Different seizure-onset patterns in mesiotemporal lobe epilepsy have a distinct interictal signature

Experimental research demonstrated that distinct underlying mechanisms go along with different seizure-onset patterns on EEG. These different mechanisms may reflect different tissue abnormalities which, we hypothesize, could also be reflected in morphological differences in the interictal epileptic and background EEG activity.

We searched our database of intracranial EEG recordings for mesiotemporal lobe epilepsy patients with either predominant low-voltage fast activity (LVF) or periodic spiking (PS). Interictal epileptiform discharges (IEDs) were characterized by the spike/polyspike amplitude, the amplitude of the post-spike slow wave, and the associated low-frequency and high-frequency power increases. The EEG background was assessed with the root mean square amplitude, the distribution of the instantaneous amplitude relative to the root mean square value, and the power spectral density.

We identified 18 patients with predominant LVF or PS. IEDs in PS were 1.7-times sharper as determined by the high-frequency power increase than IEDs in LVF. In contrast, IEDs in LVF had a 1.2-times higher post-spike slow wave amplitude, and a 1.5-times larger low-frequency power content than IEDs in PS. There was no difference in rates of IEDs/HFOs, spike amplitude, HFO co-occurrence, and EEG background.

We demonstrated an association between the morphology of IEDs and the type of the seizure-onset pattern in mesiotemporal lobe epilepsy.

Our findings therefore suggest that IED morphology is a marker of the underlying mechanisms of seizure generation.