Help for Severe Atrophy 8 months old puppy dog

Hello everyone, I kindly ask you to help me with my 8-month-old puppy. I had a blood test and everything was found to be normal, even the cpk. Today I had him do an MRI and it resulted in an atrophy of the masticatory muscles. At the time I was prescribed 25mg of cortisone divided into twice a day. On Thursday I have to do blood tests to find out if the cause is an infectious disease. I am attaching the analyses, the result of the resonance and a photo of my dog. I hope I can get as much help as possible because I am desperate, please if you can give me your opinion.

Hello - there are big scans involved here, which in the UK at least would mean that you have some pretty high-powered professionals working on this, far better qualified to answer your questions than me.  I am surprised that they have not discussed possible differential diagnoses;  am I missing a bit?   I'm afraid that we don't read other people's scans on this site, as per terms and conditions - its where our bosses draw the lines between general advice, and unethical interference with another vets' case, so we generally scroll on past. I did wonder from the signs you described, whether masticatory myositis might be on your vets' differentials list?  It an autoimmune condition that is usually diagnosed from a combination of signs / scans / biopsy, and tends to be treated with high-dose steroids as you described, so perhaps your vets had that on their lists.  Possibilities that were lower down my lists might include changes to the cranial nerves, neuromuscular junctions, the brain, other changes to the muscles themselves or neuromuscular junctions and maybe cancers.  It sounds as though your vets are checking things out thoroughly, where as I am guessing; your best next move would be to continue as directed and to wait for their verdict.   If they are not sure, questions such as : what have you learned / ruled out so far and what could it still be?  Together with:  What do you need to look up / test for in order to progress this case? -   can usually provide some insight.  I'm sorry not to read the scans and  hope that something there is useful.