## 2 Retinal Vascular Diseases
### Age-Related Macular Degeneration (AMD)
In Figure 4, an OCTA of an AMD patient showing abnormal vessels in the choriocapillaris layer and the choroid is shown. The sign of activity of CNV on FA is the leakage existence. OCTA is unable to evaluate the existence of leakage or exudation, but it can potentially differentiate active from inactive CNV by changes in the pattern of the vasculature. Active CNV appears on OCTA as a branching and well-defined anastomosing tangle of vessels surrounded by dark halo corresponding to blood, exudation or sub-retinal fibrosis. Inactive CNV appears on OCTA as large mature vessels without anastomosis [83]. These distinctions are still not entirely clear cut, and further study is ongoing. OCTA is considered to be better than FA in identifying neovascular membranes due to the absence of vascular obscuration by leakage from new vessels, which is typically seen on FA [84]. OCTA could detect CNV type 1 in 67-100% of cases in comparison with FA [85]. SS-OCTA (wavelength 1050 nm) is better than SD-OCTA (wavelength 840 nm) in visualization of CNV because of the better penetration of the former into the choroid [86]. Cases of dense subretinal hemorrhage secondary to CNV present a distinct challenge for OCTA, as well as often FA. These hemorrhages block penetration of light, and thus the underlying neovascular membranes and choriocapillaris cannot be imaged in these circumstances. Type 1 CNV appears on the en-face OCTA as a well-defined entanglement of blood vessels while it appears on cross-sectional OCTA as intrinsic flow in sub-RPE space [87], while type 2 CNV appears on cross-sectional OCTA as hyper-reflective material with the intrinsic flow in the sub-retinal space with thicker main vessel branch that is connected to the choroid. The neovascular membrane is surrounded by a dark halo corresponding to blood, exudation or sub-retinal fibrosis [88]. Mixed CNV type 1 and type 2 lesions appear on cross-sectional OCTA as abnormal intrinsic flow above and below the RPE. Mixed CNV type 1 and type 2 lesions can be visualized on en-face OCTA by varying the segmentation depth, so the component of each type can be visualized on different slabs [89]. Type 3 neovascularization appears on the en-face OCTA as a tangle of blood vessels originating from the DCP in the outer retinal slab while, it appears on the cross-sectional OCTA as a hyper-reflective vascular structure with intrinsic flow extending from the outer retina slab into the inner retina slab [88]. [PERSON] et al. [90] differentiated types of CNV using OCTA. Authors found that CNV type 1 appears larger and less demarcated with predominant visibility on the choriocapillaris slab while CNV type 2 appears smaller and better demarcated with more visibility towards the outer retina slab. Quantifying the neovascular networks extent in CNV is very important for its well-known prognostic value [91]. The study conducted by [PERSON] et al. [92] has held a comparison between the CNV type 1 size that is produced using OCTA and ICGA. Results showed that OCTA and ICGA are comparable, as they could provide information on CNV type 1 both quantitatively and qualitatively with good reproducibility and inter-user agreement. This conforms with the results of [PERSON] et al. [93] and [PERSON] et al. [9]. A potential risk exists for CNV developing in eyes of individuals who have unilateral CNV. This risk varied from 6% to 42% in different studies ([PERSON] et al. [79], [PERSON] et al. [94], [PERSON] et al. [95], [PERSON] et al. [93], [PERSON] et al. [7], [PERSON] et al. [96] [PERSON] et al. [97]). In these studies, authors explained the various factors that affect the subclinical CNV prevalence such as the duration and severity of AMD. The subclinical CNV in fellow eyes in the first year has its prevalence more than doubled as reported in the second-year follow-up [98]. [PERSON] et al. [82] studied the response of CNV membranes to anti-VEGF using OCTA. They showed that after treatment, the size and the density of vessels of the neovascular membrane were significantly reduced.