Bipolar II disorder and major depressive disorder: continuity or discontinuity?

To find if bipolar II disorder (BPII) and major depressive disorder (MDD) were distinct categories or overlapping syndromes.

308 BPII and 236 MDD outpatients, presenting for major depressive episode (MDE) treatment, were interviewed with the Structured Clinical Interview for DSM-IV. History of mania and hypomania, and hypomanic symptoms present during MDE, were systematically investigated. Presence of zones of rarity between BPII and MDD depressive syndromes was assessed. Atypical and hypomanic symptoms were chosen because atypical features and depressive mixed state (ie, MDE plus more than 2 concurrent hypomanic symptoms, according to Akiskal and Benazzi 2003) were often reported to distinguish BPII from MDD depressive syndromes (more common in BPII). If BPII were a distinct category, distributions of these symptoms should show zones of rarity between BPII and MDD depressive syndromes. Histograms and Kernel density estimate were used to study distributions of these symptoms.

BPII had significantly more atypical features and depressive mixed state than MDD. Histograms and Kernel density estimate curves of distributions of atypical and hypomanic symptoms in the entire sample did not show zones of rarity.

Finding no zones of rarity supports a continuity between BPII and MDD (meaning partly overlapping disorders without clear boundaries).