Abstracts

Rationale: The efficacy of responsive neurostimulation (RNS), as an alternative to resective surgery in patients with intractable epilepsy, is largely reliant on optimal localization of the ictal onset zone, traditionally achieved utilizing intracranial recordings. In combination with the other components of the pre-surgical evaluation, magnetoencephalography (MEG) source localization of epileptiform spikes can have a high degree of correlation with the epileptogenic zone, and can help limit the number of electrodes planned during intracranial cases, which then decreases surgical complication risks.  MEG and transcranial magnetic stimulation (TMS) motor and language mapping also provides excellent non-invasive functional mapping limiting both the need for widespread cortical mapping and helps direct decisions of surgical resection versus neurostimulation. Methods: An 18-year-old female with intractable focal onset epilepsy of right posterior frontal lobe origin with myoclonic seizures in the left lower extremity was referred for Phase 1 evaluation and assessment of candidacy for RNS placement.  Diagnostic imaging showed a focal cortical dysplasia near the left foot and leg motor cortices, and surface vEEG demonstrated frequent spikes with phase reversals at the vertex.  Non-invasive localization of both motor function and epileptiform discharges was performed through MEG and spatial extent of the primary leg motor cortex along the medial frontal gyrus was confirmed using TMS.   Following extensive monitoring and consideration of risks and side effects of potential surgical complications, including damage to the leg and foot motor cortices, the patient consented to intracranial monitoring with the goal of RNS placement. Results: Interictal discharges identified with MEG localized to the right primary leg motor cortex, overlapping with TMS locations which elicited a response in the left tibialis anterior muscle which, and found to have excellent spatial concordance with the ictal onset zone, verified by electrocorticography.  Given the location of the ictal onset zone and the cortical dysplasia itself, a decision was made by the treating epileptologists and neurosurgeon to alter the RNS electrodes to allow conformity to the orientation of the dysplasia. Electrodes were manipulated such that the 1x4 contact strips each had two electrodes diving inferiorly into the sagittal sulcus, with two electrodes remaining over the vertex, encompassing the cortical dysplasia and allowing for maximal stimulation of the ictal onset zone. Conclusions: As demonstrated here, MEG and TMS have the capacity to limit risk of surgical complications by facilitating RNS placement in patients deemed to otherwise be at high risk for functional morbidity resulting from resection of eloquent cortex.  Moreover, the observation that the patient was free of disabling seizures (Engel Class IB) following RNS placement enhances the impression that non-invasive mapping methods contribute to favorable post-surgical outcome in patients with intractable epilepsy. Funding: Le Bonheur Neuroscience Institute