[anonymously]

If it was my dog, I would take the dog to a board certified Internal Medicine Specialist (if you are not doing so already)
Excerpts from article below: http://www.marvistavet.com/html/body_masticatory_myositis.html

“Results of treatment are best early in the course of the disease. If the disease has progressed to an advanced state before treatment is initiated, there may be no response”.

“In short, treatment is suppression of the immune system usually through high doses (rather than the more commonly used lower “anti-inflammatory” doses) of prednisone or dexamethasone. High doses should be maintained until the jaw seems to open normally. After that, the dose may be gradually tapered over 6 months. In many cases the drug cannot ever be completely stopped”.