Interictal Epileptiform Discharge Dynamics in Peri-sylvian Polymicrogyria Using EEG-fMRI

Abstract

Polymicrogyria (PMG) is a common malformation of cortical development associated with a higher susceptibility to epileptic seizures. Seizures secondary to PMG are characterized by difficult-to-localize cerebral sources due to the complex and widespread lesion structure. Tracing the dynamics of interictal epileptiform discharges (IEDs) in patients with epilepsy has been shown to reveal the location of epileptic activity sources, crucial for successful treatment in cases of focal drug-resistant epilepsy. In this case series IED dynamics were evaluated with simultaneous EEG-fMRI recordings in four patients with unilateral peri-sylvian polymicrogyria (PSPMG) by tracking BOLD activations over time: before, during and following IED appearance on scalp EEG. In all cases, focal BOLD activations within the lesion itself preceded the activity associated with the time of IED appearance on EEG, which showed stronger and more widespread activations. We therefore propose that early hemodynamic activity corresponding to IEDs may hold important localizing information potentially leading to the cerebral sources of epileptic activity. IEDs are suggested to develop within a small area in the PSPMG lesion with structural properties obscuring the appearance of their electric field on the scalp and only later engage widespread structures which allow the production of large currents which are recognized as IEDs on EEG.

Keywords: EEG-fMRI; epilepsy; interictal dynamics; interictal epileptiform discharges; polymicrogyria.

Figure 1

Dual array design and concept. (A) The electrode cables are arranged into bundles according to two sets of intersecting lines: longitudinal and transverse. The bundles are grouped into two braids of cables so that the bundles from two neighboring lines travel to the different braids. Each braid is connected to a separate 32 channel referential MR-compatible EEG amplifier. In the dual array arrangement applied in this study, adjacent bundles were sent to a distal amplifier location to cause an increase in the area between electrode cables. A comparison of the bundles is used to differentiate motion artifact from brain signal. (B) This wire arrangement is used in order to reduce as much as possible the area within the loop created by two adjacent electrode wires along a single bundle (along bundles) and increase the area within the loop created by adjacent electrodes of different bundles (across bundles). Thus, while the true brain signal should be recorded similarly by adjacent electrodes, motion artifacts should differ depending on the loop created by the bundle each electrode is connected to, allowing separation between signal and noise. For a more detailed explanation of this setup see Klovatch-Podlipsky et al. (32).

Figure 2

IED and pre-IED correlated fMRI maps for patients 1-4. For each patient areas of peak activation are marked at two time points: 6 s after IED appearance on scalp EEG, showing activity correlated with IEDs according to the standard delay of the HRF peak (right), and 3 s after scalp IED appearance on EEG which shows pre-IED activity (left). All patients show weaker and earlier pre-IED activations within the PMG and later stronger activations on the border of the malformation or outside of it correlated with IED appearance.