Figure 2 shows eye movement recordings from an 11-year-old girl with intermittent exotropia who had an incomitance of 4.6°. When deviated, she could alternate fixation on visual targets, but spontaneous exotropia occurred only in the left eye. On cover testing her left exotropia was 25.5° (
Fig. 2A). Her right exotropia was larger, measuring 30.1° (
Fig. 2B). The variability in position of the deviated eye also differed, for left versus right. The position of the exotropic left eye had a standard deviation of 3.4°, whereas that of the exotropic right eye was only 1.7°.
Intermittent exotropia generally is regarded as a comitant disorder. Nonetheless, it was not uncommon to detect a difference between right and left exotropia. To address this point, strabismic deviation was plotted versus eye of fixation for all subjects (
Fig. 3). The mean exotropia was 18.2° ± 8.1°. The median exotropia was 16.0° (Q
25 – Q
75 = 13.4° – 22.4°). The scatterplot showed that the magnitude of right and left exotropia was highly correlated (
r = 0.94, Pearson correlation). However, some subjects showed a small discrepancy in exotropia, depending on the eye of fixation. Given potential tracker position error of ±1°, only an incomitance exceeding 2° was deemed meaningful. Among the 37 subjects, 16 (43%) met this threshold. For each of these 16 patients, the incomitance was significant (
P = 0.001), averaging 3.7° ± 1.5° and ranging up to 7.3°. Given that up to 2° of the difference might be explained by tracking error, the incomitance averaged at least 1.7°. There was no trend for a larger exotropia to be correlated with a greater amount of incomitance. This might be evident if our findings were an artifact, caused by inaccurate measurement of large strabismus angles.
A difference in refractive error between the eyes could explain the incomitance present in some subjects with exotropia. A switch in fixation to an eye with a more hyperopic error would result in increased accommodative demand and, hence, reduced exotropia angle. For example, the patient illustrated in
Figure 2 had a spherical equivalent of −1.00 in the right eye and −2.375 in the left eye. To focus on a target at 57 cm (−1.75 diopters) required 0.75 diopters accommodation in the right eye and none in the left eye. An incomitance of 4.6° generated by a 0.75 diopter difference in accommodative effort would correspond to an accommodative convergence/accommodation (AC/A) ratio of 6.1°/diopter (11 prism-diopters/diopter). This is 3 standard deviations greater than the average AC/A ratio, which equals 3.5 prism-diopters/diopter.
15,16 Therefore, in this patient, asymmetrical accommodative effort contributed to incomitance, but was unlikely to account for it fully.
To address this issue systematically, the accommodative effort required by each eye to focus on a target at 57 cm was calculated (
Supplementary Table). No accommodative effort was required for any eye more myopic than −1.75 diopters (1.00/0.57). A plot of difference in accommodative effort between the eyes versus amount of incomitance (
Fig. 4) showed no correlation (
r = 0.05). In fact, only 4/16 patients with detectable incomitance had an interocular difference in accommodative effort exceeding 0.25 diopters. Only 3 of these 4 patients displayed a smaller exotropia when fixating with the eye that required a greater accommodative effort.