Predominant Contribution of Superior Rectus-Levator Complex Enlargement to Optic Neuropathy and Inferior Visual Field Defects in Thyroid Eye Disease

Purpose: To compare extraocular muscle volumes in thyroid eye disease patients with and without compressive optic neuropathy. Methods: A retrospective review of 44 orbital CT scans (28 orbits without compressive disease and 16 orbits with compressive optic neuropathy) was conducted. The extraocular muscle volumes, summated soft tissue volumes, and optic nerve volumes were calculated at a section in the posterior 1/3 of the orbit. The visual fields of the orbits with compressive optic neuropathy were analyzed. Results: The mean combined extraocular muscle/summated soft tissue volume ratio and the mean superior rectus-levator complex/summated soft tissue volume ratio were greater in those with compressive optic neuropathy than in those without compressive optic neuropathy (p = 0.02, 0.008, respectively). The ratio of the mean inferior, medial, or lateral rectus/summated soft tissue volume did not differ significantly between patients with or without compressive optic neuropathy (p values of 0.315, 0.615, and 0.254, respectively). Visual field analysis of the compressive optic neuropathy group demonstrated that 58% of the orbits with visual field defects had inferior field defects. Conclusions: When measured at a section near the orbital apex, the mean combined muscle/summated soft tissue volume ratio and the mean superior rectus-levator complex/summated soft tissue volume ratio are greater in those with compressive disease than those without. This suggests that the specific enlargement of the superior rectus-levator complex makes a significant contribution to thyroid eye disease-compressive optic neuropathy and may explain the inferior visual field deficits classically found in this group of patients.