A new class of electrodes of 'intermediate invasiveness': preliminary experience with epidural pegs and foramen ovale electrodes in the mapping of seizure foci.

Non-invasive electroencephalographic (EEG) recording with scalp and sphenoidal leads is often insensitive to precise localization of ictal onset, and can be distorted by skull defects and underlying lesions or deformities of the brain. We present preliminary experience with epidural pegs and foramen ovale electrodes used in 30 cases of intractable partial epilepsy where non-invasive EEG did not define a zone of epileptogenesis with sufficient precision to recommend resection, or to dictate precise placement of depth electrodes or subdural girds. Custom designed mushroom shaped peg electrodes were implanted via 4.5 mm twist-drill skull holes for epidural recording from cortical areas of suspected epileptogenesis. Foramen ovale electrodes (flexible 3-contact leads) were introduced via percutaneous puncture of Meckel's cave cistern for recording from mesiotemporal regions. Chronic recording was performed for 4 to 26 days (mean 9.2 days). There were no serious complications and signal quality was excellent in every electrode. Based on interictal and ictal records, resective surgery was performed in 8 cases, more focused further invasive recording was recommended in 15 cases (with subsequent resective surgery), and surgical options were excluded in 7 cases. It is concluded that these electrodes of 'intermediate invasiveness' represent a safe and effective tool in the armamentarium for mapping complicated or elusive epileptic foci. They can be used in lieu of, in addition to, or prior to more invasive measures. Their greatest advantage lies in their relative safety and ability to survey extensive zones of suspected epileptogenesis so as to guide and focus further mapping and surgical intervention.