Nearsightedness, or myopia, as it is medically termed, is a vision condition
in which close objects are seen clearly, but objects farther away appear
blurred. Nearsightedness occurs if the eyeball is too long or the cornea,
the clear front cover of the eye, has too much curvature. As a result, the
light entering the eye isn’t focused correctly and distant objects look
blurred.
Nearsightedness is a very common vision condition affecting nearly 30
percent of the U.S. population. Some research supports the theory that
nearsightedness is hereditary. There is also growing evidence that it is
influenced by the visual stress of too much close work.
Generally, nearsightedness first occurs in school-age children. Because the
eye continues to grow during childhood, it typically progresses until about
age 20. However, nearsightedness may also develop in adults due to visual
stress or health conditions such as diabetes.
What causes nearsightedness?
If one or both parents are nearsighted, there is an increased chance their
children will be nearsighted.The exact cause of nearsightedness is unknown,
but two factors may be primarily responsible for its development:
There is significant evidence that many people inherit nearsightedness, or
at least the tendency to develop nearsightedness. If one or both parents are
nearsighted, there is an increased chance their children will be
nearsighted.
Even though the tendency to develop nearsightedness may be inherited, its
actual development may be affected by how a person uses his or her eyes.
Individuals who spend considerable time reading, working at a computer, or
doing other intense close visual work may be more likely to develop
nearsightedness.
Nearsightedness may also occur due to environmental factors or other health
problems:
- Some people may experience blurred distance vision only at night. This
“night myopia” may be due to the low level of light making it difficult for
the eyes to focus properly or the increased pupil size during dark
conditions, allowing more peripheral, unfocused light rays to enter the eye.
- People who do an excessive amount of near vision work may experience a
false or “pseudo” myopia. Their blurred distance vision is caused by over
use of the eyes’ focusing mechanism. After long periods of near work, their
eyes are unable to refocus to see clearly in the distance. The symptoms are
usually temporary and clear distance vision may return after resting the
eyes. However, over time constant visual stress may lead to a permanent
reduction in distance vision.
- Symptoms of nearsightedness may also be a sign of variations in blood sugar
levels in persons with diabetes or an early indication of a developing
cataract.
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How is nearsightedness diagnosed?
Testing for nearsightedness may use several procedures in order to measure
how the eyes focus light and to determine the power of any optical lenses
needed to correct the reduced vision.
A phoropter and retinoscope are often used to determine the lenses that
allow the clearest vision during a comprehensive eye exam.As part of the
testing, letters on a distance chart are identified. This test measures
visual acuity, which is written as a fraction such as 20/40. The top number
of the fraction is the standard distance at which testing is performed,
twenty feet. The bottom number is the smallest letter size read. A person
with 20/40 visual acuity would have to get within 20 feet to identify a
letter that could be seen clearly at forty feet in a “normal” eye. Normal
distance visual acuity is 20/20, although many people have 20/15 (better)
vision.
Using an instrument called a phoropter, an optometrist places a series of
lenses in front of your eyes and measures how they focus light using a hand
held lighted instrument called a retinoscope. The doctor may choose to use
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.
This testing may be done without the use of eye drops to determine how the
eyes respond under normal seeing conditions. In some cases, such as for
patients who can’t respond verbally, or when some of the eye’s focusing
power may be hidden, eye drops may be used. They temporarily keep the eyes
from changing focus while testing is performed.
Using the information obtained from these tests, along with the results of
other tests of eye focusing and eye teaming, your optometrist can determine
if you have nearsightedness. He or she will also determine the power of any
lens correction needed to provide clear vision. Once testing is complete,
your optometrist can discuss options for treatment.
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How is nearsightedness treated?
Persons with nearsightedness have several options available to regain clear
distance vision. They include:
- Eyeglasses. Eyeglasses are the primary choice of correction for persons with
nearsightedness. Generally, a single vision lens is prescribed to provide
clear vision at all distances. However, for patients over about age 40, or
children and adults whose nearsightedness is due to the stress of near
vision work, a bifocal or progressive addition lens may be needed. These
multifocal lenses provide different powers or strengths throughout the lens
to allow for clear vision in the distance and also clear vision up close.
Eyeglasses are frequently used to correct myopia.A large selection of lens
types and frame designs are now available for patients of all ages. Eye
glasses are no longer just a medical device that provides needed vision
correction, but can also be a fashion statement. They are available in a
wide variety of sizes, shapes, colors and materials that not only correct
for vision problems but also may enhance appearance.
- Contact Lenses. 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.
- Orthokeratology (Ortho-k). Orthokeratology, also known as corneal refractive therapy,
involves the fitting of a series of rigid contact lenses to reshape the
cornea, the front outer surface of the eye. The contact lenses are worn
daily for limited periods, such as overnight, and then removed. Persons with
moderate amounts of nearsightedness may be able to temporarily obtain clear
vision for most of their daily activities.
- LASIK. Nearsightedness can also be corrected by reshaping the cornea using a laser
beam of light. Two commonly used procedures are photorefractive keratectomy
(PRK) and laser in situ keratomileusis (LASIK).
In PRK, a laser is used to remove a thin layer of tissue from the surface of
the cornea in order to change its shape and refocus light entering the eye.
There is a limit to how much tissue can safely be removed and therefore the
amount of nearsightedness that can be corrected.
LASIK does not remove tissue from the surface of the cornea, but from its
inner layers. To do this, a section of the outer corneal surface is cut and
folded back to expose the inner tissue. Then a laser is used to remove the
precise amount of corneal tissue needed to reshape the eye, and then the
flap of outer tissue is placed back in position to heal. The amount of
nearsightedness that LASIK can correct is limited by the amount of corneal
tissue that can be removed in a safe manner.
- Intraocular Lenses. People who are highly nearsighted or whose corneas are too thin to allow the
use of laser procedures now have another option. They may be able to have
their nearsightedness surgically corrected by implanting small lenses in
their eyes. These intraocular lenses look like small contact lenses and they
provide the needed optical correction directly inside the eye.
- Vision therapy is an option for people whose blurred distance vision is
caused by a spasm of the muscles which control eye focusing. Various eye
exercises can be used to improve poor eye focusing ability and regain clear
distance vision.
People with nearsightedness have a variety of options to correct their
vision problem. In consultation with your optometrist, you can select the
treatment that best meets you visual and lifestyle needs.
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