"Our results dispute the notion that TBI is a one-time event with a stagnant outcome after a short period of recovery," said study author Benjamin L. Brett, PhD, of the Medical College of Wisconsin in Milwaukee. "Rather, people with TBI continue to show improvement and decline across a range of areas including their ability to function and their thinking skills."

The study involved people at 18 level 1 trauma center hospitals with an average age of 41. A total of 917 people had mild TBI and 193 people had moderate to severe TBI. They were matched to 154 people with orthopedic injuries but no head injuries. Participants were followed for up to seven years.

Participants took three tests on thinking, memory, mental health and ability to function with daily activities annually from two to seven years post-injury. They also completed an interview on their abilities and symptoms, including headache, fatigue, and sleep disturbances.

When researchers looked at all test scores combined, 21% of people with mild TBI experienced decline, compared to 26% of people with moderate to severe TBI and 15% of people with orthopedic injuries with no head injury.

Among the three tests, researchers saw the most decline over the years in the ability to function with daily activities. On average, over the course of 2 to 7 years post-injury, a total of 29% of those with mild TBI declined in their abilities and 23% of those with moderate to severe TBI.

Yet some people showed improvement in the same area, with 22% of those with mild TBI improving over time and 36% of those with moderate to severe TBI.

"These findings point out the need to recognize TBI as a chronic condition in order to establish adequate care that supports the evolving needs of people with this condition," Brett said. "This type of care should place a greater emphasis on helping people who have shown improvement continue to improve and implementing greater levels of support for those who have shown decline."

A limitation of the study was that all participants were seen at a level 1 trauma center hospital within 24 hours of their injury, so the findings may not apply to other populations.

The study was funded by the National Institute of Neurological Disorders and Stroke, National Institute on Aging, the National Football League Scientific Advisory Board and the U.S. Department of Defense.