Correction of Excyclotropia by Surgery on the Inferior Rectus Muscle in Patients with Thyroid Eye Disease: A Retrospective, Observational Study

PURPOSE To examine the characteristics of excyclotropia correction through surgery on the inferior rectus muscle in patients with thyroid eye disease. METHODS This was a retrospective, observational study at a single institution. We reviewed 36 patients who had undergone unilateral inferior rectus muscle recession, with or without nasal inferior rectus muscle transposition. The following factors were investigated as possibly influencing excyclotropia correction: inferior rectus muscle thickness, degree of adipose change in the inferior rectus muscle, smoking status, history of orbital radiotherapy, and the amount of inferior rectus muscle recession. Using T1-weighted coronal magnetic resonance imaging, we measured the cross-sectional area of the inferior rectus muscle at its largest point, as well as the bright-signal area of the inferior rectus muscle, which reflects intermuscular adipose change. We then calculated the percentage internal bright-signal area at the point of the largest inferior rectus muscle cross-sectional area. The history of orbital radiotherapy was graded using a binary system. We evaluated correlations among excyclotropia correction, the amount of nasal inferior rectus muscle transposition, and the possible influencing factors listed, using stepwise multiple regression analyses. RESULTS The multiple regression model demonstrated a significant relationship among excyclotropia correction, amount of nasal inferior rectus muscle transposition, and the amount of inferior rectus muscle recession (YCORRECTION = 8.546XTENDON WIDTH + 0.405XRECESSION- 0.908; r = 0.844; adjusted r2 = 0.695; P < 0.001). CONCLUSIONS Excyclotropia correction was correlated with the amount of nasal inferior rectus muscle transposition and the amount of inferior rectus muscle recession, but not with the other factors. The regression model presented in this study will enable us to determine more precisely the amount of nasal inferior rectus muscle transposition in patients with excyclotropia of various angles.