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Brain Injury Plus Common Weed Killer May Triple Parkinsonís Risk

Certain lifestyle factors, such as smoking, exposure to pesticides, and head injuries, can influence a person’s risk of developing Parkinson’s disease (PD).  Now researchers have shown that the combination of two of these factors, traumatic brain injury plus exposure to the weed killer paraquat, may triple a person’s risk of PD.  The study, published in the November 12, 2012 issue of Neurology, suggests that environmental factors can work together to trigger PD.

Previous studies have linked exposure to paraquat, one of the world’s most widely used herbicides, to PD.  Likewise, people with traumatic brain injuries appear to have an increased risk of PD.  But scientists still do not understand why some people with these risk factors develop PD, while most others do not.  One possible explanation is that PD develops only when people experience more than one risk factor, for example, paraquat exposure plus a head injury.

To test this so-called “multiple hit” hypothesis of Parkinson’s, Pei-Chen Lee, Ph.D., and her colleagues at the University of California at Los Angeles asked 357 people newly diagnosed with PD and 754 healthy controls to report any head injuries in which they had lost consciousness for more than five minutes.  These people lived in California’s Central Valley, an agricultural region with substantial pesticide use.  Paraquat applied to crops could have drifted to nearby homes and businesses, exposing people to low levels of the pesticide.

It is very difficult to assess a person’s environmental exposure in a lifetime.  The researchers used a computer program to estimate how much paraquat people were exposed to at their homes and workplaces.  The program used existing records that detailed pesticide use since 1974 on Central Valley crops.  The researchers then examined whether people who had head injuries and paraquat exposure were more likely to have PD than those with neither or only one risk factor.


  • People who had a traumatic brain injury were nearly twice as likely to develop PD as those with no brain injury.
  • People who were exposed to paraquat were slightly more likely (about 1.4 times more likely) to develop PD than those with no exposure.
  • People who had both a brain injury and paraquat exposure were three times more likely to develop PD than those with neither risk factor.

What Does It Mean?

PD is a complex disease.  It is likely that for different people, different risk factors contribute to the development of Parkinson’s disease.  It is also very likely that in most cases, a person needs more than one risk factor to develop PD.  Heart disease provides an example. Some diabetics, but not all, develop heart disease.  Some people with elevated cholesterol, but not all, develop heart disease.  The combination of diabetes and elevated cholesterol increases the risk for heart disease significantly.

This study provides evidence for the “multiple hit” hypothesis of PD — in other words, people may need to have more than one risk factor to develop PD.  These risk factors may be genetic (for example, a mutation in a PD-related gene) or environmental (such as a head injury or paraquat exposure).  This hypothesis explains why only a fraction of people with a particular risk factor, go on to develop PD.

The researchers here made a tremendous effort to obtain accurate information. A possible limitation of this study is that the researchers relied on study volunteers to report their head injuries.  Some people may have remembered incorrectly whether or not they had a head injury that caused them to lose consciousness for more than five minutes.  Also, the computer program used to estimate people’s paraquat exposure relied on geographical information, pesticide application records, and estimates of pesticide drift, which may not have always been accurate.

The researchers do not know exactly how head injury and paraquat might cooperate to produce PD.  However, scientists have shown that in rats, traumatic brain injuries kill the same dopamine-producing neurons that are lost in PD.  Treating rats with a small dose of paraquat a few days after the brain injury kills even more of these neurons.  If these animal studies hold true in people, it is possible that the combination of head injury with paraquat could harm enough brain cells to trigger PD.  Further research is needed to better understand these risk factors.

Reference: Pei-Chen Lee, Yvette Bordelon, Jeff Bronstein, and Beate Ritz. (2012). Traumatic brain injury, paraquat exposure, and their relationship to Parkinson disease. Neurology, 79(20), 2061–2066. doi:10.1212/WNL.0b013e3182749f28



Source Date: Nov 30 2012