SUNDAY, May 22 (HealthDay News) -- Young athletes who return to play before a concussion fully heals and sustain another head injury can suffer serious and potentially deadly brain complications, an expert warns.

Athletes under age 25 are especially vulnerable to "second-impact syndrome," according to Dr. James Kinderknecht, a sports medicine and shoulder service physician at the Hospital for Special Surgery in New York City.

But there are ways to prevent a subsequent injury, including knowing the signs of concussion. These include headache, dizziness, irritability, mood changes, vomiting, changes in vision and hearing and difficulty following instructions.

If young athletes have any of these symptoms in the hours or days immediately after a head injury, their physical activity should be restricted until they're evaluated by a doctor.

Symptoms of a concussion can disappear before the brain is fully healed, said Kinderknecht in a hospital news release. A doctor may test a young athlete's readiness to return to play by engaging the youngster in non-contact physical activity and watching for any symptoms that may not have been present when the child was at rest, Kinderknecht explained.

Parents and coaches also need to remember that young athletes may be reluctant to report symptoms of concussion, especially if it occurs during a highly competitive game.

"I always tell my patients, if it's a sore knee, it might be all right to try to play," Kinderknecht said. "But you can't take chances with what may be a brain injury. If there are any symptoms, even mild ones, it is not appropriate to participate."

"With educated parents and coaches supporting a stringent policy about keeping injured players off the field, we can all worry less about second-impact syndrome," he added.

Kinderknecht gave a presentation on concussions at a recent hospital symposium on sports medicine for young athletes.

More information

The U.S. Centers for Disease Control and Prevention has more about concussion in sports.