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.

Figure 1

Cover-induced exotropia is equal to spontaneous exotropia. A, Recordings from a 26-year-old woman (refraction: plano both eyes [OU]) during fixation on a 0.5-degree target at 57 cm. The mean position (solid line) and standard deviation (shading) are shown for the right eye (red) and left eye (blue). At t ¼ 0 seconds, a shutter occluded either the right eye or the left eye, inducing an exodeviation. n = number of events. Positive and negative values for horizontal deviation denote right and left gaze, respectively. B, Spontaneous exotropia occurring intermittently between cover-induced episodes has an amplitude nearly equal to shutter-induced exotropia, with a difference of 0.1 degree for right exotropia and 0.4 degree for left exotropia. The shapes of the mean position traces and the variability in individual position traces are also similar. C: Recordings from a 36-year-old woman (refraction: −1.00 OU), showing a larger exotropia. D: Interspersed episodes of spontaneous fusion loss are similar to those from shutter occlusion, with an amplitude difference of 0.7 degrees for right exotropia and 1.4 degrees for left exotropia.

Figure 2

Cover-induced exotropia different from spontaneous exotropia. Recordings from a 35-year-old man (refraction: −1.25 +0.75 right eye; −1.25 left eye), showing that A, right exotropia induced by occlusion was smaller and more variable than B, spontaneous right exotropia. C, Covering the dominant left eye produced a highly variable outward eye movement; spontaneous left exotropia never occurred (Supplementary Video 1, available at www.aaojournal.org).

Figure 3

Deviation amplitude is correlated with peak velocity of the deviating eye’s movement during onset of spontaneous exotropia.

Figure 4

Peak velocity of the deviating eye’s movement is similar for spontaneous exotropia compared with shutter-induced exotropia.

Figure 5

Four strategies for ocular realignment. A, A 35-year-old man with left eye dominance. Strategy 1: After the nondominant right eye was uncovered at t = 0 seconds,it made a vergence-like adducting movement. Strategy 2: After the dominant left eye was uncovered, an alternating saccade was made, followed at variable intervals by a vergence-like movement of the right eye. Blinks have been removed (Supplementary Video 1, available at www.aaojournal.org). B, A 31-year-old man with left eye dominance. After the right eye was uncovered, strategy 1 was employed. Convergence (small arrow) was faster than divergence (large arrow), which occurred with subsequent episodes of fusion loss. Strategy 3: After the left eye was uncovered, a disconjugate saccade (much larger in the adducting, left eye) was made, followed by vergence movements (arrows). The vergence movement was larger in the nondominant right eye. C, An 11-year-old girl with right eye dominance. Strategy 4: After the dominant eye was uncovered, an alternating saccade was made to bring it onto the target (first stage of strategy 2). This was followed by a disconjugate saccade and asymmetrical vergence movement (strategy 3). After the left eye was uncovered, this child used strategy 3 to recover fusion.

Figure 6

Exotropia at near and distance. The eye trackers were calibrated at distance; 0 degrees refers to fixation at 305 cm. A, Task 1: Spontaneous episodes of right exotropia (amplitude 15.0 degrees, peak velocity 20.1 degrees/second) in a 46-year-old man during fixation at 305 cm. B, Task 2: Exotropia (amplitude 14.2 degrees, peak velocity 24.5 degrees/second) after shutter occlusion of the right eye, while fixating on a crosshair at ~21 cm from the left eye along its line of sight with a distant laser spot. The intent was to mimic the eye movement in (A), by moving the right eye from −15.0 degrees to 0 degrees. The crosshair may have been placed a few centimeters closer than intended, resulting in a movement of the right eye from −18.1 degrees to −3.9 degrees. C, Task 3: Gaze shift along left eye’s line of sight from a crosshair nominally at 21 cm to a laser spot at 305 cm resulted in an outward movement of the right eye (amplitude 18.1 degrees, peak velocity 142.2 degrees/second) that combined divergence with a disconjugate saccade. Replication of the right eye’s movement in (B) was intended, but was foiled by the occurrence of a saccade. D, Task 4: Same task as (C), except that when the distant laser spot appeared, a shutter simultaneously covered the right eye. The right eye moved outward (amplitude 33.0 degrees, peak velocity 43.4 degrees/second) from −17.4 degrees to 15.3 degrees, to assume the same final position as in (A).