A seizure is a sudden burst of abnormal electrical activity in the brain that temporarily changes how you move, feel, think, or behave. Up to 10% of people worldwide will experience at least one seizure during their lifetime, and a single seizure does not necessarily mean a person has epilepsy.
What Happens in the Brain During a Seizure
Your brain runs on electrical signals. Billions of nerve cells communicate by firing tiny electrical impulses in coordinated patterns. During a seizure, a group of neurons begins firing in an uncontrolled, synchronized way, and this wave of abnormal activity spreads to neighboring groups of cells. Think of it like a power surge rippling through a circuit.
This surge happens because of an imbalance between two forces in the brain: excitation (signals that tell neurons to fire) and inhibition (signals that tell them to stop). When excitation overwhelms inhibition, the result is a seizure. The specific symptoms depend entirely on where in the brain the abnormal firing starts and how far it spreads. A seizure confined to the area that controls one hand might cause that hand to twitch. One that sweeps across both sides of the brain can cause a person to lose consciousness and convulse.
Types of Seizures
Seizures fall into three broad categories based on where the abnormal activity begins.
Focal onset seizures start in one specific area of one hemisphere. You might stay fully aware during a focal seizure, noticing odd sensations, involuntary movements, or a wave of déjà vu. Or your awareness might become impaired, leaving you confused, staring blankly, or making repetitive motions like lip smacking or hand rubbing. Because the activity starts in one spot, the symptoms often reflect whatever that brain region normally does.
Generalized onset seizures appear to involve both hemispheres from the very start. The most recognizable type is the tonic-clonic seizure (formerly called grand mal), in which the body stiffens, the person falls, and rhythmic jerking follows. But generalized seizures also include absence seizures, which look like brief staring spells lasting only a few seconds and are especially common in children. Other forms involve sudden muscle jerks, a sudden loss of muscle tone that causes collapse, or brief episodes of stiffening without jerking.
Unknown onset seizures are the third category, used when doctors can’t determine where the activity originated, often because no one witnessed the beginning or testing isn’t conclusive.
The Four Phases of a Seizure
Not every seizure follows all four phases, but understanding them helps you recognize what’s happening before, during, and after.
- Prodrome: Hours or even days before a seizure, some people notice subtle changes: mood shifts, headaches, irritability, or a sense that something is “off.” Not everyone experiences this, but those who do often learn to recognize the pattern over time.
- Aura: This is actually the very beginning of the seizure itself, though it’s often called a warning. It might feel like a rising sensation in the stomach, an unusual smell or taste, tingling, fear, or visual disturbances. Auras are most common with focal seizures.
- Ictal phase: The seizure event itself. This is the period of active abnormal electrical firing, and it can last anywhere from a few seconds to several minutes. Symptoms range from brief staring to full-body convulsions.
- Postictal phase: The recovery period after the electrical storm passes. Confusion, fatigue, headache, muscle soreness, and difficulty speaking are common. This phase can last minutes to hours. Some people fall into a deep sleep afterward.
Common Causes and Triggers
Seizures can happen to anyone, and they don’t always mean something is chronically wrong with the brain. Many seizures are “acute symptomatic,” meaning a temporary condition triggered the event.
Common causes include abnormal blood sugar (too high or too low), electrolyte imbalances like low sodium or calcium, high fevers (especially in young children), head trauma, stroke, and brain infections. Certain medications can also lower the seizure threshold, particularly at high doses or when started or stopped abruptly. Alcohol withdrawal is another well-known trigger.
For people who have epilepsy, seizures can be provoked by sleep deprivation, stress, illness, flashing lights (in a subset of people), missed medications, or hormonal changes. Sometimes no identifiable trigger exists at all.
One Seizure vs. Epilepsy
A single seizure is not the same as epilepsy. Epilepsy is diagnosed when a person has had at least two unprovoked seizures more than 24 hours apart. It can also be diagnosed after just one unprovoked seizure if testing shows a high likelihood (60% or greater) that more seizures will follow over the next decade, or if the person fits a recognized epilepsy syndrome.
The distinction matters because treatment decisions hinge on it. Someone who has a seizure caused by a correctable problem, like dangerously low blood sugar, may never have another one once the underlying issue is fixed. A person diagnosed with epilepsy typically needs ongoing management.
How Seizures Are Diagnosed
The starting point is almost always a detailed history: what happened before, during, and after the event, how long it lasted, and whether anything similar has occurred before. Eyewitness accounts are especially valuable since the person who had the seizure often can’t remember the details.
The most important test is an electroencephalogram, or EEG, which records electrical activity in the brain through sensors placed on the scalp. It can reveal abnormal patterns and help identify the type of seizure and where in the brain it originates. A routine EEG takes minutes to a couple of hours. In some cases, doctors use prolonged video-EEG monitoring, where a camera captures the person’s behavior while the EEG runs simultaneously, sometimes over days in a hospital setting.
Brain imaging, most commonly an MRI, helps identify structural causes like tumors, scarring, or blood vessel abnormalities. Other specialized scans, including PET and SPECT, can show areas of unusual metabolic activity in the brain. Neuropsychological testing may be added to evaluate how thinking and memory are affected.
What to Do If Someone Has a Seizure
If you witness a seizure, the most important thing is to keep the person safe, not to stop the seizure itself. Stay with them. Move away hard or sharp objects. If they’re lying down, gently roll them onto their side with their mouth pointing toward the ground to keep the airway clear. Put something soft under their head if possible. Loosen anything tight around the neck. Remove their glasses.
Time the seizure. If it lasts longer than 5 minutes, call 911. Also call for help if they have a second seizure shortly after the first, have trouble breathing or waking up afterward, were injured during the seizure, had the seizure in water, are pregnant, have diabetes and lost consciousness, or have never had a seizure before.
Do not put anything in their mouth. Do not try to hold them down. Do not attempt to give food or water until they’re fully alert. Once the seizure ends, stay nearby, calmly explain what happened, and help them get to a safe, comfortable spot while they recover.