The medical examiner ruled her death an accident, and doctors said she might have survived had she received immediate treatment. However, nearly four hours elapsed between her lethal fall at her admission to a hospital

The Tony-winning actress suffered from an epidural hematoma, which causes bleeding between the skull and the brain's covering, said Ellen Borakove, a spokeswoman for the New York City medical examiner's office.

Such bleeding is often caused by a skull fracture, and it can quickly produce a blood clot that puts pressure on the brain. That pressure can force the brain downward, pressing on the brain stem that controls breathing and other vital functions.

Patients with such an injury often feel fine immediately after being hurt because symptoms from the bleeding may take time to emerge.

"This is a very treatable condition if you're aware of what the problem is and the patient is quickly transferred to a hospital," said Dr. Keith Siller of New York University Langone Medical Center. "But there is very little time to correct this."

To prevent coma or death, surgeons frequently cut off part of the skull to give the brain room to swell.

"Once you have more swelling, it causes more trauma which causes more swelling," said Dr. Edward Aulisi, neurosurgery chief at Washington Hospital Center in the nation's capital. "It's a vicious cycle because everything's inside a closed space."

Richardson, 45, died Wednesday at Lenox Hill Hospital in Manhattan after falling at the Mont Tremblant resort in Quebec on Monday. Details of her treatment have not been disclosed.

It remained unclear Thursday exactly how she was injured. Resort officials have said only that she fell on a beginner's trail and later reported not feeling well.

A CT scan can detect bleeding, bruising or the beginning of swelling in the brain. The challenge is for patients to know whether to seek one.

"If there's any question in your mind whatsoever, you get a head CT," Aulisi advised. "It's the best 20 seconds you ever spent in your life."

Descended from one of Britain's greatest acting dynasties, including her mother, Vanessa Redgrave, Richardson was known for her work in such plays as "Cabaret" (for which she won a Tony) and "Anna Christie" and in the films "Patty Hearst" and "The Handmaid's Tale."

Mourning continued Thursday with Broadway theaters dimming their lights in Richardson's honor at 8 p.m., the traditional starting time for evening performances.

Praise also came from both tabloid celebrities such as "The Parent Trap" co-star Lindsay Lohan and theater artists like Sam Mendes, who directed the 1998 revival of "Cabaret."

"It defies belief that this gifted, brave, tenacious, wonderful woman is gone," said Mendes, who also directed the Academy Award-winning "American Beauty."

Charlotte St. Martin, executive director of The Broadway League, the trade organization for Broadway theaters and producers, called Richardson "one of our finest young actresses."

"Her theatrical lineage is legendary, but her own singular talent shined memorably on any stage she appeared," she said.

A spokesman for the family, Los Angeles-based Alan Nierob, said he had no information about funeral arrangements. Instead of flowers, the family asked that donations be made to the amfAR foundation for AIDS research, Nierob said. Richardson, whose father died of complications from the disease in 1991, was a longtime supporter of the charity and served on its board of trustees since 2006.

Richardson gave several memorable stage performances, more than living up to some of the theater's most famous roles: Sally Bowles of "Cabaret," Blanche DuBois of "A Streetcar Named Desire" and the title character of Eugene O'Neill's "Anna Christie," a 1993 revival in which she co-starred with future husband Liam Neeson . (They have two sons: Micheal, 13, and Daniel, 12.)

Her final feature film, "Wild Child," has been released internationally but has not been released in the U.S., and Universal Pictures said one had not been scheduled.

The death of Richardson, who was not wearing a helmet, greatly heightened the debate over skiing safety. In Quebec, officials are considering making helmets mandatory on ski hills.

Jean-Pascal Bernier, a spokesman for Quebec Sport and Leisure Minister Michelle Courchesne, said Thursday that the minister met with emergency room doctors this week and will meet with ski hill operators soon.

Emergency room doctors in the province first called for mandatory use of helmets three weeks ago.

Questions also arose about why the first ambulance called to the ski resort was turned away.

Yves Coderre, director of operations at the emergency services company that sent paramedics to the Mont Tremblant resort, told The Globe and Mail newspaper that he reviewed the dispatch records and the first 911 call came at 12:43 p.m. Monday.

Coderre said medics arrived at the hill 17 minutes later. But the actress refused medical attention, he said, so ambulance staffers turned and left after spotting a sled taking the still-conscious actress away to the resort's on-site clinic.

At 3 p.m., a second 911 call was made — this time from Richardson's luxury hotel room — as her condition deteriorated. An ambulance arrived nine minutes later.

"She was conscious and they could talk to her," Coderre said. "But she showed instability."

The medics tended to her for a half-hour before transporting her to a hospital a 40-minute drive away.

On Thursday, the ski resort where Richardson had her fatal fall was subdued, as employees refused to speak about the accident.

Still, the sunny slopes were crowded — and the gentle hill Richardson fell on was teeming with beginners, many of them children.

Dozens of skiers and snowboarders took breaks from the runs to discuss Richardson's death — and many said they bought a helmet because of Richardson.

"I bought a helmet yesterday after I heard," said Nathalie Beaulieu, 41. "My daughters always wear them, but now my husband and I will, too."

Not everyone said they would change their ways.

"I haven't worn one up to now and I'm OK," said Jacques Garnier, 45. "My kids wear them, for sure, though."