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As the name implies, alpha rhythm has activity ranging from 8 to 13 Hz (alpha frequency). The frequency is the same in both hemispheres and remains constant during the recording, except for two situations. During drowsiness, it may slow by 1 Hz, and immediately after eye closure, it may be a little faster. The latter is known as alpha squeak (Figure 11.1) Normal individuals, even the elderly, should have an alpha rhythm that is at least 8.5 Hz. Only 1% of normal individuals will have a slower frequency, so when it is seen, abnormality should be suspected (9).

Alpha activity is of highest amplitude in the occipital region. It may project to central and temporal regions, particularly in children and young adults. In an ear, reference montage frontal leads may show alpha rhythm as well due to contamination of the reference (ear) with the alpha activity.

The morphology of the alpha rhythm is usually sinusoidal and regular. It may appear peaked at the top or bottom of the waveform if there are superimposed beta frequencies; this is referred to as apiculate alpha activity (10) (Figure 11.2). Apiculate alpha activity can be differentiated from sharp waves by its association with similarly shaped waveforms (ie, “does not disrupt the background”), location, disappearance during sleep, and absence of an aftergoing slow wave.

Amplitude of the alpha activity varies during the tracing and between the hemispheres. More often the amplitude on the right side is higher; unless the amplitude on the left side is less than 50% of that on the right, it should not be consider abnormal. Amplitude differences occur because of the thickness of the occipital bone (7). Amplitude asymmetries are best assessed in an ear reference montage. Waxing and waning of the amplitude can also occur when two frequencies (ie, 10 and 11 Hz) occur together. This is referred to as “beating” of the alpha activity.

Reactivity is another important feature of alpha activity. Alpha activity is seen in relaxed wakefulness with eyes closed. The disappearance, or blocking, of the alpha activity with eye opening, stimulation, or even mental concentration is known as reactivity. Blocking of the alpha activity should occur simultaneously in both hemispheres; if it occurs only on one side, the other is abnormal. This is called Bancaud phenomenon. In some patients, alpha activity disappears with eye closure but appears with eye opening. This is known as paradoxical alpha rhythm. This does not have a pathological significance. Reactivity of the alpha activity should be tested in every patient with eye opening and closing or with other types of stimulation.

Alpha rhythm is more appropriately referred to as posterior dominant rhythm in young children as the frequency of this activity is less than alpha frequency. Between term and 3 months, a clear posterior dominant rhythm is not present. After 3 months, an anterior-to-posterior gradient appears, and the posterior dominant rhythm frequency is about 3.5 to 4 Hz. It is 4 Hz by 6 months, 6 Hz at 12 months, and 7 Hz at 18 months. At 2 years, the posterior dominant background is 8 Hz and increases to 9 Hz by 7 years. By late childhood (around 10 years), it reaches 10 Hz (Table 11.1). In older children and adolescents, high-amplitude delta waves often interrupt the posterior dominant rhythm. These posterior slow waves of youth consist of delta waves with overriding alpha activity. The delta waves and the overriding activity are reactive to eye opening and stimulation (11,12).

In the elderly, the frequency of the alpha activity slows slightly, from 10–11 Hz to 8.5–9 Hz. However, as noted earlier, a frequency of 8 Hz, even in this age group, is considered abnormal. The distribution of the alpha activity also moves more anteriorly, and is often noted more in the frontocentral regions. The persistence and voltage also decrease with age.

All these characteristics of the alpha activity should be noted. However, the absence of alpha activity is not an abnormality. A number of adults may not have an alpha rhythm; instead, their occipital rhythm is a low-amplitude activity, which will be discussed later.

Beta Activity

Beta activity is EEG activity that is greater than 13 Hz. Unlike alpha rhythm, beta activity is defined only by its frequency. Most beta activity is between 15 and 25 Hz. It commonly occurs in the frontal and central regions in awake individuals (Figure 11.3). Unilateral attenuation of this activity can be seen with movement of the contralateral limb. Beta activity is usually of low amplitude, often less than 20 µV.

