Vision disorders

Definition

Vision disorders in seniors refer to a group of eye diseases or disorders that cause vision impairment. Deterioration or loss of eyesight is a major problem for seniors as it can interfere with activities of daily living (ADLs) and be a risk factor for depression .

Description

The four most common eye disorders in seniors are (in alphabetical order) age-related macular degeneration (AMD), cataract, diabetic retinopathy , and glaucoma .

Age-related macular degeneration

Age-related macular degeneration (AMD) is the single most common cause of vision loss in seniors. It is a disorder of the macula, the central portion of the light-sensitive retina at the back of the eye. The macula is the part of the retina where one's central vision is sharpest.

There are two basic forms of AMD, wet (or exudative) and dry (or atrophic). About 90 percent of seniors diagnosed with AMD have the dry form, which accounts for only 20 percent of cases of blindness caused by AMD. The 10 percent of seniors diagnosed with wet AMD account for 80 to 90 percent of cases of total vision loss caused by the disorder.

Cataract

Cataract is an eye disorder that develops when the lens of the eye develops cloudy or opaque spots, in most cases after age 40. Cataracts are classified according to their location within the lens. Nuclear cataracts develop in the center of the lens; cortical cataracts develop along the sides of the lens; and posterior subcortical cataracts form in the central part of the lens on the posterior lens capsule.

Diabetic retinopathy

Diabetic retinopathy is an eye disorder that develops in seniors who have had diabetes for five years or longer. It accounts for almost 7 percent of cases of blindness in the United States. There are two major types of diabetic retinopathy, proliferative and nonproliferative. Nonproliferative diabetic retinopathy (NPDR) is often the first stage of the disorder. In this type, tiny blood vessels in the retina can leak or become blocked. The patient may not notice any changes in his or her vision; on the other hand, fluid leaking from the broken vessels may cause swelling of the macula and blurring of vision. NPDR can progress to proliferative diabetic retinopathy (PDR), as the lack of oxygen supply to the retina causes new blood vessels to form (proliferate) alongside the retina. These new vessels may bleed, cloud vision, or

cause the retina to pull away from underlying tissue (retinal detachment ).

Glaucoma Glaucoma is an eye disorder resulting from destruction of the cells in the optic nerve caused (in most cases) by increased intraocular pressure or IOP. Two other classic signs of glaucoma are cupping of the optic disc and loss of the visual field. There are two major types of glaucoma, open-angle and closed-angle. These terms refer to the angle inside the eye where the cornea (the clear curved structure at the front of the eye where light enters the eye) and the iris (the colored part of the eye) meet. In a normal eye, fluid from a chamber at the front of the eye flows through a meshwork of tissue in this angle to nourish the tissues of the eye. If the flow of fluid through the meshwork becomes too slow, pressure inside the eye builds up. If the optic nerve is not able to tolerate the increased pressure, its cells begin to die. This condition is called open-angle glaucoma and accounts for about 80 percent of cases of glaucoma.

In closed-angle glaucoma, which is a less common form, the edge of the iris brushes against the lens and completely closes off the chamber at the front of the eye. Fluid backs up behind the meshwork in the angle and can cause irreversible damage to the eye within 48 to 72 hours. Closed-angle glaucoma is a medical emergency; the patient needs to see an ophthalmologist (doctor who specializes in eye disorders) as soon as possible.

Demographics

Approximately one senior in every three has some eye disorder involving partial or total loss of vision by age 65. Age is considered an independent factor for vision loss in seniors; for this reason, all seniors should be screened for vision problems at least once a year. About 1 million Americans over the age of 40 are blind as of 2008; those who are considered visually impaired number about 3.5 million.

In general, blacks and Hispanics are at increased risk of age-related eye disorders. Alaska has the lowest percentage of visually impaired seniors, while North Dakota has the highest. The rate of eye disorders in all races and ethnic groups increases rapidly after age 75.

The demographics of specific vision disorders are as follows:

• AMD: About 1.6 million Americans over age 40 have a late form of either wet or dry AMD. Black women are at slightly greater risk than black men.
• Cataract: One in six Americans over the age of 40 has a cataract in one or both eyes; more than half of seniors over 80 have cataracts. Women are at slightly greater risk than men for cataract formation.
• Diabetic retinopathy: The risk of diabetic retinopathy increases the longer a person has diabetes. Twenty years after diagnosis, almost all adults with type 1 diabetes and more than 60 percent with type 2 diabetes have some degree of diabetic retinopathy. As of early 2008, about 5.3 million adults in the United States are affected by diabetic retinopathy.
• Glaucoma: About 1 in 200 adults under the age of 50 and as many as 1 in 10 over the age of 80 have some form of glaucoma. With regard to open-angle glaucoma, blacks over the age of 40, Hispanics over the age of 60, and people with a family history of glaucoma are at increased risk. Blacks in the United States are five times as likely as Caucasians to develop open-angle glaucoma. Women, persons
  
  
  of Chinese descent, Eskimos, and people who are farsighted have an increased risk of closed-angle glaucoma.

Causes and symptoms

Causes

The causes of vision disorders in seniors vary:

• AMD: Dry AMD is caused by the buildup of fatty deposits underneath the cells in the retina that sense light. The layer of supportive cells directly under the retina may also start to break down. Wet AMD develops when new tiny blood vessels form underneath the retina, leaking fluid or breaking open and causing scarring of the retina. It is not yet known what triggers the processes leading to either wet or dry AMD; however, risk factors for the disorder include Caucasian race; a history of cigarette smoking; and variations in a specific gene located at chromosome 1q31.
• Cataract: Cataract is caused by the formation of cloudy or opaque spots in the crystalline lens of the eye. Although some children are born with cataracts, in most cases cataracts develop as a result of exposure to ultraviolet light from the sun or from exposure to other forms of radiation. Other risk factors for cataract formation include malnutrition, smoking, alcoholism, treatment with corticosteroid medications, or a history of eye surgery or trauma to the eye.
• Diabetic retinopathy: Diabetic retinopathy is caused by the swelling and breaking of tiny blood vessels in the retina of the eye due to weakness of the walls of these blood vessels. This weakness is caused by high blood sugar levels—which is why it is critical for people diagnosed with diabetes to control their blood sugar.
• Glaucoma: Glaucoma is caused by the buildup of fluid in the interior of the eye resulting from slowed or blocked passage of fluid between the structures in the front of the eye. The increase in intraocular pressure (IOP) can cause irreversible damage to the cells in the optic nerve. The vulnerability of the nerve to increased IOP varies from person to person, however, and is thought to be influenced by genetic factors. Some people can have an increase in IOP without developing glaucoma.

Symptoms

The symptoms of specific vision disorders are as follows:

• AMD: Blurred vision, usually in both eyes; dark areas in the central part of the visual field; distortion of letters or images; difficulty reading. Dry AMD typically develops gradually, while wet AMD is more likely to develop rapidly.
• Cataract: Blurred vision in either one or both eyes; glare; painless; seeing double in the eye that has the cataract.
• Diabetic retinopathy: Blurred vision; floaters; loss of portions of the visual field; poor night vision.
• Glaucoma: Shrinking or narrowing of the visual field; blurred vision (in advanced glaucoma). Patients with open-angle glaucoma may not notice any changes in vision until the disorder is advanced; then they may notice that they are gradually less able to see objects out of the corner of their eye or that they are losing their side vision. Patients with closed-angle glaucoma, however, may become severely nauseated and headachy within hours of the angle closure as well as have blurred vision and pain or redness in the affected eye. Another common symptom of closed-angle glaucoma is seeing halos around lights.

Diagnosis

All seniors should have an eye examination that involves dilation at least once a year (twice a year if the senior has been diagnosed with diabetes). Dilation involves the use of eye drops that cause the pupil of the eye to open wider than usual; it allows the doctor to examine the retina and the optic nerve at the back of the eye for signs of disease.

