Trends in the Incidence of Parkinson Disease

Abstract

__To the Editor__ We read with interest the article by Savica et al, who described an increase in the incidence rate of parkinsonism in the Rochester Epidemiology Project between 1976 and 2005. This finding contradicts our recently reported observation that the incidence rate of parkinsonism was lower in a subcohort of the Rotterdam Study that was followed up between 2000 and 2011 compared with a subcohort that was followed up between 1990 and 2000.2 Similarly, a UK primary care study previously reported a significant decline in Parkinson disease (PD) incidence rates between 1999 and 2009. Savica et al1 hypothesized that the changes observed in the Rochester Epidemiology Project could be attributed to a decrease in the prevalence of smoking in the second half of the 20th century, but they were unable to test this hypothesis in their cohort. Within the Rotterdam Study, we assessed smoking habits at baseline of both subcohorts (1990 and 2000). As expected, we observed that the ageand sex-adjusted prevalence of current smoking was lower across all ages (55-106 years) in the subcohort that started in 2000. During follow-up, incident parkinsonism was diagnosed in 182 of 6752 persons in the subcohort that started in 1990, and in 28 of 2440 persons in the subcohort that started in 2000. The age- and sex-adjusted incidence rate (IR) of parkinsonism for smokers was similar in both subcohorts (IR, 0.63; 95% CI,0.43-0.91 in the 1990 subcohort; IR, 0.61; 95% CI, 0.27-1.44 in the 2000 subcohort). The age- and sex-adjusted incidence rate ratio (IRR) for parkinsonism of persons in the 2000 subcohort vs the 1990 subcohort was 0.55 (95% CI, 0.36-0.81). After additional adjustment for smoking status, the IRR remained virtually unchanged (IRR, 0.57; 95% CI, 0.37-0.84). Unfortunately, the small number of PD cases in the 2000 subcohort prevented PD-specific analyses on the effect of smoking. We conclude that it is unlikely that the decline in smoking prevalence drove a change in the incidence of parkinsonism in the Rotterdam Study. The discrepant findings of the study by Savica et al compared with previous studies, including the Rotterdam Study, highlight the lack of insight on causality of risk factors for parkinsonism and PD. For smoking in particular, causality of its inverse association with the risk for parkinsonism and PD remains highly contentious, and the inference that the increase in parkinsonism incidence in the Rochester Epidemiology Project can be attributed to a decline in smoking may shift focus from other putative etiological factors. To better understand factors that drive differential trends in the incidence of parkinsonism across populations, there is an urgent need for cross-cohort collaboration, similar to recently initiated efforts for dementia.