Normal Variants in the Electroencephalogram

Posterior Occipital Sharp Transients of Sleep

Posterior occipital sharp transients of sleep (POSTS) are one of the most common normal variants seen in the EEG and can be considered one of the normal elements of sleep. The acronym POSTS tells the story of these distinctive waveforms: POSTS are of Positive polarity, they are seen in the Occipital areas; they have a Sharp Transient waveform, and they occur in Sleep. POSTS are “triangular” or V-shaped wave that are particularly prominent in light sleep (see Figures 11-1 and 11-2 ). If not recognized as POSTS, these low-voltage discharges could potentially be mistaken for occipital sharp waves. Because POSTS are so common, the polarity of any low- to medium-voltage occipital sharp wave seen in sleep should be assessed before deciding that it is abnormal. Displaying POSTS in an appropriate referential montage should confirm their positive polarity and correctly identify them as POSTS rather than epileptiform discharges. POSTS usually appear in a bilaterally synchronous fashion, although normal POSTS may manifest asymmetrical amplitudes. POSTS may occur in brief, semirhythmic runs. Although POSTS usually consist of low-voltage, V-shaped waves, they may occasionally assume a more spiky appearance (see Figure 11-3 ).

Lambda Waves

Lambda waves are discussed with POSTS because their morphology and location are similar. The two are easily distinguished because lambda waves occur exclusively during wakefulness and POSTS are seen during sleep. Lambda waves also appear as low-voltage triangular waves in the occipital areas, reminiscent of the Greek letter λ , but they are distinctive in that they occur at the time of lateral searching eye movements. Confirmation that a low-voltage occipital sharp transient wave is a lambda wave is made easier by finding evidence of concurrent lateral eye movement artifact. Lambda waves may be either surface positive or surface negative in the occipital area (see Figure 11-4 ). They are not as common as POSTS and are seen more frequently in children than in adults. Because they are related to searching eye movements, lambda waves are generally seen when the patient’s eyes are open and the posterior rhythm is suppressed. Voltage asymmetry of lambda waves is not necessarily considered abnormal.

Small Sharp Spikes and Benign Epileptiform Transients of Sleep

The terms small sharp spikes (SSS) and benign epileptiform transients of sleep (BETS) are synonymous. These quick, low-voltage spikes are usually seen in the temporal areas with a broad gradient across the temporal chain. The upward and downward phases of the transients are usually of similar amplitude. They occur either unilaterally or bilaterally and, when bilateral, they may occur either synchronously or independently (see Figure 11-5 ). SSS are seen in drowsiness and light sleep and tend to disappear with deepening sleep. SSS are considered by many to represent a normal variant but some authors still contend that the finding suggests an increased degree of epileptogenicity.

Mu Rhythms

Mu rhythms are commonly encountered rhythms seen in the central areas during wakefulness, best recorded by the C3 and C4 electrodes. They are most often seen from later childhood into the adult years, although they are occasionally seen in very young subjects. The mu rhythm has a distinctive arciform (arch-like) or “comb-like” morphology (see Figure 11-6 ). Because the mu rhythm is suppressed by voluntary motor activity in the opposite hand, the technologist can establish that a sharp central rhythm is a mu rhythm by requesting that the patient move the contralateral hand and demonstrating that the rhythm disappears. Although classically suppressed by moving the contralateral hand, movement of the ipsilateral hand or planning to move the hand may also suppress the mu rhythm in some subjects.

Because this arciform rhythm is sharpened on one side and rounded on the other, there is some potential to mistake it for epileptiform activity. When mu rhythms occur in trains, it is not difficult to identify them correctly on the basis of their location, morphology, and suppression with movement, if necessary. Occasionally, fragments of a mu rhythm may resemble low-voltage spike activity (see Figure 11-7 ). Apparent low-voltage central spikes can be confirmed to be a mu phenomenon by showing that the morphology of the spike fragment is similar to the mu waves when they occur in trains.

Mu rhythms may be seen either unilaterally or bilaterally. They may suppress independently. Asymmetrical expression of mu rhythms is not considered abnormal. The mu rhythm tends to occur at a frequency similar to that of the patient’s posterior rhythm and therefore, varies with age. In some patients, the posterior rhythm’s field blends into the field of the mu rhythm creating large zones of alpha activity in the posterior quadrants. Because of the similar frequencies and amplitudes of the two rhythms, in such cases, it is not always clear where the posterior rhythm ends and the mu rhythm begins.

The mu rhythm and the posterior rhythm are the two main idling rhythms of the EEG: the mu rhythm is only seen during contralateral motor inactivity and suppresses with movement. Similarly, the posterior rhythm is only present during visual inactivity and suppresses with eye opening or visual attention. A mu-shaped rhythm that does not necessarily suppress with movement is occasionally seen in the central midline (Cz) and is referred to as a midline theta rhythm .

Wicket Spikes and Wicket Rhythms

Because their morphology is quite similar to that of mu rhythms, wicket rhythms are discussed with mu rhythms. Wicket rhythms differ from mu rhythms in that they are seen in drowsiness and light sleep rather than wakefulness and have a predilection for the temporal rather than the central areas (see Figure 11-8 ). Their arciform morphology is similar. Wicket rhythms range from 6 to 11 Hz with a voltage range of 60 to 200 μV ( and Lebel, 1977). Similar to the situation seen with mu rhythms, it is possible to mistake a fragment of a wicket rhythm for epileptiform activity rather than a normal variant. Such fragments are called wicket spikes . Wicket spikes are distinct from temporal spike-wave discharges in that there is no aftercoming slow wave and they do not disrupt the underlying rhythm. The confirmation that a temporal spike is a wicket spike is best made by noting that the waveform is similar to that of the spikes when they occur in a train (as a continuous wicket rhythm) found elsewhere in the same tracing.

14- and 6-Hz Positive Bursts

Often referred to simply as “14 and 6,” 14- and 6-Hz positive bursts are seen most frequently in adolescence. The term ctenoids (a word that means shaped like the teeth of a comb or like overlapping fish scales) has also been used for this phenomenon in the past but is no longer the preferred term. As the name implies, two versions of this variant are seen, one firing at a rate of 14 Hz and the other at 6 Hz. The 14-Hz form is more common (see Figures 11-9 through 11-11 ). The discharges are most prominent in the posterior temporal and occipital areas. The bursts consist of fast, arciform, or comb-shaped rhythmic discharges of low, medium, or high voltage in which the sharp phase has positive polarity and the rounded phase has negative polarity. It was initially asserted that 14 and 6 was associated with a variety of pathologic states, including epilepsy, but these bursts are now classified by most as a normal variant. The 6-Hz version of 14 and 6 is less commonly seen but felt to have the same significance; some patients manifest both forms in the same tracing.

Although 14 and 6 positive bursts may occur bilaterally, they usually do not fire synchronously. Asymmetrical occurrence of 14 and 6 positive bursts is not considered abnormal. Some authors believe that the 6-Hz component of 14 and 6 actually fires at 7 Hz and represents a subharmonic of the fundamental 14-Hz frequency, although true 6-Hz examples are seen. The bursts usually last 1 second or less, and there is no evolution in firing frequency during the burst. The distinctive wave morphology, frequency, and positive polarity help to confirm examples of 14- and 6-Hz positive bursts.