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.

Electroencephalograph (EEG) is a decades-old technology that measures electrical activity in the brain from the surface of the scalp. But using it to study mild traumatic brain injury has been a challenge, in part because the technology is highly susceptible to noise, such as head movements, and it must be performed by a trained expert.

Recently companies have developed more robust, portable devices, thanks to new sensors and advances in the algorithms used to process the data they collect. Such devices also require less training for those who use them. BrainScope, a startup based in Bethesda, MD, has developed one such device, which it is testing for athletic and military applications.

In the new study, McCrea and collaborators used the BrainScope device to analyze brain activity in nearly 400 football players at the start of the season to determine baseline brain activity. Twenty-eight of those players sustained a concussion during the study period. These players had their brain activity measured again right after the incident, as well as days later. Scientists also gave the players tests currently used to assess concussion, including tests of cognitive function and balance. They then compared changes in brain activity in injured players to both noninjured players and nonathlete controls.

“It turned out that symptoms, cognitive function, and balance had all returned to normal within the first week after a concussion,” says McCrea. “But brain electrical activity remained abnormal at day eight.” Brain activity returned to normal a month and a half later, when the next measurement was taken. McCrea says they now plan to repeat the study, assessing brain activity after two weeks to get a better handle on when the brain returns to normal. The findings, published this month in the Journal of Head Trauma Rehabilitation, suggest that the brain’s “vulnerability lasts a bit longer than we thought,” says Ross Zafonte, a physician and scientist at Spaulding Rehabilitation Hospital in Boston. Zafonte was not involved in the study.

At this point, the BrainScope device is still a research tool, rather than a diagnostic one. It’s not yet clear whether it can diagnose an individual case of concussion; in the newest study, researchers added together brain activity profiles, rather than comparing before-and-after profiles of individual players. “I do think there are EEG abnormalities, but I don’t know how specific or reliable they are,” says David Hovda, director of the Brain Injury Research Center at the University of California, Los Angeles. Hovda was not involved in the study.

It’s also unclear what the abnormalities in brain-injured players really mean–doctors don’t understand exactly what’s happening in the brain after such an injury. “Detecting an abnormality is something we’re good at, but linking it to a clinically meaningful situation is different,” says Zafonte.