A worthwhile study was recently published in the Journal of Vascular and Interventional Neurology discussing what is known and not known about CTE in athletes competing in contact sports.
In the study, titled “Chronic Traumatic Encephalopathy in Athletes Involved with High-Impact Sports” the authors reviewed major clinical studies identifying CTE cases in athletes looking for conclusions and shortcomings in the data.
Among the conclusions drawn are that, of known athletes with CTE, professional boxers demonstrated the most debilitating symptoms. Also of note the authors suggest that strengthening neck muscles (a topic previously canvassed here) can help minimize the risks of CTE for contact sports athletes.
The study is worth reading in full and can be accessed here.
In addressing the increased symptoms in boxers vs football players and discussing possible strategies to reduce risk the authors note as follows:
Clinical and pathological features of CTE can manifest differently between sports, as rTBI exposure and mechanisms of impact can vary considerably. In fact, an analysis of previously reported CTE cases by Montenigro et al.  showed a vast difference in clinical presentation. 83% (5/6) of professional boxers, who had more debilitating motor impairments, compared to 18.8% (3/16) of professional football players. In addition, severe dentate neurofibrillary tangles were present in 17% (2/12) and 80% (4/5) of professional football players and boxers, respectively, indicating a more pernicious progression in boxers . The difference in symptoms and neuropathology may be explained through the frequency of linear and rotational impact forces that occur in both sports.
Rotational forces causing angular accelerations are frequent in boxing. Boxers face their greatest danger when their opponent lands a hook punch, where impact near the lateral side of the head cause rapid outward rotation of the skull and twisting forces the brain . Lateral bending of the neck can also occur, but linear forces from a punch are often below the mTBI threshold . The rotational movement of the brain causes shearing forces that can lead to axonal damage . Shearing forces are most prominent near areas such as the midbrain section, where glial and axonal injury could result in severely debilitating consequences [29,58].
As opposed to punches, helmet-to-helmet or helmet-to-ground contact forces cause the majority of mTBI injuries in professional football players. Viano et al.  have shown that in professional football concussions, inertial forces can be up to 30% greater than inertial forces in professional boxers who endure a hook punch. The greater inertial forces correlate with a higher linear acceleration endured by football players, suggesting that linear forces are prominent in causing concussive and subconcussive impacts in professional football players. In support of this mechanism, brain modeling shows that rotational accelerations from uppercuts or hook punches are much greater than rotational accelerations in professional football helmet-to-helmet impacts . The linear to rotational force ratio difference between boxers and football players could explain the differences in clinical presentation between the two sports.
In professional football, helmet-to-helmet collisions can cause the head to move in the anterior or posterior direction. The incidence rates of mTBI have been shown to vary depending on position, with running backs and wide receivers suffering from mTBI more than linemen . Neck musculature acts to stabilize the position of the head, and a more developed musculature is directly correlated to lowered mTBI risk . Linemen have been found to have stronger necks and larger girth compared to running backs, which could act to slow linear accelerations of the head and reduce risk of mTBI . The differences in neck strength between positions may explain the varying incidence rates of mTBI. Additionally, it should be noted that different player positions may be more prone to certain types of impacts—linemen may experience more frequent subconcussive helmet-to-helmet impact, while wide receivers could endure more threatening forces while being tackled. The pathological repercussions of variations in impact type and frequency between boxing and football have yet to be elucidated in full detail, but they may partially explain the difference in clinical presentation between different types of athletes.
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