Michael Schumacher underwent a second surgery after a brain scan showed small, "surprising" signs of improvement, but doctors said on Tuesday they could offer no insight into the prognosis for the seven-time Formula One world champion.

Schumacher, who turns 45 on Friday, suffered critical head injuries when he fell and struck a rock on Sunday while skiing on a family vacation in the French Alps.

His manager confirmed the accident cracked his helmet, which doctors credited for giving him a chance at survival.