What is astigmatism?
The problem of astigmatism is caused by a difference in the focal length of the eye for two perpendicular directions, which can occur if the eyeball becomes slightly deformed, like a grape being squeezed between two fingers. The curvature of the cornea (outer eye surface) would be flattened in one plane but remain more rounded in the other one. A deformed eye lens can also cause astigmatism.
During an eye examination, the optometrist tests for astigmatism by showing the patient a diagram of straight lines radiating outward from the center of the picture. A person with normal eyes will see all the lines in focus, but someone with astigmatism will see only one line sharply focused while the other ones are fuzzy. For example, if the horizontal line is perceived to be in focus, then the vertical line will be blurred while the lines in between will be partially out of focus. Each eye must be tested individually because the amount of astigmatism can differ.
To correct for astigmatism, the optometrist can add a cylindrical correction to the eyeglass prescription, which changes the focal length of the eye in only one plane. A cylindrical lens can be pictured to be thick in the middle and thin at the edges, like a slice cut off from the outside edge of a cylindrical object.
A typical prescription for a person who is farsighted and also has astigmatism might be “+2.0D + 0.5 cyl axis 90.” The “+2.0D” is the strength of a typical converging lens for a farsighted person, expressed in diopters. Diopters are equal to the inverse of the focal length, so 2.0 diopters equals a focal length of 0.5 meter, or 50 centimeters. The correction for astigmatism here specifies a cylindrical lens of +0.5 diopters situated at an angle of 90 degrees to the horizontal axis.
Astigmatism can be corrected with hard contact lenses because the lens makes contact with the cornea over a layer of tears. The tears fill the space between the lens and the misshapen eyeball, providing the extra focusing that is required.
Laser surgery has become a highly successful procedure to correct for vision problems, including astigmatism. The most common forms of refractive surgery for astigmatism are laser in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), and laser epithelial keratomileusis (LASEK). In laser surgery, a computer-controlled laser beam is used to reshape the inner cornea to restore horizontal and vertical symmetry for the eyeball.
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