4.2 Interictal Epileptiform Discharges (IEDs)
Key Takeaways
- True Interictal Epileptiform Discharges (IEDs) are generated at the cellular level by synchronous Paroxysmal Depolarization Shifts (PDS) followed by prolonged GABA-mediated hyperpolarizing inhibitory postsynaptic potentials (IPSPs).
- The six neurophysiologic criteria for true IEDs require: 1) Di- or tri-phasic waveform with sharp/spiky morphology, 2) Distinct duration (spike 20–70 ms, sharp wave 70–200 ms), 3) Asymmetric rising and falling slopes, 4) Distinct separation from background activity, 5) Presence of an after-coming slow wave, and 6) Disruption of baseline background activity.
- In bipolar montages, the site of maximal electronegativity is identified by instrumental phase reversal (waveforms pointing toward each other at the shared electrode), whereas in referential montages it corresponds to peak referential amplitude.
- Temporal Intermittent Rhythmic Delta Activity (TIRDA) is an epileptiform equivalent exhibiting >90% specificity for Mesial Temporal Lobe Epilepsy (MTLE), whereas FIRDA is non-epileptic and reflects diffuse encephalopathy or midline dysfunction.
- Volume conduction dictates that genuine cortical discharges generate consistent potential fields across adjacent electrodes, distinguishing them from single-channel electrode pop artifacts.
Last updated: August 2026
4.2 Interictal Epileptiform Discharges (IEDs)
Interictal Epileptiform Discharges (IEDs) are transient, paroxysmal neurophysiological events generated by hypersynchronous neuronal firing within cortical networks. In Long-Term Video-EEG Monitoring (LTM), the accurate identification, morphological categorization, and spatial localization of IEDs provide definitive objective evidence supporting a diagnosis of epilepsy, define the irritative zone for presurgical planning, guide anti-seizure medication selection, and classify specific electro-clinical syndromes.
1. Cellular Neurophysiology: The Paroxysmal Depolarization Shift (PDS)
At the microscopic cellular level, an interictal epileptiform spike or sharp wave corresponds to a Paroxysmal Depolarization Shift (PDS) occurring synchronously across a large population of pyramidal neurons in cortical layers III, V, and VI.
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| CELLULAR MECHANISM: THE PAROXYSMAL DEPOLARIZATION SHIFT (PDS) |
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| 1. INTRACELLULAR EVENT: |
| Sudden, massive depolarization (+20 to +40 mV) mediated by glutamate (AMPA / NMDA receptors) |
| and inward Calcium/Sodium currents (Ca2+, Na+) -> Generates high-frequency action potentials |
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| v |
| 2. SCALP EEG CORRELATE: |
| Extracellular current sink in superficial cortical layers -> SURFACE-NEGATIVE SPIKE / SHARP WAVE |
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| 3. AFTER-HYPERPOLARIZATION: |
| Prolonged intracellular hyperpolarization mediated by GABA_A / GABA_B receptors and outward |
| Potassium currents (K+) -> Terminates the burst |
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| v |
| 4. SCALP EEG CORRELATE: |
| AFTER-COMING SLOW WAVE (Hyperpolarization wave, 200–500 ms) |
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- The Spike/Sharp Component: Driven by excessive excitatory neurotransmission (glutamate acting on AMPA and NMDA receptors) and voltage-gated inward calcium ($Ca^{2+}$) and sodium ($Na^+$) currents. Extracellularly, this creates a massive current sink, manifested on scalp EEG as a surface-negative spike or sharp wave.
- The After-Coming Slow Wave: Mediated by inhibitory postsynaptic potentials (IPSPs) driven by GABAergic interneurons ($GABA_A$ chloride influx and $GABA_B$ potassium efflux). This inhibitory refractory period prevents immediate seizure generation and is recorded on scalp EEG as the after-coming slow wave.
2. Standardized Temporal Definitions: Spikes vs. Sharp Waves
The International Federation of Clinical Neurophysiology (IFCN) and the American Clinical Neurophysiology Society (ACNS) define IEDs by precise temporal durations:
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| IED TEMPORAL CLASSIFICATION SPECTRUM |
| |
| [ < 20 ms ] --------> [ 20 – 70 ms ] --------> [ 70 – 200 ms ] --------> [ > 200 ms ] |
| Muscle Artifact TRUE SPIKE TRUE SHARP WAVE Slow Wave / Background |
| (Non-Cerebral) (Sharply pointed) (Sharply contoured) (Delta / Theta activity) |
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- Spike: A transient, sharply pointed waveform distinctly separated from background activity, with a duration strictly between 20 and 70 milliseconds (1/50th to 1/14th of a second).
