I had posted this reply on another CarePlace forum (A New Path) but perhaps it will help anyone who is still reading this board. ;-(
I hope there are members who can offer concrete information, DD. What I've learned comes via my experience with a NPD spouse and several family members diagnosed with bipolar-1 and bipolar-2.
It is my understanding that 'yes', NPD is common-enough for people who have bipolar disorder. As difficult as it might be to distinguish the difference between a personality disorder and a bipolar mania, the main distinction seems to be Consistency.
Grandiosity is a long-term trait for someone diagnosed with NPD. The bipolar person also exhibits grandiosity though it's relative to biological moods and not external circumstances.
Same thing with 'revenge' or 'envy' or other pathological traits/patterns observed in the Narcissist over their lifetime. Perhaps ego defenses increase during a 'corrosive life event' in the N's life, but the defenses were ALWAYS there though not as magnified perhaps.
Bipolar people might also be distinguised by their capacity for empathy and their TRUE remorse once a mania has run its course. This is when their depression might lead to suicidal obsessions because they DO feel terrible about the harm they caused other people or themselves.
In my experience, the X never could feel deeply remorseful for his behavior because of course, he unconsciously projected blame (justification) onto others. Not so with bipolar people. They suffer tragically for moods over which they have no control.
A few people I've encountered online had made the assumption their partner was NPD when in fact, they were eventually diagnosed with bipolar. And yes, bipolar can manifest as late as Midlife when symptoms had been more manageable at a younger age. We might consider bipolar as a potential cause of the Midlife Crisis and at this point, it appears many therapists are seeking familial history to rule out bipolar as the problem.
This is merely my perception but it might have some validity in other people's experience with bipolar and NPD: Holding the self responsible.
The more narcissistic a bipolar person may be, the less likely they will be responsible about taking medication to sooth destructive moods. In other words, the bipolar person must CARE (empathize and remorse) about the people they impact when they are unable to regulate emotions. The Narcissist uses ego defenses to deflect personal responsibility (we've all experienced this!). So the more responsible someone is about their mental illness and it's impact on others, the less likely they are to be NPD.
I could be all washed up on that point but it's something I've thought about.
My sis is not NPD but the thought sure crossed my mind prior to her diagnosis. She always takes her meds and she also sees a psychiatrist for cognitive behavioral therapy. My nephew (he's still a teen) resists medications and that's why we've tried to counteract his narcissism with activities urging him away from isolation.
The X never considered his thinking might be nutz even when he was destroying himself and others. His arrogance remained consistent over his lifetime (we met at 19 and divorced in our 50's) though it was never pathological until several uncontrollable events threatened his illusions of grandiosity at Midlife.
Did he seek help? HELK NO
Did he sympathize with his family's pain? HELK NO
Did he trust anyone to be telling him the truth? HELK NO
Not so with my sis. As soon as people started talking to her about her behavior, she sat her little be-hind in a psychiatrist's office and asked for help.
"NPD is one of the most commonly occurring personality disorders among bipolar patients (Brieger, Ehrt, & Marneros, 2003); and bipolar disorder is the third most common Axis 1 disorders found in narcissistic patients (5-11%; Ronningstam, 1996)." ~Excerpted from page 121, Elsa Ronningstam, Understanding and Identifying the Narcissistic Personality.
"Notable is the bipolar NPD patients' appreciation for mood elevation and capability to integrate high energy and activity level into periods of successful personality functioning, and even highly valuable professional or creative achievements.
"...Identifying the presence of genetic or biological communalities awaits further research." (page 124)
hugs all,
CZBZ