Continuous Distribution of Atypical Depressive Symptoms between Major Depressive and Bipolar II Disorders: Dose-Response Relationship with Bipolar Family History
Despite the categorical position of formal diagnostic approaches (i.e. ICD-10 and DSM-IV) to mood disorders, atypical depression (AD) occupies an ambiguous position between major depressive (MDD) and bipolar II (BP-II) disorders. Three hundred and eighty-nine and 261 consecutive BP-II and MDD patients, respectively, presenting for treatment of depression in an Italian private practice, were interviewed by a mood specialist psychiatrist using the Structured Clinical Interview for DSM-IV Axis I Disorders - Clinician Version as modified by the authors to improve the probing for hypomania. Familial bipolarity was measured by the Family History Screen. AD was defined, according to DSM-IV, as a major depressive episode with the 'atypical features' specifier. BP-II, versus MDD, had the usual distinguishing features (i.e. earlier age at onset, higher rate of depressive recurrences, AD symptoms, and bipolar family history). Such categorical distinction notwithstanding, the distribution of the number of AD symptoms between BP-II and MDD depressions, studied by Kernel estimate, was continuous, showing no bimodality. Furthermore, there was a dose-response relationship between such symptoms and bipolar family history. The continuous distribution of a distinct clinical feature (i.e. atypical symptoms) between BP-II and MDD supports a dimensional view of depressive disorders. Our data could also be interpreted as providing further support for the subclassification of AD within the bipolar spectrum.