Dave Mirra, the star BMX rider who killed himself in February, had CTE, chronic traumatic encephalopathy, the brain disease linked to head trauma.
The condition was diagnosed by neuropathologists at the University of Toronto and the Canadian Concussion Center, where Mirra's brain was examined after his death.
Mirra is the first action sports star to be diagnosed with CTE.
"It's assumed it is related to multiple concussions that happened years before," Dr. Lili-Naz Hazrati told ESPN.
Hazrati said the tau protein deposits found in Mirra's brain were indistinguishable from the kind found in the brains of former football and hockey players with CTE
Mirra won 14 gold medals at the X Games and helped popularize freestyle BMX, in which riders perform tricks on bicycles on the ground and soaring through the air. In 2000, he became the first person to land a double back flip in a competition. He endorsed numerous products, including a video game series.
Freestyle BMX is a dangerous sport that leads to many crashes and injuries, and Mirra was a victim of many. In a 2006 crash, he told the Washington Post, "I basically fell 16 feet straight to my head."
He was also hit by a car at age 19, suffering a fractured skull.
Mirra died from a self-inflicted gunshot in February in Greenville, N.C. He was 41.
CTE can only be diagnosed from a brain examination after death. Dozens of football players have been found to have had it, including Frank Gifford, Junior Seau and Ken Stabler.
The disease affects memory, cognitive function and mood. "I started to notice changes in his mood," his wife, Lauren, told ESPN. "And then it quickly started to get worse. He wasn't able to be present in any situation or conversation, so it was hard to be in a relationship with him to any degree."
The Mirra family decided to pursue posthumous testing.
Lauren Mirra said she planned to start "a platform for CTE awareness and research."