Electrical stimulation for cortical mapping reduces the density of high frequency oscillations

Background: High frequency oscillations (HFOs, 80-500. Hz) are EEG biomarkers for epileptogenic areas. HFOs are also indicators of disease activity as HFO rates increase after reduction of antiepileptic medication. Electrical stimulation (ES) can be used for diagnostic purposes as well as therapy in patients with refractory epilepsy. This study investigates the occurrence and changes of HFOs during ES in patients with refractory epilepsy. Objective: Analysis of the effects of ES using intracranial ES on the occurrence of epileptic HFOs. Methods: Patients underwent ES for diagnostic purposes. Ripples (80-200. Hz) and fast ripples (200-500. Hz) were visually marked in a baseline EEG segment prior to ES, after each period of ES as well as after the end of ES. In patients in whom ES triggered a seizure a pre- and postictal segment was marked. Rates of HFOs were compared for the different time periods using a Spearman's correlation and Wilcoxon rank sum test (. p<. 0.05). Results: 12 patients with 911 EEG channels were analyzed. Ripple (. r=. -0.42, p<. 0.001) as well as fast ripple (. r=. -0.21, p<. 0.001) rates decreased significantly over the course of stimulation. This phenomenon was not focal over the seizure onset or neighboring contacts but even observed over distant contacts. Conclusions: ES resulted in a gradual decrease of HFO-Rates over time. The decrease of HFOs was not limited to SOZ areas. If HFOs are considered as markers of disease activity the reduction in HFO-rates as a result of intracranial ES has to be interpreted as a reduction of disease activity.