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

BACKGROUND: 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). OBJECTIVES: To investigate long-term IEDs distribution in routine EEGs. METHODS: 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) <8 years (n = 140), (b) 9-17 years (n = 505), (c) 18-29 years (n = 1165), and (d) >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. RESULTS: 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). CONCLUSIONS: 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.