In addition to dilating the eye, the doctor will use the following tests or techniques:

• Visual acuity test. This test involves asking the senior to read an eye chart to determine how well he or she can see at various distances.
• Visual field test. The examiner asks the patient to close one eye; then moves his or her hand to one side, moves it back toward the patient, and asks when the patient can see the hand again. This test evaluates the loss of side vision.
• Tonometry. This test measures IOP. Older types of tonometry required the application of a local anesthetic to the eye followed by the use of a probe or pressure sensor that had to make direct contact with the cornea. Newer devices can measure IOP through the eyelid or by using an air puff.
• Slit lamp examination. The slit lamp is a device that contains a high-intensity light source that can be concentrated into a narrow beam or slit. It can be used to examine the structures in the front of the eye to check for cataracts, or used after dilation to examine the retina and the optic nerve.
• Gonioscopy. The gonioscope is a device containing a mirror that is used together with a slit lamp to measure the angle between the cornea and the iris in the front of the eye. This test is done to check for glaucoma.

Treatment

Treatment varies depending on the specific disorder:

• AMD: There is no generally accepted treatment for dry AMD as of the early 2000s, although quitting smoking helps to slow the progress of the disease in smokers. Wet AMD is treated with laser therapy to stop the development of new blood vessels. Other approaches that have been tried include low-level radiation therapy or transplanting a new retina from a tissue donor.
• Cataract: Patients with small and slow-developing cataracts may not need surgery for some years and usually benefit from a new eyeglass prescription. Surgery is the preferred treatment for cataracts that are large enough to interfere with driving, reading, and other daily activities. About 98 percent of surgical procedures for cataracts in the United States involve removal of the natural lens of the eye and replacing it with a clear plastic lens.
• Diabetic retinopathy: Photocoagulation (destruction of leaking blood vessels) with a laser benefits about 50 percent of patients with diabetic retinopathy. In cases of retinal detachment or clouding of the fluid inside the eye, a surgical procedure called a vitrectomy can be performed. In this procedure the surgeon removes the vitreous humor (a clear gel-like substance that lies between the lens of the eye and the retina) and replaces it with a sterile fluid. In addition to surgery, patients with diabetic retinopathy are usually treated with medications to control high blood pressure, which can make the eye disorder worse.
• Glaucoma: Open-angle glaucoma is treated with medications to lower the IOP; with laser surgery or conventional surgery to create a new opening for fluid to move through the eye; or by a combination of these approaches. Closed-angle glaucoma is treated with drugs in the emergency room to lower the IOP until an ophthalmologist can perform surgery on the eye. The most common procedure involves surgical removal of part of the iris in order to allow fluid pressure to equalize on both sides of the iris. This surgery is usually done with a laser as of 2008.

Nutrition/Dietetic concerns

Good nutrition is as important to the health of the eyes as to other parts of the body. With regard to AMD, there is some evidence that antioxidant vitamin supplements, particularly carotenoids (a group of nutrients found in carrots, orange juice, squash, egg yolk, spinach, mango, and other fruits and vegetables), may protect the health of the retina. With regard to diabetic retinopathy, careful attention to blood sugar levels can slow the progress of the disorder.

Therapy

Therapy for age-related eye disorders may involve medications, surgery, laser treatment, corrective lenses, low-grade radiation therapy, or a combination of these approaches.

Prognosis

The prognoses of the various vision disorders vary:

• AMD: Most patients with the dry form of AMD have only mild or moderate loss of vision. The prognosis is less hopeful for patients with wet AMD, as laser therapy is not effective in some patients.
• Cataract: Cataract surgery is very safe, with about 1.5 million procedures performed each year in the United States. About 95 percent of patients have better vision and improved quality of life; fewer than 1 percent have complications.
• Diabetic retinopathy: When diagnosed early, the progress of diabetic retinopathy from the nonproliferative to the proliferative form can be slowed significantly by tight control of blood sugar levels. In proliferative diabetic neuropathy, 95 percent of patients will keep their sight for at least 5 years if treated promptly. If they are treated late, only 50 percent will keep their sight for 5 years.
• Glaucoma: Open-angle glaucoma, if diagnosed early, is treatable. Vision that has already been lost cannot be restored; however, laser surgery and conventional surgery are highly effective in lowering IOP in 60 to 80 percent of patients. Medications to reduce IOP must be taken correctly and regularly. The prognosis for closed-angle glaucoma depends on speed of emergency treatment for the disorder.

QUESTIONS TO ASK YOUR DOCTOR

• How often should I have an eye examination?
• Glaucoma runs in my family. How much will that affect my risk of developing it?
• Can my eye disorder be treated with medications or will I need surgery?
• What can I do to slow the progress of my eye disorder?

Prevention

Genetic factors that increase the risk of vision disorders in seniors cannot be prevented as of the early 2000s.

Quitting smoking (or never starting in the first place) reduces the risk of most age-related vision disorders, as does limiting alcohol consumption. Good nutrition lowers the risk of cataract formation in later life, along with protecting the eyes from ultraviolet radiation by wearing sunglasses during outdoor sports or other activities.

Seniors diagnosed with either type 1 or type 2 (adult-onset) diabetes should have an eye examination twice yearly as they are at increased risk of AMD and diabetic retinopathy. They should also take extra care to control their blood sugar levels and blood pressure .

KEY TERMS

Cornea —The clear curved structure at the front of the eye that allows light to enter the eye.

Floater —An object in a person's field of vision caused by a tiny particle or deposit in the vitreous humor.

Intraocular pressure (IOP) —The pressure of the fluid inside the eyeball. An increase in IOP is a major factor in the development of glaucoma.

Iris —The colored circular membrane in front of the lens of the eye.

Macula —An area of tissue at the center of the retina that is responsible for the clearest central vision.

Ophthalmologist —A doctor who specializes in diagnosing and treating eye disorders.

Photocoagulation —The use of light from a laser to block or destroy broken blood vessels in the eye.

Retina —The innermost layer of the eye containing light-sensitive nerve cells and fibers. The retina is connected to the brain via the optic nerve.

Vitrectomy —The surgical removal of the vitreous humor. The gel is replaced with saline or another clear fluid.

Vitreous humor —The clear gel that fills the space between the lens and the retina of the eye.

Caregiver concerns

Caregivers should make sure that seniors in their care:

• Have an annual eye examination even if they haven't noticed any changes in their vision.
• Tell the doctor right away if there are changes in vision (blurring, glare, floaters, problems with night driving, etc.)
• Take all medications prescribed for eye care—particularly medications for glaucoma—and wear sunglasses when they are outside in bright sunlight.
• Eat a nutritious diet.
• Quit smoking if they are a smoker.
• See the doctor at once if they have an accident involving injury to the head or eye; if they have pain or redness in one or both eyes; or if they begin to see halos around lights.

Resources

BOOKS

Beers, Mark H., M. D., and Thomas V. Jones, MD. Merck Manual of Geriatrics, 3rd ed., Chapter 127, “Ocular Disorders.” Whitehouse Station, NJ: Merck, 2005.

Friedman, Neil J., and Peter K. Kaiser. Essentials of Ophthalmology, 1st ed. Philadelphia: Saunders Elsevier, 2007.

PERIODICALS

Green, C. M., L. S. Kearns, J. Wu, et al. “How Significant Is a Family History of Glaucoma? Experience from the Glaucoma Inheritance Study in Tasmania.” Clinical and Experimental Ophthalmology 35 (December 2007):793–799.

Hassell, J. B., E. L. Lamoureux, and J. E. Keeffe. “Impact of Age-Related Macular Degeneration on Quality of Life.” British Journal of Ophthalmology 90 (May 2006):593–596.

Quillen, David A. “Common Causes of Vision Loss in Elderly Patients.” American Family Physician 60 (July 1, 1999): 99–108.

OTHER

National Eye Institute (NEI). Age-Related Macular De-generation: What You Should Know. Bethesda, MD: NEI, 2008. Available online in PDF format at http://www.nei.nih.gov/health/maculardegen/webAMD.pdf [cited February 28, 2008].

National Eye Institute (NEI). Facts about Cataract. Bethesda, MD: NEI, 2008. Available online at http://www.nei.nih.gov/health/cataract/cataract_facts.asp [cited February 28, 2008].

