Toward a definition of a cyclothymic behavioral endophenotype: which traits tap the familial diathesis for bipolar II disorder?

BACKGROUND: Although the cyclothymic temperament appears to be related to the familial diathesis of bipolar disorder, exhibiting high sensitivity for bipolar II (BP-II) disorder, it is presently uncertain which of its constituent traits are specific for this disorder.
METHODS: In a sample of 446 major depressive patients (BP-II and unipolar), in the French National EPIDEP study, the cyclothymic temperament was assessed by using clinician- and self-rated scales. We computed the frequency of individual traits and relative risk for family history of bipolarity.
RESULTS: From both clinician- and self-rated scales, four items related to mood reactivity, energy, psychomotor and mental activity were significantly highly represented in the subgroup with positive family history of bipolarity. The item "rapid shifts in mood and energy" obtained the highest relative risk (OR=3.42) for positive family history of bipolarity.
CONCLUSION: These findings delineate those cyclothymic traits which are most likely to tap a familial-genetic diathesis for BP-II, thereby identifying traits which can best serve as a behavioral endophenotype for this bipolar subtype. Such an endophenotype might underlie the cyclic course of bipolar disorder first described in France 150 years ago by Falret and Baillarger.