Spread of ictal activity in focal epilepsy

Latencies between seizure onset, propagation of ictal activity, and initial clinical symptoms and signs are critically important for the successful implementation of detection-based intervention systems in the treatment of epilepsy. This study analyzes intracranial EEG-recordings for temporal characteristics of ictal spread and its dependence on focus localization.

Intracerebral EEG recordings of 215 seizures from 43 patients with pharmacoresistant focal epilepsy were evaluated based on site of first propagation, latencies between EEG seizure onset, early propagation, and clinical seizure onset. Seizure onset was mesial temporal in 15 patients, neocortical temporal in 15 patients, and frontal in 13 patients.

Periods during which ictal activity remained confined to the seizure onset area showed significant differences between the patient groups. Median latencies between electrographic seizure onset and early propagation were significantly longer for patients with mesial temporal (5 s in seizure-based analysis/10 s in patient-based analysis) as compared to neocortical temporal (3 s/5 s) and frontal seizure focus (1 s/2 s; p < 0.01). Concordantly, median latencies to onset of clinical symptomatology were significantly longer for patients with mesial temporal (17 s/19 s) as compared to neocortical temporal (11 s/17 s) and frontal seizure focus (4 s in seizure-based analysis and 6 s in patient-based analysis; p < 0.01).

The speed of propagation of ictal activity and the latencies until initial clinical seizure symptoms differ significantly depending on focus localization. Extended spread often occurred within the time window during which current detection systems operate. This suggests that inclusion criteria of patients suitable for testing the efficacy of detection-based seizure intervention strategies should be based on focus localization and patient-individual propagation patterns.