Females are more likely to be affected than males in dermatomyositis while females and males are affected equally in polymyositis. Polymyositis and dermatomyositis are believed to be autoimmune disorders. In autoimmune disease, the immunological mechanisms that normally provide protection against infection and foreign substances are disrupted and instead, produce antibodies that attack and injure the body's own tissues. Researchers are looking into the reasons why this immune response occurs in some people. These are not considered hereditary disorders in the strict sense.
Cases are usually sporadic and a history of the disorder in the same family is unusual.
However, a hereditary factor, possibly an abnormality of the immune system, may be implicated to some extent. Evidence pointing in this direction is the frequent occurrence of polymyositis or dermatomyositis in one person and another autoimmune connective tissue disease in another member of the same family. The entire skin may take on a reddish hue in severe cases. Dermatomyositis is seen more frequently in children than polymyositis, and it's course is often severe and rapidly progressive. In cases of acute dermatomyositis, the muscles may be tender and swollen so that movement is painful.