Hyperexplexia: A rare genetic disorder inherited in an autosomal dominant manner in which babies have an exaggerated startle reflex. Symptoms at birth may include muscle stiffness (hypertonia), an exaggerated response to being startled, and strong brain stem reflexes (especially head-retraction reflex). The startle reflex is sometimes accompanied by sudden stiffness (acute generalized hypertonia), which can cause the affected person to fall to the ground like a log. Treatment is via medications. Also known as exaggerated startle disease, hyperekplexia, Kok disease, startle disease, and stiff baby syndrome.