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Britton JW, Frey LC, Hopp JLet al., authors; St. Louis EK, Frey LC, editors. Electroencephalography (EEG): An Introductory Text and Atlas of Normal and Abnormal Findings in Adults, Children, and Infants [Internet]. Chicago: American Epilepsy Society; 2016.

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Electroencephalography (EEG): An Introductory Text and Atlas of Normal and Abnormal Findings in Adults, Children, and Infants [Internet].

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The Normal EEG

The Background

One of the initial goals for EEG interpretation is determination of the background. To gain a complete sense about the background EEG, one should employ a variety of different screening montages to enable several different perspectives of its chief frequencies, amplitude, and degree of synchrony.

Common Physiological Artifacts

Artifacts are common during the wakeful EEG, and one of the first hurdles of EEG interpretation is distinguishing these from cerebral signal. Most notable is the presence of low-amplitude, high-frequency activity arising from scalp muscles, often frontally dominant but seen throughout the tracing. Rapid eye movements (REMs), resulting from saccades and spontaneous changes of gaze, may be seen as small, rapid deflections in frontal regions. Extremely large-voltage, diphasic potentials in frontal regions result from blinks. This occurs because the eye is a dipole, relatively positive at the corneal surface and negative at the retinal surface, and the eye moves characteristically upward during a blink according to Bell phenomenon, resulting in a moving charge and potential change. Since the positivity of the cornea rotates upward toward frontal electrode sites, a transient positivity, then negativity is recorded there. Another common artifact during the waking EEG is caused by swallowing and the related movement of the tongue, which similar to the eye is a dipole and causes a slow potential with superimposed muscle artifact. See Appendix 4 for representative common EEG artifacts seen during wakefulness.

The Posterior Dominant Rhythm

Healthy adults typically manifest relatively low-amplitude, mixed-frequency background rhythms, also termed desynchronized. When the patient is relaxed with eyes closed, the background is usually characterized by the posteriorly dominant alpha rhythm, also known simply as the posterior dominant rhythm. (Figure 7). The alpha rhythm, or alpha, is attenuated in amplitude and frequency and often completely ablated by eye opening. Alpha amplitude is usually highly symmetrical, although it may be of somewhat higher amplitude over the right than left posterior head regions (greater than 50% amplitude asymmetry is considered abnormal, with the abnormality usually on the side of the lower amplitude). Alpha frequency normally remains symmetrical, so if one side is slower than the other, an abnormality of cerebral functioning exists on the slower side. The alpha generator is thought to be located within the occipital lobes. While some normal patients lack well-formed alpha activity, the frequency, symmetry, and reactivity of alpha merits special attention and comment in any EEG report. There are several variants of the alpha rhythm, and they include temporal alpha, characterized by independent alpha activity over the temporal regions seen in older patients, frontal alpha, consisting of alpha activity over the anterior head regions, which may be related to drugs, anesthesia, or following arousal from sleep (Note: When invariant and unreactive to any stimuli in a comatose patient, this variant is pathological and represents an alpha coma pattern.) or paradoxical alpha, which is a return of alpha activity with an alerting stimulus or eye opening.

Figure 7.. The posterior dominant alpha rhythm.

Figure 7.

The posterior dominant alpha rhythm. The normal background EEG during wakefulness contains posteriorly dominant, symmetrical, and reactive alpha rhythm. Alpha activity is more prominent in amplitude during relaxed, eyes-closed wakefulness and demonstrates (more...)

Other Features of the Normal Waking Background

The remainder of the normal waking EEG is usually composed of lower amplitude beta frequencies in the fronto-centro-temporal head regions (see Figure 8). Beta frequencies are generally over 13 Hz and of low amplitude. Beta is often enhanced during drowsiness, seen in a precentral distribution, and felt to be related to the functions of the sensorimotor cortex. When beta is prominent in amplitude, either in the frontal or generalized distribution, it is likely a result of the use of sedating drugs such as benzodiazepines or barbiturates. In this sense, it is a mild abnormality of the background and often referred to as “excess beta” (Figure 8).

Figure 8.. Excess beta activity.

Figure 8.

Excess beta activity. In example (a), generalized excess beta activity is shown in a modified alternating bipolar montage. In example (b), a very prominent frontally maximal beta rhythm is noted in this slightly drowsy 32-year-old woman, very likely as (more...)

