Astigmatism

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

(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.

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

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:

•

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.

Eyeglasses. These can be used to correct astigmatism caused by

•

of astigmatism than soft contact lenses.

Hard contact lenses. These usually provide more effective correction

•

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.

Orthokeratology (Ortho-K). This is a procedure in which the person

•

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

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.

Laser surgery. An ophthalmologist can use lasers to reshape the’s cornea.

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.

SEE ALSO

For more information

BOOKS

Bakri, Sophie J., ed.

Clinic, 2007.

Viegas, Jennifer.

Group, 2002.

Mayo Clinic Guide to Better Vision. Rochester, MN: MayoThe Eye: Learning How We See. New York: Rosen Publishing

WEB SITES

American Optometric Association (AOA).

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.

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.

health/astigmatism/DS00230 (updated January 18, 2007; accessed May 6,

2008).

Montreal Vision Clinic.

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).

Available online at http://www.nei.nih.gov/CanWeSee/qa_refractive.asp

(updated December 2006; accessed May 6, 2008).

TeensHealth.

.kidshealth.org/teen/your_body/take_care/vision_care.html (updated

January 2008; accessed May 6, 2008).

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.Astigmatism. Available online atEl Greco (1541–1614). Available online atAstigmatism. Available online at http://www.mayoclinic.com/EyeMotion: Astigmatism. Available online at http://Questions and Answers about Refractive Errors.Taking Care of Your Vision. Available online at http://wwwHyperopia; Myopia’s cornea and can be used to determine the proper fit for’s symptoms

Astigmatism