To investigate the pattern of changes in routine EEGs over time.

Patients with temporal lobe epilepsy (TLE) and hippocampal sclerosis (HS) present a widespread pattern of abnormalities extending beyond the temporal lobe. Although some studies have confirmed progression of structural alterations over time, fewer studies investigated the dynamic changes of interictal epileptiform discharges (IED) during the long-term follow-up.

We evaluated 315 patients, (201 females, with a mean epilepsy duration of 36 years) with 4175 routine EEGs (average of 13.3 EEGs/patient). Six patients presented bilateral HS, 165 left, and 144 right HS (groups balanced for sex, p=0.6). The EEGs were classified as: i) "Ipsilateral IEDs" (n=1661) when there was only IEDs ipsilateral to the HS; ii) "Bilateral+Contraleral IEDs" (n=617), and iii) “Normal EEGs" (n=1897). The duration of epilepsy when the EEG was performed (average=28.9 years) was divided into a) <8y: EEGs performed with less than 8 years from the first seizure; b) 8-17y from the first seizure, c) 18-29y and d) >30y. We performed an analysis of variance adjusted for multiple comparisons to investigate the differences within EEG classes over the different time categories.

While the relative numbers of Bilateral+Contraleral IEDs remained stable over time, there was a significant increase of Ipsilateral IEDs (from 22% to 41%, p<0.0001) with a decrease of Normal EEGs (from 69% to 45%; p<0.005). The pairwise comparisons of Ipsilateral IEDs were significant for <8y vs. 18-29y (p=0.005), <8y vs. >30y (p<0.0001), 8-17y vs. >30y (p=0.0002) and 18-29y vs. >30y (p=0.045), and Normal EEGs differed for most of comparisons (p<0.001).