Introduction

Writing in the late nineteenth century, the Prussian psychiatrist Ewald Hecker provided a clinical picture of a form of ‘cyclothymic’ illness manifesting in periods of depression and hypomania that bears a striking resemblance to the contemporary diagnostic category of Bipolar II Disorder (BP II) (Koukopoulos, 2003). Hecker and his senior colleague, Ludwig Kaulbaum, likely influenced Emile Kraepelin's seminal work on ‘manic-depressive insanity’ (Baethge et al., 2003). Kraepelin used the term ‘hypomania’ to refer to non-psychotic, milder forms of mania, which were expressed along a single continuum ranging from purely manic to recurring depressive states (Akiskal and Pinto, 1999; Koukopoulos, 2003). It is not difficult to imagine how a disorder similar to BP II would have fitted within this scheme (Akiskal and Pinto, 1999). Preceding such views by almost 2000 years, Aretaeus of Cappadocia is also known to have described a spectrum of bipolar illness with varying intensities of mania and depression in the first century AD (Goodwin and Jamison, 1990).

Despite these early, and perhaps even seemingly prescient advances, the modern concept of BP II was only first defined in the 1970s by Dunner and colleagues (Dunner et al., 1976). But it was not until the fourth edition of the Diagnostic and Statistical Manual for Mental Disorders (DSM–IV) in 1994 that BP II became officially recognised as a discrete, diagnostic entity (American Psychiatric Association, 1994).