In his six years with the Cincinnati Reds, Ryan Freel wowed fans with his ostentatious, fearless style, running into walls and colliding with other players. He estimated that he had sustained 10 concussions in his career, though his family said the number might have been even higher.

But off the field, he struggled with depression and substance abuse. His repeated injuries led him to retire from baseball early in 2010. One year ago, Freel, then 36, killed himself with a shotgun.

Today, he’s become the first Major League Baseball player to be diagnosed with chronic traumatic encephalopathy, or CTE, after researchers at the Boston University School of Medicine released the results of a test on his brain tissue after his death.

CNN’s Stephanie Smith and Dan Moriarty explain that Freel’s brain was in the moderate stage of the degenerative disease, which has recently become better-known for its possible connection to brain damage and suicides in former pro football players:

Testing of his brain tissue after death … found that he had Stage 2 CTE, which is associated with erratic behavior and memory loss. Stage 4, the worst possible expression of the disease, is associated with full-blown dementia, aggression and paranoia. The brain tissue of people found to have CTE displays an abnormal build-up of tau—a protein that, when it spills out of cells, can choke off, or disable, neural pathways controlling things like memory, judgment and fear.

The diagnosis of several NFL players with CTE has sparked widespread debate over the future of football. But Freel’s case is interesting because it suggests that the disease can afflict even players of less-aggressive sports.

"I cringe when I see two guys going after the same ball," Robert Stern, co-founder of the Center for the Study of Traumatic Encephalopathy at BU, told CNN. "Is baseball as significant a concussion sport compared to others? No. Is it a concussion sport? Yes."