Hippocampal dysfunction is associated with memory impairment in multiple sclerosis: A volumetric and functional connectivity study

BACKGROUND

Previous studies have suggested a relationship between neuroanatomical and neurofunctional hippocampal alterations and episodic memory impairments in multiple sclerosis (MS) patients.

OBJECTIVE

We examined hippocampus volume and functional connectivity (FC) changes in MS patients with different episodic memory capabilities.

METHODS

Hippocampal subfield volume and FC changes were compared in two subgroups of MS patients with and without episodic memory impairment (multiple sclerosis impaired (MSi) and multiple sclerosis preserved (MSp), respectively) and healthy controls (HC). A discriminant function (DF) analysis was used to identify which of these neuroanatomical and neurofunctional parameters were the most relevant components of the mnemonic profiles of HC, MSp, and MSi.

RESULTS

MSi showed reduced volume in several hippocampal subfields compared to MSp and HC. Ordinal gradation (MSi > MSp > HC) was also observed for FC between the posterior hippocampus and several cortical areas. DF-based analyses revealed that reduced right fimbria volume and enhanced FC at the right posterior hippocampus were the main neural signatures of the episodic memory impairments observed in the MSi group.

CONCLUSION

Before any sign of episodic memory alterations (MSp), FC increased on several pathways that connect the hippocampus with cortical areas. These changes further increased when the several hippocampal volumes reduced and memory deficits appeared (MSi).

Hippocampal microstructural damage correlates with memory impairment in clinically isolated syndrome suggestive of multiple sclerosis

OBJECTIVE

We investigated whether diffusion tensor imaging (DTI) could reveal early hippocampal damage and clinically relevant correlates of memory impairment in persons with clinically isolated syndrome (CIS) suggestive of multiple sclerosis (MS).

METHODS

A total of 37 persons with CIS, 32 with MS and 36 controls prospectively included from 2011 to 2014 were tested for cognitive performances and scanned with 3T-magnetic resonance imaging (MRI) to assess volumetric and DTI changes within the hippocampus, whole brain volume and T2-lesion load.

RESULTS

While there was no hippocampal atrophy in the CIS group, hippocampal fractional anisotropy (FA) was significantly decreased compared to controls. Decrease in hippocampal FA together with increased mean diffusivity (MD) was even more prominent in MS patients. In CIS, hippocampal MD was correlated with episodic verbal memory performance ( r = -0.57, p = 0.0002 and odds ratio (OR) = 0.058, 95% confidence interval (CI) = 0.0057-0.59, p = 0.016 adjusted for age, gender, depression and T2-lesion load), but not with cognitive tasks unrelated to hippocampal functions. Hippocampal MD was the only variable discriminating memory-impaired from memory-preserved persons with CIS (area under the curve (AUC) = 0.77, sensitivity = 90.0%, specificity = 70.3%, positive predictive value (PPV) = 52.9%, negative predictive value (NPV) = 95.0%).

CONCLUSION

DTI alterations within the hippocampus might reflect early neurodegenerative processes that are correlated with episodic memory performance, discriminating persons with CIS according to their memory status.

Functional MRI in investigating cognitive impairment in multiple sclerosis

There is increasing evidence that the severity of the clinical manifestations of multiple sclerosis (MS) does not simply result from the extent of tissue destruction, but it rather represents a complex balance between tissue damage, tissue repair, and cortical reorganization. Functional magnetic resonance imaging (fMRI) provides information about the plasticity of the human brain. Therefore, it has the potential to provide important pieces of information about brain reorganization following MS-related structural damage. When investigating cognitive systems, fMRI changes have been described in virtually all patients with MS and different clinical phenotypes. These functional changes have been related to the extent of brain damage within and outside T2-visible lesions as well as to the involvement of specific central nervous system structures. It has also been suggested that a maladaptive recruitment of specific brain regions might be associated with the appearance of clinical symptoms in MS, such as fatigue and cognitive impairment. fMRI studies from clinically (and cognitively) impaired MS patients may be influenced by different task performances between patients and controls. As a consequence, new strategies have been introduced to assess the role, if any, of brain reorganization in severely impaired patients, including the analysis of resting-state networks. The enhancement of any beneficial effects of this brain adaptive plasticity should be considered as a potential target of therapy for MS.

