ETIOLOGY

The etiology of juvenile dermatomyositis (JDM) is unknown. It is characterized by activation of T and B lymphocytes, leading to vasculitis affecting small vessels of skeletal muscle, with immune complex deposition and subsequent inflammation of blood vessels and muscle. JDM may follow infections, allergic reactions, or sun exposure, but no causal relationship has been shown.

EPIDEMIOLOGY

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.

CLINICAL MANIFESTATIONS

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 lift the 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.

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.

LABORATORY AND IMAGING STUDIES

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.

DIFFERENTIAL DIAGNOSIS

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).

TREATMENT

Methotrexate, supplemented by short-term use of systemic corticosteroids, is the cornerstone of therapy for JDM. Initial treatment with pulse intravenous methylprednisolone is followed by several months of tapering doses of oral prednisone. Early institution of methotrexate significantly decreases the duration of corticosteroid use and its associated toxicities. In severe or refractory cases, it may be necessary to use cyclosporine or cyclophosphamide. Intravenous immunoglobulin is a useful as adjunctive therapy. Hydroxychloroquine or dapsone has been used for the skin manifestations. These drugs do not significantly affect the muscle disease. Exposure to the sun worsens the cutaneous manifestations and exacerbates the muscle disease; sunlight may lead to flare. Patients should be advised to wear sun block and refrain from prolonged sun exposure. Accordingly, supplementation with calcium and active forms of vitamin D is also indicated.

COMPLICATIONS

The most serious complication of JDM is the development of calcinosis. Dystrophic calcification can occur in the skin and soft tissues in any area of the body; it ranges from mild to extensive (calcinosis universalis). Although it is difficult to predict who will develop calcinosis, it occurs more commonly in children with cutaneous vasculitis, prolonged disease activity, or delays in onset of therapy. Patients with JDM who develop vasculitis also are at risk for gastrointestinal perforation and gastrointestinal bleeding. JDM has been associated with lipoatrophy and insulin resistance, which can progress to type 2 diabetes. Control of insulin resistance frequently leads to improvement in muscle disease in these patients.

PROGNOSIS

The outcome of JDM depends greatly on the extent of muscle disease and the time between disease onset and initiation of therapy. JDM follows one of three clinical courses: a monophasic course, in which patients are treated and improve without significant sequelae; a chronic recurrent course; and a chronic progressive course marked by poor response to therapy and resulting loss of function. Patients who ultimately develop calcinosis are at risk for chronic loss of mobility depending on the extent of calcium deposition. The association of dermatomyositis with malignancy seen in adults does not occur in children.