A seizure is a surge of uncontrolled electrical activity in the brain. Normally, your brain cells communicate through carefully timed electrical signals, with some cells encouraging activity and others dampening it. During a seizure, that balance breaks down. Large groups of neurons start firing simultaneously and excessively, overwhelming the brain’s normal signaling and producing the physical and mental symptoms you can see from the outside. Up to 10% of people worldwide will experience at least one seizure during their lifetime.
The Balance Between Excitation and Inhibition
Your brain runs on a push-and-pull system. Some chemical messengers (neurotransmitters) tell neurons to fire, while others tell them to stay quiet. The main “quiet down” signal comes from a neurotransmitter called GABA, the most common inhibitory neurotransmitter in the central nervous system. GABA lands on receptors on a neuron’s surface and reduces that cell’s ability to receive, create, or send messages to other cells. Think of it as a brake pedal for brain activity.
On the other side, excitatory neurotransmitters like glutamate act as the gas pedal, encouraging neurons to fire. In a healthy brain, these two forces stay roughly in balance. A seizure happens when excitation wins, either because there’s too much “gas” or not enough “brake.” The result is a runaway chain reaction: one group of neurons fires too aggressively, which triggers neighboring neurons, which triggers more, spreading the abnormal electrical storm across part or all of the brain.
What Happens at the Cellular Level
The firing of a neuron depends on tiny channels in its membrane that open and close to let charged particles (ions) flow in and out. Sodium channels let positive charge rush in to trigger a signal. Potassium channels then restore the cell to its resting state by letting positive charge flow back out, a process called repolarization. This reset is what keeps neurons from firing endlessly.
In seizure-prone brains, these channels can malfunction. If potassium channels don’t work properly, the neuron struggles to reset itself after firing. Worse, when repolarization happens too quickly, it can actually prime sodium channels to fire again sooner, increasing the rate and synchronization of signals across large networks of neurons. The effect is like a crowd doing the wave in a stadium: once the pattern starts, individual cells get swept up in the collective rhythm rather than acting independently.
How a Seizure Spreads
Between each neuron is a tiny fluid-filled gap called a synapse. Neurotransmitters carry signals across that gap, landing on specific receptors on the next cell. During a seizure, this relay system gets hijacked. Neurons release excessive excitatory signals across their synapses, recruiting more and more cells into synchronized firing. Whether the seizure stays localized or spreads across the whole brain determines what type of seizure it becomes and what it looks like from the outside.
Focal vs. Generalized Seizures
Seizures fall into categories based on where and how they start. The International League Against Epilepsy recognizes four main classes: focal, generalized, unknown, and unclassified.
- Focal seizures begin in one specific area of the brain. Because only part of the brain is involved, the symptoms depend on which region is affected. A focal seizure in the area controlling your right hand might cause involuntary twitching in that hand. One in a sensory region might produce strange smells or visual disturbances. You may or may not lose consciousness during a focal seizure.
- Generalized seizures involve both sides of the brain from the start. These are the ones most people picture when they hear the word “seizure”: the body stiffens, muscles jerk rhythmically, and consciousness is lost. But generalized seizures also include absence seizures, where a person simply blanks out for a few seconds and stares into space.
Doctors classify seizures by the sequence of signs and symptoms as they unfold, not just the first thing that happens. A focal seizure can also spread to become generalized, starting with a twitch in one area and then overtaking the whole brain.
The Three Phases of a Seizure
A seizure typically moves through distinct stages, though not everyone experiences all three.
The first is the aura, which some people feel before the seizure fully takes hold. This is actually a small focal seizure in itself. It might show up as a strange taste, a feeling of déjà vu, a rising sensation in the stomach, or sudden anxiety. Not everyone gets an aura, but for those who do, it can serve as a brief warning.
The second phase is the seizure itself, called the ictal phase. This is when the abnormal electrical activity is at its peak. Depending on the type, it can involve convulsions, muscle stiffening, staring spells, repetitive movements like lip smacking, or loss of consciousness. Most seizures last between 30 seconds and two minutes.
The third phase is the postictal state, the recovery period after the electrical storm subsides. This is when the brain is essentially rebooting. Common symptoms include headache, confusion, fatigue, memory loss, muscle soreness, difficulty speaking, and mood changes like anxiety or agitation. Some people also experience nausea, abnormal heart rate, or loss of bladder control. The postictal state averages five to 30 minutes but can last anywhere from a few minutes to a few days. More severe seizures tend to produce longer, more intense recovery periods.
What Lowers the Seizure Threshold
Everyone has what doctors call a seizure threshold, the point at which normal brain activity tips over into a seizure. Some people have naturally lower thresholds due to genetics, brain injuries, or neurological conditions. But even in people with epilepsy, seizures don’t happen constantly. They occur when something pushes the brain past its tipping point.
The most well-established triggers include sleep deprivation, stress, alcohol (particularly withdrawal after heavy use), fever, and menstruation. In children, infections with fever are an especially common trigger. Flashing lights can also provoke seizures in people with photosensitivity. The most dangerous range is 5 to 30 flashes per second. This is why the Epilepsy Foundation recommends that emergency systems like fire alarms keep their flash rate below 2 flashes per second.
Certain medications can also lower the seizure threshold. These span many categories, from some antibiotics and antidepressants to certain pain medications, antipsychotics, and even some bronchodilators used for asthma. This doesn’t mean these drugs cause seizures in most people, but they can increase risk in someone already prone to them.
When a Seizure Becomes an Emergency
Most seizures stop on their own within two minutes. A seizure that lasts longer than five minutes, or multiple seizures without the person regaining normal consciousness in between, is classified as status epilepticus. This is a medical emergency because prolonged uncontrolled electrical activity can damage brain cells. The longer it goes on, the harder it becomes to stop and the greater the risk of lasting harm.
What an EEG Actually Shows
An electroencephalogram, or EEG, measures electrical activity across the scalp and is the primary tool for evaluating seizures. During a seizure, the EEG picks up characteristic “spike and wave” patterns: a sharp spike of electrical activity lasting 20 to 70 milliseconds, followed by a slower wave. These spikes must appear on at least two channels simultaneously to be considered significant, helping distinguish seizure activity from normal background brain noise. Between seizures, some people with epilepsy still show abnormal spikes on their EEG, which helps doctors confirm the diagnosis and identify where in the brain seizures originate.