Characteristics of subclinical seizures in an adult epilepsy population—a retrospective observational study

Abstract

Purpose Subclinical seizures (SCSs) are commonly detected during video-EEG monitoring but frequently overlooked in clinical practice. Their prevalence, localization characteristics, and relationship to clinical seizures (CSs) and interictal epileptiform discharges (IEDs) remain incompletely defined. This study aimed to characterize SCS prevalence, localization, frequency, and concordance with CS and IED localization in a presurgical epilepsy cohort, and to examine associations between SCS presence and neuropsychological performance. Methods A retrospective observational study was conducted in 116 adult patients undergoing inpatient scalp video-EEG telemetry and neuropsychological evaluation at King’s College Hospital between 2015 and 2023. SCS localization was compared with CS and IED localization using a five-category concordance framework and Cohen’s kappa for lateralization and temporal vs. extratemporal agreement. Results SCSs were present in 26.7% of patients. No clinical or demographic variable was significantly associated with their presence. SCSs most frequently localized to left temporal and right extratemporal regions, whereas CSs were more evenly distributed across temporal foci. Full concordance between CS and SCS localization was observed in 55.2% of co-recorded studies; where incomplete, SCS localizations were more often contained within CS localizations than the reverse. IED localizations were more widespread than both seizure types. Lateralization agreement was moderate across all comparisons (κ = 0.40–0.47). SCS presence was not significantly associated with neuropsychological test scores. Conclusion SCSs are present in approximately one quarter of presurgical epilepsy patients and show moderate concordance with CS and IED localization. Their distinct localization profile and frequent containment within CS zones support their value as complementary information in epilepsy care.