That condition differs from the damage that occurs during a stroke, when a single blood vessel is blocked, said Dr. Lee H. Schwamm, executive vice chairman of neurology at Massachusetts General Hospital.

With an injury like Mr. Warmbier’s, “blood flow is reduced everywhere — the pump is turned off,” said Dr. Schwamm, who reviewed the coroner’s report at The Times’s request.

In the United States, the most common cause is a cardiac arrest, often precipitated by a heart attack. While cardiac arrests are unlikely in healthy young men, they can happen if the victim is malnourished and suffers an imbalance of blood electrolytes, such as potassium, calcium, or magnesium, Dr. Schwamm said.

The damage is quick: It takes just four minutes without blood for brain cells to start to die.

Blood flow to the brain can be interrupted for any number of reasons. The whole-body CT scan did not find injuries associated with hanging, for example, but Dr. Schwamm said the evidence might not be visible if a bedsheet were used and the spine were not dislocated.

Mr. Warmbier had a scar at the base of his neck that was probably caused by the insertion of a tube into his trachea as he was hooked up to a ventilator. This may indicate that he was not breathing on his own for a long period of time, Dr. Sammarco said.

He was weaned from the ventilator in North Korea, though; he was not using it when he was returned.

There were few other signs of injury on Mr. Warmbier’s body. There were no bedsores, and his skin condition was excellent, Dr. Sammarco said. “His muscle volume was pretty good for someone who was bedridden for over a year,” she said.

In the United States, most families would opt for palliative care for a relative in a vegetative state resembling Mr. Warmbier’s, she added. That would consist of providing pain medications and sedatives to soothe them.