Default mode network connectivity indicates episodic memory capacity in mesial temporal lobe epilepsy

Abstract

SUMMARYPurpose: The clinical relevance of resting state functional connectivity in neurologic disorders, including mesial temporal lobe epilepsy (mTLE), remains unclear. This study investigated how connectivity in the default mode network changes with unilateral damage to one of its nodes, the hippocampus (HC), and how such connectivity can be exploited clinically to characterize memory deficits and indicate postsurgical memory change. Methods: Functional magnetic resonance imaging (fMRI) resting state scans and neuropsychological memory assessments (Warrington Recognition Tests for Words and Faces) were performed on 19 healthy controls, 20 patients with right mTLE, and 18 patients with left mTLE. In addition, postsurgical fMRI resting state and memory change (postsurgical memory performance-presurgical memory performance) data were available for half of these patients. Key Findings: Patients with mTLE showed reduced connectivity from the posterior cingulate cortex (PCC) to the epileptogenic HC and increased PCC connectivity to the contralateral HC. Stronger PCC connectivity to the epileptogenic HC was associated with better presurgical memory and with greater postsurgical memory decline. Stronger PCC connectivity to the contralateral HC was associated with less postsurgical memory decline. Following surgery, PCC connectivity to the remaining HC increased from presurgical values and showed enhanced correlation with postsurgical memory function. It is notable that this index was superior to others (hippocampal volume, preoperative memory scores) in explaining variance in memory change following surgery. Significance: Our results demonstrate the striking clinical significance of the brain's intrinsic connectivity in evaluating cognitive capacity and indicating the potential of postsurgical cognitive morbidity in patients with mTLE.

Hippocampal Atrophy Is Associated with Altered Hippocampus–Posterior Cingulate Cortex Connectivity in Mesial Temporal Lobe Epilepsy with Hippocampal Sclerosis

“…Our results are consistent with those of previous studies that reported decreased iFC between the PCC and the epileptogenic hippocampus or mesial temporal lobe in patients with left MTLEϩHS. 10,31 In addition, we observed increased iFC on the right side in the same group of patients (On-line Table). Bettus et al 32 investigated iFC between the hippocampus and other limbic structures on the healthy side in a group of patients with unilateral MTLE 32 ; they reported increased iFC between the right hippocampus and the right temporal pole and right amygdala in patients with left MTLE.…”

“…9 The resting-state fMRI studies have shown decreased iFC between the epileptogenic hippocampus and the PCC in patients with unilateral MTLEϩHS, suggesting that this functional metric represents an altered functional pathway of the epileptic network. 10,11 Taken together, hippocampus-PCC connectivity (both structural and functional) is a potential pathway of the epileptic network in unilateral MTLEϩHS. Bernasconi et al 12 reported that volume loss in the hippocampus was related to epilepsy duration in patients with unilateral MTLEϩHS.…”

Functional Connectivity of the Hippocampus in Temporal Lobe Epilepsy: Feasibility of a Task-Regressed Seed-Based Approach

“…This network is important in TLE because of its inclusion of the hippocampus (HC) and possible association with the episodic memory impairments observed in patients with refractory TLE (Holmes et al, 2012;McCormick et al, 2013). These diffuse, extratemporal reductions in the DMN agree with the diffuse structural impairments observed in the gray and white matter of patients with refractory TLE (Bernhardt et al, 2010;McDonald et al, 2008;Pittau et al, 2012).…”

Network Analysis of the Default Mode Network Using Functional Connectivity MRI in Temporal Lobe Epilepsy

“…Another method of seed analysis is by correlating the signal between two, or multiple, regions of interest. Using such a method, the connectivity was found to be reduced between the lesional hippocampus and the posterior DMN in TLE 38 . On the other hand, ICA is data driven without a pre-existing hypothesis and delineates spatio-temporally distinct networks within the resting state brain.…”