principle, where two “physical locations” are used to have one “perceptual localization”. The examples are as done on synoptophore when tested subjectively or the Hess or Lees screen. Diplopia testing: When red and green glasses are placed before the right and left eye respectively; they dissociate the two images and are seen double in cases of strabismus. Esodeviations cause uncrossed diplopia (homonymous diplopia) and exodeviations cause crossed diplopia (heteronymous diplopia). In the former the image falls on the nasal half of the retina and is projected on the temporal half of the field, and so is seen uncrossed (same side as the eye). In exodeviations the image falls on the temporal retina to produce crossed diplopia. It is preferable to use an illuminated slit target and to use the slit vertically for charting horizontal deviations and to use it horizontally to chart vertical deviations. A tilt of the image is also better appreciated with a slit target. The test can be done both for near and distance. For distance one may utilize the Maddox tangent scale or cross, to quantify the deviation otherwise prisms for deviation are used. The separation between the two images is
recorded for each eye in the nine diagnostic gaze positions. A red filter alone can also be used but the dissociation is then not complete as with red and green glasses. The test is very useful for diagnosis and follows-up of incomitant strabismus. Hess and Lees screen: Here two test objects (two locations) are shown to the patient but seen by him as one. The dissociation may be done by red-green glasses as in the Hess screen test (Fig. 23.9A) or a mirror septum as in the Lees screen (Fig. 23.9B). The Lancaster red-green test with the two Foster torches use red-green filters in the torches and green glasses. Polaroid dissociation can also used in order to have a more physiological dissociation. These haploscopic tests are very good for documentation of ocular muscle paralysis and restrictive conditions. The Hess chart has a grid pattern where each square represents 5 degree excursion for the fixating eye. Thus the inner square tests for 15
Fig. 23.9A: Hess screen
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Fig. 23.9B: Lees chart
Fig. 23.10: Maddox rod
degree eye movements from the primary position. The outer square (each side is curved inwards) represents 30 degree excursion for the fixing eye. The outer square is usually charted to mild underaction. The Hess chart is a very good test which can document the under or over action of the extraocular muscles.
the torsion in the slides. The slides used should have vertical features or one can use the after image slides. For objective evaluation of the cyclodeviations, the indirect ophthalmoscopy and fundus photography are useful methods. These are good to semi-quantify the cyclodeviations. Normally the fovea is located between the two horizontal lines, one passing through the center of the disk and the other cutting the lower pole of the disk tangentially. The usual location is in the middle of these two horizontal lines, that is, 0.3 disk diameters below the horizontal line through the center of the disk. A difference of 0.25 disk diameter or more between the two eyes is considered abnormal.
Measurement of cyclodeviations: While the objective tests are difficult, the subjective tests are very good for measuring cyclodeviations. Diplopia charting with a slit target is used to make the patient appreciate the tilt. A horizontal slit appears to be tilted in the opposite direction to the cyclodeviation in the eye. In case of excyclodeviation of the right eye, the tilt is anticlock-wise and in case of incyclodeviation it will be clock-wise. By using two Maddox rods (Fig. 23.10), preferably one white and the other red, the tilt is neutralized by rotating the Maddox rods in the requisite direction. The change of axis on the trial frame can be read to give the actual cyclodeviation. It is known as the double Maddox rod test. A vertical prism of 5 prism diopter can be added to create a separation between the two horizontal lines seen through the Maddox rods. The synoptophore is a good instrument to measure cyclodeviations, the slides can be tilted to make the patient appreciate straightening of
Limitations of movements: In addition to measuring the ocular deviations, it is valuable to note the limitations of movements. In restrictive strabismus, the limitation of movements is marked compared to the ocular deviation which is small. In contrast, in the incomitant strabismus the limitation of movement and ocular deviation collaborate each other. Both the ductions and the versions should be noted and documented. Usually a subjective assessment is made on a scale of 7 points (+3 to -3) or 9 points (+4 to -4). This is usually helpful in follow-up of cases
Comitant Strabismus: Diagnostic Methods of incomitant strabismus. It should be noted that due to variations in the adnexal structures, these cannot be fool proof method of assessing such deviations. Normally adduction is considered normal when the nasal one-third cornea crosses the lower punctum. Less than this is considered to be limited. For abduction to be considered normal, temporal limbus should touch the lateral canthus. Vertical movements are difficult to assess due to palpebral aperture. The overaction of the obliques can be assessed. Mild over action is appreciated only in sursum adduction and moderate overaction in adduction. If there is a hyper or hypotropia in primary position it signifies severe overaction. Grading oblique overactions: Another useful clinical test to grade the inferior oblique overaction is by observing the angle the adducting eye makes with the horizontal line as it elevates and abducts ( if overacting ) on lateral version to the opposite side. It may be noted that in the absence of inferior oblique overaction eye would remain in the horizontal line. Analogous to this grading the supererior oblique also can be graded. The angle adducting eye makes with the horizontal as it depresses and abducts on a lateral version, is noted (Table 23.3).
Measurement of Vergences In actual practice the manifestation of a strabismus (heterotropia) only occurs if the latent tendency for the strabismus (heterophoria) is not overcome by the fusional vergences. The measurement of vergences is very important, as it determines the capability of the motor system to cope with an induced misalignment of visual axes. If these vergence amplitudes are large, even a large angle strabismus remains asymptomatic, and if they are small, or intermittent, even a small angle strabismus manifests remains symptomatic. Vergences are usually tested in the three planes: (a) Horizontal vergences: convergence and divergence (b) Vertical vergences: sursumvergence and deorsumvergence and (c) Torsional vergences: incyclovergence and excyclovergence. In principle, to measure the vergences, the axes are misaligned artificially; and this may be done with prisms or on the synoptophore. The horizontal and vertical vergences can be measured only by prisms in this manner, as the prisms cannot induce a torsional misalignment.
TABLE 23.3: GRADING OF OBLIQUE OVERACTIONS Inferior oblique overaction
Superior oblique overaction
Grade 1+*
Up to 20 degree angle with the horizontal line
Grade 1+
Up to 15 degree angle with the horizontal line
Grade 2+
Up to 30 degree angle with the horizontal line
Grade 2+
Up to 30 degree angle with the horizontal line
Grade 3+
Up to 60 degree angle with the horizontal line
Grade 3+
Up to 60 degree angle with the horizontal line
Grade 4+
Up to 90 degree angle with the horizontal line
Grade 4+
Up to 90 degree angle with the horizontal line
* Grade 1 + inferior oblique overaction may not be easily detectable on lateral version and is better appreciated by only in tertiary position, e.g. levoelevation for right inferior oblique
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Diagnostic Procedures in Ophthalmology Horizontal Vergences Near point of convergence: The simplest way to measure the convergence is to bring a line drawn on a paper closer to the eyes, till the point, it becomes double. This determines the near point of convergence (NPC). It is important to note that the line should appear to be double, not blurred. The point at which it is blurred would determine the near point of accommodation (NPA). The test is done with each eye separately (monocular) and also with both eyes together (binocular). For testing NPC in presbyopes or ametropes, suitable glasses should be used by the subject. The point at which the line becomes double is called the break point convergence. If the line is gradually withdrawn away, at some point the line becomes single again which is the recovery point convergence. The two differ from each other. The measurements are made from the bony margin of the lateral canthus. Normally it is 8-10 cm. NPC is more than 10 cm is considered defective. Apart from the patient describing the diplopia, an objective assessment is made by the examiner by seeing one of the eyes deviating out. Near point ruler, Royal Air Force binocular gauge and Livingstone gauge are instruments based on this principle. Convergence sustenance: A further assessment needs to be made to assess the ability of the eyes to hold the convergence at the near point. This is convergence sustenance. It gives a good parameter to assess the strength of fusional convergence. Normally one should be able to hold it for 45 seconds to one minute. Less than 30 seconds is definitely poor and indicates symptomatic exodeviation.
Measurement of Vergences with Prisms Convergence and divergence can be measured both for distance (6 m) and near (33 cm) fixation with the help of a prism bar or a rotary prism.
With the patient properly seated and made to fixate at a fixation target at distance or near as desired, the prism bar is moved with its prism strength increasing. The end point is noted as break point, when one eye deviates out or diplopia is reported. The prism strength is then gradually reduced till the object is seen single again this is noted as the recovery point. The break point is usually more than the recovery point but within 3 to 5 diopter. A larger difference would indicate poor recovery as in cases of intermittent exotropia who are symptomatic. Using base out prisms the convergence amplitudes are measured and using base in prisms the divergence amplitudes are measured. The vertical vergences are similarly measured using prisms base up for deorsumvergence and prisms base down for sursumvergence. The order of testing the vergences should be convergence, deorsumvergence, divergence and sursumvergence. This is to avoid any artificial changes. The normal values of these vergences are shown in Table 23.4. TABLE 23.4: SHOWS NORMAL VALUES OF VERGENCE FOR NEAR AND DISTANCE Vergence with prisms
Distance (6 m) in pd
Near (33 cm) in pd
Convergence Divergence Vertical vergence Incyclovergence* Excyclovergence*
14-20 5-8 2-4 10-12 10-12
35-40 15-20 2-4 10-12 10-12
*Though cyclovergences (in degrees) are as good as above on synoptophore the tolerance in practice is up to 4° only.
Examination of the Sensory Status The evaluation of sensory status of a patient with strabismus is aimed to find the following facts: 1. Presence of binocularity 2. The presence of confusion and diplopia 3. Presence of suppression and its depth
Comitant Strabismus: Diagnostic Methods 4. Presence of ambylopia and its degree 5. Type of correspondence and 6. Presence of stereopsis and its grade.
Binocularity and Diplopia If binocular diplopia is present, it indicates binocularity. The presence of true diplopia should be established by differentiating it from false impression of diplopia due to blurring or elongation of an image due to astigmatism. It is necessary to establish binocular nature of diplopia by closing one eye. If, it disappears, it is suggestive of binocular diplopia. In longstanding cases of strabismus even of adult onset, patient may learn to ignore the other image. In such cases use of dissociating mechanisms like red-green goggles, Bagolini’s glasses, single or double Maddox rod are helpful in visualization of diplopia. In cases of strabismus without diplopia or in cases without strabismus, dissociation test are helpful in detecting the presence of binocular perception.
Suppression Suppression is a sensory adaptation to strabismus in children, which only occurs when the eyes are open. It is a temporary phenomenon. As soon as the fixating eye is covered, deviated eye takes up the fixation. It occurs from the active cortical inhibition of disparate and confusing retinal images originating from the retina of the deviating eye. The stimulus for suppression is diplopia, confusion or a blurred image resulting from astigmatism or anisometropia. Clinically, suppression can be classified into three types: 1. Central or peripheral: In central suppression the image from the fovea of the deviating eye is inhibited to avoid confusion, while in peripheral suppression image forming on the peripheral retina is inhibited.
2. Facultative or obligatory: Facultative suppression occurs only during binocular conditions and obligatory during monocular conditions. 3. Unilateral or alternating: Unilateral suppression is present in one eye only (unilateral) while in alternating suppression both the eyes are involved or it may alternate between two eyes (alternating). The extent (area) and depth (intensity) of suppression should be noted. The sensitive period during which suppression may develop ends after the age of 8-9 years. Once developed, suppression may persist throughout life. If due to any reason, patient loses the ability to suppress during adult age, it can never be regained and diplopia prevails. Such a thing can happen due to head trauma, spontaneous change of angle of strabismus or due to iatrogenic causes like strabismus surgery or improper orthoptic treatment.
Tests for Suppression Bagolini striated glasses: Bagolini striated glasses tests is the most physiological test for dissociation of eyes. A pair of striated glasses is used (Figs 23.11 and 23.12). The axis of striations of the two eyes is kept at right angle to each other, i.e. 45o and 135o. When viewed through the glasses, a point source of light is seen as a line which is at right angle to striation. In suppression two types of response can be seen: a. Single line response: If only one line is seen, it is suggestive of other eye scotoma, i.e. suppression response.
Fig. 23.11: Bagolini striated glasses
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Fig. 23.12: Bagolini glasses test: A Crossed response (NRC with no strabismus or harmonious ARC with strabismus), B Left suppression, C Right suppression, D Crossed response with central scotoma in Right eye E Esotropic diplopia, F Exotropic diplopia (With right and left eye lenses at 135 and 45 degrees, respectively)
b. Cross response with central gap in one: It is suggestive of central suppression scotoma in that line, i.e. eye. Interpretation of responses: 1. Symmetrical cross response: If the patient has no strabismus, it is suggestive of normal bifoveal correspondence. But if a manifest strabismus is present, it is indicative of harmonious anomalous retinal correspondence. 2. Asymmetrical cross response: Two lines intersecting each others at some other point than midline, indicates an incomitant strabismus with normal retinal correspondence, i.e. diplopia response. Worth four dot test: In the Worth four dot test (WFDT) eyes are dissociated with red-green goggles. It is more dissociating and hence less physiological as compared to Bagolini glass. This test is performed with patient wearing red lens in front of right eye which filters all color except red, and green lens in front of left eye which filters all colors except green. The patient then views a box with four lights which has one red, two green and one white light (Fig. 23.13). The result is interpreted as follows: 1. Four dots are suggestive of normal retinal correspondence (NRC) if manifest strabismus is not present. In presence of manifest deviation it suggests harmonious anomalous retinal correspondence.
Fig. 23.13: Worth four dot test: A Four dots: NRC with no strabismus or ARC with strabismus, B Left suppression, C Right suppression, D Binocular diplopia (With red and green glasses in front of right and left eye)
2. Five dots (2 vertical red, 3 green in inverted triangle form) are suggestive of normal retinal correspondence with manifest deviation. These five dots are separated differently depending on the type of deviation. The uncrossed pattern with red on right is suggestive of esodeviation, crossed pattern, i.e. red on left side is suggestive of exodeviation and if they are vertically displaced, it is suggestive of vertical anomalous retinal correspondence. 3. Three dots (green) indicate right suppression. 4. Two dots (red) indicate left suppression. It is performed at 6 meter distance and it subtends an angle of 1.2 degree. In case of central scotoma larger than this size, WFDT will not be visualized. The patient can be brought nearer to WFDT test to increase angle subtended by it. Synoptophore: The suppression scotoma can be mapped with synoptophore, at least in the horizontal meridian. One arm is rotated, and the points are noted at which the target carried by the moving arm disappears and reappear. Slides which present paramacular targets are used. After image test: After image test demonstrates the visual direction of the fovea or eccentric fixation point. It is highly dissociating orthoptic test. The right eye is flashed with a vertical bright flash of light and left by a horizontal flash. As each eye is stimulated separately, fovea of each eye or fixation point in eccentric fixation are at
Comitant Strabismus: Diagnostic Methods
Fig. 23.14: After image test: A Crossed response (NRC with no strabismus), B Asymmetric cross (ARC with strabismus, left suppression), C Esotropia, D Exotropia, E Right suppression, F Left suppression diplopia (With vertical and horizontal flash before right and left eyes, respectively)
the center of the after images. The patient is then asked to draw the relative positions of the after images (Fig. 23.14). Interpretation of the test is as follows: a. Cross response: If the two after images are seen as a cross, the patient has normal retinal correspondence. This is irrespective of the deviation of two eyes. b. Asymmetrical crossing: In this vertical and horizontal lines have their centers separated. The amount of separation is proportional to angle of anomaly. In case of esotropia with ARC, vertical after image (belonging to right eye) will be seen to the left of the horizontal after image. These findings are reversed in exotropia. A single vertical after image is suggestive of left suppression and a horizontal after image is suggestive of right suppression.
Measurement of Suppression Scotoma Suppression scotoma can be charted under binocular conditions (fixating with one eye, while the field of other eye is charted). This can be done by following methods: Use of prisms: This is a simple method based on the patient recognizing diplopia when the image falls outside the limits of scotoma. Prisms are used to displace the central object peripherally till it can be visualized in different directions.
Binocular perimetry: The testing apparatus is arranged in such a manner that fixation target is common to both eyes but the test object is seen only by the eye under examination. This can be achieved in following way: a. Lees screen or Hess screen: When one eye is charted the other eye fixates through mirror in Lees and red green dissociation in Hess charting. b. Two Bjerrum screens at 90o to each other. c. Polaroid scotometer: Using polaroid dissociation while one eye fixates, the field of other eye is charted. Depending upon the test different responses are observed. i. With more dissociating tests like prisms and Lees screen single large coarse scotoma is seen, extending from fovea to the diplopia joint. Some authors have demonstrated hemiretinal scotoma in exodeviations, whereas esodeviations showed more discreate scotoma. ii. With less dissociating test like phase difference haploscope and Polaroid scotometer two discrete scotomas are seen. These are foveal scotoma about 2o–3o in size and diplopia point scotoma. This is seen in both esodeviation and exodeviation; in exodeviation the foveal scotoma showed a vertical step-like hemiretinal scotoma of Jampolsky.
Depth of Scotoma Depth of scotoma is measured by using differential stimulation of the two eyes. This can be done with red filters of increasing density arranged in ladder pattern (Bagolini’s graded density filter bar). The patient fixates a small target and filters of increasing density are placed in front of the fixating eye until patient perceives diplopia. Greater the density of filter required to induce diplopia, greater the depth of suppression.
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Retinal Correspondence Investigation of state of retinal correspondence is indicated in all cases of constant strabismus. A bifoveal correspondence is called normal retinal correspondence (NRC). A correspondence between fovea of one eye and extrafoveal point of the other eye (deviating eye) is called anomalous retinal correspondence (ARC). It is an acquired binocular functional sensory adaptation to strabismus at the cortical level. Suppression precedes the development of ARC. At the cortical level there is a change in synoptic connections from the foveo-foveal to the foveo-extrafoveal. The extrafoveal point should have good visual potential in order to have an association with the fovea of the fixing eye. Following conditions facilitate the development of ARC: 1. Early onset strabismus: a good neural plasticity is required for the new connections, 2. Constant angle of deviation: Constancy of the stimuli favors development of new connections and 3. Small angle of deviation especially esodeviations and rarely exodeviations: It is rarely found in exotropias as they are generally intermittent or variable due to good fusional vergence. ARC allows some binocular vision with limited fusion to be maintained in the presence of heterotropia.
Diagnosis of ARC It is necessary to measure the angle of deviation by subjective and objective methods to diagnose ARC. ARC is present when there is difference in the subjective and objective deviations. In NRC objective and subjective angles are equal. If the subjective angle is zero, there is no subjective strabismus. In the presence of objective angle showing a strabismus, ARC is termed as harmonious ARC. If the subjective angle is not
zero but less than the objective angle of deviation, it is unharmonious ARC. The difference between objective and subjective angle is the angle of anomaly. Hence in harmonious ARC, subjective angle is zero and objective angle is equal to angle of anomaly. In unharmonious ARC, objective angle is greater than subjective angle and hence angle of anomaly. In NRC, objective and subjective angles are same and angle of anomaly is zero. The objective angle of deviation can be measured by prism bar cover test (PBCT) or by cover-uncover test, and on the synoptophore with alternate on-off method. The subjective angle of deviation can be measured by following methods: 1. After image test 2. Synoptophore 3. Worth four dot test 4. Maddox rod or red filter test 5. Polaroid dissociation 6. Phase difference haploscope and 7. Bagolini’s striated glasses.
Bagolini’s Striated Glasses The patient with strabismus is first evaluated with cover-uncover test and objective angle is measured with PBCT. Next with the strabismus manifested, the patient looks through the Bagolini’s striated glasses. If he sees a cross response (as seen by person with NRC with no strabismus) in the presence of manifest strabismus it implies a harmonious ARC. If the patient does not see a cross response, i.e. X, the prisms are added to get a X cross response. The prism power required is the subjective angle of deviation and it is termed non-harmonious ARC. In presence of strabismus with NRC, patient sees two oblique lines crossing asymmetrically to form a V or A instead of X response. In the presence of suppression, patient sees only one line, line of the other eye which is suppressed is not seen. In case of central scotoma, the eye with the
Comitant Strabismus: Diagnostic Methods scotoma sees a line with a break in the center (Fig. 23.12).
Worth’s Four Dot Test The objective angle is first measured by PBCT. After wearing red green glasses, patient is asked to look at the dots. The prisms are added till the patient shows a normal response that is four dots in a normal rhombic pattern. The strength of prism indicates subjective angle.
Synoptophore The objective angle is measured by patient alternately fixing till there is no movement of eyes on alternate on-off. The subjective angle is measured by the patient aligning the two images by his perception of simultaneous perception slides.
After Image Test After image test is used to measure subjective angle. Each eye is monocularity flashed with a self flash to create a horizontal after image in right eye and vertical after image in left eye (Fig. 23.14). Each after image is centered at fovea (even in cases of ARC due to central fixation). In case of ARC, patient sees an asymmetric cross. The displacement between the centers of the two after images is proportional to the angle of anomaly (tan θ =displacement/distance of testing). The angle of anomaly can thus be calculated. The strabismus can be corrected surgically if the appearance warrants it. More extensive surgery can be performed if there is established ARC with sensory and motor fusion, the correspondence should adapt to the new eye position, reducing the risk of consecutive strabismus. Treatment may not be needed on many cases of ARC because it may cause
intractable diplopia, bifoveal binocular single vision cannot be restored and many patients with ARC may have useful and symptom-free binocular single fusion.
Amblyopia Amblyopia is a condition with unilateral or bilateral decrease of visual functions caused by form vision deprivation and/or abnormal binocular interaction. It cannot be explained by a disorder of ocular media or visual pathway. It is a condition caused by abnormal visual experience during early childhood, the critical period of visual development. In appropriate cases it is reversible by therapeutic measures.
Classification of Amblyopia 1. Strabismic amblyopia 2. Anisometropic amblyopia (unilateral or asymmetric) a. Anisohyperopic b. Anisomyopic 3. Form vision deprivation amblyopia (unilateral or bilateral) a. Stimulus deprivation amblyopia or amblyopia ex-anopsia, ptosis (covering pupil), opacities in cornea, lens or vitreous, unilateral occlusion or penalization b. Ametropic amblyopia (uncorrected bilateral high refractive error) i. Hyperopia ii. Myopia iii. Astigmatism (meridional amblyopia) 4. Nystagmus related amblyopia 5. Organic amblyopia a. Subclinical macular damage b. Malorientation of cones c. Cone deficiency syndrome Amblyopia is a disorder of visual perception, only one of which is the visual acuity on the
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Diagnostic Procedures in Ophthalmology standard vision charts (Snellen acuity); but there are other visual functions too that are affected. The amblyopia syndrome shows the following abnormalities: 1. Decreased visual acuity (Snellen) 2. Decreased grating acuity (Teller) 3. Decreased vernier acuity 4. Decreased or lost stereoacuity 5. Decreased contrast sensitivity 6. Decreased brightness perception 7. Abnormal contour interaction 8. Increased perception and reaction times 9. Nasotemporal asymmetries in resolution of vertical gratings and 10. Motility defects in pursuit, saccades and fixation. While the hallmark of amblyopia is decreased visual acuity, it is important to understand that recognition acuity (Snellen or similar charts) is more affected than either resolution acuity (Teller acuity, Fig. 23.15) or detection acuity (Catford drum or Bailey-Hall cereal test). Secondly the anisometropic and strabismic amblyopes behave differently. The Snellen letter or recognition acuity is affected more in strabismic or mixed (strabismic + anisometropic) amblyopes
Fig. 23.15: Teller acuity cards
compared to anisometropic amblyopes. Both Snellen acuity and grating acuity are affected equally in anisometropic amblyopes, whereas in strabismic amblyopes the grating acuity is affected to half the extent of Snellen acuity. Thus strabismic amblyopia is underestimated on grating tests.
Diagnosis of Amblyopia For diagnosis of unilateral amblyopia difference of vision between two eyes or in case bilateral amblyopia , difference from the age-related norm is taken into consideration. Clinically, a difference of two-line on Snellen chart (one octave difference) is considered significant. Yet another well recognized feature of strabismic amblyopic vision is that it is not degraded by neutral density filters, it may even show some improvement. However, in anisometropic amblyopes, an equal deterioration is seen in amblyopic and normal eyes. Other organic retinal pathologies causing diminution of vision are susceptible to deterioration by neutral density filters. This test can thus distinguish functional amblyopia from organic ones. Abnormal contour interaction is seen in the form of degradation of visual acuity for objects placed in a row or line (linear acuity), compared to the acuity of the same object viewed separately (single letter acuity). This phenomenon has been described as the crowding phenomenon. Crowding phenomenon is present to some extent even in normal subjects (critical area of separation=1.9 to 3.8 min of arc). In amblyopes this is more pronounced, similar to the critical area of separation of peripheral retina of normal human subjects (8.4 to 23.3 min of arc).The crowding phenomenon has also been attributed to the poor visual acuity present in amblyopes. But its importance in prognosticating progress in amblyopia therapy should be remembered. The single letter acuity improves more rapidly during
Comitant Strabismus: Diagnostic Methods the course of treatment. Finally both the single letter and linear acuity should approach each other, if it is not so there is always a risk of recurrence of amblyopia. Children who cannot be tested with linear charts, single letter, optotypes with “surrounds” can be used to cause contour interaction. In the normal charting of Snellen vision, high contrast (80%) letters are used. Abnormal contrast functions have been recorded both in strabismic and anisometropic amblyopes, particularly at high spatial frequencies. At low spatial frequencies the contrast sensitivity is normal in amblyopes, but at high spatial frequencies the contrast sensitivity is deteriorated, more so with severe amblyopia. This is due to a neural loss of foveal function and not due to optical factors, or unsteady fixation movements or eccentric fixation. Contrast sensitivity has been observed to differ in strabismic and anisometropic amblyopes. It becomes normal in strabismic amblyopes when the luminance levels are reduced, while the deficit persists in anisometropic amblyopes. Other psychophysical functions are also affected. Besides the affection of form vision, brightness perception is also affected in ambylopes. Dark adaptation curves are essentially normal and even if there is an effect on the light sense, there is clearly dissociation between the effect on the light sense and the acuity. While recovery time after a glare stimulus to fovea is normal, the perception time and reaction time is 6 times longer. The critical flicker fusion frequency (rate at which a flicker just disappears) is usually normal compared to maculopathies. Pupils are generally normal and briskly reacting though afferent pupillary defect and raised edge light pupil cycle time have been reported.
Stereoacuity Stereopsis refers to our ability to appreciate depth that is the ability to distinguish the relative
distance of objects with an apparent physical displacement between the objects. It is possible to appreciate the relative location of objects using one eye (monocular clues). However, it is the lateral displacement of the eyes that provides two slightly different views of the same object (disparate images) and allows acute stereoscopic depth discrimination. Stereoacuity develops and can be tested on the preferential looking tests (PLT) at 6 months after birth. It has been used as a screening test for binocular vision anomalies in preschool children, but with difficulty. Stereopsis is an important binocular clue to depth perception. Stereopsis cannot occur monocularly and is due to binocular retinal disparity within Panum’s fusional space. Two objects stimulate disparate (noncorresponding) retinal points within Panum’s fusional area. Stereoacuity tests: The real-depth tests are not used as clinical tests. Most clinical tests are based on the haploscopic principle, using two dimensional or vectographic pictures. Some elements of the two pictures have a disparity which is fused to create a 3-D image. There are two groups of clinical tests used to measure stereopsis. These are the contour stereotests and the random-dot stereotest. Random-dot stereograms were first used by Julesz to eliminate monocular clues. As there are no contours, depth perception (stereopsis) can only be appreciated when binocular fusion occurs. Two process of stereopsis are used and these are local and global stereopsis. Local stereopsis exists to evaluate the two horizontally disparate stimuli. This process is sufficient for contour stereotests. Global stereopsis is required in random-dot stereogram when the evaluation and correlation of corresponding points and disparate points are needed over a large retinal area. An example of a contour stereotest used in the clinic is Titmus Fly stereotest. In the Titmus Fly stereotest, horizontal disparity is presented via the
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Fig. 23.16: Titmus fly stereotest with polaroid glasses
Fig. 23.17: Lang stereotest
vectographic technique (Fricke and Siderov). Examples of random-dot stereotests used in the clinic are Frisby stereotest, Randot stereotest, Random-dot E stereotest and Lang stereotest. Stereopsis can be tested by following methods: (i) Synoptophore with stereopsis slides (ii) Titmus fly stereotest with polaroid spectacles (Figs 23.16). (iii) Randot stereotest (iv) TNO test with red-green goggles (v) Frisby and Lang stereotest without using glasses (vi) Special 3-D pictures. The last two are examples in which the dissociation is not achieved by glasses which are not liked by children. The Lang test is based on the principle of “panography” where two images are printed on the same card each interrupting the other with regular linear interruptions. A prismatic film laminated over the picture ensures that one image is visible to right eye only and the other to the left eye only. The two, when fused in spite of the disparity, create a 3-D vision (Fig. 23.17). The newer special 3-D pictures, much in fashion; recently have two pictures specially merged in such a manner that if the two eyes are artificially diverged but controlling accommodation (as if looking for distance “through” the print), each
eye sees two different images creating a 3-D image.
