Capturing the Moment of Fusion Loss in Intermittent Exotropia.

Purpose: To characterize eye movements made by patients with intermittent exotropia when fusion loss occurs spontaneously and to compare them with those induced by covering 1 eye and with strategies used to recover fusion.

Design: Prospective study of a patient cohort referred to our laboratory.

Participants: Thirteen patients with typical findings of intermittent exotropia who experienced frequent spontaneous loss of fusion.

Methods: The position of each eye was recorded with a video eye tracker under infrared illumination while fixating on a small central near target.

Main Outcome Measures: Eye position and peak velocity measured during spontaneous loss of fusion, shutter-induced loss of fusion, and recovery of fusion.

Results: In 10 of 13 subjects, the eye movement made after spontaneous loss of fusion was indistinguishable from that induced by covering 1 eye. It reached 90% of full amplitude in a mean of 1.75 seconds. Peak velocity of the deviating eye's movement was highly correlated for spontaneous and shutter-induced events. Peak velocity was also proportional to exotropia amplitude. Recovery of fusion was more rapid than loss of fusion, and often was accompanied by interjection of a disconjugate saccade.

Conclusions: Loss of fusion in intermittent exotropia is not influenced by visual feedback. Excessive divergence tone may be responsible, but breakdown of alignment occurs via a unique, pathological type of eye movement that differs from a normal, physiological divergence eye movement.