Q:  Identical Twins & Mental Health Histories

Dear Dr. Phelps,

I am an identical twin who was diagnosed with Bipolar II three years ago.  My twin sister is concerned about the likelihood that she will develop the disorder as well.  Our mother, her sister, and several cousins also are bipolar. 

My sister has been taking Paxil for the past eight years, has had only one episode of major depression in her thirties and has never had an episode of mania or hypomania. In contrast, I have had several episodes of major depression, the first when I was nineteen, and was diagnosed as bipolar after an episode of mania induced by nortriptelene.  I'm curious as to why our mental health histories are so different given that we are indentical twins and presumably share the same genetic predisposition toward the disorder.

Thank you for your help.
 

Dear Ms. P'--
Your situation almost perfectly sums up some of the genetic puzzles in bipolar disorder: why are your histories so different if your genes are the same? Your story demonstrates the known fact that not all of bipolar risk is carried in genes. Something triggers an underlying susceptibility. That could be as far back as in the uterus, if somehow one twin gets more blood flow than the other or something like that. After that, as you know from your experience, there are many differences between twins which emerge over time -- yet we don't really know why (that is, what was the trigger; and what was the mechanism). So we don't really know what could lead to your different courses over time. 

Nortriptyline is an older "tricyclic" antidepressant. Most (but not all) studies show that tricyclics are more likely to induce mania than serotonin reuptake inhibitors (SRI's)  like Paxil. But Paxil is an odd bird among SRI's, with some effect on norepinephrine, another neurotransmitter, not just serotonin; and some think Paxil may be one of the least likely of the SRI's to trigger manic symptoms. So maybe that's part of the difference too. Not that I'd recommend you start taking Paxil, mind you; what to do with antidepressants in your situation is a pretty complicated issue. Good luck finding your way from here. 

Dr. Phelps

Published April, 2006