To assess the accuracy of estimating postoperative deviation in large-angle exotropia over 40 prism diopters (PD).

A retrospective analysis was performed on 61 exotropia patients with over 40 PD exotropia who had undergone 2-muscle surgery by two surgeons and with at least 6 months follow-up. The accuracy was assessed by analyzing the discrepancy between preoperatively predicted residual deviation and deviation at postoperative 6weeks. Successful surgery was defined as deviation within ± 10 PD at the last postoperative visit.

More accurate residual deviation was predicted in exotropia with prism diopters between 40 and 59 (97.8%) than in exotropia with 60 PD or more (62.5%). And there was no significant difference between two surgeons. Surgical success rates at six months and one year after surgery were 96.8%, 94.7% in exotropia with 40 to 49 PD, 71.4%, 60.0%, 50 to 59 PD, 50.0%, 45.5%, 60 PD or more, respectively. There were four patients of whom the postoperative deviation angle exceeded more than 10 PD of the estimated deviation.

The accuracy of estimating residual deviation prior to surgery was higher and more successful surgery was achieved in exotropia ranging in 40 to 59 PD than in exotropia with 60 PD or more. Therefore, 2- muscle surgery will be suitable for large-angle exotropia with less than 60 PD.