Densities of Orbital Fat and Extraocular Muscles in Graves Orbitopathy Patients and Controls

Purpose: To investigate CT densities of orbital soft tissue volumes in patients with Graves orbitopathy (GO) and to compare these with the densities of controls. Methods: Observational case series. Of 95 patients with GO and 150 controls, soft tissue volumes, mean densities, and ratios of fat volume to orbital volume and muscle volume to orbital volume were calculated with software. The 95% confidence intervals of the controls were used as reference values. The densities were plotted against age and volume ratios. For statistical analysis SPSS 16.00.2 was used. p values were calculated with the following tests: analysis of variance, Pearson correlation, Kruskal-Wallis, Mann-Whitney, and linear regression. Results: The main outcome measurements were differences in orbital soft tissue densities. In GO patients the mean orbital fat density was significantly higher than in controls (p <= 0.001) and independent of age (p = 0.23). The mean extraocular muscle density of GO patients was within the range of controls and did not decrease with age (p = 0.16) as it did in controls (p <= 0.001). Mean fat density increased with decreasing fat volume (p = 0.001). Mean extraocular muscle density increased slightly with increasing muscle volume (p = 0.09). Muscle density correlated with fat density in both controls and GO patients. Conclusions: Orbital fat density in GO patients is significantly higher than in controls and negatively correlated to fat volume but positively correlated to muscle volume and muscle density.