Tonic seizures are fairly uncommon. They 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 atonic seizures.)

When a tonic seizure occurs, the muscles in the body suddenly contract and the entire body stiffens. This occurs suddenly, without warning, and often causes the person to fall down.

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