When the former N.H.L. enforcer Todd Ewen died in September, reportedly of a self-inflicted gunshot, his brain was sent to researchers. Years of memory loss and undiagnosed depression led to speculation that Ewen, 49, had chronic traumatic encephalopathy, a degenerative brain disease caused by repeated blows to the head.
The results raise even more questions about why certain athletes get the brain disease and others do not.