Unraveling the Decline: Search for Factors Behind the Trend in Parkinsons Disease Incidence Rates: A Cohort Study of German Health Claims Data

Sporadic Parkinson's disease (PD) is the second most common neurodegenerative disease in older age. Recent studies have led to a debate regarding trends in incidence and prevalence. We aimed to investigate age-specific incidence rates of PD and possible explanations for the observed trend in Germany by studying two randomly selected longitudinal cohorts, each consisting of 250,000 individuals aged 50 and above. We started observing them at the beginning of 2004 and 2014, and followed them through the end of 2009 and 2019, respectively. We compared age-specific incidence rates for both cohorts and performed Cox regression models to calculate the hazard ratios (HR) of PD in the second period compared with the first period ten years earlier, adjusted for age, sex, and several prodromal factors, comorbidities, and risk factors of PD. For each age-group in men and women, we found lower age-specific PD incidence rates in the second period, except for the lowest age-group (50-54) in men. Cox regression analysis demonstrated an overall 20% risk reduction of PD incidence (HR=0.80, 95% confidence interval [0.75-0.86]). Mean age at diagnosis increased in men (+1.98 years) and women (+0,87 years). Stepwise adjustment for prodromal symptoms, comorbidities, and risk factors revealed a 22%-49% risk reduction of PD incidence. Sensitivity analysis considering the competing event of death showed a 48% risk reduction (HR=0.52 [0.48-0.56]), demonstrating the independence of the time trends from changes in death rates. In conclusion, our data show that the risk of PD has decreased over time, and that this decrease is independent of factors such as changes in death rates, age structure, sex, motor and sensory impairments, sleep and psychiatric conditions, comorbidities, and specific risk factors.