Subjects underwent MRI in central gaze and maximum attainable abduction and adduction. Adduction angles achieved in ET and HT were 30.6° ± 0.9° and 27.2° ± 2.3° (mean ± standard error of mean, SEM), not significantly different from normal (28.3° ± 0.7°, P = 0.11 and P = 0.72, respectively). Abduction angles achieved in XT and HT were 24.2° ± 1.1° and 21.4° ± 1.8°, also similar to normal at 22.9° ± 0.6° (P = 0.34, P = 0.36, respectively). However, adduction angle in XT and abduction angle in ET were 20.2° ± 0.7° and 19.0° ± 2.5°, both significantly subnormal (P = 0.005, for both). In all strabismic groups, small vertical ductions (range, −1.1° to 0.3°) were associated with horizontal duction, but in each group, this was similar to normal subjects (P > 0.3).
Sinuosity of the ON (
Fig. 3) in each strabismic group was similar to normal in central gaze (
P > 0.05, for all). In abduction, ON sinuosity was not significantly different from that in central gaze in any group (
P > 0.1). In adduction, the ON became significantly straighter than in central gaze in control, ET, and HT subjects (
P < 0.03, for all), but not in XT (
P = 0.99). In the group with XT, ON sinuosity in both central gaze and adduction was 103.6%, implying the absence of ON sheath tethering in adduction.
Since 5 of 11 subjects with XT had restrictive strabismus, a subgroup analysis of ON sinuosity compared restrictive versus nonrestrictive cases. The mean adduction angle of subjects with restrictive XT was insignificantly smaller than that of subjects with nonrestrictive XT (15.1° vs. 20.9°, respectively, P = 0.275). The ON was significantly more sinuous in all gaze positions in the group with restrictive XT (P < 0.01), but the ON did not straighten in adduction in either group.