Asymmetrical hippocampal connectivity in mesial temporal lobe epilepsy: evidence from resting state fMRI

Abstract

BackgroundMesial temporal lobe epilepsy (MTLE), the most common type of focal epilepsy in adults, is often caused by hippocampal sclerosis (HS). Patients with HS usually present memory dysfunction, which is material-specific according to the hemisphere involved and has been correlated to the degree of HS as measured by postoperative histopathology as well as by the degree of hippocampal atrophy on magnetic resonance imaging (MRI). Verbal memory is mostly affected by left-sided HS, whereas visuo-spatial memory is more affected by right HS. Some of these impairments may be related to abnormalities of the network in which individual hippocampus takes part. Functional connectivity can play an important role to understand how the hippocampi interact with other brain areas. It can be estimated via functional Magnetic Resonance Imaging (fMRI) resting state experiments by evaluating patterns of functional networks. In this study, we investigated the functional connectivity patterns of 9 control subjects, 9 patients with right MTLE and 9 patients with left MTLE.ResultsWe detected differences in functional connectivity within and between hippocampi in patients with unilateral MTLE associated with ipsilateral HS by resting state fMRI. Functional connectivity resulted to be more impaired ipsilateral to the seizure focus in both patient groups when compared to control subjects. This effect was even more pronounced for the left MTLE group.ConclusionsThe findings presented here suggest that left HS causes more reduction of functional connectivity than right HS in subjects with left hemisphere dominance for language.

Large‐scale brain networks are distinctly affected in right and left mesial temporal lobe epilepsy

“…Differences on connectivity alterations between left and right MTLE have been previously described in the literature [Haneef et al, 2015; Pereira et al, 2010; Waites et al, 2006]. Through diversified methodologies, these studies explored distinct aspects of brain functional connectivity using anatomical or functional relevant ROIs, independent component analysis and graph theoretical information.…”

“…These data put together implies that right and left MTLE could have different pathological mechanisms. Although previous studies have focused on the differences in functional connectivity between these groups of patients, these analyses were restricted to specific brain nodes, more often including the hippocampal region [Morgan et al, 2012; Pereira et al, 2010; Zhang et al, 2010]. However, today it is accepted that in order to understand the brain function and its disruptions, we must move forward from looking at specific brain regions and try to understand how the different areas interact and the possible abnormalities of this interaction.…”

Seizure Frequency Can Alter Brain Connectivity: Evidence from Resting-State fMRI

“…10 In contradistinction to the aforementioned results, there have been reports on increased hippocampal connectivity that was presumed to be a compensatory mechanism because it linearly correlated with a disease duration of Ͼ10 years. [1][2][3][4]6 Graph-theory analysis of resting-state fMRI data from patients with epilepsy also revealed decreased functional nodal topologic properties of the DMN that were positively correlated with disease duration. 8,[11][12][13][14] Regional homogeneity analysis of resting-state fMRI data was even used as a presurgical tool for seizure identification in patients with MR-negative focal epilepsy.…”

“…1 Application of connectivity principles to these data has promoted research in various aspects of epileptic seizures, and there has been overwhelming report of decreased connectivity around the seizure-onset zone [2][3][4][5][6] and the default mode network (DMN) by several groups. 7,8 In 2012, Jehi 7 and Morgan et al 9 reported that connectivity patterns were different in patients with right and left mesial temporal sclerosis and that there was decreased connectivity between the regions of the DMN and the hippocampus and amygdala in patients with mesial temporal sclerosis.…”

Neuroimaging the Epileptogenic Process

“…fMRI BOLD signal changes have helped identify epileptogenic networks that not only correlate with seizure foci, but which are also seen at sites distant from EEG foci [77]. Among MTLE patients, compared with resting controls, there is decreased basal functional connectivity seen in the epileptogenic medial temporal lobe [115][116][117]. Decreased functional connectivity has been described in left mesial TLE across a network that includes thalamic, brainstem, frontal, and parietal regions, as well as focally in the dorsal medial prefrontal cortex, mesial temporal lobe, and inferior temporal cortex compared with controls [7].…”