Chronic traumatic encephalopathy, a degenerative brain disorder linked to repeated head injuries, is not associated with either the duration of an athlete’s career or the position the athlete played, according to a retrospective cohort study of elite football and hockey players, contrasting previous research and adding to a heated debate over the safety of collision sports.

“Millions of individuals play contact sports in their daily lives, and many of these individuals have heard of CTE through media headlines,” said Dr. Lili-Naz Hazrati, an associate professor at the University of Toronto and an author of the study.

Writing in the journal Neurology, Hazrati and her co-authors examined the brains of 35 collision sport athletes at autopsy, including 24 former football players and 11 former hockey players. The researchers wanted to see if positions associated with frequent or severe concussions, such as the forward role in hockey, were associated with CTE. The hockey players included both forwards and defensemen, while the football players included offensive backs/receivers, defensive backs, tight ends/linebackers and offensive/defensive linemen.

The athletes’ brains were part of a larger brain bank, made up of individuals who had shown signs of mild traumatic brain injury prior to their death. 

“While research does support the hypothesis that traumatic brain injuries increase one’s risk of long-term neurological consequences, the relative risks of head injury from sport versus the benefits of physical exercise, social interactions and teamwork remain unclear,” said Hazrati.

Mild traumatic brain injury has previously been linked to CTE, which is diagnosed by the presence of accumulated tau proteins throughout the deepest grooves of the cortex. However, the authors pointed out that similar “taupathies” are seen in other neurodegenerative diseases, including Alzheimer’s disease, frontotemporal lobar dementia and age-related tau astrogliopathy, making it difficult for scientists or clinicians to differentiate the two conditions.

Twenty-four out of the 35 brains exhibited some other brain disorder, such as Alzheimer’s, with 11 of those brains also showing signs of CTE.

In evaluating the players’ positions and frequency of concussions, the researchers mined online databases to measure career duration as well as playing position and, among hockey players, penalty minutes and fighting history. They also used information from next of kin, who described the character and severity of traumatic brain injury exposure among individual athletes.

Just 48.6% of the brains showed CTE symptoms, well below the rate of 84-89% found in some previous research on football players. In addition, CTE did not correlate with career duration, player position, fighting or penalization. This finding, too, runs contrary to earlier work, which has linked years of football played with CTE risk.

“While bringing attention to the risks of concussion is important, it is critical that individuals do not self-diagnose a lifelong neurodegenerative disease based on non-specific symptoms, such as anxiety, depression, or irritability,” Hazrati added. “Indeed, many of these symptoms may manifest for a number of reasons. While CTE is most definitely a real phenomenon, it is not an inevitable consequence of playing sports.”

The authors acknowledged the limitations of their study, which involved a relatively small sample of non-representative athletes. They emphasized the risk of false negatives, or Type II errors, especially in their findings on player position and CTE.

The researchers also spotlighted the lack of a control group of elite former athletes without CTE-like neurological symptoms, noting that the reliance on a mild traumatic brain injury brain bank creates a referral bias for most such retrospective CTE studies.

“We are continuously increasing the sample size within our brain bank, and hope to publish future updates regarding the pathology of concussed brains,” said Hazrati. “While it is difficult to say where the research community stands with the implementation of longitudinal follow-up studies of patients with repeated head injuries, we believe that the increased interest from the public may increase recruitment, making this possible in the near future.”

Hazrati and her colleagues stated that factors aside from concussions, such as drug use, stress or genes, may influence the risk of CTE among elite football or hockey players.

“There is sufficient evidence to suggest that individuals with systemic disease, drug use, altered gut microbiota and several other factors are at an increased risk for neurodegenerative diseases, especially tauopathies such as Alzheimer’s disease,” said Hazrati. “I believe that all of these factors, in addition to exposure to head injury, increase one’s risk of CTE. We believe that like many diseases, this is a personalized risk.”

The research team members, who have worked together since 2009 in conjunction with the Canadian Sports Concussion Project and the Canadian Concussion Centre. noted that they intend to build on this work in future research. They are delving into the specific neurological symptoms that athletes showed during life.

“My lab is primarily interested in studying the upstream, early pathophysiological mechanisms involved in traumatic brain injury, those which precede pathological inclusions and may represent viable early intervention therapies,” said Hazrati. “We are currently looking into other pathological changes that may account for clinical changes,” she continued, pointing to DNA repair in glial cells as a possible driver of concussion-related outcomes.

The study, “Association of Position Played and Career Duration and Chronic Traumatic Encephalopathy at Autopsy in Elite Football and Hockey Players,” was published on Feb. 24 in Neurology, the medical journal of the American Academy of Neurology. The authors of the study were Nicole Schwab and Karl Grenier, of the University of Toronto, Richard Wennberg, Carmela Tartaglia and Charles Tator, of the Toronto Western Hospital Krembil Brain Institute, and Lili-Naz Hazrati, of the Hospital for Sick Children.