But cognitive difficulties weren't directly related to the number of concussions a player had suffered in his career - complicating the controversy over long-term effects of head injuries.

"Not everyone gets this problem," said Dr. John Hart, Jr. from the University of Texas at Dallas, who worked on the study. "It's a more complex issue than has just sort of been thrown out there."

The topic of concussions in the NFL and other sports leagues has been thrust into the spotlight after reports of players suffering from depression or memory trouble long after retiring. Last year, former San Diego Chargers star Junior Seau committed suicide after what some believe were years of depression related to concussions he may have suffered as a player.

Hart hopes the new findings will encourage more NFL players to get their brains checked out.

Ideally, he said, all players would be evaluated before, during and after their careers to check for brain changes. That would help doctors learn more about how head trauma is related to mental decline and dementia - and hopefully avert those problems in future athletes.

Hart has been evaluating former football players for years, both in Baltimore and Dallas.

"Starting a long time ago… guys would come in with these problems, and they'd look somewhat different than the typical types of degenerative diseases and cognitive diseases that you see from folks," he said.

For their new study, he and his colleagues evaluated the thinking and memory skills of 34 retired NFL players and also took brain scans of 26 of them.

The men were between 41 and 79 years old and had spent an average of 10 years in the league. Hart's team compared their results with those from non-football players who were similar to the original group in terms of age, education and IQ.

Of the retired players, all but two had sustained at least one concussion, with one player reporting up to 13.

Twenty of the men were cognitively normal, according to thinking and memory tests.

Two formers players had dementia and the rest had mild or fixed (non-progressing) mental impairment. The players with those problems were more likely to also show changes in their white matter on brain scans, the research team reported Monday in the journal JAMA Neurology.

White matter contains nerve fibers responsible for sending signals from the brain to the rest of the body.

Eight of the former players were depressed - a slightly higher percentage than would be expected in a normal group of men.

Hart said there are a couple of explanations for why more concussions didn't mean a higher risk of cognitive problems in his group of NFL veterans.

"Sub-concussive injuries, all the wear and tear… may be playing a role in this" - not just concussions, he said.

Second of all, "I think there are predisposing factors that if you do get an injury, some people respond differently than others," Hart added. His team has started examining whether genetic differences may influence the effect of concussions on players' mental status.

An NFL representative said the league would be sharing the report with members of its Head, Neck and Spine Committee.

"The NFL, both directly and in partnership with the CDC, NIH, and other leading organizations, is committed to supporting a wide range of medical and scientific research that will promote player safety in the long term," the representative wrote in an email to Reuters Health.

In an editorial published with the study, Dr. Ramon Diaz-Arrastia and Dr. Daniel Perl from the Uniformed Services University of the Health Sciences in Bethesda, Maryland, write, "If confirmed, the findings of studies such as these would support the use of magnetic resonance imaging (MRI)… for monitoring the cumulative burden of concussions in athletes and others who have experienced multiple mild traumatic brain injuries."

Hart pointed to the importance of further research to determine how head injuries, thinking and memory skills and physical changes in the brain are related. Researchers will need players' help for that, he said.

"I think a lot of players because of all the press are a little nervous to come in and be checked out, and if you look at the study, more than half of the guys were normal," he said.

SOURCE: bit.ly/URjDYO JAMA Neurology, online January 7, 2013.

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