|Dermatomyositis mostly affects the muscles of the hips and thighs, the upper arms, the top part of the back, the shoulder area and the neck.|
Dermatomyositis is one of the inflammatory myopathies, a group of muscle diseases that involves inflammation of the muscles or associated tissues, such as the blood vessels that supply the muscles. A myopathy is a muscle disease, and inflammation is response to cell damage.
Another word for inflammatory myopathy is myositis. The myo root means muscle, and the itis root means inflammation; so a myositis is an inflammatory muscle disease.
DM is distinct among the muscle diseases for its manifestation in the skin (“dermato”). A reddish or purplish rash, presumably due to inflammation of surface blood vessels, may occur over the face, neck and chest; on the shoulders and upper back, resembling a shawl; and/or on the elbows, knees and ankles. The skin may be scaly, dry and rough. Sometimes it looks like a sunburn.
Other symptoms of DM include a condition called calcinosis, in which calcium is deposited just under the skin in hard, painful nodules, and panniculitis, inflammation of the fat lying just under the skin.
Over time, the inflammatory process in DM leads to destruction of muscle tissue, and is accompanied by weakness and sometimes pain. There can be loss of muscle bulk (atrophy).
The muscles of the shoulders, upper arms, hips, thighs and neck display the most weakness in DM. There also can be joint pain, inflammation of the heart and lung muscle tissues, as well as inflammation of blood vessels to other organs. For more, see Signs and Symptoms .
In most cases, the cause of an inflammatory myopathy is unclear. For some reason, the body’s immune system turns against its own muscles and damages muscle tissue in an autoimmune process. In DM, these cells attack the small blood vessels that supply muscles and skin.
DM is not a genetic disorder, although there may be genetic factors that make it more or less likely that an inflammatory myopathy will develop. For more, see Causes/Inheritance .
DM's onset is in childhood or adulthood, and its progression is gradual. In children, onset is usually between the ages of 5 and 14, and girls are affected more often than boys. Although inflammatory myopathies like DM can lead to great discomfort for at least a period of time, effective treatments are available, and for the most part these conditions aren’t life-threatening. In fact, many people recover partially or completely from DM.
New research in DM  is rapidly leading to increased understanding of DM and to more successful treatments. Scientists are examining factors that may trigger DM such as viruses, certain drugs or vaccines. All these factors are being studied so inflammatory myopathies like DM can someday be better understood, treated or perhaps prevented entirely.