Parsons Diseases of the Eye PDF (23rd Edition)

Parsons’ Diseases of the Eye 23RD EDITION Editors Ramanjit Sihota, MD, DipNB, FRCOphth, FRCSEd Professor of Ophthalmology, Dr Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi Radhika Tandon, MD, DipNB, FRCOphth, FRCSEd (Gold Medalist) Professor of Ophthalmology, Dr Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi Table of Contents Cover image Title page Copyright Preface to the twenty third edition Preface to the twenty second edition Acknowledgements Reviewers Section I: Anatomy and Physiology of the Eye and Visual Pathway 1. Anatomy of the eye Suggested reading 2. Physiology of the eye Suggested reading 3. The physiology of vision Suggested reading 4. The neurology of vision Suggested reading Section II: Optics and Refraction 5. Elementary optics and optical system of the normal eye Suggested reading 6. Clinical refraction and refractive errors Suggested reading Section III: Basics of Clinical Examination and Treatment 7. Clinical history taking and interpretation of symptoms 8. Clinical examination Assessment of vision Examination of the anterior segment Examination of the posterior segment and orbit Suggested reading 9. Clinical investigations 10. Ocular therapeutics and lasers Suggested reading Section IV: Diseases of the Eye 11. Diseases of the conjunctiva Suggested reading 12. Diseases of the cornea Suggested reading 13. Diseases of the sclera 14. Diseases of the uveal tract Suggested reading 15. Diseases of the lens Suggested reading 16. Glaucoma Suggested reading 17. Diseases of the retina and vitreous Suggested reading 18. Diseases of the optic nerve Suggested reading 19. Intraocular tumours Suggested reading Section V: Eye Movements and Their Disorders 20. Anatomy and physiology of the motor mechanism 21. Squint: Comitant strabismus Suggested reading 22. Paralytic squint Section VI: Diseases of the Adnexa 23. Diseases of the eyelids 24. Diseases of the lacrimal apparatus 25. Diseases of the orbit Suggested reading Section VII: Systemic Ophthalmology 26. Diseases of the nervous system with ocular manifestations 27. Ocular manifestations of systemic disorders Section VIII: Ophthalmic Emergencies 28. Eye injuries 29. Nontraumatic ophthalmic emergencies References Section IX: Preventive Ophthalmology 30. Genetics in ophthalmology 31. Community ophthalmology Suggested reading Section X: Ophthalmic Instruments and Their Practical Usage 32. Dark room procedures in ophthalmology 33. Surgical instruments in ophthalmology Appendix I: Local anaesthesia in ophthalmology Appendix II: Radiology illustrations Appendix III: Case studies Index Copyright RELX India Pvt. Ltd. Registered Office: 818, Indraprakash Building, 8th Floor, 21, Barakhamba Road, New Delhi-110001 Corporate Office: 14th Floor, Building No. 10B, DLF Cyber City, Phase II, Gurugram-122002, Haryana, India Parsons’ Diseases of the Eye, 23rd Ed, Sihota and Tandon Copyright © 2020 by RELX India Pvt. Ltd. All rights reserved. Previous editions copyrighted 2015, 2011, 2007, 2003, 1990, 1984, 1978, 1970, 1964, 1959, 1954, 1948, 1942, 1938, 1936, 1934, 1930, 1926, 1923, 1918, 1912, 1907. ISBN: 978-81-312-5415-8 e-Book ISBN: 978-81-312-5416-5 No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notice Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds or experiments described herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made. To the fullest extent of the law, no responsibility is assumed by Elsevier, authors, editors or contributors in relation to the adaptation or for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Content Strategist: Sheenam Aggarwal Content Project Manager: Fariha Nadeem Sr Production Executive: Ravinder Sharma Sr Cover Designer: Milind Majgaonkar Typeset by GW Tech India Printed in India by... Preface to the twenty-third edition Ramanjit Sihota, Radhika Tandon It is an honour and pleasure to present yet another edition of ‘Parsons’ Diseases of the Eye’. This classic textbook with its unique features provides a comprehensive compendium of information covering all the relevant aspects of ophthalmology for thorough knowledge of the subject. This 23rd edition has been updated keeping in view the changing disease spectrum, practice patterns and advancements in technology. We are happy that the MCI has provided an updated competency-based undergraduate curriculum which is to be rolled out in August 2019 and glad to provide the information that the contents of this book have been updated in compliance to fully cover all the aspects as prescribed. Additional information has been seamlessly integrated to provide a higher level of understanding of ophthalmology for those with a higher level of interest such as medical students with special curiosity about the subject, postgraduate students to revise their own understanding and teachers and clinicians who wish to refresh and update themselves. Core skills prescribed by the MCI are also adequately covered in the text and are supported by digital material. We hope you enjoy reading it and using the online resources to enrich yourselves as much as we did in preparing it for you. Preface to the twenty-second edition Ramanjit Sihota, Radhika Tandon It is an honour and pleasure to present yet another edition of “Parsons’ Diseases of the Eye”. This classic textbook with its unique features provides a comprehensive compendium of information covering all the relevant aspects of ophthalmology for thorough knowledge of the subject. This 22nd Edition has been updated keeping in view the changing disease spectrum, practice patterns and advancements in technology. We hope you enjoy reading it and enrich your information spectrum as much as we did in preparing it for you. Acknowledgements Ramanjit Sihota, Radhika Tandon The authors remain deeply indebted to the faculty, residents and students of Dr Rajendra Prasad Centre for Ophthalmic Sciences, Chief of the Centre and Director of All India Institute of Medical Sciences for the rich academic and clinical milieu provided to nurture our work. We are also grateful to Elsevier, a division of RELX India Pvt Ltd, and its entire team of professional experts for support in finishing the book. A special thanks to Professor Pradeep Sharma who provided extremely valuable suggestions to improve the Strabismus chapters, Professor Pradeep Venkatesh who has been graciously updating the Vitreoretinal and Uvea chapters for many years, Professor Neelam Pushkar for her inputs to the Ophthalmoplasty section from her vast experience and Prof Sanjay Sharma for providing common, typical ocular Radiology images. Dr Archita Singh has enthusiastically helped collate photographs and skill-based videos that have greatly helped improve the book. The authors would also like to thank Dr. Kishan Azmira, Dr. Priyanka Ramesh and Dr. Kartikeyan for their work on Case Studies. We acknowledge and greatly appreciate the efforts and invaluable comments of our editors; we would especially like to thank Fariha for her diligent support and professional help throughout the revision process. Last but not the least, we would like to make an endearing mention of our families who with their loyal forbearance allowed us to spend our spare time and devote our attention to this work, without which it would not have been possible to achieve this. Reviewers Abadan Khan Amitava, MBBS, MS, Professor, Jawaharlal Nehru Medical College, Aligarh Muslim University, Aligarh, Uttar Pradesh B Zaibunissa, MBBS, DO, MS, Faculty for DNB Training, Rajan Eye Care Hospital, Chennai, Tamil Nadu Bhavana Sharma, MBBS, MS, Professor and Head, Department of Ophthalmology, All India Institute of Medical Sciences, Bhopal, Madhya Pradesh Sumita Sethi, MBBS, MS, Associate Professor of Ophthalmology, BPS GMC for Women, Sonepat, Haryana Yogesh Gupta, MBBS, MS, Professor, Jawaharlal Nehru Medical College, Aligarh Muslim University, Aligarh, Uttar Pradesh SECTION I: Anatomy and Physiology of the Eye and Visual Pathway OUTLINE 1. Anatomy of the eye 2. Physiology of the eye 3. The physiology of vision 4. The neurology of vision CHAPTER 1 Anatomy of the eye CHAPTER OUTLINE Development of the Eye, 2 Anatomy, 4 Cornea, 5 Sclera, 6 Anterior Chamber, 6 Lens, 7 Uveal Tract, 8 Posterior Chamber and Vitreous Humour, 9 Retina, 9 The Blood Supply of the Eye, 11 Clinical Anatomy of the Eye, 14 Learning objectives To gain basic knowledge of the development of the eye. To develop essential understanding on how abnormalities at various stages of development can arrest or hamper normal formation of the ocular structures and visual pathways. To acquire adequate information about normal anatomy of the eye and related structures and develop a strong foundation for a better understanding of common ocular problems and their consequences. Development of the eye The formation, early growth and development of the eyes and visual system in utero, is fascinating and of critical importance in understanding the aetiopathogenesis of congenital disorders, providing an insight in to genetic evolution and a better understanding of developmental disorders. The eyes and visual system develop as a part of the central nervous system with contributions from the adjacent mesodermal and ectodermal tissues (Table 1.1). FIG. 1.1 The development of the eye. In each case, the solid black is the neural ectoderm, the hatched layer is the surface ectoderm and its derivatives, the dotted area is the mesoderm: a, cavity of the forebrain; b, cavity of the optic vesicle; c, cavity of the optic cup (or secondary optic vesicle) formed by invagination. (A) Transverse section through the anterior part of the forebrain and optic vesicles of a 4-mm human embryo. (B) The primary optic vesicle. (C) The formation of the optic cup by invagination at the embryonic fissure and invagination of the surface epithelium. (D) The optic cup and lens vesicle. (E) The formation of the ciliary region and iris, the anterior chamber, the hyaloid artery and the lid folds. The lens is formed from the posterior cells of the lens vesicle. (F) The complete eye. TABLE 1.1 Summary of Ocular Embryogenesis Period after Major Milestone Conception 3rd week Optic groove appears 4th week Optic pit develops into optic vesicle Lens plate forms Embryonic fissure develops Fig. 1.1A–D 1st month Lens pit and then lens vesicle forms Hyaloid vessels develop 1½ months Closure of embryonic fissure Differentiation of retinal pigment epithelium Proliferation of neural retinal cells Appearance of eyelid folds and nasolacrimal duct 7th week Formation of embryonic nucleus of the lens Sclera begins to form First wave: formation of corneal and trabecular endothelium Second wave: formation of corneal stroma Third wave: formation of iris stroma Fig. 1.1E 3rd month Differentiation of precursors of rods and cones Anterior chamber appears Fetal nucleus starts to develop Sclera condenses Eyelid folds lengthen and fuse 4th month Formation of retinal vasculature begins Hyaloid vessels begin to regress Formation of physiological optic disc cup and lamina cribrosa Canal of Schlemm appears Bowman’s membrane develops Formation of major arterial circle and sphincter muscle of iris 5th month Photoreceptors differentiate Eyelid separation begins 6th month Differentiation of dilator pupillae muscle Nasolacrimal system becomes patent Cones differentiate Fig. 1.1F 7th month Rods differentiate Myelination of optic nerve begins Posterior movement of anterior chamber angle Retinal vessels start reaching nasal periphery 8th month Completion of anterior chamber angle formation, hyaloid vessels disappear 9th month Retinal vessels reach temporal periphery, pupillary membrane disappears After birth Macular region of the retina