The exact cause of dermatomyositis is not clearly understood, although the condition shares many characteristics with autoimmune disorders such as lupus, scleroderma, and vasculitis.

Therefore, an unknown factor may trigger the production of autoimmune antibodies that then attack the body’s tissues. One theory regarding the underlying mechanism of the disease is that autoantibodies target the endothelium of capillaries in the muscles which prevents the supply of oxygenated and nutrient-rich blood to the tissues.

It has been observed in some cases that people with dermatomyositis have also previously been diagnosed with other types of infection such as Epstein-Barr virus, chlamydia or mononucleosis. The condition has also been found to overlap with other autoimmune disorders such as lupus and scleroderma. Due to the link between dermatomyositis and autoimmune conditions, physicians may perform an antinuclear antibody test which is positive in lupus-like cases of illness.

Dermatomyositis may also occur as paraneoplastic syndrome. This means the condition may occur as a consequence of the presence of cancer, which can itself cause abnormal inflammatory reactions and the release of several immune mediators in the body.

Paraneoplastic conditions often manifest or are detected independently, before the actual cancer is diagnosed. In the case of dermatomyositis caused by underlying cancer, if remission occurs following tumor excision or aggressive treatment, the symptoms of dermatomyositis also usually resolve.

Due to the high risk of cancer in dermatomyositis patients, physicians will often run tests to check for cancer in these individuals. Furthermore, the immunosuppressant medications used to treat the severe forms of dermatomyositis can also increase the risk of cancer and screening should be continued in these patients.

A shared pathology between Lyme disease and dermatomyositis has also been suggested. In 1988, Lyme disease expert Dr Alan Steere noted that the infiltration of immune cells around blood vessels did not differ between the two conditions. He suggested that the histological derangements seen in Lyme disease are a result of immunological damage caused by persistence of the Borrelia bacteria.