Dorsey and colleagues estimated that the number of persons with Parkinson's disease (PD) in the UnitedStates was approximately 340,000 in 2005 and projected that number to reach approximately 610,000 in 2030 as a result of population aging.1 These projections were made under the simple assumption that the age-and sexspecific prevalence would remain stable over 25 years. However, the burden of diseases may be modified over time by human practices, including public health interventions and medicine.2 Rossi and colleagues have suggested that the successful implementation of smoking cessation interventions in the United States may lead to an increase in the projected number of persons who will suffer from PD by 2040.3 Their revised projections were based on a projected 46.2% increase in the population 50 years old and a 59.8% reduction in the percentage of smokers 50 years old between 2000 and 2040. They also made the fundamental assumption that smoking has a preventive effect on PD. Based on their assumptions and on the relatively conservative prevalence estimates published by Dorsey et al for 2005, 1 they projected for 2040 a total of 700,000 patients with PD because of population aging and an additional 70,000 patients (10% increase) because of the reduced frequency of smoking (total of 770,000 patients). Overall, they projected a 56% increase in the number of patients between 2005 and 2040. Rossi and colleagues argued that these revised projections are important to adequately plan for future cost and care needs related to PD.
3Whether or not smoking has a preventive effect on the risk of PD, the number of patients with PD is likely to increase by 2040 above and beyond the simple effect of population aging. There are now 3 studies from 3 continents showing an increase in the risk of PD over time (from the United States, Finland, and Taiwan).4-6 However, there are also 3 studies showing a possible decline in risk over time (from the United Kingdom, Taiwan, and the Netherlands). [7][8][9] Assuming that all 6 studies have adequate methods (no major biases) and comparable study design, these discrepant findings may suggest that the risk factors for PD vary across countries, by sex, and over time. Our study in Olmsted County, Minnesota, showed an approximately 50% increase in the incidence of PD in men and women combined over 30 years ; relative risk increase). The increase was more sizable in men age 70 years (approximate relative risk increase of 100%), and there was a significantly higher risk for men born between 1915 and 1924 (birth cohort effect).