Incidence and characteristics of bipolar disorder in middle-aged adults: A prospective population-based study

Background : Although bipolar disorder (BD) typically emerges in young adulthood, several studies have suggested that the onset of this disorder can occur later in life. However, there are hardly any studies that have established the incidence of BD in older ages and compared clinical features between later-onset and earlier-onset BD. Our study aimed to 1) assess the incidence rate of BD in a population-based prospective study of people older than 35 years, 2) clinically characterize these people with incident BD, and 3) compare their sociodemographic and clinical characteristics with those of people who had already reported lifetime BD at baseline. Methods : We included 3,709 participants from a population-based cohort study aged 35 to 75 years at the first psychiatric evaluation (mean age 51.4 years, 54.1% women) with at least two psychiatric evaluations. Those exempt from BD at baseline were followed-up (mean duration 11.3 years) to assess the incidence rate of BD. Diagnostic criteria for mental disorders were elicited according to the DSM-IV using the semi-structured Diagnostic Interview for Genetic Studies. Results : At baseline, 94 participants already met lifetime criteria for BD, whereas five developed BD during the follow-up, corresponding to an incidence rate of 12.2 per 100,000 person-years. Participants who developed BD during the follow-up had a substantially older age at the first episode compared to those who had already reported lifetime BD at the initial psychiatric evaluation (49.8 vs. 29.0 years, respectively). Those with incident BD also reported more frequent initial episodes with mixed symptoms ( p =0.003), a shorter duration of initial episodes ( p =0.005) and a higher prevalence of pre-existing or co-occurring illicit drug use disorders ( p =0.039) than those with pre-existing BD. Conclusions : Although our results support a later emergence of BD in middle-aged adults, they also suggest atypical first manifestations of this later disorder with a high proportion of mixed episodes and high comorbidity with drug use disorders. From a clinical point of view, our data highlight the necessity for a thorough screening for first manifestations of BD also in middle-aged people particularly in the presence of drug misuse, which may delay the earlier recognition of mood episodes.