Abstract:

 Objective To analyze the extraocular muscle thickening on computed tomography (CT) images using easy methods and assess its correlation with the activity of thyroid associated ophthalmopathy（TAO） to provide objective evaluation indicators. Design Retrospective case-control study. Participants 116 cases of TAO patients and 30 cases of normal control were included. Method Auto-CAD software was used to calculate the cross-sectional area of eye muscles, total orbit coronal area and exophthalmos at 2 mm behind the eyeball, and its correlation with clinical activity score（CAS） was analyzed. The receiver operating characteristic curve (ROC) was used to determine the diagnostic value of these CT parameters for the disease activity. Main Outcome Measures Orbital extraocular muscle coronal area/total orbit coronal area ratio (OM/TOA ratio), exophthalmos, CAS. Results The cross-sectional area of the superior rectus muscle, inferior rectus muscle, medial rectus muscle, lateral rectus muscle, and the OM/TOA ratio in the TAO group was 56.39±22.77 mm², 43.72±16.63 mm², 43.51±13.70 mm², 34.15±7.05 mm² and 0.21±0.043 respectively. These parameters were significantly greater than that in the normal control group (28.47±2.30 mm², 22.53±2.5 mm², 28.21±1.66 mm², 28.21±1.66 mm² and 0.15±0.017 respectively) (all P<0.001). The OM/TOA ratio was positively correlated with the CAS (r=0.593, P<0.001). There was no significant correlation between the exophthalmos and the CAS(r=0.086, P=0.361). The cutoff value of OM/TOA ratio 0.18 was the best for detection TAO as active, with sensitivity 89.0% and the specificity 53.5% (area under the curve=0.761, P<0.001). Conclusion The OM/TOA ratio at 2 mm behind the eyeball on the CT image can be used as an objective quantitative parameter for assessing the TAO activity. Combined with CAS, it can be used for accurately assessing the disease and guiding treatment. （Ophthalmol CHN, 2018, 27： 333-337）

Key words: thyroid associated ophthalmopathy, orbit computed tomography scan, orbital extraocular muscle coronal area/total orbit coronal area ratio (OM/TOA ratio), exophthalmos, clinical activity score (CAS)