What is a clonic seizure?

"Clonus" (KLOH-nus) means fast stiffening and relaxaing of a muscle that happens repeatedly. In other words, it is repeated jerking. The movements cannot be stopped by restraining or repositioning the arms or legs.

Clonic (KLON-ik) seizures are rare and most commonly occur in babies. Most often, clonic movements are seen as part of a tonic-clonic seizure.

  • Jerking movements alone, as with a clonic seizure, may last a few seconds to a minute.
  • Jerking or clonic movements that follow stiffening of muscles, as in a tonic-clonic seizure, can last seconds to 1-2 minutes.
  • A clonic seizure may sometimes be hard to distinguish from a myoclonic seizure. The jerking is more regular and sustained during a clonic seizure.

Where does a clonic seizure start in the brain?

When it starts in one area of the brain:

  • Clonic seizures can start in the motor area on one side of the brain. These are called focal clonic seizures.
  • The jerking movements would affect just one side or part of the body or face.

When it starts on both sides of the brain:

  • Clonic seizures can also affect both sides of the brain at once. These would be called generalized clonic seizures.
  • The jerking movements would affect both sides of the body or the whole body at once.

Who is at risk for clonic seizures?

  • Clonic seizures can affect people of all ages, including newborns.
  • In infants, the clonic seizures are very brief and don’t happen very often. They may also go away on their own in a short period of time.
  • Clonic seizures that don’t go away on their own will need long term treatment.

What’s it like to have a clonic seizure?

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

  • If the clonic seizure has a focal onset (starts in one part of the brain), the person may be aware during it.
  • If the clonic seizure has a generalized onset (affects the whole brain), usually the person is not aware during it.
  • If a person is standing when a clonic seizure occurs, they may fall.
  • Sometimes a focal clonic seizure affects both the motor and sensory parts of the brain. It may also include tingling or numbness of the body part with the jerking.

What happens after a clonic seizure?

After a clonic seizure, the person may simply continue what they are doing, especially if they were aware when it happened. If a person was not fully aware, they may need to rest for a few minutes before returning to usual activity.

  • The most important first aid step is to protect the person from injury when the jerking happens or if they fall.
  • Do NOT try to restrain the person’s movements.
  • If a clonic seizure goes into a tonic-clonic one, the person will most likely be tired and confused after the seizure. Let them rest as needed and follow care and comfort first aid.

If someone has clonic seizures, how often will they happen?

The frequency of seizures depends a lot on the cause and the person. Some people may have rare seizures, and others may have them more frequently.

How can I tell if someone is having a clonic seizure?

  • Clonic seizures consist of rhythmic jerking movements of the arms and legs, sometimes on both sides of the body, that can be seen by the person observing.
  • Sometimes "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. A jittery infant also will be more alert than an infant who is having a clonic seizure.
  • Children with other neurological problems can have repeated jerking that could be mistaken for clonic seizures. EEG (electroencephalogram) testing may be needed to see what happens in the brain when the symptoms occur.

How are clonic seizures diagnosed?

  • A good description of the symptoms is the first step. Written reports of what happens from the person having the seizure and from observers can be very helpful. Bring video recordings of events to the doctor’s visit if you have them.
  • An EEG is done to look at the electrical activity of the brain.
  • Video EEG telemetry that can record the electrical activity and what it looks like during an event is helpful if seizrues don’t respond to treatment. This test may also be done to sort out clonic seizures from other causes.

How are clonic seizures treated?

Certain seizure medicines help prevent clonic seizures. Knowing where seizures begin – in one area (focal onset) or both sides (generalized onset) of the brain – will help decide which medicine may be best to use.

What should I do if I think my loved or myself may have clonic seizures?

If you think you or your loved one may be having clonic seizures, talk to your doctor as soon as possible. Getting these diagnosed early can help lead to better treatment.

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Authored By: 
Elaine Kiriakopoulos MD, MSc
Patty Osborne Shafer RN, MN
Authored Date: 
Reviewed By: 
Robert Fisher MD, PhD
Joseph I. Sirven MD
Wednesday, March 15, 2017