Two research groups have demonstrated brain imaging findings in collegiate football players over the course of a season.Importantly these abnormal finding occurred in players who did not experience a concussion..While concussions occurring at all levels of football competition has finally attracted some long overdue attention,more recently the significance of so called " sub-concussive" head blows is under scrutiny.
Bazarian et al studied 10 college football players over the course of one season. ( "Persistent Long Term cerebral White Matter Changes after Sports related Repetitive Head Impacts. Plos one 9(4),e94737)
Head impacts were recorded and measured by helmet accelerometers and brain imaging was done preseason, immediately post season and six months after the end of the season. Imaging was done by the diffusion tensor technique (DTI).This MR technique can demonstrate abnormalities in the white matter and can visualize connections of fiber tracts between different parts of the brain.Current theory regarding traumatic brain injury is that axons are stretched with resultant micro damage.These changes are not detectable by conventional MR while DTI is capable of detecting abnormalities associated with so-called mild brain trauma.
White matter changes were noted in these players none of whom sustained a clinically evident concussion.
The changes in most players,but not all, were also seen in the six month followup images. Evidently in some cases the 6 month period with no head impacts allowed recovery or at least return to the pre season scan pattern.. There was a correlation between number of head impacts and DTI findings. The DTI changes were not correlated with changes in cognitive testing or tests of balance.The authors state that it is not known whether the changes noted represented damage per se or recovery and beneficial plasticity. The number of head impacts ranged from a low of 431 to 1850.Multiple head impacts occur regularly in lineman while quarterbacks and wide receivers are more at risk for more severe single hits and concussions.
A possibly encouraging finding in the Bazarian study is that none of players demonstrated any scan abnormalities at the beginning of the season.One would expect that if these findings were to persist for very long periods of time (longer than 6 months) that these players who likely played football for a number of years by this time would have shown some abnormality from repeated head blows.
Another TDI imaging study was published by TW McAllister ( Effect of head impact of diffusivity measures in a cohort of collegiate contact sports athletes, Neurol. 10:1212/01Dec 11 2013.) There was at least one important difference from Bazarian's results. Quoting the authors from this study which involved 80 varsity football and ice hockey players and 79 non contact sports participants:
"The magnitude of change in corpus callosum MD (mean diffusivity) was associated with poorer performance on a measure of verbal learning and memory."
What are the underlying tissue changes corresponding to the DTI patterns? No one knows whether they represent damage to the axon and or the myelin sheath and/or edema. See here for a comprehensive discussion of the DTI technique and findings in head trauma.
Regardless of the exact relationship between imaging findings and the tissue changes, it is difficult to argue that subjecting (allowing) young brains to sustain multiple hits can be anything other than potentially harmful.Although recent emphasis on concussion and chronic traumatic encephalopathy (CTE) has lead to some rule changes in the direction of limiting to some degree head blows and rules regarding management of concussions ( when to be allowed to return to play etc) ,every Friday night tens of thousand of young men hit each other repeatedly in the head and are cheered on by hundred of thousands of football fans and parents. For now the dogs continue to bark while the caravan moves on.
Much to no one's surprise a similar study has been published involving measurement of head impact forces and DTI changes in a high school football team over the course of a season. (Devenport,EM et al, "Abnormal white matter integrity related to head impact exposure in a season of high school varsity football"
J of Neurotrauma 2014 Jul 14, published ahead of print) .There was a significant linear relationship between their measure impact force and DTI changes as well as relationship between DTI measurements and changes in a memory test.