The development of a model of fatigue in neuromuscular disorders: a longitudinal study.

BACKGROUND Severe fatigue is reported by the majority of patients with three relatively common types of neuromuscular disorders. OBJECTIVE This study aimed to identify predictors of fatigue in a longitudinal study and to develop a model of fatigue in patients with three neuromuscular disorders. METHODS One hundred ninety-eight patients [60 facioscapulohumeral muscular dystrophy (FSHD), 70 adult-onset myotonic dystrophy (MD), and 68 hereditary motor and sensory neuropathy type I (HMSN-I) patients] were studied twice during an 18-month period. Fatigue severity was assessed by the Checklist Individual Strength. A multidimensional assessment method was used, including self-report questionnaires, a daily Self-Observation List, and physical activity (actometer). Muscle strength was determined using the Medical Research Council scale. Structural equation modeling was used to develop and test a model of factors contributing to the persistence of experienced fatigue. RESULTS Muscle strength, self-reported physical activity, sleep disturbances, and pain at baseline contributed directly or indirectly to fatigue and impairment at follow-up. Lower muscle strength contributed to lower levels of physical activity, which, in turn, contributed to fatigue severity. The model showed excellent fit for the whole group of neuromuscular disorders. In FSHD, pain also contributed to physical activity. A model with the actometer as measurement for actual physical activity instead of self-report showed an excellent model fit in FSHD and HMSN but an insufficient fit in MD. CONCLUSION The model of perpetuating factors for fatigue in FSHD and HMSN is different from the model in MD. The main difference is in physical (in)activity. These differences have implications for interventions based on these models.