Diagnostic Procedures in Ophthalmology PDF

Diagnostic Procedures in OPHTHALMOLOGY Diagnostic Procedures in OPHTHALMOLOGY SECOND EDITION HV Nema Former Professor and Head Department of Ophthalmology Institute of Medical Sciences B...

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 ® JAYPEE BROTHERS MEDICAL PUBLISHERS (P) LTD New Delhi Ahmedabad Bengaluru Chennai Hyderabad Kochi Kolkata Lucknow Mumbai Nagpur St Louis (USA) Published by Jitendar P Vij Jaypee Brothers Medical Publishers (P) Ltd Corporate Office 4838/24 Ansari Road, Daryaganj, New Delhi - 110 002, India, +91-11-43574357 (30 lines) Registered Office B-3 EMCA House, 23/23B Ansari Road, Daryaganj, New Delhi 110 002, India Phones: +91-11-23272143, +91-11-23272703, +91-11-23282021, +91-11-23245672, Rel: +91-11-32558559 Fax: +91-11-23276490, +91-11-23245683 e-mail: [email protected], Website: www.jaypeebrothers.com Branches 2/B, Akruti Society, Jodhpur Gam Road Satellite Ahmedabad 380 015 Phones: +91-79-26926233, Rel: +91-79-32988717 Fax: +91-79-26927094 e-mail: [email protected] 202 Batavia Chambers, 8 Kumara Krupa Road, Kumara Park East Bengaluru 560 001 Phones: +91-80-22285971, +91-80-22382956, +91-80-22372664 Rel: +91-80-32714073, Fax: +91-80-22281761 e-mail: [email protected] 282 IIIrd Floor, Khaleel Shirazi Estate, Fountain Plaza, Pantheon Road Chennai 600 008 Phones: +91-44-28193265, +91-44-28194897, Rel: +91-44-32972089 Fax: +91-44-28193231 e-mail: [email protected] 4-2-1067/1-3, 1st Floor, Balaji Building, Ramkote Cross Road Hyderabad 500 095 Phones: +91-40-66610020, +91-40-24758498, Rel:+91-40-32940929 Fax:+91-40-24758499 e-mail: [email protected] No. 41/3098, B & B1, Kuruvi Building, St. Vincent Road Kochi 682 018, Kerala Phones: +91-484-4036109, +91-484-2395739, +91-484-2395740 e-mail: [email protected] 1-A Indian Mirror Street, Wellington Square Kolkata 700 013 Phones: +91-33-22651926, +91-33-22276404, +91-33-22276415 Rel: +91-33-32901926, Fax: +91-33-22656075, e-mail: [email protected] Lekhraj Market III, B-2, Sector-4, Faizabad Road, Indira Nagar Lucknow 226 016 Phones: +91-522-3040553, +91-522-3040554 e-mail: [email protected] 106 Amit Industrial Estate, 61 Dr SS Rao Road, Near MGM Hospital, Parel Mumbai 400012 Phones: +91-22-24124863, +91-22-24104532, Rel: +91-22-32926896 Fax: +91-22-24160828 e-mail: [email protected] “KAMALPUSHPA” 38, Reshimbag, Opp. Mohota Science College, Umred Road Nagpur 440 009 (MS) Phone: Rel: +91-712-3245220, Fax: +91-712-2704275 e-mail: [email protected] USA Office 1745, Pheasant Run Drive, Maryland Heights (Missouri), MO 63043, USA, Ph: 001-636-6279734 e-mail: [email protected], [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 Surbhit Chaudhary MS Director, Vissum Ex-Fellow Institute of Ophthalmology of Alicante Sankara Nethralaya Alicante, Spain Chennai, Tamil Nadu, India Sonal Ambatkar DNB Glaucoma Service Taraprasad Das MS Aravind Eye Hospital Director Tirunelveli, Tamil Nadu, India LV Prasad Eye Institute Bhubaneswar, Orissa, India Francisco Arnalich MD Vissum Munish Dhawan MD Institute of Ophthalmology of Alicante Dr RP Centre for Ophthalmic Sciences Alicante, Spain AIIMS, New Delhi, India Sreedharan Athmanathan MD, DNB Virologist Lingam Gopal MS, FRCS LV Prasad Eye Institute Chairman Hyderabad, Andhra Pradesh, India Medical Research Foundation Sankara Nethralaya, Chennai Mandeep S Bajaj MD Tamil Nadu, India Professor Dr RP Centre for Ophthalmic Sciences AK Grover MD, FRCS AIIMS, New Delhi, India Chairman Department of Ophthalmology Tinku Bali MS Sir Ganga Ram Hospital Consultant New Delhi, India Department of Ophthalmology Sir Ganga Ram Hospital, New Delhi, India Roshmi Gupta MD Consultant, Narayana Nethralaya Rituraj Baruah MS Bengaluru, Karnataka, India Senior Registrar Lady Hardinge Medical College Sanjiv Gupta MD New Delhi, India Dr RP Centre for Ophthalmic Sciences AIIMS, New Delhi, India Jyotirmay Biswas MS, FAMS Head, Ocular, Pathology and Uveitis Stephen C Hilton OD Sankara Nethralaya, Chennai West Virginia University Tamil Nadu, India Morgantown, USA Ambar Chakravarty MS, FRCP Santosh G Honavar MD, FACS Honorary Professor and Head Director Department of Neurology Department of Ophthalmic Plastic Surgery and Vivekananda Institute of Medical Sciences Ocular Oncology, LV Prasad Eye Institute Kolkata, West Bengal, India Hyderabad, Andhra Pradesh, India viii Diagnostic Procedures in Ophthalmology Anjali Hussain MS A Narayanaswamy Consultant Consultant LV Prasad Eye Institute Sankara Nethralaya Hyderabad, Andhra Pradesh, India Chennai, Tamil Nadu, India Subhadra Jalali MS Rajiv Nath MS Head Professor Smt Kanuri Santhamma Retina-Vitreous Centre Department of Ophthalmology LV Prasad Eye Institute KG Medical University Hyderabad, Andhra Pradesh, India Lucknow, Uttar Pradesh, India Sadao Kanagami FOPS Professor Tomohiro Otani MD Kitasato University School of Medicine Professor Teikyo, Japan Department of Ophthalmology Gunma University School of Medicine Sangmitra Kanungo MD, FRCS Maebashi, Japan Consultant LV Prasad Eye Institute Nikhil Pal MD Hyderabad, Andhra Pradesh, India Senior Resident Dr RP Centre for Ophthalmic Sciences Shahnawaz Kazi MS AIIMS, New Delhi, India Fellow Sankara Nethralaya Chennai, Tamil Nadu, India Rajul Parikh MS Consultant, Sankara Nethralaya R Kim DO Chennai, Tamil Nadu, India Head Retina-Vitreous Service David Piñero OD Aravind Eye Hospital and Vissum Postgraduate Institute of Ophthalmology Institute of Ophthalmology of Alicante Madurai, Tamil Nadu, India Alicante, Spain Parmod Kumar OD K Kalyani Prasad MS Glaucoma Imaging Centre Consultant New Delhi, India Krishna Institute of Medical Sciences Hyderabad, Andhra Pradesh, India S Manoj MS Consultant Retina-Vitreous Service Leela V Raju MD Aravind Eye Hospital and Postgraduate Institute Monongalia Eye Clinic of Ophthalmology, Madurai, Tamil Nadu, India Morgantown, USA S Meenakshi MS VK Raju MD, FRCS, FACS