Epileptiform Normal Variants on EEG

Updated: Jun 15, 2017
  • Author: Selim R Benbadis, MD; Chief Editor: Helmi L Lutsep, MD  more...
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Overview

Overview

Epileptiform normal variants are electroencephalographic (EEG) patterns that resemble epileptogenic abnormalities. Most of these patterns initially were thought to be associated with epilepsy or other neurologic conditions but subsequently were demonstrated to have no such significance. [1] They now are considered normal variants of no clinical significance; however, their recognition is important to avoid overinterpretation or misinterpretation of their significance. [2, 3]

This article reviews the following such patterns: small sharp spikes (SSSs), wicket spikes, 14- and 6-Hz positive spikes, phantom spike and waves, psychomotor variant, subclinical rhythmic EEG discharges of adults (SREDA), and midline theta rhythm.

Most of these patterns initially were described in the 1950s. Gibbs and Gibbs described small sharp spikes in 1952, [4] and 14- and 6-Hz positive spikes were described at approximately the same time. [5, 6, 7, 8] The 6-Hz phantom spike-wave was described by Walter in 1950, [9] and the psychomotor variant was described by Gibbs and Gibbs in 1952. [4] Wickets were described in 1977 by Reiher and Lebel. [10] Midline theta rhythm was described by Ciganek in 1961. [11]

Go to Epilepsy and Seizures for an overview of these topics.

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Types of Variant Waveforms

Small sharp spikes

Small sharp spikes (SSSs), also known as benign epileptiform transients of sleep (BETS), occur in light sleep (stages I and II of nonrapid eye movement [NREM] sleep), usually sporadically. [12] Their location is temporal, either unilateral or bilaterally independent, and with a broad field of distribution. The morphology is typically monophasic, occasionally diphasic, and the decline after the first negative peak is very steep. SSSs rarely may have a single aftergoing slow-wave component but generally do not disturb the background.

The main features of SSSs are expressed in their name: their duration is short, and their amplitude is small. An easy guideline states that SSSs generally should last less than 50 milliseconds and should be smaller than 50 µV (see the images below).

Small sharp spike is present in left temporal regi Small sharp spike is present in left temporal region (modified double banana montage). Note widespread field of distribution (isopotential at F7, T1, and T3), low amplitude (< 50 µV), and short duration (< 50 ms).
Left temporal small sharp spike. Note low amplitud Left temporal small sharp spike. Note low amplitude (< 50 µV) and short duration (< 50 ms).
Small sharp spike is present in right temporal reg Small sharp spike is present in right temporal region. Note low amplitude (< 50 µV) and short duration (< 50 ms).

Wicket spikes

Wicket spikes occur both in the awake state and in light sleep. Their frequency is 6-11 Hz, and they usually occur in short runs (wicket rhythm) but may also appear as single sharp transients. The location is temporal, usually bilateral and independent. The morphology is archlike or mu-like, sharp, monophasic, and not followed by an aftergoing slow wave. The amplitude may be high, but the transient arises out of an ongoing rhythm and does not stand out (see the images below).

Wicket spikes in left temporal region. Note sharp Wicket spikes in left temporal region. Note sharp transients arising out of ongoing rhythm and symmetric upslope and downslope, giving wicket morphology.
Wicket spikes in left temporal region. Note sharp Wicket spikes in left temporal region. Note sharp transients arising out of ongoing rhythm and symmetric upslope and downslope, giving wicket morphology.
Wicket spikes in left temporal region. Note sharp Wicket spikes in left temporal region. Note sharp transients arising out of ongoing rhythm and symmetric upslope and downslope, giving wicket morphology.

14-Hz and 6-Hz positive spikes

The pattern of 14- and 6-Hz positive spikes may be observed at any age, but it is expressed maximally in adolescents, especially those aged 13-14 years. [13] The 6-Hz positive spikes predominate in children younger than 1 year and in adults older than 40 years, and the 14-Hz positive spikes predominate or combine with 6-Hz spikes in the other age groups. [14]

Both 14- and 6-Hz positive spikes are observed predominantly during light sleep. These spikes usually appear in short runs lasting less than 2 seconds, and their frequencies, as the name indicates, are 14 Hz and 6 Hz. The location is mostly posterior temporal, either unilaterally or bilaterally. The morphology is a sharply contoured positive spike alternating with rounded negative component. The amplitude is medium, around 20-60 µV.

