For years, as researchers have established a link between repeated impacts to the head and the degenerative brain disease known as CTE, one major question has loomed: Just how common is it — especially among high-risk groups, like professional football players? A new study from researchers at Mass General Brigham, published Tuesday in the prominent medical journal The BMJ, seeks to establish a more accurate baseline of CTE prevalence among retired NFL players who have died. Of football players who appeared in at least one NFL game in their careers and who died between 2016 and 2021, the study found that a minimum of around 25% had CTE upon their death.
"We've known that this disease process can be caused by, in part, repeated traumatic brain injury. But we didn't know how common it was," said Dr. Daniel Daneshvar, the study's lead author.
Chronic traumatic encephalopathy, or CTE, is a degenerative brain disease that can only be diagnosed with a brain dissection after death. Because of that, most research about the disease has relied on brains donated to researchers by families or by individuals themselves, many of whom had seen or experienced symptoms, such as memory loss or major mood swings, that led them to believe they might have the disease. That dynamic has long muddied the picture about the actual prevalence of CTE — especially for combat veterans exposed to traumatic brain injuries, or current and former elite-level athletes in contact sports like American football and ice hockey, where blows to the body and head are a regular occurrence.
Daneshvar and his colleagues drew on the large collection of samples at the UNITE Brain Bank at Boston University. The brains of more than 330 NFL players who died between the years of 2008 and 2021 have been donated to the bank. Of those, 93% have been diagnosed with CTE.
To better understand what proportion of all NFL players that sample represents, the study's authors created a list of every player who had appeared in at least one NFL game since 1949. They referenced demographic data and obtained death certificates for all deceased players from the Centers for Disease Control and Prevention to create a denominator list of 1,712 players who died in the same time period. The researchers then narrowed the sample to those who had died between 2016 and 2021, when the proportion of players whose brains were donated for study was highest.
The prevalence estimate of 25% is at the "low end," Daneshvar said. The actual figure could be higher, the paper noted, pointing to a number of CTE diagnoses made outside of the brain bank sample, along with other suspected cases of CTE in former football players whose brains were not evaluated after death. Additionally, researchers wrote, an analysis of death certificates of football players who did not donate their brains revealed another 215 whose certificates listed a neurodegenerative disease as a cause of death.
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