eye disease

eye disease

 any of the diseases or disorders that affect the human eye (eye, human).

      This article briefly describes the more common diseases of the eye and its associated structures, the methods used in examination and diagnosis, and the factors that determine treatment and prognosis. The first part deals with conditions affecting the orbit, lids, and external eye, and the second focuses on diseases of structures within the eyeball, or globe. Later sections deal with injuries, ocular conditions associated with systemic disease, disorders of vision, methods of examining the eye, and devices for correcting visual defects. For more information about the medical specialty dealing with the diagnosis and treatment of eye diseases, see ophthalmology; for more information about the health care profession concerned with the eye, see optometry.

Disorders of the outer eye and auxiliary structures

The orbit and lacrimal apparatus
      The orbit is the bony cavity in the skull that houses the globe of the eye (eyeball), the muscles (muscle) that move the eye (the extraocular muscles), the lacrimal gland, and the blood vessels (blood vessel) and nerves required to supply these structures. The remaining space within the orbit is filled with a fatty pad that acts as a cushion for the eye and allows free movement of the globe. With aging, this pad of fat tends to atrophy so that the globe recedes, causing a more sunken appearance of the eye that is often seen in elderly people.

Inflammatory (inflammation) conditions of the orbit
      Since the bone that separates the orbit from the nose and the nasal sinuses (sinus) is rather thin, infection (infectious disease) sometimes spreads from the nasal sinuses into the orbit, causing the orbital tissue to swell and the eye to protrude. This condition, called orbital cellulitis, is serious because of the possibility that the infection may spread into the cranial cavity via the pathways of the cranial nerves (cranial nerve) that reach the eye through the posterior orbit. Infections can also spread to the cranial cavity by way of the blood vessels that lie within the orbit. Prompt administration of appropriate antibiotics (antibiotic) in most cases eliminates such infections. However, surgical drainage of orbital abscesses (pockets of pus surrounded by areas of tissue inflammation) may be required. Sterile (noninfectious) inflammatory (inflammation) conditions such as Graves ophthalmopathy (eye disease caused by thyroid (thyroid gland) dysfunction) also affect the orbit.

      The lacrimal glands, the small glands that secrete the watery component of tears (tear duct and glands) and are located behind the outer part of each upper lid, are rarely inflamed but may become so as a complication of viral infection, such as in mumps or mononucleosis (caused by Epstein-Barr (Epstein-Barr virus) virus). Inflammations of the lacrimal sac are much more common. The lacrimal, or tear, sac lies in a hollow at the inner corner of the eye in the front part of the nasal wall of the orbit; under normal conditions, tears run along the margins of the eyelids (eyelid) toward the nose and are drained through two tiny holes (called puncta) connected by small tubes to the upper part of the lacrimal sac. The lower part of the sac is connected to the nose by the nasolacrimal duct, and infection may ascend this passage from the nose and cause an acute painful swelling at the inner corner of the eye (called dacryocystitis). Blockage of the nasolacrimal duct prevents the passage of tears into the nose and results in a watery eye. Such a blockage, which is often accompanied by chronic inflammation in the lacrimal sac, is usually treated in infancy with a simple massaging technique. However, if the problem persists, a procedure to open or stent the tear passageway can be performed to relieve the obstruction. If this approach also fails, a different operation can be undertaken in which a new opening from the lacrimal sac to the nasal cavity is made.

Tumours (tumour) of the orbit
      Tumours (tumour) in the orbit are comparatively rare and may arise from within the orbit, as an extension from nearby sinuses, or as a metastasis from a distant tumour. Tumours arising within the orbit include lacrimal gland tumours, lymphoid tumours, vascular tumours, and tumours of the optic nerve, among others. Such tumours may be benign or malignant. Orbital tumours can cause a slow and gradual protrusion or displacement of the eye, which may prevent ocular movements from being coordinated with those of the normal eye. If this occurs, the images of the two eyes, which are normally fused, may separate and give rise to double vision (diplopia).

Disorders of the lids (eyelid)
Inflammatory conditions
      The chronic inflammation of the lid margins known as blepharitis is a common and potentially distressing condition. The inflammation may be mild, causing simple redness of the lid margin with scaling of the skin, or it may be severe, causing intense irritation and burning along with eyelash destruction and distortion. Blepharitis tends to be associated with greasiness of the skin and with dandruff. The skin of the lids is particularly sensitive to allergic (allergy) processes, and itching, scaling, and redness of the lids are common reactions to drugs or cosmetics applied to the eye of a sensitized person.

      Another common inflammatory condition of the lid is a sty, in which inflammation of particular oil glands or lash follicles occurs along the lid margin. Sties originating in the lash follicles are usually infectious and start as a painful swelling of the lid. At first it may be difficult to find a localized lesion, but soon one area becomes more swollen, and, as pus forms, a yellow point may be seen near the lid margin. A rather similar appearance can be produced by an inflammation of the tiny glands in the inner eyelid, called meibomian glands, that open onto the lid margin. Since the glands are embedded in tough fibrous tissue, the pain and reaction may be more severe than in a sty of the lash follicle. Examination of the internal surface of the lid often shows a red, velvety area with a central yellow spot through which pus may later discharge. Sometimes the meibomian glands suffer from a chronic infection, and a painless firm lump, called a chalazion, appears in the lid and slowly increases in size. The skin can be moved freely over the surface of the lump, showing that the latter is in the deeper tissue of the lid. The inner surface of the lid will show a discoloured area surrounded by inflammation. If the chalazion persists and does not respond to warm compresses, eyelid scrubs, and massage, it can be removed through an incision made on the inner surface of the lid.

       herpes zoster (shingles) may affect the skin of the eyelids and is of particular importance because the cornea (the transparent covering of the front of the eyeball) and inner eye may also be affected. The condition often starts with pain and redness of the forehead and the eyelids of the same side. Vesicles, or small blisters (blister), form later in the affected area. The pain may be severe, and some constitutional disturbance is common.

Malposition of the lids
      Malposition of the lid is common in elderly people. Although usually not serious in itself, it can give rise to considerable discomfort, irritation, and even impairment of vision. ectropion arises when the lower lid curls away from the globe in such a way that, if significant, the tears overflow the lid. This constant wetting of the skin of the lower lid is abrasive and irritates the skin. In addition, severe cases can lead to exposure of and damage to the cornea. The opposite condition is entropion, in which the lid turns inward and the lashes cause irritation by rubbing on the eye. It may be caused by scarring of the deeper tissues of the lid or may be due to age-related changes in lid muscle tone. In ectropion and entropion, plastic surgery is usually necessary to bring the lid back to a near normal position.

      Another type of lid malposition is called blepharoptosis, in which the upper lid margin droops below its normal resting position when the eyelids are open. This can result from age-related changes, congenital abnormalities, or nerve or muscle disorders, among other causes. Severe cases of blepharoptosis can cause significant impairment of vision by blocking light from entering the pupil. In children, such vision impairment may become permanent if not treated promptly. Again, surgical intervention is usually required.

Tumours (tumour) of the lids
      Benign overgrowths of the blood vessels, called hemangiomas (hemangioma), may occur in the lids and give rise to soft, bluish swellings. They are most often present at birth and tend to grow in the first few years of life, sometimes contributing to obscuration of vision and amblyopia. Often they disappear spontaneously, but they can be treated with corticosteroids (steroid hormones (steroid hormone) such as cortisone, prednisone, or prednisolone), with interferon (potent proteins released by cells of the immune system that block cell reproduction and modulate immune response), or, rarely, by surgical removal. Simple overgrowths of skin, called papillomas (polyp), result from viral infections and are common along the lid margin. They require no special treatment except excision or ablation for cosmetic reasons. A nevus (birthmark) is a benign growth, usually pigmented and raised, that arises from pigment cells of the skin. Change in shape, size, or colour of a nevus may indicate transformation into a malignant tumour.

