Associations between carotid macrovascular and retinal microvascular alterations in diabetes: Evidence synthesis and implications for risk assessment.

Diabetes mellitus is characterized by progressive vascular injury affecting both the microcirculation and large arteries. Diabetic retinopathy is a major microvascular complication and has been associated in observational studies with several carotid vascular phenotypes, including increased carotid intima-media thickness, focal plaque burden, arterial stiffness, plaque vulnerability markers, and, in selected cohorts, carotid stenosis. However, these carotid endpoints differ in measurement protocol, biological meaning, and prognostic implications, and most available evidence is cross-sectional. Retinal imaging modalities, including optical coherence tomography and optical coherence tomography angiography, provide quantitative measures such as vessel density, foveal avascular-zone size, retinal nerve fiber layer thickness, and choroidal thickness that may parallel systemic vascular injury, but validated thresholds for carotid disease detection have not been established. Shared mechanisms, including chronic hyperglycemia, endothelial dysfunction, inflammation, oxidative stress, dyslipidemia, and hypoxia-driven angiogenic signaling, may contribute to both retinal and carotid abnormalities; insulin resistance is particularly relevant to type 2 diabetes and metabolic syndrome, whereas diabetes duration and glycemic exposure are central across diabetes types. This narrative review therefore summarizes the associations between retinal microvascular changes and specific carotid vascular changes, while emphasizing that many findings may be influenced by shared risk factors and that most studies show association rather than causation or proven clinical utility. Current evidence supports multidisciplinary awareness and comprehensive cardiovascular risk-factor management in people with diabetes, but it does not yet justify retinal findings or OCTA metrics as validated triggers for screening asymptomatic patients with carotid ultrasonography. Prospective studies are needed to show whether combined retinal and carotid assessment improves diagnosis, risk prediction, cost-effectiveness, and patient outcomes before it can be recommended as a routine screening strategy.