Abstracts

Rationale: Around 30% of the people with epilepsy remain refractory, and half of this population is potential candidate for epilepsy presurgical evaluation. Presurgical evaluation requires reliable and accurate delineation of epileptic and eloquent language and visual networks. Non-invasive and invasive investigation modalities used during presurgical evaluation include fMRI and intracranial EEG. Direct current stimulation (DCS) is traditionally applied through intracranially implanted subdural grid electrodes to map eloquent cortex. We aim to demonstrate that intracranially implanted depth electrodes (stereo-electroencephalography: SEEG) and fMRI can be used to map eloquent networks and specific areas safely and reliably during presurgical evaluation of refractory focal epilepsy. Methods: We investigated six patients with refractory focal epilepsy undergoing presurgical evaluation. All patients underwent detailed history taking, clinical examination, 3T MRI, long-term video telemetry, neuropsychological assessment, and fMRI. The SEEG implantation strategy was tailored according to the hypothesis for presumed epileptogenic zone and eloquent cortex for individual patients. Functional MRI was performed on a 3T Siemens Verio MRI scanner (scanning parameters: GRE-EPI sequence; TR 3000ms; TE 30ms; in-plane resolution 3.9mm; 64x64 matrix; FOV 250 mm; slice thickness 3mm; gap 25%). Two language paradigms: silent word generation and sentence completion and a visual paradigm, checker board, were used during fMRI acquisition. Functional MRI t-maps were generated and overlaid on T1 structural images to localise language and visual networks. Language and visual mapping, using SEEG depth electrodes DCS, was performed using the following parameters: biphasic pulse, phase duration: 150-300 microsec, intensity:1-15mA, frequency: 10Hz, duration: 2-3sec. A range of language tasks was employed including: sentence completion, picture naming, and verbal fluency. Visual mapping was performed using a whiteboard divided into four quadrants and patient reported visual symptoms after DCS. Results: Three patients had MR negative left neocortical temporal lobe epilepsy. Language fMRI showed bilateral language network lateralizing to the left with activations in left inferior frontal gyrus (IFG) and superior temporal gyrus (STG). DCS showed language areas in left STG and supramarginal gyrus (SMG) within the ictal network in two of these patients but in the contralateral hemisphere in the third patient. One patient had left mesial occipital lobe epilepsy secondary to temporo-occipital encephalomalacia and incidental hippocampal sclerosis. Language maps using fMRI lateralized to the left side. DCS localized language areas to left STG, SMG, and angular gyrus, and visual areas to mesial occipital cortex around the calcarine fissure. One patient had right mesial temporal lobe epilepsy with incidental right pareito-occipital encephalomalacia. Visual mapping using fMRI showed activations in bilateral primary visual cortices. DCS showed functional visual areas in right occipital cortex around the calcarine fissure. One patient with a VNS in-situ had diffuse left hemisphere onset but speech excluded from left hemisphere with DCS. None of the patients had any complications from SEEG or DCS.  Conclusions: Eloquent cortex can be safely and reliably localized with DCS using depth electrodes aided by fMRI. SEEG and fMRI potentially have a complimentary role in surgical planning, enabling refining of the eloquent cortex and operable epileptogenic network.  Funding: No funding