Former Formula 1 chief doctor Dr Gary Hartstein has warned that fans must prepare themselves “for the worst” news as attempts to awaken Michael Schumacher continue following his horrific ski accident.

Schumacher suffered serious brain injuries after hitting his head on a rock when skiing off-piste in the exclusive French resort of Meribel at the end of December, with attempts to bring him out of his medically-induced coma now in an eighth week.

The 45-year-old’s manager Sabine Kehm confirmed 13 days ago that attempts to awaken the seven-time world champion are continuing and will do for “as long as it takes”, warning that it can be a very long and drawn out process.

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However, Dr Hartstein – who was a key figure in the paddock until the end of the 2012 season – has admitted that the longer it takes, the less likely it is that Schumacher will ever recover.

Writing on his personal blog, the American said: “As time goes on it becomes less and less likely that Michael will emerge to any significant extent.”

Shape Created with Sketch. Moving scenes as fans hold silent vigil for Michael Schumacher's 45th birthday Show all 10 left Created with Sketch. right Created with Sketch. Shape Created with Sketch. Moving scenes as fans hold silent vigil for Michael Schumacher's 45th birthday 1/10 January 3 2014 As the former Formula 1 champion fights for his life, well-wishers and supporters have staged a vigil outside the Grenoble hospital where he is currently being treated. AFP/Getty 2/10 January 3 2014 Candles were lit in support of the F1 champion, who is currently in a medically induced coma AFP/Getty 3/10 January 3 2014 T-shirts with the slogan ‘I Heart Schumi’, Ferrari caps, banners and flowers festooned the fences surrounding the medical facility. AFP/Getty 4/10 January 3 2014 Meanwhile, Ferrari clubs from France and Italy gathered outside the hospital. AFP/Getty 5/10 January 3 2014 Michael Schumacher’s fans were determined not to let his 45th birthday pass by unmarked, and left scores of tributes. AFP/Getty 6/10 January 3 2014 In the deepest respects for his recovery, the vigil was entirely silent. AFP/Getty 7/10 January 3 2014 While a sombre and poignant occasion, those present are optimistic to hear more encouraging news on the medical condition of the German sportsman. AFP/Getty 8/10 January 3 2014 The team with whom Schumacher won a record five titles in a row with between 2000 and 2004 organised a silent march in support. AFP/Getty 9/10 January 3 2014 Schumacher is currently believed to be in a critical but stable state. AFP/Getty 10/10 January 3 2014 On his fans, the Schumacher family said in a statement: "They are giving us great support. We all know he is a fighter and will not give up. Thank you." AFP/Getty 1/10 January 3 2014 As the former Formula 1 champion fights for his life, well-wishers and supporters have staged a vigil outside the Grenoble hospital where he is currently being treated. AFP/Getty 2/10 January 3 2014 Candles were lit in support of the F1 champion, who is currently in a medically induced coma AFP/Getty 3/10 January 3 2014 T-shirts with the slogan ‘I Heart Schumi’, Ferrari caps, banners and flowers festooned the fences surrounding the medical facility. AFP/Getty 4/10 January 3 2014 Meanwhile, Ferrari clubs from France and Italy gathered outside the hospital. AFP/Getty 5/10 January 3 2014 Michael Schumacher’s fans were determined not to let his 45th birthday pass by unmarked, and left scores of tributes. AFP/Getty 6/10 January 3 2014 In the deepest respects for his recovery, the vigil was entirely silent. AFP/Getty 7/10 January 3 2014 While a sombre and poignant occasion, those present are optimistic to hear more encouraging news on the medical condition of the German sportsman. AFP/Getty 8/10 January 3 2014 The team with whom Schumacher won a record five titles in a row with between 2000 and 2004 organised a silent march in support. AFP/Getty 9/10 January 3 2014 Schumacher is currently believed to be in a critical but stable state. AFP/Getty 10/10 January 3 2014 On his fans, the Schumacher family said in a statement: "They are giving us great support. We all know he is a fighter and will not give up. Thank you." AFP/Getty

A number of tributes and messages of support have been passed on in the hope that Schumacher will recover, and the support across the globe is not a surprise for the 58-year-old Professor.

“I always knew Michael was adored,” Dr Hartstein continued.

“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.”

Reports earlier today claimed that Schumacher had lost up to 25 per cent of his body weight due to muscle atrophy, a condition common when patients are in a deep coma as their muscles are not being used.

Dr Hartstein said that it is "entirely possible and, in fact, probable” that Schumacher had lost a significant amount of weight.

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“Happily, the consequences are not particularly dramatic, at least immediately,” he added.

“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.

“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."

Dr Hartstein went on to describe Schumacher’s current state as a “persistent coma”, and mentioned the “severe ramifications” that a lengthy period of time in a coma can cause.

“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,” Dr Hartstein 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.”