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What is Refractive Error?
Laser Vision Correction treats
refractive errors of the eye. A refractive error is an incorrect focusing of an
image onto the retina by the cornea and lens of the eye. The cornea, just by its
curvature, does about 80% of the focusing of the eye. The lens within the eye
does the remaining focusing.
In myopia (nearsightedness), the cornea
is curved too steeply for the length of the eye, and the image is focused in
front of the retina. This leads to blurred vision at distance, while the vision
may be clear at a closer distance (thus "nearsighted").
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Example of a Myopic Eye and Image
In hyperopia
(farsightedness), the cornea is not curved enough to focus light onto
the retina, or the eye is too short in length. This leads to blurred vision at
both near and distance, but the vision will be less blurred at distance (thus
"farsighted"). If the lens of the eye is capable of focusing, some or all of a
farsighted refractive error may be self-corrected by the eye by focusing in the
image.
Example of a Hyperopic Eye and Image
In astigmatism, the cornea is curved more
in one direction than another. This leads to different parts of an image being
in focus at different places on the retina. Astigmatism can occur in combination
with myopia or hyperopia. In the example below, the vertical lines of the image are
blurred more than the horizontal lines. The direction of blurring can
occur at any angle from horizontal to vertical, which is the axis of
astigmatism.
Example of an Astigmatism Image
In presbyopia (the inability to focus at near),
the lens in the eye loses its near focusing ability, so reading glasses or
bifocals are needed for close vision. Presbyopia can occur in combination with
any other type of refractive error.
All of the above refractive errors can be corrected by
glasses or contact lenses, or with bifocals or reading glasses in the case of
presbyopia. Laser Vision Correction can correct from low to high levels of
myopia with or without astigmatism, and low to moderate levels of hyperopia with
or without astigmatism. Laser Vision Correction cannot restore the focusing
ability of the lens of the eye, and thus presbyopia cannot by treated at this
time. However, one eye can be left intentionally nearsighted for reading,
similar to the monovision effect with contact lenses.
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