Dermatomyositis (DM) and Polymyositis (PM) are two similar chronic diseases that affect the muscles. They are types of myositis. Their characteristic is inflammation of the muscles that leads to weakness. Sometimes they are associated with problems outside of the muscles such as a rash, lung inflammation or arthritis. They can affect any muscle and important functions such as swallowing, breathing or heart contractions can be affected.
They are both most easily detected by physical examination, checking muscles enzymes and inflammatory markers in the blood. Subsequent to this an MRI / EMG / biopsy of the muscle may be appropriate.
DM and PM can be described as autoimmune or auto-inflammatory disease’s where the body’s immune system attacks its own healthy cells and tissues. People can often feel fatigued and unwell with these conditions and it can take sometime after disease remission induction for these constitutional symptoms to improve.
Muscle inflammation can also be seen with a number of other conditions such as hypothyroidism, infection, storage disorders, vasculitis, Sjogren’s syndrome, lupus, and systemic sclerosis (scleroderma). Statin usage must be one of the most common causes of muscle inflammation. Everyone who gets myositis should be screened for cancer as there is a slight increased prevalence of cancer among people who have myositis.
While anyone can get DM or PM, 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.