      The lids and the skin of the nose near the inner margins of the lids are common sites for the development of skin cancer in older people. The most usual type, called a basal cell carcinoma (or “rodent ulcer”), starts as a small nodule in the skin that gradually enlarges and breaks down to form an ulcer with a hard base and pearly, rolled edges. Bleeding may occur from the base of the ulcer. Although basal cell carcinomas (carcinoma) are malignant in the sense that they destroy tissue locally, they do not spread to distant areas of the body by means of the lymph system (lymphatic system) or the blood vessels. Other malignant cancers affecting the eyelid include sebaceous carcinoma of the eyelid glands and melanoma, the latter of which can arise from preexisting nevi.

Strabismus (squint (strabismus))
      In the lower vertebrates (vertebrate), such as fishes (fish), the eyes are situated on either side of the head to give the maximum view of the surroundings and an early warning of the presence of predators. The field of vision of each eye is separate except for a narrow sector immediately in front of the animal, where the visual fields of each eye overlap. The improved judgment of distance obtained by viewing an object with both eyes conferred considerable biological advantage (adaptation) in the struggle for survival. In the higher animals, particularly the predatory species of birds (bird) and mammals (mammal), binocular vision became more and more important. Structural changes in the placement of the eyes in the head permitted a larger overlap of the two visual fields until the situation was reached in the higher mammals in which the visual axes of the two eyes—that is, the line of direct sight—became essentially parallel. This desirable visual arrangement is fully realized in humans. However, the structural changes necessary to bring this about seem to have lagged behind the function, and the geometrical axes of most eyes are still slightly divergent (i.e., the two eyes at rest are directed slightly away from the nose). The bony structure of the orbit has lagged even farther behind, and the axes of the two orbits make an angle of only about 45°.

      It is the function of using two eyes together that keeps the optic axes straight in a normal person. If, for example, one eye becomes blind, it tends to revert to an anatomical position of rest in line with the axis of the orbit. A blind eye will therefore often appear to be diverging under resting conditions. The visual axes can remain straight only if each eye has reasonably good vision, the ocular muscles can move the eyes in the required directions of gaze, and the complex neuromuscular inputs required to coordinate the movements of the two eyes are intact. Failure to maintain the visual axes parallel may therefore result from visual defects, muscular defects, or paralysis, which may result in loss of normal movement of the eye, or from defects in the central nervous system (nervous system, human) that affect the coordinating neural pathways. A true strabismus (also called squint) is a condition in which the visual axes are no longer parallel (i.e., one eye deviates in position from the other). A false strabismus may be seen in children as the result of prominent skin folds in the nasal portion of the eyelids, which make the eyes appear to be converging (i.e., appear to cross). These skin folds usually disappear when the bony structure of the nose has developed more fully.

      Clinically, strabismus can be divided into concomitant, in which the abnormal angle between the visual axes remains constant in all positions of gaze, and incomitant, in which the eye deviation varies with the direction of gaze. A common type of concomitant strabismus seen in young children (childhood disease and disorder) is accommodative esotropia, in which the eyes are consistently or intermittently crossed when trying to focus. It is usually first noticed between the ages of two and three and is often associated with a family history of strabismus. The etiology of this condition is tied to the “near reflex,” which has two main components. First, the visual axes must converge, so that both eyes can view the same object. Second, the focus (accommodation) of the eyes must be adjusted for near vision. The link between convergence of the eyes and focusing is very strong, and normally the two actions work in harmony, resulting in both eyes being appropriately aligned on the object of regard. However, in children with accommodative esotropia, the balance between eye convergence and accommodation is abnormal, such that the eyes cross more than is required for a given amount of focusing demand. This imbalance is more common in farsighted children, who require more focusing effort to see clearly both distant and near objects.

 Once parallelism of the visual axes has been lost, the images no longer lie on corresponding areas of each retina, and two images are perceived. This condition of double vision, or diplopia, is detrimental to the child, whose adaptive brain reacts by suppressing the image from the deviating eye. If the brain's suppression of one eye's vision is allowed to continue, the central vision of the affected eye can drop to a low level, a condition called amblyopia. Even if the original disturbance that started the strabismus is corrected, amblyopia may prevent the restoration of normal binocular vision. The longer the suppression is allowed to continue, the less likely the child is to regain normal vision in the deviating eye. Covering, or patching, the good eye will usually encourage the recovery of the suppressed vision, but it must be started as soon as possible. Any refractive error present (i.e., any defect that prevents light rays from focusing properly on the retina, such as nearsightedness (myopia) or farsightedness) must be corrected, usually by eyeglasses. Early treatment along these lines may be all that is necessary, but, if the visual axes are still abnormal, surgery of the extraocular muscles is usually required to correct the deviation.

Diseases of the conjunctiva
      A thin membrane called the conjunctiva lines the outside of the eyelids, covers the anterior surface of the eye (except the cornea), and is lubricated by the tears. This warm, moist habitat can provide a suitable environment for the growth of bacteria, viruses (virus), and other organisms, all of which may cause conjunctivitis (inflammation of the conjunctiva). Bacterial conjunctivitis starts with a feeling of grittiness and discomfort. The eye becomes red and exudes a thick discharge. The discharge is particularly noticeable after sleep, when the lids may be stuck together by the exudate on the lashes. Vision is not affected except by the strands of mucus, which can be blinked away from the cornea. Antibiotic drops usually clear the condition quickly. Vernal conjunctivitis (or spring catarrh) is, as its name suggests, an allergic condition occurring in the spring and early summer. It is more common in young people and may result from sensitivity to external irritants. It usually responds to treatment with antiallergy or anti-inflammatory medication.

      Chronic conjunctivitis also causes a gritty feeling, with redness of the eyes and a slight mucoid discharge. It is a common condition, the cause of which may be difficult to find. Often there is underlying chronic inflammation of the lid margin (such as blepharitis); however, sometimes the condition is caused by an allergic sensitivity to cosmetics or to drugs applied to the eye. An unsuspected foreign body or a deficiency of tear secretion may cause similar symptoms.

       trachoma, although rare in more-developed countries, is a significant cause of preventable blindness in the world. Widespread in some Middle Eastern countries, it has remained common in Asia, India, Central and South America, and Africa and occurs sporadically in southern and eastern Europe. The agent responsible is an intracellular bacterial organism known as Chlamydia trachomatis. The disease is contagious and thrives where populations are crowded together in poor hygienic surroundings. Shortage of water for washing and the myriads of flies (fly) attracted to human waste aid the dissemination of the disease. In some ways trachoma is more of a social problem than a medical problem. When living standards are improved, overcrowding reduced, flies discouraged, and adequate water supplies ensured, the incidence of trachoma decreases rapidly.

      The early symptoms of trachoma infection are pain, watering of the eye, and sensitivity to light. At this stage the conjunctival lining of the lids is red and velvety in appearance, and the cornea may show gray areas. Later the conjunctiva appears to have grains of sand embedded in its tissue, and blood vessels grow into the cornea, causing it to thicken and become hazy. Secondary bacterial infections are common, but the real dangers of trachoma lie in the scarring and contracture of tissue that occur when healing takes place. These changes affect the upper lid in particular, causing it to buckle inward in such a way that the lashes rub across the already diseased cornea, exacerbating the corneal scarring and potentially leading to blindness. Antibiotic treatment is usually effective at eradicating the infection, although any existing scarring will remain.

