To evaluate the amount of excyclotorsion according to degree of inferior oblique overaction (IOOA) in patients with primary IOOA.

Fifty-nine primary IOOA patients who underwent inferior oblique muscle surgery were evaluated. Visual acuity, ocular movement test, prism cover test, Bielschowsky head tilt test, fundus photograph and photographic examination for excyclotorsion were performed. The correlation analysis was performed for the excyclotorsion according to the amount of IOOA (Control group; IOOA < 1, Group 1; 1 ≤ IOOA < 2, Group 2; 2 ≤ IOOA < 3, Group 3; 3 ≤ IOOA < 4, Group 4; IOOA = 4).

Excyclotorsion was 7.37 ± 2.36° in the Control group, 9.29 ± 3.79° in Group 1, 10.04 ± 4.39° in Group 2, 17.98 ± 4.62° in Group 3, and 24.70 ± 4.61° in Group 4. The amount of IOOA and excyclotorsion showed a positive correlation (Pearson's correlation coefficient r = 0.675). Asymmetric IOOA was observed in 35 patients (59.3%) and symmetric IOOA was observed in 24 patients (40.7%). Asymmetric IOOA combined with hypertropia was presented in 9 patients (37.5%) and symmetric IOOA combined with hypertropia was presented in 3 patients (8.6%). Therefore, patients with asymmetric IOOA have a higher incidence of vertical diplopia. The amounts of IOOA and excyclotorsion were greater in primary IOOA with esotropia than in subjects with exotropia ( p = 0.001).

The excyclotorsion was proportional to the amount of IOOA in Group 3 and Group 4. The aspect of excyclotorsion was widely distributed in Group 1 and Group 2 compared to that in the Control group.