There are four major categories of idiopathic inflammatory myopathy: dermatomyositis (DM), polymyositis (PM), immune-mediated necrotizing myopathy (IMNM), and inclusion body myositis (IBM), which are clinically, histologically, and pathogenically distinct (Tables 33-1,33-2,33-3).1–12 These myositides may occur in isolation or in association with cancer, various connective tissue diseases (overlap syndromes), and autoantibodies. Other less common idiopathic myositides (i.e., granulomatous and myositis associated with infections) will also be discussed in this chapter. DM and IBM are rather homogeneous clinically and histologically. On the other hand, what has been called “PM” in the literature is likely a heterogeneous group of disorders. It is important to emphasize that not all myopathies with inflammation are classified as “inflammatory myopathies.” In this regard, various muscular dystrophies (e.g., congenital, facioscapulohumeral, and dysferlinopathies) may be associated with profound inflammation and are not uncommonly misdiagnosed as PM.
TABLE 33-1.IDIOPATHIC INFLAMMATORY MYOPATHIES: CLINICAL AND LABORATORY FEATURES |Favorite Table|Download (.pdf) TABLE 33-1.IDIOPATHIC INFLAMMATORY MYOPATHIES: CLINICAL AND LABORATORY FEATURES
|Disorder ||Sex ||Age of Onset ||Rash ||Pattern of Weakness ||Serum CK ||Muscle Biopsy ||Cellular Infiltrate ||Response to IS Therapy ||Common Associated Conditions |
|DM ||F > M ||Childhood and adult ||Yes ||Proximal > distal ||Normal or increased (up to 50× normal or higher) ||Perimysial and perivascular inflammation; increase expression of IFN-1 regulated proteins on muscle fibers and capillaries; MAC, Ig, C deposition on capillaries ||CD4+ dendritic cells; B cells; macrophages ||Yes ||Myocarditis, interstitial lung disease, malignancy, vasculitis, other connective tissue diseases (CTD) |
|PM ||F > M ||Adult ||No ||Proximal > distal ||Increased (up to 50× normal or higher) ||Endomysial inflammation ||CD8+ T cells; macrophages; plasma cells ||Yes ||Myocarditis, interstitial lung disease, other connective tissue diseases |
|IBM ||M > F ||Elderly (>50 years) ||No ||Proximal = distal; predilection for: finger/wrist flexors, knee extensors ||Normal or mildly increased (usually <10× normal) ||Endomysial inflammation; rimmed vacuoles; amyloid deposits; EM: 15–18 nm tubulofilaments ||CD8+ T cells; macrophages; plasma cells ||None or minimal ||Neuropathy autoimmune disorders - uncommon |
|NM ||M = F ||Adult and elderly ||No ||Proximal > distal ||Elevated (>10 × normal or higher) ||Necrotic muscle fibers; minimal inflammatory infiltrate; MHC1 and MAC expression on sarcolemma of non-necrotic muscle fibers ||Mainly macrophages in necrotic fibers undergoing phagocytosis ||Yes ||Malignancy, CTD, possibly triggered by statin use |
TABLE 33-2.DIAGNOSTIC CRITERIA FOR POLYMYOSITIS, DERMATOMYOSITIS, IMMUNE-MEDIATED NECROTIZING MYOPATHY, AND NONSPECIFIC/UNSPECIFIED MYOSITIS |Favorite Table|Download (.pdf) TABLE 33-2.DIAGNOSTIC CRITERIA FOR POLYMYOSITIS, DERMATOMYOSITIS, IMMUNE-MEDIATED NECROTIZING MYOPATHY, AND NONSPECIFIC/UNSPECIFIED MYOSITIS
Onset usually over 18 year (post puberty)
Subacute or insidious onset
Pattern of weakness: symmetric proximal > distal weakness
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