Beyond the Onset: The Diagnostic Value of Late Significant Patterns in Scalp Ictal EEG

Abstract

Purpose

This study aimed to systematically assess ictal scalp electroencephalography (EEG) evolution by examining both ictal onset characteristics and late significant patterns (LSPs) in adults with focal epilepsy. We aimed to investigate their individual and collective descriptive roles in seizure lateralization and localization, as well as their correlation with seizure semiology, interictal EEG findings, and structural magnetic resonance imaging (MRI) within a multimodal presurgical assessment framework.

Methods

We retrospectively analyzed 67 adults with drug-resistant focal epilepsy who underwent extended video-EEG monitoring, during which 242 seizures were documented. The ictal EEG findings were categorized based on the morphology of the onset and subsequent rhythmic patterns. The yields of lateralization and localization were evaluated independently for ictal onset patterns, LSPs, and combined ictal interpretations. Cohen's κ statistics were used to look at multimodal concordance between ictal EEG, seizure semiology, interictal EEG, and MRI findings.

Results

The most common types of ictal onset were non-rhythmic or unclassifiable activity (29.3%) and low-voltage fast activity (28.5%). Late significant patterns were detected in 36.8% of seizures, predominantly manifesting as rhythmic theta–alpha activity, followed by repetitive spikes or sharp waves. The integration of LSPs resulted in enhanced lateralization and localization yields, in contrast to the analysis of onset patterns in isolation. At the individual patient level, combined ictal interpretation lateralized seizures in 68.6% of cases and localized seizure-onset regions in 70.1% of cases. There was a high degree of agreement between ictal and interictal EEG (κ = 0.775, strict right-left), a high degree of agreement between ictal EEG and MRI (κ = 0.710), and moderate-to-high agreement between ictal EEG and seizure semiology (κ = 0.647).

Conclusion

A systematic evaluation of ictal EEG evolution, including the identification of LSPs, provides additional descriptive data that augments conventional ictal-onset analysis in focal epilepsy. The integration of ictal evolution findings with semiology, interictal EEG, and MRI validates a multimodal interpretative framework for presurgical evaluation. These results highlight the possible clinical importance of incorporating seizure evolution analysis into conventional scalp EEG interpretation, although further validation with surgical outcome data is required.