The most common form of strabismus, intermittent exotropia, is thought to become manifest when the drive to fuse is overcome by excessive divergent muscle tone. This principle is tested by examining the alignment of the eyes in the absence of vision. We compare the ocular deviation in patients with intermittent exotropia under conditions of monocular versus binocular occlusion.
This prospective study of a patient cohort referred to our laboratory enrolled 18 patients with typical findings of well-controlled intermittent exotropia. Eye positions were recorded with video eye trackers while patients looked at a fixation spot at a distance of 57 cm. One eye was occluded, and the resulting ocular deviation was measured. Both eyes were then occluded, and the ocular deviation was re-measured.
The majority of patients (11/18) had a smaller deviation when both eyes were covered. Occlusion of one eye resulted in a mean exotropia of 13.5° ± 4.7°. Occlusion of both eyes reduced the mean exotropia to 6.0° ± 6.5° (paired t-test, P < 0.001), corresponding to a 56% reduction in the ocular deviation. This reduction persisted during prolonged bilateral occlusion but reversed as soon as vision was restored.
Bilateral occlusion reveals a fixation-free state of alignment that is different from orthotropia and usually less than the exotropia that occurs spontaneously during binocular viewing. This finding demonstrates that the deviation angle in patients with intermittent exotropia is actively mediated by visual feedback, which the fixating eye is capable of providing alone.