You don’t have to be a professional football player to get a solid conk on the head. By one estimate from medical researchers, over 27 million people around the world sustain a traumatic brain injury every year. Some are from car accidents, others are from falls, or taking a header on the soccer field. But a growing body of evidence shows that even mild hits to the head can cause long-term damage and heighten the risk of neurological disease.
The brain is soft and usually cushioned from our skulls by cerebrospinal fluid. But when something hits the head hard enough, our brains get jostled and can smash into that hard bone, causing swelling or bleeding. That can lead to concussion symptoms like short-term memory loss or confusion. (Not every concussion causes people to black out or feel nauseous or dizzy.)
A new study published this month in the journal Alzheimer’s & Dementia draws from a large data pool tracking Americans whose health outcomes have been tallied for the last 25 years. The authors find that head injuries, even mild ones, are associated with a long-term increase in risk of dementia. The study also found that the more head injuries people sustain, the greater the risk of developing dementia.
Dementia is a general term for memory and cognitive losses caused by changes in the brain. The most common type is Alzheimer’s disease, a progressive and irreversible disorder in which tangles of proteins interrupt how neurons communicate with each other. But there are other types of dementia, including vascular dementia, which occurs when there isn’t enough blood flow supplying oxygen to the brain, and frontotemporal dementia, which is caused by a loss of cells in the front and side regions of the brain that can drastically alter personality and behavior.
The researchers hope that this new information will add to growing awareness about the implications of head injuries and the importance of preventing them. “That’s really one of the most important take-home messages from this study, because head injuries are something that are preventable to some degree,” says Andrea Schneider, a neurologist at the University of Pennsylvania and the lead author of the paper. “You can do practical things like wearing bike helmets or wearing your seatbelt.”
Previous studies have demonstrated a similar relationship between head injuries and dementia, but most focused on specialized populations like military veterans. Schneider says this study is one of the first to look at the relationship in a general, community-based population, which might be more representative of the average person.
Schneider and her colleagues at the University of Pennsylvania analysed data from over 14,000 participants in the Atherosclerosis in Communities study, an ongoing effort which has followed people between the ages of 45 and 65 in Minnesota, Maryland, North Carolina, and Mississippi since 1987. The study was meant to track the environmental and genetic conditions that might contribute to heart disease, but the researchers also collected medical records and asked participants to self-report any head injuries.
When the University of Pennsylvania researchers analyzed the data on traumatic brain injuries, they found that people who sustained one head injury were 25 percent more likely to develop dementia than those who did not. That risk doubled for those who had sustained two or more head injuries.
There are other health factors that could play a role, too. Genetics make some people more prone to dementia; some forms are heritable or accompany other progressive disorders like Parkinson’s and Huntington’s disease. Other risks include vascular problems like diabetes and high blood pressure, environmental influences like pollution, and lifestyle choices like smoking. But Schneider says head injury is a significant factor. “We were able to say that about 9.5 percent of all cases of dementia in our study were attributable to head injury,” she says.
“This study adds to the growing evidence that head injuries may have negative effects on the brain long after the injury has appeared to heal,” says Deborah Barnes, a professor of psychiatry at the University of California, San Francisco, who did not contribute to this study. She adds that the strength of this study lies in the self-reporting criteria, which may help catch many injuries that weren’t severe enough to merit a trip to the emergency room or to show up in medical records, but that still had a long-lasting health effect.
While it’s becoming clearer that brain injuries are related to dementia, it’s not exactly understood why that’s true. It could be that head trauma leads to a buildup of neurofibrillary plaques and tangles that are associated with Alzheimer’s disease. Or it may trigger an increase in the tau proteins that cause chronic traumatic encephalopathy, a neurodegenerative disease sometimes found in people who receive repeated blows to the head, like football players and boxers, which can cause increased aggression, depression, and memory loss. Another possibility is that when the brain rattles around in the skull, tiny blood vessels get destroyed, decreasing the amount of oxygen available to the brain. The pathology of dementia caused by head injuries is also unclear; not all head injuries lead to the same outcome.
Scientists say that because they can’t examine a patient’s brain while they’re still alive, it can be difficult to pinpoint exactly what happened and when the disease started. “Dementia is a complex disorder that has multiple, interrelated mechanisms that can cause symptoms,” says Barnes. “The brain pathology underlying dementia is occurring for decades before symptoms start to emerge, making dementia mechanisms very difficult to study.”
The University of Pennsylvania study also found that the overall risk of developing dementia following one or more head injuries is about 50 percent greater for women than for men. They also found that it was a little less than 30 percent greater for white people than for Black respondents. (The study didn’t include data for other racial groups.)
But it’s not yet clear why head injuries pose a greater risk for different demographic groups. “The association between sex and race with dementia risk after TBI has not been consistent across all studies, so more research is needed to examine this association,” says Jesse Fann, a professor of psychiatry at the University of Washington who studies dementia but did not contribute to this paper. Some studies have found that women are more likely to develop dementia than men. Others conclude the risk is the same regardless of sex.
A large study published in 2016 by researchers at the University of California, San Francisco, using data from over 200,000 Kaiser Permanente patients in six racial groups found that the risk of dementia was 65 percent higher for Black people than for the lowest risk group, Asian Americans. A 2019 study found that the incidence of Alzheimer’s disease dementia was twice as high for Black people as for white people.
“Studies draw from different populations, so part of the explanation may be inherent in the specific study sample for a given study,” Fann points out. He also notes that because dementia risk is already higher for Black patients, the added event of a head injury might not be enough to increase that baseline risk.
Barnes notes that, in the case of the University of Pennsylvania study, because the data relies on self-reporting, it’s also possible that male and Black participants were less likely to report accurately, so the data may not be conclusive.
Schneider was surprised by her study’s findings and acknowledges that the data on these racial and gender differences doesn’t tell the whole story. “There’s a great deal more work that is needed to be done to look at reasons why we may have observed these differences,” says Schneider.
The researchers hope that by understanding who is most harmed by head injuries and how it elevates the risk of dementia, there may be something scientists can do to mitigate their effects. But that goal is still far off. “First, we need to understand these short-term and long-term associations with head injury before we can really think about devising treatment strategies and interventions,” says Schneider.
Right now, if you do suffer a brain injury, there isn’t a lot that one can do to specifically reduce dementia risk. “What we’ve been recommending is that people need to pay attention to all the risk factors that they may have for dementia,” Schneider says. That means making sure patients attend to modifiable behaviors like controlling their blood pressure, cholesterol, and blood sugar levels. “All of these things that we would actually recommend for everyone, it might be that they’re actually more important for these people that have this other risk of having had a traumatic brain injury,” she says.
But Schneider says that research into brain injuries has already improved patient care by making people more aware of the risks that even mild concussions can present. “I think that what people think about as a serious head injury has actually changed over the years,” she says. “Back 50 years ago, people would just go about their business unless they had to be taken to the hospital or were exceptionally symptomatic.” Now, school and sports leagues have concussion protocols and are less likely to send players back onto the field after they’ve sustained a blow to the head.
Schneider is planning to do further research about how brain trauma might exacerbate the vascular risk factors for dementia and is hoping to learn more about the specific age, sex, and racial factors that contribute to dementia. That way, doctors can start to develop customized treatment plans for brain injury patients. At this point, she says, the biggest lesson is that the more brain trauma you have, the greater your risk of developing dementia becomes.