develops further The anterior portion of the neural tube is the precursor of the forebrain, and its lateral aspect develops a thickening (the optic plate) which then grows outwards as a diverticulum towards the surface to form the primary optic vesicle (Fig. 1.1A and B). In the developing human embryo, the two eyes develop from the neural ectoderm derived from the neural tube as a pair of diverticula (primary optic vesicle) arising from the sides of the forebrain with contributions from the adjacent mesoderm and surface ectoderm in contact with it ( Table 1.2). TABLE 1.2 Primordial Tissue and Its Derivatives Precursor Derivatives Neural ectoderm Smooth muscle of the iris Optic vesicle and cup Iris epithelium Ciliary epithelium Part of the vitreous Retina Retinal pigment epithelium Fibres of the optic nerve Surface ectoderm Conjunctival epithelium Corneal epithelium Lacrimal glands Tarsal glands Lens Mesoderm Extraocular muscles Corneal stroma Sclera Iris Vascular endothelium of eye and orbit Choroid Part of the vitreous Neural cresta Corneal stroma, keratocytes and endothelium Sclera Trabecular meshwork endothelium Iris stroma Ciliary muscles Choroidal stroma Part of the vitreous Uveal and conjunctival melanocytes Meningeal sheaths of the optic nerve Ciliary ganglion Schwann cells of the nerve sheaths Orbital bones Orbital connective tissue Connective tissue sheath and muscular layer of the ocular and orbital blood vessels a During the folding of the neural tube, a ridge of cells comprising the neural crest develops from the tips of the converging edges and migrates to the dorsolateral aspect of the tube. Neural crest cells from this region subsequently migrate and give rise to various structures within the eye and the orbit. Note: The structures are listed from anterior to posterior. The primary optic vesicle grows outwards, meets the surface ectoderm, and then invaginates from below (the optic cup), the line of invagination remaining open for some time as the embryonic fissure (Fig. 1.1C). The inner layer of the cup forms the main structure of the retina, from which the nerve fibres eventually grow backwards towards the brain. The outer layer of the cup remains as a single layer of pigment epithelium. A narrow space representing the original optic vesicle remains as a potential space between the retina and the retinal pigment epithelium, and this space may get manifest in diseases such as retinal detachment or subretinal haemorrhage. The anterior border of the optic vesicle and optic cup form parts of the ciliary body and iris, both having corresponding inner and outer layers which are contiguous with the retina or retinal pigment epithelium, respectively (Fig. 1.1E). At the point where the neural ectoderm meets the surface ectoderm, the latter thickens to form the lens plate, invaginates to form the lens vesicle (Fig. 1.1C) and then separates to form the lens (Fig. 1.1D). The hyaloid artery enters the optic cup through the embryonic fissure and grows forwards to meet the lens, bringing temporary nourishment to the developing structures before it eventually atrophies and disappears; as it does so, its place is taken by a clear jelly (the vitreous) largely secreted by the surrounding neural ectoderm. While these ectodermal events are taking place, the mesoderm surrounding the optic cup differentiates to form the coats of the eye and the orbital structures; the mesoderm between the lens and the surface ectoderm becomes hollowed to form the anterior chamber, lined by mesodermal condensations which form the anterior layers of the iris, the angle of the anterior chamber and the main structures of the cornea; whereas the surface ectoderm remains as the corneal and conjunctival epithelium. In the surrounding region, folds grow over in front of the cornea, unite and separate again to form the lids (Fig. 1.1E and F). Anatomy Knowledge of the anatomy of the eye is important for a proper understanding of ocular health and disease conditions affecting it. The eye, also known as the eyeball, is a globe. The wall is composed of an outermost coat which is a dense, imperfectly elastic supporting tissue, the transparent cornea forming the anterior one-quarter and the opaque sclera forming the remainder three-quarters of the eye (Fig. 1.2). The anterior part of the sclera is covered by a mucous membrane—the conjunctiva—which is reflected from its surface onto the lids. The external appearance of the eye as seen clinically is illustrated in Fig. 1.3. FIG. 1.2 General anatomy of the eyeball, including its tunics and chambers. Source: (From Rolston D, Nielsen C. Chapter 2: Ophthalmology. In: Rapid review USMLE step 3. St. Louis: Mosby; 2007. p. 28–44.) FIG. 1.3 The eyelids and anterior aspect of the eyeball. Source: (Adapted from Standring S. Chapter 39: The orbit and accessory visual apparatus. In: Standring S, Borley NR, Collins P, editors. Gray’s anatomy: The anatomical basis of clinical practice. 40th ed. Edinburgh: Churchill Livingstone; 2009. p. 655–74.) Inner to the sclera lies the middle layer of the wall consisting of the uveal tract comprising the choroid posteriorly and the ciliary body and iris anteriorly. The retina form the innermost layer of the posterior two-thirds to three-quarters of the eye lining the posterior part of the eye up to its anterior 360° circumferential boundary called the ora serrata. Inside the eye, the globe is broadly divided into the anterior segment and posterior segment by the lens. The iris divides the anterior segment into an anterior chamber bounded by the cornea anteriorly and posterior chamber bounded by the lens posteriorly (Fig. 1.4). The posterior segment, i.e. the part of the eye that lies behind the lens, is bounded internally by the ciliary body anteriorly and retina posteriorly. Hence, posteriorly the sclera is lined by the uveal tract and inner to that lies the retinal pigment epithelium and retina. The optic nerve head is visible as the optic disc and represents the retinal nerve fibres leaving the eye as a tract nasal to the posterior pole of the eye (Fig. 1.5). The cavity of the eye contains a clear watery fluid called aqueous humour in the anterior segment, and posterior to the lens, the cavity is filled with a transparent gel-like substance called the vitreous humour. FIG. 1.4 Optical section of the normal eye, as seen with the slit lamp. The light (arrowed) comes from the left and, in the beam of the slit lamp, the sections of the cornea and lens are clearly seen. FIG. 1.5 The normal fundus. The disc or optic nerve head is approximately 1.5 mm in diameter and the centre of the fovea or macula is located about 2 disc diameters temporal to it. Cornea The cornea is the transparent front part of the eye which resembles a ‘watch glass’ and consists of different layers and regions: Epithelium Bowman’s membrane Stroma (substantia propria) Dua’s layer (pre-Descemet’s layer) Descemet’s membrane Endothelium Transparency of cornea Transparency of the cornea is related to the regularity of the stromal components. The stromal collagen fibrils are of regular diameter, arranged as a lattice with an interfibrillar spacing of less than a wavelength of light so that tangential rows of fibres act as a diffraction grating resulting in destructive interference of scattered rays. The primary mechanism controlling stromal hydration is a function of the corneal endothelium which actively pumps out the electrolytes and water flows out passively. The endothelium is examined by a specular microscope at a magnification of ×500. Endothelial cells become less in number with age, and the residual individual cells may enlarge to compensate. Blood supply and innervation The cornea is avascular with no blood vessels with the exception of minute arcades, extending about 1 mm into the cornea at the limbus. It is dependent for its nourishment upon diffusion of tissue fluid from the vessels at its periphery and the aqueous humour. The cornea is very richly supplied with unmyelinated nerve fibres derived from the trigeminal nerve. Sclera The sclera is the ‘white’ supporting wall of the eyeball and is continuous with the clear cornea. It is a dense white tissue, thickest in the area around the optic nerve. The outer surface of the sclera is covered by the conjunctiva, beneath which is a layer of loose connective tissue called episclera and the innermost layer of the sclera consists of elastic fibres called the lamina fusca. Lining the inner aspect of the sclera are two structures—the highly vascular uveal tract concerned chiefly with the nutrition of the eye, and within this a nervous layer, the true visual nerve endings concerned with the reception and transformation of light stimuli, called the retina. Anterior chamber The anterior chamber is a space filled with fluid, the aqueous humour; it is bounded in front by the cornea, behind by the iris and the part of the anterior surface of the lens which is exposed in the pupil. Its peripheral recess is known as the angle of the anterior chamber, bounded posteriorly by the root of the iris and the ciliary body and anteriorly by the corneosclera (Fig. 1.6). In the inner layers of the sclera at this part, there is a circular venous sinus, sometimes broken up into more than one lumen, called the canal of Schlemm, which is of great importance for the drainage of the aqueous humour. At the periphery of the angle between the canal of Schlemm and the recess of the anterior chamber, there lies a loosely constructed meshwork of tissues, the trabecular meshwork. This has a triangular shape, the apex arising from the termination of Descemet’s membrane and the subjacent fibres of the corneal stroma and its base merging into the tissues of the ciliary body and the root of the iris. It is made up of circumferentially disposed flattened bands, each perforated by numerous oval stomata through which tortuous passages exist between the anterior chamber and Schlemm’s canal. The extracellular spaces contain both a coarse framework (collagen and elastic components) and a fine framework (mucopolysaccharides) of extracellular materials, which form the probable site of greatest resistance to the flow of aqueous. FIG. 1.6 The region of the angle of the anterior chamber. The endothelial cells of Schlemm’s canal are connected to each other by junctions which are not ‘tight’ but this intercellular pathway accounts for only 1% of the aqueous drainage. The major outflow pathway appears to be a series of transendothelial pores, which are usually found in outpouchings of the endothelium called ‘giant vacuoles’. The anterior chamber is about 2.5 mm deep in the centre in a normal adult; it is shallower in very young children and in old people. Lens The lens is a biconvex mass of peculiarly differentiated epithelium. It has three main parts: the lens capsule, the lens epithelium and the lens fibres. The outer capsule lined by the epithelium and the lens fibres and is developed from an invagination of the surface ectoderm of the fetus, so that what was originally the surface of the epithelium comes to lie in the centre of the lens, the peripheral cells corresponding to the basal cells of the epidermis. Just as the epidermis grows by the proliferation of the basal cells, the old superficial cells being cast off, so the lens grows by the proliferation of the peripheral cells. The old cells, however, cannot be cast off, but undergo changes (sclerosis) analogous to that of the stratum granulosum of the epidermis, and become massed together in the centre or nucleus; moreover, the newly formed cells elongate into fibres. The lens fibres have a complicated architectural form, being arranged in zones in which the fibres growing from