(CN) - Seven-eighths of the brains of 202 former football players showed evidence of chronic traumatic encephalopathy, a brain disease linked to repeated blows to the head, the Journal of the American Medical Association reported Tuesday.
The report is the largest update on chronic traumatic encephalopathy, or CTE, a progressive neurodegeneration that can cause a range of symptoms, including memory loss, erratic behavior and poor judgment. Later-stage symptoms of CTE include vertigo, dementia and suicidal thoughts.
Among the 202 football players, CTE was diagnosed in 177 players (87 percent) who had an average of 15 years of football participation. Those athletes include three of 14 high school players (21 percent); 48 of 53 college players (91 percent); nine of 14 semiprofessional players (64 percent); seven of eight Canadian Football League players (88 percent); and 110 of 111 NFL players (99 percent).
The neuropathological severity of CTE in the players reflected how long the athletes played: all three former high school players had mild pathology, while most former college (56 percent), semiprofessional (56 percent) and professional (86 percent) players had severe pathology.
"These findings suggest that CTE may be related to prior participation in football and that a high level of play may be related to substantial disease burden," the researchers write.
Among 27 players with mild CTE pathology, 96 percent had mood or behavioral symptoms or both, 85 percent had cognitive symptoms, and 33 percent had signs of dementia.
Of the 84 participants with severe CTE pathology, 89 percent had behavioral or mood symptoms or both, 95 percent had cognitive symptoms, and 85 percent had signs of dementia.
"In a convenience sample of deceased football players who donated their brains for research, a high proportion had neuropathological evidence of CTE, suggesting that CTE may be related to prior participation in football," the report states.
Led by Boston University neuroscientist Ann C. McKee, the researchers determined the neuropathological features of CTE through laboratory examination. The team identified clinical symptoms of CTE by talking to players' next of kin, collecting detailed histories of the athletes' head trauma, athletic participation and military service.
The authors said several other football-related factors may influence CTE risk and disease severity, including the total number of hits, player position, duration of participation and age at first exposure to football.
The team acknowledged that the findings are based on a skewed sample, of brains from players who experienced symptoms and signs of brain injuries before they died. That, along with growing public awareness of a possible connection between repetitive head trauma and CTE, may have motivated the players and their families to participate in the research.
The researchers add that estimates of the prevalence of CTE among football players cannot be concluded or implied based on the findings.
The average age of death of the study subjects was 66.
After years of denial, the National Football League in 2016 acknowledged a link between repeated blows to the head and brain disease. The NFL agreed to a $1 billion settlement to compensate former players who accused the league of hiding the risks.