Several studies investigating bipolar disorders have shown that polarity of onset can predict differences in symptomatology, course, and prognosis. Frequently, however, research on the topic has examined only bipolar I inpatients and has not included patients with mixed onset. The aim of the present naturalistic study was to evaluate the clinical characteristics and illness course of a consecutive sample (407 outpatients, 58.7% with bipolar I (BD-I) and 41.3% with bipolar II (BD-II) disorder) according to polarity of onset: depressive (DP-o); manic/hypomanic (HM-o); or mixed--broadly defined to include agitated depression for BD-II--onset (MX-o). As compared with patients in the other two groups: a) DP-o patients (67.3%) were more frequently affected by BD-II and had lower ratings for psychotic symptoms; b) HM-o patients (17%) had a higher rate of family history for psychosis and a lower rate of suicide attempts; and c) patients in the MX-o group (15.7%) more frequently showed substance abuse and had a higher number of mixed recurrences per year. In the BD-II group, MX-o patients more frequently attempted suicide. The present study's main limitations are those of retrospective assessment of onset polarity and lack of treatment-impact evaluations over illness course. In conclusion, we confirm clinical expression differences in bipolar disorder in function of polarity of onset and underscore the importance of carefully considering broadly defined mixed state when examining polarity of onset. Further investigations are required to confirm the present study's results.
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