Professional rugby can cause "concerning" changes to a player's brain structure, research suggests.
Heading footballs has long been linked to cognitive damage, and several of the high-profile stars of England's 1966 World Cup victory later developed dementia.
Chronic traumatic encephalopathy specifically is a progressive and life-threatening brain disorder associated with repeated traumatic brain injuries, usually diagnosed post-mortem.
With rugby famous for its scrums, scientists from Imperial College London analysed the MRI brain scans of 44 elite players.
Results – published in the journal Brain Communications – reveal a significant proportion of the players had abnormalities in their white matter, which acts as the "wiring" within the vital organ, enabling cells to communicate.
They also showed signs of "microbleeds", a marker of underlying damage.
Nevertheless, experts have stressed it is unclear if these abnormalities indicate disease, with exercise in itself lowering the risk of dementia.
"At present, the long-term consequences of these brain structure abnormalities are unknown and require further research," said Lauren Pulling, CEO of The Drake Foundation, a not-for-profit organisation investigating head injuries.
"However, taken together with existing evidence across different sports, as well as recent cases of rugby players being diagnosed with brain diseases in their 40s, they are painting a concerning picture when it comes to players' long-term brain health."
The Imperial scientists analysed 41 male and three female rugby players between July 2017 and September 2019.
At the start of the study, all of the players had an advanced MRI scan, with around half undergoing the same assessment a year later.
The scan allowed the scientists to assess the players' white matter and blood vessel structure. These were then compared against the scans of non-collision athletes and volunteers who do not play sport.
Of the rugby players, 21 were assessed shortly after a mild head injury, which often causes a concussion. These make up around one in five of the injuries that occur in England's professional rugby union.
Results reveal just under a quarter (23%) of the rugby players showed signs of abnormalities to their axons – the "wires" of cells – or small tears in their blood vessels, which can cause "microbleeds".
These abnormalities occurred regardless of whether the players had endured a recent head injury, but were not linked to a worse performance on memory tests.
"The results suggest there are changes in structure," said Dr Virginia Newcombe, from the University of Cambridge.
"What is less clear is what these changes may mean.
"However, the increased presence of small microhaemorrhages and the types of structural changes seen on the MRI makes it likely at least some of the findings may indicate damage, and potentially neurodegeneration."
Watch: What is chronic traumatic encephalopathy?
Experts have stressed the study is small, raising the risk the results were down to "pure chance".
"Brain scans can be changed by factors other than irreversible brain damage," added Professor Derek Hill, from University College London.
"For example, dehydration and some medicines can result in change in fluid balance in the brain that can be picked up by advanced MRI."
The results may also not apply to non-professional athletes.
"It is important to note our results in adult professional rugby union and league players are not directly comparable to those who play at local or youth levels," said study author Karl Zimmerman.
"The overall health benefit of participating in sports and physical exercise have been well established, including the reduction in mortality and chronic diseases such as dementia."
Nevertheless, Professor Huw Williams – from the University of Exeter – added: "Concussive blows are more common in amateur and school level rugby.
"Therefore, there is a need to also undertake related research work in these groups and, of course, whether further changes in the game may be needed to reduce risk, such as more emphasis on touch rugby at younger ages".
Dr Simon Kemp from the Rugby Football Union (RFU) agreed "it is unclear what the individual long-term implications are regarding the brain changes", but added "it is clearly a priority to investigate this further."
"The RFU is fully committed to advancing our understanding of the short, medium and long term consequences of head impacts and concussions so we can ensure we can make continued improvements in player welfare," he said.
Watch: Former rugby union player 'can't remember winning the World Cup'