A

Age-related macular degeneration (AMD) is a chronic disease that occurs when tissue in the macula (the part of the retina responsible for central vision) deteriorates, resulting in blurred central vision or a blind spot in the center of the visual field. This condition tends to develop as one grows older, hence, the name. Macular degeneration is the leading cause of severe vision loss in people over the age of fifty. The first sign may be the need for more light when close-up work is done. Fine newsprint may become harder to read and street signs are more difficult to recognize. Eventually, an object, that should be smooth, will appear distorted or crooked. Gray or blank spots may mask the center of the field of vision. The condition may progress rapidly leading to severe vision loss in one or both eyes. (See more under Macular Degeneration.)

AIDS (acquired immunodeficiency syndrome) is a disease of the immune system characterized by a deficiency of thymus-derived lymphocytes. It is caused by a virus called HIV (human immunodeficiency virus). Having the HIV virus is not the same as having the disease called AIDS. This is the last stage of an HIV infection. Visual disorders or disturbances affect over fifty per cent of AIDS patients; and one-third of these have cytomegalovirus retinitis, an infection of the retina caused by the cytomegalovirus (CMV). This virus is a member of the herpes virus family and may cause primary, latent, and persistent infections. Cytomegalovirus retinitis progressively destroys tissue that leads to scarring and possible retinal detachments. Symptoms of the condition include blind spots, blurred vision, and loss of peripheral vision. The infection may also be present without any apparent symptoms. The infection progresses rapidly and may cause blindness if left untreated. Current treatments involve the use of two antiviral drugs (ganciclovir and foscarnet). Ganciclovir stops the progression of the disease, but is contraindicated for use with AZT. Foscarnet also stops the progression of the infection and can be taken in conjunction with AZT. However, it is less effective in preventing relapses of CMV retinitis after the initial high-dose induction phase of the treatment.

Air bags are installed in many cars and credited with saving lives. However, they do pose risks, including eye injuries, especially with children. Therefore, to prevent such injuries, children should not be permitted to ride in the front seat. Some possible eye injury outcomes include cataracts, glaucoma, blood in the front chamber of the eye, alkali burns, temporary loss of consciousness and visual acuity, laceration of the eyelid, inflammation of the iris, corneal lesions and abrasions, black eye, and swelling and hemorrhage of blood vessels under the outer surface of the eyeball.

Albinism is a hereditary condition in which all or part of the body lacks pigment. Albinism may affect the skin, eyes, and hair of the individual and is thought to be caused by an enzyme deficiency involving metabolism of melanin during prenatal development. There are two types of albinism: oculocutaneous and ocular.

• Oculocutaneous albinism involves a lack of pigmentation in the eyes, skin, and hair. This condition is further subdivided into two groups: tyrosinase-positive or tyrosinase-negative albinism, depending on the presence or absence of the enzyme tyrosinase in the hair bulbs. Tyrosinase inhibits the formation of pigment in the body. Tyrosinase-negative individuals have white hair, pink skin, and pale blue eyes. Tyrosinase-positive individuals produce some melanin and vary as to physical condition and coloring.
• Ocular albinism involves the lack of, or reduced amount of, pigmentation of the eye. Individuals with ocular albinism may not lack pigmentation of the skin and hair. Those with ocular albinism and tyrosinase-negative oculocutaneous albinism usually experience severe visual disorders. These may include a visual acuity of 20/200 or less; nystagmus (an involuntary movement or jerking of the eyes), amblyopia (a condition in which the brain does not receive information from one or both eyes as a result of a failure of vision development as an infant or child); iris transillumination (the ability of light to pass through the normally pigmented iris); lack of fundus pigmentation; and photophobia (a sensitivity to light). Albinism cannot be cured, but the symptoms or eye conditions presented may be treated with therapy, surgery, corrective lenses, or light-reducing lenses.

Amaurosis is the loss of sight without apparent lesion of the eye. It is usually caused by disease of the optic nerve, spine, or brain. Leber’s congenital amaurosis is a hereditary blindness occurring at, or shortly after, birth. It is associated with an atypical form of diffuse pigmentation and commonly with optic atrophy and the thinning (attenuation) of retinal vessels.

