A Question Worth Asking

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To the Editor:
We read with interest “Diagnosis of Electrical Status Epilepticus During Slow-Wave Sleep with 100 Seconds of Sleep” by Weber et al.1 Electrical Status Epilepticus in Sleep (ESES) is an electroencephalographic (EEG) pattern characterized by almost continuous and diffuse interictal epileptiform discharges during sleep.2 Although rarely discussed, the length of the EEG epoch used for quantification of epileptiform activity varies widely among different authors and defines how we diagnose and monitor patients with ESES and the resources we use for doing it.
The group that initially described this EEG pattern suggested that, to be consistent with a diagnosis of ESES, the pattern of diffuse epileptiform activity should occur during at least 85% of overnight slow sleep and persist on three or more EEG recordings over a period of at least 1 month.2 Under this framework a diagnosis of ESES cannot be made on a single EEG but requires persistence of the pattern on three or more EEGs over a minimum period of 1 month.2 A looser version of this framework has been largely followed: it permits diagnosis on a single EEG but still requires quantification during the whole night of sleep. Other groups quantify epileptiform activity by sampling epochs of sleep, for example, during the first nonrapid eye movement (non-REM) sleep cycle or nap EEG.3 This heterogeneity in the literature is mirrored by heterogeneous EEG sampling in clinical practice. In a survey among Neurologists and Epileptologists in North America respondents reported that the segment of EEG used for calculation of epileptiform activity was: the whole night of non-REM sleep (40.2%), the whole night including all phases of sleep (27.6%), a comparison of wakefulness with sleep (11.8%), unknown (11.8%), variable (3.2%), and other selected samples (5.5%).3
The method of quantifying epileptiform activity throughout the entire night assumes that quantification fluctuates during the night and that sampling might yield counts that are misleadingly high or misleadingly low. Therefore, under this assumption, an overnight EEG is needed. In contrast, the method of sampling epochs of non-REM sleep assumes an approximately even distribution of epileptiform activity during non-REM sleep. Therefore, under this assumption, an overnight EEG might not be needed. The initial descriptions of ESES reported that epileptiform activity is more frequent during the first hours of sleep as compared to later sleep stages during the same night, although this may also reflect more frequent interruption of the ESES pattern by REM sleep during the later phases of night sleep, if assessments included REM sleep.2
Our center evaluated the usefulness of quantifying epileptiform activity only during the first 5 minutes of non-REM sleep.4 In an initial study on eight EEGs, we reported that epileptiform activity quantified during the first 5 minutes of non-REM sleep correlates well with epileptiform activity quantified overnight.5 In addition, we also compared the counts during the first 5 minutes of non-REM sleep with the counts during 3 EEG samples of 100 seconds each randomly selected throughout overnight non-REM sleep.4 We quantified the spike-wave percentage as the percentage of one-second bins with at least one spike-wave complex and the spike frequency as the number of spike-wave complexes per 100 seconds.4 The comparison in 28 EEG tracings with ESES showed a Pearson correlation coefficient of 0.972 for spike-wave percentage and of 0.946 for spike frequency.4 These correlations are concordant with the findings by Weber et al. showing a good correlation between shorter and longer sampling.1
As Weber et al.1 point out, quantification of epileptiform activity with overnight EEG is resource intensive because it needs prolonged periods of EEG recording, frequent hospital admissions, and detailed readings during many hours of EEG tracings.
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