- Sharp Wave: A transient, sharply contoured waveform distinctly separated from background activity, with a duration strictly between 70 and 200 milliseconds (1/14th to 1/5th of a second).
- Spike-and-Slow-Wave Complex: A spike immediately followed by an associated slow wave (typically lasting 200–500 ms).
- Polyspike Complex (Multiple Spike Complex): A sequence of two or more contiguous spikes (often 3 to 10+ spikes) occurring together, with or without an accompanying after-coming slow wave. Highly characteristic of idiopathic generalized epilepsies, especially Juvenile Myoclonic Epilepsy (JME).
3. The Six Neurophysiologic Criteria for True IEDs
To prevent the over-reading of sharply contoured background rhythms, vertex waves, and artifacts, the IFCN and ACNS established six mandatory neurophysiologic criteria. A genuine epileptiform discharge typically fulfills all or nearly all of these criteria:
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| THE SIX NEUROPHYSIOLOGIC CRITERIA FOR TRUE IEDs |
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| 1. Di- or Tri-phasic Waveform with Sharp / Spiky Morphology: |
| Demonstrates distinct phase deflections (often a small initial positive phase, a major sharp |
| negative peak, and a subsequent positive phase). |
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| 2. Distinct Duration within Standard Limits: |
| Must satisfy strictly defined duration thresholds: 20–70 ms for spikes, 70–200 ms for sharp waves. |
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| 3. Asymmetric Rising and Falling Phases: |
| The ascending slope is significantly steeper than the descending slope (or vice versa), unlike |
| symmetrical physiological rhythms (e.g., mu, alpha, wicket). |
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| 4. Distinct Separation from Ongoing Background Activity: |
| The discharge clearly stands out from the surrounding background in amplitude and morphology. |
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| 5. Followed by a Slow After-Wave: |
| A prominent hyperpolarizing slow wave (200–500 ms) immediately follows the sharp component. |
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| 6. Disrupts and Replaces Baseline Background Activity: |
| Abruptly interrupts and temporarily resets ongoing background rhythms rather than riding on top. |
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4. Spatial Potential Fields & Volume Conduction Principles
Genuine cortical potentials are generated by large populations of synchronously active pyramidal neurons acting as electrical dipoles. Through physical volume conduction across cerebrospinal fluid, meninges, skull, and scalp, these dipoles generate a broad, rational potential field.
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| VOLUME CONDUCTION: TRUE CEREBRAL FIELD VS. ELECTRODE POP |
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| TRUE CORTICAL DIPOLE FIELD (Volume Conduction): |
| - Maximum negative potential at epicenter (e.g., F7 = -100 µV) |
| - Physiological voltage decay across adjacent electrodes (Fp1 = -35 µV, T3 = -40 µV, F3 = -25 µV) |
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| ELECTRODE POP ARTIFACT (Contact Impedance Discontinuity): |
| - High-voltage transient present ONLY in a single electrode (F7 = -150 µV) |
| - Zero potential recorded at neighboring electrodes (Fp1 = 0 µV, T3 = 0 µV, F3 = 0 µV) |
| - Physical impossibility for cerebral volume conduction -> ARTIFACT! |
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[!CAUTION] The Single-Channel Trap: A sharp transient appearing in a single electrode without a corresponding potential gradient in adjacent scalp electrodes cannot be of cerebral origin. It is an artifactual contact impedance jump (electrode pop) or mechanical disturbance.
5. Localization Principles: Bipolar vs. Referential Montages
Accurate spatial localization of IEDs requires mastering the fundamental operational differences between bipolar and referential recording montages.
