Abstracts

A novel approach for implanting investigational RNS (NeuroPace, Inc) electrodes is presented for optimizing overlap of the epileptic circuit. White matter pathways are targeted such that the biophysical properties of myelinated axons are used to propagate electrical current distant from the source of stimulation. This hypothesis was tested post-RNS implant by reproducing a patient[apos]s typical visual aura when stimulating depth contacts implanted in parahippocampal white matter. RNS current was [apos]injected[apos] during the aura onset while capturing resulting transient blood flow changes using subtracted activated SPECT (SAS)., RK (age: 38 years) was stereotactically implanted with a 4-contact depth lead placed completely within the left parahippocampal white matter. In addition, a 4-contact subdural strip was placed in the left mesial occipital region followed by securing a skull-based RNS device. Multiple, presurgically acquired subtracted ictal SPECT studies aided localization for lead placement. An SAS study was performed at 18 months post-implant following delivery of stimulation current without producing an afterdischarge. Pre-implant diffusion tensor imaging was used to compare white matter orientation and distribution with SAS findings., Preliminary results obtained from RK during stimulation of the depth electrode suggest distant neuronal spread of current. That is, upon repetitive bipolar stimulation of posterior mesial temporal white matter depth contacts, propagation to primary visual cortex was observed by semiology and blood flow measures. Specifically, a reproducible elementary visual response was reported by the patient in the right upper visual quadrant.This sensory phenomenon overlapped the patient[apos]s typical visual aura in distribution, color, form, and movement. An unexpected finding of hypoperfusion was shown near the stimulated depth contacts and in the mesial occipital region. Distant focal blood flow changes following RNS lead stimulation in a different patient were previously reported by our center¹., Clinical manifestations of direct cortical stimulation are shown to relate to transient hypoperfusion at distant white matter propagation pathways. This information ostensibly represents the extent of cortical modulation for a given set of focal stimulation parameters passed through a specific electrode contact shape, orientation and location in white matter. Presurgical planning using axonal pathways to direct the spread of RNS current to distant epileptic sources may amplify the efficacy of the available intracranial electrode set and simplify the surgical approach.
1. Rossi, M.A., et al. (2005). Noninvasive presurgical estimation of cortical activation for optimizing intracranial electrode placement for responsive neurostimulation in refractory epilepsy, AES 3.169., (Supported by NeuroPace, Inc.)