In a move that will have ramifications for collision sports, the US National Institutes of Health has formally acknowledged a causal link between repeated blows to the head and the neurodegenerative disease chronic traumatic encephalopathy (CTE).

The NIH is the largest biomedical research agency in the world, and the decision to rewrite their official guidance on CTE has been described by campaign groups as a tipping point in the debate about the risks of playing collision sports. In the NIH’s view, research to date suggests the causal link between repeated traumatic brain injury and CTE is clear and unequivocal.

That position is at odds with the one held by the Concussion in Sport Group, which is supported by Fifa, World Rugby, and the IOC, among others. The concussion consensus documents published by CISG have consistently downplayed the connection between CTE and brain injuries sustained in sport. The most recent one, from 2017, states “to date, a cause and effect relationship between CTE and concussions or exposure to contact sports has not been established”, a position that has been cited by multiple sports federations as they defend themselves against both legal challenges and calls to reform.

The NIH’s change in guidance was made after a group of 41 leading scientists, doctors, and epidemiologists co-signed a letter to the National Institute of Neurological Disorders and Stroke (Ninds). The letter cited a recent review of the research into CTE, published in July the Frontiers in Neurology journal, which established a clear causal link with the kinds of recurrent brain injuries suffered by abuse victims, soldiers, and sportspeople in particular. There has been evidence this is the case since the disease was first recognised in the 1950s, the director of Ninds said the causal link was “pretty clear” in 2014, but their official guidance had not reflected that until now.

The change brings the NIH into line with the US Centers for Disease Control and Prevention, who stated in their advice in 2019: “Most research suggests that CTE is caused in part by exposure to repeated traumatic brain injuries.” It means two of the leading independent medical research bodies in the world are in agreement on the causes of CTE. It is to be seen whether CISG’s next concussion consensus will reflect that. The group is holding a conference in Amsterdam on Thursday and Friday to draft the latest iteration of the consensus, which will be published early next year.

CISG is already under increased scrutiny after its chair, and lead author, Dr Paul McCrory, resigned earlier this year when it was alleged there were multiple instances of plagiarism in his own work. At the time McCrory was quoted on Retraction Watch apologising, saying his failure to attribute was “not deliberate or intentional”.

“Now that causation has been established, the world has a tremendous opportunity to prevent future cases of CTE,” said a spokesperson for the not-for-profit group the Concussion Legacy Foundation. “The only known cause of CTE is an environmental exposure, and in most cases a choice – the choice to play contact sports.

“Our goal is to reform all youth sports so they no longer include preventable repetitive head impacts before age 14 – no heading in soccer, no tackling in [American] football and rugby. This change, combined with logical limits to repeated head impacts in sports for people over 14 (such as no hitting in football/rugby practice, strict limits on headers in practice) would be expected to prevent the vast majority of future CTE cases.”

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