Abstracts

Rationale: A higher number of electrodes in scalp electroencephalography (EEG) is needed to offer better spatial precision for detecting the source of single epileptic spikes in patients with known epilepsy. However, it remains controversial whether the same number of electrodes is needed to detect brainwave abnormalities in patients with altered mental status in emergency settings. Currently, half of the EEG electrodes are placed over the parasagittal regions of the brain. This process is costly and time consuming, and parasagittal coverage may not be diagnostically necessary in every case if parasagittal seizures are shown to be relatively rare and if they can be detected in lateral channels. In a seminal study, Pedley et al. (Ann Neurol 1981;9:142) showed that only 14 of 8708 adult and pediatric EEGs had focal parasagittal seizures; only three of these were recorded in adults. Gururangan et al. (Clin Neurophysiol Pract, in press) reported that reduced montage EEG covering only the lateral regions of the scalp is sufficient for detecting hemispheric seizures and rhythmic periodic patterns. In the current study, we aimed to systematically explore the frequency of occurrence of focal parasagittal seizures in adult patients undergoing long-term (>24 hours) EEG monitoring for the evaluation of altered mental status in intensive care unit (ICU) and non-ICU settings within a major medical center. Methods: We sequentially sampled a cohort of continuous EEGs performed in the ICU (n=100) and non-ICU (n=100) inpatient settings and determined the occurrence of seizures with parasagittal and non-parasagittal activity. We then created a reduced montage for each EEG with seizure activity by removing the parasagittal channels (leaving only the lateral chains of the double banana montage) and assessed if the seizures could have been detected on reduced EEG montage without displaying parasagittal channels. Results: In 200 long-term EEGs, only 16 seizures were captured. Of these, only 2 seizures were reported to be focal and concentrated in the parasagittal regions. In both cases, epileptic waveforms could be detected in the lateral channels and were visible using reduced EEG montage (see Figure). Conclusions: Our study confirms that, in adults undergoing long-term EEG monitoring for evaluation of altered mental status (rather than for seizure localization prior to surgical treatment), focal parasagittal seizures are very rare and can still be reliably detected in lateral channels. Funding: None