To perform and compare differential diagnosis of patients with thyroid-associated myopathy, idiopathic orbital myositis and normal controls based on orbital computed tomography. Orbital fat and extraocular muscle densities were quantified using Hounsfield Unit (HU) and their characteristics were compared and analyzed.

From February 2005 to January 2013, orbital computed tomography was performed on 90 eyes of 47 thyroid-associated myopathy patients, 18 eyes of 14 idiopathic orbital myositis patients and 280 eyes of 140 normal subjects. The average values of orbital fat and extraocular muscle densities were measured and compared using HU. The density differences between the patients with thyroid-associated myopathy and the normal group were analyzed by age, clinical activity score, ocular protrusion and disease duration.

In the thyroid-associated myopathy group, orbital fat and extraocular muscle densities were -87.8 ± 12.5 HU and 48.7 ± 7.1 HU, respectively. In the idiopathic orbital myositis group, the orbital fat and extraocular muscle densities were 79.9 ± 9.9 HU and 49.2 ± 9.1 HU, respectively. There was a statistically significant lower result of orbital fat in the thyroid-associated myopathy group ( p = 0.002), however, the extraocular muscle density did not show a statistically significant difference ( p = 0.775). The orbital fat and extraocular muscle densities of the normal group were -79.0 ± 11.2 HU and 54.3 ± 6.3 HU, respectively. There were significantly lower results in both orbital fat and extraocular muscle densities in the thyroid-associated myopathy group than normal group ( p = 0.000). In active cases and those accompanied by ocular protrusion, there was no significant difference in orbital fat density ( p = 0.345 and p = 0.952, respectively), while extraocular muscle density significantly decreased ( p = 0.007 and p = 0.003, respectively).

A difference between the orbital fat and extraocular muscle densities in thyroid-associated myopathy and idiopathic orbital myositis could be quantitatively found using HU and orbital computed tomography.