Former NFL star wide receiver Demaryius Thomas
(CNN) Former NFL player Demaryius Thomas was suffering from stage 2 CTE when he died late last year, according to his parents.
Katina Stuckey Smith, Bobby Thomas, said the diagnosis came after an examination of her son's brain following his death in December, according to a press release from the Concussion Legacy Foundation.
Smith and Thomas told ABC News that their 33-year-old son died of cardiac arrest caused by a seizure disorder that led to him retiring from football in June 2021.
CTE, known as chronic traumatic encephalopathy, occurs when the brain begins to degenerate likely due to repeated head trauma, according to the Mayo Clinic.
It has been found in the brains of people playing soccer and other contact sports, the clinic said, and diagnosis is currently only possible through an autopsy by studying sections of the brain.
Dr. Ann McKee, a professor of neurology and pathology at Boston University CTE Center, studied Thomas's brain and made the diagnosis.
Thomas's parents told ABC News that they saw their son face many symptoms of CTE in the months leading up to his death, including paranoia, memory loss, and frequent headaches.
He added: "CTE itself does not cause death. You don't die from CTE ... What CTE does is change your behavior and your personality."