Chapter 12: Strabismus
EXOTROPIA (Divergent Strabismus)
Exotropia is less common than esotropia, particularly in infancy and childhood. Its incidence increases gradually with age. Not infrequently, a tendency to divergent strabismus beginning as exophoria progresses to intermittent exotropia and finally to constant exotropia if no treatment is given. Other cases begin as constant or intermittent exotropia and remain stationary. As in esotropia, there may be a hereditary element in some cases. Exophoria and exotropia (considered as a single entity of divergent deviation) are frequently passed on as autosomal dominant traits, so that one or both parents of an exotropic child may demonstrate exotropia or a high degree of exophoria.
Alternative Classification of Exotropia
Constant or intermittent exotropia can also be classified on a descriptive basis as being an excess of divergence or an insufficiency of convergence. These descriptive terms do not imply that the cause of the deviation is understood.
A. Basic Exotropia:
Distance and near deviations are approximately equal.
B. Divergence Excess:
Distance deviation is significantly larger than near deviation.
C. Convergence Insufficiency:
Near deviation is significantly larger than distance deviation.
D. Pseudodivergence Excess:
Distance deviation is significantly larger than near deviation: however, use of a +3 diopter lens for near measurement will cause the near deviation to become approximately equal to the distance deviation.
Intermittent exotropia accounts for well over half of all cases of exotropia. The onset of the deviation may be in the first year, and practically all have presented by age 5. The history often reveals that the condition has become progressively worse. A characteristic sign is closing one eye in bright light (Figure 12-10). The manifest exotropia first becomes noticeable with distance fixation. The patient usually fuses at near, overcoming moderate to large angle exophoria. Convergence is frequently excellent. There is no correlation with a specific refractive error.
Since a child fuses at least part of the time, there is usually no gross sensory abnormality. For distance, with one eye deviated, there is suppression of that eye and normal retinal correspondence with little or no amblyopia.
A. Medical Treatment:
Nonsurgical treatment is largely confined to refractive correction and amblyopia therapy. If the AC/A ratio is high, the use of minus lenses may delay surgery for a while. Occasionally, antisuppression or convergence exercises may be of temporary benefit
B. Surgical Treatment:
Most patients with intermittent exotropia require surgery when their fusional control deteriorates. Deterioration of control is documented over time by an increasing percentage of time the manifest exotropia is observed, an enlarging angle of deviation, decreasing control for near fixation, and worsening in the patient's measured distance and near stereoscopic abilities. Surgery may also alleviate diplopia or other asthenopic symptoms.
The choice of procedure depends on the measurements of the deviation. Bilateral lateral rectus muscle recession is preferred when the deviation is greater at distance. If there is more deviation at near, it is best to undertake resection of a medial rectus muscle and recession of the ipsilateral lateral rectus muscle. Surgery on one or even two additional horizontal muscles may be necessary for very large deviations (> 50Δ). It is desirable to obtain slight overcorrection in the immediate postoperative period for best long-term results.
CONSTANT EXOTROPIA (Figure 12-11)
Constant exotropia is less common than intermittent exotropia. It may be present at birth or may occur when intermittent exotropia progresses to constant exotropia. Because infantile exotropia is commonly seen in children with underlying neurologic impairment, pediatric neurologic consultation is indicated in all such cases. Some cases have their onset later in life, particularly following loss of vision in one eye. Except for cases due to loss of vision, the underlying cause is usually not known.
Constant exotropia may be of any degree. With chronicity or poor vision in one eye, the deviation can become quite large. Adduction may be limited, and hypertropia also may be present. There is suppression if the deviation was acquired by age 6-8; otherwise, diplopia may be present. If exotropia is due to very poor vision in one eye, there may be no diplopia. Amblyopia is uncommon in the absence of anisometropia, and spontaneous alternation of the fixating eye is frequently observed.
Surgery is nearly always indicated. The choice and amount are as described for intermittent exotropia. Slight overcorrection in an adult may result in diplopia. Most patients adjust to this, especially if they have been forewarned of the possibility. If one eye has reduced vision, the prognosis for maintenance of a stable position is less favorable, with the strong possibility that the deviating eye will gradually become more exotropic. Botulinum toxin type A injections can be useful as primary treatment in small deviations or as supplementary treatment in significant surgical overcorrections or undercorrections.
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