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Visual Analysis of the EEG:: Wakefulness, Drowsiness, and Sleep


An orderly approach to visual analysis of the EEG is important, especially for those who are beginning to hone their EEG reading skills. Although not all EEG records necessarily lend themselves to a single reading approach, it is useful to start the process of record interpretation with a preplanned analysis strategy that is based on the findings of a typical EEG, such as the EEG of a normal adult or child. The approach can be modified from this starting point when more atypical tracings are encountered.

There are two fundamental strategies for EEG analysis and a good approach to reading includes a combination of both strategies. The first strategy consists of making a mental list of the EEG elements that one would expect to see in the EEG given the patient’s age and sleep state and identifying and analyzing each of these elements in turn. The second strategy consists of examining the array of waveforms present on the page, identifying each, and classifying each as a normal element, an abnormal element, or an artifact. In summary, the first strategy consists of making a list of “what do I expect to see?” and attempting to find each element in the list of expected findings in the EEG record. The second step is to survey the landscape of the EEG and to attempt to identify each waveform that one sees. Of course, the two strategies are complementary and can be carried out in any order; combining the two strategies ensures that the reader will consider everything that does appear on the EEG page but will also notice what is absent from the EEG record but should be there.

The purpose of this chapter is to give a brief overview of a normal EEG tracing, including transitions from wakefulness to sleep and then back to wakefulness. Next, the elements involved in these transitions are examined more closely.

QUICK TOUR: Transition of the EEG from Wakefulness to Drowsiness, Sleep, and Arousal from Sleep

Wakefulness

Figure 2-1 shows the typical appearance of the EEG in a patient who is awake with eyes closed. The basic setup of the EEG page is summarized in the figure’s caption. The most prominent rhythm on the page is denoted by the solid black arrows and is called the posterior rhythm . Note that this waveform is highly rhythmic and sinusoidal (i.e., shaped like a sine wave). It is of highest voltage in the posterior or occipital channels (black arrows) and becomes much less prominent in the anterior channels. The posterior rhythm is best seen when the patient is awake with eyes closed.

Table 2-1
Summary of Transition from Wakefulness to Sleep in the Routine EEG
Awake Eyes closed: posterior rhythm present
Eyes open: low-voltage, nondescript pattern seen posteriorly, posterior rhythm absent
Drowsy Mild slowing of the posterior rhythm
Slow roving lateral eye movements appear
Disappearance of posterior rhythm in the occipital areas, replaced by low-voltage theta activity
Diffuse increase in theta range activity, particularly at the vertex
Stage I Sleep Vertex waves of sleep
Stage II Sleep Sleep spindles
K-complexes
Arousal High-voltage hypersynchronous (rhythmic) slowing in some
Return of posterior rhythm and typical waking patterns

Figure 2-1, In this normal, awake electroencephalogram, the posterior rhythm is the most prominent waveform on the page. Each horizontal wave is generated by recording from the pair of electrodes denoted in the left margin. Each vertical division represents one second. Odd-numbered electrodes are placed on the left side of the scalp and even-numbered electrodes are on the right; those with z-subscripts (for “zero”) are in the midline. The initial letters of the electrode names represent different brain regions: (Fp) frontopolar, (F) frontal, (T) temporal, (P) parietal, (C) central, and (O) occipital.

Another typical feature of wakefulness is the presence of an anteroposterior gradient of voltage and frequency. Anteriorly, waves are generally of lower voltage and higher frequency. Posteriorly, waves are of higher voltage and lower frequency. Comparing the first line and the fourth line of this figure bears out these relationships. The top channel is relatively flat and has a lower voltage, higher frequency (faster) waveform. The fourth line has a higher voltage, lower frequency (slower) waveform. This is what is meant by the anteroposterior gradient: lower voltage, faster activity anteriorly and higher voltage, slower activity posteriorly. Additional examples of the anteroposterior gradient are given later in this chapter.

As seen in Figure 2-2 , the posterior rhythm suppresses and often disappears completely with eye opening and fixation of gaze. When the eyes are closed, the rhythm returns. In summary, the posterior rhythm is a rhythm of wakefulness that is present when the eyes are closed. During wakefulness with the eyes open, the EEG shows a lower voltage, nondescript pattern in the occipital region, as is seen in Figure 2-2 , between eye opening and closure.

Figure 2-2, EEG of the same patient shown in Figure 2-1 , awake, demonstrating the effect of spontaneous eye opening and closure on the posterior rhythm. The posterior rhythm suppresses dramatically with eye opening. Note also that the posterior rhythm actually begins to return 1.5 seconds before the eyes close, suggesting a period of relative visual inattention. The exact moment of eye closure is marked by the eye-closure artifact seen in the frontal leads (hollow arrow).