opposite directions meet in sutures. Without going into details, it is important to bear in mind that the central nucleus of the lens consists of the oldest cells and the periphery or cortex the youngest (Fig. 1.7). FIG. 1.7 The structure of the lens in an adult 40 years of age, as shown in the optical beam of the slit lamp: 1, anterior capsule; 2, cortex; 3, adult nucleus; 4, infantile nucleus; 5, fetal nucleus; 6, embryonic nucleus (see Fig. 15.9). The fibres of the lens are split into regions depending on the age of origin. The central denser zone is the nucleus surrounded by the cortex. The oldest and innermost is the central embryonic nucleus (formed during 6–12 weeks of embryonic life), in which the lens fibres meet around Y-shaped sutures. Outside this embryonic nucleus, successive nuclear zones are laid down as development proceeds, called, depending on the period of formation, the fetal nucleus (3–8 months of fetal life), the infantile nucleus (last month of intrauterine life till puberty), the adult nucleus (corresponding to the lens in early adult life) and finally and most peripherally, the cortex consisting of the youngest fibres. In this part of the lens, also the fibres meet along the sutures with a general stellate arrangement. The mass of epithelium which constitutes the lens is surrounded by a hyaline membrane, the lens capsule, which is thicker over the anterior than over the posterior surface and is thinnest at the posterior pole; the thickest basement membrane in the body, the lens capsule, is a cuticular deposit secreted by the epithelial cells, having on the outside a thin membrane, the zonular lamella. The lens in fetal life is almost spherical; it gradually becomes flattened so as to assume a biconvex shape. It is held in place by the suspensory ligament or zonule of Zinn. This is not a complete membrane, but consists of bundles of strands which pass from the surface of the ciliary body to the capsule where they join with the zonular lamella. The strands pass in various directions so that the bundles often cross one another. Thus, the most posterior arise from the pars plana of the ciliary body almost as far back as the ora serrata; these lie in contact with the ciliary body for a considerable distance and then curve towards the equator of the lens to be inserted into the capsule slightly anterior to the equator. A second group of bundles springs from the summits and sides of the ciliary processes, i.e. far forwards, and passes backwards to be inserted into the lens capsule slightly posterior to the equator. A third group passes from the summits of the processes almost directly inwards to be inserted at the equator. Uveal tract The uveal tract consists of three parts, of which the two posterior, the choroid and ciliary body, line the sclera while the anterior forms a free circular diaphragm, the iris. The plane of the iris is approximately coronal; the aperture of the diaphragm is the pupil. Situated behind the iris and in contact with the pupillary margin is the crystalline lens. Iris The iris is thinnest at its attachment to the ciliary body, so that if torn it tends to give way in this region (Fig. 1.6). It is composed of a stroma containing branched connective tissue cells, usually pigmented but largely unpigmented in blue irides, with a rich supply of blood vessels which run in a general radial direction. The tissue spaces communicate directly with the anterior chamber through crypts found mainly near the ciliary border; this allows the easy transfer of fluid between the iris and the anterior chamber. The stroma is covered on its posterior surface by two layers of pigmented epithelium, which developmentally are derived from the retina and are continuous with each other at the pupillary margin. The anterior layer consists of flattened cells and the posterior of cuboidal cells. From the epithelial cells of the former, two unstriped muscles are developed which control the movements of the pupil, the sphincter pupillae, a circular bundle running round the pupillary margin, and the dilator pupillae, arranged radially near the root of the iris. The anterior surface of the iris is covered with a single layer of endothelium, except at some minute depressions or crypts which are found mainly at the ciliary border; it usually atrophies in adult life. The iris is richly supplied by sensory nerve fibres derived from the trigeminal nerve. The sphincter pupillae is supplied by parasympathetic autonomous secretomotor nerve fibres derived from the oculomotor nerve, while the motor fibres of the dilator muscle are derived from the cervical sympathetic chain. Ciliary body The ciliary body in anteroposterior section is shaped roughly like an isosceles triangle, with the base forwards. The iris is attached about the middle of the base, so that a small portion of the ciliary body enters into the posterior boundary of the anterior chamber at the angle (Fig. 1.6). The chief mass of the ciliary body is composed of unstriped muscle fibres, the ciliary muscle. This consists of three parts with a common origin in the ciliary tendon, a structure which runs circumferentially round the globe blending with the ‘spur’ of the sclera and related to the trabecular mesh work. The greater part of the muscle is composed of meridional fibres running anteroposteriorly on the inner aspect of the sclera to find a diffuse insertion into the suprachoroid. Most of the remaining fibres run obliquely in interdigitating V-shaped bundles so as to give the impression of running in a circle around the ciliary body, concentrically with the base of the iris. The third portion of the muscle is composed of a few tenuous iridic fibres arising most internally from the common origin and finding insertion in the root of the iris just anterior to the pigmentary epithelium in close relation to the dilator muscle. The inner surface of the ciliary body is divided into two regions—the anterior part is corrugated with a number of folds running in an anteroposterior direction while the posterior part is smooth. The anterior part is therefore called the pars plicata; the posterior is called the pars plana. About 70 plications are visible around the circumference macroscopically, but if microscopic sections are examined, many smaller folds, the ciliary processes, will be seen between them. These contain no part of the ciliary muscle, but consist essentially of tufts of blood vessels, not unlike the glomeruli of the kidney. They are covered upon the inner surface by two layers of epithelium, which belong properly to the retina, and are continuous with similar layers in the iris; the outer layer, corresponding to the anterior in the iris, consists of flattened cells, the inner of cuboidal cells, but only the outer layer in the ciliary body is pigmented. The ciliary body extends backward as far as the ora serrata, at which point the retina proper begins abruptly; the transition from the ciliary body to the choroid, on the other hand, is gradual, although this line is conveniently accepted as the limit of the two structures. The ora serrata thus circles the globe, but is slightly more anterior on the nasal than on the temporal side. The ciliary body is richly supplied with sensory nerve fibres derived from the trigeminal nerve. The ciliary muscle is supplied with motor fibres from the oculomotor and sympathetic nerves. Choroid The choroid is an extremely vascular membrane in contact everywhere with the sclera, although not firmly adherent to it, so that there is a potential space between the two structures—the epichoroidal or suprachoroidal space. On the inner side, the choroid is covered by a thin elastic membrane, the lamina vitrea or membrane of Bruch. The blood vessels of the choroid increase in size from within outwards, so that immediately beneath the membrane of Bruch there is a capillary plexus of fenestrated vessels, the choriocapillaris. Upon this is the layer of medium-sized vessels, while most externally are the large vessels, the whole being held together by a stroma consisting of branched pigmented connective tissue cells. The choroid is supplied with sensory nerve fibres from the trigeminal as well as autonomic nerves, presumably of vasomotor function. Posterior chamber and vitreous humour It will be noticed that there is somewhat a triangular space between the back of the iris and the anterior surface of the lens, having its apex at the point where the pupillary margin comes in contact with the lens; it is bounded on the outer side by the ciliary body. This is the posterior chamber and contains aqueous humour. Behind the lens is the large vitreous chamber, containing the vitreous humour. This is a jelly-like material, chemically of the nature of an inert gel containing a few cells and wandering leucocytes. As in other gels, the concentration of the micellae on the surface gives rise to the appearance of a boundary membrane in sections—the so-called hyaloid membrane. The vitreous body is attached anteriorly to the posterior lens surface by the ligament of Weigert. In the region of the ora, the vitreous cortex is firmly attached to the retina and pars plana and this attachment is referred to as the vitreous base. Posteriorly, the vitreous body is attached to the margin of the optic disc and to the macula forming a ring around each structure and also to the larger blood vessels. The primary vitreous is concentrated into the centre of the globe by the secondary vitreous and forms the canal of Cloquet which contains material less optically dense than the secondary vitreous. The body of the vitreous has a loose fibrous framework of collagenous fibres, whereas its cortex is made up of collagen-like fibres and protein. Retina The retina corresponds in extent to the choroid, which it lines, although the same embryological structure is continued forwards as a double layer of epithelium as far as the pupillary margin. If the two layers of epithelium are traced backwards, the anterior layer in the iris is found to be continuous with the outer layer in the ciliary body, and this again is continued into the pigment epithelium of the retina as a single layer of hexagonal cells lying immediately adjacent to the membrane of Bruch. Similarly, the posterior layer in the iris, although pigmented, passes into the inner unpigmented layer of the ciliary body, and this suddenly changes at the ora serrata into the highly complex visual retina. The retina develops from the neural ectoderm by invagination of the optic vesicle—the outer layer forming the retinal pigment epithelium, the inner layer the transparent neurosensory retina and the cavity of the embryologic optic vesicle remains as a potential space between them. The two layers are largely lightly apposed to each other in the healthy eye except at the edges of the optic disc and the ora serrata where the two are firmly attached and inseparable. Histologically, the retina consists of 10 layers (Fig. 1.8A and B) formed by three strata of cells and their synapses, namely outermost visual cells or photoreceptors, the bipolar cells in the middle and the ganglion cells innermost. FIG. 1.8 Retina and photoreceptor cell structure. (A) Cross-section of human retina, showing retinal layers. (B) Drawing of rod photoreceptor cell, showing different portions of the cell. The photoreceptor sensory cilium is indicated. Ch, choroid; GC, ganglion cell layer; INL, inner nuclear layer; ONL, outer nuclear layer; RPE, retinal pigment epithelium. Source: (From Levin LA, Albert DM. Chapter 74: Retinitis pigmentosa and related disorders. In: Ocular disease: mechanisms and management. Edinburgh: Saunders; 2010. p. 579–89.) Retinal pigment epithelium: The pigment