While there is a general consensus that sub-sets of bipolar illness exist, there is in my view no definitive agreement upon the definition of Bipolar II Disorder or bipolar spectrum disorder. This is illustrated by the differences between the DSM–IV and ICD–10 classificatory systems. Bipolar I Disorder, BP II, cyclothymia and Bipolar Disorder Not Otherwise Specified (NOS) are itemised in DSM–IV (American Psychiatric Association, 1994) but ICD–10 (World Health Organization, 1992) only categorises different sub-sets of bipolar affective disorder and mania. DSM–IV criteria for BP II require the presence or history of one or more major depressive episodes and at least one hypomanic episode (past or present), while ICD–10 specifies two or more hypomanic/manic and depressive episodes as being necessary for bipolar affective disorder – raising the curious possibility that some might not consider first-episode mania to be part of bipolar affective disorder.

As earlier detailed in this volume, there is controversy as to whether BP II is distinct from unipolar depression, or if it exists on an overlapping continuum with unipolar depression. Benazzi (2006) reports that this depends on interpretation: BP II and unipolar depression are distinct if classic diagnostic validators are used (family history, age of onset, gender, clinical course of illness); BP II and unipolar depression are continuous if clinical features are used (lifetime manic/hypomanic symptoms, intra-depression hypomanic symptoms and intra-mania depressive symptoms). These issues have yet to be reconciled.