Quantitative computed tomography of Graves' ophthalmopathy. Extraocular muscle and orbital fat in development of optic neuropathy.

Quantitative assessment of computed tomographic (CT) scans was performed on 98 orbits of 49 patients with Graves' ophthalmopathy. Extraocular muscle volume and orbital fat volume were determined, according to whether optic neuropathy was absent, had developed during follow-up, or was present at the time of scanning. No differences in muscle volume were found between the group already having optic neuropathy and the group in which optic neuropathy developed during follow-up. Both optic neuropathy groups, however, had significant increases in extraocular muscle volume compared with the group without optic neuropathy. The absolute amount of orbital fat was the same for all groups, but the relative amount of orbital fat tended to decrease as total orbital volume increased with development or presence of optic neuropathy. This study establishes a quantitative relationship between optic neuropathy and extraocular muscle enlargement in Graves' ophthalmopathy. Orbital fat, however, does not play an important role in the disease.