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Chapter 33: Inflammatory Myopathies

INTRODUCTION

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
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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

DM, dermatomyositis; PM, polymyositis; IBM, inclusion body myositis; NM, necrotizing myopathy; F, female; M, male; IS, immunosuppressive; Ig, immunoglobulin; MAC, membrane attack complex; C, complement; CK, creatine kinase; IFN-1, interferon-1.

Modified with permission from Amato AA, Barohn RJ. Idiopathic inflammatory myopathies. Neurol Clin. 1997;15(3):615–648.

TABLE 33-2.DIAGNOSTIC CRITERIA FOR POLYMYOSITIS, DERMATOMYOSITIS, IMMUNE-MEDIATED NECROTIZING MYOPATHY, AND NONSPECIFIC/UNSPECIFIED MYOSITIS
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TABLE 33-2.DIAGNOSTIC CRITERIA FOR POLYMYOSITIS, DERMATOMYOSITIS, IMMUNE-MEDIATED NECROTIZING MYOPATHY, AND NONSPECIFIC/UNSPECIFIED MYOSITIS

  1. Polymyositis (PM)

    1. Clinical features

      1. Inclusion criteria

        1. Onset usually over 18 year (post puberty)

        2. Subacute or insidious onset

        3. Pattern of weakness: symmetric proximal > distal weakness

      2. Exclusion criteria

        1. Clinical features ...

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