Seizures are classified into four main categories based on where they start in the brain: focal onset, generalized onset, unknown onset, and focal to bilateral tonic-clonic. This system, established by the International League Against Epilepsy in 2017, replaced older terminology and gives doctors a clearer framework for diagnosis and treatment. Each type looks and feels different, and knowing which one you’re dealing with matters for both everyday management and emergency response.
Focal Onset Seizures
A focal seizure starts in one specific area of one hemisphere of the brain. Because only part of the brain is involved, the symptoms depend entirely on which region is affected. Someone having a focal seizure might experience unusual sensations, repetitive movements like lip smacking or hand rubbing, or sudden emotional changes like unexplained fear or déjà vu.
Focal seizures are further divided by whether awareness is preserved. In a focal aware seizure, the person remains conscious and can recall the experience afterward. These were previously called “simple partial seizures” and often involve sensory symptoms: a strange taste, a rising feeling in the stomach, tingling in one hand, or visual disturbances. They typically last less than two minutes.
In a focal impaired awareness seizure (formerly “complex partial seizure”), the person loses awareness of their surroundings. They may stare blankly, fumble with objects, wander, or make chewing motions without realizing it. Afterward, they’re often confused and have no memory of what happened. These seizures frequently originate in the temporal lobe, the brain region involved in memory and emotion.
Generalized Onset Seizures
Generalized seizures involve both sides of the brain from the very start, which means they affect consciousness and typically involve the whole body. They’re split into motor and non-motor subtypes, and the differences between them are dramatic.
Tonic-Clonic Seizures
These are what most people picture when they think of a seizure. Tonic-clonic seizures cause muscle stiffness followed by rhythmic jerking. The person may cry out, lose consciousness, and fall to the ground. They usually last a few minutes and leave the person confused and exhausted afterward, sometimes for hours. This recovery period is called the postictal state.
Absence Seizures
On the opposite end of the spectrum, absence seizures are so subtle they often go unnoticed. These cause brief lapses of consciousness, typically lasting only a few seconds. The person may stare into space, blink rapidly, or make small hand or chewing movements. They’re most common in children and can happen dozens of times a day, sometimes mistaken for daydreaming or inattention at school.
Myoclonic Seizures
Myoclonic seizures produce short, lightning-quick jerking movements, often in the arms or upper body. They can feel like a sudden electric shock and are over in a fraction of a second. Some people experience clusters of myoclonic jerks, particularly shortly after waking up.
Other generalized motor subtypes include tonic seizures (sustained stiffening without the jerking phase), atonic seizures (sudden loss of muscle tone causing the person to drop or collapse), and clonic seizures (rhythmic jerking without the initial stiffening).
Unknown Onset Seizures
Sometimes a seizure simply can’t be classified as focal or generalized. This happens when no one witnessed the beginning of the event, or when diagnostic tests like EEG recordings and brain imaging haven’t yet been completed. The “unknown onset” category exists as a practical placeholder rather than a permanent label.
Even without a clear origin, these seizures can still be described by their physical features. They’re classified as motor (including tonic-clonic movements or spasms) or non-motor (such as behavior arrest, where the person suddenly freezes). Once more information becomes available through testing or observation of future episodes, doctors often reclassify the seizure into the focal or generalized category.
Focal to Bilateral Tonic-Clonic Seizures
This fourth type begins as a focal seizure in one area of the brain and then spreads to involve both hemispheres, producing a full tonic-clonic seizure. Older medical literature called these “secondarily generalized” seizures. The distinction matters because the treatment approach targets the focal origin point rather than treating it as a purely generalized event.
People who experience this type often notice a warning phase, or aura, before the convulsive part begins. The aura is actually the focal seizure itself: it might involve a strange smell, a visual disturbance, or a wave of nausea. This warning can last anywhere from a few seconds to a couple of minutes before the seizure spreads and consciousness is lost. Recognizing the aura gives some people enough time to get to a safe position before the tonic-clonic phase starts.
How Seizure Type Is Determined
Doctors rely primarily on two things to classify seizures: a detailed description of what happened (ideally from a witness) and EEG recordings that measure electrical activity in the brain. Each seizure type produces distinctive brainwave patterns. Absence seizures, for example, generate a characteristic pattern of electrical spikes followed by slow waves repeating at a very regular rhythm of about 3 cycles per second. Tonic seizures show rapid bursts of activity at about 10 cycles per second. These patterns act as electrical fingerprints that help confirm the type even when the physical symptoms are ambiguous.
Brain imaging with MRI may also reveal structural causes, like a scar, tumor, or malformation, that help explain why seizures start in a particular location. This is especially important for focal seizures, where identifying the exact origin can open up targeted treatment options.
What to Do During a Seizure
For any seizure, the priorities are keeping the person safe and timing the episode. Stay calm, stay with them, and move nearby objects that could cause injury. If they’re lying down, turn them gently onto their side with their mouth pointing toward the ground to keep the airway clear.
For tonic-clonic seizures, ease the person to the ground if they’re falling, place something soft under their head, remove their glasses, and loosen anything around the neck. Never put anything in their mouth or try to hold them down.
The critical time threshold is five minutes. A seizure lasting longer than five minutes, or multiple seizures without the person regaining normal consciousness between them, is a medical emergency called status epilepticus. This requires immediate emergency care.
For focal aware seizures where the person remains conscious, your role is mostly to stay nearby, speak calmly, and make sure they’re in a safe environment until the episode passes. Once any seizure ends, help them sit somewhere comfortable, explain what happened, and offer to contact someone who can help them get home safely.