Beyond muscle strength: functional performance measures enhance assessment of disease activity in juvenile dermatomyositis.

Abstract

To evaluate the measurement properties of functional performance measures (FPMs), broad jump (BJ) and 10-m run (10MR) in JDM, and to assess their added value alongside conventional clinical tools. This longitudinal cohort study included 39 children with JDM. BJ and 10MR were assessed with Physician Global Assessment of Disease Activity (PGA), Cutaneous Dermatomyositis Disease Area and Severity Index (CDASI), Manual Muscle Testing in 8 muscles (MMT8) and the Childhood Myositis Assessment Scale (CMAS) at baseline and follow-up visits (every 3-6 months). Construct validity and responsiveness were assessed through cross-sectional and longitudinal analyses. FPMs correlated significantly with disease activity and muscle function, with faster 10MR and longer BJ performance associated with lower PGA and higher CMAS/MMT8. 10MR and BJ demonstrated moderate responsiveness (standardized response mean = -0.56 and 0.31, respectively), and longitudinal analyses confirmed that FPM changes paralleled improvements in CMAS and MMT8 over time. Within the clinically inactive disease (CID) group, functional heterogeneity was evident. Cluster analysis based on CMAS and MMT8 identified a subgroup with relatively low strength scores and poor FPM performance, while a more comprehensive four-variable clustering using BJ, 10MR, CMAS and MMT8 further stratified CID patients into three distinct phenotypes. Notably, one subgroup exhibited high CMAS/MMT8 scores but reduced FPMs, indicating residual functional impairment not captured by traditional assessments. Low internal consistency (Cronbach's α = 0.44) further suggested that FPMs capture dimensions distinct from CMAS/MMT8. FPMs are quick, feasible tools that may provide complementary information on dynamic muscle performance not fully captured by CMAS or MMT8. Their ability to identify functional variability within CID patients supports their potential utility in monitoring functional recovery in JDM.