Randot Stereotest Randot stereotest (Fig. 23.18) is the most popular clinical test and has replaced the earlier popular Titmus fly test. It uses Julesz’ random dot background to mask the monocular clues which are there with the animal tests and Wirt’s circle test. Geometric figures like square circle, triangle and star are also presented devoid of any monocular clues. But the letter type figures, though a better test, are usually not appreciated by small children. The test requires polaroid glasses to be worn by the patient. It is used at a distance of 40 cm
Fig. 23.18: Randot stereotest
Comitant Strabismus: Diagnostic Methods
Fig. 23.19: TNO test with red-green glasses
and thus tests near binocular vision, therefore, the myopes up to 3 diopters can be missed in the screening test. The Wirt circles (1-10) test has the stereoacuity from 400 arc seconds to 20 arc seconds.
TNO Test TNO test (Fig. 23.19) is also based on the randomdot background but uses red-green glasses for dissociation of the two images. It tests stereoacuity from 480 arc seconds to 15 arc seconds.
Fig. 23.20: Frisby stereotest
of the plate is closest to the observer. By altering the thickness of the plate and the distance from the subject different stereoacuities can be assessed. For 30 cm viewing distance, the 6 mm, 3 mm and 1.5 mm plates represent 600, 300 and 150 arc seconds of stereoacuity, respectively. This assumes the interpupillary distance of 60 mm, but no significant change is caused by different IPD.
Frisby Test The Frisby stereotest (Fig. 23.20) consists of three perspex plates of different thickness: 6 mm, 3 mm and 1.5 mm. On one face of each plate are found squares, three of which are filled with a random pattern of blue triangles of various sizes and the fourth of which has a central circular area that is not patterned. On the opposite side of the plate coincident with this area is a circular pattern of similar blue triangles. The plate is held in front of a white board and when viewed directly, the squares are all filled with random patterns although in one square a binocular viewer will see a circle standing up from the plate (crossed disparity) or lying below the rest of the design (uncrossed disparity) depending on which side
Distance Stereopsis Tests Stereoacuity should be tested for distance also. A projection vectographic test or Oculus distance stereotest can be used to test stereopsis for distance. A diminished stereopsis for distance may be an early sign of decompensating exophoria.
Normal Stereoacuity The adult individuals are capable of appreciating stereopsis with disparities as fine as 1520 arc seconds. The adult norm is 40 arc seconds. For children, 3-5 years old the norm is 70 arc seconds, and for 5-7 years it is 50 arc seconds. Children above 8 years have the adult norm.
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Diagnostic Procedures in Ophthalmology Lang Two Pencil Test In the absence of fine stereotests a gross estimation of stereopsis can be made by a bedside test, two pencil test popularized by Lang. A pencil is held in the examiner’s hand horizontally and the child is asked to touch the tip with the tip of another pencil rapidly, coming from one side. Care should be taken to avoid giving the end on view of the pencil, as that can be accomplished even monocularly, therefore, horizontal pencils are better, as they do not allow an end-on view. Always compare the binocular task with monocular task. The test is a gross stereopsis test of about 400 arc seconds disparity. All the tests provide a measure of stereoacuity by asking the patient to identify the correct target that has stereoscoptic depth (target with disparity). The working distance and interpupillary distance will need to be taken into consideration when calculating stereoacuity. Patients with disturbed binocular vision or different refractive error in one eye will perform poorly on depth discrimination tests. A rough estimation of visual acuity has been made on the basis of stereoacuity.
Fixation Disparity The concept of fixation disparity is important to understand the relationship of binocularity and heterophoria. Ogle et al described the fusion disparity as a physiological sensory phenomenon occurring in heterophoria, in which a deviation of the visual axes of 6-10 minutes of arc is compatible with bifoveal binocular single vision The phoria that is measured after disrupting fusion by cover- uncover test or similar methods is dissociated phoria. Fixation disparity is dependent upon the Panum’s fusional area. Under binocular conditions there may be a misalignment of the fixation points in the two eyes within the limits of the Panum’s area of fusion, which is fused and is seen as one. This
Fig. 23.21A: Fixation disparity
fusible misalignment is fixation disparity (Fig. 23.21A). This is quantified in minutes of arc. Under binocular conditions of viewing a vertical line is shown such that the upper half is seen by right eye and the lower half by the left eye, each viewed through polaroid dissociation. If there is a misalignment, prisms are used to align the two halves. This is called associated phoria. The rest of the picture shown apart from the vertical lines is seen by the two eyes and function as a fusion lock. The associated phoria is different from the phoria seen under dissociation and is, therefore, named differently. Fixation disparity curves: Under forced vergence situations, using 3, 6, 9, 12 pd prisms base-in, and base-out alternatively, the fixation disparity and the associated phoria can be charted and plotted. These plots are called fixation disparity curves. There are four common types of fixation disparity curves (Fig. 23.21B). Individuals with Type I curves are frequently asymptomatic. Those individuals having a steep slope, greater than 0.77 minute/prism diopter in esophoria and 1.06 minutes/prism diopter in exophoria are usually symptomatic. Type II curves do not intersect the
Comitant Strabismus: Diagnostic Methods
Fig. 23.21B: The four fixation disparity curve types developed by Ogle et al. The type of curve depends upon the curve shape and not on the vertical or horizontal position of the curve on the graph
fixation targets provides more natural circumstances and a fusion lock and are visible equally by both eyes. The central fixation target for vertical associated phoria has half split horizontal lines, each half visible to the right or left eye. For horizontal associated phoria, the split half lines are vertical. For measuring associated phoria the adjustment knob (to move one half line) is kept at zero and prisms are used to correct the misalignment that is reported by the patient. Both vertical and horizontal associated phorias can be measured. For measuring fixation disparity, the knob is shifted to set 10 minutes of exofixation disparity. If the patient reports misalignment the knob is taken to the other end till the patient reports misalignment in the reverse direction. Finally the misalignment is decreased till the patient reports alignment. The viewing time should be limited to a few seconds.
Wesson Card X-axis. Type IV curve is associated with unstable binocularity. The individuals with types II, III and IV are usually symptomatic. The flatter portion of the curve represents the condition of rapid adaptation to the vergence stimuli. Vision therapy may be considered to successfully flatten these curves and make the patient asymptomatic. Forced fixation disparity curves can also be plotted using different spherical lenses (in prepresbyopes) using lens power + 2.0 D to - 3.0 D in 0.5 D or 1.0 D steps. These forced fixation disparity curves are also used to measure AC/A ratio. Fixation disparity can be measured by Sheedy disparometer, Mallet unit and Wesson card.
Disparometer A close-up view of the fixation disparity targets on the disparometer is shown in the Figure 23.22. The fine reading print shown adjacent to the
Wesson card has to be viewed through polaroid glasses (Fig. 23.23). It has vertical lines in the upper half (seen by one eye) and an arrow in the lower half (seen by the other eye). The rest of the card is viewed binocularly.
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Fig. 23.22: Disparometer for measuring angular amount of fixation disparity, A Front or patient side, B Back or clinician side
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Fig. 23.23: Wesson fixation disparity card
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Bibliography 1. Adelstein FE, Cuppers C. Analysis of the motor situation in strabismus. In: Arruga A (Ed). International strabismus symposium (university of Giessen, 1966) New York, S. Karger A.G. 1968; 139-48. 2. Bagolini B. Tecnica par L’esame della visione binoculare sensa introduzone di elimenti dissicianti “test del vetro striato”. Boll Ocul 1958;37:195. 3. Bielschowsky A. Lectures on motor anomalies, Hanover NH. 1943. (reprinted 1956) Dartmouth College Publications. 4. Bixenmann WW, Noorden GK von. Apparent foveal displacement in normal subjects and in cyclotropia. Ophthalmologica 1982;89:58. 5. Brodie SE. Photographic calibration of the Hirschberg test. Invest Ophthalmol Vis Sci 1987;28: 736. 6. Broniarczyk-Loba A, Nowakowska O, Laudanska-Olszewska I, Omulecki W. Advancements in diagnosis and surgical treatment of strabismus in adolescent and adults. Klin Oczna 2003;105(6):410-3. 7. Bruckner R. Exakte strabismus diagnostik bei Yz -3 jahrigen, Kindem mit einem einfachen “Durch leuch tungs test” Ophthalmologica 1962;144:184. 8. Burian HM. Normal and anomalous correspondence in Allen JH (Ed). Strabismus Ophthalmic symposium II. St. Louis, Mosby,1958. 9. Capobianco NM. The subjective measurement of the near point of convergence and its significance
15. 16.
17. 18. 19 20.
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in the diagnosis of convergence insufficiency. Am Orthopt J 1952;2:40. Fern KD, Manny RE. Visual acuity of the preschool child: a review. Am J Optom Physiol Optics 1986;63:314-34. Filipovic T, Grzetic R, Sviderek-Stalekar D. Earlier detection of amblyopia and strabismus by ophthalmologic screening card attached to the vaccination card. Can J Ophthalmol 2003; 38(7):58792. Holmes JM, Fawcett SL. Testing distance stereoacuity with the Frisby-Davis 2 (FD2) test. Am J Ophthalmol 2005;139(1):193-95. Hussein MA, Coats DK, Muthialu A, Cohen E, Paysse EA. Risk factors for treatment failure of anisometropic amblyopia. J AAPOS 2004; 8(5):42934. Kim DS, Coats DK, McCreery KM, Paysse EA, Wilhelmus KR. Accuracy of clinical estimation of abnormal head postures. Binocul Vis Strabismus Q 2004;19(1):21-24. Leske DA, Holmes JM. Maximum angle of horizontal strabismus consistent with true stereopsis. J AAPOS 2004;8(1):28-34. Parvataneni M, Christiansen SP, Jensen AA, Summers CG. Referral patterns for common amblyogenic conditions. J AAPOS 2005; 9(1):2225. Sharma P. Strabismus Simplified. Modern Publishers, New Delhi 1999. Sokol S. Visually Evoked Potentials: theory techniques and clinical applications. Surv Ophthalmol 1976:21:18. Spielmann A. A translucent occluder to study eye position under unilateral or bilateral cover test. Am Orthopt J 1986;36:65. Teller DY, McDonald M, Preston K, Sebris SL. Dobson V. Visual acuity in infants and children: the acuity card procedure. Dev Med Child Neurol 1986:28:779. Urist MJ Pseudostrabismus caused by abnormal configuration of the upper eyelid margins. Am J Ophthalmol 1993;75:455. Veronneau-Troutman S. Prisms in the Medical and Surgical Treatment of Strabismus. St. Louis: Mosby, 1994. Von Noorden GK, Campos EC. Binocular Vision and Ocular Motility: Theory and management of strabismus. 6th edn, St Louis, Mosby, 2002. Wright KW, Walonker F, Edelman P. 10 dioptre fixation test for amblyopia. Arch Ophthalmol 1981;9:1242.
Incomitant Strabismus
S MEENAKSHI, T SURENDRAN
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Incomitant Strabismus
Incomitant strabismus comprises a large group of disorders in which the amount of deviation is different in different gaze positions. Broadly these can be categorized in paralytic and restrictive or mechanical. Paralytic strabismus includes: 1. Third cranial nerve (Oculomotor nerve) palsy 2. Fourth cranial nerve (Trochlear nerve) palsy 3. Sixth cranial nerve (Abducens nerve) palsy 4. Paralysis of nerve supplying single muscle 5. Monocular elevation deficiency 6. Monocular depression deficiency and 7. Möbius syndrome. Restrictive strabismus includes: 1. Duane’s retraction syndrome 2. Orbital blow-out fractures with muscle entrapment 3. Thyroid related strabismus 4. Congenital fibrosis and 5. Brown’s syndrome. Following important points must be considered while examining a case of incomitant strabismus: 1. As most of the incomitant strabismus are acquired a detailed history should be obtained about: (a) Trauma to the head and orbit
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(b) Previous surgery such as cataract, glaucoma implant and scleral buckle (c) Treatment for thyroid eye disease. Due to the incomitant nature of the strabismus patient may adopt an anomalous head posture for fusion. During duction and version testing, one must be aware of restrictive and paretic muscles giving the impression of an overacting yoke muscle. Primary and secondary deviations should be measured meticulously. Primary deviation is measured with the prism in front of the affected eye and secondary deviation, with the prism in front of the normal eye. Alternate prism cover testing should be done in all cardinal positions of gaze. Ability of the patient to proper fusion should be ascertained with neutralizing prisms or Fresnel prisms for both Snellen and free space. It is important to ascertain fusion for both primary and down gaze as these are the important functional gazes for activities of daily living. Head tilt measurements are important to assess cyclovertical muscle involvement.
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Diagnostic Procedures in Ophthalmology 8. The presence of subjective torsion as suspected by subjective complaints of tilting or seen on diplopia charting may be quantified by the Double Maddox Rod test. The same can also be confirmed by on fundus examination. 9. In office, forced duction and force generation testing should be carried out when possible, to assess the presence of restrictions and degree of nerve function preserved. 10. Neuroimaging of the orbits and brain is often required to make a complete diagnosis and planning appropriate intervention. 11. It is also important to consider myasthenia gravis and chronic progressive external ophthalmoplegia in the differential diagnosis.
Paralytic Strabismus Third Cranial Nerve Palsy The third cranial nerve supplies the levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique. Therefore, the patient with third nerve palsy may present with a combination of the following symptoms and signs: 1. Diplopia: horizontal and vertical 2. Ptosis 3. Exotropia and hypotropia 4. Limited ocular motility in the direction of action of affected muscles (Fig. 24.1), a partial paresis involving only the superior or inferior divisions or isolated muscle palsy may also be a presentation.
Fig. 24.1: Third cranial nerve palsy
Incomitant Strabismus 5. Paralysis of the pupillary sphincter with resultant mydriasis or pupil sparing type 6. Long-standing palsy may have aberrant regeneration in the form of ipsilateral retraction of the upper lid on attempted adduction and/or attempted infraduction, as well as pupillary constriction. If the pupil is spared the cause is most likely vascular. When the pupil is involved, the cause is likely to be an aneurysm. Patients with vasculopathy and pupil sparing third nerve palsy should be observed daily for one week, then weekly for one month, and finally monthly for six months. To rule out concurrent superior oblique palsy, patient is asked to attempt adduction and one looks for incyclotorsion by observing the conjunctival blood vessels.
Etiology The etiology of third cranial nerve palsy differs in pediatric and adult groups. Causes of pediatric onset third nerve palsy are: 1. Congenital due to birth trauma 2. Trauma 3. Inflammation 4. Neoplasm and 5. Aneurysm. Causes of adult onset third nerve palsy are: 1. Aneurysms 2. Vascular disease 3. Trauma 4. Neoplasm 5. Ideopathic.
Fourth Cranial Nerve Palsy The fourth cranial nerve supplies the superior oblique muscle. Fourth nerve palsy is the most common cyclovertical muscle palsy. Patients may present with a combination of the following symptoms and signs:
1. Head tilt in both congenital and acquired palsy 2. Facial asymmetry: The side of the face on the side of the tilt is often less developed in the congenital variety. This may be easily evaluated by looking at old photographs. 3. Torsional diplopia can be assessed subjectively and objectively. 4. The three-step test is the key to making the diagnosis of isolated cyclovertical muscle palsy. The test requires motility measurements first in the primary position. This step incriminates the depressors of the hypertropic eye and the elevators of the hypotropic eye. Next measurements are performed in the side gazes and an increase in hypertropia is noted (Fig. 24.2). This step eliminates the two muscles that do not act in the field of gaze showing the increased hypertropia. The third step, which is the head tilt to either side, exposes the weakened muscle that is unable to elevate the eye. 5. Version testing may reveal an ipsilateral inferior oblique overaction, ipsilateral superior oblique underaction and contralateral superior oblique overaction due to failure of the paretic eye to infraduct well in abduction giving the impression of overaction. 6. Bilateral superior oblique palsy has some unique features. These include history of closed head trauma, subjective torsion, objective torsion more than 10 degrees, alternating hypertropia on head tilts, Vpattern esotropia and a chin-down posture.
Etiology The etiology of fourth cranial nerve palsy may be congenital and acquired. 1. Congenital is the commonest cause of fourth cranial nerve palsy in many series. 2. Acquired fourth cranial nerve palsy may be due to trauma, tumor, aneurysm and iatrogenic.
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Fig. 24.2: Fourth cranial nerve palsy
Sixth Cranial Nerve or Abducens Palsy The sixth cranial nerve supplies the lateral rectus muscle. The constellation of signs and symptoms, which depend on the severity of the palsy are as follows: 1. Horizontal diplopia 2. Face turn is present towards the side of the paretic muscle. The face turn is present to avoid diplopia and may be subtle. 3. Esophoria is present in primary gaze in mild cases and esotropia with prominent horizontal incomitance in more severe palsy (Fig. 24.3). The esotropia is typically more for distance and in bilateral cases there may be a V-pattern. 4. Absence of vertical deviation.
Etiology The sixth cranial nerve palsy occurs due to infection, trauma, neoplasm, systemic vascular disorders, systemic hematological disorders, intracranial hypertension, raised intracranial pressure, inflammatory disorders and idiopathic.
Principles of Management 1. If the cause is vascular or postinfectious, many patients improve with time. It is prudent to wait 3 to 6 months for recovery before contemplating surgical intervention. 2. Conservative management options are available to tide over during the waiting period. Options include monocular occlusion, prisms either ground in or Fresnel to help patient fuse and carry on with activities of daily living.
Incomitant Strabismus This can be achieved by: a. Weakening the overacting antagonist b. Strengthening the paretic muscle after ascertaining the residual muscle function. This may be obtained by shortening of lateral rectus muscle in the sixth nerve palsy, or tuck of the superior oblique. c. If muscle function is very poor one must consider transposition procedures such as Jensen’s or Hummelsheim’s procedure. Full tendon transposition for the lateral rectus palsy and superior oblique transposition procedures for the third nerve palsy are recommended.
Monocular Elevation Deficiency
Fig. 24.3: Sixth cranial nerve palsy
3. Some patients are able to manage with a small head posture especially patients with mild sixth nerve palsy. 4. Botulinum toxin A injection into the antagonist muscle may prevent contracture and help with the recovery of the paretic muscle. 5. Pre-surgical in-office testing includes: a. To assess the amount of residual muscle function in the paretic muscle. This is done by assessment of saccades clinically or through saccadic velocity testing. This can also be done by the force generation test. b. To assess the presence of contracture of the antagonist muscle. This is done by the force duction test. 6. Surgical goals include fusion in primary gaze or with a minimal head posture and cosmesis.
Formerly named double elevator palsy, the terminology has been changed after studies showed that many patients have only superior rectus palsy without involvement of the inferior oblique.
Etiology Etiology may be congenital or acquired. Congenital causes include supranuclear defects, primary superior rectus paresis, primary inferior rectus restriction and inferior rectus restriction secondary to superior rectus paresis. Acquired deficiency may occur due to cerebrovascular disease, tumors, sarcoidosis and infectious diseases. Monocular elevation deficiency has following clinical presentations: 1. Unilateral limitation of up gaze above midline with accompanying ptosis 2. Hypotropia 3. Limitation of elevation in both abduction and adduction 4. Abnormal head posture in the form of chin elevation for fusion
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Diagnostic Procedures in Ophthalmology 5. Pseudoptosis in addition to true ptosis 6. Acquired deficiency presents with acute onset of diplopia in primary position and up gaze. Diagnosis of the etiology is based on certain clinical features. These include presence of Bell’s phenomenon, saccadic testing below and above midline and forced duction testing. Bell’s phenomenon is present if the cause is supranuclear and absent in the others. Upward saccades are normal in inferior rectus restriction and slowed in supranuclear monocular elevation deficiency below midline and absent above midline. In primary superior rectus paresis, the saccades are slowed both above and below midline. Forced duction testing is positive only if inferior rectus is restricted. A simple approach to management includes: 1. Forced duction testing on the table 2. If inferior rectus restriction is present then the muscle is recessed. 3. If forced duction test is negative consider transposition procedure such as Knapp’s transposition where the medial and lateral rectus are either split or in their entirety are transposed adjacent to the superior rectus insertion
or muscle aplasia is the primary event has not been established. Cranial nerves VI to XII may be involved. This may present as dysphagia, paralysis and hypoplasia of the tongue. The cranial nerve VIII may be spared and III and IV nerves are rarely involved. However, facial nerves are always involved. The VI nerve is involved in 75% of cases while the XII nerve in only in minority of cases.
Möbius Syndrome
Duane’s retraction syndrome (DRS) is a congenital ocular motility disorder characterized by limitation of abduction or/and adduction, palpebral fissure narrowing on attempted adduction (Fig. 24.4) due to retraction of globe and upshoot or downshoot on adduction due to leash effect of tight lateral rectus. It is postulated that a disturbance in normal embryogenic development causes Duane syndrome. It is further substantiated by the frequent associations of the syndrome with ocular anomalies and congenital abnormalities of the facial, skeletal, and CNS. Agenesis of the sixth nerve or nucleus and innervation of the lateral rectus muscle by the inferior division of third nerve nucleus cause simultaneous cocontracture
A child with Möbius syndrome usually presents in infancy. The child is brought by the caregiver for the lack of facial movements while crying and inability to smile. The child may be unable to close his mouth, may have a prominent upper lip and indistinct speech. No racial, sex predilection has been described, and the inheritance pattern is variable.
Clinical Features The clinical features include congenital facial diplegia, bilateral abducens nerve palsies and congenital bilateral incomplete facial palsy resulting in a mask-like face. Nerve, brainstem,
Systemic Findings Möbius syndrome is associated with congenital deformities. The most common deformity is clubfoot. Brachial deformities and pectoral muscle hypoplasia (Poland anomaly) are common. Brachydactyly and syndactyly are also described. CT or MRI shows calcification in the region of VI nerve. Management involves a multidisciplinary approach and is primarily cosmetic.
Restrictive Strabismus Duane’s Retraction Syndrome
Incomitant Strabismus Type 1: Duane is the most frequent classical form. Apart from the defective abduction, retraction of the globe, and palpebral fissure narrowing on adduction, additional abnormalities like A or V phenomenon, up drift or down drift of the affected eye on adduction, or attempted abduction may be present. Type 2: Duane is characterized by limitation or complete palsy of adduction with exotropia of the paretic eye, abduction appears to be normal or only slightly limited. As in Duane 1, distinct narrowing of the palpebral fissure and retraction of the globe on attempted adduction are also present.
Fig. 24.4: Duane’s retraction syndrome
of lateral rectus and medial rectus resulting in globe retraction on attempted adduction. DRS is more common in female, often unilateral and left eye is more affected (75%). Both sporadic and autosomal dominant inheritance have been reported. Duane' syndrome is traditionally classified into 3 types:
Type 3: Duane is a combination of limitation or absence of both abduction and adduction of the affected eye. In this form, adduction and abduction may be defective in the equal degree (affected eye in parallel position), or adduction more defective than abduction (affected eye in divergent position). Globe retraction and narrowing of the palpebral fissure on attempted adduction are also present. Duane' syndrome is associated with ocular and systemic anomalies. Ocular anomalies include optic nerve hypoplasia, morning glory syndrome, congenital ptosis, dysplasia of the iris stroma, cataracts, heterochromia, Marcus Gunn jaw-winking, choroidal coloboma, crocodile tears, and microphthalmos. Goldenhar syndrome, Klippel-Feil anomaly and congenital labyrinthine deafness are systemic anomalies.
Management Usually surgical results in Duane’s retraction syndrome are disappointing. Surgery is done for abnormal head posture, up shoot or down shoot and enophthalmos. The most common indication for surgical treatment is an unacceptable face turn to permit fusion. Patients who have Duane's syndrome with exotropia in primary position
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face turn away from the side of the affected eye. The face turn is treated with recession of the ipsilateral lateral rectus muscle. If primary position exotropia is present with a marked up shoot or down shoot, a lateral rectus recession usually is combined with a Y-splitting procedure or a posterior fixation suture. In up shoot or down shoot, recession of the lateral rectus muscle is effective. Posterior fixation suture: A posterior fixation suture on the lateral rectus muscle can effectively prevent slippage of the muscle belly over the globe. It may be used as an alternative procedure to treat up shoots and down shoots. Simultaneous recession of both lateral and medial rectus may help with up shoot and down shoot.
Brown’s Syndrome Brown’s syndrome is a restrictive strabismus marked by limitation of elevation that is worse when the eye is in adduction (Fig. 24.5). It is characterized by following features: 1. Limitation of elevation in adduction. There is some limitation of elevation in abduction but the limitation is more marked in adduction. 2. Minimal or no hypotropia in primary gaze 3. Minimal or no over action of ipsilateral superior oblique 4. Divergence in upgaze causing Y-pattern 5. Limited elevation in abduction can produce pseudo inferior oblique over action of the fellow eye 6. Intorsion on attempted up gaze 7. Compensatory head posture chin elevation and face turn to keep the affected eye in abduction 8. Good fusion and stereopsis 9. In adduction, palpebral fissure widens and there is a down shoot of the involved eye. 10. Forced duction test is positive
Incomitant Strabismus
Fig. 24.5: Brown’s syndrome
Eustin and colleagues graded Brown’s syndrome into 3 types: 1. Mild: Restriction of elevation in adduction, no hypotropia in primary gaze and no down shoot in adduction 2. Moderate: No hypotropia in primary gaze and down shoot on adduction 3. Severe: Hyportropia in primary gaze, marked down shoot on adduction and amblyopia, usually patients develop compensatory head posture and have fusion. If the patient presents with a manifest hypotropia and no compensatory head posture or have associated horizontal strabismus, there is increased risk of amblyopia. Brown's syndrome is classified into congenital and acquired forms. The congenital form is subdivided into true and pseudo. Cysticercosis is an important cause of acquired Brown’s syndrome. Differential diagnosis of the syndrome include double elevator palsy, blow-out fracture, inferior oblique palsy, superior oblique palsy and Duane’s retraction syndrome.
status should be closely monitored in young children. Spontaneous resolution may occur in some cases. Corticosteroids may be beneficial. In the severe form of the syndrome with hypotropia in primary gaze, surgery is indicated. Other indications for surgery include anomalous head posture, loss of binocularity in a child and cosmetically unacceptable down shoot in adduction. The surgery is based on the principle of lengthening the superior oblique tendon. Procedures such as tenotomy and tenectomy are not controlled to achieve a controlled elongation of superior oblique tendon a Wright superior oblique tendon expander can be used.
Congenital Fibrosis of Extraocular Muscles It is an autosomal dominant disease affecting the extraocular muscles. It is characterized by blepharoptosis (Fig. 24.6) and chin elevation, absence of elevation or depression of the globe with eyes fixed 20 to 30 degrees below the horizontal meridian, little or no horizontal movement and absence of Bell’s phenomenon. Fibrosis of extraocular muscles is seen on histopathology. Ductions are very limited. Amblyopia, refractive errors particularly hyperopia and astigmatism are common. Patients may also show esotropia in attempted elevation probably due to the adductive action of superior rectus muscle.
