To evaluate the relevance of ¹⁸F-labeled fluorodeoxyglucose-positron emission tomography/computed tomography (¹⁸F-FDG PET-CT) for discriminating polymyalgia rheumatica (PMR) and spondylarthritis (SpA) in atypical presentations.

In 2 rheumatology departments, we identified PMR and atypical SpA patients who underwent ¹⁸F-FDG PET-CT and compared the 2 groups. The relevant ¹⁸F-FDG PET-CT findings identified on univariate analyses as discriminant for both groups were entered into a multivariable logistic regression model to derive a composite musculoskeletal score.

Between September 2012 and August 2018, we enrolled 35 PMR and 27 SpA patients (median [interquartile range] age 71 years [63.5–74.5] and 54 years [41.5–63], P<0.001). ¹⁸F-FDG uptake in enthesis/bursae was more frequent in PMR than SpA (ischial tuberosities: 88.6% vs. 48.1%, P<0.001; interspinous processes: 91.4% vs. 51.9%, P<0.001). ¹⁸F-FDG uptake in sacroiliac joints was specific to SpA but rare (14.8% vs. 0 in PMR, P<0.05). The intensity of ¹⁸F-FDG uptake was similar in both conditions. The musculoskeletal score, including ¹⁸F-FDG uptake of the shoulders, ischial tuberosities and interspinous process, was higher for PMR than SpA patients (2.74 vs. 1.11, P<0.001). A score≥2 provided sensitivity and specificity of 74.1% and 77.1% for the diagnosis of PMR.

¹⁸F-FDG PET-CT patterns of atypical SpA and PMR widely overlap, so differentiating the conditions is challenging. The use of the proposed PET-CT composite score could improve the diagnostic performance of ¹⁸F-FDG PET-CT to discriminate these 2 entities in clinical practice.