Widespread beta activity may also be seen in some individuals and may be a medication effect (Figure 11.4). Benzodiazepines, barbiturates, and other sedatives cause an increase in the amplitude of beta activity, thus making it more prominent. Beta activity persists in light sleep and rapid eye movement (REM) sleep, but is less common in slow wave sleep. A particular type of beta activity, fast alpha variant, is noted in the occipital region and is discussed with normal variants.

Mu Rhythm

Mu rhythm is an arch-shaped activity seen over the central or centroparietal regions. Its frequency is similar to alpha rhythm, 8–11 Hz. Mu rhythm is asymmetric and asynchronous over the two hemispheres and is frequently interspersed with beta activity (Figure 11.5). Mu activity blocks when the individual is asked to move the contralateral extremity. Paradoxical mu rhythm is when this activity appears with contralateral limb movement. At times, the apiculate phase of the mu rhythm can resemble spikes, particularly when there is an overlying skull defect. The lack of an aftergoing slow wave, typical location, and reactivity can help differentiate benign mu activity from central spikes. Mu activity is seen more commonly in younger individuals and decreases with age.

Lambda Waves

Lambda waves are saw tooth–shaped waveforms of positive polarity seen in the occipital region. They are usually between 160 and 250 ms in duration and less than 50 µV in amplitude. Lambda waves are usually bilaterally synchronous, though they can occur asymmetrically and mimic sharp waves. They occur when an individual is scanning a complex visual image and can be eliminated when asked to look at a blank white sheet of paper (10). As they are best seen with visual scanning, eye blink artifact is often seen with lambda waves (Figure 11.6). These waves are most often seen in younger individuals, and they decrease in the elderly.

Low-Voltage EEG

In some individuals, a clear alpha rhythm cannot be identified. Instead, their background activity is a low-amplitude activity with beta, alpha, and theta frequencies. The amplitude of this activity is usually less than 20 µV (Figure 11.7). A low-voltage EEG is seen more often in older individuals than in children and is not considered an abnormality unless a previous EEG in the same patient showed clear alpha rhythm. When all activity is less than 10 µV it may be abnormal.

NORMAL SLEEP EEG

There are several characteristic EEG waveforms that occur in sleep. They occur in different stages of sleep, and their presence or absence helps determine the sleep stage. These waveforms will be discussed first, and then features of the various sleep stages will be presented.

FIGURE 11.5 Mu activity occurring asymmetrically over the central regions.

Vertex Waves

Vertex waves, also called V waves, are surface-negative, biphasic discharges that are of maximal amplitude over Cz (vertex, hence their name). They can project to Fz and Pz, as well as to parasagittal frontal and central electrodes. A phase reversal over Cz is seen in a transverse bipolar montage, which is best for identifying sleep architecture (Figure 11.8). Vertex waves decrease in amplitude with age, and in young children these waves may be of high amplitude and phase reverse over Fz. Occasionally, they can be asymmetric and resemble central spikes. Persistent asymmetry should raise suspicion of an abnormality. Vertex waves can occur in runs or with other sleep architecture. They are mostly seen in light stages of sleep (stage I) but can occur with deeper stages as well. Rarely low-amplitude vertex waves may be seen in awake individuals. A loud noise or an alerting stimulus can induce vertex waves in a sleeping individual.

Positive Occipital Sharp Transients of Sleep

Positive occipital sharp transients of sleep (POSTS) are monophasic, triangular waveforms of positive polarity. They are common and are seen in most normal individuals. As their name implies, POSTS are seen in the occipital region during light sleep. They often occur synchronously but can occur independently. POSTS usually recur at an irregular frequency; however, they can occur in trains of about 1 per second. Morphologically, they resemble lambda waves, and when they are of high amplitude, they can mimic sharp waves (Figure 11.9). Their monophasic morphology, location, and occurrence only in light sleep differentiate them from epileptic sharp waves.

