For the interpretation of EEG, the patient’s condition is described, including whether the patient is awake, has eyes closed or open, and is relaxed or tense. Activation methods are also described.

A routine EEG recording should include the name of the patient, other identification, date of the study, a clinical diagnosis, names of medications, and date of last seizure.

The following is a sample of a routine recording:

If the EEG is abnormal, the body of the report should describe the specific abnormalities and the summary should give the electrodiagnostic impression.

The EEG is interpreted in the context of the patient’s state. Slow wave activity would be normal in stages III and IV of sleep but is considered abnormal when the patient is awake. While interpreting an EEG study a physician should keep in mind the patient’s clinical state, examine the composition of frequencies, and compare the right-to-left symmetry. Background changes in the awake state should be noted. Abnormal responses to photic stimulation and HV should be noted, as should any abnormal wave, such as sharp waves and spikes.

The alpha rhythm is seen in a normal awake individual with eyes closed and in a relaxed state; it is most dominant in the occipital electrodes O1 and O2. The posterior dominant alpha rhythm is usually suppressed by eye opening but returns with eye closure, a response commonly evaluated in routine EEGs.

The amplitude of the posterior dominant alpha rhythm is usually between 15 and 50 μV; however, it may be slightly lower in older individuals. The lower amplitude is not considered abnormal if the background is normal. Asymmetry of background alpha amplitude between hemispheres should be less than 50 %. The bilateral slowing of the posterior dominant rhythm to less than 8 Hz is usually due to diffuse encephalopathy, although the underlying cause cannot be determined solely through EEG.

Interpretation of EEG recordings requires basic knowledge of the possible waveform morphologies and frequencies and the ability to develop a spatial analysis of what is being recorded. These items are discussed below.

The frequency of delta waves is less than 4 Hz, and this is considered normal in sleep. Focal subcortical lesions may demonstrate polymorphic delta activity.

Theta activity of 4–7.99 Hz is normal in drowsiness and as seen in the posterior slow waves of youth, but is abnormal when seen as temporal theta in elderly individuals with focal theta activity over a structural lesion.

Alpha activity of 8–12.99 Hz is normal as the posterior dominant rhythm. Generalized alpha activity is indicative of a poor prognosis, described as an alpha coma. This form of coma is described as anterior alpha dominance without any variation in frequency or amplitude and without reactivity.

Beta activity is more than 13 Hz and this is normal in the anterior region; however, it could be drug-induced, as when it is due to benzodiazepine or barbiturate use, or it could be a breach rhythm if there is a skull defect (the skull and scalp act as a high-frequency filter). Asymmetry in beta activity with absence on one side may be indicative of a structural cortical or extra-axial lesion on that side, such as cerebral edema or a subdural hematoma.

Slow waves, if focal within a normal background, may be due to structural lesions. Generalized slowing of the background is commonly due to an encephalopathy (Fig. 2.1). Focal slow activity may be recorded over areas of cerebral damage. Focal slowing is least evident when the patient is wakeful and is most apparent in stage I sleep.

Slowing of the background to less than 8 Hz is usually abnormal over the age of 1 year. The alpha rhythm has the highest amplitude in the O1 and O2 electrodes. Theta and delta frequencies are not usually prominent in normal awake EEG. The occipital dominant alpha rhythm attenuates on opening of the eyes or if the patient is tense. Asymmetry of the posterior dominant rhythm should not be more than 50 %; however, less than 50 % asymmetry may be normal. Decreases in the amplitude of alpha activity and slowing of the background may be seen with increasing age in adults.

A normal posterior dominant rhythm is established by the age of 1 year and rapidly gains in frequency during childhood, typically being 10 Hz in adulthood. However, up to 5 % of the adult population has minimal or no clearly identifiable posterior rhythm with alpha frequency range. The slow waves of youth are superimposed on a normal posterior dominant rhythm and are in the delta range, which may be seen up to 30 years of age. Vertex waves are broadly distributed; negative sharp wave transients are most prominent at the Cz electrode and mark the onset of stage I sleep, when they are seen most prominently. Sleep spindles, seen by age 2 years, are 11- to 14-Hz waves and typically of 1–2 s in duration, occurring bilaterally and maximally within the frontocentral region. They are most prominent at the C3 and C4 electrodes and are usually seen in stage II sleep. K-complexes are diphasic vertex waves that appear in association with sleep spindles; the K-complexes may occur before, during, or after the vertex sharp wave. Both sleep spindles and K-complexes are thought to originate within the thalamus.