My doctors never did an emg so I don't know how to interpret the findings. Muscle biopsy is the best way to determine what you have and my docs did an mri first to determine where the inflammation was active (in my case, everywhere) and to inventory the extent of the disease. If you had polymyositis the pattern would be different than for IBM which has inclusion bodies. However, IBM is often very spotty compared to the typical case of PM. Progression of the 2 diseases is quite different however. PM hits the proximal muscles symmetrically and progresses over a period of weeks or months. You will first notice difficulty getting out of chairs, going up and down stairs, and raising your hands over your head. IBM progresses over a period of years and does not often include another disease. It affects your quads, forearms, and hands and is often not symmectrical. It is an entirely different disease.
ANA patterns and results can change. I don't know the explanation for it. Don't know about RNP changing without meds...probably can.
I had a spike in BP but it was due to being on high doses of prednisone for 10 months.
My suggestion is that you get a second opinion from a rheumy, not a neuro. Make sure the rheumy is experienced in treating AI diseases. If there is a local lupus support group you might want to attend it and ask about local doctors. Rheumys take a different approach to diagnosis and are the specialists I would seek. You should not be so impatient with your docs. They are now omniscient and MCTD, IBM or PM can be difficult diseases to diagnose no matter how good the doc. I know others who had either the wrong diagnosis or no diagnosis for months, even years. One friend had a diagnosis of PM only to find out 25 years later he had IBM. I don't think that mistake would happen today but in the early 80s few docs had experience with PM or IBM.
Are you currently getting any treatment? What are your clinical, not labs, symptoms?