Treatment The mild and moderate form of the syndrome without strabismus in primary position should be left untreated. Visual acuity and binocular
Fig. 24.6: Congenital fibrosis of the extraocular muscles
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movement testing can differentiate between paretic and entrapped muscle. The associated severe globe damage is often rare since blowout is a protective phenomenon. Plain X-ray (Waters view) and CT-scan confirm diagnosis.
Management
Goals of management are: (a) clear the visual axis, (b) alleviate chin-up posture, and (c) align eyes in primary position. Large recessions after careful dissection of intermuscular septal attachments, with the use of preplaced sutures, give good results. Frontalis sling is the mainstay of ptosis repair. Absence of Bell’s phenomenon often necessitates long-term generous use of lubricants in these patients.
In initial phase of injury, surgery should be withheld until two weeks till edema subsides. Patient should be followed-up closely with serial diplopia and Hess charting. Surgery should be deferred if sufficient diplopia-free area is present in primary and down gaze. Surgery can be performed for significant enophthalmos. Teflon plate can be placed subperiostially. For troublesome diplopia, Fresnel prisms trial can be considered. Initially inferior rectus recession can be planned for surgical correction.
Orbital Blow-out Fracture
Bibliography
Management
Strabismus after blow-out fracture has been estimated to occur in about 58% of patients.
Clinical Features Immediately following injury with cricket or tennis ball or road traffic accidents, the patient presents with black eye and restriction of ocular movement in all directions of gaze which usually subsides by end of first week. It presents as specific restrictive strabismus with diplopia in up and down gaze due to entrapment of soft tissue in fractured fragment, hypoesthesia along infraorbital nerve and enophthalmos due to herniation of orbital contents into the maxillary sinus. The presence of muscle entrapment can be confirmed on force duction test (FDT). Saccadic
1. Ahluwalia BK, Gupta NC, Goel SR, et al. Study of Duane’s retraction syndrome. Acta Ophthalmologica (Copen) 1988;66:77. 2. Brown HW. True and simulated superior oblique sheath syndrome. Doc Ophthalmol 1973;34:123. 3. Kraft SP, Jacobson ME. Technique of adjustable suture strabismus surgery. Ophthalmic Surg 1990;21:633. 4. Park MM, Eustis HS. Simultaneous superior tenotomy and inferior oblique recession in Brown syndrome. Ophthalmology 1987;94:1043. 5. Sharma P. Strabismus simplified. New Delhi, Modern Publishers, 1999. 6. Smith RS, Damasat M. Acquired orbital retraction syndrome after 6th nerve paralysis. Neurology 1973;14:147. 7. von Noorden GK. Binocular Vision and Ocular Motility. 4th ed. St Louis, Mosby, 1990. 8. Wilson WE, Eustis HS, Park MM. Brown syndrome. Survey Ophthalmol 1989;34:153.
Diagnostic Procedures in Dry Eyes Syndrome
MS SRIDHAR
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Diagnostic Procedures in Dry Eyes Syndrome
Dry eye (DE) is a disorder of tear film due to tear deficiency or excessive tear evaporation causing damage to the interpalpebral ocular surface and associated with symptoms of ocular discomfort.1 Presently, DE is classified into 2 major groups: Tear deficient DE and evaporative DE. Tear deficient DE (ATD—Aqueous tear deficiency) is a disorder of lacrimal function causing decreased secretion of aqueous or can result from failure of transfer of lacrimal fluid into the conjunctival sac. In evaporative or tear sufficient DE, lacrimal function is normal and in most cases, the tear abnormality is due to increased tear evaporation. Meibomian gland (MG) dysfunction and blinking disorders are common causes for evaporative DE.
Clinical Features The usual symptoms of a patient with dry eyes caused by ATD include tearing, redness, burning, blurring of vision, fluctuating vision, itching, irritation, dryness, foreign body sensation, tired eyes and heaviness of lids. Symptoms tend to get aggravated in hot, arid climates and by certain occupations like exposure to chemicals, dust,
smoke and prolonged use of computer video terminals. Signs include presence of greasy scales on the lid margins suggestive of seborrheic blepharitis or crusts on the lid margin, the removal of which results in oozing of blood from the surface which is common is staphylococcal blepharitis. The tear film height is usually reduced and mucus debris or stringy discharge may be seen. The conjunctiva may appear lusterless. It may be thickened, edematous, hyperemic, or may show slight folding inferiorly. In advanced cases, the conjunctiva may be keratinized particularly in the exposed areas.2 Corneal examination may reveal fine punctate staining with fluorescein which in severe cases may appear as confluent patches. Epithelial defects may develop which may be slow to heal or may persist and predispose to rapid sterile corneal ulceration or secondary bacterial infection. Plaques or filaments on the surface of the cornea may be formed. Occasionally, clinical signs may be absent in a patient with dry eyes and hence clinical diagnostic tests are mandatory in the evaluation. The clinical signs of meibomian gland dysfunction include stenosed or pouting orifices, squamous metaplasia of the orifices (white shafts
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Diagnostic Procedures in Ophthalmology of keratin in the orifices), reduced expressibility of meibomian gland secretions and turbid or thick toothpaste like secretions of the meibomian glands. The presence of thickening of the lid margins, telangiectatic blood vessels and cysts on the lid margin may be noted. Transillumination may also reveal drop out of the meibomian gland ducts, which is a sign of obstructive meibomian gland disease. In patients with acne rosacea with inflammatory meibomitis, the gland architecture is distorted. Video meibography, using one-chip infrared video camera and a hand held transilluminating light source along with a video monitor, is useful for imaging the abnormal structure of the meibomian glands in chronic blepharitis.
Clinical Diagnostic Tests for Dry Eyes Tear Film Break-Up Time (TBUT) According to the diagnostic algorithm put forth by Plugfelder3 for diagnosing dry eye, fluorescein tear break-up time (TBUT) is the first diagnostic test to be done which gives information regarding tear film stability. A diagnosis of tear film instability is made when a fluorescein TBUT value of <10 seconds is obtained. Even though Decker in 1876 started research on tear film stability, it was Norn4 in 1965 who evolved a simple and convenient method to assess the tear film stability by observing the tear film using a cobalt blue filter on a slit-lamp, after instilling fluorescein stain. The interval between the last blink and the first appearance of a dry spot on the fluorescein-stained tear film was then called the corneal wetting time. Later term break-up time or tear break up time (BUT or TBUT) was used.5-7 Although different methods now exist like the invasive5 and non-invasive methods2, difference in the method of instilling fluorescein, number of readings taken4-6,8 and the type of slit-lamp
observation, with a narrow vertical slit, horizontal slit or a full beam9, 10, this test is of importance ever since it has been identified as the screening test in determination of dry eyes in the potential contact lens wearers.7 The clinical method of doing fluorescein tear break-up time (TBUT) is as follows: The subject is made to sit comfortably on a slit-lamp with forehead firmly against the forehead rest and chin resting comfortably on the chin rest. The microscope is positioned directly in front of the eye to be receiving the stain. The fluorescein strip is wetted with a drop of preservative-free saline. The strip is then applied over the inferotemporal conjunctiva. The subject is asked to blink for 3-5 times only and then is asked to stop blinking. The fluorescein staining is then viewed under a slit-lamp (full beam) using a blue filter along with a wratten filter with magnification. The time interval from the last blink to the appearance of a randomly seen dark spot is recorded with a stopwatch. This is followed by taking another 2 readings of TBUT with a gap of 3-5 blinks in between. Non-invasive break-up time (NIBUT): In this method, mire is projected using a keratometer, topography, perimeter or tearoscope. The time taken for the tear film to distort or break-up after a blink is measured. The reading of break-up time is less than that of invasive tear break-up time using fluorescein, but it is more difficult practically to do it in the clinic.
Schirmer’s Test 1 without Anesthesia This test assesses the reflex secretion and tear production potential. No anesthetic is used. Whatman filter paper #41 measuring 35 mm in length with a bend at 5 mm is used, and placed at the junction of medial 2/3 and lateral 1/3 of the lower lid in the fornix (Fig. 25.1). The patient is asked to look forward and blink
Diagnostic Procedures in Dry Eyes Syndrome Schirmer’s Test with Nasal Stimulation
Fig. 25.1: Schirmer’s test showing the Whatman filter at the junction of medial 2/3rd and lateral 1/3rd of lower lid in the fornix
normally. The test is carried out in dim illumination and under standard conditions of temperature and humidity. The length of wetting is recorded after 5 minutes. Wetting of less than 5 mm is considered abnormal.
Schirmer’s Test with Anesthesia Topical proparacaine is instilled into the conjunctiva. After 5 minutes, the excess fluid is wiped off with a Johnson’s bud. The fluorescein strip is then placed as mentioned above and the length of wetting is read after 5 min. While taking the reading when the front of the wetted area was uneven, the millimeter it crosses is recorded as the value. Schirmer’s test though categorized by many researchers as non-reliable11-14 is still widely used to assess adequacy of tear production15-19 to help in diagnosis of keratoconjunctivitis sicca, screening for dry eye in contact lens wearers12,20 and for analysis of chemical components of tear film.21-23 Lemp in the workshop on clinical trials of dry eyes puts forth the use of Schirmer’s test as the standard measure for diagnosing teardeficient dry eye. Validated by van Bijsterveld,24 this test is also recommended by the working group on diagnostic tests, and available for routine clinical practice.
After performing routine Schirmer’s test, a cotton swab is inserted into the nasal cavity towards the direction of ethmoid sinus. A 75 mm strip of Whatman filter paper #41 is placed in the conjunctival fornix and the length of wetting measured after 5 minutes. Wetting of less than 10 mm is considered abnormal. It is advisable to perform this test on a different occasion. Patients, who do not respond to nasal stimulation by an increase in the lacrimal secretion, are thought to have an invasion of lymphocytes into their lacrimal glands,5 resulting in anatomic destruction of the gland. Such patients do not show any response even on maximal stimulation. Patients who respond, the lacrimal gland is viable. When patient responds to nasal stimulation but is less responsive to conjunctival stimulation, it is postulated that the reflex circuit between the lacrimal gland and the conjunctiva is disturbed.
Diagnostic Dye Staining: Fluorescein and Rose Bengal Stain The instillation of dyes is a common method to detect ocular surface epithelial pathology associated with dry eyes. Rose bengal is a fluorescein derivative that has been used for the diagnosis of dry eye since Sjögren25 described the presence of dye staining in patients with keratoconjunctivitis sicca (KCS) in 1933. It was thought to stain only devitalized epithelial cells but it also stains healthy epithelial cells when they are not protected by a healthy layer of mucin.26 Therefore, it has the unique property of evaluating the protective status of the preocular tear film. Rose bengal also stains dead or degenerating cells, lipid-contaminated mucous strands, and corneal epithelial filaments (Fig. 25.2). Solution is preferred over impregnated
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Fig. 25.2: Diffuse slit-lamp view showing rose bengal staining (arrow) in a patient with aqueous tear deficiency (ATD)
Fig. 25.3: Diffuse slit-lamp view showing fluorescein staining with filaments in a dry eye patient
strips.27 A double vital staining technique was described at the NEI Workshop.1 A 2 μl mixture of 1% rose bengal and 1% fluorescein (preservative-free) and non-preserved saline without anesthetic, is instilled into the conjunctiva. The areas of staining are graded on slit-lamp examination. The interpretation of rose bengal staining in dry eyes is based on two factors, intensity and location. Van Bijsterveld 28 reported a grading scale that evaluates the intensity based on a scale of 0 to 3 in three areas: nasal conjunctiva, temporal conjunctiva and cornea, with a maximum possible score of 9. The classic location for rose bengal staining in aqueous tear deficiency is interpalpebral conjunctiva, which appears in the shape of two triangles whose bases are at the limbus. The NEI workshop has recommended division of nasal and temporal conjunctiva into 3 zones, each graded from 0-3, with a maximum possible score of 18. Rose bengal staining is considered more sensitive and more specific in detecting patients with dry eyes than either reduced tear breakup time or a low Schirmer’s test. Rose bengal staining may help to differentiate between ATD and lipid tear deficiency (LTD) by studying the distribution of stain in the non-exposure zone.
Preferential staining has been observed in nonexposure zones in the LTD, whereas in ATD, the staining is seen in the exposed interpalpebral areas.29 Fluorescein is another diagnostic dye commonly used for diagnosis of dry eye. The dye penetrates intercellular spaces and indicates increased epithelial permeability.26 Fluorescein generally stains the cornea more than the conjunctiva (Fig. 25.3). Lissamine green B has been investigated as a marker for ocular surface disease. It is found to detect dead or degenerated cells and it produces less irritation after topical administration than rose bengal.
Fluorescein Clearance Test Schirmer’s Strip Ten microlitre of 0.5% fluorescein and 0.4% oxybuprocaine hydrochloride are instilled into the conjunctival fornix. The eyes are kept open for 5 minutes. 35 mm long strips of Whatman filter paper #41 is placed in both the eyes. The eyes are then closed for 5 minutes. The intensity of color is compared to a standard scale. Each grade shows a 12.5% increase in the basal tear turnover and tear flow. This method may not
Diagnostic Procedures in Dry Eyes Syndrome reflect the basic tear secretion since reflex tearing may occur in response to the slit-lamp illumination or irritation by the strip. The length of wetting may affect the intensity of fluorescein color on the strip. In general the darker the color of fluorescein, the less or poor is the clearance.
Visual Scale This is a safe and inexpensive method that
corroborates with irritation symptoms. Six microlitre of 2% fluorescein is instilled into the inferior cul-de-sac. Fifteen minutes later, the color of the lower tear meniscus at the lateral 1/3 of lower lid is compared with the standardized scale (3 = Normal, >3 = disease, <3 = equivocal). A simple diagnostic algorithm is given below to place a given patient in one of the categories of dry eyes.
Diagnostic Algorithm for Dry Eyes
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Laboratory Tests
Lysozyme and Lactoferrin Assays
Tear Film Osmolarity
Though lysozyme and lactoferrin are found to be low in dry eye, the tests for their evaluation are cumbersome and expensive and hence not recommended for use in clinical practice.
Tear film (TF) osmolarity is said to represent the gold standard in the diagnosis of DE because of its greater sensitivity and specificity as a single test or in combination with other tests. Though, it is unable to distinguish between ATD DE and Evaporative DE. The osmolarity of basal tears is measured and thus reflex tearing has to be avoided. There exists a need for an instrument that is more reliable and freely available for testing TF osmolarity. Technical errors resulting in falsely abnormal values are reported.
Tear Ferning Conjunctival mucus from a normal eye crystallizes in the form of ferns when placed on a dry glass slide and observed under the microscope. The scrapings are obtained from lower nasal palpebral conjunctiva, 30 immediately following drying; the slide is evaluated under a microscope to find typical mucus arborization or ferning. Ocular ferning test from conjunctival scrapings is considered as a quantitative test for mucin deficiency. The conjunctival mucus may be reduced or absent in those patients with conditions like chemical burns, ocular cicatricial pemphigoid and Stevens-Johnson syndrome.
Conjunctival Impression Cytology The surface of the normal conjunctiva contains goblet cells that produce mucin. In cases of advanced dry eye, the epithelium undergoes pathologic changes, termed squamous metaplasia, and the density of goblet cells decreases. As a result, the tear film becomes unstable secondary to a reduction in the mucin layer of the tear film. Conjunctival impression cytology allows the evaluation of epithelium and goblet cells on the conjunctival surface.
Measurements of Immnoglobulins and Antibodies Measurements of IgA, IgG, IgM and viral antibodies 31 in tears by ELISA have been tried as laboratory tests in the diagnosis of dry eyes. Since constitutive protein concentration in the tears varies with flow rate, tear collection must be standardized and it has to be assured that only non-stimulated tears are obtained.32
Serum Autoantibodies Detection of serum autoantibodies is used to diagnose Sjögren’s syndrome ATD. One or more of the following autoantibodies may be found: Antinuclear antibodies (ANA titer ≥1:160), rheumatoid factors (RF titer ≥1:160) or Sjögren’s syndrome–specific antibodies such as anti-Ro (Sjögren-A) or anti-La (Sjögren-B). In one study33 antinuclear antibodies (ANA) were the most frequently detected antibodies in ATD being present in 80%. In contrast, positive RF was found in 65% and positive SS-A in 30% of the same group of patients.34
References 1. Lemp MA. Report of the National Eye Institute/ Industry Workshop on Clinical Trials in Dry Eyes. CLAO 1995;21:221-32. 2. Tabbara KF, Wagoner MD. Diagnosis and management of dry eye syndrome. Int Ophthalmol Clin 1996;36:61-76. 3. Pflugfelder SC, Tseng SCG, Sanabin O, et al. Evaluation of subjective assessment and objective diagnostic tests for diagnosing tear film disorders known to cause ocular irritation. Cornea 1998;17:38-56.
Diagnostic Procedures in Dry Eyes Syndrome 4. Norn MS. Desiccation of precorneal film. I Corneal wetting-time. Acta Ophthalmol (Kbh) 1969;47:865-80. 5. Lemp MA, Hamill JR. Factors affecting tear film break up time in normal eyes. Arch Ophthalmol 1973;89:103-05. 6. Rengstorf RH. The precorneal tear film break up time and the location in normal subjects. Am J Optom Physiol Opt 1974;51:765-69. 7. Holly FJ. Tear film physiology and contact lens wear. II. Contact lens tear film interaction. Am J Optom Physiol Opt 1981;58:331-41 8. Vanley GT, Leopold IH, Gregg TH. Interpretations of tear film break up. Arch Ophthalmol 1977;95:445-48. 9. Cho P, Brown B. Review of TBUT technique and a closer look at the TBUT of HK-Chinese. Optom Vis Sci 1993;70:30-38. 10. Cho P, Brown B, Chan I, Conway R, Yap M. Reliability of the tear film break up technique of assessing tear stability and the locations of tear break up in Hong Kong Chinese. Optom Vis Sci 1992; 69:879-85. 11. Wright JC, Meger GE. A review of Schirmer’s test for tear production. Arch Ophthalmol 1962;67:773-82. 12. Tabak S. A Short Schirmer’s test. Contacto 1972;16(2):38-42. 13. Hanson J, Fikentscher R, Rosenberg B. Schirmer’s test of lacrimation. Arch Ophthalmol 1975;101:293-95. 14. Henderson JW, Prough WA. Influence of age and sex on flow of tears. Arch Ophthalmol 1950;43:224-31. 15. Mishima S, Gasset A, Klyce SD, Baum JL. Determination of tear volume and tear flow. Invest Ophthalmol 1966;5(3):264-76. 16. Jones LT. Lacrimal secretory system and its treatment. Am J Ophthalmol 1966;62:47-60. 17. Shapiro A, Merin S. Schirmer test and break up time of tear film in normal subjects. Am J Ophthalmol 1979;88:752-57. 18. Hamano H, Hori M, Hamano T, Mitsunaga S, Maeshima J, Kojima S, Kawabe H, Hamano T. A new method for measuring tears. CLAO J 1983;9:281-89. 19. Rajiv, Mithal S, Sood AK. Pterygium and dry eye – a clinical correlation. Indian J Ophthalmol 1991;39:15-16.
20. Clinch TE, Benedetto DA, Feldberg NT, Laibson PR. Schirmer’s test: a closer look. Arch Ophthalmol 1983;101:1383-86. 21. Prause JU. Immunoelectrophoretic determination of tear fluid proteins collected by the Schirmer I test. Acta Ophthalmol (Kbh) 1979; 57:959-67. 22. Kiljstra A, Jeurissen SHM, Koning KM. Lactoferrin levels in human tears. Br J Ophthalmol 1983;67:199-202. 23. Stuchell RN, Feldman JJ, Farris RL, Mandel ID. The effect of collection technique on tear composition. Invest Ophthalmol Vis Sci 1984; 25:374-77. 24. Van Bijsterveld OP. Diagnostic tests in the sicca syndrome. Ann Ophthalmol 1969;82:10-14. 25. Sjögren HS. Zur kenntnis der keratoconjunctivitis sicca (keratitis folliformis ber hypofunktion der ranendrasen). Acta Ophthalmol (Copen) 1933; 11:1-151. 26. Feenstra RPG. Tseng SCG. Comparison of fluorescein and rose bengal staining. Ophthalmology 1992;101:984-93. 27. Roberts DK. Keratoconjunctivitis sicca. J Am Optom Assoc 1991;62:187-99. 28. Van Bijsterwald OP. Diagnostic tests in the sicca syndrome. Ophthalmology 1969;82:10-14. 29. Lee SH, Tseng SCG. Rose Bengal staining and cytologic changes associated with meibomian gland dysfunction. Am J Ophthalmol 1997;124: 736-50. 30. Tabbara KF, Okumoto M. Ocular ferning test. A qualitative test for mucus deficiency. Ophthalmology 1982;89:712-14. 31. Coyle PK, Sibony PA. Viral antibodies in normal tears. Invest Ophthalmol Vis Sci 1988;29:10. 32. Fullard RJ, Snyder C. Protein levels in non-stimulated and stimulated tears of normal human subjects. Invest Ophthalmol Vis Sci 1990;32:8. 33. Pflugfelder SC, Whitcher JP, Daniels TE. Sjögren’s syndrome In: Pepose J, Holland G, Whilhelmus K (Eds). Ocular infection and immunity. St. Louis, Mosby, 1997. 34. Pflugfelder SC, et al. Chronic Epstein-Barr viral infection and immunologic dysfunction in patients with aqueous tear deficiency. Ophthalmology 1990;97:313-23.
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AK GROVER, RITURAJ BARUAH
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Evaluation of Epiphora
Applied Anatomy and Physiology of the Lacrimal Apparatus Lacrimal Gland The lacrimal gland is an exocrine gland (20×15×5 mm) that lies in the lacrimal fossa formed by the frontal bone in the anterosuperior lateral orbit. It is divided into a larger orbital lobe and a smaller palpebral lobe by the fibrous extensions from the Whitnall ligament, levator aponeurosis and its lateral horn.1 It secretes tears through a series of ducts (10-12) into the conjunctival sac just in front of the superior fornix, 5 mm above the lateral tarsal border. Two to six ducts from the orbital portion run through and join the ducts of the palpebral lobe. Removal or damage to the palpebral lobe can thus lead to significant decrease in tear secretion. There are accessory exocrine glands of Krause and Wolfring that are located in the superior fornix and above the superior border of the tarsus, respectively. These glands have got no apparent nerve supply. The blood supply of the lacrimal gland is from the lacrimal branch of the ophthalmic artery. Venous blood drainage is via the ophthalmic vein.2
Nerve Supply The lacrimal gland has got sensory, secretomotor and, sympathetic supply. Sensory supply comes through the lacrimal branch of the ophthalmic division of the Vth cranial nerve. Secretomotor supply is via the parasympathetic fibers. Parasympathetic preganglionic fibers arise from the lacrimal nucleus in the pons near the glossopharyngeal nucleus. Sympathetic postganglionic fibers come from superior cervical ganglion and reach the lacrimal gland via deep petrosal and also along with the sympathetic fibers around lacrimal artery and nerve.2
Lacrimal Excretory Apparatus The lacrimal excretory apparatus consists of the upper and the lower puncta, canaliculi, tear sac and the nasolacrimal duct (Fig. 26.1). Puncta: These are small, round to oval orifices of about 0.2 mm in diameter on the summit of an elevation, the papilla lacrimalis that lies near the medial end of the eyelid margins at the junction of its ciliated and the non-ciliated parts in line with the openings of the meibomian glands. The puncta, being relatively avascular
Evaluation of Epiphora (0-5 mm long). It then enters a small diverticulum of the sac, the lacrimal sinus of Maier at a point on the posterolateral surface of the sac about 2.5 mm from the apex of the sac. The common canaliculus is directed anteriorly forming an acute angle of about 45o with the sac before entering it. This acute entry into the lacrimal sac creates a potential mucosal flap or valve across the opening, the valve of Rosenmuller.2,3 The canaliculi are lined by stratified squamous epithelium supported with elastic tissue that can be dilated to three times the normal diameter.2 Fig. 26.1. Lacrimal system
is paler than its surrounding, serving as a guide in case of finding a stenosed punctum. The upper punctum is slightly medial relative to the lower but when the eyelids are closed they appose each other. The medial ends of the lower lid retractors also help stabilize the puncta and prevent punctal eversion on blinking. The patency of the puncta is maintained by the surrounding dense fibrous tissue continuous with the adjacent tarsal plate. Canaliculi: The canaliculi are hollow tubes of 0.5 mm diameter connecting the puncta to the lacrimal sac. Each canaliculus has a vertical part, which is 2 mm in length and a horizontal part of 8-10 mm, which follows the eyelid margin converging towards the medial canthus. The canaliculi are enveloped by the orbicularis muscle fibers and elastic tissue, except on the posterior walls, which are covered by conjunctiva through which the probe can be easily seen. The upper is slightly shorter than the lower. There is a dilatation at the junction of these two parts, which is the ampulla. The canaliculi pierce the periorbita of the lacrimal sac separately, uniting at an angle of 25o to form a short common canaliculus
Lacrimal sac: The lacrimal sac lies in the lacrimal fossa formed by the lacrimal bone and the frontal process of the maxilla in the anterior part of the medial wall of the orbit which is continuous below with the nasolacrimal duct. Vertical suture line between the frontal process of the maxilla and the lacrimal bone is slightly medial to the middle of the floor of the fossa. This is of surgical importance because in dacryocystorhinostomy operation, the first bony opening is made through this line. The lacrimal sac is 12-15 mm tall, 4-6 mm anteroposteriorly and 2-3 mm wide. The sac above the junction of the common canalicular duct is known as fundus. An imaginary line drawn from the medial canthus to the first upper molar tooth that slopes downward and backward at 15-25° indicates the long axis of the sac. A portion of the periorbita, which splits at the posterior lacrimal crest, encloses the lacrimal sac and then joins at the anterior lacrimal crest forming the anterior and the posterior lacrimal fascia, respectively. Anterior ethmoidal air cells and vessels are medial to the upper part of the sac, the cribriform plate and the frontal sinus floor lie superior to the sac. Between the posterior surface of the sac and the posterior lacrimal fascia there is a vascular plexus; injury to the plexus cause troublesome bleeding. Anteriorly, the upper
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Diagnostic Procedures in Ophthalmology part of the sac is in close contact with the medial palpebral ligament so much that the ligament may have to be divided near its attachment to the anterior lacrimal crest for complete mobilization of the sac. The angular vein crosses the ligament subcutaneously 8 mm from the medial canthus; however, the position of the vein is not always constant. Incision for removal of the sac should not be more that 2-3 mm medial to the medial canthus. Nerve supply of the lacrimal sac is by the infratrochlear branch of the nasociliary nerve. Blood supply of the lacrimal sac is via the dorsalis nasi and medial superior palpebral, both being branches of the ophthalmic artery, angular branch of the facial artery, branch of the external maxillary artery and the infraorbital branch of the internal maxillary artery. Venous drainage is through the rich venous plexus surrounding the sac into the angular vein.
Hasner, which prevents air from entering the lacrimal sac on sudden blowing the nose. The duct opening varies in size, shape and also the site of opening. The duct is surrounded by a network of venous plexus. The plexus of vessels when engorged is sufficient to obstruct the duct. Nerve supply to the duct is by the infratrochlear and the anterior superior alveolar nerves. Blood supply of the nasolacrimal duct is from the palpebral branches of the ophthalmic, angular and infraorbital arteries and nasal branch of the sphenopalatine. Venous drainage is via the angular and the infraorbital vessels above and below into the nasal veins. Lymphatics of the nasolacrimal duct pass onto the submandibular and deep cervical nodes.
