Doctors say that while the bullet that struck Arizona Rep. Gabrielle Giffords did not hit any critical parts of the brain, whether she will survive and how fully she will recover are still unknown.
"This was a devastating wound that traveled the length of the brain on the left side," Dr. Peter Rhee, trauma director at University Medical Center in Tucson, said during a press conference. Giffords' family confirmed to ABC affiliate KTRK that the bullet entered the back of her head and exited through her forehead.
Giffords is currently in a medically induced coma that doctors say will help her brain rest. She had surgery to stop the bleeding and help control swelling on the left side of the brain. Doctors also had to decompress her eyes. Eyelids often swell when there is trauma to the brain.
"Brain swelling is the biggest threat at this point," said Dr. Michael Lemole, chief of the the division of neurosurgery at the University of Arizona. To help control swelling, part of Giffords skull was removed and will be reimplanted, possibly in a few months.
Giffords was awakened periodically and she has made nonverbal responses to simple commands, but Rhee said she has not spoken because she is on a ventilator.
Lemole said Giffords was able to squeeze a doctor's hand and hold up fingers when asked, and these responses are good signs.
"We take them for granted, but they [represent] a high level of function," he said.
Next Days Crucial to Recovery
The next few days and weeks will be critical to determine how much brain function Giffords has lost, if any. Doctors will keep an especially close eye on the level of brain swelling and also on her ability to recover speech and movement on the right side of her body, which are controlled by the left side of the brain.
"If we do run into trouble in the next few days, we can use medical agents to dry out the brain," said Rhee. "We can put a tube into the center of the brain to relieve the pressure, but she looks good now."
In addition to swelling, the risks to Giffords include "CSF [cerebrospinal fluid] leakage through the entry and exit wounds, cerebral artery spasm [vasospasm], seizures, and meningitis or brain abscess," said Dr. Neil Martin, professor and chair of the Department of Neurology at the Geffen School of Medicine at UCLA in Los Angeles.
Quick Response Helped Gabrielle Giffords After Gunshot
While the worst isn't yet over for Giffords, a number of factors played a role in her survival thus far. Rhee and Lemole said she benefited from an outstanding response from the medical team who treated her and the fact that they were able to preserve a lot of her brain tissue before the surgery.
Her chances of survival and recovery would be much worse if the bullet had passed through both parts of the brain or hit vital parts.
"Crossing both halves of the brain, involving more lobes of the brain, traversing the fluid-filled spaces of the brain, or injuring the major blood vessels at the base of the brain are all indicators of a worse prognosis," said Dr. Atif Haque, a neurosurgeon at the Fort Worth Brain & Spine Institute. "Crossing the 'geographic' center of the brain is essentially non-survivable."
Giffords is also extremely fortunate for defying the odds of surviving such in injury.
"About 90 percent of people with penetrating craniocerebral injuries end up dying," said Dr. Anand Germanwala, chief of the Division of Cerebrovascular and Skull Base Neurosurgery at the University of North Carolina School of Medicine in Chapel Hill. "She remains in a very special class of patients with these types of injuries."
Over the next few months, doctors hope to get a better idea of her cognitive deficits. Her recovery could take years, and she could be in the hospital for several months.
"The faster your recovery, the better your recovery will be. That's a general rule of thumb," said Lemole.
Rhee said he is optimistic based on what he has seen so far, but others say it may be too early to tell.
"This is very early to give survival prognosis. If she continues to follow commands [when she is not in a coma] over the two days then prognosis is good for survival," said Dr. Jamshid Ghajar, attending neurosurgeon at New York-Presbyterian Hospital.
Despite the unknowns, she may be in medically better shape than if she were involved in a car crash or suffered combat-related brain trauma.
"If you survive the gunshot wound to the brain itself and get through all of the potential neurologic an medical complications which can arise, those types of injuries can fare better than brain trauma that might occur from motor vehicle accidents or even military type blast injuries," said Dr. Alan H. Weintraub, medical director of the Brain Injury Program at Craig Hospital in Denver, Colo.
ABC News' Dr. Richard Besser, Courtney Hutchison, Dana Malajian, Bojana Zupan, Susan Schwartz, Emily Friedman and Dr. Mark Abdelmalek contributed to this report.