'Stigma' is the Greek
word for a spot, and 'a-' at the beginning
of a Greek word makes it negative. So 'astigma'
means 'not a spot'. Astigmatism is the kind
of eye-focusing problem in which a small
circular spot of light is focused by the
lens system of the eye in such a way that
the image of the spot on the retina is a
smeared line instead of a sharp point.
Most people have a
small degree of astigmatism and glasses
are entirely unnecessary for this. But more
severe astigmatism causes blurring of objects
oriented in a particular direction. A person
with astigmatism might, for instance, see
horizontal lines clearly while vertical
lines are blurred; or the greatest blurring
may be of lines set at an oblique angle.
Symptoms
Astigmatism of more
than minor degree causes blurred vision
that can't be corrected by ordinary equally
curved lenses. Normal minor degrees of astigmatism
do not cause headaches, and even high degrees
are no more likely to cause headaches than
any other kind of focusing error.
Causes
The outer lens, the
cornea, is the main focusing lens of the
eye. Ideally, it should be curved like the
surface of a perfect sphere, with an equal
degree of curvature along every meridian
(meridians are lines around a sphere passing
through its poles, like the lines of longitude
on the earth). In astigmatism, although
the eye is perfectly healthy, the cornea
is curved more like the surface of an egg,
with a steep curve in one meridian and a
flatter curve in others.Because the lens
has greatest power in one meridian and least
power in the curve at right angles to that,
an astigmatic cornea has a range of focus
and can't focus anything sharply. The maximum
curvature is often set vertically or horizontally,
but it can be at any axis in between.
Treatment
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Astigmatism
