Purpose. We wanted to investigate the relative significance of fat and muscle enlargement in the development of dysthyroid optic\nneuropathy (DON) in Gravesâ�� orbitopathy (GO). Methods. Preoperative coronal CT scans of 13 patients with and without DON\nwho subsequently underwent orbital decompression were retrospectively analyzed. Thirteen patients imaged for unilateral orbital\nfractures served as controls. Results.The retrobulbarmuscle volume was 2.1 �± 0.5 cm3 (mean �± SD) in controls, 4.3 �± 1.5 cm3 in GO\nwithout DON, and 4.7 �± 1.7 cm3 in GO with DON. The retrobulbar fat volume was 5.4 �± 1.6 cm3 in controls, 8.7 �± 8.0 cm3 in GO\nwithout DON, and 9.4 �± 3.1 cm3 in GO with DON. The muscle and fat volumes were higher in patients with GO than in controls\n(P < 0.001), but the volumes in orbits with and without DON were not significantly different. The volume of the optic nerve were\nsimilar in the 3 groups. The number of apical, coronal 2mm thick slices with no fat was 2.9 �± 0.9 in normal orbits, it was 4.1 �±\n1.0 in GO orbits without DON and 5.3 �± 0.8 in GO orbits with DON (P = 0.007). Conclusion. Apical muscle enlargement may\nbe more important than orbital fat enlargement in the development of DON. However, the fact that apical crowding and muscle\nenlargement also occur in orbits without DON suggests that other factors also play a role in the development of DON.
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