As the sun sets behind the mountains in an exclusive neighbourhood 30 minutes from the Las Vegas strip, I can hear a recognisable voice singing behind a closed door.

"Is that Celine?" I ask.

Her security man guarding the hotel suite nods.

I am about to interview a musical megastar, and it sounds like she is in a cheerful mood.

But the iconic voice I can hear casually singing away is one that fans feared they might never hear again.

"Ive been dealing with problems with my health for a long time," she shared in an Instagram video in December 2022.

"I have been diagnosed with a very rare neurological disorder."

After that devastating announcement, Celine Dion pulled out of the remainder of her world tour, and has rarely been seen in public since.

Signs of trouble

The medical name for her little-known illness is Stiff Person Syndrome (SPS), a neurological condition that causes muscles to spasm.

As we sit down to talk, Celine says it went undiagnosed for years.

The 56-year-old describes the distress she felt as a performer when she began to notice changes in her voice on tour.

"It was just feeling a little strange, like a little spasm," the Canadian star says.

"My voice was struggling, I was starting to push a little bit."

She demonstrates the subtle difference by singing the first few bars of her 1993 hit The Power of Love, showing how she was having to force her voice to hold the notes that once came more easily.

Occasionally, she would ask the conductor of her backing musicians to bring certain songs down a key for a few performances.

"I needed to find a way to be on stage," she explains.

She hoped singing fractionally lower might give her voice a chance to recover.

Amazon MGM Studios Celine Dion doing exercises with her arms in the airAmazon MGM Studios

Celine Dion shares how she is managing Stiff Person Syndrome in a new documentary

Audiences would never have known the struggle she was facing behind the scenes.

But Celine says at the time it felt impossible to take time off.

"These shows were sold out for a year and a half, going around the globe.

"And Im going to say to people, 'Excuse me about my spasm? Excuse me about my je ne sais quoi?'"

She put huge pressure on herself to keep the show on the road, not understanding what was causing the symptoms she was experiencing.

But it all became too much.

Her eventual diagnosis brought the realisation that this was not tour fatigue. It was permanent.

Celine's children joined her on stage at the final show of her Vegas residency in 2019

SPS is an autoimmune disease that can be debilitating. There is no known cure.

It is caused by the signals from the nerves to muscles not working properly.

During a particularly severe episode, the spasms can be so bad that she can barely move.

But having been properly diagnosed, she now understands much more about the condition, which can be managed.

"My goal is to be part of the funds and raising money for awareness and to find a cure. That would be amazing," she says.

Watch Celine Dion's extended interview with the BBC's Emma Vardy

During her time away from performing, Celine has been learning to adapt with medication, physical therapy and the help of experts like Dr Amanda Piquet.

"Now this disease is gaining public awareness, it's in the public eye," says Dr Piquet.

Dr Piquet has treated a number of patients who had also gone undiagnosed for years.

The publicity gained by Celine speaking out about her battle with the illness is something she hopes will help others.

"We need to diagnose this disease better," Dr Piquet says. "When we do, that's going to lead to more clinical trials, and more treatments that are approved."

She says that although Celine will live with this condition for the rest of her life, the therapy she is undergoing to lessen the muscle spasms will help her to sing on stage again.

"My voice will be rebuilt," says Celine. "I mean, it started a while ago already. My voice is being rebuilt as we speak, right now."

New Vegas show

Celine Dion's two previous Las Vegas residencies earned more than $680m (£530m)

Fans will be able to see how the singer's health struggles have affected her life in a new documentary titledI Am: Celine Dion.

She can now finally see a way back to performing, and has been preparing for a new show in Las Vegas.

"We have been working so hard to put this show together, because Im back," she says with a huge smile.

It is clear she has missed performing terribly.

"Ill be on stage. I dont know when exactly, but trust me I will scream it out loud.

"I cant wait."

Celine holds the record for the most successful residency of all time on the Las Vegas strip. She is extremely proud of her career, but time away has given her a chance to reflect.

After years of touring the world, she has realised she has seen very little of it.

There has been "a price to pay" for always being in tour mode while travelling, she says.

"All my days off, I wanted to be ready for the next show. I toured the world and I did not see a lot.

"But as a performer and a singer, I have received so much love from the fans."

My earliest memory of Celine Dion was one of our secondary school teachers playing her single Pour Que Tu M'aimes Encore on cassette tape to try to inspire our French studies.

She is best known to audiences as the queen of power ballads.

Her hits from the 90s like The Power of Love and Its All Coming Back to Me Now are instantly recognisable for their soaring vocals.

But at the height of her health battles, Celine admits to having felt a little envious of the breathy, 'whisperpop' style used by some modern female artists.

Singers like Billie Eilish and Lana Del Rey are achieving big hits at the opposite end of the volume register.

Celine Dion shot to fame after she won the 1988 Eurovision Song Contest for Switzerland

"I was like, well, should I just go to bed really, really late at night and start smoking?" she jokes.

"I was jealous. I was like, theyre going out, theyre partying, theyre barely cleaned up, and theyre amazing."

That's a contrast with the strict regime she has held herself to for much of her career.

"I was like, dont talk, and dont eat this, thats too much dairy, its going to produce phlegm."

She continues to joke that perhaps she will throw her classical training out of the window and adopt a new persona with a husky tone.

"How about that for a concert?" says Celine, winking at the camera.

She is managing to keep a sense of humour through her ongoing health battles.

But whatever style she chooses, fans will be extremely glad to see her back. (BBC)