Differential features between bipolar I and bipolar II disorder.

Although bipolar II disorder is generally viewed as a mild form of classic manic-depressive illness, recent investigations suggest that it could be a valid diagnostic category different from bipolar I in genetic, biological, clinical, and pharmacological aspects. Twenty-two patients fulfilling Research Diagnostic Criteria for the diagnosis of bipolar II disorder and 38 bipolar I patients were evaluated with the Schedule for Affective Disorders and Schizophrenia by two independent interviewers and compared. Bipolar II patients had significantly more previous episodes (P = .001), including both depressive (P = .003) and hypomanic (P = .006) switches, but had been hospitalized (P = .001) and presented psychotic symptoms (P < .001) less frequently. These results suggest that bipolar II disorder is less severe than bipolar I with regard to symptom intensity, but is more severe with respect to episode frequency.