JDM is a rare disease, with an incidence of less than 0.1:100,000 children. JDM can occur in all age groups with a peak incidence between 4 to 10 years. The disease is slightly more common in girls than boys.
Dermatomyositis tends to present in a slow, progressive fashion, with insidious onset of fatigue, malaise, and progressive muscle weakness, accompanied by low-grade fevers and rash. Some children present in an acute fashion, however, with rapid onset of severe disease.
The muscle disease of JDM primarily affects the proximal muscles, particularly the hip and shoulder girdles, and the abdominal and neck muscles. Children have difficulty climbing steps, getting out of chairs, and getting off the floor. The patient may have a positive Gower sign. In severe cases, the patient is not able to sit up from a supine position or even liftthe head off the examination table (see Chapter 182). If muscles of the upper airway and pharynx are involved, the patient’s voice will sound nasal and the patient may have difficulty swallowing.
A classic JDM rash occurs on the face and across the cheeks but also can be found on the shoulders and back (shawl sign). Patients may have heliotrope discoloration of the eyelids. Scaly, red plaques (Gottron papules) classically are found across the knuckles but can be found on the extensor surfaces of any joint. Patients may have periungual erythema and dilated nail-fold capillaries. Less commonly, patients develop cutaneous vasculitis, with inflammation, erythema, and skin breakdown.
Rash typical of dermatomyositis Symmetric proximal muscle weakness Elevated muscle enzymes (ALT, AST, LDH, CPK, and aldolase) EMG abnormalities typical of dermatomyositis (fasciculations, needle insertion irritability, and high-frequency discharges) Positive muscle biopsy specimen with chronic inflammation
ALT, Alanine aminotransferase; AST, aspartate aminotransferase; CPK, creatine phosphokinase; EMG, electromyography; LDH, lactate dehydrogenase. *To make a definitive diagnosis of dermatomyositis, four of five criteria are required.
At some point, 15% of patients with JDM develop arthritis, commonly affecting small joints, but any joint may be involved. Raynaud phenomenon, hepatomegaly, and splenomegaly may also occur.
Many patients with JDM have no evidence of systemic inflammation (normal blood count and erythrocyte sedimentation rate). Evidence of myositis can be identified in 98% of children with active JDM by elevated serum muscle enzymes, including aspartate aminotransferase, alanine aminotransferase, creatine phosphokinase, aldolase, and lactate dehydrogenase. Electromyography and muscle biopsy can document the myositis. Magnetic resonance imaging is a noninvasive means of showing muscle inflammation.
Diagnosis of JDM is based on the presence of documented muscle inflammation in the setting of classic rash (Table 91-1).A small percentage of children have muscle disease without skin manifestations, but polymyositis is sufficiently rare in children that they should have a muscle biopsy to exclude other causes of muscle weakness, such as muscular dystrophy (particularly boys). The differential diagnosis also includes postinfectious myositis and other myopathies (see Chapter 182).