Basic Knowledge of the EEG Wave

To interpret an EEG, the EEG frequency, ampli-tude, and waveform, as well as the distribution, form, and changes of abnormal waves, must be analyzed.

5.1.3.1 Frequency

Frequency equals the number of repetitions per sec-ond of EEG waves that share the same period. The frequency of EEG waves encountered in clinical

analysis ranges from 0.1 to 100 Hz and is mainly within the 0.3–70 Hz interval. Internationally, the range of EEG wave frequency is divided into five bands represented by five Greek letters, of which delta and theta are slow wave bands, and beta and gamma are fast wave bands.

Alpha wave: 8–13 Hz (Fig. 5.2)

Beta wave: 14–30 Hz (Figs. 5.2 and 5.3) Theta wave: 4–7.5 Hz (Fig. 5.3)

Delta wave: 0.3–3.5 Hz (Figs. 5.3 and 5.4) Gamma wave: >30 Hz

Fig. 5.2 Box A shows an alpha wave, and Box B shows a beta wave. The subject is a 53-year-old male who is awake with closed eyes

Fig. 5.3 Box A shows a delta wave, Box B shows a spindle wave in the beta band, and Box C shows a theta wave. The patient is a 69-year-old male in coma who is believed to have an intracranial infection

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5.1.3.2 Wave Amplitude

Amplitude, also called voltage, measures the potential difference between any two electrodes and is expressed in microvolts (μV). Age has a large effect on amplitude; thus, for adults:

Low amplitude: <25 μV Medium amplitude: 25–75 μV High amplitude: 75–150 μV Very high amplitude: >150 μV

For children:

Low amplitude: <50 μV Medium amplitude: 50–150 μV High amplitude: 150–300 μV Very high amplitude: >300 μV 5.1.3.3 Waveform

Whether EEG waveforms are normal depends on multiple factors including age, monitoring status, and location. Common waveforms are as follows:

Sinusoidal wave: the upward and downward branches of the wave are smooth.

Simple wave and biphasic wave: the former refers to a wave that deflects in a single direction from the baseline (up or down), whereas the latter contains one segment above the baseline and the other segment below the baseline.

Triphasic wave: this wave contains three phases;

the first phase is generally a relatively small negative wave, the second phase is positive, and the third phase is negative with an ampli-tude usually higher than that of the first phase.

It is common in metabolic encephalopathy, hepatorenal failure, and hypoxia. Positive- negative- positive triphasic waves exist.

Spike: the shape is similar to a sharp nail, with a time limit of 20–70 ms.

Sharp wave: this waveform is similar to those of spikes, with a time limit of 70–200 ms; spikes and sharp waves are usually abnormal wave-forms, but the vertex sharp waves during sleep and sharp waves in the occipital region of children and in the frontal region of neonates should not be regarded as abnormal.

Complex: contains two or more continuous wave components.

Spike (sharp) and wave: the first component is a spike (sharp wave), followed by a slow wave.

Polyspike: two or more consecutive spikes.

Polyspike and wave: two or more spikes as the initial wave, followed by a slow wave.

K-complex: one of the signs of stage 2 sleep, appearing as high-amplitude 1 Hz slow activ-ity of the brain hemisphere; under sound stim-ulation, short-range 12–14 Hz fast activity will appear.

Fig. 5.4 The boxes indicate diffuse delta waves. The patient is a 47-year-old male in a moderate coma with tuberculous meningoencephalitis

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5.1.3.4 EEG of Normal Adults

Alpha and beta waves are the basic waves in the awakened state (Fig. 5.2), and a few fast waves and slow waves are scattered during this state.

The primary brain wave is the alpha wave, which is distributed in the back of the head and is sym-metrical on both sides. On symsym-metrical sites of both hemispheres, the frequency difference of alpha waves should not exceed 20%, and the amplitude difference should be no more than 50% in the occipital region and no more than 20% in other regions. The average amplitude of alpha waves is less than 100 μV and that of beta waves less than 50 μV. During the opening and closing of the eyes, mental activity, or sensation of a stimulus, alpha waves should be attenuated.

Slow waves of normal adults are random, low- amplitude waves, mostly theta waves, and contin-uous high-amplitude beta or delta waves should not be observed at any site. During sleep, brain waves should be symmetrical. Abnormal electri-cal activity should not be detected. There should not be spikes or spike and waves during waking or sleep.

5.1.3.5 Distribution of Abnormal Waves

According to the location of the abnormal waves identified by the electrodes, the distribution of abnormal waves is usually classified as follows:

Generalized: appearing in regions of both hemispheres, basically symmetrical (Fig. 5.5).

Diffuse: appearing in regions of both hemi-spheres, similar to the generalized type but with less symmetry.

Symmetrical: the waveform, amplitude, phase, and frequency of electrical activities are essen-tially the same in both hemispheres (Fig. 5.6).

Asymmetrical: the waveform, amplitude, phase, and frequency of electrical activities differ from one hemisphere to the other (Fig. 5.7).

Lateralized: abnormal electrical activity occurs in one hemisphere or mainly in one hemisphere.

Localized: abnormal electrical activity is limited in certain regions (Fig. 5.8).

Migratory: characteristic EEG activity gradually shifts from one region to another on the same or opposite side of the brain, often manifested as a weakening waveform in one region grad-ually emerging in another region.

5.1.3.6 Patterns of Abnormal Waves The following terms are commonly used to describe abnormal waves according to the time and number of their occurrence:

Wave: wave in a single form.

Activity: several consecutive waves similar in form and prevalent within a certain range of time and space.

Fig. 5.5 Demonstrates generalized delta activity. The patient is 27 years old with autoimmune encephalitis and is in the lowest state of consciousness. The stars indicate eye-blink artifacts

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Fig. 5.6 Demonstrates discharges of symmetrical bilateral synchronous periodic spikes. The patient is 74 years old and in a deep coma after cardiopulmonary resuscitation

Fig. 5.7 Demonstrates asymmetrical electrical activity, in which the spindle wave is well developed on the left side of the brain (indicated by the box) but nonexistent on

the right side. The patient, a 59-year-old male, is in a stu-por after massive infarction of the right hemisphere

Fig. 5.8 Demonstrates localized discharge of spikes under the F4 electrode. This 49-year-old female patient with epi-lepsy is in a state of somnolence

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Rhythm: the consecutive appearance of more than three identical waves. According to the time limit of occurrence, rhythms are classified into short-range (shorter than 1 s), medium-range (longer than 1 s but shorter than 3 s), and long-range (longer than 3 s).

Random: a single wave appearing at irregular intervals in the same lead or different leads.

Episode: only one or two occurrences of a particular waveform during one period of recording.

Transient: a certain waveform stands out from the background activity, with irreg-ularity, a short range, and infrequent occurrence.

Periodicity: waves or wave groups of similar forms and phases that stand out from the back-ground activity and recur at similar intervals (Fig. 5.6).

Synchronous: bilateral EEG changes with a fixed relationship of phase and the same fre-quency (Fig. 5.6). Bilateral EEG changes may have a time difference less than 25 ms;

otherwise, they are regarded as secondary synchronization.

Asynchronous: bilateral EEG changes occur in a non-fixed relationship of phase and nonidenti-cal frequencies.

Burst: brain waves prominent against the back-ground activity with abrupt onset and termina-tion (Fig. 5.9).

Paroxysm: brain waves prominent against the background activity with milder prominent components compared to bursts.

Hypsarrhythmia: asymmetry of amplitude, fre-quency, and waveform plus asynchronicity.

5.1.4 Electrode Placement