What is the initial workup for a patient suspected of having infectious myositis?

Workup for Infectious Myositis

The initial workup for infectious myositis should include a complete clinical evaluation, laboratory studies focusing on muscle inflammation markers, microbiological testing, and appropriate imaging to identify the causative pathogen and extent of muscle involvement.

Initial Clinical Assessment

• Evaluate for key symptoms:

  • Muscle pain, tenderness, swelling, and weakness (more typical than pain in myositis) 1
  • Fever and poor general state (suggests bacterial etiology) 2
  • Diffuse muscle pain with flu-like symptoms (suggests viral etiology) 2
  • Travel history to endemic areas (suggests parasitic infection) 3

• Assess for predisposing factors:

  • Recent trauma, surgery, foreign bodies, or devitalized tissue (common in bacterial myositis) 4
  • Immunocompromised status (increases risk for fungal myositis) 4, 5
  • History of residence or travel to endemic areas (for parasitic myositis) 3

Laboratory Investigations

Basic Studies

• Complete blood count with differential (eosinophilia suggests parasitic infection) 3, 2
• Inflammatory markers: ESR and CRP 1
• Muscle enzymes:
  • Creatine kinase (CK) 1
  • Aldolase 1
  • Transaminases (AST, ALT) 1
  • Lactate dehydrogenase (LDH) 1
• Urinalysis to assess for myoglobinuria/rhabdomyolysis 1

Microbiological Testing

• Blood cultures (for bacterial and fungal pathogens) 2
• Specific serological tests based on clinical suspicion:
  • Parasitic antibody testing (for trichinosis, cysticercosis, toxoplasmosis) 3
  • Viral studies for suspected viral myositis 2

Imaging Studies

• MRI of affected muscles (preferred imaging modality):

  • Helps identify location and extent of muscle inflammation 1
  • Can guide biopsy site selection 1
  • Detects fasciitis, which may accompany myositis 1

• CT scan may be considered as an alternative when MRI is unavailable 2

Advanced Diagnostic Procedures

• Electromyography (EMG):

  • Helps differentiate myositis from other neuromuscular disorders 1
  • Shows spontaneous activity in affected muscles 1

• Muscle biopsy (when diagnosis is uncertain):

  • Histopathological examination for inflammatory changes 1
  • Microbiological cultures from tissue samples 2
  • PCR testing for specific pathogens 3

Specific Considerations by Suspected Etiology

Bacterial Myositis

• Focus on blood cultures and imaging-guided aspiration of purulent collections 2
• Consider surgical drainage for focal collections 2

Viral Myositis

• Viral studies and serology based on clinical presentation 2
• Consider testing for common viral causes like influenza and coxsackievirus 4

Parasitic Myositis

• Eosinophil count (typically elevated) 3
• Specific serological tests for parasites (trichinosis, cysticercosis) 3
• Stool examination for parasites when appropriate 3

Fungal Myositis

• More common in immunocompromised patients 4
• Fungal cultures from blood and tissue samples 5
• Consider testing for endemic fungi based on geographic exposure 5

Diagnostic Pitfalls to Avoid

• Failure to consider infectious etiology in patients with muscle symptoms and fever
• Missing predisposing factors like immunosuppression or travel history
• Overlooking the need for tissue sampling in cases with unclear etiology
• Delaying treatment while awaiting definitive diagnosis in severe cases

Remember that infectious myositis is relatively uncommon given the natural resistance of muscle tissue to infection 4. Therefore, a high index of suspicion and thorough workup are essential for timely diagnosis and appropriate management.