A study released Monday of 40 former NFL players between the ages of 40 and 65 found that those who began playing tackle football before the age of 12 faced a higher risk of altered brain development than those who waited until they were older.
The findings, by researchers at Boston University School of Medicine and Brigham and Women’s Hospital, are the first to show a link between repetitive head impacts early in life and structural brain changes later in life, researchers said.
The report comes during a period when participation in youth football has declined amid concerns about possible brain damage from repeated head trauma.
The study team’s leader, Dr. Robert Stern, cautioned against concluding from the findings that there are specific long-term dangers associated with children playing tackle football. He cited the small number of players who were examined and noted that the study group involved only individuals who went on to play professional football.
“But it is a beginning,’’ said Stern, a professor and director of the BU Alzheimer’s Disease Center and director of clinical research for the school’s Chronic Traumatic Encephalopathy Center. “What it tells us is that we need to think rationally about when is it more or less likely that sustaining repetitive hits to the head [as children] will result in later-life problems.’’
Researchers reported finding increased evidence of a “critical window’’ of brain development for children between the ages of 10 and 12, when the brain may be especially susceptible to injury.
“This development process may be disrupted by repeated head impacts in childhood possibly leading to lasting changes in brain structure,” said Julie Stamm, who conducted the study as part of her doctoral dissertation at BU and now is a post-doctoral fellow at the University of Wisconsin School of Medicine and Public Health.
The study, which appears online in the Journal of Neurotrauma, was funded by the National Institutes of Health.
Each former player who was examined had more than 12 years of organized football experience, including at least two years in the NFL. Half the players participated in tackle football before age 12 and half began at 12 or later. The number of concussions they sustained was similar between the two groups.
The former players, who were not identified, submitted to advanced magnetic resonance imaging exams of their brains. The tests looked specifically at the movement of water molecules in areas that are considered superhighways within the brain for relaying commands and information. Researchers said the results showed the NFL veterans who started playing football before age 12 were more likely to experience changes in the largest structure of the brain that connects the two cerebral hemispheres.
Yet the study’s authors acknowledged that the findings might vary if they examined a group of former youth players other than NFL veterans.
“The results of this study do not confirm a cause-and-effect relationship, only that there is an association between younger age of first exposure to tackle football and abnormal brain imaging patterns later in life,” said Martha Shenton, a professor and director of the psychiatry neuroimaging laboratory at Brigham and Women’s.
The study supports the findings of a BU research project this year that found former NFL players who participated in tackle football as children were more likely to develop problems later in life with reasoning, planning, and memory.
In 2012, youth football leagues in Greater Boston said they would sharply limit head-to-head contact during practices, following new rules by the national Pop Warner organization designed to cut the chance of concussions.
On Monday, a spokesman for Pop Warner Little Scholars, the nation’s largest youth football organization, said he forwarded an interview request from the Globe to Pop Warner’s medical director, Dr. Julian Bailes, seeking his comment on the BU study. Bailes did not respond.
The Pop Warner spokesman declined to comment on a federal lawsuit in Wisconsin that alleges a 25-year-old man killed himself in 2012 because of brain damage he suffered playing youth football. The man’s mother, Debra Pyka, states in court records that Joseph Chernach began playing Pop Warner football when he was 11.
Chernach went on to play high school football, but the lawsuit alleges he suffered his worst brain damage from concussions as a Pop Warner player. An autopsy allegedly found evidence that Chernach suffered from Chronic Traumatic Encephalopathy, or CTE, a degenerative brain disease associated with repeated head trauma.
At least 17 deceased NFL players have been diagnosed with CTE, including former Patriot Junior Seau, who shot himself to death in 2012 at age 43.
Child safety advocates said they welcome research on the possible dangers of head injuries, but some cited the limitations of the latest study. Brooke de Lench, executive director of the Waltham-based MomsTeam Institute, an advocacy group for youth sports safety, said it is “very hard, if not impossible, to draw any conclusions from this study about the level of risk to those who begin playing tackle football before the age of 12 because we are now light-years ahead of where we were 25 to 45 years ago in terms of head safety.’’
De Lench, who produced the PBS documentary, “The Smartest Team: Making High School Football Safer,” said she does not oppose children as young as 6 playing tackle football so long as they are properly trained, correctly equipped, matched with similarly sized players, and they refrain from head contact.
Stern, the BU researcher, praised the positive effects of youth sports and indicated that science has yet to determine the best age for someone to begin playing tackle football. But, he said, “Regardless of the results of this study, doesn’t it just make sense that children whose brains are rapidly developing should not be hitting their heads over and over again?’’Bob Hohler can be reached at email@example.com.