epithelium consists of a single layer of hexagonal cells lying between the retinal photoreceptor outer segments and Bruch’s membrane of the choroid. They assist the metabolism of the retina by transporting selected substances to the receptor cells. Products of metabolism are freely exchanged between the receptor cells and the pigment epithelium. The most striking inclusions in the pigment epithelium are the melanin granules responsible for its colour. Most of the light which passes through the retina and is not absorbed by the photopigments in the photoreceptor outer segments is absorbed by these granules. The cells also contain important organelles called phagosomes which are known to be discarded rod discs that have been engulfed by the pigment epithelium. The phagocytic capacity of the pigment epithelium is demonstrated in the response of the retina to injury as by laser irradiation, when the number of phagosomes in the underlying epithelial cells increases significantly. Photoreceptors (rods and cones): Most externally, in contact with the pigment epithelium is neural epithelium, the rods and cones, which are the end-organs of vision (Fig. 1.9). The microanatomy of the rods and cones reveals the transductive region (outer segment), a region for the maintenance of cellular homoeostasis (inner segment), a nuclear region (outer nuclear layers) and a transmissive region (the outer plexiform or synaptic layer). When the outer segments of the rods are sectioned parallel to their long axes, they are seen by the electron microscope to consist of a boundary or cell membrane, which encloses a stack of membrane systems. The discs in the rods are continuously renewed throughout life. New discs are formed in the region of the inner segment and are progressively displaced towards the pigment epithelium. Rod discs have a limited life and are eventually lost to the pigment epithelium. At the junction of the inner and outer segments, the cell body of both rods and cones constricts. The electron microscope reveals a connecting cilium which is always eccentric and provides the only link between the inner and outer segments. FIG. 1.9 Anatomical features of rods and cones revealed by electron microscopy. (A) Cross-section of the human retina (×2440) demonstrating rod and cone outer segments adjacent to the pigment epithelium. (B) Tangential section through the inner segments of the human photoreceptor layer (×3750). The large inner segments belong to cones, and the smaller inner segments are those of rods; note the large number of mitochondria in the inner segments. (C) Tangential section of human retina at the outer segment level, showing rod discs contained within the cell membrane (×14,110). (D) Rod outer segment showing discs contained within the cell. A phagosome within a pigment epithelial cell is on the upper right (rhesus monkey ×23,000). Source: (From Ryan S, Schachat A, Wilkinson C, Hinton D, Wilkinson C, editors. Retina. 4th ed. Edinburgh: Elsevier; 2005.) Histologically distinguishable 10 layers of retina (outermost to innermost) are listed as follows: 1. Retinal pigment epithelium 2. Outer segment of rods and cones 3. External limiting membrane 4. Outer nuclear layer: the outer nuclear layer (the nuclei of the rods and cones) 5. The outer plexiform layer consisting of synapses 6. The inner nuclear layer (the nuclei of the bipolar cells) 7. The inner plexiform layer (again synaptic) 8. The ganglion cell layer 9. The nerve fibre layer composed of the axons of ganglion cells running centrally into the optic nerve 10. The internal limiting membrane These special nervous constituents are bound together by neuroglia, the better developed vertical cells being called the fibres of Müller, which in addition to acting as a supportive framework, have a nutritive function. The structure is completed by two limiting membranes, the outer perforated by the rods and cones, and the inner separating the retina from the vitreous. To excite the rods and cones, incident light has to traverse the tissues of the retina but this arrangement allows these visual elements to approximate the opaque pigmented layer to form a functional unit, and their source of nourishment is the choriocapillaris. At the posterior pole of the eye, which is situated about 3 mm to the temporal side of the optic disc, a specially differentiated spot is found in the retina, the fovea centralis, a depression or pit, where only cones are present in the neuroepithelial layer and the other layers are almost completely absent. The fovea is the most sensitive part of the retina, and is surrounded by a small area, the macula lutea, or yellow spot which, although not so sensitive, is more so than other parts of the retina. It is here that the nuclear layers become gradually thinned out, while parts of the plexiform layers are especially in evidence. The ganglion cells too, instead of consisting of a single row of cells, are heaped up into several layers. There are no blood vessels in the retina at the macula, so that its nourishment is entirely dependent upon the choroid (see Table 1.3). TABLE 1.3 Special Features of Fovea Centralis and Macula Lutea Fovea Centralis Macula Lutea Most sensitive for vision Nuclear layers get gradually thinned out Contains only cones There are several layers of ganglion cells There are no blood vessels in this region Entirely dependent on choroid for nourishment At the optic disc, the fibres of the nerve fibre layer pass into the optic nerve (see Chapter 14, Diseases of the Uveal Tract), the other layers of the retina stopping short abruptly at the edge of the aperture in the scleral canal. This is spanned by a transverse network of connective tissue fibres containing much elastic tissue, the lamina cribrosa, through the meshes of which the optic nerve fibres pass. The optic nerve fibres which are the axons of the ganglion cells of the retina are afferent or centripetal fibres and they become surrounded by medullary sheaths as soon as they exit the eye. The blood supply of the eye The arteries of the eye are all derived from the ophthalmic artery (Fig. 1.10A and B), which is a branch of the internal carotid artery. The ophthalmic artery has few anastomoses, so that on the arterial side the ocular circulation is an offshoot of the intracranial circulation. As far as the venous outflow is concerned, most of the blood passes to the cavernous sinus by way of the ophthalmic veins. It is to be noted that these veins anastomose freely in the orbit, the superior ophthalmic vein communicating with the angular vein at the root of the nose and the inferior ophthalmic vein with the pterygoid plexus. This has important clinical implications. FIG. 1.10 (A) The retinal circulation. (B) The choroidal circulation. The retina is supplied by the central retinal artery, which enters the optic nerve on its lower surface, 15–20 mm behind the globe. The central artery divides on, or slightly posterior to, the surface of the disc into the main retinal trunks, which will be considered in detail later (Fig. 1.10A). The retinal arteries are end arteries and have no anastomoses at the ora serrata. The only place where the retinal system anastomoses with any other is in the neighbourhood of the lamina cribrosa. The veins of the retina do not accurately follow the course of the arteries, but they behave similarly at the disc, uniting on, or slightly posterior to, its surface to form the central vein of the retina, which follows the course of the corresponding artery. The blood supply of the optic nerve head in the region of the lamina cribrosa is served by fine branches from the arterial circle of Zinn but mainly from the branches of the posterior ciliary arteries (Fig. 1.11). The central retinal artery makes no contribution to this region. The prelaminar region is supplied by centripetal branches from the peripapillary choroidal vessels with some contribution from the vessels in the lamina cribrosa region. The central artery of the retina does not contribute to this region either. The surface layer of the optic disc contains the main retinal vessels and a large number of capillaries in addition to some small vessels. The capillaries on the surface of the disc are derived from branches of the retinal arterioles. In this part of the disc, vessels of choroidal origin derived from the adjacent prelaminar part of the disc may be seen usually in the temporal sector of the disc, and one of them may enlarge to form a cilioretinal artery. The capillaries on the surface of the disc are continuous with the capillaries of the peripapillary retina. These capillaries are mainly venous and drain into the central retinal vein. In the retrolaminar part of the optic nerve, blood is supplied by the intraneural centrifugal branches of the central artery of the retina with centripetal contributions from the pial branches of the choroidal arteries, circle of Zinn, central artery of the retina and the ophthalmic artery. FIG. 1.11 Blood supply of the optic nerve. Region marked: A, represents the surface of the disc and peripapillary nerve fibre layer; B, portion anterior to the lamina cribrosa; C, portion related to the lamina cribrosa; D, portion behind the lamina cribrosa; LC, lamina cribrosa; PR, prelaminar. Source: (Reproduced with kind permission from Hayreh SS. Arch Ophthalmology 1977;95:1560.) Venous drainage of the optic disc is mainly carried out by the central retinal vein. The prelaminar region also drains into the choroidal veins. There is no venous channel corresponding to the circle of Zinn. The central retinal vein communicates with the choroidal circulation in the prelaminar region. The uveal tract is supplied by the ciliary arteries, which are divided into three groups—the short posterior, the long posterior and the anterior (Figs 1.10B and 1.12). The short posterior ciliary arteries, about 20 in number, pierce the sclera in a ring around the optic nerve, running perpendicularly through the sclera, to which fine branches are given off. The long posterior ciliary arteries, two in number, pierce the sclera slightly farther away from the nerve in the horizontal meridian, one on the nasal, the other on the temporal side. They traverse the sclera very obliquely, running in it for a distance of 4 mm. Both these groups are derived from the ophthalmic artery, while the anterior ciliary arteries are derived from the muscular branches of the ophthalmic artery to the four recti. They pierce the sclera 5 or 6 mm behind the limbus, or corneoscleral junction, giving off twigs to the conjunctiva, the sclera and the anterior part of the uveal tract. The ciliary veins also form three groups—the short posterior ciliary, the vortex veins and the anterior ciliary. The short posterior ciliary veins are relatively unimportant; they do not receive any blood from the choroid, but only from the sclera. The vortex veins or venae vorticosae are the most important, consisting usually of four large trunks which open into the ophthalmic veins. They enter the sclera slightly behind the equator of the globe, two above and two below, and pass very obliquely through this tissue. The anterior ciliary veins are smaller than the corresponding arteries, since they receive blood from only the outer part of the ciliary muscle. Of these ciliary vessels, the short posterior ciliary arteries supply the whole of the choroid, being reinforced anteriorly by anastomoses with recurrent branches from the ciliary body. The ciliary body and iris are supplied by the long posterior and anterior ciliary arteries. The blood from the whole of the uveal tract, with the exception of the outer part of the ciliary muscle, normally leaves the eye by the vortex veins