Celine Dion in a shocking post on Thursday revealed her battle with stiff person syndrome.

Stiff person syndrome is a rare autoimmune movement disorder that affects the central nervous system (brain and spinal cord).

Also known as Moersch-Woltman syndrome, it is a rare neurological disorder that has features of an autoimmune disease.

People with this condition first experience a tightening of the trunk muscles followed, over time, by the development of stiffness in the legs and other muscles of the body.

Here are 8 facts to know about the condition.

1. Stiff person syndrome is a very rare condition that occurs in about one in a million people.

2. Twice as many women have Stiff Person Syndrome as men.

3. It is commonly found in middle-aged people between 30 and 60 years old.

3. The SPS was first described by Moersch and Woltman in 1956.

4. Stiff person syndrome can cause someone to have altered posture and, in severe cases, can limit the person’s ability to walk or move.

5. Researchers aren’t sure of the exact cause of stiff person syndrome. However, they believe it is an autoimmune disorder, a condition in which the immune system attacks healthy cells.

6. Symptoms include pain, muscle stiffness, painful aches, difficulty walking, depression and anxiety.

7. Because stiff person syndrome can be misdiagnosed as Parkinson’s disease, multiple sclerosis, fibromyalgia, psychosomatic illness, or anxiety and phobia, the condition is diagnosed with a blood test that measures the level of glutamic acid decarboxylase (GAD) antibodies in the blood.

8. There is no known way to prevent it.