Aim To evaluate the change in the rectus muscle volume following orbital bony wall decompression for Graves' orbitopathy.

Methods We used a computer program (syngo Volume Evaluation) to measure the rectus muscles from the digital preoperative and postoperative orbital CT.

Results Of the 25 patients (20 women and five men; mean age 46 (range 18–64) years) enrolled in the study. A significant increase (mean 0.23 ml (16.5%) of preoperative volume; p=0.005) in the volume of the medial rectus muscle (MRM) was detected postoperatively, whereas no significant changes were found in the volume of the other rectus muscles and between eyes that underwent surgery in the active and inactive phases of the disease. A significant negative association was observed between the time of postoperative CT scans and the change in the MRM volume (p=0.0004) (a mean increase of 68% of preoperative MRM volume for those measured between 3 and 9 months, and a mean decrease in the volume of 50% for those measured between 41 and 50 months).

Conclusion The change in the volume of the MRM may partly explain the variability in the proptosis reduction following orbital decompression.