      Viral conjunctivitis, caused by viruses (virus) that tend to attack the cornea as well as the conjunctiva, occurs more commonly than bacterial conjunctivitis. Viral infection is contagious and is often responsible for outbreaks of epidemic keratoconjunctivitis (called “pinkeye”), in which infection with an adenovirus causes inflammation of the cornea and conjunctiva. The onset is acute, with redness, swelling, irritation, and watering of the eye and eyelids, along with a tender swelling of the lymph node in front of the ear (ear, human). Infection frequently spreads from one eye to the other, but strict hygiene, especially hand washing, can limit its spread. Treatment relies on symptom management with artificial tears and cool compresses. Persistent symptoms and changes in the eye surface may occur.

Degenerative conditions of the conjunctiva
      Exposure to wind and dust frequently causes degenerative changes in the exposed part of the conjunctiva, particularly in older people. A yellow nodule, called a pinguecula, forms initially on the nasal side of the cornea and later on the opposite side. It is frequently unnoticed until an incidental infection such as conjunctivitis occurs simultaneously, causing the pinguecula to stand out clearly against the red background of dilated conjunctival vessels. It causes no symptoms and requires no treatment.

      A more serious degeneration, called pterygium, is found particularly in people who live in bright, sunny climates. It appears as a fleshy growth at the edge of the cornea and has a tendency to progress across the front surface of the cornea, where it may interfere with vision. Treatment consists of surgical removal, but recurrences are common.

Diseases of the cornea and sclera
      The cornea is the clear window of the eye. It covers the iris and pupil and serves as a powerful refractive surface. Any surface irregularity or scar in the substance of the cornea can affect vision. The cornea is an extremely sensitive tissue and contains many nerve fibres that respond to pain. Diseases of the cornea also elicit a flow of tears because of a specialized reflex action that is part of the protective system of the eye.

inflammation of the cornea
      As with inflammations of the conjunctiva, bacterial infection of the cornea is much less common than viral infection. Of the viruses, the herpes (herpes simplex)viruses, which cause the common “cold sore” of the lips and skin and the venereal (venereal disease) form of herpes, are a frequent cause of corneal ulceration (ulcer). Infection is most often spread by personal contact. The herpesvirus causes a typical ulcer of the cornea called, from the pattern of the lesion, a dendritic (branching) ulcer. The disease starts with an acutely painful eye, with tearing and sensitivity to light. The ulcer may heal spontaneously or after medical treatment, but the virus often lies dormant in the tissues. Recurrences are common, and with each recurrence there is danger that the virus will extend deeper into the cornea and cause further inflammation and scarring, with eventual vision impairment.

      Oral antiviral medications or application of antiviral eye drops to the cornea usually causes the ulcer to heal more rapidly. The action of these drugs (drug) limits the multiplication of the virus by interfering with the formation of virus deoxyribonucleic acid ( DNA) in the host cell.

      Bacterial infections of the cornea usually occur after injury to, or breakdown of, the corneal surface, as few bacteria have the power to penetrate the intact surface layers of the cornea. Such ulcers may be extremely severe, and there is always a danger of perforation of the eye, particularly in debilitated patients.

      Spores of fungi (fungus) are present in the atmosphere, and the normal cornea is resistant to infection by these organisms. However, a fungal infection of the cornea can develop after a corneal injury or other lesion, particularly if corticosteroid drugs have been used in the treatment of these conditions. Intensive treatment with antifungal drugs is usually effective in killing the organisms, but a dense scar may be left.

      A corneal inflammation may start in the deeper layers of the tissue, either by direct infection or from immune-related processes. One type is seen in adolescents who have congenital syphilis. Both eyes are usually attacked, although there may be an interval before the second eye is affected. As a result of inflammation, the cornea rapidly becomes hazy, and blood vessels grow in from the surrounding tissues to form a pink patch. With the decline in congenital syphilis in developed countries, the condition is becoming increasingly rare.

inflammation of the sclera
      The sclera is the fibrous covering of the eye that shows up as a dense white layer beneath the transparent conjunctiva. A relatively mild nodular inflammation, called episcleritis, sometimes occurs in the superficial layers just above the sclera. It occurs more often in young and middle-aged adults and usually improves without treatment. In more severe cases, treatment with anti-inflammatory medication may be necessary. Inflammation of the deeper sclera, called scleritis, is more severe and is often painful. It occurs more frequently in older people and may be associated with underlying disorders, such as tuberculosis or rheumatoid arthritis. However, the cause of scleritis is often not discovered.

Degenerative conditions of the cornea and sclera
      There are numerous types of corneal degeneration, many of which are rare and some of which are familial. The most common type is keratoconus, a curious condition in which the central part of the cornea, normally spherical in shape, begins to bulge and protrude forward as a cone. The only symptom is deterioration of vision due to irregular astigmatism caused by the changing corneal curvature. Contact lenses (contact lens) are often more effective than eyeglasses in treating this condition. Advanced cases of keratoconus, and most other types of corneal degeneration, may require corneal transplants.

Diseases of the inner eye

The uveal tract
      The uveal tract is a vascular layer of tissue—that is, a layer rich in blood vessels (blood vessel)—lying next to the inner surface of the sclera. It is divided into three structures: the choroid, a highly vascular layer that supplies blood to the outer layers of the retina; the ciliary body, a largely muscular tissue, which contracts and relaxes to alter the focusing of the lens; and the iris, the coloured part of the eye (eye, human), which forms the adjustable aperture of the eye, the pupil. The ciliary body, which lies behind and at the base of the iris, also functions by forming the aqueous humour, the production and drainage of which regulate intraocular pressure. The aqueous humour also is the source of nutrition to the lens and cornea, which are avascular (without blood vessels).

      Inflammations (inflammation) of the uveal tract are always potentially serious because of the secondary effects they may have on other intraocular structures. In most cases the disease affects either the anterior part of the uvea—that is, the iris and ciliary body—or the posterior part, the choroid. Inflammation affecting primarily the ciliary body and anterior vitreous area is termed intermediate uveitis and, if no other cause is identified, may also be called pars planitis. An attack of acute anterior uveitis (also called iritis) starts with deep pain, redness, and mistiness of vision. The eye is sensitive to light and may water, though there is no discharge as in conjunctivitis. The pupil tends to constrict, and the normally clear iris markings may become less distinct. In chronic anterior uveitis the main symptom is blurring of vision. Acute choroiditis (also called posterior uveitis) is characterized by a sudden onset of blurred vision with many black spots floating in the eye's field of vision.

      Many infectious conditions and systemic inflammatory and immunologic diseases are known to cause uveitis. In a large proportion of cases, however, particularly when the inflammation is confined to the anterior segment, it proves impossible to be sure of the cause. A proportion of cases of anterior uveitis are associated with ankylosing spondylitis, a chronic disease of the joints of the spine. Some cases are associated with Reiter syndrome, a condition affecting young males that usually starts as an infection of the urogenital tract (urogenital system), with the later development of joint changes, particularly in the sacroiliac joints of the lower back, and recurrent attacks of anterior uveitis. Anterior uveitis may also be caused by herpesvirus infection, arthritis associated with psoriasis, inflammatory bowel disease, and complications of severe cataracts (cataract) or lens damage.

