Dec. 3, 2012 -- On the heels of the latest NFL suicide, researchers announced today that 34 NFL players whose brain were studied suffered from CTE, a degenerative brain disease brought on by repeated hits to the head that results in confusion, depression and, eventually, dementia.
The study was released just days after the murder-suicide of Kansas City Chiefs linebacker Jovan Belcher. It's not yet known what triggered Belcher's action, but they mirror other NFL players who have committed suicide.
Researchers at Boston University's Center for the Study of Traumatic Encephalopathy published the largest case series study of CTE to date, according to the center. Of the 85 brains donated by the families of deceased veterans and athletes with histories of repeated head trauma, they found CTE in 68 of them. Of those, 34 were professional football players, nine others played college football and six played only high school football.
Of the 35 professional football players' brains donated, only one had no evidence of the disease, according to the study.
According to the new study, Boston University researchers divided CTE, or chronic traumatic encephalopathy, into four stages, the first of which involves headaches and the last of which involves "full-blown dementia." The disease involves brain tissue degeneration and a buildup of an abnormal protein called tao, which is also found in patients with Alzheimer's disease.
Kansas City police say Belcher, 25, shot and killed his girlfriend Saturday morning before going to the team stadium and committing suicide by shooting himself in the head as he was talking to coaches.
Chiefs chairman Clark Hunt said Sunday that Belcher was "a player who had not had a long concussion history," even though he was a three-time all-America wrestler and a star on the football team at his West Babylon, N.Y., high school. It is not yet clear whether his brain will be donated to the study.
However, the Boston University researchers have not yet determined how much brain trauma results in CTE.
"While it remains unknown what level of exposure to brain trauma is required to trigger CTE, there is no available evidence that occasional, isolated or well-managed concussions give rise to CTE," one of the study's co-authors, Dr. Robert Cantu, said in a press release.
It's not yet clear what prompted Belcher's actions, but his suicide closely follows those of former NFL players Junior Seau, 43, and Dave Duerson, 50, both of whom died of self-inflicted gunshot wounds to the chest in the last two years. Duerson's brain is being studied at the Boston University research center, where researchers have already learned that he had CTE.
Seau's brain was donated to a different facility and the results have not been released.
In 2006, former Pittsburgh Steelers player Terry Long killed himself by drinking antifreeze, and former Philadelphia Eagles player Andre Waters shot himself in the head. Both of them suffered from CTE.
CTE has also been found in hockey players, wrestlers and boxers. It's still not possible to diagnose while a person is alive.
Seau's death in May prompted NFL player Jacob Bell to quit the sport altogether, leaving behind his contract with the Cincinnati Bengals.
"We're getting so much money, so much glory, so much fame; we're boosting our egos so much by playing a sport that's violent and could later on risk our lives," Bell said in May.
CTE researchers from Boston University were unavailable, and do not comment on players' deaths until more research is obtainable.
The Associated Press contributed to this report.