People who suffer a traumatic brain injury (TBI) may face an increased risk of developing dementia, including Alzheimer's disease, one of the largest studies of its kind has found.

Danish and US researchers looked at almost 2.8 million people over a 36-year period - 1977 to 2013. They assessed the long-term risk of dementia in people who had suffered a TBI, compared to those without a TBI, and also those who had experienced physical traumas that did not involve the brain or spine, such as fractured bones.

An estimated 9,000-11,000 people suffer a TBI in Ireland every year. Causes include road traffic accidents, falls, sporting accidents and assaults.

The study found that overall, those who sustained a TBI were 24% more likely to be diagnosed with dementia over the study period compared to those who had not suffered such an injury.

The risk of dementia increased with the number of TBIs and the severity of these injuries. For example, a person over 50 who suffered one TBI had a 22% increased risk of developing dementia, while someone who suffered four TBIs had a 61% increased risk.

However, even a mild TBI (concussion) increased the risk by 17%.

A concussion usually occurs following a blow to the head and can temporarily affect how the brain works. Symptoms can include headache, blurred vision, a loss of consciousness, confusion and nausea.

The study also found that the younger a person was when they sustained the TBI, the higher their subsequent risk of developing dementia. For example, a person who sustained a TBI in their 20s was 63% more likely to develop dementia 30 years later compared to someone who did not sustain such an injury in their 20s.

Those who sustained a TBI in their 30s were 37% more likely to develop dementia 30 years later compared to those who did not.

Overall, the risk of dementia was slightly higher in men who had sustained a TBI compared to women.

"Individuals with a history of TBI, including those with less severe injuries have an increased risk of developing dementia, even decades after the injury. However, it's important to emphasise that although the relative risk of dementia is increased after TBI, the absolute risk increase is low," commented lead researcher, Prof Jesse Fann, of the University of Washington School of Medicine in Seattle.

The researchers noted that while previous research has suggested a link between TBI, including concussion, and subsequent dementia, earlier studies have been limited in size and details, and have had short follow-up periods.

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.

"Shedding light on risk factors for dementia is one of the most important tasks in health research. Our analysis raises some very important issues, in particular that efforts to prevent TBI, especially in younger people, may be inadequate considering the huge and growing burden of dementia and the prevalence of TBI worldwide.

"Our findings suggest that improved TBI prevention programmes may have an opportunity to reduce the burden of dementia worldwide," Prof Fann said.

Details of these findings are published in the journal, The Lancet Psychiatry.