[Epidemiology, etiology and classification]

Idiopathic inflammatory myopathies are a group of heterogeneous striated muscle acquired autoimmune diseases, characterized by progressive symmetrical muscle weakness, elevated serum levels of muscle enzymes, electromyographic abnormalities and inflammatory infiltrates on muscle biopsy. This group of diseases comprises polymyositis, dermatomyositis and inclusion-body myositis. They are considered rare autoimmune diseases, with an overall incidence range of 2 to 10 new cases per million persons at risk per year, with differences in distribution according to age, gender and race. Their etiology is largely unknown, but it likely involves both genetic and environmental factors that contribute to autoimmune disorders, with striated muscle as a common target.