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.
"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."