Celine Dion is battling stiff person syndrome, a rare neurological disorder. What is it?

Celine Dion received a standing ovation at Sunday's Grammy Awards when she made a surprise appearance on stage to present the award for album of the year, which went to Taylor Swift.

"Those who have been blessed enough to be here, the Grammy Awards, must never take for granted the tremendous love and joy that music bring to our lives and to people all around the world," Dion told the supportive crowd.

The 55-year-old music powerhouse, who has five Grammys of her own, is battling stiff person syndrome, a rare neurological disorder that affects one or two in a million people, according to Johns Hopkins Medicine.

The disease notably causes progressive muscle stiffness and painful spasms. Treatments focus on relieving symptoms.

Here's what you need to know about stiff person syndrome, the symptoms of the disease and more.

What is stiff person syndrome?

Stiff person syndrome, or SPS, is a rare "neurological disorder with features of an autoimmune disease," the National Institute of Neurological Disorders and Stroke notes.

"It's a disease that's characterized by progressive muscle stiffness, muscle spasms, rigidity – typically in the muscles of the back, specifically the lower back, as well as the upper legs," Dr. Kunal Desai, a Yale Medicine neurologist and assistant professor of neurology who specializes in neuromuscular disease, told USA TODAY.

"It can affect other muscles, including muscles in the arms as well as speaking and swallowing muscles," Desai said. "But that's less common."

With extreme muscle spasms, "the muscles can feel tight as a rock. So, it can be very, very uncomfortable and painful," Desai said, adding that other stimuli that can provoke these spasms sometimes include touching the muscles, "certain subtle movements" and stress.

What are the symptoms of SPS?

Prominent SPS symptoms include muscle stiffening and painful muscle spasms that can be provoked by emotional stress or, again, outside stimuli. It's possible for these spasms to be "so severe that they cause the person to fall down," Yale Medicine says.

"We all have muscle spasms, but these are muscle spasms that are beyond your control to the point that muscles lock and are so rigid. There's truncal dystonia," Dr. Robert Wilson, a neurologist and the director of the Autonomic Center at the Cleveland Clinic, told USA TODAY, expanding on some severe cases he's seen. "The limb can look distorted and contorted. I've seen people actually break a limb from it."

These symptoms can lead to difficulty walking, among other disabilities. People with SPS are also more likely to experience anxiety and depression, Yale Medicine notes – pointing to how unpredictable the disease can be and because patients "have lower levels of the neurotransmitter GABA, which regulates anxiety."

NINDS adds that many people with SPS "are afraid to leave the house" due to the environmental stimuli, such as loud noises, that can trigger episodes.

Are the symptoms same for each person?

Of course, SPS symptoms can range significantly and every patient's experience is unique. "It's a spectrum of severity," Desai says.

"Some people have mild spasms. Some people can have the full dystonia where a part of the body becomes almost like rigid to a board or like a statue. ... And some people can be constant, with 24/7 locking, or some people could just (have) episodes," Wilson said. "There's a variability within each patient."

In rare cases, both Desai and Wilson note, stiff person syndrome has also been associated with cancer.

How rare is SPS?

SPS is considered to be very rare because the disease is expected to affect one or two people per million, according to Johns Hopkins Medicine.

Desai adds that people who are diagnosed with stiff person Syndrome are usually in their 20s to 50s, although the disease has been found in people who are younger and older as well. The disease is also "two to three times more common in women," he said.

How is SPS diagnosed?

Diagnosing the syndrome can be difficult, especially because the disease is so rare, Desai and Wilson note.

A comprehensive review of a patient's medical history, a neurological exam and tests including blood tests, spinal fluid analysis and an EMG can be used to confirm the diagnosis.

Especially if a patient's symptoms were left unexplained or misdiagnosed in the past, Wilson says, diagnosis is crucial.

"Validation is such an important thing for a person with a neurologic disorder, because they can look good at some level and yet still suffer internally," he said. "Once you feel the validation, we can help them out."

What causes the disease?

The exact cause remains unknown. However, research suggests that SPS results from "an autoimmune response gone awry in the brain and spinal cord," NINDS writes.

Is SPS curable? Is it terminal?

There is no cure for SPS, Yale Medicine notes. But there are treatments to help relieve symptoms – including various medications and immunotherapies prescribed by doctors and different types of physical therapy.

SPS can be terminal in rare cases, Desai explained.

"It's a rare disease to begin with. And there are cases of death being reported, but ... it's rare for a rare disease," Desai said.