Increasing age of multiple sclerosis onset from 1920 to 2022: a population-based study.

Objective: To study the age at onset of relapsing-remitting multiple sclerosis (RRMS) during the past century.

Methods: This is a population-based cohort study of persons diagnosed with RRMS in Hordaland, Møre, and Romsdal counties, Western Norway, from 1920 to 2022. Individual patient data were available and assessed from previously conducted prevalence and incidence studies in addition to hospital records up until October 31, 2022. Participants were categorized according to onset period and analyzed for temporal trends in age at onset, time from onset to diagnosis, and distribution of onset over time.

Results: We identified 3364 persons with confirmed RRMS. The mean age at onset significantly increased (p < 0.001) throughout the study period, despite a decrease in time from symptom onset to diagnosis (p < 0.001). The proportion of persons with MS onset after 50 years of age increased from 2.6% before 1970 to 11.9% after 2010. We also found a trend toward a bimodal distribution of age at onset that peaked at around 30 years and 40-45 years of age in the latest period.

Conclusion: Age at onset of MS significantly increased throughout the study period. This was mainly due to an increasing number of persons with MS, predominantly female, experiencing onset after 40-45 years of age. This bimodal distribution could indicate different susceptibility periods of MS or changes in exposure to risk factors during the observation period.