(NEXSTAR) – At the opening ceremony of the 2024 Paris Olympics, Celine Dion sang atop the Eiffel Tower in her first performance since revealing her diagnosis with a rare disease known as stiff-person syndrome.

The legendary Canadian singer revealed in 2022 she would be taking a break from performing after discovering she has the neurological disorder. It affects about 1 or 2 people per million, according to Johns Hopkins Medicine.

What is stiff-person syndrome?

The disease “is characterized by fluctuating muscle rigidity in the trunk and limbs and a heightened sensitivity to stimuli such as noise, touch, and emotional distress, which can set off muscle spasms,” according to the National Institute of Neurological Disorders and Stroke (NINDS).

People with stiff-person syndrome often have a hunched-over or stiff posture, and may have trouble walking or moving. Some people may be “afraid to leave the house because street noises, such as the sound of a horn, can trigger spasms and falls,” NINDS explains.

Without “normal defensive reflexes,” people with stiff-person syndrome often suffer falls, and can injure themselves easily.

When she announced her diagnosis, Dion said she had been experiencing spasms “for a long time” and they were affecting “every aspect” of her life, making it hard for her to walk and even sing.

Since then, Dion has taken a break from performing, TODAY reports, though she has made some public appearances, including at the 2024 Grammys. When she was spotted in Paris ahead of the Olympic games, rumors began flying.

The cause of stiff-person syndrome isn’t yet understood, but research suggests it may be an autoimmune issue in the brain and spinal cord.

NINDS says the rare condition is often misdiagnosed as Parkinson’s or multiple sclerosis, among other diseases.

It can be treated with intravenous immunoglobulin (IVIg), muscle relaxants and pain relievers, but there is no known cure.