Paediatrics: Juvenile dermatomyositis

Juvenile dermatomyositis

Autoimmune inflammatory disease of skin and muscles. Rare (inci-dence 2–3 per million) and occurs between ages of 10 and 14yrs. Cause unknown; infectious and environmental triggers are likely.

Diagnostic criteria for juvenile dermatomyositis (JDM)*

Erythematous rash plus two other criteria:

•   Symmetrical weakness of proximal muscles

•   Periorbital oedema with heliotrope discoloration; scaly rash over MTPs and PIPs (Grottron’s papules)

•   Elevation of one or more muscles enzymes: CK, AST, LDH, aldolase

•   EMG changes of myopathy and denervation

•   Muscle biopsy evidence of necrosis and inflammation

•   MRI (has largely superseded EMG and muscle biopsy in children)

•   *Modified from Bohan A, Peter JB. (1975). Polymyositis and dermatomyositis. N Engl J Med

•   403–7.

Clinical features

•   Onset usually insidious, rash may precede muscle weakness.

•   Rash: periorbital oedema with heliotrope discolouration of upper lids; facial rash includes nasolabial folds (unlike SLE); erythematous maculopapular rash over extensor surfaces of MCP, PIPs, elbows, and knees (Grottron’s papules); nail-fold vasculitis.

•   Muscles: typically symmetrical proximal muscle weakness with fatiguability of arms and legs; truncal weakness (unable to sit from lying); ‘Gower’s sign’; palatal and respiratory muscles affected in severe cases with nasal speech, poor swallowing, decreased lung volume.

•   Arthritis (60%): 2/3 oligoarthritis; 1/3 polyarthritis.

•   Lung disease (uncommon): interstitial fibrosis; pulmonary vasculitis.

Diagnosis 

Diagnosis is usually made by typical rash, proximal muscle weakness, raised muscle enzymes, and typical MRI changes.

Differential diagnosis 

Infectious myositis (usually viral); overlap with autoimmune disorder (SLE and mixed connective tissue disease (MCTD)); systemic arthritis JIA.

Late complications

•   Calcinosis: linked with active myositis; occurs in skin, fascia, subcutaneous fat and muscle. Superficial lesions may be painful, erupt, and discharge. Sheets of calcification may prevent movement and usually resolve with time. Tumoural deposits may be surgically removed.

Lipodystrophy: generalized or partial; painless; generalized form associated with insulin resistance, diabetes, liver disease, and short stature.

Investigations

• Muscle enzymes: CK; LDH; aldolase; AST; ALT.

• ANA (6–60%): may be raised; Myositis-specific antibodies rarely present in children.

• ESR and CRP: variable.

• MRI (STIR or T2 fat suppressed): diffuse white signal throughout affected muscles.

• EMG: low amplitude, short-duration polyphasic potentials with early recruitment, fibrillations, and repetitive discharges.

• Muscle biopsy can show histological evidence of necrosis and inflammation.

Monitoring

• Regular examination: muscle strength testing and muscle enzymes.

• Aim to maintain function, normalize muscle enzymes, and limit steroid effects on growth.

Course and treatment 

The condition may be uniphasic, polyphasic, or continuous. Corticosteroids are the mainstay of treatment. Methotrexate is used in more severe and persistent severe cases as a steroid-sparing drug. Cyclophosphamide is used in some centres for vasculitis. Treat for 18mths after remission induced. Some evidence that aggressive treatment may minimize calcinosis.