There are five major types of idiopathic inflammatory myopathies: dermatomyositis (DM), polymyositis (PM), inclusion body myositis (IBM), autoimmune necrotizing myopathy (AINM) and inflammatory myopathies associated with connective tissue diseases (overlap myositis). DM, PM and AINM ore characterized by subacute, proximal and symmetrical weakness and respond to corticoids and immunosuppressants. Cutaneous involvement is specific for DM. IBM manifests by late onset, selective muscle weakness with early distal involvement and is unresponsive to immunosuppressants. PM is the rarest of these conditions. Histological features characterize each entity. perivascular inflammation, microangiopathy with reduced capillary density, ischemia, and perifascicular atrophy for DM; endomysial inflammation with invasion of non-necrotic fibers and diffuse expression of major histocompatibilty complex class I antigens for PM; rimmed vacuoles in IBM coexisting with immunological features similar to PM; and necrosis is the prominent feature of AINM, without inflammation but associated with microongiopathy. The risk of malignant disease is increased in DM and AINM. Myopathy associated with antisynthetase antibodies is characterized by frequent interstitial lung disease, perifascicular atrophy and prominent perimysial pathology. Myopathy associated with anti-SRP antibody is a necrotizing myopathy with rapid progression and partial resistance to corticoids. Inflammatory myopathies associated with connective tissue disease (CTD) ore heterogeneous, involving all four major types (PM, DM, AINM, and IBM) and including additional pathological features. This category of myopathies has not yet been adequately characterized, because classification is usually replaced by the term "overlap myositis".
