Interictal high frequency oscillations (HFOs) in patients with focal epilepsy and normal MRI

Abstract

Objective: We aim to analysis the relationship between HFOs-generating regions and the seizure onset zone (SOZ) in epileptic patients without a visible lesion on MRI.

Methods: Intracerebral EEGs were recorded in 17 patients with intractable focal seizures and normal MRIs. The rates of interictal HFOs and spikes inside and outside the SOZ were analyzed as well as the specificity, sensitivity and accuracy of HFOs and spikes to determine the SOZ.

Results: The mean rate of spikes, ripples and fast ripples (FR) was higher in the SOZ than in the non-SOZ channels. In regard to the identification of the SOZ the sensitivity was 91% for spikes, 91% for ripples and 66% for FR, the specificity was 30% for spikes, 42% for ripples and 80% for FR, and the accuracy was 44% for spikes, 54% for ripples and 76% for FR.

Conclusions: The rates of spikes and HFOs were higher inside than outside the SOZ. However, HFOs are also more specific and accurate than spikes to delineate the SOZ.

Significance: Analysis of interictal HFOs during 5-10 min of sleep recording is a good tool to localize the SOZ in patients with epilepsy and normal MRI, and could potentially reduce the duration of chronic intracerebral EEG recordings.

Fig. 1

Example of ictal discharge (A) recorded in wakefulness and interictal spike samples (B–D) recorded in slow wave sleep from patient 1. The electrodes were implanted orthogonally, aimed at the anterior hippocampus (H), amygdala (A), parahippocampus (P), orbito-frontal region (O) and temporal pole (S) in both hemispheres. We show only electrodes in the left hemisphere (L). In parts B–D, sections labeled (1) show the raw EEG, sections with expanded time (highlighted by gray) labeled (2) show the ripple band (high pass 80 Hz) and sections labeled (3) show the FR band (high pass 250 Hz). Examples of interictal spikes (B1), with ripples (B2) and FR (B3) on electrode contacts inside the seizure onset zone (SOZ). Examples of interictal spikes outside the SOZ (C1, D1). Some of these were associated with ripples (C2) and some were not (D2); neither were associated with FRs (C3, D3). Note different amplitude calibrations. The same channels were represented in ictal and interictal samples. LS, left temporal pole (labeled as LTP in Table 1); LA, left amygdala; LH, left hippocampus; LO, left orbito-frontal (labeled as LOF in Table 1).