Low blood flow in the brain is an early warning sign of dementia.
http://neurosciencenews.com/dementia-stiff-vessel-blood-flow-7015/
“If you have problems with the blood vessels in the brain, then you’re going to end up with difficulty with thinking skills, cognition, memory, and ultimately this can be related to other brain pathologies such as Alzheimer’s disease,” said Daniel Nation, lead author of the study and an assistant professor of psychology at USC Dornsife.

Researchers report harmful plaques associated with Alzheimer's disease may build up in the brain as a result of high blood pressure and decreased cerebral blood flow.