Tailoring epilepsy surgery with fast ripples in the intraoperative electrocorticogram

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Excerpt

Intraoperative electrocorticography (ECoG) can measure epileptiform activity directly from the cortex during epilepsy surgery, aiming to delineate the epileptogenic tissue. This so‐called “tailoring” searches for interictal spikes before and after initial resection and can be repeated until no epileptiform abnormalities remain.1 A postresection ECoG (post‐ECoG) can verify completeness of the resection and should thus indicate the prognosis toward surgical success.3 The value of tailoring has been challenged; the removal of tissue with interictal spikes in preresection recordings (pre‐ECoG) has been associated with seizure freedom by some,4 but not by others.10 Spikes in pre‐ECoG extended outside the resected tissue in around 75% of frontal lobe cases and in 91% of surgeries in the central, parietal, and occipital lobes.13 Several studies have suggested that residual spikes in the final post‐ECoG predict poor surgical outcome,7 but this is again contradicted by others.3 A main concern is that surgical manipulation of the cortex may agitate the tissue, evoking spikes in the resection margin, which are not correlated with seizure outcome.18
In the last decade, a new biomarker for epileptogenic tissue has been suggested: high‐frequency oscillations (HFOs), consisting of ripples (80–250 Hz) and fast ripples (FRs; 250–500 Hz).19 Research on HFOs has focused on chronic ECoG and depth electroencephalography (EEG) and, to a lesser extent, on fast ripples (FRs) in intraoperative pre‐ECoG.22 Incomplete removal of tissue with HFOs, especially FRs, was strongly linked to poor surgical outcome, whereas this relationship was not found for spikes.21 Recently, we demonstrated that the presence of residual FRs after resection, in post‐ECoG, predicted recurrent seizures whereas spikes and ripples did not.23 FRs occur more focally than ripples and spikes given that they probably arise from local pathological connections of neuron clusters, rather than from the larger‐scale networks in which spikes propagate. FRs are therefore considered more specific for epileptogenic tissue than spikes.21 It seems that surgical manipulation of cortex does not provoke “de novo” FRs at the resection margin,25 so FRs could be a useful clinical biomarker to tailor epileptogenic tissue.
The tailoring strategy is largely determined by the magnetic resonance imaging (MRI) findings. In case of a clear lesion, the primary goal is often a lesionectomy including corticectomy of the surrounding epileptogenic tissue based on ECoG findings. In neocortical temporal epilepsies, ECoG is used to determine hippocampal involvement and the necessity of a hippocampectomy.3
We wondered what would be the best ECoG approach to assure successful outcome. Is it time to replace spikes with FRs and would this hold for all pathologies and tailoring strategies? Can we rely on HFOs at resection borders? We attempted to provide a complete picture on the prognosis of intraoperative tailoring, including both spikes and HFOs. We compared HFOs, especially FRs, and spikes in pre‐ and post‐ECoG recordings, related them to outcome, and verified our findings for different tailoring strategies.

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