Researchers say they have found “conclusive evidence” that CTE brain disease can be caused by repetitive impacts to the head during tackling and ball-heading sports.
Key points:
- The study gathered research on CTE from around the world and compared it to criteria previously used to link smoking to lung cancer.
- Researchers say they have the ‘highest scientific confidence’ repetitive head impacts cause CTE
- They call for tackle and header bans in junior sport to limit exposure
Previous studies have observed an association between contact sports and chronic traumatic encephalopathy (CTE), but stopped short of saying that one caused the other.
CTE, which can only be diagnosed post-mortem, is associated with behavioral, mental and cognitive disorders, including memory loss, personality changes and depression.
The researchers call for repetitive head impacts and CTE in children to be treated like exposure to lead, mercury, smoking and sunburn.
Their research was published in the journal Frontiers in Neurology.
“Highest scientific confidence” in causation
Researchers from nine academic institutions in six countries, including the University of Melbourne and the University of Sydney, have brought together independent studies of CTE from around the world and in different sports.
They then checked this evidence against an established set of criteria designed to test whether there was a causal relationship between environmental exposure (i.e. repeated impacts on the head) and an adverse health effect. (ETC).
This method, known as the Bradford Hill criteria, has previously been used to establish the causal relationship between smoking and lung cancer.
In the case of CTE, the researchers were able to satisfy each of the nine criteria, said Alan Pearce, co-author of the report.
“Each institution took a section of the criteria and collated the evidence to meet the criteria,” said Dr. Pearce, a neuroscientist at La Trobe University.
“It was basically a very structured review of existing research.”
The nine criteria included showing that there was a consistent association between repeated head impacts and CTE, that it occurred after the head impact, that there was a plausible mechanism between the cause and effect, and that greater exposure leads to greater incidence.
“We looked at different sports with different characteristics, and they all showed a similar thing,” Dr Pearce said.
“Anyone who engages in repetitive head trauma…they get CTE where you don’t see it in random populations of people.”
Lead author Chris Nowinski, CEO of the Concussion Legacy Foundation, said the analysis “gives us the greatest scientific confidence that repeated head impacts cause CTE.”
Proving Causality Without Randomized Controlled Trials
Adrien Cohen, concussion expert and founder of the charity Neurosafe, said the study was “extremely powerful evidence” of the link between repetitive head impact and CTE.
“This article is fundamental because it indicates that here are the criteria by which we can say that A leads to B and it is irrefutable,” said Dr Cohen, who was not involved in the research.
He said sports administrators often argued there was not enough evidence linking repeated blows to the head to the CTE.
“It’s been the party line in sports for a long time,” he said. Because CTE can only be diagnosed post-mortem, and symptoms typically appear years or even decades after a brain injury occurs, it has been difficult to prove causation.
Randomized controlled trials, for example, are simply not possible over such a long period.
“The importance of this document from this group is that they said when we are not able to do [randomised controlled trials] To prove causation, let’s look at these criteria.
“It gives a framework for determining whether you can justify moving from observed association to causation.”
How common is CTE?
The study does not shed light on how many sports people are likely to have CTE, said Michael Buckland, head of the Australian Sports Brain Bank at the University of Sydney and co-author of the study.
“One of the problems is that CTE can only be diagnosed at autopsy and there’s a big selection bias there – usually only people who have problems raise their hands to donate their brain.
Other analyzes have estimated that the rate of CTE among professional American football players may be around 10%.
“These estimates aren’t definitive, but they tell you that at least in American football, it’s likely that a significant number of professionals have this condition,” Professor Buckland said.
The most recent study’s causation will prove a “turning point” and provide “a shot in the arm for any class action lawsuits that are looming”, he said.
“Change can’t come soon enough.”
Calls to the head, fight bans to high school
The Australian Institute of Sport’s most recent position statement on concussion in sport, last updated February 2019, states that “the link between sport-related concussion and CTE remains tenuous” .
It was outdated, Dr. Pearce suggested.
“We hope sports will take notice and now recognize the risks more seriously,” he said.
This could include changing contact sport rules, more consistent enforcement of existing safety rules and restricting tackling and heading in youth sports.
“We need to modify children’s sport to try to reduce that exposure the same way we smoke or drink,” Dr Pearce said.
“The risk of CTE doubles every three years of exposure to repetitive head trauma.”
CTE has been diagnosed in people as young as 18 to 20, he said.
“It’s not just an old person’s disease.”
Earlier this month, the Football Association of the United Kingdom announced that it was testing a ban on deliberately directing children under 12, which will come into force in two seasons.
In the United States, similar rules have been in effect since 2015.
Restrictions on tackles and caps should apply around the start of high school to limit the risk of CTE, Dr. Cohen said.
“When brain maturation has taken place, children will be more resilient,” he said.
