Normal EEG variants and artifacts

Question 1

Fast and slow alpa variants

Answer 1

Harmonics of the posterior background rhythm: twice as fast (fast alpha variant) or half as fast (slow alpha variant).

Reactive to eye opening and closure.

Notched appearance can resemble Rhythmic Mid-Temporal Theta Bursts of Drowsiness (RMTTBD) except that it occurs over the posterior head regions.

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.

Question 2

What is shown here?

Answer 2

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

Question 3

What is shown here?

Answer 3

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

Question 4

What is shown here?

Answer 4

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

Question 5

Alpha squeak

Answer 5

Transient increase in frequency immediately after eye closure.

Assessment of the frequency of the posterior background rhythm should not include the first 0.5–1s after eye

Question 6

What is shown here?

Answer 6

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

Question 7

Rhythmic Mid-Temporal Theta Bursts of Drowsiness (RMTTBD)

Answer 7

Also known as Rhythmic Mid-Temporal Discharges (RMTD) and psychomotor variant.

Composed of rhythmic bursts or trains of theta waves (5–7Hz) 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.

Question 8

What is shown here?

Answer 8

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

Question 9

Midline theta rhythm

Answer 9

Also known as Ciganek rhythm.

Most prominent in the central vertex lead.

Consists of a rhythmic train of 5–7Hz smooth, sinusoidal, arciform, spiky, or mu-like activity.

Occurs during wakefulness and drowsiness. Variable reactivity to eye opening and alerting.

Question 10

What is shown here?

Answer 10

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

Question 11

What is shown here?

Answer 11

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

Question 12

Subclinical rhythmic electrographic discharge in adults (SREDA)

Answer 12

Uncommon pattern.

Seen in people older than 50years.

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 5–7Hz lasting from 20s 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).

Question 13

What is shown here?

Answer 13

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

Question 14

14- and 6-Hz positive bursts

Answer 14

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 14Hz; the 6Hz can occur either independently or in association with 14Hz.

Maximal amplitude over the posterior temporal region.

Better seen in a referential montage (ear references).

Peak at the age of 13–14and decrease in incidence with increasing age.

May be enhanced in Reye’s syndrome.

Question 15

What is shown here?

Answer 15

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

Also known as phantom spike-and-wave

Question 16

6 Hz spike-and-wave

Answer 16

Also known as phantom spike-and-wave.

Consist of 5–7Hz 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.

Question 17

What is shown here?

Answer 17

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

Question 18

Benign sporadic sleep spikes (BSSS)

Answer 18

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 (<50µV) and short-duration (<50ms) 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.

Question 19

What is shown here?

Answer 19

Left wicket rhythm (arrows); sensitivity 7 μV/mm, LFF 0.5 Hz, HFF 70 Hz

Question 20

Wickets

Answer 20

Intermittent trains of monophasic arciform waveforms or single spike-like waveforms.

Occur exclusively on one side (left>right) or bilaterally with shifting predominance.

Frequency of 6–11Hz and possibly represent fragments of temporal alpha activity or the third rhythm.

Seen during wakefulness, drowsiness, and light sleep, and disappear in deeper sleep.

Should not be mistaken for a temporal seizure discharge or spikes; if a single spike is found, it should be compared with a train of wicket spikes on other pages.

Not associated with a slow wave and do not distort the background.

Question 21

What is shown here?

Answer 21

The third rhythm (arrows); sensitivity 7 μV/mm, LFF 1 Hz, HFF 70 Hz

Question 22

The third rhythm

Answer 22

Also called temporal alphoid rhythm.

Rhythmic activity in the alpha and upper theta range over the mid-temporal region.

Rarely detected in the scalp EEG and more commonly seen when there is a local bone defect or recorded from epidural electrodes.

The origin and function remain debatable, with some related it to cortical auditory function.

Question 23

What is shown here?

Answer 23

Frontal arousal rhythm (FAR) (arrows); sensitivity 7μV/mm, LFF 1Hz, HFF 70Hz

Question 24

Frontal arousal rhythm (FAR)

Answer 24

Trains of 7–20Hz waveforms that occur predominantly over the frontal regions lasting up to 20s.

May be notched in appearance with varying harmonics.

Seen mainly in children following arousal from sleep and disappears with full wakefulness.

Question 25

What is shown here?

Answer 25

Mu-rhythm (arrows); sensitivity 7 μV/mm, LFF 1 Hz, HFF 70 Hz

Question 26

Mu-rhythm

Answer 26

Archiform 7–11Hz waveforms occurring independently over the central head regions.

Functionally related to the sensorimotor cortex and is attenuated by touch, active, or passive movement of the extremities, or thought of such movement.

Question 27

What is shown here?

Answer 27

Lambda waves (arrows); sensitivity 7 μV/mm, LFF 1 Hz, HFF 70 Hz

Question 28

Lambda waves

Answer 28

Resemble the Greek letter λ with monophasic or diphasic waveforms with prominent surface-positive waveform.

