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Even Veterans With mTBI More at Risk for Dementia

by U.S. Medicine

April 12, 2011

WASHINGTON, DC—Since an Institute of Medicine report in 2008, there has been significant consensus that penetrating and severe traumatic brain injury (TBI) increases the risk for dementia later in life. The evidence was not as strong linking mild TBI (mTBI) to dementia. More research was needed.

VA researchers found themselves in a unique position to answer that call, with access to medical data for a patient population that was both older than the general population and had a higher rate of TBI.

“We talk a lot about TBI in young veterans, but less so in older veterans,” said Deborah Barnes, PhD, an epidemiologist working at the San Francisco VA, and a member of the dementia epidemiology research group at the University of California, San Francisco. “Our objectives were to examine the prevalence of TBI diagnoses in older veterans and determine whether those diagnoses are associated with increased risk for dementia.”

Confirming the Link

Barnes, in describing her research at the annual VA Health Services R&D conference, said the study looked at 281,540 veterans over age 55. All of them had at least one baseline visit for some form of care, not necessarily for TBI, between 1997 and 2000 and one follow-up between 2001 and 2007. The study excluded anyone with dementia prior to follow-up.

“We looked at TBI diagnosis during baseline and dementia diagnosis during follow-up. We also looked at medical and psychiatric comorbidities,” Barnes said. “We were interested in a dementia diagnosis at least two years after the TBI diagnosis.”

The researchers found that risk of dementia was approximately double for veterans with TBI (7% compared to 15%). For those patients who had mTBI, the risk was slightly less (12%), but still greater than the 7% non-TBI average.

“We were restricted to ICD-9 diagnoses, and we did not have diagnoses outside of VA,” Barnes said. “Our most important limitation is that the date of the TBI was not known. We don’t know if these happened during combat, or a car accident, or a fall that happened a year ago.”

Possible Mechanisms

Regardless, Barnes said the research indicates a definitive link between TBI and dementia and described several potential mechanisms for this. The most likely is diffuse axonal injury: damage over a widespread area of the brain that is a common occurrence with head trauma, she said.

Another possibility is that the injury causes swelling of axons and an accumulation of amyloid-beta. Amyloid-Beta plaques were found in 30% of TBI patients who died due to their injuries. Some researchers believe that amyloid-Beta leads to axonal degeneration over the long-term.

While there is also a possibility that neuronal loss may lead to earlier symptoms of dementia. Barnes suggested that diffuse axonal injury is the likely culprit. “I think the clinical implications of this mean that clinicians should consider monitoring older patients who have a TBI diagnosis for onset of dementia symptoms.”

Barnes pointed out that TBI could exacerbate other ongoing pathologies that could lead to an earlier diagnosis of dementia. The research showed that those veterans with TBI had a higher rate of comorbid conditions, such as diabetes, hypertension, substance abuse, and depression.

“I think there’s a lot more that we can do with the data,” she said. “I’d like to drill down a little bit and see how far back the diagnosis goes in the medical record to better determine what the timing was.”

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