Graves' orbitopathy: extraocular muscle/total orbit area ratio is positively related to the Clinical Activity Score

Both extraocular muscle (EOM) and orbital fat are involved in Graves' orbitopathy (GO) but their enlargement might occur with a different temporal pattern. Two GO subtypes have been described, one with predominant EOM enlargement and the other with prevalent fat tissue involvement. We longitudinally investigated the EOM in patients with GO and their relationship with clinical activity.

By using commercial software with a segmentation technique, we calculated from computed tomography (CT) scan EOM coronal area (CA) and total orbit coronal area (TOA) in 23 control subjects and in 32 patients with GO. The latter were studied both at presentation and 18 months later. Superior, lateral, inferior, and medial EOM areas and TOA were selected by 3 different contiguous CT slices: A, B, and C, chosen at globe pole tangent and 2 and 4 mm backward. The Clinical Activity Score (CAS) was also measured.

Orbital EOM CA/TOA ratio (OM/TOA ratio) after 18 months decreased in most patients with GO, indicating that EOM area decrement contributed significantly to OM/TOA ratio reduction. Clinical Activity Score decrease was significantly correlated to the OM/TOA ratio decrease.

An easy method to measure CA of EOM and orbit allowed us to observe that in most patients with GO the OM/TOA ratio decreases with time, suggesting that macroscopic EOM involvement occurs initially and resolves as the other clinical signs and symptoms of the disease resolve, as indicated by the significant OM/TOA ratio correlation with CAS.