Accurate Localization of the Seizure Onset Zone by Electric Source Imaging May Correlate With Favorable Outcome in Resective Surgery

In a study evaluating the accuracy of electric source imaging (ESI) of interictal spikes in delineating the seizure onset zone (SOZ) defined by intracranial electroencephalography (EEG), investigators at Geneva University Hospitals (GUH) in Switzerland and elsewhere found that localization of interictal spikes by ESI in individual epilepsy patients’ MRI scans provided an accurate estimate of the SOZ. The results, published in the January issue of the Journal of Neurology, Neurosurgery & Psychiatry, support the role of interictal-spike ESI as a reliable method for delineating the SOZ and, according to the authors, “add to the evidence that ESI has an important role to play in defining the strategy for the implantation of intracranial electrodes and for resective surgery.”

For patients experiencing drug-resistant focal epilepsy, surgical resection of the epileptogenic zone is a therapeutic option that has led to seizure freedom in a substantial proportion of cases. Accurate localization of the cortical area generating seizures is critical to such surgical success, and intracranial EEG has been the gold standard for this purpose—albeit an invasive one, among other limitations. Of the noninvasive approaches that have been investigated, EEG-based ESI of interictal spikes has proven to be among the most accurate for localizing the epileptogenic zone. What remains unclear, however, is whether the irritative zone—the source of interictal spikes—co-localizes with the SOZ and, if so, whether ESI can be reliably accurate in detecting it. Previously, no study had compared ESI of interictal spikes to the localization of the SOZ by intracranial EEG.

In the GUH study, the investigators prospectively recruited 44 consecutive patients with drug-resistant epilepsy who had undergone high-density EEG recordings prior to intracranial EEG monitoring. In the 38 patients in whom EEG monitoring recorded epileptic activity, surgery was carried out in 32 (84%). In all 38 patients, the researchers performed interictal-spike ESI, measuring the distance between the ESI maximum and the nearest intracranial electrodes in the SOZ and the irritative zone, and then correlated resection of the region containing the ESI maximum to surgical outcome.

Finding the median distance from the ESI maximum to the nearest SOZ electrode to be 17 mm, the investigators co-localized the SOZ and irritative zone in most patients, supporting the observation that localizing interictal spikes is a valid surrogate for the SOZ. In the 32 patients who had resective surgery, inclusion of the ESI maximum in the resection correlated with favorable outcome.

—Fred Balzac

In a study evaluating the accuracy of electric source imaging (ESI) of interictal spikes in delineating the seizure onset zone (SOZ) defined by intracranial electroencephalography (EEG), investigators at Geneva University Hospitals (GUH) in Switzerland and elsewhere found that localization of interictal spikes by ESI in individual epilepsy patients’ MRI scans provided an accurate estimate of the SOZ. The results, published in the January issue of the Journal of Neurology, Neurosurgery & Psychiatry, support the role of interictal-spike ESI as a reliable method for delineating the SOZ and, according to the authors, “add to the evidence that ESI has an important role to play in defining the strategy for the implantation of intracranial electrodes and for resective surgery.”

For patients experiencing drug-resistant focal epilepsy, surgical resection of the epileptogenic zone is a therapeutic option that has led to seizure freedom in a substantial proportion of cases. Accurate localization of the cortical area generating seizures is critical to such surgical success, and intracranial EEG has been the gold standard for this purpose—albeit an invasive one, among other limitations. Of the noninvasive approaches that have been investigated, EEG-based ESI of interictal spikes has proven to be among the most accurate for localizing the epileptogenic zone. What remains unclear, however, is whether the irritative zone—the source of interictal spikes—co-localizes with the SOZ and, if so, whether ESI can be reliably accurate in detecting it. Previously, no study had compared ESI of interictal spikes to the localization of the SOZ by intracranial EEG.

In the GUH study, the investigators prospectively recruited 44 consecutive patients with drug-resistant epilepsy who had undergone high-density EEG recordings prior to intracranial EEG monitoring. In the 38 patients in whom EEG monitoring recorded epileptic activity, surgery was carried out in 32 (84%). In all 38 patients, the researchers performed interictal-spike ESI, measuring the distance between the ESI maximum and the nearest intracranial electrodes in the SOZ and the irritative zone, and then correlated resection of the region containing the ESI maximum to surgical outcome.

Finding the median distance from the ESI maximum to the nearest SOZ electrode to be 17 mm, the investigators co-localized the SOZ and irritative zone in most patients, supporting the observation that localizing interictal spikes is a valid surrogate for the SOZ. In the 32 patients who had resective surgery, inclusion of the ESI maximum in the resection correlated with favorable outcome.

—Fred Balzac

In a study evaluating the accuracy of electric source imaging (ESI) of interictal spikes in delineating the seizure onset zone (SOZ) defined by intracranial electroencephalography (EEG), investigators at Geneva University Hospitals (GUH) in Switzerland and elsewhere found that localization of interictal spikes by ESI in individual epilepsy patients’ MRI scans provided an accurate estimate of the SOZ. The results, published in the January issue of the Journal of Neurology, Neurosurgery & Psychiatry, support the role of interictal-spike ESI as a reliable method for delineating the SOZ and, according to the authors, “add to the evidence that ESI has an important role to play in defining the strategy for the implantation of intracranial electrodes and for resective surgery.”

For patients experiencing drug-resistant focal epilepsy, surgical resection of the epileptogenic zone is a therapeutic option that has led to seizure freedom in a substantial proportion of cases. Accurate localization of the cortical area generating seizures is critical to such surgical success, and intracranial EEG has been the gold standard for this purpose—albeit an invasive one, among other limitations. Of the noninvasive approaches that have been investigated, EEG-based ESI of interictal spikes has proven to be among the most accurate for localizing the epileptogenic zone. What remains unclear, however, is whether the irritative zone—the source of interictal spikes—co-localizes with the SOZ and, if so, whether ESI can be reliably accurate in detecting it. Previously, no study had compared ESI of interictal spikes to the localization of the SOZ by intracranial EEG.

In the GUH study, the investigators prospectively recruited 44 consecutive patients with drug-resistant epilepsy who had undergone high-density EEG recordings prior to intracranial EEG monitoring. In the 38 patients in whom EEG monitoring recorded epileptic activity, surgery was carried out in 32 (84%). In all 38 patients, the researchers performed interictal-spike ESI, measuring the distance between the ESI maximum and the nearest intracranial electrodes in the SOZ and the irritative zone, and then correlated resection of the region containing the ESI maximum to surgical outcome.

Finding the median distance from the ESI maximum to the nearest SOZ electrode to be 17 mm, the investigators co-localized the SOZ and irritative zone in most patients, supporting the observation that localizing interictal spikes is a valid surrogate for the SOZ. In the 32 patients who had resective surgery, inclusion of the ESI maximum in the resection correlated with favorable outcome.

—Fred Balzac