4.1 Normal Variants and Benign Epileptiform Patterns
Key Takeaways
- Normal variants and benign epileptiform patterns of uncertain significance (BEPUS) are non-epileptic physiological waveforms that mimic true epileptiform discharges or seizures, but lack after-coming slow waves, focal background disruption, and association with clinical epilepsy.
- Wicket spikes are temporal, arciform/mu-like 6–11 Hz monophasic transients seen during drowsiness and light sleep that occur in trains or isolated spikes without disrupting the ongoing background.
- 14 and 6 Hz positive spikes (ctenoids) exhibit surface-positive polarity over posterior temporal regions during drowsiness and light sleep, and are maximally visualized on long-distance referential montages due to bipolar phase cancellation.
- Small Sharp Spikes (SSS / BETS) display ultra-short duration (<50 ms), low amplitude (<50 µV), an oblique transverse dipole field across hemispheres, and characteristically disappear in deep slow-wave sleep (Stage N3).
- Lambda waves and POSTS share identical triangular surface-positive occipital morphology; however, Lambda waves occur during active awake visual scanning with eyes open, whereas POSTS occur during NREM sleep.
4.1 Normal Variants and Benign Epileptiform Patterns
In Long-Term Video-EEG Monitoring (LTM) and Epilepsy Monitoring Units (EMUs), the misinterpretation of normal physiological variants and benign sharp transients as epileptiform abnormalities represents one of the most prevalent causes of diagnostic error. Known clinically as "over-reading," mistaking a benign variant for an interictal epileptiform discharge (IED) or electrographic seizure leads to catastrophic clinical consequences: erroneous epilepsy diagnoses, inappropriate anti-seizure medication (ASM) toxicity, driving license revocations, psychological morbidity, and inappropriate surgical referrals.
For the Certified Long Term Monitoring Technologist (CLTM), recognizing the distinct morphological signatures, polarity distributions, montage-dependent appearances, and state-dependencies of benign variants and physiological sleep transients is a mandatory core competency.
1. Principles Differentiating Benign Variants from True IEDs
To distinguish benign variants from true epileptiform discharges, technologists and electroencephalographers systematically evaluate six foundational physiological rules:
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| BENIGN NORMAL VARIANTS VS. TRUE EPILEPTIFORM DISCHARGES |
| |
| Diagnostic Feature Benign Physiological Variant True Epileptiform Discharge (IED) |
| +----------------------------+ +---------------------------------+ +---------------------------------+
| | After-Coming Slow Wave | | Absent (or tiny shallow notch) | | Prominent hyperpolarizing wave |
| +----------------------------+ +---------------------------------+ +---------------------------------+
| | Background Disruption | | None; blends into rhythm | | Abrupt disruption / asymmetry |
| +----------------------------+ +---------------------------------+ +---------------------------------+
| | Waveform Symmetry | | Symmetric, monophasic, arch-like| | Asymmetric (steeper rise phase) |
| +----------------------------+ +---------------------------------+ +---------------------------------+
| | State Dependency | | Drowsiness (N1) / Light sleep | | Persists/activates in N2/N3/Wake|
| +----------------------------+ +---------------------------------+ +---------------------------------+
| | Polarity | | Often surface-positive or neutral| | Predominantly surface-negative |
| +----------------------------+ +---------------------------------+ +---------------------------------+
| | Clinical Correlation | | Zero association with seizures | | High specificity for epilepsy |
| +----------------------------+ +---------------------------------+ +---------------------------------+
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2. Wicket Spikes and Wicket Rhythms
Wicket spikes (or Wicket rhythms) are benign temporal arch-shaped waveforms that mimic anterior or mid-temporal epileptiform spikes.
[ WICKET SPIKE MORPHOLOGY ]
Negative Peak (Sharp)
/\
/ \
/ \
/ \________ Baseline Unaltered (No Slow Wave)
___________/ Rounded
Positive Base
- Frequency: 6–11 Hz (typically 8–9 Hz)
- Monophasic / Arch-like ("Croquet Wicket")
- Mid-to-Anterior Temporal (T3/T4, F7/F8, T7/T8)
- Occurs in trains or isolated single transients
Electrographic Features
- Frequency & Morphology: Monophasic, sharply contoured waveforms occurring at 6–11 Hz (most commonly 8–9 Hz). They exhibit a distinctive mu-like or arcade-like shape resembling a croquet wicket, featuring a sharp negative apex and a rounded positive base.
