Can the brain shrink without it being alzheimer's?

My mum's been diagnosed with having post coritcal atrophy, however, she was also depressed which I know can shrink the brain due to stress etc. Anyway, she's cheered up a fair amount and has re-remembered all the things she'd forgotten, such as how to tie a shoe lace (without having to be taught, she just remembered again) etc. Is there any chance she doesn't have alzheimer's and that while her brain has shrunk, it is not degenerating? Her lumber puncture results came back clear too..