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 (rs = 0.74; CI 0.55-0.85) and moderately with the MMT (rs = 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.