Nasolacrimal duct: The nasolacrimal duct is a continuation of the lacrimal sac. There is only a slight constriction at the junction of the nasolacrimal duct and the sac. The long axis is along the line joining the medial canthus to the first molar. It can be identified from outside by the more numerous and prominent vein surrounding the duct than the sac and from inside by the focal narrowing (valve of Krause) at the junction. The duct also has a thicker wall which becomes apparent on incision. The nasolacrimal duct can be divided into two parts, an interosseous part (12 mm approximately) and an intermeatal part (5 mm approximately). It lies embedded in a bony canal formed medially by the maxillary bone and laterally by the lacrimal bone above and the inferior conchea below. The nasolacrimal duct opens on the lateral wall of the nasal cavity about 10 mm posterior to the anterior end of the inferior conchea and 30 mm from the external nares. The duct opening has a mucosal fold, the valve of
Orbicularis oculi is the muscle that acts as the protector of the eyes through its blinking action. It has got two main parts—the orbital and the palpebral part. Our main concern here is the latter. The palpebral part is again divided into pretarsal—that part of orbicularis lying over the tarsus and the preseptal part over the orbital septum. The insertions of the orbicularis at the medial canthus around the lacrimal sac are called heads. The preseptal part has its superficial head inserted into the medial canthal tendon and the deep head into the fascia on the dome of the lacrimal sac and into the upper part of the posterior lacrimal crest. The pretarsal part has its circumferential fibers oriented over the superior and inferior tarsal plate. Laterally, it originates from the horizontal raphe and also from the lateral orbital tubercle. Medially, it is inserted into the anterior and posterior lacrimal crest by its two heads—the superficial and the deep head.
Orbicularis Oculi
Evaluation of Epiphora Two other muscle strips, the marginal preciliary and the retrociliary (muscle of Riolan) exist that are part of the pretarsal part of the orbicularis oculi. Nerve supply to the orbicularis oculi muscle is by the facial nerve.
Tear Secretion and Elimination4-6 Tear secretions are mainly by the two sets of glands—lacrimal and the accessory glands of Wolfring and Krause. The accessory glands are the basal secretory that give a constant supply of tears as they lack any known innervations. The lacrimal gland is the reflex secretor. Autonomic stimulation; emotional stimulation; conjunctival, corneal or uveal irritation or irritative foci in the sinus, mouth, ear, or teeth lead to reflex tearing. It can accompany yawning, laughing, sneezing and coughing. Tears are distributed along the conjunctival fornices, precorneal tear film and the marginal tear strips. Approximately 25% of the secreted tears are lost by evaporation. Rest are drained through the lacrimal drainage system via the punctum, the canaliculi, the sac, the nasolacrimal duct and ultimately into the inferior meatus of the nose. About 60% of the tears are drained via the lower punctum but in case of abnormalities the upper punctum can function efficiently without overflow.6 The blinking action of the eyelids helps in driving the tears forward into the drainage system. Blinking displaces the upper as well as the lower lid medially due to the firm attachment of the orbicularis to the medial canthal tendon. Moreover, with each blink the upper and lower lid approximates at the lateral canthal area and then proceeds towards the medial canthus displacing the tear film towards the puncta. Even in the absence of blinking, low flow of tears occurs through the puncta due to the Krehbiel phenomenon, capillary action and the
normal downhill slope of the eyelids but along with a passive reflux into the lacrimal lake. With the start of the blink the tears are propelled towards the puncta. As the process continues the open puncta move towards each other and occlude. Due to the orientation of the superficial and the deep heads of the orbicularis around the canaliculi and their firm attachment to the bone, the eyelid is pulled medially on contraction, the canaliculi are shortened squeezing the tear already in the ampulla and the canaliculi into the sac. The attachment of the deep head of the preseptal part of the orbicularis into the fascia on the dome of the lacrimal sac pulls the sac laterally on contraction enabling the sac filling (negative pressure). The valve of Rosenmuller reduces backflow, once the tears are in the sac. On opening the eyes, the muscle around the canaliculi relaxes leading to the flow of tears into canaliculi again due to the reduced intracanalicular pressure. Tears can also gravitate down the nasolacrimal duct passively. But active drainage into the nose is by the complex action of the Horner muscle.
Evaluation of Epiphora Tearing can broadly be grouped under two main headings: 1. Lacrimation (Hypersecretion of tears) 2. Epiphora (Impairment of drainage) It is essential to differentiate between two in planning the management. • Epiphora is a condition where there is excessive tearing due to reduced tear outflow, i.e. defective tear drainage. Obstruction at any point along the lacrimal drainage pathway, from the punctum to the nose can cause epiphora. This can lead to epiphora varying in severity from intermittent epiphora with a partial block to tears
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History A meticulous history taking is vital to the evaluation. Patient‘s symptoms, past ophthalmic, nasal and medical history should be elicited. A history of allergic diathesis and use of drugs should be obtained. In case of congenital tearing, parents may complain of constant tearing with minimal or no mucopurulent discharge suggesting upper
system block (punctal or canalicular dysgenesis). Constant tearing with frequent mucopurulent discharge and matting of the lashes suggest nasolacrimal duct block. Intermittent tearing with mucopurulence may suggest intermittent obstruction of the nasolacrimal duct (impaction of a swollen inferior nasal turbinate associated with an upper respiratory tract infection).
Examination The lacrimal examination can be divided under three heading: 1. Periorbital, lid and lacrimal system assessment General examination of the face, periorbital and medial canthal areas and eyelids is essential. It includes: • Slit-lamp examination of the puncta, external eye and tear meniscus, • Syringing, • Diagnostic probing and • Fluorescein dye test. 2. Examination of the nasal cavity 3. Radiological examination 4. Newer modalities
Periorbital, Lid and Lacrimal System Assessment8 General examination of the face and periorbital region: Examination of these parts with relevance to the symptoms help in establishing a diagnosis. Eyelid malposition, facial and periorbital asymmetry should be looked for. Lacrimal sac swelling: A lump over the medial canthal area below the medial palpebral ligament strongly indicates to a lacrimal sac swelling (Fig. 26.2). Evidence of inflammation: Fistula (Fig. 26.3) and inflammation over the sac area need to be further evaluated.
Evaluation of Epiphora
Fig. 26.2: Swelling above the area of the lacrimal sac in a child
Fig. 26.3: Acute dacryocystitis with fistula
Shortening of anterior lamella: Vertical eyelid tightness should be checked by asking the patient to look up at the ceiling. If there is short anterior lamella, the ectropion will be exacerbated. Assessment of puncta: All the four puncta should be looked for the presence of any stenosis or membrane blocking them. They should face towards the lacrimal lake. The relative position of the upper and the lower puncta to each other and to the caruncle should also be assessed. Eyelid laxity: The eyelid can in itself be a cause of epiphora. Involutional ectropion often progresses from punctal eversion to involve the medial third, then the medial half of the lower eyelid and eventually the entire lid. Examination
of the lid with utmost care is needed to diagnose the condition. Horizontal laxity of the eyelid can be estimated by Pinch test and snap back test: • Pinch test: Using the thumb and the index finger, the lid is pulled firmly away from the globe, the distance between the lid and the eye is measured and the laxity is documented as: None 5 mm Minimal 5-7 mm Mild 8-9 mm Moderate 10-12 mm Severe >12 mm • Snap back test: The speed with which the lower lid settles back against the globe after being pulled down and released is to be observed, as well as whether there is a short gap between the lid and globe once settled and before the first blink. • Medial canthal tendon laxity: It is always to be assessed while evaluating a case of epiphora. It is graded with the lateral distraction test and by noting the position of the lower punctum in relation to the upper. These tests depend on the fact that the lower puncta normally lies at the plica at rest and also when pulled laterally. The patient is made to sit in front of the examiner at arm length distance with their eyes at the same level. The patient is asked to look at the bridge of the examiner’s nose and without inducing accommodative convergence by moving too close, the lower punctum position is noted relative to the upper. -1 Punctal medialization 0 Normal +1 Midway between the plica and the medial limbus +2 In line with the medial limbus +3 - +6 Beyond the medial limbus
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Diagnostic Procedures in Ophthalmology • Lateral distraction test: After noting the resting position of the lower punctum, the lower lid is pulled laterally and the position of the punctum is noted again. The test is graded as: 0 No distraction at all +1 Punctum reaches midpoint of plica and medial limbus +2 Punctum reaches medial limbus +3 Punctum reaches midpoint of medial limbus and pupil line +4 Punctum reaches pupil line +5 Punctum reaches midpoint of pupil line and limbus line +6 Punctum reaches lateral limbus
Fig. 26.4: Involutional ectropion
Slit-lamp Examination The slit-lamp examination is an essential part of evaluation. • Punctum should be evaluated for patency, size, position and discharge. • Mild degrees of ectropion (Fig. 26.4) and entropion (Fig. 26.5) that are not apparent to gross external examination may be revealed on the slit-lamp biomicroscopy. Small lesions of eyelid margins like papillomas, molluscum contagiosum, chalazia, nevi and carcinoma are best detected with the slit-lamp. • Pressure over the lacrimal sac may cause discharge from the punctum, suggesting nasolacrimal duct obstruction. • Presence of inflammation on the area overlying the canaliculus and discharge from the punctum on pressure over the area may suggest canaliculitis (Fig. 26.6). • Examination for the signs of blepharitis (Fig. 26.7) as well as dry eye syndrome which lead to hypersecretion of tears should be looked for. Conjunctival lesions particularly pinguecula and pterygium may induce tearing. The forniceal and palpebral conjunctiva should be inspected for follicles
Fig. 26.5: Congenital entropion
and papillae of reactive inflammatory disorders and allergic conjunctivitis. • Cornea should be examined for any irregularities, features of dry eye syndrome or epithelial dystrophies. These examinations help to rule out causes of hyperlacrimation. • The vertical height of the tear meniscus is to be measured prior to instillation of any eyedrops. Staining the tear film with a small amount of fluorescein aids in assessing the volume of the tear lake.
Evaluation of Epiphora
Fig. 26.6: Canaliculitis
Fig. 26.7: Blepharitis
Schirmer Test Schirmer test (Fig. 26.8) helps us to exclude pseudoepiphora. For this test white filter paper strips (41 Whatman) of 35 mm in length and 5 mm width are used. They are folded 5 mm at one end and inserted into the inferior fornix at the junction of the middle and lateral third of the lid and allowed to remain in this position for 5 minutes with the eyes open. The patient should be comfortably sitting in a dimly lit room away from direct air source as the fan. Moreover there should not be any kind of verbal stimulation. After the end of the 5 minutes, the wetting of the filter paper is measured.
Fig. 26.8: Schirmer test
Schirmer test is basically of three types: Schirmer I test, Basic secretion test and Schirmer II test. Schirmer I test is performed without topical anesthetic. Ten mm or more wetting is taken as normal. Excessive wetting can be due to pseudoepiphora or hypersecretion. Basic secretion test is done as the Schirmer I test but after instillation of topical anesthetic into the lower fornix. This anesthesia eliminates the local source of irritation as by the Schirmer test strips and gives an estimate of the basic tear
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Diagnostic Procedures in Ophthalmology secretion by the glands of Krause and Wolfring.9,10 Wetting of less than 10 mm after 5 minutes indicates deficiency of basic tear production. A tearing patient with patent lacrimal drainage system with deficient basic tear production indicates towards reflex hypersecretion. Schirmer II test measures the reflex tearing from the main lacrimal gland after eliminating the local causes of irritation. After anesthetizing the conjunctival sac, the trigeminal nerve is stimulated either with a cotton-tipped applicator applied to the nasal mucosa or with ammonium chloride on a cotton pledget held at the external nares. The amount of excess wetting in addition to that of the basic secretion test is the reflex secretion.
Fig. 26.9: Instruments used for syringing and probing
Syringing Syringing the canalicular system provides information regarding the patency status. One to two drops of topical anesthesia (proparacaine or tetracaine) is instilled into the conjunctival sac. The punctum is dilated gently by advancing the Nettleship dilator (Figs 26.9 and 26.10), first
Fig. 26.10: Dilatation of punctum and syringing
Evaluation of Epiphora vertically for about 2 mm and then horizontally with a twisting movement. Simultaneously, lateral traction is applied to the eyelid. With the eyelid stretched, the dilator is withdrawn and the lacrimal cannula attached with syringe filled with normal saline is advanced horizontally through the punctum and the canaliculus (Fig. 26.10). No resistance should be felt in its entire path. Irrigation is then done and the patient is asked to respond if fluid passes into the oropharynx or nose. If there is resistance to irrigation, obstruction is present. Regurgitation of fluid from the same punctum indicates that there is a canalicular block. Regurgitation of fluid from the upper punctum indicates blockage at the level of common canalicular duct, lacrimal sac or nasolacrimal duct. Immediate regurgitation of clear fluid usually suggests a common canalicular obstruction. Relatively delayed regurgitation of fluid mixed with mucus or pus usually indicates NLD blockage.
Diagnostic Probing Probing the canaliculi provides information regarding the site of obstruction, which is necessary for decision-making. It is performed only after obstruction is demonstrated by other tests. After topical anesthesia of the conjunctival sac, the canaliculi are also irrigated with anesthetics. A probe of appropriate size is inserted into the punctum after dilatation and advanced till it meets obstruction. First it is passed vertically through the punctum, turned medially and advanced until it encounters the lacrimal bone (Fig. 26.11A). Through out the procedure the lid should be firmly pulled laterally so that there is no kinking of the canaliculi. It is then withdrawn a few millimeters and rotated inferiorly and slightly posterolaterally until the proximal part of the nasolacrimal duct is felt. The probe is then
passed until it strikes the floor of the nose in the inferior meatus (Fig. 26.11B). If in between any obstruction is felt, the site of obstruction is noted by grasping the probe with a forceps at its entrance before withdrawing.
A
B Figs 26.11A and B: Dilatation of punctum and probing
Obstruction can be felt as a “soft stop” in case of a stenosis of the canaliculus or as a “hard stop” as the probe hits the bone at the medial wall of the lacrimal sac. Obstruction at less than 8 mm indicates a canalicular block, 8-10 mm indicates a common canalicular obstruction and distal to that if the probe passes more than 10 mm. While probing a child, a few considerations should be noted. Probing is usually recommended through the upper canaliculi as the lower canaliculus carries more tear flow than the upper and it is wise to avoid the possibility of injury to it. Up to 1 year of age, the distance from the punctum to the nasolacrimal duct is approximately 12 mm, whereas, to the floor of the nose, it is approximately 20 mm.11
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Diagnostic Procedures in Ophthalmology Fluorescein Dye Test Dye disappearance test or fluorescein dye retention test: This is a semiquantitative test of delayed or obstructed tear outflow. It is of particular importance for the evaluation of congenital dacryostenosis in infants and toddlers where lacrimal irrigation is impossible without anesthesia and deep sedation. One drop of 2% fluorescein is instilled into the unanesthetized conjunctival sac of both the eyes. The volume of the tear lake is then noted preferably under the cobalt blue light. The patient is instructed not to wipe the eyes. The tear lakes are examined 5 minutes later, and the relative volume is determined. Persistence of significant dye and especially asymmetric clearance of the dye from the tear meniscus over a 5 minutes period indicates a relative obstruction of the side retaining the dye.12,13 Jones tests: 14,15 The Jones tests are dye tests for functional epiphora where the lacrimal drainage system observed to be patent on syringing. There are two types of Jones tests (Figs 26.12A and B). • Jones tests I: It investigates the lacrimal outflow under normal physiological conditions. Fluorescein (2%) is instilled into the conjunctival sac and presence of the dye at the inferior meatus is noted at 2 minutes and 5 minutes with the help of a cotton tip applicator. Rate of false negative is very high with this test. • Jones tests II: It is a nonphysiological test that determines the presence or absence of fluorescein in the irrigating saline fluid retrieved from the nose. Flushing of the residual dye (of the unsuccessful Jones test I) from the conjunctival sac is done and after that topical anesthesia is instilled into the conjunctival sac. Patient is seated with head tilted forward and a transcanalicular irrigation with saline is done. Patient is then asked to blow or spit
Figs 26.12A and B: Jones test I and II
the fluid onto a paper tissue and fluorescein dye is looked for. A positive test is with the presence of the dye on the tissue paper suggesting that the dye had reached the lacrimal sac but in the presence of a narrowed nasolacrimal duct or a nonfunctioning lacrimal pump requiring the syringing pressure to force it down. The test is said to be negative when the tissue is clear of any dye indicating that it did not get into the lacrimal sac with the Jones I test as in eyelid malposition, lacrimal pump failure, punctal or canalicular stenosis. A positive Jones test II confirms anatomical patency with a high-pressure wash out of fluorescein.
Modifications of Tests Taste test: One drop of saccharin is instilled into the conjunctival sac and one gets the taste of
Evaluation of Epiphora it after several minutes in case of a patent lacrimal drainage system. Endonasal dye test: This is done as the Jones test I and presence of the dye is seen through an endoscope inserted into the nares. Oropharynx dye appearance test: Fluorescein 2% is instilled into the conjunctival sac of one side at a time and the oropharynx is checked periodically for the appearance of the dye. This test is of particular importance in infants where sedation or anesthesia is otherwise needed.
Examination of Nasal Cavity The key to success of a dacryocystorhinostomy surgery lies in the intact anatomy of the nasal cavity. Moreover, pathology of the structures around the opening of the nasolacrimal duct itself may be the cause of epiphora. Examination of the nasal cavity can be done either with a nasal speculum or more completely with a rigid nasal endoscope. Treatment of the existing pathology is necessary before contemplating surgical intervention.
Fig. 26.13: Dacryocystography showing passage of contrast into the nasal cavity
Ancillary Radiological Investigations Radiological tests help in confirming the site of obstruction or stenosis in case of blocked syringing, confirm a functional cause of epiphora and delineate the anatomical as well as the pathological process pertaining to the problem. Dacryocystography Dacryocystography (Figs 26.13 and 26.14) is of importance in case of blocked syringing to locate the site of obstruction. Moreover, it gives additional information regarding any fistula or intrasac pathology. After instillation of local anesthesia, a fine catheter is introduced into the canaliculus (preferably the superior one) and 0.5-2 ml of water soluble iodinated contrast medium is injected
Fig. 26.14: Dacryocystography showing pooling of contrast in the lacrimal sac (NLD obstruction)
continuously during either conventional tomography or CT acquisition. CT dacryocystography is considered superior to conventional one as it provides useful anatomical information about the orbital wall, sinus as well as allowing evaluation of the nasolacrimal duct. MRI dacryocystography provides the same information as the conventional studies, without the use of catheterization and contrast medium. Both the sides are preferably done simultaneously
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underlying cause of epiphora which may be due to craniofacial injury, congenital deformities or lacrimal sac neoplasia. The paranasal sinuses, especially the maxillary sinuses are imaged for any abnormalities that might be affecting the nasolacrimal duct. Preoperative assessment of the cribriform plate is noted for any abnormal position to avoid a possible cerebrospinal leak at the time of surgery.
Newer Modalities Chemiluminescence test 22 : Cyalume, a chemiluminescent material is injected with a sialography catheter to demonstrate the patency of outflow passages. Dacryoscopy: Dacryoscope, a mini rigid endoscope allows the direct visualization of the interior and the lining of the lacrimal passages.23,24 Fig. 26.15: Dacryocystography after digital subtraction
Dacryoscintigraphy18-20 Functional epiphora becomes difficult to differentiate from partial block of the lacrimal drainage system. Dacryoscintigraphy assesses the lacrimal drainage system under physiological condition.Technetium-99, a gamma ray-emitting radionuclide in saline or technetium sulfur colloid are instilled into the conjunctival sac and imaged with a gamma camera at fixed interval. Delay in the passage of the dye may occur at any site as in the region of the conjunctival sac or the canaliculi, which may be due to lid or canalicular diseases. Apart from being a noninvasive technique, radiation exposure to the lens is minimal compared to that of dacryocystography. The disadvantage of dacryoscintigraphy is that it lacks in showing finer anatomical detail. Computer Tomography (CT)21 The role of CT scan comes when anatomical or pathological abnormalities are suspected as the
Standarized echography: Gross anatomical structural defects can be evaluated with the standardized echography.25 Thermography: Thermographic evaluation of the lacrimal passage used in conjunction with routine lacrimal irrigation to visualize the tear ducts in normal subjects and in a patient with obstructive epiphora has been described.26
References 1. Jane Olver J. Colour Atlas of Lacrimal Surgery. London, Butterworth- Heinemann 2002;11-26. 2. Bron AJ, Tripathi RC, Tripathi B. Wolff’s Anatomy of the eye and orbit. 8th edn. Edinburgh, Chapman & Hall Medical Publication 1997;72-84. 3. Tucker NA, Tucker SM, Linberg JV. The anatomy of the common canaliculus. Arch Ophthalmol 1996;114:1231-34. 4. William MH Jr. (Ed). Adler’s Physiology of the Eye: Clinical Application 9th edn. Harcourt Brace Asia 1992.
Evaluation of Epiphora 5. Doane MG. Blinking and the mechanics of the lacrimal drainage system. Ophthalmology 1981; 88:844-50. 6. Becker BB. Tricompartmental model of the lacrimal pump mechanism. Ophthalmology 1992; 99:1139-45. 7. Basic and clinical science course, American Academy Ophthalmology, 2005; Orbit, eyelids and the lacrimal system. Chapter 14-Evaluation and management of the tearing patient, 272. 8. Conway ST. Evaluation and management of “functional” nasolacrimal blockage: results of a survey of the American Society of Ophthalmic Plastic and Reconstructive surgery. Ophth Plastic Reconstr Surg 1994;10:185-87. 9. Krupin T, Cross D A, Becker B. Decreased basal tear production associated with general anesthesia. Arch Ophthalmol 1977;95:107. 10. Lamberts DW, Foster CS, Perry HD. Schirmer test after topical anesthesia and the tear meniscus height in normal eye. Arch Ophthalmol 1979;97:1082. 11. Nesi FA, Lisman RD, Levine MR. Smith’s Ophthalmic plastic and reconstructive surgery. 2nd edn. St Louis, Mosby 649-60. 12. Flack A. The fluorescein appearance test for lacrimal obstruction. Ann Ophthalmol 1979; 11:237. 13. MacEwen CJ, Young JDH. The effect of fluorescein disappearance test (FDT): an evaluation of its uses in infants. J Paed Ophthal Strab 1991; 28:305. 14. Zappia RJ, Milder B. Lacrimal drainage function.I. The Jones fluorescein test. Am J Ophthalmol 1972; 74:154-59. 15. Zappia RJ, Milder B. Lacrimal drainage function.I. The fluorescein dye disappearance test. Am J Ophthalmol 1972;74:160-62.
16. Galloway JE, Kavic TA, Raflo GT. Digital substraction macrodacryocystography. Ophthalmology 1984;91:956-62. 17. Lloyd GAG, Welham RAN. Substraction macrodacryocystography. Br J Radiol 1972;47: 379-82. 18. Rossomondo RM, Carlton WH, Trueblood JH. A new method of evaluating lacrimal drainage. Arch Ophthalmol 1972;88:523. 19. Hurwitz JJ, Maisey MN, Welham RAN. Quantitative lacrimal scintillography. Br J Ophthalmol 1975;59:308-12. 20. Jedrzynski MS, Bullock JD. Radionuclide dacryocystography. Orbit 1998;17:1-25. 21. Kallman JE, Foster JA, Wulc AE, et al. Computer tomography in lacrimal outflow obstruction. Ophthalmology 1997;104:676-82. 22. Raflo GT. Assessment of efficacy of chemiluminance evaluation of lacrimal drainage system. Ophthalmic Surgery 1982;13:36. 23. Coehn SW, Prescott R, Sherman M. Dacryoscopy. Ophthalmic Surgery 1979;10:57. 24. Tsugihisa Sasaki, Yuuko Nagata, Kazuhisa Sugiyama. Nasolacrimal duct obstruction classified by dacryoendoscopy and treated with inferior meatal dacryorhinostomy. Part I: Positional diagnosis of primary nasolacrimal duct obstruction with dacryoendoscope. Am J Ophthalmol 2005;140:1065-69. 25. Dutton JJ. Standardised echography in the diagnosis of lacrimal drainage dysfunction. Arch Ophthalmol 1989;197:1010. 26. Raflo GT, Chant P, Hurwitz JJ. Thermographic evaluation of lacrimal drainage system. Ophthalmic Surgery 1982;13:119.
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MANDEEP S BAJAJ, SANJIV GUPTA
27
Diagnostic Techniques in Proptosis
Introduction Proptosis is defined as an anterior displacement of the globe from its normal position in the orbit. The orbit is a unique area packed with numerous vital structures which are delicately poised in a dynamic equilibrium. Even a minute alteration in this balance can lead to clinically significant ramifications. Orbit is a closed cavity which usually does not allow for direct evaluation of any pathological process developing inside, and is often referred to as a Pandora box. It is a watershed area, being the meeting ground of many specialities and, therefore, a collaborative approach is required in the diagnosis and management of orbital disorders. A wide variety of disease processes can involve the orbit such as inflammations, parasitic infestations, metabolic and endocrine disturbances (Fig. 27.1), vascular anomalies, primary and metastatic tumors (Figs 27.2 and 27.3), depending on the age group and other predisposing factors. Common orbital tumors encountered are cavernous hemagioma, lacrimal pleomorphic adenoma, meningioma, dermoid cysts, optic nerve glioma and lymphoma, to name a few. Parasitic involvement of the orbit, especially cysticercosis and occasionally hydatid cyst are
Fig. 27.1: A patient with thyroid ophthalmopathy with bilateral exophthalmos
Fig. 27.2: Clinical photograph of a patient showing proptosis of the right eye with marked downward and outward displacement of the globe
Diagnostic Techniques in Proptosis important clinical parameters to be taken into consideration are the laterality, direction of globe displacement, characteristics of the mass if palpable, visual status and posterior segment evaluation. A vast array of diagnostic techniques have evolved over the years to confirm the presumptive clinical diagnosis. This chapter describes techniques which complement the process of diagnosis in a case of proptosis and are crucial for appropriate management.
Diagnostic Techniques Fig. 27.3: A child with acute onset proptosis of the right eye suggestive of an orbital malignancy
not uncommon in developing countries. Endocrine disturbances, such as thyroid dysfunction, could have some of their earliest manifestations in the orbit and adnexa. In a case of proptosis, as in any other clinical situation, the diagnostic work-up begins with a careful history and clinical examination. The degree of proptosis is quantified by performing an exophthalmometry. The most commonly used instrument is the Hertel’s exophthalmometer (Fig. 27.4), in which the position of the anterior corneal surface is recorded, taking the lateral orbital rim as a reference point. An absolute reading greater than 21 mm or a relative difference of more than 2 mm between the two eyes is used as a cutoff value for diagnosing proptosis. Some of the
Fig. 27.4: Hertel’s exophthalmometer
A large number of diagnostic techniques are available for evaluation of a case of proptosis. However, as a general principle, one should follow a graded approach in employing these techniques, starting with the less invasive ones and going on to the more invasive ones, only if indicated. One should also be able to distinguish as to which group of investigations would be relevant in a particular case. The noninvasive techniques include imaging studies, which are the cornerstone in reaching a diagnosis in a case of proptosis. Invasive techniques are aimed mainly on efforts to reach a tissue diagnosis, which entails harvesting of tissue and subjecting it to routine and specialized histopathological tests.
Imaging Techniques Standard Roentgenography (Plain X-ray) Standard X-rays of the orbit were a useful imaging technique for initial screening before the advent of CT. They are of value in demonstrating bony changes and particularly fractures and foreign bodies in the orbit. Special optic foramen views can be obtained to visualize enlargement which can denote apical tumors. There are a variety of views of the orbit that can be requested, each
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Diagnostic Procedures in Ophthalmology with its own specific benefits. These include the Caldwell view (general view), Water’s view (orbital view), Rhese view (optic foramen) and Lateral view/axial basal view (paranasal sinuses). Important findings to look out for include orbital enlargement (trauma, benign tumor), orbital wall erosion (benign pathology), orbital wall destruction (malignant pathology), calcification (phlebolith, meningioma, lacrimal gland carcinoma, retinoblastoma), hyperostosis (meningioma, Paget’s disease, malignant osteoblastic secondary, fibrous dysplasia), enlargement of the optic foramen (optic nerve glioma, meningioma) and enlargement of the superior orbital fissure (aneurysm or tumor with posterior extension).