Amaurosis fu’gax is a sudden, temporary, or fleeting blindness.

Ametropia is a vision problem caused by the eye’s inability to focus properly. Ametropia occurs when the cornea and the lens of the eye cannot focus light effectively, and the resulting image, onto the retina. This focusing impairment is also called a “refractive error.” Refractive errors as hyperopia (farsightedness), myopia (nearsightedness), astigmatism (distorted image), or presbyopia (aging eyes) result in blurred vision. Ametropia can be diagnosed in a routine ophthalmologic examination and is usually corrected with eyeglasses or contact lenses.

Amsler grid is a specialized testing device, invented by Professor Marc Amsler and used to detect disorders and scotoma (blind spots) in the central field of vision. It is routinely used to screen patients for age-related maculopathy during the ophthalmologic exam. The Amsler grid is a black-on-white grid of 400 squares and composed of twenty horizontal squares by twenty vertical squares. A black dot is in the center of the grid where the two central horizontal and vertical lines intersect. With one eye covered, the patient is asked to look at the center dot and then asked if the dot is visible, all the sides are visible, the entire grid is visible, any lines are wavy or bent, and if there are any blurs, color differentiations, or distortions within the grid. Any change viewed on the grid could indicate scotomas or age-related maculopathy.

Aneurysm is a diseased or weakened section of a blood vessel that fills with blood and dilates. Aneurysms may occur within any blood vessel throughout the body. Within the blood vessels of the brain, an aneurysm may be the result of atheriosclerosis. These may rupture and bleed into the cerebral fluid, a condition called a subarachnoid hemorrhage. This may cause increased pressure within the brain and a decreased blood supply resulting in a stroke. Paralysis of the limbs, sudden changes in the field of vision, speech, memory disorders, and thinking or reasoning may follow. Aneurysms of the eye’s retina may occur as a result of another disease or disorder as diabetic retinopathy. As the disease progresses, neovascularization occurs producing weakened blood vessels that rupture, hemorrhage, and may cause retinal detachment and scar tissue resulting in permanent vision loss.

Aniridia, or irideremia is the absence of the iris within the eye. It is a congenital or acquired condition and usually bilateral. In many cases of a congenital condition, a vestigial portion of the iris root or margin is present. It is often seen in conjunction with congenital cataracts. Glaucoma tends to develop in early adolescence in approximately twenty-five percent of the congenital cases, because of the abnormal angle of the anterior chamber and blockage by the iris root. Those affected are photophobic, and may experience amblyopia (loss of vision in one eye due to disuse) and nystagmus (involuntary jerking of the eyes). Associated retinal abnormalities or malformations may also exist. Very rare, congenital aniridia develops in about the twelfth week of pregnancy, affecting one in 500,000 to a million children. Acquired aniridia occurs as a result of trauma or injury. The iris becomes dislodged or torn from the ciliary body and contracts into a small pellet. It moves to the outlet of the anterio chamber where it may form a blockage and cause secondary glaucoma. The glaucoma of both types of aniridia is treated with medications or with such surgical procedures as goniotomy or trabeculotomy if medications prove ineffective. The prognosis for vision is generally poor in either case.

Anisocoria is a condition in which the pupils differ in size. Average-sized pupils range from three to four millimeters. All pupils are small at birth. Differences in size become more apparent as the individual grows and achieves maximum pupil size by adolescence. The pupil may become smaller with advancing age. Approximately seventeen percent of the population have minor amounts of anisocoria, but only four percent have pronounced cases. The difference in size may vary from minute measurements to noticeable degrees and may vary during the course of a day. On average, women tend to have larger pupils than men. Blue, or less pigmented irises, are generally larger than brown. Myopic (nearsighted) individuals usually have larger pupils than those with hyperopia (farsightedness). Anisocoria may be congenital or acquired, as from ingestion of oral contraceptives. The condition may be a first indication of pupillary defects but may also exist in the absence of any ocular disorder. One or both anisocoric pupils that fail to properly react to light may indicate neurological problems.

