Causes

The eye has two structures with curved surfaces that bend (refract) light onto the retina, which makes the images:

• The cornea, the clear front surface of the eye along with the tear film
• The lens, a clear structure inside the eye that changes shape to help the eye focus on near objects

In a perfectly shaped eye, each of these elements has a round curvature, like the surface of a smooth ball. A cornea or lens with such curvature bends (refracts) all incoming light equally to make a sharply focused image directly on the retina at the back of the eye.

A refractive error

If either the cornea or the lens is egg-shaped with two mismatched curves, light rays aren't bent the same, which means that two different images form. These two images overlap or combine and result in blurred vision. Astigmatism is a type of refractive error.

Astigmatism occurs when the cornea or lens is curved more steeply in one direction than in another. You have corneal astigmatism if your cornea has mismatched curves. You have lenticular astigmatism if your lens has mismatched curves.

Either type of astigmatism can cause blurred vision. Blurred vision may occur more in one direction: horizontally, vertically or diagonally.

Astigmatism may be present from birth, or it may develop after an eye injury, disease or surgery. Astigmatism isn't caused or made worse by reading in poor light, sitting too close to the television or squinting.

Other refractive errors

Astigmatism may occur in combination with other refractive errors, which include:

• Nearsightedness (myopia). This occurs when the cornea is curved too much or the eye is longer than usual. Instead of being focused precisely on the retina, light is focused in front of the retina, making distant objects seem blurry.
• Farsightedness (hyperopia). This occurs when the cornea is curved too little or the eye is shorter than usual. The effect is the opposite of nearsightedness. When the eye is in a relaxed state, light never comes to a focus at the back of the eye, making nearby objects seem blurry.