Long before experts started to diagnose Trump with dementia, he displayed all of the signs of ADHD. Unlike dementia, which isn't a definitive diagnosis without a brain biopsy or autopsy (or maybe, finally recent biochemical markers to confirm it), ADHD is defined completely by objective, observable behaviors. Trump unequivocally meets at least 13 of 18 criteria, and only 5 are needed for diagnosis.

His ADHD diagnosis is important, because he makes no sense if you consider him just a narcissist. He so often blurts out comments, doesn't do his homework, doesn't follow through, rambles on, jumps into distractions, none of which help convey the image he craves as being a "stable genius".

He may also be a narcissist, but that requires knowing his motivations. To overgeneralize, we blame narcissists for their "bad" behavior but tolerate the same actions if they come from ADHD-generated "mistakes". But we simply don't know how much of Trump's aberrant behavior comes from ADHD or from narcissism. Was his largely ignoring the COVID crisis primarily from inattention (ADHD)? or from not caring at all as hundreds of thousands died (narcissism)? https://dockruse.medium.com/donald-trumps-adhd-shapes-the-coronavirus-crisis-cf11de5e7fef

Attempts to explain his behavior should not be interpreted as excuses for his behavior. But whatever else his is, Mr. Trump is a human being.