In the largest study of its kind, researchers have found that traumatic brain injury is associated with an increased risk of dementia.
The survey of 36 years' worth of data - collected from the Danish national patient register - found that the risk of dementia rose with the number and severity of brain injuries, a team wrote in The Lancet Psychiatry, a medical journal.
"While there is growing interest in the question of whether collisions in sports like rugby or football might affect dementia risk, this study only looked at head injuries that required hospital treatment and doesn't tell us anything about the impacts you'd normally expect to see on the sports field". Those affected should avoid certain behaviors, researchers suggest.
'There are 850,000 people with dementia - this number is set to rise to 1 million by 2021 and more research is urgently needed to fill the gaps in our understanding of lifestyle factors that increase dementia risk'.
A new way to rapidly assess levels of consciousness in people with head injuries could improve patient care. For example, individuals having a TBI in their 20s were 63 per cent more likely to develop dementia about 30 years later compared to those who didn't sustain a TBI in their 20s.
According to the UN's World Health Organisation, about 50 million people worldwide suffer from dementia, including Alzheimer's Disease - the most common form with about two-thirds of cases.
Prior studies of traumatic brain injury and dementia have struggled to establish a link because of low sample sizes or study intervals that were too short to observe dementia development.
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A traumatic brain injury can be caused by a fall, a traffic accident, a sports accident or a violent attack.
"These uncertainties notwithstanding, this study reinforces the importance of trying to prevent injury to the brain", he said.
"There are some cognitive rehabilitation strategies that may decrease the cognitive deficits associated with a brain injury", he said. For their first T.B.I. diagnosis, 85 percent were this mild type.
Between 1999 and 2013, 4.5 per cent of the study population aged 50 and older were diagnosed with dementia.
For years, scientists have studied the effects of traumatic brain injuries that appear in professional athletes, looking for ties to dementia later in life.
Writing in a linked comment, Professor Carol Brayne from the Cambridge Institute of Public Health, University of Cambridge, Cambridge, UK says, "Now we need to tease out what is happening in terms of traumatic brain injury, wider spectrum exposures and how these occur across different ages, by gender, and also by community within societies".