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. 2011 Aug 9;77(6):524-31.
doi: 10.1212/WNL.0b013e318228bee2. Epub 2011 Jul 13.

Interictal scalp fast oscillations as a marker of the seizure onset zone

L P Andrade-Valenca  1 , F Dubeau, F Mari, R Zelmann, J Gotman
Affiliations

Interictal scalp fast oscillations as a marker of the seizure onset zone

L P Andrade-Valenca et al. Neurology. .

Abstract

Objective: This study aims to identify if oscillations at frequencies higher than the traditional EEG can be recorded on the scalp EEG of patients with focal epilepsy and to analyze the association of these oscillations with interictal discharges and the seizure onset zone (SOZ).

Methods: The scalp EEG of 15 patients with focal epilepsy was studied. We analyzed the rates of gamma (40-80 Hz) and ripple (>80 Hz) oscillations, their co-occurrence with spikes, the number of channels with fast oscillations inside and outside the SOZ, and the specificity, sensitivity, and accuracy of gamma, ripples, and spikes to determine the SOZ.

Results: Gamma and ripples frequently co-occurred with spikes (77.5% and 63% of cases). For all events, the proportion of channels with events was consistently higher inside than outside the SOZ: spikes (100% vs 70%), gamma (82% vs 33%), and ripples (48% vs 11%); p < 0.0001. The mean rates (events/min) were higher inside than outside the SOZ: spikes (2.64 ± 1.70 vs 0.69 ± 0.26, p = 0.02), gamma (0.77 ± 0.71 vs 0.20 ± 0.25, p = 0.02), and ripples (0.08 ± 0.12 vs 0.04 ± 0.09, p = 0.04). The sensitivity to identify the SOZ was spikes 100%, gamma 82%, and ripples 48%; the specificity was spikes 30%, gamma 68%, and ripples 89%; and the accuracy was spikes 43%, gamma 70%, and ripples 81%.

Conclusion: The rates and the proportion of channels with gamma and ripple fast oscillations are higher inside the SOZ, indicating that they can be used as interictal scalp EEG markers for the SOZ. These fast oscillations are less sensitive but much more specific and accurate than spikes to delineate the SOZ.

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Figures

Figure 1
Figure 1. Patient 10: Examples of artifacts and ripple oscillations
1) Short EMG bursts. 2) Ripples co-occurring with sharp wave. (A) Raw EEG. (B) EEG filtered with high-pass filter of 80 Hz. Gray section in A is expanded in time and amplitude in B. Note that for this and subsequent figures the calibration is different in the left and right part of the figure, but is the same for the top and bottom parts. Ripple oscillations are underlined. The waveform morphology of nonartifactual fast oscillations is more rhythmic and regular in amplitude and frequency than artifactual oscillations.
Figure 2
Figure 2. Patient 11: Examples of artifacts and ripple oscillations
1) Short EMG bursts. 2) Ripples independent of epileptiform discharge. (A) Raw EEG. (B) EEG filtered with high-pass filter of 80 Hz. The ripple oscillations are underlined.
Figure 3
Figure 3. Patient 4: Examples of artifacts and gamma oscillations
1) Short EMG bursts. 2) Gamma co-occurring with polyspikes. (A) Raw EEG. (B) EEG filtered with high-pass filter of 40 Hz. The gamma oscillations are underlined.
Figure 4
Figure 4. Examples of interictal gamma
1) Gamma co-occurring with spike with oscillations visible during spike. 2) Gamma co-occurring with spike with oscillations not visible during spike, but visible after filtering. 3) Gamma independent of spike. (A) Raw EEG. (B) Raw EEG with expanded time. (C) EEG filtered with high-pass filter of 40 Hz. Note different amplitude calibrations.
Figure 5
Figure 5. Examples of interictal ripples
1) Ripples co-occurring with spike with oscillations visible during spike. 2) Ripples co-occurring with spike with oscillations not visible during spike, but visible after filtering. 3) Ripples independent of spike. (A) Raw EEG. (B) Raw EEG with expanded time. (C) EEG filtered with high-pass filter of 80 Hz. Note different amplitude calibrations.
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