Startle response: An exaggerated startle response to sudden noises, movements, or touch Muscle stiffness: Extreme muscle tension that prevents voluntary movement Hypertonia: Increased muscle tone Hypnagogic myoclonus: Muscle twitches when falling asleep Sleep movements: Movements of the arms or legs while asleep Seizures: Rarely, infants with hereditary hyperekplexia experience recurrent seizures