Fast oscillations associated with interictal spikes localize the epileptogenic zone in patients with partial epilepsy

Abstract

Although interictal epileptic spikes are defined as fast transient activity, the spatial distribution of spike-related high-frequency power changes is unknown. In this study, we localized the sources of spike-locked power increases in the beta and gamma band with magnetoencephalography and an adaptive spatial filtering technique and tested the usefulness of these reconstructions for determining the epileptogenic zone in a population of 27 consecutive presurgical patients with medication refractory partial epilepsies. The reliability of this approach was compared to the performance of conventional MEG techniques such as equivalent current dipole (ECD) models. In patients with good surgical outcome after a mean follow-up time of 16 months (Engel class I or II), the surgically resected area was identified with an accuracy of 85% by sources of spike-locked beta/gamma activity, which compared favorably with the accuracy of 69% found for ECD models of single spikes. In patients with a total of more than 50 spikes in their recordings, the accuracies increased to 100% vs. 88%, respectively. Imaging of spike-locked beta/gamma power changes therefore seems to be a reliable and fast alternative to conventional MEG techniques for localizing epileptogenic tissue, in particular, if more than 50 interictal spikes can be recorded.

Figure 1

MEG results in 2 typical patients. A, C. Short-time Fourier transforms reveal transient power increases time-locked to the peak of interictal epileptic spikes at 0 seconds in a broad frequency band ranging from delta to gamma rhythms, with high-frequency changes being more focal than low-frequency changes. B, D. An adaptive spatial filter localizes spike-locked beta and gamma power increases (squares in upper row) to the zone that was later surgically resected (lower row), and, in case of patient 5, to the area of clusters of corresponding ECD (yellow dots). Images are presented in neurological convention (left is left). ECDss refers to equivalent current dipoles of single spikes; SpiFi, spike-locked dual-state adaptive spatial filtering in the beta/gamma frequency band.

Figure 2

The sensitivity, accuracy, and positive predictive value as defined in the Method section are shown for conventional MEG analysis techniques and for the new method introduced here. ECDss refers to equivalent current dipoles of single spikes; ECDav, ECD of averaged spikes; SAM(g2), synthetic aperture magnetometry and excess kurtosis; SpiFi, spike-locked dual-state adaptive spatial filtering in the beta/gamma frequency band.