Astigmatism Health Article

Definition

Astigmatism results from an inability of the cornea to properly focus an image as a single point onto the retina, instead focusing the image on two different locations. The result is a blurred image.

Description

The cornea is a transparent layer that bends light and helps to focus it onto the retina, where light is detected. Any incorrect shaping of the cornea results in an incorrect focusing of the light that passes through it. Usually the cornea is spherically shaped, like a baseball. However, in astigmatism the cornea is elliptically shaped with 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 90° apart. A point of light, therefore, will have two points of focus instead of one. This causes blurry vision.

Some astigmatism is caused by problems in the eye's lens. Minor variations in the curvature of the lens can produce minor degrees of astigmatism (lenticular astigmatism). Infants generally have the least amount of astigmatism. Astigmatism may increase during childhood as the eye develops.

Causes and symptoms

The main symptom of astigmatism is blurred vision. Patients may also experience headaches and eyestrain. Astigmatism is suspected 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 upon the eyeball, creating distortion; surgical incisions in the cornea; trauma or scarring to the cornea; tumors of the eyelid; or a developmental anomaly. Irregular astigmatism can be caused by scarring or keratoconus, a condition in which the cornea thins and becomes cone shaped. Although the etiology of keratoconus is unknown, it may be hereditary and may be worsened by chronic eye rubbing. Diabetes can also play a role in astigmatism. High blood sugar levels can cause shape changes in the intraocular lens. This usually occurs slowly and is often noticed only when the diabetic starts treatment. The return to a more normal blood sugar allows the lens to return to normal; this is sometimes manifested as farsightedness. Diabetics should wait until their blood sugar is under control for at least one month before being refracted for eyeglasses.