An estimated 75% of professional football players who died over a 15-year period had signs of chronic traumatic encephalopathy, or CTE, according to a study released Wednesday.
This is the highest estimate to date. More recently, a study published in August by the same group estimated that a quarter of NFL players who died during a five-year period suffered from CTE. The new study, published in JAMA Network Open, estimates that figure three times higher.
Not everyone with signs of CTE in the brain experiences cognitive decline or dementia, and some people may never experience symptoms. According to the study, a smaller proportion of former players surveyed had dementia, around 27%.
The results were surprising, said study leader Yorghos Tripodis, director of statistics and data management at Boston University’s CTE and Alzheimer’s Disease Center. Boston University is home to the UNITE Brain Bank, which studies the effects of repetitive head trauma on donated brains, including those of athletes, military personnel and others.
“I had to double-check the numbers,” Tripodis said. Still, he said, that figure is just an estimate. “Right now, we don’t know how to diagnose CTE when people are alive, so we’re trying to make an estimate.”
In an emailed statement, an NFL spokesperson said, in part, that the league “continually works to make football safer, including implementing strategies to reduce concussions and head impacts.”
Repeated hits and biases
When a person’s head shakes during an impact, the brain — which is suspended in fluid inside the skull — can hit the skull or twist, said Dr. Julian Bailes, chairman of the department of neurosurgery at Endeavor Health in the Chicago area. Each hit is an opportunity for proteins called tau to build up in the brain.
“If this happens over and over again, the damage accumulates,” said Bailes, who was not involved in the new research.
The disease can affect parts of the brain involved in regulating behavior and emotions, which can lead to memory loss, depression, violent mood swings and other symptoms. These symptoms may also be linked to other illnesses.
Although CTE can cause symptoms similar to other neurodegenerative diseases, such as dementia and Parkinson’s disease, the signature it leaves on the brain is unique, Bailes said.
One of the main obstacles to studying the prevalence of CTE is that microscopic changes do not show up on brain scans. This means that the disease can only be diagnosed during an autopsy.
Previous research has been criticized for bias because people with cognitive decline and their families are much more likely to donate their brains to brain banks after death or request autopsies. An earlier study by Boston University researchers found that 99% of brains donated to research by former NFL players showed signs of CTE.
In the new study, Tripodis and his team attempted to eliminate as much of this bias as possible to determine a more accurate estimate of CTE rates in former NFL players. They combined data from the UNITE Brain Bank with national death records of other former NFL players who died between 2008 and 2023.
In total, researchers examined data from 396 donated brains, as well as information from 1,966 other deceased former NFL players. They took into account age of death as well as estimated rates of mild cognitive impairment and dementia among former NFL players aged over 50.
About 93% of donated brains showed signs of CTE, which results in structural damage in the brain. The research team calculated that families of gamers with symptoms of cognitive decline were about five times more likely to donate brains than families of gamers without symptoms, then performed a statistical analysis to correct for this bias.
From there, they estimated that of the approximately 2,360 former players who died during the 15-year period, about 1,760, or nearly 75 percent, suffered from CTE. By comparison, only about 1% of brains donated to banks in the United States, Austria and Brazil show signs of the disease, researchers say.
But many people with signs of CTE never develop symptoms, Tripodis said.
Researchers estimated that 27% of the 2,360 players studied had dementia.
Dementia “can arise from any cause,” Tripodis said. “But it is very likely that in this specific population, CTE causes dementia.”
Reduce impacts
Until doctors find a way to diagnose CTE while people are still alive, any study quantifying the number of people with the disease will be an estimate, said Dr. Jim Kelly, professor emeritus of neurology at the University of Colorado Anschutz.
“These are just estimates. They haven’t done autopsies on five times as many deceased players as they did autopsies on,” said Kelly, who was not involved in the research.
Some autopsies may also have revealed one or two lesions associated with CTE, “and it seems unlikely that this could lead to anything in terms of behavioral change or dementia,” he said.
These are difficult studies to do, Bailes acknowledged. They seek to determine “how many people have disease when not all of them are symptomatic,” he said, adding that the findings call for more research into diagnosing CTE in people who are still alive.
“If you can’t know if someone has a certain illness until they die, you really can’t help them,” he said.
In the meantime, prevention is key, Tripodis said: “If you reduce the number of brain injuries and brain impacts, you significantly decrease the risk. »
The NFL has already implemented measures to try to reduce the number of high-impact hits players face. They include redesigned helmets – such as the Guardian Cap – and the dynamic launch rule, which leads to lower collision speeds.
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