Normal EEG Variants and Artifacts

Fig. 3.1

Fast alpha variant (arrow); sensitivity 7 μV/mm, low frequency filter (LFF) 1 Hz, high-frequency filter (HFF) 70 Hz

Fig. 3.2

Slow alpha variant (arrows); sensitivity 7 μV/mm, LFF 1 Hz, HFF 70 Hz

Harmonics of the posterior background rhythm: twice as fast (fast alpha variant) or half as fast (slow alpha variant).
Notched appearance can resemble Rhythmic Mid-Temporal Theta Bursts of Drowsiness (RMTTBD) except that it occurs over the posterior head regions.
Reactive to eye opening and closure.
Fast alpha variant is similar to beta rhythms except that it is located in occipital rather than in frontal, central, and parietal regions.
Slow alpha variant is more difficult to discern without clear reactivity to eye closure and opening.

Alpha Squeak (Fig. 3.3):

Fig. 3.3

Alpha squeak (arrows); sensitivity 7 μV/mm, LFF 1 Hz, HFF 70 Hz

Transient increase in frequency immediately after eye closure.
Assessment of the frequency of the posterior background rhythm should not include the first 0.5–1 s after eye closure in order to avoid overestimation.

Rhythmic Mid-Temporal Theta Bursts of Drowsiness (RMTTBD) (Fig. 3.4):

Fig. 3.4

Rhythmic Mid-Temporal Theta Bursts of Drowsiness (RMTTBD) (arrows); sensitivity 7 μV/mm, LFF 1 Hz, HFF 70 Hz

Also known as Rhythmic Mid-Temporal Discharges (RMTD) and psychomotor variant.
Composed of rhythmic bursts or trains of theta waves (5–7 Hz) usually with a notched appearance that is maximal in mid-temporal regions.
Occurs bilaterally with a shifting emphasis from side to side.
It is monomorphic and monorhythmic and does not evolve into other waveforms or frequencies.
Occurs during relaxed wakefulness and drowsiness.

Midline Theta Rhythm (Fig. 3.5):

Fig. 3.5

Midline theta rhythm (arrow); sensitivity 7 μV/mm, LFF 1 Hz, HFF 70 Hz

Also known as Ciganek rhythm.
Most prominent in the central vertex lead.
Consists of a rhythmic train of 5–7 Hz smooth, sinusoidal, arciform, spiky, or mu-like activity.
Occurs during wakefulness and drowsiness.
Variable reactivity to eye opening and alerting.

Subclinical Rhythmic Electrographic Discharge in Adults (SREDA) (Fig. 3.6a–d):

Fig. 3.6

Consecutive EEGs showing subclinical rhythmic electrographic discharge in adults (SREDA)

Uncommon pattern.
Seen in people older than 50 years.
Occurs at rest or drowsiness or during hyperventilation.
Abrupt onset of mixed frequencies in the delta and theta ranges that evolve into a rhythmic pattern consisting of sharp-contoured components 5–7 Hz lasting from 20 s to a few minutes.
Widespread distribution with maximal amplitude over parietal-posterior temporal head regions.
Usually bilateral but may be asymmetric.
May resemble a subclinical EEG seizure discharge but typically does not correlate with clinical seizures (this is however controversial).

14- and 6-Hz Positive Bursts (Fig. 3.7):

Fig. 3.7

14- and 6-Hz positive bursts (arrow); sensitivity 7 μV/mm, LFF 1 Hz, HFF 70 Hz

Also known as ctenoids.
Occur during drowsiness and light sleep.
Consist of short trains of arch-shaped waveforms with alternating positive spiky components and a negative, smooth, rounded waveform that resembles a sleep spindle with a sharp positive phase.
Mostly asynchronous and occurs bilaterally with shifting predominance.
Predominantly 14 Hz and the 6 Hz can occur either independently or in association with 14 Hz.
Maximal amplitude over the posterior temporal region.
Better seen in a referential montage (ear references).
Peak at the age of 13–14 and decrease in incidence with increasing age.
May be enhanced in Reye’s syndrome.

6-Hz spike-and-wave bursts (Fig. 3.8):

Fig. 3.8

6-Hz spike-and-wave bursts (arrow); sensitivity 7 μV/mm, LFF 1 Hz, HFF 70 Hz

Also known as phantom spike-and-wave.
Consist of 5–7 Hz brief bursts of a subtle low-amplitude spikes followed by a more prominent slow wave.
Occurs during relaxed wakefulness and drowsiness disappearing with deep sleep (unlike spike-and-wave discharges which persist during sleep).
Usually occurs bilaterally and synchronously.
Two types have been described: FOLD (Female Occipital Low-amplitude and Drowsiness) and WHAM (Wake High-amplitude Anterior and Male).
FOLD is considered to be benign, whereas WHAM is more likely to be associated with seizures.

Benign Sporadic Sleep Spikes or (BSSS) (Fig. 3.9):

Fig. 3.9

Benign sporadic sleep spikes (BSSS) (arrows); sensitivity 7 μV/mm, LFF 1 Hz, HFF 70 Hz

Also known as small sharp spikes (SSS) or benign epileptiform transients of sleep (BETS).
Seen in adults during drowsiness and light sleep and disappear with deeper sleep.
Low-voltage (<50 µV) and short-duration (<50 ms) monophasic or diphasic spike with abrupt ascending limb and a steep descending limb.
Usually do not have a slow-wave component and do not occur in repetitive trains.
Commonly occur unilaterally but can independently involve the opposite hemisphere.