Benign EEG Patterns
This chapter is devoted to several EEG patterns that are seemingly abnormal because the waveform resembles epileptiform activity, the discharge has an asymmetric distribution, or the rhythmic pattern mimics an ictal (seizure) discharge. Correctly recognizing these variants is important in order to avoid overinterpreting their clinical significance. There may be some controversy and difference of opinion about these patterns, but most are in agreement that they are patterns of normal variants.
14- and 6-Hz Positive Spike Bursts
Historically, 14- and 6-Hz positive spikes have been the most extensively discussed by many investigators and have created much controversy in regard to their clinical significance. The pattern has a characteristic waveform, frequency, and distribution—comb-shaped positive spikes maximum at the posterior temporal/occipital electrodes with a frequency of 13 to 17 Hz and/or 5 to 7 Hz, mostly consisting of 14 Hz and/or 6 Hz. They occur predominately in light sleep. The pattern may appear as a 14- and 6-Hz positive spike complex (Fig. 14-1), 14-Hz positive spikes alone (Fig. 14-2), or 6-Hz positive spikes alone (Fig. 14-3). The 6-Hz positive spikes tend to appear in early childhood and in adults, while 14-Hz positive spikes are more common in older children and adolescents.1 The pattern is well visualized with a circumferential montage which includes occipital and posterior temporal electrodes. The pattern is also well demonstrated with an ear reference recording. However, ear reference ipsilateral to the side of the spikes is often active and contamination of the spike discharge results in seemingly “negative” spikes at the frontal region (Fig. 14-4A). An alternative montage is to use the contralateral ear reference, which increases the interelectrode distance and eliminates the contamination (Fig. 14-4B).
Since the discovery of this pattern by Gibbs and Gibbs,2 who regarded it as the evidence of thalamic and hypothalamic epilepsy, there have been numerous studies correlating 14- and 6-Hz positive spikes with autonomic nervous system dysfunctions and psychiatric and/or behavioral disorders.3,4,5,6,7 However, the enthusiasm for these clinical correlates faded when it became evident that the pattern was often seen in normal individuals, especially in adolescents.8,9 One study showed that the incidence of 14- and 6-Hz positive spikes was 40% to 60% in 14- to 16-year-old adolescents.8 The pattern is rarely seen in individuals before the age of 5 or after the age of 25.
The 14- and 6-Hz positive spike burst can be readily identified because of its characteristic waveform, distribution, and polarity. In some cases, 14- and 6-Hz positive spike bursts precede generalized slow waves, making the complex look abnormal (Fig. 14-5). A more difficult situation may be when the 14- and 6-Hz positive spike bursts precede a negative spike and wave burst. It is debatable if this should be regarded as abnormal.
One interesting clinical correlation of 14- and 6-Hz positive spikes is a high incidence of the discharge in patients with Reye’s syndrome10 (see Fig. 11-17A and B). The discharges appear during the acute coma state and disappear after recovery from coma.11 Also, 14- and 6-Hz positive bursts have been reported in an adult patient with hepatic coma12 and children with diverse encephalopathies including Reye’s syndrome.13
Small Sharp Spikes
Small sharp spikes (SSS) are also known as benign epileptiform transients of sleep (BETS). The pattern is characterized by a small, usually less than 50 µV, mono- or diphasic spike of short duration (<50 ms). In a bipolar derivation, it often appears as a small “needle-like” spike (Fig. 14-6A). Recording with an ear reference montage (Fig. 14-6B) will show more widely distributed, often larger amplitude spikes than those recorded with it a bipolar montage. Because of the wide spread and fairly even distribution of the spike, bipolar recording tends to be canceled out, often making it appear to be more focal than if seen in a referential recording. The morphology of SSS may vary from predominantly negative to diphasic or to predominantly positive polarity within the same individual. SSS also tends to shift from side to side (Fig. 14-7A and B). This is essentially a pattern in adulthood with a peak age between 20 and 25 years,14 and it is extremely rare before the age of 10.
SSS are regarded as a normal pattern, but the differentiation from a genuine epileptiform spike is sometimes difficult. The following points aid in differentiating between the two:
1. Because SSS tends to show a variable morphology and a side-to-side shift, a consistent localization and morphology are likely to indicate epileptiform activity.
2. Since SSS appears only in stage I or II sleep, spikes appearing in the awake state or in deep sleep are more likely to be epileptiform activity.
FIGURE 14-1 | An example of 14- and 6-Hz positive spike bursts in an 8-year-old boy. Note positive phase reversal at T6 and T4 electrodes (shown in oval circle) and 6-Hz positive spikes (shown in rectangular box) with equipotential at T6 and T4.
FIGURE 14-2 | An example of 14-Hz positive spikes in a 10-year-old girl. Note the difference of waveform and distribution between positive spikes (shown in rectangular box) and sleep spindles (shown in oval circle).
FIGURE 14-3 | An example of 6-Hz positive spikes in a 4-year-old boy (shown in box). Note positive phase reversal at P3 electrode.
3. SSS usually appears as a single transient and does not occur in rhythmic trains; thus, rhythmically recurring spikes are likely abnormal discharges.
4. SSS are not usually followed by a prominent wave. Therefore, a spike associated with a spike-wave complex is likely epileptiform activity.
5. SSS does not disturb the background activity.
It is generally agreed that SSS has no correlation with epilepsy and is a normal pattern,14,15,16,17 though some investigators regard the pattern with a “moderate epileptogenic property.”18,19