Seizures fall into three broad categories based on where they start in the brain: focal onset (starting in one hemisphere), generalized onset (starting in both hemispheres at once), and unknown onset (when the starting point can’t be determined). Within those categories, there are many specific types, each with distinct symptoms and behaviors. Understanding the differences matters because the type of seizure directly shapes treatment and what to expect.
Focal Onset Seizures
Focal seizures begin in one specific area of the brain. They’re sometimes called partial seizures, though that older term is falling out of use. Focal seizures split into two subtypes based on whether you stay aware during the event.
Focal Aware Seizures
During a focal aware seizure, you remain conscious and can often talk and remember the episode afterward. Symptoms depend on which part of the brain is involved. You might experience a sudden wave of nausea, an unusual smell or taste, a rising feeling in the stomach, tingling in one hand, or intense emotions like fear or déjà vu. These episodes are sometimes called “auras,” but they’re actually seizures in their own right. They typically last under two minutes.
Focal Impaired Awareness Seizures
These seizures cause a noticeable loss of awareness. You may stare blankly and not respond to people around you. A hallmark feature is involuntary repetitive movements called automatisms: lip smacking, chewing motions, picking at clothing, rubbing the hands together, or fumbling with nearby objects. When seizures involve the frontal lobes of the brain, more dramatic movements can appear, including leg bicycling or pelvic thrusting. Some people repeat words or phrases, laugh, scream, or cry during these episodes. Afterward, there’s usually a period of confusion, and most people don’t remember the seizure at all.
Focal to Bilateral Tonic-Clonic Seizures
A focal seizure can spread from its starting point to involve both sides of the brain, producing a full-body convulsion. This was previously called a “secondarily generalized seizure.” The person may initially have focal symptoms (like a strange sensation or staring spell) before progressing into the stiffening and jerking that characterize a tonic-clonic seizure. Recognizing that early focal phase is important because it tells clinicians exactly where in the brain the seizure originates, which can guide treatment decisions.
Generalized Onset Seizures
Generalized seizures engage both hemispheres of the brain from the very start. They always involve some change in awareness, though the visible symptoms vary widely depending on the specific type.
Tonic-Clonic Seizures
This is the type most people picture when they hear the word “seizure.” It unfolds in two distinct phases. In the tonic phase, the muscles suddenly stiffen, the person loses consciousness, and they may fall to the ground. A groan or cry at the onset is common, caused by air being forced past the vocal cords. This phase lasts roughly 10 to 20 seconds. Then comes the clonic phase: rhythmic jerking of the arms and legs, alternately flexing and relaxing. Convulsions typically last one to two minutes. Afterward, the person is often confused, exhausted, and may sleep for an extended period.
Absence Seizures
Absence seizures look nothing like convulsions. The person suddenly goes blank, staring into space for about 3 to 15 seconds, then snaps back as if nothing happened. There may be subtle eyelid fluttering or small mouth movements like lip smacking. These seizures are far more common in children, and they can be remarkably frequent. Some people report 10 to over 30 episodes a day, and in children the frequency can reach into the hundreds. Because the episodes are so brief and quiet, they’re easy to mistake for daydreaming, which can delay diagnosis for months or years.
Myoclonic Seizures
Myoclonic seizures produce sudden, brief muscle jerks, almost like being startled. They last only a second or two and can affect the whole body or just the arms and legs. You stay conscious during the jerk itself, but the sudden movement can cause you to drop things or fall. These seizures often cluster in the morning shortly after waking.
Tonic Seizures
During a tonic seizure, muscles stiffen without the jerking phase that follows in a tonic-clonic seizure. The stiffening often affects the back, arms, and legs and can cause a fall if you’re standing. These episodes are typically short, lasting under 20 seconds, but the risk of injury from falling makes them particularly dangerous.
Atonic Seizures
Atonic seizures are essentially the opposite of tonic seizures. Instead of stiffening, muscles suddenly lose all tone, causing a collapse. They’re sometimes called “drop attacks” because the person goes limp and falls without warning. The seizure itself is brief, but the falls can cause head injuries, so people with frequent atonic seizures sometimes wear protective helmets.
Clonic Seizures
Clonic seizures involve rhythmic jerking movements without an initial stiffening phase. They’re relatively rare as a standalone type and more commonly seen in infants and young children.
Unknown Onset Seizures
Sometimes there isn’t enough information to determine whether a seizure started in one spot or both sides of the brain. This happens when nobody witnessed the beginning of the seizure, or when test results are inconclusive. Unknown onset seizures can still be classified by their main feature: motor (involving movement, such as tonic-clonic convulsions or spasms) or non-motor (such as a sudden pause in behavior, called behavior arrest). The “unknown” label isn’t permanent. As more information becomes available through brain wave monitoring or witness accounts, the seizure can often be reclassified as focal or generalized.
How Seizure Types Are Identified
The primary tool for identifying seizure types is an electroencephalogram, or EEG, which records electrical activity across the brain using sensors on the scalp. Different seizure types produce recognizable electrical signatures. Absence seizures, for example, create a distinctive, highly regular wave pattern cycling at about 3 times per second. Myoclonic seizures show rapid clusters of electrical spikes at around 20 cycles per second. Tonic seizures produce fast bursts at about 10 cycles per second. These patterns help clinicians pinpoint the seizure type even when the outward symptoms are ambiguous.
Witness descriptions also play a major role. Details like whether the person was aware during the event, which body parts moved first, how long it lasted, and what the recovery period looked like all help narrow the classification. Keeping a seizure diary with these details, or having someone record a video of an episode, can be extremely useful at medical appointments.
How Common Each Type Is
An analysis of over 370,000 people with epilepsy in the U.S. found that focal seizures accounted for about 25% of diagnosed cases, generalized seizures with convulsions for about 24%, and generalized seizures without convulsions (like absence seizures) for about 9%. Notably, the largest group, at nearly 37%, had seizures that weren’t clearly classified into a specific type. This highlights how often the starting point of a seizure remains uncertain, especially early in diagnosis.
What to Do During a Seizure
For any seizure, the core steps are the same: stay calm, stay with the person, move nearby objects that could cause injury, and time the seizure. If it lasts longer than 5 minutes, or if a second seizure begins before the person regains consciousness, that’s a medical emergency called status epilepticus, and you should call 911 immediately.
For tonic-clonic seizures specifically, ease the person to the ground, place something soft under their head, turn them gently on their side with their mouth pointing toward the ground to keep the airway clear, remove glasses, and loosen anything tight around the neck. Never put anything in the person’s mouth. For focal seizures where the person is conscious but confused, stay nearby, speak calmly, and gently guide them away from hazards like traffic or stairs until the episode passes and full awareness returns.