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Astigmatism

Astigmatism

Definition

Astigmatism is the result of an inability of the cornea to properly focus an image onto the retina. The result is a blurred image.

Description

The cornea is the outermost part of the eye. It is a transparent layer that covers the colored part of the eye (iris), pupil, and lens. The cornea bends light and helps to focus it onto the retina where specialized cells (photo receptors) detect light and transmit nerve impulses via the optic nerve to the brain where the image is formed. The cornea is dome shaped. Any incorrect shaping of the cornea results in an incorrect focusing of the light that passes through that part of the cornea. The bending of light is called refraction and focusing problems with the cornea are called diseases of refraction or refractive disorders. Astigmatism is an image distortion that results from an improperly shaped cornea. Usually the cornea is spherically shaped, like a baseball. However, in astigmatism the cornea is elliptically shaped, more like a football. There is a long meridian and a short meridian. These two meridians generally have a constant curvature and are generally perpendicular to each other (regular astigmatism). Irregular astigmatism may have more than two meridians of focus and they may not be 908 apart. A point of light, therefore, going through an astigmatic cornea will have two points of focus, instead of one nice sharp image on the retina. This will cause the person to have blurry vision. What the blur looks like will depend upon the amount and the direction of the astigmatism. A person with nearsightedness (myopia ) or farsightedness (hyperopia ) may see a dot as a blurred circle. A person with astigmatism may see the same dot as a blurred oval or frankfurter-shaped blur.

Some cases of astigmatism are caused by problems in the lens of the eye. Minor variations in the curvature of the lens can produce minor degrees of astigmatism (lenticular astigmatism). In these patients, the cornea is usually normal in shape. Infants, as a group, have the least amount of astigmatism. Astigmatism may increase during childhood, as the eye is developing.

Causes and symptoms

The main symptom of astigmatism is blurring. People can also experience headaches and eyestrain. Parents can notice that a child may have astigmatism when the child can see some part of a pattern or picture more clearly than others. For example, lines going across may seem clearer than lines going up and down.

Regular astigmatism can be caused by the weight of the upper eyelid resting on the eyeball creating distortion, surgical incisions in the cornea, trauma or scarring to the cornea, the presence of tumors in the eyelid, or a developmental factor. Irregular astigmatism can be caused by scarring or keratoconus. Keratoconus is a condition in which the cornea thins and becomes cone shaped. It usually occurs around puberty and is more common in women. Although the causes of keratoconus are unknown, it may be hereditary or a result of chronic eye rubbing, as in people with allergies. The center of the cone may not be in line with the center of the cornea. Diabetes can play a role in the development of astigmatism. High blood sugar levels can cause shape changes in the lens of the eye. This process usually occurs slowly and, often, is only noticed when the diabetic has started treatment to control their blood sugar. The return to a more normal blood sugar allows the lens to return to normal and this change is sometimes noticed by the patient as farsightedness. Because of this, diabetics should wait until their blood sugar is under control for at least one month to allow vision to stabilize before being measured for eyeglasses.

Diagnosis

Patients seek treatment because of blurred vision. A variety of tests can be used to detect astigmatism during the eye exam. The patient may be asked to describe the astigmatic dial, a series of lines that radiate outward from a center. People with astigmatism will see some of the lines more clearly than others. One diagnostic instrument used is the keratometer. This measures the curvature of the central cornea. It measures the amount and direction of the curvature. A corneal topographer can measure a larger area of the cornea. It can measure the central area and mid-periphery of the cornea. A keratoscope projects a series of concentric light rings onto the cornea. Misshapen areas of the cornea are revealed by noting areas of the light pattern that do not appear concentric on the cornea. Because these instruments are measuring the cornea, it is also important to have a refraction in case the lens is also contributing to the astigmatism. The refraction measures the optics or visual status of the eye and the result is the eyeglass prescription. The refraction is when the patient is looking at an eye chart and the doctor is putting different lenses in front of the patient's eyes and asks which one looks better.

