Abstracts

Rationale: Epileptic areas in pre-surgical evaluation of epilepsy patients are usually identified by visual inspection of the electro-corticogram. Recent studies suggest that single pulse electrical stimulation evoked late potentials (SPES) correlate with the seizure onset zone. We tried to reproduce this finding by systematically stimulating all electrodes over the presumed epileptic area. Furthermore, we investigate if SPES can be used to identify not only pathological but also functional networks, such as the ones involved in language. Methods: We performed systematic bipolar stimulation of all electrodes by administering a single pulse electrical current (10mA, 0.5Hz, 0.2msec pulse width, 20 trials per electrode pair) on four patients undergoing intracranial monitoring for intractable epilepsy. Electrodes were localized using post-operative CT and MRI and aligned upon the reconstructed cortical surface of a pre-operative MRI scan. Responses to electrical stimulation were averaged at all electrodes. The stimulation response at each electrode was considered significant if the late response (100-1000ms) exceeded 3 standard deviations from the baseline activity (-500 to 0ms). Language areas were identified using standard functional stimulation mapping. Results: Stimulating electrodes over Broca s area in two patients showed significant responses in other electrodes considered part of the language network as defined with functional stimulation mapping. Responses to stimulation in the seizure onset zone were mainly found in electrodes over well circumscribed areas involved in seizure onset and early spread. Conclusions: Our results suggest that single pulse electrical stimulation evoked potentials correlate reasonably well with clinically defined epileptic areas. Furthermore, the same method might prove to be useful to not only delineate pathological but also functional networks. We believe that single pulse cortico-cortical electrical stimulation is a promising technique in delineating the seizure onset zone and eloquent cortex.