Eye Anatomy

A Brief Anatomy of the Eye
Gray's Anatomy 39th

The eyeball, the peripheral organ of vision, is situated in a skeletal cavity, the orbit, the walls of which help to protect it from injury. The orbit also has a more fundamental role in the visual process itself, in providing a rigid support and direction to the eye and in forming the sites of attachment for its external muscles. This setting permits the accurate positioning of the visual axis under neuromuscular control, and determines the spatial relationship between the two eyes - essential for binocular vision and conjugate eye movements.

The eyeball is embedded in orbital fat, separated from it by a thin fascial sheath. It is composed of the segments of two spheres of different radii. The anterior segment, part of the smaller sphere, is transparent and forms c.7% of the surface of the whole globe. It is more prominent than the posterior segment, which is part of a larger sphere and opaque, and forms the remainder of the globe. The anterior segment is bounded by the cornea and the lens, and is incompletely subdivided into anterior and posterior chambers by the iris. These chambers are continuous through the pupil. The anterior chamber is slightly overlapped by the sclera peripherally. The angle between the iris and cornea therefore forms an annulus of greater diameter than the limbus, the junction between the sclera and cornea. The difference between these two varies from 1 to 2 mm, the angle being deeper above and below than at the sides of the eyeball. The posterior chamber lies between the posterior surface of the iris and the anterior aspect of the lens and its supporting ligament, the zonule, and is triangular in section. The apex of the triangle is the point where the iris touches the lens, and the base, or zonular region, extends among the collagenous bundles of the zonule, sometimes even into a retrozonular space between the zonule and the vitreous humour in the posterior segment of the eyeball. The posterior segment consists of the parts of the eye posterior to the zonule and lens.

The anterior pole is the centre of the anterior (corneal) curvature, and the posterior pole is the centre of its posterior (scleral) curvature; a line joining these two points forms the optic axis. (By the same convention, the eye has an equator, equidistant between the poles: any circumferential line joining the poles is a meridian.) The optic axes of the two eyes, in their primary position, are parallel and do not correspond with the orbital axes, which diverge anterolaterally at a marked angle to each other . The optic nerves follow the orbital axes and are therefore not parallel; each enters its eye c.3 mm medial (nasal) to the posterior pole. The ocular vertical diameter (23.5 mm) is rather less than the transverse and anteroposterior diameters (24 mm); the anteroposterior diameter at birth is c.17.5 mm and at puberty 20-21 mm; it may vary considerably in myopia (c.29 mm) and in hypermetropia (c.20 mm). In females all diameters are on average slightly less than in the male.

Figure 1 The organization of the eye, viewed from above. In this illustration the left eye and part of the lower eyelid are depicted in horizontal section and also cut away to show internal structure.

OCULAR FIBROUS TISSUE

The eye has three layers enclosing its contents. From the outer surface these are a fibrous layer, which consists of the sclera behind and the cornea in front; a vascular, pigmented layer which consists of (from behind forwards) the choroid, ciliary body and iris, collectively termed the uveal tract; and a neural layer, known as the retina.

The fibrous layer of the eyeball (Fig. 1) has an opaque posterior sclera and a transparent anterior cornea. Together these form the protective enclosing capsule of the eye, a semi-elastic structure which when made turgid by intraocular pressure, determines with great precision the optical geometry of the visual apparatus. The sclera also provides attachments for the extraocular muscles which rotate the eye, its smooth external surface rotating easily on the adjacent tissues of the orbit. The cornea admits light, refracts it towards a retinal focus, and plays an important role in the image-processing mechanism

OCULAR VASCULAR TUNIC

The vascular tunic, or uveal tract (Fig. 2), consists of the choroid, ciliary body and iris (Fig. 3), which collectively form a continuous structure. The choroid covers the internal scleral surface, and extends forwards to the ora serrata. The ciliary body continues forward from the choroid to the circumference of the iris, which is a circular diaphragm behind the cornea and in front of the lens. It presents an almost central aperture, the pupil

Figure 2 The vascular arrangements of the uveal tract. The long posterior ciliary arteries, one of which is visible (A), branch at the ora serrata (b) and feed the capillaries of the anterior part of the choroid. Short posterior ciliary arteries (C) divide rapidly to form the posterior part of the choriocapillaris. Anterior ciliary arteries (D) send recurrent branches to the choriocapillaris (e) and anterior rami to the major arterial circle (f). Branches from the circle extend into the iris (g) and to the limbus. Branches of the short posterior ciliary arteries (C) form an anastomotic circle (h) (of Zinn) round the optic disc, and twigs (i) from this join an arterial network on the optic nerve. The vorticose veins (J) are formed by the junctions (k) of suprachoroidal tributaries (l). Smaller tributaries are also shown (m, n). The veins draining the scleral venous sinus (o) join anterior ciliary veins and vorticose tributaries. (By permission from Hogan MJ, Alvarado JA, Weddell JE 1971 Histology of the Human Eye. Philadelphia: WB Saunders.)

Figure 3. Composite view of the surfaces and internal strata of the iris. In a clockwise direction from above, the pupillary (A) and ciliary (B) zones are shown in successive segments. The first (brown iris) shows the anterior border layer and the openings of crypts (c). In the second segment (blue iris), the layer is much less prominent and the trabeculae of the stroma are more visible. The third segment shows the iridial vessels, including the major arterial circle (e) and the incomplete minor arterial circle (f). The fourth segment shows the muscle stratum, including the sphincter (g) and dilator (h) of the pupil. The everted 'pupillary ruff' of the epithelium on the posterior aspect of the iris (d) appears in all segments. The final segment, folded over for pictorial purposes, depicts this aspect of the iris, showing radial folds (i and j) and the adjoining ciliary processes (k). (By permission from Hogan MJ, Alvarado JA, Weddell JE 1971 Histology of the Human Eye. Philadelphia: WB Saunders.)

RETINA

The retina is the sensory neural layer of the eyeball. It is a most complex structure and should be considered as a special area of the brain, from which it is derived by outgrowth from the diencephalon . It is dedicated to the detection and early analysis of visual information and is an integrated part of the much larger apparatus of visual analysis present in the thalamus, cortex and other areas of the central nervous system.

Layers of Retina

The retina is organized into layers or zones where distinctive components of its cells are clustered together or in register to form continuous strata. These layers extend uninterrupted throughout the photoreceptive retina except at the exit point of the optic nerve fibres at the optic disc, although certain layers are much reduced at the foveola where the photoreceptive elements predominate. The names given to the different layers reflect in part the components present within them, and also their position in the thickness of the retina. Conventionally, those structures furthest from the vitreous (i.e. towards the choroid) are designated as outer or external, and those towards the vitreous are inner or internal.

Customarily, ten retinal layers are distinguished (Fig. 4), beginning at the choroidal edge and passing towards the vitreous. These are: retinal pigment epithelium; layer of rods and cones (outer segments and inner segments); external limiting membrane; outer nuclear layer; outer plexiform layer (OPL); inner nuclear layer (INL); inner plexiform layer (IPL); ganglion cell layer; nerve fibre layer; internal limiting membrane. Some of these are subdivisible into substrata, and an innermost plexiform layer between layers 8 and 9 has also been demonstrated.


The composition of the different retinal layers is as follows:

Layer 1: Pigment epithelium

This is a simple low cuboidal epithelium which forms the back of the retina, and, therefore forms the boundary with the choroid, from which it is separated by a thick composite basal lamina.

Layer 2: Rod and cone cell processes

This contains the photoreceptive outer segments and the outer part of the inner segments of rod and cone cells.

Layer 3: External limiting membrane

This layer appears as a distinct line by light microscopy. It consists of a zone of intercellular junctions of the zonula adherens type (p. 7) between the processes of radial glial cells and photoreceptor processes.

