Our lovely old boy (16 years old) started bobbing/swaying his head in his sleep late last week. It was a very weird movement so we took him to the vet who said it was probably neurological as he has no problem with his ears. He's acted mildly confused for a while, staring into space and yowling so it was not a huge surprise. They said they could do a brain scan but he'd need sedation and they wouldn't want to risk that at his age. They gave him some steroid tablets which they said might slow it down and he's had those for a couple of days.
But, I've just got home and he's was doing an odd thing with his back leg...stretching it out fully, then twitching it back uncontrollably. It was not pleasant to watch him but he didn't seem distressed or in pain. I took him to his food bowl and he ate and drank , then I gave him a big cuddle as being held seemed to ease the twitching. He walked himself back to his food bowl and has now gone upstairs for a nap. We'll phone the vet in the morning to update them but it's not good is it? There's nothing else they can do for him and if symptoms like this carry on we'll have to let him go. 
Has anyone else had experience of this and how quickly he might deteriorate?