New insight of the physiopathology of Interictal Epileptic Spikes (IES) using Time-Frequency Analysis

Objectives To better define the physiopathology of Interictal Epileptic Spikes (IES) and specially: (i) the mechanisms that propel neurons to hypersynchronizations, and (ii) the local and distant dynamic changes of neuronal population activities, we performed a time frequency analysis (TFA) of the surrounding activation of the IES. Methods HD EEG (64 and 128 electrodes) of 19 children (10 males; mean age 9.6 years), with refractory partial epilepsy of different aetiology, were analysed. Two types of IES were analyzed: spikes and poly-spike waves. TFA was performed on HD EEG epochs around the peak of IES (± 1000 ms). TFAs were calculated for frequencies between 4 and 50 Hz. Source localization was then performed from averaging spikes on EEG, from hypersynchronization, and from changes in TFA before hypersynchronization. Results Two time-frequency patterns were individualized: one specific for spike, the other specific for poly-spike-wave. TFA also revealed desynchronization around (before and after) hypersynchronization of IES (± 400 ms) in the epileptogenic zone, as already described in BECTS and in epileptic rats. At the lobar level the source localization was more accurate when performed during desynchronization (100%) than when performed during hypersynchronization (90%), and also than when performed on the peak of IES (74%) in the time domain. Discussion TFA with HD EEG in a children population of refractory partial epilepsy indicate complex changes in the dynamics of neuronal networks, not only during, but also before and after IES that is not specific to the type of epilepsy but to the type of IES. This provides new insight in the mechanism that propels neurons to synchronize. Source localization of IES should be performed during desynchronization, suggesting that the pathological activation already spread before the neuronal hypersynchronization of the IES.