Introduction

As discussed elsewhere in this book, bipolar disorder is categorically distinguished from unipolar recurrent depression by the presence of mania or hypomania. Bipolar illness is further divided into Bipolar I Disorder (BP I) and Bipolar II Disorder (BP II) generally on a dimensional basis of severity and symptom duration. While clinically useful, such dimensional distinction has limited the capacity to identify differing neurobiological markers, thereby contributing to a common non-specific approach to conceptualising and managing the two bipolar disorders.

Of the growing number and variety of studies pursuing neurobiological markers, most have not considered BP I and BP II separately and, of those that have attempted this distinction, few have achieved satisfactory partitioning. Consequently, there is a paucity of relevant information especially in regards to the neurobiology of BP II. Hence, this chapter focuses on the few promising findings that may eventually provide insights into the neural underpinnings of bipolar disorder, and especially BP II.

The approaches that investigators have used can, broadly, be considered chronologically. Neurobiological approaches that have been employed most widely and thus far yielded interesting results in patients with bipolar disorder include the direct sampling of blood and brain chemistry, and the assessment of brain function using neurocognitive tests coupled with neuroimaging. Post-mortem studies have also been useful, as have studies exploring the complex genetics of bipolar disorder.