Fast frequencies dominate the normal adult waking EEG. The posterior rhythm is recorded in the awake state with the eyes closed. Eye opening attenuates the posterior dominant rhythm. There is an anterior-posterior gradient, with faster frequencies seen over the frontal regions, usually in the beta range. These faster frequencies are of low amplitude, however.
Anterior cerebral activity
The background activity of the frontal region is composed predominantly of low- voltage fast activity with superimposed eye movement artifact. Rhythmic beta may be recorded from the frontal and central regions, especially when sedatives are used. Drug enhanced beta is more commonly seen after
benzodiazepine and barbiturate sedation than following chloral hydrate.
Theta and delta activity are not prominent in the normal awake adult EEG. However, digital EEG signal analysis shows a small amount of bihemispheric theta in most patients. Posterior cerebral activity
In the awake state with the eyes closed, the predominant rhythm from the occipital region
Figure 4-1: Normal waking EEG.
Normal EEG recorded using the left parasaggital portion of the
longitudinal bipolar montage. There is a posterior dominant rhythm of 9- 10 Hz which attenuates to eye opening (signified as “OE!”)
is about 10 Hz. The range of normal is 8.5 to 11 Hz, with frequencies slower than 8.5 Hz being abnormal, usually indicating dementia or encephalopathy. There is some oscillation of this rhythm, but it still has a very regular appearance.
The posterior dominant rhythm is usually symmetric, but asymmetries of up to 25% are frequently seen. Asymmetry should not be interpreted as abnormal unless it is at least 50%. The amplitude from the left hemisphere is often lower than the right, so this should be considered when interpreting amplitude asymmetries.
Increasing age results in the posterior dominant rhythm being lower amplitude and less well organized. While slowing of the background is common in the elderly, it is still abnormal. We use a rigid cutoff of 8 Hz; any frequency less than this is abnormal at any adult age.
Tense state can suppress the posterior dominant rhythm even with the eyes closed, so relaxation is important to recording an adequate response.
Drowsiness (Sleep stages 1A and 1B)
As the patient progresses from the relaxed awake state to drowsiness, the first sign is attenuation of the posterior dominant rhythm with some spread anteriorally, plus slight slowing of the background. This is stage 1A of sleep. As the patient sinks further into stage 1B, the posterior dominant rhythm is reduced to less than 20% with theta activity becoming more prominent.
Vertex waves may be seen at stage 1B, but are more a sign of stage 2 sleep. Differentiation of stages 1A and 1B is not important for routine EEG.
Sleep
Sleep patterns
POSTs
Positive occipital sharp transients of sleep (POSTS) are surface positive potentials with maxima at O1 and O2. They may occur as single waves or in trains. They resemble lambda waves except that they are present only in the sleeping state, whereas lambda waves are seen only in the waking state and with the eyes open.
POSTS may be related to replay of visual information during sleep, but this hypothesis is not universally accepted. They are not seen in patients who are blind or are severely visually impaired.
POSTS are not a consistent feature of sleep and have no diagnostic significance, unless they are seen only from one side. This asymmetry is unlikely to be the only sign of focal abnormality in an EEG record.
BETS
Benign epileptiform transients of sleep (BETS) are very small spike-like potentials that occur in the temporal regions during drowsiness and light sleep. They are less than 50 µV with a duration of less than 15 ms. Also called small sharp spikes, BETS are differentiated from
epileptiform pikes by their small amplitude, short duration, lack of a slow wave, and normal EEG background.
Vertex waves
Vertex waves are surface- negative potentials with a maximum amplitude on either side of the midline (C3 and C4). They are most common in stage 2 sleep and often appear at times of partial arousal. The vertex waves are a biphasic sharp wave with an initial negative deflection followed by a positive deflection. The sharp wave may be followed by a slow wave or a spindle, the latter being termed a K complex.
The vertex waves can be asymmetric, especially in children, and high amplitude in younger patients. The asymmetry should be abnormal if more than 25% and if consistent between the hemispheres. Vertex waves first appear at approximately 8 weeks of age. In children, vertex waves that appear in trains may be mistaken for seizure activity.
Figure 4-2: POSTs
Positive occipital shart transients of sleep (POSTs) recorded using the left parasaggital ortion of the longitudinal bipolar montage. The upward deflections in the last channel are due to positive potentials from the O1 electrode.
