Abstracts

RATIONALE: EEG and magnetoencephalography (MEG) source imaging techniques provide non-invasive localization of the seizure focus. We sought to evaluate the characteristics, localization and outcome predictive value of interictal and ictal magnetic sources in a series of Temporal Lobe Epilepsy (TLE) patients evaluated for epilepsy surgery.
METHODS: We performed simultaneous scalp EEG/MEG recordings on a consecutive series of 26 TLE patients being evaluated for epilepsy surgery at the Northern California Comprehensive Epilepsy center at the University of California, San Francisco between 1996 and 1997. Scalp EEG was obtained from 21 channels (10-20 international system), whereas MEG was recorded from two 37-channel sensors. Subsequently, we performed source analysis of the spike magnetic fields and early magnetic seizure discharges by using a single equivalent dipole model and coregistered modeling dipoles to the brain MRI. We assessed spike and seizure magnetic field evolution as well as the modeling dipole location and orientation and correlated these findings with intracranial EEG, neuroimaging and 2 year post-operative outcome.
RESULTS: Twelve patients had predominantly basal vertical or antero-mesial oblique dipoles underlying early MEG spike activity. Mesial temporal onset was recorded in 2/12 patients during invasive EEG monitoring. All 10/12 patients who underwent surgery had successful outcome after selective amygdalohippocampectomy (AHC) or standard anteromedial temporal lobectomy (AMTL). Eleven patients demonstrated anterior horizontal or tangential dipoles to the antero-lateral inferior temporal tip modeling early spike activity. Temporal mesial or entorhinal onset was recorded in 2/11 patients on intracranial EEG. In those 10 patients undergoing surgery, successful ourcome was observed in 9 patients after AMTL. The other patient failed selective AHC, but became seizure free after AML as well. Three TLE patients demonstrated predominantly lateral vertical tangential dipoles. Intracranial EEG onset in all 3 patients was localized to the temporal neocortex and all patients had successful outcome after temporal neocortical lesional or non-lesional resection.
We recorded ictal MEG in 2/26 patients and this included 12-32 seconds of the ictal onset. Ictal MEG lateralized the seizure onset and source analysis was concordant with interictal MEG localization in both patients. In one patient who underwent invasive EEG recording, ictal MEG source localization concorded with intracranial EEG localization to the entorhinal cortex.
CONCLUSIONS: Early spike and seizure MEG source modeling reveals specific dipole patterns that provide clinically useful information in TLE. Interictal as well as ictal MEG source localization predicts intracranial EEG onset and can optimize surgical outcome after epilepsy surgery for intractable TLE. MEG is a useful functional and non-invasive technique in localizing the seizure onset particularly in complex intractable TLE or when planning more restricted resections for controlling temporal lobe seizures.
[Supported by: NIH-ROI-NS31966-01.]