What is Astigmatism?

Astigmatism is a type of refractive error that occurs when either the cornea or lens are not evenly curved. A distorted shape of the cornea is called corneal astigmatism, whereas a distorted shape of the lens is termed lenticular astigmatism. Both types of astigmatism cause blurred vision.

The cornea and the lens are the vital parts of the eye responsible for focusing images. In a perfect eye, both the lens and cornea have a smooth curvature, causing all incoming light to focus images sharply on the back of the retina. Unsmooth curvatures in the cornea or the lens cause incoming light to be refracted incorrectly. With astigmatism, the lens or cornea is more steeply curved in one or another direction causing blurred vision. Depending on the improper curvature, astigmatism causes blurred vision more prominently in one direction.

Astigmatism is one type of refractive error. Others include myopia (nearsightedness) and hyperopia (farsightedness). In myopia, light is refracted in front of the retina, instead of on it. In hyperopia, light is focused behind the retina, instead of on it. Astigmatism is often diagnosed in combination with these other refractive errors.

Are you at risk?

Most people are born with some degree of astigmatism. Signs of astigmatism include distortion in areas of the visual field, and the blurring of horizontal, vertical or diagonal lines. Many people incorrectly assume that sitting too close to the TV or reading in inadequate light causes or can worsen astigmatism. This is not the case. There are only a few situations where astigmatism can be caused or worsened, including;

1) disease
2) eye injury
3) eye surgery

Diagnosing Astigmatism

Astigmatism is an easy refractive error to diagnose. A comprehensive eye examination, using specialized tools to view the inside of the eye, can provide a diagnosis of astigmatism. Along with the routine procedures used in an eye exam, the following tools may also be used.

1) Keratometer

A keratometer is an instrument used to measure reflected light from the corneal surface. This helps a doctor assess the exact orientation and amount of corneal astigmatism.

2) Keratoscope

A keratoscope is employed to assess the curvature of the corneal surface. Using light to project rings on the cornea, the keratoscope then measures the spacing and shape of the reflection of the rings on the cornea. This allows an assessment of the degree of astigmatism present.

3) Videokeratoscope

A videokeratoscope is simply a keratoscope that is fitted with a small video camera and is the most widely used instrument to assess irregularities in the corneal surface. The process is typically referred to as corneal topography.

Astigmatism is a common, and highly treatable, type of refractive error. Treatment options for astigmatism include eyeglasses, contact lenses, and some types of vision surgery. Often, astigmatism is treated alongside other common refractive errors, including both near and farsightedness.

Resources: Understand how astigmatism is diagnosed. Get a detailed description of astigmatism and its treatment.