Increased delta power as a scalp marker of epileptic activity: a simultaneous scalp and intracranial EEG study.

BACKGROUND: To evaluate whether interictal epileptiform discharges (IEDs) that are not visible on the scalp are associated with changes in frequency spectrum on scalp EEG.

METHODS: We recorded simultaneous scalp high-density EEG and intracranial EEG recordings in 9 patients undergoing presurgical invasive recordings for pharmaco-resistant temporal lobe epilepsy. We selected epochs with hippocampal IED visible on intracranial EEG (ic-IED) but not on scalp EEG, as well as control epochs without intracranial EEG IED. We computed Welch's power spectral density (PSD) for each scalp electrode and for each subject, the PSD was further averaged across the canonical frequency bands and compared between the two conditions with and without ic-IED. For each patient we determined the peak frequency in the delta band (the significantly strongest frequency band in all patients) during periods of ic-IED. We also determined the five electrodes showing strongest power at the peak frequency.

RESULTS: We found that IEDs are associated with an increase in delta power on scalp EEG, in particular at frequency ≥ 1.4Hz. Electrodes showing slow frequency power changes associated with IEDs were consistent with the hemispheric lateralization of IED. Electrodes with maximum power of slow activity were not limited to temporal regions but also involved frontal (bilateral or unilateral) regions.

CONCLUSIONS: In patient with clinical picture suggestive of temporal lobe epilepsy, the presence of delta slowing ≥1.4Hz in anterior-temporal regions can represent a scalp marker of hippocampal IEDs. To our best knowledge this is the first study that demonstrates the co-occurrence of intracranial IED and increased delta power.