Given the widespread under-reporting of sport-related concussion the association between TBI and dementia may be even stronger, warned Associate Professor Michael Buckland at the University of Sydney's Brain and Mind Centre.
A link between brain injury and dementia has been confirmed by a study of nearly three million people.
For example, one traumatic brain injury is associated with about a 22 percent increased risk of dementia.
They did, however, caution that the findings do not suggest that everyone who suffers a traumatic injury will go on to develop dementia. The risk of dementia increased with the number and severity of injuries, and even concussion was linked with a higher risk of dementia.
Dementia remained relatively rare: only 4.7 percent of study participants developed dementia at all, a total of 126,734 people.
The risk of dementia increased 33 percent higher for two or three TBIs, 61 percent higher for four TBIs, and 183 percent higher for five or more TBIs.
Dementia affects up to 55,000 people in Ireland and Alzheimer's disease is the most common cause of dementia, accounting for 60-70 per cent of all cases. From 1999 to 2013, 4.5 percent of the patients over age 50 years developed dementia.
Over 36 years, 132,093 individuals had at least one TBI, and most cases were categorised as mild.
Previous research on links between brain injury and dementia has produced conflicting results, said the study authors writing in the journal The Lancet Psychiatry. The risk of dementia was highest among people who had suffered multiple T.B.I.s.
Still, the lead author, Dr. Jesse R. Fann, a professor of psychiatry at the University of Washington School of Medicine, said that the absolute risk of getting dementia as young as 50 is quite low. The mean age at first diagnosis of dementia was 80.7 years.
The new study took account of other influences on dementia risk including diabetes, heart disease, depression and substance abuse.
"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", Fann said. And they looked at other types of trauma, such as broken bones, and found that brain injuries were more closely tied to dementia. "The attributable risk of traumatic brain injury to different exposures and how these change across time needs policy attention, given it is likely that prevention of these need be considered at societal, community, and local levels".