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Polymyositis (PM) and Dermatomyositis (DM) are two similar and rare chronic (long-term) diseases that affect the muscles. They cause inflammation in the muscles that leads to weakness but not usually pain or swelling. The most significant difference between PM and DM in terms of symptoms is that a rash also occurs with DM that does not occur with PM.

When polymyositis and dermatomyositis are severe, they can cause problems with things like swallowing. This is serious because it can cause a person to choke or breathe things into their lungs. In the rarest and worst of cases, it can even impact the muscle of the heart and the muscles used for breathing.

PM and DM are autoimmune diseases, meaning that they occur when the body’s immune system attacks its own healthy cells and tissues. The reason why it does this is not well understood. When the body’s immune system is “activated” in this way, it can make a person feel very tired, similar to when they have the flu.

PM and DM are types of inflammatory myopathies, which refer to diseases that cause inflammation of the muscles.

For simplicity, this article will refer to them as myositis. There are other types of myositis that are not related to the autoimmune disorders PM and DM discussed in this article.

Myositis can exist on its own, or it can sometimes be seen along with other rheumatic diseases. Muscle inflammation can be seen with Sjogren’s syndrome, lupus, and systemic sclerosis (scleroderma). In a small number of people, myositis can be accompanied by a cancer that somehow triggers the immune system and causes the condition. Everybody who gets myositis should be screened for cancer.

While anyone can get PM or DM, they most often affect two age groups: children aged 10 to 15 and adults aged 45 to 60. Myositis is very rare: it affects only about 10 people in every million. Women are about 2 times more likely to get myositis than men.