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.

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.

Each of the four major chains of electrodes can be examined from the anterior to posterior direction, as indicated by the four vertical arrows on the left side of the page. The posterior rhythm, a well-formed sinusoidal rhythm, becomes more prominent as each chain is scanned from front to back and is best defined in the channels that include the occipital electrodes, O1 and O2 (solid arrows). In this patient, as in most, the posterior rhythm is of slightly higher voltage over the right occipital area (bottom two solid arrows) compared with the left (top two solid arrows).

The normal awake EEG tracing often manifests two types of voltage and frequency transitions seen between anterior and posterior head regions, termed the anteroposterior gradient of the EEG: going from front to back, the amplitude of waves generally increases and the frequency of waves decreases, paralleling the shading of the arrows. Note that anteriorly in the brain (the top channels of each set of four), voltages are low, and more fast activity is seen. Posteriorly (bottom channels of each set of four, indicated by solid arrows), voltages are higher and waves are slower. In this example, posterior waves are higher because of the presence of the posterior rhythm, approximately 10 Hz in this sample.

Table 2-1 Summary of Transition from Wakefulness to Sleep in the Routine EEG

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).

Related posts:

The Structure and Philosophy of the EEG Report Introduction EEG Patterns in Stupor and Coma The Abnormal EEG Filters in the Electroencephalogram Introduction to Commonly Used Terms in Electroencephalography

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