Traumatic brain injuries increase the risk of a dementia diagnosis for more than 30 years after a trauma, though the risk of dementia decreases over time, according to a new study published this week in PLOS Medicine by Anna Nordström and Peter Nordström of Umeå University in Sweden.

Traumatic brain injury (TBI) has been associated with dementia, but the details of that risk over time and in different TBI types have not been well studied. In the new study, the researchers tracked all diagnoses of dementia and TBI in Swedish nationwide databases from 1964 through 2012.

The aim of the present study was to examine whether different types of dementia diagnoses are associated with previous TBI and whether any observed association is time-dependent, in a nationwide cohort.

In a retrospective cohort, 164,334 individuals with TBI were matched with control participants who did not have TBI. In a case-control cohort, 136,233 individuals diagnosed with dementia at follow-up were matched with control participants who did not develop dementia; and in a third cohort, the researchers studied 46,970 sibling pairs with one individual having a TBI.

In the first year after TBI, the risk of dementia is increased by 4- to 6-fold, the researchers found. After that, the risk decreased rapidly but was still significantly more than 30 years after the TBI. Overall, the risk of dementia diagnosis was increased by about 80% 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's likely that in some cases, the onset of dementia preceded the TBI, so the researchers caution against making causal inferences.

The associations found between TBI and the risk of subsequent dementia diagnosis could also be influenced by familial factors, such as upbringing conditions, education, and genetic factors. To our knowledge, no previous population-based study with a long follow-up period has evaluated these potential influences.

In the present study, we thus examined the association in about 47,000 full sibling pairs with discordant TBI status during follow-up. The risk of dementia diagnosis during follow-up was almost doubled in siblings with TBI compared with their counterparts without TBI, and it remained increased more than 10 years after TBI.

These results are similar to those obtained for the other cohorts, suggesting that familial factors cannot explain the association between TBI and dementia.

"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."