How can a person be fine a few minutes after having a fall and in critical condition an hour later?

That's one of the questions being asked after actress Natasha Richardson died following a fall at the Mont Tremblant ski resort in Quebec.

Richardson, 45, was removed from life support Wednesday, three days after her seemingly harmless tumble. The New York City medical examiner's office says the actress died of an epidural hematoma "due to blunt impact to the head."

Her death was a shock -- especially since her fall was described by witnesses as "unspectacular." Richardson reportedly got right up after the fall, joked about her clumsiness, and said she was fine.

She asked to be taken back to her room but about an hour later, she complained of a severe headache and was whisked to hospital, where she died.

Experts say a blow to the head can cause one of several brain injuries: brain swelling, a tear to a blood vessel to the brain, called an arterial dissection, or a blood clot that leads to a pooling of blood, called a hematoma.

Dr. Cameron Guest, an intensive care physician at Sunnybrook Health Sciences Centre in Toronto told CTV.ca shortly after initial reports about the accident trickled out that that a blood clot sounded the most likely, based on reports about Richardson's symptoms.

Guest said a tear to the main artery leading to the brain -- the carotid artery for example -- is rare. Such an injury would normally cause symptoms such as weakness on one side of the body, not a severe headache.

Brain swelling also sounded unlikely to Guest, since such an injury doesn't usually allow someone to appear normal and wide awake right after the trauma - a period called the "lucid phase".

"So the blood clot possibility sounds like the most likely one, based on what has been described," he said, stressing that his observations are merely speculation.

While the brain is covered by a leathery skin called the dura and then a thick skull, it can be extremely fragile, Toronto neurosurgeon Dr. Charles Tator told Canada AM Wednesday.

"So this is an example that even so-called minor falls can result in serious damage - either from bleeding, I suspect, or from swelling," Tator said.

Doctors can effectively treat many brain injuries, Tator says, but not every one.

"If it's a blood clot, we can remove it, and if it's removed quickly, then people can recover very well," he explained.

"If there's brain swelling, it's much more difficult to treat. We do have some drugs that reduce brain swelling. We have ways of putting in extra oxygen for recovery of the brain. But for major brain swelling, sometimes it's fatal. Modern medicine just hasn't gone far enough for a lot of those people," he said.

A blood clot in the skull can cause either an epidural or subdural hematoma if an artery inside the skull tears and begin to bleed.

As blood accumulates between the brain and the skull, the skull can't expand, and the brain gets compressed. The increasing pressure causes the severe headache, and as pressure builds. The brain is pushed downward. If the pressure is not released, the brain stem becomes compressed, leading to an eventual loss of consciousness.

Arteries, which are under high pressure, can release a lot of blood in a short period of time into the skull, and a person's condition can deteriorate rapidly.

"If the pressure in the skull is higher than the pressure in the bloodstream trying to push blood into the brain, blood won't flow. And that's when brain death happens," explained Guest.

Treatment usually involves either a craniotomy -- the surgical opening of the skull - or more often, a burr hole, which is a small hole drilled into the skull to relieve pressure. There would also need to be surgery to stop the bleeding and the removal of any clot.

In some cases, there is simply no time for treatment, said Guest. Once someone starts to show deterioration through loss of consciousness, the time available to deal with it and have a good outcome is "pretty short," he said - on the order of minutes to a few hours -- depending on how fast the hematoma is growing.

Tator says it's important that someone who may have had a brain injury not be left alone.

"In this case, the ski instructor did the right thing: accompanied her to the hotel room and apparently stayed with her," he says.

He says it is possible for someone with a traumatic brain injury to be able to talk afterward but then deteriorate quickly an hour or so later.

"We call it 'talk and die,' and the way to catch it is to monitor a person after a head injury," he said.

Tator, who is the founding president of Think First Canada, a non-profit organization dedicated to the prevention of brain and spinal cord injuries, says the best way to avoid such brain injuries is to wear a helmet when participating in potentially dangerous activities, such as skiing, biking, scootering, skateboarding.

"Helmets are important. A helmet might have saved her life. We need to stress helmets for all skiers -- all ages," he said.

The Canadian Institute for Health Information's reports that 138 people were hospitalized across Canada in 2005-2006 because of a head injury sustained while skiing or snowboarding. The figures do not include those from Quebec.

Although head injuries during skiing are considered rare, an estimated 60 per cent of skiing fatalities involve a head injury. Many others lead to paralysis or other life-changing consequence.

A Norwegian study published in February 2006 in the Journal of the American Medical Association found that six out of 10 head injuries in alpine skiers and snowboarders can be prevented by using helmets.