Adding to the growing evidence of the link between football and CTE, samples from the brains of dead pro and amateur players showed the risk for CTE went up with playing time.
"While we don't yet know the absolute risk of developing CTE among football players, this study found that the risk of developing CTE increased by 30% per year played," said researcher Dr. Ann McKee, director of Boston University's CTE Center.
For every three years played, the odds for CTE doubled, and for those with CTE, every five years they played doubled the risk for severe CTE, the researchers found.
"This study provides compelling evidence that one of the strongest risk factors between CTE pathology and contact sports-associated repetitive head trauma is duration," said Kevin Bieniek, director of the Glenn Biggs Institute for Alzheimer's and Neurodegenerative Diseases at the University of Texas Health Science Center in San Antonio.
The longer contact sport athletes participate in a setting where they were exposed to brain injuries of different intensities, frequencies and directional forces, the more likely they were to develop this CTE brain pathology. "These findings will be an important benchmark as researchers look at different sports and different athletes across various levels of play," said Bieniek, who wasn't involved with the study.
CTE is a chronic degenerative brain disease possibly the result of sustaining a number of concussions, which is why it's been linked to football. Its symptoms are like dementia, severely affecting memory and the ability to function. CTE can only be diagnosed after death, and no treatments or cures are available.
Several well-known players have suffered from CTE, including NFL Hall of Fame players Frank Gifford, who died in 2015, and Junior Seau, who killed himself in 2012.
"The easiest way to reduce the risk for CTE, without making any intrinsic modifications to the rules of the game, is to reduce the number of football playing years - starting later or finishing sooner," McKee said.
For the study, McKee and colleagues analyzed the brains of 223 former players with CTE and 43 without the condition from the Veterans Affairs-Boston University-Concussion Legacy Foundation and Framingham Heart Study Brain Banks.
Specifically, those who played less than five years were 10 times less likely to develop CTE, compared with those who played longer. But several men who played four years or less were diagnosed with CTE.
Men who played more than 15 years were 10 times more likely to develop CTE than those who played less. Several men who played 15 or more years, however, had no signs of CTE, the researchers noted.
Factors that the researchers didn't take into account that might affect the findings include the number of concussions suffered, the position played and the age at which someone started playing tackle football. Also, playing other contact sports may have an effect.
"There is a growing body of research connecting repeated head trauma, including the kind that happens in contact sports like football, to CTE," said Heather Snyder, who reviewed the study. She is vice president for medical and scientific relations at the Alzheimer's Association.
Head trauma can affect a person's cognitive abilities, including learning and thinking skills, Snyder said. "There is also a link between head injury and future risk of dementia."
Not everyone who experiences a head injury develops dementia, and not everyone who develops dementia experienced a head injury, she said.
More research is needed to understand the link between head injury and dementia, Snyder said.
She noted that professional football players aren't the only ones who suffer traumatic brain injuries.
Falls are the most common cause of traumatic brain injury, and falling poses an especially serious risk for older adults. Car crashes are another common cause of traumatic brain injury, Snyder said.
"Anyone who experiences an impact to the head and develops any symptoms of traumatic brain injury should seek medical attention, even if symptoms seem mild. Call emergency services for anyone who's unconscious for more than a minute or two, or who experiences seizures or symptoms that seem to worsen as time passes," Snyder advised.
The report was published online Oct. 7 in the Annals of Neurology.
Sources: Ann McKee, M.D., chief, neuropathology, VA Boston Healthcare System, and director, Boston University CTE Center; Heather Snyder, Ph.D., vice president, medical and scientific relations, Alzheimer's Association; Kevin Bieniek, Ph.D., director, brain bank core, Glenn Biggs Institute for Alzheimer's and Neurodegenerative Diseases, University of Texas Health Science Center, San Antonio; Oct. 7, 2019, Annals of Neurology, online