Celine Dion revealed to her fans Thursday she has been diagnosed with stiff-person syndrome, a rare neurological disorder that affects about 1 or 2 people per million, according to Johns Hopkins Medicine. 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...