A relative who has had bipolar manic depression for the past 16 years, has become fixated with the idea that her spouse is cheating on her. He has told her several times that he is not being unfaithful. She is obsessing on this one idea and is staying up at night to try to "catch" him leaving the house. We are concerned about her. She had her lithium levels checked about a month ago and the results were normal. I would like to know what course of action if any do I have. Thank you for your time.
 

Dear Maria -- 
That belief of "infidelity" can be a manic symptom, all right (not common, but definitely recognized as such).  It means her current treatment is not holding her mood disorder stable.  Read our Webmaster's Summary about what you can do.  

Dr. Phelps