It happened to my father, who was suffering a severe mental decline because of hydrocephalus. After weeks of trying to figure out what happened, one day he was sitting in the hospital perfectly fine, talking normally. I remember being so relieved. But then the next day he was back to not knowing anything and being half aware. He died a few weeks later.

I saw it with a family member that had glioblastoma. It was weird, one of the nurses had seen it quite a few times apparently, it's also known as rallying. Family member's situation was similar to sibling comment's family, it wasn't right before death, just in the days and weeks leading up to it. It was like a sudden burst of vitality that came of nowhere, like their condition had dramatically reversed itself.

I'm sure it's a little different for everyone though.

Lost my grandfather to it in his late 80's. He did indeed become more responsive and engaged the day (maybe two) before he died, but he was non-verbal at that point. I recalled my mother telling me he was having a surprisingly good day on Sunday and seemed to recognize people, then getting the call from her the following evening that he had passed.

One of my grandparents had AD and became strangely lucid one day, a couple weeks before her death. It was like she finally woke from a years long fog.

I remember it vividly because I got to tell her I love her and she smiled and said it back to me. She seemed to understand me for the first time in years.

It was short lived but I'm very grateful for that single exchange it gave me.