Distorted vision caused by an uneven curvature of the transparent cornea at the front of the eye
In people with astigmatism, the transparent cornea at the front of the eye is unevenly curved and refracts (bends) the light rays striking different parts of it to differing degrees. The lens of the eye is then unable to bring all the rays into focus on the light-sensitive retina at the back of the eye, and vision becomes blurred. Astigmatism can run in families and often occurs in combination with myopia or hypermetropia.
The most common form of astigmatism is present from birth and is due to a slight buckling of the cornea in both eyes. Instead of being round like a football, the cornea is shaped like a rugby ball, with a steep curvature in one direction and a shallow curvature in the other. This type may worsen slowly with age. Less often, astigmatism is due to an eye disorder such as keratoconus or an eye injury that causes a corneal ulcer to develop.
The majority of people have a slight degree of astigmatism. If you are only slightly affected, you probably will not notice much wrong with your vision. More severe astigmatism, however, may lead to significant visual problems.
Astigmatism can affect vision in a number of different ways. Symptoms may include the following:
Blurring of small print, causing difficulty in reading.
Inability to see both near and distant objects clearly.
If you are experiencing difficulty in seeing objects clearly at any distance, it is important to visit your optometrist as soon as possible to have a vision test.
In people with astigmatism, vision can usually be corrected by glasses that have special lenses that compensate for the unevenly shaped cornea. Rigid contact lenses are also effective because they smooth out the surface of the cornea. Conventional soft contact lenses mould to the shape of the cornea and can normally correct only mild astigmatism. However, soft contact lenses known as toric lenses, which are specially designed to correct astigmatism, are also available.
In some people, astigmatism may be corrected permanently by laser treatment that alters the shape of the cornea. This procedure leaves only minimal scarring (see Surgery for refractive errors).
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.