Longitudinal analysis of interictal electroencephalograms in patients with temporal lobe epilepsy with hippocampal sclerosis

•Interictal epileptiform discharges (IEDs) remain predominantly ipsilateral to the hippocampal sclerosis (HS) over decades.•The frequency of routine EEGs with IEDs increases over time in patients with TLE-HS.•The frequency of contralateral IEDs remains stable over decades.•Contralateral IEDs could reflect early abnormalities and not epilepsy progression.•The persistence of mostly ipsilateral IEDs in TLE-HS has important clinical implications. While studies have shown the progression of atrophy in temporal lobe epilepsy (TLE) with hippocampal sclerosis (HS), little is known about the long-term dynamics of interictal epileptiform discharges (IEDs). To investigate long-term IEDs distribution in routine EEGs. We evaluated 314 patients with TLE and MRI signs of HS (TLE-HS). Six had bilateral, 163 had left, and 145 had right HS. We analyzed 3655 routine EEGs (average 11.6 EEGs/patient). The EEGs were classified into four groups: (i) ipsilateral-IEDs (n = 1485), EEGs with only IEDs ipsilateral to the HS; (ii) bilateral-IEDs (n = 390); (iii) contralateral-IEDs (n = 186); and (iv) normal-EEGs (n = 1594). The duration of epilepsy at the time of the EEG (average 27.9 years) was divided into four groups: (a) 30 years (n = 1845). We performed ANOVA with Tukey’s pairwise comparisons and linear regression analysis between the duration of epilepsy and the EEG groups. The ANOVA showed a difference in the distribution of IEDs over time (p < 0.0001). While there were no significant changes in the relative numbers of bilateral and contralateral-IEDs combined, there was a significant increase in ipsilateral-IEDs (p < 0.0001) and a decrease in normal-EEGs (p < 0.0001) over time. The linear regression analysis confirmed that the proportion of ipsilateral-IEDs (p < 0.0001), and to a lesser extent, bilateral-IEDs (p = 0.0002), increased over time, while contralateral-IEDs were unchanged (p = 0.923). Contrary to our expectations, contralateral-IEDs remained stable over time, whereas normal-EEGs decreased and ipsilateral-IEDs increased. Contralateral-IEDs may reflect early abnormalities and not epilepsy progression.