I have also had the muscle biopsy at the same time as my sural nerve biopsy (many years ago).

Mine showed denervation atrophy (where the nerve severs from the muscle). But it also showed a process in some areas of reinnervation of previously denervated fibers (which helps to resolve the atrophy in those areas). According to their comments, it is common in some disease processes to have denervation, then reinnervation, then sometimes even going back to denervation. So there can be active and/or inactive process, all depending on whether it is in the denervation or reinnervation stage.

Not sure if any of this is helpful for you. Are the doctors thinking a biopsy is the next step for you? That is odd if you have obvious atrophy, but the MRI doesn't show it. Maybe Glenntaj has some input that would be helpful in this case.