Retinal nerve fibre layer thickness reflects characteristics of brain grey and white matter.

The retina is an optically accessible part of the central nervous system. Using high-resolution optical coherence tomography, we explored the relationship between retinal thickness and structural features of the brain obtained with magnetic resonance imaging. We hypothesized that there are associations between circumpapillary (i.e., around optic disc) retinal nerve fibre layer thickness and structural features of (i) brain areas and pathways related to visual information processing and (ii) more widespread brain areas affected by the consequences of cardiovascular risk factors and/or age-related neurodegeneration. In a population-based sample of over 500 subjects, in support of the first hypothesis, we showed associations of circumpapillary retinal nerve fibre layer thickness with visual cortex grey matter density and with optic radiation fractional anisotropy. These correlations were stronger for the right eye, possibly reflecting right ocular dominancy. Regarding the second hypothesis, while we confirmed the broad impact of cardiovascular risk factors such as body mass index, diabetes, and hypertension on brain structure, we did not find (adequate) significant partial correlations between circumpapillary retinal nerve fibre layer thickness and cardiovascular risk factors. Consequently, we were unable to confirm an association between circumpapillary retinal nerve fibre layer thickness and the impact of cardiovascular risk factors on brain structure especially on grey matter rather than white matter. However, even when the effects of cardiovascular risk factors were accounted for statistically, circumpapillary retinal nerve fibre layer thickness (particularly on the right side) was associated with fractional anisotropy of limbic system tracts, that is, the fornix and stria terminalis including hippocampus and amygdala, areas which are commonly affected by Alzheimer's disease. To further explore the structural associations between eye and brain, in terms of a possible common underlying pathology related to cardiovascular risk factors and progressive neurodegenerative diseases on the central nervous system, longitudinal and interventional studies are necessary.