National Eye Institute (NEI). Facts about Diabetic Retinopathy. Bethesda, MD: NEI, 2008. Available online at http://www.nei.nih.gov/health/diabetic/retinopathy.asp [cited February 28, 2008].

National Eye Institute (NEI). Facts about Glaucoma. Bethesda, MD: NEI, 2006. Available online at http://www.nei.nih.gov/health/glaucoma/glaucoma_facts.asp [cited February 28, 2008].

Prevent Blindness America and the National Eye Institute (NEI). Vision Problems in the U.S.: Prevalence of Adult Vision Impairment and Age-Related Eye Disease in America. Chicago, IL: Prevent Blindness America, 2002.

Available online in PDF format at http://www.nei.nih.gov/eyedata/pdf/VPUS.pdf [cited February 27, 2008].

ORGANIZATIONS

American Academy of Ophthalmology (AAO), P.O. Box 7424, San Francisco, CA, 94120, (415) 561-8500, (415) 561-8533, http://www.aao.org/aao/.

Macula Vision Research Foundation (MVRF), Five Tower Bridge, 300 Barr Harbor Drive, Suite 600, West Conshohocken, PA, 19428, (610) 668-6705, (866)-4MAC-ULA, (610) 667-1459, lea@mvrf.org, http://www.mvrf.org/.

National Eye Institute (NEI) Information Office, 31 Center Drive, MSC 2510, Bethesda, MD, 20892, (301) 496 5248, 2020@nei.nih.gov, http://www.nei.nih.gov/index.asp.

Prevent Blindness America, 211 West Wacker Drive, Suite 1700, Chicago, IL, 60606, (800) 331-2020, http://www.preventblindness.org/.

Wills Eye (formerly Wills Eye Hospital), 840 Walnut Street, Philadelphia, PA, 19107, (215) 928-3000, (877) AT-WILLS, http://www.willseye.org/.

AMD Alliance International, 1929 Bayview Avenue, Toronto, Ontario, Canada, M4G 3E8, (416) 486-2500 ext. 7505, (877) AMD-7171, (416) 486-8574, info@amdalliance.org, http://www.amdalliance.org/.

Rebecca J. Frey Ph.D.

Vision Disorders

Refractive errors

Hyperopia/presbyopia

Myopia

Astigmatism

Other refractive errors

Strabismus

Nonparalytic strabismus

Paralytic strabismus

Cataracts

Lens displacement

Glaucoma

Chronic simple glaucoma

Acute glaucoma

Secondary glaucomas

Fundus disorders

Vascular conditions

Degeneration of the macula

Detached retina

Fundus inflammation and tumors

Benign and malignant tumors

Retinal dystrophies

Other causes

Resources

Vision disorders are irregularities or abnormalities either of the eye, visual pathway, or brain, which affect one’s ability to see. In healthy vision, visual acuity— often referred to as “20/20 vision”—develops rapidly by three to six months of age and generally decreases rapidly as people approach 45 years of age. Poor visual acuity is often correctable with glasses or contact lenses. However, many other factors affect human’s ability to see—some preventable or correctable and others not. Vision disorders may manifest from refractive errors, defective eye muscles, cataracts, lens displacement, glaucoma, fundus conditions, color vision deficits, eyelid conditions, orbital diseases, eye injuries, and optic nerve and visual pathway damage.

In 2003, the U.S. National Institutes of Health (NIH) reported that over nine million people in the United States are considered visually impaired. People who are 45 years or older report are reported to have some type of vision impairment in 17% of that group. When the age group goes to 75 years or older, the percentage goes up to 26%. In 2006, according to the American Foundation for the Blind, roughly about ten million people in the United States were considered blind or visually impaired. According to an U.S. Congressional study, announced in 2005, about 80 million Americans have a disease that could potentially blind them. The results of the federal study further stated that even though one-half of all blindness can be medically prevented, by the year 2030 the number of blind or visually impaired Americans is projected to double in numbers.

Refractive errors

When parallel rays of light enter the eye they are refracted (bent) by the crystalline lens and projected into the eye. The healthy eye adjusts for distance, focusing each image perfectly on a minute hollow in the retina at the back of the eye called the fovea. In refractive errors, the image either falls short of the fovea or lands behind it. This causes blurred vision as a result of inadequate adjustment of the eye, irregular axial length (distance from the front to the back of the eyeball), or incorrect curvature of the cornea. These problems can usually be fixed with corrective lenses or surgery, which adjusts the shape of the cornea.

Hyperopia/presbyopia

When the eye’s axial length is shorter than normal, close objects appear blurry. This is called hyperopia or presbyopia, commonly termed long-sightedness. It may be latent—meaning the eye can compensate through accommodation, the ability to adjust; or it may be absolute—in which case correction requires convex or positive lenses. Around middle-age, the eye’s ability to accommodate deteriorates, which is why almost all people over 45 or 50 years of age require glasses for close-up vision. Irregular curvature of the cornea or lens will produce the same deficit.

Myopia

when the axial length is longer than normal, distant objects appear blurry. This error is called myopia, or short-sightedness. Myopia can also be caused by irregular curvature of the cornea or lens, and is correctable with concave or negative lenses.

Astigmatism

astigmatism, irregular curvature of the cornea, causes blurred vision of objects both near and far. It may be regular, which means light rays fall on the retina in two different areas, or irregular, resulting from corneal damage which projects light onto many different points on the retina. The former is correctable with lenses, the latter is not. Sometimes the faulty component is the crystalline lens and in these instances the error is called lenticular astigmatism.

Other refractive errors

While the above refractive errors may be congenital (originating at birth) or occur gradually, fast changes in the eyes’ focusing ability should be investigated immediately by an ophthalmologist. Some causes may be senile cataracts (described below); diabetes mellitus—in which fluctuating blood sugar levels cause corresponding fluctuations in focus; external pressure from lumps or cysts on the eyelids distorting the shape of the cornea; subluxation (partial dislocation) of the crystalline lens, causing hyperopia when it moves backward and myopia when it moves forward; and keratoconus, the thinning of the cornea allowing irregular curvature of the eyeball.

Strabismus

strabismus comes from the Greek strabismos meaning twisted, and results from a lack of parallelism of the visual axes of the eyes. Often, the cause of strabismus is not known, but it appears to be hereditary and is usually obvious soon after birth. Cosmetically, strabismus causes a squint—convergence or divergence of one or both eyes from the parallel line. Sometimes the term cross-eyed is used to describe one or both eyes turning toward the nose, and wall-eyed when one or both eyes turn outward. It may be concomitant (nonparalytic), in which the divergence or convergence remains the same no matter what way the eyes turn; or noncomitant (paralytic), which means that the deviation is more noticeable when the eyes look in one direction than in another. Strabismus is often correctable if treated before the age of four or five years.

Nonparalytic strabismus

Nonparalytic strabismus is thought to be due to underdeveloped binocular reflexes within the brain resulting in diplopia, the projection of a double image of a single object to the brain. The brain does not see both images, however—particularly if onset is at a young age and remains untreated—because the visual system develops an adaptive sensory mechanisms to deal with the confusion. This mechanism is called suppression, in which the brain suppresses the image from the weaker eye and neurons associated with the dominant eye take over.

The most common problem associated with suppression is strabismic amblyopia, or lazy eye, which affects more than four million people in the United States and causes blindness in more people under 45 years of age than any other ocular disease and injury combined. If it is present at birth or occurs within the first few months, vision never develops in the affected eye. This is called amblyopia of arrest. When the problem appears during the first two or three years, it is termed suppression amblyopia.

Paralytic strabismus

Paralytic strabismus is caused by some form of interference in the transmission of motor impulses from the brain to the eye muscles. This paralysis results in limited eye movement, particularly when the eyes turn in the direction of the paralyzed muscle. When it occurs one year or more after birth, it is usually the result of diseases such as meningitis, encephalitis, and multiple sclerosis and is termed acquired paralytic strabismus. Some congenital forms are evident at birth, perhaps caused by developmental abnormalities in the neural pathway or head injury during birth.

