Encyclopedia - Astigmatism

What causes astigmatism?
How do you know you have astigmatism?
How is astigmatism treated?

Astigmatism is a vision disorder that occurs when the cornea of the eye is uneven in shape. More rarely, it can result from the way in which the eye's natural crystalline lens refracts light. Either condition causes a distorted image to fall on the retina.

The human eye works much like a camera with two lenses -- the cornea, which is a clear membrane that covers the front of the eye, and the natural crystalline lens, which is located behind the pupil. These two lenses work together to focus light on the retina, which is the membrane that covers the back two-thirds of the eye and works like the film in a camera. A normal cornea should be curved equally in all directions, allowing light to focus exactly on the surface of the retina. Most vision problems result from an irregularity in the curvature of the cornea or in the shape of the eye. (See Anatomy of the Eye.)

What causes astigmatism?

Nearsightedness, or myopia, occurs when the cornea is too steeply curved or when the eye is more elongated than normal, causing light rays to focus in front of the retina. With farsightedness, or hyperopia, the cornea itself is flatter than normal or the eye is shorter from front to rear, resulting in too short a distance from the retina. If the cornea is uneven in shape, the result is astigmatism, which causes light rays to be bent out of focus, either horizontally or vertically, resulting in distorted vision at all distances.

Astigmatism is prevalent and, in most cases, if you have it, you were born with it. Just as your hands are shaped differently from other people's hands, so are your eyes. Eyelid swelling, corneal scars, and keratoconus, a rare condition that causes the cornea to be misshapen, can also cause astigmatism.

How do you know you have astigmatism?

Very mild astigmatism may cause no visual symptoms because the muscles of the eye will compensate for the uneven curvature of the cornea. If the eye has to work too hard to compensate, however, eyestrain and headaches can result. In addition, mild astigmatism can cause eye fatigue or blurry vision at certain distances. Severe astigmatism will usually cause distorted, double, or blurry vision.

An eye doctor detects astigmatism during the course of a regular eye examination.

How is astigmatism treated?

Astigmatism can be treated surgically or nonsurgically. Prescription eyeglasses and contact lenses or laser vision correction surgery correct most cases of astigmatism. The most prevalent nonsurgical correction is a prescription for rigid gas permeable (RGP) contact lenses. Because it is rigid, an RGP lens will fill in the irregular areas of the cornea with tears, creating a smooth spherical surface and correcting astigmatism. Special soft contact lenses called torics also compensate for the astigmatic shape of the corneas. In those cases where the astigmatism arises from the eye's natural crystalline lens rather than the cornea, a special bitoric contact lens may be prescribed. It offers refracting surfaces on the front and back to correct the problem in much the same way that eyeglasses do.

If you are contact lens-intolerant or just want to be free from glasses or contacts, you may opt to have some form of vision correction procedure performed by a qualified eye surgeon. LASIK, the most popular form of laser vision correction, can provide correction for relatively high degrees of nearsightedness and astigmatism as well as some cases of farsightedness and astigmatism.