Gelastic cataplexy is a sudden loss of muscle tone without loss of consciousness triggered by laughter. It’s a characteristic sign of a rare genetic disease called Niemann Pick disease type C.

Spastic cerebral palsy is a disorder that involves muscle stiffness. Symptoms and signs may include difficulty walking, trouble manipulating objects, and vision, hearing, or speech challenges.

Benign hereditary chorea is a rare disorder that affects movement. The condition starts in infancy or childhood. Although it affects movement, it can also cause additional problems in other organs, ...