Drivers of Rising Prevalence in Major Motor Neurodegenerative Diseases: Temporal Trends in Sweden and France (2003-2022).

Abstract (English)

BACKGROUND AND OBJECTIVES: The prevalence of Parkinson disease (PD), multiple sclerosis (MS), and motor neuron diseases (MNDs) is rising globally. However, it is unclear to what degree this is related to an increase in incidence or to improved survival after diagnosis. METHODS: We performed 2 nationwide, population-based, retrospective cohort studies, including all individuals living in Sweden between 2001 and 2016 and living in France between 2009 and 2022, respectively. Pooled mixed-effects regression models, with country as a random effect, were used to determine temporal trends in prevalence, crude and age-standardized and sex-standardized incidence, and age and life expectancy at diagnosis. RESULTS: Annualized prevalence of PD, MS, and MNDs increased significantly between 2003 and 2022 in the pooled model (PD: prevalence ratio [PR] per year = 1.014, <i>p</i> < 0.001; MS: PR = 1.029, <i>p</i> < 0.001; MND: PR = 1.028, <i>p</i> < 0.001). While the crude incidence of both PD and MS remained nearly stable over time (PD: incidence rate ratio [IRR] per year = 0.998, <i>p</i> < 0.001; MS: IRR = 0.992, <i>p</i> < 0.001), the standardized incidence showed a more marked decrease for PD (IRR = 0.986, <i>p</i> < 0.001) while remaining almost unchanged for MS (IRR = 0.995, <i>p</i> < 0.001). For MNDs, both the crude and standardized incidence increased over time (crude IRR = 1.018, <i>p</i> < 0.001; standardized IRR = 1.008, <i>p</i> < 0.001). Life expectancy at diagnosis of PD increased between 2003 and 2013 (+0.95 months per calendar year, <i>p</i> < 0.001) and then decreased between 2013 and 2022 (-1.20 months, <i>p</i> = 0.002), while it increased significantly over the entire study period for MS (+2.35 months, <i>p</i> < 0.001) and MNDs (+0.34 months, <i>p</i> = 0.01). DISCUSSION: These findings indicate that the rising MS prevalence is largely survival-driven and the rising MND prevalence reflects a true increase in incidence, whereas PD prevalence grows modestly, largely independent of incidence. Depending on the mechanism that drives prevalence, whether increased incidence reflecting changing risk factor exposures, improved survival due to therapeutic advances, or demographic aging of the population, inferences about underlying causes differ substantially between PD, MS, and MNDs, with direct implications for health care planning and etiologic research.