A study by researchers at Umeå University in Sweden found that traumatic brain injuries (TBI) heighten the risk of dementia for more than 30 years after the trauma – although the risk reduces over time.

In the first year after TBI, the risk of dementia increases four to six-fold, the researchers found.

Beyond that, the risk decreased rapidly but was still significant more than 30 years after the TBI. Overall, the risk of dementia diagnosis was increased by around 80 per cent during a mean follow-up period of 15 years.

The risk of dementia was higher for those with a severe TBI or multiple TBIs and was similar in men and women.

Because the development of dementia can be a risk factor for accidents resulting in TBI, it is likely that in some cases, the onset of dementia preceded the TBI, so the researchers caution against making causal inferences.

“The findings of this study suggest an existence of a time and dose-dependent risk of developing dementia more than 30 years after TBI,” the authors say.

“To our knowledge, no previous prospective study with similar power and follow-up time has been reported,” they add.

The researchers tracked all diagnoses of dementia and TBI on Swedish nationwide databases from 1964 to 2012.

From all inhabitants living in Sweden above 50 years of age, three cohorts were formed.

In the first, 164,334 individuals with TBI were matched with controls; in the second case-control cohort, 136,233 individuals with dementia were matched with controls; and in the third cohort, 46,970 full sibling pairs discordant for TBI were evaluated for dementia during follow-up.