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Astigmatism is a refractive error which causes blurred or distorted vision. It is one of the most common vision problems. It usually occurs when the front surface of the eye, the cornea, has an irregular curvature. This irregular shape prevents light from focusing properly on the back of the eye, the retina. As a result, a person’s vision may be blurred at all distances. Normally the cornea is smooth and equally curved in all directions. With astigmatism, the cornea is “warped”, meaning it curves more in one direction and objects appear somewhat indistinct and slanted. It can occur with or without short-sightedness and long-sightedness.

Astigmatic Distortion
Left: chart seen with no astigmatism (or corrected astigmatism) ; Right: chart seen with uncorrected astigmatism

Astigmatism usually occurs when the front surface of the eye, the cornea, has an irregular curvature. Normally the cornea is smooth and equally round in all directions so that light entering the cornea is focused equally in all planes, or in all directions. In astigmatism, the front surface of the cornea is curved more in one direction than in the other. This abnormality may result in vision that is much like looking into a distorted mirror. The distortion results because of an inability of the eye to focus light to a point focus.

If the corneal surface has a high degree of variation in its curvature, corrective lenses are needed to help focus light better. If this is the case, only a small proportion of the rays are focused and the remainder are not, so that the image formed is always blurred. Astigmatism causes blurred vision at all distances.

Astigmatism is very common. Some experts believe that almost everyone has a degree of astigmatism, often from birth, which may remain the same throughout life. The exact reason for differences in corneal shape remains unknown, but the tendency to develop astigmatism is inherited. For that reason, some people are more prone to develop astigmatism than others. Gravity pulls your cornea in one direction most of the time, and your lids massage the cornea constantly, so it is not surprising that there is some degree of corneal curvature.

Topographic maps of a normal and an astigmatism cornea
Left: a map of the curves (and so focus power) of a normal spherical cornea. Right: a map of the curves (and so focus powers) of an astigmatic cornea. notice it has a rugby ball or saddle like shape.
  • Distortion or blurring of images at all distances
  • Double vision
  • Headache and fatigue
  • Squinting and discomfort or eye irritation

The symptoms described above may not necessarily mean that you have astigmatism. However, if you experience one or more of these symptoms, contact your eye doctor for a complete exam. Patients with astigmatism have blurred or distorted vision at all distances. It can also cause images to appear doubled, particularly at night. Patients have inconsistent difficulty focusing on various objects such as finely printed words and lines. Headache and fatigue are common as the person tends to strain their eyes. Squinting, eye discomfort and irritation are also frequent.

If the degree of astigmatism is slight and you have no other refractive problems, such as nearsightedness or farsightedness, corrective lenses may not be needed. If the degree of astigmatism is enough to cause eyestrain, headache, distorted or blurred vision, prescription lenses are be needed for clear and comfortable vision.

In general, normal astigmatism can be corrected with:

In very extreme cases of astigmatism, rigid contact lenses may provide a more satisfactory result than glasses but this is uncommon. Modern soft contact lenses are able to correct mild to moderate astigmatism very effectively, but do need to be fitted appropriately by a contact lens practitioner.