Bipolarity might be underdiagnosed by the DSM-IV. This could particularly pertain for young adults, when the disorder is in early stages and the manifestations of hypomania are not typical. Additionally, the current DSM-IV-based instruments are not sufficiently sensitive for detecting hypomania. Support comes from epidemiological and clinical studies, which showed that subthreshold BPD is common and clinically significant. In the National-Comorbidity-Survey-Replication (NCS-R) the overall prevalence of BPD increased from 2.1% up to 4.4% if subthreshold BPD was considered. Role impairment, comorbidity (alcohol abuse, panic disorder, impulse control), and family history of mania are the validators of subthreshold bipolarity. In the small prospective Zurich Study a stepwise broadening of the criteria for hypomania allocated almost half of the subjects with MDE to a broadly defined bipolar group. In bigger samples, such as 2300 patients presenting recurrent or resistant depressions, the hypomania rate, when dimensionnaly rated (by using the hypomania checklist) was more than 60%.

DSM-IV “Bipolar II disorder” and “Cyclothymia” are still ill defined with a great confusion among the territory of “Soft Bipolar Spectrum”, and clinicians need a more practical approach.

The Kahlbaum-Hecker syndrome of cyclothymia includes depressive (dysthymia), hypomanic (hyperthymia), and mixed hypomanic-depressive phases. According to French experts, Deny and Kahn (1909) supported the hypothesis that Cyclothymia represents a special constitution, which can be considered nowadays as particular affective temperament with close continuum with subtrheshold bipolarity.

The great idea of Kahlbaum, Hecker, Deny is to combine the following variables: 1) premorbid temperament; 2) age of onset, 3) time course of an illness (cyclicity / intermittence). Applying this combination, bipolar spectrum could be currently divided into 2 categories: “Typical BP-I/II” (episodic mania or hypomania with free intervals) versus “Cyclothymia” (radically instable form with continuous ups and downs). This intra-bipolar dichotomy seems to be closer to the clinical reality of mood disturbances. Cyclothymia is probably the most frequent expression of bipolar disorder (more than 30% of major depressions). Repeated brief swings with high mood instability and rapid switching seemed to be a distinct entity with early onset, irritable (“dark”) hypomania and high suicide risk. This condition emerged as the most prevalent and severe expression of the bipolar spectrum.