Many late former NFL players had CTE. What to know about the disease.

The family of Demaryius Thomas revealed Tuesday that the former star wide receiver had stage 2 CTE at the time of his death. Thomas was found dead in his Georgia home in December, just months after he retired from the NFL. He was 33 years old. Authorities have yet to release a cause of death.

CTE is at the heart of an ongoing discussion about player safety and head injuries in American football. Thomas is the latest in a long string of ex-professional athletes, including many former NFL players, to receive a CTE diagnosis, which can only be made posthumously.

The NFL has been embroiled in multiple head injury lawsuits in recent years. In 2015, a judge approved a $1 billion settlement plan in which the league would compensate former players who suffered from brain trauma, the Associated Press reported.

Former Broncos WR had CTE: Ex-NFL wide receiver Demaryius Thomas had stage 2 CTE at the time of his death

'Race-norming' addressed: Judge approves fix to stem racial bias in NFL concussion deal

What is CTE?

CTE stands for chronic traumatic encephalopathy, a progressive degenerative brain disease, according to Boston University’s CTE Research Center, which has led much of the research on the disease. The condition is caused by a history of repeated hits to the head and emerges months or even years after the head injuries were sustained, according to Boston University.

Athletes in a number of contact sports, including football, hockey and boxing have been diagnosed with CTE over the years, according to Boston University. It was first observed in boxers in the early twentieth century and was known as punch drunk symptom, according to Boston University. Official diagnoses can only be made posthumously, upon close examination of the brain.

In 2017, Boston University researchers diagnosed CTE in 99% (110 of 111) of former NFL players whose brains were donated for study, USA TODAY reported.

The same study examined the brains of 202 football players at different levels of the game, including high school and college. It found CTE in 87% of the players studied, USA TODAY reported.

'Got to find better ways to help': Former NFL player Greg Clark had CTE when he killed himself

Vincent Jackson: Former NFL receiver had stage 2 CTE

What are CTE symptoms?

Symptoms include memory loss, difficulty thinking and behavioral changes like impulsivity and aggressiveness, according to the Mayo Clinic. Depression, substance abuse and suicidal thoughts and behavior are also associated with CTE, according to the Mayo Clinic.

How many stages of CTE are there?

There are four stages of CTE. Stage 4 is the most severe. People with stage 1 CTE are often asymptomatic or report mild short-term memory loss and depressive symptoms, according to a report published on the National Institutes of Health website.

Stage 2 CTE can manifest in severe depressive symptoms and “behavioral outbursts,” according to the report.

At stage 3 and 4, patients experience cognitive impairments like memory loss and poor spatial awareness, according to the report. Patients begin to struggle with speech, lose motor skills and experience severe psychotic symptoms at stage 4, according to the report.

CTE in American Soccer: First known case diagnosed in former MLS player Scott Vermillion

Demaryius Thomas had stage 2 CTE

Thomas had stage 2 CTE. According to a news release by the Concussion Legacy Foundation, which assisted with diagnosis, Thomas experienced depression, panic attacks, anxiety and memory issues in the years before his death. The Fulton County Medical Examiner’s office has yet to release a cause of death or an autopsy report, USA TODAY reported.

Thomas had begun to experience frequent seizures in the time leading up to his death, and a family member told the Associated Press last year that his family believed that was his cause of death. The Concussion Legacy Foundation’s Tuesday news release noted that doctors believed his seizures were unrelated to his CTE diagnosis and were more likely the result of injuries sustained from a car accident and a fall years prior to his death.

"He had two different conditions in parallel," Ann McKee, the director of the CTE Center, told the New York Times.

What sport is CTE most common in?

American football players and boxers account for most confirmed CTE cases, according to a 2016 medical journal article published on the NIH website.

Is CTE curable?

There is currently no cure for CTE, but symptoms of depression and anxiety can be treated individually, according to Boston University’s CTE Center.

This article originally appeared on USA TODAY: What is CTE? Symptoms, stages and more on the diagnosis