Introduction: When electrically stimulated above a threshold, the neocortex will fire a series of electrographic spikes called an afterdischarge (AD). The AD threshold varies considerably over the cortex of a single individual, and this threshold has been suggested to reflect the general cortical excitability. Some have proposed using the thresholds of electrical stimulation-evoked afterdischarges for seizure focus mapping. We sought to describe the association between seizure onset zone and features of cortical afterdischarges.
Methods: We examined the records of 16 patients with drug resistant focal epilepsy who were undergoing extraoperative monitoring with subdural electrode grids. We used the surgeon’s stimulation notes, which recorded the different currents used at each electrode pair, and the electrocorticographic (ECoG) trace during the bedside mapping session to determine AD threshold and AD intensity for each electrode pair. The neurology report reviewing the subdural grid recording during ictal events was examined to determine the electrodes representing the seizure onset zone. We analyzed the correlation between the seizure onset zone, the area of lowest AD threshold, and the surgical outcomes.
Results: The average age of patients was 24 years old, and the most frequent epileptic etiologies were cortical dysgenesis, low grade glioma, and idiopathic (N = 7, 6, and 3, respectively). Of the 808 electrodes that were implanted, 49 (6%) belonged to the seizure onset zone. Only 7 (17%) of the 41 seizure onset zone electrodes that were stimulated had AD thresholds in the lowest quartile of stimulation for a given patient. There was no association between a patient having seizure onset zone electrode with a low AD threshold and the patient’s seizure reduction post-operatively or epileptic etiology.
Conclusions: Cortex of the seizure onset zone does not have lower AD threshold than surrounding cortex, and therefore AD threshold may be of limited utility in the localization of seizure foci.
Patient Care: Seizure reduction after resection depends largely on the accurate localization of the epileptogenic region. While there are several techniques for seizure focus localization, the results of these different methods can be discordant. Patients with focal epilepsy may be denied resection if their epileptogenic region cannot be well delineated. In response to these shortcomings, we sought to evaluate the potential of a novel method of seizure focus mapping. While researchers have known about the afterdischarge phenomenon for decades, there has be scant research into its relationship with the seizure onset zone. Our conclusion that AD threshold has little correlation with the seizure onset zone will help guide surgeons as they plan their resection. Patients with medically resistant focal epilepsy will benefit from focus mapping that is more evidence based.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe the rationale for using responses of cortex to electrical stimulation for seizure focus mapping, 2) Describe the correlation between seizure onset zone and afterdischarge threshold, 3) Discuss, in small groups, other stimulation-based modalities for seizure focus localization