Brooke Shield's mom's passing

I know this was weeks ago but I wondered what she died of specifically? All the news article I read said complications from Dementia but what exactly is that?

It was just a few years ago that she was in the news for having lunch with a friend who worked for the National enquirer that angered Brooke Shields. My mom got dementia a little later than her so I'm worried now. How was it that she was healthy enough to get up and go and then fast forward a few years and she's ill and dead?