Abstracts

According to the literature, the localization of MEG dipoles to mesial temporal structures in patients with temporal lobe epilepsy (TLE), does not exceed 40% and many of them do not present spikes during the MEG examination. We hypothesized that this limitation could be improved by analyzing the slow wave activity so often seen in EEG recordings obtained in patients with TLE.
MEG recordings were performed in 19 patients (7 females, mean age: 30 years) who remained seizure free after anterior temporal lobectomy including amygdalectomy and hippocampectomy. We performed a dipolar density analysis (DDA) of the delta and theta activity recorded during MEG tracings from the entire brain using the method described by Fehr et. al. (2001), which expresses results in dipoles/sec. The dipole lateralization was calculated according to a logistic regression method. Traditional dipole source localization of the slow wave activity was also calculated and the results of both methods were compared with the dipole localization of the interictal epileptic discharges and with the MRI results.
In 17/19 patients (89%) the DDA of the slow wave activity agreed with the traditional MEG dipole source localization of this activity and with the MRI findings. The most salient results were obtained in 8/19 patients without MEG interictal spikes, in which the DDA and the source localization of the slow wave activity were concordant with the MRI lesion. This method was also useful in 5/19 patients (26%) presenting with bilateral slow wave activity or with bitemporal epileptiform discharges in which the lateralization provided by DDA was concordant with the MRI.
The morphological appearance of the slow wave activity was either monomorphic or polymorphic. A strong correlation of the slow wave activity with the MRI lesion and not with the interictal spiking was documented.
Although the DDA does not discern between mesial and neocortical dipole localization, traditional dipole source analysis documented a mesial temporal location of slow wave dipoles in 42% of patients. Thus both methods are complementary.
Dipole density analysis and logistic regression of the slow wave activity, significantly improves the localizing yield of MEG in TLE reaching 89%. This is very useful, particularly in patients without interictal spiking during the MEG recording. The slow wave activity seems to correlate more with the MRI lesion than with the interictal epileptic disturbance.