Neuromagnetic Recordings in Temporal Lobe Epilepsy

Abstract

The introduction of whole-head magnetoencephalography (MEG) systems facilitating simultaneous recording from the entire brain surface has established MEG as a clinically feasible method for the evaluation of patients with temporal lobe epilepsy (TLE). In mesial TLE, two types of MEG spike dipoles could be identified: an anterior vertical and an anterior horizontal dipole. Dipole orientations can be used to attribute spike activity to temporal lobe subcompartments. Whereas the anterior vertical dipole is compatible with epileptic activity in the mediobasal temporal lobe, the anterior horizontal dipole can be explained by epileptic activity of the temporal tip cortex. In nonlesional TLE, medial and lateral vertical dipoles were found which could distinguish between medial and lateral temporal seizure onset zones as evidenced from invasive recordings. In lesional TLE, MEG could clarify the spatial relationship of the structural lesion to the irritative zone. Evaluation of patients with persistent seizures after epilepsy surgery may represent another clinical important application of MEG because magnetic fields are less influenced than electric fields by the prior operation. Simultaneous MEG and invasive EEG recordings indicate that epileptic activity restricted to mesial temporal structures cannot reliably be detected on MEG and that an extended cortical area of at least 6 to 8 cm2 involving also the basal temporal lobe is necessary to produce a reproducible MEG signal. In lateral neocortical TLE MEG seems to be more sensitive than scalp-EEG which further underlines the potential role of MEG for the study of nonlesional TLE. Whole-head MEG therefore can be regarded as a valuable and clinically relevant noninvasive method for the evaluation of patients with TLE.

Surgical Implications of Neuromagnetic Spike Localization in Temporal Lobe Epilepsy

“…The perilesional irritable area was successfully localized by MEG in our patients with neocortical lesion, as found previously (20). Additionally, we found extensive spike localization in two patients.…”

“…Classification of temporal lobe MEG spikes into anterior temporal horizontal (ATH), anterior temporal vertical (ATV), and posterior temporal vertical (PTV) dipoles (18,19) and comparison with ECoG findings showed that dominant ATH and ATV dipoles were correlated with seizure onset from the medial temporal structures, whereas PTV dipoles were correlated with onset from the lateral temporal neocortex or nonlocalized onset. Investigation of the relation between the dipole pattern and clinical diagnosis found that patients with medial TLE showed ATH or ATV dipoles, whereas patients with nonlesional TLE (no MRI abnormalities) showed dipoles localized on the anterior medial or posterior lateral temporal lobe (20). In the present study, all five patients with…”

“…Previously, anterior temporal spikes in scalp EEG and MEG were found to be highly specific to medial TLE (7,(18)(19)(20). Classification of temporal lobe MEG spikes into anterior temporal horizontal (ATH), anterior temporal vertical (ATV), and posterior temporal vertical (PTV) dipoles (18,19) and comparison with ECoG findings showed that dominant ATH and ATV dipoles were correlated with seizure onset from the medial temporal structures, whereas PTV dipoles were correlated with onset from the lateral temporal neocortex or nonlocalized onset.…”

“…Excellent surgical outcome was achieved when spike source localization was concordant between MEG and EEG (16), and the reliability of interictal MEG localization was verified in patients with extratemporal neocortical epilepsy followed by postoperative good outcome (13). However, little is known about the correlation between MEG findings and surgical outcome in patients with TLE, although MEG may provide the basis for the diagnosis, such as medial or lateral TLE, based on the spike localization (18)(19)(20).…”

Reconstruction of Subcortical Brain Activity by Spatially Filtered MEG During Epileptic Seizures

“…Depth electrode recordings and lesion studies in epilepsy patients have shown that deep temporomesial structures like the hippocampus (HC) are important for the pathophysiology of epileptogenesis. However, depth electrode recordings show that with conventional dipole localization approaches activity originating from deep brain structures is likely to escape detection [2]. To noninvasively elucidate time course and location of these sources during epileptiform discharges we used a localization approach based on spatial filtering taking into account the spatio-temporal information of the measured MEG signal.…”