      Inflammations of the choroid (the posterior portion of the uveal tract) and the retina are frequently infectious in origin. One of the organisms more commonly involved is Toxoplasma gondii (toxoplasmosis), a protozoan of worldwide distribution among domestic animals (such as cats), small mammals, and humans. Although antibodies (antibody) to the organism can be found in a high proportion of most populations, overt signs of disease are rarely seen. Most people can acquire the infection without being aware of any systemic disturbance at all, and only in special circumstances does the organism cause disease. One of these special circumstances is pregnancy. If a woman with no previous exposure to toxoplasmosis (i.e., she lacks immunity) becomes infected during pregnancy, it is possible for the organism to pass through the placenta and infect the unborn child. In severe cases the child may be stillborn or may be born with congenital (congenital disorder) toxoplasmosis, a serious disease affecting many organs of the body, particularly the brain and the eyes. In less-serious cases small foci of infection are left in the nervous system and the retina of the eye. These may not be apparent at birth and may remain quiescent, only to become active 15 or 20 years later in the form of an inflammation of the choroid and the retina. Children of subsequent pregnancies are unaffected.

      Other causes of posterior uveitis include viruses (virus) such as herpes, cytomegalovirus (seen more frequently in AIDS patients), and rubella as well as fungal diseases and immunologic diseases such as systemic lupus erythematosus.

      A panuveitis refers to an inflammation of all parts of the uvea, often affecting both eyes and involving other ocular structures. Possible causes include syphilis, tuberculosis, Lyme disease, and sarcoidosis. The treatment of uveitis has been transformed by the advent of corticosteroid (corticoid) drugs. Even when a specific cause cannot be discovered, therapy with corticosteroids is usually successful in controlling the worst ravages of the inflammation.

Tumours (tumour) of the uveal tract
      Pigmented tumours (tumour) are the most common tumours arising from the uveal tract. They may be benign (such as a nevus or a mole) or malignant (such as melanoma). The choroid is a common site for these lesions, which can push the retina forward and possibly cause a retinal detachment. Disturbances of vision are the most common symptom, but, if the tumour is neglected, choroidal melanomas may enlarge and cause inflammation and raised pressure within the eye. Small portions of the tumour can enter the bloodstream and settle in distant organs, particularly the liver. The growth of these secondary deposits is often slow, and they may not be apparent until many years after the diagnosis of the tumour in the eye. Treatment options for melanoma vary and include local radiation (radiation therapy) treatment or removal of the eye (called enucleation).

Diseases and disorders of the lens
      The lens is a transparent, avascular organ surrounded by an elastic capsule. It lies behind the pupil and is suspended from the ciliary body by a series of fine ligaments (ligament) called zonular fibres. Its transparency is the result of the regular arrangement of the internal lens fibres, which form continuously throughout life. Interference with the growth or maintenance of lens fibres can result in the formation of abnormal fibres or fibre arrangements that cannot transmit light as well as the normal lens fibres. An opacity is thus seen in the lens. Minor irregularities are common in otherwise perfectly normal eyes. If the opacity is severe enough to affect vision, it is called a cataract.

      Congenital lens opacities of many varieties have been recognized and described since the early days of ophthalmology, but they remained curiosities until the work of an Australian ophthalmologist, Norman M. Gregg, threw new light on their cause—and, indeed, on that of many other congenital defects. In 1941 Gregg noticed that, after an epidemic of German measles ( rubella), many of the children whose mothers had contracted the disease in the first two months of pregnancy were born with cataract, sometimes associated with deafness and congenital heart disease. Congenital cataracts can also be inherited, can be associated with genetic, metabolic, or other infectious diseases and disorders, or may have no known cause or association.

      Cataract in the adult may be the result of injury to the lens by a perforating wound, exposure to radiation such as X-rays (X-ray), chronic inflammation such as uveitis, or ingestion of toxic substances or even of some drugs. The most common form of cataract is age-related cataract, so called because it becomes progressively more common with advancing age (human aging). Various types of age-related cataracts—called nuclear, cortical, and posterior subcapsular—are distinguished by the portion of the lens they involve, their natural course of development, and the somewhat differing symptoms they elicit. The most common type, nuclear sclerotic cataract, forms as the centre, or nucleus, of the lens slowly undergoes compression and hardening, turns yellowish or brown in colour, and becomes less transparent. Typical symptoms include cloudy vision, poor colour discrimination, and changes in distance vision. Mature, more severe cortical cataracts can cause the whole lens to appear white. Posterior subcapsular cataracts tend to occur in younger people and can be troublesome even when small, depending on their particular location on the back surface of the lens.

      In the early stages of cataract development, some visual improvement can usually be obtained with eyeglasses, but, as the cataract progresses, the visual deterioration becomes sufficiently severe to warrant surgical treatment. Cataract surgery involves removing the cloudy lens and, in most cases, placement of an artificial lens within the eye.

The retina
 The retina is a thin transparent membrane that lines the inner eye. Its outermost layer, the pigment epithelium, consists of pigmented cells that are closely adherent to the underlying blood vessels of the choroid. The layer of rods (rod) and cones (cone) is more loosely attached to the pigment epithelium and has complicated interconnecting networks that culminate in the innermost layer of nerve fibres. These fibres run back through the optic nerve to the brain. The inner portion of the retina derives its blood supply from a special complex of vessels, called the retinal vessels, that enter the eye through the optic nerve.

Retinal detachment (detached retina)
      A retinal detachment (detached retina) occurs when the main vision-sensing part of the retina becomes separated from the pigment epithelium. This may result from an injury to the eye, a tumour within the eye, or inflammation of the underlying choroid. The most common type of detachment, however, has no such predisposing factors; instead, the distinctive feature is the formation of a small hole or tear in the retina, usually at its periphery. In most cases the tear is caused by normal forces acting on attachments between the retina and the gel-like substance called the vitreous humour that fills the interior of the eye. Sudden movement of the eye, age-related changes within the vitreous, or an injury can cause the vitreous to pull on the retina, thus creating a tear. When this has happened, fluid can pass through the hole and strip the retina off the pigment epithelium. Myopic (myopia) (nearsighted) eyes are particularly prone to retinal detachment because they are larger than normal, and the coats of the eye are thinned and stretched. The periphery of the retina in particular often shows weak areas.

      The history of a retinal detachment is often quite typical, with the pull of the vitreous on part of the retina creating a sensation of brief flashes of light noticed by the person as the eye is moved. When an actual tear has developed, the retina may start to become detached, and the person has the sensation of a shadow slowly coming across the vision of the affected eye.

 The approach to the repair of a retinal detachment is variable and dependent on the patient and the particular features of the retinal break and detachment. The essential factor in early treatment is to seal off the tear in the retina if there is danger of detachment. The part of the retina containing the tear must be brought into close contact with the underlying pigment epithelium and choroid. The retina is then made to stick to the underlying choroid by means of heat (via a laser) or cold (via cryotherapy), thus sealing off the leak. Remaining fluid under the retina can be drained away, allowing the retina to fall back into place. A scleral buckle is a flexible device that is fitted surgically around the outside of the mid-portion of the eyeball within the orbit and secured so that the retina is flattened back onto the pigment epithelium and choroid. Other techniques are available to accomplish this, including the use of air bubbles or silicone within the eyeball.

