student Université Catholique de Louvain Brussels, Belgium
Rationale: Stereotactic-EEG (SEEG) is often needed to characterize involvement of the insula in the epileptogenic zone (EZ). Early definition of suspected insular epileptogenic foci is crucial to optimize SEEG electrode placement. Electrical Source Imaging (ESI) may offer a solution to localize interictal epileptiform discharges (IEDs) to the insula. In this study, we evaluate the accuracy of automated low-density interictal ESI to characterize the insular irritative zone (IZ). Methods: The database of St. Luc Hospital, Brussels, was retrospectively searched for consecutive patients with: (1) SEEG with at least one depth electrode exploring operculo-insular region(s); (2) concomitant scalp EEG with ≥19 electrodes; (3) pre-operative 3T structural MRI. SEEG recordings were reviewed to identify patients with IEDs involving insular contacts (SEEG insular-IZ). Patients were further classified as having: i) primary-IZ (seizure onset involving the insular contacts); ii) secondary-IZ/dominant (no insular seizure onset and prevalence of dominant insular IEDs); iii) secondary-IZ/propagated (no insular seizure onset and prevalence of IEDs propagating to the insula). Scalp EEG recordings underwent automated spike detection, clustering and subsequent ESI, using Epilog PreOp® (Epilog NV, Belgium). ESI was performed with patient-specific head model constructed from MRI and sLORETA as inverse solution. For the four most represented clusters, ESI was analyzed at the half-rising and peak time of the averaged waveform. Patients had ESI-insular-IZ if at least one ESI cluster was localized to the operculo-insular sublobes. ESI/SEEG concordance analysis was performed using SEEG-insular-IZ as reference standard (e.g., true positives: ESI-insular-IZ confirmed by ipsilateral SEEG-insular-IZ). Sensitivity, specificity, positive and negative predictive values (PPV, NPV) were calculated accordingly. Results: Thirty patients with verified SEEG insular contacts (mean insular contacts per patient= 6.2) and concomitant scalp EEG (mean hrs analyzed per patient = 143) were included. Twenty-four of 30 patients showed insular IEDs and were classified in the SEEG-insular-IZ group. Amongst them, 4/24 (17%) showed insular primary-IZ, 9/24 (37%) secondary-IZ/dominant and 11/24 (46%) secondary-IZ/propagated insular IEDs. Overall accuracy of interictal ESI was 63%. Fourteen of 24 patients showed a concordant ESI-insular-IZ (58% sensitivity). Specificity, PPV and NPV were respectively 83%, 93% and 33%. Two of the four patients (50%) with insular primary-IZ showed an interictal ESI insular cluster. Insular clusters were detected in 78% of the secondary-IZ/dominant insular IEDs group, against 45% in the secondary-IZ/propagated insular IEDs group. Conclusions: Automated low-density interictal ESI shows good accuracy in localizing insular IEDs from scalp EEG. Interestingly, higher sensitivity is found in patients who have IEDs with genuine insular dominance confirmed by SEEG. The technique showed high specificity and high PPV, suggesting its usefulness to confirm a suspected insular irritative focus. Further validation of the results is warranted, particularly by direct comparison of ESI/SEEG single spike localization. Funding: Please list any funding that was received in support of this abstract.: No specific funding was received in support of this abstract.
