Dr. Mark Halstead of Washington University School of Medicine in St. Louis said an explosion in research about concussions in the past five years has increased understanding of how serious concussions may be.

"There is a common misconception that an athlete only has a concussion if he or she loses consciousness," Halstead said in a statement. "In fact, most athletes may only suffer from a mild headache or feeling confused or foggy. Concussions may even occur without impact, from the head being shaken."

It is important for coaches, parents and athletes to recognize the symptoms of a concussion because athletes may only have one symptom, Halstead said. No athlete should be returned to play while still experiencing symptoms of a concussion.

Common symptoms of a concussion include: headache, dizziness, changes to the vision such as blurry vision or double vision, upset stomach or vomiting, feeling tired, moodiness, confusion and loss of memory of events before and after the concussion.