A seizure is a burst of uncontrolled electrical activity in the brain. Normally, your brain cells communicate through small, carefully timed electrical signals. During a seizure, large groups of neurons fire simultaneously and excessively, overwhelming the brain’s normal signaling and temporarily disrupting how it functions. Up to 10% of people worldwide will experience at least one seizure during their lifetime, and most of those people will never have another.
What Happens Inside the Brain
Your brain runs on a balance between two types of chemical signals: ones that excite neurons (tell them to fire) and ones that inhibit them (tell them to stay quiet). The main excitatory chemical is glutamate, and the main inhibitory one is GABA. A seizure begins when that balance tips sharply toward excitation, either because there’s too much glutamate activity, too little GABA putting the brakes on, or both.
When inhibition drops or excitation spikes, neurons in one area start firing together in lockstep. Each firing neuron triggers its neighbors through a web of connections, creating a chain reaction. The voltage inside individual neurons shifts dramatically, a phenomenon researchers call a “paroxysmal depolarizing shift,” which is essentially the electrical fingerprint of a neuron being overwhelmed. On a brain wave recording (EEG), this shows up as sharp spikes or spike-and-wave patterns that look nothing like normal brain activity.
This cascade can stay contained in one region of the brain or spread across both sides. How far it spreads, and which parts of the brain it reaches, determines what the seizure looks like from the outside.
Focal vs. Generalized Seizures
Seizures fall into two main categories based on where the abnormal electrical activity starts.
Focal seizures begin on one side of the brain. Because only part of the brain is affected, symptoms depend on what that region normally controls. A focal seizure starting in the area that governs movement might cause rhythmic jerking in one hand or one side of the face. One starting in a sensory region might produce strange smells, a rising feeling in the stomach, or a sudden wave of déjà vu. Some focal seizures leave you fully aware the entire time. Others alter your consciousness, causing you to stare blankly, fumble with your clothes, or smack your lips without realizing it.
Generalized seizures begin on both sides of the brain at once, or at least appear to. They almost always involve a loss of consciousness. The most dramatic type is the tonic-clonic seizure (formerly called grand mal): the body stiffens, then jerks rhythmically for one to three minutes. But generalized seizures can also be subtle. Absence seizures, common in children, look like brief staring spells lasting only a few seconds, sometimes with fluttering eyelids, and can happen dozens of times a day without anyone noticing.
What Causes Them
Seizures have a wide range of triggers and underlying causes. The International League Against Epilepsy groups them into several categories:
- Structural: Physical changes in the brain, such as tumors, traumatic brain injuries, strokes, or abnormal blood vessel formations.
- Genetic: Inherited mutations that affect how ion channels or neurotransmitter receptors work, making neurons more excitable from birth.
- Metabolic: Disruptions in blood sugar, sodium, calcium, or other substances the brain depends on. Severe kidney or liver failure can also trigger seizures.
- Infectious: Brain infections like meningitis or encephalitis, or in some parts of the world, parasitic infections.
- Immune: Conditions where the immune system mistakenly attacks brain tissue.
- Unknown: In many cases, no clear cause is found despite thorough testing.
A single seizure can also be provoked by temporary factors like high fever (especially in young children), alcohol withdrawal, extreme sleep deprivation, or certain drugs. These provoked seizures don’t necessarily mean a person has epilepsy. Epilepsy is diagnosed when someone has recurrent, unprovoked seizures, and it affects roughly 4 to 10 out of every 1,000 people at any given time.
What It Feels Like
The experience varies enormously depending on the type. Some people have a warning sensation seconds before a seizure begins, sometimes called an aura. This is actually a focal seizure in its earliest stage and might feel like a sudden metallic taste, a wave of fear, tingling in one arm, or a visual disturbance like flashing lights.
During a generalized tonic-clonic seizure, the person is unconscious and has no awareness of what’s happening. They won’t remember the event afterward. During a focal seizure with preserved awareness, on the other hand, people sometimes describe feeling “trapped,” fully conscious but unable to speak or stop the involuntary movements.
The Recovery Phase
What happens after a seizure is often as disorienting as the seizure itself. This recovery period, called the postictal state, typically lasts between 5 and 30 minutes but can stretch to a few days after severe seizures.
The most common symptoms include confusion, extreme fatigue, headache or migraine, muscle soreness, memory gaps, and difficulty speaking. Some people experience mood changes like anxiety, agitation, or sudden sadness. More severe seizures can cause temporary loss of consciousness, hallucinations, or loss of bladder control. The brain essentially needs time to restore its normal chemical balance and electrical rhythm after being flooded with unregulated activity.
Seizures vs. Non-Epileptic Events
Not everything that looks like a seizure involves abnormal electrical activity. Psychogenic non-epileptic seizures (PNES) can closely resemble epileptic seizures but are driven by psychological factors rather than electrical misfiring. They’re surprisingly common, accounting for a significant portion of cases seen at epilepsy centers.
There are subtle physical differences that help clinicians distinguish the two. During epileptic seizures, the eyes are almost always open. In a study using video EEG monitoring, 97% of patients with epileptic seizures had their eyes open at seizure onset, while 96% of patients with non-epileptic events had their eyes closed. Epileptic seizures also tend to start abruptly, while non-epileptic events more often build gradually. Side-of-the-tongue biting is common in epileptic seizures, while tip-of-the-tongue biting is more associated with non-epileptic events. People with PNES also tend to reorient quickly afterward, whereas epileptic seizures typically produce that prolonged postictal confusion.
Distinguishing between the two matters because the treatments are completely different. PNES doesn’t respond to anti-seizure medication but often improves with specialized psychological therapy.
How Seizures Are Detected
The primary tool for confirming seizure activity is an EEG, which records electrical patterns across the surface of the brain using sensors placed on the scalp. During a seizure, the EEG picks up characteristic spikes, sharp waves, or spike-and-wave discharges that look distinctly different from normal brain rhythms. In focal seizures, these abnormal patterns cluster in one specific area, which can help pinpoint where in the brain the seizures originate. In generalized seizures, the discharges appear across both sides of the brain simultaneously.
An EEG can also pick up epileptiform abnormalities between seizures, subtle electrical patterns that suggest the brain has a tendency toward seizure activity even when one isn’t actively happening. Brain imaging, blood work, and other tests help identify the underlying cause, whether structural, metabolic, or something else.