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Unknown: Astigmatism

Unknown: Astigmatism

Definition
Description
Demographics
Causes and Symptoms
Diagnosis
Treatment
Prognosis
Prevention
The Future
For more information

Definition

Astigmatism is a condition in which either the cornea or the lens of the eye has an abnormal curvature, causing out-of-focus vision.

Description

Astigmatism is a common refractive error in which a person cannot see objects with complete clarity. In some cases the person can see vertical lines more clearly than horizontal lines; this is called with-the-rule astigmatism. In other cases the person can see horizontal lines clearly while vertical lines look blurred; this is called against-the-rule astigmatism. Children are more likely to have with-the-rule astigmatism while adults are more likely to have against-the-rule astigmatism.

Demographics

Astigmatism is a very common visual defect. It often exists together with nearsightedness (myopia) or farsightedness (hyperopia). It is estimated that as many as a third of the general American population has some degree of astigmatism. Mild astigmatism is considered normal and may not require corrective lenses.

Astigmatism is equally common in men and women and in all races and ethnic groups.

Causes and Symptoms

The exact cause of astigmatism is not known; it is usually present from birth. In a few cases, astigmatism may result from an injury to the eye or from keratoconus, an eye disorder that causes the tissue of the cornea to become thinner over time. In an astigmatic eye, the irregularly shaped cornea or lens prevents light from focusing properly on the retina, the light-sensitive tissue layer that lines the back of the eye. When the cornea is irregularly curved, the eye focuses light on two different points inside the eye rather than one. If the person is nearsighted, one of the two focal points will lie in front of the retina; if the person is farsighted, the second focal point will lie behind the retina.

In simple astigmatism, one of the two focal points will lie on the retina itself. If the person has compound myopic astigmatism, however, both focal points lie in front of the retina. Similarly, in compound hyperopic astigmatism, both focal points lie behind the retina. A few people have what is called mixed astigmatism, in which one focal point falls in front of the retina and the other behind it.

The basic symptom of astigmatism is that objects look blurred at any distance. People who are severely astigmatic may also develop headaches, eye strain, and fatigue along with blurring of vertical, diagonal, or horizontal lines if their refractive error is not corrected.

Diagnosis

Astigmatism and other refractive errors are evaluated by a series of vision tests. After the examiner takes a history of the patient's symptoms (including a family history of eye problems), the patient is usually asked to read the letters on an eye chart known as a Snellen chart. The examiner may also shine lights into the eyes or administer eye drops that allow him or her to see all the structures inside the eye clearly. This part of the

examination allows the doctor to evaluate the patient for nearsightedness or farsightedness.

Was El Greco Astigmatic?

El Greco, or “The Greek,” was the nickname of Domenikos Theotokopoulos (1541–1614), a great artist of the seventeenth century. Born on the island of Crete, he studied art in Italy with Titian, another famous painter, and moved to Spain in 1577. As he grew older, the human figures in his paintings became progressively more elongated, almost looking unnatural. This lengthening is one of the most distinctive characteristics of his style, making it relatively easy for art historians to tell the difference between his paintings and those produced by other artists of the period.

Some art historians suggested that his distortion of the human figure might have been the result of astigmatism. They theorized that El Greco had a type of astigmatism that caused him to see elon-gated human figures as normally proportioned. Though this theory is interesting, researchers generally believe his style was purposeful, not the result of a refractive error. As evidence they state that some of the figures in his paintings have normal body proportions, and x-ray studies of his paintings reveal that he sketched normally proportioned figures on his canvas before starting to

add color and then painted his elongated figures over the sketched outlines.

To determine the degree of astigmatism, the examiner will use a keratometer, a device that measures the curvature of the cornea. The examiner will measure the steepest and flattest curves on the cornea. Another technique that can be used to evaluate astigmatism is called corneal topography and uses a device called a videokeratoscope. The keratoscope part of the machine projects rings of light onto the cornea while the examiner studies the pattern of the reflected light and records it with a video camera. Corneal topography provides a more detailed picture of the shape of the patient's cornea and can be used to determine the proper fit for contact lenses.

Treatment

Mild astigmatism may not need corrective treatment. A person should see their eye doctor, however, if they are developing headaches or eye strain, or if blurry vision is interfering with daily activities.

