More severe brain trauma made the risk of contracting the disease later in life even more likely. Veterans with a moderate to severe traumatic brain injury saw an 83 percent increased risk.
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"This is not the first study to show that even mild traumatic brain injury increases the risk for Parkinson's disease. But we were able to study every single veteran across the U.S. who had been diagnosed at a Veterans Affairs hospital, so this is the highest level of evidence we have so far that this association is real," Dr. Raquel Gardner, the study's lead author, who works for the San Francisco Veterans Affairs (VA) Medical Center, told Reuters.
Kristine Yaffe, another author of the study from the University of California, San Francisco (UCSF) and the VA, said that most of the former soldiers who were diagnosed with Parkinson's actually got their head injuries during civilian life.
"While the participants had all served in the active military, many if not most of the traumatic brain injuries had been acquired during civilian life," she explained.
But overall, the number of veterans who were diagnosed with Parkinson's was quite small. Only one in 212 veterans who had experienced a concussion developed the disease. The rate was slightly higher, at one in 134 among those who reported a more serious moderate-to-severe traumatic brain injury.
Dr. Michael Silver, an assistant professor at Emory University's Department of Neurology who was not involved in the research, called the data "robust."
"This has been a controversial issue but most studies that have looked at this have found a correlation between traumatic brain injury (TBI) and the subsequent development of Parkinson's Disease. This is of course a difficult topic to study since if you would like to start with a cohort of patients that have suffered TBI, you have to wait and track a subject for years," Silver said.
"With this robust VA data, specifically the fact that the system reliably codes for TBI's, we are able to put the pieces together years later," he said.
Although Silver said the study was well done and controlled for many factors, he suggested a longer follow-up on patients would have made the research more helpful.
"I would have liked a longer follow-up on the subjects since the average age was only 48, and the usual age of onset for Parkinson's Disease is in the sixties," he said. "This is an intriguing study and as we gather more data going forward, can make more conclusive links between TBI and Parkinson's disease."
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According to the Centers for Disease Control and Prevention, Parkinson's is the most common neurodegenerative disease after Alzheimer's. Risk of the disease increases with age, from about 1 percent at age 60 to around 4 percent at 80.
Silver said that as of now, doctors don't have a way of intervening to prevent Parkinson's. He said that he recommends a "healthy diet and exercise" to patients who have experienced head trauma, as previous studies suggest this could reduce the risk of dementia (which Parkinson's can lead to).
The authors of the study have similar advice for individuals concerned about developing Parkinson's later in life. Gardner told CNN that a healthy diet, regular exercise and keeping medical conditions under control are the best ways to avoid any neurodegenerative disease.
"If anyone is worried, do a little bit better to live more healthily," she said.
Read the new study at n.neurology.org.