Interictal epileptiform discharges (IEDs) — spikes, sharp waves, polyspikes, and spike-and-wave complexes — are the EEG signature of epileptogenicity. Recognizing them confidently is the central skill of epilepsy EEG interpretation. The diagnostic challenge is that IEDs share waveform features with several benign normal variants, with artifacts, and with subclinical patterns of unclear significance. Overcalling IEDs leads to incorrect epilepsy diagnoses and unnecessary treatment; undercalling them misses real disease. This page covers the canonical IED morphologies, the criteria that distinguish them from non-epileptic mimics, and the syndromic patterns they suggest.

Definition and Criteria

The 2017 International Federation of Clinical Neurophysiology (IFCN) consensus criteria require at least four of six features for a discharge to be classified as an IED:

Di- or triphasic waves with sharp or spiky morphology.
Wave duration different from the ongoing background (usually shorter).
Asymmetry of the waveform — a steep ascending phase and slower descending phase, or vice versa.
An aftercoming slow wave, often more prominent than the spike itself.
Distribution following a physiologic field (not confined to a single electrode).
Disturbance of the surrounding background activity.

A discharge meeting all six is unequivocally epileptiform. Four or five features make it likely. Three or fewer suggest benign or artifact.

Spike vs Sharp Wave

Spike: 20–70 ms duration, very brief, “spiky” appearance.
Sharp wave: 70–200 ms duration, slightly broader but still distinct from background.
Polyspike: multiple spikes in rapid succession.
Spike-and-wave: spike followed by a slow wave; the slow wave is often the most prominent feature.
Polyspike-and-wave: multiple spikes followed by a slow wave; common in generalized epilepsies.

Canonical Patterns by Syndrome

Focal Epileptiform Patterns

Temporal Lobe Spikes/Sharp Waves

Most common location for IEDs in adult epilepsy.
Maximum at F7/F8 (anterior temporal) or T3/T4 (mid-temporal) or T5/T6 (posterior temporal).
Phase reversal in longitudinal bipolar montage between adjacent electrodes — pinpoints the source.
Aftercoming slow wave, often with broader distribution than the spike itself.
Frequently activated by sleep.
Mesial temporal sclerosis often shows F7 or T3 maximum left, F8 or T4 right.
Bilateral independent temporal IEDs (BITS) suggest bilateral temporal disease, but can also occur in unilateral disease.

Centrotemporal Spikes (Rolandic)

Childhood: classic of benign epilepsy with centrotemporal spikes (BECTS, also called benign Rolandic epilepsy).
High-amplitude di- or triphasic sharp waves over central/temporal regions (C3/C4, T3/T4).
Increase markedly during drowsiness and sleep.
Typical clinical presentation: nocturnal focal motor seizures (face, oral region) in school-age children.

Occipital Spikes

Childhood occipital epilepsy syndromes (Panayiotopoulos, Gastaut type).
Posterior dominant spikes or polyspikes.
Activated by eye closure.

Frontal Spikes

Frontal lobe epilepsies, both lesional and genetic.
Maximum at Fp1/Fp2, F3/F4, or F7/F8.
Activated by sleep.
Often nocturnal seizures.

Generalized Epileptiform Patterns

3-Hz Spike-and-Wave (Typical Absence)

Bilateral synchronous spike-and-wave at exactly 3 Hz (slowing to 2–2.5 Hz at end of burst).
Generalized but often anterior maximum.
Burst duration typically 5–20 seconds.
Triggered by hyperventilation in childhood absence epilepsy.
Stops abruptly with clinical and electrical end of seizure.

4–6 Hz Polyspike-and-Wave (JME)

Juvenile myoclonic epilepsy.
Bilateral synchronous polyspike-and-wave at 4–6 Hz.
Often during awakening or after sleep deprivation.
Associated with morning myoclonic jerks.

1–2.5 Hz Slow Spike-and-Wave (Lennox-Gastaut)

Bilateral synchronous slow spike-and-wave at 1–2.5 Hz.
Generalized, sometimes shifting maximum.
Background often slow.
Multiple seizure types: tonic, atonic, atypical absence.

