Hippocampal-related memory network in Multiple Sclerosis: a structural connectivity analysis (P6.138)

OBJECTIVE: To quantify structural connectivity integrity of the hippocampal-related episodic memory network and its association with memory performance in multiple sclerosis (MS) patients. BACKGROUND: Episodic memory is typically affected in MS patients. DESIGN/METHODS: Brain diffusion tensor and 3D T1-weighted sequences were obtained from 71 MS patients and 50 healthy volunteers (HV). Thirty gray matter regions (selected a priori as part of the hippocampal-memory network) were segmented and used as seeds to perform probabilistic tractography and create a connectivity network matrix for each subject. Global, nodal and edge network metrics were calculated. Verbal and visuo-spatial memory were evaluated with the Paired-Associate Word Learning Test, Short Story Test, and Rey-Osterrieth Complex Figure delayed recall task. A memory cognitive impairment index (MCII) was also calculated. RESULTS: Compared to HV, MS patients showed significantly decreased (p<0.01) strength, assortativity, transitivity, global efficiency and increased average path length of the whole network. Significant correlation was found between global efficiency and strength vs memory scores. The thalamus and parietal cortex were the nodes with highest betweenness centrality (BC) and strength of the network both in patients and HV. Several nodes showed increased BC and decreased strength in MS patients compared to HV (p<0.01). Thirty-nine/435 edges had decreased communicability, whereas 108/435 edges had decreased fiber count (p<0.01, FDR correction). The thalamus and amygdala had increased communicability with other nodes in MS patients vs HV. Performance at single cognitive tests correlated with node and edge properties. CONCLUSIONS: The hippocampus-related memory network is globally impaired in MS patients, even if it maintains a configuration in which the thalamus and parietal lobes show the highest strength and BC. The integrity of this network seems relevant for preserving memory functions. Partially supported by a grant from Fondazione Italiana Sclerosi Multipla (FISM 2012/R/8).