Cataracts

a cataract is the clouding or opacity of the crystalline lens that alters the amount of light entering the eye. Developmental cataracts are relatively minor and harmless, primarily present at birth or developing in early childhood, but that can occur later in life. No reduction of vision is noticeable with this type of disorder, which is thought to be hereditary. Congenital cataracts are present at birth and may appear as a white or gray pupil; they create a squint like that apparent in strabismus and cause visual impairment. In most instances, the cause is unknown; however, diseases and disorders contracted by the child’s mother during pregnancy—such as rubella, syphilis, diabetes mellitus, or birth defects as a result of Down syndrome, are thought to be causative. Systemic disease-associated cataracts develop as a result of such disorders as diabetes, high doses of steroids over long periods, and a dysfunctioning thyroid. Some cataracts develop secondary to ocular diseases like keratitis (inflammation of the cornea), iritis (inflammation of the iris), radiation (either from radiation therapy or the sun), or from trauma, such as intraocular surgery or penetration of the eye.

The most common cataracts are senile cataracts, a phenomenon of aging that occurs in almost all people over 65 years of age. Sometimes they develop in younger people whose nutrition is poor and are then termed pre-senile cataracts. Both types are probably caused by changes in or loss of enzyme production, which synthesizes nutrients that in turn feed the layer of cells on the lens surface. Senile cataracts grow slowly over months or years, cause no pain, usually affect both eyes, and gradually reduce visual acuity. Symptoms include two or more images from one eye, dark spots in the center of vision, extreme sensitivity to the glare of bright lights or sunshine, and reduced color vision. If not removed surgically, they eventually cause blindness.

Lens displacement

In a normal eye, the crystalline lens fits snugly behind the iris. Either complete (dislocated) or partial (subluxated) lens displacement causes the iris to wobble as the lens is no longer supporting it, or the edge of the lens may be visible through the pupil. If the lens moves forward into the anterior (frontal) chamber or backward into the posterior (rear) chamber, interrupted flow of fluid may cause glaucoma. Immediate rectification is necessary to prevent blindness in the affected eye. Posterior subluxation will also cause cataracts, and— because the lens moves about—the patient experiences rapid and frequent changes in visual acuity.

Glaucoma

Glaucoma is a serious condition that, left undiag-nosed and treated, will cause permanent blindness. Glaucoma is the unrelieved increase of intraocular pressure that acts like air in a tire. In a normal eye, pressure is maintained by the continuous production of aqueous humor, its movement through the pupil into the anterior chamber, and its drainage out of the chamber. When drainage is decreased or prevented, pressure inside the chamber increases.

Chronic simple glaucoma

Because this type occurs gradually and there is no pain or evident loss of visual acuity in the early stages, it causes more blindness word-wide than any other disease. It affects perhaps as many as 2% of the population over the age of 40 years, with the highest incidence after 60 years. This disease is inherited by approximately 10% of all children whose parents are known to be affected. Symptoms are gradual loss of close-up vision but occurring more quickly than presbyopia, and partial loss of visual field, noticeable because the patient frequently bumps into objects.

Acute glaucoma

Acute glaucoma may happen in a matter of hours and causes very fast vision loss, severe pain, and a bloodshot and watery eye. Colored haloes around lights may be visible prior to onset. It primarily affects one eye at a time, usually begins in the evening, and often after a physical or emotional trauma. It is most common in people over the age of 40 years, particularly in the 60 to 70 year age group. Immediate medical intervention is imperative to prevent damage to the optic nerve and subsequent blindness.

Secondary glaucomas

These are complications commonly caused by other eye diseases such as iritis, injury, blockage to the retinal vein, or long-term use of steroid eye drops for other disorders.

Fundus disorders

The fundus is the lining inside the eye, which includes the retina and its blood vessels, the macula, and the optic disc. Viewed through an ophthalmoscope—a special instrument for examining the interior of the eye—the fundus appears a reddish brown in color, the macula in the center of the retina appears as a darker red spot, and the optic disc located toward the nose side of center a lighter red. Many general health and vision-related abnormalities can be detected by examining the fundus. Congenital myopia appears as a pale crescent moon in the vicinity of the optic disc, while cloudy nerve fibers—which should be the same color as the retina—appear as a whitish patch. Both these conditions are harmless. Others, however, are not.

Vascular conditions

Blood vessels in the fundus affected by hypertension (high blood pressure) are narrow and appear quite bright. Rapid-onset or severe hypertension, which can cause strokes, may first be diagnosed when a patient seeks an eye examination for blurred vision caused by the disorder. Arteriosclerosis (hardening of the arteries) is a normal process of aging and poses no danger to vision. Central retinal artery occlusion and central retinal vein occlusion can cause sudden loss of vision in the affected eye when the blood supply to the retina is obstructed for longer than two hours, or if hemorrhaging occurs within the retina. Vision is then severely limited to light only and damage is virtually irreversible. Causes include hypertension, diabetes mellitus, narrowing of a carotid artery (the primary arteries supplying blood to the head), and blood disorders such as anemia, leukemia, and sickle-cell anemia.

Degeneration of the macula

This is primarily an age-related disorder resulting in deterioration and often distortion of vision. It is one of the leading causes of blindness in developed countries. There is no treatment; however, if detected early, laser treatment may prevent further degeneration.

Detached retina

This condition manifests with the sudden appearance of bright, floating specks and flashes of light in the affected eye with a shadow or curtain appearing later, which may gradually cover the entire visual field. It occurs when part of the retina falls away from the outer layer of cells to which it is normally attached. Breaks or tears in the retina also cause similar symptoms. Although most common in people over 50 years of age, especially those with a high degree of myopia, detachment can also occur in younger people, particularly following trauma. Surgery is the only form of treatment for detachment but laser surgery for a tear or break will sometimes prevent full detachment.

Fundus inflammation and tumors

The choroid is the layer of cells over which the retina lies. Inflammation of this layer is visible through the retina and manifests as rapid onset of blurred vision—usually only in one eye—and haziness and spots over the entire field of vision. It may be caused by a parasitic protozoan infection in the bloodstream; larvae of dog tapeworms (the eggs of which may be accidentally ingested through contact with dog feces); a seriously compromised immune system—from AIDS (acquired immune deficiency syndrome), for example; a fungus infection called Histoplasmosis; or syphilis passed on to unborn babies.

Benign and malignant tumors

Benign melanomas produce no symptoms and usually cause no loss of vision. Malignant melanomas

KEY TERMS

Astigmatism —Irregular curvature of the cornea causing distorted images.

Cataract —Eye disease characterized by the development of a cloudy layer in the lens of the eye.

Diplopia —Double image.

Dystrophy —Atrophy, deterioration.

Fundus —Layers lining the inside of the eye.

Glaucoma —A disease of the eye in which increased pressure within the eyeball can cause gradual loss of vision.

Hyperopia/presbyopia —Far-sighted; close objects are out of focus.

Keratoconus —Thinning of the cornea.

Moypia —Near-sighted; distant objects out of focus.

Stab Sismus —Non-parallel eye axes.

of the choroid occur primarily around middle-age, producing symptoms similar to those caused by a detached retina—which can occur secondary to the tumor itself—and may result in spread of cancer to the bloodstream. When localized treatment with radiation therapy is not effective, the removal of the entire eye is recommended. Retinoblastoma usually affects children within the first two years of life, is closely related to astrocytoma—a tumor of the brain—and is the only primary tumor that occurs on the retina. It manifests as a white pupil or squint and, if treatments with radiation therapy are ineffective, the affected eye is removed.

Retinal dystrophies

Retinitis pigmentosa and macular dystrophy are degenerative disorders that appear to be hereditary, generally affect young children and adolescents, and cause night blindness, tunnel vision, slow deterioration of central vision, and increasing loss of sight. As yet, effective treatment has not been found.

Other causes

Many people, especially males, experience color blindness, varying degrees of which range from inability to differentiate between red and green to total inability to see one or more colors. Damage to any area in the visual pathway—including the optic nerve, optic tract, optic chiasm, optic radiations, or visual cortex—will cause vision deficits or loss. Although not vision disorders themselves, diseases, disorders, and damage of the eye lid, eye, and bony orbit (eye socket) may—if severe and left untreated—result in reduced, impaired, or lost vision.