- Temporal Presentation: May emerge as sustained rhythmic trains (Wicket rhythm) or as isolated, single, sharp transients (Wicket spikes).
- Topography: Localized to the temporal regions, maximal at anterior and mid-temporal derivations (T3/T4, F7/F8, T7/T8). They can occur unilaterally, bilaterally synchronously, or independently over both temporal lobes.
- State: Predominantly observed during relaxed wakefulness, drowsiness (Stage N1 sleep), and light sleep (Stage N2 sleep). They typically attenuate or fragment in deeper slow-wave sleep.
- Demographics: Most prevalent in adults older than 30–40 years of age.
Critical Diagnostic Distinctions
- Absence of After-Coming Slow Wave: Unlike true anterior temporal spikes seen in Mesial Temporal Lobe Epilepsy (MTLE), wicket spikes are not followed by an after-coming slow wave.
- Preservation of Background Architecture: Wicket spikes do not disrupt or attenuate the surrounding background activity; they emerge smoothly from and blend directly back into ongoing temporal rhythms.
- Monophasic Contour: True epileptiform spikes typically exhibit a di- or tri-phasic morphology with an asymmetric slope (rapid rise and slower fall), whereas wickets are monophasic and symmetrical.
[!WARNING] Clinical Trap: The Wicket vs. MTLE Trap: When an adult patient with non-epileptic spells has wicket spikes occurring as isolated single transients in the left anterior temporal region, inexperienced reviewers frequently misinterpret them as left temporal sharp waves, prompting incorrect initiation of ASMs. Always verify whether an after-coming slow wave and focal background disruption exist.
3. Subclinical Rhythmic Electrographic Discharges of Adults (SREDA)
Subclinical Rhythmic Electrographic Discharges of Adults (SREDA) is a dramatic, rhythmic normal variant that closely mimics a focal electrographic seizure or non-convulsive status epilepticus.
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| SREDA: SUBCLINICAL RHYTHMIC ELECTROGRAPHIC DISCHARGES OF ADULTS |
| |
| [ ABRUPT / GRADUAL ONSET ] -> Rhythmic 5–7 Hz parieto-temporal theta sharp activity |
| | |
| v |
| [ EVOLUTIONARY PATTERN ] -> Evolves into high-voltage rhythmic sinusoidal slowing or sharp-slow |
| discharges lasting 20 seconds to 2+ minutes |
| | |
| v |
| [ ABRUPT / GRADUAL OFFSET] -> Immediate return to baseline wakefulness background |
| * NO POST-ICTAL SLOWING * NO POST-ICTAL SUPPRESSION * |
| | |
| v |
| [ CLINICAL CORRELATE ] -> Patient remains fully conscious, oriented, and asymptomatic during burst|
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Detailed Electrographic Characteristics
- Morphology & Progression: SREDA begins as a repetitive sequence of sharp contours or rhythmic theta waves (5–7 Hz) that rapidly coalesce into a sustained, rhythmic, high-amplitude sinusoidal or notched theta/delta discharge. The pattern often appears to "evolve" in frequency and voltage, mimicking ictal progression.
- Topography: Typically maximal over the parietal and posterior temporal regions (P3/P4, T5/T6, P7/P8), distributed bilaterally and symmetrically, or with asymmetric prominence.
- Duration: Typically lasts between 20 seconds and 2 minutes (occasionally longer).
- Demographics: Classically observed in adults over 50 years of age.
- Activation Triggers: Often triggered by hyperventilation (HV) or relaxed drowsiness.
Clinical and Diagnostic Pitfalls
Despite looking identical to an electrographic seizure, SREDA is strictly subclinical and non-epileptic:
- No Semiology: The patient exhibits zero clinical manifestations, maintaining full orientation, intact speech, and normal memory encoding during the discharge.
- No Post-Ictal Slowing: When the pattern terminates, the EEG background immediately returns to baseline without the post-ictal polymorphic delta slowing or voltage suppression that follows true epileptic seizures.