Sleep Spindles

Sleep spindles are a series of rhythmic waves occurring at a frequency of 12 to 14 Hz and lasting at least 0.5 second. They are best seen over the vertex, but have a wide distribution. In adults, sleep spindles are symmetric and synchronous (Figure 11.10). They are a hallmark of stage II sleep, but can be seen in deeper stages. In deeper stages of sleep, sleep spindles occur at a slightly slower frequency and are more prominent over Fz.

K Complex

A K complex is a biphasic with an initial sharp component, followed by a slow wave. Its distribution is similar to that of vertex waves. Sleep spindles usually follow the slow wave of a K complex (Figure 11.10). They are seen in stage II sleep and auditory stimuli can induce K complexes in a sleeping individual. It should be noted that although there is an amplitude criteria for K complexes in polysomnography, similar criteria do not apply in EEG.

FIGURE 11.8 Vertex wave seen over the vertex in a transverse montage.

Delta Waves

REMs are not an EEG waveform, rather they are a useful biologic artifact. Eye movement artifact is seen most prominently in the frontal region. REM are differentiated from slower eye movements by the upslope of the deflection, which is less than 300 ms (16) (Figure 11.11). REMs are usually lateral eye movements. Lateral eye movements will show out-of-phase deflections in the temporal chain (F7, F8) of an anterior posterior bipolar (double banana) montage. REMs are seen in wakefulness as well as REM sleep.

Slow Eye Movements

Slow eye movements (SEM) are differentiated from REM by an upslope that is greater than 500 ms. The distribution of SEM is similar to that of REM. SEM, however, are seen in drowsiness and light sleep, and they disappear in deeper stages of sleep.

Sleep Stage I

The lightest sleep is stage I, also referred to as stage non-REM 1 (N-1). In early stage I, or drowsiness, there is slowing of the alpha rhythms, persistence of frontocentral beta frequencies, and appearance of SEM. Once the alpha rhythm disappears, the EEG consists of low-amplitude, mixed-frequency activity. Vertex waves and POSTS also appear in this stage. Stage I is a short-lived stage, often quickly transitioning to stage II.

FIGURE 11.11 REM noted best in Fp1/Fp2 – F7/F8 channels.

Sleep Stage II

Hyperventilation for 3 to 5 minutes is commonly performed in EEG laboratories. Hypocarbia induced by hyperventilation and the resulting cerebral vasoconstriction and hypoperfusion is thought to be responsible for the changes that occur. A normal response consists of gradually increasing theta frequencies, followed by rhythmic delta bursts, and finally generalized, continuous, rhythmic delta activity. This activity is initially noted in the frontal region in adolescents and adults (Figure 11.12). Sixty to ninety seconds after stopping hyperventilation, the slow activity begins to subside. Hyperventilation-induced slowing is more prominent if the individual’s blood sugar is low (long time since last meal) or if significant cerebral ischemia occurs. It is much more remarkable in younger patients and is difficult to elicit in the elderly.

Abnormal responses to hyperventilation include generalized spike and wave discharges, focal spikes, or lateralized slowing. Lateralized slowing may be more evident in the posthyperventilation period when the generalized delta activity is subsiding. If epileptiform discharges or focal slowing is not seen, a hyperventilation response should be considered normal, even if it induces remarkable rhythmic delta slowing. Because it induces hypocarbia and cerebral vasoconstriction, hyperventilation should not be performed in patients with significant cardiopulmonary disease, acute stroke, sickle cell disease, or pregnancy.

Photic Stimulation

Photic stimulation consists of brief bursts of light applied at frequencies of 1 to 30 Hz. The light produces a visual evoked potential that can be recorded best in the occipital area. At frequencies close to an individual’s alpha rhythm, each flash evokes a time-locked response. This is known as photic driving. Photic driving can also occur at subharmonic or harmonic frequencies of the stimulus (Figure 11.13). The probability that such a driving response will be seen can be increased when the eyes are closed and the stimulator is less than 30 cm from the patient. The amplitude of photic driving can be different on the two sides; even an amplitude asymmetry of 50% is not abnormal (7). Another type of normal response to photic stimulation is a photomyoclonic response. This consists of contractions of the frontalis or periocular muscles at the same frequency as the stimulus. The artifact created by muscle twitching is noted in the frontal leads 50 to 60 ms after the stimulus.