People whose astigmatism is severe enough to require correction have several options:

  • Eyeglasses. These can be used to correct astigmatism caused by uneven curvature of the lens as well as the cornea. Eyeglasses are prescribed by an optometrist or ophthalmologist but made and fitted by an optician.
  • Hard contact lenses. These usually provide more effective correction of astigmatism than soft contact lenses.
  • Orthokeratology (Ortho-K). This is a procedure in which the person wears hard contact lenses for several hours overnight in order to gradually correct the curvature of the cornea. The lenses are removed during the day. Ortho-K is also referred to as corneal molding. It does not permanently improve vision; if the patient stops wearing the retainer lenses, their vision may return to its original condition.
  • Laser surgery. An ophthalmologist can use lasers to reshape the cornea either by making a flap in the surface of the cornea and reshaping the tissue of the cornea under the flap, or by completely removing the upper layer of tissue in the cornea before reshaping the lower layers of tissue.

There are drawbacks to surgical correction of refractive errors, however. These include the risks of infection, development of haze in the cornea, or dry eyes. In some cases the surgeon may need to perform a second operation if the first one either overcorrected or undercorrected the shape of the patient's cornea.

It is important for a patient with astigmatism to discuss all the treatment options with the optometrist or ophthalmologist, as no two people will have exactly the same degree of visual blurring or the same lifestyle.

Prognosis

Astigmatism may either improve over time or grow worse; it also changes from with-the-rule astigmatism in childhood to against-the-rule astigmatism in adulthood in some people. Most people with astigmatism who need corrective lenses or other types of treatment, however, do very well.

About 20 percent of people with keratoconus, however, will eventually need a corneal transplant.

Prevention

Astigmatism is largely a matter of heredity and cannot be prevented. People can, however, live comfortably with it by visual screening in childhood, regular eye checkups at all ages, and wearing corrective lenses if necessary.

The Future

Astigmatism is such a common refractive error in the general population that it is not likely to disappear in the near future. It is possible that the present variety of treatment options will be improved or expanded by further research.

WORDS TO KNOW

Against-the-rule astigmatism: A type of astigmatism in which the eye sees horizontal lines more clearly than vertical lines.

Astigmatism: A refractive error caused by irregularities in the shape of the cornea or the lens of the eye.

Cornea: The transparent front part of the eye where light enters the eye.

Hyperopia: The medical term for farsightedness.

Keratoconus: An eye disorder in which the tissue of the cornea grows thinner over time.

Myopia: The medical term for nearsightedness.

Ophthalmologist: A doctor who specializes in diagnosing and treating eye disorders and can perform eye surgery.

Optician: An eye care professional who fills prescriptions for eyeglasses and corrective lenses.

Optometrist: An eye care professional who diagnoses refractive errors and other eye problems and prescribes corrective lenses.

Orthokeratology: A treatment for astigmatism that consists of wearing hard contact lenses overnight to reshape the cornea during sleep. The lenses are removed during the day.

Refractive error: A general term for vision problems caused by the eye's inability to focus light correctly.

Retina: The light-sensitive layer of tissue at the back of the eyeball.

With-the-rule astigmatism: A type of astigmatism in which the eye sees vertical lines more clearly than horizontal lines.

SEE ALSO Hyperopia; Myopia

For more information

BOOKS

Bakri, Sophie J., ed. Mayo Clinic Guide to Better Vision. Rochester, MN: Mayo Clinic, 2007.

Viegas, Jennifer. The Eye: Learning How We See. New York: Rosen Publishing Group, 2002.

WEB SITES

American Optometric Association (AOA). Astigmatism. Available online at http://www.aoa.org/x4698.xml?Play=true (accessed May 6, 2008). The page includes three animations about astigmatism that have a total playing time of about 90 seconds.

Art, Vision, & the Disordered Eye. El Greco (1541–1614). Available online at http://www.psych.ucalgary.ca/PACE/VA-Lab/AVDE-Website/ElGreco.html (accessed May 7, 2008). This is a fascinating website maintained by the Vision and Aging Laboratory of the Department of Psychology at the University of Calgary in Canada. Viewers can study eight different artists from Mary Cassatt to Vincent van Gogh from the standpoint of the influence of vision disorders on their art.

Mayo Clinic. Astigmatism. Available online at http://www.mayoclinic.com/health/astigmatism/DS00230 (updated January 18, 2007; accessed May 6, 2008).

Montreal Vision Clinic. EyeMotion: Astigmatism. Available online at http://www.eyemotion.com/eyemotion/library/_montrealeyefr.php?src=1 (accessed May 6, 2008). This is a one-minute animation of the refractive errors involved in astigmatism.

National Eye Institute (NEI). Questions and Answers about Refractive Errors. Available online at http://www.nei.nih.gov/CanWeSee/qa_refractive.asp (updated December 2006; accessed May 6, 2008).

TeensHealth. Taking Care of Your Vision. Available online at http://www.kidshealth.org/teen/your_body/take_care/vision_care.html (updated January 2008; accessed May 6, 2008).

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