Abstracts

Rationale: Epilepsy surgery aims at removing the epileptogenic zone to achieve seizure freedom. Currently interictal epileptiform discharges (IEDs) in intra-operative electrocorticography (iECoG) can be used to tailor the resection. High frequency oscillations (HFOs; ripples: 80-250 Hz and fast ripples: 250-500 Hz) seem more specific for the epileptogenic zone than IEDs. Removal of tissue with HFOs preresection is correlated to seizure freedom. However, some HFOs, especially ripples, are also present outside the resection area. Knowledge is lacking on the effect of resection on these distant HFOs and how this relates to surgical outcome. Therefore, we compared the occurrence of HFOs in the iECoG preresection to postresection.Methods: We selected two patients with refractory temporal epilepsy who underwent iECoG during epilepsy surgery with high sampling rate (2048 Hz). 4x5 electrode grids and 1x5 electrode strips were placed on the cortex at different positions. Electrode placement was based on clinical information like structural abnormalities and video-EEG investigations. Tailoring was based on IEDs in the iECoG while propofol anesthesia was interrupted. Ripples and fast ripples were detected in one-minute epochs. HFO detection was performed using the automatic MNI detector optimized for iECoG recordings in a bipolar montage [1]. To compare the number of HFOs at each location pre- and postresection, the position of the grids and strips was schematized guided by photos during the OR. The scheme was optimized so each box represents a bipolar electrode pair. The resected area was included in the scheme as well (Figure 1A).Results: In the first patient, three grid electrode positions were recorded before resection and one after. In the second patient there were two grid electrode positions before and two after resection. We found 477 ripples and 40 fast ripples before resection and 28 ripples and no fast ripples after. In patient 1, ripples as well as fast ripples were found in the resection area and at 5 cm distance from this resection area before resection. After resection all ripples and fast ripples disappeared, even at distant locations. This patient became seizure free (Engel 1A, Figure 1). In patient 2, only ripples were found before resection: in the resection area and at 3 cm distance. Postresection most locations showing ripples before resection did not show them anymore. At some locations ripples remained, though at a lower rate. This patient did not become seizure free (Engel 2A).Conclusions: Our results show that resection of epileptogenic brain tissue can affect the occurrence of HFOs at distant locations. This could mean that HFOs can spread or result from epileptic networks and removing part of the network yields seizure freedom. Recording of post resection HFOs might be useful to predict seizure freedom. [1] Zelmann, R., et al., A comparison between detectors of high frequency oscillations. Clinical Neurophysiology, 2012. 123(1): p. 106-116.