What are atonic seizures?

Loss of tone. Some complex epileptic have drop attacks=fall to the floor because of loss of muscle tone in legs usually have other seizures like tonic= totally stiff or starring spells in same child--or myoclonic seizures like simple jerks of trunk or limb.
Loss of tone. Most seizures involve a tonic phase and clonic phase. This is just the reaction of muscles that stiffen and then jerk. In some seizures the muscle lose tone, and the patient falls abruptly unable to move. These may also be called drop attacks. These may occur if with the patient fully aware and unable to control the event, but are usually associated with loss of consciousness.