Intracranial correlates of small sharp spikes

ObjectiveTo identify cortical correlates of scalp small sharp spikes (SSS) using simultaneous scalp and intracranial EEG recordings.MethodsPatients were retrospectively evaluated based on a database of intracranial long-term recordings at the Epilepsy Center Freiburg. Inclusion criteria were: simultaneous recordings with intracranial and scalp EEGs and the presence of at least five unequivocal SSS in the scalp EEG.Intracranial recordings were analyzed regarding the co-occurring intracranial potentials during scalp SSS.Results33 patients, aged 9-60y, 17 females, fulfilled the above-mentioned criteria. Almost all patients had intracranial SSS correlates in the form of spike/polyspike-waves in the temporal lobe, predominantly in the hippocampus (24/28), less frequently involving the amygdala (5/29), temporal basal (3/18), lateral neocortical (4/32), entorhinal cortices (1/12), and the parietal lobe (2/13). Amplitudes of intrahippocampal spikes or polyspikes co-occurring with SSS were significantly higher than intracranial discharges without scalp correlates. In 45% of patients, intracranial spikes accompanying SSS were located within the seizure onset zone (SOZ).ConclusionsOur results strongly support an epileptic origin of SSS and provide evidence about their heterogenous generators.SignificanceThis study suggests that SSS cannot with certainty be classified as “benign” but rather considered as one of the EEG manifestations of focal epilepsy.