Rhythmic temporal theta bursts of drowsiness (RTTBD) were previously referred to as psychomotor variant and rhythmic midtemporal discharges (RMTD). As the name implies, this patterns consists of 5 to 7 Hz discharge in the temporal regions that occurs during relaxed wakefulness or drowsiness. The theta waves have a flat-topped, sharp, or notched appearance (Figure 11.14). RTTBD occur bilaterally or independently over the two hemispheres and are seen mostly in adolescents and young adults. The discharge is monomorphic and does not evolve in frequency, differentiating it from a seizure discharge. This resemblance with a temporal lobe seizure discharge was why it was initially called psychomotor variant. It is no longer considered to have clinical significance and is seen in approximately 2% of normal adults (18).

Midline Theta Rhythm

Midline theta rhythm is a 4 to 7 Hz discharge seen most prominently over Cz but also spreading to parasagittal leads. The discharge can have an archiform, sinusoidal, or mu-like appearance. It has variable reactivity to eye opening and limb movement (Figure 11.15). Like RTTBD, this rhythm is seen in relaxed wakefulness and drowsiness. When originally described, midline theta rhythm was thought to be associated with epilepsy, though now it is considered a normal variant (10).

A number of variants of the alpha rhythm have been described. All have the same distribution and reactivity as normal alpha rhythm and often occur admixed with it. The slow alpha variant has a frequency of 4 to 5 Hz and can have a notched appearance (Figure 11.16). The fast alpha variant occurs at a harmonic of the underlying alpha rhythm, usually 16 to 20 Hz (Figure 11.17). Both slow and fast alpha variants are normal physiologic rhythms.

Subclinical Rhythmic Electrographic Discharge in Adults

Subclinical rhythmic electrographic discharge in adults (SREDA) is an uncommon discharge seen mostly in older adults. It occurs in relaxed wakefulness or drowsiness and consists of rhythmic theta and delta waves that evolve in frequency. Usually, it is a generalized pattern but can be more prominent focally. The usual duration is 20 to 40 seconds, but it can last several minutes. The onset of an SREDA pattern is either with a run of monomorphic sharp waves or high amplitude delta waves that suddenly interrupt the background (Figures 11.18A and 18B). Though this pattern resembles a seizure discharge, there is no alteration of consciousness or changes in cerebral blood flow (19). Consequently, SREDA is considered a benign EEG phenomenon.

Small Sharp Spikes

Small sharp spikes (SSS), also known as benign epileptiform transients of sleep (BETS), are common transients seen in sleep stages I and II in about 25% of adults (20). They are monophasic or biphasic and have an amplitude less than 50 µV and duration less than 50 ms. There may be an aftergoing dip in the background (slow wave), however its amplitude is less than that of the spike. SSS are unilateral, though can be reflected on the contralateral hemisphere. At times their polarity is complex, with an oblique dipole extending over both hemispheres (Figure 11.19). They are best seen in a referential montage with long interelectrode distances. SSS do not occur in runs, nor is there associated focal slowing. It is important not to confuse SSS with epileptiform spikes as they are not associated with epileptogenicity and are considered a normal variant.

FIGURE 11.18B This is the page after Figure 11.18A. The rhythmic discharge continues with evolving frequencies for about 90 seconds.

Fourteen- and six-hertz positive bursts, previously called ctenoids, consist of short (0.5 to 1 second) runs of positive sharp waves that are best seen over the temporal areas but have a widespread field. The bursts have either a 14-Hz (13 to 17 Hz) or a 6-Hz (6 to 7 Hz) frequency, though the 14-Hz discharges are more common. They are best visualized in a referential montage with long interelectrode distances. Between the spiky positive components is a rounded negative phase (Figure 11.20). These discharges are seen mostly in drowsiness and light sleep and occur mostly in adolescents and young adults and decrease with age. They can be differentiated from epileptiform spikes by their distribution, lack of aftergoing slow wave, and typical morphology.

