Normal Variants

Normal Variants

There is a set of EEG waveforms that appear either sharp in contour or rhythmic in nature and may be mistakenly described as epileptiform or abnormal features. However, based on many years of experience and large studies of normal EEG, these patterns, though not evident on every normal EEG, are accepted as benign normal EEG variants. The importance of recognizing and describing these normal variants lies in not mistaking them for findings that raise suspicion of dysfunction.

Alpha Squeak This finding occurs immediately after eye closure during the waking state. The posterior dominant rhythm appears to transiently increase in frequency by approximately by 1 Hertz over the normal posterior rhythm.

Slow alpha variant This finding is a subharmonic of the normal posterior dominant rhythm. This rhythm is approximately half the normal frequency. The morphology of this waveform is typically notched, indicating a superimposition of the subharmonic rhythm and not a replacement of the normal occipital rhythm with slow frequencies that would suggest pathology.

Fast alpha variant This finding is a super harmonic of the normal posterior dominant rhythm and typically is about twice the normal frequency.

Mu rhythm This monomorphic rhythm is seen over the central region (here most prominent in the F3-C3 and F4-C4 derivations). Analogous to the resting occipital rhythm, this is an “idling” rhythm of sensorimotor cortex. It is arciform in morphology and can sometimes appear sharp in contour. The frequency is usually in the alpha range, and it can be seen in either hemisphere. This rhythm is reactive, attenuated by movement or thought of movement of the contralateral hand.

Central/Midline Theta Rhythm of Ciganek is a normal drowsy pattern seen most in children and adolescents. These regular, sinusoidal, theta range rhythms wax and wane in amplitude, and do not evolve in frequency or distribution (unlike electrographic seizures).

Breach rhythm Regionally increased waveform amplitude and increased frequency bands suggest that there has been a breach or a break in the skull as a result of a fracture or surgical procedure. An intact skull acts as a natural filter of EEG signals, reducing their amplitude and attenuating faster frequencies. When the integrity of the skull has been disrupted these waveforms are more easily detected at the scalp. Though the skull breach may be an abnormal phenomenon, the accentuation of normal underlying brain rhythms is not.

Lambda waves These waveforms are seen in the occipital regions of the brain. They are typically triangular, biphasic, positive in polarity and are seen in people who are actively scanning with their eyes during the EEG. They are thought to be related to a visual evoked potential as they are sensitive to visual stimuli. When the subject is asked to close their eyes or to defocus their vision, these waveforms disappear.

Positive Occipital Transients of Sleep (POSTS) These waveforms are also seen in the occipital regions but during sleep rather than wakefulness. They are indicative of stage 1 sleep, are positive in polarity and often occur in runs. The phenomenon of a positive signal displayed as an upward deflection on the EEG occurs on bipolar montages when the signal arises from an end-of-chain electrode. The POSTS on this example are surface positive at O1 and O2, and not surface negative at T5, T6, P3 or P4.

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Small Sharp Spikes (SSS) also called Benign Epileptiform Transients of Sleep (BETS). As their name implies, these waveforms are seen during drowsiness and light sleep. They are temporal in location, are low voltage (<100 microvolts), needle-like in morphology (<100 msec) and spiky, sometimes multiphasic, and are distributed across a broad field with a shallow voltage gradient. They are often difficult to see in a bipolar montage but can be more easily detected in a referential montage. In contrast to temporal epileptiform discharges, they are not associated with any after going slow wave.

Wickets This alpha or theta (6-7 Hz) rhythm is seen in the temporal regions of adults during drowsiness. These monomorphic waveforms are arciform or notched in appearance and can occur in runs bilaterally or independently in each temporal region. When they occur singly (“wicket spikes”) they can appear sharp and high in voltage, but usually have a characteristically curved up- and down-slope, do not stand out from the background frequencies, and unlike epileptiform sharp waves, do not have after going slow waves.

