Muscle Strength and Muscle Endurance During the First Year of Treatment of Polymyositis and Dermatomyositis: A Prospective Study

Objective. To investigate muscle impairment (isometric and dynamic) and disease activity during the first year after diagnosis of polymyositis (PM) and dermatomyositis (DM), and to study the relationship between muscle impairment, patient-reported health, and disease activity. Methods. Seventy-two patients enrolled in the Swedish Myositis Register, 2003-2010, were followed prospectively. The Manual Muscle test (MMT-8; isometric muscle strength), the Functional Index of myositis test (FI-2; dynamic, repetitive muscle function), and disease activity (6-item core set) were retrieved at the time of diagnosis, and after 6 and 12 months. Self-reported health (Medical Outcomes Study Short Form-36; SF-36) was retrieved at 12 months. Results. At the time of diagnosis, median (Q1-Q3) for the FI-2 was 27.2% (7.9-60.5%) of maximal score compared to 93.8% (92.5-98.8%) of maximal MMT-8. At 12 months, the FI-2 and the MMT-8 improved to 29.4% (16.5-60.7%; p < 0.05) and 96.1% (88.1-99.4%), respectively (p < 0.01). At 12 months, 45% of patients improved >= 20%, and 27% worsened >= 20% in FI-2 score, while 10% improved >= 20% in MMT-8. Physician's global visual analog scale (VAS), Health Assessment Questionnaire, and creatine phosphokinase levels improved significantly at 12 months (p < 0.05-0.001) while patient's global and extramuscular VAS remained unchanged. The SF-36 physical function correlated strongly with the FI-2 (r(s) = 0.74; CI 0.55-0.85) and moderately with the MMT (r(s) = 0.54; CI 0.27-0.73), with lower correlations between muscle function and other SF-36 domains. Conclusion. Patients with PM/DM were characterized by impaired dynamic repetitive muscle function (DRMF) that correlated well with patient-reported physical function. Assessment of DRMF adds information regarding muscle impairment in these patients.