What is the initial evaluation and treatment approach for a patient with suspected myositis?

Initial Evaluation and Treatment Approach for Suspected Myositis

The initial evaluation for suspected myositis should include a complete rheumatologic and neurologic history and examination, blood testing for muscle enzymes (CK, aldolase), inflammatory markers (ESR, CRP), autoantibody testing, and consideration of EMG, MRI, and/or muscle biopsy when diagnosis is uncertain. 1

Diagnostic Workup

Clinical Assessment

• Complete rheumatologic and neurologic history focusing on pattern of muscle weakness (proximal vs. distal), presence of skin findings, and extramuscular manifestations 1
• Thorough muscle strength examination, particularly of proximal muscle groups 1
• Assessment for characteristic skin findings in dermatomyositis (Gottron papules, heliotrope rash, periungual telangiectasias) 1

Laboratory Testing

• Muscle enzymes: creatine kinase (CK), aldolase, transaminases (AST/ALT), and lactate dehydrogenase (LDH) 1
• Inflammatory markers: erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) 1
• Autoantibody testing:
  • Myositis-specific antibodies (anti-Jo-1, anti-Mi-2, anti-SRP, anti-HMGCR) 1, 2
  • Testing for overlap with myasthenia gravis (anti-AChR and anti-striational antibodies) 1
• Urinalysis to evaluate for rhabdomyolysis 1

Cardiac Evaluation

• Troponin to assess for myocardial involvement 1
• ECG and echocardiogram or cardiac MRI if cardiac involvement is suspected 1

Advanced Diagnostics

• Electromyography (EMG) to identify myopathic patterns 1
• Magnetic resonance imaging (MRI) of affected muscles to identify inflammation and guide biopsy site 1, 3
• Muscle biopsy (gold standard) to confirm diagnosis and determine specific subtype of myositis 1, 4

Treatment Approach

Initial Treatment

• For adult patients with idiopathic inflammatory myositis, begin with high-dose corticosteroids (prednisone 1-2 mg/kg/day) concurrent with a steroid-sparing agent such as methotrexate, azathioprine, or mycophenolate mofetil. 1, 5
• Corticosteroids are FDA-approved for systemic dermatomyositis (polymyositis) 5

Treatment Algorithm Based on Severity

Grade 1 (Mild weakness with or without pain)

• Continue immune checkpoint inhibitors if that's the cause 1
• If CK and/or aldolase are elevated and patient has muscle weakness, offer oral corticosteroids starting at 0.5 mg/kg/day 1
• Provide analgesia with acetaminophen or NSAIDs for myalgia if no contraindications 1
• Consider holding statins if patient is taking them 1

Grade 2 (Moderate weakness limiting instrumental ADLs)

• Hold immune checkpoint inhibitors temporarily if that's the cause 1
• Refer to rheumatologist or neurologist 1
• If CK is elevated (≥3× ULN), initiate prednisone at 0.5-1 mg/kg/day 1
• May require permanent discontinuation of immune checkpoint inhibitors with objective findings of muscle involvement 1

Grade 3-4 (Severe weakness limiting self-care ADLs)

• Hold immune checkpoint inhibitors and permanently discontinue if any evidence of myocardial involvement 1
• Consider hospitalization for patients with severe weakness affecting mobility, respiration, or swallowing 1
• Urgent referral to rheumatologist and/or neurologist 1
• Initiate prednisone 1 mg/kg/day or equivalent 1
• For severe cases, consider IV methylprednisolone 1-2 mg/kg or higher-dose bolus 1
• Consider intravenous immunoglobulin (IVIG) therapy, especially for refractory cases 1, 2
• Consider plasmapheresis in patients with acute or severe disease 1

Refractory Disease Management

• If symptoms and CK levels do not improve or worsen after 4-6 weeks, consider other immunosuppressant therapy 1:
  • Methotrexate (15-20 mg/m²/week, subcutaneous preferred) 1, 6
  • Azathioprine 1, 6
  • Mycophenolate mofetil 1, 6
  • Rituximab (especially for specific autoantibody-positive cases) 1, 7
  • Cyclophosphamide (for severe cases with organ involvement) 1, 6
  • IL-6 inhibition with tocilizumab 1

Monitoring and Follow-up

• Regular monitoring of muscle enzymes (CK, aldolase) 1
• Assessment of muscle strength using validated measures 6
• Tapering of corticosteroids as clinical improvement occurs 6
• Consider discontinuation of immunosuppressive therapy after at least 1 year of remission 6

Special Considerations and Pitfalls

• Early recognition and treatment is critical to prevent permanent muscle damage 1
• Check IgA levels before administering IVIG to prevent severe anaphylactic reactions in IgA-deficient patients 2
• Do not administer IVIG immediately before plasmapheresis as it will be removed 2
• Caution with rechallenging immune checkpoint inhibitors in patients who developed myositis 1
• Permanently discontinue immune checkpoint inhibitors if there is evidence of myocardial involvement 1
• Consider combining pharmacological treatment with individualized and supervised exercise to improve muscle performance 7
• Subgrouping patients based on clinical and serological features may help predict response to specific therapies 7, 8