Objective: Whether bipolar II disorder(BP-II) is simply a milder form of bipolar I disorder(BP-I) or a distinct diagnostic category is still under controversy. Investigations exploring differential features of the two conditions are needed. This study aimed to obtain a comprehensive view of differences in clinical characteristics between BP-I and BP-II.

Methods: 71 BP-I, 34 BP-II patients were assessed using Diagnostic interview for Genetic Studies(DIGS), Korean version. Demographic data, age at onset, number of (hypo)manic/depressive episode, duration of illness, polarity at onset, seasonality, rapid cycling, atypical depression, symptom profiles of episode, and comorbidity of other psychiatric disorders and conditions were evaluated.

Results: BP-I and BP-II had similar demographic characteristics and ages of onset. The frequency of the depressive episode after illness onset was higher in BP-II(U=429.50, p<0.0001). Seasonality and rapid cycling course were more common in BP-II. Among symptoms of depressive phase, leaden paralysis, psychomotor agitation, guilty feeling and suicidal idea were more frequently observed in BP-II. BP-II patients showed higher axis I comorbidity, especially with phobia and eating disorder. Suicidal behavior was also more frequently reported in BP-II.

Conclusion: BP-I and BP-II showed different clinical characteristics. BP-II seems to be more severe with regard to the course of mood episodes and psychiatric comorbidity. This result provides additional evidence supporting the distinction of two groups as separate diagnostic categories that might have somewhat different biological basis.