Ultrasonography (USG) Ultrasonography is a rapid noninvasive tool for the evaluation of orbital lesions causing proptosis. As most USG machines are compact and portable, it can be performed in an office setting as well as peroperatively. It gives useful information about the characteristics of the lesion and can even clinch the diagnosis when done by an experienced observer. Despite being inferior to CT-scan and MRI in depicting the bony wall, orbital apex, adjacent sinuses and intracranial compartments, ultrasound is arguably a better imaging modality in the detection of subtle changes of the soft tissues within the orbit, and the differentiation of extraocular muscles and optic nerve lesions. The machine basically consists of a transducer at the tip of a probe which emits ultrasonic waves by the vibration of a piezo-electric crystal inside the probe. These waves are reflected, scattered and absorbed by the medium. The reflected waves are then processed in a computer to generate a single or multidimensional picture on a screen. Ophthalmic USG uses frequencies ranging
between 6 and 20 MHz (typically 10 MHz). The speed of sound varies with the medium, and in the orbit it is usually 1550 m/s. The lower frequencies provide better penetration but lower resolution and vice-versa. Ultrasound is, however, of limited value in assessing lesions of the posterior orbit; (sound waves at 8-10 MHz do not penetrate beyond the mid-orbit) or the sinuses or intracranial space. Standardized A-scan is a time-amplitude display mode where we get one dimensional display of vertical (amplitude) spike and the horizontal axis which is modified to display the distance in millimeters. The A-scan gives us important information regarding the internal structure of a lesion. For example clear cysts and homogenous solid lesions (e.g. lymphoma) typically produce low amplitude internal spikes (reflectivity) whereas heterogeneous lesions (e.g. hemangioma and dermoids) produce higher amplitude echoes within the normal orbital pattern. B-scan is a two dimensional intensity modulated display. It is seen as a funnel-shaped display on the screen, the mouth of the funnel being on the right, the probe position (transducer band) is on the right and the horizontal extent on the right gives the depth of penetration of sound beam. The vertical axis represents the segment of the eye being scanned. B-scan allows a real time evaluation of any lesion and successive cross sections are displayed on the monitor. The signal amplitude is displayed as dots whose brightness gives an idea of the strength of the returning echoes, which is referred to as the Gray scale. Orbital B-scan can be transocular where the beam crosses the globe, which is then seen in front behind which the orbital shadow is displayed, or par-ocular, which bypasses the globe and is used mainly for anterior orbital lesions. B-scan shows rather characteristic alterations of the normal orbital pattern in various lesions such as tumors, cysts and inflammation.
Diagnostic Techniques in Proptosis
Fig. 27.5: Ultrasound picture in a case of thyroid ophthalmopathy showing enlargement of the belly of an extraocular muscle with tendon sparing
For example, one can differentiate thyroid orbitopathy from pseudotumor by demonstrating “tendon sparing” thickening of extraocular muscles in thyroid ophthalmopathy (Fig. 27.5) as contrast to tendonitis and posterior scleritis which typically occur in idiopathic orbital inflammatory disease (IOID) or pseudotumor. In addition certain lesions can be definitely diagnosed on USG like cavernous hemangioma (Fig. 27.6), cysticercosis and hydatid cyst (Figs 27.7 and 27.8). Another important application of USG is for serial measurements of size of lesions and evaluation of response to therapy like in the case of orbital cysticercosis, dysthyroid ophthalmopathy, and optic nerve thickness in optic neuritis. Color Doppler (CD) imaging is one of the most important developments of the last decade (Figs 27.9A and B). It allows evaluation of blood flow along with simultaneous B scan imaging of the lesion and can definitely diagnose lesions such as orbital varices, A-V malformations and carotidcavernous sinus fistulas. The patterns obtained
Fig. 27.6: Ultrasound A- and B-scan of the orbit showing a well demarcated, intraconal lesion with high internal reflectivity and moderate sound attenuation, suggestive of cavernous hemangioma
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Fig. 27.7: Ultrasound picture showing an orbital cyst with scolex suggestive of cysticercosis
A
Fig. 27.8: Ultrasound picture showing an orbital cyst with “double wall sign” typical of hydatid cyst
B
reveal information on the extent of arterial or venous flow in the substance of the lesion. Apart from these, there are other methods of USG like C-scan which depicts orbital lesions in a coronal plane and D-scan which provides a three dimensional display. Three-dimensional ultrasound (3D USG) imaging is a novel way of imaging ophthalmic pathologies in vivo, revealing valuable topographic information in ways more familiar and recognizable to the untrained eye, where surfaces can be perceived and their approximate relationships in three-dimensions can be presented (e.g. to determine the contour and size
Figs 27. 9A and B: Color Doppler examination in a case of orbital varices. A Before and B After Valsalva maneuver showing low blood flow velocity on dynamic evaluation
of tumors, to ascertain the shape and relative configurations of tissues and structures in the eye). Three-dimensional USG imaging allows volumetric and topographic reconstruction of the vitreous, retina, choroid, sclera, and orbital structures. Volumetric reconstruction is valuable in tumor growth assessment, while topographic mapping provides a more comprehensive quantitative description of the surface and marginal parameters responsible for volumetric changes.
Diagnostic Techniques in Proptosis
Fig. 27.10: Clinical and CT picture in a case of orbital lymphangioma. CT shows a diffuse, poorly defined, heterogenous lesion with minimal enhancement
Computed Tomography Computed tomography (CT) is one of the most important investigations in a case of proptosis as it gives anatomic details par excellence. It has revolutionized the management of orbital disorders and is valuable for delineating the shape, locations, extent, and characteristics of lesions of the orbit. Furthermore, current CT- scan administers a dose of radiation which compares favorably with an X-ray of the skull. Its spatial resolution is 0.5 mm. Eight slices are required to perform an orbital scan which extend from the maxillary sinus below to inferior part of the frontal lobe above, and include the optic chiasm and pituitary area. Axial-scan is done in supine position and coronal in prone position. For sagittal views, re-formating of images is required as they cannot be done directly. ‘Bone windows’ are available to enhance bony changes and three dimensions reconstruction is possible to aid in surgical planning. Suspected orbital disease associated with paranasal sinus disease, thyroid ophthalmopathy, foreign bodies, hemorrhage, or orbital trauma is evaluated using noncontrast CT, while the visualization of tumors that are well supplied with blood vessels (e.g. meningioma) or whose blood vessels leak is improved by the use of IV contrast enhancing
agents. CT-scan has resolution and tissue contrast capabilities allowing for the imaging of soft tissues, intracranial structures, masses or processes suspected of calcification such as lymphangioma (Fig. 27.10), bones (e.g. sinus anatomy) or bony destruction (e.g. leukemia, lymphoma, histiocytosis, and rhabdomyosarcoma), contrast containing blood vessels and foreign bodies. Coronal sections with 2-3 mm slices should be specifically asked for in cases of blow-out fractures and for assessing extraocular muscle size in Graves ophthalmopathy. High resolution CT with 1 mm cuts is useful for studying optic nerve lesions. Axial sections show both globes, the horizontal rectus muscles, optic nerve, other orbital soft tissue and bony structures. Coronal section, anteriorly, shows globe with relation to recti muscles and posteriorly, all four rectus muscles, oblique muscles, optic nerve and soft tissue of the orbit. At the apex it also shows the optic foramen. CT adequately documents findings such as the extent of proptosis, muscle enlargement, location (intraconal or extraconal) and size of a lesion, compression of the globe or optic nerve and presence or absence of bone erosion, as well as the condition of adjacent sinuses and the presence of intracranial involvement. It also shows the internal characteristics of the lesion—
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Fig. 27.11: CT-scan (axial view) showing a well delineated, fusiform, intraconal mass isodense to the optic nerve, suggestive of glioma
whether it is homogeneous or heterogeneous, solid or cystic, presence of calcification and the effect of contrast enhancement. Benign tumors such as cavernous hemangioma, neurilemoma, dermoids and gliomas (Fig. 27.11) usually have rounded well circumscribed borders. Malignant lesions on the other hand have diffuse, irregular boundaries. Important features of thyroid ophthalmopathy include swelling of muscles maximally in the mid-portion (Fig. 27.12) (relative sparing of the tendons), slight uveo-scleral thickening, apical crowding, increase in the diameter of the retrobulbar optic nerve sheath, increased density of orbital fat, and anterior
Fig. 27.12: CT-scan (axial view) showing significant enlargement of extraocular muscles with sparing of tendons in a case of thyroid exophthalmos
Fig. 27.13: CT-scan (coronal view) showing an infiltrative lesion in the lacrimal gland fossa with irregular internal structure suggestive of a malignant lacrimal gland tumor
displacement of the lacrimal gland. CT is a useful modality for the evaluation of lacrimal fossa masses, especially epithelial tumors (Fig. 27.13). CT can adequately depict osseous alterations and calcifications, and can differentiate a group of epithelial tumors from inflammatory and lymphoproliferative conditions. Features specific of orbital pseudotumor include a poorly defined intra- or extraconal mass close to the surface margin of the globe. In the myositic type one may get enlargement of one or more muscles close to their insertion, with ill-defined margins. Other features of orbital pseudotumor are that it typically involves muscles and tendon insertions, there is increased density of retroorbital fat, thickening and enhancement of sclera near Tenon’s capsule and enlargement of the lacrimal gland. Lymphangioma may be diagnosed if there is a multi-lobulated pattern on CT-scan (Fig. 27.10) and a cystic internal structure in standardized ultrasound evaluation. Cavernous hemangiomas show as well circumscribed, solid, masses involving the intra or extraconal compartment. On CT-scan lymphoproliferative tumors typically show up as a localized or diffuse mass with moulding to the orbital structures.
Diagnostic Techniques in Proptosis
Fig. 27.14: A patient undergoing an MRI-scan
Magnetic Resonance Imaging Magnetic resonance imaging (MRI) is a noninvasive imaging technique which does not employ ionizing radiation and has no known adverse biological effects. The process involves a strong magnetic field which is applied to the body (Fig. 27.14). It excites protons in the body tissues and causes them to align in a particular orientation in relation to the magnetic field. When the magnetic field is switched off, the protons relax to their original alignment and re-emit the energy gained. The signal is recorded in terms of intensity and location. T1 weighting and T2 weighting refer to two methods of measuring the relaxation times of the excited protons after the magnetic field is switched off. The various body tissues have different relaxation times and a given tissue may be T1 or T2 weighted, implying that is best visualized on that particular type of image. Coronal, sagittal and axial images can be directly obtained. A surface coil is used for ophthalmic purposes to enhance spatial resolution. Four basic parameters can be adjusted to identify different tissues: proton density of tissue, bulk motion of protons (flow), spin lattice
relaxation time (T1) and spin-spin relaxation time (T2). Tissues with high proton (hydrogen nuclei) density (e.g. fat) emit a high signal as does low proton flow (coagulated blood). Low signal is produced by bone, sclera and sinus air and faster proton flow like in flowing blood. In T1 image (short TR and TE, i.e. relaxation time and echo time, respectively), the fat is bright and vitreous dark, and is reverse in T2 image (long TR and TE). In proton density image (long TR and short TE), the vitreous is intermediate density as seen in muscle, and the fat is seen brightly. Paramagnetic substances like melanin and methemoglobin alter the signal character of the image causing relative brightness on T1 weighted image. Similarly, gadolinium, a paramagnetic substance is used as a contrast agent (coupled with diethylenetriaminepenta acetic acid or DTPA). Using this, false negative tests have been vastly removed and imaging of meningiomas, demyelination, metastasis, meningeal lesions, ventricular abnormalities and pituitary masses have been greatly enhanced. A few contraindications are there to the administration of contrast, the relative ones being severe hepatic or renal dysfunction and absolute ones include sicklecell anemia and hemolytic anemia. Mild allergic reactions may still occur. Technical advances in MRI include the use of various surface coils, motion artifact and fat suppression techniques which greatly enhance visualization of orbital images. Contrastenhanced MRI (using IV gadolinium) is helpful in the evaluation of orbital lesions such as cavernous hemangiomas, high-flow vascular malformations (IV gadolinium enhancement brightens vascularized lesions so that they exhibit the same density as fat), nonthyroid related extraocular muscle enlargement, which includes myositis or metastases, or processes that potentially extend into the cavernous sinus.
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Fig. 27.15: MRI-scan of the orbit (axial view, T1 weighted) showing a hyperintense lesion at the posterior pole suggestive of choroidal melanoma
On MRI, melanin within melanomas typically gives such tumors a hyperintense signal on T1weighted scans (Fig. 27.15) and hypointense signal on T2-weighted scans (Fig. 27.16) relative to the vitreous. Non-contrast, fat-suppression studies help to determine the extension of ocular melanoma into the orbit and optic nerve. Subretinal hemorrhage may be differentiated from choroidal melanoma by MRI when visualization is poor and ultrasound inconclusive. While MRI is a more useful diagnostic modality in lymphoangiomas (better anatomical illustration
Fig. 27.16: MRI-scan of the orbit (sagittal view, T2 weighted) showing a hypointense lesion at the posterior pole suggestive of choroidal melanoma
Fig. 27.17: MRI-scan of the orbit (sagittal view, T1 weighted) showing orbital metastasis
of the cystic nature of the lesion and the hemorrhages in lymphangiomas), it is interesting to note that these lesions typically do not enhance with gadolinium. In thyroid ophthalmopathy one notices a high signal intensity in enlarged eye muscles on T2W1. In orbital pseudotumor the lesion is isodense to fat on T2W1. MRI of the orbit is especially useful in optic nerve lesions or trauma, unusual orbital inflammation, orbital metastasis (Fig. 27.17) and tumors extending to the orbital apex or having suspected intracranial extension. Salient contraindications to performing an MRI scan include the presence of ferrous metallic foreign bodies (even mascara which contains ferrous compounds cause artifacts), aneurysm clips, cardiac pacemaker and cochlear implants. In addition, claustrophobic and obese patients may pose problems. Other limitations of MRI are lack of bone visualization, higher cost and longer time of scan. An interesting advancement in MRI is Magnetic Resonance Angiography (MRA) in which special software is used to suppress normal soft tissue to enhance vascular structures (Fig. 27.18). This is analogous to bone window setting on CT-scan. Gadolinium enhancement is needed
Diagnostic Techniques in Proptosis
Fig. 27.18: Magnetic resonance angiography (MRA) scans of the brain
for visualizing venous structures but is not required for the arterial system. It allows noninvasive visualization of the large- and medium-sized vessels of the arterial system but does not provide as fine a detail as direct arteriography. This modality is still evolving and angiography remains the gold standard in imaging of vascular structures of the orbit.
Orbital Venography Orbital venography or phlebography is a technique in which contrast is introduced in the frontal or angular veins and sequential X-rays are taken in the AP view. It is useful in cases of orbital varices and changes in superior ophthalmic vein, whose obstruction or distortion by a mass lesion can be made out. Subtraction and magnification techniques have been used to increase the resolution of venography. A
relative disadvantage of orbital venography is that apart from the adverse effects of the contrast agent, it cannot pick up small lesions. Also, larger lesions obstructing dye flow in the superior ophthalmic vein do not allow the rest of the venous system to be visualized. Prior to CT-scan and MRI, orbital venography was used in the diagnosis and management of orbital varices and in the study of the cavernous sinus. With the advent of MRA, orbital venography has fallen from favor and is more or less obsolete in the present era.
Orbital Arteriography In orbital arteriography a suitable contrast material is injected into the ipsilateral common or internal carotid artery and then appropriate X-rays are taken. It is useful in demonstrating rare cases of A-V malformations, carotid-cavernous fistulas,
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Diagnostic Procedures in Ophthalmology aneurysms and hemangiopericytoma. Maximum visualization can be obtained by the use of magnification to allow viewing of the smaller caliber vessels, and subtraction techniques to radiologically eliminate bone. With the advent of MRI and CT, particularly MRA, its role and utility are gradually fading out.
Blood Tests The nature of the blood investigations performed will depend to a large degree upon the clinical findings of the patient. Given herein are some of the more commonly utilized blood investigations to assist in the evaluation of a patient with proptosis.
Total and Differential Blood Counts This test is particularly useful in evaluating patients with leukemia and lymphomas.
Thyroid Function Tests Thyroid function tests include tests of T3, T4 and TSH. These tests will be abnormal in a majority of patients with thyroid ophthalmopathy. However, if thyroid disease is strongly suspected and these tests are normal, additional endocrine studies can be considered. Further tests which can be done include the antithyroglobulin and antimicrosomal antibodies, which are abnormal in nearly 70% of patients with Graves disease.
Antineutrophil Cytoplasmic Antibody (cANCA) Serum Assay Diagnosis of Wegener’s granulomatosis should be considered in patients with scelrokeratitis or coexisting sinus disease and orbital mass lesions. The antineutrophil cytoplasmic antibody (cANCA) serum assay is a very sensitive test for the presence of this rare disease.
Serum Angiotensin Converting Enzyme The diagnosis of sarcoidosis may be assisted by testing for serum angiotensin converting enzyme (ACE). This multi-system granulomatous inflammatory condition may present with lacrimal gland enlargement.
ELISA for Cysticercosis Elisa test is used for evaluating the presence of an orbital cyst, if cysticercosis is suspected. However, it needs to be corroborated with clinical and imaging findings due to a high percentage of both false positive and false negative results.
Biopsy Techniques Although imaging techniques can help us in making a provisional diagnosis and are indicative in nature, a definitive diagnosis can only be made by obtaining a tissue specimen and subjecting it to routine and specialized histopathological techniques. Biopsy techniques which are commonly employed are described below.
Fine Needle Aspiration Cytology Fine needle aspiration cytology (FNAC) is employed for rapid diagnosis of suspected malignant orbital lesions. It is minimally invasive in nature and can be performed in an office setting. Although strict asepsis is mandatory, a full fledged operative set up is not required. It is done with the help of a hand held gun with 22 to 25 gauge needle (Fig. 27.19). After localizing the mass by palpation (for anterior orbital lesions) or under ultrasound or CT guidance (for relatively posterior lesions), the needle is introduced into the mass and the material is aspirated by using negative pressure. The aspirate is then spread over a slide, air dried, fixed in 95% alcohol and
Diagnostic Techniques in Proptosis Core Biopsy
Fig. 27.19: Instrument used for performing fine needle aspiration (FNAC gun)
finally stained with hematoxylin and eosin. The slide needs to be examined by a trained cytologist for accurate interpretation. The accuracy has been reported to be more than 80%. The principal disadvantage of this technique is that scanty cellular material is obtained from a limited region of the mass which may be difficult to evaluate and interpret. Secondly it uses cytology technique rather than routine histopathology that fails to detect tissue or tumor architecture. The main use of FNAC is in cases of suspected lymphoma metastatic tumors or orbital recurrence of retinoblastoma or melanoma, which may require to be treated by chemotherapy or radiotherapy. It has also been used for diagnosing optic nerve sheath meningioma. Fine needle capillary sampling (FNCS) is another similar technique in which instead of aspiration with a syringe, a 25-gauge needle is introduced in the mass after stabilizing it manually. Gentle to and fro movement is performed and the needle is withdrawn without any aspiration. The material is then processed like FNAC. Reported sensitivity of this technique is said to be in the range of 90-95%. The complications of these procedures include globe perforation, retrobulbar hemorrhage and rarely intracranial penetration. Transient visual loss, diplopia and ptosis have also been reported.
A somewhat more invasive technique than FNAC is core biopsy that uses a trephine which is 24 mm in diameter. It is passed with a gradual rotatory motion into the lesion after exposure under local infiltration, and an adequate specimen is obtained. The advantages are that it is a rapid, out-patient procedure with lesser morbidity and much better yield of tissue than FNAC, for a more accurate diagnosis. Its main limitation is that posterior lesions are difficult to access. An endoscopic biopsy can be performed for the posterior lesions but requires greater expertise to yield credible results.
Incisional Biopsy Incisional biopsy is a surgical technique where partial removal of the tumor is done under local or general anesthesia. The purpose of this biopsy is to obtain adequate tissue for histopathological examination. Imaging studies should be done for accurate localization of the lesion before undertaking the biopsy procedure. These are useful in planning the surgical approach. Care should be taken to obtain tissue from the main mass itself, because biopsy from superficial or adjacent structures will give false results.
Excisional Biopsy Imaging and supportive investigations certainly help in establishing a good differential diagnosis, but a definitive diagnosis is sometimes established only after complete removal of the mass and subjecting it to histopathology. This is achieved by performing an orbitotomy procedure through one of the surgical approaches to the orbit. The principles of localization and surgical planning are similar to the ones described above. This, along with incisional biopsy, is the gold standard for diagnosis and
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Pathology Techniques This area is the most important part of any diagnostic process as it provides actual tissue diagnosis, which may have therapeutic and medico-legal importance. It is imperative to have proper communication with the pathologist preoperatively, to facilitate and plan the appropriate histopathologic technique for a given case.
Fig. 27.20: Gross specimen of an orbital cavernous hemangioma
Cytology As already stated under the section on FNAC, cytology is a low cost technique for rapid diagnosis. The aspirate is spread over a slide and air dried followed by alcohol fixation and stained by Papanicolaou technique and H&E or May-Grunwald-Giemsa stain; mainly used for suspected malignant lesions. Cytology has its limitations as discussed earlier.
Gross Examination The gross excised specimen is inspected for shape, size, consistency (firm/hard/cystic/nodular), and whether the capsule is intact or broken. Measurements are made in three dimensions. Then it is cut to see the internal architecture – color, areas of necrosis, calcification and inner structure (solid or cystic). For example, on gross examination, pleomorphic adenoma of the lacrimal gland displays an intact capsule, with firm, bosselated appearance, and on cut section it has whitish, firm solid areas with some interspersed friable areas. Cavernous hemangioma on the other hand has a reddish-bluish color
Fig. 27.23: Gross section of an orbital cavernous hemangioma (gross specimen)
and has a firm to soft spongy consistency (Fig. 27.20). On cut section, it has a typical honeycomb pattern of innumerable cystic spaces (Fig. 27.21), which can be very well appreciated on H&E stain (Fig. 27.22). Parasitic cysts, such as hydatid cyst are seen as a thin walled translucent fluid-filled structure (Fig. 27.23). The gross specimen is sent to the pathologist in a labeled bag filled with 10% formalin solution in adequate quantity.
Routine Histopathology The biopsy or excised specimen is further processed in the pathological laboratory by
Diagnostic Techniques in Proptosis
Fig. 27.22: Histopathological picture (H & E) of cavernous hemangioma showing large blood-filled spaces
Fig. 27.24: Histpathological picture (H & E) of pleomorphic adenoma of the lacrimal gland
Histochemistry
Fig. 27.23: Gross specimen of a hydatid cyst of the orbit
dehydrating in alcohol. Then it is embedded in paraffin and sectioned with a microtome knife. This is followed by removal of paraffin and staining with hematoxylin and eosin stain (Fig. 27.24). Hematoxylin is a basic dye that binds acidic structures like DNA and nuclei in cells while eosin is an acidic dye that stains basic structures like proteins. This gives clues about the nature of the lesion. For example, cells with prominent nuclei and scanty cytoplasm will stain blue as in lymphoma, retinoblastoma, inflammatory lesions and basal cell carcinoma. On the other hand, cells with abundant cytoplasm like epithelial cells and connective tissue as in squamous cell carcinoma and amyloidosis, will stain pink.
In situations where the routine histological process is difficult to interpret, various histochemical and immunohistochemical techniques provide assistance. For example, Oil red O or Sudan black are used to stain fat in cases of sebaceous gland carcinoma or xanthomatous tumors. Similarly, Alcian blue is used for mucinous substances and PAS (Periodic acid Schiff) for glycogen and some fungal hyphae. Fontana is used for staining melanin, esterase for cytoplasmic granules in leucocytes and Bodian for nerve fibers.
Immunohistochemistry Immuno-histo-chemistry is a highly sensitive technique, which utilizes the principle of antigenantibody reaction to capture certain specific proteins in specific tissues, which can then point out to the correct diagnosis. This reaction is coupled by an enzyme, which then generates a color reaction when combined with certain chemicals called chromogens. Monoclonal antibodies are directed against an important group of cytoskeletal components called intermediate filaments. These are specific for
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Electron Microscopy Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) are sometimes used in evaluating unusual lesions. TEM can be vital in the diagnosis of certain tumors such as leiomyoma, neurilemoma, neurofibroma and amelanotic melanoma and also in certain poorly differentiated tumors like alveolar rhabdomyosarcoma and alveolar soft part sarcoma. It is an expensive and time consuming process. With the advent of the above mentioned immunohistochemical stains, it is rarely used. SEM can be used to see the three dimensional ultrastructure of lesions and can provide elemental details of retained foreign bodies.
Additional Investigations Once the initial cause of the proptosis has been determined, it is often necessary to undertake additional investigations to further determine the full extent of the pathology. This is particularly true in the cases where the cause of the proptosis is found to be a tumor. For example patients with hematological and lymphoproliferative tumors require the following additional investigations: X-ray chest, blood
counts, serum immunoglobulin electrophoresis, bone marrow aspiration and biopsy, bone scan, liver and spleen scan and abdominal and pelvic CT.
Conclusion Diagnosis of a case of proptosis requires a systematic approach through a proper clinical evaluation coupled with appropriate investigative techniques. If used effectively, these techniques can guide the clinician in achieving an accurate diagnosis and optimal management in this rather challenging field.
Bibliography 1. Aburn NS, Sergott RC. Orbital Color Doppler Imaging. Eye 1993;7:639-47. 2. Aviv RI, Miszkiel K. Orbital imaging: Part 2. Intraorbital pathology. Clin Radiol 2005;60:288307. 3. Bartley GB, Gorman CA. Diagnostic criteria for Graves ophthalmopathy. Am J Ophthalmol 1995;119:792-95. 4. Bilaniuik CT. Vascular lesions of the orbit in children. Neuroimaging Clin N Am 2005;15:107-20. 5. Devis PC, Newman NJ. Advances in neuroimaging of visual pathways. Am J Ophthalmol 1996;121:690-705. 6. Dutton JJ, Byrne SF, Proia AD. Diagnostic Atlas of Orbital Diseases. Philadelphia, Saunders, 2000. 7. Newton TH, Bilaniuk LT (Eds). Radiology of Eyes and Orbit. New York, Raven, 1990. 8. Rootman J (Ed). Diseases of Orbit: A Multidisciplinary approach. Philadelphia, Lippincott William & Wilkins, 2003. 9. Shields JA, Shields CL. Atlas of Orbital Tumors. New York, William & Wilkins, 1999. 10. Shields JL, Shields JA, Honavar, SG, et al. Clinical spectrum of primary ophthalmic rhabdomyosarcoma.Ophthalmology 2001;108:2284-92. 11. Wiersinga WM, Prummel MF. Pathogenesis of Graves ophthalmopathy—current understanding (Editorial). J Clin Endocrinol Metab 2001;86:501-03.
