August 2017
Volume 58, Issue 10
Open Access
Research Highlight  |   August 2017
The Riddle of Intermittent Exotropia
Author Affiliations
  • Michael C. Brodsky
    Department of Ophthalmology, Mayo Clinic, Rochester, Minnesota, United States; brodsky.michael@mayo.edu.
Investigative Ophthalmology & Visual Science August 2017, Vol.58, 4056. doi:10.1167/iovs.17-22644
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      Michael C. Brodsky; The Riddle of Intermittent Exotropia. Invest. Ophthalmol. Vis. Sci. 2017;58(10):4056. doi: 10.1167/iovs.17-22644.

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      © ARVO (1962-2015); The Authors (2016-present)

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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.7 
Acknowledgments
Supported by a grant from The Knights Templar Eye Foundation and by a departmental grant from Research to Prevent Blindness, Inc. 
References
Posner A. Divergence excess considered as an anomaly of the postural tonus of the muscular apparatus. Am J Ophthalmol. 1944; 27: 1136–1142.
Wiggins RE, von Noorden GK. Monocular eye closure in bright sunlight. J Pediatr Ophthalmol Strabismus. 1990; 27: 16–20.
Knapp P. Divergent deviations. In: Allen JH, ed. Strabismus Ophthalmic Symposium II. St. Louis, MO: CV Mosby; 1958: 354.
Kushner BJ, Morton GV. Distance/near differences in intermittent exotropia. Arch Ophthalmol. 1998; 116: 478–486.
Jampolsky A. Treatment of exodeviations. In: Pediatric Ophthalmology and Strabismus: Transactions of the New Orleans Academy of Ophthalmology. New York, NY: Raven Press; 1986: 201–234.
Adams DL, Economides JR, Horton JC. Incomitance and eye dominance in intermittent exotropia. Invest Ophthalmol Vis Sci. 2017;58:4049–4055.
Brodsky MC, Marshall M. Parks memorial lecture: ocular motor misbehavior in children: where neuro-ophthalmology meets strabismus. Ophthalmology. 2017; 124: 835–842.
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