"Both stroke and traumatic brain injury are common, costly, and leading causes of severe disability in adults, and approximately 20 percent of strokes occur in adults under age 65," said study author James F. Burke, MD, MS, of the University of Michigan and the Ann Arbor VA Healthcare System and a member of the American Academy of Neurology. "A large proportion of stroke risk is unexplained, especially in the young, so if we can identify new risk factors, we have the potential to prevent more strokes and improve outcomes."

For the study, researchers looked at the records of adults who went to the emergency department or were admitted to a hospital for TBI or other trauma with no brain injury in the state of California during a five-year period.

A total of 435,630 people with traumatic brain injury were studied, along with 736,723 people with trauma with no brain injury. Over an average of 28 months following the injury, 11,229 people, or 1 percent, had an ischemic stroke. A total of 1.1 percent of those with TBI suffered a stroke, compared to 0.9 percent of those with trauma with no brain injury. With an ischemic stroke, blood flow to part of the brain is blocked. Eighty percent of strokes are ischemic.

After adjusting for factors that can affect stroke risk, such as age, high blood pressure and high cholesterol, as well as other disorders such as heart disease and the severity of the trauma, the researchers found that people with traumatic brain injury were 30 percent more likely to develop a stroke than those with trauma with no brain injury.

"While the stroke risk of one person with TBI is small, the overall link between TBI and stroke was substantial -- as large as the link between the strongest stroke risk factor, high blood pressure, and stroke," Burke said. "If further research establishes TBI as a new risk factor for stroke, that would stimulate research to help us understand what causes stroke after TBI and help us learn how to prevent these strokes." The study was supported by an advanced fellowship through the Department of Veterans Affairs.