This is a topic that has come to my mind mostly from observation of people throughout my life and some reading on psychology. It has been a point of interest to me to see how these two states of mind can interact and in some cases be confused. In both conditions, the concept of "self" and "other" becomes blurred. Those two concepts are the foundation of how we relate to self and the people around us. Our understanding of self and its equivalence to others is what we base our morals, socialization, and self-worth upon.
When the line between self and other disintegrates, everything is experienced as part of the "self". For the narcissist, this can mean that the suffering of others only has value to the extent it can be internalized and experienced as the "self". I suspect it is even possible for an individual with narcissistic leanings to see themselves as an empath because when they encounter a person with migraine problems, suddenly, they have the problem as well. If someone is sensitive to stress, has digestive problems, etc., they internalize it as their own experience.
The empath can also internalize these external cues and literally feel the pain of others, but the distinction comes in the manner in which the individual resolves the pain. The narcissist will require additional resources to sooth self and the empath will extend resources to sooth the other person in pain. The narcissist can live with someone who suffers from muscle pain, start to experience it for themselves, and then require the original person with the pain to give them more to help with the narcissists imaginary pain. The narcissist sees every want as their own and as yet another excuse to take more from others. The empath sees every want as a reason to eliminate pain in others, sometimes as great cost to self. When self suffers in the empath, they can sometimes not see it if they are too overcome with the pain of another person.
Have you ever encountered either of these conditions in yourself or others? Have you noticed how the issue of the original pain is resolved for the narcissist or the empath?