Abstracts

Patients with epilepsy and bilaterally synchronous epileptiform discharges may be good surgical candidates if a focal origin for such discharges can be demonstrated. It is unclear whether magnetoencephalography (MEG) is helpful in lateralizing or localizing spike sources to focal area(s) in such patients.
We analyzed 13 patients with medically refractory epilepsy who had bilaterally synchronous epileptiform discharges on EEG. Patients with additional epileptiform discharges occurring focally, regionally or in either hemisphere were included in the study. The syndromic classification of patients was as follows: cryptogenic partial epilepsy 9, lesional partial epilepsy 3, and unknown whether partial or generalized 1. The latter patient was diagnosed with Landau-Kleffner syndrome. Among the 12 patients with partial epilepsy, 2 had temporal lobe epilepsy while the remaining 10 had extratemporal lobe epilepsy. Among the 3 patients with lesional epilepsy, 2 had large encephalomalacia and 1 had glioma.
MEG studies were performed by using a whole-head magnetometer system containing 148 detector channels (BTI, San Diego, USA).
Based on the presence of asymmetry (differences in the amplitude between the hemispheres) and overall predominance of number of discharges and independent focal discharges in one hemisphere, EEG findings were classified as follows: 1) no lateralization; 2) predominant lateralization to one hemisphere; and 3) possible lateralization to one hemisphere. Both bilateral synchronous and independent focal discharges were assessed by single equivalent current dipole (ECD) model to calculate the spike sources underlying the magnetic field pattern.
Interictal EEG revealed bilateral synchronous epileptiform discharges without any lateralization in 4 patients (all with cryptogenic etiology); with predominant lateralization to one hemisphere in 4 patients (3 with lesional and 1 with cryptogenic etiology); and with possible lateralization to one hemisphere in 5 patients (4 with cryptogenic and 1 with LKS etiology).
MEG spike sources were lateralized to one hemisphere in 9 patients and were present in homotopic regions of both hemispheres in 4 patients. Both focal and bilaterally synchronous epileptiform discharges in 3 patients with lesional epilepsy and 1 patient with LKS showed spike sources lateralized to one hemisphere. These dipole sources were colocalized with the lesions in patients with lesional epilepsy and present in the left perisylvian region in the patient with LKS.
MEG can lateralize spike sources to one hemisphere in some patients with epilepsy despite the presence of bilateral synchronous epileptiform discharges. In lesional epilepsy MEG spike sources are consistently colocalized with the lesions.