Causes, effects and connectivity changes in MS-related cognitive decline

Cognitive decline is a frequent but undervalued aspect of multiple sclerosis (MS). Currently, it remains unclear what the strongest determinants of cognitive dysfunction are, with grey matter damage most directly related to cognitive impairment. Multi-parametric studies seem to indicate that individual factors of MS-pathology are highly interdependent causes of grey matter atrophy and permanent brain damage. They are associated with intermediate functional effects (e.g. in functional MRI) representing a balance between disconnection and (mal) adaptive connectivity changes. Therefore, a more comprehensive MRI approach is warranted, aiming to link structural changes with functional brain organization. To better understand the disconnection syndromes and cognitive decline in MS, this paper reviews the associations between MRI metrics and cognitive performance, by discussing the interactions between multiple facets of MS pathology as determinants of brain damage and how they affect network efficiency.

Hippocampal-DMN disconnectivity in MS is related to WM lesions and depression

The hippocampus is part of the default-mode network (DMN) and is functionally hit early in multiple sclerosis (MS). Hippocampal and DMN dysfunctions have been associated with depression, both in patients with MS and in major depressive disorders. We hypothesized that white matter lesions may contribute, through a disconnection mechanism, to hippocampal dysfunction. To test this, we assessed the relationship between hippocampal resting-state (RS) functional connectivity (FC) abnormalities with brain T2 lesion volumes and the presence and severity of depression. Structural and RS fMRI images were acquired from 69 patients with cognitively intact MS and 42 matched healthy controls (HC). Depression was quantified using the Montgomery-Asberg Depression Rating Scale. Seed-voxel hippocampal RS FC was assessed. SPM8 was used for between-group comparisons and correlation analysis between RS FC abnormalities with clinical and structural MRI variables. Compared to HC, patients with MS showed a significant atrophy of the whole brain and left hippocampus (P < 0.001), and a distributed pattern of decreased RS FC between the hippocampi and several cortical-subcortical regions, which were mostly located within the DMN. Reduced hippocampal RS FC with regions of the DMN was strongly correlated with higher T2 lesion volume, longer disease duration, and the severity of depression and disability. In patients with cognitively preserved MS, brain focal WM lesions are related to the functional integration of the hippocampus to other brain regions of the DMN, suggesting a disconnection syndrome. Such a disruption of hippocampal RS FC is likely to contribute to the occurrence of depression and to clinical disability.

Fronto-limbic novelty processing in acute psychosis: disrupted relationship with memory performance and potential implications for delusions

Recent concepts have highlighted the role of the hippocampus and adjacent medial temporal lobe (MTL) in positive symptoms like delusions in schizophrenia. In healthy individuals, the MTL is critically involved in the detection and encoding of novel information. Here, we aimed to investigate whether dysfunctional novelty processing by the MTL might constitute a potential neural mechanism contributing to the pathophysiology of delusions, using functional magnetic resonance imaging (fMRI) in 16 unmedicated patients with paranoid schizophrenia and 20 age-matched healthy controls. All patients experienced positive symptoms at time of participation. Participants performed a visual target detection task with complex scene stimuli in which novel and familiar rare stimuli were presented randomly intermixed with a standard and a target picture. Presentation of novel relative to familiar images was associated with hippocampal activation in both patients and healthy controls, but only healthy controls showed a positive relationship between novelty-related hippocampal activation and recognition memory performance after 24 h. Patients, but not controls, showed a robust neural response in the orbitofrontal cortex (OFC) during presentation of novel stimuli. Functional connectivity analysis in the patients further revealed a novelty-related increase of functional connectivity of both the hippocampus and the OFC with the rostral anterior cingulate cortex (rACC) and the ventral striatum (VS). Notably, delusions correlated positively with the difference of the functional connectivity of the hippocampus vs. the OFC with the rACC. Taken together, our results suggest that alterations of fronto-limbic novelty processing may contribute to the pathophysiology of delusions in patients with acute psychosis.