In a propensity-matched cohort study of more than 350,000 veterans with and without traumatic brain injuries (TBI), mild traumatic brain injury (mTBI) without loss of consciousness was associated with more than a twofold increase in the risk of a dementia diagnosis, even after adjusting for medical and psychiatric co-morbidities. This large epidemiological study was recently published in JAMA Neurology.  Approximately 2.8 million TBIs occur each year in the United States; approximately 80% are in the “mild” category.

Although prior studies of the association between mTBI and dementia have been mixed, this study, among the largest epidemiological studies to date, adds to the weight of evidence suggesting that even mild TBI is associated with an increased dementia diagnosis risk.

Consistent with prior studies focused on moderate and severe TBI, the study found a dose-response association between TBI severity and dementia diagnosis. In other words, with more severe TBIs, the risk of a dementia diagnosis was higher. To develop a comparison sample of veterans without TBI, the researchers selected a 2% random sample of all patients who received VHA care during the same timeframe and then used propensity matching to select one veteran without TBI for each veteran with TBI.

One of the strengths of this study is the large cohort, which provided the researchers with ample power to detect associations and to adjust for a wide range of potential confounders. Another strength was the close propensity matching of the control group.

There are some limitations in applying the study results to the population at large.

• The database could not differentiate subjects with single TBIs from subjects with multiple TBIs – other data indicates that there is a high prevalence of multiple TBIs in the veteran population, a factor that could distinguish them from civilians.
• The data also did not identify the mechanism of injury. The authors describe the high number of blast-related mTBI among military personnel; some researchers have suggested that this mechanism may be unique in its potential to cause long term consequences. An editorial in the same May, 2018 issue of JAMA Neurology suggests, however, that the mechanism of injury may not be as significant as the authors suggest, referring to Department of Defense data indicating that more than 75% of TBIs among their personnel occur in garrison or training; in other words, they do not involve blasts.

The authors of the study note that the young mean age of veterans in this study (50 years) raises concerns that this problem will increase as TBI-exposed veterans age. “The implications for the military health system, VA health care, and society”, they say, “are profound,” highlighting the need to further understand what physiological processes increase the risk of dementia and how this risk can be reduced.