There are 4 major types of epileptiform variant patterns:
1. 14- and 6-Hz positive bursts.
2. Small sharp spikes (benign epileptiform transients of sleep [BETS]).
3. 6-Hz spike and wave (phantom spike and wave).
4. Wicket spikes.
Previously called "14- and 6-Hz positive spikes" or "ctenoids," these variants occur as bursts of rhythmic arched waves, similar to sleep spindles, with a smooth negative component and a spike-like positive component (Fig. 4). As the name implies, these trains occur at 14 and 6 Hz. The bursts last only 0.5 to 1 s. Such bursts are best captured on referential montages (because of the greater interelectrode distances). They are maximal at the posterior temporal head regions and usually occur independently from bilateral hemispheres with shifting predominance. This variant appears in 10 to 58% of healthy subjects, but is influenced by age, montage, and duration of drowsiness and sleep. This pattern is more prevalent in children and adolescents.
2.2. Small Sharp Spikes/BETS
As these names imply, small sharp spikes or BETS are low in amplitude (~50 ^V) and brief (~50 ms). Their morphology can be monophasic or diphasic. When diphasic, the ascending limb is quite abrupt and the descending limb slightly less so. They may exhibit a subtle following slow wave. BETS are isolated and sporadic. They appear during drowsiness and light sleep in adults. They are usually unilateral but can appear independently (and rarely synchronously) from bilateral regions (Fig. 5). On a transverse montage, their field often illustrates a transverse oblique dipole (opposite polarities across the opposing hemispheres), an atypical finding in bona fide epileptiform discharges. Other distinguishing features
between BETS and epileptiform activity are that BETS do not run in trains, distort the background, or coexist with rhythmic slowing, and BETS diminish with deepening sleep, whereas epileptiform discharges worsen with deeper sleep stages. White et al. reported the incidence of BETS to be comparable in healthy subjects (24%) as in symptomatic patients (20%). Thus, BETS seem unrelated to the diagnosis of epilepsy.
2.3. Six-Hertz Spike and Wave ("Phantom" Spike and Wave)
These rhythms have a frequency ranging from 5- to 7-Hz. They occur in brief bursts lasting 1 to 2 s, rarely up to 3 to 4 s. The spike component can be difficult to recognize because it is not only very brief but also of very low amplitude and, thus, has a fleeting quality. This subtle characteristic has given rise to the pithy term, "phantom" spike and wave. By contrast, its slow-wave component is broader in duration, higher in amplitude, and more widespread in distribution.
This pattern appears in waking or drowsiness in adolescents and adults. However, it is absent from slow-wave sleep. Silverman reported its overall incidence to be 2.5%. It usually has a diffuse, bilaterally synchronous distribution. At times, it is asymmetric or more regional.
Phantom spike and wave can appear quite similar to the 6-Hz positive spike burst. Infrequently, a transition may be seen on the same subject's EEG between these two types of variants. This pattern is thought by most to represent a benign finding. However, its morphology may be easily confused with an epileptiform pattern. Its failure to persist into slow wave sleep and its monomorphic quality permit its distinction from bona fide epileptiform discharges.
This variant pattern appears as single spike-like waves or as intermittent trains of arc-like monophasic waves at 6- to 11-Hz (Fig. 6). Amplitudes range from 60 to 200 ^V. Wicket spikes commonly appear from temporal channels and can be bilateral and synchronous or with shifting predominance. They occur mainly in drowsiness and light sleep in adults older than 30 yr of age.
When wicket spikes occur in isolation, they may be mistaken for an epileptiform discharge. Several features help differentiate isolated wicket spikes from pathological spikes. A similar morphology of the isolated wicket spike to those in a later train or cluster argues for the variant pattern and against an epileptiform discharge. The absence of a following slow-wave argues for the variant and against an epileptiform discharge. An unchanged background also argues more for the variant and against an epileptiform event.
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