# Clinical Applications of Optical Coherence Angiography Imaging in Ocular Vascular Diseases
## 4 Retinal Artery Occlusion
### Role of OCTA in Central Retinal Artery Occlusion  
OCTA technology provides us with a means to assess the microvascular structures across all retinal layers and has a potential role in diagnosing, assessing the extent of macular ischemia, and monitoring disease progression in CRAO [64; 69; 14]. Currently, there are only a few studies on the role of OCTA in CRAO.  
[PERSON] et al. found that areas of acute retinal ischemia appeared as areas of non-perfusion in the SCP, DCP, outer retina, and choriocapillaris layers. This was accompanied by a loss of capillary density, which was most marked in the SCP and DCP. Capillary flow was found to be below the limit of detection, but arteriole flow remained detectable. In contrast, well-perfused areas had larger retinal arteries and veins in the superficial plexus. They also showed that the choriocapillaris demonstrated reduced signal intensity in the area of occlusion that correlated to areas of inner retina edema and hyperreflectivity. Findings in the outer retinal were confounded by the altered flow projections from the inner retina [67].  
[PERSON] et al. reported on four cases of RAO in children using OCT and OCTA. In the acute stage, OCTA images showed ischemic changes in the SCP and DCP. OCTA images on follow-up revealed increasing areas of ischemia that appeared as a darker, smooth background, persistent narrowing of arteries, and loss of the capillary network (Figure 7) [70].  
[PERSON] et al. conducted a case study of 7 patients with acute or chronic CRAO and BRAO. They found that OCTA showed decreased vascular perfusion in both the SCP and DCP that corresponded to areas of delayed perfusion on FA. In patients with acute and chronic CRAO, they found equal areas of decreased vascular perfusion in the SCP and DCP (Figure 7B,C). However, in a patient with chronic CRAO with clioretial sparing there was focal restoration of the deep capillary plexus perfusion in the macula region. 75% of their cases of acute and chronic BRAO showed focal restoration of the deep capillary plexus perfusion where the superficial capillary plexus perfusion is abnormal. Hence, with OCTA imaging, which allows the ability to differentiate between the superficial and deep vascular networks, it is now possible to image in subacute RAO cases, recanalization and reorganization of vascular interconnections that may contribute to partial restoration of the deep capillary plexus in areas where the superficial capillary plexus is still abnormal. A limitation of this study is that they did not segregate acute and chronic patients when analyzing OCTA findings. In addition, this study did not correlate re-perfusion with visual function [63].  
OCTA is limited in retina ischemia as it only allows the evaluation of a snapshot of blood flow over time. Artefacts from the inner retina layers in acute ischemia also affect visibility of the outer retina [67]. The role of OCTA in chronic RAO is also limited due to difficulty with auto-segmenting layers of atrophic and thinned retina accurately [63]. In many cases, recanalization of the large vessels may occur and abnormal areas of flow may not be clearly evident in the presence of the thinned inner retina and OCTA images of cases with chronic RAO may even appear normal [71].