Bipolar disorder diagnosis: challenges and future directions

Abstract

Bipolar disorder refers to a group of affective disorders, which together are characterised by depressive and manic or hypomanic episodes. These disorders include: bipolar disorder type I (depressive and manic episodes: this disorder can be diagnosed on the basis of one manic episode); bipolar disorder type II (depressive and hypomanic episodes); cyclothymic disorder (hypomanic and depressive symptoms that do not meet criteria for depressive episodes); and bipolar disorder not otherwise specified (depressive and hypomanic-like symptoms that do not meet the diagnostic criteria for any of the aforementioned disorders). Bipolar disorder type II is especially difficult to diagnose accurately because of the difficulty in differentiation of this disorder from recurrent unipolar depression (recurrent depressive episodes) in depressed patients. The identification of objective biomarkers that represent pathophysiologic processes that differ between bipolar disorder and unipolar depression can both inform bipolar disorder diagnosis and provide biological targets for the development of new and personalised treatments. Neuroimaging studies could help the identification of biomarkers that differentiate bipolar disorder from unipolar depression, but the problem in detection of a clear boundary between these disorders suggests that they might be better represented as a continuum of affective disorders. Innovative combinations of neuroimaging and pattern recognition approaches can identify individual patterns of neural structure and function that accurately ascertain where a patient might lie on a behavioural scale. Ultimately, an integrative approach, with several biological measurements using different scales, could yield patterns of biomarkers (biosignatures) to help identify biological targets for personalised and new treatments for all affective disorders.

Figure 1. Mood changes over time in bipolar disorder type I, bipolar disorder type II, and recurrent unipolar depressive disorder

M=mania. m=hypomania. D=depression.

Figure 2. Conventional diagnostic classification criteria for affective disorders compared with neurobiologically defined classes of illness

(A) Conventional and overlapping diagnostic categories of affective disorders: bipolar disorder type I, bipolar disorder type II, cyclothymia, bipolar disorder not otherwise specified, and recurrent unipolar depressive disorder. The diagnostic criteria for these disorders are based on observable symptoms and mood episodes. (B) Examples of an integrated biological systems approach to affective disorders. New neurobiologically defined classes of affective disorders could be based on biosignatures of dimensions of pathology expressed at genetic, molecular, neural circuitry, and behavioural levels.