Anisometropia is a common condition in which the eyes have unequal refractive power or unequal ability to focus and see. The degree of anisometropia can be determined during an ophthalmologic examination. It is corrected by prescribing a stronger prescriptive lens for one eye than the other to equalize the eyes’ refractive powers. Contact lenses often can greatly reduce the effects of the condition. Without correction, the condition can cause amblyopia in children. The weaker anisometropic eye sends less reliable information to the brain than does the stronger eye. The visual cells of the brain ignore the information of this eye, become dysfunctional, and cause the eye, however healthy, to become amblyopic.

Anterior Chamber is an area of the eye between the cornea and the lens. The chamber is filled with a clear liquid called aqueous fluid. The cornea and the lens of the eye lack blood vessels, which deliver nutrients and carry away waste. The aqueous fluid, produced by the ciliary body, provides these services as it circulates within the anterior chamber. The aqueous fluid leaves the chamber through the Schlemm’s canal, a channel where the iris and the cornea meet. If the channel becomes blocked, fluid builds up in the eye and causes a rise in the intraocular pressure, a condition known as glaucoma.

Aphakia refers to an eye without a lens. After cataract surgery, during which time the lens of the eye is removed, the eye is aphakic. A monocular aphake is a person without the lens from one eye. A bilateral aphake is a person missing both lenses. Without the lens, the eye is unable to accommodate or adjust to focus on an object. Commonly, elderly cataract surgery patients experience limited accommodation before surgery and therefore, surgery-induced aphakia minimally affects their vision. People with aphakia are at a greater risk for some types of retinal detachment than those who are not affected. Aphakia also distorts the size of objects, makes images appear larger, and may bend or curve straight lines. Distance judgment problems may result. Changes in color-perception may also occur whereby colors may appear brighter, pinker, or bluer. Aphakia can be corrected with glasses, contact lenses, or intraocular lenses (IOLs).

Appetite supressants include amphetamines, dextroamphetamines, methamphetamines, and phenmetrazine compounds. Systematic use of these drugs can affect the eyes, causing such side effects as pupil dilation and impaired vision. These drugs can also cause difficulty with accommodation and convergence, which can make reading difficult. Persistent use can lead to closed-angle glaucoma in some people.

Aqueous Fluid/Aqueous Humor is a clear liquid that fills the anterior chamber of the eye, located between the cornea and the lens. The cornea and lens must be transparent to function and, therefore, contain no blood vessels. The aqueous fluid functions as blood vessels in the anterior chamber by delivering the nutrients and antibodies to the area while carrying away the waste materials from the avascular cornea and lens. This clear, thin fluid is produced by the ciliary body at a rate of approximately two cubic millimeters (about one drop) per minute. The nutrients and antibodies are pumped from blood vessels within the ciliary body and mix with the aqueous which flows from the ciliary body, behind the iris, across the lens, through the pupil into the anterior chamber, and along the back surface of the cornea. The aqueous fluid drains from the eye at the base of the cornea, an outflow channel called the Schlemm’s canal. The fluid is strained of waste and collected in the canal and flows to small veins in the sclera and joins the body’s blood stream. When draining channels become blocked or if the eye overproduces aqueous, the fluid may build up. Intraocular pressure builds within the eye, damaging the nerve endings of the retina, producing a condition known as glaucoma.

Arcus senilis, or cornea ring is a grey or white ring or arc that appears on the edge of the iris (colored part of the eye) where it borders the sclera (white part). The ring is acually located on the margin of the cornea, the clear, protective covering of the eye. This condition is caused by ageing and is most often noticed in dark-eyed people. Arcus senilis is a harmless condition that does not affect vision and is not a sign of another disorder.