Neurological Disorders of Pupil
AMBAR CHAKRAVARTY
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Neurological Disorders of Pupil
Introduction Pupillary examination is a powerful tool in the neurological evaluation. The key in understanding the significance of pupillary findings is to know the anatomy of the system and to recognize the various reactions of the pupil. It is further important to correlate historical information with clinical findings in the context of known anatomy to arrive at a cogent diagnosis. Studies on pupil have also figured significantly in advances of autonomic physiology.1-4
Anatomy and Physiology The normal pupil is slightly situated inferomedial to the center of cornea. When viewed in the natural state, the iris and pupil appear slightly larger (12.5%) because of the corneal magnification. The sphincter muscle is located at the pupillary border and is more powerful than the dilator muscle. Blood supply of iris is through the radially arranged vessels arising from the major arterial circle at the iris base. Pupillary control is essentially a balance between parasympathetic and sympathetic system. Although pupillary size and reactivity, as well as ciliary muscle tone, are basically controlled
by the autonomic nervous systems, the major role is played by the parasympathetic system due to the mechanical superiority of the sphincter muscle. Parasympathetic impulses arise in the Edinger-Westphal Complex (EWC), a central paired subnucleus of the oculomotor nerve in the midbrain. Light directed into either eye usually produces bilateral pupillary constriction. The pupillary light reflex (Fig. 28.1) begins with hyperpolarization of the retinal photoreceptors. Ultimately the retinal ganglion cells are activated. The retinal ganglion cells send their axons through the optic nerve, chiasm and optic tract to synapse in the pretectal nuclei. Interneurons then connect the pretectal nuclei to the EWC. Efferent pupillary fibers arise from the EWC and traverse the mesencephalon in the rostral fascicles of the third cranial nerve. It enters the orbit as part of the inferior division of the nerve, and arrives at the ciliary ganglion by means of the motor nerve to the inferior oblique muscle. Most of the pupillomotor fibers synapse in the main or accessory ciliary ganglion and reach the iris sphincter muscle via the short ciliary nerves. Interruption in this pathway, EWC to the sphincter muscle will cause pupillary dilatation and decreased reactivity. Afferent
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Fig. 28.1: Pupillary light reflex pathway from retina through optic pathway to lateral geniculate body and then on to Edinger-Westphal nucleus complex and then along the third nerve trunk to the iris sphincter
visual system pathology does produce difference in pupillary size. Pupillary constriction is also a component of a number of synkinetic reactions involving parasympathetic activity–near reflex (miosis, accommodation, and convergence), Bell’s phenomenon (levator inhibition, superior rectus muscle stimulation, and miosis) and Westphal-Piltz reaction (orbicularis spasm and miosis). The cortical region responsible for supranuclear generation of the near response remains uncertain. It probably rises from diffuse cortical projections. Ultimately, supranuclear inputs for the near reflex converge upon the rostral superior colliculus. From here connections are made to the mesencephalic reticular formation, pretectum and EWC to generate the near triad— pupillary constriction, lens accommodation and convergence. First order neuron of sympathetic pathway (Fig. 28.2) arises in the hypothalamus and descends through the reticular substance to synapse in the intermediolateral gray substance of the lower cervical and upper thoracic spinal cord (ciliospinal center of Budge Waller, C8-T1). Second order neuron arises in the intermediolateral gray column and then ascends without synapse through the sympathetic paraspinal chain to the superior cervical ganglion. From
Fig. 28.2: Sympathetic pathway to the iris dilator muscle
the superior cervical ganglion the postganglionic or third order neuron travels on the surface of the common carotid artery. At the bifurcation of the internal and external carotid arteries, fibers controlling facial sweating follow the external carotid artery, while those destined for the eye and lid follow the internal carotid artery. In the cavernous sinus these eye and lid fibers join the fifth and sixth cranial nerves and enter the orbit via the superior orbital fissure. Fibers destined for the dilator muscle enter the eye via the long posterior ciliary nerves or short posterior ciliary nerves.
Examination of Pupil The most important evaluation technique for pupil is the history. A careful history of known pupillary disorders is vital to establish whether a pupillary sign has any meaning in the context of the disorder under consideration. Rarely, a
Neurological Disorders of Pupil patient will present with complaint of abnormality of pupil as a presenting symptom hence the presenting complaints should be stressed upon to get a proper clue into the diagnosis of pupillary abnormality such as associated history of trauma, features of raised ICT, irritative lesions, poor visual acuity, lid lag, double vision and use of drugs. Old photographs provide significant information and should be evaluated in all cases of long-standing or asymmetry documented on examination. Bedside evaluation of pupil and its reactions require a good visibility with comfort of patient and examiner. Examination of pupil requires a good illuminator which provides bright, even beam without hot spots or dim areas. Evaluation should begin in darkness or in very dim light as this allows the pupils to start their constriction from a bigger size and increases the amplitude of the pupillary movement making it easier to see. There are three stages in the examination of the pupils.
Evaluation of Anisocoria Pupillary inequality is usually due to an iris innervation problem. The best way to decide whether it is the sphincter muscle or the dilator muscle that is weak is to compare the amount of anisocoria in darkness and in light. No anisocoria in darkness or in light indicates an intact efferent arm of the light reflex arc. Virtually everyone has a measurable pupillary size difference if sensitive enough techniques are used; however, only 20% of normal individuals have enough asymmetry to be recognized clinically (i.e., 0.4 mm or more). Age plays a major role in pupillary size. Newborns have small hyporeactive pupils, young children have larger, briskly reactive pupils and as the age progress the normal pupillary size and reactivity diminishes such that older individuals have miotic, relatively slowly reactive pupils.
Evaluation of the Afferent Arm of the Light Reflex Arc Swinging-Light Pupil Test The swinging-light pupil test is a rapid, low cost, accurate and objective method of identifying asymmetric optic nerve disease but it is useless unless proper technique is used. The idea is to look for a Relative Afferent Pupillary Defect (RAPD) in one eye compared to the other by alternate projection of light over each eye (Fig. 28.3). Technique 1. The room illumination should be dim. Unless the test is performed in darkness, the amplitude of pupil constriction will be too low. 2. The patient should fixate on a distant target. This provides maximal relaxation of the iris sphincter muscle. A near target would evoke miosis associated with the synkinetic near reflex. 3. Use a bright light stimulus. Dim lights do not produce enough pupil constriction. If neither pupil constricts very much to
Fig. 28.3: Left afferent pupillary defect (RAPD)
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Diagnostic Procedures in Ophthalmology flashlight illumination, use a more potent light source, such as the indirect ophthalmoscope. Avoid a light so bright that it causes photophobia or extreme miosis. 4. Direct the light from below the level of the patient’s eyes. This is done so as not to provoke miosis from the patient’s fixing on the light. 5. Move the light briskly and rhythmically from eye to eye several times. If you move the light across the nose too slowly, you will evoke too much constriction and miss a subtle relative afferent pupil defect. Make about five swings. This repetition is necessary to be sure that any pupillary dilation on one side does not reflect merely the adventitious sphincter movement of physiologic pupillary unrest.
mild or severe, but a visual field abnormality can almost invariably be detected by a perimetry. The optic disk may appear normal or acutely swollen but will later develop pallor. Examples of optic nerve disorders include optic neuritis, ischemic optic neuropathy, hereditary optic neuropathy, compressive lesions, toxins, trauma, and cellular infiltration. 1. The largest afferent defects occur in association with unilateral optic nerve disorders. 2. “Resolved” optic neuritis may result in optic disk pallor and RAPD despite recovery to normal visual acuity and normal visual field. 3. The extent of damage in bilateral optic nerve disorders is rarely symmetrical. Therefore, a RAPD will be found on the side with greater damage on carefully examination.
Interpretation
Optic Chiasm 1. Compressive lesions of the optic chiasm can produce asymmetric visual loss and, therefore, a RAPD. Commonly, a junctional scotoma is found. 2. Symmetric bitemporal hemianopsia is not associated with a RAPD, because injury to the visual and pupillary pathways is symmetric.
A relative afferent pupillary defect (RAPD) is a sensitive indicator of unilateral or asymmetric injury to the afferent pupillary pathway. If a RAPD is found, it needs to be investigated. In general, the size of the RAPD correlates with the asymmetry of visual field loss and the resultant asymmetry of pupillomotor input. It also tends to vary with the location of the lesion within the afferent pathway. Retina 1. Large unilateral retinal lesions, i.e. retinal detachment or central retinal artery occlusion produce a clear RAPD. Visual acuity might be good if the macula is spared. A careful dilated funduscopic examination is usually diagnostic, and a consultation with an ophthalmologist/neurologist is important. 2. Cataracts and corneal opacities do not cause afferent pupillary defects. Optic Nerve Damage to the optic nerve almost always produces a RAPD. Visual acuity loss may be
Optic Tract A pure optic tract lesion will produce a small RAPD in the contralateral eye. Thus, a complete homonymous hemianopsia with an afferent defect in the eye with the temporal field loss should raise the possibility of a tract lesion as the cause of visual loss. Pretectal Nucleus 1. The pretectal nucleus in the dorsal midbrain is the final synapse site of pupillary fibers coming from tract via brachium of the superior colliculus. Visual fibers, however, have separated off to go on to the lateral geniculate nucleus. Therefore, a dorsal midbrain lesion can produce a small contralateral RAPD and no visual loss.
Neurological Disorders of Pupil 2. Afferent pupillary defects form optic tract or midbrain injury are usually small and are fairly rare.
Further Observations The intensity of the RAPD, which is related more closely to differences in visual field loss than visual acuity loss in the two eyes, can be quantitatively measured by placing progressively higher neutral-density filters over the normal eye until the RAPD is eliminated.5,6 The filters are particularly useful when the diagnosis of RAPD is equivocal. The examiner places the 0.3-log unit filter over each eye consecutively and performs the swinging-light test. If no RAPD is present, the pupil of the eye covered by the filter dilates slightly as the light is swung towards it. When a RAPD is minimal, the filter placed over the affected eye makes the pupil dilation in that eye more obvious. The swinging-light pupil test is useful even when only one iris sphincter muscle is operational. Constriction of the pupil in the unaffected eye as the light is swung toward it is equivalent to pupillary dilation in the eye with the suspected RAPD. This phenomenon is called (misleadingly) a “reverse RAPD”, it is merely a different way to elicit a standard RAPD.
Evaluation of Near Response The pupillary response to near effort must be checked. If the light reaction seems a little weak, the examiner should look to see if the pupils constrict better to a near stimulus than to light. If they better constrict to light this is called ‘lightnear dissociation’ and may indicate underlying pathology like neurosyphilis, lesions of the dorsal midbrain (obstructive hydrocephalus, pineal region tumors) and aberrant regeneration (oculomotor nerve palsy, Adie’s tonic pupil).
Features of common causes of pupillary asymmetry in neuro-ophthalmology have been given in Table 28.1 and the sites of lesions causing pupillary abnormalities are shown in Figure 28.4.
Pupillary Abnormalities Anisocoria Local Ophthalmologic Conditions Typically, patients with anisocoria due to local causes have a painful red eye with a small pupil and visual disturbance. a. Any condition resulting in an inflammatory response within the anterior chamber may cause spasm of the sphincter muscle, resulting in anisocoria. b. Acute closed-angle glaucoma results in a red painful eye and visual disturbance. In this condition, the pupil tends to be dilated, with an impaired light reflex that may simulate interruption of the parasympathetic nervous system. c. Prosthetic eyes have yet to show a brisk light reflex. d. Other important causes of irregular pupils with poor light reflex are congenital malformation of the iris, postoperative changes, and posttraumatic mydriasis due to tears in the iris and its sphincter muscle.
Episodic Anisocoria Either parasympathetic or sympathetic paresis or over activity may produce intermittent anisocoria. The common causes of episodic anisocoria due to parasympathetic paresis include uncal herniation, seizure disorder and migraine. Parasympathetic hyperactivity conditions like cyclic oculomotor paresis and parasympathetic spasm7 are known to add to
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General characteristics Round, regular
Small, round, unilateral Usually larger in bright light, sector pupil palsy, vermiform movement unilateral or less often bilateral Small, irregular, bilateral Mid dilated, may be oval, bilateral Mid dilated, unilateral, rarely bilateral Very large round, unilateral
Condition
Essential anisocoria
Horner’s syndrome
Adie tonic pupil
Argyll-Robertson pupils
Midbrain pupils
Oculomotor palsy
Pharmacologcally dilated pupils
Fixed
Fixed
Poor to light, better to near
Lighted
—————-
Dilates
Dilates
No change Lighted
Poor
Dilates
Dilates
Dilates
Response to mydriatics
No change
Lighted
Absent to light, tonic to near; tonic redilation
Poor to light, better to near
Darkened
No change
Room condition in which anisocoria is greater
Both brisk
Both brisk
Response to light and near stimuli
No
Constricts
Constricts
Constricts
Constricts
Constricts
Constricts
Response to miotics
TABLE 28.1: CHARACTERISTICS OF PUPILS ENCOUNTERED IN NEURO-OPHTHALMOLOGY
Pilocarpine 1% does not constrict
———————
——————-
——————-
Pilocarpine 0.1% constricts
Cocaine 4% poor dilatation
Normal and rarely needed
Response to pharmacologic agents
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Neurological Disorders of Pupil
Fig. 28.4: Sites of lesions in pupillary abnormalities
the episodic anisocoria. Other conditions producing the episodic anisocoria include sympathetic hyperactivity conditions, sympathetic dysfunction producing alternating anisocoria and pupillary dilatation.
Dilated Pupil (anisocoria that increases in bright illumination) The patient has anisocoria that increases in bright light. Differential diagnoses of a dilated pupil include Adie tonic pupil, III cranial nerve palsy, and pharmacologic blockade.
Adie Tonic Pupil Adie tonic pupil predominately occurs in females aged 20-50 years. Patient may complain of
photophobia, episodes of blurred near vision or blurred vision when switching from near to far viewing and may even complain of unequal pupils. Typically, the involved pupil displays a poor response to light, with a relatively preserved response to sustained near fixation but an abnormally slow or tonic contraction. Slitlamp examination often reveals sector palsies of the iris. The parasympathetic defect in Adie pupil8 is believed to occur after the fibers leave the ciliary ganglion. As a result of denervation supersensitivity, the affected eye displays an abnormally brisk response to dilute (1/8%) pilocarpine and this test (Fig. 28.5) has been suggested as a way of differentiating preganglionic and postganglionic parasympahetic lesions. Recent literature reports many patients who have mydriasis due to oculomotor nerve
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Fig. 28.5: Mid pilocarpine test for right Adie’s pupil
compression and have displayed reactivity to dilute pilocarpine, should show other signs of third nerve dysfunction. The combination of an idiopathic tonic pupil with decreased deep tendon reflexes and/or orthostatic hypotension is termed Holmes-Adie syndrome. The condition is common in young women. Adie pupil is commonly unilateral, but may become bilateral in 10% cases. The symptoms of a tonic pupil tend to be self-limited. Adie pupil is believed to be of uncertain etiology. However, neurosyphilis, diabetes, herpes zoster, giant cell arteritis, and alcoholism have been incriminated. A closely related rare condition is the Ross syndrome characterized by the triad of segmental anhidrosis, hyporeflexia and tonic pupils. Only
a handful of cases have been described in the world literature so far.9 Harlequin syndrome refers to segmental anhidrosis only without any ocular manifestation. In fact, it is reasonable to assume that all these dysautonomic syndromes (Horner, Adie, Ross and Harlequin) represent clinical manifestations of a generalized autonomic injury with or without somatic nervous system involvement (e.g. areflexia).
Third Cranial Nerve Dysfunction The most worrisome cause of an enlarge pupil is oculomotor nerve dysfunction. Anisocoria in the setting of a head injury with decreased level of consciousness may be due to uncal herniation.
Neurological Disorders of Pupil An expanding supertentorial lesion forces the inner basal edge of the uncus towards the midline and over the edge of the tentorium, thereby compressing the adjacent midbrain and oculomotor nerve. This results in ipsilateral third nerve palsy and decreased level of consciousness. In addition, the contralateral cerebral peduncle is compressed against the free edge of the tentorium, resulting in the ipsilateral hemiparesis. Such patients require urgent intervention to lower intracranial pressure and have a poor prognosis without surgical intervention. When a parasympathetic pupillary defect coexists with ptosis and extraocular muscle palsies in a patient with a normal level of consciousness, the diagnosis is third nerve palsy with pupillary involvement. In these situations, one needs to exclude the posterior communicating artery aneurysm. Emergency neuroimaging is indicated unless another etiology is apparent.
Nonisolated Third Cranial Nerve Palsy If other neurologic symptoms or signs are present, they will, in most cases, direct the clinician to the site along the oculomotor nerve pathway where the responsible lesion is likely to be residing. For example, a patient with contralateral hemiparesis has a lesion in the midbrain ipsilateral to the III nerve palsy, while a patient with additional involvement of the VI cranial nerve probably has a lesion in the ipsilateral cavernous sinus. In some cases, the presence of other neurologic or systemic features will suggest the specific disorder responsible for the ophthalmoplegia, as in an elderly patient with headaches, weight loss, transient visual loss and tenderness of the superficial temporal arteries who is most likely harboring giant cell arteritis.
Isolated Third Cranial Nerve Palsy Many times, however, a patient develops III cranial nerve palsy without other neurologic or systemic symptoms or signs. In such patient, consideration of specific characteristics of the ophthalmoplegia (e.g. status of the pupil) and historical details (e.g. static versus progressive) will help guide one to the likely disorder responsible for the presentation. There are five important causes of acute third nerve palsy seen in routine clinical practice. 1. Infarction of the peripheral cranial nerve 2. Compression by tumor 3. Compression by aneurysm 4. Trauma 5. Brainstem stroke. The list should probably also include TolosaHunt syndrome or painful ophthalmoplegia syndrome when considering clinical practice in the Indian context. However, the overall impression on the frequency of occurrence of this condition in India is probably somewhat overestimated and certainly the condition would not appear to be as prevalent if strict diagnostic criteria (neuroradiological) are employed (vide infra). Most of the less frequent disorders causing oculomotor nerve palsy as well as trauma and brainstem stroke listed above, occur in patients with historical information or physical signs that implicate the underlying cause of ophthalmoplegia (i.e. nonisolated). Therefore a neurologist evaluating a patient with acute and neurologically-isolated III cranial nerve palsy faces following dilemmas: • Is it due to infarction or compression (by aneurysm)? • Can one wait and watch, or do one need to image emergently? • Should one go directly to catheter angiography, or screen using a noninvasive test following imaging to exclude aneurysm?
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Diagnostic Procedures in Ophthalmology Ischemic III cranial nerve palsy is the most common pupil-sparing oculomotor palsy in middle-aged or older adults. It is usually the result of infarction of the extra-axial segment of the oculomotor nerve,10 although patients with similar clinical characteristics have been described who have documented midbrain strokes.11 This condition has traditionally been referred to as “diabetic III nerve palsy” or “diabetic ophthalmoplegia” since it frequently occurs in patients with established diabetes. However, the same condition may occur in patients who are not diabetic but who have other overt or unrecognized vascular risk factors, most notably left ventricular hypertrophy (LVH) associated with hypertension and polycythemia. Patients with the ischemic III nerve palsy have the following clinical profile: • Age usually greater than 45 years, occasionally, younger and those with longstanding diabetes. • Abrupt onset. • Often times associated with a dull, continuous pain around the ipsilateral brow, eye, or temple lasting a week or two, at most. • Pupil usually normal, about 1/3 of patients demonstrate small anisocoria (one millimeter or less), but the pupil characteristically remains reactive to light. Anisocoria greater than one millimeter, or a “blown” pupil, is not consistent with ischemic injury, and usually indicates that a mass lesion is compressing the oculomotor nerve. • Neurologically isolated, bilateral III nerve, or ipsilateral IV or VI nerve, involvement is not consistent with ischemic III nerve palsy. • Progression of external ophthalmoplegia during the subsequent one to two weeks occurs in about 2/3 of acutely-evaluated patients who have incomplete deficits at their first visit. Progression of deficits beyond this period indicates that the initial diagnosis was
in error and that a mass lesion is compressing the third nerve. • Excellent prognosis or spontaneous recovery (without signs of aberrant regeneration) is expected within 3 months in 90% of patients. Recurrent events involving the same or other ocular motor nerves occur in at least 15% of patients.
Work-up for a Patient with Pupilsparing Complete Third Nerve Palsy Following investigation should be performed: • Fasting and 2 hour postprandial glucose estimation and, if known diabetes, hemoglobin Alc assay (It is not uncommon to identify unsuspected diabetes in a patient presenting with ischemic ocular motor nerve palsy. In those with established diabetes, poor glycemic control may be a predisposing factor). • Serial blood pressure measurement and ECG looking for LVH (As with diabetes, it is not uncommon to identify previously unsuspected hypertension in this population). • Hemoglobin and hematocrit (for polycythemia). • Other evaluations for vascular risk factors, (as appropriate.) • Erythrocyte sedimentation rate and/or creactive protein if patient is older than 55 years to screen for giant cell arteritis. • Neuroimaging is generally not necessary unless: • Age less than 45 years • Vascular risk factors not present • No recovery within 3 months • Signs of aberrant regeneration develop • Pupil becomes involved with anisocoria > one millimeter • Other neurologic signs develop. Tumors that compress the III cranial nerve in ambulatory outpatients are usually located
Neurological Disorders of Pupil in the parasellar, cavernous sinus or orbital apex region. Common offenders include pituitary adenomas, meningiomas, craniopharyngiomas and chordomas. They often injure additional neighboring structures, resulting in a combination of IV or VI cranial nerve palsy, trigeminal neuropathy, post-ganglionic Horner syndrome, or produce visual loss due to compression of the optic nerve or chiasm. The pupil is usually dilated and poorly reactive when the III cranial nerve is compressed by tumor. When signs of aberrant regeneration are present, a chronic mass lesion within the cavernous sinus (e.g. meningioma or aneurysm) should be suspected. When a patient develops oculomotor palsy following seemingly minor head trauma, neuroimaging is indicated to exclude the presence of a previously unsuspected mass lesion at the base of the skull. Aneurysms compressing the III cranial nerve classically produce acute, painful ophthalmoplegia with pupil involvement. One must remember that chronic progressive, painless, and pupil-sparing/relative pupil-sparing presentations are not inconsistent with aneurysm. Three aneurysmal sites are most often encountered: 1. Posterior communicating-internal carotid junction: This is the most common site of aneurysm that causes III cranial nerve injury. Unruptured aneurysm in this location usually does not produce other neurologic symptoms apart from ipsilateral headache. The pupil is “blown” (i.e. dilated and unresponsive to light) in 50% to 75% of patients, but may be normal in 14% especially if external ophthalmoplegia is incomplete.12 2. Basilar tip: The posterior circulation is an often forgotten source of aneurysms that can compress the III nerve. They are often giant or fusiform in appearance. These aneurysms may compress the oculomotor nerve from below; sparing the pupillomotor fibers that
are concentrated along the superior segment of the nerve, producing pupil-sparing or relative pupil-sparing ophthalmoplegia.13 3. Intracavernous carotid: Aneurysms are often giant or dolichoectatic, and often produce other signs of the cavernous sinus syndrome, such as IV or VI nerve palsy, trigeminal neuropathy, or Horner syndrome.14 If the pupillomotor fibers have been injured, the pupil may still appear normal in room light due to aberrant regeneration or superimposed oculosympathetic paresis. Evaluating the size of the pupils in bright light, however, will usually reveal that the affected pupil is larger than the fellow pupil because of iris sphincter paresis. In addition, evaluating the size of the pupils in darkness will reveal that the affected pupil is smaller than the fellow pupil if affected by either aberrant regeneration or Horner syndrome. These aneurysms have a relatively lower risk of rupture and, when they do, often cause signs of high-flow carotidcavernous fistula rather than subarachnoid hemorrhage.
Ischemic vs Aneurysmal Damage to Oculomotor Nerve What elements of the history and physical examination can be used to differentiate between ischemic and aneurysmal injury to the oculomotor nerve? A complete history of the patient should be recorded. The usual details include age, presence of vascular risk factors, pain and progression of ophthalmoplegia in the acute setting are common enough in both disorders that none provide sufficient clinical clue. The status of the pupil, on the other hand, is variable. The discriminating power of the pupil to differentiate III nerve infarction from compression by a mass lesion has become formalized into a clinical
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Diagnostic Procedures in Ophthalmology dictum, the “rule of the pupil”. While this “rule” will work most of the time, there are important pitfalls that must be recognized in order to avoid missing aneurysms. The “rule” states that a normal pupil implies infarction while a dilated pupil implies compression (by aneurysm) of the oculomotor nerve. What is the anatomic basis of this “rule”? The pupillomotor fibers are concentrated peripherally along the superior to medial longitudinal segment of the 3rd nerve as it course from the interpeduncular fossa to the cavernous sinus. Aneurysms arising from the junction of the posterior communicating and internal carotid arteries typically expand downward and laterally, preferentially compressing the pupillomotor fibers as the III nerve becomes injured (producing pupil-involving third nerve palsy). In contrast, the core of the III nerve receives its blood supply via the vaso vasorum. Accordingly, the peripherally located pupillomotor fibers tend to be spared when the core of the III nerve is injured by ischemia (producing pupil-sparing third nerve palsy). How reliable is the “rule” in clinical practice? In regards to aneurysms, Kissel and colleagues12 reviewed the course of III nerve angiographically in 51 patients with proven aneurysm at the junction of the posterior communicating and the internal carotid arteries. A similar study was reported in a series of patients evaluated by a single investigator. 15 The main clinically relevant caveats to be derived from these studies include: • None of the patients in either series has pupilsparing complete III nerve palsy; pupilsparing complete III nerve palsy is generally not a sign of aneurysm. However, the exceptional reports exit. • Kissel and coworkers12 observed that 14% of the patients had pupil-sparing, but the ophthalmoplegia was incomplete in all.
Therefore, pupil-sparing can be an important sign of aneurysmal compression in patients with incomplete ophthalmoplegia. • Complete third nerve palsy was observed in 5 of 7 patients with pupil-sparing, within 5 days. Therefore, always reevaluate those patients with pupil-sparing incomplete III nerve palsy within a week to identify pupil involvement, a sign signifying aneurysm. • In Kissel and coworkers series 24% of patients had partial involvement (i.e., relative pupilsparing III nerve palsy), the majority of whom had incomplete ophthalmoplegia. • In the above series, 63% of patients had complete pupillary involvement (i.e. pupilblown third nerve palsy) and the majority (47%) of whom had complete III nerve palsy. Indeed one does not have to see a pupil-blown complete III nerve palsy to implicate aneurysmal compression. There are four important settings where a patient with aneurysm and III cranial nerve palsy may have a normal appearing pupil. If one does not consider these traps, one may be at risk of missing an aneurysm: 1. When external ophthalmoplegia is incomplete, it usually refers to a patient with all III nerve innervated extra ocular muscles affected, but not fully. Incomplete ophthalmoplegia may also refer to the situation where not all of the extraocular muscles are affected. 2. When the inferior division of the third nerve is spared. The oculomotor nerve travels within the dural lateral wall of the cavernous sinus where it bifurcates into a superior and inferior division near the superior orbital fissure. The superior division carries fibers that innervate the superior rectus and levator palpebrae, while the inferior division carries fibers that innervate the inferior rectus, medial rectus, inferior oblique, and iris sphincter muscles. Intracavernous carotid or basilar tip aneurysms not uncommonly preferentially
Neurological Disorders of Pupil injure the superior division of the III cranial nerve, producing ptosis and paresis of ocular elevation, but no anisocoria. 3. When 3rd nerve palsy is combined with Horner syndrome. Giant intracavernous carotid artery aneurysms commonly injure the oculomotor nerve and, less often, the oculosympathetic pathway. If III nerve palsy and Horner syndrome occur in the same patient, the size of the resulting pupil looks fairly similar to the other pupil in room light. But, observing the pupils with added light will usually expose the paretic iris sphincter, the affected pupil looks slightly larger than the fellow pupil. In addition, the affected pupil usually dilates so poorly that it appears smaller than its fellow pupil if observed in darkness. 4. When the injured third nerve has undergone aberrant regeneration.16 In some cases of chronic compression of the oculomotor nerve by a giant aneurysm of the cavernous sinus, regenerating fibers may become miss-wired, a process called aberrant regeneration. Fibers originally destined to innervate certain extraocular muscles become re-routed into pupillomotor fibers that innervate the iris sphincter. The enhanced tone of the iris sphincter that results from this process produces a pupil that is smaller than normal, reacts poorly to light but better to near, and dilates poorly in darkness. The evaluation of a patient with III cranial nerve palsy must proceed urgently because of the threat of cerebral aneurysm, which may be as frequent as 30% in some series of isolated cases. As discussed, the relationship between the degree of internal and external ophthalmoplegia is the best clinical predictor of whether neurologically isolated and acute III cranial nerve injury is due to compression or infarction. Except in those patients with pupil sparing complete III nerve palsy, neuroimaging, preferably using
MRI, is indicated to identify a mass lesion compressing the oculomotor nerve (e.g. pituitary apoplexy) or some other explanation for the presentation (e.g. midbrain stroke). If the study is unrevealing, one must then proceed emergently to exclude aneurysm. While catheter angiography remains the “gold standard” for identifying aneurysm, it is not without risk. The complication rate is higher in certain patients in particular, those greater than 70 years of age, as well as those with symptomatic atherosclerotic cerebrovascular disease, significant cardiovascular or renal disease, or Ehlers-Danlos syndrome. Threedimensional magnetic resonance angiography (MRA) is a tempting alternative to catheter angiography. How sensitive is this screening test for detecting aneurysms causing III cranial nerve palsy? The answer depends, in part, on considering the ability of MRA to detect aneurysms of various sizes and the proportion of aneurysms in each size class associated with III nerve palsy. A recent metaanalysis disclosed estimates regarding aneurysms at the junction of the posterior communicating and internal carotid arteries.17 With aneurysm size more than or equal to 5 mm, the miss rate was 3% whereas with sizes less than 5 mm, the miss rate exceeded 45%. Because the sensitivity of aneurysmal detection using MRA has become sufficiently high, one may now substitute it for catheter angiography in the diagnostic evaluation of some, but not all, patients with III cranial nerve palsy. However, it should be considered a screening test to exclude aneurysm under certain clinical circumstances, namely when the likelihood of aneurysm is relatively high. The use of MRA to detect aneurysm is subject to the following two crucial caveats: 1. Most importantly, the skill of the interpreting neuroradiologist must be first rate.