Layer 4: Outer nuclear layer

This consists of several tiers of rod and cone cell bodies and their nuclei, the cone nuclei lying outermost. Mingled with these are the outer and inner fibres from the same cell bodies, directed outward to the bases of inner segments, and inwards towards the outer plexiform layer.

Layer 5: Outer plexiform layer

This is a region of complex synaptic arrangements between the processes of the cells whose cell bodies lie in the adjacent layers. The outer plexiform layer contains the synaptic processes of rod and cone cells, bipolar cells, horizontal cells, and some interplexiform cells (which in this account are grouped with the amacrines).

Layer 6: Inner nuclear layer

This is composed of three nuclear strata. Horizontal cell nuclei form the outermost zone, then in sequence inwards, the nuclei and cell bodies of bipolar cells, radial glial cells, and the outer set of amacrine cells, including the interplexiform cells whose dendrites cross this layer.

Layer 7: Inner plexiform layer

This is divisible into three layers depending on the types of contact occurring. The outer or 'OFF' layer contains synapses between 'OFF' bipolar cells, ganglion cells and some amacrines; a middle or 'ON' layer contains synapses between the axons of 'ON' bipolars and the dendrites of ganglion cells and displaced amacrines; and an inner 'rod' layer contains synapses between rod bipolars and displaced amacrines. (Refer to Wässle & Boycott 1991 for an explanation of the 'OFF' and 'ON' cell designations.)

Layer 8: Ganglion cell layer
This layer contains the nuclei of the displaced amacrine cells. Its inner regions consist of the cell bodies, nuclei and initial segments of retinal ganglion cells of various classes.

Layer 9: Nerve fibre layer

This contains the unmyelinated axons of retinal ganglion cells. It forms a zone of variable thickness over the inner retinal surface, and is the only component of the retina at the point where the fibres pass into the nerve at the optic disc. The inner aspect of this layer contains the nuclei and processes of astrocytes which, together with radial glial cells, ensheath the nerve fibres. Between the nerve fibre layer and the ganglion cells there is another narrow innermost plexiform layer where neuronal processes make synaptic contact with the axon hillocks and initial segments of ganglion cells.

Layer 10: Internal limiting membrane

This is a glial boundary between the retina and the vitreous body. It is formed by the end feet of radial glial cells and astrocytes, and is separated from the vitreous body by a basal lamina.

Figure 4 The layered arrangement of neuronal cell bodies in the retina and the interconnections of their processes in the intervening plexiform layers. Also shown are the two principal types of neuroglial cell in the retina; microglia are also present but not shown.

Optic disc

The optic disc is the region where retinal tissues meet the neural and glial elements of the optic nerve and the connective tissues of the sclera and meninges. It is the exit point for the optic nerve fibres, and a point of entry and exit for the retinal circulation. It is the only site where anastomoses occur with other arteries (the posterior ciliary arteries). It is visible, by ophthalmoscopy, and is a region of much clinical importance, since it is here that the central vessels can be inspected directly: the only vessels so accessible in the whole body. Oedema of the disc (papilloedema) may be the first sign of raised intracranial pressure, which is transmitted into the subarachnoid space around the optic nerve and compresses the central retinal vein where it crosses the space.

The optic disc is superomedial to the posterior pole of the eye, and so lies away from the visual axis. It is round or oval, usually c.1.6 mm in transverse diameter and 1.8 mm in vertical diameter, and its appearance is very variable (for details see Jonas et al 1988). In light-skinned subjects, the general retinal hue is a bright terracotta-red, with which the pale pink of the disc contrasts sharply; its central part is usually even paler and may be light grey. These differences are due in part to the degree of vascularization of the two regions, which is much less at the optic disc, and also to the total absence of choroidal or retinal pigment cells, since the retina is represented in the disc by little more than the internal limiting membrane. In subjects with strongly melanized skins, both retina and disc are darker . The optic disc does not project at all in many eyes, and rarely does it project sufficiently to justify the term papilla. It is usually a little elevated on its lateral side, where the papillomacular nerve fibres turn into the optic nerve. There is usually a slight depression where the retinal vessels traverse its centre.

RETINAL VASCULAR SUPPLY

The central retinal artery enters the optic nerve as a branch of the ophthalmic artery, c.1.2 cm behind the eyeball. It travels in the optic nerve to its head, where its fascicles traverse the lamina cribrosa. At this level, which is usually not visible to ophthalmoscopy, the central artery divides into two equal branches, superior and inferior. After a few millimetres, these divide into superior and inferior nasal, and superior and inferior temporal, branches. Each of these four supplies its own 'quadrant' of the retina, although each territory is much more than a quadrant, since the branches ramify as far as the ora serrata. Corresponding retinal veins unite to form the central retinal vein. However, the courses of the venous and arterial vessels do not correspond exactly, and arteries often cross veins, usually lying superficial to them. In severe hypertension the arteries may press on the veins and cause visible dilations distal to these crossings. Arterial pulsation is not visible by routine ophthalmoscopy without higher magnification.

The branching of the artery is usually dichotomous, and equal rami diverge at angles of 45-60°. Smaller branches may leave singly and at right angles. Arteries and veins ramify in the nerve fibre layer, near the internal limiting membrane, which accounts for their clarity when seen through an ophthalmoscope . Arterioles pass deeper into the retina and may penetrate to the internal nuclear lamina, from which venules return to larger superficial veins. The question of whether or not the dense capillary bed is diffusely organized or layered is unsettled. Some lamination has been identified, most noticeably at the interface between the inner nuclear and outer plexiform layers. The structure of the blood vessels resembles that of vessels elsewhere, except that the internal elastic lamina is absent from the arteries, and muscle cells may appear in their adventitia. Capillaries have a non-fenestrated endothelium.

OCULAR REFRACTIVE MEDIA

The components of the eye that transmit and refract light are the cornea, the aqueous humour, the lens and the vitreous body. Of these, only the refracting power of the lens can be varied.

Aqueous humour

To satisfy the requirements of vision the eye has its own circulatory system. Aqueous humour is secreted into the posterior chamber by the non-pigmented epithelium of the ciliary processes. It passes into the anterior chamber through the pupil and drains to the scleral venous sinus at the iridocorneal angle through the spaces of the trabecular tissue. It is responsible for maintaining the metabolism of the avascular transparent media, vitreous, lens and cornea, and it also maintains and regulates the relatively high intraocular pressure (c.17 mmHg), and hence the constancy of the ocular dimensions of the eyeball, via the balance between production and drainage. Depth of the anterior chamber may be assessed using slit-lamp biomicroscopy, and the filtration angle may be viewed directly by gonioscopy. Any interference with its drainage into the sinus increases intraocular pressure leading to the condition of glaucoma.

Lens

The lens is a transparent, encapsulated, biconvex body, which lies between the iris and the vitreous body. Posteriorly, the lens contacts the hyaloid fossa (p. 719) of the vitreous body. Anteriorly, it forms a ring of contact with the free border of the iris, but further away from the axis of the lens the gap between the two increases to form the posterior chamber of the eye (p. 708). The lens is encircled by the ciliary processes, and is attached to them by the zonular fibres which issue mainly from the pars plana of the ciliary body. Collectively, the fibres form the zonule which holds the lens in place and transmits the forces which stretch the lens (except in visual accommodation).

The lens has a characteristic shape. Its anterior convexity is less steep, and has a greater radius of curvature, than the posterior, which has a more parabolic shape. The central points of these surfaces are the anterior and posterior poles; a line connecting these is the axis of the lens. The marginal circumference of the lens is its equator. In fetuses the lens is nearly spherical, has a slight reddish tinge, and is soft, such that it breaks up on application of the slightest pressure. A hyaloid artery from the central retinal artery traverses the vitreous body to the posterior pole of the lens, whence its branches spread as a plexus. This covers the posterior surface and is continuous round the capsular circumference with the vessels of the pupillary membrane and iris.

