The nine Amish girls shot in the head Monday by a deranged milk truck driver faced long odds: National statistics show that only 5 percent of people who sustain such gunshot wounds survive them.
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.
Children generally fare better than adults, and patients who are somewhat responsive do better than those who arrive at the hospital in a deep coma.
Key factor A 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 damage The 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.
Consciousness When 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.
Looking at indicators such as whether the patient can move limbs or respond to someone's voice, they rate the victim's consciousness on a so-called Glasgow scale of 3 to 15.
A CT scan reveals important features of the injury - such as major bleeding or damage to the brain stem, both of which bode poorly for the patient. Located in the back of the head and just above the spinal cord, the brain stem controls involuntary functions such as breathing and heartbeat.
"You knock that out and you don't have much hope," said Nance.
Generally, patients transported to trauma centers with gunshot injuries to the head spend an hour being resuscitated, evaluated and stabilized before they're wheeled into surgery.
Surgeons have many goals, but removing the bullet is not one of them. "One, you can't really find it, and you don't want to go digging in the brain to find it," Nance said. "You'll injure tissue to get to it."
Instead, surgeons concentrate on reducing as much of the bullet's damage as they can. They remove dead tissue - a process called debridement - and relieve pressure and swelling that can occur in the injury's aftermath. In some cases, surgeons temporarily remove a portion of the patient's skull to give the swelling brain a safe outlet.
Without such an outlet, the brain has a tendency to herniate into the only natural opening - at the base of the skull.
"It's a terminal event," said Nance.
Sometimes, doctors place a drain inside the brain so excess fluids can escape.
Though the circumstances of the Amish shootings were unusual and terrible, doctors at Hershey and Children's Hospital of Philadelphia said they're accustomed to treating injuries of this sort.
Children's serves a city with a high homicide rate, and Hershey draws patients from the wide territory between Philadelphia and Pittsburgh.
"This has garnered national attention for days and days," said Nance. "I'm not saying that's wrong, but this happens every day. This just happened in a place where you'd never expect it, and the details are horrible.
"We lose kids all the time from firearm injuries," he said. "It's a horrible problem."