Is impulsivity a common trait in bipolar and unipolar disorders?

Bipolar disorder (BD) is commonly associated with increased impulsivity, particularly during manic and depressed episodes (1, 2), and unipolar depressive disorder (UP) is also associated with increased impulsivity during depressive episodes (3). In BD, impulsivity also remains elevated during euthymic phases, suggesting that it is a trait-like personality characteristic (4). Increased impulsivity adversely affects the illness course of BD and UP by increasing suicide risk (1, 3, 5, 6) and mood instability (7). In BD, elevated impulsivity during euthymic periods contributes to disruptive behaviors such as reckless driving (8), substance abuse disorder (9), and poor adherence to treatment (10). However, there is a lack of studies assessing impulsivity in euthymic UP patients, so it is not known whether impulsivity is related to poor illness course and risk of substance abuse as with BD. Increased impulsivity in BD and UP during illness episodes as well as in euthymic states supports the suggestion that impulsivity is related to mood disorders in general (4, 7). If this is confirmed, then therapies targeting impulsivity could represent novel interventions for mental disorders with impulsivity at their core, as suggested by Pattij and Vanderschuren (11). In this study, we compared questionnaire-measured impulsivity in euthymic BD and UP patients, healthy controls (HC), and a group of unaffected relatives (NAR) of BD patients. The NAR were included because impulsivity may be an inherited trait that could predispose vulnerable individuals to affective disorders in general (12). We also compared impulsivity in patients with and without history of suicide attempts and substance abuse to determine whether impulsivity during the euthymic interval predicts these outcomes. We hypothesized that: (i) euthymic BD and UP subjects have higher impulsivity than NAR and HC; (ii) NAR have higher impulsivity than HC; and (iii) euthymic BD and UP patients with past suicide attempts and substance abuse history have higher impulsivity than BD and UP subjects without such histories.