A study of American football players has found that even seemingly minor hits to the head – ones that don’t cause concussion – can bring long-term problems.
They damage a brain area linked to chronic traumatic encephalopathy (CTE), a condition that leaves some sportspeople with thinking problems, mood disorders and even dementia.
A team led by Adnan Hirad, from the University of Rochester Medical Centre, US, put sensors in the helmets of 38 male football players to monitor the force of head impacts across a season of the notoriously high-contact sport.
The sensors were primed to measure two kinds of impact: linear movements, where the brain bounces forwards and backwards after a hit, and rotational movements, where it twists around on the central stalk known as the brainstem.
The players also got MRI brain scans before the season started and just after it finished.
When the results came in, the players had sustained a colossal 19,128 head hits, averaging more than 500 impacts each.
Nearly 60% of those happened during practice and 37% were during games. The typical force of linear hits was just over 25gs: astronauts lifting off into space experience 3gs, and race car drivers around 6gs.
Strikingly, however, those linear impacts were not the smoking gun in this study.
When the researchers compared the pre- and post- season brain scans, they found players had damage to the midbrain, a part of the brain stem also affected in CTE. But it was the rotational hits, not the linear ones, that were linked to the midbrain damage.
“The twisting aspect of the force is what is more correlated with the changes that we are observing in these players,” says Hirad.
“It makes sense because this part of the brain has what we call ‘biomechanical susceptibility’ because it’s really narrow; you know, you have this big dome sitting on a very narrow cone and that predisposes it to twisting,” he says.
Critically, the vast majority of those hits were not bad enough to cause concussion, which can involve loss of consciousness, confusion and amnesia. Only two of the 38 players got concussed in the study.
“There’s been an emphasis on concussion and there’s been an emphasis on the big hits that everybody notices,” says Brad Mahon, from Carnegie Mellon University, US.
“What our study indicates is that that’s just the tip of the iceberg. There are all of the other hits that players sustain in the course of playing a typical game of football without concussion, and all of those sub-concussive hits seem to add up to cause damage to the brain,” he says.
The researchers made a final sobering discovery. They looked back at a separate group of 29 contact sport athletes who were monitored after a concussion. Those people had MRI scans done and also a blood test for tau, a protein released when the brain gets damaged.
The team found that higher tau levels in those athletes were linked to changes in the same brain area, the midbrain, affected in footballers in the current study. Tau has also been implicated in CTE.{%recommended 7603%}
“The brain accumulates a whole variety of toxic, like poisonous proteins,” says Jeffrey Bazarian an emergency physician at the University of Rochester Medical Center.
“When those nerve cells break after each head hit it’s almost like garbage builds up in the brain. And the brain has a system for getting rid of that, but if it’s overwhelmed because it’s getting hit 70, 80, 90 times a day, those proteins just build up,” he says.
A question remains as to whether the findings, particularly the focus on rotational injury, carry lessons for other sports. Rotational impacts have been linked to concussion in hockey, soccer and indeed Australia’s AFL football, and so the current study would appear to have relevance beyond the gridiron field.
Ultimately, it raises the disturbing possibility that apparently minor head injuries could put a player at risk of CTE down the track, something that might be picked up on MRI scan of the midbrain. Which is not, of course, a technology likely to be open to coaches on the sidelines.
“We also need to re-evaluate how we make return-to-play decisions,” Hirad says.
“Right now, those decisions are made based on whether or not a player is exhibiting symptoms of a concussion like dizziness or loss of consciousness. Even without a concussion, the hits players are taking in practice and games appear to cause brain damage over time.”
The study appears in the journal Science Advances.
