After one of the largest ever investigations into the link between traumatic brain injury (TBI) and cognitive decline in later life, Danish and United States researchers concluded that the younger a person was when sustaining a head injury, the higher the risk of developing Alzheimer's and other forms of dementia.
Researchers found the "fully adjusted risk of all-cause dementia in people with a history of TBI was higher than in those without a history of TBI, as was the specific risk of Alzheimer's disease". "Our findings do not suggest that everyone who suffers a traumatic brain injury will go on to develop dementia in later life".
Dementia affects 47 million people worldwide, and that number is expected to double in the next 20 years, the researchers said. A study of 3.3 million people in Sweden earlier this year showed similar results. TBI occurs when an external force such as a bump or blow to the head disrupts the normal function of the brain. The leading causes include falls, motor vehicle accidents and assaults. Whether TBI among veterans and in contact sports such as football and boxing increases the risk of dementia has been hotly debated.
Responding to the study, Dr Mahmoud Maina, a research associate at the University of Sussex said: "The findings are truly novel due to the large sample size employed, in-depth history collected and follow-ups".
For their study, the researchers evaluated data from the Danish patient registry of almost three million people.
The risk of dementia increased with the number of TBIs and the severity of injury. But the risk increased significantly for people with multiple brain injuries, and for people who were in their 20s at the time for their first brain injury. From 1999 to 2013, 4.5 percent of the patients over age 50 years developed dementia, of those, 5.3 percent had sustained at least one TBI during the observation period, which began in 1977. Even a single mild T.B.I. increased the risk by 17 percent.
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The authors found that someone with one TBI had a risk of developing dementia after age 50 that was 22 per cent higher than someone with no diagnosed brain injury, this was 33 per cent higher with two TBIs or 200 per cent higher with five or more. "Greater efforts to prevent TBI and identify strategies to ameliorate the risk and impact of subsequent dementia are needed".
Among men and women with TBI histories, men had slightly higher rate of developing dementia (30 percent vs. 19 percent).
The association between TBI and dementia held true even when comparing people with a history of TBI to those with non-TBI fractures not involving the skull or spine.
The researchers note that the absolute risk remains low, but one must remain especially mindful nevertheless.
Fann said future research trying to narrow down why some people with brain injuries get dementia while other don't is important.