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Chapter 46: The Inflammatory, Metabolic, and Toxic Myopathies

Cuixia Tian; Alexander M. Zygmunt

INTRODUCTION

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.

INFLAMMATORY MYOPATHIES

ESSENTIALS OF DIAGNOSIS AND TYPICAL FEATURES

  • 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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Table 46–1. Classic clinicopathologic phenotypes of juvenile idiopathic inflammatory myopathies (JIIMs).
Clinicopathologic Phenotype Historical, Epidemiologic, and Physical Examination Notes Muscle Biopsy Outcomes and Potential Complications
Juvenile dermatomyositis (JDM)

  • ~85% of JIIM

  • Youngest age of onset (median, 7.5 years)

  • Clinical features include symmetric proximal muscle weakness, Gottron papules or heliotrope rash, small vessel vasculopathy (periungual), and other photosensitive rash

  • Perifascicular muscle atrophy

  • Inflammatory infiltrate in perimysium and pericapillary

  • Reduction in capillaries

  • 2%-3% mortality

  • 24%-40% monocyclic course

  • 50%-60% chronic course

  • 20%-40% calcinosis

  • ~10% lipodystrophy

  • Rare GI ulcerations and pulmonary or GI air leaks
Overlap myositis

  • 6%-11% of JIIM

  • Meet criteria for JIIM plus another autoimmune disease

  • Systemic lupus erythematosus, juvenile idiopathic arthritis, systemic sclerosis, scleroderma

  • Clinical features include Raynaud phenomenon, interstitial lung disease, Malar rash, and arthritis

  • More commonly has findings of JDM but can have histopathologic appearance of JPM

  • Higher mortality than JDM

  • Interstitial lung disease is often associated with overlap myositis
Juvenile polymyositis (JPM)

  • 4%-8% of JIIM

  • Older age of onset (median, 11 years old)

  • Clinical features include proximal and distal muscle weakness in absence of typical rash, weight loss, rapid and severe onset, myalgias

  • Often misdiagnosed in patients who have noninflammatory myopathies

  • Endomysial inflammation

  • CD8+ T cells

  • Mortality intermediate between JDM and overlap myositis

  • Cardiac involvement ...

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