Accurate recognition and distinction of benign variants in the EEG are essential to avoid over interpretation of such findings as epileptiform, and subsequent errant diagnosis of epilepsy. The range of benign variants includes wicket waves, rhythmic mid-temporal theta of drowsiness (RMTD or RTTD), benign small sharp spikes (BSSS, sometimes denoted BSST for benign small sleep transients), 14 and 6 positive spikes, 6-Hz “phantom” spike and wave, subclinical rhythmic EEG discharge of adults (SREDA), and the midline theta rhythm of Ciganek.

Wicket waves are by far the most commonly encountered benign variant and a frequent source of overinterpretation and mistaken diagnosis of epilepsy on EEG. Wicket waves are single waveforms that occur in brief trains or clusters. In distinction to true epileptiform spikes, however, wicket waves have a more arciform appearance and earn their name therefore by looking like “wicket” (see Figure 48). Wickets are most frequent in the temporal regions, occurring bilaterally or unilaterally, have a frequency of 6 to 11 Hz and amplitude ranging from 60 to 200 μV. Wickets are not accompanied by after-going slow waves and are mainly seen in older adults during drowsiness and light sleep, becoming apparent when the alpha and other wakeful patterns drop out.

Figure 48.

Typical wicket waves. Note the arciform appearance, lack of after-going slow wave, and lack of background disruption or disturbance. The wicket waves are seen in the left temporal region with phase-reversal at T7 in seconds 3 and 4 of the tracing, longitudinal (more...)

A common normal variant finding, often mistaken for pathological activity during drowsiness, is RMTD. This activity was previously known as “psychomotor variant” because of the similarity of this phenomenon to the focal rhythmic activity of a seizure discharge (Figure 49). In contrast to focal epileptiform activity, RMTD rhythms usually have a flat-topped or notched morphologic appearance, thought to result from a combination of two more different frequencies in the alpha and theta range. Also distinct from epileptiform activity, RMTD rhythms do not evolve or spread to other electrode sites over time and are typically very short-lived, lasting on the order of 5 to 15 seconds in duration. In additional, RMTD tends to become less prominent during increasing levels of drowsiness and disappears during light NREM sleep, which is the opposite for epileptiform activity. RMTD may be either unilateral or bilateral and independent.

Figure 49.

Rhythmic temporal theta of drowsiness. Bitemporal left greater than right, longitudinal bipolar montage. Noted are notched rhythmic waveforms localized to the temporal regions, some of which are sharply contoured. This rhythm was formerly referred to (more...)

BSSS are monophasic or diphasic spikes with steep ascending and descending limbs (see Figure 50). BSSS occur mainly during drowsiness and light sleep in the adult age group and may have a wide electrographic field of distribution but are best seen in temporal and ear leads. Distinguishing small sharp spikes from more pathologic temporal spike or sharp-wave discharges is usually not difficult; BSSS have a characteristic sharp ascending and descending limb morphology of the waveform, an exquisitely brief duration and low amplitude, often bilateral distribution, and lack of a disturbance of the background or associated focal abnormality in the EEG. Another distinction between BSSS and epileptiform spikes is the tendency of BSSS to disappear during deeper levels of sleep, while interictal spikes are often further activated during deeper sleep and often occur during wakefulness as well (while BSSS may occur in drowsy wakefulness, they do not occur during periods of normal vigilance).

Figure 50.

BSSS (aka BETS) on ipsilateral ear reference (left) and longitudinal bipolar montage (right). Note steep descending slope of low-amplitude spike and small after-coming slow wave, particularly at T7 to TP11. BSSS are a common feature in adult EEGs during (more...)

The benign variants, 14 and 6 positive spikes, occur in 20 to 60 percent of the normal population, predominantly in adolescents and younger adults aged 12 to 20 years, especially during drowsiness and light NREM sleep. The 14 and 6 positive spikes are so named because of their tendency to occur in bursts at a rate of 14 Hz or 6 to 7 Hz (range, 0.5–1 second; see Figure 51). The 6-Hz positive spikes predominate in very young infants (under 1 year old) and in some adults. The 14-Hz positive spikes occur more frequently in adolescents. These bursts typically consist of “negative” arciform waveforms located over the posterior temporal head regions, with alternating “positive” spiky components, and may be independent over the two hemispheres. The 14 and 6 positive spikes are best appreciated by using long interelectrode distances and referential montages.

Figure 51.

Example of 14- and 6-Hz positive spikes in a 13-year-old girl. Note the 14-Hz positive spiky waveforms best appreciated over the posterior temporal and biposterior head regions, best seen in the third second.

The 6-Hz phantom spike and wave (see Figure 52) usually have a mitten-like morphology, with a very small or absent spike component, and a more apparent slow wave. The 6-Hz phantom spike and wave pattern may be observed in both adolescents and adults and is another pattern seen predominantly during drowsiness and light NREM sleep, vanishing in N3 and REM. The 6-Hz phantom spike and wave are diffuse or, alternatively, anteriorly or posteriorly predominant bursts.

Figure 52.

Example of 6-Hz “phantom” spike and wave in an adolescent patient. There are bilaterally symmetrical diffuse tiny spikes with prominent wave components (“mitten-like” morphology) in seconds 3 through 6 below. Longitudinal (more...)

SREDA (aka SCREDA) is another benign variant pattern often mistaken for a subclinical electrographic seizure. SREDA typically occurs in normal older and elderly adults, especially over the age of 50 and is characterized by a sharply contoured theta frequency (between 5 and 6 Hz) discharge most often seen diffusely but maximally over the parietal and posterior head regions, and it may last a few seconds to approximately 1 or 2 minutes in duration. Again, SREDA is seen more during drowsiness. Typically, there may be mild frequency evolution but no spatial or topographic evolution to this EEG discharge (i.e., no spread to other brain regions), and unlike most partial seizures, there is no clinical accompaniment if response or interaction testing is performed. Sometimes SREDA has an abrupt offset, which may help distinguish this activity from partial seizures (see Figure 53).

Figure 53.

SREDA pattern, longitudinal bipolar montage. Black brackets show onset of periodic posterior-predominant sharply contoured waveforms, becoming rhythmic, then resolving at latter portion of figure. This normal elderly adult was asymptomatic during the (more...)

A relatively rare benign variant again often confused with seizure activity is the midline theta rhythm (midline theta of Ciganek). Previously thought to potentially correlate with underlying epilepsy, the rhythm appears to be another nonspecific benign rhythm of drowsiness. Prominent theta frequency activity is seen confined to the vertex and midline derivations during drowsiness.