A progressive, degenerative brain disease known as chronic traumatic encephalopathy (CTE) was found in 99 percent of former National Football League (NFL) players whose brains were donated for research after their deaths, a new study said Tuesday.
The study, published in the Journal of the American Medical Association (JAMA), suggested that CTE may be more common among football players than previously thought.
"The data suggest that there is very likely a relationship between exposure to football and risk of developing the disease," Jesse Mez, a Boston University School of Medicine (MED) assistant professor of neurology and lead author on the study, said in a statement.
In the study, Mez and colleagues examined the donated brains of 202 former football players for evidence of CTE, which has been linked to repeated blows to the head.
Researchers believe CTE is caused by the buildup of an abnormal form of a protein called tau in the brain that can lead to brain cell deaths and a variety of clinical symptoms, including memory loss, confusion, impaired judgment, depression, suicidality, and, eventually, progressive dementia.
Of the 202 brains studied, the group diagnosed 177 with CTE, including 110 of 111 from the NFL players; seven of eight from the Canadian Football League; nine of 14 semi-professional players; 48 of 53 college players, and three of 14 high school players.
The study has several limitations, including that it is a skewed sample based on a brain donation program because public awareness of a possible link between repetitive head trauma and CTE may have motivated players and their families with symptoms and signs of brain injury to participate in this research.
As a result, the researchers urged caution in interpreting the high frequency of CTE in this study, stressing that estimates of how prevalent CTE may be cannot be concluded or implied.
Despite these limitations, they noted that the study -- the largest and most methodologically rigorous CTE case series ever published -- offers important information and direction for further research.
"I think the data are very surprising," said corresponding author Ann McKee, a MED professor of neurology and pathology and director of BU's CTE Center. "We've sort of become accustomed to it, but it is very shocking."
In a statement widely cited by U.S. media, the NFL said it appreciates the work done by McKee and her colleagues "for the value it adds in the ongoing quest for a better understanding of CTE."
"The NFL will continue to work with a wide range of experts to improve the health of current and former NFL athletes," the statement said, noting that "there are still many unanswered questions relating to the cause, incidence and prevalence of long-term effects of head trauma such as CTE."
The NFL added that it contributed 100 million U.S. dollars to neuroscience research last year in addition to the 100 million dollars that the league and its partners are already spending on this area.
