## 2 Retinal Vascular Diseases
### Retinal Artery Occlusion (RAO)  
During the RAO acute phase, the capillary flow collapse results in intracellular edema, axoplasmic stasis, and retinal inner layers ischemic necrosis [6]. Usually, edema resolution is accompanied by the recanalization and reperfusion of the the obstructed retinal artery over the course of several weeks, but the infarction of the inner retina results in profound vision loss. Ultimately, the retina is left atrophic and with persistent vessel narrowing [29].  
The retinal arterioles, which are arranged in two layers that are morphologically distinct, have a vital role in autoregulating blood flow in the retina. The superficial retinal capillary plexus exists at the nerve fiber layer close to the disk (radial parapapillary capillaries); however, it is present predominantly inside the ganglion cell layer in the central macular region. The DCP consists of intermediate and deep plexuses existing at inner and outer planes of the inner nuclear layer, respectively. Perpendicularly oriented vessels connect these capillary layers, which could be affected disproportionately by the ischemic vascular disease. Capillary plexus ischemia in RAO might occur in one or more inner layers of the retina resulting in ischemic edema, axoplasmic stasis and even death of the cells in the nonperfused region. These vascular layers could be affected differently by the severity of intracellular edema (there is capillary leakage or edema on OCT in RAOs), and the depth and area of capillary nonperfusion of the retina. In addition, the visual prognosis could be affected by the breadth of retinal capillary ischemia [139].  
Branch retinal artery occlusion (BRAO) usually develops during the sixth or seventh decade of life as unilateral, painless vision loss in the visual field corresponding to the territory of the obstructed artery [140]. Montage OCTA is a technique that unites OCTA images of adjacent retinal regions to provide a wide field of view while preserving the microvascular details [6]. [PERSON] used montage OCTA to evaluate a case with BRAO. Montage of OCTA comprise OCT angiograms segmented between Bruch's membrane and the inner limiting membrane (ILM). This uncovered severe decreased flow distal to the obstruction and a very distinct border of capillary non-perfusion. Capillary non-perfusion in the retina could be easily discerned in OCTA more than in FA due to the OCTA absence of choroidal flush [141].  
[PERSON] et al. [142] studied the retina's microvasculature in nonarteritic RAO eyes using OCTA and FA. They found that OCTA imaging observed a vascular perfusion decrease in deep and superficial retinal capillary plexus, which corresponded to the delayed perfusion areas in FA imaging. They also revealed that it is sensitive for the characterization of the extent of macular ischemia and for monitoring changes in the vascular flow within the course of the disease.  
Cases with central retinal artery occlusion (CRAO) are at a higher risk of ocular neovascularization, so there is a recommendation for the retinal vasculature follow-up [143]. This is one of the important values of OCTA in follow up of cases with CRAO.