Long‐term strength and functional status in inclusion body myositis and identification of trajectory subgroups

INTRODUCTION: Objective information on longitudinal disease progression in inclusion body myositis (IBM) is lacking. METHODS: Longitudinal dynamometry and functional status data were collated from a cohort of IBM patients. Annual change was calculated by means of linear modeling. Trajectories of change in grip, knee extension, IBM Functional Rating Scale (IBM‐FRS) and Neuromuscular Symptom Score (NSS) were identified by means of latent growth mixture modeling. RESULTS: Data were collated from 75 IBM patients (348 person‐years follow‐up). Annual strength loss was greatest for pinch (−10%) and knee extension (−4%). Functional deterioration was greatest for males. Three distinct trajectory groups were identified. Rapid deterioration trajectory for grip strength was associated with younger diagnosis age. Rapid deterioration for knee extension strength was associated with older age of diagnosis. DISCUSSION: This study has quantified strength change in IBM and identified distinct trajectory groups, which will aid prognostication and stratification for inclusion into future clinical trials.