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 |Favorite Table|Download (.pdf) 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 || |
|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 |
Epidemiological studies suggest that the incidence of IM grouped together is greater than 4 cases per 100,000 ...