Sometimes, a prominent alpha-range frequency of 8 to 12 Hz is seen over the central head regions, termed the mu rhythm (Figure 9). Mu is seen in between 20 and 40 percent of normal adults, is characterized by arch-shaped (arciform) waves occurring either unilaterally or bilaterally over the central regions, and is prominent during drowsiness. Mu is unrelated to eye opening or closure and reacts to movement, somatosensory stimulus, or the thought of movement. It is thought to be generated in the rolandic region of the frontal and parietal lobes in relation to functions of the sensorimotor cortices. The technologist should instruct the patient to wiggle their thumb to distinguish mu from alpha; mu will attenuate, whereas alpha is unchanged, by movement or intention to move.

Figure 9.. Mu rhythms.

Figure 9.

Mu rhythms. (a) A prominent Mu rhythm is seen over the right central region. Note the arciform waves of approximate alpha-range frequency of 8 to 12 Hz. Mu is reactive to movement or the thought of movement, unlike alpha activity, which is reactive instead (more...)

Slower Background Rhythms

Occasional slower theta (4–7 Hz) or even delta (1–3 Hz) frequencies transiently may be seen during normal wakefulness, but usually these slower activities only become prominent during drowsiness (Figure 10). In children, adolescents, young adults, and some elderly individuals, it is frequent and entirely normal for there to be “drowsy bursts” of generalized theta–delta frequency activity on the EEG (Figure 10). Intermittent or pervasive, focal or generalized, theta or delta frequency, range slowing of the background in a vigilant adult is abnormal and indicates either focal, regional, or generalized cerebral dysfunction (see section on Abnormal Background for further discussion on the significance of background slowing and for example Figures). An additional normal background phenomenon is the occurrence of lambda waves (Figure 11). Lambda is elicited by pattern viewing, having the configuration of the Greek letter lambda (Λ) and is a surface positive, occipitally predominant waveform.

Figure 10.. Background in drowsiness.

Figure 10.

Background in drowsiness. Normal EEG during drowsiness in an 8-year-old child, illustrating background theta and delta frequency slowing and a “drowsy burst” of frontally dominant theta activity in the third and fourth seconds. Such findings (more...)

Figure 11.. Lambda waves.

Figure 11.

Lambda waves. Lambda waves over posterior head regions, elicited by complex pattern viewing. Note the surface positive waveforms over both occipital regions. Longitudinal bipolar montage. Copyright 2013. Mayo Foundation for Medical Education and Research. (more...)

Provocation Techniques

During the wakeful EEG recording, provocative maneuvers are usually administered in an effort to produce possible background or epileptiform abnormalities, including hyperventilation and photic stimulation. In adults, hyperventilation often produces minimal change, but in children, adolescents, and young adults, a prominent high-amplitude or hypersynchronous background-slowing phenomena termed “buildup” is often seen and is considered a normal finding in these age groups. The expected normal findings during photic stimulation are either no change in the background, or a symmetrical “photic driving” response, consisting of entrainment of the background alpha rhythm to the same or a harmonic frequency variant of the administered flashing lights (see Figure 12, below). A similar finding is sometimes seen over the frontal head regions induced by photic stimulation, but this represents instead evoked responses from retinal neurons, the electroretinogram (ERG), which is distinguished by its purely anterior (rather than posteriorly predominant photic stimulation) response (see Figure 13, below). See the section on Abnormal Background for further discussion concerning typical abnormalities induced by activating procedures during EEG.

Figure 12.. Photic stimulation.

Figure 12.

Photic stimulation. Photic stimulation responses include either no change in the background or, as shown below, symmetrical entrainment of the background posterior rhythms over the occipital region. Longitudinal bipolar montage. Photic stimulus marked (more...)

Figure 13.. Photic stimulation may also induce an anteriorly dominant frequency in the EEG, but this emanates from evoked retinal neuronal responses, the ERG.

Figure 13.

Photic stimulation may also induce an anteriorly dominant frequency in the EEG, but this emanates from evoked retinal neuronal responses, the ERG. The ERG artifact is caused by retinal depolarization induced by photic stimulus shown in FP1 and FP2 derivations (more...)