Treatment

Astigmatism can be treated by the use of cylindrical lenses. They can be in eyeglasses or contact lenses. The unit of measure describing the power of the lens system or lens is called the diopter (D). The lenses are shaped to counteract the shape of the sections of cornea that are causing the difficulty. Because the correction is in one direction, it is written in terms of the axis the correction is in. On a prescription, for example, it may say 1.00 × 180°. Cylinders correct astigmatism, minus spheres correct myopia, and plus spheres correct hyperopia.

There is some debate as to whether people with very small amounts of astigmatism should be treated. Generally, if visual acuity is good and the patient experiences no overt symptoms, treatment is not necessary. When treating larger amounts of astigmatism, or astigmatism for the first time, the doctor may not totally correct the astigmatism. The cylindrical correction in the eyeglasses may make the floor appear to tilt, thus making it difficult for the patient at first. Generally, the doctor will place lenses in a trial frame to allow the patient to try the prescription at the exam. It may take a week or so to get used to the glasses, however, if the patient is having a problem they should contact their doctor, who might want to recheck the prescription.

KEY TERMS

Meridian A section of a sphere. For example, longitude or latitude on the globe. Or, on a clock, a section going through 12:00-6:00 or 3:00-9:00, etc.

Refraction The turning or bending of light waves as the light passes from one medium or layer to another. In the eye it means the ability of the eye to bend light so that an image is focused onto the retina.

Contact lenses that are used to correct astigmatism are called toric lenses. When a person blinks, the contact lens rotates. In toric lenses, it is important for the lens to return to the same position each time. Lenses have thin zones, or cut-off areas (truncated), or have other ways to rotate and return to the correct position. Soft toric lenses are available in a variety of prescriptions, materials, and even in tints. Patients should ask their doctors about the possibility of toric lenses.

In 1997, the Food and Drug Administration (FDA) approved laser treatment of astigmatism. Patients considering this should make sure the surgeon has a lot of experience in the procedure and discuss the possible side effects or risks with the doctor. In the case of keratoconus, a corneal transplant is performed if the astigmatism can not be corrected with hard contact lenses.

Prognosis

Astigmatism is a condition that may be present at birth. It may also be acquired if something is distorting the cornea. Vision can generally be corrected with eyeglasses or contact lenses. The major risks of surgery (aside from the surgical risks) are over and under correction of the astigmatism. There is no cure for over correction. Under correction can be solved by repeating the operation.

Resources

BOOKS

Berkow, Robert, editor. Merck Manual of Medical Information. Whitehouse Station, NJ: Merck Research Laboratories, 2004.

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Astigmatism

Astigmatism

Definition

Astigmatism is a problem of visual distortion caused by a misshapen cornea. The cornea acts as a focusing lens for the eye. If the cornea does not have the proper shape, the eye is unable to properly focus an image. Most people have a certain degree of astigmatism. Corrective measures are necessary only in cases where the distortion is severe.

Description

Light rays entering a normal eye come to a point of focus on the retina through a transparent, dome-shaped layer called the cornea. In astigmatism there is unequal curvature of the cornea, and the light rays come to focus at more than one point on the retina. This causes the person to see a blurred or doubled image. Astigmatism is usually present at birth and may increase during childhood as the eye tissue develops. Usually the degree of astigmatism remains fairly constant throughout adulthood.

Causes & symptoms

It is unknown why some people develop a misshapen cornea. It is possible that astigmatism is an inherited trait. Factors such as stress , continual reading in dim lighting, or excessive close-up work may also contribute to the development of astigmatism. It is sometimes caused by pressure from chalazion, a condition that causes the eyelid to swell; from scars on the cornea; or from keratoconus, a condition that involves swelling of the cornea. The main symptom of astigmatism is blurred or distorted vision. There may also be a history of headaches, eye strain, fatigue , and double vision.

Diagnosis

The standard eye examination with a refraction test, given by an optometrist or opthalmologist, is

used to determine the presence of astigmatism. An instrument called a keratometer is used to measure the cornea and calculate the shape of the required corrective lens.

