What is the diagnosis and treatment of myositis (inflammatory muscle disease)?

From the Guidelines

The diagnosis of myositis involves a combination of clinical evaluation, blood tests for muscle enzymes, autoantibody testing, electromyography, MRI, and muscle biopsy, and treatment typically begins with corticosteroids, with the most recent evidence suggesting high-dose glucocorticoids at 1-2 mg/kg/day for severe cases 1. The diagnosis of myositis is complex and requires a multifaceted approach, including:

• Clinical evaluation to assess muscle weakness, pain, and inflammation
• Blood tests for muscle enzymes, such as creatine kinase, to evaluate muscle damage
• Autoantibody testing to identify specific autoantibodies associated with myositis
• Electromyography to assess muscle function and identify patterns of muscle damage
• MRI to visualize muscle inflammation and damage
• Muscle biopsy to confirm the diagnosis and evaluate the extent of muscle damage

Treatment of myositis depends on the severity and subtype of the disease, but generally involves:

• Corticosteroids, such as prednisone, at a dose of 0.5-1 mg/kg/day for mild to moderate cases
• High-dose glucocorticoids at 1-2 mg/kg/day for severe cases, as recommended by the most recent evidence 1
• Immunomodulatory treatments, such as methotrexate, azathioprine, or mycophenolate mofetil, for patients who do not respond to corticosteroids or require steroid-sparing options
• Intravenous immunoglobulin (IVIG) at 2 g/kg divided over 2-5 days for severe or rapidly progressive cases
• Physical therapy to maintain muscle strength and prevent contractures

The specific treatment approach depends on the myositis subtype, including dermatomyositis, polymyositis, inclusion body myositis, or immune-mediated necrotizing myopathy, and disease severity. Treatment effectiveness is monitored through clinical improvement in muscle strength, normalization of muscle enzyme levels, and reduction in inflammatory markers. The most recent evidence suggests that prompt recognition and early management of myositis is imperative, with discontinuation or interruption of checkpoint inhibitors (CPIs) and initiation of high-dose systemic glucocorticoids being crucial for severe cases 1.

From the FDA Drug Label

1. Collagen Diseases During an exacerbation or as maintenance therapy in selected cases of: Systemic lupus erythematosus Systemic dermatomyositis (polymyositis)

• The diagnosis of myositis (inflammatory muscle disease) is not directly addressed in the provided drug label.
• However, Systemic dermatomyositis (polymyositis) is mentioned as an indication for prednisone therapy.
• Treatment of myositis may involve prednisone, but the label does not provide specific guidance on diagnosis 2

From the Research

Diagnosis of Myositis

• Myositis is a group of inflammatory muscle diseases that include dermatomyositis (DM), polymyositis (PM), necrotizing myopathy (NM), and inclusion body myositis (IBM) 3
• Diagnosis of myositis is based on a combination of clinical features, muscle biopsy, magnetic resonance imaging of skeletal muscles, and detection of myositis-specific autoantibodies 3
• Muscle biopsy is indispensable for making a reliable diagnosis of myositis 3, 4
• Clinical features of myositis can be distinct, but can also overlap with other muscle disorders, making accurate diagnosis challenging 4

Treatment of Myositis

• Treatment of myositis typically involves immunosuppressive therapy, such as prednisone, azathioprine, or methotrexate 5, 6
• The use of corticosteroid therapy in polymyositis-dermatomyositis (PM-DM) requires careful management, including an adequate initial dose, continuation of the initial dose until serum creatine kinase (CK) levels normalize, and a slow taper rate 6
• In some cases, myositis can resolve spontaneously without treatment 5
• Treatment of myositis can be challenging, and classification schemes may need to be updated to reflect current research findings on pathogenesis 7

Differential Diagnosis

• The differential diagnosis of myositis is large, and accurate diagnosis requires careful clinical assessment and detailed laboratory investigations 4
• Muscle biopsy can help confirm or exclude differential diagnostic possibilities, and electron microscopy may be valuable in selected cases 4
• Close collaboration between clinicians and muscle pathologists is essential for accurate interpretation of myopathologic findings in the clinical context 4