Review provides ‘conclusive proof’ that repetitive head impacts cause CTE

Neurodegenerative disorders

By Michael Woodhead

10 Aug 2022

A new analysis by Australian and international experts has found there is now conclusive evidence to support a causative link between repetitive head impacts (RHI) and chronic traumatic encephalopathy (CTE).

While sports bodies have long played down a cause and effect relationship between sports concussion and CTE, the new analysis using trusted ‘Bradford Hill criteria’ found robust evidence of a causal relationship, sufficient to confirm the hypothesis, its authors say.

The researchers from the University of Sydney and University of Melbourne, as well as academics from Harvard and Oxford say their findings show there is a need for urgent action from sports organisations, government officials and parents on sports concussion.

“We have the highest confidence in the conclusion that RHI causes CTE. We encourage the medical, scientific and public health communities to now act under the premise of a causal relationship and take immediate action to prevent CTE, minimise risk, and develop therapeutics to slow or stop disease progression,” they write.

In their paper, published in Frontiers of Neurology, they say that until now, policies on RHI and CTE have been guided by consensus statements from the controversial Concussion in Sport Group (CISG), chaired by Dr Paul McCrory. This group, which has questioned or dismissed the quality of evidence around repetitive head injury and long-term neurodegeneration, has been accused of bias through conflicts of interest and funding from corporate sporting groups.

“Developing consensus on CTE causation has important public health and medico-legal implications,” the authors said.

“If RHI causes CTE, then it becomes an environmentally caused brain disease that is preventable. If CTE is environmentally caused, then settings with exposure to RHI, which could include participation in some sports, may become regulated by governmental organisations that oversee workplace and public safety, and individuals and organisations could become financially liable for the care of those who develop CTE.”

In their analysis, the researchers examined the literature on RHI and CTE through the Bradford Hill criteria, which were developed to determine if one can justifiably move from an observed association to a verdict of causation.

The nine criteria include viewpoints such as strength of association, consistency, specificity, temporality, biological gradient, plausibility, coherence, experiment, and analogy.

While acknowledging the criteria could not be used as “indisputable evidence”, the review concluded that the incredible strength of association found, combined with robust evidence in all nine benchmarks “suggests an extremely high likelihood of a causal relationship”.

“That conclusion is bolstered by the complete absence of evidence for plausible alternative hypotheses. Besides RHI, there does not appear to be any other common variable or possible mechanism that would explain why so many contact sport athletes, in diverse sports, and in multiple countries, are being diagnosed with CTE, while individuals without RHI exposure are not.”

The authors said the findings had important implications because international sporting organisations including FIFA, World Rugby, the IOC, as well as sporting codes such as AFL and NRL have thus far refused to publicly acknowledge a causal relationship.

“This innovative analysis gives us the highest scientific confidence that repeated head impacts cause CTE,” said study lead author Dr Chris Nowinski, Concussion Legacy Foundation CEO.

“Sport governing bodies should acknowledge that head impacts cause CTE and they should not mislead the public on CTE causation while athletes die, and families are destroyed, by this terrible disease,” he added.

The findings also have major implications beyond professional sport, particularly for children, the authors said.

“This analysis shows it is time to include repetitive head impacts and CTE among child protection efforts like exposure to lead, mercury, smoking, and sunburns,” said co-author Dr Adam Finkel, Clinical Professor of Environmental Health Sciences at the University of Michigan School of Public Health and a former Director of Health Standards, US Occupational Safety and Health Administration (OSHA).

“While we call for more research, we also believe that the strength of the current evidence compels us to move past a scientific discussion focused solely on filling gaps in the evidence to focus on immediately implementing aggressive CTE mitigation programs, especially for children,” the authors said.

“We encourage awareness efforts so parents, athletes, and policymakers can better understand the risks associated with RHI, change how games are played to reduce or eliminate RHI — especially for children — and make more informed decisions regarding participation in contact sports.”

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