Six-Hertz Spike and Wave Bursts

Six-hertz spike and wave bursts are runs of bilaterally synchronous bursts of spike and slow wave discharges occurring at a frequency of 5 to 7 Hz, mostly 6 Hz. The spike is often low amplitude and buried in the slow wave, hence the previous name, phantom spike and wave. The bursts last 1 to 2 seconds and occur in relaxed wakefulness and drowsiness, disappearing in deeper stages of sleep (Figure 11.21). They are mostly seen in adolescents and young adults, becoming less common in older adults.

Two types of 6 Hz spike and wave bursts have been described (21). The FOLD (female, occipital, low amplitude, drowsy) variety is thought to be a benign variant. The WHAM (wake, high amplitude, anterior, male) variant is more likely to be associated with epilepsy. Slower discharges with a high-amplitude spike component are more likely to be abnormal and associated with epilepsy.

Wicket Spikes

Wicket spikes are sharp waves that are usually between 90 and 150 ms in duration and less than 200 µV in amplitude. They occur in runs or in isolation independently in both temporal regions during drowsiness and light sleep. Wicket spikes can be differentiated from epileptiform spikes by the lack of aftergoing slow waves, no associated slowing, disappearance in deeper stages of sleep, and no disruption of underlying background (Figure 11.22). Isolated wicket spikes can be correctly recognized by comparing their morphology to that of a train of wicket spikes, which will be similar. It is widely thought that wicket spikes are fragmented temporal alpha rhythm and a normal variant (10).

Breach rhythm is seen over areas of a skull defect. Since bone acts as a high-frequency filter, EEG overlying a skull defect has higher amplitude and faster frequencies compared to the other side (Figure 11.23). Highest amplitude breach rhythm is seen over the central region, where the amplitude may be three times as high as the other side. This may make underlying mu rhythm and wicket spikes look deceptively abnormal. Similarly, single sharply contoured waveforms may look epileptiform. The absence of an aftergoing slow wave, lack of spread to adjacent areas, and disappearance of this activity in deeper stages of sleep should alert one to their benign nature. Breach rhythm is not abnormal and does not signify epilepsy or other pathology. It should be noted, however, that breach rhythm is often associated with focal slowing from underlying brain injury. In such a case, the focal slowing is considered abnormal but the breath rhythm is not.

ARTIFACTS

There are many types of artifacts that can occur in an EEG, and the most common ones will be discussed here. Artifacts can be divided into biological (arising from within the patient, but not the brain) and nonbiologic (arising from the environment). Attention to technical detail can minimize the occurrence of artifacts, but, sometimes, despite careful preparation, they are still seen. One of the foremost responsibilities of the electroencephalographer is to recognize these artifacts and not misinterpret them as abnormalities. To this end, understanding that abnormalities have a typical topographic field and particular characteristics is of critical importance. In addition, the technologist’s notations can be extremely helpful in identifying artifacts.

Electrocardiographic Artifact

The electrocardiogram (ECG) is usually recorded in a dedicated channel in most EEG. However, it often appears in EEG channels as a biologic artifact. When the ECG is recorded in a separate channel, the artifact is easy to identify by noting its occurrence with the ECG. The ECG artifact is seen most often in montages with long interelectrode distances, such as referential montages, particularly when A1 or A2 are used as the reference. One ear represents the negative end (usually A2) of the cardiac dipole and the other ear the positive end (usually A1) (5). Obese patients, those with short necks, and neonates are more likely to have prominent ECG artifacts. This artifact can be reduced by linking A1 and A2, by using Cz as a reference, or by changing the position of the head (ie, extension).