Consultant Clinical Professor Pediatric Ophthalmology Sankara Nethralaya Department of Ophthalmology Chennai, Tamil Nadu, India West Virginia University Morgantown, USA Amit Nagpal MS Consultant LS Mohan Ram D Opt, BS Sankara Nethralaya, Chennai LV Prasad Eye Institute Tamil Nadu, India Hyderabad, Andhra Pradesh, India  Contributors ix R Ramakrishnan MS Yog Raj Sharma MD Professor and CMO Professor Aravind Eye Hospital Dr RP Centre for Ophthalmic Sciences Tirunelveli, Tamil Nadu, India AIIMS, New Delhi, India Manotosh Ray MD, FRCS Deependra Vikram Singh MD Associate Consultant Senior Resident National University Hospital Dr RP Centre for Ophthalmic Sciences, AIIMS Singapore New Delhi, India Pukhraj Rishi MD Consultant Devindra Sood MD Sankara Nethralaya Consultant, Glaucoma Imaging Centre Chennai, Tamil Nadu, India New Delhi, India MS Sridhar MD Monica Saha MBBS Consultant Department of Ophthalmology LV Prasad Eye Institute KG Medical University Hyderabad, Andhra Pradesh, India Lucknow, Uttar Pradesh, India S Sudharshan MS Chandra Sekhar MD Fellow Director Sankara Nethralaya LV Prasad Eye Institute Chennai, Tamil Nadu, India Hyderabad, Andhra Pradesh, India Kallakuri Sumasri B Optm Harinder Singh Sethi MD, DNB, FRCS Retina-Vitreous Centre Senior Research Associate LV Prasad Eye Institute Dr RP Centre for Ophthalmic Sciences Hyderabad, Andhra Pradesh, India AIIMS, New Delhi, India T Surendran MS, M Phil Pradeep Sharma Vice Chairman and Director Professor Pediatric Ophthalmology Dr RP Centre for Medical Sciences Sankara Nethralaya AIIMS, New Delhi, India Chennai, Tamil Nadu, India Rajani Sharma MD (Ped) Garima Tyagi B Opt Senior Resident Retina-Vitreous Centre Department of Pediatrics LV Prasad Eye Institute AIIMS, New Delhi, India Hyderabad, Andhra Pradesh, India Savitri Sharma MD Vasumathy Vedantham MS, DNB, FRCS Head Consultant, Retina-Vitreous Service Jhaveri Microbiological Centre Aravind Eye Hospital and Postgraduate LV Prasad Eye Institute Institute of Ophthalmology Hyderabad, Andhra Pradesh, India Madurai, Tamil Nadu, India Tarun Sharma MD, FRCS L Vijaya MS Director Head Retina Service, Sankara Nethralaya Glaucoma, Sankara Nethralaya Chennai, Tamil Nadu, India Chennai, Tamil Nadu, India  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 follow- up 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. xii 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 xiii 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  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 (Director- Publishing) 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 xviii 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 1 STEPHEN C HILTON, LEELA V RAJU, VK RAJU Visual Acuity 1 Vision is the most important of all senses. human optic system to identify two points as Approximately 80% of the information from the different stimuli is defined as the threshold of outside world is incorporated through the visual resolution. Visual acuity is the reciprocal of the pathway. Loss of vision has a profound effect threshold of resolution.2 Clinically, discrimina- on the quality of life. ting letters in a chart determine this, but this The process of vision includes: task also requires recognition of the form and 1. Central resolution (visual acuity) shape of the letters, which are processes that 2. Minimal light sensitivity also involve higher centers of visual perception. 3. Contrast sensitivity Discrimination at a retinal level may, there- 4. Detection of motion fore, be determined by less complex stimuli, such 5. Color perception as contrast sensitivity gratings. Theoretically, the 6. Color contrast maximum resolving power of the human retina 7. Peripheral vision (spatial, temporal and could be derived from an estimate of the angle motion detection). of approximately 20 seconds of arc because this In the normal clinical settings, we measure represents the smallest unit distance between only one of these functions – central resolution two individually stimulated cones. Thus the at high contrast (visual acuity).1 resolving power of the eye could be much greater than what is measured by visual acuity charts.3 Cones have the highest discriminatory Definition and Terminology of capacity, but rods can also achieve some Visual Acuity resolution. The greater the distance from the fovea The most basic form of visual perception is the level of visual acuity falls off rapidly. At a detection of light. Visual acuity is more than just 5° distance from the foveal center, visual acuity detecting light. It is the measurement of the ability is only one quarter of foveal acuity.4 Luminance to discriminate two stimuli separated in space of test object, optical aberrations of the eye and at high contrast compared with the background. the degree of adaptation of the observer also The minimal angle of resolution that allows a influence the visual acuity.5 2 Diagnostic Procedures in Ophthalmology Visual thresholds can be broadly classified contains mainly rods, may be assessed by into three groups: peripheral visual field.1 1. Light discrimination (minimum visible, Visual acuity is the first test performed after minimum perceptible) obtaining a careful history. Measurement of the 2. Spatial discrimination (minimum separable, central visual acuity is essentially an assessment minimum discriminable) of the function of the fovea centralis. An object 3. Temporal discrimination (perception of must be presented so that each portion of it is transient visual phenomena such as separated by a definite interval. Customarily, this flickering stimuli). interval has become one minute of an arc, and Many clinical tests can assess many visual the test object is one that subtends an angle of functions simultaneously. In a healthy observer five minutes of an arc. A variety of test objects in best focus, the resolution limit, or as it is has