Biopsy and EMG studies are frequently avoided in diagnosing children with juvenile dermatomyositis. Other diagnostic studies, including MRI, are used more frequently.

A diagnosis of juvenile dermatomyositis should be considered for patients presenting with the following criteria:

  • Symmetrical muscle weakness in the shoulders/upper arms or hips/upper legs and trunk
  • Characteristic dermatomyositis skin rash:
    • 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
  • Elevated muscle enzymes (aldolase, creatine kinase, transaminase)
  • Myopathic changes on EMG (if done)
    • Short, small, low-amplitude polyphasic motor unit potentials
    • Fibrillation potentials, even at rest
    • Bizarre high-frequency repetitive discharges
  • Muscle pain on grasping or spontaneous pain
  • 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)
  • Abnormalities on MRI suggestive of inflammatory myositis
  • Skin biopsy (if done) demonstrates 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
  • Other findings (calcinosis, dysphagia, dysphonia, skin ulcerations)

Further testing

Other blood tests may be used to diagnose or monitor juvenile myositis:

  • Antinuclear antibody (ANA)
  • Myositis-specific antibodies (MSA), i.e. anti-Jo-1, anti-SRP
  • Myositis-associated antibodies (MAA), i.e. anti-PM-Scl
  • Flow cytometry
  • Neopterin (a marker for immune system activation)
  • von Willebrand factor antigen (factor VIII-related antigen)

Other diagnostic tools may include:

  • MRI to identify signs of inflammation and edema
  • Nailfold capillaroscopy to study possible decreased density of nailfold capillary loops (dropout) or dilation of the capillaries
  • Muscle ultrasound
  • Manual muscle testing (MMT) and other tests of muscle strength
  • Tests for physical function, such as Childhood Health Assessment Questionnaire (CHAQ) and Childhood Myositis Assessment Scale (CMAS)