THURSDAY, Jan. 3 (HealthDay News) -- Having a traumatic brain injury at some time in your life doesn't raise the risk of dementia in old age, but it does increase the odds of re-injury, a new study finds.

"There is a lot of fear among people who have sustained a brain injury that they are going to have these horrible outcomes when they get older," said senior author Kristen Dams-O'Connor, assistant professor of rehabilitation medicine at the Icahn School of Medicine at Mount Sinai Medical Center in New York City.

"It's not true," she said. "But we did find a risk for re-injury."

The 16-year study of more than 4,000 older adults also found that a recent traumatic brain injury with unconsciousness raised the odds of death from any cause in subsequent years.

Those at greatest risk for re-injury were people who had their brain injury after age 55, Dams-O'Connor said. "This suggests that there are some age-related biological vulnerabilities that come into play in terms of re-injury risk," she said.

Dams-O'Connor said doctors need to look out for health issues among older patients who have had a traumatic brain injury. These patients should try to avoid another head injury by watching their balance and taking care of their overall health, she said.

To investigate the consequences of a traumatic brain injury in older adults, the researchers collected data on participants in the Adult Changes in Thought study, conducted in the Seattle area between 1994 and 2010. The participants' average age was 75.

At the start of the study, which was published recently in the Journal of Neurology, Neurosurgery & Psychiatry, none of the participants suffered from dementia. Over 16 years of follow-up, the researchers found that those who had suffered a traumatic brain injury with loss of consciousness at any time in their lives did not increase their risk for developing Alzheimer's or other forms of dementia.

The risk of another traumatic brain injury, however, more than doubled if the first injury occurred before age 25 and almost quadrupled if the injury happened after age 55. Similarly, a recent traumatic brain injury more than doubled the odds of death from any cause, the study found.

Dams-O'Connor's group plans to look at risk factors to try to understand why some people have poor long-term prognosis after a brain injury.

One expert said genetics may play a role. "My guess is that the risk for post-traumatic-brain-injury Alzheimer's disease has a genetic component with some genes increasing risk and others offering protection," said Dr. Sam Gandy, associate director of the Mount Sinai Alzheimer's Disease Research Center in New York City.

These findings should not be confused with those regarding athletes who suffer brain injuries, Gandy said.

"The dramatic examples of former [National Football League] players, hockey players and wrestlers who have an unusual illness, marked by depression, agitation and psychosis are quite different from Alzheimer's disease patients who tend to be apathetic," he said.

"Much remains to be discovered about the role of lifelong traumatic brain injury history, including severity and nature of torque and other physical factors, and late-life mental decline," Gandy said.

Another expert, Dr. Danny Liang, a neurosurgeon at North Shore-LIJ Cushing Neuroscience Institute in Manhasset, N.Y., thinks these findings are too narrow to say much about the risk of dementia as a result of traumatic brain injury.

"The study is restricted to a limited population so it's hard to extrapolate these findings to other populations," he said. "It is also possible that there were people who had traumatic brain injury who did develop dementia before age 65, so they were not included in the study," Liang said.

There also was no data on injury severity or duration of unconsciousness, he said. Brain injuries differ, and knowing the severity is important to determine the ultimate outcome, he said.

More information

For more information on traumatic brain injury, visit the U.S. Centers for Disease Control and Prevention.