6-Hz phantom spike-wave

The 6-Hz phantom spike-wave pattern may be observed in both adolescents and adults. It generally occurs during relaxed wakefulness and stage I sleep and disappears during deeper levels of sleep.

The frequency is 6 Hz, as the name indicates, and the bursts last 1-2 seconds. The location is usually diffuse, bisynchronous, and relatively symmetric. This pattern may predominate in the anterior and posterior head regions. The morphology typically consists of a small (< 30 µV and < 30 ms), evanescent diphasic spike followed by a higher (50-100 µV) slow wave component. Thus, the spike component may be difficult to see.

Psychomotor variant

A more useful and descriptive term for psychomotor variant is rhythmic midtemporal theta of drowsiness (RMTD). The frequency is theta (4-7 Hz). The location is maximum midtemporal, unilateral or bilaterally independent or bisynchronous. The morphology typically is notched, flat-topped, or sharply contoured (see the image below). Bursts may last 1-10 seconds or longer and thus resemble temporal lobe seizures. The amplitude is medium to high. [15, 16]

Rhythmic midtemporal theta of drowsiness (psychomo Rhythmic midtemporal theta of drowsiness (psychomotor variant). Note seizurelike rhythmicity and notched morphology.

Subclinical rhythmic EEG discharges of adults

Subclinical rhythmic electroencephalographic (EEG) discharges of adults (SREDA) is an uncommon pattern observed mainly in older persons (>50 y). It may occur at rest or during drowsiness.

Superficially, SREDA resembles an EEG seizure pattern. The frequency is typically 5-6 Hz. The location is widespread or bilateral with a posterior maximum. The morphology is seizurelike (ie, rhythmic sharply contoured theta). Abrupt onset and termination may help distinguish SREDA from an EEG seizure. The duration ranges from 20 seconds to minutes (average, 40-80 s).

Midline theta rhythm

Midline theta rhythm (ie, Ciganek rhythm) may be observed during wakefulness or drowsiness. The frequency is 4-7 Hz, and the location is midline (ie, vertex). The morphology is rhythmic, smooth, sinusoidal, arciform, spiky, or mu-like.

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Clinical Presentation

As a whole, these normal variants must be differentiated from epileptiform discharges (see Generalized EEG Waveform Abnormalities). In general, the benign patterns lack the characteristics of pathologic epileptiform discharges—that is, the high amplitude and aftergoing slow wave or suppression that make epileptiform discharges “disturbing” to the background activity. By default, assume that sharp transients are benign variants, and consider them epileptiform and abnormal only if they do not meet criteria for any benign transients.

Small sharp spikes (SSSs) are generally easy to distinguish from spikes because of their short duration and small amplitude.

Wicket spikes commonly are misinterpreted as sharp waves, especially when they occur as single sharp transients. Examining the context and determining whether they arise out of an ongoing rhythm are important. Wickets predominate in adults older than 30 years and have an incidence of 0.9%. [10]

The 14- and 6-Hz positive spikes may be distinguished from temporal spikes by their characteristic polarity (epileptiform spikes are almost always surface negative in polarity) and typical frequency. [17]

The 6-Hz phantom spike-waves may be difficult to distinguish from the definitive clinically significant spike-wave complexes. A helpful distinguishing feature is the tendency of benign 6-Hz phantom spike-waves (6 Hz) to disappear during sleep; epileptiform discharges (spike-wave complexes) tend to persist or become more prominent with deeper levels of sleep. [18, 19, 20]

Psychomotor variant (rhythmic midtemporal theta of drowsiness [RMTD]) differs from a seizure discharge in that it is usually a monomorphic or monorhythmic pattern that does not evolve into other frequencies or waveforms, as usually occurs during seizures.

The subclinical rhythmic electroencephalographic (EEG) discharges of adults (SREDA) pattern is never associated with symptoms, in contrast to a seizure pattern. [21]

Midline theta (Ciganek) rhythm does not have any clinical significance. Like many other patterns, this pattern initially was believed to occur predominantly in patients with temporal lobe epilepsy. Later reviews have shown that the Ciganek rhythm represents a nonspecific variant of theta activity. [22, 23]

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