The study, published Aug. 25 in the journal The BMJ, examined donated brain tissue from 338 former NFL players who died between 2008 and 2021. A total of 1,712 former players died in that time frame, so the analysis included only about 20% of the players' brains that could theoretically have been examined. Then, the researchers used that subset of brains to estimate the likely prevalence of CTE in the overall group.
These physical signs of CTE aren't always associated with overt symptoms, such as cognitive decline, mood and behavioral changes, or dementia. Only about 60% of the individuals with CTE-related brain changes were also diagnosed with dementia in this study. That said, those with stage IV CTE were more likely to have dementia, with 90% having a diagnosis.
What can this study tell us about the CTE risk that current football players face, from youth sports to the professional level? To put the research into context, Live Science spoke with Thayne Munce, an associate professor and director of the Jayhawk Athletic Performance Laboratory at the University of Kansas, who was not involved in the BMJ study. Munce researches brain health and concussion in contact sports, primarily among young athletes. He cautioned that the BMJ study's findings shouldn't be extrapolated to all levels and ages of play, or even to today's NFL players.
Thayne Munce: It's not surprising to me. This group and their colleagues have published previous work from donors — so these are former players who have donated their brains for research. They have reported even higher rates than 25% among just their donor sample [in this study]. This really high rate — over 90% of CTE cases in the donor group — is in line with what previous research has shown for these brain banks. So that's not terribly surprising.
What's different in this study is they compared the donors over a certain period of time versus all of the deaths among NFL players during that period of time and estimated the range based on how representative that donor sample was to the overall group of NFL players.
So that's where you get this big range. [The true prevalence] really depends on how representative that donor sample was to the peer group of players from that era.
TM: I think even more problematic than that big range is to understand that this is a historical study.
This is a different era of football. So we should not be making any estimations about what current CTE rates are based on this study, because a lot has changed in 50 years. It [the CTE rate] could be the same. It could be greater. It could be less. We really don't know.
But that is the challenge of this study. It's interesting [in that] it gives us a range of what CTE may be in that group of former players. But it doesn't give us a lot of information about what rates are in current players in the NFL. And even more so, whatever the CTE rate is in professional NFL players does not translate to former collegiate, high school or youth football players, because CTE is an exposure-related disease. For players who have less exposure to these repetitive head impacts, it should follow that they have a considerably lower risk of CTE.
Cleveland Browns quarterback Bill Nelson hands off to running back Leroy Kelly during the 1969 NFL Championship Game against the Minnesota Vikings on January 4, 1970. Many safety precautions that are applied to today's NFL players did not exist at that time. (Image credit: James Drake via Getty Images)
NL: You mentioned that football has become safer over time. Could you spell out how the game has changed?
There are changes in equipment, for sure. Modern football helmets — the material, the padding, the way in which these manufacturers are using head impact data to construct position-specific helmets and materials that are designed now to not just prevent catastrophic injuries, like skull fracture, but also to minimize the force from having repetitive head impacts — that's come a long way.
Virginia Tech has a helmet rating system where they test football helmets, and every year, the helmets score better on these tests. They now have star ratings for youth helmets, so manufacturers of youth football helmets are now being held to account, as well.
You look at the way in which practice was conducted, from full contact, two-a-day practices, which were common even in high school 15, 20 years ago. Now it's really not allowed at any level of play. So the amount of hitting that they do, the amount of contact that players are exposed to in practice and in the preseason is far less.
TM: Theoretically, yes. There's evidence that if the brain is injured and then it's exposed to a second insult or injury before it's properly recovered, that makes the condition even worse.
Even if you don't get another concussion, just those nonconcussive repetitive head impacts on top of an injured brain would not be good. It certainly is only going to delay recovery, likely is going to compound your inflammation and all of the downstream consequences of that initial injury.
Now whether that leads to CTE, we can't really say. But I think it's reasonable to suspect that it probably would increase that risk.
Recent brain scans of NFL hall-of-fame football player Joe DeLamielleure, who was diagnosed with CTE. (These are not from the new study.) (Image credit: Charlotte Observer via Getty Images)TM: So CTE is a neurodegenerative disease that can only be diagnosed by neuropathologists — people that look at slices of the brain and look for particular proteins that are present in certain regions of the brain. [This examination is done after death.] It's a pathological marker. It means that there are changes in the structure of the brain.
Even in the study, they showed that they [the CTE brain changes and symptoms] weren't necessarily linked. Now I believe they showed that for those who had more severe CTE — like the stage IV — that there was a pretty high rate of those individuals who also had subsequently diagnosed dementia. They had these clinicians that looked at the records and diagnosed them. That's a little bit of a limitation of the study: that a lot of the dementia cases were diagnosed after the person had passed away, based on medical records.
