An atonic seizure is a sudden loss of muscle tone in the muscles that hold the body and head upright.

• The seizure occurs without warning and usually causes the person to fall down.
• Some atonic seizures may be more limited, only causing the person's head to drop for a moment.

Atonic seizures are fairly uncommon and occur mostly in people with Lennox-Gastaut syndrome, a severe form of generalized epilepsy that begins during early childhood. (Children with Lennox-Gastaut syndrome may also have tonic seizures.)

People who have atonic or tonic seizures are likely to be injured when they fall. Children may have to wear helmets and restrict their activities to prevent serious injury.