Drowsiness

One of the first EEG changes seen in drowsiness is a subtle slowing of the posterior rhythm. In Figure 2-3 , the posterior rhythm is seen to slow over the course of the page from 10 Hz in the first second of the page to 8 Hz in the seventh second, an early indication of drowsiness in this patient. (This brief period of posterior rhythm slowing is not always identifiable; sometimes the posterior rhythm simply “drops out” without an observable period of slowing.) Another, more subtle finding is that of slow roving lateral eye movements of drowsiness, which are indicated by the shaded rectangles (see figure caption for further explanation). Such slow roving eye movements are commonly detected by the EEG but are not actually visible on casual observation of the patient because they are hidden by the patient’s eyelids. Although the appearance of slow roving eye movements in the EEG technically represents an artifact (because they are not actual brain waves), they still provide useful information to the reader regarding onset of drowsiness. The EEG appearance of slow roving eye movements is discussed in more detail in Chapter 6 .

Figure 2-3, The initial transition to drowsiness is marked by a slowing of the posterior rhythm. A frequency of 10 Hz can be counted over the black brace in the first second of this page. The frequency falls to approximately 8 Hz by the seventh second over the blue brace. Artifact from slow roving eye movements can also be seen: note the subtle spreading apart of the waveforms of the two channels that include F7 (top blue rectangle) compared with a relative narrowing together of the two channels that include F8 (bottom blue rectangle). The appearance of this artifact is caused by a slow roving movement of the eyes to the left, a sign of early drowsiness, and is described in more detail in Chapter 6 .

The next EEG page ( Figure 2-4 ) in this example shows two additional important changes that mark advancing drowsiness: first, the posterior rhythm has dropped out nearly completely. Second, there is an increase in theta-range (slow) activity throughout the tracing. Most characteristically, theta activity has appeared at the vertex, particularly in the midline central (Cz) electrode, although it may be seen in other locations as well.

Figure 2-4, This page shows the transition from deepening drowsiness to light sleep. The posterior rhythm completely disappears after the first second and vertex activity increases (black arrow) in the form of theta waves. More low voltage theta range (slow) activity is seen in other brain areas as well.

On the following EEG page ( Figure 2-5 ), the first true vertex waves of sleep are seen. These midline sharp waves mark the onset of Stage Ia sleep and may occur in dramatic bursts. After they are established, assuming no subsequent arousals, vertex-wave bursts continue in a repetitive fashion through Stage II sleep.

Figure 2-5, In this figure, vertex waves become well established. Note that the vertex wave voltage is highest in the C3, Cz, and C4 electrodes (as evidenced by phase reversals in those locations—see black arrows). A second set of vertex waves is seen near the end of the page. Rhythmic vertex theta can be seen between the two larger vertex-wave bursts. The appearance of vertex waves marks the onset of Stage Ib sleep.

Stage II Sleep

The onset of Stage II sleep is defined by the appearance of sleep spindles. The sleep spindles that occur early in Stage II sleep are usually of maximum voltage in the central electrodes (C3 and C4) and at the central vertex (Cz), as is seen in this example. They consist of lower voltage, regular 14-Hz waves lasting from 1 to a few seconds. In deeper Stage II sleep, the field of sleep spindles may include both the frontal and central areas. Figure 2-6 shows the appearance of the first, bicentral sleep spindles in this patient, intermixed with vertex waves. By the next page, the sleep spindles become more sharply defined (see Figure 2-7 ) and continue to be intermixed with repetitive vertex waves. The combination of repetitive vertex waves and spindles marks well-established Stage II sleep. An example of the fields of spindles and vertex waves is shown in Figures 2-8 and 2-9 and schematically in Figures 2-10 and 2-11 .

Figure 2-6, Transition from Stage Ia to Stage II sleep is marked by the appearance of sleep spindles. The sleep spindles are the lower voltage, sinusoidal 14-Hz waves that are following close on the heels of the vertex waves (a portion of the spindles is highlighted by the blue rectangles). Sleep spindles are also seen surrounding the vertex wave on the right side of the page (arrows). The spindles become better defined in the next examples.

Figure 2-7, Well-established sleep spindles (arrows) following a vertex wave. These spindles are maximum in the frontocentral regions and in the midline. Repetitive vertex waves continue. Note that spindles often follow vertex waves, although the link between the two is not always consistent.

Figure 2-8, The blue rectangles highlight the brain areas in which vertex waves and spindles are seen most prominently, here displayed in a bipolar montage. Note that the temporal chains, represented by the four-channel groupings at the top and bottom of the page not included in the blue rectangles, are relatively uninvolved with the vertex wave and spindle waveforms.