In infants and adults the lens is avascular, colourless and transparent, but still quite soft in texture. In old age, the anterior surface becomes a little more curved, which pushes the iris forward slightly. It becomes less clear, with an amber tinge, and its nucleus is denser. In cataract, the lens gradually becomes opaque, causing blindness.

The dimensions of the lens are optically and clinically important, but they change with age as a consequence of continuous growth. Its equatorial diameter at birth is 6.5 mm, increasing rapidly at first, then more slowly to 9.0 mm at 15 years of age, and even more gradually to reach 9.5 mm in the ninth decade. Its axial dimension increases from 3.5-4.0 mm at birth to 4.75-5.0 mm at age 95. The radii of curvature reduce throughout life; the anterior surface shows the greater change as the lens thickens (Brown 1974). Average adult radii of the anterior and posterior surfaces are 10 mm and 6 mm respectively; the reduction during accommodation occurs mainly at the anterior surface.

Vitreous body

The vitreous body fills the vitreous chamber, and occupies about four-fifths of the eyeball. It is hollowed in front as a deep concavity, the hyaloid fossa, which is adapted to the lens. It is colourless, consisting of c.99% water, but not entirely structureless. At its perimeter it has a gel-like consistency (100-300μm thick) and is firmly attached to the surrounding structures of the eye; nearer the centre it has a more liquid zone in the form of long glycosaminoglycan chains, fills the whole vitreous. In addition, the peripheral gel or cortex contains a random loose network of type II collagen fibrils which are occasionally grouped into fibres. The cortex also contains scattered cells, the hyalocytes, which possess the characteristics of mononuclear phagocytes. They are responsible for the production of . Whilst they are normally in a resting state, they have the capacity to be actively phagocytic in inflammatory conditions. Hyalocytes are not present in the cortex bordering the lens. The liquid vitreous is absent at birth, appears first at 4 or 5 years, and increases to occupy half the vitreous space by the seventh decade. The cortex is most dense at the pars plana of the ciliary body adjacent to the ora serrata, where attachment is strongest, and this is often referred to as the base of the vitreous. Here the vitreous is thickened into a mass of radial (zonular) fibres which form the suspensory ligament of the lens

A narrow hyaloid canal runs from the optic nerve head to the central posterior surface of the lens. In the fetus this contains the hyaloid artery which normally disappears about 6 weeks before birth. It persists as a very delicate fibrous structure and is of no functional importance.

Retinal Detachment

Retinal Detachment and Related Retinal Degenerations
Vaughan & Asbury's General Ophthalmology 17th Edition

Retinal detachment is the separation of the sensory retina, ie, the photoreceptors and inner tissue layers, from the underlying retinal pigment epithelium. There are three main types: rhegmatogenous, traction, and serous or hemorrhagic detachment.

Rhegmatogenous Retinal Detachment

The most common type of retinal detachment, rhegmatogenous retinal detachment is characterized by a full-thickness break (a "rhegma") in the sensory retina, variable degrees of vitreous traction, and passage of liquefied vitreous through the break into the subretinal space. A spontaneous rhegmatogenous retinal detachment is usually preceded or accompanied by a posterior vitreous detachment and is associated with myopia, aphakia, lattice degeneration, and ocular trauma. Binocular indirect ophthalmoscopy with scleral depression reveals elevation of the translucent detached sensory retina with one or more full-thickness sensory retinal breaks, such as a horseshoe tear, round atrophic hole, or anterior circumferential tear (retinal dialysis). The location of retinal breaks varies according to type; horseshoe tears are most common in the superotemporal quadrant, atrophic holes in the temporal quadrants, and retinal dialysis in the inferotemporal quadrant. When multiple retinal breaks are present, the defects are usually within 90 degrees of one another.

Figure 1. Retinal Detachment

Treatment

The principal aims of detachment surgery are to find and treat all the retinal breaks, cryotherapy or laser being applied to create an adhesion between the pigment epithelium and the sensory retina, thus preventing any further influx of fluid into the subretinal space, to drain subretinal fluid, internally or externally, and relieve vitreo-retinal traction. Various surgical techniques are employed.

In pneumatic retinopexy air or expandable gas is injected into the vitreous to maintain the retina in position, while the chorioretinal adhesion induced by laser or cryotherapy achieves permanent closure of the retinal break. It has a lower success rate than other methods and is used only when there is a small accessible single retinal break, minimal subretinal fluid, and no vitreo-retinal traction.

Scleral buckling maintains the retina in position, while the chorioretinal adhesion forms, by indenting the sclera with a sutured explant in the region of the retinal break. This also relieves vitreo-retinal traction and displaces subretinal fluid away from the retinal break. The success rate is 92–94% in suitably selected cases. Complications include change in refractive error, diplopia due to fibrosis or involvement of extraocular muscles in the explant, extrusion of the explant, and possibly increased risk of proliferative vitreoretinopathy.

Pars plana vitrectomy allows relief of vitreo-retinal traction, internal drainage of subretinal fluid, if necessary by injection of perfluorocarbons or heavy liquids, and injection of air or expandable gas to maintain the retina in position, or injection of oil if longer-term tamponade or the retina is required. It is used if there are superior, posterior, or multiple retinal breaks, when visualization of the retina is inhibited, such as by vitreous hemorrhage, and if there is significant proliferative vitreoretinopathy. Vitrectomy induces cataract formation and may be contraindicated in phakic eyes. Postoperative posturing may be required.

The visual results of surgery for rhegmatogenous retinal detachment primarily depend on the preoperative status of the macula. If the macula has been detached, recovery of central vision is usually incomplete. Thus, surgery should be performed urgently if the macula is still attached. Once the macula is detached, delay in surgery for up to 1 week does not adversely influence visual outcome.

Traction Retinal Detachment

Traction retinal detachment is most commonly due to proliferative diabetic retinopathy. It can also be associated with proliferative vitreoretinopathy, retinopathy of prematurity, or ocular trauma. In comparison to rhegmatogenous retinal detachment, traction retinal detachment has a more concave surface and is likely to be more localized, usually not extending to the ora serrata. The tractional forces actively pull the sensory retina away from the underlying pigment epithelium toward the vitreous base. Traction is due to formation of vitreal, epiretinal, or subretinal membranes consisting of fibroblasts and glial and retinal pigment epithelial cells. Initially the detachment may be localized along the vascular arcades, but progression may spread to involve the midperipheral retina and the macula. Focal traction from cellular membranes can produce a retinal tear and lead to combined traction-rhegmatogenous retinal detachment.

Proliferative vitreoretinopathy is a complication of rhegmatogenous retinal detachment and is the most common cause of failure of surgical repair in these eyes.

Figure 2. Retinal Detachment

Treatment

Pars plana vitrectomy allows removal of the tractional elements followed by removal of the fibrotic membranes. Retinotomy and/or injection of perfluorocarbons or heavy liquids may be required to flatten the retina. Gas tamponade, silicone oil, or scleral buckling may be used.

Serous & Hemorrhagic Retinal Detachment

Serous and hemorrhagic retinal detachment occurs in the absence of either retinal break or vitreoretinal traction. They form as a result of accumulation of fluid beneath the sensory retina and are caused primarily by diseases of the retinal pigment epithelium and choroid. Degenerative, inflammatory, and infectious diseases, including the multiple causes of subretinal neovascularization, may be associated with serous retinal detachment and are described in an earlier section of this textbook. This type of detachment may also be associated with systemic vascular and inflammatory disease, or intraocular tumors.

