Can routine hits from playing American football cause damage to the brain?

Researchers discovered that routine hits experienced during contact sport can lead to long-term brain damage even when athletes are not diagnosed with concussion.

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Researchers at Carnegie Mellon University (PA, USA) and The University of Rochester Medical Center (NY, USA) discovered that concussions are not the sole cause of damage to the brain in contact sports. 

The first part of the study, which looked at college American football players, revealed that typical hits sustained in just one season can cause structural changes to the brain. 

The team of researchers studied 38 players, by placing devices that measured accelerative force in their helmets for every practice and game. MRI scans of the players’ brains from before and after the season of play were compared for structural changes. 

Out of the 38 players only two suffered a clinically diagnosed concussion during the season, however, the MRI scans revealed that two-thirds of the players experienced a decrease in structural integrity of their brain. 

Specifically, the researchers discovered reduced white matter integrity in the midbrain after the season compared with before, suggesting the injury was associated with playing football. Furthermore, researchers discovered the amount of white matter damage correlated with the number of hits to the head the player sustained. 

Senior study author, Brad Mahon (Carnegie Mellon University), commented: “The big hits are definitely bad, but with the focus on the big hits, the public is missing what's likely causing the long-term damage in players' brains. It's not just the concussions. It's everyday hits, too.”

As the midbrain is relatively rigid it absorbs forces differently to surrounding softer tissue making it biochemically susceptible to damage from head hits. The researchers decided to focus on this structure, hypothesizing that the midbrain would be the ‘canary in the coal mine’ for sub-concussive hits. 

Lead author, Adnan Hirad (University of Rochester) commented: “What we cataloged in our study are things that can't be observed simply by looking at or behaviorally testing a player, on or off the field. These are 'clinically silent' brain injuries.”

Astronauts on the space shuttle experience 3 gs during lift off and race car drivers feel the effects of 6gs. The players in this study together experienced nearly 20,000 hits in total across all practices and games, with the median force being 25gs. 

“We measured the linear acceleration, rotational acceleration and direction of impact of every hit the players sustained. This allowed us to create a three-dimensional map of all of the forces their brains sustained,” Hirad explained.

From the MRI scans, researchers discovered that rotational acceleration (causing the head to twist) more so than linear acceleration (head-on impact) is correlated with the observed changes in the midbrain. 

"This study suggests that midbrain imaging using diffusion MRI might be a way in the future to diagnose injury from a single concussive head hit and/or from repetitive sub-concussive head hits," commented co-author, Dr. Jeffrey Bazarian (University of Rochester).  

The second part of the study focused on athletes from various other contact sports, including 29 players who had clinically defined concussion and 58 who did not. 

Like the American football cohort these players displayed reduced structural integrity in the midbrain. Furthermore, they also exhibited increased tau proteins. 

The findings from this study suggest that repetitive sub-concussive hits from playing contact sport are associated with long-term brain damage in players. Further larger-scale longitudinal studies of athletes are needed to look at the effects on athletes in various age groups. 

“We also need to re-evaluate how we make return-to-play decisions. 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,” concluded Hirad. 

Sources: Hirad AA, Bazarian JJ, Merchant-Borna K, et al. A common neural signature of brain injury in concussion and subconcussion. Sci. Adv. doi:10.1126/sciadv.aau346 (2019) (Epub ahead of print); 

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Kimberley Ndungu

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