been constructed on this principle, so that usually called, the minimum angle of resolution an angle of five minutes is at distances varying (MAR), is between 30 seconds of arc and one from a few inches to many feet5 (Figs 1.1 and minute of arc. Clinically, we use Landolt C and 1.2). The most familiar examination chart is Snellen E to assess visual acuity. The minimum Snellen chart (Fig. 1.3). Conventionally, reading discriminable hyper-acuity or vernier-acuity is vision is examined at 40 cm (16 inches). The another example of spatial discrimination. The testing distance of a preferred near distance chart 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 Fig. 1.1: Snellen letters subtend one minute of arc in The function of the eye may be evaluated by a each section, the entire letter subtends five minutes of number of tests. The cone function of the fovea arc 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 Fig. 1.2: Each component of Snellen letters subtend one is also a measurement of foveal function. minute of visual angle the entire letter subtends five minutes The function of the peripheral retina which of visual angle at stated distance  Visual Acuity 3 Fig. 1.4: ETDRS chart 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. Fig. 1.3: Snellen chart In the United States these numbers are given in inches or feet, whereas in the Europe the should be observed accurately. The Snellen designation is in meters. notation is simply an equivalent reduction for The test chart commonly used in the United near, maintaining the same visual angle. Most States has its largest test object one that subtends of the Snellen-based distance acuity charts are an angle of five minutes at a distance of 200 also commercially available as ‘pocket’ charts feet (6 m). Then there are test objects of 100, 80, to check the near acuity at a preferred distance 70, 60, 50, 40, 30, 20 and 15 feet. If the individual for every patient or at a defined distance for is unable to recognize the largest test object, then clinical trial purposes including ETDRS (Fig. 1.4) he or she should be brought closer to it, and and Snellen letter “E”. the distance at which he or she recognizes it The Jaeger notation is a historic enigma and should be recorded. Thus, if the individual Jaeger never committed himself to the distance recognizes the test object that subtends a five at which the print should be used. The numbers minute angle at 200 feet when he or she is at on the Jaeger chart simply refer to the numbers 12 feet, the visual acuity is recorded as 12/200. on the boxes in the print shop from which Jaeger This is not a fraction but indicates two physical 4 Diagnostic Procedures in Ophthalmology Fig. 1.5A: Jaeger's type near vision chart  Visual Acuity 5 Fig. 1.5B Fig. 1.5B: Near vision chart: Music type and numericals 6 Diagnostic Procedures in Ophthalmology 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 move- ments 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 Fig. 1.6: Broken C, letter E and pictures of familiar objects for testing visual acuity in illiterates and children conditions. The contrast sensitivity function of a subject may be affected even when Snellen measurements, the test distance and the size of acuity is normal. The contrast sensitivity tests are more accurate in quantifying the loss of vision the test object. in cases of cataracts, corneal edema, neuro- The most familiar test objects are letters or numbers. Such tests have the disadvantage of ophthalmic diseases, and retinal disorders. A patient with a low contrast threshold has a high requiring some literacy on the part of the patient. degree of sensitivity; therefore, a healthy young Additionally, there is a variation in their ability to be recognized. “L” is considered the easiest subject may have a threshold of 1%, and a contrast sensitivity of 100% (inversely proportional). It letter in the alphabet to read and “B” is considered is important to have adequate lighting when the most difficult. To obviate this difficulty, broken rings (Fig. 1.6) have been devised in which the testing visual acuity so that it does not become a test of contrast sensitivity. 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 Factors Affecting Visual Acuity test objects are easier to see than letters, eliminate Factors affecting visual acuity may be classified some of the difficulties inherent in reading, and as physical, physiological and psychological.  Visual Acuity 7 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 A one wants to evaluate best visual acuity at the fovea. Physiological conditions include pupil size, accommodation, light-dark adaptation and age.2 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, B increasing depth of focus, and modifying the diameter of the blur circle on the retina. Figs 1.7A and B: Occluder with multiple holes Many patients have been referred for neuro- Accommodation -ophthalmologic consultation because of An accommodation creates miosis, which could painless loss of vision in one eye only. The best account for small hyperopic prescriptions being visual acuity may be 20/60 in the affected eye rejected for distance viewing in younger but when properly tested with the pinhole, the individuals. acuity may improve to 20/20. This indicates that It is worth while to discuss the role of a pinhole the macula and optic nerve are functioning in obtaining the best visual acuity in the clinical normally. When the patient’s vision is improved setting. The optimum pinhole is 2.5 mm in with pinhole one knows the problem is a refractive diameter. A pinhole in an occluder (Fig. 1.7) may one and simply need the change in glasses. If be introduced in a trial frame with the opposite the patient’s vision is less when looking through eye occluded. Single pinhole device is not the pinhole; it indicates that the patient has either adequate. The patient must be able to find a hole, an organic lesion at macula, or a central scotoma, therefore, multiple pinholes are preferred. If the or functional amblyopia. A patient with 20/400 patient is older or infirm, or has tremors, he is vision that improves with pinhole to 20/70 asked to read only a single letter from each line indicates that the improvement is refractive, but as we proceed down the chart to record the vision. some pathology may also be present. 