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FORMULA 1 legend Michael Schumacher remains in a critical condition as French doctors work '"hour-by-hour" to save him.
The 44-year-old has been placed in an artificial coma while neurosurgeons try to reduce the pressure on his brain.
He is in an induced coma being treated for an intracranial haematoma after a "high speed" impact, say his surgeons.
Without a helmet, the German racing star would have died, doctors say.
Contrary to earlier reports, Schumacher has been operated on only once, reporters were told at a press conference in Grenoble.
A spokesman for the hospital said: "Michael Schumacher was the victim of very serious trauma. He was very agitated when he arrived and we decided he was in a critical situation and he quickly went into a coma.
Michael Schumacher pictures while announcing his retirement from Formula 1 at the end of the 2012 season.
"The neurosurgical treatment he received brought us quite a lot of information. We had to operate urgently to release some pressure in his head. Unfortunately, he has some lesions within his brain."
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Professor Jean-Francois Payen told reporters: "I'd say this accident happened in the right place because he was taken into hospital immediately and operated on as soon as he arrived, this meant his state is critical and he is still in a coma and he will be kept in a coma.
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"Everything that needed to be done has been done at the moment we can't really say when he will recover, we cannot answer this yet."
Professor Payen said Schumacher was "fighting for his life".
"We judge him to be in a very serious situation. We cannot tell what the outcome will be yet. We are working hour-by-hour but it's too early to say what is going to happen and to have a prognosis.
"We think his helmet did help, without a helmet he wouldn't be here now."
Schumacher's wife Corinna and the rest of his family thanked well-wishers and gave a special nod to the doctors treating him.
Jean Marc Grenier, the deputy director of the hospital, speaks to the press.
"We would like to thank the medical team who, we know, do everything possible to help Michael," Corinna said in a statement.
Schumacher had been skiing off-piste in the upmarket Meribel resort when he fell and hit his head on a rock, mountain police who gave him first aid said.
Schumacher, who lives with his family in Switzerland, was on a private stay in Meribel, according to his spokeswoman.
Meribel is part of an extensive ski region with about 180 lifts connecting three alpine valleys.
Schumacher was reportedly skiing with his 14-year-old son at the time of the accident.
A helicopter in front of the Grenoble hospital, French Alps, where Michael Schumacher is being treated after he sustained a head injury during a skiing accident in Meribel, France.
He was airlifted to a local hospital, then to the Grenoble facility. A specialist neurosurgeon from Paris was rushed in to oversee his treatment.
The director of the Meribel resort, Christophe Gernigon-Lecomte, had said just after the accident that Schumacher had been wearing a helmet and was "conscious but a little agitated'' just after the accident, with early reports suggesting there was no cause for alarm.
But when Schumacher then fell into coma, doctors realised the damage was worse than initially feared.
The two mountain police officers who gave first aid said Schumacher was suffering "severe cranial trauma'' when they got to him and a helicopter was brought in to evacuate him within 10 minutes.
A renowned Paris surgeon, Dr Gerard Saillant - who also operated on Schumacher when he broke his calf and shinbone during his most serious racing crash at the 1999 British Grand Prix - was brought to the Grenoble hospital in a police car to take charge of the famous patient.
A file photo shows Michael Schumacher skiing in the Italian Alps in 2006. Schumacher is in a critical condition after hitting his head in a fall while skiing.
Police officers have been stationed to guard the hospital's entrances.
Dr Gary Hartstein, former Medical Delegate for the Formula 1 World Championship, took to Twitter to attempt to explain the medical situation Schumacher finds himself in.
"It's quite well known that extradural hematomas, a kind of cerebral hemorrhage, can leave a lucid interval after injury," he wrote, referring to reports that Schumacher was conscious when he was attended to after the accident.
"Then as the hematoma forms, the sudden increase in pressure causes sudden and dramatic symptoms. Pressure must be relieved rapidly.
"This is done with a neurosurgical intervention. Then the victim is observed in an ICU environment.
A file pitcure of Michael Schumacher from his Ferrari days, Schumacher is in a critical condition after hitting his head in a fall while skiing.
"Quality of recovery depends on: 1) severity of initial injury, 2) acuteness and amplitude of pressure rise when hematoma forms, 3) rapidity with which it is drained, 4) quality of neuro intensive care and rehab."
Schumacher retired from Formula 1 for a second time at the end of the 2012 season. He won two world titles with Benetton and five in a row with Ferrari.
Schumacher has had accidents before, including a motorcycle crash in February 2009 in Cartagena, Spain, where he damaged a vertebra, a rib and the bottom of his skull.
At the time, his doctor, Johannes Peil, said it had caused the racer the most serious long-term harm of his career.
That accident prevented him taking the place of Felipe Massa at Ferrari after the Brazilian suffered severe head injuries in a crash at the Hungarian Grand Prix in 2009.
He recovered from the injuries to make a shock return in 2010 at Mercedes alongside Nico Rosberg.
Though his countryman largely outshone the former champion in their three seasons together, we did see flashes of the old Schumacher.
At Monaco in 2012, the then 43-year-old five-time winner at the principality scored a popular pole position, although he would not start there on race day thanks to a 10-spot grid penalty earnt at the previous race.
That lap, along with a podium finish at Valencia the same year, were the high watermarks of his otherwise ill-fated comeback.
Formula One drivers and fans rushed to wish Schumacher a quick recovery.
"Like millions of Germans, the chancellor and members of the government were extremely dismayed when they heard about Michael Schumacher's serious skiing accident," German Chancellor Angela Merkel's spokesman, Steffen Seibert, said in Berlin.
Sebastian Vettel, for whom Schumacher was a boyhood idol, told German news agency dpa: "I am shocked and hope that he will get better as soon as possible."
Ferrari, which Schumacher raced for, expressed its concern in a statement.
"Everyone at Ferrari has been in a state of anxiety since hearing about Michael Schumacher's accident," it said, adding that company president, Luca di Montezemolo, and race team leader, Stefano Domenicali, were in contact with the family.
British former world racing champion Jenson Button said his "thoughts are with Michael Schumacher at this tough time. ... Michael more than anyone has the strength to pull through this."
