Clinical information from other informants, such as close friends or relatives, is often use- ful in establishing the diagnosis of bipolar II disorder.
A hypomanic episode should not be confused with the several days of euthyrnia and re- stored energy or activity that may follow remission of a major depressive episode. Despite the substantial differences in duration and severity between a manic and hypomanic episode, bi- polar II disorder is not a ”milder form" of bipolar I disorder. Compared with individuals with bipolar I disorder, individuals with bipolar II disorder have greater chronicity of illness and spend, on average, more time in the depressive phase of their illness, which can be severe and / or disabling. Depressive symptoms co-occurring with a hypomanic episode or hypomanic symptoms co-occurring with a depressive episode are common in individuals with bipolar H disorder and are overrepresented in females, particularly hypomania with mixed features. In- dividuals experiencing hypomania with mixed features may not label their symptoms as hy- pomania, but instead experience them as depression with increased energy or irritability.
A common feature of bipolar II disorder is impulsivity, which can contribute to suicide at- tempts and substance use disorders. Impulsivity may also stem from a concurrent person- ality disorder, substance use disorder, anxiety disorder, another mental disorder, or a medical condition. There may be heightened levels of creativity in some individuals with a bipolar disorder. However, that relationship may be nonlinear; that is, greater lifetime creative accomplishments have been associated with milder forms of bipolar disorder, and higher creativity has been found in unaffected family members. The individual’s attach- ment to heightened creativity during hypomanic episodes may contribute to ambivalence about seeking treatment or undermine adherence to treatment.
The 12-month prevalence of bipolar II disorder, internationally, is 0.3%. In the United
States, 12-month prevalence is 0.8%. The prevalence rate of pediatric bipolar II disorder is difficult to establish. DSM-IV bipolar I, bipolar II, and bipolar disorder not otherwise spec- ified yield a combined prevalence rate of 1.8% in US. and non-U.S. community samples, with higher rates (2.7% inclusive) in youths age 12 years or older.
Although bipolar II disorder can begin in late adolescence and throughout adulthood, av- erage age at onset is the mid-205, which is slightly later than for bipolar I disorder but ear- lier than for major depressive disorder. The illness most often begins with a depressive episode and is not recognized as bipolar II disorder until a hypomanic episode occurs; this happens in about 12% of individuals with the initial diagnosis of major depressive disor- der. Anxiety, substance use, or eating disorders may also precede the diagnosis, compli- cating its detection. Many individuals experience several episodes of major depression prior to the first recognized hypomanic episode.
The number of lifetime episodes (both hypomanic and major depressive episodes) tends to be higher for bipolar II disorder than for major depressive disorder or bipolar I disorder. However, individuals with bipolar I disorder are actually more likely to experi- ence hypomanic symptoms than are individuals with bipolar II disorder.The interval between mood episodes in the course of bipolar II disorder tends to decrease as the indi- vidual ages. While the hypomanic episode is the feature that defines bipolar II disorder, depressive episodes are more enduring and disabling over time. Despite the predomi- nance of depression, once a hypomanic episode has occurred, the diagnosis becomes bi- polar II disorder and never reverts to major depressive disorder.