What are they like?

Clonic seizures consist of rhythmic jerking movements of the arms and legs, sometimes on both sides of the body.

How long do they last?

The length varies.

Tell me more

"Clonus" (KLOH-nus) means rapidly alternating contraction and relaxation of a muscle -- in other words, repeated jerking. The movements cannot be stopped by restraining or repositioning the arms or legs. Clonic (KLON-ik) seizures are rare, however. Much more common are tonic-clonic seizures, in which the jerking is preceded by stiffening (the "tonic" part). Sometimes tonic-clonic seizures start with jerking alone. These are called clonic-tonic-clonic seizures!

Who gets them?

Clonic seizures are not seen very often. They can occur at various ages, including in newborns.

What's the outlook?

Brief and infrequent clonic seizures in infants usually disappear on their own within a short time. Other types may need prolonged treatment.

What else could it be?

Occasionally "jitteriness" in a young infant can be mistaken for a clonic seizure, especially if it is severe (during crying, for instance). Changing the position of the baby's arms or legs should reduce or stop jitteriness. The jittery infant also will be more alert than an infant who is having a clonic seizure.

Children with neurological impairments sometimes have repetitive movements that could be mistaken for clonic seizures.

How is the diagnosis made?

The doctor should recognize the appearance of a clonic seizure if he or she witnesses an episode. The EEG pattern will change during a seizure, so video-EEG is very useful. A factor distinguishing clonic from tonic-clonic seizures is that clonic seizures are not followed by a period of tiredness or confusion. Tonic-clonic seizures usually are.

Topic Editor: Orrin Devinsky, M.D.
Last Reviewed:2/11/04