Abstracts

Rationale: High resolution dense array scalp EEG consists of 128 or 256-channels sensors with vertex reference. The electrodes are quickly applicable using cap system so that it is utilized in initial routine outpatient EEG evaluation. While conventional 10-20 System uses 21 electrodes with inter-electrode distance 7 cm, 128 channel system has better spatial resolution with 2.1 cm in inter-electrode distance. Taking the advantage of superior resolution and easiness in cap application, we use the system for all of outpatient EEG recordings and evaluated the efficacy to detect interictal discharges and their spatial distribution. Methods: A 128 channel high resolution dense array EEG system was utilized in outpatient clinic. Indication of EEG was episodic mental status change including epileptic syndrome. The electrodes were applied using 128-channel Geodesic Sensor Net (EGI, Eugene OR, USA). It was applied to each person for a routine EEG session of 25 to 30 minutes. Recording sampling frequency was 1000 Hz, HFF 120 Hz and LFF 0.5 Hz. All of the recordings were first interictal recording. Interictal epileptiform discharges can be single focus or multiple foci, and their locations were recorded in electrode numbers which were supplied by EGI System manufacturer. Rhythmic and evolutional discharges in frequency and amplitudes were counted as electrographic seizures. Multi-focal electrographic seizures were counted as independent phenomena in different electrodes. Results: 1159 EEG recordings were performed in our outpatient clinic since August 2013 to May 2016. Age distribution was between 25 to 89 year old. Among them, interictal discharges without electrographic seizure pattern were seen in 342 EEGs (29.5 %) which is comparable with conventional 10-20 recording. On the other hand, interictal electrographic seizure patterns were seen in as many as 181 EEGs (15.6 %). Distribution of majorities of electrographic seizure foci predominated in frontal regions, but may be grouped into two triangular regions in each hemisphere: (1) frontal-infrafrontotemporal region, and (2) post-Rolandic region. Small number of foci distributed in midline structure at Fz, Cz and Pz. A few cases of occipital electrographic evolving discharges were also noted. Conclusions: Van Donselaar et al. (1992) prospectively examined the EEG findings with first seizures and found that only 12% had epileptiform discharges on the first EEG. In our study, dense array HR-EEG with 128-channel recording revealed up to 45% positivity in either interictal discharges or interictal electrographic seizure pattern in non-selected patient population of outpatient neurology clinic. It is therefore concluded that a single scalp dense array EEG recording has greater chance to detect epileptic abnormality than single routine conventional EEG. The distribution pattern of the interictal electrographic seizures were grouped in two triangular area anterior: frontotemporal region, and posterior: post-Rolandic region. Smaller number of electrographic seizure falls in midline and occipital electrodes. The findings may give new insight of electrographic seizure patterns. This study received no funding from any organization. Funding: (1) High resolution dense array EEG recordings detected subclinical electrographic seizure patterns in 15% of outpatient population of epilepsy clinic, and inter-octal discharges in 30%. (2) Distributions of electrographic seizure foci may be grouped into triangular areas in frontotemporal and post-Rolandic regions in each hemisphere.?