Mississippi–Ohio River Valley Linked to Higher Risk of Parkinson's Disease

Medicare beneficiaries living in the Mississippi–Ohio River valley had a higher risk of developing Parkinson's disease, compared with other regions of the United States, according to findings from a study that was released ahead of its scheduled presentation at the annual meeting of the American Academy of Neurology.

The association was attributed to concentrations of particulate matter (PM) _2.5 in the Mississippi–Ohio River valley, which was on average higher than in other areas, but that didn't entirely explain the increase in Parkinson's disease in that region, Brittany Krzyzanowski, PhD, a postdoctoral research fellow in the neuroepidemiology research program of the department of neurology at Barrow Neurological Institute, Dignity Health St. Joseph's Hospital and Medical Center, Phoenix, said in an interview.

"This study revealed Parkinson's disease hot spots in the Mississippi–Ohio River valley, a region that has some of the highest levels of air pollution in the nation," she said, "but we also still find a relationship between air pollution and Parkinson's risk in the regions in the western half of the United States where Parkinson's disease and air pollution levels are relatively low."

Dr. Krzyzanowski and colleagues evaluated 22,546,965 Medicare beneficiaries in 2009, using a multimethod approach that included geospatial analytical techniques to categorize their exposure to PM_2.5 based on age, sex, race, smoking status, and health care usage. The researchers also performed individual-level case-control analysis to assess PM_2.5 results at the county level. The Medicare beneficiaries were grouped according to average exposure, with the lowest group having an average annual exposure of 5 mcg/m³ and the group with the highest exposure having an average annual exposure of 19 mcg/m³.

In total, researchers identified 83,674 Medicare beneficiaries with incident Parkinson's disease, with 434 new cases per 100,000 people in the highest exposure group, compared with 359 new cases per 100,000 people in the lowest-exposure group. The relative risk for Parkinson's disease increased in the highest quartile of PM_2.5 by 25%, compared with the lowest quartile after adjusting for factors such as age, smoking status, and health care usage (95% confidence interval, 20%–29%).

The results showed the nationwide average annual PM_2.5 was associated with incident Parkinson's disease, and the Rocky Mountain region carried a strong association between PM_2.5 and Parkson's disease with a 16% increase in risk per level of exposure to PM_2.5. While the Mississippi-Ohio River valley was also associated with Parkinson's disease, there was a weaker association between PM_2.5 and Parkinson's disease, which the researchers attributed to a "ceiling effect" of PM_2.5 between approximately 12-19 mcg/m³.

Dr. Krzyzanowski said that use of a large-population-based dataset and high-resolution location data were major strengths of the study. "Having this level of information leaves less room for uncertainty in our measures and analyses," she said. "Our study also leveraged innovative geographic information systems which allowed us to refine local patterns of disease by using population behavior and demographic information (such as smoking and age) to ensure that we could provide the most accurate map representation available to date."

A focus on air pollution

Existing research in examining the etiology of Alzheimer's mainly focused on exposure to pesticides, Dr. Krzyzanowski explained, and "consists of studies using relatively small populations and low-resolution air pollution data." Genetics is another possible cause, she noted, but only explains some Parkinson's disease cases.

"Our work suggests that we should also be looking at air pollution as a contributor in the development of Parkinson's disease," she said.

Ray Dorsey, MD, professor of neurology at the University of Rochester (N.Y.), who was not involved with the study, said that evidence is mounting that "air pollution may be an important causal factor in Parkinson's and especially Alzheimer's disease."

"This study by a well-regarded group of researchers adds epidemiological evidence for that association," he said. Another strength is that the study was conducted in the United States, as many epidemiological studies evaluating air pollution and Parkinson's disease have been performed outside the country because of "a dearth of reliable data sources."

"This study, along with others, suggest that some of the important environmental toxicants tied to brain disease may be inhaled," Dr. Dorsey said. "The nose may be the front door to the brain."

Dr. Krzyzanowski said the next step in their research is further examination of different types of air pollution. "Air pollution contains a variety of toxic components which vary from region to region. Understanding the different components in air pollution and how they interact with climate, temperature, and topography could help explain the regional differences we observed."

One potential limitation in the study is a lag between air pollution exposure and development of Parkinson's disease, Dr. Dorsey noted.

"Here, it looks like (but I am not certain) that the investigators looked at current air pollution levels and new cases of Parkinson's. Ideally, for incident cases of Parkinson's disease, we would want to know historical data on exposure to air pollution," he said.

Future studies should include prospective evaluation of adults as well as babies and children who have been exposed to both high and low levels of air pollution. That kind of study "would be incredibly valuable for determining the role of an important environmental toxicant in many brain diseases, including stroke, Alzheimer's, and Parkinson's," he said.

Dr. Krzyzanowski and Dr. Dorsey reported no relevant financial disclosures. This study was supported by grants from the Department of Defense, the National Institute of Environmental Health Sciences, and The Michael J. Fox Foundation for Parkinson's Research.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

Lead Image: Alessandro Grandini/Dreamstime
Image 1: Brittany Krzyzanowski/MDedge
Image 2: Ray Dorsey/MDedge

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Cite this: Mississippi–Ohio River Valley Linked to Higher Risk of Parkinson's Disease - Medscape - Apr 12, 2023.