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.

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.

Astigmatism is 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. (Reproduced by permission of

Electronic Illustrators Group

)

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

Hyperopia or farsightedness is a condition in which vision is better for distant objects than for close objects. (Reproduced by permission of

Electronic Illustrators Group

)

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,

Myopia or nearsightedness is a condition in which vision is better for near objects than for distant objects. (Reproduced by permission of

Electronic Illustrators Group

)

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).