The DSM-IV and ICD-10 categories of recurrent [major] depressive and bipolar II disorders: evidence that they lie on a dimensional spectrum.J Affect Disord. 2006 May; 92(1):45-54.JA
Presently it is a hotly debated issue whether unipolar and bipolar disorders are categorically distinct or lie on a spectrum. We used the ongoing Ravenna-San Diego Collaboration database to examine this question with respect to major depressive disorder (MDD) and bipolar II (BP-II).
The study population in FB's Italian private practice setting comprised consecutive 650 outpatients presenting with major depressive episode (MDE) and ascertained by a modified version of the Structured Clinical Interview for DSM-IV. Differential assignment of patients into MDD versus BP-II was made on the basis of discrete hypomanic episodes outside the timeframe of an MDE. In addition, hypomanic signs and symptoms during MDE (intra-MDE hypomania) were systematically assessed and graded by the Hypomania Interview Guide (HIG). The frequency distributions of the HIG total scores in each of the MDD, BP-II and the combined entire sample were plotted using the kernel density estimate. Finally, bipolar family history (BFH) was investigated by structured interview (the Family History Screen).
There were 261 MDD and 389 BP-II. As in the previous smaller samples, categorically defined BP-II compared with MDD had significantly earlier age at onset, higher rates of familial bipolarity (mostly BP-II), history of MDE recurrences (>or=5), and atypical features. However, examining hypomania scores dimensionally, whether we examined the MDD, BP-II, or the combined sample, kernel density estimate distribution of these scores had a normal-like shape (i.e., no bimodality). Also, in the combined sample of MDE, we found a dose-response relationship between BFH loading and intra-MDE hypomania measured by HIG scores.
Although the interviewer (FB) could not be blind to the diagnostic status of his private patients, the systematic rigorous interview process in a very large clinical population minimized any unintended biases.
Unlike previous studies that have examined the number of DSM-IV hypomanic signs and symptoms both outside and during MDE, the present analyses relied on the more precise hypomania scores as measured by the HIG. The finding of a dose-response relationship between BFH and HIG scores in the sample at large strongly suggests a continuity between BP-II and MDD. Our data indicate that even in those clinically depressed patients without past hypomanic episodes (so-called "unipolar" MDD), such scores are normally rather than bimodally distributed during MDE. Moreover, the absence of a 'zone of rarity' in the distribution of hypomanic scores in the combined total, MDD and BP-II MDE samples, indicates that MDD and BP-II exist on a dimensional spectrum. From a nosologic perspective, our data are contrary to what one would expect from a categorical unipolar-bipolar distinction. In practical terms, intra-MDE hypomania and BFH, especially in recurrent MDD, represent strong indicators of bipolarity.