Application of a novel Musculoskeletal Ultrasound Sum Score (MUSS) in the follow-up of patients with juvenile idiopathic arthritis.

Abstract

The objective of this study was to evaluate the novel Musculoskeletal Ultrasound Sum Score (MUSS) as a longitudinal marker of disease activity (DA) in patients with JIA within a treat-to-target management approach. Demographics, clinical, laboratory and US (B Mode, BM; Power Doppler Ultrasound, PDU; scored 0-3, using the paediatric OMERACT score) findings were recorded over a 1-year follow-up in the PRO-KIND cohort. The US imaging of joints was individual to the patient, determined by the treating clinicians (unaware of the study aims) as routine. MUSS (a combination of the highest single BM grade of all examined joints + the highest single PDU grade of all examined joints) was calculated (maximum score 6). Spearman correlations with clinical activity measures and ROC-derived MUSS cut-offs for clinical inactive disease were calculated, using pooled visit-level data. Thirty-three JIA patients were included (oligoarthritis: n = 12, median age 4.4 years; polyarthritis: n = 21, 5.3 years). The median baseline MUSS was similar between subtypes. MUSS values decreased significantly over follow-up, paralleling improvements in JADAS-10 and global assessment scores. MUSS correlated strongly with JADAS-10, physician and parent/patient global scores (Ρ = 0.68-0.81; all P < 0.001) and with ESR in polyarthritis (Ρ = 0.57; P < 0.001). ROC analysis identified a MUSS cut-off of ≥1 as optimal for detecting active disease (specificity 100%, sensitivity 55%). MUSS is a promising, feasible sonographic score for longitudinal monitoring of disease activity in JIA, showing strong correlations with clinical measures and excellent specificity for active disease at a cut-off of ≥1. Validation in independent, prospective cohorts is warranted.