Time trends of incidence and prevalence of severe mental illness in Danish adults: a nationwide register study

PURPOSE: Severe mental illness (SMI) accounts for over a third of all mental disorders globally, significantly reducing life expectancy and quality of life. Trends in SMI vary, with recent Danish studies reporting both stable and rising rates in the 2000s. This study examines the prevalence (1996-2018) and incidence (2000-2018) of SMI in Denmark, analyzing age- and sex-specific trends using nationwide health registry data. MATERIAL AND METHODS: The study included individuals aged ≥18 years with a psychiatric hospital diagnosis of SMI, recorded in the Danish National Patient Registry (1995-2018). SMI was defined as moderate to severe depression, bipolar disorder, or psychotic disorder based on ICD-10 classifications. We estimated five-year prevalence (2000-2018) and one-year incidence (1996-2018), stratified by age and sex. RESULTS: Between 2000 and 2018, the crude prevalence of depression more than doubled, while bipolar and psychotic disorders increased by 89.6% and 35.0%, respectively. All SMIs increased among individuals aged 18-29. From 1996 to 2018, depression and bipolar disorder incidence rose by 137.5% and 45.1%, respectively, while psychotic disorder incidence declined by 9.5%. The largest incidence increase occurred in the 18-29 age group. Bipolar and psychotic disorder prevalence declined in those ≥70 years. Depression was the most prevalent disorder among individuals ≥90 years. CONCLUSION: SMI prevalence and incidence in Denmark have risen over the past two decades, particularly among younger adults. However, professional, cultural, administrative, and societal factors must be considered before concluding an actual increase in SMI cases.