Voltage sources in mesial temporal lobe epilepsy recorded with foramen ovale electrodes

Objective We introduce a monopole model to examine the sources of ictal and interictal activity in mesial temporal lobe epilepsy (MTLE) recorded using foramen ovale electrodes (FOE). Methods Classical electrostatic theory was applied to derive mathematical expressions. Interictal and ictal activity was acquired using FOE and scalp video-electroencephalography (v-EEG) during awake and sleep states. A total of 2057 interictal spikes and 712 ictal spikes were analyzed. Thirty-five seizures from several consecutive episodes were examined. MRI and clinical data were correlated with voltage source localization. Results Patients (20) were grouped according to the spatial distribution of voltage sources of interictal activity. Voltage sources were located over 3.4 and 21.6 mm in the anterior-to-posterior axis of mesiotemporal structures and separated no more than 7 mm from this axis. In most patients (16), sources were restricted to 11.1 ± 1.5 mm, whereas other patients (4) exhibited a wider distribution (29.6–43.5 mm). Sources of ictal and interictal activity partially overlapped, with ictal sources exhibiting a posterior localization at 20–40 mm. Both interictal and ictal sources were anterior to MRI atrophy. No difference between awake and sleep states were found, neither correlation between source scattering and history of epilepsy. Conclusions Voltage source analysis applied to FOE suggests that, in most MTLE patients, interictal activity emerges from very restricted areas. Some patients, however, exhibited sources which are distributed all along the mesiotemporal structures. Our data suggest an anterior-to-posterior alignment of the irritative, ictal and atrophic zones. Significance The voltage source model applied to FOE can help to map the extension of the irritative and ictal areas in mesiotemporal structures.