Figure 2-9, The same vertex waves and spindles displayed in a referential montage. Because referential montages dedicate one channel to each active electrode (compared to a pair of active electrodes per channel in bipolar montages), the individual electrodes that pick up the spindle and vertex-wave discharges are easier to discern. The blue rectangles highlight the field of these waves, which primarily include the central, frontal, and midline regions.

Figure 2-10, A schematic of the approximate field of vertex waves of sleep is shown. The maximum activity of vertex waves is at the Cz electrode with lesser voltages measured at the adjacent C3 and C4 electrodes.

Figure 2-11, Classically, spindles are centered over the central areas, particularly the C3 and C4 electrodes. In many examples such as the EEG traces shown in Figures 2-8 and 2-9 , spindles are also seen to spread frontally (F3 and F4 electrodes). The field of spindles only occasionally spreads laterally to the midtemporal electrodes where they should be of lower voltage compared with the central electrodes or not seen at all.

Bursts of high voltage waves occurring across nearly all channels may be seen sporadically in sleep. These discharges, called K-complexes, can be dramatic and are sometimes mistaken for spike-wave discharges, an epileptiform abnormality. K-complexes may be set off by stimuli (such as a noise) in the environment of the sleeping patient that cause a mild subarousal (an increase in the level of arousal or a lightening of the sleep state that is not strong enough to awaken the patient fully). In fact, EEG technologists often demonstrate K-complexes in the EEG by tapping a pencil on the EEG instrument while the patient is in light sleep. The tapping sound may elicit a subarovsal and an associated K-complex. Most K-complexes, however, appear to occur spontaneously without an obvious trigger. The field of a K-complex differs from that of sleep spindles or sleep vertex waves and is shown in Figure 2-12 . K-complexes may or may not be intermixed with a sleep spindle, as occurs in the example shown in Figure 2-13 .

Figure 2-12, The field of a K-complex is diffuse and may include all brain areas. This helps differentiate it from simple spindles, which are maximum frontocentrally and concentrated in the midline and parasagittal areas as described in Figure 2-11 . A sleep waveform of an intensity just as strong in the temporal areas as in the midline is not likely to represent a sleep spindle or vertex wave but may represent a K-complex.

Figure 2-13, A K-complex is seen during Stage II sleep (blue rectangle). At first glance, the K-complex resembles a vertex wave followed by a sleep spindle. Note, however, that the high-voltage waves preceding the spindles in this example do not have the typical distribution of a vertex wave of sleep. Although vertex waves are maximum in C3, Cz, and C4, the example of this particular wave shows frontal voltages that are just as high in the temporal chains (Fp1-F7, F7-T3, Fp2-F8, F8-T4) as in the parasagittal chains. The broad field extending to the temporal chains of the high-voltage wave that precedes the spindles is the tip-off that this complex is not simply a combination of a vertex wave and sleep spindles but a K-complex.

Arousal from Sleep

Arousal from sleep may occur uneventfully with a simple return of the posterior rhythm and other patterns of wakefulness described earlier. At other times, arousal from sleep may be marked by a dramatic run of diffuse, high-voltage rhythmic waves called an arousal hypersynchrony . Figure 2-14 shows a fairly simple arousal with a brief increase in rhythmic slowing followed by high-voltage motion artifact generated from the patient stirring in bed. This is followed by a return of the posterior rhythm.

Figure 2-14, Several elements of this EEG page signal an arousal from sleep. Diffuse rhythmic slow waves, as seen in the second second of this tracing, are typical of arousal and sometimes much more dramatic than in this example. The very high-voltage deflections that cross into neighboring channels are large motion artifacts, often seen at the time of arousal from sleep, which is typically associated with body movements. The very fast waves that turn some channels dark black, most prominent in the temporal chains at the top and bottom of the page, represent muscle artifact also associated with movements related to arousal. Note the reappearance of the posterior rhythm in the second half of the page following the high-voltage motion artifact.

The sequence of wakefulness to drowsiness to sleep is shown in a second patient in Figures 2-15 through 2-20 , with fewer figure markings to help the render practice identification of normal sleep waveforms.

Figure 2-15, An example of a normal, awake patient. The posterior rhythm is seen well in the posterior channels: T5-O1, P3-O1, Cz-Pz, P4-O2, and T6-O2. The frontal channels in each chain of four—Fp1-F7, Fp1-F3, Fp2-F4, and Fp2-F8—are darker than others because of artifact from the frontalis muscle. Bobbing waves seen in those channels represent artifact related to vertical eye movements made under closed eyelids (given the presence of the posterior rhythm).