      Provided that the detachment is not long-standing, retinal function often recovers quite well once the retina has been reattached. The small central area of retina that subserves the most acute vision, called the macula lutea, has only one source of blood supply, the underlying choroid. Once it is separated, some permanent damage usually ensues, even if the retina is subsequently replaced in its correct position. Thus, it is most important that retinal detachments be treated early, before the central area of the retina becomes detached.

Inherited retinal degenerations and age-related macular degeneration
      Although inherited retinal degenerations are relatively uncommon, their unusual affects on the retina and the inexorable advance of this diverse group of diseases have stimulated a considerable amount of research in this area. As a result, a large number of genetically determined degenerations of the retina have been described. These conditions are typified by the category of retinal degenerative diseases collectively known as retinitis pigmentosa and the retinitis pigmentosa-like disorders. The earliest symptom is night blindness, which may first be noticed in childhood and is due to alteration in the function of the rods, which are the visual receptors used in dim light. The more peripheral parts of the retina are affected first, and, while central vision may be good, the field of vision often progressively decreases until only “tunnel vision” remains. Genetic studies have linked many forms of retinitis pigmentosa to abnormalities in genes (gene) coding for proteins (protein) involved in light sensation. The disease is often recognizable on examination by the narrowing of retinal vessels and the scattering of clumps of pigment throughout the retina.

      Age-related macular degeneration (ARMD), unlike many other retinal disorders, affects central vision and spares peripheral vision. This common disease occurs in people over the age of 50 and is a major cause of severe vision loss. ARMD is classified into two types: dry macular degeneration (lacking abnormal blood vessels) and wet macular degeneration (containing abnormal blood vessels). The majority of afflicted people have dry ARMD, in which yellowish deposits (drusen) and other abnormalities associated with the retinal pigment epithelium develop. Vision can remain relatively good, provided it does not progress to the neovascular, or wet, type or undergo extensive atrophy.

      People with ARMD who experience quick and severe vision loss typically have wet ARMD, in which new blood vessels from the choroid beneath the retina grow up into the space between the pigment epithelium and sensory retina. These vessels may bleed and detach overlying tissues or lead to destruction and scarring of the central retina. In a certain subset of people with ARMD, laser treatment of the affected retina or injection of certain drugs directly into the eye may help slow the degenerative process. While degeneration of the macula does not cause total blindness, it is extremely disturbing because it affects central visual acuity and makes reading or fine work difficult or impossible.

      The causes of ARMD are under investigation, as are a variety of treatment regimens. However, while awaiting breakthroughs in the understanding and management of this debilitating disease, many people with ARMD benefit from the use of special magnifying devices and other low-vision aids. The retinal changes that may occur in diabetes mellitus, arteriosclerosis, and vascular hypertension are described below in the section Complications of systemic disease (eye disease).

Disorders of the optic nerve
 The optic nerve, which carries about one million nerve fibres, leaves the globe from the back of the eye and passes through the apex of the orbit into the cranial cavity. It is surrounded by an extension of the membranes that surround the brain. This connection with the intracranial cavity is important because some intracranial diseases cause increased pressure within the skull. Increased pressure is transmitted along the covering of the optic nerve, causing swelling of the optic nerve head, a condition that is visible inside the eye. This swelling of the nerve head of each eye (called papilledema) is one of the most important signs of increased intracranial pressure. If the swelling persists, damage to the fibres of the optic nerve can take place, with subsequent loss of vision.

      Swelling of the optic nerve may also be caused by inflammatory changes in the nerve, a condition known as optic neuritis. The symptoms are loss of vision in or near the central part of the visual field, pain behind the eye, and pain when moving the eye. The condition is most common in young adults and may be a manifestation of multiple sclerosis, a condition in which the sheaths of the nerves become altered and interfere with the transmission of nervous impulses. This disease may strike any part of the nervous system, but the optic nerve is a common site, and vision loss or eye pain are often the first symptoms to be noticed by the patient. The disease is characterized by long periods of remission from symptoms, and, after an episode of optic neuritis, it may be 10 years or more before other neurologic signs are apparent. Usually the function of the optic nerve recovers after an attack of optic neuritis; however, residual visual disturbance often remains.

      Optic nerve damage or atrophy may result from glaucoma or any serious disease of the retina in which a large amount of neural tissue has been destroyed. It may also follow damage to the optic nerve within the skull or the optic chiasm, where the optic nerves crisscross (close to the pituitary gland). For example, tumours of the pituitary gland often compress the optic nerve fibres and cause some degree of atrophy with loss of vision in that part of the visual field subserved by the fibres affected. Usually it is the fibres on the inner side of the optic nerve and those that cross at the chiasm that are most involved. These fibres supply the half of the retina nearer the nose, which receives images from the outer part of the visual field. Thus, in pituitary lesions it is common to find that the outer parts of both visual fields are abnormal, a condition called bitemporal hemianopia.

      Certain chemicals, drugs, and nutritional deficiencies can also cause optic nerve damage or atrophy. If the underlying cause is corrected in time, some vision improvement may occur.

      The thin coats of the eye are not sufficiently rigid in themselves to withstand distortion following the pull of the extraocular muscles when the eye is rotated. The eyeball is kept rigid by the action of the ciliary body, which secretes sufficient amounts of aqueous humour fluid to maintain the pressure of the eye at a level above atmospheric pressure. Aqueous humour is constantly being formed and drains away at the base of the iris through specialized drainage channels. Should these channels become blocked, the pressure within the eye rises to abnormally high levels. If the intraocular pressure remains raised for a long period of time, some retinal nerve fibres will atrophy, causing loss of visual function.

      Glaucoma is the name given to a group of diseases that cause a particular type of optic neuropathy (i.e., optic nerve disease or abnormality) that results in visual field loss. Increased pressure within the eye is one of several important risk factors for development of glaucoma, but no one particular pressure is indicative of the disease. The susceptibility of an individual's optic nerve, and of the retinal cells whose fibres make up the optic nerve (called ganglion cells), to damage at a particular eye pressure varies widely. Typically, a “normal” eye pressure range is between 10 and 21 mm (0.4 and 0.82 inch) of mercury, but glaucoma can arise in people with pressure below 21 mm (normal-tension glaucoma, or low-tension glaucoma). In addition, people with pressures above 21 mm (ocular hypertension) may never show signs or symptoms of glaucoma. A person's eye pressure is determined by the rate of aqueous humour formation by the ciliary body and the resistance to outflow through various pathways. Two major classes of glaucoma are primary open angle glaucoma (POAG) and angle closure glaucoma.

      Primary open angle glaucoma is a common disease and a leading cause of vision loss in older individuals. Although the actual cause is not known, it may be due to degenerative changes in the outflow channels for aqueous fluid. It is rare below age 40, but after this its incidence increases. Genetic influences are important, and relatives of patients with glaucoma are more likely than others to develop the disease, as are individuals of African descent, who also tend to be affected more severely.

      The symptoms of POAG are slight or absent in the early stages. The slow progressive optic nerve damage does not cause pain, and the early visual loss is in the peripheral parts of the visual field, affecting central vision only late in the disease. Both eyes are usually involved, although one may be more severely affected than the other. Since vision loss from glaucoma cannot be restored, successful treatment can only prevent further loss of vision. It is of great importance, therefore, that the disease be diagnosed as early as possible. Measurement of the intraocular pressure is of great value in the diagnosis of glaucoma. This is a simple test that can be applied as a screening method for surveys of the normal population.

