Utility of CT imaging in differentiating sacroiliitis associated with spondyloarthritis from gouty sacroiliitis: a retrospective study.

Sacroiliitis is one of the criteria for classification as spondyloarthritis (SpA), though not unique to SpA. Other conditions including gout may be erroneously diagnosed as SpA due to sacroiliitis. The objective was to identify specific CT findings in sacroiliitis associated with SpA and gout. In this retrospective study, CT images of patients with sacroiliitis and clinical diagnosis of gout or SpA from 2010 to 2015 were independently reviewed by two radiologists, blinded to diagnosis. Axial and coronal oblique images were analyzed for characteristics of erosions. The receiver operator characteristic curve was constructed to analyze the discriminating ability of radiological findings. CT SI joint images of 11 patients with gout and 224 patients with SpA were re-analyzed. There was excellent agreement between the radiologists (ICC from 0.78 to 1). Erosions were more numerous in SpA. Erosions in gout were associated with tophi in 65.7% (73/111). Erosions in gout were para-articular and had sclerotic margins, overhanging edges, and multilobulated base (P < 0.0001 for all). Length and depth of erosions were more in gout as compared to SpA. AUCs for length, depth of erosions, and subchondral sclerosis were 0.665, 0.694, and 0.991, respectively. Subchondral sclerosis ≤ 4.5 mm had a sensitivity and specificity of 100 and 96%, respectively, for diagnosis of gout. In addition to known radiological features of gout, multilobulated base of erosions and absence of subchondral sclerosis could possibly distinguish sacroiliitis in SpA from gout. Our limited analysis suggests that CT imaging could help in differentiating the two.