AIM:To observe the effect of inferior oblique belly transposition(IOBT)in unilateral mild inferior oblique overaction with small angle vertical stabismus.

METHODS: The data of patients who underwent IOBT in our hospital from September 2019 to August 2021 were analyzed retrospectively. Inclusion criteria targeted patients with mild inferior oblique overaction(2+ and below)and small angle incomitant vertical strabismus(4-9PD). The horizontal deviation and vertical deviation in both primary and lateral gazes were measured, and the degree of inferior oblique overaction and fovea-disc angle(FDA)were also evaluated preoperatively and postoperatively.

RESULTS: A total of 16 cases(16 eyes)were included, aged 4-39 years. One case was 5a postoperative congenital esotropia with secondary unilateral inferior oblique overaction by mild superior oblique palsy, whereas 15 patients had monocular primary inferior oblique overaction with horizontal strabismus. The follow-up was 3-6mo. The mean improvement of inferior oblique overaction was 2.00(1.25, 2.00)grade from +2.00(2.00, 2.00)preoperatively to 0.00(0.00, 0.00)postoperatively, the difference was statistically significant(Z=-3.70, P<0.001). The horizontal strabismus decreased from 69.13±25.86PD preoperatively to 2.75±2.59PD postoperatively(t= 9.929, P<0.001). The vertical strabismus in the primary position decreased from preoperative 7.44±1.32PD to 1.00±1.21PD postoperatively(t=22.335, P<0.001), mean corrected hypertropia 6.44±1.15PD, and vertical strabismus in lateral gazes decreased from preoperative 12.44±2.73PD to 3.00±2.13PD postoperatively, mean corrected hypertropia 9.44±2.73PD, these differences were statistically significant(t=13.819, P<0.001). The FDA decreased from -8.85°±6.53° preoperatively to -6.49°±7.01° postoperatively, the difference was statistically significant(t=-2.384, P<0.001), with a mean reduction of 2.36°. No postoperative complications such as postoperative overcorrection or inferior oblique underaction were observed.

CONCLUSION:IOBT is safe and effective in correcting unilateral mild inferior oblique overaction with small angle vertical strabismus.