Comparing the phenomenology of depressive episodes in bipolar I and II disorder and major depressive disorder within bipolar disorder pedigrees.

Objective: In a prior study of bipolar disorder pedigrees, we demonstrated distinct clinical differences between depressive episodes in bipolar disorder and major depressive disorder (MDD), including differentiation between these conditions using the Probabilistic Approach to Bipolar Depression. The aim of this retrospective study was to compare the phenomenology of the most severe lifetime depressive episodes between bipolar I (BP-I) and II (BP-II) disorder subtypes and MDD in these pedigrees.

Method: Patients with DSM-IV diagnoses of BP-I (n = 202), BP-II (n = 44), and MDD (n = 120) from bipolar disorder pedigrees were assessed using the Diagnostic Interview for Genetic Studies between 1998 and 2012. Multivariate logistic regression was used to identify distinguishing clinical features. The utility of the Probabilistic Approach in distinguishing BP-I and BP-II depression from MDD was assessed.

Results: BP-I differed from MDD in terms of greater rates of psychomotor retardation (P < .05) and psychotic features (P < .05). BP-II was distinguished from MDD (P < .01) by the greater likelihood of mixed features. Patients with BP-II had a greater likelihood of mixed features (P < .001) and a lesser likelihood of psychomotor retardation (P < .05) compared to those with BP-I. The Probabilistic Approach significantly differentiated both BP-I and BP-II from MDD (P < .01 to P < .001, depending on cutoff) but did not robustly distinguish between BP-I and BP-II.

Conclusions: First, the differentiation of BP-II from both BP-I depression and MDD in terms of the presence of mixed symptoms is of particular interest given the current debate over "mixed specifiers" for these conditions in DSM-5. Second, the Probabilistic Approach to Bipolar Depression was demonstrated for the first time to significantly distinguish both bipolar disorder subtypes from MDD.