That may be true, but it may also be that it's due to old age, or it could be due to genetics or some other condition.
Football players at Muhlenberg High School in Pennsylvania do drills during practice. These young players likely don't have the same degree of risk as professional players when it comes to CTE. (Image credit: MediaNews Group/Reading Eagle via Getty Images via Getty Images)
NL: Since there was a higher likelihood of dementia in the stage IV cases, it seems like there's a stronger link between the pathological and clinical diagnoses at advanced stages. Is that a fair read?
NL: What should current NFL players take from this study?
If anything, I think it probably validates all of these efforts that we discussed to try to improve brain health and minimize risk of exposure.
So let's take steps to reduce that risk — and I think it validates all of the changes that have taken place over the last 50 years. I mean, a lot of them have taken place over the last 10 to 15 years. That's probably the biggest message: to say we were right to do these things and we need to continue to see if there are further opportunities to make the game safer.
NL: Looking forward, what further improvements could be made?
We know that sleep is really powerful and could learn more about that in and of itself. And what about nutrition? What about pharmacological interventions? Wouldn't it be great if we could develop a medicine, a drug at some point that could be taken preventatively to reduce athletes' inflammation? Really looking into ways in which we can optimally improve brain health as a countermeasure to the damage or the trauma that is occurring.
And then, certainly, diagnostic techniques. We talked about CTE only being able to be diagnosed at death. It would certainly be a game changer if we could diagnose it convincingly in the living so it could be recognized early. People are going to have different risks, and some of that is probably genetically determined; some people are going to have the same level of exposure and develop CTE while some aren't. If we could identify individuals who are at higher risk or in the earliest stages of the development of the disease, you can then change lifestyle, behavior, exposure — that would be huge as well.
NL: To shift gears slightly, could you describe your own research in youth sports?
That ties into the CTE question in the sense that CTE is thought to be due to cumulative exposure to brain trauma — repetitive head impacts — and certainly most of the work, like in this study, has been done in NFL players. The question for a lot of the general population is, "What about youth players, high school players, that make up 90% of all football players?" So from a public health perspective, it's those younger players that I think we really need to be studying and concerned about.
NL: With recent research, do we better understand how those nonconcussive impacts affect brain health?
And then, over time, this recognition of what has been termed subconcussive or nonconcussive brain injury has emerged, as there's been some imaging studies in collegiate athletes, and even high school athletes, that have found that players who don't get concussed sometimes have changes in their brain, even over the course of the season.
With CTE, it's always looking backwards, because it's something that's diagnosed at death. For researchers like myself who study concussion, or study active football players, that has also then been a reaction of saying, "Well, if it is repetitive trauma, if it's repetitive head impacts, that is a risk factor for neurodegenerative disease such as CTE, then that makes it even more important to study it in active players." What does that head impact exposure look like at different ages, and how can we reduce that exposure? How can we reduce the number and severity of head impacts that players are experiencing?
That's why there's so much concern about repetitive head impacts and why there have been rule changes at the collegiate and high school level about the amount of contact and limiting those types of activities, with the idea of reducing that overall head-impact burden.
TM: Yep — so every hit would be considered a head impact, and then certain impacts are strong enough, or hit you in the right spot, to cause a concussion. The vast majority of impacts are nonconcussive.
TM: When I do talk to players and coaches and parents, I often like to remind them that, generally, the benefits of playing sports, including tackle football, outweigh the risk. Oftentimes we get so concerned about the risk, which is real, that we forget about the benefits and about the alternatives of not playing. Almost always, the benefits — from the physical, the psychological and social components, the mental health components — are going to outweigh the risks. That's one thing to keep in mind.
[With parents], I'm reassuring them that the risk is far less for the younger players than what they hear about in this type of study. We're talking about an apples-to-oranges comparison.
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TM: I played for a Division II school in South Dakota called — it's Augustana University now; it was Augustana College when I played there.
Even at the collegiate level, the way in which collegiate practices are conducted and the safety measures that are in place for current collegiate players is nothing like what we had when I was playing. So I like to reassure players that this is probably the safest time to be playing the sport of football.
Unfortunately, we don't know what that risk is [currently]. We can't tell parents, "Here's objectively what your risk of injury is going to be." It makes it tough for parents. So I like to reassure them that a lot of the concerns that are expressed for NFL players are greater than they should have as parents of youth players.
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