Abnormal Synchronization Between Cortical Delta Power and Ripples in Hippocampal Sclerosis

Discriminating between epileptogenic and physiological ripples in the hippocampus is important for identifying epileptogenic (EP) zones; however, distinguishing these ripples on the basis of their waveforms is difficult. We hypothesized that the nocturnal synchronization of hippocampal ripples and cortical delta power could be used to classify epileptogenic and physiological ripples in the hippocampus.

We enrolled 38 patients with electrodes implanted in the hippocampus or parahippocampal gyrus between April 2014 and March 2023 at our institution. We divided 11 patients (11 hippocampi) who were pathologically diagnosed with hippocampal sclerosis into the EP group and five patients (six hippocampi) with no epileptogenicity in the hippocampus into the nonepileptogenic (NE) group. Hippocampal ripples were detected using intracranial electroencephalography with hippocampal or parahippocampal electrodes. Cortical delta power (0.5-4 Hz) was assessed using cortical electrodes. The Pearson correlation coefficient between the ripple rates and cortical delta power (Corr-RD) was calculated on the basis of the intracranial electroencephalographic signals recorded each night.

Although hippocampal ripples were similar among the EP and NE groups based on their waveforms and frequency properties, the Corr-RDs in the EP group (mean [standard deviation]: 0.20 [0.049]) were significantly lower than those in the NE group (0.67 [0.070]). On the basis of the minimum Corr-RDs, the two groups were classified with 94.1% accuracy.

Our results demonstrate that the Corr-RD is a biomarker of hippocampal epileptogenicity.