Figure 2-16, During the course of this page, the posterior rhythm drops in frequency by 1 Hz and becomes less prominent (seen best in T5-O1, P3-O1, P4-O2, and T6-O2). Slow roving lateral eye movements are seen as evidenced by approximation of the Fp1-F7 and F7-T3 channels at the same time as a mild “bulging apart” of the Fp2-F8 and F8-T4 channels (arrows). The reason that lateral eye movements create this appearance is explained in more detail in Chapter 6 . An increase in theta activity, another sign of drowsiness, is seen at the vertex in the seventh and eighth seconds in the Fz-Cz channel (blue rectangle).

Figure 2-17, The first vertex waves appears at the beginning of the fourth second exclusively in Cz, visible in the Fz-Cz and Cz-Pz channels (small blue rectangle). The second vertex wave appears in the seventh second and now includes both Cz and the central electrodes, seen in the channels that include C3 and C4 (large blue rectangle).

Figure 2-18, Cascades of vertex waves of sleep are seen regularly in the midline channels (Fz-Cz and Cz-Pz) as stage 1b sleep becomes well-established. Overall, the background shows an increased number of low-voltage slow waves.

Figure 2-19, Vertex waves continue, and sleep spindles now make their appearance (arrows), marking the onset of Stage II sleep.

Figure 2-20, In deeper Stage II sleep, spindle activity becomes more prominent. Vertex waves also continue, seen in the last 2 seconds of the page.

Natural Sleep

The foregoing example represents a quick tour through wakefulness, drowsiness, Stage Ia sleep, and Stage II sleep, followed by an arousal and return to wakefulness. The reader may ask why this sequence does not include examples of deeper sleep such as sleep Stages III and IV and REM sleep. In practice, these deeper sleep stages are not typically encountered during routine EEG recordings. The amount of sleep recorded during a routine EEG is usually less than 20 to 30 minutes and sometimes just a few minutes if much time has been spent getting the patient to fall asleep for the test. During routine EEG testing, few patients have the time to enter Stage III, IV, or REM sleep during routine EEG recordings. Examples of these patterns are seen in Figures 2-21 through 2-23 .

Figure 2-21, A sample of Stage III sleep is shown containing approximately 50% delta activity. Some sleep spindle activity persists (arrows) but is more difficult to appreciate against the backdrop of the slow wave activity.

Figure 2-22, By definition, 50% or more of Stage IV sleep samples consist of delta activity, as is seen in this sample.

Figure 2-23, Rapid eye movement (REM) sleep, or dream sleep, is also referred to as paradoxical sleep because of the overall decrease in voltages that somewhat resembles an awake pattern. REMs are best seen in specialized channels designed to pick up eye movements but may also be detected in the frontal electrodes that are near the eyes, such as the large deflection seen in the four frontal channels in this example (arrow).

During natural sleep, normal individuals sequentially cycle through Stages I through IV and then back up to Stage I, after which they may enter a brief REM stage after 1 to 2 hours. The first third of the night’s sleep is dominated by slow-wave sleep, whereas REM sleep is most plentiful during the early morning hours before awakening, at which time the portions of the cycle devoted to REM sleep are lengthier.

VISUAL ANALYSIS OF THE EEG: Identification of Expected Elements

Wakefulness

The Posterior Rhythm

A good first step in the interpretation of the awake EEG is identification of the posterior rhythm, often the most distinctive and easily identifiable element of the waking EEG. The posterior rhythm is, as the name implies, best seen in the posterior head regions, although how far the field of the posterior rhythm spreads forward varies among patients. In some patients, the posterior rhythm is confined to the occipital areas, but, in many, the posterior rhythm spreads forward to include the whole of the posterior quadrants (see Figure 2-15 ), and at times the field may even reach the superior frontal electrodes (F3 and F4), as in Figure 2-1 . The posterior rhythm should never be visible in the frontopolar electrodes, however.

In addition to its location on the head, two other distinctive qualities define the posterior rhythm. First, it is a rhythm of wakefulness. Indeed, the posterior rhythm is the hallmark of EEG wakefulness in those patients who manifest such a rhythm. Second, the posterior rhythm is predominantly seen when the subject’s eyes are closed or when visual attention is lacking. The posterior rhythm may dramatically suppress when the eyes are opened, as was seen in Figure 2-2 . The posterior rhythm suppresses equally well when patients open their eyes spontaneously as when they open their eyes in response to a request. Although visual attention and eye opening usually go together, during certain intervals, a patient may have the eyes open but may not be attending visually (fixating) on a target. Because it is specifically visual attention rather than eyelid opening that causes suppression of the posterior rhythm, the posterior rhythm is occasionally seen at times when the eyes are open but the subject is visually inattentive. Although the posterior rhythm can be identified in the great majority of patients, in a small number of normal individuals, no posterior rhythm is seen on scalp-recorded EEG. Absence of the posterior rhythm as an isolated finding is not necessarily considered an abnormality.

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