Argon laser was invented in 1964 by William Bridges, a scientist at Hughes Aircraft. It is an instrument often used to treat diabetic retinopathy, a complication of diabetes and a leading cause of blindness in the US. The retinal tissue loses oxygen normally brought in by the vessels and develops new vessels (neovascularization) that tend to be weak and bleed. Continuous leaking and the formation of retinal scars may lead to blindness. If neovascularization has occurred, unhealthy, oxygen-deprived tissue is treated with the laser. The directed energy of the laser causes scarring and prohibits the tissue from forming new, weak vessels.The argon laser may also be used to treat subretinal neovascularization, a disorder caused by age-related maculopathy (ARM), a major cause of blindness associated with ageing, and producing scarring of the macula. Subretinal neovascularization is caused by a break in the pigment epithelium, a layer just below the retina. A collection of blood vessels below the retina bleeds into the retina through the break and causes scarring. During argon laser treatments, burns are placed around the neovascularization areas to cauterize the bleeding. However, the bluegreen light of the argon laser is absorbed by the pigment of the inner retina, above the deeper affected layers. This may cause damage to the inner retina.The red krypton laser is often substituted for the argon on this procedure since it is absorbed only in the melanin pigment of the deepest layers of the retina where the neovascularization occurs. Treatment with the krypton laser is preferred since it allows the inner, unaffected retinal layers to remain untouched while treating only the deep, affected layers.Laser therapy, called laser trabeculoplasty, may be an alternative to surgery in the treatment of glaucoma. When medication fails to lower the intraocular pressure, laser trabeculoplasty with the argon may be used to open the drainage area located where the cornea meets the iris. A series of burns (1000 to 2000) are placed in the drainage area, which causes scarring and openings within the meshwork of drainage channels. The fluid drains more easily from the eye and intraocular pressure is reduced.

Arterial occlusion is a blockage of the arteries of the retina. When this occurs, blood is prevented from reaching the retina, and vision is threatened. Arterial occlusions may be caused by carotid artery stenosis, retinal artery emboli, retinary artery occlusion, and temporal (giant cell) arthritis. Arterial occlusion occurs most frequently in people between the ages of 50 and 80.

• Carotid artery stenosis occurs when one of the carotid arteries, which lie on each side of the neck and supply blood to the brain, becomes narrowed with arteriosclerotic plaque. The blood flow to the brain and the eye is decreased, and a TIA (trans ischemic attack) or ministroke may occur. TIAs are warning signs of more serious impending strokes and include such symptoms as a tingling in the face or limbs, difficulty speaking, headaches, dizziness, vision disturbances, inability to swallow, mental confusion, and loss of memory. During a carotid artery stenosis (TIA), vision may suddenly be lost in one eye. This loss is usually painless and generally lasts for a short period of time. Temporary loss of vision of this type may be accepted as a symptom or warning sign of a TIA. Individuals with one or more of these symptoms should be examined immediately.
• Retinal arterial occlusion is a blockage of the retinal artery that does not involve emboli. This is most common in hypertension patients as the result of arteriosclerosis. The sight loss is usually sudden, painless, and permanent. Retinal artery emboli are small particles of blood clots that enter the blood’s circulation and become wedged in a retinal artery. These emboli may cause obstructions that result in vision loss, albeit usually swift and painless. If treatment is not begun within minutes, vision may be permanently lost. However, results from current treatments to restore vision are often disappointing and therapy should include treatment for the underlying cause, as arteriosclerosis, hypertension, and heart disease.
• Temporal, or giant-cell arteritis, is a chronic condition that involves inflammation of all the body’s arteries. This inflammation causes the artery walls to thicken, which may block the flow of blood to vital organs, including the eyes. Symptoms include dizziness, headache, weight loss, scalp tenderness, jaw pain, depression, low-grade fever, and vision loss which may be sudden and permanent. Temporal artheritis may last from two to three months and is rarely seen in patients under the age of fifty.

Artificial tears are used to treat dry eyes, a condition that can occur with allergies, keratoconjunctivitis (corneal and conjunctival inflammation), extreme vitamin A deficiency, trachoma, chemical burns, eye disorders, and such diseases as Bell’s palsy. There are over-the-counter artificial tears, as well as those prescribed by physicians. Artificial tears are known by such generic names as hydroxyethylcellulose, hydroxyproplycellulose, hydroxypropl methylcellulose, methylcellulose, polyvinyl alcohol and other polymeric solutions. There are also a myriad of trade names.

