Seasonality and its distinct clinical correlates in bipolar II disorder

Seasonality is one of the key features in subjects with mood disorders and is involved in the multi-faceted nature of the clinical course. However, few studies have explored the clinical implications of seasonality in bipolar disorders. We examined the differential effects of seasonality on clinical variables between bipolar I and II disorder (BD I and II). Seasonality was assessed using the Seasonal Pattern Assessment Questionnaire (SPAQ) in 204 subjects with BD I and 308 with BD II. Following the comparisons between BD I and II groups, clinical characteristics related to seasonality were explored. Next, to predict the presence of seasonality, a logistic regression model was applied. The global seasonality score on the SPAQ was significantly higher in the BD II group than in the BD I group. In the BD I group, seasonality was associated with suicide attempt history. In the BD II group, on the other hand, seasonality was associated with female gender, depressive predominance, and premenstrual dysphoric disorder (PMDD). In the regression models, the presence of PMDD and female gender was significantly associated with seasonality in the BD II group. Our findings suggest that high seasonality tendency, a vulnerability maker for cyclic worsening, may contribute to a differential pattern of clinical characteristics in BD II.