Rhythmic Midtemporal Theta of Drowsiness (RMTD) Also known as psychomotor variant, this is another rhythm that is widespread over the temporal region in the 4-7 Hz theta range. They were thought to be related to seizures in the past but are now accepted as a benign variant seen during drowsiness or light sleep in children or adults. These monomorphic waveforms appear notched or are flat-topped and can be seen bilaterally or independently in the temporal regions. In contrast to the rhythmic discharges of a seizure, they do not evolve in frequency or spatial distribution.

14 and 6 Positive Bursts These bursts of activity are seen during drowsiness and are of positive polarity in the posterior or midtemporal regions bilaterally or independently. They tend to build in amplitude and are composed of two frequency bands of both 6 Hz and/or14 Hz.

6 Hertz Phantom Spike Wave Discharges These waveforms are low voltage 6 Hz discharges of small spike and higher voltage slow wave morphology that are best seen during drowsiness. They can be anteriorly or posteriorly predominant and are usually symmetric. In contrast to epileptic spike-wave discharges, they tend to build up and then taper gradually in amplitude.

Subclinical Rhythmic Epileptiform Discharges in Adults (SREDA) is a rhythmic, non-evolving discharge that begins and ends abruptly, can be unilateral or bilateral and is superimposed on a normal background. It is quite rare and of unclear significance.

Common Normal Variants

Pattern	Frequency	Morphology	Distribution	Duration	Age	State
14-and-6 Hz positive bursts	14 and 6 Hertz	Repetitive arch-shaped positive spikes	Posterior temporal/parietal; bilaterally synchronous or independent	<1-2 seconds	Adolescence (10-58%)	Stage I & II sleep
Midline theta rhythm (of Ciganek)	4-7 Hertz	Rhythmic trains of sinusoidal, spiky or arciform shape	Midline, usually central	4-20 seconds	Children and adults	Awake, drowsy
Phantom 6 Hz spike and wave	5-7 Hertz	Diphasic, small spike (<40 μV)and large aftergoing slow wave	FOLD: Female, Occipital, Low amp, Drowsiness WHAM: Waking, High-amp Anterior, Males†	<1 second	Adolescence/adults (2-5%)	Drowsy/awake, Stage I (not II)
Positive occipital sharp transients of sleep (POSTS)	Sporadic	Surface positive focal spikes and sharp waves up to 200 msecs	Occipital: unilateral or bilateral	Sporadic	Children	Drowsy, asleep
Rhythmic midtemporal discharge (RMTD) / “Psychomotor variant”	4-7 Hertz	Notched harmonic; abrupt onset and end	Midtemporal; unilateral, bilateral, independent or bisynchronous	Few-10 seconds	Young adult, middle aged females (1-2%)	Stage I sleep
Small sharp spikes (SSS) / Benign epileptiform transients of sleep (BETS)	Sporadic	<50 μV, <50 msec short spikes; broad shallow local gradient	Mid-/anterior temporal, shifting distribution, widespread bifrontal or bilaterally independent	Sporadic	Adults/Adolescents (20-25%)	Stage I & II sleep
Subclinical rhythmic EEG discharges of adults (SREDA)	5-6 Hertz	Mono-/biphasic sharp waves followed by rhythmic 4-7 Hertz waves – abrupt onset/offset	Symmetrical, posterior temporal/parietal maximally; may be unilateral or asymmetric	40-80 seconds	Older adults	Awake, Stage I, HV
Wicket spikes	6-12 Hertz	Monophasic arciform; no slow waves; notched harmonic	Midtemporal, bilateral independent	Rhythmic up to a few seconds	Adult (0.9%)	Awake, Stage I sleep
Hypnogogic/Hypnapompic hypersynchrony	3-5 Hertz	Moderate to high amplitude rhythmic bursts with intermixed spikes	Generalized, maximum anterior or posterior	1-6 seconds	Children	Drowsy/ArousalsState transitions

†NOTE: Anteriorly predominant phantom 6Hz spike-wave variants have been reported by some as less than benign variants, and while a nonspecific finding, may have an association with a predisposition to seizures.