Evolving epileptic activity under focal rhythmic Delta activity: insights from simultaneous intracranial and scalp EEG

Objective: Focal rhythmic delta activity (RDA) is a subtype of lateralized RDA (LRDA). Although several studies have compared scalp and intracranial electroencephalography (EEG) recordings to determine whether LRDA observed on scalp EEG reflects epileptic abnormalities occurring intracranially during the acute phase, to our knowledge, no studies have examined focal RDA. Therefore, we investigated the extent to which focal RDA on scalp EEG during the acute phase reflects epileptic activity observed on intracranial EEG. Methods: We retrospectively reviewed cases with simultaneous intracranial and scalp EEG (January 2003-August 2024). Patients who developed an intracranial hematoma during electrode placement were identified and included. For each case, we evaluated whether RDA, spikes, polyspikes, electrographic seizures (ESz), electrographic status epilepticus (ESE), and high-frequency oscillations (HFOs) observed on intracranial EEG near the hematoma were reflected on scalp EEG in terms of appearance, timing, frequency, and amplitude. Results: Of the 116 patients, six developed intracranial hematoma and were included in the analysis. Focal RDA was observed on scalp EEG in all six cases, whereas intracranial RDA appeared earlier than focal RDA on scalp EEG. In all cases, spikes and polyspikes were superimposed on the RDA in the intracranial EEG, whereas these findings were observed on scalp EEG in only three cases. ESz, ESE, and HFOs were observed in all intracranial recordings but were not detected on scalp EEG. Significance: Focal RDA observed on scalp EEG during the acute phase may reflect growing epileptic activity and serve as a potential target for antiseizure medication therapy.