A gunman's shooting distance and angle on a target can make all the difference between a victim's tenuous life, death or general recovery.

Before the crucial quick steps that aides and surgeons took to help save U.S. Rep. Gabrielle Giffords (D–Ariz.) soon after she was shot in the head on Saturday, the series of initial conditions that led to the bullet's trajectory through the brain—from the shooter's planning to his pacing—might have helped to spare her life and ultimately her functioning. And as past cases demonstrate, a shift of inches or a varied assault type can change everything for an individual patient, determining whether key brain regions are shattered or spared.

Although Giffords remains in critical condition, doctors say they are increasingly optimistic that she will survive. Details about the bullet's path have not been released per the family's request, but doctors have confirmed that it passed through the left hemisphere of her brain from back to front, fully exiting the head.

"This wasn't a little grazing wound," Peter Rhee, chief of the University of Arizona's University Medical Center (UMC) Division of Trauma, Critical Care and Emergency Surgery who has helped with Giffords's case, noted in a briefing on Sunday. "This was a devastating wound that traveled the entire length of the brain on the left side."

Motor skills and speech centers are located in the brain's left hemisphere in most people, and "depending on where it entered the brain in the back, it could affect some of her visual field," explains Marc Nuwer, a critical care expert and neurologist at the University of California, Los Angeles and member of the American Academy of Neurology.

"It's unclear whether she has damage to critical areas," Nuwer says, but because Gifford has been responding to commands, basic motor skills and "the understanding speech part seems to be working okay."

The bullet, which is reported to have come from a 9-millimeter Glock 19, is large compared with those that come from .22-caliber weapons. And bullet size is "relevant because very small caliber bullets, like a .22, cause much less damage than a larger bullet," says Nuwer, who has seen many people survive gunshot wounds to the head from these smaller bullets.

That the bullet cleared the skull helps to limit the opportunity for infection, but it also means that the congresswoman's brain bore less of the brunt of the bullet's full energetic load. Nuwer notes that if she had been hit on the head with a baseball that contained the same amount of force that the bullet contained, the brain would have likely sustained much more damage. But in Giffords's case, the energy was dispersed along the bullet's full track as well as being carried off away on the bullet's continued trajectory.

Unknown damage
Although researchers are still parsing out how different areas of the brain contribute to normal function, neuroscience has evolved a long way from the days of lobotomies to stave off seizures.

Surgeons can now often remove brain tumors and nearby damaged sections "and the patient will do marvelously after surgery," Nuwer says. But of course, Giffords's injury was nothing like a carefully mapped surgery.

The end result will depend largely on which regions are affected. "There are parts of the brain that do control specific functions," Nuwer says. "Other parts are a little more nebulous as to how they fit into the big picture." Some of these parts, he notes, "can be damaged and removed without causing noticeable impairment of function."

Surgeons working on the case at UMC noted in the Sunday briefing that they had found only minimal amounts of "devitalized" brain matter and had already removed it. Nevertheless, "as time goes on, the cavitation effects from the bullet itself" are likely going to continue to kill off brain cells, Rhee noted.

As doctors dial down anesthetics and remove the patient's ventilator tube, more will be apparent about Giffords's potential for recovery. However, "in neurosurgery, we talk about recovery on the orders of months to years," Michael Lamole, chief of neurosurgery at UMC, said on Sunday. "In fact, we don't even close the book on it until we're several years out."

But chances of survival even in the minutes and hours after a gunshot wound to the head are typically slight. "Overall, this is about as good as you're gonna get," Rhee said. "When you get shot in the head, and the bullet goes through your brain, the chances of you living are very small, and the chances of you waking up and following commands are actually much smaller than that."

View slide show of lessons learned from infamous brain injuries