ASTIGMATISM
[a-not + stigma-, dot]
Astigmatism
is a Greek word meaning "not like a dot". The name
of this refractive error comes from the fact that astigmatic people
can see dots like a line. This is true for moderate
to severe astigmatism, since a survey of normal eyes shows that almost
every human eye has a baseline corneal astigmatism of at least 0.25
to 0.50 diopters. Astigmatism exceeding 1 diopter is about 20% among
the general population.
In
order to have clear vision, the cornea has to have a smooth surface
and curvature. In astigmatism natural corneal curvature is distorted
in a meridian (egg shape). This distorted corneal meridian(s)
scatter(s) light rays. In other words, an astigmatic eye bends light
rays different at different meridians, resulting in many images
with many focal points (inability to focus on a single plane). This causes blurry, ghosted, or smudged vision.
If you examine the image focused by an astigmatic eye on its retina
you may notice many superposed off-focus images on a well-focused
image. As you can guess, people with astigmatism have a constant blur
for both near and distant vision.
Sometimes astigmatism may not stem from the cornea but from the lens
or from other refractive media in the eye.
The severity of astigmatism can be classified as follows:
Astigmatic
people may suffer from headaches, tired eyes and sensitivity to bright
lights. Sometimes, like nearsighted people, they may squint in order
to improve their vision.
Classifications of Astigmatism
Astigmatism has many classifications, but in general there are two
types of astigmatism: regular astigmatism and irregular astigmatism.
If a dot is seen like two perpendicular lines, it is regular astigmatism. These lines are produced by the steepest
and flattest meridians of the cornea. Cylindrical lenses are used
to compensate for regular astigmatism. On the other hand, if the astigmatic eye sees
many lines, it is irregular astigmatism.
This means that the cornea has many irregularly shaped meridians at
different angles. Unfortunately, people with irregular astigmatism
cannot see well with glasses, but they can benefit from hard contact lenses.
Your eye doctor may tell you that the condition of your eyes
is not merely astigmatism, but a combination of refractive errors.
The following list will provide you with a more detailed explanation about classes
of astigmatism to help you understand your condition better. Regular
astigmatism may have 5 sub-classes.
1. Simple hyperopic astigmatism: One meridian is emmetropic (normal),
while the other is hyperopic.
2. Simple myopic astigmatism: One meridian is emmetropic (normal),
while the other is myopic.
3. Compound hyperopic astigmatism: Both meridians are hyperopic but
one is more severe.
4. Compound myopic astigmatism: Both meridians are myopic but one
is more severe.
5. Mixed astigmatism: One meridian is myopic, while the other one
is hyperopic.
Astigmatism
can be treated with LASIK. Sometimes when the astigmatism
is severe, astigmatic keratotomy (AK) is performed after LASIK treatment.
For irregular
astigmatism, it's better to measure the eyes' refractive power point-by-point with aberrometers (wavefront devices) rather than relying
only on the conventional refractive measurements. Custom LASIK treatment
of irregular astigmatism with wavefront and corneal topography data
is a quantum leap in the correction of the refractive errors.
