NM

(also known as necrotizing autoimmune myopathy or immune-mediated necrotizing myopathy)

A diagnosis of necrotizing myopathy should be considered for patients presenting with no skin symptoms and 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 that include many necrotic muscle fibers as the predominant abnormal histological feature. Inflammatory cells are or only sparse perivascular, perimysial infiltrate is not evident. Membrane attack complex deposition on small blood vessels or pipestem capillaries on endomysium may be seen, but tubuloreticular inclusions in endothelial cells are uncommon or not evident.