4. Small Sharp Spikes (SSS) / Benign Epileptiform Transients of Sleep (BETS)
Small Sharp Spikes (SSS), historically designated as Benign Epileptiform Transients of Sleep (BETS) or Benign Sporadic Sleep Spikes (BSSS), represent one of the most common benign variants encountered during sleep recordings.
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| SMALL SHARP SPIKES (SSS / BETS) - DIAGNOSTIC CRITERIA |
| |
| Criteria Physiological Value Clinical Significance |
| +---------------------------+ +--------------------------------+ +----------------------------------+|
| | Duration | | < 50 milliseconds (20–40 ms) | | Ultra-short spike duration ||
| +---------------------------+ +--------------------------------+ +----------------------------------+|
| | Amplitude | | < 50 µV (typically 20–40 µV) | | Low-voltage transient ||
| +---------------------------+ +--------------------------------+ +----------------------------------+|
| | After-Coming Slow Wave | | Absent or minuscule notch | | No prominent delta slow wave ||
| +---------------------------+ +--------------------------------+ +----------------------------------+|
| | Topography | | Shifting, bilateral temporal | | Unilateral or independent sites ||
| +---------------------------+ +--------------------------------+ +----------------------------------+|
| | Dipole Vector | | Oblique / Transverse Dipole | | Negative temporal, Pos contralat ||
| +---------------------------+ +--------------------------------+ +----------------------------------+|
| | Sleep Stage Dynamics | | Drowsiness (N1) & N2 ONLY | | DISAPPEAR in deep N3 sleep ||
| +---------------------------+ +--------------------------------+ +----------------------------------+|
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Electrographic Features
- Duration: Extremely brief, strictly less than 50 milliseconds (typically 20–40 ms), distinguishing them from broader epileptogenic sharp waves (70–200 ms).
- Amplitude: Low-voltage, strictly less than 50 µV (rarely up to 60 µV).
- Morphology: Monophasic or diphasic, very sharply contoured needle-like spikes. They may be followed by a tiny, shallow slow-wave notch, but never a prolonged, high-amplitude polymorphic slow wave.
- Topography & The Oblique Dipole: SSS are distributed over the temporal and fronto-temporal regions (T3/T4, F7/F8, T7/T8). A characteristic feature supporting SSS is their transverse/oblique dipole across the head: a sharp negativity over one temporal region is accompanied by a concurrent, broad, low-amplitude positivity in the contralateral hemisphere.
- State Dependency: SSS emerge exclusively during drowsiness (Stage N1) and light NREM sleep (Stage N2).
[!IMPORTANT] The Deep Sleep Rule for BETS: True epileptogenic interictal spikes in temporal lobe epilepsy characteristically activate and increase in frequency during deep slow-wave sleep (Stage N3). In sharp contrast, Small Sharp Spikes (SSS/BETS) completely disappear in Stage N3 sleep. If a spike-like transient persists or proliferates in deep N3 sleep, it is NOT BETS.
5. 14 and 6 Hz Positive Spikes (Ctenoids)
14 and 6 Hz positive spikes (also referred to as ctenoids or comb rhythms) are distinctive physiological bursts that appear during drowsiness and light sleep.
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| 14 & 6 Hz POSITIVE SPIKES (CTENOID PATTERN) |
| |
| - Dual Frequency: 13–15 Hz (14 Hz burst) or 5–7 Hz (6 Hz burst) |
| - Polarity: SURFACE POSITIVE (Downward deflection in referential montage to ear/Cz) |
| - Location: Posterior Temporal / Parieto-Occipital (T5/T6, P7/P8, O1/O2) |
| - Optimal Montage: Long-distance referential (Contralateral Ear A1/A2 or Cz) |
| - State: Drowsiness (N1) and Light Sleep (N2) in children and adolescents |
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Electrographic Features
- Dual Frequencies: The pattern manifests in two harmonically related frequencies: a fast component at 13–15 Hz (commonly 14 Hz) and a slower component at 5–7 Hz (commonly 6 Hz). The 14 Hz and 6 Hz components may occur independently or intermixed within the same burst.
- Polarity: Unlike virtually all true cortical epileptiform discharges (which are surface-negative), 14 & 6 Hz bursts possess a surface-positive polarity.
