How to diagnose autoimmune myopathy?

From the Guidelines

Diagnosing autoimmune myopathy requires a comprehensive approach combining clinical evaluation, laboratory testing, imaging, and often muscle biopsy. The diagnostic process typically begins with a thorough clinical assessment looking for characteristic symptoms such as proximal muscle weakness, difficulty climbing stairs or rising from chairs, and sometimes muscle pain or tenderness. Laboratory tests are essential, particularly creatine kinase (CK) levels, which are usually markedly elevated (often 10-50 times normal) 1. Additional blood tests include myositis-specific antibodies (MSAs) like anti-Jo-1, anti-SRP, anti-HMGCR, and anti-Mi-2, which can help identify specific subtypes. Electromyography (EMG) typically shows myopathic changes with spontaneous activity. Magnetic resonance imaging (MRI) of affected muscles can reveal edema, inflammation, and atrophy patterns. Muscle biopsy remains the gold standard, showing characteristic findings such as necrotic muscle fibers, inflammatory infiltrates, and MHC-I upregulation depending on the subtype. For necrotizing autoimmune myopathy, MAC deposition on capillaries may be seen. A comprehensive evaluation is necessary as presentations can overlap with other neuromuscular disorders, and early diagnosis enables prompt treatment with immunosuppressive therapy to prevent irreversible muscle damage and disability.

Key Diagnostic Steps

• Clinical assessment for characteristic symptoms
• Laboratory tests: CK levels, MSAs, and other inflammatory markers
• Imaging: MRI of affected muscles
• Electromyography (EMG) for myopathic changes
• Muscle biopsy for definitive diagnosis

Important Considerations

• Early diagnosis is crucial to prevent irreversible muscle damage and disability
• Presentations can overlap with other neuromuscular disorders, making a comprehensive evaluation necessary
• Treatment with immunosuppressive therapy should be initiated promptly after diagnosis 1

Recent Guidelines

• The American Society of Clinical Oncology (ASCO) recommends a comprehensive diagnostic work-up, including laboratory tests, imaging, and muscle biopsy, for patients with suspected autoimmune myopathy 1
• The European League Against Rheumatism (EULAR) and the American College of Rheumatology (ACR) have developed classification criteria for adult and juvenile idiopathic inflammatory myopathies, which can aid in diagnosis 1

From the Research

Diagnostic Approach for Autoimmune Myopathy

To diagnose autoimmune myopathy, a combination of clinical, serological, and pathological data is essential. The following steps can be taken:

• Collect a thorough medical history to identify symptoms and potential underlying causes
• Perform a neuromuscular examination to assess muscle weakness and other clinical features
• Conduct serologic studies to identify specific autoantibodies associated with unique clinical phenotypes, such as myositis-specific autoantibodies 2, 3
• Use imaging techniques, such as muscle magnetic resonance and chest ultrasound, to aid in diagnosis and classification 4
• Perform a muscle biopsy to examine inflammatory infiltrates and other pathological features 3, 4

Laboratory Tests and Biomarkers

Laboratory tests can help diagnose and classify autoimmune myopathy:

• Creatine kinase (CK) levels can be elevated in patients with autoimmune myopathy, particularly in those with immune-mediated necrotizing myopathy 5
• Autoantibody testing, such as anti-Jo1 antibody, can help identify specific subtypes of autoimmune myopathy and assess the risk of coexisting malignancy 3, 6
• Other biomarkers, such as capillaroscopy and serological biomarkers, can also be useful in diagnosing and managing autoimmune myopathy 4

Clinical Scenarios and Special Considerations

Certain clinical scenarios require special consideration:

• Interstitial lung disease, associated cancer, and pregnancy can complicate the diagnosis and management of autoimmune myopathy 4
• Statin-induced autoimmune necrotizing myopathy (SINAM) is a rare but potentially fatal complication that requires prompt recognition and treatment 5
• A combination of therapies, including corticosteroids, immunosuppressive agents, and intravenous immunoglobulins, may be necessary to manage autoimmune myopathy, particularly in patients with severe or refractory disease 2, 5