After one of the largest ever investigations into the link between traumatic brain injury (TBI) and cognitive decline in later life, Danish and US 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. 

They looked at the health records of 2.8 million people over 36 years and found that individuals who sustained a TBI in their 20s were 63 per cent more likely to develop dementia in the next three decades than someone who did not sustain one.

But if they experienced the same head injury in their thirties, the risk over the same time period was 37 per cent.

A TBI is classified as a blow to the head which disrupts the normal functioning of the brain. The leading causes include falls, motor vehicle accidents and assaults.

Concussion is classed as a mild TBI.

Even a single incidence of concussion was found to lead to a significantly higher risk (17 per cent) of dementia.

The findings are particularly worrying in light of recent attention around the risk of concussion in contact sports like boxing, rugby and football.

“Our analysis raises some very important issues, in particular that efforts to prevent traumatic brain injury, especially in younger people, may be inadequate considering the huge and growing burden of dementia and the prevalence of TBI worldwide,” said Jesse Fann, professor of psychiatry and behavioural sciences at the University of Washington School of Medicine.

More than 50 million people experience a TBI each year and Professor Fann said tackling this could help lower the overall risk of dementia which already affects 47 million people worldwide, but is expected to double in the next 20 years as life expectancies increase.

In total, 5.3 per cent of participants with dementia had a history of TBI compared with 4.7 per cent of those without the condition.

While 85 per cent of the TBIs diagnosed were classed as mild, researchers said they are still likely to have been more serious concussions as the symptoms had to be severe or persistent enough to warrant a trip to hospital.

The study also examined the impact of sustaining multiple separate brain injuries and the likelihood of subsequently developing dementia.

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.

The findings also show that men with a history of TBI had a slightly higher risk of developing dementia than women.

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.

He added: “This study certainly reinforces the fact that sports in which head injury occurs are dangerous and may make us susceptible to dementia. However, it is important to note that TBI is not specific to head injuries from sports. Previous studies on war veterans with head injuries came to a similar conclusion that about its risk for dementia.”

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.

“These uncertainties notwithstanding, this study reinforces the importance of trying to prevent injury to the brain,” he said.

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