- Morphology: The waveform displays an arch-like or comb-like appearance (from Greek ctenoid, meaning comb-like), where the sharp spiky component is positive and the rounded component is negative.
- Topography: Maximally expressed over the posterior temporal and adjacent parietal-occipital areas (T5/T6, P7/P8, O1/O2). They occur bilaterally or with shifting asymmetry.
- Demographics: Highly prevalent in children, adolescents, and young adults (peak incidence between 12 and 16 years of age); rare in older adults.
- Montage Sensitivity: Because 14 & 6 Hz positive spikes generate a broad, widespread positive potential field across adjacent scalp regions, bipolar (double banana) montages frequently cancel the signal out due to common mode rejection. They are best demonstrated on referential montages utilizing a distant reference such as the contralateral earlobe (A1/A2), mastoid, or Cz.
6. 6 Hz Spike-and-Wave ("Phantom Spike-and-Wave")
The 6 Hz Spike-and-Wave pattern consists of brief, rhythmic bursts of miniature spike-and-wave discharges occurring at a repetition rate of 5–7 Hz (predominantly 6 Hz).
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| 6 Hz PHANTOM SPIKE-AND-WAVE DICHOTOMY: FOLD VS. WHAM |
| |
| Clinical Parameter FOLD (Benign Variant) WHAM (Potentially Epileptogenic) |
| +---------------------------+ +---------------------------------+ +---------------------------------+
| | Gender Association | | Female predominance | | Male predominance |
| +---------------------------+ +---------------------------------+ +---------------------------------+
| | Spatial Topography | | Occipital dominance (O1, O2) | | Anterior / Frontal (Fp1, Fp2, Fz)|
| +---------------------------+ +---------------------------------+ +---------------------------------+
| | Voltage / Amplitude | | Low amplitude (<40–50 µV) | | High amplitude (>50–100 µV) |
| +---------------------------+ +---------------------------------+ +---------------------------------+
| | State of Vigilance | | Drowsiness / Relaxed wake | | Wakefulness |
| +---------------------------+ +---------------------------------+ +---------------------------------+
| | Clinical Significance | | Completely benign normal variant| | Borderline / Seizure association|
| +---------------------------+ +---------------------------------+ +---------------------------------+
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Morphology and The "Phantom" Concept
The spike component of the 6 Hz complex is extremely brief (<30 ms) and of low amplitude compared to the accompanying 6 Hz slow wave, giving the visual impression that the spike is faint, diminutive, or "ghostly" (hence phantom).
The FOLD vs. WHAM Rule
To determine clinical relevance, electroencephalographers apply the classic FOLD vs. WHAM dichotomy:
- FOLD (Female, Occipital, Low amplitude, Drowsiness): This pattern is maximal over posterior/occipital derivations, low in amplitude, occurs in drowsy females, and represents a purely benign normal variant with zero association with epilepsy.
- WHAM (Wakefulness, High amplitude, Anterior, Male): This pattern displays frontal/anterior dominance, high voltage, occurs during wakefulness, and is seen predominantly in males. WHAM carries a higher clinical correlation with seizures and may represent a true epileptiform predisposition.
7. Rhythmic Mid-temporal Theta of Drowsiness (RMTD)
Rhythmic Mid-temporal Theta of Drowsiness (RMTD), historically known as the "psychomotor variant", is a benign rhythmic burst seen during sleep transition.
- Frequency & Morphology: Rhythmic 5–7 Hz theta activity composed of notched, flat-topped, or trapezoidal waves. The notched contour frequently gives the false impression of a spike-wave burst.
- Location: Strictly localized to the mid-temporal electrodes (T3/T4, T7/T8), occurring unilaterally or shifting independently between hemispheres.
- Duration: Bursts typically persist for 2 to 10 seconds or longer.
- State: Exclusively recorded during drowsiness (Stage N1 sleep).
- Distinguishing Features: Unlike focal temporal seizures, RMTD exhibits no progressive evolution in frequency, morphology, or spatial distribution, and produces no post-burst slowing or cognitive disturbance.
8. Breach Rhythm (Skull Defect Variant)
A Breach rhythm is a normal physiological alteration of the EEG background caused by a cranial bone defect (e.g., prior craniotomy, burr hole, traumatic skull fracture, or craniectomy).
