The study, reported Monday at the Alzheimer's Association International Conference in France, challenges the current view that only moderate or severe brain injuries predispose people to dementia.
"Even a concussion or a mild brain injury can put you at risk," said Laurie Ryan, a neuropsychiatrist who used to work at Walter Reed Army Medical Center and now oversees Alzheimer's grants at the U.S. National Institute on Aging.
The study was led by Dr. Kristine Yaffe, a University of California professor and director of the Memory Disorders Clinic at the San Francisco VA Medical Center. The U.S. Department of Defense and the National Institutes of Health paid for the work.
"It's by far the largest" study of brain injury and dementia risk, she said. "It's never been looked at in veterans specifically."
Researchers reviewed medical records on 281,540 veterans who got care from Veterans Health Administration hospitals from 1997 to 2000 and had at least one follow-up visit from 2001-2007. All were at least 55 and none had been diagnosed with dementia when the study began. This older group was chosen because dementia grows more common with age, and researchers needed enough cases to compare those with and without brain injuries.
Records showed that nearly 5,000 of the veterans had suffered a traumatic brain injury, or TBI, ranging from concussions to skull fractures. Researchers don't know how long ago the injuries occurred. Many participants were Vietnam War vets and their injuries were during active duty. None were due to strokes -- those cases were weeded out.
Over the next seven years, more than 15 percent of those who had suffered a brain injury were diagnosed with dementia versus only 7 percent of the others -- a more than doubled risk. Severity of the injury made no difference in the odds of developing dementia.
"It's not just one kind of TBI or super-severe TBI" that poses a danger, Yaffe said.
That worries Ryan Lamke, 26, a medically retired Marine who lives in suburban Washington, D.C. He suffered a traumatic brain injury from multiple blast exposures in 2005 in Iraq. "I'm diagnosed as a moderate (brain injury) but it feels like a mild," said Lamke, who relies on electronic calendars and other gadgets to stay organized. He's a student at Georgetown University and works part-time as a government relations intern for a private firm.
"I have to read for twice as long as my classmates" to accomplish what's needed, he said. "I've not found a doctor so far who can give me a true understanding of what's going to happen 20 or 30 years down the road."
Troops will need close monitoring in the years ahead and treatment for post-traumatic stress, depression and other conditions that can lead to cognitive problems, experts said.
"While we don't want people frightened to think they're going to be permanently impaired, a mild traumatic brain injury does not necessarily mean" no long-term problems, said Dr. Gregory O'Shanick, a psychiatrist and chairman of the board of the advocacy group Brain Injury Association of America.
