Researchers say they have found "conclusive evidence" that the brain disease CTE can be caused by repetitive head impacts during sports with tackling and ball heading.
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 behavioural, mental and cognitive impairments, including memory loss, personality changes and depression.
The researchers are calling for repetitive head impacts and CTE among children to be treated like exposure to lead, mercury, smoking and sunburn.
Their research has been published in the journal Frontiers in Neurology.
'Highest scientific confidence' in causation
Researchers from nine academic institutions in six countries, including University of Melbourne and University of Sydney, collated independent CTE studies from around the world, and across different sports.
They then checked this evidence against an established set of criteria designed to test whether there was a causal relationship between an environmental exposure (i.e. repetitive head impacts) and an adverse health outcome (CTE).
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, a 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 essentially a very structured review of the existing research."
The nine criteria included showing there was a consistent association between repetitive head impacts and CTE, that it occurred after the head impact, that there was a plausible mechanism between the cause and the effect, and that greater exposure lead to greater incidence.
"We looked at different sports with different characteristics, and they were all showing 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 highest scientific confidence that repeated head impacts cause CTE".
Proving causation without randomised controlled trials
Adrien Cohen, a 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 paper is seminal because it says here are the criteria where we can say A leads to B and it's irrefutable," said Dr Cohen, who was not involved in the research.
He said that sports administrators often argued there was not enough evidence linking repeated head blows to CTE.
"That's been the party line across sport for a long time," he said. Because CTE can only be diagnosed post-mortem, and symptoms typically appear years or even decades after brain injury is sustained, proving causation has been difficult.
Randomised controlled trials, for instance, are simply not possible over such a long period of time.
"The importance of this paper from this group is that they've said when we are not able to do [randomised controlled trials] to prove causation, let's look at these criteria.
"It gives a framework to determine if you can justify the move from observed association to causation."
How common is CTE?
The study doesn't shed light on how many sports players might have CTE, said Michael Buckland, head of the Australian Sports Brain Bank at the University of Sydney, and a co-author of the study.
"One of the problems is CTE can only be diagnosed at autopsy and there's a big selection bias in that — usually only people who have problems put up their hand to donate their brain.
Other analyses have estimated the rate of CTE among professional American football players could be about 10 per cent.
"These estimates are not hard and fast but they tell you that at least in American football it's likely to be a significant number of professionals that have this disease," Professor Buckland said.
The most recent study's causal link will prove a "turning point" and provide a "shot in the arm for all of the looming class actions", he said.
"Change can't come soon enough."
Calls for heading, tackling bans until high school
The Australian Institute of Sport's most recent position statement on concussion in sport, last updated in February 2019, states that, "the link between sport-related concussion and CTE remains tenuous."
This was now out of date, suggested Dr Pearce.
"We're hoping that the sports will take notice and hoping they will now acknowledge more seriously the risks," he said.
This could include changing the rules of contact sports, enforcing the existing safety rules more consistently, and restricting tackling and heading in junior sports.
"We need to modify children's sport to try to reduce that exposure the same way we would smoking or drinking," 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 years old, he said.
"It's not just an old person's disease."
Earlier this month, the UK's Football Association announced it was trialling a ban on deliberate heading by children under 12, to come into effect in two season's time.
In the US, similar rules have been in force since 2015.
Restrictions on tackling and heading should apply to around the start of high school to limit the risk of CTE, said Dr Cohen.
"When brain maturation has occurred, kids will be more resilient," he said.