8 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 Fig. 1.8: Preferential looking test chart 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. Fig. 1.9: OKN drum 1.9) 3. Recording visual evoked potentials (VEP In this group of preschool children, visual acuity Fig. 1.10). testing is easier to perform with the use of the It is desirable to measure the visual acuity following charts: of children sometime during their third year to 1. Allen and Osterberg charts (Fig. 1.11) detect strabismic or sensory amblyopia and to 2. Illiterate E chart recognize the presence of severe refractive errors. 3. Landolt broken ring.  Visual Acuity 9 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 Fig. 1.10: VEP testing 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 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 Fig. 1.11: Allen and Osterberg chart acuity loss, therefore, is the aspect that must be addressed in individual rehabilitation plans. Colenbrander9 subdivides several components Contrast Sensitivity of visual loss into impairment aspects (how the A general definition of spatial contrast is that eye functions), visual ability (how the person it is a physical dimension referring to the light- functions in daily living), and social/economic dark transition at a border or an edge of an image aspects (how the person functions in society that delineates the existence of a pattern or object. (Table 1.1).  10 TABLE 1.1: RANGES AND ASPECTS OF VISION LOSS Impairment aspects Visual ability aspects/functional vision Social and economic aspects (how the eye function) (how the person functions-daily living skills) (how the person functions in society) Ranges Visual Newsprint Statistical estimate VAS Comments (ICD-9-CM) acuity (1 M) of reading ability Visual aids Normal vision 20/12.5 63in Normal reading speed None 110 Note that normal adult 20/16 50in Normal reading distance 105 vision is better than 20/20 20/20 40in Reserve capacity for 100 20/25 32in small print 95 Mild vision loss 20/32 25in Normal reading speed 90 Many functional criteria 20/40 20in Reduced reading distance 85 (whether for a driver’s 20/50 16in No reserve for small 80 license or for cataract 20/63 12.5in 75 surgery) fall within the range Diagnostic Procedures in Ophthalmology Moderate vision loss 20/80 10in Near-normal with Vision 70 In the United States, 20/100 8in appropriate reading aids enhancements 65 children in this range qualify for 20/125 6in Low-power magnifiers aids 60 special educational assistance 20/160 5in and large-print books 55 Severe vision Loss 20/200 4in Slower than normal with 50 In the United States, 20/250 3in reading aids 45 persons in this range 20/320 2.5in High-power magnifiers 40 are considered legally 20/400 2in (restricted field) 35 blind and qualify for tax-break disability benefits. Profound vision loss 20/500 1.6in Marginal with aids 30 In the EU, many benefits 20/630 1.2in Uses magnifiers for spot 25 start at this level. The 20/800 1in reading, but may prefer 20 WHO includes this range 20/1000 talking books for leisure 15 in its blindness category. Near-blindness 20/1250 1cm No visual reading Vision 10 In this range, residual vision 20/1600 1cm must rely on talking substitution 5 tends to become unreliable, 20/2000 1cm books or other aids 0 though it nonvisual sources may still be used as an adjunct to vision substitution skills. Total Blindness NLP (From Colenbrander A. Preservation of vision or prevention of blindness [editorial]? Am J Ophthalmol 2002;133:2. p.264.)  Visual Acuity 11 some of the factors that can lead to erroneous Summary results. A little care in ensuring the proper Both distance and near visual acuities are environment for testing can significantly improve recorded for each eye with and without spectacles. accuracy. 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 References can be recorded using reduced Snellen or 1. Newell FW. Ophthalmology Principles and equivalent cards at 40 cm. Acuity performance, Concepts. St Louis, Mosby, 1969. like any other human performance, is subject 2. Moses RA (Ed). Adlers Physiology of the Eye. to impairment depending on ocular and general St Louis, Mosby, 1970. health, emotional stress, boredom, and a variety 3. Scheie H. Textbook of Ophthalmology. of drugs acting both peripherally and centrally. Philadelphia, WB Saunders, 1977. 4. Duane TD. Clinical Ophthalmology. New York, The examiner must provide encouragement and Harper and Row, 1981. must have patience. 5. Michaels DD. Visual Optics and Refraction. St For clinical studies the ETDRS charts are Louis, Mosby, 1985. recommended because near vision is often more 6. Vander J. Ophthalmology Secrets. Hanley and Belfus. important in the daily life of older or infirm 7. Borish I. Clinical Refraction. Professional patients. Reading charts or other near vision Publisher, 1970. testing charts should be used as part of the routine 8. Owsley C. Contrast Sensitivity. Ophthalmic assessment of the visual acuity. Visual acuity Clinics of North America 2003;16:173. measurement is often taken for granted. Many 9. Colebrander A. Preservation of Vision or Prevention of Blindness? Am J Ophthalmol 2003; pitfalls make this most important assessment 133:263. subject to variability.10Ambient illumination, 10. Kniestedt, Stamper RL. Visual Acuity and its aging bulbs, dirty charts or slides, small pupils, Measurements. Ophthalmic Clinics of North and poorly standardized charts are just America 2003; 16:155. 