WASHINGTON - High school athletes who suffer more than one concussion are more likely to show severe symptoms, including loss of consciousness, a study finds. Researchers say the study is the first to show that the effects of concussion accumulate in high schoolers.

The study is based on assessments by trainers and team physicians right after a player suffered a concussion. Researchers compared athletes who had three or more previous concussions with those who had just suffered their first.

Athletes with more of the jarring injuries to the brain had a risk of symptoms that was nine times higher than athletes with their first concussions.

"It does look like these athletes are more vulnerable," said Michael W. Collins of the University of Pittsburgh Medical Center's Sports Medicine Concussion Program. Collins was lead author of the report in the November issue of the journal Neurosurgery.

The researchers looked at data on 98 athletes, more than three-quarters of them football players, in Pennsylvania, Michigan, Illinois, Oregon and Maine. All the schools already were using a database that tracked concussions. Sixty students had three or more concussions before the one analyzed in the study. The rest of the athletes had no such previous history.

Players in the repeat-concussions group were 9.3 times more likely to have at least three of four problems examined. Those markers were on-field loss of consciousness, inability to recall things before the injury, inability to recall things after the injury, and generalized confusion. To separate more severe injuries from minor ones, any symptom had to last at least five minutes.

Team officials have used three concussions in a season as a rule-of-thumb marker for benching a player for the rest of the season, so the study was an attempt to see if the rule had value, Collins said.

The study indicates that concussion damage adds up, but the findings alone should not be used as justification for benching a player, Collins said. The decision has to be made on a player-by-player basis by people who have examined the athlete, he said.

The finding also does not mean athletes are fine until they've had three or more concussions. The study was not able to measure the rate at which damage accumulates. It is known that some athletes need only a couple of days to recover, while others may take weeks, Collins said.

"This is a first-attempt study; no previous studies examined this issue," Collins said. "We need to do further study looking at how many injuries are too many."

The researchers could not tell exactly how much damage the players' brains suffered. But Collins believes players with histories of concussion were more vulnerable.

"We do know clinically that seemingly mild blows to the head are causing more problems in people who had more concussions in the past," Collins said.

The study also could not rule out the possibility that the high-concussion athletes had, for some reason, a lower threshhold for concussion damage, Collins said.

The new report supports the idea that three concussions is a big sign of trouble, said head injury researcher Kevin M. Guskiewicz of the University of North Carolina, Chapel Hill, who was not involved in the research.

NCAA players who had three concussions have been found to have a slightly slower recovery following a subsequent concussion, Guskiewicz said. And he said retired pro football players with three or more concussions had a higher risk of clinical depression later in life.

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National Institute of Neurological Disorders and Stroke Traumatic Brain Injury Information Page: http://www.ninds.nih.gov/health_and_medical/disorders/tbi_doc.htm

National Information Center for Children and Youth with Disabilities Traumatic Brain Injury Fact Sheet: http://www.nichcy.org/pubs/factshe/fs18txt.htm