More than a billion people tuned into the 2014 FIFA World Cup to see the most beautiful game played at the highest level.

Something else the audience saw? Soccer players colliding with each other, the ball, sometimes both — and unknowingly suffering from debilitating concussions.

While this may seem like a common — even inevitable — part of the game, concussions are a form of traumatic brain injury that can come with irreversible, long-term health repercussions.

In recent years, sports doctors, parents of young players, and others have pushed for more stringent concussion protocol on and off the field. But Michael Cusimano, a neurosurgeon at the University of Toronto, says that protocol isn’t actually being followed.

In a study published Tuesday in the Journal of the American Medical Association, Cusimano and his team show that the majority of head collisions were not properly treated in the 2014 FIFA World Cup.

Concussions are, at their most basic level, a jostling of the brain. Typically, they’re caused by a blow to the head. In professional soccer, that often happens when players clash — an elbow to the head or two heads colliding at high velocity can cause serious trauma. That forcible shaking of the brain can cause a release of chemicals and strain or injure brain cells.

For all that action inside the brain, it can outwardly be difficult to feel like something’s seriously wrong. In the past, athletes were often told they’d know a concussion when they saw stars or blacked out, but more recent research shows that many people can be concussed without losing consciousness, memory, or even knowing they have a concussion at all. And still other times, players continue to compete despite being concussed.

That last part — returning to play despite being concussed — is troubling, because doing so worsens symptoms and prevents recovery. In rare cases, players can be severely disabled or even die if they receive a second concussion before the first one is healed. Plus, repeated concussions can increase the chance of dementia later in life. Thanks to continued research and analysis, concussion protocol, or the means by which coaches and team doctors determine if a player is concussed, has gotten more precise.

That doesn’t mean it’s being followed, though. Cusimano’s team studied footage of all 64 games of the 2014 FIFA World Cup to assess players for concussions according to officially decreed protocol. During the entire series, the team identified 81 head collisions, based on common concussion symptoms like disorientation and sudden physical weakness.

They found that in 67 of those 81 collisions, the athletes in question exhibited at least two signs of a concussion — a pretty good indication they’d been injured. But only four of those players were sidelined. The rest went back into the heat of competition, some with a concussion assessment and some without. Ultimately, the study showed that concussion protocol wasn’t followed in a whopping 63 percent of FIFA collisions.

“I had a sense it was going on, but I didn’t have a sense it was going on to this degree,” Cusimano says of the results. In the entire 64-game series, the neurosurgeon says he saw only two concussion checks that met his standard. The assessments, he emphasized, lasted an average of 84 seconds. That’s far below the gold standard of concussion testing, the Sport Concussion Assessment Tool, or SCAT, which can take 10 minutes to complete.

Cusimano believes this poor adherence to the rules is a problem not just for the FIFA players potentially put in harm’s way, but for soccer fans around the world and players at all levels. “The World Cup is actually an event that’s watched by literally billions of people around the world,” he says. “So what goes on there is very public and it’s seen by a lot of people and what goes on there has trickle down effects to other levels of soccer.”

Photos via Getty Images / Amin M. Jamali, Getty Images / Matthias Hangst