Radiographic analysis of extraocular muscle volumetric changes in thyroid-related orbitopathy following orbital decompression

Purpose: To evaluate radiographic volume changes in extraocular muscles (EOM) following orbital decompression for thyroid-related orbitopathy (TRO). Methods: Medical records of 22 orbits in 12 patients undergoing postoperative orbital CT after orbital decompression for TRO were retrospectively reviewed. All orbits demonstrated no signs of clinical reactivation of TRO. EOM volumes were determined by the summation of each EOM's cross-sectional area in the coronal plane of the CT scans and multiplying the sum by the slice thickness. Main outcome measure was a comparison of EOM volumes preoperatively and postoperatively. Results: All orbits demonstrated proptosis reduction postoperatively with a mean of 4.8 mm ± 2.1 mm (p < 0.0001). The significant increase in the medial rectus muscle (p = 0.0010) postoperatively accounted for the primary change in the postoperative enlargement of total EOM volume (p = 0.028). The medial rectus muscle increased 27% from the preoperative volume. The lateral rectus, superior rectus/levator complex, inferior rectus, and superior oblique muscles did not demonstrate a significant volume increase postoperatively (p = 0.23, 0.21, 0.17, 0.40, respectively). Two patients who underwent unilateral orbital decompression demonstrated EOM enlargement postoperatively in the operative orbit only. Conclusions: There is a significant increase in medial rectus muscle volume postoperatively in orbits undergoing orbital decompression for TRO despite lacking clinical evidence of disease reactivation.