Introduction: Identification of the seizure onset zone (SOZ) is a crucial step in the surgical care of patients with partial epilepsy. The SOZ is usually identified through manual analysis of intracranial electrode recordings during spontaneous seizures. Although this approach is the “gold standard”, it is costly, time-consuming, and imperfect. Single pulse electrical stimulation (SPES) has been explored as a means of SOZ identification, but previous SPES protocols are unsuitable for clinical care. We are developing a novel SPES technique to identify the SOZ in an automated and reproducible manner.
Methods: SPES is applied to all implanted electrodes using a pseudorandom routine with multiple repetitions. Trial-averaged spectrograms that are masked for significance (p<0.05) are used to assign values to each electrode based on response strength and time-frequency pattern. Maps are generated from these values to provide an estimate of the SOZ.
Results: We have developed a unique protocol of SPES application and analysis over the last 2 years. We first explored low-level stimulation parameter space, using 4 mm circular subdural electrodes, in multiple patients, and found unique responses in the SOZ required a stimulation intensity of 3.5 mA using 1 ms pulses. We then identified two unique response patterns in 5 patients with focal epilepsy. Stimulation of electrodes over the clinically determined SOZ showed suppression of broadband (10-250 Hz) power lasting as long as 1 second. In one patient, an alternate pattern of broadband burst in power 150-400 ms after stimulation was shown. Response maps developed from these feature sets for all 5 patients demonstrate high specificity for the SOZ. Our SPES protocol does not induce after-discharges or seizures.
Conclusions: Our results, while preliminary, are encouraging. Identification of the SOZ is rapid, reproducible, and specific, which suggests that our approach, if validated, may be useful for clinical care.
Patient Care: Potentially reduce time and cost of seizure onset zone diagnosis.
Learning Objectives: Participants should be able to discuss the development of a novel stimulation paradigm to identify seizure onset zone.