Bell’s phenomenon in thyroid-associated inferior rectus myopathy

Purpose To examine Bell’s phenomenon in patients with unilateral thyroid-associated inferior rectus myopathy and changes in this phenomenon after inferior rectus muscle recession. Methods This prospective interventional study included 12 patients who underwent inferior rectus muscle recession with or without nasal transposition. Bell’s phenomenon was examined before and 3 months after surgery. The upper eyelid was held open by a finger to prevent complete eyelid closure. Then, the distance of upward excursion of the inferior corneal limbus or the corneal light reflex was measured during voluntary maximum forced eyelid closure. The pre- and postoperative distances of upward excursion on the affected side were statistically compared with the preoperative distance on the unaffected side using the Mann-Whitney U test, and the pre- and postoperative distances on the affected side were statistically compared using paired t -test. The relationships among postoperative changes of Bell’s phenomenon, patient age, the amount of recession and nasal transposition of the inferior rectus muscle, postoperative angle of ocular deviation, and reduction in the angle after surgery were analyzed using stepwise multiple regression analyses. Results The preoperative measurement of Bell’s phenomenon was significantly shorter on the affected side (1.6 ± 1.6 mm) than the unaffected side (4.3 ± 1.6 mm; P = 0.001). However, the distance on the affected side significantly increased after surgery (4.1 ± 1.9 mm; P < 0.001), compared to the preoperative distance on the unaffected side ( P = 0.843). Using a stepwise method, all variables were deleted from the regression equation. Conclusions Bell’s phenomenon decreased on the affected side, which improved after inferior rectus muscle recession.