Despite a high risk of brain injury, military personnel rarely develop chronic traumatic encephalopathy (CTE), a disabling condition often found in former boxers and football players.
Fewer than 5% of 225 brains from deceased service members showed evidence of CTE, a team reports in the June 9 issue of The New England Journal of Medicine.
In contrast, a 2017 study of brains from deceased college and NFL football players found that 87% had signs of CTE.
Even service members who had experienced concussions from bomb blasts were unlikely to develop CTE. Just 6.7 percent of the brains from 45 people exposed to blasts were diagnosed with the condition.
The results suggest that "serving in the military and being exposed to blast is probably not a significant risk factor for developing CTE," says Dr. Daniel Perl, a professor of pathology at the Uniformed Services University in Bethesda and one of the study's authors.
CTE can only be diagnosed after a person has died. During an autopsy, a pathologist looks for areas of the brain that have high concentrations of a toxic form of the protein tau.
The condition is associated with dementia, mood problems, and a range of psychiatric disorders.
The symptoms of CTE overlap those seen in military personnel exposed to bomb blasts.
So some doctors have worried that CTE might be partly responsible for the high rates of suicide and post traumatic stress disorder (PTSD) among veterans who served in Iraq and Afghanistan.
"Their families were saying that their personalities had changed, that they were having trouble sleeping," Perl says.
To see whether CTE was a factor, researchers turned to the Brain Tissue Repository, which is operated by the Department of Defense and the Uniformed Services University under Perl's direction.
"We said this was the opportunity to look at those brains and see how much CTE played a role in this problem," Perl says.
The team found that of the 10 brains with CTE, only three came from service members who had been exposed to bomb blasts.
"Then we found that all ten had played contact sports," Perl says.
The results add to the evidence that bomb blasts and sports impacts affect the brain in different ways.
Brain injuries in football or boxing are caused by an impact that pushes the brain against the skull. In a bomb blast, a pressure wave passes through brain tissue, causing it to stretch and deform.
"The physics are different," Perl says. "And apparently the pathology that comes from it is different."
But impacts and blasts can both do lasting damage, Perl says.
"One shouldn't go away thinking that because we didn't find CTE, the brains are normal," he says. "That's clearly not the case."
Also, most of the brains in this study came from relatively young people, Perl says. So it's possible more of them would have gone on to develop CTE as they got older.
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