What are the diagnostic criteria and treatment options for polymyositis?

Diagnostic Criteria for Polymyositis

The diagnosis of polymyositis requires a comprehensive evaluation including muscle enzyme testing, autoantibody panels, electromyography, MRI, and muscle biopsy, with muscle biopsy being the gold standard for definitive diagnosis.

Key Diagnostic Elements

Clinical Features

• Symmetric proximal muscle weakness (hallmark feature)
• Subacute to chronic onset over weeks to months
• Difficulty with activities like climbing stairs, rising from chairs, lifting arms
• Absence of skin rash (distinguishes from dermatomyositis)
• Possible extramuscular manifestations (fever, weight loss)

Laboratory Investigations

1. Muscle Enzymes:

   • Creatine kinase (CK) - often elevated 5-50 times normal
   • Other enzymes: LDH, AST (SGOT), ALT (SGPT), aldolase

2. Inflammatory Markers:

   • ESR (or plasma viscosity)
   • CRP (often elevated)

3. Autoantibody Testing:

   • Myositis-specific antibodies (MSAs)
   • Myositis-associated antibodies (MAAs)
   • ANA, RF (to rule out overlap syndromes)

4. Other Blood Tests:

   • Full blood count
   • Renal and liver function tests
   • Thyroid function (to exclude endocrine myopathies)
   • Vitamin D levels

Imaging Studies

• MRI of muscles:
  • T2-weighted/STIR sequences to detect muscle inflammation
  • Helps identify suitable biopsy sites
  • Shows increased signal intensity in affected muscles 1

Electrophysiological Studies

• EMG findings:
  • Polyphasic motor unit action potentials of short duration and low amplitude
  • Increased insertional and spontaneous activity
  • Fibrillation potentials and sharp waves
  • Helps differentiate myopathy from neuropathy 1

Muscle Biopsy (Gold Standard)

• Characteristic findings:
  • Endomysial inflammatory infiltrates (predominantly CD8+ T cells)
  • Invasion of non-necrotic muscle fibers expressing MHC-I antigen
  • Muscle fiber degeneration, regeneration, and necrosis 1

Differential Diagnosis Algorithm

1. Rule out other inflammatory myopathies:

   • Dermatomyositis (check for characteristic rash)
   • Inclusion body myositis (asymmetric weakness, distal involvement)
   • Immune-mediated necrotizing myopathy

2. Rule out non-inflammatory myopathies:

   • Drug-induced myopathies (especially statins)
   • Endocrine myopathies (thyroid disorders, hyperparathyroidism)
   • Metabolic myopathies
   • Muscular dystrophies
   • Infectious myopathies 1

3. Rule out systemic conditions:

   • Malignancy (particularly in adults >40 years)
   • Other connective tissue diseases
   • Neurological disorders (myasthenia gravis, motor neuron disease)

Treatment Approach

First-Line Therapy

• Corticosteroids:
  • Initial high-dose prednisone (0.5-1 mg/kg/day)
  • Methylprednisolone IV pulses for severe cases
  • Gradual taper over months based on clinical response 1

Steroid-Sparing Agents (initiated concurrently with steroids)

• Methotrexate: 15-25 mg weekly
• Azathioprine: 2-3 mg/kg/day
• Mycophenolate mofetil: 2-3 g/day 1

For Refractory Cases

• IVIG: 2 g/kg divided over 2-5 days
• Rituximab: especially for cases with specific autoantibodies
• Cyclophosphamide: for severe cases with organ involvement
• Cyclosporine: alternative for refractory cases 1

Monitoring Disease Activity

• Regular assessment of muscle strength
• Serial measurement of muscle enzymes (CK, LDH)
• Inflammatory markers (ESR, CRP)
• Repeat MRI to assess response to treatment 1

Common Pitfalls to Avoid

1. Misdiagnosis: Pure polymyositis is rare; ensure thorough evaluation to exclude mimics
2. Inadequate initial treatment: Insufficient steroid dosing can lead to poor outcomes
3. Premature steroid tapering: May cause disease flare
4. Overlooking malignancy: Screen appropriately, especially in adult patients
5. Missing cardiac or pulmonary involvement: Perform baseline cardiac and pulmonary assessments 1

Special Considerations

• Cardiac evaluation (ECG, echocardiogram) should be performed in all patients
• Pulmonary function tests including CO diffusion to assess for interstitial lung disease
• Swallow function assessment to identify dysphagia
• Age-appropriate cancer screening, particularly in adults 1

Remember that polymyositis is increasingly considered a diagnosis of exclusion, and thorough evaluation is essential to rule out other conditions that may mimic its presentation.