Dermatomyositis (DM) is classified both as a neuromuscular disease and an autoimmune disease. It is thought that the inflammation resulting in cell damage is created when the immune system attacks healthy muscle tissue and blood vessels under the skin. DM can affect both the muscles and the skin.

DM is one of several diseases in the Idiopathic Inflammatory or Acquired Myopathy category. The term “idiopathic” means no known cause, however some cases are known to have been caused by medications, viruses, bacteria, trauma, toxins, cancer or other illness.