A new study of nearly 200,000 older military veterans has found that those with traumatic brain injuries are 60% more likely to develop Alzheimer's disease or other forms of dementia.
The study, published online Wednesday in the journal Neurology, adds to a growing body of evidence that traumatic brain injuries, or TBIs, can have a wide range of consequences long after a patient appears to recover.
But the study did not answer the key question of whether the risk of dementia is elevated in cases of mild brain trauma — a common injury from roadside bombs in Afghanistan and Iraq as well as in football and other contact sports.
"It's still controversial," said Donald Stein, a brain injury expert at Emory University School of Medicine who was not involved in the study.
The study was based on medical records of 188,764 veterans ages 55 and older who had been patients in the Veterans Affairs health system from 2000 to 2003 and did not have a diagnosis of dementia during that time.
Of those, 1,229 had a TBI diagnosis.
The researchers used medical records from 2003 to 2012 to compare the fates of the veterans with TBIs to the much larger group without brain injuries.
Among the veterans with TBIs, 16% went on to develop various types of dementia. That figure was 10% in the comparison group.
Dementia also tended to develop sooner in the TBI patients — two years earlier, on average.
A statistical analysis ruled out the possibility that a history of depression, post-traumatic stress disorder and diseases of the blood vessels supplying the brain were responsible for the higher rates of dementia, said Deborah Barnes, an epidemiologist at UC San Francisco who led the study.
Still, it was not clear that brain injuries were the cause of the dementia. It is possible that other unexplored variables — genetic predisposition or alcohol abuse, for example — could be the drivers.
Although the study looked at veterans, their injuries weren't necessarily related to military service, and experts said the results could just as easily apply to the civilian world.
"These injuries are very similar to what you would find in industrial accidents, automobile accidents and sports," Stein said.
The researchers did not classify the brain injuries by severity or determine when or how they occurred, leaving open the question of whether mild TBIs — which involve relatively brief periods of disorientation and sometimes loss of consciousness — are associated with increased risk of dementia.
John Corrigan, a brain injury expert at Ohio State University, said scientific evidence of a link is stronger for moderate and severe TBIs, which often involve penetrating wounds, skull fractures and extended black-outs.
There is also growing evidence that repeated mild brain injuries can have a variety of long-term effects, including impaired cognitive ability and an increased risk of dying from neurodegenerative diseases.
"What the field is coming to is that TBI is a chronic condition," Corrigan said.