Seasonal variation of mixed and pure episodes of bipolar disorder

Background: Although seasonal patterns of manic episodes have been reported, the seasonal variation of mixed states of bipolar disorder has received little attention. In the current report we address that concern as well as the overall seasonality of manic episodes. Methods: The seasonal pattern of 304 psychiatric hospital admissions for treatment of mixed or manic bipolar episodes over a 3-year period were analyzed employing two definitions of mixed manic states: DSM-III-R and an ROC derived definition. Results: The frequency of all manic episodes combined peaked in early spring, with a nadir in late fall. Pure manic admissions showed a similar pattern. Mixed manic admissions had a significantly different pattern, with a peak in late summer and a nadir in November. The differences between pure and mixed manic admissions were demonstrated with the use of the ROC definition for mixed states. Limitations: Effects of medications and medication non-compliance may dampen natural seasonal patterns of episodes. Conclusions: The different seasonal pattern of mixed and pure manic episodes support the separation of mixed episodes as a distinct clinical subtype.