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ASTIGMATISM


What is astigmatism?

The cornea is responsible for about 2/3 of the eye’s refractive power needed to focus on the retina; the lens provides the other 1/3. If the surface of either of these optical components is not smoothly spherical, (i.e., less curved across some orientations than others) some orientations will be better focused than others. This visual defect, normally due to asymmetries in the curvature of the cornea is termed astigmatism. Clinicians distinguish between two general types of astigmatism: with-the-rule and against-the-rule. In with-the-rule astigmatism, the eye has more refractive power along the vertical axis and the patient has difficulty resolving targets with horizontal lines (e.g., letters such as E or F). A patient with against-the-rule astigmatism has the opposite problem; they have difficulty focusing vertically oriented targets.

The visual effect of with-the-rule astimatism can be seen in the following simulation using a Green's Astigmatic Chart, a test used to detect astigmatism. As can be seen, due to greater refractive power allong the vertical axis, the lines along the XII/VI orientation are more sharply focused. Some other common symptoms of astigmatism include headaches, and fatigue.

If you would like to see if you might have astigmatism, look directly at the center of the chart below, using the eye you want to test (close the other eye). Without shifting your gaze, note whether the lines along some orientations look lighter or more blurry than others. If they do, you may have astigmatism and may want to consult your eye-care practitioner (i.e., optometrist or ophthalmologist).

 

 

Astigmatism in Early Development

Severe astigmatism, if untreated during early development can lead to a permanent reduction of good vision along the affected orientation. This type of deficiency is called meridional amblyopia. Interestingly, there seems to be some developmental differences in astigmatism associated with race. For example, caucasian babies seem more likely to show against-the-rule astigmatism whereas oriental babies more frequently manifest with-the-rule astigmatism.


Treatment

Almost all eyes have some degree of astigmatism. Correction is needed only when it is severe enough to interfere with normal visual function. Typically, astigmatism is treated by the prescription of cylindical lenses that supplement the eye's refractive power only for the orientation where is is needed. Astigmatism can also be treated using photorefractive surgery (e.g., LASIK or PRK).

 


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