[Pathogenesis of polymyositis and dermatomyositis]

The idiopathic inflammatory myopathies, collectively called myositis, are a heterogeneous group of diseases of which polymyositis and dermatomyositis are the best known. These heterogeneous group of chronic disordes sharing the clinical symptom of muscle weakness and, in typical cases, inflammatory cell infiltrates in muscle tissue. There are four major types of idiopathic inflammatory myopathies: dermatomyositis, polymyositis, inclusion body myositis and immune-mediated necrotizing myopathies (autoimmune necrotizing myopathies). Clinical and histopatological distinctions between these conditions suggest that different pathogenic processes underline each of the inflammatory myopathies.