Role of specific interictal and ictal EEG onset patterns

The objective of this review is to present the role of specific interictal and ictal EEG onset patterns during scalp video-EEG monitoring. Specific non-epileptiform abnormalities include temporal intermittent rhythmic delta activity (TIRDA) and temporal intermittent rhythmic theta activity (TIRTA) indicating a temporal lobe seizure onset, while interictal rhythmical midline theta activity occurs more frequently in frontal epilepsy. Specific interictal epileptiform abnormalities comprise Type 1 spikes pointing towards a mesial and Type 2 spikes indicating a lateral temporal irritative zone. Unilateral temporal interictal epileptiform discharges (IEDs) are predictive for a good surgical seizure outcome in temporal lobe epilepsy. Small sharp spikes (SSS) named Benign Epileptiform Transients of Sleep (BETS) in the past represent scalp EEG markers of hippocampal epileptic activity. While the localizing value of IEDs in extratemporal epilepsies is often limited, a consistently localized spike focus predicts a good surgical seizure outcome in non-lesional extratemporal patients. A specific ictal EEG pattern for mesial temporal lobe epilepsy consists of a 5–9 Hz rhythmic temporal activity which also predicts a good surgical outcome. In extratemporal epilepsies, ictal scalp EEG frequently is non-localized. Concerning the correspondence of ictal scalp-EEG and intracranial EEG (iEEG) patterns there is no simple one-to-one relationship. Scalp-EEG and iEEG patterns correspond closer to each other when there is no delay between clinical and scalp-EEG onset. Paroxysmal fast activity on scalp-EEG matches with low-voltage fast activity on iEEG. Repetitive epileptiform discharges on scalp EEG indicate an underlying focal cortical dysplasia.