Dermatomyositis (DM) is a connective-tissue disease related to polymyositis (PM) that is characterized by inflammation of the muscles and the skin. While DM most frequently affects the skin and muscles, it is a systemic disorder that may also affect the joints, the esophagus, the lungs, and the heart.
The main symptoms include skin rash and symmetric proximal muscle weakness (in over 90% of patients) which may be accompanied by pain and tenderness. Some dermatomyositis patients have a little pain, while in others the pain may be severe.
The cause is unknown, but it may result from an initial viral or bacterial infection (even after treatment or cure of the infection). Regardless of the trigger, DM may behave as a systemic autoimmune disease. Many people diagnosed with DM were previously diagnosed with infectious mononucleosis, Epstein-Barr virus, Chlamydia pneumoniae, psittacosis, and other types of infections. Some cases of DM "overlap" (i.e. co-exist with or are part of a spectrum that includes) other autoimmune diseases such as Sjögren's syndrome, lupus, scleroderma, or vasculitis.
Many cases of DM are a paraneoplastic phenomenon, indicating the presence of cancer.