1  What is a Seizure?

A seizure is a transient occurrence of symptoms and/or signs due to abnormal excessive or synchronous neuronal activity in the brain.

Think of it like an electrical storm — instead of brain cells communicating in their usual, organized way, many neurons fire at once in an abnormal pattern.

Important Distinction

Having a seizure does not necessarily mean that a person has epilepsy, unless the specific criteria for diagnosis of epilepsy are met. Many people have a single seizure in their lifetime and never have another. See the next chapter for details on how diagnosis of epilepsy is made.

1.1 The Brain’s Normal Electrical Activity

Your brain’s cells communicate via electrical signals. Billions of nerve cells (neurons) are constantly sending messages to each other, coordinating everything from breathing, to thinking, to moving. This activity is carefully controlled and organized.

During a seizure, this control breaks down. Groups of neurons begin firing excessively and synchronously — like a crowd of people all shouting at once instead of having an orderly conversation.

1.2 What Happens During a Seizure?

The features of a seizure depend entirely on which part of the brain is affected and how the abnormal electrical activity spreads. For example:

  • If the seizure involves the movement areas of the brain, you might see rhythmic jerking or stiffening of the arms or legs
  • If it involves the vision area, the person might see flashing lights or colors that aren’t there
  • If it affects areas controlling awareness, the person may stare blankly and become unresponsive

1.3 Classification of Seizures

Epileptic seizures fall broadly into three main categories based on where they start in the brain:

  1. Focal onset seizures — start in one area of the brain
  2. Generalized onset seizures — involve both hemispheres of the brain from the start
  3. Unknown onset seizures — the beginning of the seizure wasn’t witnessed or seen on testing

1.3.1 Focal Onset Seizures

Focal onset seizures (previously called partial seizures) begin in networks limited to one hemisphere of the brain. They can then be further classified based on the level of awareness:

Focal Aware Seizures

  • The person remains awake and aware during the seizure
  • They can usually remember what happened
  • Features might include:
    • Jerking or stiffening of one arm or leg
    • Unusual sensations (tingling, numbness, strange smells or tastes)
    • Sudden emotions (fear, déjà vu, pleasure)
    • Visual changes (flashing lights, distortions)
    • Speech difficulties while remaining aware
    • Autonomic features (heart racing, sweating, goosebumps)

Focal Impaired Awareness Seizures

  • The person’s awareness or consciousness is affected
  • They may seem confused or “out of it”
  • They typically don’t remember the seizure afterward
  • Common features include:
    • Staring blankly
    • Automatisms (lip smacking, chewing, picking at clothes, fumbling)
    • Wandering or performing repetitive movements
    • Not responding when spoken to

Focal to Bilateral Tonic-Clonic Seizures

  • Starts as a focal seizure in one hemisphere
  • Then spreads to involve both hemispheres
  • Evolves into a bilateral tonic-clonic seizure
  • Previously called “secondarily generalized seizures”

Focal seizures can also be described by their motor or non-motor features, which helps doctors understand which brain regions are involved.

1.3.2 Generalized Onset Seizures

Generalized onset seizures involve bilateral networks from the beginning — meaning both hemispheres of the brain are involved from the start. Awareness is typically impaired in most generalized seizures.

Motor Generalized Seizures:

Tonic-Clonic Seizures

  • Has two phases:
    • Tonic phase: Body suddenly stiffens, person falls if standing, may cry out as air is forced out
    • Clonic phase: Rhythmic jerking of arms and legs
  • Usually lasts 1-3 minutes
  • Person may bite tongue, lose bladder control
  • Followed by postictal confusion and drowsiness

Tonic Seizures

  • Sudden bilateral stiffening of muscles
  • If standing, the person usually falls
  • Brief, typically lasting less than 20 seconds
  • Most common during sleep

Clonic Seizures

  • Bilateral rhythmic jerking movements
  • Less common as an isolated seizure type
  • Usually brief

Myoclonic Seizures

  • Brief, shock-like jerks of muscles
  • Can affect arms, legs, or the whole body
  • Like a sudden jolt or twitch
  • Very brief (a fraction of a second to 1-2 seconds)
  • Person remains conscious
  • Can occur in clusters, especially in the morning

Atonic Seizures

  • Sudden loss of muscle tone
  • Person collapses or “drops”
  • Also called “drop attacks”
  • Can cause injuries from falling
  • Very brief, lasting only 1-2 seconds
  • Awareness quickly returns

Non-Motor Generalized Seizures:

Absence Seizures

  • Brief episodes of impaired awareness (usually 3-10 seconds)
  • The person suddenly stops activity and stares
  • May have subtle eye fluttering or slight facial movements
  • Quickly returns to normal with no memory of the episode
  • Can happen many times per day (even hundreds)
  • Often mistaken for daydreaming or inattention
  • Can be typical (more common, abrupt start/stop) or atypical (longer, slower onset/offset)

1.3.3 Unknown Onset Seizures

Sometimes the beginning of a seizure is not witnessed or captured on video/EEG. These are classified as unknown onset until more information becomes available. They may later be reclassified as focal or generalized if additional features or testing provide clarity.

1.4 Epilepsy Imitators

It’s important to note that not all events that look like seizures are actually seizures. There are many conditions that can cause paroxysmal (sudden, episodic) events that can mimic seizures. These include:

  • Syncope (fainting)
  • Movement disorders
  • Sleep disorders (like night terrors or sleepwalking)
  • Migraine with aura
  • Panic attacks
  • Non-epileptic seizures (psychogenic or dissociative seizures)
  • Cardiac arrhythmias

Your doctor will carefully evaluate your symptoms to distinguish true epileptic seizures from these epilepsy imitators.

1.5 After a Seizure: The Postictal Period

After the abnormal electrical activity passes, the brain needs time to recover. This recovery period is called the postictal period. Features during this time might include:

  • Confusion or disorientation
  • Profound tiredness and need to sleep
  • Difficulty speaking (postictal aphasia)
  • Weakness on one side of the body (Todd’s paralysis)
  • Headache

These postictal features are completely normal and usually resolve within minutes to hours. The duration and features of the postictal period can sometimes provide clues about where the seizure started in the brain.

1.6 What Caused the Seizure?

An important distinction that your doctor will make is whether the seizure was provoked (meaning triggered by some external cause) or unprovoked (no clear cause).

Provoking factors can include:

  • Fever (especially in young children — febrile seizures)
  • Sleep deprivation
  • Metabolic disturbances (low blood sugar, low sodium)
  • Medications or drug/alcohol withdrawal
  • Head injury
  • Stroke or other acute brain injuries

1.7 The Bottom Line

A seizure is a transient event caused by abnormal excessive or synchronous neuronal activity in the brain. Understanding the type of seizure and its features is essential for accurate diagnosis and appropriate treatment. While seizures can be frightening to witness, most are brief and self-limited.

In the next chapter, we’ll discuss the difference between having a seizure and having epilepsy, and when the diagnosis of epilepsy is made.