What is stiff-person syndrome, the disorder impacting Céline Dion's ability to sing?

Céline Dion revealed in an emotional and tear-filled announcement on Thursday that she had to postpone her upcoming European tour after being diagnosed with a "very rare neurological disorder": stiff-person syndrome. 

The debilitating illness has impacted her ability to sing and walk. 

According to the National Institute of Neurological Disorders and Stroke, stiff-person syndrome, otherwise known as Moersch-Woltman syndrome, is a "rare neurological disorder with features of an autoimmune disease." The illness causes the body to become rigid and more sensitive to noise, touch and emotional distress. That heightened sensitivity can cause muscle spasms, as well as "hunched over and stiffened" postures," according to the institute.