Relevance of subcortical visual pathways disruption to visual symptoms in dementia with Lewy bodies

Visual hallucinations represent a core diagnostic criterion for dementia with Lewy bodies (DLB). We hypothesized that thalamic regions, which are critically involved in the modulation of visual transmission, may be differentially disrupted in DLB as compared to Alzheimer's Disease (AD) and that these deficits could relate to visual dysfunction in DLB patients. Magnetic Resonance and Diffusion Tensor Imaging (DTI) were performed with a 3 T scanner on a sample population of 15 DLB patients, 15 AD patients and 13 healthy volunteers. Regional thalamic micro-structural changes were assessed by parcelling the thalamus based on its connectivity to cortex and to amygdala and by measuring the mean diffusivity (MD) in each connectivity-defined sub-region. Micro-structural grey matter damage associated to higher MD values was found bilaterally in DLB compared to controls in the sub-regions projecting from thalamus to prefrontal and parieto-occipital cortices. Right thalamic sub-region projecting to amygdala and left thalamic sub-region projecting to motor cortex were also affected in DLB compared to controls. Higher MD values were found bilaterally in AD compared to controls in the thalamic sub-regions projecting to temporal cortex. Specific comparison between the two forms of dementia found differences: the sub-regions which project from thalamus to parieto-occipital cortex and to amygdala showed higher MD values in DLB compared to AD patients. In DLB patients, correlation analysis showed a significant correlation between NPI hallucinations item scores and MD values in the right thalamic sub-regions projecting to parietal and occipital cortices. The present study demonstrates how thalamic connectivity alterations between higher and lower visual areas may be relevant in explaining visual hallucinations in DLB.