Ultrasound-detected bilateral subacromial-subdeltoid bursitis exceeding 3 mm differentiates polymyalgia rheumatica from rotator cuff tendinopathy: a cross-sectional observational study

Polymyalgia rheumatica (PMR) is a common inflammatory disorder in the elderly, characterized by pain and stiffness in the shoulder and pelvic girdles. The diagnosis of PMR may be challenging due to its nonspecific clinical features and overlap with other musculoskeletal conditions, such as degenerative rotator cuff tendinopathy (dRCT). Although subacromial-subdeltoid bursitis (bSAD) is included in the 2012 ACR/EULAR classification criteria for PMR, its diagnostic specificity is limited unless further ultrasound (US) parameters are considered. In this cross-sectional study, 20 patients with newly diagnosed PMR and 27 patients with dRCT were prospectively enrolled across two rheumatology centers in Italy. Bilateral shoulder US was performed prior to treatment initiation. Patients with full-thickness rotator cuff tears were excluded. Maximum extension of bSAD (mexSAD), power Doppler signal (pdSAD), and inflammatory markers (ESR, CRP) were assessed. Statistical comparisons were performed using t-tests and Spearman correlation analyses. Patients with PMR demonstrated significantly higher mexSAD values compared to those with dRCT (6.9 ± 3.9 mm vs. 2.3 ± 0.4 mm; p < 0.0001). A moderate positive correlation was observed between mexSAD and pdSAD (Rs = 0.38; p < 0.01), and between mexSAD and CRP (Rs = 0.46; p < 0.01). No significant correlation was found between mexSAD and ESR (Rs = 0.33; p = 0.06). Bilateral bSAD with mexSAD > 3 mm yielded a specificity of 96.3% and a sensitivity of 84.2% for diagnosing PMR. Our findings support the diagnostic utility of detailed ultrasound evaluation of bSAD in PMR. A bilateral bSAD thickness > 3 mm may serve as a reliable sonographic cut-off to differentiate PMR from dRCT, even in the absence of typical clinical or laboratory findings. To our knowledge, this is the first study to report these parameters. Further validation in larger cohorts is warranted.