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Intro
Astigmatism is present in nearly 100% of the population to some
degree.
Optics
 Astigmatism
is usually caused by an abnormal corneal curvature that resembles
the shape of a football rather than that of a basketball. Light
rays focus at different places depending on their orientation, and
objects at all distances appear blurred. Astigmatism may be accompanied
by myopia or hyperopia.
For example, this picture demonstrates that the vertical light
beams are focused in front of the retina, while light beams oriented
90 degrees away (horizontal) are focused at or near the retina.
The result is two different focal planes 90 degrees apart, instead
of a sharp focal point.
Spectacle Correction
A special type of lens is needed to correct astigmatism. Soft contact
lens are not able to correct astigmatism unless they are a special
type known as toric lens. Large amounts of astigmatism often can
only be treated by rigid contact lenses.
Surgical Correction
Most commonly, moderate astigmatism (1 to 4 diopters) is corrected
using a diamond scalpel by making accurate incisions across the
steep meridian of the cornea. This flattens the steep areas of the
cornea, giving it a more spherical shape. Currently, the excimer
lasers approved in the United States do not correct astigmatism.
However, software which allows the ablation to be oval instead of
circular, is being tested and refined in Europe and South America
and will soon be available in the United States.
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