Purpose: To evaluate the relationship between orbital bony geometry and the volume of the intraorbital structures in predicting compressive dysthyroid optic neuropathy (DON). Design: Retrospective, consecutive case series. Participants: Images of 198 orbits from 99 patients suffering from thyroid-related orbitopathy were reviewed. Methods: Clinical examination and computed tomography of the orbits, including volumetric analysis, were performed on all patients. Main Outcome Measures: Clinical and radiologic predictors of DON. Results: Significant volumetric univariate predictors of compressive optic neuropathy included medial rectus volume (P = 0.005), lateral rectus volume (P = 0.011), superior muscle group volume (P = 0.04), and total rectus muscle volume (P = 0.015). Inferior rectus muscle volume, orbital volume, bony orbital apex angle, globe diameter, and bony medial wall contour were not associated with optic neuropathy. Multivariate modeling found medial rectus volume the only independently significant predictor. Univariate modeling of simple rectus diameter measurements found medial rectus axial diameter (P = 0.003) and total recti diameter (P = 0.016) predictive of optic neuropathy. Lateral rectus, superior rectus, and inferior rectus diameters were not predictive. Multivariate modeling found only medial rectus diameter to be a significant independent predictor of optic neuropathy. The area under the receiver operating characteristic curve was not different between the volumetric and rectus diameter curves. Conclusions: Medial rectus size was found to be the most important quantifiable predictor of compressive optic neuropathy in patients with optic neuropathy. In this study, simple maximum medial rectus diameter, as measured on axial scans, was equally predictive of compressive optic neuropathy as the more involved volumetric calculation. Assessment of orbital geometry, including bowing of the medial wall, orbital apex angle, globe diameter, and orbital volume, were not found to be predictive of DON. Financial Disclosure(s): The authors have no proprietary or commercial interest in any of the materials discussed in this article.
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