Abstracts

Rationale: While benign epileptiform transients of sleep (BETS) have been well known to clinical neurophysiologists, understanding of their neuroanatomical and behavioral correlates is lacking. Localization of BETS generators to hippocampus by electromagnetic source imaging (Wennberg et al., 2020) was collaborated by polygraphic demonstration (Matsuo, 2021 AES Annual Meeting Abstract. AESnet.org). Investigation into variations among increasing numbers of BETS revealed waveform complexity in posterior temporal derivation indicative of contribution of multiple generators. This study was focused on polygraphic correlates of basal generator involved at BETS initiation.

Methods: EEG electrode placements consisted of 19 according to 10-20 International System and two basal pairs (zygomatic and mastoid, notated as F11-F12 and T11-T12, respectively). EEG was digitized at 200/s. Conventional BETS waveform criteria were applied to initial off-line EEG review of anterior-to-posterior serial bipolar derivations (Figure A). Entire BETS samples were re-examined in 25-channel common reference derivations (Figure D for right-sided BETS), designed for display-analysis of subtle phase and latency differences, not apparent on default common reference display (Figure B). Reference (AV10) was average of 10 electrodes on temporal plane.

Results: Index and five additional EEGs yielded 206 BETS. Biphasic waveform (Fig C) was found in 79 BETS, of which 58 were right-sided. Right- and left-sided BETS were symmetrical (Figure A and B). Illustrative BETS (E1-145936) was chosen for robust waveform against attenuated background of early sleep and had peak voltage difference of 61.8 uv at phase reversal between F8 and P7 derivations. BETS segment of interest was highlighted by 35-ms window with F8 peak segment at center (Figure D). Inter-peak latency difference of biphasic BETS in T12 derivation is 20 ms. Downward peak T12 is in phase with peaks at T8 and P8, and reveals phase reversal against left parasagittal and midline peaks of lesser amplitude (with minimal amplitude difference among them); consistent with deep basal source location. Right parasagittal peaks, longer in duration and higher in amplitude anteriorly, form frontotemporal peak. BETS peak can then be followed along temporal-electrode chain to P8, revealing phase reversal against left temporal derivations (Fig C and D).

Conclusions: BETS served as exemplars of focal epileptifom transients associated with deep generators. Polygraphic reformatting in common reference derivations has advantages: 1) availability for noninvasive EEG screening, 2) flexibility to accommodate for neuroanatomical target location and electrode placements, and 3) estimation of source depth and equivalent current dipole magnitude from peak voltages at phase reversal. Polygraphic channel-overlay and duplication are helpful reformatting options to facilitate analysis of subtle EEG phase relationship. While still to be considered preliminary, demonstrated polygraphic features of peak movement differentiate BETS from clinical basal temporal epileptiform transients (Matsuo, 2017 AES Annual Meeting Abstract).

Funding: None