Abstracts

RATIONALE: In addition to the conventional EEG bandwidth ([gt]0.5Hz), experimental studies on sleep have indicated prominent brain activity at much lower frequencies, which is often called direct current (DC) potential shifts. Faithful detection of these patterns requires a genuine DC coupled EEG amplifier and non-polarizable electrodes. This study was set out to characterize this very slow EEG activity during human sleep, and to explore the possibility that interictal epileptic events could be associated with the sleep-related DC potential shifts.
METHODS: We recorded bedside non-epileptic and epileptic patients (n=15) during afternoon naps and overnight sleep. Recordings were performed with a custom made 16 channel DC-EEG amplifier and Ag/AgCl electrodes (see Vanhatalo et al, this meeting). Due to the high global synchrony of slow activity we used referential recording with calculated linked mastoid reference. Only non-REM sleep was analyzed and it was divided into two parts: non-slow wave sleep (phases I-II) and slow wave sleep (phases III-IV). Visual and spectral analysis was performed. Interictal epileptiform events were identified from a simultaneously recorded conventional EEG, and their timing was compared to the phase of the very slow EEG oscillations (10-40s per cycle).
RESULTS: Visual inspection revealed very prominent patterns of oscillating DC potential shifts with 10-40s cycle. The cycle length was irregular, and became shorter with gradual development of slow wave sleep. Spectral analysis confirmed prominent activities at 0.02-0.5Hz. On visual inspection, there was an impression that the conventional EEG activities ([gt] 0.5Hz; K-complexes, delta frequency bursts, etc) were unequally distributed in reference to the oscillating DC potential shift, suggesting that the latter might have a functional significance in the brain mechanisms related to sleep. In addition, visual comparison of the interictal epileptiform activity and the phase of the oscillating DC potential shifts showed that the vertex negative (referred to mastoid) phase of oscillating DC potential shift was significantly less likely to involve epileptiform events than the other phases.
CONCLUSIONS: Our findings demonstrate that human sleep involves very prominent EEG patterns at frequencies that are much lower than those recorded with conventional (AC) EEG. These can be recorded at bedside, and their temporal relation to physiological EEG phenomena suggests a significance in sleep mechanisms. Furthermore, our findings of a temporal relationship of interictal epileptiform activity to the phase of the oscillating DC potential shifts may help in understanding the mechanisms of sleep-epilepsy interactions.
[Supported by: Finnish Academy, Finnish Cultural Foundation, Arvo and Lea Ylppo Foundation, the University of Washington Regional Epilepsy Center]