Click on to find out more about chronic traumatic encephalopathy, the progressive brain disease at the center of a controversy over concussions in sports.
Chronic traumatic encephalopathy (CTE) is a progressive degenerative disease, diagnosed post-mortem in individuals with a history of multiple concussions and other forms of head injury.
Symptoms of CTE include: headaches, disorientation, confusion, poor judgment, overt dementia, slowed muscular movements, staggered gait, tremors and deafness.
Researchers say CTE has a clear environmental cause (repeated brain trauma) rather than a genetic cause. In other words, CTE is the only preventable form of dementia.
In 2008, the Sports Legacy Institute joined with the Boston University School of Medicine to form the Center for the Study of Traumatic Encephalopathy (CSTE), where many brain trauma cases are studied.
CTE has been most commonly found in professional athletes participating in football, boxing, professional wrestling and other contact sports.
The condition is also found in professional athletes who have played ice hockey and, to a lesser extent, those involved in military service.
NFL great Junior Seau's suicide at age 43 in May 2012 prompted discussions about brain trauma. Studies after his death by the National Institutes of Health concluded that Seau had suffered from CTE.
Reports say that Seau had acted erratically in the years before his death, getting arrested and driving an SUV off a 100-foot cliff. According to CNN, he'd told a friend he was worried about football-related brain damage.
John Mackey, one of NFL football's great tight ends, died in 2011 of frontotemporal dementia, a syndrome similar to CTE that involves degeneration of the brain.
The NFL Players Association initially refused to pay Mackey a disability income due to there not being a proven link between brain injury and playing football. The league and the NFL Players Association have responded with the "88 plan" –- named after Mackey's number.
The plan provides $88,000 per year for nursing home care and up to $50,000 annually for adult day care. Since, 2007, the plan has distributed $32 million to ex-players with dementia, Alzheimer's and Parkinson's, according to the league.
In 2007, WWE wrestler Chris Benoit had his brain examined after he killed his wife and son before committing suicide. Originally the tragedy was thought to be due to the abuse of anabolic steroids. However, a brain biopsy identified pathognomonic brain tissue changes of CTE.
In 2011, former Chicago Bears safety Dave Duerson, 50, shot himself in the chest, leaving a suicide note donating his brain to science. Boston University pathologists found lesions in Duerson's brain.
Duerson's was the 15th NFL brain they dissected. They found similar damage in all but one of the 15.
An autopsy performed one year after Kansas City Chiefs linebacker Jovan Belcher fatally shot his 22-year-old girlfriend and then killed himself in front of his coach and the team's general manager on Dec. 1, 2012, found signs of CTE.
San Francisco 49ers linebacker Chris Borland announced his retirement from football on March 16, 2015, due to worries about head trauma. The team's leading tackler in the 2014-15 season as a rookie, Borland told ESPN that even though he was diagnosed with only two concussions, both while in high school, he will not change his mind about retiring.
In the fall of 2013, former NFL quarterback great Brett Favre said he had been suffering memory lapses and that he feared it was related to the multiple concussions he suffered throughout his career. Memory loss can be a sign of CTE.
The family of Frank Gifford said in a November 2015 statement that the former NFL running back and television analyst suffered from chronic traumatic encephalopathy before his August 2015 death. Gifford's family said they decided to have his brain studied "in hopes of contributing to the advancement of medical research concerning the link between football and traumatic brain injury." And they decided to make his diagnosis public to honor Gifford's commitment to promoting player safety, dating back to his involvement in creating the NFL Players Association, a union representing players' interests, in 1956.
Former Oakland Raiders quarterback Ken Stabler, the late NFL and Super Bowl MVP who was elected to the Pro Football Hall of Fame in early 2016, was posthumously diagnosed with the brain disease CTE, Boston University announced in February 2016. Stabler, who died of colon cancer at 69 in July 2015, had Stage 3 chronic traumatic encephalopathy, Dr. Ann McKee told The Associated Press.
On March 15, 2016, an NFL official for the first time publicly acknowledged a connection between football and CTE. Answering a question posed at a roundtable discussion hosted by a U.S. House committee, Jeff Miller, the NFL's senior vice president of health and safety policy, said there "certainly" is a link and pointed to Dr. McKee's research. The NFL followed that up by issuing a statement the next day, saying "The comments made by Jeff Miller yesterday accurately reflect the view of the NFL."
Speaking at the NFL owners meetings on March 23, 2016, Dallas Cowboys owner Jerry Jones said it is "absurd" to believe there is a connection between playing football and developing ailments such as CTE. "There's no data that in any way creates a knowledge," Jones said, according to the Washington Post. "There's no way that you could have made a comment that there is an association and some type of assertion. In most things, you have to back it up by studies. And in this particular case, we all know how medicine is. Medicine is evolving. I grew up being told that aspirin was not good. I'm told that one a day is good for you. ... I'm saying that changed over the years as we've had more research and knowledge."
On March 25, 2016, the New York Times ran a front-page story scrutinizing the NFL's handling of concussions among its players, comparing the league's response to the crisis "to that of the tobacco industry, which was notorious for using questionable science to play down the dangers of cigarettes." A few days later, the NFL demanded a retraction and the league's attorney appeared to suggest that the NFL could take legal action.
The NFL's settlement of nearly $1 billion with retired players who filed concussion lawsuits was upheld April 18, 2016, by a three-judge panel of the Third U.S. Circuit Court of Appeals. The settlement resolves thousands of lawsuits and covers more than 20,000 NFL retirees for the next 65 years.