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A small study of professional soccer players found that even those who have never experienced a concussion still have changes in the white matter of their brains, likely from routine and unprotected headers.

The brain‘s white matter is made up of nerves and their myelin protective coating (similar to the insulation that blankets electrical wire) that play a significant role in connecting brain regions and establishing neural networks that are critical to cognition. Previous studies have investigated how concussions lead to changes in this white matter, but a new study lead by Dr. Inga K. Koerte of Harvard Medical School in Boston, is one of the first to look at how even blows to the head that aren’t considered concussions may lead to traumatic brain injury.

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In the study, the researchers compared bran scans of 12 male soccer players from German elite-level soccer clubs who had not experienced a concussion, to brain scans of 11 competitive swimmers who had similarly never experienced repetitive brain trauma. The research team used high-resolution diffusion tensor imaging (DTI), which looks at the brain microscopically and is much more effective at catching white matter changes than the standard MRI.

The researchers found surprising alterations in the white matter that were “consistent with findings observed in patients with mild TBI, and suggesting possible demyelination [nerve disorder].” Even though the players had no concussions, their brains told a different story of damage, including changes to the myelin sheaths surrounding nerves.

“Although our study is small, it is the first to look at soccer players with no symptoms and no diagnoses of concussions,” says Koerte. “We think it is an important finding not just for soccer players, but other athletes of other sports too.”

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While concussions have long been a part of professional sports such as boxing and football, researchers are still struggling to define concussions clinically, and research into brain changes resulting from repetitive blows to the head is a relatively new area of research. “[Brain damage from repetitive blows] would have tremendous public health implications,” says Dr. Jeffrey Bazarian, an associate professor of emergency medicine at URMC. “If players are damaging their brains, it is a large public health issue because everyone, even at a young age, hits their head like this. But right now we really don’t have enough information.” Bazarian was not involved with the study, but has also used DTI to assess mild brain injury in high school football and hockey players.

Although Bazarian credits the study authors for using DTI, he says that the lack of strict criteria defining concussions means they are difficult to diagnose. And the culture of professional sports means it is unlikely players will admit to having a concussion, especially when their pay depends on their staying in the game. “The athlete themselves will know if they’ve had a concussion or not, but there is no way to call them out on that,” he says.

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Koerte says more research involving more players at different ages is needed to get a better idea of how potentially damaging head trauma can be at young ages. If it turns out repetitive head bumps are harming cognition, Koerte says regulations like sitting out after a single or a designated number of hits may be needed. Or heading shouldn’t be allowed at all until a certain age.

The study is published in the Journal of the American Medical Association (JAMA).