Drowsiness and Sleep

During drowsiness, the first discernible change is gradual loss of the frequent muscle and movement artifacts and reduction of blinks and rapid lateral eye movements. Instead, a very slow frequency of 0.25 to 1.0 Hz in the frontal and lateral frontal channels emerges. These are slow rolling eye movements, or SEMS (slow eye movements of sleep), which begin in drowsiness and persist through stage 1 sleep, until gradually being lost with deeper stages of non-rapid eye movement (NREM) sleep. The EEG during drowsiness contains slower, synchronous frequencies of theta and delta throughout the background (see Figure 14).

Figure 14.. Example of drowsiness from a normal adult EEG recording.

Figure 14.

Example of drowsiness from a normal adult EEG recording. Note the prominent theta and delta activity, lack of eye movements or blinks, lack of muscle or movement artifact, and early suggestion of slow lateral rolling eye movements best seen in the F7 (more...)

Defining features of sleep stages are listed in Table 1. NREM sleep is classified as light NREM (stages 1–2; now termed N1–2) and deeper slow wave sleep (SWS, formerly known as stages 3–4; N3–4), as well as REM sleep. Typically, approximately 75% of the night is spent in NREM sleep and up to 25% in REM sleep. Stage 1 (N1) sleep is contiguous with drowsiness and is characterized by SEMS and slower theta and delta EEG frequencies of 1 to 7 Hz, with less than 50% alpha frequency activity in a 30-second epoch. It is easily marked by the appearance of vertex waves (V-waves); sharply contoured, fronto-centrally predominant waves (Figure 15). During stage 2 (N2) sleep, more delta frequency background begins to emerge, and the defining features of sleep spindles, K-complexes, and posterior occipital sharp transients of sleep (POSTS) are seen (Figures 16, 17). Sleep spindles are thought to reflect the synchronous activity mediated by thalamo-cortical neuronal networks. SWS (N3) has similar features, but less spindles, K-complexes, and POSTS are seen and even more delta frequency activity emerges (Figure 18).

TABLE 1.

TABLE 1.

Defining Features of Sleep Stages on the EEG

Figure 15.. Stage 1 (N1) sleep.

Figure 15.

Stage 1 (N1) sleep. Characterized by slow rolling eye movement artifacts, and slower theta and some delta frequencies in the EEG background. V-waves (V) also typically occur. Copyright 2013. Mayo Foundation for Medical Education and Research. All rights (more...)

Figure 16.. Stage 2 (N2) sleep.

Figure 16.

Stage 2 (N2) sleep. Slower theta and some delta (by definition, less than 20% of background of delta range slowing) frequencies in the EEG background. K-complexes and sleep spindles are the hallmarks of N2 architecture. Copyright 2013. Mayo Foundation (more...)

Figure 17.. Slow wave sleep (N3) contains greater than 20% high-voltage (>75 μV crest to crest) delta frequencies and fewer K-complexes and spindles.

Figure 17.

Slow wave sleep (N3) contains greater than 20% high-voltage (>75 μV crest to crest) delta frequencies and fewer K-complexes and spindles. The figure below was taken from a full EEG recording during a polysomnogram, since N3 sleep is typically (more...)

Figure 18.. REM sleep is characterized by a more typically wake-appearing, desynchronized, mixed-frequency background, which may contain alpha frequencies, characteristic centrally dominant sharply contoured sawtooth waves, and rapid eye movement artifacts in lateral frontal electrode sites.

Figure 18.

REM sleep is characterized by a more typically wake-appearing, desynchronized, mixed-frequency background, which may contain alpha frequencies, characteristic centrally dominant sharply contoured sawtooth waves, and rapid eye movement artifacts in lateral (more...)

REM sleep was previously known as paradoxical sleep, because REM actually resembles the waking EEG more closely than NREM sleep, having a desynchronized, low-voltage background. There are also fronto-central, sharply contoured theta frequencies called sawtooth waves, as well as REM artifacts seen in lateral frontal sites (Figure 18). Proper sleep-staging criteria also require features of very low-voltage chin electromyography (EMG) and eye movements recorded by electrooculogram (EOG) channels, but these polysomnographic channels are not routinely recorded during outpatient EEGs.

Copyright ©2016 by American Epilepsy Society.

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Bookshelf ID: NBK390343

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