See also Blindness and visual impairments; Radial keratotomy.

Resources

BOOKS

Goldberg, Stephen, M.D. Ophthalmology Made Ridiculously Simple. Miami, FL: MedMaster, 2005.

Denniston, Alistair. Oxford Handbook of Ophthalmology. Oxford, UK, and New York: Oxford University Press, 2006.

Fekrat, Sharon and Jennifer S. Weizer, eds. All About Your Eyes. Durham, NC: Duke University Press, 2006.

Madge, Simon N. Clinical Techniques in Ophthalmology. Edinburgh, UK: Churchill Livingstone, 2006.

Marie L. Thompson

Vision disorders

Vision disorders are irregularities or abnormalities either of the eye , visual pathway, or brain , which affect one's ability to see. In healthy vision, visual acuity—often referred to as "20/20 vision"—develops rapidly by three to six months of age and generally decreases rapidly as people approach 45. Poor visual acuity is often correctable with glasses or contact lenses. However, many other factors affect human's ability to see—some preventable or correctable and others not. Vision disorders may manifest from refractive errors, defective eye muscles, cataracts, lens displacement, glaucoma, fundus conditions, color vision deficits, eyelid conditions, orbital diseases, eye injuries, and optic nerve and visual pathway damage.

Refractive errors

When parallel rays of light enter the eye they are refracted (bent) by the crystalline lens and projected into the eye. The healthy eye adjusts for distance, focusing each image perfectly on a minute hollow in the retina at the back of the eye called the fovea. In refractive errors, the image either falls short of the fovea or lands behind it. This causes blurred vision as a result of inadequate adjustment of the eye, irregular axial length (distance from the front to the back of the eyeball), or incorrect curvature of the cornea. These problems can usually be fixed with corrective lenses or surgery , which adjusts the shape of the cornea.

Hyperopia/presbyopia

When the eye's axial length is shorter than normal, close objects appear blurry. This is called hyperopia or presbyopia, commonly termed long-sightedness. It may be latent—meaning the eye can compensate through accommodation, the ability to adjust; or it may be absolute—in which case correction requires convex or positive lenses. Around middle-age, the eye's ability to accommodate deteriorates, which is why almost all people over 45 or 50 years of age require glasses for close-up vision. Irregular curvature of the cornea or lens will produce the same deficit.

Myopia

When the axial length is longer than normal, distant objects appear blurry. This error is called myopia, or short-sightedness. Myopia can also be caused by irregular curvature of the cornea or lens, and is correctable with concave or negative lenses.

Astigmatism

Astigmatism, irregular curvature of the cornea, causes blurred vision of objects both near and far. It may be regular, which means light rays fall on the retina in two different areas, or irregular, resulting from corneal damage which projects light onto many different points on the retina. The former is correctable with lenses, the latter is not. Sometimes the faulty component is the crystalline lens and in these instances the error is called lenticular astigmatism.

Other refractive errors

While the above refractive errors may be congenital (originating at birth ) or occur gradually, fast changes in the eyes' focusing ability should be investigated immediately by an ophthalmologist. Some causes may be senile cataracts (described below); diabetes mellitus—in which fluctuating blood sugar levels cause corresponding fluctuations in focus; external pressure from lumps or cysts on the eyelids distorting the shape of the cornea; subluxation (partial dislocation) of the crystalline lens, causing hyperopia when it moves backward and myopia when it moves forward; and keratoconus, the thinning of the cornea allowing irregular curvature of the eyeball.

Strabismus

Strabismus comes from the Greek strabismos meaning twisted, and results from a lack of parallelism of the visual axes of the eyes. Often, the cause of strabismus is not known, but it appears to be hereditary and is usually obvious soon after birth. Cosmetically, strabismus causes a squint—convergence or divergence of one or both eyes from the parallel line. Sometimes the term "cross-eyed" is used to describe one or both eyes turning toward the nose, and "wall-eyed" when one or both eyes turn outward. It may be concomitant (nonparalytic), in which the divergence or convergence remains the same no matter what way the eyes turn; or noncomitant (paralytic), which means the deviation is more noticeable when the eyes look in one direction than in another. Strabismus is often correctable if treated before the age of four or five years.

Nonparalytic strabismus

Nonparalytic strabismus is thought to be due to underdeveloped binocular reflexes within the brain resulting in diplopia, the projection of a double image of a single object to the brain. The brain does not see both images, however—particularly if onset is at a young age and remains untreated—because the visual system develops an adaptive sensory mechanisms to deal with the confusion. This mechanism is called suppression, in which the brain suppresses the image from the weaker eye and neurons associated with the dominant eye take over.

The most common problem associated with suppression is strabismic amblyopia, or lazy eye, which affects more than four million people in the United States and causes blindness in more people under 45 years of age than any other ocular disease and injury combined. If it is present at birth or occurs within the first few months, vision never develops in the affected eye. This is called amblyopia of arrest. When the problem appears during the first two or three years, it is termed suppression amblyopia.

Paralytic strabismus

Paralytic strabismus is caused by some form of interference in the transmission of motor impulses from the brain to the eye muscles. This paralysis results in limited eye movement, particularly when the eyes turn in the direction of the paralyzed muscle. When it occurs a year or more after birth, it is usually the result of diseases such as meningitis , encephalitis , and multiple sclerosis and is termed acquired paralytic strabismus. Some congenital forms are evident at birth, perhaps caused by developmental abnormalities in the neural pathway or head injury during birth.

Cataracts

A cataract is the clouding or opacity of the crystalline lens which alters the amount of light entering the eye. Developmental cataracts are relatively minor and harmless, primarily present at birth or developing in early childhood, but which can occur later in life. No reduction in vision is noticeable with this type, which are thought to be hereditary. Congenital cataracts are present at birth and may appear as a white or gray pupil; they create a squint like that apparent in strabismus and cause visual impairment. In most instances, the cause is unknown; however, diseases and disorders contracted by the child's mother during pregnancy—such as rubella, syphilis, diabetes mellitus , or birth defects as a result of Down syndrome , are thought to be causative. Systemic disease-associated cataracts develop as a result of such disorders as diabetes, high doses of steroids over long periods, and a dysfunctioning thyroid. Some cataracts develop secondary to ocular diseases like keratitis (inflammation of the cornea), iritis (inflammation of the iris), radiation (either from radiation therapy or the sun ), or from trauma, such as intraocular surgery or penetration of the eye.

The most common cataracts are senile cataracts, a phenomenon of aging that occurs in almost all people over 65 years of age. Sometimes they develop in younger people whose nutrition is poor and are then termed presenile cataracts. Both types are probably caused by changes in or loss of enzyme production, which synthesizes nutrients that in turn feed the layer of cells on the lens surface. Senile cataracts grow slowly over months
or years, cause no pain , usually affect both eyes, and gradually reduce visual acuity. Symptoms include two or more images from one eye, dark spots in the center of vision, extreme sensitivity to the glare of bright lights or sunshine, and reduced color vision. If not removed surgically, they eventually cause blindness.

Lens displacement

In a normal eye, the crystalline lens fits snugly behind the iris. Either complete (dislocated) or partial (subluxated) lens displacement causes the iris to wobble as the lens is no longer supporting it, or the edge of the lens may be visible through the pupil. If the lens moves forward into the anterior (frontal) chamber or backward into the posterior (rear) chamber, interrupted flow of fluid may cause glaucoma. Immediate rectification is necessary to prevent blindness in the affected eye. Posterior subluxation will also cause cataracts, and—because the lens moves about—the patient experiences rapid and frequent changes in visual acuity.

Glaucoma

Glaucoma is a serious condition which, left undiagnosed and treated, will cause permanent blindness. Glaucoma is the unrelieved increase of intraocular pressure which acts like air in a tire. In a normal eye, pressure is maintained by the continuous production of aqueous humor, its movement through the pupil into the anterior chamber, and its drainage out of the chamber. When drainage is decreased or prevented, pressure inside the chamber increases.

