Reflection of the Ictal Electrocorticographic Discharges Confined to the Medial Temporal Lobe to the Scalp-Recorded Electroencephalogram

Abstract

Objective: Previous reports on the simultaneous recording of electroencephalography (EEG) and electrocorticography (ECoG) have demonstrated that, in patients with temporal lobe epilepsy (TLE), ictal ECoG discharges with an amplitude as high as 1000 μV originating from the medial temporal lobe could not be recorded on EEG. In contrast, ictal EEG discharges were recorded after ictal ECoG discharges propagated to the lateral temporal lobe. Here, we report a case of TLE in which the ictal EEG discharges, corresponding to ictal ECoG discharges confined to the medial temporal lobe, were recorded. Case report: In the present case, ictal EEG discharges were hardly recognized when the amplitude of the ECoG discharges was less than 1500 μV. During the evolution and burst suppression phase, corresponding to highly synchronized ECoG discharges with amplitudes greater than 1500 to 2000 μV, rhythmic negative waves with the same frequency were clearly recorded both on the lateral temporal lobe and scalp. The amplitude of the lateral temporal ECoG was approximately one-tenth of that of the medial temporal ECoG. The amplitude of the scalp EEG was approximately one-tenth of that of the lateral temporal ECoG. Conclusions: Highly synchronized ictal ECoG discharges with high amplitude of greater than 1500 to 2000 μV in the medial temporal lobe could be recorded on the scalp as ictal EEG discharges via volume conduction.

Mandibular and chin electrodes as a supplemental recording for detection of epileptiform discharges in mesial temporal lobe epilepsy

“…In terms of the ictal discharges, the MA and CH electrodes also could detect ictal events from the hippocampal onset, the same as other extracranial electrodes. Our previous report[ 7 ] demonstrated that highly synchronized intracranial ictal discharges with high amplitudes >1500–2000 μV in the mesial temporal lobe could be recorded by the extracranial electrodes as ictal extracranial EEG discharges through volume conduction. It is considered that the sensitivity of ictal discharge detection of MA and CH electrodes is also comparable to other extracranial electrodes.…”

“…MTLE patients undergo epilepsy surgery without invasive examination in our institution and we rarely have the chance to perform simultaneous intra- and extracranial recordings. [ 7 ] Second, we examined the IEDs detected in extracranial EEG monitoring during 100 consecutive IEDs and only two ictal discharges. Hence, we could only discuss the sensitivity but not the specificity of MA and CH electrodes.…”

“…Two major reasons for the limited sensitivity of the extracranial recording to IEDs from the mesial temporal lobe are due to the rapid decay of the hippocampal IEDs due to the volume conduction and smearing effect of the skull, as we demonstrated in our previous reports. [2,[7][8][9] While MA and CH electrodes can be attached easily without the need for measurement, they are at a greater distance from the hippocampus than the other extracranial electrodes and the volume-conducted hippocampal IEDs should be greatly attenuated on the MA bone. In the present study, however, the MA and CH electrodes had nearly the same detection rate of the hippocampal IEDs as other extracranial electrodes.…”

“…He underwent simultaneous recordings of extra-and intracranial EEG as part of a presurgical evaluation, as described previously. [2,[7][8][9] EEG recordings were collected using a digital EEG recorder (Neurofax: Nihon Kohden, Tokyo, Japan).…”