      The medical treatment of POAG currently consists of the use of eyedrops that lower the intraocular pressure. Inhibitors of the enzyme carbonic anhydrase, when taken by mouth, reduce the formation of aqueous humour and are used as an additional measure when necessary. If the pressure remains raised in spite of all medical treatment, then surgical methods or laser treatments must be used to increase the drainage of fluid from the eye. Treatments aimed at other potential mechanisms involved in glaucoma are under investigation.

      Another common type of glaucoma is called angle closure glaucoma. It can be caused by mechanisms that either push the iris forward from behind or pull it forward to block the outflow of aqueous humour through the trabecular meshwork. The trabecular meshwork is located in the anterior chamber angle formed at the insertion (far periphery) of the iris. The aqueous fluid formed in the ciliary body behind the iris flows forward through the pupil to the angle of the anterior chamber. In one form of angle closure glaucoma, called pupillary block glaucoma, the lens seals against the iris and blocks the flow of aqueous humour through the pupil. The root of the iris (which is rather thin) is then pushed forward because of increased posterior pressure, which closes the angle and prevents outflow of aqueous humour. The angle may eventually become completely closed, causing intraocular pressure to rise rapidly. The eye then becomes red, hard, and painful, and vision deteriorates. The pain may be so severe as to cause vomiting. Urgent treatment is required to lower the pressure and prevent damage to the optic nerve that could lead to permanent vision loss.

      In some cases an acute attack such as this heralds the onset of glaucoma; however, most people experience minor subacute attacks that are relieved by rest and sleep and usually occur for months or years. Modern methods of medical and surgical treatment are usually effective in lowering the pressure in an acute attack and preventing recurrences. Other causes of glaucoma include chronic inflammatory disease of the eye, tumours within the eye, and congenital afflictions of the eye. Congenital glaucoma usually is found in the neonatal or infantile period and is heralded by tearing, aversion to light, eyelid spasms, and clouding and enlargement of the cornea. Treatment is aimed at medically or surgically reducing intraocular pressure, but long-term visual prognosis is often poor.

Ocular injuries
      The bony orbit provides excellent protection for the eye (eye, human), especially from blunt injuries. A blow to the front of the orbit with a rounded instrument such as a fist or a tennis ball, however, can cause a shock wave to travel through the eye, damaging many structures along the way, including the retina. Central vision may be reduced after such injuries without any obvious changes in the appearance of the eye. In severe cases the bones (bone) of the orbit may be fractured. Perforating wounds from glass, sharp metal fragments, and so on are always serious. Injuries to the lens will result in the formation of a cataract, and often after penetrating injuries the eye remains inflamed for a considerable time.

      One rare type of inflammation following injury, called sympathetic ophthalmia, is of particular importance. In this condition an injured eye causes the other, previously normal eye to take part in the inflammation, with resulting impairment of vision. Sympathetic ophthalmia can occur weeks, months, or years after the initial injury. The cause of sympathetic ophthalmia is not fully known, but if an injured eye is removed within 10 days of injury, sympathetic ophthalmia almost never occurs in the other eye. In the past there was little effective treatment for the condition, but therapy with corticosteroids and other immunomodulatory agents has proved effective in controlling inflammation in many cases.

Foreign bodies
      Most foreign bodies that contact the eye remain on or near the surface. When they touch the cornea, they cause intense pain and a flow of tears. The tears may be sufficient to wash the foreign body out of the eye, but, if it becomes embedded in the cornea, it may have to be removed surgically. Many small foreign bodies lodge in the undersurface of the upper lid in such a way that every time the eye blinks, the foreign body rubs on the cornea, causing pain and irritation. Metallic foreign bodies embedded in the cornea often leave rust rings, which should be removed to aid in proper healing.

      Small foreign bodies traveling at high speeds may penetrate into the interior of the eye with remarkably few symptoms, and their presence may not be recognized until weeks or months later when inflammatory changes occur. The most common foreign bodies to enter the eye in this way are fragments of metal from hammer-and-chisel accidents or from moving parts of machinery. Whenever such injuries are suspected, it is important to locate the position of the fragment as carefully as possible and to remove it by surgery. If the foreign body is magnetic, a magnet can be used to attract the foreign body to the site of entry into the eye, permitting extraction. Safety goggles or glasses equipped with safety lenses are of utmost importance in the prevention of such accidents during high-risk activities.

Chemical and radiation injuries
      Strong acids (acid) and alkalis (alkali) cause severe injury if they contact the eye. Alkalis, such as lye, ammonia, and lime, are particularly damaging in that they tend to rapidly melt and penetrate the cornea, producing extensive tissue destruction within the eye. Speed is the vital factor in first-aid treatment, and copious irrigation with water is the first essential step. Delay of first-aid treatment in the hope of finding a neutralizing substance is a serious error, as strong acids and alkalis quickly become bound to the ocular tissues and cause severe damage.

      Except for extremely intense light, such as that from a laser or from prolonged staring at the Sun, the visible wavelengths of the electromagnetic (electromagnetic radiation) spectrum—i.e., visible light rays—rarely cause ocular injury. Retinal damage from staring at the Sun causes visual impairment that often improves after months, although residual deficits can remain. Ultraviolet light (ultraviolet radiation) (UV), however, is strongly absorbed by the cornea surface and is the cause of a condition known as snow blindness (so called because it can occur in skiers from UV light reflected off snow) and a closely related condition called arc eye (also called welder's flash; caused by the intense flash of UV light produced when using a welding rod). Symptoms, consisting of intense pain and copious flow of tears, may not occur until some time after exposure. Treatment consists of temporary eye patching and application of cold compresses and soothing artificial lubricants to the eye. Usually the eyes recover without any permanent damage.

      Extensive exposure to ionizing radiation (radiation injury), without adequate protection for the eyes, can cause cataract formation as well as corneal and conjunctival damage. The lens is also susceptible to X-rays (X-ray), and the eyes require shielding when therapeutic irradiation is used for growths around or near the eye.

Complications of systemic disease

The central nervous system (nervous system, human)
      Since the optic nerve and retina are, embryonically, an extension of the brain, it is not surprising that central nervous system diseases (nervous system disease) frequently affect the eye (eye, human). As a result, visual defects may be the earliest evidence (diagnosis) of general nervous system disease. The nerve supply to the ocular muscles, particularly the extraocular muscles, may also be involved early in some diseases of the central nervous system. This will result in defective movement of the eyes, causing lack of coordination between the two eyes and diplopia, or double vision.

 The nerve fibres that connect the retina with the site of primary visual sensation in the occipital cortex (part of the occipital lobe, located in the rear of the brain) travel across the brain in a regular pattern, and many lesions of the brain, such as tumours (tumour), impinge on part of this pathway. From a detailed examination of the sensitivity of different parts of the retina (visual field testing), it is often possible to localize the site of an intracranial lesion. A frequent first symptom of multiple sclerosis is sudden onset of loss of central vision in one eye caused by optic neuritis. Detailed ophthalmic examination is therefore essential in any patient suspected of having disease of the central nervous system.

      The eye is the one structure in the body in which the blood vessels (blood vessel) are easily visible to the examiner. Changes observed in the retinal vessels mirror changes that are taking place in other parts of the body, particularly those in the brain. In arteriosclerosis, degenerative changes occur in the walls of arteries (artery) that lead to thickening of arterial walls and narrowing of blood vessels and may give rise to complete occlusion (blockage) of a vessel. If the central retinal artery that supplies blood to the inner retina is affected, loss of vision is profound and sudden and, unless the obstruction can be relieved right away, permanent. Occlusion of the retinal veins (vein) is more common than arterial occlusion and also has dramatic effects caused by the damming up of blood in the eye. Blockage of retinal veins results in the bursting of small vessels, retinal swelling, and multiple hemorrhages scattered over the retina. Some degree of recovery of vision is usual but depends on whether a branch of the central vein or the central vein itself is occluded.