only. The two long posterior ciliary arteries pass forward between the choroid and the sclera, without dividing, as far as the posterior part of the ciliary body. Here each divides into two branches (Fig. 1.12); they run forward in the ciliary muscle, and at its anterior part bend round in a circular direction, anastomosing with each other and thus forming the circulus arteriosus iridis major. This is situated in the ciliary body at the base of the iris; from it, the ciliary processes and iris are supplied. Other branches from the major arterial circle run radially through the iris, dividing dendritically and ending in loops at the pupillary margin. A circular anastomosis takes place a little outside the pupillary margin, the circulus arteriosus iridis minor. FIG. 1.12 The ciliary circulation. The tributaries of the vortex veins, which receive the whole of the blood from the choroid and iris, are arranged radially, the radii being bent, so as to give a whorled appearance—hence their name. The veins of the iris are collected into radial bundles which pass backwards through the ciliary body, receiving tributaries from the ciliary processes. Thus reinforced, they form an immense number of veins running backwards parallel to each other through the smooth part of the ciliary body. After reaching the choroid, they converge to form the large anterior tributaries of the vortex veins. The veins from the outer part of the ciliary body, on the other hand, pass forward and unite with others to form a plexus (the ciliary venous plexus), which drains into the anterior ciliary veins and the episcleral veins. These vessels communicate directly with the canal of Schlemm, which is intimately connected with the anterior chamber by means of numerous tortuous channels through the loose tissue of the trabecular meshwork. From this canal, the efferent channels form a complex system (Fig. 1.13); some of them drain into efferent ciliary veins in the sclera while others traverse the sclera and only join the venous system in the subconjunctival tissues (aqueous veins). FIG. 1.13 The exit channels of the aqueous humour in man: C, cornea; S, sclera; I, iris; and CB, ciliary body. The primitive drainage channels of lower animals are seen in VP, the ciliary venous plexus, draining by EV, the ciliary efferent veins into ACV, and the anterior ciliary veins. Superimposed on this is the drainage system peculiar to primates, represented by T, the trabeculae; SC, the canal of Schlemm; IP, the intrascleral plexus; and AV, an aqueous vein emptying into the anterior ciliary veins. The marginal loops of the cornea and the conjunctival vessels are branches of the anterior ciliary vessels (Fig. 1.12). Clinical anatomy of the eye On clinical examination, the parts of the external surface of the eye appear as shown in Fig. 1.3. The appearance of the anterior segment from the cornea to the lens is as shown in Fig. 1.4 and the posterior segment behind the lens as shown in Fig. 1.5. Summary The human eye and its adnexal structures develop from the neuroectoderm of the neural groove and the adjoining surface ectoderm, mesoderm and cells of neural crest origin. Though the development takes place by a predetermined sequence of events, local interactions and trophic influences affect the chain of interrelated processes which take place both simultaneously and sequentially. Teratogenic influences like intrauterine infections, noxious stimuli, maternal intake of drugs or alcohol and exposure to radiation can affect the normal course of development leading to abnormalities and congenital deformities. The milestones in embryological development are not absolute and are more representative of a time period than an actual finite time. Any disruption in that period will have an effect on the structures forming at that particular phase of development. As expected, abnormal developmental influences have a more severe impact if they occur early when the system is more immature and more prone to major developmental defects. The eye is a complex anatomical structure consisting of delicate tissues. It is a sense organ which is designed to capture and focus light to form a retinal image which is translated into electrical signals and transmitted to the central nervous system via the optic nerve. The eye is protected from the environment by the eyelids, lashes and the orbital wall. The extraocular muscles function in a synchronized fashion to stabilize the globes, enable binocular vision and the full functional field of vision by allowing the full range of ocular movements. The blood supply of the eye and orbit is derived from the ophthalmic artery. There are certain facts which are noteworthy as clinically relevant. The major part of ocular development occurs from week 3 and week 10. The neural tube ectoderm is the precursor of the retina, optic nerve, epithelium of iris and ciliary body, and smooth muscles of the iris. The surface ectoderm gives rise to the lens, the epithelium of cornea and conjunctiva, eyelids, and lacrimal system. The other ocular structures form from mesenchyme. The PAX 6 gene plays a significant part in ocular development.. Suggested reading 1. Beauchamp GR, Knepper PA. Role of the neural crest in anterior segment development and disease. J Pediatr Ophthalmol Strabismus 1984;21:209–14. 2. Duke-Elder S, Cook C. Normal and abnormal development: Part 1, Embryology. In: Duke-Elder S, editor. System of ophthalmology, vol. III. London: C.V. Mosby; 1963. p. 23–4. 3. Gray H. Gray’s anatomy. 38th ed. Edinburgh: Churchill Livingstone; 1989. 4. Noden DM. Periocular mesenchyme. Neural crest and mesodermal interactions. In: Jakobiec FA, editor. Ocular anatomy, embryology, and teratology. Philadelphia: Harper & Row; 1982. p. 97–119. 5. Ozaincs V, Jakobiec F. Prenatal development of the eye and its adnexa. In: Jakobiec FA, editor. Ocular anatomy, embryology, and teratology. Philadelphia: Harper & Row; 1982. p. 11–96. 6. Snell RS, Lemp MA. Clinical anatomy of the eye. Hong Kong: Blackwell Scientific; 1989. 7. Wolff E. Anatomy of the eye and orbit. 7th ed. London: HK Lewis; 1976. CHAPTER 2 Physiology of the eye CHAPTER OUTLINE Blood–Retinal Barrier, 15 Nature and Formation of Intraocular Fluid, 16 Circulation of Aqueous Humour, 16 Intraocular Pressure, 17 Metabolism of Ocular Tissues, 17 Vascularized Tissues of the Eye, 17 Nonvascularized Tissues of the Eye, 17 Learning objectives To assimilate important facts about ocular physiology and inculcate the skill of correlating common ocular diseases with disruption of normal physiology To grasp the main principles in applying knowledge of basic sciences in planning and instituting therapy To understand how disruption of normal functions, for example tear film and aqueous formation or drainage abnormalities, lead to pathology The human eye is a precise system, which comprises components that must be optimally maintained, so that a clear image is seen. Transparency, surface regularity and smoothness, and a stable ocular anatomy are important for sight. The stable shape of the eye is due to the structure of the sclera and a stable intraocular pressure, higher than the atmospheric pressure. In the anterior segment of the eye, between the cornea and lens, the anterior and posterior chambers of the eye are filled with aqueous humour, a fluid with an ionic composition very similar to blood plasma and with two main functions—(1) to supply nutrients to the avascular structures of the eye, cornea and lens and (2) to maintain intraocular pressure within its physiological range. In the posterior segment, in the space between the lens and retina, lies the vitreous humour also known as the vitreous gel or simply the vitreous. It is a transparent stagnant gel formed by a network of collagen type II fibres with the glycosaminoglycan hyaluronic acid, contains few cells (mostly phagocytes to clear debris and hyalocytes on the surface of the vitreous), no blood vessels, and 98%–99% of its volume is water. Blood–retinal barrier In the eye, semipermeable membranes separate the blood from the ocular cavity and comprise the blood–ocular barrier, the composition of which is shown in Fig. 2.1. The blood–ocular barrier is formed in the posterior segment of the globe by (1) the walls of retinal vessels, which like those of the central nervous system are nearly impermeable, (2) Bruch’s membrane and (3) the retinal pigment epithelium. In the ciliary region, it is formed by the two-layered ciliary epithelium, through which fluid must traverse before the posterior chamber is reached. In the iris, it is formed by walls of the iris capillaries, which are freely exposed to the anterior chamber through the crypts and spongy stroma. FIG. 2.1 The effective blood–ocular barrier. A: In this zone, the barrier is formed by retinal capillaries, Bruch’s membrane and retinal pigment epithelium. B: In this zone, the barrier consists of the ectodermal layer formed by the anterior part of the retina and its prolongation as the two-layered ciliary epithelium. C: In this zone, the barrier is formed by the uveal capillaries only. The arrows indicate that in A and C, two-way traffic exists; in the ciliary region, fluid traffic is essentially into the cavity of the eye, determining a circulation through the pupil and out at the angle of the anterior chamber. Capillaries in the retina (like those in the central nervous system) are relatively impermeable, so that practically no colloid molecules can pass into the cavity of the eye. Fluorescein dye injected in the bloodstream is bound to albumin, making a larger molecular complex. The blood–retinal barrier, by preventing leakage of this dye in the physiological state, results in a clear outline of retinal vessels of all calibres. In the choroidal circulation, fluorescein passes freely across vessels to the extravascular spaces. There is, however, a physiological barrier to the passage of dye from the choroidal spaces, across Bruch’s membrane and the intact retinal pigment epithelium into the subretinal space. The peculiar impermeability of the retinal capillaries and of the Bruch’s membrane–pigment epithelial barrier, while necessary from the optical point of view, prevents the ready passage of large-sized molecules of any kind into the eye. Antibiotics, when administered systemically, are often of little value in ocular therapeutics. Substances with a high lipoid solubility, however, which easily penetrate living cells, traverse the barrier much more readily, such as sulphonamides and chloramphenicol. If the permeability of the capillaries is increased, large molecules will be able to pass through their walls, so that a turbid fluid, rich in protein is formed leading to plasmoid aqueous in the anterior chamber or exudates in the retina or choroidal effusion. This increase in permeability may be brought about by vasodilator drugs, in inflammatory conditions such as iridocyclitis or choroiditis, and also if the capillary walls are mechanically stretched by suddenly lowering the intraocular pressure and removing their external support. This occurs when the globe is suddenly opened as by paracentesis or when the intraocular pressure is lowered by vigorous massage of the globe. Nature and formation of intraocular fluid Normal aqueous is a transparent, colourless, low refractive index medium formed continuously from plasma by the ciliary epithelial cells. Gap junctions between the nonpigmented and pigmented ciliary epithelial cells allow free communication between these cells, whereas tight junctions between the nonpigmented epithelial cells form the blood–aqueous barrier. Formation of the aqueous humour (Fig. 2.2) is known to involve a number of mechanisms—ultrafiltration, diffusion