CausesBy Mayo Clinic staff
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|Anatomy of the eye|
Your eye has two parts that focus images — the cornea and the lens. In a perfectly shaped eye, each of these focusing elements has a perfectly smooth curvature, like the surface of a smooth ball. A cornea or lens with such a surface curvature bends (refracts) all incoming light the same way and makes a sharply focused image on the back of your eye (retina).
However, if your cornea or lens isn't evenly and smoothly curved, the light rays aren't refracted properly. This causes a refractive error. Astigmatism is one type of refractive error. In astigmatism, your cornea or lens is curved more steeply in one direction than in another. When the cornea has a distorted shape, you have corneal astigmatism. When the lens is distorted, you have lenticular astigmatism. Either type of astigmatism can cause blurred vision. Blurred vision may occur more in one direction — either horizontally, vertically or diagonally.
Astigmatism may occur in combination with other refractive errors, which include:
- Nearsightedness (myopia). This occurs when your cornea is curved too much or your eye is longer than normal. Instead of being focused precisely on your retina, light is focused in front of your retina, resulting in a blurry appearance for distant objects.
- Farsightedness (hyperopia). This occurs when your cornea is curved too little or your eye is shorter than normal. The effect is the opposite of nearsightedness. When your eye is in a relaxed state, light is focused behind the back of your eye, making nearby objects blurry.
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.
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- Preferred practice pattern: Comprehensive adult medical eye evaluation. San Francisco, Calif.: American Academy of Ophthalmology. http://one.aao.org/ce/practiceguidelines/ppp_content.aspx?cid=64e9df91-dd10-4317-8142-6a87eee7f517. Accessed Nov. 21, 2010.
- Preferred practice pattern: Refractive errors and refractive surgery. San Francisco, Calif.: American Academy of Ophthalmology. http://one.aao.org/ce/practiceguidelines/ppp_content.aspx?cid=e6930284-2c41-48d5-afd2-631dec586286. Accessed Nov. 21, 2010.