Radiographic evidence of prominent retro and suborbicularis oculi fat in thyroid-associated orbitopathy

Purpose: To compare the radiological differences in retro-orbicularis oculi fat (ROOF) and suborbiculars oculi fat (SOOF) among patients with thyroid-associated orbitopathy (TAO) and normal subjects using computed tomography (CT).

Methods: A retrospective analysis of orbital CTs was performed in 39 consecutive patients, who were imaged between October 2005 and June 2009. Bilateral orbital CTs of 16 patients with a final report significant for thyroid orbitopathy and 23 normal subjects were evaluated. All of the CTs consisted of 0.75 mm thick axial slices with 1.5 mm coronal reconstructions. Using the axial soft tissue windows, the ROOF and SOOF tissues were identified. The maximum ROOF thickness was measured perpendicular to the frontal bone, immediately superior to the supraorbital rim. Similarly, the maximum SOOF thickness was measured perpendicular to the zygomatic bone, immediately inferolateral to the infraorbital rim. The radiologist was blinded to the CT reports while conducting the measurements. Multivariable analysis of the two groups was then performed for comparison.

Results: Seventy-eight ROOF and SOOF measurements were obtained from 16 TAO patients with and 23 patients without TAO. The female-to-male ratio was 6:1 in the TAO group and only 3:2 among the normal subjects. The mean axial ROOF thicknesses was 3.8 ± 1.9 for TAO patients and 2.8 ± 1.0 mm in patients without TAO, while the SOOF thickness was 4.4 ± 1.0 and 3.4 ± 1.0 mm in the 2 groups, respectively. To account for bilateral measurements, the mixed model analysis was used. After controlling for age, gender, and ethnicity, the mean TAO ROOF and SOOF thicknesses were 1.0 mm (p = 0.04) and 0.9 mm (p < 0.01) greater than the control group, respectively.

Conclusion: Retro-orbicularis oculi fat (ROOF) and suborbicularis oculi fat (SOOF) hypertrophy are two additional radiologic CT measurements that may be valuable in establishing the diagnosis of thyroid-associated orbitopathy.