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ESSENTIALS OF DIAGNOSIS

  • Symmetric proximal muscle weakness progressing over weeks to months.

  • Elevated muscle enzymes, including creatine kinase (CK), aldolase, aspartate aminotransferase (AST), and alanine aminotransferase (ALT).

  • An “irritable myopathy” shown by electromyography (EMG).

  • Magnetic resonance imaging (MRI) of affected muscles reveals evidence of edema, fasciitis, or both.

  • Heliotrope rash or Gottron sign/papules are pathognomonic of dermatomyositis.

  • Muscle biopsy in dermatomyositis and polymyositis frequently reveals endomysial, perimysial, and perivascular lymphocytic infiltrates. Perifascicular atrophy is pathognomonic of dermatomyositis.

  • Muscle biopsy with necrotizing and regenerating muscle fibers is characteristic of the immune-mediated necrotizing myopathies, including statin-associated autoimmune myopathy.

  • A careful family history, medication list review, physical examination, laboratory evaluation, and muscle biopsy are all critical parts of the evaluation.

  • Exclusion of alternative diagnoses, such as an inherited muscle disease or toxic myopathy, is essential.

General Considerations

Polymyositis, dermatomyositis, and the immune-mediated necrotizing myopathies comprise a group of rare, heterogeneous autoimmune myopathies with an approximate incidence of 1 case per 100,000 per year. Although polymyositis is virtually unheard of in children, juvenile dermatomyositis is well described and occurs most commonly between the ages of 10 and 15 years. Immune-mediated necrotizing myopathy has also been described in children. In adults, the autoimmune myopathies can occur at any age but appear to peak between the ages of 45 and 60 years.

Muscle, skin, and lung are the organs most commonly affected in the autoimmune myopathies. Dermatomyositis is typically distinguished from polymyositis by the presence of a distinct rash, although occasionally a diagnosis of dermatomyositis can be made in the absence of a rash if the classic muscle biopsy features of dermatomyositis are present. These patients are said to have “dermatomyositis sine dermatitis” (Table 25–1). Although most patients with dermatomyositis have both skin and muscle involvement, patients occasionally have only the skin manifestations and are classified as having “dermatomyositis sine myositis” or amyopathic dermatomyositis.

Table 25–1.Classification of the autoimmune myopathies.

In both polymyositis and dermatomyositis, muscle biopsies are characterized by lymphocytic infiltrates. Perifascicular atrophy is pathognomonic of dermatomyositis. The immune-mediated necrotizing myopathies, which include statin-associated autoimmune myopathy, have a distinct appearance on muscle biopsy: prominent myofiber degeneration, muscle necrosis, and a paucity of inflammatory cells.

CLINICAL FINDINGS

A. Symptoms and Signs

Symmetric proximal muscle weakness evolving over weeks to months is the presenting symptom in most patients. Typical complaints include difficulty rising from a low chair, walking up steps, and washing one’s hair. In more severe cases, weakness of the neck flexors, pharyngeal weakness, and diaphragmatic weakness can cause head drop, dysphagia, and respiratory compromise, respectively. On physical examination, weakness of the ...

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