Treatment

The Bates method or other type of visual training may be helpful in improving vision and reducing symptoms. The homeopathic remedies Ruta graveolens (from common rue) and Apis mellifica (from the honey bee) can be used to relieve eyestrain, one of the main symptoms and possible contributors to astigmatism.

Acupuncture treatment or Traditional Chinese Medicine may help, as the liver system is connected to eye functions. Certain treatments can strengthen and correct the skewing of the Liver qi. (Qi is the flow of energy in the body. It is sometimes associated with certain organs.)

Allopathic treatment

Astigmatism can be most simply treated with either eyeglasses or contact lenses. The lenses are made so as to counteract the shape of the sections of cornea that are causing difficulty. Contact lenses that are used to correct astigmatism are called toric lenses. Hard contact lenses may be better for correcting astigmatism than soft contact lenses. In the case of keratoconus, a corneal transplant is performed if the astigmatism cannot be corrected with hard contact lenses.

Refractive surgery can be performed to correct the curvature of the cornea. In radial keratotomy (RK) for astigmatism, cuts are made into the cornea with a diamond blade instrument. In photorefractive keratectomy (PRK), a laser is used to improve the shape of the cornea by removing micro-thin slices. Laser assisted in situ keratomileusis (LASIK) is the most recently developed type of refractive surgery. A flap of the cornea is cut with a laser and then the corneal tissue underneath is shaved to improve the shape.

Doctors continue to improve LASIK techniques. In 2002, a cross-cylinder method was developed in refractive surgery to help treat mixed astigmatism, one of the most difficult types to treat. Refractive surgery requires a high level of expertise. Anyone considering it should make sure that the surgeon has a lot of experience in the procedure. There should also be an in-depth discussion of the possible side effects and risks of the procedure. For instance, patients with flatter corneas seem to come out of surgery with more light distortion than those with curved corneas. Pupil size also may affect surgery outcome.

Expected results

Effects of astigmatism can generally be greatly improved with eyeglasses or contact lenses. Refractive surgery may diminish the need for lenses or make them unecessary altogether. The major risks of surgery include chronic visual problems, injury to the eye tissue, infection, and over- or under-correction, which would still leave some astigmatism. Complications may require the use of medication or further surgery.

Resources

BOOKS

Lininger, Skye, ed. The Natural Pharmacy. Rocklin, Calif.: Prima Health, 1998.

Newell, Frank W. Ophthalmology, Principles and Concepts. 8th ed. St. Louis: Mosby, 1996.

PERIODICALS

"Clues to LASIK Vision Problems (Risk of 'Starbursts')." Prevention (June 2002): 161.

Meszaros, Liz."Cross-Cylinder Technique Offers Many Benefits, Little Trauma: Method to Correct Mixed Astigmatism Minimizes Tissue Removal, not Ideal for High-Correction Cases." Ophthalmology Times (May 15, 2002): 41.

OTHER

"Refractive Surgery." The Merck Manual Online. (December 2000).<http://www.merck.com/pubs/mmanual/section8/chapter102/102c.htm/>.

Patience Paradox

Teresa G. Odle

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astigmatism

astigmatism (əstĬg´mətĬz´əm), type of faulty vision caused by a nonuniform curvature in the refractive surfaces—usually the cornea, less frequently the lens—of the eye. As a result, light rays do not all come to a single focal point on the retina. Instead, some focus on the retina while others focus in front of or behind it. The condition may be congenital, or it may result from disease or injury; it can occur in addition to nearsightedness or farsightedness. The spherical lenses used to correct nearsightedness and farsightedness must be specially adapted to correct the out-of-focus plane of vision of the astigmatic eye. When the patient observes a pattern of straight lines placed at various angles, those running in one direction appear sharp while those in other directions (particularly at right angles to the sharp lines) appear blurred. A special cylindrical lens is placed in the out-of-focus axis to correct the condition. In many cases contact lenses are the most effective means of correcting astigmatism.

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Astigmatism

Astigmatism

Who Gets Astigmatism?

What Happens to Vision When People Have Astigmatism?

What Is the Treatment for Astigmatism?

