Study limitations
- No cause and effect relationship established: While the study findings "add to our understanding of the effects of heading on brain structure and function beyond the recognized role of heading as a relatively uncommon cause of concussion," Lipton and his colleagues write, "an important limitation ... is its cross-sectional nature, which does not allow us to infer a causal relationship between heading and imaging or functional changes."
- Relied on self-reporting over short period of time. The study relied on self-reporting by athletes of the number of times they headed a soccer ball over the prior 12-month period as a proxy for lifetime exposure, which is likely most important, as suggested by emerging research on the long-term effects of concussive brain injury such chronic traumatic encephalopathy (CTE). As with any self-reporting measure, researchers were depending on the players' ability to recall the number of heading events, which was subject to bias, and has been criticized as unreliable. [20] It thus did "not address the role of lifetime (or even relatively recent) cumulative exposure, as players typically begin as children." Webbe agrees that, in his experience, players rarely estimate heading with great accuracy. But a 2009 study of which he was co-author found that, relative to each other, at least with children, players' estimates when compared to direct obervation do properly categorize them as frequent versus infrequent headers. [25]
- No information on individual heading events: The study was also "not able to characterize features of individual heading events, such as site of impact, velocity, rotation, and other features." Because of the likelihood that heading events vary greatly, this data could only be captured with prospective use of biomechanical instrumentation (eg. impact sensors), which would still be affected by the limitation on assessment of cumulative lifetime exposure.
- Other measures used in DTI not considered: The findings were based on analysis of only one diffusion measure called fractional anisotropy (FA) because it is the measure that has been most widely studied in TBI and, particularly, in association with cognitive outcomes in TBI patients. Other measures could shed additional light on the brain injuries seen, but the authors did not expect that the basic associations they reported would change significantly.
Injury thresholds impractical?
"Characterizing the dose-response curve linking heading and TBI toward understanding threshold effects might facilitate safety guidelines that could help minimize the risk of adverse effects on the brain," Lipton writes. "Prospective monitoring of exposure at the team level, perhaps to be termed head counts, could identify a point at which a player's heading should be curtailed for a specific recovery period, also to be defined through further research. ... [which] is thus essential to develop evidence-based protective strategies that can ensure the future of safe soccer play."
"What can't be directly concluded from this study is the issue of the existence of a frequency 'threshold' above which subconcussive blows are clearly damaging to the integrity of the brain," says Florida Tech's Webbe. "Too many factors are not controlled, including the intensity of the heading episodes, the use of proper technique, the interaction with previous documented concussions, and the biggest to me, idiopathic differences between people with respect to propensity to be concussed. We know that people differ in this regard, but we don't know why. Current work looking at genetic markers and other predisposing factors is critical in this regard."
Webbe said that, "I have a hard time with the threshold idea just from a practical standpoint. There is likely no firm threshold that could be applied to all individuals, so then we get to a statistical prediction where we can predict that N% of individuals who exceed a given threshold will develop brain impairments. It won't matter where the threshold is set, some number of individuals will be affected."
"I am intrigued by this study, and any study that proposes to give some aspect of concrete recommendations for potential overuse or cumulative injury," said Koutures. "Right now, the only real evidence-based recommendations we have to prevent injury are the baseball pitch counts, so I applaud any effort to give more evidence and numbers-based information to help families and medical professionals make more informed decisions."
"With that being said, given the multiplicity of factors that contribute to risk for concussion and the recovery spectrum after a concussion, it may be very difficult to come up with a uniform set of recommendations for preventing head injuries, be it number of headers, number of blows to head on the football field, or even magnitude of blows to head. From professional experience, seems that every person has some form of cognitive threshold (some are higher, some are lower) which once exceeded, can increase the risk for concussion and complicated recovery. Several factors may contribute to a lower threshold (see table at the end of this article), such as history of learning issues/anxiety/depression, past headaches, past concussion, family history of concussion, etc. There are athletes which obvious factors that lower their risk, but many who either we don't appreciate the higher risk or don't have easy to identify higher risk concerns."
"Thus, having a one-size-fits all approach to declaring a heading threshold is in theory a commendable and possibly very useful idea, but in reality, will likely be difficult and impractical to impliment given the amazing heterogenicity of how concussions are caused, how athletes react to concussion, and the short-term and long-term results of suffering a concussion," said Koutures.
Caution ahead
"As we see more and more scientific data that reflects true structural, pathological changes in the brain due to heading combined with some level of cognitive change, heading [may] assume[ ] a position in medicine similar to tobacco," suggests Webbe. "We know clearly that some number of people who smoke cigarettes will develop lung cancer and that this number will be much higher than in those who do not smoke. In our society we have chosen not to ban tobacco products as yet, even though everyone is cautioned against smoking."
"I think that it is time to caution against heading until future research shows that it is perfectly safe or until the soccer community accepts that it is a harmful practice, and legislates it out of existence."
