(Reuters Health) - People who have a mild traumatic brain injury - even without loss of consciousness - may be more than twice as likely to develop dementia than individuals who don’t have these injuries, a study of U.S. veterans suggests.

While severe and even moderate brain injuries have long been linked to an increased risk of dementia, research to date on the link between milder brain injuries and cognitive decline has been mixed, particularly for cases when patients didn’t lose consciousness.

For the current study, researchers examined data on almost 358,000 veterans, about 90 percent male. Half of them were diagnosed with brain injuries between 2001 and 2014, and half were a control group of individuals who were similar to the injured veterans but without a history of brain injury.

Overall, 10,835, or 6 percent, of the veterans with brain injuries were diagnosed with dementia during the study, compared with 4,698, or less than 3 percent, of those without past brain injuries.

Compared to veterans without brain injuries, participants with a mild traumatic brain injury who didn’t lose consciousness were 2.4 times more likely develop dementia, and when they had lost consciousness, their odds of a dementia diagnosis were 2.5 times higher. Moderate to severe brain injuries carried an almost quadrupled dementia risk.

“This data suggests that there is a higher incidence of dementia after a head injury of any severity,” said Dr. Jack Tsao, a neurology researcher at the University of Tennessee Health Science Center and Memphis Veterans Affairs Medical Center.

“However, much remains unknown about risks for why these individuals developed dementia,” Tsao, who wasn’t involved in the study, said by email.

When they joined the study, participants were 49 years old on average.

A dementia diagnosis typically occurred about 1.5 years sooner in people with brain injuries than people without brain injuries, Dr. Kristine Yaffe of the University of California, San Francisco, and colleagues note in JAMA Neurology.

Head injuries were tied to an increased risk of dementia even after accounting for medical and psychiatric conditions that might independently make people more likely to experience cognitive decline.

However, the study wasn’t a controlled experiment designed to prove whether or how brain injuries cause or accelerate the development of dementia. It’s also possible that medical records used in the analysis didn’t reflect mild or early stages of dementia in some participants, the authors note.

Veterans are also exposed to more brain injuries than other people, both because of combat and training activities as well as a tendency to participate in more sports and leisure activities that can lead to head injuries, said Dr. Ramon Diaz-Arrastia, a neurology researcher at the University of Pennsylvania in Philadelphia and director of the Penn Clinical TBI Initiative.

“The severity of the mild TBIs and the number of mild TBIs that veterans are exposed to is higher than in the general civilian population,” Diaz-Arrastia, author of an accompanying editorial, said by email.

“It is not uncommon for military veterans to report 3, 5 or even 10 or more mild traumatic brain injury exposures,” Diaz-Arrastia added. “That is more than in civilian populations (at least those who do not play contact sports professionally) and clearly the total number of exposures matters.”

For soldiers and civilians alike, getting proper treatment and taking precautions to avoid multiple hits to the head are crucial, Tsao said.

“There is unfortunately no way to minimize dementia risk after a traumatic brain injury,” Tsao said. “However, with a concussion or mild TBI, rest - cognitive and physical - as well as avoiding re-injury are keys to immediate clinical recovery.”

SOURCE: bit.ly/2FXSf6y and bit.ly/2ruFE6w JAMA Neurology, online May 7, 2018.