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| LOCALIZATION IN BIPOLAR VS. REFERENTIAL MONTAGES |
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| BIPOLAR MONTAGE (Differential Amplifier Rule: Grid 1 - Grid 2): |
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| Channel 1: Fp1 - F7 ---> (Fp1 = -20 µV) - (F7 = -100 µV) = +80 µV ---> DOWNWARD DEFLECTION |
| Channel 2: F7 - T3 ---> (F7 = -100 µV)- (T3 = -30 µV) = -70 µV ---> UPWARD DEFLECTION |
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| Result: Pen deflections point TOWARD each other at F7 = **INSTRUMENTAL PHASE REVERSAL** |
| Rule: For negative potentials, phase reversal points together at the channel sharing maximal focus. |
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| --------------------------------------------------------------------------------------------------- |
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| REFERENTIAL MONTAGE (Active Electrode - Inactive Common Reference): |
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| Channel 1: Fp1 - Cz ---> -20 µV (Deflection Upward) |
| Channel 2: F7 - Cz ---> -100 µV (LARGEST UPWARD DEFLECTION = **PEAK REFERENTIAL AMPLITUDE**) |
| Channel 3: T3 - Cz ---> -30 µV (Deflection Upward) |
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| Rule: Site of maximal negative voltage displays the MAXIMUM ABSOLUTE DEFLECTION AMPLITUDE. |
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Bipolar Montages and Phase Reversal
- In a bipolar montage (e.g., longitudinal anterior-to-posterior "double banana"), each channel measures the potential difference between two adjacent scalp electrodes ($V_{out} = V_{Grid1} - V_{Grid2}$).
- Standard neurophysiology convention dictates that a negative charge in Grid 1 produces an UPWARD deflection, while a negative charge in Grid 2 produces a DOWNWARD deflection.
- When an epileptogenic spike with surface negativity is maximal at an electrode shared between two consecutive channels (e.g., F7 in
Fp1-F7andF7-T3):- In
Fp1-F7, F7 is in Grid 2, so the negative potential drives the pen DOWNWARD. - In
F7-T3, F7 is in Grid 1, so the negative potential drives the pen UPWARD. - The resulting waveforms point directly toward each other, creating an instrumental phase reversal that pinpoints F7 as the focus of maximal electronegativity.
- In
- Positive Phase Reversal: If a focus is surface-positive (e.g., POSTS, Lambda, or a positive spike), the deflections in the shared channel point away from each other (phase divergence).
Referential Montages and Peak Amplitude
- In a referential montage, every scalp electrode (Grid 1) is connected against a single common reference electrode (Grid 2, such as Cz, contralateral ear A1/A2, or average reference).
- The channel exhibiting the highest amplitude (peak referential amplitude) represents the true epicenter of the epileptogenic focus, provided the reference electrode itself is electrically inactive and free of contamination.
6. Rhythmic Interictal Slow Patterns: TIRDA vs. FIRDA vs. OIRDA
Intermittent rhythmic delta slowing patterns provide crucial diagnostic information regarding focal epileptogenesis versus generalized encephalopathy.
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| RHYTHMIC INTERMITTENT DELTA ACTIVITY COMPARISON |
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| Pattern Frequency / Morphology Topography State / Reactivity Clinical Significance |
| +--------+ +-----------------------+ +---------------+ +-----------------+ +------------------------+|
| | TIRDA | | 2–3 Hz rhythmic, | | Anterior | | Drowsiness / | | Highly specific (>90%) ||
| | | | sinusoidal/sawtooth | | Temporal | | Light NREM | | for Temporal Lobe ||
| | | | runs (1–3 seconds) | | (F7/T3, T1/T2)| | sleep | | Epilepsy / MTS ||
| +--------+ +-----------------------+ +---------------+ +-----------------+ +------------------------+|
| | FIRDA | | 1.5–3 Hz sinusoidal, | | Frontal | | Awake / Drowsy; | | Non-specific diffuse ||
| | | | rhythmic bursts | | Bilateral | | Attenuates with | | encephalopathy, toxic- ||
| | | | (high amplitude) | | Synchronous | | eye opening | | metabolic, midline ICP ||
| +--------+ +-----------------------+ +---------------+ +-----------------+ +------------------------+|
| | OIRDA | | 2–3 Hz rhythmic delta | | Occipital | | Pediatric sleep | | Childhood Absence ||
| | | | bursts | | Bilateral | | / wake; blocks | | Epilepsy; benign ||
| | | | | | Synchronous | | with eyes open | | pediatric generalized ||
| +--------+ +-----------------------+ +---------------+ +-----------------+ +------------------------+|
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Temporal Intermittent Rhythmic Delta Activity (TIRDA)
- Morphology: Short runs (lasting 1 to 3 seconds) of rhythmic, 2–3 Hz sinusoidal or saw-toothed delta waves.
- Topography: Strictly localized to the anterior and inferior temporal electrodes (F7/T3, F8/T4, T1/T2), occurring unilaterally or independently bilaterally.