Artificial vision system uses technology to produce images that can be detected and recognized by blind people. Developed by Dr. William H. Dobelle, the device is battery operated and weighs about ten pounds. It works through a miniature digital video camera and an ultrasonic distance sensor, which are built into a pair of sunglasses. The sensor connects through a cable to a small computer, which the blind person wears around his/her waist. The computer processes the video and distance signals, then uses computer-imaging technology to transmit the image to the person wearing the device. After the image has been transmitted, the computer triggers a second computer, which sends pulses to 68 platinum electrodes that have been surgically implanted into the part of the brain that controls vision. The second computer is screwed into the wearer’s skull bone, with its shaft protruding through the skin. When the electrodes are stimulated, they generate a display of phosphenes, which are bursts of light caused by pressure on the eyeball. Phosphenes appear as black-and-white images, and the wearer of the device possibly can learn to recognize patterns and distinguish images. It is reported that one blind man who received the artificial vision system made such progress that he was able to recognize different character sets commonly used in visual acuity tests. The system also has an electronic interface that is a radio frequency. It can replace the camera and allow patients to watch television and use a computer. The artificial vision system, also called a visual prosthesis, is still in the testing stages; but it is expected to be available on a limited commercial basis by the year 2002. Dr. Dobelle, who heads the Dobelle Institute at Columbia Presbyterian Medical Center in New York City and the Institute Dobelle AG in Zurich, Switzerland, has been working on the vision system for about thirty years.

A-scans measure the length of the eye by using ultrasonic sound waves. An A-scan is performed prior to cataract surgery to predetermine the power of an intraocular lens implant or to ascertain whether an implant is even necessary. Almost all cataract surgery involves not only the removal of the cloudy lens, but replaces it with a clear, plastic lens (an implant or intraocular lens) which will serve the same focusing function as the natural lens. Since the eye cannot be measured with a ruler, the A-scan machine uses sound waves to measure the axial length. The length of the eye is measured from the cornea, through the pupil, through the lens (or cataract), through the vitreous, and all the way through to the retina. The normal length is about 24 millimeters, or approximately one inch. A shorter eye usually results in farsightedness, or hyperopic. A longer eye usually means nearsightedness, or myopic. A shorter eye needs a stronger lens to focus properly while a longer eye requires a weaker lens in order to focus. An extremely long eye may not require an implant. The length of the eye has no correlation with a person’s height, only with the eyeglass prescription. In severe myopia, the strength of the implant, as calculated by the A-scan, may be so weak as to make an implant unnecessary. Although the A-scan is quite accurate in measuring the length of the eye and determining optimum power of an implant, it is by no means 100% accurate in determining whether one will need glasses for reading or distance after cataract surgery. Most patients will still need glasses for one or the other or both after surgery since small amounts of astigmatism, nearsightedness, or farsightedness often remain. The information from this painless test is analyzed by a computer, along with the results of keratometry.

Astigmatism results when an image is not focused to a point because of an irregularly shaped cornea. The word “astigmatism” comes from the Greek a, meaning “without”, and stigma, meaning “point.” The result is a distortion of an image typically seen in one direction more than in others, either horizontally, vertically, or diagonally. In the normal eye, the dome of the cornea is curved evenly and smoothly in all directions allowing for the image seen to be focused perfectly on the retina. Some corneas, however, are not evenly and smoothly curved, but are curved more steeply in a particular direction. For example, a round ball viewed through such a cornea, will be seen as oblong. Thus, a basketball could look more like a football. In most instances, astigmatism is inherited; but it can also develop after an injury or a disease. About half of nearsighted people will also have some astigmatism, which does not change throughout life. More serious astigmatism can be corrected with a cylindrical lens that counteracts the uneven curvature of the cornea. The same lens used to correct nearsightedness or farsightedness can also be made to neutralize the astigmatism.