      Vascular hypertension, or raised blood pressure, usually occurs in association with arteriosclerosis. Typical changes can be recognized in the small vessels of the fundus (the back portion of the interior of the eyeball). In severe cases, multiple hemorrhages, exudates (leaking proteinaceous fluid), and swelling of the optic disk (the head of the optic nerve) may be present. As with arteriosclerosis, hypertension can lead to vascular occlusions of the retina. The presence of hypertension typically worsens the effects of diabetic eye disease in patients having both afflictions.

      Diabetic eye disease is a major cause of vision loss and blindness. It occurs more commonly with increasing duration of the disease and increasing patient age. People with advanced diabetic retinal disease are at increased risk of heart, kidney, and peripheral vascular disease. The actual cause of the changes in the retinal vessels is not clear, but the natural history of the disease is well recognized. Two general types of diabetic eye disease are known and are characterized as nonproliferative diabetic retinopathy (which lacks abnormal blood vessel growth) and proliferative diabetic retinopathy (in which abnormal blood vessels are present in the retina and sometimes the iris). Each type possesses different levels of severity, although one common cause of vision loss in diabetes, macular edema, can occur in either type at any level of severity. Nonproliferative diabetic retinopathy features changes due to damaged, weakened blood vessels, in which tiny aneurysms (aneurysm) form and small hemorrhages and swelling within the retina can be seen. Areas of retinas that no longer receive appropriate blood flow, a condition called ischemia, can also appear. Visual loss at this stage may be absent or caused by retinal swelling or ischemia. If unchecked, nonproliferative diabetic retinopathy can lead to worsened blood flow to the retina, more severe damage, and the appearance of new abnormal blood vessels on the optic disk, retina, and even iris (proliferative diabetic retinopathy). Changes in these abnormal vessels can cause hemorrhage into the vitreous cavity, retinal detachment (detached retina), and glaucoma.

      Prevention or control of diabetic retinopathy relies on control of blood glucose levels. Various types of laser photocoagulation of the retina are used in certain forms of diabetic retinopathy in an attempt to halt or slow its progression. In cases of retinal detachment or persistent or recurrent hemorrhage within the vitreous gel, more extensive surgical treatments are employed. Glaucoma stemming from diabetic eye disease is often difficult to treat, but both medical and surgical approaches can be attempted.

Thyroid (thyroid gland) disease
      Graves ophthalmopathy (an eye disease related to thyroid (thyroid gland) dysfunction) usually occurs in people with hyperthyroidism, although it can occur in people with normal or even reduced thyroid function. It is characterized by swelling and inflammation of the orbital tissues, including the extraocular muscles, that may lead to retraction of the eyelids (eyelid), restriction of eye movement (causing double vision), and bulging forward of the eyeball (called proptosis). If severe, this could lead to corneal exposure, ulceration, and, in rare cases, loss of the eye. In addition, pressure on the optic nerve behind the eyeball from swollen tissues could cause vision loss. In most uncomplicated situations treatment is conservative, relying only on artificial lubrication, but in severe cases the lids may need to be partially sutured together or surgery may be required to relieve pressure in the orbit. Further eye muscle and lid surgeries may also be needed to correct persistent eye problems related to Graves ophthalmopathy.

      The ocular complications of rheumatoid arthritis involve the sclera and cornea and can cause dry eye. inflammation of the sclera, called scleritis, can cause intense, boring pain and, if severe, could be associated with life-threatening systemic disease. Treatment varies, depending on the disease severity, but generally includes anti-inflammatory and immune-modulating agents.

Visual disorders

Floaters, blind spots, and flashes
      One of the most common visual symptoms is the sensation of small black objects floating in front of the eye (eye, human), known as “floaters.” These move with the eye but lag slightly at the beginning of an eye movement and overshoot when the movement stops. They are due to proteins (protein), cells (cell), and fragments of debris in the vitreous cavity of the eye. In certain conditions, as when looking at a blue sky, almost everybody can perceive them, and they are normal phenomena. However, a sudden increase in their number may indicate degenerative changes in the vitreous, which are particularly likely to occur in nearsighted (myopia) individuals and in older people. These changes, although annoying, are of no serious import. The appearance of many floaters, however, may be associated with inflammation, bleeding in the eye, or a retinal tear and should be evaluated urgently.

      Blind areas in the field of vision, called “blind spots (blind spot),” occasionally force people to seek medical advice. Any condition that causes failure of function of part of the retina, the optic nerve, or the optic pathway to the brain can cause such a blind spot, and the symptom requires careful investigation. There is a naturally occurring blind spot in each visual field that corresponds with the lack of retinal elements where the optic nerve leaves the eye. The brain is so skillful in filling in the visual pattern that the normal blind spot can be detected only by special methods.

      Flashing lights in the field of vision are caused by stimulation of the retina by mechanical means. Most commonly this occurs when the vitreous degenerates and pulls slightly on its attachments to the retina. Similar symptoms also arise when the retina becomes torn or detached, causing brief flashing lights to be seen. The combination of the abrupt onset of multiple flashes and floaters with a sensation of a shadowy “curtain” or “veil” coming across the vision strongly suggests the presence of a retinal tear and detachment.

night blindness and defects of colour perception
      Defective vision under reduced illumination may reflect the congenital or hereditary condition known as retinitis pigmentosa or may be acquired as a result of severe deficiency of vitamin A.

      Defective colour vision affects men more often than women. Total colour blindness is extremely rare and is nearly always associated with poor vision in ordinary light and with nystagmus. Individuals who are partially colour-defective, such as those with red-green colour blindness, may not be aware of the disability until special instruments are used to test the person's ability to distinguish between hues in one or another part of the visible light spectrum. With these less-severe colour deficiencies, other visual functions are usually normal.

      Eyestrain, or asthenopia, is the term used to describe subjective symptoms of fatigue, discomfort, lacrimation (tearing), and headache following the use of the eyes. Such symptoms may result from intensive, prolonged close work. In people with perfectly normal eyes, eyestrain may indicate abnormalities of muscle balance or refractive errors. Eyestrain is more likely to be manifest during periods of fatigue or stress and is common among students studying for examinations. Refractive errors require correction, and muscle imbalance may require treatment. Psychological factors can be more important to address than physical factors.

Refractive (refraction) errors
      In a normal eye, rays of light from distant objects come to a focus on the retina. In near vision the refractive power of the eye is increased by altering the shape of the lens (i.e., causing it to become thicker) to focus the image on the retina. This ability to alter the shape of the lens decreases with age until fine print cannot be read at a normal reading distance. This condition is known as presbyopia and usually becomes increasingly problematic after age 40. It is corrected by the use of convex lenses for reading.

  In some eyes, rays of light from distant objects are not brought to a focus on the retina but are focused on a plane in front of the retina, as in myopia (nearsightedness), or behind the retina, as in hyperopia (farsightedness). In myopia, near objects are brought into focus on the retina but distant objects can be seen clearly only with the aid of concave lenses. In hyperopia, distant objects can sometimes be brought into focus by using the accommodative power of the lens, and in young people there is usually sufficient accommodation to enable them to see close up as well. The constant accommodative effort required, however, may result in problems such as asthenopia or esotropia, and the necessity for accommodating for distance can be overcome by wearing convex glasses.

