Hi Menon.

My Mum has had dementia for 6 years. During that time she has gone from a lively and sociable woman to an incontinent, mute and helpless person who cannot walk.
I like to think that her 'spirit' has left her body already. It might occasionally revisit for the very rare lucid moments that she has, but these have become so rare that I believe my Mum has already departed her body.
I have no real reason to believe this (hell - I'm not sure I even believe in spirits anymore!), but it makes me feel better.