Patterns of Extraocular Muscle Enlargement in Graves’ Orbitopathy and Acromegaly

<b><i>Purpose:</i></b> Extraocular muscle (EOM) enlargement occurs in both acromegaly and Graves' disease, but the degree and pattern of enlargement have not been directly compared in these patient groups. This study investigated whether acromegaly and Graves' orbitopathy (GO) are associated with different patterns of EOM enlargement at the time of diagnosis. <b><i>Study Design:</i></b> Retrospective cohort. <b><i>Methods:</i></b> All new patients with a diagnosis of acromegaly or GO who presented to St Joseph's Health Care in London, Ontario, between January 1, 2015, and July 1, 2020, and who underwent computed tomography (CT) scanning with adequate orbital imaging were considered for inclusion. We included age- and sex-matched control patients with pituitary macroadenomas without thyroid or growth hormone abnormalities. Orbital CT scans were analyzed by a single neuroradiologist, who measured the maximum diameter and cross-sectional area of each EOM. The relative likelihood of involvement of each rectus EOM was analyzed separately using an analysis of variance test. <b><i>Results:</i></b> We included 16 patients with GO, 17 with acromegaly, and 18 controls. Ages (mean ± standard deviation) of groups were 55.6 ± 18.0, 50.2 ± 15.7, and 49.3 ± 14.0 years. The mean maximum diameter of EOMs in GO, acromegaly, and controls was inferior rectus (IR) = 4.77 ± 1.53, 4.66 ± 0.61, and 3.68 ± 0.61 mm; medial rectus (MR) = 5.35 ± 2.23, 4.84 ± 0.81, and 3.65 ± 0.42 mm; superior rectus (SR) = 4.94 ± 1.84, 4.88 ± 0.91, and 3.68 ± 0.61 mm; and lateral rectus (LR) = 3.91 ± 1.59, 4.55 ± 0.60, and 3.20 ± 0.43 mm. The IR, MR, and SR muscles were significantly larger in the GO group compared with controls (IR, <i>p</i> = 0.020; SR, <i>p</i> = 0.004; MR, <i>p</i> < 0.001; and LR, <i>p</i> = 0.166), and all four EOMs were larger in acromegaly compared with controls (IR, <i>p</i> = 0.039; SR, <i>p</i> = 0.006; MR, <i>p</i> = 0.006; and LR, <i>p</i> = 0.001). There was no significant difference between the GO and acromegaly groups (IR, <i>p</i> = 0.959; SR, <i>p</i> = 0.987; MR, <i>p</i> = 0.408; and LR, <i>p</i> = 0.250). <b><i>Conclusions:</i></b> GO and acromegaly groups demonstrated the enlargement of the IR, MR, and SR muscles when compared with controls. The GO group did not show significantly larger EOM sizes compared with the acromegaly group. In the GO group, the IR, MR, and SR were similarly affected and did not follow previously described patterns of enlargement in GO.