A blow to the head can damage a person’s chances of staying cognitively healthy in the future, confirms one of the largest studies to ever examine this issue. After poring over medical records for nearly 190,000 military veterans, researchers from the University of California, San Francisco, report in the June 25 Neurology online that traumatic brain injury (TBI) led to a 60 percent spike in the odds of subsequently developing dementia.
Many previous studies have linked head injury to increased risk for dementia, as well as Parkinson’s disease and amyotrophic lateral sclerosis (Mortimer et al., 1991; Bower et al., 2003; Chen et al., 2007). However, results are mixed, noted first author Deborah Barnes (see Jan 2014 news story). "Our study adds to the weight of evidence suggesting that TBI is associated with an increased risk of developing dementia,” she wrote in an email to Alzforum. The study results certainly apply to civilian TBIs, given the similarities between those and military head injuries, Brenda Plassman of Duke University in Durham, North Carolina, commented in an email to Alzforum (see full comment below).
Alzforum covered related results, from a similar but separate cohort, presented by senior author Kristine Yaffe in 2011 (see Aug 2011 news story). In the current work, Barnes and colleagues studied records from 188,764 people, aged 55 or older, who had been treated in the Veterans Health Affairs system between 2000 and 2013. Of those, 1,229 had a TBI diagnosis during the first three years, the study’s baseline period. The records offer little detail; the brain injury could have occurred during military service or thereafter, and could have resulted from a single incident or multiple mild traumas.
During the follow-up period, 10 percent of those who reported no TBI developed some kind of dementia, including Alzheimer’s, vascular dementia, or dementia with Lewy bodies, while 16 percent of the TBI cases did. “The authors convincingly showed that mild trauma has a role in increasing the risk of dementia,” wrote Rodolfo Savica of the University of Utah, Salt Lake City, in a Neurology editorial. The risk was highest in those with TBI plus depression, post-traumatic stress disorder, or cerebrovascular disease. That suggests people who had TBIs can still try to stave off dementia by dealing with those mental or physical issues, Barnes wrote.
How does physical damage translate into future dementia? Barnes suggested that the trauma might be one of multiple factors that deplete a person’s “cognitive reserves.” Alternatively, TBI could precipitate buildup of Aβ or tau (see Jan 2014 news story). Other possibilities include disrupted axon transport or damage to the blood-brain barrier, suggested Plassman, who was not involved in the study.
Brain imaging, such as for tau, and biomarkers in blood or cerebrospinal fluid should help scientists understand the process, Sam Gandy of the Icahn School of Medicine at Mount Sinai in New York and Steven DeKosky of the University of Pittsburgh Medical School commented in an email to Alzforum. Genetics and epigenetics may also contribute to the link between head trauma and dementia risk, Savica suggested.
It will be important to understand the relative risks of different activities, Gandy and DeKosky wrote. In war, head injuries from explosives are common, but contact sports, falls, car accidents, and other trauma can also be bad for the brain. Barnes next plans to investigate the specific types of TBI and their severity with respect to dementia risk.—Amber Dance
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