Variability of Ocular Deviation in Strabismus

Abstract

Importance: In strabismus, the fixating eye conveys the direction of gaze while the fellow eye points at a peripheral location in space. The stability of the eyes may be reduced by the absence of a common target.

Objective: To quantify the stability of eye position in strabismus and to measure variability in the ocular deviation.

Design, setting, and participants: From 2010 to 2014, a prospective comparative case study of 25 patients with alternating exotropia with normal visual acuity in each eye and 25 control individuals was conducted in a laboratory at a tertiary eye center. A video eye tracker was used to measure the position of each eye while participants alternated fixation on the center of a cross under dichoptic conditions or scanned pictures of natural scenes.

Main outcomes and measures: Spatial and temporal variability in the position of the fixating eye and the nonfixating eye in patients with strabismus and control individuals, quantified by the log area of ellipses containing 95% of eye positions or mean SDs of eye position.

Results: In the 25 patients with strabismus, the mean (SD) age was 28 (14) years (range, 8-55 years) and the mean (SD) ocular deviation was 14.2° (5.9°) (range, 4.4°-22.4°). In the patients with strabismus, the mean position variability (1.80 log units; 95% CI, 1.66-1.93) for the deviating eye was greater than for the fixating eye (1.26 log units; 95% CI, 1.17-1.35) (P < .001). The fixating eye of patients with strabismus was more variable in position than the fixating eye of individuals without strabismus (0.98 log units; 95% CI, 0.88-1.08) (P < .005).

Conclusions and relevance: In patients with strabismus, even without amblyopia, the deviated eye is more variable in position than the fixating eye. Both eyes are less stable in position than the eyes of control individuals, which indicates that strabismus impairs the ability to fixate targets steadily. Saccades contribute to variability of the deviation angle because they are less conjugate in patients with strabismus.

Figure 1. Variability of Eye Position

A, Participant with normal alignment showing positions of the left eye (blue dots) and the right eye (red dots) while fixating on a cross. Points for the deviating eye (the eye to which the fixation cross is invisible) are plotted on top of those for the fixating eye. B, Patient with esotropia showing positions of the deviating eye and fixating eye; ellipses contain 95% of points. C, Patient with exotropia. In both patients with strabismus, there is increased scatter in the position of the deviating eye compared with the fixating eye. The numbers equal the log of the ellipse area. For illustration, only 1/50 position points is plotted.

Figure 2. Comparison of Eye Position Variability

Ellipse areas for patients with strabismus show greater variability in the position of the nonfixating eye compared with the fixating eye. For control participants, areas for the nonfixating eye and the fixating eye are similar and less than the corresponding values for the patients with strabismus.

Figure 3. Horizontal and Vertical Eye Position Variability

A, Mean SD for the vertical vs horizontal components of eye position for the nonfixating eye of each participant. Patients with strabismus show more variability in eye position, especially in the horizontal domain. B, Fixating eye shows more position variability in patients with strabismus compared with control individuals. The black cross represents tracker noise (0.24°) determined using an artificial eye.

Figure 4. Temporal Changes in Variability of the Ocular Deviation

Difference in log ellipse area for the nonfixating eye minus the fixating eye analyzed for different time windows reveals how the variability in ocular deviation varies over time. For control participants, this variability is negligible for all time windows. For patients with strabismus, the variability rises over a time scale of 1 to 100 seconds. The error bars indicate SDs.

Figure 5. Variability of Eye Alignment While Viewing a Natural Scene

A, Fixations of a patient with exotropia recorded while scanning a picture of a deer; each white line pairs left eye position (blue dot) and right eye position (red dot) during a single fixation. B, Horizontal left eye (blue) and right eye (red) positions and horizontal deviation angle (right eye – left eye) recorded while the patient viewed the picture in panel A. C, Ellipses containing 95% of position points for each eye during the 25 individual left eye fixations. The ellipses for the left eye have been centered at the origin, and each corresponding ellipse for the right eye shifted by the same amount. The large red dashed ellipse encompasses 95% of right eye positions during the 10-second viewing period. D, Ellipses containing 95% of positions recorded while the patient performed the task of alternating fixation on a central cross. Only left eye fixations are shown and, for clarity, only 1/50 points is plotted.