The three major groups of seizures are focal onset, generalized onset, and unknown onset. This classification, established by the International League Against Epilepsy (ILAE), is based on where abnormal electrical activity begins in the brain. Focal seizures start in one area, generalized seizures engage both sides of the brain simultaneously, and unknown onset seizures are those where the starting point can’t be determined. Focal seizures account for roughly 60% of new epilepsy cases, with generalized seizures making up the remaining 40%.
Focal Onset Seizures
Focal onset seizures begin in a specific region on one side of the brain. Because they start in a defined area, the symptoms often reflect the function of that brain region. A seizure starting in the area that controls movement might cause jerking in one hand, while one starting in a sensory area might produce tingling, unusual smells, or visual disturbances.
One of the most important distinctions within this group is whether you remain aware during the seizure. In a focal aware seizure, you know what’s happening around you and can interact with your environment, even though something clearly abnormal is occurring. In a focal impaired awareness seizure, your consciousness is disrupted during some or all of the event. You may stare blankly, seem confused, or perform repetitive movements without realizing it.
Focal seizures are further divided into motor and nonmotor types. Motor symptoms include sustained muscle stiffening, rhythmic jerking, sudden loss of muscle tone in a limb, or automatisms like lip-smacking, tapping, or swallowing. Nonmotor symptoms are more subtle and varied: sudden waves of fear or anxiety, déjà vu, changes in heart rate or blood pressure, altered vision or hearing, or a complete pause in movement called behavioral arrest.
A focal seizure can also spread. When abnormal electrical activity recruits enough surrounding neurons, it can travel through brain pathways and eventually affect both sides of the brain. This process, called propagation, can turn a focal seizure into one that looks identical to a generalized tonic-clonic seizure. This is why someone might experience an “aura” (really a brief focal seizure) seconds before losing consciousness and convulsing.
Generalized Onset Seizures
Generalized onset seizures appear to engage both sides of the brain from the very beginning. They typically cause loss of consciousness or awareness and affect the entire body. Within this group, there are several distinct types that look and feel very different from one another.
Tonic-Clonic Seizures
These are what most people picture when they think of a seizure. Previously called “grand mal” seizures, they involve two phases: a tonic phase where the muscles stiffen, followed by a clonic phase of rhythmic jerking. They usually last a few minutes and are followed by confusion, fatigue, and sometimes headache. The person has no memory of the event.
Absence Seizures
Absence seizures, formerly known as “petit mal,” are brief lapses in consciousness that often go unnoticed. They typically last only a few seconds. A person may stare into space, blink rapidly, or make small chewing or hand movements. These are most common in children and can happen dozens of times a day, sometimes mistaken for daydreaming. On an EEG, they produce a characteristic burst of spike-and-wave patterns generated by circuits connecting the brain’s cortex and thalamus.
Myoclonic, Tonic, and Atonic Seizures
Myoclonic seizures cause sudden, lightning-quick jerks of the arms or legs. They’re so brief that they can feel like an electric shock. Tonic seizures involve sudden muscle stiffening without the jerking phase, which can cause falls if you’re standing. Atonic seizures are the opposite: a sudden loss of muscle tone that can cause the head to drop or the entire body to collapse. These last type are sometimes called “drop attacks” because of how abruptly they cause falls.
Unknown Onset Seizures
When a seizure isn’t witnessed from the beginning, or when testing can’t pinpoint where it started, it’s classified as unknown onset. This isn’t a permanent label. It’s a practical category that allows doctors to describe what they observed, such as tonic-clonic movements or behavioral arrest, without guessing at the origin. As more information becomes available through brain monitoring or eyewitness accounts, unknown onset seizures are often reclassified into the focal or generalized group.
This category matters because treatment depends on accurate classification. Medications that work well for focal seizures don’t always work for generalized seizures, and vice versa. Keeping the “unknown” option prevents premature decisions that could lead to the wrong treatment approach.
Why Classification Matters for Treatment
The type of seizure directly shapes which medications are most likely to work. For focal onset seizures, the typical first-line options target the specific brain circuits involved. For generalized tonic-clonic seizures, broader-acting medications are preferred. Absence seizures respond to a different set of drugs entirely, and myoclonic seizures have their own preferred treatments.
Getting the classification wrong can mean prescribing a medication that not only fails to control seizures but potentially worsens them. Some drugs effective for focal seizures can aggravate certain generalized seizure types. This is one reason doctors rely heavily on EEG patterns, though interpretation isn’t always straightforward. More than 60% of people with certain generalized epilepsies show atypical EEG features that can complicate the picture.
How to Help During a Seizure
For any seizure, the basics are the same: stay calm, stay with the person, and move anything nearby that could cause injury. Time the seizure. If it lasts longer than five minutes, call emergency services.
Generalized seizures that cause falls or convulsions require extra steps. Ease the person to the ground, turn them gently onto their side with their mouth pointing downward to keep the airway clear, and place something soft under their head. Remove glasses and loosen anything tight around the neck. Never put anything in their mouth or try to restrain them.
Focal seizures where awareness is impaired call for a calmer approach. The person may wander, perform repetitive movements, or seem confused. Gently guide them away from danger without restraining them. Once the seizure ends, explain what happened, since they likely won’t remember it.
The 2025 Classification Update
The ILAE released an updated classification in 2025, building on the 2017 framework. The core structure of focal, generalized, and unknown onset remains intact, but the update now includes four main classes and 21 seizure types. One notable change is clearer language around consciousness during focal seizures, incorporating both awareness and responsiveness into the definitions to better align with how consciousness is assessed in other neurological conditions.