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Myopathies are a heterogeneous group of disorders that affect the structure, function, or both structure and function of muscle fibers manifested most commonly by weakness in the patient. They may also present with either positive myopathic symptoms, such as cramps, contracture, hypertrophy, myalgia, or myoglobinuria, or negative myopathic symptoms, such as exercise intolerance, fatigue, muscle atrophy, or weakness.

There are a wide range of potential etiologies. This chapter will address inflammatory, metabolic, and toxic myopathies.



  • Juvenile idiopathic inflammatory myopathies are a rare and heterogeneous group of disorders, the most common clinical phenotype of which is juvenile dermatomyositis.

  • Clinically, these myopathies are characterized by weakness, chronic inflammation of skeletal muscles, and typical skin rashes (Gottron papules or heliotrope rash) with onset during childhood,

  • Serum autoantibody classification and clinicopathologic phenotype are essential for prognosis and treatment of these diseases.

Although classically diagnosed via clinicopathologic criteria first established in 1975 by Bohan and Peters, the juvenile idiopathic inflammatory myopathies (JIIMs) have recently undergone a paradigm shift in classification with the rise of antibody testing. In 2017, the European League Against Rheumatism (EULAR) and the American College of Rheumatology (ACR) introduced a new methodology to classify JIIMs based on physical examination signs and ancillary testing, specifically antibody testing. In contrast, the clinicopathologic criteria subdivided inflammatory myopathies into several discrete phenotypic groups, including juvenile dermatomyositis (JDM), juvenile polymyositis (JPM), overlap myositis, immune-mediated necrotizing myositis (IMNM), and hypomyopathic dermatomyositis (HD) (Table 46–1).

Table 46–1.Classic clinicopathologic phenotypes of juvenile idiopathic inflammatory myopathies (JIIMs).

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