How can a person be fine one minute and brain dead the next?

Natasha Richardson was reported to be "fine at first" after she hit her head in a skiing accident, but the 45-year-old actress' health began to deteriorate within an hour.

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The Tony award-winning actress died Wednesday at Lenox Hill Hospital in New York City, where she was transported to Tuesday after Canadian officials diagnosed her as brain dead.

“I can only speculate, but it sounds like something we call the ‘talk and die’ syndrome,” said Dr. Steven Flanagan, director of Rusk Institute of Rehabilitation Medicine at New York University’s Langone Medical Center.

“What this implies is that someone hits their head and they are seemingly OK initially," he told FOXNews.com. "But then they get a rapid collection of blood — usually called epidural hemorrhage — and that means bleeding between the skull and the brain.”

Richardson was reportedly suffered a head injury after falling on a beginner's trail Monday during a ski lesson at the Mont Tremblant ski resort in Quebec, Canada.

"She did not show any visible sign of injury, but the ski patrol followed strict procedures and brought her back to the bottom of the slope and insisted she should see a doctor," the resort said in a statement.

Mont Tremblant spokeswoman Catherine Lacasse said Richardson was getting a private lesson and that she said she was "fine at first."

"An hour later she said she didn't feel well. She had a headache, so we sent her to the hospital," Lacasse said. "There were no signs of impact and no blood, nothing."

Flanagan, who has not treated Richardson, said a person doesn't always show outward signs of trauma when suffering a head injury.

“When someone has bleeding between the skull and the brain, it basically presses on the brain, and if it presses enough… it can cause substantial damage and even death,” Flanagan told FOXNews.com.

“And presumably that’s what probably happened to her — but again, we’re speculating.”

Symptoms of an epidural hemorrhage include:

— Headache;

— Loss of consciousness;

— Weakness on one side of the body;

— A change in mental status

If the condition is not treated immediately, the person will fall into a coma and “it’s downhill from there,” Flanagan said.

“So you need to get the injury treated immediately,” he said.

“First we would do an emergency CAT scan to find out exactly where the hemorrhage is, and then the patient would need immediate surgery.”

While Flanagan said he doesn’t know exactly what kind of injury Richardson sustained, he said it would have to be a substantial blow to the head.

“If you bumped your head getting out of the car, this wouldn’t happen to you,” he said. “It would have to be a significant injury.”

The Associated Press contributed to this report.