Chronic traumatic encephalopathy after repeated soccer headings

Methodological and societal barriers

Posted by Benjamin Gravesteijn on Sunday, October 25, 2020


This weekend, the NRC, an important Dutch newspaper, published an article concerning the influence of major sports organizations (UEFA, FIFA, AFL) on research concerning chronic traumatic encaphalopathy (CTE). CTE has seen more and more public interest in the last couple of decades. This disease has been described to occur after a career in sports such as soccer, football, or boxing: these are all associated with a substantial amount of repeated mild concussions. This article discusses how research about CTE is being influenced by major sports institutions, much like how the tobacco industry influenced research about smoking for their own interest.

These institutions are described to influence research via two different strategies. On the one hand, they finance and help writing guidelines and recommendations which disregard low-tier evidence such as mouse models, and pathological case reports. Although it is common in academia to focus on “higher-tier” evidence, evidence from exactly these sources are quite consistent in their findings: repeated mild concussions lead to an early-onset version of Alzheimer’s disease, which is called CTE. On the other, they are described to disrupt financial streams to objective, independent researchers who investigate CTE. Rather, they apparently want to control research by directly financing their own researchers.

Finally, the newspaper implies, by comparing this situation to the tobacco industry, that CTE is real, and that institutions try to hide the skeletons in their closets. However, I want to underscore that evidence concerning CTE is actually quite thin, and there is a good reason why.

Methodological barriers

Let us go back to the basics. In epidemiological research, we often want to assess whether a specific “exposure”, in this case repeated mild concussions, is causally related to an “outcome”, in this case CTE. If we focus on soccer, this means that we need to know whether repeated heading is causally related to CTE. This was exactly the subject of my bachelor’s thesis, so I have read most of the existing literature on this topic.

There are multiple limitations as to why the exposure in this question is hard to measure. Heading is not just the cumulative frequency of heading someone performs in their career. Rather, the “density” of headings per unit of time might also influence the risk of CTE. For example, 10 headings evenly spread out over a year might be less harmful than 4 consecutive headings in one game: the brain might have less time to recover in between these mild concussions, and therefore be extra vulnerable. Moreover, not only the frequency, but the severity of the impact is likely relevant. And this, is of course, much harder to measure. Finally, it might be easier to measure the frequency of headings in professional, recorded matches. But the training sessions between matches also include headings, which of course could also contribute in the end to CTE.

However, there are also multiple limitations to address when measuring the outcome. First and foremost, the outcome only occurs after twenty to forty years. We would therefore need to follow the observed soccer players for at least thirty years to observe the outcome of interest. Secondly, the most objective way to observe the outcome is by dissecting the brains of those affected and observe the pathological consequences of CTE: accumulation of certain proteins, beta-amyloid plaques, in specific parts of the brain. Of course, the easier way to objectify CTE is by neuropsychological examination at different follow up moments. However, there are other reasons for a decline in neuropsychological functioning than repeated headings that need to be ruled out, such as alcohol or drug consumption or psychiatric illnesses. The major limiation is that these “other” factors for declining neuropsychological functioning are not all perfectly measurable, or simply unknown.

Finally, there are even more limitations of current studies, which include a highly selective sample of players, small sample sizes, and missing follow-up data. Describing these methodological barriers hopefully convinces you that future studies need to bring their “A-game” (to remain in sports discourse) to “tackle” this problem. And unfortunatly, that requires a lot of financial resources.


I cannot refute that sports institutions might have conflicts of interest concerning research in CTE, which should make us more critical about studies that investigate this important relationship. However, the evidence concerning the relationship between repeated mild concussions and CTE is thin, and rightly so. We need better evidence, which requires investments. And these investments should, as the article by NRC also says, come from governments, enabling a stable structure of objective and high-quality research.

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