Astigmatism is a vision condition that causes blurred vision due either to
the irregular shape of the cornea, the clear front cover of the eye, or
sometimes the curvature of the lens inside the eye. An irregular shaped
cornea or lens prevents light from focusing properly on the retina, the
light sensitive surface at the back of the eye. As a result, vision becomes
blurred at any distance.
Astigmatism is a very common vision condition. Most people have some degree
of astigmatism. Slight amounts of astigmatism usually don't affect vision
and don't require treatment. However, larger amounts cause distorted or
blurred vision, eye discomfort and headaches.
What causes astigmatism?
When the cornea or lens of an eye is irregularly shaped, vision may be out
of focus at any distance.
Astigmatism occurs due to the irregular shape of
the cornea or the lens inside the eye. The cornea and lens are primarily
responsible for properly focusing light entering your eyes allowing you to
see things clearly.
The curvature of the cornea and lens causes light entering the eye to be
bent in order to focus it precisely on the retina at the back of the eye. In
astigmatism, the surface of the cornea or lens has a somewhat different
curvature in one direction than another. In the case of the cornea, instead
of having a round shape like a basketball, the surface of the cornea is more
like a football. As a result, the eye is unable to focus light rays to a
single point causing vision to be out of focus at any distance.
Sometimes astigmatism may develop following an eye injury or eye surgery.
There is also a relatively rare condition called keratoconus where the
cornea becomes progressively thinner and cone shaped. This results in a
large amount of astigmatism resulting in poor vision that cannot be clearly
corrected with spectacles. Keratoconus usually requires contact lenses for
clear vision, and it may eventually progress to a point where a corneal
transplant is necessary.
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How is astigmatism diagnosed?
A phoropter and a retinoscope are instruments commonly used by optometrists
to measure refraction.Astigmatism can be diagnosed through a comprehensive
eye examination. Testing for astigmatism measures how the eyes focus light
and determines the power of any optical lenses needed to compensate for
reduced vision. This examination may include:
- Visual acuity - As part of the testing, you'll be asked to read letters on
a distance chart. This test measures visual acuity, which is written as a
fraction such as 20/40. The top number is the standard distance at which
testing is done, twenty feet. The bottom number is the smallest letter size
you were able to read. A person with 20/40 visual acuity would have to get
within 20 feet of a letter that should be seen at forty feet in order to see
it clearly. Normal distance visual acuity is 20/20.
- Keratometry - A keratometer is the primary instrument used to measure the
curvature of the cornea. By focusing a circle of light on the cornea and
measuring its reflection, it is possible to determine the exact curvature of
the cornea's surface. This measurement is particularly critical in
determining the proper fit for contact lenses. A more sophisticated
procedure called corneal topography may be performed in some cases to
provide even more detail of the shape of the cornea.
- Refraction - Using an instrument called a phoropter, your optometrist
places a series of lenses in front of your eyes and measures how they focus
light. This is performed using a hand held lighted instrument called a
retinoscope or an automated instrument that automatically evaluates the
focusing power of the eye. The power is then refined by patient’s responses
to determine the lenses that allow the clearest vision.
Using the information obtained from these tests, your optometrist can
determine if you have astigmatism. These findings, combined with those of
other tests performed, will allow the optometrist to determine the power of
any lens correction needed to provide clear, comfortable vision, and discuss
options for treatment.
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How is astigmatism treated?
Persons with astigmatism have several options available to regain clear
vision. They include:
- eyeglasses
- contact lenses
- orthokeratology
- laser and other refractive surgery procedures
Eyeglasses are a common form of correction for persons with
astigmatism.Eyeglasses are the primary choice of correction for persons with
astigmatism. They will contain a special cylindrical lens prescription to
compensate for the astigmatism. This provides for additional lens power in
only specific meridians of the lens. An example of a prescription for
astigmatism for one eye would be -1.00 -1.25 X 180. The middle number
(-1.25) is the lens power for correction of the astigmatism. The "X 180"
designates the placement (axis) of the lens power. The first number (-1.00)
indicates that this prescription also includes a correction for
nearsightedness in addition to astigmatism.
Generally, a single vision lens is prescribed to provide clear vision at all
distances. However, for patients over about age 40 who have the condition
called presbyopia, a bifocal or progressive addition lens may be needed.
These provide different lens powers to see clearly in the distance and to
focus effectively for near vision work.
A wide variety of lens types and frame designs are now available for
patients of all ages. Eyeglasses are no longer just a medical device that
provides needed vision correction. Eyeglass frames are available in a many
shapes, sizes, colors and materials that not only allow for correction of
vision, but also enhance appearance.
For some individuals, contact lenses can offer better vision than
eyeglasses. They may provide clearer vision and a wider field of view.
However, since contact lenses are worn directly on the eyes, they require
regular cleaning and care to safeguard eye health.
Soft contact lenses conform to the shape of the eye, therefore standard soft
lenses may not be effective in correcting astigmatism. However, special
toric soft contact lenses are available to provide a correction for many
types of astigmatism. Because rigid gas permeable contact lenses maintain
their regular shape while on the cornea, they offer an effective way to
compensate for the cornea’s irregular shape and improve vision for persons
with astigmatism and other refractive errors.
Orthokeratology (Ortho-K) involves the fitting of a series of rigid contact
lenses to reshape the cornea, the front outer cover of the eye. The contact
lenses are worn for limited periods, such as overnight, and then removed.
Persons with moderate amounts of astigmatism may be able to temporarily
obtain clear vision without lenses for most of their daily activities.
Orthokeratology does not permanently improve vision and if you stop wearing
the retainer lenses, your vision may return to its original condition.
Astigmatism can also be corrected by reshaping the cornea using a highly
focused laser beam of light. Two commonly used procedures are
photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK).
PRK removes tissue from the superficial and inner layers of the cornea.
LASIK does not remove tissue from the surface of the cornea, but only from
its inner layer. To do this, a section of outer corneal surface is cut and
folded back to expose the inner tissue. Then a laser is used to remove the
precise amount of tissue needed and the flap of outer tissue is placed back
in position to heal. Both procedures allow light to focus on the retina by
altering the shape of the cornea.
Individuals with astigmatism have a wide range of options to correct their
vision problem. In consultation with your optometrist, you can select the
treatment that best meets your visual and lifestyle needs.
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