Ultrasound clusters of joint inflammation in systemic lupus erythematosus: a cross-sectional study

Despite the high frequency of Systemic Lupus Erythematosus (SLE)-related joint involvement, few studies investigated the anatomical distribution of this manifestation. In the present study, by applying musculo-skeletal ultrasound (US), we aimed at mapping joint involvement in a large cohort of SLE patients. We enrolled SLE patients with past or present joint involvement. US evaluation was performed at level of metacarpophalangeal joints (MCPs), proximal interphalangeal (PIPs) joints, wrists, knees, and metatarsophalangeal (MTPs) joints. Grey scale synovitis and Power Doppler were scored according to the EULAR-OMERACT ultrasound scoring system (range 0-3). Principal Component Analysis (PCA) and Unsupervised Hierarchical Cluster Analysis (CA) were performed to define the presence of clusters. As control we evaluated patients with RA. We evaluated 119 SLE patients and consequently 4046 joints. US synovitis ≥ 1 was detected in 375 joints (9.3%). The wrist was the most commonly involved joint [right (R): 63.5%, left (L) 54.2%]. Ninety-six joints (2.3%) had at least grade 2 synovitis, mainly at wrists (R 8.40%, L 7.56%) and knees (R 6.72%, L 5.8%). PCA identified four clusters: medium-large joints (wrists and knees), MTPs; PIPs and MCPs. This result was confirmed by applying the correlation matrix. Finally, a similar clusters distribution was observed when using CA. Our analysis demonstrated as wrists and knees were the most commonly involved joints according to US assessment. Through a multi-statistical approach, we demonstrated the presence of at least three different US-detected clusters.