Does age at onset support a dimensional relationship between Bipolar II disorder and major depressive disorder?

The current categorical splitting of bipolar and depressive disorders has been questioned. Age at onset is an important variable used to support such a division. Study aim was to assess the distribution of age at onset between bipolar II disorder (BP-II) and major depressive disorder (MDD), and onset age-bipolar family history, onset age-BP-II diagnosis dose-response relationships. No bi-modal distribution and no presence of dose-response relationships would not support a categorical distinction between BP-II and MDD. Consecutive 389 BP-II and 261 MDD major depressive episode (MDE) outpatients were interviewed with the DSM-IV Structured Clinical Interview and the Family History Screen, by a mood specialist psychiatrist in a private practice. Age at onset was defined as age at onset of the first MDE. Distribution of age at onset between BP-II and MDD was studied by Kernel density estimate and histogram methods, dose-response relationships by ROC analysis. BP-II, versus MDD, had significantly lower age at onset, more recurrences, and more bipolar family history. Kernel density estimate and histogram distributions of age at onset showed no bi-modality. Likelihood ratios between age at onset and bipolar family history loading, and between age at onset and BP-II diagnosis, showed dose-response relationships. The bi-modality and dose-response approaches, versus classic diagnostic validators, seem to support a dimensional relationship between BP-II and MDD.