It makes sense that narcissists are more likely to develop dementia because psychopaths already HAVE brain damage or disfunction in the frontal lobes. (No, narcissists aren't necessarily psychopaths, but the two groups do share important characteristics, including egocentricity and a lack of empathy, conscience, and remorse -- and thus many diagnosed as narcissists are probably also psychopaths).

Interesting topic-adjacent book:

The Psychopath Inside: A Neuroscientist's Personal Journey into the Dark Side of the Brain

From the Amazon description (bold added):

For his first fifty-eight years, James Fallon was by all appearances a normal guy. A successful neuroscientist and professor, he’d been raised in a loving family, married his high school sweetheart, and had three kids and lots of friends. Then he learned a shocking truth that would not only disrupt his personal and professional life, but would lead him to question the very nature of his own identity.

While researching serial killers, he uncovered a pattern in their brain scans that helped explain their cold and violent behavior. Astonishingly, his own scan matched that pattern. And a few months later he learned that he was descended from a long line of murderers. Fallon set out to reconcile the truth about his own brain with everything he knew as a scientist about the mind, behavior, and personality.