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Justin Bieber appears to have suffered "quite severe loss" of movement in his face and could be left with paralysis "long term", a neurological expert has told Sky News.
Late on Friday, the 28-year-old singer posted a video on Instagram revealing he has been diagnosed with a rare condition called Ramsay Hunt syndrome.
It has resulted in complete paralysis of the right side of his face, he said, and will mean his upcoming tour dates will have to be postponed.
Professor Derick Wade, an expert in neurological rehabilitation at Oxford Brookes University, told Sky News that although most people recover fully, Bieber appears to have a serious case of the virus.
"I noticed that there wasn't any moment, so that is quite severe loss," he said.
"How well will it recover? He's already told you the answer to that."
In the three-minute clip, the Canadian star said: "It's pretty serious, I wish this wasn't the case but obviously, my body's telling me I gotta slow down.
"We don't know how much time it is going to be but it's going to be okay."
Ramsay Hunt syndrome is a complication of the shingles virus - which can emerge in people who have suffered chicken pox as a child.
Chances of recovery improve if antiviral treatment is given within the first 72 hours of symptoms - with 70% of that group recovering fully.
But this drops to just 50% if the patient does not receive rapid treatment.
Symptoms include a painful rash or blisters inside the mouth, on the ear, scalp and hairline.
Patients will also experience a loss of taste on the affected side of the tongue and deafness in the affected ear.
Bell's palsy 'quite common' - but Ramsey Hunt is rarer
Professor Wade explained: "Paralysis of the face like this is quite common.
"It's normally termed Bell's Palsy and that is presumed to be due to a virus infection affecting a facial nerve as it travels through a very narrow canal in the skull.
"What he has had is shingles, the chickenpox virus affecting the same nerve in the same position.
"So it arises because of the information in the nerve causes a swell and there's no space to swell, so it stops functioning while swollen and inflamed."
Asked about recovery times, he added: "Some people [recover] very quickly - in three weeks - and then other people can take several months. So it's a very unpredictable exercise.
"Some people are left with it long term and there are some residual effects."
Bieber said he has been doing facial exercises to try to regain movement.
Fans have flooded social media with well wishes, alongside his wife Hayley who wrote: "I love you baby."