الفهرس 
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Abstract
E
xodeviation is a manifest outward deviation of the visual axes relative to each other, according to fusion control, exodeviation could be XT; being present all the time or X(T),where large exophoria intermittently breaks down to XT. Mechanisms responsible for these variations include the degree of fusional control with varying levels of alertness, the convergence-accommodation relationship and the change of the angle of deviation at different fixation distances.
Exodeviation which is considered normal has a small angle of deviation, usually less than 10 PD, Most normal adults have a small exodeviation when fully dissociated. Exodeviations are controlled with our innate strong fusional convergence. Intermittent exotropia expresses early in childhood and rarely associated with neurological anomalies.
The amblyopia is infrequent unless the disease progresses to constant XT. Intermittent exotropia is usually first observed by the parents in early childhood or late infancy as an infrequent drifting or squinting of one eye. Patients with X(T) tend to manifest their deviation when they are tired, have a cold or flu, which express as asthenopia, visual fatigue, and diplopia in older children and adults. many patients with X(T) have photophobia (squinting to bright light) which originally thought to be a way for eliminating diplopia or confusion.
As a rule, during the phoric phase of X(T), the eyes are perfectly aligned and the patient has bifoveal fusion with excellent stereoacuity. This excellent bifoveal fusion develops because the eyes are well aligned in early infancy when the critical binocular cortical connections are being established. The poor fusion in these cases is associated with a predominance of the tropic phase.
Intermittent exotropia can be further classified into four types based on the difference between the distance and near deviation: (1) basic, where the distance and near deviations are similar, within 10 PD and the target angle is the distance deviation. (2) divergence excess where the exotropia is larger for distance fixation than near fixation (≥10 PD). True divergence excess is when the divergence excess persists even after prolonged binocular dissociation by monocular patching (i.e., Patch test). (3) Pseudo-divergence excess is when the near deviation increases after the patch test, so the distance and near deviations are similar.(4) convergence insufficiency when the near deviation is 10 PD or larger than the distance deviation.
In our study about the prevalence of different types of X(T) among exotropic children, by cross sectional study of 139 exotropic children (6_12 years old) attending to the ophthalmology outpatient clinic of Ain Shams University. We found that X(T) is the most type of exodeviation by (86.3%).
According to types of X(T), the ‘‘basic type’’ was the most common type of X(T) by (84.2%), the true divergence excess was the second most common type by (7.5%),pseudo divergence excess was (5.8%) and the convergence insufficiency was the least common type by (2.5%).