Occurs over the occipital regions when subject is visually scanning.

Bilateral and synchronous but may be asymmetric.

Possibly represent an evoked cerebral response to visual stimuli.

Question 29

What is shown here?

Answer 29

Positive occipital sharp transients (POSTs) (arrows); sensitivity 7μV/mm, LFF 1Hz, HFF 70Hz

Question 30

Positive occipital sharp transients (POSTs)

Answer 30

Sharp-contoured, positive transients over the occipital regions.

Bilateral synchronous but may be asymmetric.

Occur during light sleep.

Question 31

What is shown here?

Answer 31

Frontal mitten pattern (arrows); sensitivity 7 μV/mm, LFF 1 Hz, HFF 70 Hz

Question 32

Mitten pattern

Answer 32

Seen during sleep.

Consists of fast-wave and slow-wave components and resembles a mitten with the thumb of the mitten formed by the last wave of a spindle and the hand portion by the slower wave component.

Variant of a vertex wave or K-complex.

Question 33

What is shown here?

Answer 33

Breach rhythm in the left centro-parietal region (arrows); sensitivity 7μV/mm, LFF 1Hz, HFF 70Hz

Question 34

Breach rhythm

Answer 34

High-voltage activity over a skull defect.

Consists of a spiky appearance and sharply contoured arciform 6–11Hz waveforms.

Most prominent in temporal and central regions and can usually represent wickets and mu-rhythms depending on the location of the skull defect.

Question 35

What is shown here?

Answer 35

Vertical eye movements: (a) up gaze, (b) down gaze; sensitivity 7μV, LFF 1Hz, HFF 70Hz

Question 36

What is shown here?

Answer 36

Horizontal eye movements: (a) left gaze, (b) right gaze; sensitivity 7μV, LFF 1Hz, HFF 70Hz

Question 37

What is shown here?

Answer 37

Oblique eye movement: up and to the left (arrow); sensitivity 7μV, LFF 1Hz, HFF 70Hz

Epilepsy Board Review (p. 87). Springer New York. Kindle Edition.

Question 38

Eye movements on EEG

Answer 38

All eye movements are generated by corneal and retinal potentials.

It is a direct current represented by a dipole whose positive pole localizes to the cornea and negative pole localizes to the retina.

The electrodes involved are closest to the eyeball: Fp1, Fp2, F7, and F8.

The electrodes surrounding the eyeball detect a positive potential which voltage is usually greater than the cerebral potential.

For example, when the eyes are closed, the eyeballs move upward to their natural position (Bell’s phenomenon) and this upward movement is detected by a positive potential recorded at Fp1 and Fp2 (Fig.3.21). This activity is then followed by a falloff recorded at the next electrodes: F3 and F4. When the eyes are open again, the inverse occurs. If these movements happen repetitively, they will result in a blink artifact.

Another example is when eyes move to the left, the activity at Fp1 and Fp2 remains steady, with no change in potential. However, the positive potential is detected by the electrode F7 and it becomes more positive than other electrodes. Because the eyes move conjugately, the cornea is moving away from F8 and it becomes less positive, or more negative, because the retina is now closer to this electrode. This horizontal movement to the left produces a positive phase reversal with a maximal positive potential at F7 and a negative phase reversal with a maximal negative potential recorded at F8. Oblique eye movements are more difficult to interpret and constitute a combination of both vertical and horizontal movements. An eye movement upward and to the left would generate an equal positive potential recorded in a bipolar montage recording from electrodes Fp1–F7 and a large upward deflection on the channel recording Fp2–F8. This occurs because the positive potential involves both Fp1 and F7 relatively equally, and the potential difference recorded with the differential amplifier approximates to zero. The potential difference recorded from Fp2–F8 is negative at Fp2 and positive at F8, creating an upward deflection in that channel. This is due to the rules of localization (if input 1 is more negative than input 2, an upward deflection will be recorded).

Eyelid flutter produces low-voltage slow activity and is often limited to Fp1 and Fp2 electrodes.

Horizontal nystagmus is usually detected unilaterally, and the movement is recorded by the electrode on the side of the fast phase of the nystagmus because of the larger positive voltage of the cornea near that electrode. Vertical nystagmus is rarely detected because of the low voltage and the distance of these electrodes from the eyeball.

Question 39

What is shown here?

Answer 39

The lateral rectus artifact is a low-voltage motor unit potential recorded from the F7 and F8 electrodes. It appears as a sharp positive deflection of very short duration with a slow falloff as the muscle relaxes.

Question 40

Myogenic artifact

Answer 40

Electromyographic—lateral rectus, single motor units, frontalis, temporalis, swallowing, and chewing:

Single motor units appear as repetitive or single negative or positive deflections that have a comb-like appearance.

The frontalis electromyogram (EMG) is seen in frontal electrodes, as when tightly closing eyes. This can also be seen during photic stimulation, a photomyoclonic response.

