A narcissist's "self absorption" is absorption in a false self. An external entity. His false image. This accounts for the strangest thing of all about malignant narcissists: they are as out of touch with themselves as they are with the rest of the world!
All people can get out of touch with themselves and lose track of how they feel. This comes from being too intensely focused too much of the time. Especially with sedentary work. In this state, the brain learns to filter out sensation. One of the benefits of Eastern practices like Yoga and Tai Chi is that it helps us get back in touch with ourselves.
What goes on in a narcissist is essentially the same thing, I suppose, but to a bizarre degree.
How bizarre? Well, I'm afraid you'd have to see it to believe it, but here goes.
One narcissist I knew was so dissociated from himself that he never knew when he was ill. Subconsciously he must have known, because he lost his appetite and went to bed. But his wife had to tell him he had a cold or the flue. He could never answer the simple question Where does it hurt? If asked, "Does it hurt here?" he would answer, "I don't think so." He felt nothing short of extreme pain. By old age, he had made his brain so numb he didn't even feel that. Rough handling by his caretaker had dislocated his shoulder. On seeing this, the person who discovered his injury was astonished that he was sitting there as if in no pain. He said it didn't hurt. Or at least he didn't think it hurt. (No abused little child wants to know his mommy hurt him.) When told that a dislocated shoulder made big, strong football players lie on the ground writhing in agony, he reconsidered. Within minutes, he began complaining that it hurt. But he never did seem to feel severe pain.
Another narcissist I know of was as hypertensive as a shrew. Astronomical blood pressure had enlarged her heart. The doctor asked how long she had felt it beating against her rib cage. "What?" "You must have felt it beating against your rib cage. How long have you felt that happening?" It is almost certain that her heart had been throbbing against her ribs, 24-7 for well over a year. Yet she had never been aware of that.
How do narcissists get so far out of touch with themselves? I suspect they do it simply by keeping focused on something all the time. They fill the gaps between daily work and appearances before mirrors with distractions. One thing I've noticed about narcissists is that they must always be busy, busy, busy. And when there's nothing to do, they bury their nose in a book or newspaper, or they glue themselves to a TV -- anything to avoid a moment of quiet reflection. Blaring radios, sleep, TV, aimless busywork, useless reading — anything to keep burying that repressed corpus delicti (their true self) so it doesn't surface to consciousness in a moment of self awareness.