Dermatomyositis is one of a group of muscle diseases known as the inflammatory myopathies, which are characterized by chronic muscle inflammation accompanied by muscle weakness. Dermatomyositis’ cardinal symptom is a skin rash that precedes, accompanies, or follows progressive muscle weakness. The rash looks patchy, with purple or red discolorations, and characteristically develops on the eyelids and on muscles used to extend or straighten joints, including knuckles, elbows, knees, and toes."
Source : US-NINDS
The cause is unknown. Current Theories include that it is an autoimmune disease or a result of a viral infection or cancer.
At present there is no cure, but the condition is treatable with medication and physical therapy.