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Diagnostic Procedures in Ophthalmology 2. Furthermore, the detection rate is dependent upon review of all imaging data, including source images, maximum intensity projections (the “angiogram”), multiplanar reformatted images, and spin echo images. If any shortcuts are taken, the failure rate will increase. Before ordering MRA to evaluate patients with third nerve palsy, one must first discuss its potential role with his/her radiologist. If there is any doubt about the quality of the study or its interpretation, stick with catheter angiography as the definitive study to rule out aneurysm. Lumbar puncture is generally not necessary when evaluating patients with acute and neurologically isolated III cranial nerve palsy. In India, isolated third nerve palsy may be an uncommon manifestation of CNS tuberculosis. CSF study is often warranted if third nerve palsy is preceded by systemic symptoms (fever, headache, vomiting, etc.), associated with other cranial nerve palsies (where imaging had been negative or non-contributory) and in immunocompromised subjects. The third cranial nerve palsy (with or without pupillary involvement) is a prominent feature of Tolosa-Hunt syndrome.
Tolosa-Hunt Syndrome and Painful Ophthalmoplegia18 Almost any process causing ophthalmoplegia can be painful, with the possible exceptions of myasthenia gravis and chronic progressive external ophthalmoplegia. The physician should always be concerned about infection and tumor. However, there are a group of patients who present with painful, combined ophthalmoplegia due to a granulomatous inflammatory process that affects the carvernous sinus, extending forward to the superior orbital fissure, and orbital apex. Called the Tolosa-Hunt syndrome, it is
usually a disease of middle or later life that may spontaneously remit and relapse. The presenting complaints are steady, retro-orbital pain and diplopia. The third, fourth, sixth or a combination of ocular motor nerve may be affected. Visual impairment occurs in some patients. There is some overlap with orbital pseudotumor. Sensation supplied by the ophthalmic and maxillary trigeminal divisions may be impaired. The pupil may be either constricted or dilated, depending on whether the sympathetic or parasympathetic innervation is involved, respectively. Pathologic examination has shown a low-grade, noncaseating, granulomatous, inflammatory response in the cavernous sinus encroaching on the carotid artery and nerves. Diagnosis is by imaging, which demonstrates soft-tissue infiltration in the cavernous sinus, sometimes with extension into the orbit apex, but without erosion of bone. The infiltrate is either hypointense on T1-weighted images and isointense on T2-weighted images; or hyperintense on T1-weighted and intermediate weighted images. Angiography may show narrowing of the carotid siphon, occlusion of the superior orbital vein, and non-visualization of the cavernous sinus. It has been suggested that the Tolosa-Hunt syndrome is a variant of a larger syndrome of recurrent multiple cranial neuropathies. There is also an association with other forms of vasculitis, such as lupus or Wegener’s granulomatosis. Patients with the Tolosa-Hunt syndrome usually respond promptly to corticosteroid treatment. However, caution is required in attributing diagnostic value to a positive response, since tumors in the cavernous region may respond similarly to steroids, or even resolve spontaneously. Thus, serial MRIs to monitor such patients are advisable. The differential diagnosis of Tolosa-Hunt syndrome includes orbital myositis that may
Neurological Disorders of Pupil usually be distinguished by swelling and erythema of the eyes. The combination of painful palsies of the ocular motor nerves associated with Horner syndrome is so-called Reader paratrigeminal syndrome and usually reflects coexistent involvement of the oculosympathetic fibers in the cavernous sinus, usually due to mass lesions. Ophthalmoplegic migraine is reported to affect each of the ocular motor nerves. Distinction may sometimes be difficult from Tolosa-Hunt syndrome. Years ago, Mathew and Chandy19 and then Mathew20 described a similar syndrome which they considered akin to the Tolosa-Hunt syndrome and this accounted for nearly half of all cases of acute ophthalmoplegia encountered at Vellore, South India. CT or MRI was not available at that time but catheter angiography did demonstrate intercavernous carotid artery narrowing in one case. Over the years, most neurologists in India, must have encountered many such angio-negative, steroid responsive acute painful ophthalmoplegias (though not well documented in literature) and given the diagnosis of Tolosa-Hunt syndrome. With ready availability of MR-scan now, personal experience of the author is that radiological evidence of cavernous sinus-orbital fissure inflammatory lesion can be detected in about half of such cases. The nosology of the Indian variant, therefore, remains a little uncertain and needs studying a large series.
Optic Nerve Pathology The commonest condition is optic neuritis and in fact any lesion of optic nerve might cause pupillary dilatation.
Pharmacologic Mydriasis The final common entity accounting for an isolated dilated pupil is pharmocologic
mydriasis. If high suspicion exists for the use of mild dilating drops, 1% pilocarpine in the affected eye confirms the diagnosis (Fig. 28.6). Eyes exposed to parasympatholytic medications do not respond to 1% pilocarpine. Sympathomimetics that commonly are used to facilitate nasotracheal intubation or ophthalmologic examination also cause mydriasis. In one study, dilute pilocarpine (1/8%) was used to differentiate between pathologic and sympathomimetic mydriasis; 1% pilocarpine overcomes sympathomimetic mydriasis. Inadvertent ocular exposure to anticholinergic agents also has been reported. Patients using scopolamine patches have been noted to have self-limited mydriasis, which has been dubbed as cruise ship anisocoria.
Miosis Increases in Dim Illumination Horner Syndrome21-23 Hallmark features of Horner syndrome include: (1) unilateral miosis, (ii) ptosis and (iii) anhidrosis. It is the result of disruption of the sympathetic innervation to the eye at any place along the pathway. The affected eye has a delayed response – ‘dilation lag’ to reduced illumination, as a result, the anisocoria of Horner syndrome is greater seconds after entering a dark environment than it is after 15-30 seconds. In patients with an unestablished diagnosis, instillation of 4-10% cocaine solution is indicated. Cocaine inhibits the reuptake of norepinephrine, causing more norepinephrine to be available at the neuromuscular junction of the iris dilator muscle. One should assess the pupils at baseline and at 40-60 minutes. In a positive test, the sympathetically impaired pupil fails to dilate, and the degree of anisocoria increases. In some studies, this test has been both sensitive and specific for Horner syndrome (Fig. 28.7). The delineation of the level of a lesion causing Horner syndrome has proved more problematic. This becomes
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Fig. 28.6: Pilocarpine (1%) test for right pharmacologic pupil
Fig. 28.7: Cocaine test for right Horner syndrome
Neurological Disorders of Pupil important, as patients with postganglionic Horner syndrome tend to have a very good prognosis, while preganglionic lesions are often the hallmark of myelopathy or malignancy. The exception is in patients with carotid dissection, which may result in postganglionic Horner syndrome. Fortunately, such lesions are associated with acute neck pain or other neurologic deficits. Therefore, in cases of painful Horner syndrome, emergent evaluation of the anterior cerebral circulation is indicated. In patients with Horner syndrome without pain, a chest X-ray to exclude a Pancoast tumor probably is indicated, but further work-up may be pursued on an outpatient basis. Horner syndrome also can occur in incipient transtentorial herniation. Such patients typically experience a rapid deterioration in brainstem function and have a decreased level of consciousness. Due to the proximity of the carotid sheath, Horner syndrome may occur as a complication of an inferior alveolar nerve block. Causes of Horner syndrome are detailed in Table 28.2.
Hydroxyamphetamine Test Hydroxyamphetamine test is employed to differentiate between a preganglionic and a postganglionic Horner syndrome. The importance of such distinction has already been mentioned. Hydroxyamphetamine enhances the release of norepinephrine from the third order terminal. If the postganglionic neuron is injured, the pupil will not dilate or will dilate poorly. Cremer et al23 found that a 1 mm increase in the amount of anisocoria is associated with 85% probability that the lesion is postganglionic. 2 mm increase is associated with a probability of 99% that postganglionic defect exists. However, the hydoxyamphetamine test is not perfect. Cremet et al23 found that anisocoria increased in 93% of postganglionic cases. The anisocoria did not change in 90% preganglionic cases. Therefore, one has to assume an approximately 10% error rate with this test. In case with non-availability of hydroxyamphetamine, one may substitute with adrenaline. Instillation of adrenaline (1:1000) in a case of postganglionic Horner syndrome will
TABLE 28.2: COMMON CAUSES OF HORNER SYNDROME Central
Preganglionic
Postganglionic
Dorsolateral medullary infarct (Wallenberg’s syndrome) Hypothalamic, thalamic, or mesencephalic infarct, hemorrhage, tumor or demyelination Multiple system atrophy Cervicothoracic spinal cord lesions Cervicothoracic paraspinal mass (including neuroblastoma) Cervical disk herniation Apical lung cancer Cervical sympathectomy Neck injury during forceps delivery Internal carotid artery dissection Cervical adenopathy Cervical tumors Neck trauma Neck injury during forceps delivery Internal carotid artery dissection Cervical adenopathy Cervical tumors Neck trauma Otitis media Cavernous sinus lesion Cluster headache
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Simple Anisocoria Simple anisocoria may be found in approximately 20% of the general population and may vary from day to day in the same individual. In most patients, the degree of anisocoria is less than 1 mm, and not associated with ptosis, dilation lag, or vasomotor dysfunction. In some patients, simple anisocoria may be provoked by oral medications (e.g. pseudoephedrine, selective serotonin reuptake inhibitors). Installation of 4-10% cocaine solution causes dilation of both eyes. Old photographs can provide evidence that the anisocoria had been present for some time. Simple anisocoria may be the result of a variety of cholinergic antiglaucoma medications. A small pupil may be the result of a chance exposure to cholinergic agents. Suspect pharmacologic miosis in an otherwise asymptomatic patient who wears contact lenses and has experienced acute onset of miosis. Implicated agents include anticholinesterases (e.g. flea-collar anisocoria) and inhaled anticholinergics (ipratropium bromide). In such case, withdrawal of exposure to the agent confirms the diagnosis.
Bilateral Constricted Pupils The constricted pupil indicates lesion in the various circuitous pathway taken by the sympathetic supply to the dilator muscle. The lesion may be in the hypothalamus, brainstem, lateral aspect of the spinal cord, the sympathetic chain, the cervical sympathetic ganglia, the pericarotid plexus, or in the sympathetic fibers, which run to the orbit by accompanying the ophthalmic division of the trigeminal nerve. The common causes for bilateral constricted pupils
include pontine hemorrhage, primary or secondary tumors involving the cervical sympathetic chain, vascular lesions of the carotid artery or its sheath and toxins. Bilateral spontaneous miosis means almost invariably an upper brainsteam lesion.
Argyll-Robertson Pupils Argyll-Robertson pupils are classically associated with neurosyphilis. The exact location of the pathologic lesion is hotly debated. Consensus places the lesion in the dorsal midbrain interrupting fibers serving the light reflex with sparing of the ventral accommodative pathways. Clinical features include: (a) small pupils nonreactive to light stimulation with an intact near response (Fig. 28.8), (b) irregular pupils, (c) pupils that dilate poorly in the dark and to mydriatic agents. Similar pupillary findings may be seen in diabetic patients. Other causes of light-near dissociation are: 1. Dorsal midbrain syndrome: Besides light-near dissociation these patients have other clinical features like eyelid retraction, convergence, retraction-nystagmus and decreased up gaze. 2. Severe bilateral visual loss of optic nerve or retinal origin: These patients would have dilated pupils nonreactive to light but nearconvergence reaction with pupillary constriction may be preserved by proprioceptive input to the brain.
Bilateral Dilated Pupils Differential diagnosis of the dilated pupil is relatively small. Once angle-closure glaucoma and a mechanically damaged sphincter pupillae muscle are eliminated from the possible etiologies, dysfunction of the parasympathetic nervous system clearly remains the possibility. Such pupils are dilated and show poor reactivity.
Neurological Disorders of Pupil
Fig. 28.8: Light and near response of Argyll-Robertson pupil
Dilated pupils are caused by the paralysis of the parasympathetic fibers either at their origin form the pretectal nuclei and the EdingerWestphal nucleus in the midbrain, during their course with the oculomotor nerve or at the ciliary ganglion in the orbit. The common causes of the dilation include vascular accidents in the midbrain, tentorial herniation or aneurysms of the carotid artery.
Conclusion Almost all pupillary conditions would be diagnosed with the above approaches delineated. Further work-up will depend on the specific
diagnosis and requires investigations like chest X-ray, CT/MRI-scan of the head and cervical spine. Episodic conditions may be difficult to diagnose at first evaluation and requires repeated evaluations to make a diagnosis. The management of pupillary abnormalities will depend upon the cause of asymmetry, which may include the management of raised intracranial pressure and removal of irritative cause local or distant.
References 1. Lowenfeld IE. The pupil: anatomy, physiology and clinical application. Ames: Iowa State University Press, 1993.
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Diagnostic Procedures in Ophthalmology 2. Burde RW, Savino PJ, Trobe JD. Clinical decision in Neuro-Ophthalmology (2nd ed). St. Louis: Mosby, 1998;221-45. 3. Kordon RH, Thompson HS. The pupil. In. Rosen ES, Thompson HS, Cumming WJ, Eustace P. Eds. Neuro-ophthalmolog. London, Mosby 1998;13.1-13.19. 4. Muller NR, Newtron NJ (Eds). In: Walsh and Hoyts Clinical Neuro-ophthalmology. 5th ed. Vol 1. Baltimore. William and Wilkins 1998; 8271042. 5. Thompson HS, Montague P, Cox TA, et al. The relationship between visual acuity, pupillary defects and visual field loss. Am J Ophthalmol 1982;93:681-86. 6. Johnson IN, Hill RA, Bartholomew MJ. Correlation of afferent pupillary defect with visual loss on automated perimetry. Ophthalmology 1988; 95:1649-55. 7. Thompson HS, Corbett JJ. Spasms of the iris sphincter. Ann Neurol 1980;8:547-49. 8. Adler FW, Scheie HG. The site of disturbance in tonic pupils. Trans Am Ophthalmol Soc 1940; 38:183-88. 9. Chakravarty A, Mukherjee A, Roy D. Ross syndrome – a case documentation. Acta Neurol Scand 2003;107:72. 10. Asbury AR, Alderedge H, Hersberg R, Fisher CM. Oculomotor palsy in diabetes mellitus: a clinicopathological study. Brain 1970;93:555-56. 11. Breen LA, Hopf HC, Farns BK, Gutman L. Pupilsparing oculomotor palsy due to midbrain infarction. Arch Neurol 1995;48:10-16. 12. Kissel JT, Burde RM, Kingele TG, et al. Pupil sparing oculomotor palsies with internal carotid-
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18. 19. 20. 21. 22.
23.
posterior communicating artery aneurysms. Ann Neurol 1983;13:149-54. Guy JR, Day AL. Intracranial aneurysms with superior division paresis of the oculomotor nerve. Ophthalmology 1989;96:1071-76. Linskey ME, Sekhar LM, Hirsch W, et al. Aneurysms of the intracavernous carotid artery: clinical presentation, radiographic features and pathogenesis. Neurosurg 1990;20:71-79. Keane JR. Aneurysms and third nerve palsies. Ann Neurol 1983;14:696-97. Ford FR, Walsh FB, King A. Clinical observations on the pupillary phenomenon resulting from regeneration of the third nerve. Bull Johns Hopkins Hosp 1941;68:309-18. Jacobson DM, Trobe JD. The emerging role of magnetic resonance angiography in the management of patients with third cranial nerve palsy. Am J Ophthalmol 1999,128:94-96. Kline LB, Huyt WF. The Tolosa-Hunt syndrome. J Neurol Neurosurg Psychiatry 2002,71:577-82. Mathew NT, Chandy J. Painful ophthalmoplegia. J Neurol Sci 1970;11:243-56. Mathew NT. Painful ophthalmoloplegia. In Tropical Neurology. Ed. Spiliane JD. London, Oxford University Press, 1972;120-23. Tompson HS. Diagnosing Horner’s syndrome. Trans Am Acad Ophthalmol Otolaryngel 1977; 82: 840-48. Malmey WF, Younge BR, Mayer NJ. Evaluation of the causes and accuracy of pharmacological localization in Horner’s syndrome. Am J Ophthalmol 1980;90:394-420. Cremer SA, Thompson HS, Digre KB, et al. Hydroxyamphetamine mydriasis in normal subjects. Am J Ophthalmol 1990;100:66-70.
Index
Index A A- and B-scan 240 A and V syndromes 369 Aberropia 69 Abnormal fluorescence angiography 190 blocked choroidal fluorescence 191 blocked retinal fluorescence 190 hyperfluorescence 194 hypofluorescence 190 Acanthamoeba 320, 321 Acanthamoeba cysts 322 Acanthamoeba keratitis 318 Accommodation 7, 442 Accommodative convergence 375, 378 Acquired deficiency of color vision 18 Acridine orange 323 Acute dacryocystitis 417 Acute posterior multifocal placoid pigment epitheliopathy 212, 213 Adenoviral keratoconjunctivitis 328 Adie tonic pupil 446, 447 Adverse reactions to intravenous fluorescein angiography 185 anaphylaxis 186 local tissue necrosis 186 nausea 185 pruritus 186 shock and syncope 186 vasovagal attacks 186 vomiting 185 Age-related macular degeneration 205, 254 CNVM 206 hemorrhagic PED 207 hot spot 206, 208 laser photocoagulation 207 photodynamic therapy 207 pigment epithelial detachment 206 polypoidal choroidal vasculopathy 209 retinochoroidal anastomosis 207 transpupillary thermotherapy 207 Alternate cover test 374
Amblyopia 171, 374, 387, 403 classification 387 contrast sensitivity 389 crowding phenomenon 388 diagnosis 388 disparometer 393 distance stereopsis tests 391 fixation disparity 392 fixation disparity curves 392 Frisby stereotest 390, 391 Lang stereotest 390 Lang two pencil test 392 normal stereoacuity 391 preferential looking tests 389 random-dot E stereotest 390 random-dot stereograms 389 special 3-D pictures 390 stereoacuity tests 389 stereopsis 389 Teller acuity 388 Titmus Fly stereotest 389 TNO test 391 Wesson card 393 Wesson fixation disparity card 394 American optical company plates 25 A-mode (Amplitude modulation) 217 A-mode ultrasonography 272 Amplitude modulation scan 240 Anderson’s criteria 142, 146 Aneurysmal damage to oculomotor nerve 451 Aneurysms 397, 451-453 Angle kappa 372 Angle-closure glaucoma 113, 458 Anisocoria 443, 445, 446 Anomalies of color vision 16 acquired 16 congenital 16 Anomaloscope Nagel 28 Pickford-Nicolson 29 Anterior chamber estimation method 39 Anterior chamber paracentesis 337 Anterior chamber tap 336 Anterior ischemic optic neuropathy 124, 125, 306, 310
Anterior segment evaluation— immersion technique 220 Anterior segment photography 179 Anteroior chamber paracentesis 337 Antibiotic susceptibility 325 Antimicrobial susceptibility 321 Antineutrophil cytoplasmic antibody 436 Applications of indocyanine green angiography 205 Aqueous tear deficiency 405 Arden index 282 Argyll-Robertson pupils 446, 458,459 Arrangement tests 25 Edridge-Green Lantern test 30 Farnsworth D-15 test 27 Farnsworth Lantern test (Falant) 29 Farnsworth-Munsell 100-hue test 25 Holmes-Wright Lantern 29 Lantern tests 29 Lanthony desaturated D-15 test 27 A-scan ultrasonography 216, 240, 428 A-scan versus B-scan 237 Asteroid hyalosis 242, 243 Asymmetry of optic disk cupping 118 Atrophic bulbi 256 Atypical retinal pigmentary dystrophies 296 Audiometry 349 Automated perimetry 115 Automated perimetry fixation 136 Autonomic nervous systems 441 A-V patterns 377
B Bacterial colonies 320 Bacterial keratitis 317 Bagolini’s striated glasses Basic perimetry 128 Bebie’s curve 140, 145 Behçet’s syndrome 350
370
461
462
Diagnostic Procedures in Ophthalmology Bell’s phenomenon 400, 404, 442 Bests’ vitelliform macular dystrophy 282 Bilateral superior oblique palsy 397 Biometry in ocular pathologies 233 congenital glaucoma 233 myopia 234 nanophthalmos 234 tumor height 234 Biopsy 339, 436 Black and white films 174 Blepharitis 405, 418, 419 Blood vessels 113 Blow-out fracture 403, 404 B-mode (Brightness modulation) 218 Botulinum toxin 399 Bracketing (staircase technique) 130, 131 Brightness 12 Brightness modulation scan 240 Brown’s syndrome 395, 402, 403 B-scan ultrasonography 239, 240, 259, 428 B-wave amplitude 289
C Calcium alginate swab 317 Calcofluor white 319, 322, 323 Calibration Goldmann applanation tonometer 104 Camera 35 mm 165 35 mm SLR 173 CCD 201 Canaliculi 413, 419 Carrier stage detection 305 Catheter angiography 453, 454 Causes of Horner syndrome 457 Cavernous hemangioma 432, 438 C/D ratio 162 Central areolar atrophy 312 Central scotoma 7 Central serous chorioretinopathy 210, 211 Central serous retinopathy 185, 197 Chloroquine 93, 304 Choroidal and retinochoroidal biopsy 340 Choroidal coloboma 255 Choroidal detachment 250, 365 Choroidal hemangioma 253 Choroidal inflammatory conditions 212 Choroidal melanomas 282
Choroidal neovascular membrane 185, 203, 206, 214 Choroidal pigment 179 Choroidal thickening 249 Choroidal tumors 213 choroidal hemangiomas 213 choroidal metastasis 213 Choroideremia 282 Ciliary block glaucoma 264 Ciliary body tumor 267 Clinical uses of EOG 282 Clinical uses of visual electrophysiological tests 295 Closed circuit TV 136 Closed-angle glaucoma 445 CNVM 203, 214 Cocaine 455 Cocaine test 456 Collection of samples 316, 326 Color blindness 12 Color coded cells 15 Color constancy 13 Color contrast simultaneous 13 Color contrast successive 13 Color Doppler imaging 429, 430 Color performance 31. Color photography 184 Color triangle 14 Color vision 12 Color vision testing 19, 348 City University test 20 color confusion tests 19 color matching tests 20 Edridge-Green Lantern 21 FALANT 21 Farnsworth-Munsell dichotomous-15 test 20 FM-100 test 20, 21 Hardy-Rand-Rittler plates 23 Ishihara pseudoisochromatic plates 20 Lantern tests 21 Nagel anomaloscope test 21 pseudo-isochromatic plates 19 Comitant strabismus 369 Complete third nerve palsy 450 Compression gonioscopy 112 Computed tomography 431 Condensing lens 154 Cone dystrophies 300 Cones 15 Condensing lens 20D 153 Confocal microscopy 84, 85 Congenital color vision deficiency 16 anomalous trichromats 17 blue deficient 17 dichromats 17
green deficient 17 monochromats 17 red deficient 17 X-linked 16 Congenital fibrosis 395 Congenital fibrosis of extraocular muscles 403 Congenital glaucomas 110 Congenital optic disk pit 124 Congenital ptosis 401 Congenital stationary night blindness 282 Conjunctival and lacrimal gland biopsy 344 Conjunctival impression cytology 410 Constricted pupil 458 Contact lenses 115, 152, 93 Contact lenses for gonioscopy 107 Barkan 107 Goldmann 107 Koeppe 107 Layden 107 Sussman 107 Swan-Jacob 107 Thorpe 107 Zeiss and Posner 107 Contact tonometers 96 applanation 96 indentation 96 Contrast sensitivity 9 Contrast-enhanced MRI 433 Convergence 442 Convergent strabismus 369 Core biopsy 437 Corneal aberrometry 63 astigmatism 65 coma 65 high order aberrations 65 low order aberrations 65 measuring corneal wavefront aberration 64 measuring total wavefront aberration 63 optical and image quality 66 ray tracing system 64 Shack-Hartmann method 63 spatially resolved refractometer 64 trefoil 65 Tscherning technique 63 wavefront maps 65 Zernike polynomials 64 Zernike terms 65 Corneal astigmatism 50 Corneal biopsy 317, 318 Corneal dystrophies 89 Fuchs endothelial dystrophy 89 granular dystrophy 89
Index posterior polymorphous dystrophy 89 Corneal grafts 91 Corneal reflection tests 378 double Maddox rod test 380 grading oblique overactions 381 Hess chart 380 Hess screen 379 Hirschberg’s test 378 Krimsky test 378 Lees chart 380 Lees screen 379 Maddox tangent scale 379 measurement of cyclodeviations 380 synoptophore 378 Corneal samples 317 Corneal scraping collection 318 Corneal scrapings 317, 322 Corneal topography 46, 52 absolute scale 55 artificial intelligence programs 62 average corneal power 62 axial map 56 color-coded scales 54 corneal eccentricity index 62 corneal indexes 60 corneal maps 56 difference map 56 diffuse reflection techniques 52 elevation map 56 interferometric method-based systems 52 irregularity map 59 Moire deflectometry-based systems 52 normalized scale 55 placido disk system 52 Placido’s rings 47 raw photokeratoscope image 53 refractive map 56 relative map 56 simulated keratometry reading 61 specular reflection techniques 52 surface asymmetry index 62 surface regularity index 62 tangential curvature map 56 techniques using scattered lightslit-based systems 52 videokeratoscopy 47 Corneal ulceration 405 Corneal wetting time 406 Corrected comparison 140 Corrected pattern standard deviation 137 Cover test 372 Cover-uncover test 372, 374
Cryotherapy for retinopathy of prematurity 358 Crystalline lens 12 CT-scan 349, 404, 428, 432, 435, 436 Culture media 320 Culture methods 319 Cultures 324 Cup-disk ratio 116, 126 Cyclovertical muscle palsy 397 Cysticercosis 249, 403, 426, 430 Cytology 438 Cytopathic effect 329