12 Diagnostic Procedures in Ophthalmology HARINDER SINGH SETHI Color Vision and 2 Color Blindness Color vision examination is an essential part main characteristics of color namely hue, of screening before a person is taken up for a saturation, and brightness. Hue is a function job. A person who is color vision defective may of wavelength. It depends on what the eye and go through life quite unconscious of his color brain perceive to be the predominant wavelength deficiency and without making any incrimi- of the incoming light. An object’s “hue” is its nating mistakes, differentiating objects by their “color.” Saturation refers to the richness of a size, shape and luminosity, using all the time hue as compared to a gray of the same brightness. a complete color vocabulary based on his Saturation is also known as “chroma.” Brightness experience which teaches him that color terms correlates to the ease with which a color is seen, are applied with great consistency to certain other factors being equal. Brightness is a objects and to certain achromatic shades, until subjective term referring to the sensation circumstances are arranged to eliminate these produced by a given illuminance on the retina. accessory aids and then he realizes that his The spectral wavelengths of different colors sensations differ in some way from the normal. are as follows: violet 430 nm; blue 460 nm; green Various tests have been developed to enable 520 nm; yellow 575 nm; orange 600 nm and red screening of anomalous subjects with color 650 nm. The concept of white light is vague, deficiency from a much larger group of normal most agreeable definition is, white surface is one subjects. 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 Factors Affecting Color Vision of an object. Color is what we see and is result Crystalline Lens of stimulation of retina by radiant energy in a small band of wavelengths of the electromagnetic The lens absorbs shorter wavelengths; in young, spectrum usually considered to span about one wavelengths of less than 400 nm and in old octave, from 380 nm to 760 nm. There are three people up to 550 or 600 nm are absorbed by  Color Vision and Color Blindness 13 the lens resulting in defective color vision on testing color vision. An illuminance of 400 lux shorter wavelength side. (± 100 lux) would be practical value for most clinical applications. Retinal Distribution of Color Vision Bezold-Burcke Effect The center of the fovea (1/8 degree) is blue blind. Trichromatic vision extends 20-30° from the point von Bezold (1873) and Burcke (1878) discovered of fixation. Peripheral to this red-green become independently the phenomenon named after indistinguishable up to 70-80° and in far them, that variation of the luminance levels peripheral retina all color sense is lost although modifies hues. cones are still found in this region. In the central 5°, macula contains carotenoid pigment, Color Constancy; Aperture Colors and xanthophyll. The molecules of the pigment are Surface Colors arranged in such a way that they absorb blue light polarized in the radial direction. If one looks Color constancy is a phenomenon in which color at a white card through linear polarizer, one of the objects can be recognized unchanged in will see two blue sectors separated by two yellow spite of possible differences in the illumination. sectors the figure is called Haidinger’s brushes. Aperture colors are colors that alter due to change Macular pigment may also be seen as in in illumination. Surface colors do not vary with homogeneity in the field of blue or white light illumination. Extrafoveal vision favors the called Maxwell’s spot. appearance of aperture colors and foveal vision that of surface colors. Wavelength Discrimination Complementary Wavelengths The normal observer is able to detect a difference between two spectral lights that differ by as little Complementary wavelengths are those which, as 1 nm in wavelength in the regions of when mixed in appropriate proportions, give 490 nm and 585 nm. In the region of violet and white. red a difference of greater than 4 nm is necessary. Simultaneous Color Contrast Hue, Saturation and Lightness Color contrast is visually demonstrated by Hue is the extent to which the object is red, green, observing the color of a spot in a surround. The blue or yellow. Saturation is the extent to which general rule is that the color of the spot tends a color is strong or weak. Lightness is self toward the complementary of the color of the explanatory attribute, for example, yellow by color surround. is light. Successive Color Contrast Illumination Successive color contrast is more commonly Illumination affects color vision of low described as colored after images, when one illuminances, the errors increase due to poorer stares at a red spot for several seconds and then discrimination for most of the hue range while looks at a gray card one sees a green spot on 14 Diagnostic Procedures in Ophthalmology the card. The after image tends toward the b. Each of these receptors is characterized from complementary of the primary image (Stiles- the spectral point of view by particular Crawford effect). The light entering near the edge function of wavelengths which may be of the pupil is less effective than light entering denoted by G and the response G1 of a at the center of the pupil because of the shape receptor for radiation with a spectral energy of the receptors and the fact that they are distribution Eλ may be supposed to have embedded in a medium of different refractive the form. index. This effect is wavelength-dependent. G1 = Sgi Eλ dλ. c. Sensation of color is a function of the relative values of the three responses G1. Color Triangle d. Sensation of light is a function of a linear Color triangle can be drawn