Generalized Paroxysmal Fast Activity (GPFA)

Bilateral synchronous bursts of fast (10–25 Hz) low-amplitude activity, 1–10 seconds duration.
Common in Lennox-Gastaut.
Often associated with tonic seizures during sleep.

Field Pattern and Localization

Phase Reversal in Bipolar Montage

When source lies between two adjacent electrodes, both record the field with opposite polarity.
In longitudinal bipolar (e.g., F7-T3, T3-T5), a temporal spike maximal at T3 shows:
- F7-T3: negative (T3 is the active electrode and more negative).
- T3-T5: positive (T3 is the reference, less negative than T5 below).
This phase reversal pinpoints T3 as the source.
Source is between the two channels showing maximum opposite polarity.

Phase Reversal in Transverse Bipolar

For lateralizing focal sources.
F7-Fp1 vs Fp1-Fp2 vs Fp2-F8: phase reversal at Fp1 or Fp2 locates the source.

Topographic Maps

Modern digital EEG software displays voltage topography at instantaneous time points.
Helps confirm whether scalp field is consistent with focal cortical source.
Best used in addition to, not instead of, classic montage review.

Distinguishing IEDs from Mimics

From Wickets

Wickets are arciform (arch-shaped); IEDs are di- or triphasic with sharp slope changes.
Wickets do not have an aftercoming slow wave; IEDs do.
Wickets occur in trains within rhythmic temporal alpha; IEDs occur in isolation.

From BETS

BETS are small, stereotyped, occur only in sleep; IEDs occur in wake and sleep.
BETS have no aftercoming slow wave; IEDs have one.
BETS are bilateral or asynchronous; IEDs often have a clear unilateral focus.

From Vertex Waves

Vertex waves are at Cz only, stereotyped, common in drowsy/N1.
IEDs follow neuroanatomic patterns, not strictly vertex.

From POSTS

POSTS are occipital, positive polarity, common in light sleep.
Occipital IEDs are negative polarity and follow distinct epileptic patterns.

From Muscle Artifact

Muscle bursts are very brief (<20 ms), high-frequency, irregular shape.
IEDs are longer (20–200 ms), more stereotyped.
Muscle bursts often concentrated at frontal and temporal regions.

From Electrode Pop

Electrode pop is limited to one channel.
IEDs have a field — affect multiple channels.

From Eye Movement

Eye movements have characteristic frontal-symmetric pattern with downgoing deflection.
Horizontal eye movements have opposite polarity at F7 vs F8.
Concordant with EOG channel.

Significance of IED Frequency

Frequent IEDs (>1 per minute) suggest active epilepsy.
Rare IEDs (1 per 24 hours) still indicate epileptogenic potential but lower clinical activity.
Increase in IED frequency during medication taper: high risk for seizure recurrence.
Decrease in IED frequency with treatment: marker of treatment response.

Bilateral Independent Temporal IEDs (BITS)

Independent epileptiform discharges at both left and right temporal regions.
Often seen in bilateral mesial temporal sclerosis.
Can also occur in unilateral disease (mirror foci).
Diagnostic of bilateral disease only with intracranial confirmation.

Special IED Patterns

Hypsarrhythmia

Pattern of infantile spasms (West syndrome).
Disorganized, high-amplitude (>200 μV) background with multifocal spikes.
Chaotic appearance: spikes occur asynchronously across regions.
Often persists in slow-wave sleep.
“Modified hypsarrhythmia” if focal predominance or some preserved organization.

Burst-Suppression

Bursts of high-amplitude activity (spikes, sharp waves, or polyspikes) separated by periods of suppression.
Severe encephalopathy (toxic/metabolic, post-cardiac arrest, advanced epileptic encephalopathy).
Pediatric early myoclonic encephalopathy and Ohtahara syndrome.
Pharmacologic burst-suppression: induced by anesthetic infusion in refractory status.

Continuous Spike-Wave of Sleep (CSWS)

Electrical status epilepticus during slow-wave sleep.
Continuous bilateral spike-wave occupying >85% of slow-wave sleep.
Associated with regression of language or cognition in childhood.
Landau-Kleffner syndrome: specific form with language regression.