Lattice Degeneration

Lattice degeneration is the most common vitreoretinal degeneration. The estimated incidence in the general population is 6–10%, of which up to 50% have bilateral disease. It is more commonly found in myopic eyes with some familial tendency. It produces localized round, oval, or linear areas of retinal thinning, with pigmentation, branching white lines, and whitish-yellow flecks, and firm vitreoretinal adhesions at its margins. Lattice degeneration results in retinal detachment in only a small percentage of affected eyes, but 20–30% of eyes with retinal detachment have lattice degeneration. Strong family history of retinal detachment, retinal detachment in the fellow eye, high myopia, and aphakia require the patient to be informed of the risks of retinal detachment and the relevant symptoms but rarely warrant prophylactic treatment with cryosurgery or laser photocoagulation.

Peripheral Chorioretinal Atrophy

Peripheral chorioretinal atrophy (paving stone degeneration) is a common benign chorioretinal degeneration found in nearly one-third of adult eyes. It is thought to be due to choroidal vascular insufficiency and is associated with peripheral vascular disease. The lesions appear as isolated or grouped, small, discrete, yellow-white areas with prominent underlying choroidal vessels and pigmented borders.

Retinoschisis

Degenerative retinoschisis is a common acquired peripheral retinal disorder that is believed to develop from coalescence of preexisting peripheral cystoid degeneration. The cystic elevation is most commonly found in the inferotemporal quadrant, followed by the superotemporal quadrant. It develops into one of two forms, typical or reticular, although clinically the two are difficult to differentiate.

Typical degenerative retinoschisis forms a round or ovoid area of retinal splitting in the outer plexiform layer. Posterior extension and hole formation in the outer layer is uncommon and therefore poses low risk of progression to retinal detachment.

Reticular degenerative retinoschisis is characterized by round or oval areas of retinal splitting in the nerve fiber layer forming a bullous elevation of an extremely thin inner layer. Retinal holes occur in 23%, and posterior extension or progression to rhegmatogenous retinal detachment may occur and requires treatment.

Natural History

Degenerative retinoschisis is present in about 4% of the population and is bilateral in approximately 30% of affected individuals. Spontaneous regression occurs in up to 9% of cases. Progression to retinal detachment occurs in up to 2%, with increased risk for those with a family history of retinal detachment. Whether cataract extraction increases the risk of retinal detachment is uncertain. Retinal detachment occurs in one of two ways. A hole in the outer but not the inner retinal layer allows the cystic fluid through the defect. This type is usually not or is only slowly progressive, and therefore a demarcation line forms. It rarely requires treatment. In the second type, holes form in both the inner and the outer layers. This causes collapse of the schisis and full retinal detachment forms. Progression is quick, and treatment is required by pneumatic retinopexy, scleral buckle, or vitrectomy, depending on the size and position of the retinal holes and whether there is any proliferative vitreoretinopathy.

Differentiation from Retinal Detachment

Retinoschisis causes an absolute scotoma in the visual field, whereas retinal detachment causes a relative scotoma. The cystic elevation of retinoschisis is usually smooth with no associated vitreous pigment cells. The surface of retinal detachment is usually corrugated with pigment cells in the vitreous ("tobacco dust"). Longstanding retinal detachment produces atrophy of the underlying retinal pigment epithelium, resulting in a pigmented demarcation line. As the retinal pigment epithelium is healthy in retinoschisis, there is no demarcation line. If argon laser photocoagulation to the outer retinal layer, aimed through an inner layer break, creates an equal gray response as in an adjacent area of normal retina, this is thought to be diagnostic of retinoschisis.

Cataract

Cataract
Vaughan & Asbury's General Ophthalmology 17th Edition

A cataract is any opacity in the lens. Aging is the most common cause of cataract, but many other factors can be involved, including trauma, toxins, systemic disease (such as diabetes), smoking, and heredity. Age-related cataract is a common cause of visual impairment. Cross-sectional studies place the prevalence of cataracts at 50% in individuals aged 65-74; the prevalence increases to about 70% for those over 75.

The pathogenesis of cataracts is not completely understood. However, cataractous lenses are characterized by protein aggregates that scatter light rays and reduce transparency. Other protein alterations result in yellow or brown discoloration. Additional findings may include vesicles between lens fibers or migration and aberrant enlargement of epithelial cells. Factors thought to contribute to cataract formation include oxidative damage (from free radical reactions), ultraviolet light damage, and malnutrition. No medical treatment has been found that will retard or reverse the underlying chemical changes that occur in cataract formation. However, some recent evidence suggests a protective effect from dietary carotenoids (lutein), but studies evaluating the protective effect of multivitamins have yielded conflicting results.

A mature cataract is one in which all of the lens protein is opaque; the immature cataract has some transparent protein. If the lens takes up water, it may become intumescent. In the hypermature cataract, cortical proteins have become liquid. This liquid may escape through the intact capsule, leaving a shrunken lens with a wrinkled capsule. A hypermature cataract in which the lens nucleus floats freely in the capsular bag is called a morgagnian cataract.

Most cataracts are not visible to the casual observer until they become dense enough to cause severe vision loss. The ocular fundus becomes increasingly more difficult to visualize as the lens opacity becomes denser, until the fundus reflection is completely absent. At this stage, the cataract is usually mature, and the pupil may be white.

The clinical degree of cataract formation, assuming that no other eye disease is present, is judged primarily by the Snellen visual acuity test. Generally speaking, the decrease in visual acuity is directly proportionate to the density of the cataract. However, some individuals who have clinically significant cataracts when examined with the ophthalmoscope or slitlamp see well enough to carry on with normal activities. Others have a decrease in visual acuity out of proportion to the degree of lens opacification. This is due to distortion of the image by the partially opaque lens. The Cataract Management Guideline Panel recommends reliance on clinical judgment combined with Snellen acuity as the best guide to the appropriateness of surgery but recognizes the need for flexibility, with due regard to a patient's particular functional and visual needs, the environment, and other risks, all of which may vary widely.

AGE-RELATED CATARACT

(Figures 1, and 2)

The normal condensation process in the lens nucleus results in nuclear sclerosis after middle age. The earliest symptom may be improved near vision without glasses ("second sight"). This occurs from an increase in the focus power of the central lens, creating a myopic (near-sighted) shift in refraction. Other symptoms may include poor hue discrimination or monocular diplopia. Most nuclear cataracts are bilateral but may be asymmetric.

Figure 1. Age-related cataract. A and B: "Coronary" type cortical cataract (frontal and cross-sectional views): club-shaped peripheral opacities with clear central lens; slowly progressive. C: "Cuneiform" type cortical cataract: peripheral spicules and central clear lens; slowly progressive. D: Nuclear sclerotic cataract: diffuse opacity principally affecting nucleus; slowly progressive. E: Posterior subcapsular cataract: plaque of granular opacity on posterior capsule; may be rapidly progressive. F: "Morgagnian" type (hypermature lens): the entire lens is opaque, and the lens nucleus has fallen inferiorly.

Figure 2. Age-related cataract. In the photo at right the scene shown at left is reproduced as if seen by a person with a moderately advanced senile cataract (opacity denser centrally).

Cortical cataracts are opacities in the lens cortex. Changes in the hydration of lens fibers create clefts in a radial pattern around the equatorial region. They also tend to be bilateral, but they are often asymmetric. Visual function is variably affected, depending on how near the opacities are to the visual axis.

Posterior subcapsular cataracts are located in the cortex near the central posterior capsule. They tend to cause visual symptoms earlier in their development owing to involvement of the visual axis. Common symptoms include glare and reduced vision under bright lighting conditions. This lens opacity can result also from trauma, corticosteroid use (topical or systemic), inflammation, or exposure to ionizing radiation.

