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  • 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).
  • MRI of affected muscles reveals evidence of edema or fasciitis or both.
  • A heliotrope rash, Gottron sign/papules are pathognomonic for dermatomyositis.
  • Muscle biopsy findings frequently reveal endomysial, perimysial, and perivascular lymphocytic infiltrates. Except for perifascicular atrophy, which is pathognomonic for dermatomyositis, muscle biopsy findings are variable and nonspecific.
  • A careful family history, medication list review, physical examination, laboratory evaluation, and muscle biopsy are critical and help exclude an alternative diagnosis, such as an inherited muscle disease or toxic myopathy.

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Polymyositis and dermatomyositis are a rare, heterogeneous group of 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 frequently between the ages of 10 and 15 years. In adults, the autoimmune myopathies can occur at any age but seem to peak between the ages of 45 and 60 years.

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In polymyositis and dermatomyositis, muscle biopsies are characterized by lymphocytic infiltrates. Recently, however, a form of autoimmune muscle disease with prominent myofiber degeneration and necrosis with minimal, if any, inflammatory cells has also been recognized. This form of myositis, termed “immune-mediated necrotizing myopathy,” has been included in the most contemporary diagnostic classification schemes of the autoimmune myopathies.

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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. Conversely, in rare instances, the classic muscle biopsy features of dermatomyositis are observed in the absence of rash. These patients are said to have “dermatomyositis sine dermatitis” (Table 27–1).

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Table Graphic Jump Location
Table 27–1. Classification of the Autoimmune Myopathies.
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Symptoms and Signs

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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 proximal arm muscles, especially the deltoids, but often including the biceps and triceps, is expected. Hip flexors are the most commonly affected leg muscles but the hamstrings and quadriceps are also frequently weak. Subtle leg weakness can be detected by having the patient attempt to rise from a 6-inch high stool without using the arms.

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As a general rule in the autoimmune myopathies, distal weakness should only occur in the presence of severe proximal ...

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