Identification of pre-spike network in patients with mesial temporal lobe epilepsy.
BACKGROUND
Seizures and interictal spikes in mesial temporal lobe epilepsy (MTLE) affect a network of brain regions rather than a single epileptic focus. Simultaneous electroencephalography and functional magnetic resonance imaging (EEG-fMRI) studies have demonstrated a functional network in which hemodynamic changes are time-locked to spikes. However, whether this reflects the propagation of neuronal activity from a focus, or conversely the activation of a network linked to spike generation remains unknown. The functional connectivity (FC) changes prior to spikes may provide information about the connectivity changes that lead to the generation of spikes. We used EEG-fMRI to investigate FC changes immediately prior to the appearance of interictal spikes on EEG in patients with MTLE.


RESULTS
Fifteen patients with MTLE underwent continuous EEG-fMRI during rest. Spikes were identified on EEG and three 10 s epochs were defined relative to spike onset: spike (0-10 s), pre-spike (-10 to 0 s), and rest (-20 to -10 s, with no previous spikes in the preceding 45s). Significant spike-related activation in the hippocampus ipsilateral to the seizure focus was found compared to the pre-spike and rest epochs. The peak voxel within the hippocampus ipsilateral to the seizure focus was used as a seed region for FC analysis in the three conditions. A significant change in FC patterns was observed before the appearance of electrographic spikes. Specifically, there was significant loss of coherence between both hippocampi during the pre-spike period compared to spike and rest states.


CONCLUSIONS
In keeping with previous findings of abnormal inter-hemispheric hippocampal connectivity in MTLE, our findings specifically link reduced connectivity to the period immediately before spikes. This brief decoupling is consistent with a deficit in mutual (inter-hemispheric) hippocampal inhibition that may predispose to spike generation.
Centre for Advanced Imaging, University of Queensland , Brisbane, QLD , Australia.
Nahla L
NL
Faizo
Centre for Advanced Imaging, University of Queensland , Brisbane, QLD , Australia ; ARC Centre of Excellence in Cognition and its Disorders, Macquarie University , Sydney, NSW , Australia.
Hana
H
Burianová
Centre for Advanced Imaging, University of Queensland , Brisbane, QLD , Australia.
Marcus
M
Gray
Centre for Advanced Imaging, University of Queensland , Brisbane, QLD , Australia ; School of Psychology and Counseling, Queensland University of Technology , Brisbane, QLD , Australia.
Julia
J
Hocking
Centre for Advanced Imaging, University of Queensland , Brisbane, QLD , Australia.
Graham
G
Galloway
Centre for Advanced Imaging, University of Queensland , Brisbane, QLD , Australia ; Royal Brisbane and Women's Hospital , Brisbane, QLD , Australia.
David
D
Reutens
EEG-fMRI; functional connectivity; hippocampus; interictal spikes; mesial temporal lobe epilepsy; network