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The former chief doctor of Formula One believes fans of Michael Schumacher must prepare for "really bad news" following his horrific ski accident.

The racing driver has been in an artificial coma since December 29 after suffering terrible brain injuries in a crash at a French ski resort.

Writing on his blog, Gary Harstein said: "As time goes on it becomes less and less likely that Michael will emerge to any significant extent."

He believes the world must brace itself for the worst possible outcome for the Ferrari legend.

"I always knew Michael was adored," he wrote.

"I spent years at circuits drenched in red by the Ferrari caps, flags, and shirts, and all of that for Michael. I'm still staggered by the depth and persistence of his fans' love for him.

"And whereas I worried more than a bit about what was going to happen when and if really bad news got announced, I've realised that perhaps the lack of status updates has given us all a chance to move on a bit, to process what's happening, and to start to... detach."

Commenting on recent reports that Schumacher has lost 25 per cent of his body weight, Dr Hartstein said it is "entirely possible and, in fact, probable."

"Happily, the consequences are not particularly dramatic, at least immediately," he wrote.

"To be blunt, a patient in coma doesn’t really NEED his or her muscles . . . with the exception of the diaphragm. The diaphragm, which like the heart is pretty much always active, resists atrophy rather better than other muscles, but it does atrophy.

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"And having a machine doing the breathing for you is one of the best ways to see how disuse atrophy affects the diaphragm too. Unfortunately, and assuming (as I have until now) that Michael is being ventilated by a respirator, there is probably some degree of diaphragmatic atrophy at this point."

Speculating on Schumacher's current condition, Dr Hartstein said that he is in a "persistent coma".

"As mentioned previously, the longer one remains in a vegetative state, the less the likelihood of emerging, and the higher the chances of severe ramifications if the patient does in fact emerge," he wrote.

"Most definitions consider the vegetative state to be permanent one year after the injury.

"Patients who are in a persistent/permanent vegetative state have lifespans that are measured in months to a few years. This depends on baseline function (extraordinary in the case of Michael, of course), the quality of nursing care, and other imponderables. They usually die of respiratory or urinary infections. Longer survivals have been described, but are exceptional."

Schumacher's spokeswoman said only last week that the waking-up phase of Michael is continuing and said that it will "take as long as it takes."