to describe the combination of the three responses. trichromacy of color mixtures and is useful for Fundamental sensations deciding which bands of wavelength are By determining approximately the coordinate of indistinguishable from each other. Three reference wavelengths are chosen, i.e. 450 nm, 520 nm the confusion points of dichromats Arthur Konig in1893 established a system of fundamental and 650 nm and are placed at vertices of X, Y sensations and identified red, green and violet and Z of a triangle, the position of other wavelengths is determined. A color triangle does as fundamental colors. Blue was also identified as fundamental color in addition to red and green not describe the color of a band of wavelengths by Gothelin. unless other circumstances are defined. Theories of Color Vision Granit’s Theory of Color Vision Granit divides retina into receptor units, each This is a complex topic as no theory explains unit comprising groups of cones and rods which the phenomenon of color vision fully. Few important theories are given below: are connected with a single ganglion cell or several ganglion cells which synchronize their Young-Helmholtz Theory (Trichromatic discharges. These units are classified as Theory) “dominators” or “modulators”. The dominators which are numerous have a spectral sensitivity Young’s concept is that there are three types of curve which indicates that they are responsible retinal receptors with different spectral for the sensations of luminosity. Modulators sensitivities. Young’s principal colors are red, show a selective sensitivity which makes them green and violet. Young’s hypothesis was not responsible for color discrimination. Granit’s followed up until it was revived by Helmholtz theory does not explain the fact of trichromatism. in 1852. The Young’s theory may be summarized as follows: a. At some stage of visual receptor mechanism Hering’s Theory of Color Vision there are three different types of sensory (Opponent Color Theory) apparatus G1, G2, G3. These receptors must Hering assumed six distinct sensations arranged be same for everyone but they may not be in three opposing pairs: white-black; yellow-blue same at the fovea as at the periphery. and red-green; he explains three pairs as being  Color Vision and Color Blindness 15 due to opposing actions of light on three catching”) contain the pigment chlorolabe, which substance of the retina, a catabolism producing has a maximal sensitivity to a wavelength near warm sensation (white, yellow, red) and an 543 nm. Short wavelength-sensitive (SWS) cones anabolism the cold ones. This theory is clearly (“blue” or “blue-catching”) contain cyanolabe, a psychological concept and aims at explaining which have maximal sensitivity at 445 nm. The complex percepts than the intermediate effect of blue cones are absent in the center of the macula. the stimuli. Trichromatic vision perception occurs in central 30º field. It is not uncommon to hear the cones Anatomy of Color Vision referred to as blue, green, and red cones, but such nomenclature is misleading because the The understanding of visual pathway is complex L-cones are more sensitive to blue lights than and not evident fully. There are two types of they are to red lights. The spectral sensitivities photoreceptors in the retina: rods and cones. of the three cone pigments overlap somewhat. Approximately 120 million rods are responsible For example, light of 540 nm and 590 nm for night and peripheral vision. Rods contain stimulate both green (MWS) and red (LWS) a photopigment called rhodopsin, a chemical receptors yet we can easily distinguish between variant of vitamin A and a protein called opsin these two wavelengths as “green” and “yellow.” that serves at very low levels of illumination. If the human retina contains all three cone Rods have their maximum density about 5 pigments in normal concentrations, and has degrees from the fovea and cannot distinguish normal retinal function, the subject is a one color from another. The fovea itself is trichromat. Any color the trichromat sees can essentially rod-free containing only cones. be matched with a suitable mixture of red, green, Approximately 7 million cones are responsible and blue light. for central and color vision. Cones have their maximum density within 2 degrees of the center Color Coded Cells of the fovea. Both types of receptors diminish in number toward the retinal periphery. Two types of color coded cells are found at peripheral levels (ganglion cells and lateral geniculate body) of the visual system and they Cones have been named opponent color cells and double In the retina three types of cones responsible opponent color cells. More complex types are for the red, green and blue sensations have been found at more central levels (striate cortex). isolated. Three types of cone pigments in the Opponent color cells: An opponent color cell is human retina absorb photons with wavelengths one that gives only polarity of response for some between 400 nm and 700 nm. Color vision is wavelengths and opposite polarity of response mediated by these three cone photoreceptors for other wavelengths. Opponent color cells are referred to as long, middle, and short wavelength- concerned with successive color contrast. sensitive (LWS, MWS, SWS) cones. The long wavelength-sensitive (LWS) cones (sometimes Double opponent color cells: These are cells called “red” or “red-catching”) contain a pigment opponent for both color and space. The response called erythrolabe, which is best stimulated by may be onto red light, off to green light in the a wavelength near 566 nm. Medium wavelength- center of the receptive field and off to red light, sensitive (MWS) cones (“green” or “green- onto green light in the periphery of the receptive 16 Diagnostic Procedures in Ophthalmology field. Double opponent cells are concerned with Congenital