Methods of evaluation of microvascular structure: state of the art

Cardiovascular diseases represent the leading cause of death in Western Countries. Among them, a key role is played by arterial hypertension, which causes macro- and microvascular alterations. Specifically, hypertension is associated with structural alterations in the microvessels, such as an increased ratio of the tunica media thickness to internal lumen (M/L ratio) in small resistance arteries and a reduction of capillary density. In order to evaluate the small resistance artery structure, the direct measurements of M/L ratio through wire or pressure micromyography has been considered the gold-standard method. Despite the availability of convincing evidence about the prognostic relevance of the M/L ratio, the invasiveness of these methods has limited its implementation in the daily clinical practice. Therefore, non-invasive techniques have been developed to evaluate microvascular morphology, particularly in the retina, since it is perhaps the most accessible microvasculature. Scanner laser Doppler flowmetry (SLDF) and adaptive optics (AO) represent the most promising approaches for the evaluation of morphological characteristics of retinal arterioles, in particular for the measurement of their wall-to-lumen ratio (W/L ratio). The possibility to evaluate microvascular morphology by non-invasive techniques represents a major clinical advancement, with possibly favorable implications in research and in stratification of cardiovascular risk. In this review we will address the different methods to investigate the microcirculation as well as their clinical usefulness.