Astigmatism
Astigmatism is the result of the inability of the cornea to properly focus an image on the retina of the eye. The result is a blurred image.
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.
Astigmatism is an image distortion that results from an improperly shaped cornea. Usually the cornea is spherical, like a baseball. In astigmatism, the cornea is elliptically shaped, more like a football. This causes a point of light will have two points of focus, instead of one nice sharp image on the retina.
This double focus results in blurry vision. What the blur looks like depends 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. In addition to blurring, people with astigmatism may also experience headaches and eyestrain.
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 called 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.
Regular astigmatism may 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, or the presence of tumors in the eyelid. 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.
Diabetes plays 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 noticed only when the diabetic has started treatment to control his blood sugar. The return to more normal blood sugar levels 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.
A variety of tests can be used to detect astigmatism during an 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. 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 important to have a refraction in case the lens is also contributing to the astigmatism. A refraction is when the patient looks at an eye chart and the doctor puts different lenses in front of the patient's eyes and asks which one looks better.
Astigmatism can be treated by the use of cylindrical eyeglasse lenses or contact lenses. Lenses are shaped to counteract the shape of the sections of cornea that are causing the difficulty. There is some debate as to whether people with very small amounts of astigmatism should be treated. Generally, if vision 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.
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. Hard and soft toric lenses are available in a variety of prescriptions, materials, and even in tints.
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.
Astigmatism is a condition that may be present at birth. It may also be acquired if something distorts 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.
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