vs Acquired Color simultaneous color contrast. Deficiencies Simple, complex and hypercomplex cells: In rhesus Congenital color vision deficiencies can be monkey striate cortex there are a variety of cells distinguished functionally from acquired that are specific for both color and orientation. deficiencies in a number of ways. Congenital They have been categorized as color sensitive deficiencies typically involve red-green confu- simple, complex and hypercomplex cells. Simple sions, whereas acquired deficiencies, more often cells have a bar-flank double opponent arrange- than not, are a blue-yellow (Köllner’s rule). Also, ment to their receptive fields. Complex color coded because some of the most common congenital cells respond to color boundaries of the appro- defects are linked to the X-chromosome, they are priate orientation and the response is indepen- more prevalent in males than females. Acquired dent of the part of the receptive field being sti- defects, in contrast, are not related to gender mulated. The edge of hypercomplex cells must except by gender differences to trauma or toxic be short. exposure. Acquired color deficiencies are more Opponent color cells are found among likely to be asymmetric between the two eyes ganglion cells of the retina and lateral geniculate than are hereditary defects; they are also less body. Double opponent cells with center- likely to be stable with time. Congenital defects surround or flank receptive fields are present are usually easier to detect with standard clinical in the input layer IV of the striate cortex. Complex color vision tests, but some acquired ones can and hypercomplex color coded cells are also be more subtle and thus are difficult to diagnose. found in the striate cortex in layers II, III, V and Finally, those with acquired color deficiencies VI. Vaetichin in 1953 recorded a negative slow are also more likely to display color-naming potential from fish retinae called “S-potential” errors because, unlike those with congenital of two types: L-type (luminosity type) and C- deficiencies, they lack the life-long experience type (chromaticity type). Mitarai in 1961 regarded with defective color perception. horizontal cells as responsible for S-potentials of L-type and Muller’s fibers for those of C-type. Congenital Color Vision Deficiency The properties of S-potentials support the Herings opponent color theory more than the trichromatic The color vision anomalies commonly being theory of Young. 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 Anomalies of Color Vision one of the pigments in photoreceptors. Congenital Deficiency of color vision first was described by color deficits may be divided into classes Dalton in1794, the founder of the atomic theory, according to whether the patients are red deficient who himself was color blind; hence the term (protans), green deficient (deuterans) or blue daltonism was coined. The color deficiency is of deficient (tritans). The term anopia is used for two types: (1) congenital and (2) acquired. In absolute deficiency and anomaly for relative clinical evaluation of color vision it is important deficiency (Tables 2.1 and 2.2). to distinguish between acquired and congenital Anomalous trichromats are people who defects. generally require three wavelengths to match  Color Vision and Color Blindness 17 TABLE 2.1: CLASSIFICATION OF COLOR BLINDNESS Congenital: Males (8%), Females (0.4%) Acquired classically X-linked recessive inheritance Unilateral Red-green defect pattern, always bilateral Blue-Yellow defect (a) Achromatopsia Bilateral Red-green defect Cone monochromats Blue-Yellow defect Rod monochromats (b) Dyschromatopsia Dichromats - Deuteranopia Disease Acquired defect - Protanopia Glaucoma Blue-Yellow - Tritanopia Hypertensive retinopathy Blue-Yellow Diabetic retinopathy Blue-Yellow Anomalous trichromats AMD Blue-Yellow - Protanomaly Lesions of visual pathway Red-Green - Deuteranomaly Alcohol-nicotine Red-Green - Tritanomaly TABLE 2.2: VARIOUS TYPES OF COLOR DEFICIENCY Red deficient Green deficient Blue deficient Anomalous trichromats Protanomaly Deuteranomaly Tritanomaly Dichromats Protanopia Deuteranopia Tritanopia Monochromats Rod monochromat Blue monochromat 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. Fig. 2.1: Inheritance pattern of congenital color Deuteranopes have normal red and blue cones vision defects and tritanopes normal red and green cones. 18 Diagnostic Procedures in Ophthalmology Protans color deficient subjects are easier to Factors Responsible for Deficiency of test and classify than deuterans and tritans; Color Vision because the red cone pigment is quite sensitive to green wavelengths and both red and green Ocular Diseases cone pigments are quite sensitive to blue a. Squint amblyopia: Francois by means of clini- wavelength covering the green and blue range, cal tests stated that color vision deficiencies in deuterans and tritans, as the sensitivity of visual pigment does not fall off sharply on the in squint amblyopia do not correspond to short wavelength side of the peak. the classical type of acquired deficiencies Monochromatics can be blue cone mono- but rather approximate the normal color chromatics and rod monochromatics. Blue cone sense of eccentric retinal positions. monochromatics have normal blue cone pigment b. Glaucoma: Primary glaucoma and ocular but no red or green cone pigment. In rod hypertension cause tritan-type of defect. monochromatism only 500 nm pigment is present c. Diabetic retinopathy: Diabetic retinopathy in the retina and all three cones pigments are may cause color deficiency which may vary absent. from a mild loss of hue discrimination to moderate blue-yellow color vision defi- Genetics of congenital color deficiencies ciency. In severe cases of diabetic retinopathy The protans and deuterons are commonly sex- the defect may resemble