(CNN) One time MVP and Oakland Raiders quarterback Ken Stabler, who died in July, suffered from chronic traumatic encephalopathy, or CTE, researchers at Boston University said Wednesday.
Scientists believe repeated head trauma causes CTE, a progressive degenerative brain disease that can manifest in depression, disorientation and aggression. So far, it's only diagnosable after death. Dozens of ex-professional football players have been shown to have struggled with CTE.
Dr. Ann McKee, a neuropathologist and expert in neurodegenerative disease at Boston University School of Medicine, was part of the team that analyzed Stabler's brain.
With the naked eye, researchers immediately noticed that his brain had atrophied and was shrunken in the temporal lobe, or front of the brain, she said. His hippocampus, which governs memory and learning, was small, she said. There is a curtain that divides the brain's two hemispheres which, in healthy brains, is thick. That curtain in Stabler's brain was torn.
87 of 91 tested ex-NFL players had brain disease linked to head trauma
"We see it frequently in the former NFL players with CTE," she said.
Stabler died in July at age 69 from cancer, and had requested that his brain be removed during an autopsy and taken to researchers in Massachusetts.
He felt compelled to do that because he was having difficulty with impulse control in his 50s and developed memory problems and suffered from headaches in his 60s, McKee said.
Athletes and CTE
Former pro football player Kevin Turner, shown here during a 1998 NFL game,
had the most advanced stage of CTE when he died in March at the age of 46. Dr. Ann McKee of Boston University and the Concussion Legacy Foundation said that Turner's CTE brought on amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease.
CTE stands for chronic traumatic encephalopathy, a neurodegenerative disease associated with repeated head trauma. Scientists believe repeated head trauma can cause CTE, a progressive degenerative disease of the brain. Symptoms include depression, aggression and disorientation, but scientists can definitively diagnose it only after death.
Charles "Bubba" Smith, a former football player and actor who died in 2011, was also diagnosed with CTE. Smith played for the Baltimore Colts, the Oakland Raiders and the Houston Oilers.
Former Minnesota Vikings linebacker Fred McNeill died in November 2015 due to complications from ALS. However, an autopsy confirmed that he suffered from CTE. What makes
McNeill's case even more remarkable, though, is that he was potentially the first to be diagnosed while alive. Doctors used an experimental new technology to examine his brain.
Oakland Raiders quarterback Ken Stabler, a former NFL MVP who died in July 2015,
suffered from CTE, researchers at Boston University said.
Kansas City Chiefs linebacker Jovan Belcher shot and killed his girlfriend before killing himself in December 2012.
Pathology reports show he probably had CTE.
Hall of Fame offensive lineman Mike Webster was the first former NFL player to be diagnosed with CTE. After his retirement, Webster suffered from amnesia, dementia, depression, and bone and muscle pain.
The death of 36-year-old Pittsburgh Steelers offensive lineman Justin Strzelczyk put the link between football and CTE in the national spotlight. Strzelczyk was
killed in a 2004 car crash crash after a 40-mile high-speed chase with police in New York.
Evidence of CTE was found in the brain of football player Lew Carpenter after his death in 2010 at the age of 78.
Pro Football Hall of Famer Louis Creekmur, who played for the Detroit Lions from 1950 to 1959, suffered decades of cognitive decline before his death.
Linebacker John Grimsley of the Houston Oilers died of an accidental gunshot wound to the chest in 2008. Analysis of his brain tissue confirmed damage to the neurofibrillary tangles that had begun to affect his behavior and memory.
Offensive lineman Terry Long of the Pittsburgh Steelers committed suicide by drinking antifreeze. Although the antifreeze caused swelling of the brain, football-related brain injuries were a contributing factor to his death.
Pro Football Hall of Famer John Mackey suffered from dementia for years before dying at the age of 69.
Ollie Matson, who played 14 NFL seasons starting in the 1950s, suffered from dementia until his death in 2011.
Andre Waters spent most of his 12 seasons with the Philadelphia Eagles before his suicide at age 44.
Junior Seau
took his own life in 2012 at the age of 43. The question of CTE came up immediately after his death; scientists at the National Institutes of Health confirmed the diagnosis in January 2013.
Pro wrestler Chris Benoit was found dead at his suburban Atlanta home along with his wife, Nancy, and son in an apparent murder-suicide. Testing found that the damage to his brain was similar to that of an elderly Alzheimer's patient.
Reggie Fleming, who played for six NHL teams, was the first hockey player to be diagnosed with CTE.
Hockey player Bob Probert was found to have CTE after dying of heart failure at the age of 45.
Ryan Freel became the first Major League Baseball player to be diagnosed with CTE nearly a year after he committed suicide at age 36.
Had he lived longer, the physician said he would have almost certainly developed dementia.
She noted that studying the quarterback's brain was informative particularly because some assume that quarterbacks received fewer hits so they are at a lesser risk. "It shows that even playing quarterback -- and if you play a number of years -- that you're at risk for developing this disease."
NFL and CTE
In October, Boston University and the Department of Veterans Affairs researchers said 87 out of 91 former NFL players who donated their brains to science after death tested positive for CTE.
Opinion: Concussions and NFL: How the name CTE came about
It's not clear why some players develop the disease and others do not. The study results don't necessarily mean that 96% of all NFL players are at risk for CTE, Dr. Robert Cantu told CNN. The players who donated their brains were concerned about the disease, he said.
Stabler part of class-action lawsuit
In 2015, the league and thousands of former players settled a lawsuit that provides up to $5 million per retired player for serious medical conditions associated with repeated head trauma. The lawsuit charged that the NFL hid the dangers of concussions.
Could veterans have concussion-related CTE?
The brain tissue of people found to have CTE shows an abnormal buildup of tau, a protein that can, when it leaves cells, disable neural pathways that control memory, judgment and fear.
Cantu points out that CTE is not unique to athletes.
"There are a number of cases in people who never saw an athletic field," he said.
The doctor provided examples of former military members, people who have suffered from grand mal seizures, autistic children who rocked and banged their heads, abuse victims, and even people who were shot out of a cannon as part of a circus act.
A previous version of this story incorrectly reported an honor Ken Stabler received. He was named the National Football League MVP in 1974, according to NFL.com
CNN's Kevin Flower and Debra Goldschmidt contributed to this report.