What is Astigmatism?
Astigmatism usually occurs when the front surface of your eye, the cornea, has an irregular curvature. Astigmatism is one of a group of eye conditions known as refractive errors and these errors cause a disturbance in the way that light rays are focused within your eye. Astigmatism often occurs with nearsightedness and farsightedness, conditions also resulting from refractive errors. Astigmatism is not a disease nor does it mean that you have "bad eyes." It simply means that you have a variation or disturbance in the shape of your cornea.
Symptoms
- Distortion or blurring of images at all distances
- Headache and fatigue
- Squinting and eye discomfort or irritation
The symptoms described above may not necessarily mean that you have astigmatism. However, if you experience one or more of these symptoms, contact your ophthalmologist for a complete exam.
Causes
Normally, the cornea is smooth and equally curved in all directions and light entering the cornea is focused equally on all planes or in all directions. In astigmatism, the front surface of the cornea is curved more in one direction than in the other. This abnormality may result in vision that is much like looking into a distorted, wavy mirror. The distortion results because of an inability of the eye to focus light rays to a single point.
If the corneal surface has a high degree of variation in its curvature, light refraction may be impaired to the degree that corrective lenses are needed to help focus light rays better. At any time, only a small proportion of the rays are focused and the remainder are not, so that the image formed is always blurred. Usually, astigmatism causes blurred vision at all distances.
Risk Factors
A small amount of astigmatism is very common and the tendency to develop astigmatism is inherited. A larger amount of astigmatism can be associated with diseases such as keratoconus.
Tests and Diagnosis
The amount of astigmatism in the eye can be measured in various ways. The autorefraction or the subjective refraction—based on the patient’s response—that are done at the beginning of an eye exam is one way to measure astigmatism. The amount of astigmatism caused by the cornea is measured in the clinic by a diagnostic instrument called a keratometer.
Treatment and Drugs
If the degree of astigmatism is slight and there are no other problems of refraction, such as nearsightedness or farsightedness, corrective lenses may not be needed. If the degree of astigmatism is great enough to cause eye strain, headache, or distortion of vision, corrective lenses will be needed for clear and comfortable vision.
The corrective lenses needed for astigmatism are called toric lenses and they have an additional power element called a cylinder. They have greater light-bending power in one axis than in others. Your ophthalmologist will perform precise tests during your exam to determine the ideal lens prescription. Refractive surgery also may be an option for correcting some forms of astigmatism.
Astigmatism may increase slowly. Regular eye care can help to insure that proper vision is maintained. You may have to adjust to wearing eyeglasses or contact lenses if you do not wear them now. Other than that, astigmatism probably will not significantly affect your lifestyle.
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Reviewed by Jill E. Bixler, M.D.