Atonic: Without normal muscle tone or strength. An atonic seizure is one in which the person suddenly loses muscle tone and strength; the person cannot sit or stand upright and, unless supported, falls down. From Brain and Nervous System Resources Essential Tips for Myasthenia Gravis Understanding Myasthenia Gravis TreatmentCIDP: Reviewing Your Treatment Choices Featured Centers What Are the Best PsA Treatments for You?Understanding Biologics 10 Things People With Depression Wish You Knew