## 1 Introduction  
Optical coherence tomography angiography (OCT-A) is a technique that arose in 2013 as a non-invasive diagnostic imaging technique, which can provide data from microvascular retinal flow in a few seconds [1]. To date, there are two gold-standard procedures that are used to study retinal and choroidal circulation: Fundus fluorescein angiography (FFA) [2; 3] and indocyanine green angiography (ICGA) [2]. Both procedures require the administration of an intravenous dye with the consequent risk of allergic reactions [2; 3]. They are time-consuming [3], and discrimination of the different layers of the retinal and choroidal microvasculature is limited due to the generation of two-dimensional images [4]. On the contrary, OCT-A provides images of the retinal vasculature in a few seconds without the need to use a dye, eliminating the risk of allergic reactions and patient discomfort [2]. In light of the foregoing,OCT-A could be an excellent ancillary test for the diagnosis and follow-up of several pediatric retinal and optic nerve head pathologies. OCT-A obtains images of the retinal vessels by comparing the signals of a series of consecutive B-scans performed at the same location [5]. The temporal evolution of the optical coherence tomography (OCT) signal, caused by the motion of scattering particles such as erythrocytes within vessels, allows the visualization of functional blood vessels [2; 5; 6; 7]. Furthermore, OCT-A allows a separate visualization of different capillary networks (inner retina, outer retina, and choriocapillaris) [4], which are allocated to the superficial or deep retinal slabs depending on the segmentation method and retinal layer designation used by each device [2]. Moreover, in the inner retinal vascular plexus two different plexuses can be recognized: A superficial and a deep vascular plexus [2]. The former would supply the retinal nerve fiber and ganglion cell layers and the latter would supply the inner plexiform, inner nuclear, and outer plexiform layers [2; 8; 9]. Furthermore, OCT-A is able to examine the size and the contour of the foveal avascular zone (FAZ) [10; 11]. The FAZ is an avascular area that results from the absence of terminal capillaries of both superficial and deep plexuses [8]. The FAZ is considered to be of special relevance for the appropriate development of the fovea and it is thought to remain avascular throughout ocular development [12], being already visible at the 25 th week of gestational age. In fact, the process of foveal formation begins at the 12 th week and continues its development until the individual is 13-16 years old [4]. Such development of the fovea is a sequential process in which several stages can be distinguished. Some of the most relevant stages are: Centrifugal displacement of the inner retinal layers, centripetal displacement of the photoreceptors, and specialization of the cones [8; 12; 13; 14; 15; 16]. The aim of these changes in the foveal area, in which the FAZ is involved, is to facilitate light transmission to the photoreceptors and avoid as much as possible any potential interference [17; 18]. In this regard, some studies have shown that the size and morphology of the FAZ correlates with the visual acuity in some vascular retinal pathologies [19; 20; 21]. Another advantage of OCT-A is its ability to provide quantitative information [22; 23]. Therefore, it can be very useful to describe the status of the retinal vasculature and help during the follow-up of retinal and optic nerve disorders in which microcirculation is affected. Several quantitative OCT-A parameters have been reported in previous studies such as: FAZ size, vessel density, area of non-perfusion, blood flow index, skeletonized vessel density or vessel length density, and fractal dimension [22; 23]. OCT-A has also been shown to have good reproducibility between different operators, as well as good reliability and sensitivity both in the study of healthy subjects and in the study of individuals who have different retinal and/or optic nerve pathologies [1; 24].