Resources

Astigmatism (a-STIG-ma-tiz-um) is an eye condition that causes objects to appear blurry because the front part of the eye is misshapen.

KEYWORDS

for searching the Internet and other reference sources

Ophthalmology

Optometry

Vision

The eyeball usually is round and nearly the same size as a ping-pong ball. The front part contains a clear layer of tissue called the cornea and the lens that help to focus the light that enters the eye. When people have astigmatism, the cornea and/or the lens is misshapen and their curved surfaces are unequal. If the curve is only slightly off shape, then only objects at a distance might appear blurry. People with more serious astigmatism, however, may see the world as if they were looking in a funhouse mirror that distorts all images.

Who Gets Astigmatism?

Astigmatism is a common problem. Many people may have slight variations in the shape of their cornea or lens that do not cause problems with their eyesight. In others, however, these structures are shaped in ways that distort the light that enters the eye. The cornea usually is smooth and rounded, like the surface of a ping-pong ball that is cut in half. A person with astigmatism, however, might have a cornea that is curved more like the top of a football, as if the ping-pong ball were pulled out from its edges. Or the cornea may have peaks and valleys on its surface, instead of a smooth, rounded covering, and that too will distort vision.

No one is sure what causes astigmatism. It usually is present at birth and often is found in several members of the same family. This means that in some cases the trait is inherited, like hair color and eye color.

What Happens to Vision When People Have Astigmatism?

These variations in the shape of the cornea or lens cause the images a person with astigmatism sees to be out of focus when they reach the retina. The retina is made of layers of light-sensitive cells at the back of the eyeball that act like the film in a camera. The distorted image is projected onto the retina and transmitted to the brain for processing through the optic nerve.

The first signs of astigmatism depend on how severely the cornea or lens is misshapen. If it is only a mild problem, people with astigmatism may find that they have headaches or tired eyes at times, or distorted vision at certain distances. Those with more severe astigmatism may find they have blurry vision that makes reading, playing sports, and other activities difficult. Often, the problem is found during an eye exam in school, at the doctors office during a check-up, or when a parent notices that a child is having trouble seeing well.

What Is the Treatment for Astigmatism?

Astigmatism is managed with prescription eyeglasses or contact lenses. These help change the way that images are focused as they pass through the cornea and the lens. With eyeglasses, images can appear clear and undistorted when they reach the retina. Usually, astigmatism does not get worse as people get older.

See also

Farsightedness

Nearsightedness

Resources

Book

Cassel, Gary H., M.D., Michael D. Billig, O.D., and Harry G. Randall, M.D. The Eye Book: A Complete Guide to Eye Disorders and Health. Baltimore: Johns Hopkins University Press, 1998. A good general reference on eye problems.

Organization

The U.S. National Eye Institute posts a resource list of eye health-related publications and organizations at its website. http://www.nei.nih.gov/publications/sel-org.htm

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astigmatism

a·stig·ma·tism / əˈstigməˌtizəm/ • n. a defect in the eye or in a lens caused by a deviation from spherical curvature, which results in distorted images, as light rays are prevented from meeting at a common focus. DERIVATIVES: as·tig·mat·ic / ˌastigˈmatik/ adj.

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astigmatism

astigmatism (ă-stig-mă-tizm) n. a defect of vision in which the image of an object is distorted because not all the light rays come to a focus on the retina. This is usually due to irregular curvature of the cornea and/or lens, whose surface resembles part of the surface of an egg (rather than a sphere).
astigmatic (ass-tig-mat-ik) adj.

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astigmatism

astigmatism A lens defect in which rays in one plane are in focus when those in another plane are not. The eye can suffer from astigmatism, usually when the cornea is not spherical. It is corrected by using an anastigmatic lens, which has different radii of curvature in the vertical and horizontal planes.

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astigmatism

astigmatism Defect of vision in which the curvature of the lens differs from one perpendicular plane to another. It can be compensated for by use of corrective lenses.

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astigmatism

astigmatism defect in the eye preventing exact focusing. XIX. f. A-4 + Gr. stigmat- point (see STIGMA) + -ISM.
So astigmatic XIX.

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