Intermittent exotropia is a benign condition wherein either eye spontaneously drifts outward in the absence of diplopia.
1 Affected patients have a heightened sensitivity to light, and the condition may be first identified by a child's tendency to keep one eye closed in bright sunlight.
2 Once attributed it to abnormal divergence innervation,
3 it is now postulated that the two eyes are anatomically divergent, requiring excessive convergence to maintain binocular alignment.
4 It is assumed that strabismus surgery resets the baseline anatomical position eliminating the need for excessive convergence.
After a century of dedicated study, the protean clinical findings of intermittent exotropia remain difficult to reconcile. When innervation is suspended by nondepolarizing paralyzing anesthesia, two eyes show a divergent deviation even when forced duction testing is negative,
5 suggesting possible anatomical laxity of the medial rectus muscles. Yet intermittent exotropia is controlled optimally during near fixation. Primary extraocular muscle laxity also fails to explain why intermittent exotropia can be relentlessly progressive, and why it so often recurs following restoration of binocular alignment with strabismus surgery.
Despite this symmetrical exodeviation under general anesthesia, both eyes never drift out simultaneously when the patient is awake. Fixation seems to be required to induce the exodeviation, a finding that would not be expected if the eyes were simply reverting to a divergent anatomical position. If fixation is required, why does intermittent exotropia become worse with fatigue (when fixation is more likely to be suspended). Why does the frequency bear no relationship to the angle of exodeviation? At the sensorimotor level, the exodeviating eye shows facultative suppression of the temporal retina, so why do affected patients habitually close the one eye in bright sunlight, and how do the eyes realign instantaneously with a blink?
To these discordant clinical findings,
Adams et al.6 add their video-oculographic observation that intermittent exotropia may vary in size depending upon which eye is fixating. We have incidentally noted this binocular difference in our tracings as well, suggesting that it is unlikely to represent a measurement artifact. Whether this difference reflects an underlying innervational or muscle-tension asymmetry remains unclear. The riddle of intermittent exotropia will ultimately be solved at the molecular level, where it may come to be reclassified, along with other paroxysmal ocular motor disorders, as a channelopathy.
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