Changes in brain structure and function after a concussion remained visible in university athletes even after they were medically cleared to resume competitive sports, researchers reported Thursday.
Using advanced magnetic resonance imaging (MRI), scientists detected lingering alterations in the brain's white matter, and changes in zones linked to vision and planning, they reported in the journal Scientific Reports.
White matter consists of nerve fibres, called axons, that allow various parts of the brain to interact.
In athletes that took longer than usual to recover, the tests also spotted anomalies in areas of the brain associated with movement.
"This is the first concrete evidence we have that the brain is lagging behind in terms of recovery from a concussion," said lead author Nathan Churchill, a post-doctoral fellow at St. Michael's Hospital in Toronto, Canada.
"Our study shows that the neurobiological consequences of concussion may outlast the symptoms we're typically looking for when determining whether an athlete is ready to return to play," he said in a statement.
In making a "return-to-play" determination, trainers and doctors monitor symptoms reported by the athlete, perform fitness tests, and follow a step-by-step protocol for the resumption of intense physical activity.
The findings, however, suggest that longer recovery times may be needed to reduce the chances of repeat injuries in athletes who have been knocked out.
Concussion in sport and recreation is a growing health concern worldwide.
- Dementia risk -
In the United States alone, up to 3.8 million such injuries occur each year in professional, amateur and school sports.
The study, based on a sampling of 27 college varsity athletes from unnamed universities who had suffered concussions, showed that the problem is not confined to violent contact sports such as American football or boxing.
The 27 cases -- which were compared in the study to 27 other athletes who had not endured any head injuries -- also included rugby (9 cases), volleyball (4), lacrosse (3), basketball (3), and ice hockey (2).
The participants in the trial were scanned twice -- just after their injuries (up to seven days), and at medical clearance for return-to-play, typically three-to-four weeks later.
Further research is needed to determine the implications of the observed changes in the brain for concussion management, said the researchers.
"We want to emphasise that, in general, the health benefits of sport participation still outweigh the risk of concussion," said co-author Tom Schweitzer, head of the Neuroscience Research Programme at St. Michael.
The US National Football League has faced growing scrutiny in recent years linked to the issue of concussions and head trauma, with the league agreeing in 2015 on a $1 billion settlement to resolve thousands of lawsuits by former players suffering from neurological problems.
A recent British study concluded that professional soccer players are also at heightened risk of developing a brain disease that can lead to dementia.