The link between football and traumatic brain injury continues to strengthen. Now, one of the largest studies on the subject to date finds that 110 out of 111 deceased NFL players had chronic traumatic encephalopathy (CTE), a degenerative brain disorder associated with repetitive head trauma.

Several studies have linked CTE to suicidal behavior, dementia and declines in memory, executive function and mood. Professional athletes may be at higher risk for CTE because of their high likelihood for concussions and other traumatic brain injuries; up to 3.8 million sports-related concussions occur in the United States each year. In 2016, a health official with the NFL acknowledged the link between football and CTE for the first time.

In the new study, published in the Journal of the American Medical Association, researchers looked at the brains of 202 deceased people who had played football at various levels, from high school to the NFL. (The brains had been donated to a brain bank at Boston University for further study.) The researchers analyzed the brains for signs of CTE and also spoke to family members about the players’ histories.

They diagnosed CTE in 87% of the players. Among the 111 NFL players, 99% had CTE.

MORE: 40% of Former NFL Players Had Brain Injuries

“This study more than doubles the number of cases reported in the literature of CTE,” says study author Dr. Jesse Mez, an assistant professor of neurology at Boston University School of Medicine. “It suggests, with a lot of caveats, that this is probably not a rare disease—at least among those who are exposed to a lot of football.”

The severity of CTE symptoms appeared to progress the more a person played the sport. High school players included in the study tended to have mild disease, and most college, semi-professional and professional players had severe symptoms. The study authors also found that mood, behavior and cognition problems were common among the players with mild to severe CTE.

Among players with severe CTE, 85% had signs of dementia, and 89% had behavioral or mood symptoms, or both. They were also likely to have issues in brain regions associated with depressive symptoms, impulsivity and anxiety. 95% had cognitive symptoms, like issues with memory, executive function and attention.

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The study has key limitations. Researchers studied a limited and possibly skewed sample of brains; news about repetitive head trauma and CTE has become increasingly prevalent, and families of players with symptoms of brain injury may have felt more motivated to participate in the brain bank study. It’s also still difficult to say how common CTE is among all football players.

“The numbers are not meant to represent the prevalence of CTE in football players,” says Mez. “But it does begin to suggest a relationship between football and this disease, and that’s an important step for research that will look at this in the future.”

Mez says the brain bank, which is ongoing, receives between 50 to 100 donations every year. Having access to brain tissue allows the researchers to study possible mechanisms for CTE, and why some players develop it while others do not. “We are really early in understanding this disease,” says Mez.

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