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Dr. Amanda Piquet, who serves as director of the autoimmune neurology program at the University of Colorado Anschutz Medical Campus and is the physician who diagnosed Dion, tells PEOPLE in this week’s issue that the disease is widely unknown — even in the medical community. Stiff-person syndrome (SPS) is an incurable neurological disorder that affects the central nervous system, specifically the brain and spinal cord. It’s associated with slurred speech, double vision and painful muscle contractions that can become so severe that patients lose their ability to walk or speak.