Welcome! I would be happy to assist you. I am a 2003 graduate from UC Davis and a Medical Director of a veterinarian practice.
Well the masticatory myositis cannot be ruled-out, even with the test. I would ask your vet if doing a trial with steroids is still worthwhile.
Also, if there is some typeo of jaw or TMJ problem, it may be best to get into a veterinary dental
specialist at least for an exam. Dental Specialists
If not a dental specialist, at least a internist, since this is a very concerning, but difficult problem. Find Various Specialists
--> Under "specialty" select SAIM = Small Animal Internal Medicine
--> Or, you can search for a Neurologist, Cardiologist or Oncologist
But, it does sound well worth still doing steroids just in case this is due to some autoimmune condition.
Hope that info helps
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