Abstract
Epilepsy surgery can be a very effective therapy in medication refractory patients. During patient evaluation intracranial EEG is analyzed by clinical experts to identify the brain tissue generating epileptiform events. Quantitative EEG analysis increasingly complements this approach in research settings, but not yet in clinical routine. We investigate the correspondence between epileptiform events and a specific quantitative EEG marker. We analyzed 99 preictal epochs of multichannel intracranial EEG of 40 patients with mixed etiologies. Time and channel of occurrence of epileptiform events (spikes, slow waves, sharp waves, fast oscillations) were annotated by a human expert and non-linear excess interrelations were calculated as a quantitative EEG marker. We assessed whether the visually identified preictal events predicted channels that belonged to the seizure onset zone, that were later resected or that showed strong non-linear interrelations. We also investigated whether the seizure onset zone or the resection were predicted by channels with strong non-linear interrelations. In patients with temporal lobe epilepsy (32 of 40), epileptic spikes and the seizure onset zone predicted the resected brain tissue much better in patients with favorable seizure control after surgery than in unfavorable outcomes. Beyond that, our analysis did not reveal any significant associations with epileptiform EEG events. Specifically, none of the epileptiform event types did predict non-linear interrelations. In contrast, channels with strong non-linear excess EEG interrelations predicted the resected channels better in patients with temporal lobe epilepsy and favorable outcome. Also in the small number of patients with seizure onset in the frontal and parietal lobes, no association between epileptiform events and channels with strong non-linear excess EEG interrelations was detectable. In contrast to patients with temporal seizure onset, EEG channels with strong non-linear excess interrelations did neither predict the seizure onset zone nor the resection of these patients or allow separation between patients with favorable and unfavorable seizure control. Our study indicates that non-linear excess EEG interrelations are not strictly associated with epileptiform events, which are one key concept of current clinical EEG assessment. Rather, they may provide information relevant for surgery planning in temporal lobe epilepsy. Our study suggests to incorporate quantitative EEG analysis in the workup of clinical cases. We make the EEG epochs and expert annotations publicly available in anonymized form to foster similar analyses for other quantitative EEG methods.
Highlights
Surgical removal of seizure-generating brain tissue is an established and often beneficial treatment option for patients with drug-resistant epilepsy
The main objective of this study was to investigate the association between the channel-wise occurence of epileptiform events identified by expert EEG reading and sets of Intracranial EEG (iEEG) channels defined by the seizure onset zone” (SOZ), the resected brain tissue (RBT), and the core channels of non-linear excess interrelation, which either require information aggregation, surgical intervention or quantitative analysis
We found no outcome-dependent differences in the absolute numbers of artifact-free EEG channels, epoch duration, number of channels constituting the SOZ, RBT, or core channels of non-linear excess interrelation between iEEG channels
Summary
Surgical removal of seizure-generating brain tissue is an established and often beneficial treatment option for patients with drug-resistant epilepsy. The decision on which area to resect and how this will putatively influence epileptic activity is individually determined for each patient, taking various diagnostic information sources into consideration (including scalp EEG, structural and functional MRI, psychological assessments and intracranial EEG if necessary) These assessments usually follow established concepts like the importance of the “seizure onset zone” (SOZ), which is used as a proxy for the EZ, the procedure suffers from a considerable amount of subjectivity. The limitations of the current approaches regarding reliable prediction of the patients’ benefit from epilepsy surgery are apparent, since only about half of all patients undergoing surgery become permanently seizure free, a rate that has practically not improved over decades [3,4,5,6,7,8,9,10]. Due to the lack of methodologies allowing to record brain activity with simultaneously high spatial and temporal resolution and full coverage, seizure dynamics are still not understood in full detail
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