- Clinical Significance: TIRDA is considered an epileptiform equivalent with greater than 90% specificity for Mesial Temporal Lobe Epilepsy (MTLE) and hippocampal sclerosis. Even in the absence of sharp spikes, TIRDA strongly supports an ipsilateral temporal seizure focus.
Frontal Intermittent Rhythmic Delta Activity (FIRDA)
- Morphology: High-amplitude, monomorphic, rhythmic 1.5–3 Hz delta waves occurring in episodic, bilaterally synchronous bursts.
- Topography: Symmetrically maximal over frontal regions (Fp1/Fp2, F3/F4, Fz).
- Clinical Significance: FIRDA is non-epileptogenic. It reflects subcortical gray matter dysfunction, toxic-metabolic encephalopathy (uremia, hepatic failure), increased intracranial pressure, or structural midline lesions.
Occipital Intermittent Rhythmic Delta Activity (OIRDA)
- Morphology: High-voltage, rhythmic 2–3 Hz delta bursts over posterior regions (O1, O2, Oz).
- Topography & Demographics: Bilaterally synchronous occipital dominance in children aged 4–12 years.
- Clinical Significance: Strongly associated with Childhood Absence Epilepsy (CAE). OIRDA is blocked by eye opening and frequently co-occurs with classic 3 Hz spike-and-wave discharges.
7. Major Syndrome-Specific Interictal Patterns
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| SYNDROME-SPECIFIC INTERICTAL DISCHARGE PATTERNS |
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| Epilepsy Syndrome Defining Interictal Signature Clinical Features |
| +-------------------------------+ +--------------------------------------+ +-------------------------+|
| | Childhood Absence Epilepsy | | Generalized 3 Hz (2.5–3.5 Hz) | | Brief stare, behavioral ||
| | (CAE) | | Spike-and-Wave, Frontal Dominance | | arrest; HV activation ||
| +-------------------------------+ +--------------------------------------+ +-------------------------+|
| | Juvenile Myoclonic Epilepsy | | Generalized 4–6 Hz Polyspike-and- | | Morning myoclonic jerks;||
| | (JME) | | Wave bursts; Photoparoxysmal Resp. | | sleep deprivation trig ||
| +-------------------------------+ +--------------------------------------+ +-------------------------+|
| | Lennox-Gastaut Syndrome | | Slow (<2.5 Hz) Spike-and-Wave & | | Multiple seizure types, ||
| | (LGS) | | Generalized Paroxysmal Fast Activity | | cognitive impairment ||
| +-------------------------------+ +--------------------------------------+ +-------------------------+|
| | West Syndrome | | Hypsarrhythmia (chaotic, high-volt, | | Infantile spasms; ||
| | (Infantile Spasms) | | multifocal spikes, disorganized) | | developmental arrest ||
| +-------------------------------+ +--------------------------------------+ +-------------------------+|
| | Benign Rolandic Epilepsy | | Centrotemporal sharp waves with | | Nocturnal hemifacial ||
| | (SeLECTS / BECTS) | | Tangential Dipole (C3/T3 neg, Fz pos)| | motor seizures ||
| +-------------------------------+ +--------------------------------------+ +-------------------------+|
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Test Your Knowledge
A technologist reviews an LTM recording and notes a high-amplitude, sharply pointed transient appearing exclusively in channel F7-T3 in a longitudinal bipolar montage. Further inspection reveals that adjacent channels (Fp1-F7, T3-T5, F3-C3) show completely flat, normal baseline activity without any voltage deflection or field gradient. According to the six neurophysiologic criteria, what is the most appropriate evaluation of this transient?
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Test Your Knowledge
A 32-year-old patient evaluated in the EMU for medically refractory focal seizures demonstrates recurring 2- to 3-second bursts of rhythmic 2.5 Hz sinusoidal and saw-toothed delta waves strictly localized to the left anterior and inferior temporal electrodes (F7/T3, T1). No spikes are embedded within the runs, and the background between bursts is intact. What is this electrographic pattern and what is its clinical significance?
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Test Your Knowledge
In a longitudinal bipolar montage, a sharp wave produces a downward deflection in the Fp1-F7 channel and an upward deflection in the F7-T3 channel. In a referential montage to Cz, the sharp wave produces the largest upward negative voltage deflection at electrode F7. What electrophysiological principle explains these findings?
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Test Your Knowledge
At the cellular level, what physiological mechanism is directly responsible for generating the after-coming slow wave that characteristically follows an interictal epileptiform spike on the scalp EEG?
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