The electroencephalogram (EEG) is the depiction of the electrical activity occurring at the surface of the brain. This activity appears on the screen of the EEG machine as waveforms of varying frequency and amplitude measured in voltage (specifically microvoltages).
EEG waveforms are generally classified according to their frequency, amplitude, and shape, as well as the sites on the scalp at which they are recorded. The most familiar classification uses EEG waveform frequency (eg, alpha, beta, theta, and delta). <1, 2, 3>
Examples of alpha, beta, theta, and delta electroencephalography frequencies.
Information about waveform frequency and shape is combined with the age of the patient, state of alertness or sleep, and location on the scalp to determine significance.
Normal EEG waveforms, like many kinds of waveforms, are defined and described by their frequency, amplitude, and location. <4>
Some waves are recognized by their shape, scalp location or distribution, and symmetry. Certain patterns are normal at specific ages or states of alertness and sleep.
The morphology of a wave may resemble specific shapes, such as vertex (V) waves seen over the vertex of the scalp in stage 2 sleep or triphasic waves that occur in the setting of various encephalopathies.
The frequencies most brain waves range from are 0.5-500 Hz. However, the following categories of frequencies are the most clinically relevant:
Alpha waves generally are seen in all age groups but are most common in adults. They occur rhythmically on both sides of the head but are often slightly higher in amplitude on the nondominant side, especially in right-handed individuals. A normal alpha variant is noted when a harmonic of alpha frequency occurs in the posterior head regions. They tend to be present posteriorly more than anteriorly and are especially prominent with closed eyes and with relaxation.
Alpha activity disappears normally with attention (eg, mental arithmetic, stress, opening eyes). In most instances, it is regarded as a normal waveform.
An abnormal exception is alpha coma, most often caused by hypoxic-ischemic encephalopathy of destructive processes in the pons (eg, intracerebral hemorrhage). In alpha coma, alpha waves are distributed uniformly both anteriorly and posteriorly in patients who are unresponsive to stimuli.
Theta waves normally are seen in sleep at any age. In awake adults, these waves are abnormal if they occur in excess.
This section identifies some normal waveforms, including K complex, V waves, lambda waves, positive occipital sharp transients of sleep (POSTS), spindles, mu rhythm, spikes, sharp waves, and certain delta waves (polyphasic and monophasic shapes).
These waves are recognized by their shape and form and secondarily by their frequency. They include waves that may be normal in some settings and abnormal in others (eg, spikes, sharp waves).
Example of a K complex.
They can occur throughout the brain and usually are higher in amplitude and more prominent in the bifrontal regions.
K complexes sometimes are followed by runs of generalized rhythmic theta waves; the whole complex is termed an arousal burst.
V waves are sharp waves that occur during sleep. They are largest and most evident at the vertex bilaterally and usually symmetrically.
Often, they occur after sleep disturbances (eg, brief sounds) and, like K complexes, may occur during brief semiarousals.
Example of either lambda or positive occipital sharp transients of sleep (POSTS).
They occur in the awake patient and are said to be most evident when the subject stares at a blank, uniform surface.
POSTS are triangular waves that occur in the bilateral occipital regions as positive (upgoing) waves.
POSTS occur in sleeping patients and are said to be most evident in stage 2 of sleep, although they are not uncommon in stage 1.
POSTS are similar if not identical to lambda waves both morphologically and in the occipital distribution.