Chronic simple glaucoma

Because this type occurs gradually and there is no pain or evident loss of visual acuity in the early stages, it causes more blindness word-wide than any other disease. It affects perhaps as many as 2% of the population over the age of 40 years, with the highest incidence after 60 years. This disease is inherited by approximately 10% of all children whose parents are known to be affected. Symptoms are gradual loss of close-up vision but occurring more quickly than presbyopia, and partial loss of visual field, noticeable because the patient frequently bumps into objects.

Acute glaucoma

Acute glaucoma may happen in a matter of hours and causes very fast vision loss, severe pain, and a bloodshot and watery eye. Colored haloes around lights may be visible prior to onset. It primarily affects one eye at a time, usually begins in the evening, and often after a physical or emotional trauma. It is most common in people over the age of 40 years, particularly in the 60-70 year age group. Immediate medical intervention is imperative to prevent damage to the optic nerve and subsequent blindness.

Secondary glaucomas

These are complications commonly caused by other eye diseases such as iritis, injury, blockage to the retinal vein, or long-term use of steroid eye drops for other disorders.

Fundus disorders

The fundus is the lining inside the eye which includes the retina and its blood vessels, the macula, and the optic disc. Viewed through an ophthalmoscope—a special instrument for examining the interior of the eye—the fundus appears a reddish brown in color, the macula in the center of the retina appears as a darker red spot, and the optic disc located toward the nose side of center a lighter red. Many general health and vision-related abnormalities can be detected by examining the fundus. Congenital myopia appears as a pale crescent moon in the vicinity of the optic disc, while cloudy nerve fibers—which should be the same color as the retina—appear as a whitish patch. Both these conditions are harmless. Others, however, are not.

Vascular conditions

Blood vessels in the fundus affected by hypertension (high blood pressure) are narrow and appear quite bright. Rapid-onset or severe hypertension, which can cause strokes, may first be diagnosed when a patient seeks an eye exam for blurred vision caused by the disorder. Arteriosclerosis (hardening of the arteries ) is a normal process of aging and poses no danger to vision. Central retinal artery occlusion and central retinal vein occlusion can cause sudden loss of vision in the affected eye when the blood supply to the retina is obstructed for longer than two hours, or if hemorrhaging occurs within the retina. Vision is then severely limited to light only and damage is virtually irreversible. Causes include hypertension, diabetes mellitus, narrowing of a carotid artery (the primary arteries supplying blood to the head), and blood disorders such as anemia , leukemia , and sickle-cell anemia.

Degeneration of the macula

This is primarily an age-related disorder resulting in deterioration and often distortion of vision. It is one of the leading causes of blindness in developed countries. There is no treatment; however, if detected early, laser treatment may prevent further degeneration.

Detached retina

This condition manifests with the sudden appearance of bright, floating specks and flashes of light in the affected eye with a "shadow" or "curtain" appearing later which may gradually cover the entire visual field. It occurs when part of the retina falls away from the outer layer of cells to which it is normally attached. Breaks or tears in the retina also cause similar symptoms. Although most common in people over 50 years of age, especially those with a high degree of myopia, detachment can also occur in younger people, particularly following trauma. Surgery is the only form of treatment for detachment but laser surgery for a tear or break will sometimes prevent full detachment.

Fundus inflammation and tumors

The choroid is the layer of cells over which the retina lies. Inflammation of this layer is visible through the retina and manifests as rapid onset of blurred vision—usually only in one eye—and haziness and spots over the entire field of vision. It may be caused by a parasitic protozoan infection in the bloodstream; larvae of dog tapeworms (the eggs of which may be accidentally ingested through contact with dog feces); a seriously compromised immune system—from AIDS , for example; a fungus infection called Histoplasmosis; or syphilis passed on to unborn babies.

Benign and malignant tumors

Benign melanomas produce no symptoms and usually cause no loss of vision. Malignant melanomas of the choroid occur primarily around middle-age, producing symptoms similar to those caused by a detached retina—which can occur secondary to the tumor itself—and may result in spread of cancer to the bloodstream. When localized treatment with radiation therapy is not effective, the removal of the entire eye is recommended. Retinoblastoma usually affects children within the first two years of life, is closely related to astrocytoma—a tumor of the brain—and is the only primary tumor which occurs on the retina. It manifests as a "white pupil" or squint and if treatments with radiation therapy are ineffective, the affected eye is removed.

Retinal dystrophies

Retinitis pigmentosa and macular dystrophy are degenerative disorders which appear to be hereditary, generally affect young children and adolescents, and cause night blindness, tunnel vision, slow deterioration of central vision, and increasing loss of sight. As yet, effective treatment has not been found.

Other causes

Many people, especially males, experience color blindness , varying degrees of which range from inability to differentiate between red and green to total inability to see one or more colors. Damage to any area in the visual pathway—including the optic nerve, optic tract, optic chiasm, optic radiations, or visual cortex—will cause vision deficits or loss. Although not vision disorders themselves, diseases, disorders, and damage of the eye lid, eye, and bony orbit (eye socket) may—if severe and left untreated—result in reduced, impaired, or lost vision.

See also Blindness and visual impairments; Radial keratotomy.

Resources

books

Bankes, James L. Kennerley. Clinical Ophthalmology—A Text and Colour Atlas. London: Churchill Livingstone, 1994.

Chawla, Hector Bryson. Ophthalmology. London: Churchill Livingstone, 1993.

Stein, Harold A., Bernard J. Slatt, and Raymond M. Stein. Ophthalmic Terminology-Speller and Vocabulary Builder. St. Louis: Mosby, 1987.

Marie L. Thompson

KEY TERMS

Astigmatism

—Irregular curvature of the cornea causing distorted images.

Cataract

—Eye disease characterized by the development of a cloudy layer in the lens of the eye.

Diplopia

—Double image.

Dystrophy

—Atrophy, deterioration.

Fundus

—Layers lining the inside of the eye.

Glaucoma

—A disease of the eye in which increased pressure within the eyeball can cause gradual loss of vision.

Hyperopia/presbyopia

—Far-sighted; close objects are out of focus.

Keratoconus

—Thinning of the cornea.

Moypia

—Near-sighted; distant objects out of focus.

Stabismus

—Non-parallel eye axes.

Visual Impairment

Definition

Total blindness is the inability to tell light from dark, or the total inability to see. Visual impairment or low vision is a severe reduction in vision that cannot be corrected with standard glasses or contact lenses and reduces a person's ability to function at certain or all tasks. Legal blindness (which is actually a severe visual impairment) refers to a best-corrected central vision of 20/200 or worse in the better eye or a visual acuity of better than 20/200 but with a visual field no greater than 20° (e.g., side vision that is so reduced that it appears as if the person is looking through a tunnel).

Description

Vision is normally measured using a Snellen chart. A Snellen chart has letters of different sizes that are read, one eye at a time, from a distance of 20 ft. People with normal vision are able to read the 20 ft line at 20 ft-20/20 vision—or the 40 ft line at 40 ft, the 100 ft line at 100 ft, and so forth. If at 20 ft the smallest readable letter is larger, vision is designated as the distance from the chart over the size of the smallest letter that can be read.

Eye care professionals measure vision in many ways. Clarity (sharpness) of vision indicates how well a person's central visual status is. The diopter is the unit of measure for refractive errors such as nearsightedness, farsightedness, and astigmatism and indicates the strength of corrective lenses needed. People do not just see straight ahead; the entire area of vision is called the visual field. Some people have good vision (e.g., see clearly) but have areas of reduced or no vision (blind spots) in parts of their visual field. Others have good vision in the center but poor vision around the edges (peripheral visual field). People with very poor vision may be able only to count fingers at a given distance from their eyes. This distance becomes the measure of their ability to see.

The World Health Organization (WHO) defines impaired vision in five categories:

• Low vision 1 is a best corrected visual acuity of 20/70.
• Low vision 2 starts at 20/200.
• Blindness 3 is below 20/400.
• Blindness 4 is worse than 5/300
• Blindness 5 is no light perception at all.
• A visual field between 5° and 10° (compared with a normal visual field of about 120°) goes into category 3; less than 5° into category 4, even if the tiny spot of central vision is perfect.

