Distinct seasonality of depressive episodes differentiates unipolar depressive patients with and without depressive mixed states

Background: The bipolar nature of unipolar depression with depressive mixed sates (DMX) needs further validation studies. The seasonality of depressive episodes is indicated to be different between unipolar and bipolar depressions. We therefore explored the seasonal pattern of depressive episodes in unipolar depressive patients with DMX. Methods: The subjects were 958 consecutive depressive inpatients for a 6-year period. For defining DMX, previously validated operational criteria were used (2 or more of 8 manic or mania-related symptoms: flight of idea, logorrhea, aggression, excessive social contact, increased drive, irritability, racing thoughts, and distractibility). Onsets of the index depressive episodes during each of the 12 calendar months were summed up over the 6-year for bipolar depressive patients (N= 95), and unipolar depressive patients with (N= 77) and without DMX (N= 786) separately. An appropriate statistic was used for testing seasonality. Results: A significant seasonal variation with a large peak in spring was recognized in unipolar depression without DMX, while both bipolar depression and unipolar depression with DMX had a significant fall peak. The monthly distribution of depressive episodes was significantly different between unipolar depression without DMX and other 2 diagnostic categories. Similar results were obtained in separate analyses for each gender. Limitations: Further replication study using an epidemiological or outpatient sample is needed. Bipolar I and II patients were combined due to a small number of bipolar II patients in this sample. Conclusion: Unipolar depression with DMX has a seasonal pattern similar to bipolar depression. The finding provides further evidence of the bipolar nature of unipolar depression with DMX.