Wicket Waves

Wicket waves are sharply contoured, arciform or “mu”-shaped waveforms which most commonly are localized to the midtemporal regions. Wicket waves are characteristically present during drowsiness and resolve with wakefulness and deeper levels of sleep. However, they have also been described in rapid eye movement (REM) sleep. The importance of identifying wicket waves is to avoid incorrectly labeling them as temporal sharp waves. The prevalence of wicket waves in an analysis of findings listed in EEG reports ranges from 0.03 to 1.0 percent. Others have estimated the prevalence to be higher, perhaps reflecting differences in the documentation practices of different laboratories with respect to the rate at which benign waveforms are specified in reports.

Wicket waves may predominate in one temporal region, but are often bilateral. They typically appear in clusters lasting for 1 to 2 seconds, occurring at theta frequency. Wicket-wave clusters often exhibit a “crescendo–decrescendo” onset and offset, which aids in their identification. They may be sharply contoured and characteristically show phase reversal on bipolar montages over the midtemporal region, giving rise to a “vampire” or “piranha teeth” appearance (see Fig. 5-1 ). Occasionally, isolated wicket spikes may be present and can be difficult to differentiate from temporal sharp waves. In such cases, resemblance of the isolated wicket spike to waveforms found in more typical wicket-wave clusters in the same patient, and the lack of a succeeding slow wave, help distinguish them from epileptogenic temporal sharp waves.

Rhythmic Temporal Theta of Drowsiness

Rhythmic temporal theta of drowsiness (RTTD), also known as rhythmic midtemporal discharges and the “psychomotor variant”, consists of rhythmic theta discharges localized to the midtemporal region. Bursts of RTTD range from a few seconds to a few minutes in duration. They may be unilateral, bilateral independent, or bisynchronous. RTTD characteristically resolves during deeper levels of sleep and upon arousal. Magnetoencephalography suggests the generators of RTTD may be localized to the posterior inferior temporal region.

RTTD has characteristic morphologic features (see Fig. 5-1 ). The individual waveforms may be sharply contoured and sometimes show a “notched” appearance. RTTD discharges typically stand out prominently on the EEG, which may result in their being mistaken for paroxysmal epileptogenic activity. Features that allow distinction from seizure discharges include resolution upon arousal, absence of clinical disturbance during the discharge, the notched waveform morphology when present, and lack of evolution of the discharge to other frequencies or spread to other regions. These attributes contrast with electrographic seizures, which tend to show evolution from one frequency to another and propagation to other brain regions.

Slow Alpha Variant

Slow alpha variant consists of theta activity present over the posterior head regions, with a frequency typically measuring 50 percent of the alpha rhythm. The slow alpha variant is thought to represent a frequency subharmonic of the alpha rhythm. Slow alpha variant waveforms may be notched in appearance, similar to RTTD. Failure to recognize slow alpha variant may lead to a misdiagnosis of a posterior cerebral dysrhythmia.

Subacute Rhythmic Epileptiform Discharges of Adults

Subacute rhythmic epileptiform discharges of adults (SREDA) are rhythmic EEG discharges of moderate to high amplitude that produce an abrupt change on the EEG that may be mistaken for electrographic seizure activity. Clinical testing during SREDA, however, shows no evidence of neurologic impairment during the discharge, and clinical follow-up does not suggest an association with epilepsy. Although SREDA is less common than the other benign variants discussed in this chapter, it is important to recognize it as distinct from epileptogenic activity.

SREDA typically consists of a mixture of frequencies, with theta activity predominating. The bursts last from 20 seconds to a few minutes. It is characteristically widespread in distribution, but is often maximal in amplitude over the parietal and posterior temporal regions. The discharges are usually bisynchronous, but unilateral SREDA has been described. In contrast to other benign variants, SREDA can be seen during wakefulness, and it can also be seen in REM sleep. While it often begins with the abrupt onset of diffuse, sharply contoured activity of mixed, but predominantly theta, frequency, it may also begin with a single bisynchronous sharp complex followed by semirhythmic sharp activity, which eventually merges into a sustained discharge. The onset of a SREDA discharge is depicted in Figure 5-2 . Clinical testing during the discharge by a technologist or clinician helps to confirm the benign nature of this variant rhythm.

