My dogs jaw is inflamed and she can barely open her mouth. The vet has no idea what's wrong? Anyone had this?

They have done x-rays, etc. but don't know what's causing it. They put her on a really high dose of prednisone and after 2 days all of the muscles in her face have wasted away and it is just a bony skull. She seems to be completely fine otherwise, but it has me very worried. If we can't fix her I will have to put her down and I can't do that! Has anyone experienced this or have any idea what's causing it? PLEASE HELP!!!