Mortality and secular trend in the incidence of bipolar disorder.

Background: The world-wide interest in bipolar disorder is illustrated by an exponential increase in publications on the disorder registered in Pubmed since 1990. This inspired an investigation of the epidemiology of bipolar disorder.

Methods: This was a register-based cohort study. All first-ever diagnoses of bipolar disorder (International Classification of Diseases-10: F31) were identified in the nationwide Danish Psychiatric Central Research Register between 1995 and 2012. Causes of death were obtained from The Danish Register of Causes of Death. Age- and gender standardized incidence rates, standardized mortality ratio (SMR) and Kaplan-Meier survival estimates were calculated.

Results: We identified 15,334 incident cases of bipolar disorder. The incidence rate increased from 18.5/100,000 person-years (PY) in 1995 to 28.4/100,000 PY in 2012. The mean age at time of diagnosis decreased significantly from 54.5 years in 1995 to 42.4 years in 2012 (p<0.001). The mean time from first affective diagnosis to diagnosis of bipolar disorder was 7.9 years (SD 9.1). The SMR was 1.7 (95%-CI 1.2-2.1). Causes of death were mainly natural; 9% died from suicide.

Limitations: Only patients in psychiatric care were included. The outpatient registry opened in 1995. Patients treated solely in outpatient units are not recorded previously. Systematic studies validating all the clinical diagnoses of the registry do not exist.

Conclusions: The incidence of bipolar disorder has increased in the last 10 years. The SMR was significantly increased. Half of the patients were known to have another affective disorder. This should be considered in future decisions regarding the healthcare organization.