Challenging the unipolar–bipolar division: Does mixed depression bridge the gap?

BackgroundMixed states, i.e., opposite polarity symptoms in the same mood episode, question the categorical splitting of mood disorders in bipolar disorders and unipolar depressive disorders, and may support a continuum between these disorders. Study aim was to find if there were a continuum between hypomania (defining BP-II) and depression (defining MDD), by testing mixed depression as a ‘bridge’ linking these two disorders. A correlation between intradepressive hypomanic symptoms and depressive symptoms could support such a continuum, but other explanations of a correlation are possible.MethodsConsecutive 389 BP-II and 261 MDD major depressive episode (MDE) outpatients were interviewed, cross-sectionally, with the Structured Clinical Interview for DSM-IV, the Hypomania Interview Guide (to assess intradepressive hypomanic symptoms) and the Family History Screen, by a mood disorders specialist psychiatrist in a private practice. Patients presented voluntarily for treatment of depression when interviewed drug-free and had many subsequent follow-ups after treatment start.Mixed depression (depressive mixed state) was defined as the combination of MDE (depression) and three or more DSM-IV intradepressive hypomanic symptoms (elevated mood and increased self-esteem were always absent by definition), a definition validated by Akiskal and Benazzi.ResultsBP-II, versus MDD, had significantly lower age at onset, more recurrences, atypical and mixed depressions, bipolar family history, MDE symptoms and intradepressive hypomanic symptoms. Mixed depression was present in 64.5% of BP-II and in 32.1% of MDD (p = 0.000). There was a significant correlation between number of MDE symptoms and number of intradepressive hypomanic symptoms. A dose–response relationship between frequency of mixed depression and number of MDE symptoms was also found.ConclusionsDifferences on classic diagnostic validators could support a division between BP-II and MDD. Presence of intradepressive hypomanic symptoms by itself, and correlation between intradepressive hypomanic symptoms and depressive symptoms could instead support a continuum. Other explanations of such a correlation are possible. Depending on the method used, a BP-II–MDD continuum could be supported or not.