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- Eye Care Treatment
- Eye Disease
- Lasik Surgery
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Treating Astigmatism
Many nearsighted or farsighted patients also have
what is called “astigmatism”, which is a refractive error whereby
light is not focused to a single point.
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Vision is indistinct at every distance
because the cornea - which should be dome-shaped - is
"out-of-round" or shaped rather like the back of a spoon.
Such a cornea is more steeply curved in one direction than
the other, having two "axes" that are usually perpendicular
to each other. Depending on the person, 0.5 diopters or more
of astigmatism starts to cause noticeably blurred vision.
Laser surgery can smooth out the astigmatic cornea's
curvature, changing the shape of the central cornea (mainly
the central 6 or 7 millimeters) to look more like a
symmetrical soup bowl than the back of a lopsided spoon. To
correct astigmatism surgically, which is more difficult than
treating nearsightedness, physicians selectively remove
tissue to make the curvature of the steepest and flattest
"meridians" (the corneal curves of greatest and smallest
refractive power) more alike. |
For example, if you have simple myopic
astigmatism, the curvature of the meridians can be evened out by
lasering tissue from the steepest meridian to make it flatter -
thereby moving the light-bending effect of this forward axis
back toward the retina. Less tissue would be removed from the
other, flatter meridian.
Some refractive surgeons target astigmatism with a specially
"masked" broad-beam laser that is driven by customized computer
software. Since the surface topography of the cornea is
digitized, or put in numerical form, the computer knows which
areas are elevated. During a more modern surgical technique
using a "flying-spot" laser, a fast, small-beam laser dances
around the cornea, chipping away at the higher spots and
avoiding the flatter areas.
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There are other causes of astigmatism besides
irregularities on the front surface of your cornea. The back
of your cornea also can be malformed. Furthermore, since
your eye is a two-lens system, problems with the front and
back curvature of your crystalline lens inside your eye can
cause astigmatism. In fact, any misalignment of the internal
components of your eye can blur images placed on your
retina. A few people have astigmatism because of a disparity
between their line-of-sight (the visual axis which goes from
what they are looking at to the light-sensing photoreceptors
on the retinal fovea) and their optic axis (the way the
light strikes the retina). Densely packed with receptors,
the fovea - a tiny depression in the retina - is the area of
clearest vision. In these patients, the rays of light fail
to strike the fovea correctly. Laser surgery is unable to
treat these problems.
Currently, refractive surgeons can only correct the part of
your astigmatic error caused by irregularities in the
curvature of the front surface of your cornea. |
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Consequently, the more disparity between treatable surface corneal
astigmatism and total astigmatism, the less predictable the surgical
result. To even out the cornea's front surface, the laser can be
programmed to make the cornea more bowl-shaped. Unfortunately, some
patients have noticeable astigmatism caused by problems inside their
eyes. These patients will still have residual astigmatism after
surgery, even if the front of the cornea is treated. In fact, many
eyes that manifest no refractive astigmatism (as measured during an
eye exam) actually have astigmatic defects inside the eye, but they
are counterbalanced by astigmatism on the surface of the cornea.
This is one instance in which two wrongs do make a right, because
the error in one lens offsets that in the other to provide clear
focus. |
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