Midline Rhythmic Theta

Midline rhythmic theta was originally described as a potentially epileptogenic abnormality correlating with the diagnosis of temporal lobe epilepsy. It was determined subsequently to be a nonspecific finding, occurring in patients without a clear diagnosis of epilepsy. As its name suggests, this benign variant consists of moderate-amplitude theta activity present over the central midline vertex region (see Fig. 5-2 ). The waveform morphology can be rounded, arciform, mu-shaped, or sinusoidal in appearance. Although this discharge is an unreliable indicator of seizures, parasagittal lesions can give rise to rhythmic slowing in this same region and should be considered when this waveform is encountered on the EEG.

Breach Rhythm

“Breach rhythm” refers to the EEG alterations involving derivations from electrodes placed over the region of a skull defect. In a breach rhythm, the EEG characteristically shows an increase in background amplitude and an increased representation of lower-frequency waveforms in the affected area. The waveforms in a breach rhythm also appear sharply contoured, which may cause confusion with epileptogenic sharp waves. Figure 5-2 shows a breach rhythm in a patient who had undergone temporal lobectomy previously; the increased amplitude, slow-wave changes, and sharp contour of the electrocerebral activity in the left temporal derivations are evident. The EEG interpreter should be cautious not to overcall the epileptogenicity of sharply contoured waveforms in patients with a breach rhythm, as doing so may overestimate their seizure potential.

Small Sharp Spikes

Small sharp spikes, also known as benign sporadic sleep spikes and benign epileptiform transients of sleep, are monophasic or biphasic spikes localized to the temporal regions. On ipsilateral ear-reference recordings, the field may involve the bifrontal regions. The waveforms sometimes show a downward deflection on ear-referential montages due to surface negativity at the ear reference, giving a “stalactite” or “icicle” appearance. They may be unilateral or bilateral, and are typically present in drowsiness and light sleep, resolving upon arousal and during deeper levels of sleep. Their reported prevalence ranges from 2.5 to 24 percent. Although some investigators have reported small sharp spikes to be more prevalent in patients with focal epilepsy than in nonepilepsy populations, the general consensus is that they are nonspecific and not uncommon in the nonepileptic population.

The morphologic features of small sharp spikes are well described. They are typically low-amplitude waveforms (less than 50 μV) of short duration (less than 50 msec), with a monophasic or biphasic morphology. Biphasic waveforms typically show a steep descending slope (see Fig. 5-3 ). In contrast to epileptogenic temporal spike-wave discharges, the slow waves associated with small sharp spikes (if present at all) are much lower in amplitude and brief (less than 100 msec duration).

14- and 6-Hz Positive Spikes (Ctenoids)

Discharges of 14- and 6-Hz positive spikes, previously referred to as “ctenoids”, consist of arciform discharges featuring surface-positive spikes which are usually maximal in amplitude over the temporal and posterior head regions (see Figure 5-3 ). They are best seen in montages utilizing long interelectrode distances. The discharges typically last for 1 to 2 seconds and are most often bilateral in distribution. This EEG finding at one time was thought to correlate with juvenile delinquency, as well as a number of other neurologic conditions including migraine and attention deficit–hyperactivity disorder. However, no clinical condition has been found to be associated specifically with 14- and 6-Hz activity, and they can be found in the normal population. Discharges of 14- and 6-Hz positive spikes are most common in adolescents and young adults, and have a reported prevalence ranging from 0.5 to 5.7 percent.

6-Hz (“Phantom”) Spike-Wave Activity

“Phantom” or 6-Hz spike-wave activity consists of 1- to 2-second clusters of moderate-amplitude theta activity having a frequency ranging from 5 to 7 Hz. These complexes characteristically are associated with a low-amplitude spike, referred to as a “phantom” spike. Occasionally, the spike cannot be seen at all, resulting in a rhythmic theta discharge over the posterior or anterior head regions. As is typical for benign EEG variants, 6-Hz spike-wave complexes are characteristically present in drowsiness and resolve in deeper levels of sleep and upon arousal. Their prevalence in the EEG laboratory has been reported to be 2.5 percent.

There are two forms of 6-Hz spike-wave activity, known as the anterior and posterior variants. The posterior variant is known by the acronym FOLD (female, occipital, low amplitude, and drowsiness), and the anterior variant has been referred to as WHAM (wake, high amplitude, anterior, and male). Over the years, it has become less clear that there is a significant gender difference between the two forms. However, the distinction between anterior and posterior variants still has some clinical merit, as the anterior variant may be seen in patients with idiopathic generalized epilepsy on antiepileptic medication, in whom generalized spike-wave activity may emerge upon medication withdrawal. An example of the posterior variant of 6-Hz spike-wave activity is shown in Figure 5-3 .