The temporalis EMG is recorded from F7, F8, T7, T8, P7, and P8. This is typically seen with jaw clenching or chewing.

High-frequency filters should not be used to eliminate the EMG artifact because they alter its appearance from a sharp wave to a more sinusoidal frequency that resembles cerebral beta activity.

Question 41

What is shown here?

Answer 41

Electrocardiographic artifact (arrows); sensitivity 7 μV, LFF 1 Hz, HFF 70 Hz

The QRS complex is easily monitored by applying electrodes to the chest. The generated signal is high voltage generated by the heart. This activity when recorded at the scalp constitutes a far-field potential. It is often picked up by montages using ear electrodes as a reference. It is prevalent in obese patients, and patients with short necks and babies.

Question 42

Pulse artifact

Answer 42

Pulse artifact is usually confined to a single electrode and appears as a slow-wave potential. It occurs when an electrode is placed over a surface artery. The electrocardiogram signal will be time locked to the slow wave and always occurs at the same location.

Question 43

Cardioballistic artifact

Answer 43

A cardioballistic artifact is rhythmic delta activity, usually widespread in distribution, which represents head movement with each pulse. The relationship between the cardiac signal and these pulsations is not always time locked to any particular phase of the signal.

Question 44

What is shown here?

Answer 44

Glossokinetic and chewing artifacts; sensitivity 7 μV, LFF 1 Hz, HFF 35 Hz

Movement of the tongue creates a direct current potential where the tip of the tongue is negative with respect to its base. They are frequently recorded as slow activity from the temporal electrodes. This can be reproduced by having the patient repeat words or phrases that produce active tongue movements, such as la-la-la or ta-ta-ta. This artifact may resemble resemble generalized spike-and-wave discharges when filtered.

Question 45

What is shown here?

Answer 45

Galvanic aka diffuse sweat artifact; sensitivity 7 μV, LFF 1 Hz, HFF 70 Hz This artifact is secondary to perspiration and results in high-amplitude slow-wave potentials. Standard low-frequency filters reduce this artifact.

A salt bridge may be formed shorting two electrodes contacting the perspiration. (Salt bridges are important in a galvanic cell because salt bridges keep both solutions at the anode and cathode neutral. Without a salt bridge, the continuous buildup of positive charge solution at the anode and the continuous negative charge solution at the cathode would prevent an electron pulling force because electrons would no longer want to travel to a more negative charged side. This buildup would eventually prevent the flow of electrons and our battery would not work. This is why a salt bridge is necessary.)

Question 46

What is shown here?

Answer 46

Tremor artifact; sensitivity 7 μV, LFF 1 Hz, HFF 70 Hz

Tremor is often between 4 and 6Hz and localized to the body region involved. It is often seen in the head and upper limbs.

Question 47

Jerk artifact

Answer 47

Jerks produce enough body movement to move the electrodes or the head creating a potential in the recording.

Question 48

What is shown here?

Answer 48

60-Hz artifact; sensitivity 7 μV, LFF 1 Hz, HFF 70 Hz

Epilepsy Board Review (p. 95). Springer New York. Kindle Edition.
Sixty-cycle interference results from poor electrode application. It is produced by high-impedance electrodes that affect the input circuitry of the amplifier and also common-mode rejection when impedances are not equal (Fig.3.27).

Question 49

Capacitative and electrostatic artifacts

Answer 49

Capacitative and electrostatic artifacts are related to movement of wires. An example is when someone steps on the input cable. The cable acts as a capacitor because of multiple insulated wires enclosed in the cable. Moving or stepping on the cable causes the capacitor to discharge and results in high-voltage transient recorded on the EEG.

Question 50

Electrode artifacts

Answer 50

These are mostly related to poorly attached electrodes, high resistance, a broken wire, or changes in the lead–scalp interface (change in the gel used to complete this interface). They are usually restricted to one electrode (Fig.3.28). These may resemble repetitive discharges in a longitudinal montage.

An electrode pop is a high-voltage deflection that exceeds the limits of the individual channel
channel sensitivity and blocks or squares off at the top.

Repetitive and rhythmic electrode artifact can be produced by tapping the electrodes when the mother of the patient pats the baby’s back.

Question 51

Environmental artifacts

Answer 51

Radiofrequency waves are high-frequency signals that may be continuous or intermittent and affect some or all recording channels. This often results from being in the vicinity of machines such as microwaves or the operating room.

Question 52

Digitization

Answer 52

This often results from failure in the components used to acquire the EEG data.

One example is related to aliasing, which is sampling at a rate that is less than twice the frequency of the high-frequency filter. This
is uncommon since most EEG instruments sample at a rate of at least 200samples per channel per second. With the ability to make changes in sensitivity, filtering, and montages, many artifacts can be created where the EEG may appear very different from the true signal. This often results in misinterpretation of the EEG as possibly ictal in nature.