Well I'm sure they'll never admit to shit officially diagnosis wise but...
Alzheimer's kills the entire brain causing the person to fade away into a shell. Sounds like Sleepy Joe to me.
Frontotemporal dementia selectively destroys the frontal lobes where self control lives. This people lose self regulation and become bigger bolder less reserved versions of themselves as they progress. Kind of like getting slowly drunker and never sobering up ever again.