Abstracts

Responsive neurostimulation (RNS) directed toward optimizing containment and control of the epileptic circuit is the next generation of sophisticated surgical technologies aimed toward minimizing invasive surgical intervention while potentially maximizing clinical efficacy. A pivotal and unique approach to optimize the yield of stimulating a small set of chronically implanted electrodes currently available for RNS involves targeting activation of critical juxtacortical white matter pathways with cylindrical depth electrode contacts. Capitalizing on the biophysical properties of white matter, spread of current can be dramatically augmented. Moreover, directing the spread of current to an independent distant mirror epileptic source contralaterally can potentially amplify the efficacy of the available intracranial electrode set and simplify the surgical approach. Dipole source localization of the ictal onset for each seizure captured during presurgical video-EEG monitoring was performed using high density scalp electrodes (n=75) and a realistic head model for noninvasive determination of epileptic sources (Curry^R v 4.6). The electrode positions with respect to head anatomy were digitized using a Polhemus^R scalp electrode digitizer. Transient blood flow changes occurring during a stereotypic ictal onset was concurrently captured utilizing subtracted ictal SPECT co-registered to MRI (SISCOM) using Analyze^R. Source localization of equivalent dipoles was compared with the SISCOM data. A 3-D multi-compartment cable model was used to model the probable neuronal pathways activated. Data from a patient implanted with an RNS depth electrode in the right hippocampal angular bundle revealed distant neuronal propagation of current with physiological latencies. On repeated stimulation of bipolar contacts, contralateral homotopic propagation of a moving dipole was seen traveling by way of the anterior commissure. The cable properties of this neuronal pathway were modeled using biophysical properties of a cortical pyramidal neuron. SISCOM identified transient alterations in blood flow near the electrode contact and at distant propagation pathways related to the epileptic network. This information represents the volume of cortical activation for a given set of stimulation parameters passed through a specific electrode contact geometry and orientation. Optimal RNS depth electrode placement oriented in relation to critical white matter pathways potentially achieves maximized activation of critical nodes within the mapped ictal onset zone. This strategy impacts on early seizure termination following successful detection.