How Do Astigmatism Contacts Work?
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What is Astigmatism
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Astigmatism is a vision problem caused by an irregularly shaped cornea. The cornea, or the clear cover over the front of the eye, should be rounded. However, in people with astigmatism, the cornea is oval or oblong rather than round. This difference in curvature skews the light rays coming into the eye and the rays focus on two or more points on the retina instead of one. Astigmatism is the most common vision problem, although in many people, it is so slight they do not notice. It is hereditary, but it may also be caused by an eye injury, certain types of eye surgeries or a disease called keratoconus that affects the cornea.
How Contact Lenses Correct Astigmatism
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Contact lenses correct astigmatism by reshaping the cornea. The lenses, called toric lenses, must be specially designed for each person to correct the specific degree of curvature in addition to coexisting farsightedness or nearsightedness. Although a variety of toric lenses are available, the lens itself must be firm enough to shape the cornea. For example, with regular soft lenses, the cornea shapes the lens, rather than the lens shaping the cornea. Because toric lenses correct a specific corneal irregularity, they must sit on the cornea in a certain way to be effective. Regular contact lenses rotate and slide with blinking. Toric lenses are weighted at the bottom to help keep the lens in the correct orientation. The type of toric lens (rigid gas permeable, disposable or daily wear) depends on the level of astigmatism. That is, some types of astigmatism are more difficult to correct and may require nondisposable lenses.
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Options to Correct Astigmatism
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People with astigmatism have options other than eyeglasses or contact lenses. A noninvasive procedure called orthokeratology (ortho-k) uses a series of specially designed lenses that gradually reshape the cornea to the correct curvature. Retainer lenses are typically worn at night. Ortho-k is not a permanent solution and the cornea may revert to its natural curvature if the retainer lenses are not worn. Another, permanent option is laser surgery, which reshapes the eye by removing small layers of tissue. Laser surgery is not effective for all types of astigmatism, and it does not always restore vision to 20/20.
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