tritanopia. linked recessive. About 1% males are protanopes, d. Retinal disorders: Blue-yellow deficits are 1% protanomalous, 1% deutaranopes and 5% found in senile macular degeneration, deuternomalous. The incidence of color vision myopia, retinitis pigmentosa, siderosis bulbi deficiency (red-green) in females is 0.4%. The and chorioretinitis. gene for tritans is autosomal incompletely e. Optic nerve disorders: In one study about 57% dominant. Rod monochromatism is very rare; of patients with resolved optic neuritis occurs 1 in 30,000, autosomal recessive and thus were found to have color vision defects. an increased incidence is seen in consanguineous Red-green defects have been found in cases offsprings. of multiple sclerosis and optic atrophy. Tobacco amblyopia causes red-green Acquired Deficiency of defect. Color Vision f. Color vision after laser photocoagulation: After argon-laser photocoagulation there may be Koellner formulated that lesions in the outer overall loss of hue discrimination and color layers of the retina give rise to a blue-yellow deficiency, mostly of 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 Drugs cerebral cortex may have color deficits. These Many drugs are known to cause deficiency of may involve naming of the colors or perception color vision. They can cause more than one type of colors. of color deficiency (Table 2.3).  Color Vision and Color Blindness 19 TABLE.2.3: DRUGS CAUSING COLOR DEFICIENCY Drugs Type of color deficiency Chloroquine, Indomethacin, Blue-yellow oral contraceptives, antihistaminics, estrogens, digitalis and butazolidin. 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 A vision. Following diseases may cause color deficiency: a. Cardiovascular disease: Patients with heart diseases have been found to have blue- yellow deficiency. b. Turner’s syndrome: Red-green color deficiency is usually encountered in the syndrome. Color Vision Testing The main objective for testing the color blindness is to determine the exact nature of the defect and B 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. 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 C figure and background are isochromatic for the color-defective person. PIC tests are used Figs 2.2A to C: A Ishihara pseudo-isochromatic plates, B Transformation plate seen as “3” by patients with primarily as screening tests to identify those with anomalous red-green color defect, C “Vanishing” or an inherited color defect, although, some of the “disappearing” digit type 20 Diagnostic Procedures in Ophthalmology 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 Fig. 2.3: City University test discrimination based on arrangement tests utilizing color chips mounted in a circular cap tests permit a diagnosis of type and severity. that subtend exactly 1.5 degrees at a test distance Because the inventory of PIC tests is extensive, of 50 cm. This ensures that the observations of only the more commonly used tests are described the subject are made with the central rod free here. retina. The D-15 contains 15 colored chips and The most widely used test, Ishihara pseudo- the FM-100 contains 85 chips. The chips have isochromatic plates, is a screening test used to identical brightness and saturation and differ determine the presence of X-linked congenital from one another. Farnsworth-Munsell tests (red/green) color deficiency. Most screening tests reveal the type of defect, but not the severity. are designed to give a quick, accurate assessment of red/green deficiencies. The Ishihara test is Color Matching Tests not designed to detect tritan disorders or acquired color defects unless the optic neuropathy is severe. The spectral anomaloscope and Pickford- Nicolson anomaloscope are used for color matching examinations. They can provide the Arrangement Tests examiner with information on the severity of a The arrangement tests require the observer to particular color vision defect. The Nagel anoma- place colored samples in sequential order on the loscope is the most widely used. It consists of basis of hue, saturation, or lightness or to sort a spectroscope in which two halves of a circular samples on the basis of similarity. One of the field are illuminated respectively by monochro- earliest tests of this nature that is still available matic yellow (589 nm) and a mixture of but is rarely used today is the Holmgren Wool monochromatic red and green (670 nm and 546 test. In this matching test, 46 numerically coded nm, respectively). The observer is asked to match comparison schemes of yarn are selected to match the two halves of the circle with the three primary three test colors: yellow-green, pink, and dark colors available. red. The comparison schemes differ from the test The most widely used color vision tests are schemes in being lighter or darker. The test is the pseudo-isochromatic plates and the D-15  Color Vision and Color Blindness 21 panel due to their ease of use and relative low between 250 and 350 lux (approximately 1.5 cost. The Nagel anomaloscope and FM-100 tests meters below twin fluorescent globe). A failed are usually only found in academic or research Ishihara test under incandescent globe is a failure settings. of the examiner to observe basic principles, not All color vision tests have specific require- a failure of the subject. A pass on the other hand ments for lighting, viewing distance, and viewing is still a pass and is statistically the more likely time. It is important for the examiner to be familiar outcome. with the test requirements and score sheets before The viewing geometry should be with the conducting a color vision test, otherwise the light 45 degrees to the surface and the subject results may be inaccurate. viewing the pages at 90 degrees to the surface. Newly printed books sometimes have differential Lantern Tests reflectance between pigments so when tilted back