A seizure is a sudden burst of uncontrolled electrical activity in the brain. Normally, brain cells communicate through small, organized electrical signals. During a seizure, large groups of neurons fire at the same time in an abnormal, synchronized pattern, temporarily disrupting how the brain controls movement, sensation, behavior, or consciousness. Up to 10% of people worldwide will experience at least one seizure during their lifetime.
What Happens in the Brain During a Seizure
Your brain runs on a constant balance between signals that excite neurons and signals that calm them down. A seizure starts when that balance tips too far toward excitation. This can happen because excitatory signaling ramps up, inhibitory signaling weakens, or both occur at once. The result is a cascade: neurons begin firing in rapid, high-frequency bursts and recruit neighboring neurons to join in, creating a wave of synchronized electrical activity that overwhelms the brain’s normal function.
Under normal circumstances, surrounding inhibitory neurons act like a firebreak, preventing bursting activity from spreading. When that containment fails, the electrical storm spreads across larger brain regions, and the seizure’s visible symptoms appear. What those symptoms look like depends entirely on which part of the brain is affected and how far the abnormal activity spreads.
Types of Seizures
Seizures fall into two broad categories based on where the abnormal electrical activity begins: focal onset seizures, which start in one specific area of the brain, and generalized onset seizures, which involve both sides of the brain from the start.
Focal Onset Seizures
These used to be called “partial seizures.” Because they start in one brain region, the symptoms reflect whatever that region controls. A focal seizure in the area that governs hand movement might cause one hand to twitch. One in a sensory area might produce tingling, strange smells, or visual disturbances. The key distinction within this group is awareness: some people remain fully conscious throughout (focal aware seizures), while others become confused or unresponsive (focal impaired awareness seizures). A focal seizure can sometimes spread and become a generalized seizure.
Generalized Seizures
These affect the entire brain at once and come in several forms:
- Tonic-clonic seizures: The most recognized type. The body stiffens (tonic phase), then the arms and legs jerk rhythmically (clonic phase). The person loses consciousness, may have difficulty breathing, and their lips can turn blue. The jerking gradually slows and stops, often followed by a deep sigh as normal breathing resumes.
- Absence seizures: Sometimes mistaken for daydreaming. The person suddenly stops what they’re doing, stares blankly for about 15 seconds or less, then snaps back to full alertness with no memory of the episode. These are most common in children.
- Tonic seizures: The muscles stiffen without the jerking phase. The eyes may roll back, the back arches, and chest muscle contractions make breathing difficult.
- Clonic seizures: Repeated jerking movements of the face, neck, and arms without the initial stiffening.
- Atonic seizures: Sometimes called “drop attacks.” Muscle tone suddenly disappears, causing the person to go limp and collapse, which can lead to injuries from the fall.
The Four Phases of a Seizure
Not every seizure follows every phase, but a full seizure event can have four distinct stages. Recognizing them helps you understand what the person is experiencing before, during, and after.
The prodromal phase can begin hours or even days before the seizure itself. People often report vague warning signs like anxiety, irritability, headache, or mood changes. Next comes the aura, which is actually the very beginning of seizure activity in the brain. Common aura experiences include déjà vu, a strange taste or smell, dizziness, nausea, tingling, visual changes, or a sudden rush of emotion like fear or joy. Not everyone gets an aura, but those who do often learn to recognize it as a signal that a seizure is coming.
The ictal phase is the seizure itself, the part most people picture when they think of the word. Depending on the type, it can involve stiffening, jerking, staring, confusion, loss of bladder control, drooling, repetitive movements like lip-smacking, or a racing heart. It typically lasts from a few seconds to a couple of minutes.
The postictal phase is the recovery period afterward. It can last minutes to hours and commonly brings confusion, drowsiness, headache, body soreness, difficulty finding words, memory loss, and feelings of fear or sadness. Some people feel wiped out for the rest of the day.
Common Causes and Triggers
Anything that disrupts the brain’s balance between excitation and inhibition can cause a seizure. Some of the most common triggers include high fever (especially in young children, called febrile seizures), head injuries, stroke, brain infections like meningitis, and brain tumors. Low blood sugar is a well-known trigger because glucose is the brain’s primary fuel source, and when supply drops, neurons become unstable.
Electrolyte imbalances, particularly low sodium levels, can also provoke seizures, as can kidney or liver disease. Sleep deprivation, heavy alcohol use or sudden withdrawal from alcohol, and certain medications lower the seizure threshold in some people. Flashing lights trigger seizures in a small subset of people with photosensitive epilepsy, but this is far less common than pop culture suggests.
In many cases, a seizure has a clear, identifiable cause. These are called “provoked” seizures. When no obvious cause can be found, the seizure is classified as “unprovoked,” and that distinction matters for diagnosis.
Seizures vs. Epilepsy
Having a seizure does not mean you have epilepsy. A single seizure can be a one-time event triggered by something temporary like a high fever, a medication reaction, or extremely low blood sugar. Epilepsy is generally diagnosed after a person has had two or more unprovoked seizures separated by at least 24 hours. The distinction is important: about 1 in 10 people will have a seizure at some point, but only a fraction of them will go on to develop epilepsy.
How Seizures Are Diagnosed
After a first seizure, doctors typically order two key tests. An EEG (electroencephalogram) measures electrical activity in the brain and can detect abnormal patterns even between seizures. An MRI of the brain looks for structural issues like tumors, scar tissue, or malformations that could be causing the seizures. Blood tests check for metabolic causes, including blood sugar levels, electrolyte imbalances, and signs of infection or organ dysfunction. If an infection in the brain is suspected, a lumbar puncture (spinal tap) may be needed to test the fluid surrounding the brain and spinal cord.
How to Help Someone Having a Seizure
If you see someone having a tonic-clonic seizure, the most important thing is to keep them safe and let the seizure run its course. Ease them to the ground if they’re falling. Clear away furniture, sharp objects, or anything they could hit. Place something soft under their head, like a folded jacket. Gently turn them onto their side with their mouth pointing toward the ground to keep the airway clear. Loosen anything tight around the neck. Remove their glasses if they’re wearing any.
Time the seizure from the moment it starts. If it lasts longer than five minutes, call 911. Also call for emergency help if the person doesn’t regain consciousness afterward, has a second seizure shortly after the first, is injured during the seizure, has never had a seizure before, or is pregnant.
What you should never do: don’t hold the person down or try to restrain their movements. Don’t put anything in their mouth. The old idea about swallowing the tongue is a myth, and putting objects in the mouth can break teeth or cause a jaw injury. Don’t attempt mouth-to-mouth breathing during the seizure; normal breathing almost always resumes on its own. Don’t offer food or water until the person is fully alert, as they could choke.
Once the seizure ends, stay with the person. They’ll likely be confused and disoriented. Calmly explain what happened, help them sit somewhere safe, and offer to call someone who can help them get home.