As the US population continues to age, the number of people living with Parkinson disease (PD) continues to grow and is expected to double between 2010 and 2040.1 Alarming findings recently published by Rodolfo Savica and colleagues indicate that this growth could be underestimated.2
The study, published online in JAMA Neurology last month, revealed a significant increase in the incidence of PD and parkinsonism over 30 years, a trend largely driven by men 70 years of age and older. “These trends may be associated with the dramatic changes in smoking behavior that took place in the second half of the 20th century or with other lifestyle or environmental changes,” the authors speculated.
The authors identified cases of parkinsonism diagnosed in Olmsted County, Minnesota, between 1976 and 2005. In all identified cases, diagnosis was validated through the review of the full medical records by a movement disorder specialist, which strengthens the findings and constitutes an important advantage of this study over most comparable studies.
Of 906 patients with parkinsonism included in the analysis, roughly half (n = 464) had PD. Over the 30-year study period, the incidence rates increased significantly in all men. Men with PD were at a higher relative risk (relative risk [RR], 1.24 per decade; 95% confidence interval [CI], 1.08-1.43) than those with parkinsonism (RR, 1.17 per decade; 95% CI, 1.03-1.33). This increase was attributed to men 70 and older: in this cohort, the incidence rates were increased for both PD (RR, 1.35 per decade; 95% CI, 1.10-1.65) and parkinsonism (RR, 1.24 per decade; 95% CI, 1.07-1.44). In contrast, the incidence rates did not increase in men younger than 70 or in women of all ages.
These findings lend additional support to the protective effect of smoking in PD. In the past several decades, the prevalence of smoking declined substantially for men. For women, the prevalence of smoking has never been as high and hence declined less. This may explain the lack of noticeable increase in PD incidence rate in women. Of other environmental and lifestyle factors that changed dramatically during the past several decades, pesticide use, coffee consumption, and head trauma may be implicated as well.
In addition to secular trends, trends by birth cohort have also been analyzed. The incidence rates were the highest in men born between 1915 and 1924. The risk was also increased, albeit not significantly, in women in this birth cohort. The prenatal and early-life environmental factors that affected this cohort, eg, exposure to influenza virus, could have increased the incidence rate in this cohort.
Possible pitfalls of the study include changes in diagnostic criteria, coding practices, and authorization to use individual medical records in research. In addition, the small size of the study population prohibited the analysis of the trends in less common subtypes of parkinsonism, such as multiple system atrophy.
Despite the limitations, the study’s great methodological precision and unique advantages of the studied population lend additional validity to the findings. The results of this study contrast with previously published data, which show either no change or decrease in the incidence rate of PD. If confirmed, the present findings will have major public health implications.
1. Kowal SL, et al. The current and projected economic burden of Parkinson's disease in the United States. Mov Disord. 2013;28(3):311-318.
2. Savica R, et al. Time trends in the incidence of Parkinson disease. JAMA Neurol. 2016 June 20. [Epub ahead of print]