A type of brain disease associated with repeated hits to the head was found in the brains of nearly all of the deceased football players involved in a recent study.

The disease, known as chronic traumatic encephalopathy, or CTE, was identified in 177 of 202 former football players who played at various levels including high school, college, semi professional, and professional in the United States or Canada. Of all the brains studied, 111 of them were from former NFL athletes and all but one of those brains had the disease.

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“There’s no question that there’s a problem in football. That people who play football are at risk for this disease,” coauthor of the study Dr. Ann McKee told CNN. “And we urgently need to find answers for not just football players, but veterans and other individuals exposed to head trauma.”

Dr. McKee and her colleagues at CTE Center at Boston University and VA Boston Healthcare System, received the former players’ brains through a research donation program that their families elected to participate in. The brains were examined in a lab to look for neuropathological features of the very rare condition. Additionally, the researchers talked to the players’ next of kin to get a glimpse into their history involving head trauma, athletic participation, and military service.

The severity of CTE can vary, but 86 percent of the professional players in the study were found to have the most severe pathology. A majority of those with severe pathology had behavioral or mood symptoms or both, cognitive symptoms, and signs of dementia, according to the findings published in the medical journal JAMA.

CTE is still not completely understood by health professionals, but other possible signs and symptoms may include depression or apathy, substance abuse, suicidal thoughts or behavior, irritability, aggression, trouble swallowing, vision and focusing problems, and trouble with sense of smell, according to Mayo Clinic.

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Although the study revealed a high proportion of the players had CTE, the authors point out several limitations of the study.

“There’s a tremendous selection bias,” McKee told The New York Times, explaining that a lot of the families donated brains because they noticed previous signs and symptoms of CTE exhibited by the former player.

Despite the skewed sample, based on previous research, the NFL openly acknowledged a link between football and the brain disease in 2016. The league has even begun to encourage children to practice safer tackles and play flag football, The New York Times reported.

See also: Changes In Human White Brain Matter Seen After One Season Of Youth Football

Head Trauma From Playing Football Cause Brain Changes Even When There's No Concussion