Abstracts

Rationale: Intracranial single pulse electrical stimulation (SPES) is an increasingly used method for mapping the epileptogenicity of various brain areas during presurgical evaluation of patients with refractory epilepsy. In combination with stereoelectroencephalography (SEEG), SPES represents a powerful tool for exploring the networks within the epileptic brain in the interictal period, providing valuable information about the epileptogenicity of structures involved in seizure onset and the propagation pathways.Methods: We have designed a stimulation protocol that consists of a sequence of single pulses having variable amplitude in the range 0 to 5 mA, with 0.25 mA step. Compared to standard constant-amplitude protocols used in other investigations, this protocol allowed us to map not only the propagation of the stimulation through the epileptogenic network, but to also the cortical excitability by analyzing the stimulus-response curves. The pulses (biphasic, 3ms pulse width, 15 seconds interval) were applied in a pseudo-random amplitude sequence, in order to allow decoupling of the time and amplitude factors during subsequent data analysis, using a programmable stimulator (Guideline LP+, FHC Inc, Bowdoin, ME) that allows definition of complex waveforms. We recorded fast responses (100 msec window) to variable amplitude SPES in 7 subjects undergoing presurgical evaluation by means of SEEG method. We used the third quartile value of pooled responses recorded for all stimulated sites as a threshold for calculating the individual contact activation. For responses on each contact, we calculated a 50% activation threshold based on the stimulus-response curve obtained by fitting the data points using LOWESS (locally weighted scatterplot smoothing) non-parametric regression. All patients were operated and rendered seizure free for a period of at least 6 months. Results were retrospectively analyzed and were correlated with the standard method for establishing seizure onset zone (SOZ) and ictal propagation networks.Results: The activation current threshold was minimal (0.3 to 0.7 mA) when stimulation was performed within SOZ or adjacent epileptogenic regions in 5 out of 7 patients (71%). The number of activated contacts was maximal when stimulation was applied within epileptogenic areas within 5 of the 7 patients (71%). This shows that automated fast responses analysis and stimulus-response threshold calculation have a good specificity for the localization of the SOZ, however additional information, including presence of delayed responses and stimulation-evoked high-frequency oscillations has to be considered for a more accurate localization.Conclusions: Evaluation of thresholds from stimulus-response curves can provide valuable complementary information to recording spontaneous activity and responses to standard stimulation protocols for better defining the epileptogenic network, reducing the duration of the invasive monitoring phase.