Letter to the Editor: Telling the Whole Story: Articles Linking Chronic Traumatic Encephalopathy and Repetitive Head Impacts Have Higher Journal Impact Factors

To the Editor:

Debate continues regarding the relationship between repetitive head impact (RHI) exposure and chronic traumatic encephalopathy (CTE). Eagle and Okonkwo sought to identify potential inequality in the dissemination and consumption of this debate. ¹ They employed a bibliometric analysis to identify differences between scientific articles that reported an association between contact sports or military service and CTE (CTE+) and those that reported no association (CTE-). They identified significant inequality in citation count, media dissemination, and popular consumption between groups. Significant limitations associated with their findings exist, which impact interpretation.

Their analyses compared the 10 most-cited articles from each group disregarding the effect of journal impact factor on outcomes. Table 1 shows an average impact factor of 24.4 and 5.5 for the CTE+ and CTE- articles, respectively. Unsurprisingly, articles published in higher-tier journals are typically more noteworthy. Additionally, the average publication year for CTE+ and CTE- articles were 2012 and 2018, respectively. Although the authors noted differences in publication years across groups, they omitted average publication year, and instead reported several studies “had comparable publication years.” ¹

Further, methodological differences existed among studies in each group. As Eagle and Okonkwo reiterate, ¹ CTE can only be diagnosed post-mortem. ² Although all of the CTE+ articles include pathology, five of the 10 CTE- articles lack pathology, and hence are unable to identify CTE. At least one CTE- survey-based study reported CTE-associated symptoms in a high RHI exposure cohort; Grashow and coworkers ³ found that 12.6% of former professional football players reported cognitive impairment, a symptom believed to be associated with CTE.

Of the five remaining CTE- articles, two appear misclassified as CTE-. Hazrati and coworkers diagnosed CTE in 50% of included former professional football players. ⁴ Noy and coworkers ⁵ found that “combined history of head injury and alcohol and/or drug abuse was a significant predictor of any CTE-like changes.”

The remaining three CTE- articles with neuropathological evaluation of CTE include two articles with incomplete contact and collision sport information: Postupna and coworkers identified RHI exposure from obituaries. ⁶ These are an unlikely comprehensive source of RHI history and were only available in 52% of cases. Iverson and colleagues ⁷ only asked whether decedents took “part in headinjury associated sports” but did not ask about specific sports, which is particularly notable given that soccer is the most common youth sport in Finland, where the study was conducted. Soccer, is associated with RHI and CTE, but many would not consider it “head-injury associated.” Additionally, four experts in age related tau astrogliopathy (ARTAG) have expressed that the pathology in this Iverson et al., paper represents ARTAG, not CTE. ⁸ Tripathy and coworkers ⁹ reported on 18 former military service members without CTE, similar to a recent study in which 0/165 military personnel had CTE, while 10/60 military personnel with contact sport exposure had CTE. ¹⁰

The aforementioned points show that (1) CTE+ articles are published in higher- impact journals with more years since publication, (2) half of these CTE- articles cannot assess CTE, and (3) several purportedly CTE- articles support a link between RHI and CTE. Predictably, the CTE+ articles had more citations, and more public reach.

Recently, an international group of scientists evaluated the strength of the scientific evidence linking RHI exposure and CTE pathology and found that evidence supports a causal link between RHI and CTE. ¹¹ When evaluating whether literature received “disproportionate media attention,” strength of evidence must factor into the analysis. Comparing articles irrespective of the strength of design or results disparages scientific debate.