It's fairly common and well publicized when a sports hero gets a concussion: Red Sox star Johnny Damon recently admitted he's still feeling the aftereffects of last year's collision with a teammate; Dallas quarterback Troy Aikman's football career ended because of the risk of repeat concussions.
But it is not just professional athletes who have to worry about serious concussions. Neuroscientists say many brain injuries, particularly in children, go undiagnosed because educators, coaches, and parents simply don't know when a bump on the head is something to be concerned about.
"Most parents and children don't know the symptoms," said Robert Cantu, codirector of the Neurological Sports Injury Center at Brigham and Women's Hospital. Symptoms can include loss of consciousness, headaches, nausea, difficulty concentrating, lethargy, mood swings, or blurred vision. Even those who work in the medical sports field may not be as aware of the problem as they should be, Cantu said.
Nationwide, "traumatic brain injury is the leading cause of death and disability in people under 21 years old," said Wayne Gordon, a professor of rehabilitation medicine at Mount Sinai School of Medicine in New York.
According to the national Centers for Disease Control and Prevention, 1.5 million people sustain a traumatic brain injury annually; more than a quarter of those victims are age 14 and under. In a concussion -- which can vary greatly in severity depending on how hard the head is hit -- the brain's soft tissue is damaged by the impact against the inside of the skull.
Though the top cause of brain injuries is car accidents, an estimated 300,000 sports-related brain injuries -- from mild to moderate -- occur annually in the United States, the CDC reported.
In Massachusetts in 2002, there were 33,000 emergency room visits for traumatic brain injuries, 9,359 for children ages 5 through 19.
Those figures probably underestimate the number of concussions kids receive on the playground or the playing fields. They don't "include mild traumatic brain injury -- where someone goes to the ER and never comes back, or is seen in a doctor's office," said Jim Enders of the CDC's Injury Center.
And they don't count the injuries that are never recognized as concussions.
The good news is that about 85 percent of all people who suffer from a concussion recover fully, according to the CDC. But even minor concussions can have long-lasting symptoms, said Cantu, who is also chief of neurosurgery at Emerson Hospital in Concord.
There is no medical way to repair a brain damaged by concussion, which makes it crucial to notice and handle possible brain injuries as soon as possible, doctors say. For both children and adults, it is important to take it easy -- physically and mentally -- for several days after the blow, refrain from drinking alcohol, and only take drugs approved by a doctor, neurologists say.
The Brain Injury Association of America sells wallet-sized cards designed for use by coaches or athletic trainers to help determine, among other things, the degree of a concussion, what kind of medical attention is needed, and how long the player should be sidelined.
Extra caution is in order after a first concussion, because a second or third can compound the damage, even years after the first one has been forgotten.
That's what happened to Andrew Early, 16, of Holden. When he was 9, he bounced off a raft in an above-ground pool and landed facedown on the concrete. He was dazed and confused for a few hours, but a trip to the emergency room didn't find anything serious, and his family didn't think much more about it.
"That was in fourth grade. In fifth grade his grades started plummeting," said his mother, Maggie Curran Early. "I had him tested . . . I thought it might be the beginning of a learning disability."
In sixth grade Andrew went snowboarding with friends, sailed down a hill, and crashed headlong -- and helmetless -- into a tree.
"He got up at 2 a.m. and vomited for 16 hours," his mother said.
During a subsequent trip to the emergency room, Andrew slipped in and out of consciousness.
He recovered, but his grades kept falling and he became more "irascible," his mother said. Although he eventually got academic and emotional help from his school, it took a while to connect his brain injuries with his problems and figure out how to address them.
"I never knew that concussions left a calling card," his mother said.
Doctors emphasize that helmets -- worn properly -- are a key element in preventing serious brain trauma, but Cantu said that those wearing helmets should not assume they are invincible. Standard helmets are made to prevent bleeding in the brain and skull fractures, not concussions.
The most important thing, Gordon said, is being aware of how fragile that precious gray matter is.
"We need to approach life knowing that the brain can't be fixed," Gordon said. "Let's not be so cavalier."