Asymmetry of eye movement artifact can occur due to several reasons. Placement of frontal leads that are not symmetric is a common cause of such asymmetry. Enucleation of one eye will result in absence of artifact noted on that side (Figure 11.24). A skull defect over one frontal region will cause the eye movement artifact on that side to have higher amplitude (22).

When persistent frontal delta activity is seen, such as frontal intermittent rhythmic delta activity (FIRDA), additional leads should be placed below the eyes (eye leads). These leads will help differentiate frontal delta activity from eye movements (such as flutter). Eye movement activity will be out of phase in the eye and frontal leads referenced to ipsilateral ear; when the eyes move upward, a positive deflection is seen in the frontal leads and a negative deflection in the eye leads. On the other hand, FIRDA will be in phase in both leads as both leads will have the same polarity. Whereas most activity that is out of phase in the eye and frontal leads arises from the eyes, an exception is frontopolar spikes or slow waves. These will have opposite polarity in the eye and frontal leads, leading to out-of-phase activity.

Electromyographic (EMG) artifact is another common biologic artifact. It often occurs as repetitive single motor units that have either a positive or negative deflection; this is referred to as a comb-like appearance. This type of EMG artifact is usually confined to one electrode as nearby electrodes will not display the same motor unit. It is a low-amplitude potential that has a duration typically less than 50 ms (Figure 11.25). If the high-frequency filter is reduced, it can make the EMG artifact look like epileptiform spikes; this is discouraged. EMG artifact is commonly seen in the temporal leads (from the underlying temporalis muscle). Asking the patient to relax her/his jaw often eliminates it. Another common location of this artifact is the frontal leads (from the underlying frontalis muscle). Photic stimulation can produce contraction of the frontalis muscle that will be recorded from the frontal leads. This produces the photomyoclonic response described previously. Persistent contraction of underlying muscles can produce excessive EMG artifact that can make the underlying EEG unable to be interpreted.

Lateral Rectus Spikes

Lateral rectus spikes are a type of EMG artifact that occur with contraction of the lateral rectus muscle. The spikes are low-amplitude, short-duration discharges that have a rapid upslope and a slower downslope. They are best seen in frontal electrodes and often occur with eye movements (Figure 11.26). They must be differentiated from frontal spikes; lateral rectus spikes do not have an aftergoing slow wave, are often seen with eye movements, are limited to frontal electrodes, and disappear in sleep.

Glossokinetic Artifact

Glossokinetic artifact occurs when there is tongue movement. The tip of the tongue is negatively charged compared to its base. Thus, when the tongue moves, artifact can be seen on scalp electrodes. The artifact is best seen along the temporal chain; however, it can vary depending on the position of the tongue. It consists of a burst of delta activity accompanied by EMG artifact.(Figure 11.27)

Electrode artifact, also known as electrode pop, is a type of nonbiologic artifact that produces discharges that look remarkably different from cerebral potentials. They are typically confined to the single electrode that is at fault. At times, it can produce a high-amplitude negative phase reversal between two channels in a bipolar montage, resembling a spike (Figure 11.28). It should be differentiated from a spike by its very restricted field (confined to a single electrode), its association with other bizarre-looking discharges in the same channels, and disappearance with reapplication of the electrode. These artifacts most commonly result from poorly applied electrodes but can also occur due to a broken electrode wire, drying of electrode gel, or change in the scalp–lead interface (22). When an electrode artifact is seen, the technologist should reapply the electrode, and if it does not disappear, replace it.

Environmental Artifact

Environmental artifacts are another type of nonbiologic artifact that can be very challenging to isolate. They are often seen in hostile recording environments such as intensive care units or the operating room. Common examples include 60-Hz line artifact, drip artifact from intravenous bags, respirator artifact, and rhythmic artifacts generated by percussion beds. The technologist is instrumental in helping correctly identify these artifacts (ie, by noting the cycles of the respirator or percussion bed, by applying additional electrodes to an intravenous solution line to directly record that artifact, and displaying it in a separate channel).

 

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