Similar findings have been observed previously, "but never on this scale, and never so systematically", said Brody, who was not involved with this study but has collaborated with McKee on other research.
Research on 202 former football players found evidence of a brain disease in almost all of them, from athletes in the NFL down to high school.
In the new report, McKee and colleagues found the most severe disease in former professional players; mild disease was found in all three former high school players diagnosed with the disease.
The vast majority of donated brains - 177 in all - bore the telltale clusters of a protein called tau, the sign of CTE.
"In this study", Daneshvar noted, "we more than double the total number of cases of CTE in the world's literature". The problems can arise years after the blows to the head have stopped.
The disease "causes myriad symptoms, including memory loss, confusion, depression and dementia", according to the NYT story. The study also found CTE in 91 percent of college players and 21 percent of high school players.
The new report was published Tuesday in the Journal of the American Medical Association. Either mild or severe pathology produced frequent mood, behavior and cognitive, symptoms.
Of the 202 brains studied, 87 percent were found to have C.T.E.
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Most brains from players at advanced levels showed signs of severe CTE, including 86 percent of professional players and 56 percent of semi-professional or college players. The disease was not found in brains from two younger players.
Researchers warned that their study cannot be used to assess how many people who play football develop CTE, because their sample was not random.
Of the 111 former National Football League players she examined, 110 players were found to have CTE. "It's unclear there was a position that an athlete could play that could not develop CTE".
It can only be formally diagnosed with an autopsy, and most cases, although not all, have been seen in either veterans or people who played contact sports, particularly American football. "The patients were almost all impaired during life". "Families don't donate brains of their loved ones unless they're concerned about the person".
The NFL said the player's claim "relied on no evidence at all". "So we are looking at a sample of some of the sickest individuals who likely were exposed to a very high burden of traumatic brain injury".
Since Webster's death at age 50 in 2002, other high-profile players - including Seau, Frank Gifford, John Mackey and Kenny Stabler - were diagnosed with CTE after their deaths. "It's like a storm that you can't quite get out of", his wife said.
While new methods for CTE diagnosis are in the works, "nothing is ready for prime time yet", he said.
But Cullum, a neuropsychologist with the O'Donnell Brain Institute at University of Texas Southwestern Medical Center who has studied concussions at all levels of sport for almost three decades, says it's still too soon to definitively declare CTE a problem in football.