Inflammatory myopathies

The idiopathic inflammatory myopathies are a heterogenous group of disorders characterized by muscle weakness, inflammation in muscle tissue, and with frequent extramuscular involvement. Autoantibodies are common, supporting the notion of these disorders being autoimmune. Typically, inflammatory cell infiltrates are found in muscle biopsies. Other organs are frequently involved such as skin, lungs, joints, gastrointestinal tract, and the heart. These heterogenous disorders can be subclassified based on clinical and histopathological features, or by autoantibody specificities. The idiopathic inflammatory myopathies have traditionally comprised polymyositis (PM), dermatomyositis (DM), juvenile DM, PM/DM overlapping with another connective tissue disease, and inclusion body myositis. More recently a subgroup with similar clinical features but with no or scarce inflammation and with pronounced muscle fibre necrosis has been identified and termed immune-mediated necrotizing myopathy.