Abstracts

A novel implantable closed-loop Responsive Neurostimulation System (RNS, Neuropace Inc, Mountainview, CA) consisting of pulse generator, subdural strip/depth electrodes and programmer is undergoing a prospective multicenter clinical trial. RNS continuously analyzes electrocorticograms (ECoGs) and automatically applies electrical stimulation when specific epileptiform activity is detected. This study has shown that RNS is safe for patients and can reduce partial seizure frequency and severity. However, the exact mechanism of action of RNS is unknown.
There have been no published frequency analyses of EEG with RNS. Power spectra demonstrate relative power of regional cortical activity at a broad range of frequencies. Coherence, an index of functional connectivity, is the cross-correlation of inter-regional frequency content. We analyze acute effects of RNS on interictal ECoG power spectra and coherence., Three adult patients with refractory seizures were admitted to the epilepsy monitoring unit for intracranial EEG monitoring for epilepsy surgery evaluation. ECoG was recorded with subdural electrodes placed over the most active cortical regions. An externalized RNS (eRNS) device was connected to the most active intracranial electrodes for epileptiform activity detection. Subjects received neurostimulation in response to epileptiform discharges. Continuous video-ECoG was recorded, prior to turning RNS ON, then for 12-24 hours afterwards. ECoG during awake periods was analyzed, immediately before and after the RNS ON period. Power and coherence spectra during these epochs were compared for regions with greatest interictal epileptiform activity., In all patients a broad reduction in ECoG power occurred in the most active electrodes after RNS ON. In 2/3 patients coherence between adjacent electrode pairs was decreased for lower frequencies (1-5, 1-20 Hz) after RNS ON and increased at higher frequencies (45-100 Hz). In the 3rd patient the coherence was increased for lower frequencies (1-5 Hz) and decreased for higher frequencies (15-100 Hz)., 1. RNS had similar effects on ECoG power spectra for all patients.
2. ECoG coherence changed after RNS at most frequencies for all patients, although direction was different for 1/3. This may reflect different mechanisms of action for RNS.
3. The change in intra- and inter-regional ECoG frequency content could reflect neuromodulatory effects from neurostimulation.
4. Further ECoG frequency analyses to determine different patterns of change with RNS in regions adjacent to and remote from epileptic foci could be useful. These results will be presented.
5. Further analyses could elucidate what response patterns can predict RNS efficacy., (Supported by Neuropace Inc, Mtn View, CA.)