Table 4-1: Sleep rhythms.
Pattern Description
Vertex waves Negative potentials with a maximum at Cz. Occur in stage 2 sleep and during arousal.
Sleep spindles 11-14 Hz waves of 1-2 sec duration. Maximum at C3 and C4.
Most prominent in stage 2 sleep.
K complexes Fusion of a vertex wave and a sleep spindle. Prominent in stage 2 sleep and partial arousal.
Positive sharp transients of sleep
Sleep spindles
Sleep spindles are rhythmic 11-14 Hz waves whose duration is typically 1-2 sec, with a minimum of 0.5sec, and whose amplitude is at least 25 µV. They are most prominent in the central regions during stage 2 sleep. Unlike vertex waves, the maximum amplitude of sleep spindles is typically seen lateral to the midline (C3 and C4). Asymmetry in the abundance of sleep spindles is normal unless sleep spindles fail to appear from one hemispheres.
Sleep stages
Stage 1
Stage 1 is drowsiness, and is divided into stage 1A and 1B. Stage 1A is characterized by attenuation of the posterior dominant rhythm and some change in field of distribution so that it is seen more anteriorally than in the fully awake state.
Stage 1B is characterized by progressive loss of the posterior dominant rhythm, with less than 20% of the background composed of the alpha rhythm. Theta activity becomes more prominent. Vertex waves may be present in stage 1B, however, this is more commonly seen in stage 2. Differentiation between stages 1A and 1B is not important for routine EEG.
Stage 2
Stage 2 sleep is characterized by sleep spindles, vertex waves, increased theta, and the appearance of delta. However, less than 20% of the record contains delta. Since some vertex waves can be seen in stage 1B, the primary differentiating feature is the
appearance of sleep spindles.
Stage 3
Stage 3 sleep is characterized by increasing delta content and reduction in faster Table 4-2: Sleep stages
Stage Features
1 – drowsiness Attenuation of alpha, slight slowing, spread of alpha anteriorally. 1A = light drowsiness.
1B = deep drowsiness.
2 - light sleep Sleep spindles, vertex waves, increased theta activity, and the appearance of some delta.
3 – slow wave sleep
More delta activity, comprising 20-50% of the record. Sleep spindles and vertex waves are less prominent.
4 – slow wave sleep
More delta activity, comprising >50% of the record. Sleep spindles and vertex waves are often absent.
frequencies. Delta comprises 20-50% of the record.
Stage 4
Stage 4 sleep is characterized by a further increase in delta content, so that delta comprises more than 50% of the record. Vertex waves and sleep spindles are less prominent and are often absent.
REM
Rapid eye movement (REM) sleep is characterized by a low-voltage background
composed of predominantly fast frequencies. It can be difficult to distinguish REM sleep from light drowsiness. Rhythmic 6-8 Hz activity which is called sawtooth waves because of its unusual morphology, may appear in the frontal regions and the vertex.
Typically, REM sleep follows after progression from sleep stages 1 through 4.
Progression from drowsiness to REM sleep without passing through other stages, termed REM-onset sleep, occurs in the following conditions:
• Narcolepsy
• After sleep deprivation
• After alcohol or drug-induced REM deprivation sleep
The features of REM sleep which distinguish it from drowsiness are the following: • Rapid and chaotic eye movements (drowsiness has slow roving eye movements) • Hypotonia on submental EMG
• Irregular respiratory rate
Sequence of sleep stages
Patients progress from relaxed
wakefulness through stage 1A and 1B into stage 2 sleep. During a routine office EEG, deeper stages of sleep are seldom seen. In fact, when seen they may be misinterpreted as abnormal rhythms. Progression into slow-wave sleep,
Figure 4-3: Maturation of the posterior dominant rhythm.
The posterior dominant rhythm gradually increases during childhood years. Reaching adult frequencies by 4-5 years of age. However, there are other differences which differentiate a child’s EEG from an adult EEG.
stages 3 and 4 occurs later. The patient descends into deeper stages of sleep then ascends to drowsiness or wakefulness during a night's sleep. The cycle is repeated three to four times per night, although all stages may not be seen in all cycles. REM sleep occurs usually after at least one sleep cycle and increases in duration with subsequent cycles. Respiratory pattern is usually regular in all sleep stages except REM. Submental EMG activity declines progressively as sleep becomes deeper, until
disappearing during REM sleep.