C h a p t e r 3 8
Disorders of Special Sensory Function: Vision, Hearing, and Vestibular Function
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overactivity or underactivity of the extraocular muscles
in some fields of gaze.
The disorder may be nonaccommodative,
accommodative, or a combination of the two.
Infantile esotropia is the most common cause of
nonaccommodative strabismus. It occurs in the first 6
months of life, with large-angle deviations, in otherwise
developmentally and neurologically normal infants.
Eye movements are full, and the child often uses each
eye independently to alter fixation (cross-fixation).
The cause of the disorder is unclear. Research suggests
that idiopathic strabismus may have a genetic basis,
since siblings often present with similar disorders.
Accommodative strabismus is caused by disorders such
as uncorrected hyperopia of a significant degree, in
which the esotropia occurs with accommodation that
is undertaken to focus clearly. Onset of this type of
esotropia characteristically occurs between 18 months
and 4 years of age because accommodation is not well
developed until that time. The disorder most often is
monocular but may be alternating.
Paralytic Strabismus.
Paralytic strabismus results from
paresis (i.e., weakness) or plegia (i.e., paralysis) of one
or more of the extraocular muscles. When the normal
eye fixates, the affected eye is in the position of primary
deviation. In the case of esotropia, there is weakness of one
of the lateral rectus muscles. When the affected eye fixates,
the unaffected eye is in a position of secondary deviation.
39
Paralytic strabismus is uncommon in children but
accounts for nearly all cases of adult strabismus; it can
be caused by infiltrative processes (e.g., Graves disease;
see Chapter 32), myasthenia gravis, stroke, and direct
optical trauma.
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In infants, paralytic strabismus can
be caused by birth injuries affecting the extraocular
muscles or the cranial nerves supplying these muscles.
In general, binocular vision can be maintained when
paralytic strabismus is corrected.
Treatment.
Treatment of strabismus is directed toward
the development of normal visual acuity, correction of the
deviation, and superimposition of the retinal images to
provide binocular vision. Early and adequate treatment
is crucial because a delay in or lack of treatment can lead
to amblyopia and permanent loss of vision.
Treatment includes both surgical and nonsurgical
methods. Infantile esotropia is usually treated surgically
by weakening the medial rectus muscle on each eye while
the infant is under general anesthesia. Recurrences are
common with infantile esotropia, and multiple surgeries
are often required.
Nonsurgical treatment includes glasses, occlusive
patching, and eye exercises (i.e., pleoptics). Glasses are
often used in the treatment of accommodative esotropia
that occurs with hypermetropia. Because accommodation
is linkedwith convergence, focusingdrives the eyes inward,
producing esotropia. In infants and toddlers, intermittent
exotropia is commonly treated with patching for 1 to 2
hours daily for several months. The use of over-minus
glasses stimulates accommodative convergences, which
contracts the exotropic drift. Vision therapy involves
exercises to stimulate convergence and techniques to train
the visual system to recognize the suppressed images.
Surgical treatment of intermittent exotropia is indicted
when conservative methods fail to correct the deviation.
Early treatment of children with intermittent exotropia is
not as crucial as it is for those with constant deviations
because stereopsis can still develop.
A relatively new form of treatment involves the
injection of botulinum toxin type A (Botox) into
the extraocular muscle to produce paralysis of that
extraocular muscle.
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Paralysis of the muscle shifts the
eye into the field of action of the antagonist muscle.
During the time the eye is deviated, the paralyzed
muscle is stretched, whereas the antagonistic muscle is
contracted. Usually two or more injections of the drug
are necessary to obtain a lasting effect.
Amblyopia
Amblyopia, sometimes called
lazy eye,
describes a decrease
in visual acuity resulting fromabnormal visual development
in infancy or early childhood.
42–44
It is the most common
cause of monocular visual impairment, affecting 1% to
4% of the population. With early detection and treatment,
most cases of amblyopia are reversible and the most severe
forms of the condition can be prevented.
Normal development of the thalamic and cortical
circuitry necessary for binocular visual perception
requires simultaneous binocular use of each fovea during
a critical period early in life (0 to 5 years). Amblyopia
can result from visual deprivation (e.g., cataracts, ptosis)
or abnormal binocular interactions (e.g., strabismus,
anisometropia) during visual immaturity. In infants with
unilateral cataracts that are dense, central, and larger
than 2 mm in diameter, this period is before 2 months
of age.
15,21
In conditions causing abnormal binocular
interactions, one image is suppressed to provide clearer
vision. In esotropia, vision of the deviated eye is
suppressed to prevent diplopia. A similar situation exists
in anisometropia, in which the refractive indexes of the
two eyes are different. Although the eyes are correctly
aligned, they are unable to focus together, and the image
of one eye is suppressed.
The reversibility of amblyopia depends on the matu-
rity of the visual system at the time of onset and the
duration of the abnormal experience. Occasionally in
strabismus, some persons alternate eye fixation and do
not experience significant amblyopia or diplopia. With
late adolescent or adult onset, this habit pattern must
be unlearned after correction. Amblyopia is remark-
ably responsive to treatment if the treatment is initiated
early in life; thus, all infants and young children should
be evaluated for visual conditions that could lead to
amblyopia.
The American Academy of Pediatrics in association
with the American Association of Certified Orthoptists,
American Association of Pediatric Ophthalmology and
Strabismus, and American Academy of Ophthalmology
recommends that all newborn infants be examined in
the nursery for structural abnormalities and have a red