D Dacryocystography 423 chemiluminescence test 424 CT dacryocystography 423 CT scan 424, 428 dacryoscintigraphy 424 dacryoscopy 424 MRI dacryocystography 423 standardized echography 424 thermography 424 Dark adaptation 281 Dark trough 281 Diabetic III nerve palsy 450 Diabetic maculopathy 189 Diabetic retinopathy 18,212 Diagnosis of uveitis 333 angiotensin converting enzyme 334 antineutrophil cytoplasmic antibody test 334 antinuclear antibody 333 basic investigations 333 fluorescent treponemal antibody absorption test 334 human leucocyte antigens 334 rheumatoid factor 333 serological tests 333 serological tests for syphilis 334 venereal disease research laboratory test 334 Wegener’s granulomatosis 334 Diagnostic biopsies 335 Diagnostic vitrectomy 338 Differential blood counts 436 Diffuse lamellar keratitis 91 Digital subtraction ICGA 214 Digital angiography 183 Digital camera 370 Digital imaging 181 Digital stereo imaging 214
Dilated pupil 447, 458 Diplopia 396, 400 Diplopia testing 379 Direct ophthalmoscope 115, 161, 370 Direct ophthalmoscopy 151, 160 Direct smear examination 318, 326 Disciform macular scar 254, 255 Disk-diffusion tests 321 Dislocated lens 251 Dissociated vertical deviations 376 Divergent strabismus 369 Double elevator palsy 403 Double Maddox rod set 370, 396 Double opponent color cells 15 Draeger applanation tonometer 103 Drug or metal toxicity 280 Drugs causing color deficiency 19 Dry eye 405 Dry eyes syndrome 405 Duane’s retraction syndrome 395, 400, 401, 403
E Ectropion 418 Edinger-Westphal complex 441 Edinger-Westphal nucleus 459 Electrode placement 286 Electrooculogram 279, 280 clinical uses 282 limitations 282 Electrophysiological tests 279 Electroretinogram 279, 283 ELISA test 328, 410, 436 Emmetropic eye 151 Endonasal dye test 423 Endophthalmitis 244, 245, 249 Endothelium corneal 87 Enophthalmos 404 Entropion 418 EOG recording procedure 281 Epicanthus 372 Epiphora 412, 415 Episcleritis 268 Episodic anisocoria 445 Epithelium corneal 86 ERG 305 ERG amplitudes and latency 289 ERG response 287 Esotropia 369 ETDRS chart 3, 11 Evaluation of the retina 244 Evaluation of the vitreous 242 Evaluation of traumatized eye 250 Evaporative DE 405
463
464
Diagnostic Procedures in Ophthalmology Examination of eye in nine gaze positions 377 Examination of nasal cavity 423 Examination of pupil 442 Examination of the sensory status 382 ambylopia 383 binocularity 382, 383 diplopia 382, 383 stereopsis 383 suppression 382, 383 Excisional biopsy 437 Exophthalmometry 427 Exotropia 369, 371 External ophthalmoplegia 450 External photography 173 Exudative retinal detachment 247, 248 Eyelid laxity 417
F Fabry’s disease 93 Failure of filtering surgery 265 False negatives 136, 138 False positives 136, 138, 143 Fast oscillations of EOG 282 Fine needle aspiration cytology (FNAC) 436 Fine-needle aspiration biopsy 341 corneolimbal-zonular approach 342 limbal approach 341 pars plana approach 341 subretinal approach 342 Fixation target positions 377 Flash stimulus characteristics 286 Flicker cone response 30Hz 289 Fluorescein angiogram phases 187 arteriovenous phase 188 prearterial phase 187 recirculation phase 189 transit phase 189 venous phase 188 Fluorescein angiography 177, 181, 200 arterial and venous phase 178 film type and development 177 late phase 178 mid-phase 178 preinjection or control photograph 178 principle 177 procedure 184 Fluorescein dye test 416, 422 Fluorescein stain 407 FNAC 437 Focal ERG 283 Focal macular ERG 312
Force duction test 400, 404 Foreign body 43 Fourth cranial nerve palsy 397 Foveal threshold 136 Full-field flash ERG 283 Functional epiphora 416 Fundus camera 166, 182 digital hand-held fundus camera 167, 168 hand-held Kowa genesis camera 167 mydriatic fundus camera 165 non-mydriatic fundus camera 166 photo slit-lamp 167 portable slit-lamp with video camera 168 Fundus drawing sheet 154 Fundus drawing: color code 156 color code black 157 color code blue 156 color code brown 156 color code green 156 color code red 156 color code yellow 157 cross lines 156 interrupted lines 156 solid 156 Fundus fluorescein angiography 165, 181,345 Fungal hyphae 322 Future applications of indocyanine green angiography 214
G Ganglion cells 280 Ganzfeld bowl 283, 287 Gaze monitoring 134 Genetics of congenital color deficiencies 18 Giant retinal break 247 Giemsa stain 323, 326 Giemsa-stained smears 324 Glaucoma 263 Glaucoma hemifield test 134, 137, 138, 141, 143 Glioma 432 Global indices 134, 137 Goblet cells 410 Goldenhar syndrome 401 Goldmann contact lens 117 Gonioscopic landmarks 109 Gonioscopic lenses 168
Gonioscopy 106, 108, 151 Grading of anterior chamber angle 38, 111 Shaffer’s grade 111 Spaeth’s system 111 Gram stain 323 Gram-stained smears 322 Graves disease 436 Graves ophthalmopathy 431 Gray scale 137, 138
H Haidinger brushes 13, 378 Haidinger brushes and after images 370 Hand-held Goldmann-type tonometers 100 Draeger tonometer 100 Mackay-Marg tonometer 100 Maklakov applanation tonometer 101 Perkins tonometer 100 tonopen 101 Handling of biopsy material 342 Harlequin syndrome 448 Head posture 370 Head tilt 370 Head tracking 134 Heijl-Krakau method 136 Hemosiderosis 93 Herings opponent color theory 16 Herpes simplex virus 326 Herpetic epithelial keratitis 329 Hertel’s exophthalmometer 427 Hess chart 370, 404 Histochemistry 439 Histopathology 438 HLA association with uveitis 335 Holmes-Adie syndrome 448 Homonymous hemianopia 370 Horner syndrome 453, 455, 446, 457 HRT print out 118 Hruby lens direct ophthalmoscopy 162 HSV keratitis 328 HSV type 1 and 2 330 Hue saturation 13 Humphrey field analysis 132, 133 Humphrey single field printout 134, 135 Hydatid cyst 426, 430, 438, 439 Hydoxyamphetamine test 457 Hydroxychloroquine 304 Hypercomplex cells 16 Hyperfluorescence 194 autofluorescence 194
Index pigment epithelial window defect 195 pre-injection fluorescence 194 pseudofluorescence 194 Hyperlipidemia 93 Hypertelorism 372 Hypotropia 399
I Identification of fungal species 321 Idiopathic orbital inflammatory disease 429 III cranial nerve palsy 453 IMAGE-net digital imaging system 183 Imaging system 169 Imaging techniques 427 Immersion B-scan 257 Immunofluorescence assay 327 Immunoglobulins 410 Immunohistochemistry 343, 439 Incisional biopsy 437 Incomitant strabismus 395 Indications of diagnostic paracentesis 336 amyloidosis 336 Behçet’s disease 336, 337 endophthalmitis 336 hemorrhagic glaucoma 336 leukemia 336 malignant melanoma 336 persistent hyperplastic primary vitreous 336 phacolytic glaucoma 336 retinoblastoma 336 sarcoidosis 336 toxocara canis 336 toxoplasma gondii 336 Indications for diagnostic biopsies 335 Indications for electroretinography 296 Indications for vitreous tap 337 amyloidosis 337 asteroid hyalosis 337 Behçet’s disease 337 CMV retinitis 337 endophthalmitis 337 reticulum cell sarcoma 337 retinoblastoma 337 sympathetic ophthalmia 337 Indications of A-scan 222 anterior chamber depth 222 asteroid hyalosis 223 biometry 222, 231 choroidal detachment 230 choroidal hemangioma 226 choroidal hemorrhage 226
choroidal melanoma 226 choroidal thickening 230 congenital glaucoma 222 corneal thickness 222 dislocated lens in vitreous 231 endophthalmitis 224 foreign body localization 230 intraocular tumors 226 measurement of the axial length 222 metastatic carcinoma 226 ocular trauma 230 phthisis bulbi 231 posterior vitreous detachment 224 preretinal foreign bodies 230 retinal detachment 224, 231 retinoblastoma 227 retinoschisis 226 vitreous floaters 223 vitreous hemorrhage 224 Indirect immunofluorescence 328 Indirect immunoperoxidase 328 Indirect ophthalmoscope 153, 154, 370 Indirect ophthalmoscopy 151, 158, 181 head mounted indirect 152 modified monocular indirect 152 monocular indirect 152 penlight ophthalmoscopy 152 slit-lamp indirect 152 Indirect ophthalmoscopy in operating room 157 Indocyanine green 214 Indocyanine green angiography 165, 200, 345 advantages 203 adverse reactions 201 limitations 203 procedure 202 Infectious keratitis 280 Infectious keratitis: diagnostic procedures 316 Infrathreshold 130 Inner retinal dysfunction 300 Instant type film 174 Intermediate uveitis 348 Intermittent exotropia 375 International Society for Clinical Electrophysiology of Vision 280, 314 Interpretation of A-scan 222 Interpupillary distance 371, 377 Intervention for ROP 358 cryotherapy 358 laser ablation 359 parental counseling 360 scleral buckling 359 surgical intervention 359 vitrectomy 359
Intracorneal deposits 93 Intranuclear intracytoplasmic inclusions 326 Intraocular foreign body 243, 251, 362 binocular indirect ophthalmoscopy with scleral indentation 363 corneal wound 363 foreign body in the angle 363 gonioscopy 363 initial fundus examination 363 intralenticular foreign body 363 intraretinal hemorrhage 364 iris hole 363 lens opacity 363 metallic 362 non-metallic 362 siderosis bulbi 363, 364 signs of double perforation 364 slit-lamp examination 362 vitreous hemorrhage 364 vitreous track 364 Intraocular lenses 263 Intraocular tumors 252 Iris and ciliary body biopsy 339 Iris cyst 267 Iris fluorescein angiography 198 Iris melanomas 266 Iris neovascularization 198 Iris nevi 266 Iris thickness 262 Iris-ciliary process distance 262 Iris-lens angle 262 Iris-zonule distance 262 Ischemic III cranial nerve palsy 450 Ischemic vascular retinal disorders 301 Ishihara pseudo-isochromatic plates 22 ISNT rule 118 Isolated rod response 287 Isolated third cranial nerve palsy 449
J Jaeger notation 3 Jaeger’s charts 3 Jones tests 422 Jones tests I 422 Jones tests II 422
K Keratitis 326 Keratoconjunctivitis sicca
407
465
466
Diagnostic Procedures in Ophthalmology Keratoconus 88 Keratometer 46, 47 Keratometry 62 Keratoplasty 261 Kinetic echography 222 Kinyoun method 324 Knapp’s transposition 400 Köllner’s rule 16 Kowa genesis with slit-lamp attachment 168 KOWA VK-2 system 170 Krukenberg’s spindle 40
L Lacrimal excretory apparatus 412 Lacrimal gland 412 Lacrimal sac 413 Lacrimal sac swelling 416 Lacrimation 415 Lactoferrin assays 410 Lactophenol cotton blue 323 Laser in situ keratomileusis 90 LASIK surgery 91, 157 Latex agglutination 328 Leak 195 choroidal leak 197 disk edema 196 retinal leak 196 vitreous leak 196 Leakage of fluorescein 189 Lebers congenital amaurosis 305 Lebers hereditary optic neuropathy 307 Lees screen 370 Lens rim artifact 146 Letter “E” 6 Leukemic infiltration of iris 266 Light adaptation 281 Light and electron microscopy 343 Light peak 281 Light source 153 Light-induced rise of the resting potential 281 Light-near dissociation 445 Limbal dermoid 262 Limitations of ERG 290 Lister’s perimeter 377 Loss variance 140 Low speed angiography 201, 204 Low-coherence interforometry 269 Lumbar puncture 349, 454 Luminescence 181 Lymph node biopsy 345
Lymphangioma 431 Lysozyme assays 410
M Mackay-Marg tonometer 103 Macula 7 Macular edema 347, 348 Macular photoreceptor function 280 Magnetic resonance angiography (MRA) 434, 435, 453 Magnetic resonance imaging 433 Magnification 34 continuous zoom 34 flip type 34 Malignant lacrimal gland tumor 432 Malingering 308 Mantoux test 349 Marcus Gunn jaw-winking Maximal combined response 288 Maxwell spot 13 Mean defect 140 Mean sensitivity 140 Measurement of ocular deviation 375 Measurement of vergences 381 convergence sustenance 382 horizontal vergences 381 near point of convergence 382 torsional vergences 381 vertical vergences 381 Medial canthal tendon laxity 417 Meibomian gland dysfunction 405 Melanoma 252 Metastatic choroidal carcinoma 253 Methods for localization of IOFB 364 Berman and Roper-Hall localizers 364 Bromley’s method 366 combined B- and vector A-scan 364 computerized tomographic scan 367 Dixons’ method 366 Mac Kenzie’s method 366 magnetic resonance imaging 367 Mc Rigor’s method 366 plain X-ray 365, 366 radio opaque markers 366 Sweet’s method 366 ultrasonography 364 ultrasound biomicroscopy 365 use of contrast material 366 use of limbal ring 366 Microbial keratitis 316 Microbiological cultures 343
Microbiology 339 Mild pilocarpine test 448 Minimal inhibitory concentrations 321 Minimum angle of resolution 2 Minimum criteria for the diagnosis of glaucoma 141 Miosis 442, 455 Möbius syndrome 395, 400 Modified monocular indirect ophthalmoscopy 159 Molecular microbiology 322 Monochromatic fundus photography 179 Monocular elevation deficiency 399 Monocular indirect ophthalmoscopy 158, 159 Morning glory syndrome 124, 401 MRA 454 MRI 349, 428, 434, 435, 436, 453 Mucosal biopsy 344 Multifocal ERG 283, 308, 311 Multifocal VEP 308 Multinucleated giant cells 326, 327 Multiple evanescent white dot syndrome 212 Multiple pinholes 7 Myasthenia gravis 396 Mydriasis 397 Myopia 214 Myopic disk 123
N Nasalization of the vessels 121 Nasolacrimal duct 414 Near blindness 10 Near vision chart 5 Necrotizing scleritis 268 Negative a-wave 288 Negative ERG 300 Nerve fiber layer 179 Nerve supply 412 Neurological disorders of pupil 441 Neuroretinal notch 120 Neuroretinal rim 118, 121, 123 Neuroretinal rim notch 119 Nikor Medikor lens 173 Nodular scleritis 267 Noncom robo 170 Noncontact lenses 115, 116, 152 Noncontact tonometer 96 Non-invasive break-up time 406 Nonisolated third cranial nerve palsy 449
Index Non-viral keratitis : bacterial, fungal and acanthamoeba 316, 319, 322 Normal cornea 85 Normal cornea, shape 48 Normal fundus fluorescein angiography 187 Normal globe 242 Normal macula 270 Nystagmus 370
O Occluder 7, 370 Occult choroidal neovascular membranes 200 OCT in macular diseases 270 central serous chorioretinopathy 275, 276 diabetic macular edema 273 diabetic retinopathy with cystoid macular edema 274 diabetic retinopathy with serous retinal detachment 274 foveal retinal detachment 273 high myopic eyes 272 juvenile retinoschisis 277 macular hole 270 normal macula 271 posterior staphyloma 272 preretinal macular fibrosis 272, 273 retinoschisis 273 rhegmatogenous retinal detachment 276 vitelliform macular dystrophy 277 Octopus 139 Octopus field analyzer 132 Octopus single field printout 138 Ocular albinism 305, 307 Ocular anatomy on ultrasound biomicroscopy 261 Ocular deviation 371 Ocular ischemic syndrome 302 Ocular microbiology 316 Ocular movements 171 Ocular trauma 266 Oculomotor palsy 446 Open-angle glaucoma 123, 133 Ophthalmic imaging systems 170 Ophthalmic photography 165 Ophthalmoplegia 396, 452 Ophthalmoplegic migraine 455 Ophthalmoscopy 151 Opponent color cells 15 Optic chiasm 444
Optic coherence tomography 269 Optic disk cupping 142 Optic disk evaluation in glaucoma 115 Optic nerve 7, 444 Optic nerve avulsion 252 Optic nerve demyelination 306 Optic nerve drusen 255 Optic nerve head coloboma 124, 257 Optic nerve head drusen 256 Optic nerve hypoplasia 404 Optic tract 444 Optical coherence tomography 115, 269, 346 Optical principles 106 Optical system of fundus camera 174 Optico kinetic nystagmus drum 370 Optics 84 Optics of slit-lamp 34 clinical procedure 35 Haag-Streit type illumination 34 illumination system 34 observation system 34 Zeiss type illumination system 34 Orbicularis oculi 414 Orbital arteriography 435 Orbital blow-out fractures 395 Orbital venography 435 Oropharynx dye appearance test 423 Oscillatory potentials 288 Overlay technique 214
P Painful ophthalmoplegias 455 Pair of scleral depressors 153 Papanicolaou stain 326, 327, 330, 331 Papilledema 306 Parallelopiped 39, 40, 41, 43, 110 Paralytic strabismus 395 Pattern deviation plot 137 Pattern electroretinogram 280, 283, 290 clinical uses 292 evaluation of macular function 292 ganglion cell dysfunction 292 Pattern standard deviation 137 PCR technique 329 Pediatric ERG recording 290 Pediatric visual assessment 280 Pediatric visual impairment 305 Penlight ophthalmoscopy 160 Perimeter 370 Perimetry 128, 151 Peripapillary atrophy 121, 122 Peripheral anterior synechia 112
Peripheral choroidal tumors 267 Peripheral iridoplasty 264 Perkins applanation tonometer 103 Pharmacologic miosis 458 Pharmacologic mydriasis 455 Pharmacologically dilated pupils 446 Phases of ICGA 204 arteriovenous phases 204 between 2 and 5 seconds 204 between 5 seconds and several minutes 204 beyond several minutes 204 early phase 204 first 2 seconds 204 late phase 205 middle phase 205 prearterial and arterial phases 204 Photography in operation theatre 168 Photography of face 172 Photography of pupil 172 Photoreceptor dysfunction 295 Phthisis bulbi 254, 255 Physics of ultrasound 217 Physiological cupping 123 Pigmentary glaucoma 265 Pigmentation 113 Pilocarpine (1%) test 456 Pilocarpine 455 Pinch test 417 Pinhole 7 Pitfalls of A-scan 234 artifacts 235 cataract 236 errors in the axial length measurement by biometry 235 intraocular foreign bodies 235 low reflective spike 235 methylcellulose 236 misalignment 236 multiple reflection artifacts 234 posterior staphyloma 236 refractive errors 236 tumors 235 vitreoretinal diseases 235 Pituitary tumor 125 Plateau iris syndrome 263 Pneumatic tonometer 103 Poland anomaly 400 Polaroid or Fuji film 174 Polymerase chain reaction 322, 325, 339, 343 Poor vision in infants 280 Positive b-wave 288 Posterior globe rupture 252 Posterior staphyloma 255, 272
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468
Diagnostic Procedures in Ophthalmology Posterior vitreous detachment 243, 365 Potassium hydroxide 323 Pretectal nucleus 444 Prethreshold ROP 359 Primary angle-closure glaucoma 263 Principles of ophthalmoscopy 151 Prism bar 370 Prism bar cover test 375 Probing 416, 421 Procedures in uveitis 333 Proliferating vitreous membrane 246 Proliferative diabetic retinopathy 243 Proliferative vitreoretinopathy 245 Properties of sodium fluorescein 182 Proptosis 426 Pseudoepiphora 416 Pseudofluorescence 202 Pseudoptosis 400 Pseudostrabismus 372 Pseudotumor 434 Pterygium 43, 418 Ptosis 396, 401 Pulzone-Hardy rule 371 Puncta 412, 416, 417, 418 Pupil: Neurological disorders 7, 281, 441 Pupillary abnormalities 445 Pupillary light reflex 441
Q Quantitative echography Quinine 304
221
R Radio immunoassay 328 Radiological studies 349 Radionucleotide studies 349 RAPD 443 Reader paratrigeminal syndrome 455 Reading charts 11 Record visual acuity 2 Recording electrode 284 Burian-Allen 284 contact lens 5 gold foil 285 ground 286 LVP-Zari 285 reference 285 Red and green goggles 370 Red-free photography 179 Reference values for corneal aberrations 68
Reflection 239 Refraction 239 Refractive surgery 74, 93, 263 postastigmatic keratotomy 75 postintrastromal corneal rings implantation 77 postkeratoplasty 79 postlaser in situ keratomileusis 77 postlaser thermal keratoplasty 77 postphotorefractive keratotomy 75 radial keratotomy 74 Relative afferent pupillary defect 443 Relative pupil block glaucoma 263 Reliability factors 138 Reliability indices 134 Reproducibility 134, 138 Retcam 358 Retinal correspondence 377, 386 after image test 387 anomalous retinal correspondence 386 Bagolini’s striated glasses 386 diagnosis of ARC 386 harmonious ARC 386 normal retinal correspondence 386 unharmonious ARC 386 Worth four dot test 387 Retinal detachment 224, 231, 243, 244, 246, 348 Retinal fundus cameras 181 Retinal nerve fiber layer abnormalities 122 diffuse areas 122 slit-like defects 122 wedge-shaped 123 Retinal photoreceptors 280 Retinal pigment epithelium 280 Retinal tear 246 Retinitis pigmentosa 282, 295 Retinoblastoma 253 Retinochoroidal anastomosis 207 Retinopathy of prematurity 353 arrested vasculogenesis 353 classification 353 early treatment 353 etiology 353 international classification 354 stage 1: dermarcation line 354 stage 2: dermarcation ridge 354 stage 3: extraretinal fibrovascular proliferation 354 stage 4: partial retinal detachment 354 stage 5: total retinal detachment 354 management 353 multicenter trial of cryotherapy 353
pathogenesis 353 plus disease 355 prethreshold 355, 356 risk factors 353 birth weight 353 multiple birth 353 respiratory distress syndrome 353 young gestational age 353 Rush disease 355 screening 353, 357 screening procedure 357 threshold 355, 356 zones 353 zone-I 354 zones II and III 354 Retinoschisis 247, 273 Retroillumination 44 Rhegmatogenous retinal detachment 157 Ring scotoma 146 Rod monochromatism 305, 309 Rod-photoreceptor disorders 282 Rose Bengal stain 407, 408 Ross syndrome 448 Royal air force binocular gauge 382
S Sampaolesi line 110 Scanning electron microscopy 440 Scanning laser ophthalmoscope 202 Scattering 239 Schiøtz tonometer 95, 102 Schirmer I test 406, 419 Schirmer II test 407, 420 Schirmer’s strip 408 Schirmer’s test 408, 419 Schirmer’s test with nasal stimulation 407 Scleral depression 154, 157 Scleral staphyloma 268 Scotoma suppression 385 Sensors 136 Serum angiotensin converting enzyme 436 Serum autoantibodies 410 Seven in one printout 138, 139 Seven in one single field printout 145 Shell vial technique 329 Short term fluctuation 137, 140 Simple anisocoria 458 Single-flash cone response 289 Sixth cranial nerve palsy 398, 399 Sjögren’s syndrome 410
Index Slit-lamp 33, 259, 317, Slit-lamp attachments 44 digital camera 44 Goldmann tonometer 44 gonioscope 44 Hruby lens 44 pachymeter 44 Slit-lamp biomicroscopy 151, 181, 270 Slit-lamp examination 33, 36, 418 Smears 322 Snap back test 417 Snellen chart 2, 3, 370, 388 Specialized types of ERG 284 Specular microscopy 169 Specular photography 169 Spielmann occluder 370 Spielmann translucent occluder 373 Splinter hemorrhages 121 Split limbal technique 38 Stargardt’s macular dystrophy 305, 313 Statistical analysis 134 Statpac program 134 Stereophotography 185 Sterilization of different tonometers 102, 104 Steriopsis 33 Stevens-Johnson syndrome 410 Strabismus 365,369, 395 Stroma 87 Subepithelial nerve plexus 86 Superior oblique palsy 397 Suprathreshold 130 Suprathreshold screening 132 Swedish interactive thresholding algorithm (SITA) 132 Swinging-light pupil test 443, 445 Sympathetic pathway 442 Sympathomimetic mydriasis 455 Synoptophore 370, 371, 377, 384, 387 Syringing 416, 420
T Table-top retinal cameras 174 Taste test 422 Tear deficient DE 405 Tear ferning 410 Tear film break-up time 406 Tear film osmolarity 410 Tear secretion 415 Techniques of slit-lamp examination 36 broad beam 39
conical beam 39 diffuse illumination 36 direct 36 direct focal illumination 37 indirect illumination 40 narrow beam 37 oscillatory illumination 43 retroillumination 41 sclerotic scatter 42 specular reflection 42 tangential illumination 43 Telecanthus 372 Teleophthalmology 170 Teller acuity cards 370 Temporal hemianopia 125, 147 Tendency oriented perimetry 133 Test programs 133 Humphrey 10-2, 30-2, 24-2 133 macular grid program 133 macular program M2X 133 octopus G1X, G2 133 Testing strategy 132 Tests for suppression 383 after image test 384, 385 Bagolini glasses test 384 Bjerrum screens 385 binocular perimetry 385 depth of scotoma 385 Hess screen 385 Lees screen 385 suppression scotoma 385 synoptophore 384 Worth four dot test 384 Theories of color vision 14 Granit’s theory 14 Hering’s theory 14 Young-Helmholtz theory 14 Thioridazine 304 Third cranial nerve dysfunction 448 Third cranial nerve palsy 396 Three-dimensional ultrasound tomography 240 Three-step test 397 Threshold 130 Threshold determination 132 Thyroid function tests 436 Thyroid ophthalmopathy 426, 432 Time-gain compensation 259 Tissue culture methods 328 Tolosa-Hunt syndrome 449, 454 Tonomerty 95, 151 Goldmann applanation tonometer 95 Schiøtz tonometry 95 Tonometry on irregular corneas 103 Tonometry over gas filled eyes 103
Tonometry over soft contact lens 103 Topographic echography 220 Total deviation plot 137 Trabecular-iris angle 262 Traction retinal detachment 247, 248 Transillumination 158 Transmission electron microscopy 440 Transport of corneal samples 317 Trichromatic theory of Young 16 Trophozoites 323 Tumors of uvea 266 Types of gonioscopy 106 direct 106 indirect 108 Types of perimetry 129 automated 129 computerized 129 Goldmann perimeter 129, 130 kinetic 129 manual kinetic 129 static 129, 130 tangent screen 129
U Ultrasonography 151, 428, 430 Ultrasound 346, 430 Ultrasound biomicroscopy 259, 262, 346 Ultrasound unit 240 Uses of corneal topography 69 keratoconus 69 keratoglobus70 pellucid marginal degeneration 70 pterygium 74 Terrien’s marginal degeneration 71 Uveitis diagnostic procedures 333
V Van Herick’s technique 38, 39 Varicella zoster virus 328 Vascular filling defect 192 choroidal vascular filling defect 192 retinal vascular filling defects 192 vascular filling defects of the disk 192 Vector A-scan 240 Vector A-scan display 219 VEP 308 Viral antigens in corneal scrapings 328 Viral corneal ulcers 316 Viral keratitis 326
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470
Diagnostic Procedures in Ophthalmology Virus isolation 327 Vision 1 color contrast 1 Vision loss 10 Visual acuity 1 Visual acuity assessment 307 Visual acuity in low vision 9 Visual acuity testing in young children 8 Allen and Osterberg charts 8 binocular fixation pattern 8 illiterate E chart 8 Landolt broken ring 8 objective retinoscopy 8 occlusion 8 optokinetic nystagmus 8 preferential looking test chart 8 sensory amblyopia 8 visual evoked potentials 8 Visual electrophysiology tests 279 Visual evoked potential 279, 293 flash 293, 295 limitations 295 normal waveforms 295 pattern-onset 293
pattern-onset/offset 295 pattern-reversal 293, 295 Visual field defect 118 Visual field indices 140 Visual field testing 349 Visual function assessment 279 Visual loss assessment in infants and children 308 Visual pathway 279 Visual scale 409 Visual thresholds 2 light discrimination 2 spatial discrimination 2 temporal discrimination 2 Vitrectomy 275 Vitreoretinal surgery 93 Vitreous hemorrhage 214, 243, 244, 250 Vitreous tap 338 Vogt-Koyanagi-Harada disease 247, 249 Volk superfield lens 117 Vortex keratopathy 93 VZV infections 330
W Wegener’s granulomatosis 436, 454 Westphal-Piltz reaction 442 Wide-angle angiography 214 Wide-angle viewing system 163 panoret 163 retcam 163 Wilson’s disease 93 Worth four dot test 387
X X-ray 404, 427, 431
Z Zeiss fundus camera 201 Zernike polynomials 64 Ziehl-Neelsen 323 Ziehl-Neelsen technique 324
