Some 43 percent of retired American football players studied show signs of traumatic brain injury, raising new concerns about the long-term effects of hitting and tackling, researchers said Monday.
The study involved 40 retired National Football League (NFL) players, who underwent thinking and memory tests as well as brain scans.
"This is one of the largest studies to date in living retired NFL players and one of the first to demonstrate significant objective evidence for traumatic brain injury in these former players," said study author Francis Conidi of the Florida State University College of Medicine.
"The rate of traumatic brain injury was significantly higher in the players than that found in the general population."
The players ranged in age from 27 to 56 and played an average of seven years in the league.
Most had been out of the NFL for fewer than five years.
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They reported an average of eight concussions.
Around a third of the players said they had sustained several hits that were not strong enough to be diagnosed as concussions.
The findings, based on sensitive MRI scans called diffusion tensor imaging, were released ahead of a presentation at the American Academy of Neurology's 68th Annual Meeting in Vancouver, Canada.
The advanced MRI scans measured the movement of water molecules in the brain's white matter -- which controls how the brain functions -- to determine the amount of damage.
"Seventeen players, or 43 percent, had levels of movement 2.5 standard deviations below those of healthy people of the same age, which is considered evidence of traumatic brain injury with a less than one percent error rate," the study said.
"On the tests of thinking skills, about 50 percent had significant problems on executive function, 45 percent on learning or memory, 42 percent on attention and concentration."
Longer careers tended to put players at higher risk of TBI, but researchers established no link between higher concussion numbers and brain injury.
The findings could help scientists better understand the brain condition known as chronic traumatic encephalopathy, which is associated with aggression, dementia, depression and suicide.
"This research in living players sheds light on the possible pathological changes consistent with chronic traumatic encephalopathy that may be taking place," Conidi said.
The latest evidence of widespread brain injury among football players, the findings may increase pressure on the NFL, which has long been under fire for downplaying the effects of concussions in the most popular US sport.