I actually have polymyositis which is a sister disease to dm and also have lupus. My case was extreme and a long story but it was finally treated successfully by IVIG when prednisone failed. I have been in full remission for 7 years and off all controling meds for over 2 years. Life is different but good so myositis is treatable.
In 2005 I did have an mri and a muscle biopsy and many other tests due to the extreme and complicated nature of my case. If there is uncertainty, a muscle biopsy will clarify it if it is taken from the right muscle. Some cases can be patchy and docs will do an mri first to find the inflammation. My biopsy took a long time to heal but I was near death and on high doses of prednisone....not healing very fast. Taking a hunk of muscle tissue out of an inflammed area adds insult to injury for that muscle. Contact your doctor if you think it is infected.
There is a Facebook closed group called dermatomyositis and polymyositis patients that is pretty good. I am on there as well. Ask to join, it is only for patients, no friends or caregivers and is a closed group.
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.