Falls are by far the most common cause of head injuries among younger children, a new study that ranked the cause of serious incidents causing significant brain injuries has found.

For teenagers, sports injuries, auto accidents and assaults move to the top of the list, the study that looked at visits to 25 hospital emergency facilities from 2004 to 2006 determined.

Although traumatic brain injury has always been a leading cause of death or disability in children from one year of age, few details as to cause or types of incidents has been available up to now, the researchers noted.

The study in which more than 40,000 children were analyzed for head trauma has been published in The New England Journal of Medicine.

In the study, the researchers analyzed data on children with head injuries of all severities, from normal neurological status to deep coma, and categorized them as mild, moderate or severe.

The vast majority of injuries seen in emergency rooms, 98 percent, are classified as "mild," says Dr. John Kuluz, head of traumatic brain injury and neurorehabilitation for Miami Children's Hospital.

For children younger than two years of age, falls accounted for 77 percent of the injuries, a figure that drops to 38 percent from age 2 up to age 12, study co-author Kimberly Quayle of Washington University in St. Louis says.

Causes of head trauma in teenagers age 13 to 17 proved more varied, she says.

"Head injuries in adolescents most often were caused by assaults, sports activities and motor-vehicle crashes," she says.

Twenty-four percent of head injuries in teens were the result of assaults; 19 percent were related to sports participation; and 18 percent were the result of car accidents, they researchers reported.

In such car accidents, children suffering brain injuries were not wearing their seatbelts more than half the time.

More than 80 percent of children suffering head trauma in bicycle accidents were riding without an approved bike helmet, the study found.

The study findings could aid policy makers move toward steps to help reduce such injuries, while improving doctors' ability to safely diagnose such trauma, the researchers said.

"We have distilled a wide range of important features regarding blunt head trauma in children," Quayle says. "The findings may provide reliable guideposts in developing injury-prevention measures and should help physicians in diagnosing and treating these injuries based on strong evidence."

The study did not include concussions, which are injuries that shake the brain, but rather focused on injuries that cause bleeding in the brain.