Adults who reported ever having had a head injury and who were exposed to the herbicide paraquat had nearly a three-fold increased risk of developing Parkinson's disease, according to a survey published Monday in the journal Neurology.
Researchers at UCLA's Fielding School of Public Health surveyed more than 1,000 adults ages 35 and older who lived in central California. Some 357 of the participants were diagnosed with Parkinson's disease, a movement disorder characterized by tremors and loss of coordination.
Participants with the disease were nearly twice as likely as those without the disease to report having had a head injury in which they lost consciousness for more than five minutes.
Using a geographical tracking system, the researchers also found that those with Parkinson's disease were also more likely to live within 500 meters of a spot where paraquat was used. Paraquat is a chemical liquid commonly used to kill plants and weeds.
"While each of these two factors is associated with an increased risk of Parkinson's on their own, the combination is associated with greater risk than just adding the two factors together," Dr. Beate Ritz, lead author of the study, said in a public statement.
The trauma from the brain injury may leave brain cells more vulnerable to the exposure of the toxic pesticide, Ritz said.
While there are no definitive causes for Parkinson's disease, the study is one of many to suggest that environmental factors, not just genetic variations, may be likely triggers in some cases, many experts said.
"This demonstrates the importance of considering multiple risk factors in combination when assessing a person's risk of [Parkinson's disease]," said Dr. David Simon, associate professor of neurology at Beth Israel Deaconess Medical Center in Boston.
The study also adds to growing data on the harmful effects of pesticide exposure.
"Based on the current study, this recommendation to avoid heavy pesticide exposure may be particularly important for people who have a history of significant head trauma with loss of consciousness for more than 5 minutes, as they may be particularly susceptible to the subsequent effects of pesticide exposure," said Simon.
But besides recommending that adults avoid brain injury and exposure to chemicals -- which doctors would do anyhow -- "it would be inappropriate for clinicians to do anything with this information, other than be aware," said Dr. David Cifu, chairman of the department of physical medicine and rehabilitation at the Virginia Commonwealth University School of Medicine in Richmond.
The study is not strong enough to suggest that pesticides or even a traumatic brain injury can cause Parkinson's disease, only that in this group, some kind of link can be made between the three.
"There are likely hundreds of such linkages that may be found in any data set," said Cifu.
However, many experts said the findings warrant further study, since more definitive identification of triggers can lead to tailored treatments.
"Understanding the critical role of environmental and genetic factors will help us to gauge better, and more directed treatments for this disease," said Dr. Michael Okun, medical director of the National Parkinson Foundation.