Introduction

In the last decade many commentators have stated that Bipolar II Disorder (BP II) is under-diagnosed. Over the same period, many others have stated that it is over-diagnosed. Such contradictory views hint at problems with definition and diagnosis – and reflect the controversy as to whether it is a true condition or more a personality style. As it is also commonly viewed as lying along a spectrum of mood disorders bounded by Bipolar I Disorder (BP I) and unipolar depressive conditions (see Chapter 2), we can assume that ‘boundary’ problems (in differentiating one condition clearly from the other) also contribute to difficulties in diagnostic delineation.

In Chapter 2, Phelps well argued the salience of a ‘spectrum’ model. In this chapter, an opposing categorical model is offered for consideration. This does reflect a personal view that bipolar disorder is a categorical condition or an ‘entity’, and further, that if distinctive sub-set diagnoses (of BP I and BP II) exist, distinction between them must be meaningful (in inferring differing clinical pictures and causes, and/or salient differential treatments). Medicine advanced by distinguishing differing clinicopathological expressions of the ‘pox’ (i.e. smallpox and chickenpox), differing causes of ‘dropsy’ (e.g. renal and cardiac) and in distinguishing Type I and Type II diabetes. All three examples (i.e. the ‘pox’, ‘dropsy’ and diabetes) could have been dimensionally modelled, but explanatory power would have been less, while aetiological and treatment implications would have been obscured.