A team from the University of Washington has found that the risk of dementia is significantly higher for people with a history of traumatic brain injuries (TBI) than for people with no history of TBI.
Professor Jonathan Schott of UCL's Institute of Neurology said the research provides "perhaps the best evidence yet that traumatic brain injury is a risk factor for dementia", but said further work was required to differentiate types of injuries, such as sport-related concussions, and how they affect the brain.
A single traumatic brain injury characterized as severe increases the risk by 35 percent, while a mild traumatic brain injury or concussion increased the risk by 17 percent.
After adjusting for medical, neurological and psychiatric illnesses, they found that compared with people who had never had a T.B.I., those who had had any were at a 24 percent increased risk for dementia, and those who had had five or more had almost triple the risk.
The risk of dementia increased 33 per cent higher for two or three TBIs, 61 per cent higher for four TBIs, and 183 per cent higher for five or more TBIs.
Even decades after an injury, the risk of dementia remains high.
The study also found that the younger a person was when they sustained the TBI, the higher their subsequent risk of developing dementia.
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An estimated 9,000-11,000 people suffer a TBI in Ireland every year.
A TBI is classified as a blow to the head which disrupts the normal functioning of the brain.
However, this study is one of the first to have a sufficient sample size and follow-up period to assess the effect of TBI in younger adults on the long-term risk of dementia. Causes include road traffic accidents, falls, sporting accidents and assaults.
According to Fann, most people who sustain a single concussion do not develop dementia and the findings do not suggest that every person who sustains a severe TBI will develop dementia later in life. But he said the findings might lead people with TBI histories to change their behaviors toward other potential risk factors for dementia, such as limiting alcohol and tobacco use, engaging in regular exercise, preventing obesity, and treating hypertension, diabetes, and depression. Between 1999 and 2013, 126,734 people (4.5%) aged 50 or older were diagnosed with dementia.
"TBI was associated with an increased risk of dementia both compared with people without a history of TBI and with people with non-TBI trauma", the authors write.
Importantly, the younger the individual sustaining a TBI the higher the risk of subsequent dementia, when taking time since TBI into account. TBI correlated with elevated dementia risk compared to individuals with a non-TBI fracture not involving the skull or spine (hazard ratio, 1.29).
"However, it's important to emphasize that although the relative risk of dementia is increased after traumatic brain injury, the absolute risk increase is low", Fann noted in a journal news release.