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| PATHOPHYSIOLOGY OF THE BREACH RHYTHM |
| |
| INTACT CRANIUM (High Impedance Bone Filter): |
| [CORTEX] ---> [CSF] ---> [DURA] ---> [SKULL BONE (Low-Pass Filter)] ---> [SCALP ELECTRODE] |
| - Skull acts as a natural resistor/capacitor: attenuates voltage and smooths sharp fast frequencies |
| |
| BONE DEFECT / CRANIOTOMY (Breach Area): |
| [CORTEX] ---> [CSF] ---> [DURA] ---------------------------------------> [SCALP ELECTRODE] |
| - Loss of bone filter -> Marked amplitude elevation (2–3x) and unmasking of spiky beta/mu fast waves|
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Electrographic Signature
- Morphology: Markedly elevated amplitude (often 2 to 3 times higher than the contralateral intact hemisphere), sharply contoured, spiky waveforms. Fast beta activity (18–30 Hz) and mu rhythms appear unusually prominent, spiky, and jagged.
- Topography: Strictly confined to electrodes overlying or immediately adjacent to the bone defect.
- Distinguishing from Epileptiform Discharges:
- A breach rhythm consists of normal physiological rhythms (beta, alpha, mu) that appear spiky solely due to the absence of skull filtering.
- It is not accompanied by polymorphic delta slowing or focal background disruption.
- It does not have an after-coming hyperpolarizing slow wave.
9. POSTS and Lambda Waves
Positive Occipital Sharp Transients of Sleep (POSTS) and Lambda waves share nearly identical triangular, surface-positive occipital morphologies, but occur in opposite behavioral states of vigilance.
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| POSTS VS. LAMBDA WAVES COMPARISON |
| |
| Feature POSTS Lambda Waves |
| +---------------------------+ +---------------------------------+ +---------------------------------+
| | Morphology | | Triangular, sail-shaped diphasic| | Triangular, sawtooth diphasic |
| +---------------------------+ +---------------------------------+ +---------------------------------+
| | Polarity | | SURFACE-POSITIVE | | SURFACE-POSITIVE |
| +---------------------------+ +---------------------------------+ +---------------------------------+
| | Spatial Topography | | Occipital (O1, O2, Oz) | | Occipital (O1, O2, Oz) |
| +---------------------------+ +---------------------------------+ +---------------------------------+
| | State of Vigilance | | NREM Sleep (Stages N1 & N2) | | Awake with Eyes OPEN |
| +---------------------------+ +---------------------------------+ +---------------------------------+
| | Provocative Maneuver | | Sleep onset / transition | | Saccadic visual scanning |
| +---------------------------+ +---------------------------------+ +---------------------------------+
| | Response to Eye Closure | | Emerges during sleep | | IMMEDIATELY DISAPPEARS |
| +---------------------------+ +---------------------------------+ +---------------------------------+
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Positive Occipital Sharp Transients of Sleep (POSTS)
- Morphology: Triangular, diphasic, sail-shaped or checkmark-shaped waveforms with an initial sharp deflection.
- Polarity: Strictly surface-positive at occipital electrodes (O1, O2, Oz).
- Repetition Rate: Often occur in rhythmic, repetitive runs at 4–5 Hz resembling electrographic lambdoid waves of sleep.
- State: Emerge during NREM sleep (Stages N1 and N2) and disappear in deep slow-wave sleep (N3) and REM.
- Symmetry: Usually bilateral and synchronous, but amplitude asymmetries of up to 20–30% are entirely normal and must not be misinterpreted as focal occipital pathology.
Lambda Waves
- Morphology: Triangular, saw-tooth shaped transients with surface-positive polarity maximal over occipital derivations (O1, O2, Oz), typically lasting 100–200 ms.
- Physiological Mechanism: Elicited by saccadic eye movements during active visual exploration of a complex, well-illuminated patterned field (e.g., reading, looking around a bright room, viewing pictures).
- Distinguishing Features: Lambda waves occur exclusively in the awake state with eyes open. They immediately disappear when the patient closes their eyes, looks at a blank featureless white sheet, or when the room lights are turned off.
