Epidural and foramen-ovale electrodes in the diagnostic evaluation of patients considered for epilepsy surgery

To evaluate the clinical utility of epidural and foramen-ovale recordings and associated morbidity in the pre-surgical evaluation of epilepsy.

We retrospectively analysed 59 epilepsy patients, who underwent recordings with epidural (n = 59) and foramen-ovale electrodes (n = 46) as part of their pre-surgical evaluation between 1990-1999. The epidural and foramen-ovale evaluation was based on the results of the non-invasive EEG-video recordings in patients, in whom non-invasive evaluation failed to localise seizure onset (75%, 44 patients) or where EEG, and imaging studies were discrepant (25%, 15 patients) but allowed a testable hypothesis on the seizure onset zone.

Most patients (n = 57) were evaluated between 1990-1994. Only two patients were evaluated later. The results of the epidural (n = 559) and foramen-ovale (n = 83) electrode recordings allowed us to proceed to resective epilepsy surgery in 31% (n = 18) and to exclude further invasive evaluation in 15% (n = 9) of the patients. In 49% (n = 29) of the patients the results guided further invasive recordings using subdural and/or depth electrodes. For only three patients no additional information was gained by the electrode recordings. Temporary morbidity included local infection (epidural; n = 1) and facial pain (foramen ovale; n = 1) but no permanent complication occurred.

Epidural and foramen-ovale electrodes have almost been abandoned in recent years, most likely because of the improvement of neuroimaging techniques such as MRI, PET and ictal SPECT. However, in selected patients, epidural electrodes and foramen-ovale electrodes are either useful as a measure to avoid invasive evaluation or serve to guide invasive evaluation.