Football players who suffer a concussion on the field may not have fully healed even after their outward symptoms, such as memory or balance problems, have disappeared. The findings come from a study of nearly 400 high school and college football players using a new portable device for assessing brain injury.

Researchers hope the findings, and some form of portable brain-monitoring device, will help physicians determine when it is safe for players to return to the field.

"There has long been speculation that even after symptoms resolve, there is a period of vulnerability at which the brain has not completely healed," says Michael McCrea, a neuropsychologist at Waukesha (WI) Memorial Hospital, who led the study. "This study provides some preliminary support for that theory."

Last fall, the National Football League instituted new rules requiring players who have suffered head trauma to get permission from an independent neurologist before returning to play. Diagnosing brain trauma accurately is difficult. Also, while the issue is still controversial, many scientists and physicians think a blow to the head while the brain is still healing from an earlier blow might significantly worsen damage, especially in the long-term.

The danger of repeated concussions has become a major issue in professional football, thanks to a number of high-profile cases of ex-players suffering early dementia and severe psychological problems. Autopsies of at least six former professional players who donated their brains to research revealed chronic traumatic encephalopathy, a degenerative brain disease caused by head trauma.

An estimated 1.6 million to 3.8 million sports-related traumatic brain injuries occur each year. One of the biggest challenges in studying concussion, and both the long-term and short-term effects of repeated concussions, is finding a reliable way to assess brain injury. The damage that results from concussions is typically too subtle to be detected with traditional brain imaging technologies. So physicians diagnose it based on characteristic symptoms, such as nausea and headache, as well as through cognitive and neurological tests.

Many football players, eager to return to the field, also underreport injuries and their symptoms. According to anonymous surveys of football players, about 50 percent sustain a concussion during the season, many more than are actually reported, says Chris Nowinski, president of the Sports Legacy Institute, a nonprofit organization based in Waltham, MA, that studies brain injury in athletes. A noninvasive, simple device that could be used immediately after the injury occurred would provide a way to objectively measure a player's symptoms.