To evaluate the radiographic volume change in extraocular muscles (EOM) following orbital wall decompression for Nunery type 1 and type 2 thyroid-associated orbitopathy (TAO).

Medical records of 31 orbits in 20 patients undergoing postoperative orbital CT after orbital decompression for TAO were retrospectively reviewed. The patients were divided according to Nunery classifications. A type 1 classification was assigned to patients who had no diplopia and essentially normal versions. A type 2 classification was assigned to patients with restrictive motility loss and diplopia within 20 degrees of the primary position. EOM volumes were determined by the summation of each EOM's cross-sectional areas in the coronal plane of the CT scans and multiplying the sum by the slice thickness. Main outcome measure was a comparison of EOM volume changes between types 1 and 2 TAO and a relationship between EOM volume and change in proptosis.

In type 2 TAO, a significant increase in the volume of the medial rectus muscle, lateral rectus, and total EOM was detected postoperatively ( p = 0.044, 0.022, 0.049), whereas no significant changes were found in the EOM's volume changes in type 1 TAO. The reduction of proptosis after orbital decompression in type 1 TAO was significantly greater than in type 2 TAO ( p = 0.025). A significant positive association was observed between the preoperative EOM volumes and the reduction of proptosis following orbital wall decompression (r = 0.505).

The postoperative EOM volumes following orbital wall decompression was significantly increased in Nunery type 2 patients who had restrictive myopathy.