and secretion. The secretory process is powered by the metabolic activity of the cells of the ciliary epithelium and probably accounts for some 95% of the total quantity of aqueous. The entire mechanism is not understood, but it is known that a watery fluid rich in sodium and containing small quantities of ascorbic acid and other substances is secreted into the posterior chamber. FIG. 2.2 Formation of aqueous humour. Having this varied origin, the aqueous humour thus consists of a dilute solution of all the diffusible constituents of the plasma, in addition to substances specifically secreted. Since entry into the eye across the blood– aqueous barrier is difficult and exit through the drainage channels is easy, many of the constituents of the aqueous humour are in deficit, in comparison with blood, with the exception of those secreted. There is, however, an excess of lactic acid in aqueous compared with blood, due to the formation of this substance as an end product of the metabolism of the lens. Circulation of aqueous humour Circulation of aqueous is necessary both for metabolic purposes and to regulate the intraocular pressure. As the greater part of the fluid is formed in the ciliary region, it flows from the posterior chamber through the pupil into the anterior chamber and escapes through the drainage channels at the angle, i.e. trabecular meshwork, Schlemm’s canal, collector channels and aqueous veins, and from there into the episcleral veins. In addition to this, there is a second accessory exit (the uveoscleral outflow) which relatively allows aqueous to flow through the ciliary body into the choroid and suprachoroid and from there into the episcleral tissue (see Chapter 16, Glaucoma). Intraocular pressure The major factor controlling intraocular pressure is the dynamic balance between aqueous humour production in the ciliary body and its elimination via the canal of Schlemm. Other factors like choroidal and vitreous blood volume and the extraocular muscle tone can also affect intraocular pressure, generally in the short term. Prolonged changes in intraocular pressure are essentially caused by two factors: 1. An alteration in the forces determining the formation of the aqueous 2. Alterations in the resistance to its outflow From the clinical point of view the latter is more important. A rise in intraocular pressure may be caused either by a process which blocks the passage of aqueous into the canal of Schlemm, such as sclerosis of the trabeculae or their obstruction by exudates or organized tissues, or by an increase in pressure of the episcleral veins, into which the aqueous drains. In either event, glaucoma is the result. If the drainage channels to the canal of Schlemm are blocked, some drainage of intraocular fluid will take place through the uveoscleral outflow. Inefficiency of the drainage channels, therefore, causes either a cumulative rise of pressure or transient increments. While these are the principal factors determining prolonged changes in the intraocular pressure, other factors can exert temporary effects. 1. Variations in the hydrostatic pressure in the capillaries 2. An increase in permeability of the capillaries, allowing the formation of a plasmoid aqueous with high protein content, will increase its osmotic pressure relative to that of the blood and thus raise the pressure in the eye, a process accentuated if the drainage channels become clogged. This occurs particularly in inflammations. 3. A change in the osmotic pressure of the blood will be reflected in the intraocular pressure by altering the process of diffusion across the capillary walls, hypotonicity inducing a rise in intraocular pressure as in the water-drinking test and hypertonicity a fall, as induced by the use of glycerol by mouth or mannitol intravenously. 4. Volumetric changes within the globe are immediately transformed into pressure changes owing to the indistensibility of the sclera; if extra fluid, such as extensive vitreous haemorrhage, is forced into the eye, its pressure could rise abruptly. 5. A blockage of the circulation of aqueous has a profound effect in raising the ocular tension. Such a block may occur in two places: (1) at the pupil where the flow of fluid from the posterior to the anterior chamber may be impeded and (2) at the angle of the anterior chamber. Obstruction in situation (1) is usually due to one of the two causes. The first arises in eyes with a shallow anterior chamber—a lax iris has a larger area of apposition to the anterior surface of the lens, causing the condition of ‘relative pupillary block’ with the aqueous being dammed in the posterior chamber. The iris billows forward to reach the cornea and blocks the angle of the anterior chamber, leading to an attack of primary angle-closure glaucoma. Obstruction in situation (2) is due to organic adhesions between the peripheral iris and cornea, when the iris becomes adherent to the anterior capsule of the lens in primary angle closure, inflammatory conditions or fibrosis after neovascularization, when secondary glaucoma occurs. The intraocular pressure within the eye normally varies from 10 to 20 mm Hg. It is most accurately measured by manometry, wherein a small cannula is inserted into the anterior chamber and connected with a small- bore mercury or saline manometer. Such a technique is used experimentally on animals but its clinical application is obviously impossible. The sclera is only very slightly elastic and is rendered tense by the internal pressure, allowing the intraocular pressure to be measured by the degree to which it can be indented on the application of a standard weight or flattened by a measured pressure with considerable accuracy. Such a method is used clinically in tonometry (see Tonometry in Chapter 8, Clinical Examination of the Eye). The result thus obtained, usually recorded as millimetre of mercury (mm Hg), by standardization with a manometer on experimental animals is referred to as the intraocular pressure. Metabolism of ocular tissues Vascularized tissues of the eye The vascularized tissues of the eye, particularly the uveal tract, are similar in their general metabolism to other tissues in the body. Nonvascularized tissues of the eye The nonvascularized tissues of the eye—the cornea and the lens—depend for their energy requirements essentially on carbohydrates, which are utilized by phosphorylation and auto-oxidative mechanisms. The cornea The cornea has low energy requirements, which are necessary for the replacement of its tissues and the maintenance of transparency. Transparency depends essentially on its state of relative dehydration, which is maintained by an active transference of fluid outwards through the epithelium and endothelium, particularly the latter. A fall in metabolic activity or an increase in the permeability of its membranes thus leads to oedema and opacification. The essential physiological differences between the cornea and the sclera are that in the cornea, the fibrils are arranged in a regular latticework, in a ground substance of mucopolysaccharide, whereas the fibres of the sclera are irregularly arranged, and that the former tissue is bound by cellular membranes which control the traffic of fluid. The cornea derives its nourishment from three sources—oxygen directly from the air, solutes from the perilimbal capillaries and the aqueous humour. The first is an active process undertaken by the epithelium, and in an atmosphere of nitrogen, lactic acid collects rapidly in this layer of cells. The importance of diffusion from the limbal capillaries is seen clinically in the relative resistance of the peripheral parts of the cornea to degenerative changes, but at the same time, if these vessels are experimentally cut, corneal transparency is maintained. Similarly, if the aqueous is replaced by nitrogen, the cornea remains transparent; it turns opaque, if both these sources of nutrition are cut off. Metabolic activity exhibits a high rate of aerobic glycolysis as described below. The lens The lens derives its nourishment entirely from the aqueous humour in which it is immersed, the fluid traffic being regulated by the semipermeability of the capsule and the subcapsular epithelium. If this membrane is disrupted, the whole tissue like the cornea tends to adsorb fluid and turns opaque. Active transport takes place between the lens and the aqueous owing to the activity of the subcapsular epithelium. The capsule itself is permeable to water and electrolytes as well as colloids of small molecular size, the posterior part being more permeable than the anterior. The permeability of the capsule decreases with age. There is a small amount of oxygen in the aqueous derived from the blood, but the main metabolic process is probably anaerobic and in the lens, there are a number of enzymes which break down pyruvate to lactic acid and water. Lactic acid is found in considerable quantity in the aqueous humour when the lens is present; this is not so in the aphakic eye. Agents which appear to participate in this process are glutathione and ascorbic acid (vitamin C) which, reacting together, probably participate in an internal auto-oxidative system. The former, both as reduced and oxidized glutathione, occurs in very high concentration in the lens, particularly in the cortex; the latter is specially secreted by the ciliary body. Neither is present in a cataract. The metabolic activity of the lens is largely confined to the cortex; the older nucleus is relatively inert. The metabolism of the lens is fairly complex but can be simplified for an overview (Fig. 2.3). The energy requirements are met by various pathways of glucose metabolism, namely the glycolysis (Embden–Meyerhof–Parnas [EMP]) pathway, pentose phosphate pathway and the tricarboxylic acid (TCA) cycle. Energy is stored in the form of adenosine triphosphate (ATP). Most of the glucose which enters the normal lens, 90%–95%, is converted into glucose-6-phosphate by the enzyme hexokinase and a small quantity is converted by the enzyme aldose reductase into sorbitol and enters the sorbitol pathway. The latter pathway becomes more important when there is excess glucose in the lens as in diabetes. About 80% of the glucose is then metabolized by the EMP pathway which is the anaerobic pathway of glycolysis. Around 10% of glucose is utilized by the pentose phosphate pathway which, in addition to generating reduced nicotinamide adenine dinucleotide phosphate (NADPH)—an important reducing substance required for the biosynthesis of many vital cellular components such as reduced glutathione—also generates the building blocks for synthesis of nucleic acids, proteins and components of cell membranes. As the oxygen content of the lens is low, only about 3% of glucose is metabolized aerobically by the TCA cycle. However, as the latter is more efficient in the generation of ATP, it produces about 25% of the ATP present in the lens, with 70% being produced by anaerobic glycolysis. The TCA cycle occurs more prominently in the lens epithelium. FIG. 2.3 Overview of the major pathways of glucose metabolism in the lens. Percentages represent the estimated amount of glucose used in the different pathways. Source: (From Yanoff M, Duker JS. Basic science of the lens. In: Schell J, Boulton ME, editors. Ophthalmology. 