Myositis Treatment Guidelines

Initial Treatment Approach

The American College of Rheumatology recommends that all patients with newly diagnosed idiopathic inflammatory myopathy (IIM) should receive high-dose corticosteroids (prednisone 0.5-1 mg/kg/day) combined with a steroid-sparing agent, with the choice of agent based on patient-specific factors including comorbidities, potential side effects, and organ involvement 1, 2, 3
Steroid-sparing agents are typically slow-acting and may take 3-6 months to reach full efficacy, and the three most commonly used agents are methotrexate (MTX), azathioprine (AZA), and mycophenolate mofetil (MMF) 1, 2, 3

Methotrexate should be started at 15 mg orally once weekly with 1 mg/day folic acid supplementation, with a target dose of 25 mg once weekly within 3-6 months, and is preferred in patients without significant lung disease 1, 2, 4
Azathioprine should be started at 25-50 mg/week with increments of 25-50 mg/week, with a target dose of 2 mg/kg of ideal body weight in divided doses, and may be preferred in patients with interstitial lung disease 1, 2
Mycophenolate mofetil should be started at 500 mg twice daily and increased by 500 mg weekly, with a target dose of 1000 mg twice daily (2 g/day), and is particularly effective for patients with severe skin disease in dermatomyositis 5

For patients with significant lung involvement, MMF or AZA are preferred over MTX due to the association of MTX with pneumonitis 2
For patients with predominant skin disease in dermatomyositis, MMF is considered the first choice due to its efficacy in severe dermatomyositis skin disease 5
For patients with liver disease or abnormal liver function, AZA or MMF are preferred over MTX, with close monitoring of liver function regardless of the agent chosen 2
For women of childbearing potential, AZA is preferred if pregnancy is a possibility, while MTX and MMF are teratogenic and require reliable contraception 5

Regular assessment of muscle strength, skin disease, and major organ involvement is recommended, with monitoring of appropriate lab tests including CBC, liver function tests, and consideration of switching agents if there is an inadequate response or intolerable side effects 1, 2, 5, 6

If first-line treatment fails, consideration should be given to switching to another steroid-sparing agent, adding IVIG, or using rituximab, cyclosporine, or cyclophosphamide for severe refractory disease 5, 6, 7