Abstracts

Rationale: Ictal seizure onset in mesial temporal sclerosis (MTS) associated epilepsy may occur ipsi or contralaterally to the lesion. Seizures may also initiate on the MTS side and propagate to involve contralateral mesial temporal structures. It is not known whether contralateral seizure onset or involvement is associated with structural lesion in the contralateral hippocampus. Presence of a structural lesion on the contralateral hippocampus may negatively impact on seizure outcome after epilepsy surgery or on cognition. We investigated hippocampal volume in patients with concordant (C) or discordant (D) ictal onset in relation to the lesion in unilateral MTS patients. Methods: Adult patients with unilateral MTS underwent video-EEG monitoring with registration of at least three seizures with an EEG correlate. Patients also underwent a 3T MRI study, with whole brain and hippocampal volumetrics, with Freesurfer. Cases were classified as concordant if all seizures EEG onset were ipsilateral to the MTS or discordant if seizure onset occurred contralaterally to the MTS or if seizure onset was ipsilateral to the MTS, with later propagation of ictal discharges to the contralateral side. Corrected hippocampal volumes (hippocampus/whole brain volume) were calculated for right (R) and left (L) C and D cases. Results: We studied 36 patients: 26 were C (16 left; 10 right) and 10 were D (5 L; 5 R) and 36 healthy controls. Groups did not differ in relation to age (LC=32.8+/-9.3; LD=34.8+/-10.8; RC=33.9+/-10.5; RD=30.6+/-9.1 years), gender (%women = LC=62.5% LD=40% RC=40% RD=60%) education (LC=10.1+/-1.5 LD=12.6+/-2.2 RC=11.1+/-2.5 RD=10.8+/-0.4) and epilepsy duration (LC=25.2+/-11.8 LD=25.0+/-16.5 RC=26.3+/-12.6 RD=29.0+/-8.2). Mean Hippocampal volume was significantly lower on the MTS side (LC=0.250; LD=0.238; RC=0.256; RD=0.271, compared with the unaffected side LC=0.339 LD=0.346 ; RC=0.355; RD=0.348 and controls L=0.342 R=0.355 p<0.001). Considering contralateral hippocampal volumes, we did not find any statistically significant differences between L and R concordant and L and R discordant cases. Conclusions: Seizure onset or ictal discharge propagation to the contralateral hippocampus did not affect hippocampus volume. This finding speaks in favor of lack of structural damage in the contralateral hippocampus in unilateral MTS cases that present contralateral seizure onset or ictal involvement of the contralateral mesial temporal structures. This finding is in agreement with other studies that have indicated a favorable seizure outcome after epilepsy surgery in discordant cases.