Role of single pulse electrical stimulation (SPES) to guide electrode implantation under general anaesthesia in presurgical assessment of epilepsy.

Purpose: Single-pulse electrical stimulation (SPES) during intracranial recordings is part of the epilepsy presurgical evaluation protocol at King's College Hospital (London). Epileptiform responses correlated to the stimulus (delayed responses - DRs) tend to occur in areas of seizure onset, thereby allowing interictal identification of epileptogenic cortex in patients suffering refractory epilepsy. This preliminary study investigated the validity of SPES in the operating theatre under general anaesthesia (GA) during the implantation procedure, aiming to improve the positioning of intracranial electrodes.

Methods: Twelve drug-resistant epilepsy patients implanted with depth and/or subdural electrodes were studied. SPES (1 ms pulses, 4-8 mA, 0.2 Hz) was performed during both intra-operative electrode implantation under GA and chronic intracranial ECoG recordings, and the two recordings were compared in terms of cortical responses produced by stimulation and their electrode location.

Results: In 8/12 patients, SPES during chronic recordings produced DRs positively correlated to seizure onset and/or early seizure propagation areas. Of those eight patients, four showed DRs during electrode implantation under GA over the same electrode contacts. Among the four patients without DR during GA, three had continuous localized spontaneous epileptiform discharges, which made interpretation of SPES responses unreliable.

Conclusion: This study showed that, under GA, DRs can be reliably replicated, without false positive epileptiform responses to SPES, although the method's sensitivity is greatly reduced by spontaneous discharges. Results support SPES as a complementary technique that can be used to improve electrode placement during epilepsy surgery when no profound interictal activity is present.