The growing evidence of a link between head trauma and long-term, degenerative brain disease was amplified in an extensive study of athletes, military veterans and others who absorbed repeated hits to the head, according to new findings published in the scientific journal Brain.
The study, which included brain samples taken posthumously from 85 people who had histories of repeated mild traumatic brain injury, added to the mounting body of research revealing the possible consequences of routine hits to the head in sports like football and hockey. The possibility that such mild head trauma could result in long-term cognitive impairment has come to vex sports officials, team doctors, athletes and parents in recent years.
Of the group of 85 people, 80 percent (68 men) — nearly all of whom played sports — showed evidence of chronic traumatic encephalopathy, or C.T.E., a degenerative and incurable disease whose symptoms can include memory loss, depression and dementia.
Among the group found to have C.T.E., 50 were football players, including 33 who played in the N.F.L. Among them were stars like Dave Duerson, Cookie Gilchrist and John Mackey. Many of the players were linemen and running backs, positions that tend to have more contact with opponents.
Six high school football players, nine college football players, seven pro boxers and four N.H.L. players, including Derek Boogaard, the former hockey enforcer who died from an accidental overdose of alcohol and painkillers, also showed signs of C.T.E. The study also included 21 veterans, most of whom were also athletes, who showed signs of C.T.E.
The study was conducted by investigators at the Boston University Center for the Study of Traumatic Encephalopathy and the Veterans Affairs Boston Healthcare System, in collaboration with the Sports Legacy Institute. It took four years to complete, included subjects 17 to 98 years old, and more than doubled the number of documented cases of C.T.E. The investigators also created a four-tiered system to classify degrees of C.T.E., hoping it would help doctors treat patients.
The volume of cases in the study “allows us to see the disease at all stages of severity and how it starts and spreads in the brain, which gives us an idea of the mechanism of the injury,” said Ann McKee, the main author of the study, who is a professor of neurology and pathology at Boston University School of Medicine and works at the V.A. Boston.
Those categorized as having Stage 1 of the disease had headaches and loss of attention and concentration, while those with Stage 2 also had depression, explosive behavior and short-term memory loss. Those with Stage 3 of C.T.E., including Duerson, a former All-Pro defensive back for the Chicago Bears who killed himself last year, had cognitive impairment and trouble with executive functions like planning and organizing. Those with Stage 4 had dementia, difficulty finding words and aggression.
Despite the breadth of the findings, the study, like others before it, did not prove definitively that head injuries sustained on the field caused C.T.E. To do that, doctors would need to identify the disease in living patients by using imaging equipment, blood tests or other techniques. Researchers have not been able to determine why some athletes who performed in the same conditions did not develop C.T.E.
The study also did not demonstrate what percentage of professional football players were likely to develop C.T.E. To do that, investigators would need to study the brains of players who do not develop C.T.E., and those are difficult to acquire because families of former players who do not exhibit symptoms are less likely to donate their brains to science.
“It’s a gambler’s game to try to predict what percentage of the population has this,” said Chris Nowinski, a co-author of the study and a co-director of the Center for the Study of Traumatic Encephalopathy at Boston University School of Medicine. “Many of the families donated the brains of their loved ones because they were symptomatic. Still, this is probably more widespread than we think.”
Researchers expected the details in the study to dispel doubts about the likelihood that many years of head trauma can lead to C.T.E. The growing connections between head trauma and contact sports, though, have led some nervous parents and coaches to assume that any concussion could lead to long-term impairment. Some doctors say that oversimplifies matters. Rather, the total amount of head trauma, including smaller subconcussive hits, as well as how they were treated, must be considered when evaluating whether an athlete is more at risk of developing a disease like C.T.E.
“All concussions are not created equal,” said Robert Cantu, a co-author of the study and a co-director of the encephalopathy center. “Parents have become paranoid about concussions and connecting the dots with C.T.E., and that’s wrong. The dots are really about total head trauma.”
