Sustaining a traumatic brain injury is associated with an increased risk of developing both dementia and Alzheimer’s disease, according to one of the largest studies of its kind.
It found that people sustaining a traumatic brain injury were 24% more likely to be diagnosed with dementia than those without a history of traumatic brain injury over the study period.
“Shedding light on risk factors for dementia is one of the most important tasks in health research”
In the study of 2.8 million people over 36 years, 5.3% of participants with dementia had a history of traumatic brain injury versus 4.7% of participants without dementia.
The risk of dementia increased with the number of brain injuries and severity of injury, and even a single, mild concussion was linked with a higher risk of dementia, said Danish and US researchers.
They found 4.3% of participants with dementia had at least one mild traumatic brain injury versus 4.0% of participants without dementia.
This study, published in The Lancet Psychiatry journal, is one of the first to have a sufficient sample size and follow-up time to assess the effect of traumatic brain injury in younger adults on long-term dementia risk.
“Individuals with a history of traumatic brain injury have an increased risk of developing dementia, even decades after the injury”
Previous evidence has suggested a link between traumatic brain injury, including concussion, and subsequent dementia.
Whether such injuries sustained among army veterans and in contact sports, such as football and boxing, increases the risk of dementia has been hotly debated, noted the study authors.
They highlighted that previous research had produced conflicting results and the relationship between the two remained poorly understood due to study limitations including small sample sizes, short follow-ups, and limited information about the number, severity and timing of head impacts.
For their new study, the researchers tracked all recorded diagnoses of traumatic brain injuries in Denmark from 1977 to 2013 and compared dementia risk against time since the injury, its severity, number of injuries and person’s sex.
They adjusted for other factors that could affect the risk of dementia including diabetes, heart disease, depression, and substance abuse.
Over 36 years, 4.7% of patients had at least one traumatic brain injury diagnosis, of which most were mild. Between 1999 and 2013, 4.5% of those aged 50 or older were diagnosed with dementia.
Compared with individuals without a history of injury, the risk of dementia after the age of 50 increased consistently with the number of injuries – 22% higher risk with one, 33% higher with two or three, 61% higher with four, and nearly three times the likelihood of dementia with five or more.
“It’s important to emphasise that although the relative risk of dementia is increased, the absolute risk increase is low”
The researchers also found that a single severe traumatic brain injury increased the risk of dementia by 35%, while one mild injury increased the risk by 17%.
Importantly, the younger the individual sustaining a traumatic brain injury the higher the risk of subsequent dementia, when taking time since the injury into account, said the researchers.
For example, individuals having an injury in their 20s were 63% more likely to develop dementia about 30 years later compared to those who did not sustain one in their 20s.
In contrast, individuals sustaining a traumatic brain injury in their 30s were 37% more likely to develop dementia 30 years later compared with those without an injury in their 30s.
The findings also showed that men with a history of traumatic brain injury had a slightly higher risk of developing dementia than women, noted the researchers.
Lead study author Professor Jesse Fann, from the University of Washington School of Medicine, said: “Individuals with a history of traumatic brain injury, 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 traumatic brain injury, the absolute risk increase is low,” she said. “Our findings do not suggest that everyone who suffers a traumatic brain injury will go on to develop dementia in later life.”
“It is likely that prevention needs to be considered at societal, community, and local levels”
Professor Fann added: “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 traumatic brain injury, especially in younger people, may be inadequate considering the huge and growing burden of dementia,” she said.
“Our findings suggest that improved traumatic brain injury prevention programmes may have an opportunity to reduce the burden of dementia worldwide,” said Professor Fann.
Professor Carol Brayne from the University of Cambridge, said: “Now we need to tease out what is happening in terms of traumatic brain injury, wider spectrum exposures, and how these occur across different ages, by gender, and also by community within societies.
“The association of traumatic brain injury with different causes and how these change across time needs policy attention, as it is likely that prevention needs to be considered at societal, community, and local levels,” she added.