A new study of hundreds of thousands of U.S. armed forces veterans concludes that banging your head severely enough to lose consciousness can dramatically increase the risk of Parkinson's, the brain disease marked by tremors, slow movements, balance problems and difficulty walking.
While the overall numbers are relatively small, former members of the military who had received a concussion at some point in their lives were 56 percent more likely to develop Parkinson's than veterans who had not been knocked out, who had not experienced an altered state of consciousness or who had not had amnesia for as long as 24 hours.
When the brain trauma was more severe, the risk was even higher.
"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," lead author Dr. Raquel Gardner of the San Francisco VA Medical Center told Reuters Health.
The importance of the discovery goes beyond veterans, the research team writes in the journal Neurology.
Each year roughly 42 million people worldwide experience a concussion, also known as mild traumatic brain injury. Such injuries are especially common among athletes, in the military and constitute "a growing epidemic among the elderly," the authors note.
"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," senior study author Kristine Yaffe of the University of California, San Francisco (UCSF) and the VA said in a news release. "As such, we believe it has important implications for the general population."
The findings mean that "we need to dig in and figure out what the mechanisms are," Gardner said in a telephone interview.
The researchers could not tell from their database analyzes whether cases of Parkinson's disease related to head trauma are somehow different from other cases of the disease. That's in part because the cause of Parkinson's, which afflicts more than a million people in North America, usually after age 60, remains a mystery.
The 325,870 veterans included in the study, ranging in age from 31 to 65, were typically followed for 4.6 years but the head trauma often occurred long before that, and it is not known if the brain injury accelerated the process, said Gardner, who is also affiliated with UCSF.
But she stressed that while the increased risk seems high - it was 83 percent higher for veterans who had experienced more serious head injury - the actual number of cases is relatively small.
Only 1 in 212 veterans who had experienced a concussion was found to have Parkinson's. The rate was 1 in 134 among people who reported a more serious moderate-to-severe traumatic brain injury at some point in their lives.
"Even in our study, the vast overwhelming majority of veterans with a traumatic brain injury did not get Parkinson's," Gardner emphasized. "The risk is just a little higher among those who had TBI. It's not a huge number of people because Parkinson's doesn't affect a huge number of people. That's why it took a big study like this to undercover this association."
"While on an individual level the actual amount of risk is pretty low, on a population level this could translate into a lot of people being affected" with Parkinson's disease, Dr. Gardner said.
People who had experienced head injury tended to be diagnosed with Parkinson's two years earlier than people whose brains had not been injured, the study also found.
A recent tally by the Department of Defense (bit.ly/1BfUZbS) estimated that nearly 380,000 people on active military duty have been diagnosed with a traumatic brain injury in the past 18 years; 82 percent of those servicemen and women experienced a mild concussion.
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