Aim of the study was to characterize the pathological basis of rhythmic epileptic discharge (RED) in focal cortical dysplasia (FCD) patients. 

This is a retrospective study on a cohort of patients with pathologically proven FCD type II. Twenty-seven subjects with video-EEG recordings and surgical specimen were included. We grouped the patients by the presence of interictal REDs and the RED subtype (type 1 and type 2). Markers of potassium-chloride channels and inhibitory subtypes were quantitatively investigated. 

50% of patients with FCD type II showed interictal RED on EEG. 75% of REDs were quasicontinuous, slower, rhythmic 2–7Hz sharp waves (type 2 RED). There was a substantial increase of Na-K-Cl cotransporter (NKCC1) expression in patients with interictal REDs (P<0.01), whereas the decrease of K-Cl cotransporter type 2 (KCC2) expression remained similar. We also found a marked increase of somatostatin (SST) expression in dysplastic regions of patients with REDs (P<0.05), whereas the parvalbumin expression was equally increased regardless of the presence of REDs. With subgroup analysis, we found that only patients with type 2 REDs had a significant increase of NKCC1 and SST expression in dysplastic regions, whereas those with type 1 REDs did not. 

Our study suggests a potential correlation between RED and abnormal GABAergic signalling in patients with FCD type II.