For a diagnosis of dermatomyositis, patients must present with at least one of the following skin symptoms:

  • Heliotrope rash – a red/purple rash with swelling on the upper eyelid
  • Periungual telangiectasias – dilated capillaries at the skin folds around the fingernails, appearing as small red dots
  • Gottron’s sign or papules – red/purple hardened or eroding areas of skin on the upper surface of finger joints or over the elbows or knees
  • Shawl sign, V-sign, and holster sign – characteristic red, sometimes raised rashes distributed in a “shawl” pattern over the shoulders, arms, and upper back, in a V-shaped pattern over the front of the neck and chest, and over the outside of the hip
  • Skin biopsy demonstrating reduced capillary density, deposits of membrane attack complex on small blood vessels along the dermal-epidermal junction, and within the walls of dermal blood vessels

A diagnosis of dermatomyositis must also have at least 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:
    • Perivascular inflammation with predominantly B-cells (with smaller numbers of CD4-positive T-cells) accumulated around blood vessels
    • Perifascicular atrophy
    • Vasculitis involving endomysial and perimysial capillaries and arterioles
    • Active phagocytosis
    • Central nuclei, or
    • Active regeneration

Amyopathic dermatomyositis (that is, dermatomyositis without symptoms of muscle pain and weakness) has some unique diagnostic considerations.