What is unresponsive wakefulness?

"They just have no recognition," he said. "They don't speak. There's nothing that is, sort of, voluntary in terms of movements, even though their eyes are open."

What's the cause?

A magnetic resonance imaging, or MRI, scan of Warmbier showed "extensive loss of brain tissue in all regions of the brain," said Dr. Daniel Kanter, professor of neurology and director of the Neurocritical Care Program at the University of Cincinnati Medical Center.

This type of brain injury is usually seen as a result of cardiopulmonary arrest, meaning heart or lung malfunctions may have prevented oxygen from reaching the brain through a person's blood supply, causing brain tissue to die, he explained.

When the brain is deprived of oxygen for longer than four minutes during a cardiopulmonary arrest, the risk of brain injury increases significantly, said Dr. Robert Glatter, an emergency physician at Lenox Hill Hospital in New York.

"Brain cells require oxygen, carried by red blood cells, to survive, so when the heart or lungs are compromised, the cells may not survive," he said.

"The result of such an insult to the brain is a variable loss of brain function, including the capacity for (movement), language, vision, memory, comprehension, as well as the ability to maintain a general state of awareness," Glatter said.

Does the brain function at all?

During unresponsive wakefulness, the brain functions that control spontaneous breathing, heartbeat and blood pressure, urine production, metabolism and digestion continue, Glatter explained.

These routine functions of the body sometimes continue, as they are coordinated by different areas of the brain, he said.

However, while a person may sleep and wake as usual and may open their eyes spontaneously, that person is not truly conscious or aware of the environment as a neurologically normal individual would be, Glatter said.

How are patients treated?

Some patients who suffer an anoxic brain injury -- in which the brain is deprived of oxygen -- require care to restore normal blood pressure and heartbeat, and to ensure a continuous supply of oxygen to the brain, Glatter said.

"Patients are placed on a ventilator and require medicine to reduce the risk of seizures, which are quite common after such an injury," he said. This was not the case for Warmbier, who could breathe on his own.

The majority of patients require chronic long-term care to address nutritional needs; prevent wound infections, urinary tract infections and pressure sores that can lead to sepsis; and avoid aspiration pneumonia, in which food, saliva or liquids are breathed into the lungs or airways, Glatter said.

What are the chances of recovery?

The chances of full neurological recovery after the brain has been deprived of oxygen for prolonged periods of time -- greater than four minutes -- are poor, Glatter said.

The long-term effects depend on how long the brain lacked an adequate oxygen supply and the amount of permanent brain damage that resulted from it.

"Otto Warmbier, has completed his journey home," his parents said Monday. "We thank everyone around the world who has kept him and our family in their thoughts and prayers. We are at peace and at home too."