Scientists looking for a link between repeated brain trauma and lasting neurological damage typically study the brains of soldiers or football players. But it’s unclear whether this damage—known as chronic traumatic encephalopathy (CTE)—is prevalent in the general population. Now, a new study reports those rates for the first time.
To conduct the research, neuropathologist Kevin Bieniek, then at the Mayo Clinic in Jacksonville, Florida, and colleagues sorted through nearly 3000 brains donated to the clinic's Rochester, Minnesota, tissue registry between 2005 and 2016. Then, by scanning obituaries and old yearbooks, the researchers narrowed the group to 300 athletes who played contact sports and 450 nonathletes. The scientists removed all infants under age 1, brain samples with insufficient tissue, and brain donors without biographical data attached to their samples. Finally, they collected medical records and looked under a microscope at tissue from up to three sections of each brain for signs of CTE. Those signs include lesions and buildup of tau, a protein associated with neurodegenerative disorders such as Alzheimer’s disease.
Six percent of the brains showed some or all signs of CTE, Bieniek and his colleagues report in Brain Pathology. Not all the people experienced symptoms associated with CTE, at least according to their medical records. Those symptoms include anxiety, depression, and drug use. However, people with CTE were about 31% more likely to develop dementia and 27% more likely to develop Alzheimer’s disease than those without CTE.
People who played contact sports were more likely to have signs of CTE. Nine percent of athletes had evidence of CTE, compared with just over 3% of nonathletes.
The highest rate of CTE was in football players who participated beyond high school: Ten of 15 collegiate and professional players showed either some features of CTE or definitive diagnoses. The likelihood of developing CTE was 2.6 times as high for football players as for nonathletes, the researchers found, but more than 13 times as high for football players who continued beyond the high school level, compared with nonathletes.
“Parents need to understand that playing tackle football does increase your risk of developing CTE, and it is correlated to how many years you play,” says Chris Nowinski, the CEO of the Concussion Legacy Foundation, a nonprofit in Boston that advocates for concussion prevention among athletes. “That’s an important message if we ever want to prevent this disease.”
Only one of the 273 women in the sample exhibited signs of CTE. She was not an athlete. Bieniek says this could be because of the subjects’ somewhat advanced average age of 67. That would mean most of the women in the group were at least in their early 20s before Title IX, which prohibits sex discrimination in education and school activities, became law in 1972; they likely had fewer chances to play competitive sports than men.
The team also found that individuals with CTE were diagnosed with traumatic brain injuries no more often than people whose brains lacked CTE. The authors suggest repeated trauma is key to developing CTE; a single hit to the head may cause a concussion and concussion-related symptoms without turning into a CTE diagnosis.
Still, experts note that medical records can be incomplete, and people don’t always seek treatment for disorders such as depression and drug use. As a result, the findings could underestimate the proportion of people with CTE who develop these issues.
The results provide a good overview of CTE prevalence in the general population, says Kristen Dams-O'Connor, a clinical neuropsychologist at Mount Sinai Hospital in New York City who was not involved with the study. “Most of the research on CTE so far has been done in highly selected [groups] of people with generally very high levels of exposure to head trauma.”
Neuropathologist Daniel Perl of the Uniformed Services University of the Health Sciences in Bethesda, Maryland, also cautions that many of the brains in the study show only mild signs of CTE and should not be conflated with more extreme signs seen in professional football players’ brains. “I think we have to be very careful how we interpret this study and others like it.”
Bieniek acknowledges that the brain donors were predominantly white, raising questions about whether the findings apply to everyone. He hopes to conduct further research with a more racially diverse set of samples at his new position at the University of Texas Health Science Center in San Antonio.
*Correction, 8 July, 1:55 p.m.: An earlier version of this article misstated the location of Kevin Bieniek's previous position at Mayo Clinic.