[MRI study of rectus extraocular muscles in concomitant exotropia with small angle vertical deviation]

Objective: To observe the changes of four pulley locations of extraocular rectus muscles and rectus muscle volumes, as well as superior and inferior part ratio of horizontal rectus muscles, in concomitant exotropia with small-angle vertical deviation in primary gaze. Methods: Cross-sectional study. Data was collected from January 2018 to December 2019 in Tianjin Eye Hospital. The pulley position changes and volumes of four extraocular rectus muscles in the coronal position of patients who had concomitant exotropia without vertical deviation (group A) or with small-angle vertical deviation in primary gaze (<5 prism diopter; group B) and normal controls (group C) were observed using MRI, and the changes of the volume ratio of the superior and inferior parts of the horizontal rectus muscle were calculated. One way analysis of variance and Kruskal-Wallis test were used for statistical analysis. Results: There were 19 patients (38 eyes; 10 males, 9 females) aged (30±7) years in group A, ten patients (20 eyes; 4 males, 6 females) aged (27±6) years in group B, and 20 healthy volunteers (40 eyes) in group C. Age and gender distribution were matched among the three groups (all P>0.05). Among the three groups, the pulley locations of the four extraocular rectus muscles were not significantly different (all P>0.05). The medial rectus muscle volume in groups A [(358.6±44.9) mm³; t=6.405, P<0.01] and B [(334.7±35.6) mm³; t=6.025, P<0.01] was significantly smaller than group C [(437.5±49.3) mm³]. There was no statistically significant difference in the superior and inferior muscle volume ratio of the lateral rectus in three groups (all P>0.05), while the ratio in group B was more dispersive. Conclusions: The pulley location changes of four extraocular rectus muscles of patients with concomitant exotropia and small-angle vertical deviation in primary gaze are not significant, but the medial rectus volume in patients who had concomitant exotropia with vertical deviation or not is significantly smaller. The dispersive superior and inferior muscle volume ratio of the horizontal rectus muscles may be related to the small angle of vertical deviation in clinical examinations. (Chin J Ophthalmol, 2021, 57: 223-227).