Experiencing just one traumatic brain injury (TBI) can increase the risk of dementia, according to a new study of nearly 2.8 million people. The results of the study, published online this month in The Lancet Psychiatry, showed that individuals who suffered one or more TBIs had a 24 percent higher risk of dementia.

TBIs may occur as the result of a violent blow or penetrating injury to the head, and their severity can range from mild to severe. While most TBIs are mild and can be managed at home or with a short hospital stay, the most serious TBIs can require lengthy hospitalizations and specialized treatment.

About 1.7 million individuals sustain a traumatic brain injury in the United States each year, according to the Centers for Disease Control and Prevention (CDC). Of these, 52,000 die and 275,000 are admitted to the hospital. Emergency departments treat and release 1.365 million cases of TBI each year, which is about 80 percent of all TBIs.

Researchers have been working to determine the correlation between TBI and dementia for the past 30 years. The Alzheimer's Association discusses several previous studies that show an association between the two. One study found that older adults with a history of moderate TBI had a 2.3 times greater risk of developing Alzheimer's disease than seniors without a history of head injury, and that older adults who had suffered a severe TBI had a 4.5 times greater risk.

In the latest study, Jesse R. Fann, M.D., from the University of Washington in Seattle, and a team of researchers studied the association between TBI and the long-term risk of subsequent dementia. The scientists found that those with a history of TBI had a higher adjusted risk of all-cause dementia compared with study participants without traumatic brain injuries.

The researchers used the Danish Civil Registration System to create a population-based cohort that included everyone born in Denmark who was still living in the country on Jan 1, 1995, and who was at least 50 years old at any time during the follow-up period between 1999 and 2013. Some of the participants had had a TBI in their lifetime at the time of enrollment into the study and the others did not.

The researchers followed a total of 2,794,852 male and female participants for a mean 9.89 years per patient. Of these, 132,093 had at least one TBI between 1977 and 2013; 126,734 had incident dementia between 1999 and 2013 study. The average age at the time of the diagnosis was 81. The risk of dementia was greatest within the first six months of the TBI and was slightly higher for severe TBIs.

The risk for dementia increased with the number of TBI events. People who have had one TBI are 1.22 times more likely to suffer dementia than are those without a traumatic brain injury, according to the study's findings, while the hazard ratio leaps to 2.83 with five TBIs.

"TBI was associated with an increased risk of dementia both compared with people without a history of TBI and with people with non-TBI trauma," the authors write in the study interpretation. "Greater efforts to prevent TBI and identify strategies to ameliorate the risk and impact of subsequent dementia are needed."