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.
"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.
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.
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.
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.
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