 Another type of refractive error is astigmatism. In this condition the refractive power of the eye varies in different axes, depending on the path the light takes through the cornea. This is due to the presence of nonuniform corneal curvature and results in the distortion of vision at all viewing distances. Astigmatism is a common condition and can be corrected with the use of cylindrical lenses in eyeglasses or contact lenses.

      In general, refractive errors are easily corrected with glasses and are rarely accompanied by any serious disease of the eyes. However, hyperopia is a factor in the development of some kinds of strabismus and vision loss (amblyopia) in children, and high degrees of myopia may be associated with serious degenerative changes within the eye, particularly the retina.

Ophthalmological examination and corrective devices

Ophthalmological (ophthalmology) examination
      An ophthalmological examination includes a thorough history, with a particular focus on a patient's symptoms. The ophthalmologist physically examines the eyes with special devices and does various tests to determine visual function. The most important subjective test is for visual acuity. This is usually performed by having the patient read, from a set distance, an eye chart with a series of letters of graded sizes, which become increasingly smaller as the chart is read from top to bottom. The person is asked to read the lowest line legible, and visual acuity is then expressed in terms of the size of the letter and the distance at which it is read, relative to a person with normal vision.

      The visual field can be assessed by many methods. The confrontation visual field exam is the most basic test and involves a simple assessment of peripheral vision. In this test one eye of the patient is covered, the ophthalmologist presents one or more fingers in the peripheral visual field, and the patient indicates the number of fingers displayed. The Goldmann visual field exam is another manual test, in which the patient focuses straight ahead on a central point while an object is moved inward from the periphery. The ophthalmologist performs this several times, testing different areas of the visual field and drawing a map of the visual field for each eye. Another technique, called automated perimetry, uses a bowl-shaped instrument into which the patient looks, focusing on a central point. The instrument sends out random computer-generated flashes of light of varying durations, intensities, and locations that may or may not be within the patient's visual field. When the patient sees a flash, he presses a button, sending a signal that is automatically detected and incorporated into a map of his visual field. Automated visual field testing is commonly used today, especially in assessing glaucoma.

      Other subjective examinations include colour vision testing and tests of visual perception under reduced illumination. Examination of the external eye and part of the anterior segment is facilitated by the use of a binocular microscope attached to a slit lamp (a variable source of light that projects the image of a slit onto the eye). The direct and indirect ophthalmoscope has an illuminating system that lights up the interior of the eye and a viewing system through which the fundus can be observed. Photography of the anterior part of the eye and of the fundus is also widely used.

      Other specialized methods of examination include examination of the angle of the anterior chamber by means of a specially designed mirrored contact lens (goniolens), which is used in conjunction with the slit-lamp microscope. The electrical responses of the retina and brain to light entering the eye can also be recorded and are of great value in certain conditions.

      Estimation of the intraocular pressure is an important part of an ophthalmological examination and is accomplished by an instrument called a tonometer. This instrument is designed specifically to measure the tension or pressure that exists within the eyeball. Many types of tonometers are used, each of which has unique advantages and disadvantages.

      The refractive state of the eye can be measured objectively or subjectively or by a combination of methods. The simplest method is subjective, using lenses of different powers to give a trial-and-error estimate of the best correcting lenses. More accurate results can be obtained by using an instrument known as a retinoscope, which gives an objective assessment of the refraction that can subsequently be modified by subjective methods to suit the individual requirements of the patient.

      Many other tools are used to examine the eye and aid in the diagnosis of eye diseases, including ultrasound, retinal angiography, and devices to measure corneal curvature and topography. New devices and techniques are constantly being introduced, advancing the eye care specialist's ability to diagnose and treat eye disorders.

Optical aids
      The most widely used optical aids are eyeglasses (spectacles), and the technical design of eyeglass lenses has advanced considerably. A simple biconcave or biconvex lens causes considerable vision distortion if objects are viewed through the periphery of the lens, but, if the back surface of the lens is made concave and the required power is attained by altering the curvature of the front surface, improvement in peripheral vision results. Modern eyeglass lenses are of this form.

      Most older people require an additional lens for reading, which can be incorporated with the distance correction in the form of a bifocal lens. In some occupations an intermediate distance is also required, and a third segment can be added, forming a trifocal lens. The complete range of correction from distance to near can be achieved by means of a progressive lens, in which lens power increases as the eye moves downward, with the upper segment of the lens providing the correction for distance and the lowest segment of the lens representing the reading correction. By slightly tilting the head, it is possible to find the optimum correction for intermediate distances.

      The distortion of peripheral view when using conventional eyeglasses occurs because the correcting lens does not move when the eye moves. This problem can be completely overcome by the use of contact lenses (contact lens), which fit the anterior surface of the cornea and thus move with the eye. The earliest types were larger than the cornea and were uncomfortable to wear, but the development of smaller “hard” lenses greatly increased the scope and usefulness of contact lenses. Even so, the length of time for which they could be worn was limited until rigid gas (oxygen)-permeable lenses were introduced, which allow oxygen to pass through to the cornea much more effectively. Flexible “soft” lenses, made of water-absorbing plastic gel, also allow oxygen to reach the surface of the cornea reasonably well. The type of contact lens that best suits an individual is dependent upon the refraction and any coexisting corneal problems.

      For those persons who cannot obtain useful vision with ordinary eyeglasses or contact lenses, much can still be done through the use of special lens systems known as low-vision aids. These devices provide a magnified image but reduce the visual field. Their main value is to enable a person to read normal print that would otherwise be difficult to read. They can be of use for distance, particularly when viewing conditions are relatively static, as with the cinema, theatre, or television. Large-print materials, video magnifiers, closed-circuit television, and optimal lighting and contrast conditions are also of great benefit. For those with profound or complete vision loss, enhancement of other skills that help substitute for vision is crucial to optimizing function.

      Vision loss exists in a continuous spectrum from near-normal vision to total blindness. The definition of legal blindness can vary considerably and is a designation usually used to qualify individuals for government aid. Total blindness refers to a person who cannot perceive light whatsoever.

      There is wide variation in the causes of blindness in different parts of the world. This is partly due to geographic and climatic conditions, but, more important, it is also due to differences in standards of hygiene and the availability of medical care. Infections, particularly trachoma, spread most easily in warm countries where the population is often crowded into small villages lacking in adequate hygienic facilities. cataract is still high on the list of causes of blindness in many countries, even though it is curable by surgical means. As the standards of general medical care increase and life expectancy increases, the pattern of blindness changes and degenerative conditions, diabetic disorders of the retina, and genetically determined diseases become more prevalent. Advances in the prevention and the medical and surgical treatment of blindness can be of benefit only to a population that has access to medical care. Until the nutritional and hygienic standards of a large part of the world's population are improved, preventable blindness will remain a problem. Fortunately, charitable efforts exist to bring modern eye care to disadvantaged areas of the world.

Edward S. Perkins Daniel M. Albert David M. Gamm

Additional Reading
Gary H. Cassel, Michael D. Billig, and Harry G. Randall, The Eye Book: A Complete Guide to Eye Disorders and Health (1998); Daniel M. Albert and Frederick A. Jakobiec (eds.), Principles and Practice of Ophthalmology, 2nd ed., 6 vol. (2000); Daniel M. Albert and Diane D. Edwards, The History of Ophthalmology (1996).David M. Gamm

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Universalium. 2010.

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