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Overview
Amblyopia, also known as "lazy
eye," is a condition characterized by diminished
vision in one eye. It is not correctable by
eyeglasses or contact lenses and is not usually
triggered by an eye disease. Instead, amblyopia
can develop when:
-
the extraocular muscles fail to
align the eyes properly and the part of the
brain that controls vision "favors" one eye over
the other;
-
an eye with a significant
refractive error in one eye goes uncorrected for
a period of time; or
-
there is a large difference in
the refractive power, and one eye is favored.
All babies are born with poor
eyesight that normally improves as they grow. In
amblyopia, one eye becomes stronger. If the weaker
eye is untreated, eyesight will progressively
worsen.
Incidence and Prevalence
Amblyopia is the most common cause of visual
impairment restricted to one eye in children and
young to middle-aged adults. About 5% of children
in the United States have amblyopia.
Types
The two most common types of amblyopia are
strabismic and anisometropic. In
strabismic amblyopia, strabismus is present and
the eyes are not aligned properly resulting in one
eye being used less than the other. The
nonpreferred eye is not adequately stimulated and
the visual brain cells do not develop normally.
With anisometropic amblyopia, the eyes have
different refractive powers. For example, one eye
may be nearsighted and the other farsighted. It
may be difficult for the brain to balance the
difference and it favors the stronger eye.
Risk Factors
Anything that interferes with equal
development of vision in both eyes between birth
and about 6 years can result in amblyopia.
Strabismus and anisomtreopia are the most common
causes of amblyopia. Other risk factors include
congenital cataracts,
something that blocks the
cornea or
lens, and a
droopy eyelid that obstructs the field of vision
in one eye.
Signs and Symptoms
Amblyopia may not produce symptoms
that are obvious to a parent or the affected
child. Amblyopia caused by an undetected
refractive error may go unnoticed for years, due
to the fact that one of the eyes is functioning
normally. As a result, many children remain
unaware of vision problems, especially before they
begin school. The condition is often diagnosed
during the first eye examination at a later age,
when improvement in vision to its fullest
potential may no longer be possible.
Sometimes, though, a child may
squint or close one eye, which indicates a visual
problem. A child old enough to verbalize may
complain of headaches or eyestrain. In strabismic
amblyopia, the crossed eye is an obvious sign.
Diagnosis
A diagnosis of amblyopia is made
with a complete eye examination including a
physical exam of the eye, a medical history, and a
vision test. An
optometrist
or an
ophthalmologist
can perform the examination.
A
basic eye exam
usually begins with a medical history and
questions about the patient’s health and past and
current eye problems. The parent of an infant or
young child answers these questions. School-aged
children may be able to specify if they have
problems seeing out of one eye, or if they have
problems reading or seeing the blackboard.
Eye movement is tested by moving a
light or object through the patient’s field of
vision and assessing the eye’s ability to follow
it. This allows the doctor to determine if the eye
muscles are working properly. Tests that access
binocular vision — how the eyes work together —
are also done. In strabismus, the stronger eye may
do all the looking, or the weaker eye may align
itself intermittently.
If the patient is old enough, a
vision test involving reading letters from an eye
chart may also be utilized. This helps determine
if either eye has a
refractive error.
If the patient is not old enough to know letters,
other objective methods are used to determine the
refractive error.
Other tests may be performed if
disease is the suspected cause. These tests depend
on the results of the initial exam, symptoms, and
the appearance of the eye.
Treatment
Treatment depends on the underlying
cause and how early the condition is diagnosed. It
was previously believed that only young children
could benefit from treatment, because the brain’s
vision system is completely developed by age 8 to
10. Detection and correction before the age of 2
offers the best chance for normal vision. However,
current research has concluded that effective
treatment can take place at any age. The longer
the condition goes uncorrected, the longer the
length of treatment required. Treatment can be as
simple as a pair of eyeglasses or as complex as
surgery.
Surgery
is used to treat amblyopia caused by strabismus,
cataracts, and other blockages on the cornea. For
strabismus, surgery is performed on the eye muscle
to force the eyes into alignment. Sometimes
surgical results are cosmetic; the eyes look
straighter, but they are still not aligned and
require further treatment. Early surgery is often
recommended, so that infants can develop normal
sight as their eyes mature.
Eyeglasses
are used to correct the visual imbalance if
amblyopia is caused by a refractive error. Glasses
or bifocals are also sometimes used to straighten
strabismic eyes.
Vision therapy
uses exercises to strengthen the eye muscle and
help both eyes work together. It also trains the
brain to use the amblyopic eye, improving its
vision. Vision therapy can be used alone, before
or after surgery, and with eyeglasses.
Patching
involves covering the good eye and forcing use of
the amblyopic eye. The weaker eye becomes stronger
with use. The problem with eye patches is that
some children do not like wearing them. A patch
can be worn all day or for a few hours a day,
depending on the child’s age and vision. It is
used until the eye has strengthened and vision has
normalized or until improvement plateaus.
Prevention
Amblyopia is caused by another eye
condition, so diagnosing and treating visual
problems as early as possible can prevent its
onset. An initial eye examination is recommended
at age 3 to 6 months and then periodically. An eye
problem detected in infancy, such as strabismus,
can be corrected before it causes amblyopia.
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