What are the diagnosis and treatment options for polymyositis?

Polymyositis Diagnosis and Treatment

High-dose corticosteroids (prednisone 0.5-1 mg/kg/day) combined with immunosuppressive agents, particularly methotrexate, are the cornerstone of polymyositis treatment according to current guidelines. 1

Diagnostic Approach

Clinical Presentation

• Subacute-chronic symmetric proximal muscle weakness (hallmark feature)
• Muscle fatigue and possible myalgia
• Constitutional symptoms: fever, malaise, weight loss
• Potential extramuscular manifestations affecting:
  • Lungs (interstitial lung disease)
  • Heart (subclinical cardiac involvement)
  • Joints (arthralgia/arthritis)
  • Gastrointestinal system

Diagnostic Workup

1. Laboratory Tests:

   • Muscle enzymes: Creatine kinase (CK) - often elevated up to 50-fold 2
   • Inflammatory markers: ESR, CRP
   • Autoantibody testing: ANA, myositis-specific antibodies
   • Complete blood count and comprehensive metabolic panel
   • Thyroid function tests

2. Imaging:

   • MRI of muscles using T2-weighted/STIR sequences to detect inflammation 1

3. Electrophysiologic Testing:

   • EMG to show muscle fibrillations indicative of myopathy 3

4. Muscle Biopsy (Gold Standard):

   • Endomysial inflammatory infiltrate consisting predominantly of CD8+ T cells
   • Invasion of healthy muscle fibers expressing MHC-I antigen 2

Treatment Algorithm

First-Line Treatment

• Corticosteroids: Prednisone 0.5-1 mg/kg/day (typically 40-60 mg/day) 1, 4
  • Continue initial dose until CK normalizes
  • Slow taper only after CK normalization
  • Monitor CK levels closely during taper 5

Second-Line/Steroid-Sparing Agents

• Methotrexate: First-line steroid-sparing agent (up to 25 mg/week) 1, 6

  • Faster onset of action compared to azathioprine
  • Caution: Avoid in patients with interstitial lung disease or anti-Jo-1 antibodies 7

• Alternative Immunosuppressants:

  • Azathioprine: Alternative to methotrexate, especially with lung involvement 1, 7
  • Mycophenolate mofetil: For patients intolerant to methotrexate or azathioprine 1
  • Hydroxychloroquine: Particularly helpful if skin manifestations are present 6

Refractory Disease Options

• IVIG: 2 g/kg divided into multiple doses 6

  • Particularly effective for patients with:
    • Esophageal/throat muscle involvement
    • Pulmonary manifestations
    • Disease resistant to conventional therapy

• Advanced Therapies:

  • Rituximab (anti-B cell therapy) for refractory cases 3, 1
  • Cyclosporine (especially for pulmonary involvement) 6
  • Tacrolimus as an alternative calcineurin inhibitor 1
  • TNF-α or IL-6 antagonists in selected refractory cases 3, 1

Monitoring Treatment Response

• Regular assessment of muscle strength
• Serial CK measurements (target low-normal range) 5
• Monitor for steroid-related side effects
• Annual assessment of disease damage using validated indices 1

Special Considerations

Immune Checkpoint Inhibitor-Induced Myositis

For severe cases (G3-4) with significant weakness:

• Hold immune checkpoint inhibitor therapy
• Consider hospitalization for severe cases
• Initiate prednisone 1 mg/kg/day or IV methylprednisolone 1-2 mg/kg
• Consider plasmapheresis for acute/severe disease
• Consider IVIG (noting slower onset of action)
• Consider rituximab for refractory cases 3

Common Pitfalls and Caveats

• Failure to distinguish polymyositis from inclusion body myositis, which responds poorly to immunosuppression
• Inadequate initial corticosteroid dosing or premature tapering before CK normalization 5
• Overlooking associated conditions such as interstitial lung disease or malignancy 1
• Insufficient monitoring of CK levels during treatment taper
• Neglecting physiotherapy, which is essential for recovery of muscle strength during remission 6

Polymyositis is a rare condition that requires prompt diagnosis and aggressive immunosuppressive therapy to prevent long-term disability. The combination of high-dose corticosteroids with appropriate steroid-sparing agents offers the best chance for disease control and improved quality of life.