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Chapter 358: Inflammatory Myopathies

Steven A. Greenberg; Anthony A. Amato

INTRODUCTION

This chapter focuses on the major types of inflammatory myopathies (IM), including dermatomyositis (DM), polymyositis (PM), immune-mediated necrotizing myopathy (IMNM), antisynthetase syndrome (ASS), and inclusion body myositis (IBM) (Table 358-1). Other IM include those caused by infection, eosinophilic myositis and granulomatous myositis. Of note, inflammatory cell infiltrates can also be occasionally seen in muscle biopsies in hereditary (e.g., muscular dystrophies, metabolic myopathies) and toxic myopathies.

TABLE 358-1Inflammatory Myopathies: Clinical and Laboratory Features
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TABLE 358-1 Inflammatory Myopathies: Clinical and Laboratory Features
Disorder Sex Age of Onset Rash Pattern of Weakness Laboratory Features Muscle Biopsy Cellular Infiltrate Response to IS Therapy Common Associated Conditions
DM F > M Childhood and Adult Yes Proximal > distal Normal or increased CK (up to 50× normal or higher); Various MSAs (anti-MDA5, anti-TIF1, anti-Mi-2, anti-NXP2) Perimysial and Perivascular Inflammation; IFN-1 regulated proteins (MHC-1, MxA), MAC deposition on capillaries CD4+ Dendritic cells; B cells; macrophages Yes Myocarditis, ILD, Malignancy, Vasculitis, Other CTDs
PM F > M Adult No Proximal > distal Increased CK (up to 50× normal or higher) Endomysial and perivascular inflammation; ubiquitous expression of MHC-1 CD8+ T-cells; Macrophages; plasma cells Yes Myocarditis, ILD, Other CTDs
NM M = F Children and adults No Proximal > distal Elevated CK (> 10× normal or higher); anti-HMGCR or anti-SRP antibodies Necrotic muscle fibers; minimal inflammatory infiltrate Macrophages in necrotic fibers undergoing phagocytosis Yes Malignancy, CTD, HMGCR antibody cases can be triggered by statin use
ASS F > M Children and adults Sometimes Proximal > distal Elevated CK (>10× normal or higher); antisynthetase antibodies Perimysial and Perivascular Inflammation; perimysial fragmentation with alkaline phosphatase staining; perimysial muscle damage with necrosis CD4+ Dendritic cells; B cells; macrophages Yes Non-erosive arthritis, ILD, Raynaud phenomenon, mechanic hands, and fever
IBM M > F

Older adults

(>50 yrs)

 No Proximal and distal; predilection for: finger/wrist flexors, knee extensors Normal or mildly increased CK (usually <10× normal); anti-cN-1A antibodies; large granular lymphocytes on flow cytometry and reduced CD4/CD8 ratio with increased CD8 count Endomysial and perivascular inflammation; ubiquitous expression of MHC-1; Rimmed Vacuoles; p62, LC3, TDP-43 aggregates; EM: 15–18 nm tubulofilaments; ragged red and COX negative fibers CD8+ T-cells; Macrophages; plasma cells; myeloid dendritic cells; large granular lymphocytes None or Minimal Granular lymphocytic leukemia/lymphocytosis, sarcoidosis, SICCA or Sjogren syndrome

Abbreviations: CK, creatine kinase; cN-1A, cytosolic 5′-nucleotidase 1A; CTDs, connective tissue diseases; COX, cytochrome oxidase; DM, dermatomyositis; F, female; g, immunoglobulin; IBM, inclusion body myositis; IFN-1, type 1 interferon; ILD, interstitial lung disease; IS, immunosuppressive; M, male; MAC, membrane attack complex; MDA5, melanoma differentiation antigen; MHC-1, major histocompatibility antigen 1; NCP2, nuclear matrix protein 2 (NXP2); NM, necrotizing myopathy; PM, polymyositis; TIF1, transcriptional intermediary factor 1.

Source: Modified from AA Amato, JA Russell (eds): Neuromuscular Disorders, 2nd ed. New York, McGraw-Hill Education; 2016, Table 33-1, p. 824, with permission.

Epidemiological studies suggest that the incidence of IM grouped together is greater than 4 cases per 100,000 with ...

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