Age-related cataract is usually slowly progressive over years, and death may occur before surgery becomes necessary. If surgery is indicated, lens extraction definitely improves visual acuity in over 90% of cases. The remainder of patients either have preexisting retinal damage or develop serious postsurgical complications that prevent significant visual improvement, eg, glaucoma, retinal detachment, intraocular hemorrhage, or infection. Intraocular lenses have made adjustment following cataract operation much easier than when only thick cataract glasses or aphakic contact lenses were available.

CHILDHOOD CATARACT

(Figures 3 and 4)

Childhood cataracts are divided into two groups: congenital (infantile) cataracts, which are present at birth or appear shortly thereafter, and acquired cataracts, which occur later and are usually related to a specific cause. Either type may be unilateral or bilateral.

Figure 3. Congenital cataract.

Figure 4. Congenital cataract, zonular type. One zone of lens involved. The cortex is relatively clear.

About one-third of cataracts are hereditary, while another third are secondary to metabolic or infectious diseases or associated with a variety of syndromes. The final one-third result from undetermined causes. Acquired cataracts arise most commonly from trauma, either blunt or penetrating. Other causes include uveitis, acquired ocular infections, diabetes, and drugs.

Clinical Findings

Congenital Cataract

Congenital lens opacities are common and often visually insignificant. A partial opacification or one out of the visual axis—or not dense enough to interfere significantly with light transmission—requires no treatment other than observation for progression. Dense central congenital cataracts require surgery.

Congenital cataracts that cause significant visual loss must be detected early, preferably in the newborn nursery by the pediatrician or family physician. Large, dense white cataracts may present as leukocoria (white pupil), noticeable by the parents, but many dense cataracts cannot be seen by the parents. Unilateral infantile cataracts that are dense, central, and larger than 2 mm in diameter will cause permanent deprivation amblyopia if not treated within the first 2 months of life and thus require surgical management on an urgent basis. Even then there must be careful attention to avoidance of amblyopia related to postoperative anisometropia. Symmetric (equally dense) bilateral cataracts may require less urgent management, although bilateral deprivation amblyopia can result from unwarranted delay. When surgery is undertaken, there must be as short an interval as is reasonably possible between surgery on the two eyes.

Acquired Cataract

Acquired cataracts do not require the same urgent care (aimed at preventing amblyopia) as infantile cataracts because the children are older and the visual system more mature. Surgical assessment is based on the location, size, and density of the cataract, but a period of observation along with subjective visual acuity testing can be part of the decision-making process. Because unilateral cataracts in children will not produce any symptoms or signs parents would routinely notice, screening programs are important for case finding.

Treatment

Surgical treatment of infantile and early childhood cataracts involves lens extraction through a small limbal incision utilizing a mechanical irrigation-aspiration handpiece. Phacoemulsification is rarely required. In contrast to the procedure used for adult lens extraction, the posterior capsule and anterior vitreous are removed by many surgeons using a mechanical vitreous suction-cutting instrument. This prevents formation of secondary capsular opacification or after-cataract (see below). Primary removal of the posterior capsule thus avoids the necessity for secondary surgery and enhances early optical correction.

Using today's sophisticated surgical techniques, operative and postoperative complications are similar to those reported with adult cataract procedures. Optical correction can consist of spectacles in older bilaterally aphakic children, but most childhood cataract operations are followed by contact lens correction. The use of intraocular lenses in early childhood is becoming increasingly frequent. It may lessen the difficulty of optical rehabilitation associated with contact lenses in children, but there are difficulties calculating the appropriate power of intraocular lens, which may need to be changed as the eye develops.

Prognosis

The visual prognosis for childhood cataract patients requiring surgery is not as good as that for patients with age-related cataract. The associated amblyopia and occasional anomalies of the optic nerve or retina limit the degree of useful vision that can be achieved in this group of patients. The prognosis for improvement of visual acuity is worst following surgery for unilateral congenital cataracts and best for incomplete bilateral congenital cataracts that are slowly progressive.

TRAUMATIC CATARACT

Traumatic cataract (Figures 5) is most commonly due to a foreign body injury to the lens or blunt trauma to the eyeball. Air rifle pellets and fireworks are a frequent cause; less-frequent causes include arrows, rocks, contusions, overexposure to heat ("glassblower's cataract"), and ionizing radiation. Most traumatic cataracts are preventable. In industry, the best safety measure is a good pair of safety goggles.

Figure 5. A. Traumatic "star-shaped" cataract in the posterior lens. This is usually due to ocular contusion and is only detectable through a well-dilated pupil. B. Traumatic cataract with wrinkled anterior capsule. C. Imprint of iris pigment on anterior surface of lens.

The lens becomes white soon after the entry of a foreign body, since interruption of the lens capsule allows aqueous and sometimes vitreous to penetrate into the lens structure. The patient is often an industrial worker who gives a history of striking steel upon steel. A minute fragment of a steel hammer, for example, may pass through the cornea and lens at a tremendous rate of speed and lodge in the vitreous or retina.

CATACARACT SECONDARY TO INTRAOCULAR DISEASE ("COMPLICATED CATARACT ")

Cataract may develop as a direct effect of intraocular disease upon the physiology of the lens (eg, severe recurrent uveitis). The cataract usually begins in the posterior subcapsular area and eventually involves the entire lens structure. Intraocular diseases commonly associated with the development of cataracts are chronic or recurrent uveitis, glaucoma, retinitis pigmentosa, and retinal detachment. These cataracts are usually unilateral. The visual prognosis is not as good as in ordinary age-related cataract.

CATARACT ASSOCIATED WITH SYSTEMIC DISEASE

Bilateral cataracts may occur in association with the following systemic disorders: diabetes mellitus (Figure 6), hypocalcemia (of any cause), myotonic dystrophy, atopic dermatitis, galactosemia, and Lowe's, Werner's, and Down's syndromes.

Figure 6. Punctate dot cataract. This type of cataract is sometimes seen as an ocular complication of diabetes mellitus. It may also be congenital.

AFTER-CATARACT (SECONDARY MEMBRANE)

After-cataract (Figure 7) denotes opacification of the posterior capsule following extracapsular cataract extraction. Persistent subcapsular lens epithelium may favor regeneration of lens fibers, giving the posterior capsule a "fish egg" appearance (Elschnig's pearls). The proliferating epithelium may produce multiple layers, leading to frank opacification. These cells may also undergo myofibroblastic differentiation. Their contraction produces numerous tiny wrinkles in the posterior capsule, resulting in visual distortion. All of these factors may lead to reduced visual acuity following extracapsular cataract extraction.

Figure 7. After-cataract.

After-cataract is a significant problem in almost all pediatric patients unless the posterior capsule and anterior vitreous are removed at the time of surgery. In the past, up to one-half of all adult patients developed an opacified posterior capsule after extracapsular cataract extraction. However, improved surgical techniques and new intraocular lens materials have significantly reduced the incidence of posterior capsule opacity.

The neodymium:YAG laser provides a noninvasive method for discission of the posterior capsule. Pulses of laser energy cause small "explosions" in target tissue, creating a small hole in the posterior capsule in the pupillary axis. Complications of this technique include a transient rise in intraocular pressure, damage to the intraocular lens, and rupture of the anterior hyaloid face with forward displacement of vitreous into the anterior chamber, potentially leading to rhegmatogenous retinal detachment or cystoid macular edema. The rise in intraocular pressure is usually detectable within 3 hours after treatment and resolves within a few days with treatment. Rarely, the pressure does not return to normal for several weeks. Small pits or cracks may occur on the intraocular lens but usually have no effect on visual acuity. No significant damage seems to be done to corneal endothelium with the neodymium:YAG laser.

