Head injury, pesticides tied to Parkinson's disease
By Genevra Pittman
NEW YORK | Tue Nov 13, 2012 3:54pm EST
(Reuters Health) - The combination of a past serious head injury and pesticide exposure may be linked to an extra-high risk of developing Parkinson's disease, a new study suggests.
The findings don't prove being knocked unconscious or exposed to certain chemicals directly causes Parkinson's, a chronic movement and coordination disorder.
But they are in line with previous studies, which have linked head trauma and certain toxins - along with family history and other environmental exposures - to the disease.
"I think all of us are beginning to realize that there's not one smoking gun that causes Parkinson's disease," said Dr. James Bower, a neurologist from the Mayo Clinic in Rochester, Minnesota who wasn't involved in the new research.
"There might be many paths to the ultimate development of Parkinson's disease," he told Reuters Health.
For example, Bower said, some people who are genetically predisposed might need just one "environmental insult" - such as a blow to the head - to set them up for Parkinson's. Others who aren't naturally susceptible to the disorder could still develop it after multiple exposures.
Head trauma and contact with pesticides "may not be directly related, and may be two independent stresses," Columbia University neurologist David Sulzer, who also wasn't part of the study team, told Reuters Health in an email.
About 50,000 to 60,000 older adults in the U.S. are diagnosed with Parkinson's disease each year, according to the National Parkinson Foundation.
For the new study, researchers led by Pei-Chen Lee from the University of California at Los Angeles compared 357 people with a recent Parkinson's diagnosis to a representative sample of 754 people without the disease, all living in central California, which is a major agricultural region.
The study team asked all of them to report any past traumatic head injuries - in which people had been unconscious for at least five minutes - and used their home and work addresses to determine their proximity to pesticide sprayings since 1974.
Those surveys showed that close to 12 percent of people with Parkinson's had been knocked unconscious, and 47 percent had been exposed to an herbicide called paraquat near both their home and workplace.
That's in comparison to almost seven percent of control-group participants with a history of head injury and 39 percent with pesticide exposure.
On their own, traumatic brain injury as well as living and working near pesticide sprayings were each tied to a moderately increased risk of Parkinson's disease. Combined, they were linked to a tripling of that risk, the researchers reported Monday in the journal Neurology. That was after taking into account people's baseline risk based on their age, gender, race, education, smoking history and family history of Parkinson's.
Lee's team didn't know which came first in people who'd had both head trauma and paraquat exposure.
It makes sense, the researchers noted, that a head injury would increase inflammation in the brain and disrupt the barrier that separates circulating blood and brain fluid. Those changes could then make neurons in the brain more vulnerable to the effects of pesticides, ultimately increasing the risk of Parkinson's.
But that's just a theory.
"There are all kinds of hypotheses," Bower said. But the study "is more evidence that traumatic injury to the brain can lead to later problems that are usually neurodegenerative," he added. "We need to be increasingly careful about preventing these traumatic brain injuries."
SOURCE: bit.ly/TD3OA9 Neurology, online November 12, 2012.
Traumatic brain injury, paraquat exposure, and their relationship to Parkinson disease
Pei-Chen Lee, PhD, Yvette Bordelon, MD, PhD, Jeff Bronstein, MD, PhD and Beate Ritz, MD, PhD
Neurology November 13, 2012 vol. 79 no. 20 2061-2066
From the Department of Epidemiology, Fielding School of Public Health (P.-C.L., B.R.), and Department of Neurology, School of Medicine (Y.B., J.B., B.R.), University of California at Los Angeles, Los Angeles.
Correspondence & reprint requests to Dr. Lee: firstname.lastname@example.org
View Complete Disclosures
Objectives: Traumatic brain injury (TBI) increased risk of Parkinson disease (PD) in many but not all epidemiologic studies, giving rise to speculations about modifying factors. A recent animal study suggested that the combination of TBI with subthreshold paraquat exposure increases dopaminergic neurodegeneration. The objective of our study was to investigate PD risk due to both TBI and paraquat exposure in humans.
Methods: From 2001 to 2011, we enrolled 357 incident idiopathic PD cases and 754 population controls in central California. Study participants were asked to report all head injuries with loss of consciousness for >5 minutes. Paraquat exposure was assessed via a validated geographic information system (GIS) based on records of pesticide applications to agricultural crops in California since 1974. This GIS tool assesses ambient pesticide exposure within 500 m of residences and workplaces.
Results: In logistic regression analyses, we observed a 2-fold increase in risk of PD for subjects who reported a TBI (adjusted odds ratio [AOR] 2.00, 95% confidence interval [CI] 1.28?3.14) and a weaker association for paraquat exposures (AOR 1.36, 95% CI 1.02?1.81). However, the risk of developing PD was 3-fold higher (AOR 3.01, 95% CI 1.51?6.01) in study participants with a TBI and exposure to paraquat than those exposed to neither risk factor.
Conclusions: While TBI and paraquat exposure each increase the risk of PD moderately, exposure to both factors almost tripled PD risk. These environmental factors seem to act together to increase PD risk in a more than additive manner.
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