Astigmatism
is a condition where the optical power of the eye varies depending on the angle
of light passing through it. Astigmatism produces blurred vision at all
distances.
It is usually due to the shape of the cornea (the front surface
of the eye). If the curvature of the cornea is not the same in all directions
it will bend the light passing through it by different amounts depending on the
direction of the light, producing astigmatism.
Astigmatism is a focusing error which causes asymmetric blur.
Some directions in an image are more out of focus than others. This can be
contrasted with short-sightedness (myopia) where all directions are uniformly
blurred.
What
Does It Look Like?
Astigmatism
causes different amounts of blur in different directions. This causes images to
appear distorted, or sometimes even double. Certain letters may be more
difficult to read than others, depending on the orientation of the lines within
them.
One type of chart used to detect astigmatism uses a series of
lines arranged in a fan shape – if you have astigmatism, some lines will appear
clearer than others.
If you don’t have astigmatism (or it is corrected), you can see
what it is like by looking at the chart while squinting your eyes. When you do
this, the pressure from your eyelids distorts your corneas, causing some
temporary astigmatism. When you open your eyes again, your cornea returns to
its normal shape.
Most
astigmatism is caused by the shape of the front surface of the eye (the
cornea). It can also be caused by slight tilting of the lens inside the eye. It
may be an inherited characteristic or a normal variation accompanying growth.
Can
You Describe the Shape?
A
magnifying glass focuses the sun to a point image because its two surfaces are
spherical, each like the surface of a basketball. Now imagine a transparent
surface shaped like the side of an rugby football. It has two different
curvatures. These result in light focusing at two different locations. The
image does not focus to a point and so is blurred.
How
Does Astigmatism Affect Me?
Objects
at all distances are indistinct or blurred and the eye cannot focus. Even
slight degrees may encourage headaches, fatigue and reduce concentration. This
is because the eyes may try, without success, to correct the blur, and because
there is a tendency to screw up the eyes to try to see better, producing
discomfort in the muscles of the eyelid and face.
How
is Astigmatism Corrected?
Spectacles
and contact lenses (hard and soft) can correct astigmatism. Sometimes
correction of astigmatism can cause change in the apparent size and shape of
objects and may affect judgement of distance. A patient may feel taller or
shorter, or walls may appear to slope and floors curve.
In most cases, adjustment to these side effects takes only a
week or so. Astigmatism correction may involve a compromise between optimal
clarity and visual discomfort.
How
is Astigmatism Detected?
Some
people notice blur themselves. Only a proper eye examination will determine for
certain if you have astigmatism.
Astigmatism is not an eye disease and any changes are generally
gradual and not necessarily for the worse. Most people have at least very
slight astigmatism.