Cataract Surgery

Cataract surgery has undergone dramatic change during the past 30 years with the introduction of the operating microscope and microsurgical instruments, the development of intraocular lenses, and alterations in techniques for local anesthesia. Further refinements continue to occur, with automated instrumentation and modifications of intraocular lenses allowing surgery through small incisions.

The generally preferred method of cataract surgery in adults and older children preserves the posterior portion of the lens capsule and thus is known as extracapsular cataract extraction. Intraocular lens implantation is part of this procedure. An incision is made at the limbus or in the peripheral cornea, either superiorly or temporally. An opening is formed in the anterior capsule, and the nucleus and cortex of the lens are removed. The intraocular lens is then placed in the empty "capsular bag," supported by the intact posterior capsule. In the nuclear expression form of extracapsular cataract extraction, the nucleus is removed intact, but this requires a relatively large incision. The cortex is removed by manual or automated aspiration. The technique of phacoemulsification is now the most common form of extracapsular cataract extraction. It utilizes a handheld ultrasonic vibrator to disintegrate the hard nucleus such that the nuclear material and cortex can be aspirated through an incision of approximately 3 mm. This same incision size is then adequate for insertion of foldable intraocular lenses. If a rigid intraocular lens is used, the wound needs to be extended to approximately 5 mm. The advantages of small-incision surgery are more controlled operating conditions, avoidance of suturing, rapid wound healing with lesser degrees of corneal distortion, and reduced postoperative intraocular inflammation—all contributing to more rapid visual rehabilitation. The phacoemulsification technique does, however, entail a higher risk of posterior displacement of nuclear material through a posterior capsular tear, which generally necessitates complex vitreoretinal surgery. After all forms of extracapsular cataract surgery, there may be secondary opacification of the posterior capsule that requires discission using the neodymium:YAG laser (see After-Cataract, above). Lens extraction through the pars plana during posterior vitrectomy is called pars plana lensectomy or phacofragmentation. This type of cataract removal is usually performed in conjunction with the removal of an opaque or scarred vitreous.

Intracapsular cataract extraction, consisting of removal of the entire lens together with its capsule, is rarely performed today. The incidence of postoperative retinal detachment and cystoid macular edema is significantly higher than after extracapsular surgery, but intracapsular surgery is still a useful procedure, particularly when facilities for extracapsular surgery are not available.

Intraocular Lens

There are many styles of intraocular lenses, but most designs consist of a central biconvex optic and two legs (or haptics) to maintain the optic in position. The optimal intraocular lens position is within the capsular bag following an extracapsular procedure. This is associated with the lowest incidence of postoperative complications, such as pseudophakic bullous keratopathy, glaucoma, iris damage, hyphema, and lens decentration. The newest posterior chamber lenses are made of flexible materials such as silicone and acrylic polymers. This flexibility allows the lens implant to be folded, thus decreasing the required incision size. Lens designs that incorporate multifocal optics have also been produced. The goal of this design is to provide the patient with good vision for both near and distance without glasses, which current monofocal designs are unable to do.

After intracapsular surgery—or if there is inadvertent damage to the posterior capsule during extracapsular surgery—intraocular lenses can be placed in the anterior chamber or sometimes fixated in the ciliary sulcus.

Methods of calculating the correct dioptric power of an intraocular lens are discussed in other chapter. If an intraocular lens cannot be safely placed or is contraindicated, postoperative refractive correction generally requires a contact lens or aphakic spectacles.

Postoperative Care

If a small-incision technique is used, the postoperative recovery period is usually shortened. The patient is usually ambulatory on the day of surgery but is advised to move cautiously and avoid straining or heavy lifting for about a month. The eye may be patched on the day of surgery. Protection at night by a metal shield is often suggested for several days after surgery. Temporary glasses can be used a few days after surgery, but in most cases the patient sees well enough through the intraocular lens to wait for permanent glasses (usually provided 4-8 weeks after surgery).

Blindness

BLINDNESS
Vaughan & Asbury's General Ophthalmology> 17th Edition> Chapter 23. Blindness >

Introduction

In this chapter we will discuss blindness as a worldwide health problem, summarizing information about its epidemiology, emphasizing the value of community-based methods to prevent or treat its causes, and outlining resources available in more developed countries for rehabilitation of the blind. All of the disorders that may cause blindness are discussed more fully in other parts of this book.

Definition of Blindness

The World Health Organization (WHO) defines visual impairment as shown in Table 1. WHO officials encourage investigators and reporting agencies in all countries to report blindness and visual disability according to the categories defined in this table.

Table 1. Categories of Visual Impairment (Adapted from International Classification of Diseases, World Health Organization, 1977).

Category of Visual Impairment Visual Acuity (Best Corrected) Low Vision 1 6/18 3/10 (0.3) 20/70 2 6/60 1/10 (0.1) 20/200 Blindness 3 3/60 (finger counting at 3 m) 1/20 (0.05) 20/400 4 1/60 (finger counting at 1 m) 1/50 (0.02) 5/300 5 No light perception

Visual field

Patients with a visual field radius no greater than 10 degrees but greater than 5 degrees around central fixation should be placed in category 3 and patients with a field no greater than 5 degrees around the central fixation in category 4—even if the central acuity is not impaired.

In the United States, the most widely used definition of partial blindness is that used by the Internal Revenue Service for the purpose of determining who is eligible for tax deductions on that basis: central visual acuity 20/200 or less in the better eye with best correction, or widest diameter of visual field subtending an angle of no greater than 20 degrees. An alternative functional definition is loss of vision sufficient to prevent one from being self-supporting in an occupation, making the individual dependent on other persons, agencies, or devices in order to live.

"Industrial blindness" is said to be present when a worker can no longer pursue an occupation because of poor vision; "automobile blindness" when vision is so poor that the responsible licensing agency in that state will not issue a driver's license. The term color blindness is a misnomer since this genetically transmitted disorder is not true blindness as the term is generally understood and implies only a minor visual aberration. Loss of vision may affect only the central fields, only the peripheral fields, or only specific portions of the peripheral fields in one or both eyes. Total loss of vision in one eye is said to reduce visual capacity by only 10%, although it makes the other eye infinitely more valuable.

Prevalence of Blindness Throughout the World

WHO estimates that there are more than 50 million blind people in the world today, with at least 135 million suffering significant visual disability. Even where health statistics are most reliable, the methods of counting the blind are often crude and may be applied according to different criteria in different places and at different times within any extensive geographic area. Because of these limitations, extrapolations are often made from small sample studies to large populations. Ninety percent of the world's blind live in developing countries, mostly in Asia (approximately 20 million) and Africa (approximately 6 million), clustered largely in disadvantaged communities in rural areas and urban slums. The risk of blindness in many of these neglected communities is 10–40 times higher than in the industrially developed regions of Europe and America.

Table 2 lists some countries where fairly reliable data are available about the prevalence of blindness.

Table 2. Approximate Prevalence of Blindness (%). (Estimates Based on WHO Surveys.)1

Chad 3.2–5 Malawi 1 Liberia 3.2 Brazil 0.3 Egypt 2.6 Mexico 0.3 Philippines 2.1 Australia 0.2 Afghanistan 2 Japan 0.2 Bangladesh 2 USA 0.2 Pakistan 2 UK 0.18 Saudi Arabia 2 Canada 0.15 India 1.5 USSR 0.12 Indonesia 1.3 China 0.1 Chile 1 Germany 0.1

¹Based on available data, 1969–1980. Some data were only rough estimates when obtained and may have changed markedly since then. In some cases, the survey criteria used did not correspond to WHO definitions. Data taken from Maitchouk IF: Data on blindness: Prevalence and causes throughout the world. In: Lim ASM, Jones BR (editors); World's major blinding conditions. Vision 1982;1:99. (International Agency for the Prevention of Blindness.)