Color blindness is the reduced ability to perceive certain colors, usually red and green. It is a hereditary defect and affects very few tasks. Contrast sensitivity describes the ability to distinguish one object from another. A person with reduced contrast sensitivity may have problems seeing things in the fog because of the decrease in contrast between the object and the fog.

According to the WHO there are over forty million people worldwide whose vision is category 3 or worse, 80% of whom live in developing countries. Half of the blind population in the United States is over 65 years of age.

Causes and symptoms

The leading causes of blindness include:

• macular degeneration
• glaucoma
• cataracts
• diabetes mellitus

Other possible causes include infections, injury, or nutrition.

Infections

Most infectious eye diseases have been eliminated in the industrialized nations by sanitation, medication, and public health measures. Viral infections are the main exception to this statement. Some infections that may lead to visual impairment include:

• Herpes simplex keratitis. A viral infection of the cornea. Repeated occurrences may lead to corneal scarring.
• Trachoma. This disease is responsible for six to nine million cases of blindness around the world, of the third of a billion who have the disease. Trachoma is caused by an incomplete bacterium, Chlamydia trachomatis, that is easily treated with standard antibiotics. It is transmitted directly from eye to eye, mostly by flies. The chlamydia gradually destroy the cornea.
• Leprosy (Hansen's disease). This is another bacterial disease that has a high affinity for the eyes. It, too, can be effectively treated with medicines.
• River blindness. Much of the tropics of the Eastern Hemisphere are infested with Onchocerca volvulus, a worm that causes "river blindness." This worm is transmitted by fly bites and can be treated with a drug called ivermectin. Nevertheless, twenty-eight million people have the disease, and 40% of them are blind from it.

Other causes

Exposure of a pregnant woman to certain diseases (e.g., rubella or toxoplasmosis ) can cause congenital eye problems. Injuries to the eyes can result in blindness. Very little blindness is due to disease in the brain or the optic nerves. Multiple sclerosis and similar nervous system diseases, brain tumors, diseases of the eye sockets, and head injuries are rare causes of blindness.

Nutrition

Vitamin A deficiency is a widespread cause of corneal degeneration in children in developing nations. As many as five million children develop xerophthalmia from this deficiency each year. Five percent end up blind.

Diagnosis

A low vision exam is slightly different from a general exam. While a case history, visual status, and eye health evaluation are common to both exams, some things do differ. Eye charts other than a Snellen eye chart will be used. Testing distance will vary. A trial frame worn by the patient is usually used instead of the instrument containing the lenses the patient sits behind (phoropter). Because the low vision exam is slightly more goal oriented than a general exam, for example, what specifically is the patient having trouble with (reading, seeing street signs, etc.) different optical and nonoptical aids will generally be tried. Eye health is the last thing to be checked so that the lights necessary to examine the eyes will not interfere with the rest of the testing.

Treatment

There are many options for patients with visual impairment. There are optical and nonoptical aids. Optical aids include:

• Telescopes. May be used to read street signs.
• Hand magnifiers. May be used to read labels on things at the store.
• Stand magnifiers. May be used to read.
• Prisms. May be used to move the image onto a healthy part of the retina in some eye diseases.
• Closed circuit television (CCTV). For large magnification (e.g., for reading).

Nonoptical aids can include large print books and magazines, check-writing guides, large print dials on the telephone, and more.

For those who are blind, there are enormous resources available to improve the quality of life. For the legally blind, financial assistance for help may be possible. Braille and audio books are increasingly available. Guide dogs provide well-trained eyes and independence. Orientation and mobility training is available. There are special schools for blind children and access to disability support through Social Security and private institutions.

Prognosis

The prognosis generally relates to the severity of the impairment and the ability of the aids to correct it. A good low vision exam is important to be aware of the latest low vision aids.

Prevention

Regular eye exams are important to detect silent eye problems (e.g., glaucoma). Left untreated, glaucoma can result in blindness.

Corneal infections can be treated with effective antibiotics. When a cornea has become opaque beyond recovery it must be transplanted. Good hygiene (e.g., washing hands frequently) to prevent infection, proper use of contact lenses, and not sharing makeup are just some ways to guard against corneal infections.

Cataracts should be removed when they interfere with a person's quality of life.

Primary prevention addresses the causes before they ever begin. Fly control can be accomplished by simple sanitation methods. Public health measures can reduce the incidence of many infectious diseases. Vitamin A supplementation (when appropriate) will eliminate xerophthalmia completely. It is possible that protecting the eyes against ultraviolet (UV) light will reduce the incidence of cataracts, macular degeneration, and some other eye diseases. UV coatings can be placed on regular glasses, sunglasses, and ski goggles. Patients should ask their eye care professional about UV coatings. Protective goggles should also be worn in certain situations (e.g., certain jobs, sports, even mowing the lawn).

Secondary prevention addresses treating established diseases before they cause irreversible eye damage. Having general physical checkups can also detect systemic diseases such as diabetes or high blood pressure. Control of diabetes is very important in preserving sight.

Resources

ORGANIZATIONS

American Academy of Ophthalmology. 655 Beach Street, P.O. Box 7424, San Francisco, CA 94120-7424. 〈http://www.eyenet.org〉.

American Foundation for the Blind. 11 Penn Plaza, Suite 300, New York, NY 10001. (800) 232-5463.

Guide Dogs for the Blind. P.O. Box 1200, San Rafael, CA 94915. (415) 499-4000.

International Eye Foundation. 7801 Norfolk Ave., Bethesda, MD 20814. (301) 986-1830.

Lighthouse National Center for Education. 111 E. 59th Street. New York, NY 10022. (800) 334-5497. 〈http://www.lighthouse.org〉.

National Association for the Visually Handicapped. 22 West 21st St., New York, NY 10010. (212) 889-3141.

National Center for Sight. (800) 221-3004.

National Children's Eye Care Foundation. One Clinic Center, A3-108, Cleveland, OH 44195. (216) 444-0488.

National Eye Institute. 2020 Vision Place, Bethesda, MD 20892-3655. (301) 496-5248. 〈http://www.nei.nih.gov〉.

National Federation of the Blind. 1800 Johnson St., Baltimore, MD 21230. (301) 569-9314.

Prevent Blindness America. 500 East Remington Road, Schaumburg, IL 60173. (800) 331-2020. 〈http://www.preventblindness.org〉.

Research to Prevent Blindness. 598 Madison Ave., New York, NY 10022. (212) 363-3911.

KEY TERMS

Cornea— The clear dome-shaped structure that is part of the front of the eye. It lies in front of the colored part of the eye (iris).

Diabetic retinopathy— Retinal disease caused by the damage diabetes does to small blood vessels.

Phoropter— The instrument used to measure refractive status of the eyes. It contains many lenses which are then changed in front of the eyes while the patient is looking at an eye chart. This is when the doctor usually asks, "Which is better, one or two?"

Xerophthalmia— A drying of the cornea and conjunctiva.

VISION DISORDERS

DEFINITION

Vision disorders are conditions caused by the eye's inability to properly focus light rays. Three major types of vision disorder are hyperopia (pronounced HY-puh-ROH-pee-uh), or farsightedness; myopia (pronounced my-OH-pee-a) or nearsightedness; and astigmatism, which causes the eye to form a blurred image of an object.

DESCRIPTION

Light that strikes the eyeball first passes through the cornea. The cornea is the tough, transparent covering at the front of the eye. It is shaped like a dome, with the top of the dome facing outward. Light then passes through the lens. The lens is located just inside the eyeball, behind the cornea. The lens has a double-convex shape. That is, it bulges outward in the middle.

The cornea and lens bend light as light waves pass through them. This process is known as refraction. In a normal eye, the light waves are brought to focus on the retina. The retina is a thin membrane at the back of the eye. Light waves cause chemical changes in the retina. These chemical changes set off a signal that passes through the optic nerve to the brain. The brain reads that signal as a visual image.

Muscles in the front of the eye can change the shape of the lens. These muscles adjust the lens shape to see objects close at hand or far away. This process is known as accommodation.

