Multiple sclerosis in Denmark (1950-2023): mean age, sex distribution, incidence and prevalence.

With rising life expectancy and advancements in disease management, we expect the multiple sclerosis population is getting older. However, evidence supporting this hypothesis remains sparse. Our study aimed to determine whether the mean age of the Danish multiple sclerosis population has increased and to analyse the developments in sex distribution, incidence, and prevalence, all of which affect age composition. We conducted a cohort study by linking nationwide data from the Danish Multiple Sclerosis Registry to the Population Statistics Registry, the Danish Cause of Death Registry, and the Historical Migration Registry. We included all living patients with a confirmed multiple sclerosis diagnosis who lived in Denmark on the 1st of January each year from 1950 to 2023. We calculated the mean and median age, age distribution, sex distribution, incidence, and prevalence of the Danish MS population annually from 1950 to 2023. We included 28,145 individuals with multiple sclerosis. The mean age of the Danish multiple sclerosis population increased until the late 1970s to around 52.5 years, where it stabilised until 1990. The mean age experienced a slight decline to 51.2 years in 2005, followed by a subsequent rise to its peak of 54.2 years in 2023. In 1975, females comprised 58.7% of the multiple sclerosis population, increasing to 65.7% by 2000 and 68.5% in 2023. The incidence of multiple sclerosis remained stable at around 3.5 per 100,000 until 1975 and steadily increased by more than 2.5 times in 2000 to 11.4 per 100,000. Despite fluctuations, it remained relatively stable from 2000 until 2022, showing a slight decrease in 2022 compared to the previous two decades. Both overall and sex-specific prevalence exhibited an upward trend, particularly among females. Our study demonstrates that the mean age of the Danish multiple sclerosis population has increased, although not as decisively as expected. The female proportion has grown in tandem with prevalence, while incidence appears to have stabilised in recent decades after years of increase. Denmark's robust registry data and universal healthcare system offer a unique opportunity for reliable epidemiological analysis. Our results establish a benchmark for future demographic studies in the field of multiple sclerosis.