Relationship between change in physical activity and in clinical status in patients with idiopathic inflammatory myopathy: A prospective cohort study

Objective: This study aimed to investigate the relationship between changes in clinical status on daily life physical activity (PA) in patients with idiopathic inflammatory myopathy (IIM).

Methods: Patients with dermatomyositis (DM), immune-mediated necrotizing myopathy (IMNM) or overlap myositis (OM) who presented either a new-onset or relapsing IIM, stable disease on maintenance therapy or were undergoing immunosuppressant tapering were included. Patients were evaluated at inclusion (V0), and at two follow-up visits (V1, 94±12 days from V0; V2, 96±17 days from V1). The American College of Rheumatology/European League against Rheumatism (ACR/EULAR) response criteria was recorded. PA assessed using 14-days raw accelerometry data gathered using a wrist-worn accelerometer after each visit (mean daily Euclidean norm minus 1 g (ENMO) was computed).

Results: Fifty-five patients (16 OM, 27 IMNM and 12 DM) were included. At baseline, 67% of patients had an ENMO Z-score less than 1. At inclusion, ENMO mainly correlated with health assessment questionnaire score (HAQ, ρ=-0.51, p<0.01), manual muscle testing 8 (MMT8, ρ=0.42, p<0.01), creatinine level (ρ=0.41, p<0.01), and SF-36 physical functioning score (ρ=0.38, p<0.002). At follow-up, ENMO changes mainly correlated with changes in MMT8, HAQ, SF-36 fatigue, and depression score (all ρ>0.43, all p<0.001). Level of agreement between ACR/EULAR response criteria and changes in PA was 15, 45, and 90% for minimal (n = 13), moderate (n = 20), and major (n = 10) improvements, respectively.

Conclusion: Baseline PA levels and change in PA correlated with muscle strength and function, yet changes in PA were also influenced by psychological status. Only patients with major improvements on the ACR/EULAR criteria had significant increase in PA. Accelerometer may serve as an objective tool to define a clinically relevant real-life outcome.

Keywords: Accelerometer; Autoimmune myopathy; Idiopathic inflammatory myopathy; Myositis; Outcome measure; Physical activity.