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In a survey of 8,900 students in grades seven through 12, close to six percent reported losing consciousness for at least five minutes or being hospitalized for head trauma in the past year, and 20 percent reported ever having that type of injury.

Researchers found that students with poor grades and those who drank and smoked marijuana were especially likely to say they'd suffered a traumatic brain injury. Just under half of those injuries occurred outside of sports.

"Traumatic brain injury is preventable," said Gabriela Ilie, who led the study at St. Michael's Hospital in Toronto.

"If we know who's more likely to be vulnerable, if we know under what conditions those injuries are occurring, we can talk to the parents, we can talk to the students, we can talk to hospitals, we can talk to communities, and together we can almost change our mindset in terms of how we see (this) injury," she told Reuters Health.

Ilie, whose findings were published on Tuesday in the Journal of the American Medical Association, said she was "very surprised" at how many students reported a history of serious head trauma.

Close to half a million kids age 14 and under are seen in U.S. emergency rooms for a traumatic brain injury each year, according to the Centers for Disease Control and Prevention. For all types of traumatic brain injuries, including milder ones that don't require hospitalization, the rates among U.S. kids are between 6 and 8 per 1,000, according to CDC.

It's possible the new study's results were skewed by having adolescents recalling their own injuries, the researchers said.

"The rates that they found are much, much higher than I think has previously been reported in any study," said Dr. Matthew Eisenberg, who has studied concussions at Boston Children's Hospital.

What's more, the definition of head injury used here "gives you a pretty severe subset of injuries," Eisenberg, who wasn't involved in the new research, told Reuters Health.

He said the study will have to be repeated in other groups of teenagers to see if future findings match up. And there's still the question of how drug use and poor school performance are related to concussions, he said, as this study doesn't get at which came first.

"It doesn't answer the question of cause and effect. It certainly makes sense to think that kids who are drinking are more likely to be injured. On the flip side, maybe if you've had a bad head injury, we know that things like depression and poor school performance go along with that."

CONCUSSIONS IN POP WARNER

Earlier this month, results were released separately suggesting that concussion rates among U.S. 8-to-12-year old football players may be just as high as those on high school and college teams.

Researchers tracked injuries on 18 Pop Warner youth football teams and found 20 out of 468 players were diagnosed with concussions during a single season.

That injury rate - one for every 568 practices and games in which an athlete participates - is comparable with what researchers have reported for older players, Anthony Kontos of the University of Pittsburgh Medical Center and his colleagues said.

All but two of the concussions came during games - suggesting that efforts to prevent injuries in Pop Warner by limiting contact in practice may not have the intended effect, the researchers wrote in The Journal of Pediatrics.

Eisenberg pointed out that there were more people watching young athletes during games - including the researchers - which would make it easier to catch a concussion in that setting.

"On the other hand, it makes sense that you wouldn't get as injured at practice," he said. "Practice is in general probably less intense than a game."

Studies have been piling up showing the potential harms of concussions among kids, including headaches and memory problems. But questions remain about the long-term impacts of suffering a serious head injury during childhood, when the brain is still developing.

SOURCES: bit.ly/JjFzqx Journal of the American Medical Association, online June 25, 2013 and bit.ly/10QQtip The Journal of Pediatrics, online June 10, 2013.