10. Comprehensive Comparison Matrix of Normal Variants
| Variant Name | Typical Frequency | Spatial Topography | Defining Polarity | Typical State of Vigilance | Critical Differentiating Feature |
|---|---|---|---|---|---|
| Wicket Spikes | 6–11 Hz (8–9 Hz) | Temporal (T3/T4, F7/F8) | Surface-Negative | Drowsiness (N1), Light sleep (N2) | Monophasic, arcade-like; no after-coming slow wave; background intact. |
| SREDA | 5–7 Hz theta | Parieto-temporal (P3/P4/T5/T6) | Negative / Sinusoidal | Drowsiness, Wake, Hyperventilation | Subclinical evolving rhythm in elderly; no post-ictal slowing or semiology. |
| Small Sharp Spikes (BETS) | <50 ms duration | Shifting Temporal / Widespread | Diphasic / Transverse Dipole | Drowsiness (N1), N2 sleep | Ultra-short (<50 ms), low voltage (<50 µV); disappears in N3 sleep. |
| 14 & 6 Hz Positive Spikes | 14 Hz and 6 Hz | Posterior Temporal (T5/T6, O1/O2) | Surface-Positive | Drowsiness (N1), Light sleep (N2) | Surface-positive polarity; best seen on contralateral ear referential montage. |
| 6 Hz Phantom Spike-Wave (FOLD) | 5–7 Hz (6 Hz) | Occipital (O1/O2) | Faint negative spike | Drowsiness, Relaxed wake | Low amplitude, female predominance, occipital dominance, completely benign. |
| 6 Hz Phantom Spike-Wave (WHAM) | 5–7 Hz (6 Hz) | Frontal (Fp1/Fp2/Fz) | Negative spike | Wakefulness | High amplitude, male predominance, anterior dominance, borderline significance. |
| RMTD (Psychomotor Variant) | 5–7 Hz | Mid-temporal (T3/T4) | Notched / Flat-topped | Drowsiness (N1) | Non-evolving notched theta trains; no clinical correlate or post-burst slowing. |
| Breach Rhythm | Fast beta / mu | Overlying craniotomy defect | Sharply contoured | All states | High-amplitude spiky fast activity due to lack of bone filter; no focal slowing. |
| POSTS | 4–5 Hz runs | Occipital (O1/O2/Oz) | Surface-Positive | Early NREM sleep (N1/N2) | Triangular sail shape; positive polarity; up to 20% asymmetry is normal. |
| Lambda Waves | 100–200 ms transient | Occipital (O1/O2/Oz) | Surface-Positive | Awake with Eyes OPEN | Elicited during saccadic scanning of visual scene; disappears with eye closure. |
An 18-year-old patient undergoing continuous video-EEG monitoring in the EMU exhibits brief, low-amplitude runs of sharply contoured 14 Hz and 6 Hz waveforms over the posterior temporal and occipital regions during Stage N1 drowsiness. The technologist switches from a longitudinal bipolar montage to a contralateral ear referential montage, causing the discharges to become dramatically more prominent with a clear surface-positive deflection. What is the correct interpretation of this pattern?
A 52-year-old patient admitted to the EMU for spell characterization displays isolated, sharply contoured transients over the left mid-temporal region (T3) during drowsiness. The transients have a monophasic arch-like contour, a duration of 110 ms, an amplitude of 75 µV, and merge seamlessly into the patient's ongoing background rhythm without any after-coming slow wave or focal slowing. Which pattern is this?
A 68-year-old patient undergoing LTM suddenly develops a 75-second burst of rhythmic 6 Hz sharp-and-slow theta activity over the bilateral parietal-temporal regions during hyperventilation. The technologist immediately tests the patient at the bedside; the patient is fully alert, accurately repeats words, and demonstrates intact recall. When the discharge abruptly ceases, the EEG background returns immediately to normal baseline wakefulness without slowing. What does this represent?
A technologist reviews a sleep recording and notes very brief (<40 ms), low-amplitude (<45 µV) diphasic spikes occurring sporadically over the right and left temporal regions during Stage N1 and N2 sleep. When the patient transitions into deep Stage N3 slow-wave sleep, the transients completely disappear. What is the most likely electrographic finding?