Muscle biopsy is the gold standard test for the diagnosis of polymyositis and dermatomyositis. Necrosis, increase in endomysial and perimysial connective tissue, atrophy and degeneration of both type I and II fibers, especially in a perifascicular distribution might be seen in muscle biopsy. The muscle biopsy should be performed when a patient presented with symptoms of muscle weakness and skin rash or elevated level of muscle enzymes.

• Muscle biopsy is the gold standard test for the diagnosis of polymyositis and dermatomyositis.^{[1][2][3][4][5]}
• The following result of muscle biopsy is confirmatory of polymyositis and dermatomyositis:
  • Necrosis
  • Phagocytosis
  • Regenerative activity reflected by basophilia, large, vesicular nuclei and prominent nucleoli
  • Atrophy and degeneration of both type I and II fibers, especially in a perifascicular distribution
  • Internal migration of nuclei
  • Vacuolization
  • Fiber-size variation
  • Mononuclear inflammatory infiltrate, often most prominent in a perivascular location
  • Increase in endomysial and perimysial connective tissue
• The muscle biopsy should be performed when:
  • The patient presented with symptoms of muscle weakness and skin rash
  • An elevated level of muscle enzymes is detected in the patient.