Tonic seizures are fairly uncommon. They occur mostly in people with
Lennox-Gastaut syndrome. This is a severe form of generalized
epilepsy that begins in early childhood. (Children
with Lennox-Gastaut syndrome may also have atonic seizures.)
When a tonic seizure occurs, the muscles in the body contract and the entire body stiffens. This occurs suddenly and without warning. And it often causes the person to fall down.
Epilepsy is a general term for the tendency to have seizures. Epilepsy is usually diagnosed only after a person has had more than one seizure.
When identifiable, the causes of epilepsy usually involve some form of injury to the brain. For most people, though, epilepsy's causes aren't known.