Abstracts

Rationale: Intracranial EEG studies are widely used in the presurgical evaluation of drug-refractory patients with partial epilepsy. Because chronic implantation of intracranial electrodes carries a risk of infection, haemorrhage and oedema, it is best to limit the number of electrodes used without however compromising the ability to localize the epileptogenic zone (EZ) adequately. As such, there is always a risk that an intracranial study may fail to identify the EZ from suboptimal coverage. We present a new subdural electrode design which will hopefully allow large sampling of suspected areas of epileptogenicity at lower risk. Methods: The mould for the novel subdural mini grid electrodes were fabricated using a computer numerical controlled (CNC) machine. Impedance of proposed electrodes was characterized in vitro using an electrochemical impedance spectroscopy. MRI appearance of the novel electrodes was tested by placing the electrodes into a gel solution (0.9% NaCl with 14gm gelatin) and scanning them using a 1.5 T Phillips MR scanner. In vivo neural recordings were performed in male Sprague Dawley rats. Performance comparison was done with microelectrode recordings from rat cortex and subdural/depth recordings from epileptic patients. Histological examinations of rat brain following three-week icEEG recordings were performed.Results: The new subdural electrodes are designed with the following attributes: (a) smaller but connectable grids with the intent to provide better conductivity with tissues in curvature regions and reduce electrode displacement or overlap; (b) only two exit cables in order to reduce the risk of infections, (c) notches to provide better flexibility, (d) holes to potentially reduce the risk of oedema and hemorrhage; (e) a numbering system and edge markers to facilitate their identification on post-implantation MRI. Pure Gold was chosen as the material for wiring and contacts as it provided lower impedance (~116 k?) compared to commercial subdural or depth electrode contacts. Electrode contacts, edge markers and the numbering system were adequately identified on MRI. Long-term in vivo neural recording in the rat showed excellent signal stability. Compared to human icEEG recordings with depth and subdural electrode contacts, noises were reduced by two-fold and signal-to-noise ratio was better (~3 dB). Histological examination of rat cortex following icEEG recording using novel electrodes showed no tissue damage. Conclusions: The proposed subdural electrode system features attributes which could potentially translate into better icEEG recordings and allow sampling of large of areas of epileptogenicity at lower risk. Further validation is required.