Differences between bipolar I and bipolar II disorders in clinical features, comorbidity, and family history.

Background: The present study was designed to investigate whether bipolar II disorder (BP-II) has different characteristics from bipolar I disorder (BP-I), not only in manic severity but also in clinical features, prior course, comorbidity, and family history, sufficiently enough to provide its nosological separation from BP-I.

Methods: Comprehensive clinical evaluation was performed based on information available from ordinary clinical settings. Seventy-one BP-I and 34 BP-II patients were assessed using the Diagnostic Interview for Genetic Studies, Korean version. Psychiatric assessment for first-degree relatives (n=374) of the probands was performed using the modified version of the Family History-Research Diagnostic Criteria.

Results: The frequency of depressive episodes was higher in BP-II (p=0.009) compared to BP-I. Further, seasonality (p=0.035) and rapid-cycling course (p=0.062) were more common in BP-II. Regarding manic expression, 'elated mood' was predominant in BP-II whereas 'elated mood' and 'irritable mood' were equally prevalent in BP-I. With regard to depressive symptoms, psychomotor agitation, guilty feeling, and suicidal ideation were more frequently observed in BP-II. BP-II patients exhibited a higher trend of lifetime co-occurrence of an axis I diagnosis (p=0.09), and a significantly higher incidence of phobia and eating disorder. The overall occurrence rate of psychiatric illness in first-degree relatives was 15.4% in BP-I and 26.5% in BP-II (p=0.01). Major depression (p=0.005) and substance-related disorder (p=0.051) were more prevalent in relatives of BP-II probands.

Conclusion: Distinctive characteristics of BP-II were identified in the current study and could be adopted to facilitate the differential diagnosis of BP-I and BP-II in ordinary clinical settings.