Hi Annette:  I'm sorry to hear about your mom and hope your are coping with her passing; I know how hard it is to lose your mother.  Unfortunately I have no idea why her dowagers hump would go away while she was in the coma, but you should get a sense of great relief just knowing that while she was in this difficult situation she was finally able to lie on her back, after years of not being able to, and that in itself seems like a miracle to me, but there is probably a medical explanation for it, I just don't know what it is.

I personally know how difficult it is to not be able to lie on your back, because it affects so many different things that most aren't aware of that makes life difficult; you can't recline in car seats, or on a nice comfy reclining chair, and the list goes on and on.  Just be happy that for whatever reason, this happened and she was finally able to relax and lie flat after years of pain and not being able to do so.

Hang in there, you're a great daughter!!