Intraoperative hippocampal electrocorticography frequently captures electrographic seizures and correlates with hippocampal pathology

•Intraoperative hippocampal electrocorticography frequently captures spontaneous seizures.•Electrographic seizures are more likely in patients without disruption of hippocampal architecture.•Intraoperative hippocampal seizures may result from deafferentation from the temporal neocortex. Relationship between electrographic seizures on hippocampal electrocorticography (IH-ECoG) and presence/type of hippocampal pathology remains unclear. IH-ECoG was recorded for 10–20 min from the ventricular surface of the hippocampus following removal of the temporal neocortex in 40 consecutive patients. Correlation between intraoperative hippocampal seizures and preoperative MRI, hippocampal histopathology, and EEG from invasive monitoring was determined. IH-ECoG captured electrographic seizures in 15/40 patients (in 8/23 with abnormal hippocampal signal on MRI and 7/17 patients without MRI abnormality). Hippocampal neuronal loss was observed in 22/40 (Group 1), while 18/40 had no significant neuronal loss (Group 2). In Group 1, 4/22 had seizures on IH-ECoG, while 11/18 had electrographic seizures in Group 2. In 24/40 patients who underwent prolonged extraoperative intracranial EEG (IC-EEG) recording, hippocampal seizures were captured in 14. Of these, 7 also had seizures during IH-ECoG. In 10/24 IC-EEG patients without seizures, 3 had seizures on IH-ECoG. IH-ECoG frequently captures spontaneous electrographic seizures. These are more likely to occur in patients with pathologic processes that do not disrupt/infiltrate hippocampus compared to patients with intractable epilepsy associated with disrupted hippocampal architecture. Intraoperative hippocampal seizures may result from deafferentation from the temporal neocortex and disinhibition of the perforant pathway.