The understanding I have from reading & from person experience is that bi-polar can be misdiagnosed as narcissism when only viewed in the manic state. Which on the surface looks like a person being self absorbed/self centered. I realize this is not how it seen when being given diagnosis BP, but on the surface gives this appearance & impression of a self centered person. (You can search the internet for many example of this so I will not spend time of that here)

Accordingly when I was younger, I grew with 3 individuals who were either treated or untreated for bi-polar for religious reasons. And anecdotally they seemed very self absorbed & had a reputations as such.