First of all, I'm so sorry about this. But the good news is it doesn't always keep progressing. Lyric had this....or a similar but opposite problem, trigeminal neuropathy where the trigeminal nerve is affected and causes the same atrophy but as it progresses, the jaw doesn't close, not like the mm where it doesn't want to open. In Lyric's case, it didn't seem to bother him to open his mouth wide. He yawned fine and I even opened it to test him out. No problem. But I noticed he had a little trouble eating. (which I think it turned out to be his liver disease, which made him feel yucky, which was the next thing that came up) Anyhow, he was quite sunken in in the cheeks and in the temple area. He was on pred for a while and these sunken in areas actually filled out to a degree. He went off the prednisone and it never progressed anymore after that. It was quite a while before he got cancer and that is what killed him, not his liver disease.
I talked to a specialist at Washington State University, which is one of the most reputable veterinary schools in the country....and he said they don't know what causes it in all cases. Sometimes it could be a virus. It can come, cause some damage and then leave. So, not all hope is lost for Illsa. It may come and go as it did in Lyric.