Reduced interhemispheric hippocampal BOLD signal coupling related to early epilepsy onset

In order to investigate the factors influencing interhemispheric hippocampal connectivity we used fMRI to assess the time course of hippocampal activity during a spatial memory task. Data were obtained from 14 patients with unilateral hippocampal sclerosis and focal unilateral medial temporal lobe epilepsy. Correlation coefficients between left and right hippocampal BOLD signal fluctuations were determined in each patient as a measure for interhemispheric hippocampal coupling. On the group level a non-parametric Spearman correlation analysis was performed to assess relations of hippocampal connectivity to age, age at epilepsy onset, disease duration, and task performance. In all except for one patient correlation coefficients between left and right hippocampal time courses reached significance. Interhippocampal connectivity exhibited the strongest correlation to age at onset, i.e. patients with later onset showed greater connectivity. It was negatively correlated with disease duration, but not with age. Our findings underline the malicious effect of epileptic seizures on brain structures inevitably engaged with core cognitive functions. The results suggest that a measure of interhemispheric interplay - even at a coarse time-scale - might reflect the functional status of brain structures.