Seizures fall into three broad categories based on where they start in the brain: focal onset (starting in one area), generalized onset (starting in both sides at once), and unknown onset (when the beginning wasn’t observed). Within those categories, there are more than a dozen specific types, each with distinct physical signs and levels of awareness. Up to 10% of people worldwide will experience at least one seizure in their lifetime, though that doesn’t necessarily mean they have epilepsy.
Focal Onset Seizures
Focal seizures begin in one hemisphere of the brain. Because they start in a specific region, their symptoms depend heavily on which part of the brain is involved. Someone might experience a sudden wave of fear, a strange taste, or jerking in one hand. These seizures split into two main types based on whether the person stays aware of what’s happening.
Focal aware seizures (formerly called simple partial seizures) don’t cause any loss of awareness. You stay conscious the whole time, can often talk during the episode, and typically remember it afterward. Symptoms vary widely. You might feel a rising sensation in your stomach, see flashing lights, or notice tingling in one part of your body. These episodes are sometimes called “auras,” especially when they serve as a warning sign before a larger seizure.
Focal impaired awareness seizures (formerly complex partial seizures) involve some loss of awareness at any point during the episode. You may appear confused or dazed, stare blankly, smack your lips, or pick at your clothes. You generally can’t respond to questions or follow directions for a few minutes. Afterward, most people don’t remember the seizure clearly. These are among the most common seizure types in adults with epilepsy.
A focal seizure can also spread to involve both sides of the brain. When this happens, it’s called a focal to bilateral tonic-clonic seizure. It starts with focal symptoms, then progresses into full-body stiffening and jerking.
Generalized Onset Seizures
Generalized seizures appear to engage both hemispheres of the brain from the very start. They’re further divided into motor seizures (involving movement) and non-motor seizures (primarily involving changes in awareness or sensation).
Motor Types
Tonic-clonic seizures are what most people picture when they think of a seizure. The tonic phase involves sudden stiffening of the body, often with a cry as air is forced past the vocal cords. The clonic phase follows with rhythmic jerking of the arms and legs. The whole episode typically lasts one to three minutes. Anything longer than five minutes is classified as status epilepticus, a medical emergency.
Tonic seizures involve only the stiffening phase. Your muscles suddenly tense up, usually in the back, arms, and legs, which can cause you to fall if you’re standing. These tend to be brief, often lasting less than 20 seconds.
Atonic seizures are essentially the opposite. Your muscles suddenly go limp, causing a head drop or a full collapse to the ground. They’re sometimes called “drop attacks” because of how abruptly they happen. They’re short but carry a high risk of injury from falling.
Myoclonic seizures cause quick, shock-like jerks of a muscle or group of muscles, usually in the arms or upper body. They often happen in clusters, particularly shortly after waking up. Each jerk lasts only a fraction of a second. You typically stay fully conscious.
Clonic seizures involve repeated rhythmic jerking without the initial stiffening phase. These are less common on their own and occur more frequently in infants and young children.
Non-Motor Types (Absence Seizures)
Absence seizures are brief episodes of staring and unresponsiveness, most common in children between ages 4 and 14. During the seizure, a child stops what they’re doing and stares straight ahead. Their eyelids may flutter, and they might make chewing or smacking motions with their mouth. The episode typically lasts about 15 seconds, then ends as abruptly as it started, with the child returning to normal as if nothing happened.
These seizures are easy to mistake for daydreaming or not paying attention, which is why they often go undiagnosed for months. One key difference: a daydreaming child will respond when you snap your fingers or call their name. A child having an absence seizure will not. If that pattern happens repeatedly, it warrants a medical evaluation. Some children experience dozens or even hundreds of absence seizures per day, which can significantly affect learning and school performance.
Unknown Onset Seizures
Sometimes no one witnesses the beginning of a seizure, making it impossible to classify as focal or generalized. This commonly happens with seizures that occur during sleep or when someone lives alone. Unknown onset is a temporary label. Once more information becomes available, through witness accounts, video recordings, or brain monitoring, the seizure can usually be reclassified into one of the other categories.
Seizures That Aren’t Caused by Electrical Activity
Not all seizures involve abnormal electrical firing in the brain. Functional seizures, also called psychogenic nonepileptic seizures (PNES), look and feel remarkably similar to epileptic seizures but have a different underlying cause. Instead of abnormal brain waves, they’re a physical reaction of the nervous system to stress, past trauma, pain, or other triggers.
PNES can involve full-body shaking, loss of awareness, rapid head movements, or unresponsiveness with eyes closed. Some features are more common in PNES than in epileptic seizures: episodes lasting longer than 10 minutes, out-of-sync limb movements, pelvic thrusting, or shaking episodes where the person retains some awareness. But these aren’t reliable enough for self-diagnosis. The only definitive way to distinguish PNES from epilepsy is through video EEG monitoring, which records both brain activity and physical symptoms simultaneously. If the recording shows seizure-like movements with normal brain wave patterns, the diagnosis is PNES.
Underlying mental health conditions often play a role, including PTSD, anxiety disorders with panic attacks, dissociative disorders, and depression. Treatment focuses on therapy rather than anti-seizure medication, since the episodes aren’t driven by electrical misfiring.
How Seizure Types Are Identified
Figuring out which type of seizure someone has is essential because different types respond to different treatments. The primary tool is an EEG, which measures brainwave patterns and helps classify seizure activity. In some cases, you’ll be monitored on video during the EEG so that physical symptoms can be matched to specific electrical changes in the brain. For people with seizures that happen at night or are hard to catch during a short appointment, ambulatory EEG devices can record brain activity over several days at home.
Brain imaging fills in the other half of the picture. An MRI can reveal structural abnormalities like scarring, tumors, or developmental differences that may be triggering seizures. Functional MRI goes a step further by mapping which parts of the brain control speech, movement, and other critical functions. This becomes particularly important if surgery is ever considered.
A detailed description of what happens before, during, and after a seizure is just as valuable as any scan. If you witness someone having a seizure, noting what happened first (did it start in one part of the body or affect everything at once?), how long it lasted, and whether the person seemed aware can help a doctor narrow down the seizure type far more quickly.