Michael Schumacher’s agent said doctors treating the seven-time Formula One champion have started to lower his sedation level to wake him up from an artificial coma.

Schumacher has been in an induced coma since December 29, 2013, following a head injury in a skiing accident in the French Alps. The former F1 driver slammed his head on a rock while skiing off-piste in the Méribel resort. French police said the slope was properly marked, with investigators still trying to determine how fast Schumacher was skiing when he hit the rock.

Michael Schumacher has been in a stable but critical condition for more than four weeks in a hospital in the French city of Grenoble. “Michael’s sedation is being reduced in order to allow the start of the waking-up process, which may take long time,” his agent, Sabine Kehm, said in a statement cited by Reuters.

The hospital declined to make any further comments on the driver’s condition. Schumacher has undergone two operations since the accident. The process of reducing sedation may take days or even weeks, depending on the strength of medication used.

It remains unclear, however, to what extent Schumacher can fully recover, as he has stayed in a coma for a relatively long period of time. Peter Andrews, professor of intensive care at Edinburgh University, is quoted as saying by The Guardian that Schumacher may experience severe swelling, probably relating to severe damage, since he has been kept in a coma for so long. Most patients with head injuries are kept in a coma for a week to 10 days.

Schumacher, who turned 45 earlier this month, won a record 91 Grand Prix victories. He quit Formula One in 2012 after a disappointing three-year comeback with Mercedes. The German’s first retirement from the sport took place at the end of the 2006 season while he was driving for Ferrari.

By Dan Mihalascu

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