Abstracts

Rationale: Stereo electroencephalography (sEEG) combined with video monitoring is becoming widely used for presurgical evaluation of patients with refractory focal epilepsy. Functional cortical mapping with single-pulse electrical stimulation or high frequency trains of stimuli can be used to identify eloquent cortical areas prior to surgical resection or ablation. Despite the widespread use of functional mapping in sEEG patients, standardized methods have not been developed and their safety has not been validated. The goal of the present study is to evaluate the safety and operational parameters of motor, sensory, language and/or visual functional mapping with the use of both the Ojemann Cortical Stimulator (OCS) and a custom-made Fast Frequency Stimulator (FFS) in sEEG patients.Methods: Nine patients ages 27-51 with refractory focal epilepsy were implanted with 4-16 sEEG electrodes between 2013-2015 at Emory University Hospital as part of intracranial monitoring prior to surgical resection, ablation, or responsive neurostimulation (RNS) placement. The seizure onset zone obtained during sEEG monitoring included hippocampus (2), posterior temporal cortex (2), frontal (1), occipital (1), parietal (1), and operculum (2). Functional mapping of language (5), motor (2), or visual (2) cortex alone or in combination with sensory cortex (2) was performed in each patient. Four patients underwent testing using the OCS with 60-Hz trains of 0.5 ms bipolar pulses, and 5 underwent testing using FFS with bursts of five, 500-Hz bipolar pulses each lasting 0.5 ms, repeated every 0.5 s. The maximum tested current intensity, the current increments, and the presence of evoked afterdischarges or seizures were compared between the two groups.Results: The maximum current intensities were 12 and 5 mA in the OCS and FFS groups, respectively. The current increments were 1 and 0.5-1mA, respectively. Three out of 4 patients in the OCS group developed visual, motor or sensory symptoms during stimulation with maximum current. Two out of 5 patients in the FFS group developed motor symptoms during stimulation with current strengths ranging 1-4 mA. Stimulation-induced afterdischarges occurred in all patients from the OCS group and in one patient from the FFS group. None of the FFS patients had seizures as a result of stimulation; two patients from the OCS group developed seizures with stimulation.Conclusions: These findings suggest that preoperative functional mapping in refractory focal epilepsy patients using sEEG can be performed safely with either OCS or FFS. The safety of FFS may be superior to that of OCS; fewer afterdischarges and seizures were elicited with FFS. Stimulation of depth electrodes in proximity to eloquent cortex with either OCS or FFS produced the expected disruption of language, motor, sensory or visual function. Further studies are needed to assess these methods and related functional outcomes in larger subsets of patients undergoing cortical resection, laser ablation, and RNS placement.