Incidence rates and risk factors of bipolar disorder in the general population: a population-based cohort study

To estimate the incidence rates (IRs) of bipolar I and bipolar II disorders in the general population according to sociodemographic population characteristics.

A cohort study (during the years 1996-2007) was conducted in a general practitioners research database with a longitudinal electronic record of 800000 patients throughout the Netherlands [the Integrated Primary Care Information (IPCI) database]. Cases of bipolar disorder were identified and classified by systematic review of medical records. Age- and gender-specific IRs were calculated per calendar year, degree of urbanization, and degree of deprivation.

The overall IR of bipolar disorder was 0.70/10000 person-years (PY) [95% confidence interval (CI): 0.57-0.83]; the IR of bipolar I disorder was 0.43/10000 PY (95% CI: 0.34-0.55) and the IR of bipolar II disorder was 0.19/10000 PY (95% CI: 0.13-0.27). Two peaks in the age at onset of the disorder were noticed: one in early adulthood (15-24 years; 0.68/10000 PY) and a larger peak in later life (45-54 years; 1.2/10000 PY). In bipolar II disorder, only one peak, in the 45-54 year age category (IR 0.42/10000 PY), was found. The IRs of bipolar disorder were significantly higher in deprived areas. Similar rates were found for men compared to women and in urban compared to rural areas. No association was found between the onset of first (hypo)manic episode and month or season of birth.

We found two peaks in the age at onset of bipolar disorder, one in early adulthood and one in later life, the former consisting mainly of bipolar I disorder subjects. The incidence of bipolar disorder is higher in deprived areas. The onset of bipolar disorder is not associated with gender, urbanization, or month or season of birth.