Practical Approach to Reading for IEDs

Review wake background first.
Look for sharp transients with the IFCN 6-feature criteria.
Check field pattern (phase reversal localization).
Check stereotypy within the recording.
Check for aftercoming slow wave.
Compare across drowsiness, sleep, hyperventilation, photic stimulation.
Distinguish from variants (wickets, BETS, etc.).
Distinguish from artifacts.
Categorize by location and pattern → syndromic classification when possible.

🔍 Did You Know?

The presence of even a single, well-defined IED has substantial diagnostic value for epilepsy — but its absence does not exclude the disease. Quantitative studies have established that a single definite IED on the first EEG has specificity of approximately 95% for epilepsy, meaning ~5% of patients with that finding will turn out not to have epilepsy. The sensitivity of a single routine 20-minute EEG, however, is only ~50%. This asymmetry — high specificity, modest sensitivity — drives the clinical workflow: a normal first EEG should be followed by a sleep-deprived study, then ambulatory or video EEG if needed, before concluding that “no IEDs were ever captured.” Conversely, a definite IED on first EEG strongly supports an epilepsy diagnosis, but the discharge must meet the IFCN 6-feature criteria — a single sharp transient that meets only 3 criteria is not enough. The diagnostic implication for clinicians is that reading “IED” should require rigorous criteria, but capturing one really does mean something. The same principle applies in reverse: a patient with classic seizure semiology but consistently negative EEGs across multiple studies still likely has epilepsy — they may have deeply seated cortical sources invisible at the scalp, or their IEDs may be very rare. The EEG is a diagnostic tool, not the gold standard; the clinical syndrome is the gold standard.

Pitfalls and Pearls

IFCN 6 features: di-/triphasic, asymmetric, duration different from background, aftercoming slow wave, distribution with field, disturbance of background.
Aftercoming slow wave: the most reliable distinguishing feature from benign mimics.
Spike (20–70 ms) vs sharp wave (70–200 ms); polyspike and spike-and-wave variations.
Phase reversal in bipolar: localizes the source between channels.
Temporal IEDs: most common in adult focal epilepsy; mesial temporal sclerosis.
Centrotemporal spikes: BECTS in school-age children.
3-Hz spike-wave: childhood absence epilepsy; triggered by HV.
4–6 Hz polyspike-wave: juvenile myoclonic epilepsy.
1–2.5 Hz slow spike-wave: Lennox-Gastaut.
GPFA (generalized paroxysmal fast activity): 10–25 Hz bursts; LGS.
Hypsarrhythmia: West syndrome; chaotic high-amplitude background with multifocal spikes.
CSWS: continuous spike-wave of sleep; associated with language/cognitive regression.
BITS: bilateral independent temporal IEDs; suggest bilateral disease but can occur in unilateral.
Distinguish from wickets, BETS, vertex waves, POSTS, muscle, pop, eye movement.
Single definite IED: specificity ~95% for epilepsy.
Routine EEG sensitivity: ~50%; rises to 80%+ with sleep + activation.
If clinical suspicion high but EEG negative: repeat with sleep, ambulatory, or video EEG.

References

Kane N, Acharya J, Beniczky S, et al. A revised glossary of terms most commonly used by clinical electroencephalographers and updated proposal for the report format of the EEG findings. Revision 2017. Clin Neurophysiol Pract. 2017;2:170-185.
Ebersole JS, Husain AM, Nordli DR Jr, eds. Current Practice of Clinical Electroencephalography. 4th ed. Wolters Kluwer; 2014.
Pillai J, Sperling MR. Interictal EEG and the diagnosis of epilepsy. Epilepsia. 2006;47 Suppl 1:14-22.
Salinsky M, Kanter R, Dasheiff RM. Effectiveness of multiple EEGs in supporting the diagnosis of epilepsy: an operational curve. Epilepsia. 1987;28(4):331-334.
Niedermeyer E, Lopes da Silva F, eds. Electroencephalography: Basic Principles, Clinical Applications, and Related Fields. 5th ed. Lippincott Williams & Wilkins; 2004.