MRI scans of professional soccer players reveal changes in nerves and white matter consistent with mild traumatic brain injuries.

In the 123rd minute—the very last of extra overtime—star forward Alex Morgan hit a dramatic header, sending the ball soaring over the hands of the opposing goalkeeper and securing astunning victoryfor the U.S. women’s soccer team against Canada in this year’s Olympic semifinals.

Morgan and her teammates, embracing in the midst of a frenzied pileup, had no idea that her celebrated goal may have compromised the integrity of the white matter in her brain. White matter is responsible for regulating neural signals and ensuring that different regions of the brain work together.

Researchers at Harvard University and Ludwig Maximilian University in Germany scanned the brains of professional soccer players who had never experienced a concussion. They observed changes in white matter consistent with mild traumatic brain injuries and possible damage to the protective coverings of their nerve cells.

“We found that soccer players showed changes in white matter in the brain compared with professional swimmers,” said Dr. Inga Koerte, M.D., lead study author and Senior Research Fellow at the Psychiatry Neuroimaging Laboratory at Harvard Medical School. “This is the first study to show subtle changes in the brain’s white matter architecture in athletes who did not experience a concussion, but instead experienced frequent subconcussive blows to their heads.”

An estimated 250 million people worldwide play soccer. About 18 million Americans play, and 78 percent of U.S. players are under 18. A scientific consensus about the harmful effects of heading on brain health could lead to changes in the rules and regulations governing the sport, especially for children’s teams.

Although it’s possible that the brain damage researchers detected was caused by players’ lifestyles off the pitch, Koerte and her colleague Dr. Martha Shenton, senior author, Veteran’s Affairs investigator, and Professor of Psychiatry and Radiology at Harvard Medical School, think it’s the result of repeated blows from headers like Morgan’s.

“We believe that based on these results there may be an effect of frequent, subconcussive brain trauma,” Koerte said in an interview with Healthline. “Additional research is needed to confirm the results we observed in this small sample of soccer players, and to help clarify the effects that alterations of white matter have on behavior and health.”

Koerte says that awareness is key, both for soccer stars and for the physicians who treat them.

“Given that soccer is the most popular sport in the world, soccer players and sports doctors should be aware that this research shows changes in the architecture of the brain,” said Koerte. “While we don’t yet know what this means, players may be putting themselves at some risk for developing brain injuries. Especially children may be more vulnerable.”

Following research into high rates of shoulder injuries in Little League baseball pitchers, the rules of the game were amended to reduce the number of pitches any one child performs. Koerte believes that officials should consider similar adjustments for childhood soccer leagues.

“Future studies should be encouraged to evaluate the clinical impact of our findings. And safety precautions, such as limiting the number of headers—similar to the daily pitch limit in youth baseball—should be discussed,” said Koerte.

Twelve right-handed male players from a competitive soccer club in Germany, along with 11 right-handed male professional swimmers, had their brains scanned via a specialized magnetic resonance imaging (MRI) technique called diffusion tensor imaging (DTI). None had previously suffered a concussion.

Koerte’s team found widespread “radial diffusivity” in the soccer stars’ brains, suggesting that the structure of their white matter had been altered in ways similar to a mild traumatic brain injury, such as a minor concussion.

A study conducted last year at the Albert Einstein College of Medicine at Yeshiva University concluded that repeatedly heading a soccer ball not only causes mild brain injury, but also cognitive impairment.

Researchers rated the 38 amateur soccer players based on how often they tended to head the ball. They then used DTI scans to analyze brain matter changes, and found evidence of injuries similar to mild concussions in players who headed frequently. Significantly, the researchers determined that players reached the threshold for injury at about 1,000 to 1,500 headers per year, which is only a few per day for full-time players.

When the 38 players then underwent neuropsychological testing, patients who headed most often performed worse than their peers on measures of verbal memory and mind-body coordination. The study authors recommended that coaches and regulators use their research to help develop guidelines for safer soccer play, especially for kids.