Understanding EEG Results for Tonic‑Clonic Seizures

Understanding the EEG results for tonic‑clonic seizures can feel like decoding a foreign language, but with the right roadmap you’ll know what each spike, wave, and pause really means.

• Identify the three EEG phases that surround a generalized tonic‑clonic seizure.
• Learn the hallmark ictal pattern-high‑frequency polyspikes followed by a burst‑suppression.
• Distinguish true epileptiform activity from common artifacts.
• Apply ACNS‑recommended reporting standards.
• Use the EEG findings to guide treatment decisions.

What the test actually records

Electroencephalogram (EEG) is a non‑invasive neurophysiological test that records electrical activity of the brain via scalp electrodes, typically sampled at 500-1000Hz and displayed as voltage‑time traces. The system captures the summed postsynaptic potentials of cortical pyramidal cells, which means the signal reflects large‑scale neuronal synchrony rather than single‑neuron spikes.

Why tonic‑clonic seizures look the way they do on EEG

Tonic‑clonic seizure is a generalized epileptic event characterized by a tonic phase of muscle stiffening followed by a clonic phase of rhythmic jerking. During the tonic phase, cortical neurons enter a high‑frequency, low‑amplitude firing mode that translates on EEG to a burst of polyspikes-often 20-30Hz-lasting 0.5-2seconds. The subsequent clonic phase produces a rhythmic spike‑and‑wave pattern around 3Hz that mirrors the visible jerks.

Ictal EEG patterns - the “signature” of a tonic‑clonic event

The EEG interpretation of a generalized tonic‑clonic seizure hinges on three hallmarks:

1. Polyspike burst: rapid series of spikes (often >10spikes/sec) that signal the onset of the tonic phase.
2. Generalized spike‑and‑slow wave: 3‑4Hz spikes followed by a slow wave, coinciding with the clonic movements.
3. Post‑ictal suppression: a period of low amplitude (<10µV) activity lasting 10‑30seconds as the brain recovers.

These features are distinct from focal seizures, which usually start with a localized rhythmic discharge that spreads gradually.

Polyspike burst is a rapid series of high‑frequency spikes that marks the onset of the tonic phase in a generalized seizure.

Interictal epileptiform discharges - what you see when the seizure isn’t happening

Interictal epileptiform discharge (IED) is a brief (<100ms) spike or sharp wave that occurs between seizures and signals an increased likelihood of future events. In patients with generalized tonic‑clonic seizures, IEDs typically appear as generalized 2-3Hz spike‑and‑wave bursts, even during wakefulness. Their presence supports a diagnosis of generalized epilepsy and can guide medication choice.

Post‑ictal EEG - the brain’s wind‑down period

Post‑ictal EEG is a recording captured after a seizure, showing a suppression of background activity followed by a gradual return to baseline. The depth and duration of suppression correlate with seizure severity; longer suppression often predicts a transient cognitive “recovery” phase. Recognizing this pattern prevents mislabeling the low‑voltage segment as a technical failure.

Guidelines that keep reports consistent

American Clinical Neurophysiology Society (ACNS) guideline is a consensus document that standardizes terminology, electrode placement, and reporting criteria for EEG in epilepsy. Since 2020, the guideline recommends explicitly noting the three phases (ictal, interictal, post‑ictal) and using the term “generalized polyspike burst” rather than vague descriptors. Following ACNS standards improves inter‑rater reliability and helps neurologists compare studies across centers.

Where the seizure starts - the concept of a seizure onset zone

Seizure onset zone is a brain region that first generates the abnormal electrical discharge during a seizure. In true generalized tonic‑clonic seizures, the onset zone is essentially the whole cortex, which is why the EEG shows simultaneous bilateral activity. Differentiating this from a rapidly propagating focal seizure can affect whether a patient receives broad‑spectrum antiepileptic drugs (AEDs) or considers surgical evaluation.

Common pitfalls - artifacts that mimic seizures

Artifact is a non‑physiological signal, such as muscle movement, eye blink, or electrode malfunction, that contaminates the EEG recording. Muscle (EMG) bursts during a tonic phase can look like high‑frequency polyspikes, but they usually have a broader spectral range and are localized near temporal electrodes. Eye‑blink artifacts generate slow waveforms that can be mistaken for post‑ictal suppression if the montage is limited. Knowing these red flags saves time and avoids unnecessary medication adjustments.

Putting it together - how clinicians act on the EEG

When a neurologist reads an EEG that shows the classic polyspike‑burst → generalized spike‑and‑wave → post‑ictal suppression sequence, they can confidently label the event as a generalized tonic‑clonic seizure. The report then informs several downstream decisions:

• Medication selection: drugs that enhance generalized inhibition (e.g., valproate, levetiracetam) are preferred.
• Risk stratification: frequent IEDs or prolonged post‑ictal suppression may signal higher seizure burden, prompting a dose increase.
• Driving clearance: a documented generalized seizure mandates a minimum three‑month seizure‑free period in most jurisdictions.
• Family counseling: visualizing the EEG pattern helps explain the condition to patients and caregivers.

Related concepts you’ll encounter next

Understanding EEG results opens doors to deeper topics such as sleep‑deprived EEG, which can unmask interictal spikes; quantitative EEG (qEEG), a computer‑based analysis that measures power spectra; and magnetoencephalography (MEG), an imaging technique that pinpoints the seizure onset zone with millimeter precision. Each builds on the basics covered here.

Frequently Asked Questions

Can a single EEG capture a tonic‑clonic seizure?

Yes, but the chance is low because generalized seizures are brief and often occur unpredictably. Long‑term video‑EEG monitoring or a sleep‑deprived study increases the odds of catching an event.

What does “burst‑suppression” mean on a post‑ictal EEG?

Burst‑suppression refers to alternating periods of high‑amplitude activity (bursts) and flat, low‑voltage background (suppression). In the post‑ictal state it reflects neuronal exhaustion after a massive synchronous discharge.

How can I tell muscle artifact from true polyspikes?

Muscle artifact usually shows a broader frequency range (up to 100Hz), appears most prominently in temporal or frontal leads, and often coincides with visible movement. Polyspikes are more uniform, confined to 20-30Hz, and are present across all channels simultaneously.

Why are interictal spikes important if the patient isn’t seizing?

Interictal spikes indicate an underlying hyper‑excitable cortex and predict future seizure risk. Their presence can justify initiating or adjusting antiepileptic medication even in the absence of recent clinical events.

Do antiepileptic drugs change the EEG pattern?

Effective AEDs often reduce the frequency and amplitude of interictal spikes and may shorten the post‑ictal suppression phase. However, some drugs (e.g., benzodiazepines) can introduce diffuse slowing, which must be distinguished from pathology.