A clonic seizure is a type of seizure that causes rhythmic, sustained jerking movements on both sides of the body, along with a loss of consciousness. The jerking typically repeats at a rate of about 1 to 3 times per second, with each individual jerk lasting less than 100 milliseconds. Clonic seizures are distinct from the more commonly known tonic-clonic seizure, which has two phases: first a stiffening of the body, then the rhythmic jerking. In a purely clonic seizure, the stiffening phase is absent.
What Happens During a Clonic Seizure
The hallmark of a clonic seizure is bilateral, rhythmic muscle contractions. Unlike random twitches or isolated jerks, the movements follow a steady, repetitive pattern. The person loses consciousness at the onset and is unaware of what’s happening during the event. The jerking can involve the entire body, though it’s often uneven. In young children and infants, for example, the contractions may be distributed across the whole body but predominate on one side or in one limb.
The intensity can also vary between muscle groups during a single episode. Facial muscles might twitch rapidly with small movements while the arms produce larger, more forceful jerks. The legs sometimes contract so rapidly that they appear almost rigid, even though the underlying activity is actually fast rhythmic jerking rather than true sustained stiffness.
Clonic vs. Tonic-Clonic Seizures
The difference comes down to phases. A tonic-clonic seizure starts with a tonic phase, where every muscle tenses and the body goes rigid, sometimes for several seconds to a minute. Then comes the clonic phase, with the rhythmic jerking. A purely clonic seizure skips that initial stiffening entirely. There are also variations where the phases occur in different combinations, such as clonic-tonic-clonic sequences, but these are less common.
It’s also worth distinguishing clonic seizures from repetitive myoclonic seizures, which can look similar at first glance. The key differences: clonic seizures involve regular, rhythmic jerking and come with a loss of consciousness, while myoclonic seizures tend to produce irregular jerks and the person may retain partial awareness.
Focal vs. Generalized Clonic Seizures
Clonic activity can be either generalized (affecting both sides of the body) or focal (limited to one area). In a focal clonic seizure, the rhythmic jerking is confined to the opposite side of the body from where the abnormal brain activity originates. If the seizure starts in the part of the motor cortex that controls the hand, for instance, the jerking begins in the hand on the opposite side.
A classic pattern called a Jacksonian march occurs when focal clonic activity spreads progressively through the motor cortex. The jerking might start in the fingers, then move to the wrist, arm, shoulder, and face as the electrical activity travels along the brain’s motor strip. This spreading pattern is a useful clue for neurologists trying to pinpoint where seizures originate. In patients who undergo epilepsy surgery, the pattern of movements during a seizure correctly identifies the brain region involved about 77% of the time.
What Causes the Rhythmic Jerking
During a clonic seizure, waves of abnormal electrical activity sweep through the brain’s outer layer in a rhythmic pattern. These waves propagate at roughly 20 to 100 millimeters per second. What creates the on-off rhythm of the jerking is a tug-of-war between excitatory and inhibitory brain signals. Inhibitory circuits try to restrain the spread of electrical activity, causing it to stall momentarily before breaking through again. Once a region of the brain is fully recruited into the seizure, however, that inhibitory restraint fails and the electrical waves pass through without pausing.
As the seizure continues, several processes work to slow it down and eventually stop it. Calcium builds up inside neurons and activates channels that produce an opposing electrical current. Ion pumps ramp up their activity. Chemical messengers at the synapses become depleted. Together, these mechanisms gradually exhaust the seizure activity.
How Long Clonic Seizures Last
Most generalized motor seizures, including clonic seizures, last a few minutes. Any seizure lasting longer than five minutes is a medical emergency and requires immediate help, as prolonged seizure activity can cause brain injury. If a person doesn’t fully wake up between repeated seizures, that also warrants a 911 call.
The Recovery Period
After a clonic seizure ends, the brain enters a recovery phase called the postictal state. This is not just “feeling tired.” It’s a distinct set of symptoms that can include confusion, headache, muscle soreness, exhaustion, memory gaps, difficulty speaking, and mood changes like anxiety or agitation. Some people experience nausea, changes in blood pressure or heart rate, or loss of bladder control.
The postictal state typically lasts between 5 and 30 minutes, though it can extend to hours or even days after a severe seizure. More intense or prolonged seizures tend to produce more severe recovery symptoms, potentially including delirium or hallucinations. The severity depends partly on which brain regions were involved and how long the seizure lasted.
How Clonic Seizures Are Treated
Clonic seizures are managed with anti-seizure medications, and the choice depends on whether the seizures are generalized or focal in origin. For generalized seizures, valproate is often considered the first option because it works across a wide range of seizure types. Other commonly used medications include lamotrigine, levetiracetam, and topiramate, all of which have broad effectiveness against generalized motor seizures. For focal clonic seizures, additional options are available depending on the specific epilepsy syndrome.
Many of these medications can be taken once or twice daily, and the goal is to prevent seizures from occurring in the first place rather than stopping them once they’ve started. Finding the right medication and dose often takes some trial and adjustment, since people respond differently and side effects vary.
What to Do If You Witness One
If someone near you has a clonic seizure, the most important steps are straightforward:
- Stay with them and stay calm.
- Ease them to the ground if they’re standing, and clear away nearby objects that could cause injury.
- Place something soft under their head, like a folded jacket.
- Turn them gently onto their side with their mouth angled toward the ground to keep the airway clear.
- Loosen anything tight around the neck and remove eyeglasses.
- Time the seizure. If it goes past five minutes, call 911.
Equally important is what not to do. Never hold the person down or try to restrain their movements, as this risks injury to both of you. Don’t put anything in their mouth. Don’t attempt mouth-to-mouth breathing during the seizure, as breathing typically resumes on its own once the seizure ends. Wait until the person is fully alert before offering water or food, then help them sit somewhere safe to recover.