Mood disorders are among the most common diagnoses in psychiatry. Mood is a persistent emotional state (as differentiated from affect, which is the external display of feelings). There are three major categories of mood disorders according to the Diagnostic and Statistical Manual of Mental Disorders, 4th edition: unipolar mood disorders (major depressive disorder, dysthymic disorder), bipolar mood disorders (bipolar I disorder, bipolar II disorder, and cyclothymic disorder), and mood disorders having a known etiology (substance-induced mood disorder and mood disorder due to a general medical condition) (Table 2-1).

The best available evidence suggests that mood disorders lie on a continuum with normal mood. Although mania and depression are often viewed as opposite ends of the mood spectrum, they can occur simultaneously in a single individual within a brief period, giving rise to the concept of mixed mood states.

Mood disorders are characterized by a disturbance in the regulation of mood, behavior, and affect. Mood disorders are subdivided into (1) depressive disorders, (2) bipolar disorders, and (3) depression in association with medical illness or alcohol and substance abuse. Depressive disorders are differentiated from bipolar disorders by the absence of a manic or hypomanic episode. The relationship between pure depressive syndromes and bipolar disorders is not well understood; depression is more frequent in families of bipolar individuals, but the reverse is not true. In the Global Burden of Disease Study conducted by the World Health Organization, unipolar major depression ranked fourth among all diseases in terms of disability-adjusted life years and was projected to rank second by year 2020. In the United States, lost productivity directly related to depression has been estimated at $44 billion per year.