Atonic seizure (Human Phenotype Ontology entry HP_0010819) is a type of motor seizure characterized by a sudden loss or diminution of muscle tone without apparent preceding myoclonic or tonic event lasting about 1 to 2 seconds, involving head, trunk, jaw, or limb musculature. Also known as: Astatic seizure, Astatic seizures, Atonic seizures, Drop attacks, Drop seizures, Hypotonic seizure.