Abstracts

Rationale: Recording High Frequency (HF) EEG activity by scalp electrodes is challenging. The recorded activities may be already attenuated by electrical resistance of skull and electrical capacitance of scalp tissue and subarchnoid space, and may be contaminated with movement and EMG artifacts. Moreover, scalp EEG recording wave forms represent non-linear summation of cortical IPSPs from underlying gyri. Nevertheless, understanding the limitations, recording of scalp permeating HF activities has been discussed in several papers. We performed high frequency sampling EEG recording to evaluate the usefulness of scalp recorded ictal HF recordings in the first phase noninvasive EEG monitoring studies in EMU. Methods: Among patients admitted to the EMU for non-invasive EEG evaluations for intractable partial complex seizure, seven patients who experienced clinical seizure and lateralized interictal epileptiform discharges were recorded with higher sampling frequency by 2000Hz using Nihon-Kohden CCTV-EEG units only during sleep state after sleep deprivation. HF discharge was searched upon the electrical seizure onset. Seizure onset during awake state was excluded due to our technical limitation to eliminate movement artifacts. The recording High frequency filter was raised up to 300 Hz and recording sensitivity was up to 1 microvolt/mm. Muscle activities were ruled out by lack of specific frequency band, and scattered random distribution. Results: Four patients who demonstrated seizure onset during non-REM sleep state with sustained frequency specific γ range HF activity were identified with reasonable reproducibility. Patient #1: Onset HF was characterized as 45 Hz γ sustained activity at Right Sp2. IID were at F4. MRI showed Right hippocampal atrophy. Patient #2: 100-175 Hz γ at seizure onset. IID are frequent at T1, T3. MRI was non-lesional. Patient #3: 300 Hz γ-blush overriding sharp discharge at Right T4 upon seizure onset. IID were bilateral independent spikes at Sp1 and Sp2. MRI was non-lesional. FDG-PET was symmetrical. Patient #4: HF was characterized as 100-120 Hz γ sustained discharge at onset. IID are at T1 and T3. MRI showed bilateral hippocampal atrophy. In this small case series, lateralization of the HF correlated to within the epileptogenic hemisphere. Conclusions: Despite of technically difficulties due to amplitude attenuation and abundant artifacts during awake state, ictal high frequency EEG is reasonably readible if it is captured in sleep state with sustained frequency specific discharge pattern and concomitant lateralization with conventional localization of interictal epileptiform discharges. If recorded in reproducible manner, scalp permeating HF findings may assist seizure localization in the non-invasive EEG recordings. The limitations of the scalp HF recording is that it was only done in asleep state to avoid movement artifacts.