"Only a very few medical centers have these [MRI machines] and very rarely in a case of traumatic brain injury you can see something that can't be detected on examination," said Bernat.
In one of the most famous cases, the family of Terry Schiavo, a Florida woman who was kept alive for 15 years, insisted she was fully conscious.
Schiavo, who suffered a profound brain injury, was at the center of a seven-year legal tug-of-war that involved Congress, the U.S. Supreme Court and even President George W. Bush before a judge granted her husband the right to allow her to die in 2005. She was 41.
An autopsy later revealed she had been in a vegetative state.
Patients can live in a vegetative state for decades, "with adequate nursing care and feeding and hydrating them and treating complications like infections."
Young patients can be kept alive "for decades," but older patients, such as Sharon, usually don't live as long, even with the best of care, said Bernat.
"There are cases of people coming out of it, but that usually happens a short time after the brain injury," said Bernat. "It's rare to regain awareness after six months. And recovery isn't to normal but a severely disabled state.
"Maybe they gain awareness, but not independence," he said. "One might argue it's worse to be aware."