4th ed. Oxford: Saunders; 2014.) Summary The important physiological processes necessary for the functioning of the eye relate to the blood–ocular barrier, formation and circulation of intraocular fluid, maintenance of intraocular pressure and transparency, and metabolism of the different ocular tissues. Physiology and homeostasis of the tear film is critical for a healthy ocular surface. Stasis of tears due to obstruction or disruption of tear film drainage makes the eye prove to discomfort, inflammation and increased risk of infections. Aqueous formation occurs largely by two mechnanisms ultrafiltration through ciliary process stromal capillaries that have many fenestrations and lack tight junctions active secretion using Na/K ATPase, to pump out Na+ and water follows, and carbonic anhydrase which leads to bicarbonate ions leave with Na+. Corneal transparency – depends on its relative dehydration, relative acellularity and avascularity, uniform sized collagen fibrils arranged in a lattice pattern, with spaces between them that are less than the wave length of light. Lens – The physiological functions of the lens are refraction, accommodation and absorption of damaging ultraviolet light 50 Source: Modification of the technique originally described by Hogan MJ, Kimura SJ, Thygeson P. Signs and symptoms of uveitis. Anterior uveitis. Am J Ophthalmol 1959;47:155–70.d6/6000 = hand motion at 2 feet. Contents Protein transudation from the iris or ciliary vessels produces an opalescence of the aqueous, an aqueous flare (Fig. 8.21A), which may be visible on the slit lamp when its beam is narrowed to 2 × 1 mm. FIG. 8.21 (A) Aqueous flare, showing particles suspended in the anterior chamber as seen by a slit lamp (light coming from the right) in a case of cyclitis. (B) Slit-lamp image showing fine keratic precipitates on the posterior surface of the cornea in a case of nongranulomatous anterior uveitis. It is graded as being: 1+ if barely present 2+ if moderate 3+ if it obscures visualization of the iris pattern 4+ if fibrin is present in the anterior chamber Floating cells in the aqueous are an indication of active uveitis and are graded by counting the number seen in a 2 × 1-mm beam on the slit lamp. The aqueous cells are recorded as: Trace if 1–5 are present 1+ if 5–10 2+ if 10–20 3+ if 20–50 4+ if more than 50 The posterior surface of the cornea to detect whether any protein material or cells are deposited on it (keratic precipitates, Fig. 8.21B). Hypopyon: In infected wounds and ulcers of the cornea, and occasionally in iridocyclitis, there is a collection of lymphocytes in the anterior chamber forming a sediment at the bottom, the surface of which is level (hypopyon). Hyphaema: A similar collection of blood may occur after contusions or spontaneously (hyphaema). Microfilariae may be observed in the anterior chamber in eyes with onchocerciasis. The iris The colour of the iris and the clarity of its pattern should first be examined. The two irides or parts of the same iris may be of different colours, conditions which are known as heterochromia iridium or iridis, respectively. A dull iris with an ill-defined pattern or ‘muddiness of the iris’ suggests atrophy from iridocyclitis, and sectoral patches of atrophy suggest an acute angle-closure glaucoma or herpes zoster. Tremulousness of the iris or iridodonesis is seen when the eyes are moved rapidly if this tissue is not properly supported by the lens. This occurs in the absence, shrinkage or subluxation of the lens and is best appreciated in a dark room with oblique illumination on asking the patient to move his eye. Freckles are darkly pigmented spots in the iris which are not raised above the surface. Brushfield spots are seen in Down syndrome and pedunculated nodules (Lisch) in neurofibromatosis. Flat nodules at the pupillary margin (Koeppe nodules) or at the peripheral base of the iris (Busacca nodules) or a ‘muddy’ iris with a small, irregular pupil and sluggish reaction to light are indicative of uveitis. The position of the iris must be examined next, especially the plane in which it lies. Special attention should be paid to any adhesions or synechiae, anterior to the cornea or posterior to the lens capsule. The pupil Abnormal size of the pupil The size of the pupil is determined by the afferent and efferent pathways for pupillary light reflexes, as well as the function of the sphincter and dilator pupillae muscles. Dilatation of the pupils with retained mobility is found sometimes in myopia and in conditions of impaired tone or nervous excitement. Conversely, the pupils are small in babies and in old people. Many pathological entities can lead to disturbances of the pupil (see Chapter 31, Diseases of the Nervous System with Ocular manifestations). Very large, nonreactive pupils suggest that a mydriatic has been used, perhaps inadvertently, as when a patient has been using an ointment containing atropine and has rubbed (usually) his right eye with soiled fingers. The pupils are usually immobile, and the patient complains of dimness of vision, especially for near work. The pupils are also large and immobile in bilateral lesions affecting the retina and the optic nerve causing blindness (see Fig. 4.8), as in optic nerve atrophy. Unilateral blindness due to these diseases never produces a dilated pupil because of the consensual light response elicited by the normal, fellow eye. Bilateral dilated pupils, in bilateral blindness, can be distinguished from a bilateral efferent pupillary defect, pupilloplegia, by eliciting the near reflex. Although the patient cannot see his thumb held close to the face, by proprioception, he attempts to accommodate and causes constriction of both pupils. It is equally important to remember that the presence of a direct reaction to light does not eliminate the possibility of the patient actually being blind due to a central lesion affecting the visual pathways above the level of the lateral geniculate body (postbasal meningitis, haemorrhage, uraemia, bilateral occipital lobe infarction). Opacities in the media such as dense cataracts or vitreous haemorrhage never lead to absent pupillary reflexes if tested with bright illumination. Dilated and immobile pupils also result from third nerve palsies (absolute paralysis of the pupil); if the paralysis also affects the third nerve fibres to the ciliary muscle, accommodation is also paralysed (ophthalmoplegia interna). This results in lesions affecting the third nerve nucleus, diseases such as meningitis, encephalitis and cerebral syphilis, diphtheria, lead poisoning and orbital disease, or trauma affecting the third nerve, ciliary ganglion or the eye itself. Unilateral dilatation may result from irritation of the cervical sympathetic nerves. This may be due to conditions such as swollen lymph nodes in the neck, apical pneumonia, apical pleurisy, cervical rib and thoracic aneurysm. It may also be due to syringomyelia, acute anterior poliomyelitis and meningitis affecting the lower cervical and upper thoracic parts of the spinal cord and pressure on the sympathetic fibres leaving the cord in the lower cervical and upper thoracic ventral roots. Most of the conditions causing an irritative dilatation lead eventually to constriction from sympathetic paralysis. When all sympathetic function on one side is lost, resulting in miosis, a narrowed palpebral fissure and slight enophthalmos (due to loss of tone of the Muller muscle), sometimes associated with unilateral absence of sweating, the condition is called the Horner syndrome. Small, immobile pupils suggest the use of drugs, either locally (miotics) or systemically (morphine). A small, sluggish pupil with ‘muddiness’ of the iris is associated with an active iritis. A small, immobile pupil suggests old iritis with posterior synechiae and should be investigated with a mydriatic such as cyclopentolate to ascertain if the pupil dilates regularly. Bilateral, small pupils may be due to irritation of the third nerves, arousing suspicion of a central nervous disease in their vicinity. The condition can also be due to palsy of the sympathetic system, as in pontine haemorrhage. In acute angle-closure glaucoma, the pupil is usually large, immobile and oval, with the long axis vertical. Pupillary reflexes During routine examination of the eyes, the pupils should be examined at an early stage before any mydriatic is employed. Such an examination should be careful, detailed and best carried out with low background illumination using a bright focussed light, with the patient looking into the distance. While testing the pupillary reflexes, the following points should be kept in mind: Illumination in the examination room should be low. The patient should look into the distance. The light used should be focussed and bright. The patient should face a diffuse light so that both pupils are equally illuminated. The patient is asked to look into the distance to prevent accommodative constriction of the pupil. Note the size, shape and contour of each pupil and then test the pupillary reflexes. These reflexes are (i) constriction of the pupil to direct or consensually presented light and (ii) accommodation—constriction on viewing a near target. To elicit the direct reaction to light, cover both the eyes with the palms of the hands, preferably without touching the face. While the patient looks straight ahead, remove one hand and watch the pupil, noting if its constriction to light is well maintained. Replace this hand and remove the other, observing the other pupil. The consensual reaction to light is determined by removing one hand so that this eye is exposed to light (it should be shaded from intense light) and watching the pupils as the hand is removed from the other eye. The process is repeated while observing the other pupil. This method of examination is not always possible due to an absence of natural light or diffuse illumination. Moreover, when the reaction to light is feeble and the pupils are already small, it is difficult to be certain of the results in bright, diffuse daylight. In such cases, the examination should be carried out in a dark room and light concentrated upon one pupil by focal illumination so that it shines upon the macula, the most sensitive area from which to elicit the light reflex. By slight lateral movements, the focus of light can be moved on or off the pupil, the pupillary movements being observed constantly. Still finer observations can be made with the slit lamp, when the microscope is focussed on the papillary margin and the beam is abruptly switched from the side into the pupillary aperture. If there is no movement in these conditions, it may be concluded that the reaction to light is absent. The same method will elicit the hemianopic pupillary reaction (Wernicke) in the rare cases (lesion of one optic tract) in which it is present. The light is focussed first on the nasal side of the retina and the pupil is observed and then the light is focussed on the other side of the retina. The best source of illumination for this purpose is the focal beam of the slit lamp reduced to a spot. If the reaction is present, the pupil will react briskly when one half of the retina is illuminated but very slightly when the other half is illuminated. This is so because it is impossible to prevent diffusion of light onto the sensitive half of the retina, so the test is rarely unequivocal. ‘Swinging-flashlight’ test: A bright light is shone onto one pupil and a constriction is noted. After 2–3 seconds, the light is rapidly transferred to the opposite pupil. This swinging to-and-fro movement of the light is repeated several times while observing the response of the pupil to which the light is transferred (Fig. 8.22). When the input through both optic nerves to the midbrain is equal, the pupil to which the light is transferred will remain tightly constricted since the consensual response has the same magnitude as a direct response. Should there be a lesion of one optic nerve, the input from that side is less than that from the normal side. In that case when the light is transferred to the diseased eye, both pupils will dilate and on swinging back to the normal side both the pupils will constrict. The dilatation or ‘escape’ that occurs is commonly called the Marcus Gunn pupil or an afferent pupillary defect and may be the earliest indication of an optic nerve disease such as retrobulbar neuritis. FIG. 8.22 Swinging flashlight test. Source: From Harold A. Stein, Raymond M. Stein, Melvin I. Freeman, eds. The Ophthalmic Assistant. 