But with four of the girls still hanging on last night, surgeons who treat penetrating head injuries said several factors help determine whether a victim lives or dies.
Key factorA third factor, however, rises above the rest.
"I have to say it would be location, location, location" of the wound, said Dr. Michael Nance, director of the pediatric trauma program at Children's Hospital of Philadelphia, where three of the girls are being treated.
The fourth is at Penn State's Hershey Medical Center. Officials at both hospitals refused to discuss the cases at the families' request.
There is no precise formula for survival, but Nance and other surgeons said the bullet's direction plays a major role in the extent of injury. Patients who are shot from the front to the back of the head often have a better chance than those shot from side to side.
This is because a bullet traveling from front to back generally destroys just one of the brain's two hemispheres. "A front-to-back injury can wipe out one hemisphere while leaving the other intact," said Nance.
Extent of damageThe exception is the bullet that travels along the brain's midline, where it can damage both hemispheres, along with the brain's central core, which controls many of the body's functions.
A bullet that damages the patient's right hemisphere can leave the victim with weakness on the left side, and vice versa. But many other functions, such as cognition, memory and speech, are controlled by both sides of the brain.
As a result, damage to one hemisphere can leave a patient impaired but still able to perform those functions on some level.
With each hemisphere divided into four lobes, the "best-case scenario" is a bullet that injures one hemisphere and a single lobe - limiting the functions lost to the injury.
"As soon as the bullet crosses the midline, involving more than one lobe, it's very deadly," said Dr. Bizhan Aarabi, director of neurotrauma at the Maryland Shock Trauma Center.
According to Aarabi, 20,000 people in the United States die each year from gunshot wounds to the head. The survival rate is about 5 percent, with only 3 percent achieving a good quality of life afterward.
In 2000, Maryland recorded 235 penetrating brain injuries - 208 of them lethal.
ConsciousnessWhen a patient arrives at Shock Trauma with this type of injury, the staff's first priority is to gauge the person's level of consciousness.