Capturing the Moment of Fusion Loss in Intermittent Exotropia

Abstract

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.

Bilateral Occlusion Reduces the Ocular Deviation in Intermittent Exotropia

“…This point is illustrated in Figure 2 A and is supported by comparison of spontaneous exotropia with occluder-induced exotropia. 12 …”

“…This point is illustrated in Figure 2A and is supported by comparison of spontaneous exotropia with occluderinduced exotropia. 12 Occlusion of both eyes gives rise to a third sensory state. The resulting deviation is generally smaller than the exotropia that is present during binocular or monocular viewing.…”

“…In a recent study, we captured eye movement recordings of spontaneous episodes of intermittent exotropia. 12 The eye movements were indistinguishable from those induced by covering one eye. In both situations, a divergent muscle imbalance was unleashed, causing one eye to become exotropic.…”

Incomitance and Eye Dominance in Intermittent Exotropia

“…Recently, it has been shown that the deviation angle measured during a brief cover test matches the amplitude of exotropia present with both eyes open, with few exceptions. 38 Therefore, it seems doubtful that incomitance was an artifact of making measurements with one eye occluded, and indeed, the cover–uncover test is used routinely in the clinic to measure exotropia magnitude.…”

Ocular Motor Behavior Monitor in Intermittent Exotropia by Eye Tracker

“…With the assistance of an eye tracker device, the different gaze states of the intermittent exotropia patients and their conversion process can be accurately and continuously recorded, which might provide ophthalmologists with a better understanding of intermittent exotropia . 17 To achieve a more comprehensive strabismus evaluation in intermittent exotropia patients, a desktop eye movement recorder (SR Eyelink1000) equipped with a high-speed digital camera and well-designed automated system was employed to monitor ocular motor behavior and assess the exodeviation degree of intermittent exotropia patients over a continuous period of time. This study presents an approach that might facilitate exploring the neural mechanisms underlying intermittent exotropia and provides a more objective and comprehensive examination basis for the diagnosis and treatment of patients with intermittent exotropia.…”