The retinal and cerebral microvasculatures share many morphological and physiological properties. Assessment of the cerebral microvasculature requires highly specialized and expensive techniques. The potential for using noninvasive clinical assessment of the retinal microvasculature as a marker of the state of the cerebrovasculature offers clear advantages, owing to the ease with which the retinal vasculature can be directly visualized in vivo and photographed due to its essential two-dimensional nature. The use of retinal digital image analysis is becoming increasingly common, and offers new techniques to analyse different aspects of retinal vascular topography, including retinal vascular widths, geometrical attributes at vessel bifurcations and vessel tracking. Being predominantly automated and objective, these techniques offer an exciting opportunity to study the potential to identify retinal microvascular abnormalities as markers of cerebrovascular pathology. In this review, we describe the anatomical and physiological homology between the retinal and cerebral microvasculatures. We review the evidence that retinal microvascular changes occur in cerebrovascular disease and review current retinal image analysis tools that may allow us to use different aspects of the retinal microvasculature as potential markers for the state of the cerebral microvasculature.

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Complexity of the retinal vascular network is quantified through the measurement of fractal dimension. A computerized approach enhances and segments the retinal vasculature in digital fundus images with an accuracy of 94% in comparison to the gold standard of manual tracing. Fractal analysis was performed on skeletonized versions of the network in 40 images from a study of stroke. Mean fractal dimension was found to be 1.398 (with standard deviation 0.024) from 20 images of the hypertensives sub-group and 1.408 (with standard deviation 0.025) from 18 images of the non-hypertensives subgroup. No evidence of a significant difference in the results was found for this sample size. However, statistical analysis showed that to detect a significant difference at the level seen in the data would require a larger sample size of 88 per group.

Introduction: Discovering non-invasive and easily acquired biomarkers that are conducive to the accurate diagnosis of dementia is an urgent area of ongoing clinical research. One promising approach is retinal imaging, as there is homology between retinal and cerebral vasculature. Recently, optical coherence tomography angiography (OCT-A) has emerged as a promising new technology for imaging the microvasculature of the retina. Methods:A systematic review and meta-analysis was conducted to examine the application of OCT-A in dementia.Results: Fourteen studies assessing OCT-A in preclinical Alzheimer's disease (AD), mild cognitive impairment, or AD were included. Exploratory meta-analyses revealed a significant increase in the foveal avascular zone area and a significant decrease in superficial parafoveal and whole vessel density in AD, although there was significant heterogeneity between studies.Discussion: Although certain OCT-A metrics may have the potential to serve as biomarkers for AD, the field requires further standardization to allow conclusions to be reached regarding their clinical utility.