EEG rhythms are classified into four frequency bands. No individual band is normal or abnormal by definition. All are interpreted in the context of the topographic location and age and conscious state of the patient.
Figure 3-1: Organization of the cerebral cortex.
A: Some of the connections of the cortical and subcortical tissues is shown here. Projections from one section to another can produce potentials recorded during EEG and EPs.
B: Columnar organization of the cerebral cortex, with the large cortical efferents oriented so that the dendritic arborization is near the surface.
Alpha rhythm
The alpha rhythm is usually seen in normal, relaxed individuals who are awake with their eyes closed. It is approximately 10 Hz in adults with the maximum voltage originating from the occipital electrodes, O1 and O2. The term alpha rhythm is used by some physiologists to signify any posterior dominant rhythm regardless of frequency, but this is an improper use off the term.
In children, the dominant posterior rhythm is slower and frequency and may not attain the minimal 8.5 Hz until 12 years of age. Slower frequencies in a 12-year-old would be interpreted as abnormal and would most likely indicate a diffuse encephalopathy or, if unilateral, suggest a structural lesion.
Table 3-1: EEG rhythms
Rhythm Description Normal Abnormal
Alpha 8-13 Hz Posterior dominant
rhythm in older children and adults.
Diffuse alpha in alpha coma.
Can signify seizure activity, especially in neonates.
Beta >13 Hz Normal in sleep,
especially in infants and young children.
Drug-induced frontal beta.
Breach rhythm over a skull defect.
Theta 4-7 Hz Drowsiness and sleep.
Posterior slow waves of youth may have a theta component
Temporal theta in the elderly.
Focal theta over a structural lesion.
Delta <4 Hz Sleep Intermittent rhythmic
delta activity. Polymorphic delta activity with focal lesions.
Spikes and
sharp waves Spike: 25-70msec duration. Sharp wave: 70- 200 msec duration.
Vertex waves and frontal sharp transients in neonates.
Positive occipital sharp transients of sleep. Benign epileptiform transients of sleep 6/sec phantom spike and wave.
14-and-6 positive spikes
Focal and generalized epileptiform activity.
The posterior dominant rhythm is suppressed by eye opening and promptly returns when the eyes are closed. This reactivity of the posterior alpha rhythm should be routinely tested during EEG recording. The posterior rhythm is suppressed if the patient is tense during the recording. The lack of posterior rhythm should not be interpreted as abnormal in this situation. Other EEG features that suggest a tense state include frequent eye blinks and muscle artifact in frontal and temporal leads.
The amplitude of the posterior rhythm is 15-50 µV in young adults. Older individuals often have lower amplitude, but the frequency is the same. Low amplitude should not be interpreted as abnormal if the frequency composition is normal. Slowing of the posterior dominant rhythm is not a normal part of aging. Amplitude asymmetries of the dominant rhythm are common. The amplitude is usually higher from the non-dominant hemisphere, but the difference should not exceed 50%.
A prominent alpha rhythm can be recorded during anesthesia and coma, but the
distribution is different from the normal posterior alpha rhythm. In anesthesia and coma, the alpha rhythm is generalized with an anterior predominance. This alpha activity is invariant and monotonous, lacking the usual modulation in frequency and amplitude of an occipital alpha. The appearance of alpha coma in a patient signifies a poor prognosis for good neurologic recovery.
Beta rhythms
EEG activity with frequencies faster than 13 Hz occurs in all individuals but is usually of low amplitude and often overlooked in favor of slower frequencies during wakefulness and sleep. Beta activity is normally distributed maximally over the frontal and central regions. A low-amplitude high-frequency beta is especially prominent during normal sleep in infants and children and is enhanced by several sedatives, especially barbiturates and benzodiazepines. In some children, the beta activity is so prominent as to dominate the record.
People with hyperthyroidism may accelerate their posterior rhythm from 10-14 Hz or more. This is technically in the beta range, but the rhythm continues to react like an occipital, awake, resting rhythm, and should be considered no different than the alpha rhythm in this context.
Alterations in the frequency, amplitude, and abundance of beta activity should be commented on in the description of the record but interpreted with caution. Marked asymmetry in beta activity suggests the possibility of a structural lesion on the side lacking the beta. Focal, high-amplitude beta activity can be recorded over a skull defect, which might be a burr hole or fracture site. This is termed breach rhythm.
Theta rhythms
EEG activity with a frequency between 4 Hz and 8 Hz is seen in normal drowsiness and sleep in adults. Young children may show theta activity during the waking state, which
makes interpretation of encephalopathy particularly difficult. Digital frequency analysis does reveal a small amount of theta in the waking EEG of adults, but the content quite small and the amplitude is low. The detection of this theta usually requires high- sensitivity recordings or digital frequency analysis.
Posterior slow waves of youth may be in the theta or delta range. Theta activity in the temporal region in older individuals has been ascribed to vascular disease. While the significance of temporal theta is controversial, we suspect that that it is not part of normal aging. We suggest commenting on the presence of temporal theta in the body of the report and interpreting it as a mild abnormality.
Delta rhythms
Delta activity is not normally recorded in the awake adult but is a prominent feature of sleep and becomes increasingly abundant during the progress from stage 2 to stage 4 sleep. Focal polymorphic delta activity may be recorded over localized regions of cerebral damage. Intermittent rhythmic delta activity is recorded when there is dysfunction of the relays between the deep gray matter and cortex. The activity has a frontal predominance in adults and is called frontal intermittent rhythmic delta activity, while in children the activity has an occipital predominance and is called occipital intermittent rhythmic delta activity or posterior intermittent rhythmic delta activity.