Alex Zanardi's brain injuries do not compare with those suffered by fellow former F1 driver Michael Schumacher.

That is the claim of Giuseppe Olivieri, the doctor who operated on Zanardi following his head-on crash with a truck during a hand cycling race in Italy last weekend.

Former Williams driver Zanardi, who lost his legs in a 2001 Champ Car crash, has spent a third night in a coma and on a ventilator, with Olivieri saying his condition is "serious".

"We won't see what his neurological state is until he wakes up - if he wakes up," he said. "Serious condition means it's a situation when someone could die."

However, Olivieri has told La Gazzetta dello Sport that Zanardi's injuries - to his forehead, face and eyes - "are less than those of Michael Schumacher".

Almost nothing is known to the outside world about Schumacher's condition, but the seven time world champion has not been publicly seen or heard from since striking his head on a rock while skiing in 2013.

"Diffuse axonal damage is a very extensive injury throughout the cerebral cortex, from which there is little chance of returning to normal cognitive and motor functions," Olivieri said.

"At least from what we as doctors can understand for now, Zanardi does not suffer from this injury," he added.

"The point of impact is not what counts. What matters is the internal injuries and how much damage has spread within the skull."

Olivieri said it is still too early to attempt to wake the 53-year-old Italian.

"In injuries of this type, it takes a few days to start waking up the patient and assessing the damage. A week or maybe two," he said.

"We are talking about a fragile patient who has suffered a major head injury, so you have to be very careful. The fact that he is an athlete and in such good physical condition is a cause for optimism."