Abstracts

Rationale: Single pulse cortical electrical stimulation (SPES) and its responses yield information about the epileptic brain. Research has mainly focused on delayed SPES responses which are assumed to be associated with the underlying pathology. However, early SPES responses can map effective brain connectivity networks. SPES can be rapidly applied during surgery and might help to predict seizure spread without the need of an actual seizure. Accurate predictions of seizure spread may be useful for decision making in focal resections and evaluating the effectiveness of surgery. This could be specifically useful in cases when subpial transsections are preferred over resection for example when the seizure onset zone lies in an eloquent area. We aim to use clinical SPES data to assess the feasibility of predicting ictal propagation. Methods: In this study, data from pre-surgical evaluation of 5 patients suffering from focal epilepsy with a marked gamma onset on chronic electrocorticograpy were used. SPES (bipolar monophasic 0.2Hz stimulation with 0.1ms pulse width) was applied to electrodes in the initial seizure onset zone. An automatic detection algorithm for early responses, using Wiener filtering to correct for stimulus artifacts and response amplitude offset, was developed. Based on standardized early response amplitude thresholds we predicted which electrodes could be involved in seizure spread. The resulting electrode positions were then compared to those involved in actual seizure spread as indicated by an experienced electroencephalographer. This was used as the reference for calculating sensitivity and specificity of predicted seizure spread. Furthermore, we assessed if the delay of the early responses was correlated to the timing of seizure spread for each individual patient using Spearman's rank correlation (p<0.05). Results: The 5 patients analyzed in this study had 16-30 (mean 23) electrodes marked as being involved in seizure spread. SPES early responses were able to predict seizure spread to these electrodes with an average sensitivity of 69% (range 57-82%) and specificity of 83% (range 80-88%). No significant correlations between response delay and seizure spread delay were found in all but one patient (rho = 0.63, p = 0.02). Conclusions: Our results show that SPES early responses can be a valuable tool to peroperatively map the epileptic brain. SPES early responses were able to predict seizure spread of the patients' dominant seizure type based on stimulation of the epileptic focus. Future research should be performed to further improve this prediction and evaluate the clinical use.