Visual disorders develop when the cornea and/or the lens do not properly focus light waves. The light waves do not come to a focus (that is, they do not all reach the same point) on the retina. They may come to a focus in front of the retina or behind the retina.

When light waves come to a focus in front of the retina, the person has myopia, or nearsightedness. People who are nearsighted can see objects close at hand, but they cannot see objects far away clearly. When light waves come to a focus behind the retina, the person has hyperopia, or farsightedness. People who are farsighted (hyperopic) can see objects far away but cannot clearly see objects close at hand.

In healthy eyes the cornea has a smooth, regular shape. Some people have corneas with an ellipsoidal shape, like a football or a squashed baseball. Such corneas scatter light waves across the retina, causing objects to appear as blurred images. This condition is called astigmatism.

Vision Disorders: Words to Know

Accommodation:

The ability of the lens to change its shape in order to focus light waves from distant or near objects.

Astigmatism:

A condition in which light from a single point fails to focus on a single point of the retina. The condition causes the patient to see a blurred image.

Cornea:

The clear, dome-shaped outer covering of the front of the eye.

Diopter:

The unit of measure used for the refractive (light bending) power of a lens.

Hyperopia:

Farsightedness. A condition in which vision is better for distant objects than for close ones.

Lens:

In the eye, a transparent, elastic, curved structure that helps focus light on the retina.

Myopia:

Nearsightedness. A condition in which far away objects appear fuzzy, because light from a distance doesn't focus properly on the retina.

Optic nerve:

A bundle of nerve fibers that carries visual signals from the retina to the brain.

Radial keratotomy (RK):

A surgical procedure in which the shape of the cornea is changed in order to correct myopia.

Refraction:

The bending of light waves as they pass through a dense substance, such as water, glass, or plastic.

Retina:

A membrane at the back of the eye that is sensitive to light and that converts light waves into signals sent to the brain by way of the optic nerve.

CAUSES

Most vision disorders are thought to be inherited. That is, the disorders are passed down from one generation to the next. Most babies are born slightly hyperopic. However, this pattern changes as they grow older. In most cases, children develop normal vision. They may, however, become more hyperopic, or they may develop myopia.

Some eyecare specialists argue that vision disorders may be caused by a number of factors. For example, they think that factors in the environment, such as work conditions, stress, and eye strain, can contribute to the development of vision disorders.

The development of astigmatism has been attributed to a number of factors. For example, people with allergies often rub their eyes repeatedly. This constant rubbing may cause damage to the cornea. Diabetes (see diabetes mellitus entry) is also known to change the shape of the cornea over time.

SYMPTOMS

The primary symptom of any vision disorder is the inability to see objects clearly. Objects near by or far away will seem blurry to a person with farsightedness or nearsightedness. Objects at almost any distance will seem blurry to a person with astigmatism.

Blurry vision may lead to other problems. People who have trouble seeing clearly may begin to squint in order to see better. Constant squinting can lead to discomfort, headaches, and eyestrain.

DIAGNOSIS

Vision disorders are sometimes difficult to diagnose. Many people with vision disorders are not aware of their problem. They have often grown up seeing the world slightly out of focus and their view of objects may seem perfectly normal.

As a result, many vision problems are detected by others. For example, school teachers may realize that a pupil is unable to read the blackboard. Or a parent may notice that a child is constantly squinting while trying to read. In such cases, the individual is referred to an eye specialist for examination.

Many tests are available for detecting vision disorders. The most common tests involve the use of the familiar eye chart. The eye chart contains rows of letters of decreasing size going from top to bottom. The patient is asked to read each row on the chart. The last row the patient is able to read tells how accurate is his or her vision.

The results of an eye chart test are usually indicated by a pair of numbers, such as 20/50. These numbers show how well a person can see an object compared to a person with normal eyesight. A person with 20/50 vision can see at 20 feet from an eye chart what a person with normal vision can see at 50 feet from the chart.

Vision disorders are measured by having a patient look through various lenses at an eye chart. The lens are changed until the patient is able to see the letters on the chart clearly. The degree of error in the patient's eye is determined by the lens needed to produce perfect vision. This degree of error is measured in units called diopters (abbreviated: D). The prescription a doctor writes for glasses will indicate the shape of the lens (the diopter measurements) needed to provide perfect vision.

A simple test for astigmatism consists of a dial containing lines that radiate outward from the center. A person with astigmatism will be able to see some lines more clearly than other lines.

Doctors may also use direct tests on the eye. For example, a device known as a corneal topographer can be used to measure the shape of the cornea and determine whether the shape is correct or not. Doctors can also shine a light directly at the retina in a person's eye to look for any damage.

TREATMENT

Traditionally, the usual treatment for vision disorders has been fitting the patient with eyeglasses. Eyeglasses consist of glass or plastic lenses set in a

frame. These corrective lenses compensate (make up) for any errors in the patient's cornea and lens. If the patient's lens is too flat, for example, the lenses in the eyeglasses will be more round. Hyperopia, myopia, and astigmatism can all be treated very effectively, easily, and inexpensively with eyeglasses.

Some patients prefer to use contact lenses instead of eyeglasses. The principle behind contact lenses is the same as that behind eyeglasses. In the case of contact lenses, however, the corrective lens is placed directly on the eye.

Many vision disorders can now be treated surgically. An example is radial keratotomy (RK). Radial keratotomy (pronounced RAY-dee-uhl KARE-uh-TOT-uh-mee) is now a common procedure for the treatment of myopia. In RK, a doctor makes very small incisions (cuts) in the cornea to change its shape. The goal is to produce a cornea that has exactly the right shape to produce normal vision.

Traditional RK surgery is done with a very small diamond-tipped blade. Another approach now being studied involves the use of a laser beam to make the necessary incision. By 1999 this procedure had not yet received government approval in the United States for general use.

A form of RK surgery with lasers has been approved for treatment of astigmatism. The procedure is still quite new, however, and a patient should consider the risk carefully before choosing RK surgery.

PROGNOSIS

Most cases of vision disorder can be treated successfully with eyeglasses or contact lenses. Surgical procedures are often successful, as well. However,

many of surgical procedures are still in the early stages of development and carry considerably more risk than the use of eyeglasses or contact lenses.

PREVENTION

All forms of vision disorder are regarded as genetic. To the extent that is true, there is nothing a person can do to avoid vision problems. However, some experts believe that environmental factors may determine whether or not vision disorders develop. They suggest that maintaining good nutrition, reading and working in good light, and wearing corrective lenses as prescribed can reduce the risk of eye problems.

Regular eye examinations are also important. Eye characteristics change over time, so new corrective lenses may be needed occasionally. Also, eye examinations detect other types of eye disorders, such as glaucoma (see glaucoma entry), that are more serious than vision disorders.

See also: Color blindness.

FOR MORE INFORMATION

Books

Rosanes-Berrett, Marilyn B. Do You Really Need Eyeglasses? Barrytown, NY: Station Hill Press, 1990.

Zinn, Walter J., and Herbert Solomon. Complete Guide to Eyecare, Eyeglasses, and Contact Lenses. Hollywood, FL: Lifetime, 1996.

Periodicals

"Insight on Eyesight: Seven Vision Myths: Blind Spots about Vision Can Cause Needless Worry, Wasted Effort, and Unnecessary Treatment." Consumer Reports on Health (April 1997): p. 42.

Organizations

American Academy of Ophthalmology. PO Box 7424, San Francisco, CA 94120–7424. (800) 222–EYES. http://www.eyenet.org.

American Optometric Association. 243 North Lindbergh Boulevard, St. Louis, MO, 63141. (314) 991–4100. http://www.aoanet.org.

International Myopia Prevention Association. RD No. 5, Box 171, Ligonier, PA 15658. (412) 238-2101.

Myopia International Research Foundation. 1265 Broadway, Room 608, New York, NY 10001. (212) 684-2777.

National Eye Institute. 2020 Vision Place, Bethesda, MD 20892–3655. (301) 496-5248. http://www.nei.nih.gov.

Web sites

The Refractive Surgery Patient Resource Center. [Online] http://www.eyeinfo.com (accessed on November 1, 1999).