Abstracts

Rationale: We compared interictal magnetoencephalography (MEG); chronically-implanted subdural electrocorticogram (ECoG) over the lateral, basal, and mesial temporal area; and intraoperative hippocampal electrogram of mesial temporal lobe epilepsy (mTLE). We evaluate the characteristics of interictal spikes, and the sensitivity of extracranial magnetoencephalography (MEG) for epileptic spikes in mesial temporal lobe epilepsy. Furthermore, we estimate the surgical prognosis of Intractable mTLE. Methods: Twenty five patients with intractable temporal lobe epilepsy were followed up for more than one year after resection surgery. MEG and subdural ECoG were simultaneously measured at a magnetic shield room in interictal period. Additionally, hippocampal signals were also measured using intraventricular hippocampal surface electrode (IVHSE) during selective amygdalohippocampectomy or multiple subpial transaction of hippocampus. Results: 1. In 10 cases, MEG detected no spikes whereas intracranial EEG detected. In 2 cases, interictal MEG appeared exclusively contralateral to the ictal ECoG hemisphere. The Spike detective ratio by MEG was less than 10%. 2. There was no correlation of postoperative seizure outcome (Engel's class) between cases with and without MEG. 3. Most of the subdural ECoG spikes showed negative peaks, whereas those obtained with IVHSE showed positive peaks. This may show the direction of interictal discharge is from mesial temporal surface to ventricle. Conclusions: 1.MEG hardly detects mesial temporal lobe epileptic spikes probably because their signal sources are too deep and not constantly horizontal to the scalp surface. If the back ground activity is high in comparison with MEG spikes, it becomes difficult to detect these spikes. Our results correspond to the previous reports. On the other hand, some spikes could be detect in MEG of 10 cases, it means that we may omit invasive surgery for some cases. 2.There was no correlation of seizure outcome between cases with and without MEG. In our series, MEG cannot provide the prognosis of intractable mTLE, further more studies are needed. 3.Our studies show that SE spikes showed negative peaks, and IVHSE did positive peaks. One of the reasons may come from the complicated hippocampal construction. For example, when the current occurs from the temporal base to the intraventricle, negative charge is induce on the cortical surface and positive charge on the ventricle ependymal surface.