Hyperekplexia is an infantile-onset disorder characterized by an exaggerated startle response and intense, generalized hypertonia in response to sudden, unexpected auditory or tactile stimuli. The disorder can be fatal in neonates as the prolonged periods of rigidity can lead to sudden death from apnea or aspiration. Affected infants look alert, tense and frightened and may also suffer from congenital dislocation of the hip. While hypertonia diminishes after the first year of life, patients suffer from startle-induced falls or nocturnal myoclonic muscle jerks. Patients may also have umbilical and inguinal hernias and hyperactive brainstem reflexes such as head retraction, palmomental and snout reflexes.