Analysis of electrical discharges made with the foramen ovale electrode recording technique in mesial temporal lobe epilepsy patients.

Foramen ovale electrode (FOE) implantation is usually indicated in patients with mesial temporal lobe (MTL) epilepsy. The authors conducted this study to characterize the electrical discharge patterns obtained using FOE recording and evaluated their clinical significance. Interictal and ictal discharge patterns and their temporal relationships to EEGs were examined in 16 patients who had FOEs implanted bilaterally and had good recovery from surgery. After long-term telemetry recording with FOEs, all patients displayed clear seizure onsets originating in one or both sides of the MTL and underwent an anterior temporal lobectomy. Interictal epileptiform discharges (IEDs) were lateralized to the operated side in 12 patients based on scalp EEGs and in 8 patients based on FOE recordings. Among the various possible IED patterns observed with the FOE recordings, a periodic pattern was dominant in the operated side (P = 0.001), whereas a polyspike pattern was dominant in the nonoperated side (P = 0.002). The number of patients with ictal onsets in the operated side was five based on the scalp EEG and 10 based on FOE recordings. Paroxysmal fast frequency activity, rhythmic spikes (>2 Hz), or sharp waves were dominant over other FOE seizure onset patterns in the operated side. By contrast, delta and theta slowing and positive spikes were prevalent in the nonoperated side. The start-stop-start pattern, previously described using subdural electrodes, was also found in FOE recordings and was concordant with the operated side in all cases. The interhemispheric propagation time recorded with bilateral FOEs was 22.6 +/- 20.0 seconds for ictal discharges initiated in the operated side and 7.0 +/- 8.4 seconds for those in the nonoperated side. Among various IED spreading patterns, the most prevalent progression was FOE --> contralateral FOE --> ipsilateral temporal lobe, or --> contralateral temporal lobe. In this series, FOE implantation caused no serious complications. FOE recording appears to be useful for confirming lateralization in patients with MTL epilepsy previously determined using clinical and neuroimaging methods. A thorough analysis of FOE recordings can provide information that may facilitate the differentiation of primary from secondary (nonepileptogenic) temporal lobes.