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).
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.
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).
