The recent COVID-19 pandemic has significantly increased the workload on medical practitioners due to the treatment of severe cases, prompting a renewed examination of the virulence and transmission abilities of the coronavirus. Additionally, it has highlighted the impact of underlying diseases on the risk and progression of pneumonia, particularly in relation to widespread metabolic disorders such as diabetes mellitus (DM), hypertension, cardiovascular disease, and cerebrovascular diseases, etc. (Stefan, 2022). The pandemic has coincided with an unprecedented rise in the incidence of metabolic diseases, driven by substantial lifestyle changes (Joseph et al., 2022). Despite advances in genomics and emerging molecular biology techniques over recent decades, our understanding of the critical pathophysiological mechanisms and fundamental causes of these diseases remains incomplete. The most effective approach to mitigating the socioeconomic burden of these conditions involves early detection and intervention. However, the potential of emerging biomarkers for this purpose is often undermined in clinical settings by various factors. Therefore, the development of a natural biomarker that is easily accessible and requires minimal invasiveness could significantly alter the trajectory of this ongoing battle against metabolic diseases.

Encouragingly, recent developments have highlighted the significance of examining the microcirculation in the eye fundus due to its direct link with systemic circulation (Wu and Liu, 2022; Wagner et al., 2020; Rizzoni et al., 2022). This focus initially appears to contradict prior understanding, which emphasizes the anatomical and functional differences between microvascular and macrovascular systems, suggesting distinct characteristics (Feuer et al., 2022). Yet, an expanding body of clinical observations and research studies support the notion that these vascular systems might indeed represent complementary components of a unified framework, especially in metabolic disorders such as diabetic micro- and macrovascular diseases (Hanssen et al., 2022; Dal Canto et al., 2019), coronary vascular diseases (Godo et al., 2021), and small vessel cerebral diseases (Cabrera DeBuc et al., 2017; Houben et al., 2017). These emerging findings have prompted a reevaluation of established strategies for the prevention and management of cardiovascular complications, including stroke, acute coronary syndrome, and cognitive impairment.

Accordingly, the assessment of retinal microcirculation presents novel opportunities for the early detection, prognosis, and monitoring of chronic metabolic diseases. This review aims to elucidate the properties of the retina and its circulatory mechanics, with a concentrated focus on discussing the physiological and pathophysiological interconnections between microcirculation and macrocirculation, as well as exploring the recent advances in clarifying the influence and consequences of systemic chronic metabolic diseases, along with their risk factors, on retinal microcirculation.