Bipolar disorder is a serious disorder of mood characterized by recurrent episodes of depression alternating with periods of hypomania and/or mania that are usually separated by periods of relatively normal mood and functioning. It is a heterogeneous diagnosis that probably encompasses a broad range of cyclical mood disorders that may differ somewhat in terms of presentation, etiology, and pathogenesis. A diagnosis of bipolar disorder is made on the basis of evidence of both acute major depressive episodes and acute manic episodes (hypomania is considered a milder form of mania that is not necessarily associated with functional impairment). In the Diagnostic and Statistical Manual, 4th edition (DSM-IV: American Psychiatric Association, 1994), bipolar disorder type 1 (BP-I) is distinguished from bipolar disorder type 2 (BP-II) by severity of the manic episode. BP-I is characterized by manic presentations leading to significant social or occupational impairment (sometimes requiring admission to hospital or with psychotic symptoms); BP-II is characterized by hypomanic episodes, which are by definition not associated with social/occupational impairment. Although most epidemiological studies have identified that BP-I affects around 1% of the adult population, more recent work suggests that up to a further 4% may be affected by bipolar spectrum disorders such as BP-II and cyclothymia. BP-I affects an equal proportion of males and females but BP-II (as with recurrent major depressive disorder) appears to be more common in females. Most people with bipolar disorder date the onset of their illness to adolescence and usually experience depression as their first mood episode.
Bipolar disorder is highly morbid. It has been ranked sixth in a list of the top 10 causes of medical disability worldwide for individuals between the ages of 15 and 44. For affected individuals and their families, it causes considerable social and occupational impairment and is thought to end in the suicide of 20% of the most severely affected. Previous clinical impressions of complete recovery between mood episodes have been challenged by evidence to suggest that most bipolar individuals continue to suffer from subsyndromal symptoms (mostly depressive in nature) between frank mood disturbance episodes that can have a considerable impact on their everyday functioning. Psychiatric comorbidity is extremely common in bipolar disorder. At least 60% of bipolar patients have an additional Axis-I diagnosis, the most common of which is an alcohol or drug misuse disorder.