Relationship between high-frequency oscillations and spikes in a case of temporal lobe epilepsy

OBJECTIVE The aim of this case report was to study the relationship between high-frequency oscillations (HFOs), spikes, and seizures in a patient with temporal lobe epilepsy. INTRODUCTION During intracranial electroencephalography (EEG), HFOs are thought to be a marker for the seizure onset zone (SOZ). High-frequency oscillations are classified into ripples with frequencies of 70-200 Hz and fast ripples with frequencies of 200-500 Hz. Although HFOs are thought to be a marker for the SOZ, their relationship to spikes has not been studied in detail, especially within the SOZ. METHODS We studied the time course of ripples and spikes in a patient undergoing intracranial EEG. Medications were discontinued on day one. She suffered three seizures on day three. Her SOZ was in the left hippocampus, which displayed abundant ripples and spikes. Ripples, spikes with simultaneous ripples, and spikes without ripples were counted for this study. RESULTS We found that ripples and spikes in the SOZ had a marked diurnal variation. Ripples, spikes with ripples, and spikes without ripples increased and decreased in concert until just before seizure onset, when ripples and spikes with ripples increased markedly. Spikes without ripples did not increase. CONCLUSIONS These results support ripples as a marker for SOZ and show that they co-occur with spikes. Seizure onset was heralded by an increase in ripples and spikes with ripples, without an increase in spikes without ripples. We hypothesize that spikes associated with ripples may have a somewhat different pathophysiological mechanism than spikes not associated with ripples, differences that may be relevant for the timing of seizure onset.