The smashed window of a vehicle that hit the bicycle of American motorcycle racer Nicky Hayden while he was training in Rimini, Italy, Wednesday, May 17, 2017.
Hayden was joined at his bedside on Thursday by his mother Rose and younger brother Tommy.
The Bufalini Hospital in Cesena said in a statement on Thursday: "Nicky Hayden's clinical condition remains extremely critical".
Casey Stoner has sent his best wishes to MotoGP racer Nicky Hayden who is in a critical condition with severe brain damage.
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Polytrauma is a medical term which means that a person has suffered multiple traumas to the body.
Honda said he was taken initially by ambulance to a hospital near the coastal resort of Rimini before being transferred to Cesena.
Italy's Valentino Rossi, the man Hayden edged to win his world title, led the tributes to him on social media.
The smashed window of the auto that hit Nicky Hayden.
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He won three races in a career from 2003-2015, before switching to Superbike full-time, although he made two starts in MotoGP previous year to fill in for injured Honda rider Jack Miller.
"The most lovely thing I have about him is when he gave his hand to me after the unlucky race of Valencia 2015, on the lap of honour". This incident took place in Rimini in Italy, this Wednesday afternoon, and the site of a auto after that collision, shows the horrific intensity of this crash. He competed in Moto GP from 2003-2015.
"After a few years we fought for the title down to the last race in Valencia and, unfortunately for me, he won and became MotoGP world champion". He further said "Come on Nicky, we're all with you".
The dangers of training on public roads have been highlighted in recent weeks by other accidents affecting professional athletes, according to the Reuters report.
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Michele Scarponi, the 2011 Giro d'Italia cycling champion, was killed last month after being hit by a van while training close to his home in Filottrano in eastern Italy.