Causes of Blindness & Methods of Prevention & Treatment

The relative importance of various causes of blindness differs according to the level of social development in the geographic area being studied. In developing countries, cataract is the leading cause, with trachoma, glaucoma, leprosy, onchocerciasis, and xerophthalmia also being important. Corneal ulceration is also a significant cause of monocular blindness in the developing world. In more developed countries, blindness is to a great extent related to the aging process. Cataract is still important despite the availability of facilities for its treatment, along with age-related macular degeneration and glaucoma. Other causes are diabetic retinopathy, herpes simplex keratitis, retinal detachment, and inherited retinal degenerative disorders.

In terms of the worldwide prevalence of blindness, the vastly greater number of people in the developing world and the greater likelihood of their being affected mean that the causes of blindness in those areas are numerically more important. Cataract is responsible for more than 22 million cases of blindness and glaucoma 6 million, while leprosy and onchocerciasis each blind approximately 1 million individuals worldwide. Interestingly, the number of individuals blind from trachoma has dropped dramatically in the past 10 years from 6 million to 1.3 million, putting it in seventh place on the list of causes of blindness worldwide. Xerophthalmia is estimated to affect 5 million children each year; 500,000 develop active corneal involvement, and half of these go blind. Central corneal ulceration is also a significant cause of monocular blindness worldwide, accounting for an estimated 850,000 cases of corneal blindness every year in the Indian subcontinent alone. As a result, corneal scarring from all causes now is the fourth greatest cause of global blindness.

WHO estimates that up to 80% of blindness in developing countries is avoidable, ie, preventable or treatable. The worldwide eradication of smallpox demonstrates what can be achieved in the area of infectious disease and the superiority of prevention over treatment. Similar efforts are being made to prevent the infectious diseases trachoma, leprosy, and onchocerciasis as well as the noninfectious xerophthalmia. The sheer numbers of individuals blinded by cataract continues to overwhelm the resources available. In all programs to reduce blindness in the developing world, cooperation between governments and nongovernmental charitable organizations has proved to be essential. The WHO Prevention of Blindness Program has established centers in about 60 developing countries to undertake collaborative studies, particularly generating epidemiologically sound information to form the basis for rational planning, implementation, and proper evaluation of programs for prevention of blindness.

In more developed countries, the causes of blindness are less amenable to prevention. In general, it is necessary to rely on recognition and treatment of the early stages of the disease. This depends on education of ophthalmologists, nonophthalmologic medical personnel, and lay people about the necessity for screening for glaucoma and diabetic retinopathy and about the importance of the early symptoms of retinal detachment, age-related macular degeneration, and herpes simplex keratitis. The inherited conditions are amenable to prevention by genetic counseling.

Cataract

Cataract accounts for at least 50% of cases of blindness worldwide. As life expectancy increases, there is a continuing rise in the total number of people affected. In many parts of the developing world, the facilities available for treating cataract are grossly inadequate, hardly sufficient to cope with the new cases arising and completely inadequate for dealing with the backlog of existing cases, which is conservatively estimated to be 10 million worldwide.

It is not clearly understood why the frequency of cataract in different geographic areas varies so greatly, although exposure to ultraviolet radiation and recurrent episodes of dehydration, such as occur in severe diarrheal diseases, are thought to be important. If medical means could be found to delay the development of cataract by 10 years, it is estimated that this would reduce the number of individuals requiring surgery by 45%. Unfortunately, there is no method of preventing or retarding the growth of cataracts. Although the oral administration of antioxidants was considered promising, clinical studies have now shown conclusively that they have no effect on cataract growth. Mobile eye camps have aided in identifying patients for surgery, but surgery is no longer performed in a camp setting and there are too few well-equipped hospitals and trained surgeons in many developing countries to keep up with the load. In a number of blindness surveys, the problem of uncorrected aphakia is particularly apparent. It is now accepted that intraocular lens implantation at the time of surgery, although requiring greater expertise, is a better solution than relying on the subsequent provision of spectacles.

Trachoma

Trachoma causes bilateral keratoconjunctivitis, generally in childhood, that leads in adulthood to corneal scarring, which, when severe, causes blindness. About 400 million people have trachoma, most of them in Africa, the Middle East, and Asia. Trachoma can be treated with various antibiotics, including tetracyclines and erythromycin, but azithromycin is proving to be the drug of choice. It is estimated that 70 million individuals currently require treatment, but in the past 10 years, the number of individuals blind from trachoma has dropped from 6 million to 1.3 million. This is an obvious tribute to current WHO-supervised treatment programs and probably to the effectiveness of azithromycin. However, to eliminate the disease will depend on global implementation of WHO's SAFE strategy (surgery for trichiasis, antibiotic treatment, face washing, and environmental changes such as latrine building). Prevention of spread of infection will require provision of proper sanitary facilities, including clean water for drinking and washing, waste disposal, fly control, and behavioral change in hygiene.

Leprosy

Leprosy (Hansen's disease) affects 14 million people in the world and has a higher percentage of ocular involvement than any other systemic disease. Up to 10% of leprosy patients are blind or visually impaired from the disease. The social stigma attached to leprosy has greatly hindered its treatment, but there are now highly effective chemotherapeutic agents that in most cases eradicate the infection. Effective treatment programs using triple drug therapy (dapsone, clofazamine, and rifampin) have markedly reduced the number of cases of leprosy worldwide as well as prevented the deformity and morbidity associated with the disease.

Onchocerciasis

Onchocerciasis is transmitted by bites of the blackfly, which breeds in clear running streams (hence the name river blindness). It is endemic in the greater part of tropical Africa and Central and South America. The most heavily infested zone is the Volta River basin, which extends over parts of Dahomey, Ghana, Ivory Coast, Mali, Niger, Togo, and Upper Volta. Worldwide, 15–20 million people are affected by onchocerciasis, with 20% of individuals in hyperendemic areas blinded by the disease.

The major ophthalmic manifestations of onchocerciasis are keratitis, uveitis, retinochoroiditis, and optic atrophy. The disease is prevented by insect eradication and personal protection by screening. Treatment with ivermectin is extremely effective in killing the microfilaria and sterilizing the adult females residing in nodules in the body. The effect of the mass distribution of ivermectin in areas where onchocerciasis is endemic is a public health success story. Like leprosy, onchocerciasis is definitely decreasing in its importance as a worldwide cause of blindness because of successful treatment programs.

Xerophthalmia

Xerophthalmia is due to hypovitaminosis A. Clinically, there is xerosis of the conjunctiva with characteristic Bitot's spots and softening of the cornea (keratomalacia), which may lead to corneal perforation. Protein malnutrition exacerbates the condition and renders it refractory to treatment. Xerophthalmia is a common cause of blindness in infants, particularly in India, Bangladesh, Indonesia, and the Philippines. Affected infants often do not reach adulthood, dying from malnutrition, pneumonia, or diarrhea.

Xerophthalmia can be prevented by general dietary improvement or vitamin A supplementation. If the problems of distribution and administration were solved, the cost of a quantity of the vitamin sufficient to prevent blindness in 1000 infants would be only about $25.00. Measles immunization is also important in this regard because of the close association of measles epidemics with the blinding complications of xerophthalmia.

Other Causes

Glaucoma, retinal detachment, diabetic retinopathy, and herpes simplex keratitis are discussed in greater detail elsewhere in this text. The incidence of blindness due to glaucoma has decreased in recent years as a result of earlier detection, improved medical and surgical treatment, and a greater awareness and understanding of the disorder by the lay population. However, in many developing countries, glaucoma is the second most common cause of blindness after cataract. This is especially the case in West Africa, where untreated open-angle glaucoma is extremely common. In China and Southeast Asia, there appears to be a preponderance of narrow-angle glaucoma. Glaucoma now blinds 6 million individuals worldwide, and a simple easy method of detecting patients at risk still does not exist. Treatment is also a major problem because of the poor compliance of most patients for taking daily eye drops. A simple but safe surgical procedure may ultimately be the only solution for reducing the needless burden of blindness from this disease. More research in this area is essential.

