A while back I was diagnosed with an autoimmune disease called Dermatomyositis (I have had another one called Hashimoto’s thyroiditis for around a decade). There are three primary types of inflammatory myositis, polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM). Each is a rare condition that is at least partly autoimmune. While PM and IBM mainly affect adults, DM has a bimodal distribution affecting adolescents and middle age to older adults. Myositis affects approximately 5-15 people per one million in the population (though estimates vary).
The symptoms of Myositis can range from mild muscle pain and weakness to full immobility from muscle collapse and painful calcification of muscle tissue and skin. Both PM and DM have available therapies, but they come with significant side effects such as immune suppression and lung/liver damage. IBM has no known effective treatment and is slowly progressive over the life time of the patient. Though the conditions can be fatal, typically through lung fibrosis or pulmonary hypertension, this is rare, typically people survive normal lifespans, though a significant percentage become legally disabled.
At a cellular level the diseases are all different. DM causes a vasculitis in the vessels of the skeletal muscle causing necrosis of muscle tissue and other effects (see Histology image). There seems to be multiple targets in the system that the immune system is recognizing but the exact distribution of these targets and their individual effects on the pathology is not well understood due to lack of funding. Another problem in DM is that it is statistically correlated to the presence on visceral cancers in adults. No one knows why, but every adult DM patient (thankfully the correlation is not found in juvenile patients) has to undergo extensive cancer screening on an annual basis for the first three years post-diagnosis. The most common cancers found are colon, reproductive system in women and lung cancer.
I am lucky. I have had DM for going on my second year and my case has remained mild. Yeah, I can’t stay outside long (DM causes UV sensitivity), I can’t carry Zachary for very long without hurting, and stairs suck, but I am functional, working, and have a family who understands my bad days. Though I spend a fair amount of time in bed recovering from bad muscle flares and am immune compromised due to the suppressants I take to control my immune system, most people don’t notice the rash on my knuckles or elbows and would never guess, but it is out there.