It is estimated there are about 50,000 patients with one or other of the myositis diseases: pm, dm, sIBM, and hIBM. Although there are no official estimates sIBM seems the most prevalent, then dm, then pm, and finally hIBM which is very rare. I personally believe there are more but they are misdiagnosed or have an overlap syndrome and not classified with a myositis disease.
I have never heard of someone actually diagnosed by a myositis expert with both pm and dm. They are entirely different diseases...one B cell the other T cell. IBM is often misdiagnosed as PM only to be changed after not responding to treatment. There is no effective treatment for IBM at this time.
The Myositis Association has a website and community forums as well as a FB page. There are a number of FB groups for myositis if you are interested.
MCTD (SLE Lupus, polymyositis, scleroderma). Diagnosed 2005. Kidney, liver, GI tract, dysphagia, raynauds, barretts esophagus, quadriplegic in 2005. Recovered and now active in downhill skiing, golf, hiking. Meds: amlodipine, benezapril, omeprazole, potassium, folic acid, vitamins, maxide.
Remain Optimistic and you can overcome.