WEDNESDAY, April 18 (HealthDay News) -- Professional fighters may hit a threshold -- a specific number of fights and years in the ring -- where they can no longer take blows to the head without brain damage, a new study suggests.

But, once they cross that threshold, years might pass before symptoms show.

"The brain can tolerate or absorb a certain amount of trauma and repair itself," explained study author Dr. Charles Bernick. The findings raise the question of whether -- and when -- fighters should be medically screened, so changes could be caught earlier and available treatment offered, he said.

It's already well documented that "the more exposure you have to head trauma, the higher your risk of developing long-term complications. Primarily, this condition is chronic traumatic encephalopathy," said Bernick, associate director of the Lou Ruvo Center for Brain Health at the Cleveland Clinic in Ohio.

Also known as Boxer's Syndrome, chronic traumatic encephalopathy is a degenerative brain disease that causes the same kinds of thinking difficulties and personality changes seen with Alzheimer's disease.

As part of an ongoing study on brain health, the researchers divided 109 licensed boxers and mixed martial artists into three groups: those who had fought for less than six years, six to 12 years or more than 12 years. Their average age was about 29.

Participants underwent MRI scans to measure their brain volume and tests of their thinking and memory.

"In those that fought less than six years, we didn't find any changes," Bernick said. For that group, he said, "the more you fought didn't seem to make any differences in the size of brain structure or their performance on some of the tests like reaction time."

But for the other two groups of boxers and combat athletes, "the greater number of fights, the sizes of certain volumes of the brain were decreasing," he said. "But, it was only in those that fought more than 12 years that we could detect the changes in performance in reaction time and processing speed."

Women made up about 10 percent of the fighters in the study, too small a number to make any comparisons for now, Bernick said.

The study, released April 18, will be presented at the American Academy of Neurology's annual meeting in New Orleans, held from April 21 to 28.

"This is a cross-sectional study -- just one point in time in all these fighters' lives," Bernick said. "It needs to be substantiated and confirmed, but it's biologically plausible and it makes sense, and we're going to be following up on that."

With repetitive head trauma, Dr. Howard Derman, medical director at the Methodist Concussion Center in Houston, said "boxing is clearly more dangerous than football, because the number of the hits to the head is greater," and no headgear is used at the professional level.

"The initial presentations may begin with things like deterioration in attention, concentration, memory, disorientation, confusion and then they get much bigger issues with dementia, and then it even progresses to parkinsonian features," Derman said. Parkinsonian features include rigidity and tremor.

Derman isn't convinced that a time lag always exists between early head injuries from sports and measurable brain changes.

"Most of us believe that there is the period of quiescence, which is why you're seeing a lot of these [retired] football players in their 40s and 50s developing an issue," Derman said. "The disconcerting feature is that there are multiple cases of athletes who are very young" showing signs of chronic traumatic encephalopathy on autopsy, for instance after dying in vehicle accidents, he added.

"Clearly, everyone who plays football or is a boxer does not develop a dementia-like picture. It is fortunately a small minority of the athletes that do," said Derman, adding that genetic predisposition likely plays a role.

Study author Bernick said that if further research confirms the findings, "it may gave regulatory agencies, boxing athletic commissions, guidelines on how to protect their athletes, when to perhaps require evaluation."

But that would be a hard sell, in Derman's view.

"That probably won't happen because the professional athletes -- they don't want to know they have a problem, as funny as that seems. That's part of the issue," Derman said.

"The big thing we can do is some kind of baseline testing on all athletes, so we can compare where they are [after a head injury] with where they were," he said. "I think X-ray and MRI scans are a larger leap, and players' associations of all the leagues would really [be opposed]."

Data and conclusions presented at medical meetings should be viewed as preliminary until published in a peer-reviewed journal.

More information

Learn about sports-related brain damage at Boston University's Center for the Study of Traumatic Encephalopathy.