In the largest study of its kind, researchers have found that traumatic brain injury is associated with an increased risk of dementia. The risk of dementia was highest among people who had suffered multiple T.B.I.s. But even a single mild T.B.I. was tied to an increased risk of dementia.
T.B.I. has a wide range of severity. It extends from a mild sports concussion — an elbow to the head in a basketball game, for example — that results in very brief or no unconsciousness and no structural harm to the brain, to the most severe brain injuries that can cause extended unconsciousness, coma or even prove fatal.
The study, in Lancet Psychiatry, used Danish health databases that included all residents as of Jan. 1, 1995, who were at least 50 years old at some time during the 36-year follow-up, from 1977 to 2013. Among 2,794,852 people, they found 132,093 who had had at least one T.B.I.
After adjusting for medical, neurological and psychiatric illnesses, they found that compared with people who had never had a T.B.I., those who had had any were at a 24 percent increased risk for dementia, and those who had had five or more had nearly triple the risk. Even a single mild T.B.I. increased the risk by 17 percent. For their first T.B.I. diagnosis, 85 percent were this mild type.
Still, the lead author, Dr. Jesse R. Fann, a professor of psychiatry at the University of Washington School of Medicine, said that the absolute risk of getting dementia as young as 50 is quite low.
“I don’t want people to think that just because you’ve had a head injury you’re bound to get dementia,” he said.
An earlier version of this article misstated the number of traumatic brain injuries found in the study. It was 132,093, not 258,827.