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What is Strabismus?
Strabismus is a misalignment of the eyes
where the two eyes are pointed in different directions. Though it is
a common condition which affects about 4% of children, it may appear
later in life. The misalignment may be permanent and always
noticeable, or it may come and go appearing normal at times and
abnormal at others. One eye may be directed straight ahead while the
other is turned inward (esotropia), outward (exotropia), upward
(hypertropia) or downward (hypotropia). In other cases, the turned
eye may straighten at times and the straight eye may turn.
Eye Muscles
There are six eye muscles attached to the
outside of each eye which control its movement. In each eye, two
muscles move the eye right or left; there other four muscles move the
eye up or down and control tilting movements. In order to line up and
focus both eyes on a target, all eye muscles of each eye must be
balanced and working together with the corresponding muscles of the
other eye. When the eye muscles do not work together, misalignment of
the eyes, or strabismus, results.
The eyes are designed to focus images
clearly on the retina and then to relay them to the brain. If both
eyes are lined up on the same target, the visual portion of the brain
can fuse the two pictures into a single, 3-dimensional image. This
creates depth perception and binocular vision, which helps the eyes
work together to transmit one ãpictureä to the brain. When one eye
turns, as in strabismus, two different pictures are sent to the
brain. In the young child the brain learns to ignore the image from
the misaligned eye and see only the image from the straight or better
seeing eye. This causes loss of depth perception. Adults who develop
strabismus usually have double vision because their brain is already
trained to receive images from both eyes and cannot ignore the image
from the turned eye.
Normal alignment of both eyes during
childhood allows good vision to develop in each eye. Abnormal
alignment as in strabismus may cause reduced vision or amblyopia.
Amblyopia occurs in approximately one half of children with
strabismus. The brain will recognize the image of the better seeing
eye and ignore the image of the weaker eye. Amblyopia can often be
reversed by patching the preferred eye in order to strengthen and
improve the vision of the weaker one. If amblyopia is detected within
the first few years of life, treatment is often successful. If
adequate treatment is delayed until later, amblyopia generally becomes
permanent. As a rule, the earlier amblyopia is treated, the better the
visual result.
Causes and Symptoms
Strabismus is cause by misaligned eye
muscles. However, the exact reason for the misalignment of the eyes
leading to strabismus is not fully understood. It is known that
strabismus runs in some families. However, in many patients, there
are no relatives with the problem. The condition occurs equally in
males and females.
The brain controls the eye muscles. This
explains why children with cerebral palsy, Downâs Syndrome, and
hydrocephalus often have strabismus. Even a brain tumor may cause
strabismus. If the vision of one eye is cloudy because of a cataract
or injury, then the eye can frequently develop strabismus.
The primary symptom of strabismus is an
eye that is not straight. Sometimes a youngster will squint one eye in
bright light. Faulty depth perception may be present. Some children
turn or tilt their heads in a specific direction in order to use their
eyes together.
Parents often get the false impression
that a child may ãoutgrowä the problem. Though fatigue or illness may
worsen strabismus, children do not outgrow strabismus. Once a child
has a suspected turning of an eye, an examination by an
ophthalmologist is necessary to determine the cause and to begin
treatment.
Detection and Diagnosis
A child should be examined by the family
doctor, pediatrician or ophthalmologist (a medical eye doctor) during
infancy and preschool in order to detect any potential eye problem,
particularly if a relative has had strabismus or amblyopia. Even the
most observant parent may not discover strabismus without a doctorâs
help. It is often difficult to determine the difference between eyes
that appear crossed (pseudostrabismus) and true strabismus. Young
children usually have a wide nasal bridge, flat nose and redundant
fold of skin at the inner eyelid that tends to hide the eye during
side gaze and cause concern about strabismus. An ophthalmologist can
readily distinguish this from true strabismus.
It is never too early to have a childâs
eyes examined. Fortunately, an ophthalmologist can test even a
newbornâs eyes. If the eye examination is delayed until the child
enters school, it may be too late to properly correct strabismus and
amblyopia. Occasionally, a misaligned eye may be cause by a cataract
or tumor in the eye. It is important to know about such conditions as
early as possible so both the underlying condition and the resulting
strabismus can be corrected.
Treatment
The goals of treatment are to preserve
vision, straighten the eyes and restore binocular vision. Treatment
of strabismus depends upon the exact cause of the misaligned eyes. It
can be directed towards unbalanced muscles, cataract removal, or other
conditions which are causing the eyes to turn. After a complete eye
exam, including a detailed study of the inner parts of the eyes, an
ophthalmologist can recommend appropriate medical, optical or
surgical therapy. Covering or patching the good eye to force use of
the amblyopic eye may be necessary to ensure equal vision.
Infantile Esotropia
The most common type of strabismus in
infants is esotropia, where and eye turns in. Infants born with
esotropia will not learn to use their two eyes together and may lose
vision in the weaker eye. In most cases, early surgery is needed to
align the eyes in an effort to obtain binocular vision and prevent
permanent vision loss. The aim of the surgery is to adjust the muscle
tension on one or both eyes, on order to pull the eyes straight.
(Please see section on strabismus surgery.)
Accommodative Esotropia
Another common form of esotropia that
occurs in children usually after the age of one or two is caused by a
need for glasses. These children are farsighted. They have the
ability to focus the eyes enough to adjust for the farsightedness,
which allows them to see well for both distance and near. Some
children excessively cross their eyes when they focus, which cause the
eyes to turn in. Wearing glasses in equal strength to their
farsightedness reduces the need to focus and straightens their eyes.
Sometimes the addition of bifocals is necessary to further reduce the
need to focus when looking up close. Occasionally eye drops and
special lenses called prisms can be used to focus the eyes properly.
Exotropia
Exotropia or an outward turning of an eye
is another common type of strabismus. Most commonly this occurs when
a child is focusing at distant objects. Often the exotropia will
occur intermittently particularly when the child is daydreaming, ill
or tired. Parents often notice that the child squints one eye in the
bright sunlight. Although glasses and prism therapy may reduce the
amount of outward turning in some patients, surgery is usually needed.
Strabismus Surgery
Strabismus surgery is usually a safe and effective treatment but
is not a substitute for glasses or amblyopia therapy. During surgery
the eyeball is not removed from the socket. The selection of the
muscles to be operated is dependent on the type of strabismus in each
individualâs case. More than one surgery is often necessary to get
the eyes straight. Please see our section on strabismus surgery for
more information.
Who can treat strabismus?
An ophthalmologist
is the medical doctor who is educated trained and licensed to provide
total eye care of the eyes including the diagnosis and treatment of
strabismus. Total eye care includes performing comprehensive medical
eye examinations, prescribing corrective lenses, diagnosing disease
and disorders of the eye, and using the appropriate medical and
surgical procedures necessary for their treatment. An ophthalmologist
should be consulted if a child has a family history of amblyopia or
strabismus, if the eyes
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