Brain disease is pervasive in NFL, new estimate suggests

This summer, a scientific study sent reverberations through the football world, making headlines and deeply worrying players and families.

At least 25% of retired National Football League (NFL) players who died over a recent six-year period, the study found, had CTE, or chronic traumatic encephalopathy, the neurodegenerative brain disease associated with repeated hits to the head.

But most of the other 75% of dead players were never tested. That left a complex, high-stakes question – the actual prevalence of CTE among dead former NFL players – open for analysis.

Now, the same research team has deployed statistical techniques to home in on a more precise number.

In a new study published on Oct 28, they estimate that three out of four former NFL players who died between 2008 and 2023 most likely had CTE in their brains at death.

By contrast, the same researchers studied brain banks that reflect the general population and estimated that only 0.6% of people have CTE.

This remarkable disparity sharpens the question facing current players, NFL retirees, football fans and parents: Just how dangerous is football?

Because CTE can be diagnosed only by autopsy, the quest for an answer has revolved around brain banks that include donations from former NFL players.

Brains are sliced, stained and studied under a microscope for signs of CTE, which causes brown tangles of protein to build up around the deep grooves of the brain.

These donations have taught doctors and scientists about the biology and stages of CTE, which is progressive and linked to dementia and behavioural changes – but does not always cause symptoms, particularly in the early stages of the disease.

Across nearly 400 brains of former NFL players that were donated and studied, the statistics look impossibly grim: more than 90% showed signs of CTE.

But researchers have long cautioned that this is most likely an overestimate.

Brains are usually donated because former players or their families saw symptoms that aligned with CTE, including headaches, memory loss and unrecognisable shifts in personality.

Since families that did not see those symptoms are far less likely to donate a player’s brain, banks provide a distorted snapshot of the overall population.

Extrapolating broader lessons, policy recommendations and prevention strategies from this sample has turned into an urgent, fraught statistical puzzle.

“Symptoms really drive donations,” said Yorghos Tripodis, a professor of biostatistics at Boston University School of Public Health, who was a leader of the new study published in the journal JAMA Network Open.

In the new study, the researchers tried to quantify selection bias and analyse the likelihood that untested brains had CTE.

They estimated that among dead players, those with obvious cognitive symptoms were about five times more likely to have their brains donated to a bank than those without.

They reviewed death records for the 2,362 former NFL players who retired between 1951 and 2021 and died from 2008 to 2023, studying causes of death and mentions of dementia or other neurodegenerative diseases that might have been misdiagnosed.

They studied the ages at death, the years of NFL experience and even the positions played.

They arrived at the estimate that three out of four NFL players who died between 2008 and 2023 would have had some level of CTE if autopsies were performed.

Zachary Binney – an epidemiologist at Oxford College of Emory University, whose previous study, published in Neurology, found a minimum of one in 10 dead players had CTE pathology in their brain – said in an e-mail that the work was “pretty rigorous, and the methods seem reasonable and valid”.

“It is higher than I would have guessed previously,” he said. “But this paper has a larger dataset and applies stronger methods than previous work,” he added.

In a statement, the NFL said it “strives to make the game of football safer, including by implementing strategies to reduce concussions and head impacts”.

In the decade since the NFL first acknowledged the risk of CTE, it has altered rules and safety protocols – at the same time that it has increased the number of regular season and post-season games.

The overall impact is unclear, and until CTE can be tested with certainty in the living, no one can be sure whether today’s players are more or less susceptible to CTE than those who played in previous eras.

CTE symptoms can include memory loss, headaches, mood swings and a loss of impulse control. Addiction issues and a reduction in executive function are not unusual.

In the new study, researchers estimated that nearly three in 10 of the players had dementia but underscored that this was a minimum and that the true number could be higher.

For years, studies have been showing that dementia is about four times more common in NFL retirees than in the general male population, Binney said.

Several outside researchers cautioned against the temptation to extrapolate the frightening statistic to all former NFL players.

There are even more caveats when trying to understand the implications for people at lower levels of the sport.

Sebastien Haneuse, a biostatistician at the University of California, Irvine, said: “This is specifically people who have died and doesn’t say anything about people who are still alive.”

In its statement, the NFL said it was committed to ensuring access to resources to support players’ physical and mental well-being.

“We encourage former players to utilise these resources to identify and seek treatment when they are concerned about their health,” it said.

Researchers do not know why some people get CTE, and others do not, despite similar histories of head trauma.

They believe that the likelihood of CTE grows with each year of exposure (how many seasons of tackle football someone plays, for example, including as a child) and with the cumulative force of the hits over a lifetime.

The biological signs of CTE can appear in the brain without symptoms, especially when the damage to the brain is more minimal.

In the earlier study, for example, 93% of donated brains showed CTE pathology, but only 60% of the people had a diagnosis of dementia.

In a published response to the August study that made headlines, a group of researchers led by Marc Weisskopf at Harvard T.H. Chan School of Public Health pointed out that most former NFL players were still alive.

If the known cases of CTE cited in the study were compared with the full population of NFL players (not just those who died in those six years), he and colleagues argued, the lower bound of risk among all NFL players could be one in 83.

Weisskopf added in an interview that the people who have died may have been in worse health and more likely to have CTE than those who are still alive.

That group’s research receives funding from the NFL Players Association, but the association has no say in where or what researchers publish.

Some researchers argue that attention to CTE itself has become a negative influence on player health.

A study in the Journal of Neurotrauma examined suicide rates in professional athletes in the NBA, Major League Baseball and the NFL and found that football players’ suicide rates alone rose sharply after 2010, once the CTE risk began to be widely recognised.

Rachel Grashow, a senior research scientist at Harvard, said: “People care about players – they care about NFL players’ health, future players, current players and former players.”

Grashow said she hoped that the conversation could shift towards interventions that focus on treating and preventing symptoms in living players.

Christopher Nowinski, co-founder and executive director of the Concussion & CTE Foundation, and one of the study authors, played football at Harvard and recalled that the captain of his team had Stage 2 CTE and pronounced changes in his behaviour.

“I would say that none of us wants this to be true,” Nowinski said.

“As former college football players, none of us want to think (that) a lot of us may have this. It’s important to get it right.” NYTIMES