In fact, narcissists can be huge performers in their professional field, because their inflated sense of self-importance drives them on, to show the world just how important they really are. As society commonly associates personality disorders and psychiatric conditions with the inability to perform and function normally, these high performers may remain undiagnosed for years, and sometimes even for their whole lives.

Source: Identifying pathological narcissism in high-functioning individuals – Big ego or severe disorder? | Brain Blogger