Diabetic retinopathy is an increasingly more common cause of blindness everywhere in the world. Recent advances in surgical treatment (vitrectomy, laser therapy) are of some help, but many patients still suffer from proliferative retinopathy, recurrent vitreous hemorrhages, and eventual bilateral blindness. A vast research effort directed at all aspects of diabetes is in progress, and there is justification for hoping that the next generation of diabetics will benefit greatly from what is being done now.

Hereditary conditions are important causes of blindness but should gradually decrease in incidence in response to the efforts of genetic counselors to increase public awareness of the preventable nature of these disorders.

As is true also in other countries where medical care and social services are widely available, blindness in the United States is to a great extent related to the aging process, and about half of the legally blind people in this country are over age 65. The leading causes of blindness in this age group are degenerative retinal disorders, glaucoma, diabetes, and vascular diseases.

Costs of Avoiding Blindness

Some examples of what can be achieved for modest outlays of scarce funds are as follows:

1. To cure one person of trachoma in Saudi Arabia: $1.25.

2. To restore vision to one person in India blinded by cataracts: $30.00.

3. To prevent blindness due to xerophthalmia in one infant in Indonesia: 30 cents.

On the advice of WHO experts, the World Council for the Welfare of the Blind and several international professional ophthalmic societies and agencies agreed to take the initiative, which led to the establishment in 1974 of the International Agency for Prevention of Blindness (Vision International), with Sir John Wilson, a blind barrister, as president. The aim of this agency is to work with groups formed for the purpose of preventing blindness. Its theme, Foresight Prevents Blindness, was brought into prominent display when WHO celebrated the first World Health Day on April 7, 1976. Its goal was stated as follows: "In every donor country during 1976, every family should be asked—in thanksgiving for sight—to give $10.00 to save the sight of its fellow countrymen or of the millions in the third world

Rehabilitation of the Blind

Although no completely reliable statistics are available, the most widely used estimates place the legally blind population of the United States at 2.24 per thousand (ie, approximately 500,000). Approximately 50,000 become legally blind annually, and many others have enough visual loss to constitute a serious employment problem.

Blindness does not necessarily imply helplessness. Individual adjustment to marked visual impairment or total blindness varies with age at onset, temperament, education, economic resources, and many other factors. The older patient, for example, may accept blindness quite stoically, whereas for the younger patient, the vocational or social impact of blindness is often catastrophic. Blindness is accepted more easily by persons who are born blind and by persons of any age who lose their vision gradually rather than suddenly.

The aim of rehabilitation is to enable the patient to lead as nearly normal a life as possible. Approximately 5000 blind persons in the United States are rehabilitated and obtain paid employment each year. An additional larger number of blind homemakers are able to perform their household duties without assistance or are able to live independently of others.

Rehabilitation must be individualized. Many special services (see Appendix III) and increasingly complex optical and nonoptical aids are available, but they are not universally helpful. Different categories of the blind have different needs, and some blind people simply cannot benefit from a number of services or aids available. It has been said that over half of the blind people in the United States are over age 65. The elderly widowed housewife may need or want no more than mobility training in home care and a steady supply of Talking Books. A young person facing blindness in later life due to retinitis pigmentosa requires the full range of social services, including educational assessment, job rehabilitation, and psychological counseling as well as a number of sophisticated aids.

The responsibility of the physician clearly does not end with the diagnosis, prevention, and treatment of ocular disorders that might result in blindness. The physician caring for the patient who is suddenly faced with actual or imminent blindness is in a position to be of great assistance. When blindness is a possibility but is not inevitable (eg, during acute ocular inflammation), optimism and reassurance are warranted. However, it is unwise to offer false hope or to delay "breaking the news" when blindness is inevitable. If it is certain that blindness will occur, it is important to extend to the distraught patient as well as to the patient's family the warmth, understanding, encouragement, and assistance so desperately needed. The physician should be alert to the severe depressive reactions that may occur.

It is especially important to assist the patient in making the adjustment to blindness while some vision is still present. Early referral to rehabilitation agencies is essential for recently blinded adults and those with irreversible progressive visual loss. Training programs or reeducation for the many changes involved in daily living and employment are greatly simplified if the patient has the partial support provided by even limited vision.

The physician should work actively with both the patient and the family and with other professional people concerned with rendering services to the blind. The physician must know what referral sources are available and how to use them skillfully. Medical social workers, public health nurses, and counseling services and agencies serving the blind and visually handicapped are common sources of reliable information. It may be valuable to have the patient talk with a blind person who has made a satisfactory adjustment to blindness.

Mobility Training & Guide Dogs

Mobility training is most important in rehabilitation of the blind. Many state commissions for the blind offer a wide variety of mobility training courses, either directly or in cooperation with private agencies. The courses are offered on an outpatient and residential basis and have varied objectives according to the special needs of the people who apply for help. The curriculum commonly includes self-care, home functions, and mobility within the community. Several universities¹ have undergraduate and postgraduate programs in mobility training for the blind.

The usefulness of guide dogs is limited by their daily care needs and the physical strength required to hold them in check. They are most useful for students and professional men and women in good health who lead fairly well-organized lives. At this time, less than 2% of blind people in the United States use guide dogs. Sonar sensor canes may ultimately be a better answer to the mobility problem even for those who are now using a dog successfully.

¹Undergraduate-level programs are offered at Cleveland State University in Ohio, Florida State University in Florida, and Stephen F. Austin University in Texas. Graduate programs are available at Boston University, California State University (Los Angeles), Northern Colorado University, San Francisco State University, University of Arkansas, University of Wisconsin, and Western Michigan State University.

Braille

This remarkably effective system of reading for the blind was introduced in 1825. The braille characters consist of raised dots arranged in two columns of three. The system is so simple that a blind child can quickly learn to read braille, and proficient readers can learn to read braille as fast as they can talk. The system has been adapted to musical notation and technical and scientific uses also. An international braille code was introduced in 1951.

Braille is used less commonly now than formerly, since many blind people prefer auditory aids both for informational and recreational purposes. But the recent availability of portable data storage systems with braille-encoded input and conventional or braille form printed output has brought about a resurgence of interest. Braille continues to be essential on tags attached to items in common personal use even for people who do not wish to use it for reading.

All paper money in the Netherlands and Switzerland is braille-printed to show the denomination.

Electronic Devices

Optacon is an electronic device that converts visual images of letters into tactile forms. It is easily portable and can be used with almost any kind of reading matter. Auditory aids are becoming increasingly important (eg, talking calculators, clocks, paper money identifiers).

Financial Assistance Programs

It is unfortunate that over half of the blind people in the United States are essentially dependent on Social Security and whatever local supplemental aid may be available to them. For the younger blind population, rehabilitation programs are commonly administered at the state level by a division of the department of education specifically set up to serve blind people in the state. Some of these programs are better than others, and all physicians should support efforts to increase the effectiveness of such programs in their geographic area of influence. The programs are of wide scope and offer preliminary counseling followed by academic or vocational training as the circumstances warrant. Once a realistic vocational objective has been established, full financial support is commonly available. This single resource is probably the most crucial referral available to the ophthalmologist, particularly in the case of young patients. Counseling services are available as early as the junior high school years to ensure compliance with a curriculum consistent with measured aptitudes and interests. In many states, such rehabilitation programs as mobility training are administered under state auspices but contracted to private agencies for operational purposes.

In many countries, the blind receive no financial or other support from their governments and are either cared for by their families or left to manage by themselves in any way they can.

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