Abstracts

Rationale: It is often stated that epileptogenic lesions located deep in the brain do not generate Interictal Epileptic Discharges (IEDs) on scalp EEG, but it is also frequent that such lesions result in IEDs on scalp. We investigated whether the depth of a lesion has an influence on the likelihood that scalp IEDs are present. As High Frequency Oscillations (HFOs) are thought to be generated near the epileptic focus, we also investigated if lesion depth influences the chances of seeing HFOs on scalp EEG. Methods: Between September 2011 and August 2016, we selected all patients with lesional epilepsy who underwent EEG-telemetry studies in our monitoring unit. We marked a maximum of 50 IEDs during deep NREM sleep over the second night of monitoring, and included only samples more than 2 h before and after a seizure. We counted spikes, sharp waves, and bursts as one IED. We excluded bursts without side predominance. We analyzed HFOs in a subset of studies showing at least 50 IEDs in order to increase the probability of finding HFOs, as these are infrequent and known to occur predominantly around IEDs. Ripples (>80 Hz Superficial: lesions involving no more than 3 cm from the skull; deep: orbito/medial frontal, medial/inferior temporal, periventricular, diencephalon, basal ganglia, and corpus callosum; and, intermediate: not fitting with previous categories. We performed a permutation test in which the null hypothesis was that the probability of detecting IEDs or ripples does not depend on the location of the lesion. Results: From 192 studies, the differences in the percentage of studies showing IEDs in each depth-related group were not statistically significant (60% in superficial, p=0.35; 77% in the intermediate, p=0.27; and, 72% in the deep lesions, p=0.52) (fig.1). We analyzed HFOs in 55 tracings. There were statistically significant differences in studies showing HFOs (deep lesions 5%, p=0.007; intermediate 14%, p=0.53; and, superficial 82%, p=5. 10 -5) (fig.1). Conclusions: In scalp EEG, the depth of the lesion clearly influences the likelihood of detecting ripples but not IEDs. One interpretation of these results is that HFOs are only generated near the epileptogenic region, do not propagate and are only visible with superficial lesions, whereas IEDs, equally likely to be visible in deep and superficial lesions, can result from a near-by focus or from a deep lesion, in this case resulting from propagation. This provides an explanation for the high localization value of HFOs and poor value of IEDs.. Funding: This work was supported by grant FDN 143208 of the Canadian Institutes of Health Research.