A diagnosis of polymyositis should be considered for patients presenting with no skin symptoms and four of the following criteria:

  • Symmetrical muscle weakness in the shoulders/upper arms or hips/upper legs and trunk
  • Elevation of serum levels of skeletal muscle-associated enzymes: CK, aldolase, lactate dehydrogenase (LD or LDH), transaminases (ALT/SGPT and AST/SGOT)
  • Muscle pain on grasping or spontaneous pain
  • The triad of muscle-related changes on EMG:
    • Short, small, low-amplitude polyphasic motor unit potentials
    • Fibrillation potentials, even at rest
    • Bizarre high-frequency repetitive discharges
  • Positive for any of the myositis-specific autoantibodies
  • Nondestructive arthritis or arthralgias
  • Signs of systemic inflammation
    • Fever
    • Elevated serum C-reactive protein (CRP) level, or
    • Accelerated erythrocyte sedimentation rate (ESR)
  • Muscle biopsy findings compatible with inflammatory myositis:
    • Endomysial inflammatory cell infiltrate (I-Cells) surrounding and invading non-necrotic muscle fibers
    • Active phagocytosis
    • Central nuclei, or
    • Active regeneration