9th ed. London: Saunders; 2013. Pp. 111–142. The reaction to convergence and accommodation is determined by asking the patient to look at the far end of the room. While he does so, an accommodation target is suddenly held up vertically at about 15 cm from the patient’s nose and he is told to look at it. The movement of the pupils is studied while the patient converges. When properly conducted, the aforementioned method provides reliable information as to the shape and relative size of the pupils and their reactions. A few of the common conditions are considered here. Abnormal reactions of the pupil These are equally important (see Fig. 26.10). As mentioned earlier, loss of light reflexes results from a lesion in the retina or optic nerve causing blindness while a hemianopic reaction results from lesions in the tract (Fig. 4.8). A lesion in the third nerve abolishes both light and convergence reflexes. More complex lesions may result from damage to the relay paths in the tectum between the afferent and efferent tracts. The most important of these is the Argyll Robertson pupil, usually caused by a lesion, almost invariably syphilitic, in this region. In this condition, the pupils are small (spinal miosis) and do not react to light but the contraction on convergence is retained. This is referred to as light near dissociation. The tonic pupil (of Adie) somewhat resembles the Argyll Robertson pupil; it is of unknown aetiology, not associated with syphilis, occurs usually in young women, is often unilateral and is associated with absent knee-jerks. This pupil is slightly dilated and always larger than its counterpart; the unilateral Argyll Robertson pupil is always smaller. Although in the tonic pupil the reaction to light seems absent at first, careful examination shows it to be present as a vermiform, slight constriction. The reaction of the pupil on convergence is sluggish, with a long latent period and is unduly sustained. The tonic pupil dilates well with atropine, while the Argyll Robertson pupil does not. Finally, the tonic pupil constricts with 0.1% pilocarpine. Symptoms of the Adie pupil fall into two groups: those due to the pupil size, which do not last longer than a few weeks, and those due to ciliary dysfunction. The affected eye usually has a slight accommodative paresis, and asthenopia is often induced by near effort. Many patients can never get the two eyes to work together when reading and are best advised to use dilute pilocarpine and fix to work with the other eye. The lens The lens cannot be examined thoroughly without the assistance of a slit lamp and an ophthalmoscope. Any opacities in the pupillary area can be seen by inspection, aided with focal illumination. By direct slit-lamp examination, through the pupil of a young person’s eye, the lens substance seems almost perfectly clear; at the most, a faint bluish haze is seen. The haze is much more pronounced in an old person and the lens looks slightly milky due to sclerosis of the nucleus. It is probable that the patient has a cataract, but examination by distant direct ophthalmoscopy shows a clear red reflex. The explanation is that the refractive index of the lens substance increases with age, and scattering of light from its surface is greater. The milkiness is due to rays of light which are reflected from the lens and enter the observer’s eye. Opacities in the lens itself are seen by oblique illumination as grey, white or brown–yellow areas; by retroillumination or distant direct examination with the ophthalmoscope, they appear black. Various forms of cataract are diagnosed according to their distribution and nature, but observation must always be confirmed by ophthalmoscopic examination and the opacities localized with the help of the slit lamp (see Figs 8.30 and 15.3). A spot in the centre of the pupil, looking as if it were on the surface of the lens, may be a pupillary exudate or an anterior polar cataract. Triangular spokes of opacity with their apices towards the centre are indicative of a cuneiform senile cataract. A white appearance over the whole pupillary area suggests a total or mature cataract; if it is yellowish-white, with white spots of calcification and the iris is tremulous, a shrunken calcareous lens should be suspected. Finally, the pupil may be blocked with uveal exudates forming an inflammatory pupillary membrane. The posterior chamber The posterior chamber lies between the posterior surface of the iris and the anterior surface of the lens, is filled with aqueous and is not readily visible by direct observation due to the opaque iris. Slit-lamp biomicroscopy Slit-lamp biomicroscopy is a dynamic examination in which the eye is scanned anteroposteriorly and horizontally. Ocular problems can be identified by different methods of examination, which differ in the positioning of the illuminating light and the angle between the illumination and observation arms. Various permutations and combinations of these techniques are used, some simultaneously and others sequentially. Diffuse illumination Diffuse illumination allows an observer to obtain a direct and tangential view of the anterior segment of the eye. A direct, diffuse illumination examination with low magnification is undertaken first, which is later replaced by higher magnification for viewing areas of interest (Fig. 8.23). Diffuse illumination allows determination of general features such as colour, size and relative position of structures. This is followed by tangential illumination with a large angle of illumination which helps increase contrast and highlight the texture of ocular tissues. FIG. 8.23 Slit-lamp view of a corneal opacity (adherent leucoma) in diffuse illumination. Focal illumination Focal illumination is used for direct observation of the illuminated point, direct focal examination, or to allow observation of an adjacent area, indirect focal viewing. This permits the observer to cut an optical section of the anterior segment at any angle. Fig. 8.24 shows a general view of the eye illuminated by a slit-lamp beam of light of moderate width, entering the eye from the left side. Optically, the homogeneous media appear quite black; structures such as the cornea, lens and suspended particles in the aqueous scatter light and appear opalescent. On the left of both Fig. 8.24A and B is seen the illuminated portion of the cornea forming a parallelepiped, the brighter areas corresponding to the surfaces and the darker to the section of the cornea. The black space on the right is the anterior chamber, then follows the ‘phantom’ of the lens. A dim central interval can be distinguished, formed by the embryonic nucleus with its Y-sutures. Outside this are the successive ‘zones of discontinuity’—the fetal nucleus, the infantile nucleus, the adult nucleus and the cortex. Focal illumination permits an assessment of the depth of any ocular abnormality. FIG. 8.24 (A) Slit-lamp examination with direct focal illumination where the slit is placed over the specific area of interest. (B) Slit-lamp examination with indirect focal illumination where the slit is placed adjacent to the specific area of interest, and retroillumination (inset) where the beam is aligned to allow the light to be reflected back from behind the area of interest. Retroillumination In this form of examination, the illuminating and viewing arms of the slit lamp are placed along the same axis, coaxially, or nearly the same axis, paraxially. (i) Direct retroillumination uses an axial or paraxial light beam that shines into the pupil and reflects off the retina. The fundal glow highlights the presence of opacities in the media such as cataracts (Fig. 8.24B), corneal scars, deep corneal vessels (Fig. 8.18B), transparent cysts and refractile bodies. It also highlights the presence of defects in the integrity of the normally opaque iris, e.g. an iris hole. (ii) Indirect retroillumination from the iris is achieved by directing the light beam to the iris at an angle of 45° and focussing on the cornea. The light reflected off the iris allows visualization of subtle, transparent corneal irregularities such as ghost vessels or keratic precipitates. Specular reflection Specular reflection allows the observer to visualize the corneal endothelium by viewing light reflected back from this interface. The illuminating and viewing arms are adjusted so that each forms an angle of about 30° with the central perpendicular; the slit-lamp beam is narrowed to a height of 2 mm and focussed onto the central corneal endothelium. This is placed immediately adjacent to the reflection of the slit-lamp bulb on the cornea. A golden sheen with darker lines outlining the hexagonal endothelial cells is seen (Fig. 8.25). An approximate count of the endothelial cells is possible using an Eisner grid. FIG. 8.25 Slit-lamp view of examination in the zone of specular reflection showing hexagonal endothelial cells and central guttate spots in a patient with Fuchs endothelial dystrophy. Scleral scatter This is an indirect form of illumination, created by decentring the beam after releasing the central locking screw and directing a broad beam to the temporal limbus. This light is totally internally reflected through the thickness of the cornea, like a fibreoptic light pipe, and emerges at the opposite limbus. Any opacities in the central cornea are highlighted, e.g. nebular corneal opacities, vortex dystrophies and early corneal oedema. Tonometry Tonometry is the assessment of the intraocular pressure of the eye and is one of the cornerstones of diagnosis of glaucoma. It is also essential in the monitoring of antiglaucoma medications. Subjective method: It may be done digitally in the same manner as testing for fluctuations in other parts of the body, i.e. by two fingers placed a short distance from one another above the superior tarsal plate. Instruments known as tonometers have been devised for measuring the intraocular pressure of the intact eye and are of two types. Indentation tonometer The indentation tonometer of Schiøtz measures the depth of the indentation of the anaesthetized cornea, produced by a weighted stylet and is measured by a lever which travels over a scale (Fig. 8.26). The depth and the volume of the indentation are dependent on the intraocular pressure and the distensibility of the ocular walls. There are four weights (5.5, 7.5, 10 and 15 g) that can be applied, and the greatest accuracy is attained with the weight by which the lever is deflected by 2–4 mm. FIG. 8.26 Tonometry: palpation and Schiøtz techniques. Source: From James R. Roberts. Chapter 62: Ophthalmologic Procedures. In: Kevin J. Knoop, William R. Dennis, eds. Roberts and Hedges’ Clinical Procedures in Emergency Medicine. 6th ed. Philadelphia: Saunders; 2014. pp. 1259–1297. The instrument is calibrated so that the equivalent readings in millimetres of mercury can be read off a chart. The Schiøtz tonometer is often inaccurate, largely because of wide individual variations in the rigidity of the corneoscleral coats. However, the tonometer is useful for obtaining approximate readings, particularly for comparative measurements such as between the two eyes or for successive measurements on th

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