Wearing a helmet may have “minimised the severity” of injury suffered by racing driver Michael Schumacher following his skiing accident in France, a leading surgeon has said.
But the immediate neurosurgical treatment given to the seven-time F1 champion "suggests that something very serious has happened", he added.
Chris Chandler, consultant neurosurgeon at King's College Hospital in London, told Sky News: "Certainly after blunt trauma, which is what you would term his injury, the brain does swell and that swelling contained within the rigid box of the skull can cause dangerous pressure on the vital structure to the brain.
"That brain swelling needs to be controlled.
"The fact that he was wearing a helmet simply means that it has minimised the severity of the injury but still it is possible to sustain a serious injury even with the helmet."
Mr Chandler said the fact that Schumacher, 44, was in a coma meant he could have suffered a number of different injuries.
Career in Pictures: Michael Schumacher
Career in Pictures: Michael Schumacher
1/17 February 2006
Michael Schumacher is seen in the pits during a training session at the Ricardo Tormo racetrack in Cheste near Valencia in 2006
2/17 November 1994
Hugging two team mechanics in delight for becoming World Champion Driver after the Australian Grand Prix at the Adelaide circuit in November 1994
3/17 October 1995
Flavio Briatore carries a jubilant Michael after he wins the Pacific F1 Grand Prix at Aida in Japan, and wins his second World Drivers Championshp
4/17 February 1996
Schmacher pushes his Ferrari back to the pits after engine trouble left him stranded at the entrance of the pitlane during pre-season testing at Estoril, Portugal
5/17 March 1996
During the Australian Grand Prix in Melbourne
6/17 March 1997
Michael Schumacher in the pit lane during the Australian Grand Prix at Albert Park in Melbourne, Australia
7/17 October 2000
Celebrating on the podium after winning the formula one world championship at the Japanese Grand Prix at Suzuka, Japan
8/17 July 2006
Michael Schumacher of Germany and Ferrari leads the pack into the first corner at the start of the French Formula One Grand Prix at the Nevers Magny-Cours Circuit
Lars Baron/Bongarts/Getty Images
9/17 October 2010
Michael Schumacher drives during the Korean Formula One Grand Prix at the Korea International Circuit
Clive Mason/Getty Images
10/17 October 2011
Michael Schumacher and Mercedes GP waves to the crowd as he attends the drivers parade before the Indian Formula One Grand Prix at the Buddh International Circuit on October 30, 2011 in Noida, India
11/17 December 2011
Sebastian Vettel and Michael Schumacher celebrate after winning the Nations Cup at day one of the race of champions event at the Esprit Arena
12/17 June 2012
Michael Schumacher drives at the Valencia Street Circuit on June 22, 2012 in Valencia during the first practice race of the European Formula One Grand Prix
13/17 September 2012
Michael Schumacher crashes into the back of Jean-Eric Vergne of France and Scuderia Toro Rosso during the Singapore Formula One Grand Prix in September 2012
14/17 November 2012
Schumacher drives in his last race during the Brazilian Formula One Grand Prix at the Autodromo Jose Carlos Pace
Mark Thompson/Getty Images
15/17 December 2013
Deputy director Jean Marc Grenier talks to media outside the CHU Nord hospital in Grenoble, French Alps
16/17 December 2013
Members of the staff are seen outside the emergency services at the CHU Nord hospital in Grenoble, French Alps; this is the hospital where retired seven-times Formula One world champion Michael Schumacher is reported to be hospitalized after a ski accident
17/17 December 2013
A Scuderia Ferrari and Michael Schumacher fan waits, on December 29, in Grenoble, in front of the emergency department of the Centre Hospitalier Universitaire hospital
"He could have suffered a diffuse injury to his brain which can then result in brain swelling," he said.
"He could have sustained some sort of brain haemorrhage and if there was a blood clot within his brain or on the surface of his brain underneath his skull, that might need to be removed.
"Sometimes there is nothing actually to remove but you put in an intracranial pressure monitor, which basically is an operation that requires drilling a hole in the skull and putting a fine probe inside.
"Or if there is diffuse swelling of the brain, sometimes surgeons remove a large piece of the skull...so there is space for the brain to swell to minimise the pressure on vital structure.
"It's not clear from the reports that I've heard exactly what the nature of this emergency operation was. But the fact that they undertook something almost immediately, that he was admitted to the surgical unit, suggests that something very serious has happened."