Role of OCT Angiography OCTA in the Diagnosis of Macular Diseases

*Sherin Sadek and Ragai Hatata*

### **Abstract**

OCT becomes an indispensable tool in everyday practice. OCTA is the functional extension that provides cross-sectional information on retinal and choroidal circulations without dye injection. It allows visualization of abnormal flow in areas with no flow and abnormal vessels (like CNVM). In ARMD, it can detect active membranes before being leaky in FFA. In diabetic retinopathy, OCTA can diagnose abnormal areas of non-perfusion in the superficial plexus, deeper capillary structures, or neovascularization. OCTA can detect focal dilation and foveal capillaries alterations in macular telangiectasia. It is useful in the diagnosis of inherited retinal diseases such as retinitis pigmentosa. OCTA has many challenges including longer acquisition times and motion artifacts. Longer wavelength SS-OCT may provide a solution for imaging through media opacities and a wider field of view. OCTA does not give full details about the retinal periphery, also, it gives no information about blood-retinal barrier (no dye to leak); an important sign in many retinal diseases.

**Keywords:** OCTA, macular diseases, retinal circulation, choroidal circulation, CNVM, ARMD

## **1. Introduction**

With the introduction of OCTA, a new era of technological applications in the field of retinal pathologies has been opened. OCTA is a safe, fast imaging tool that allows better delineation of the retinal microvascular and choroidal vascular abnormalities without the risks of dye injection and morbidity hazards. It can show both blood flow and structural changes within the macular area. OCTA also helps to quantify vascular impairment according to the severity of the retinopathy. It is a useful modality for better understanding the real pathology of retinal diseases especially the retinal vascular occlusions, pathological myopia, inherited retinal disorders, and age-related macular degeneration, that opened the way for evaluating the effect of different treatment modalities, and monitoring of disease progression [1]. OCTA, in order to construct a blood flow, compares the differences in the backscattered signal intensity (decorrelation signal) between sequential OCT b-scans taken at a fixed point at a time (representing erythrocyte movement in retinal blood vessels). Incorporation of the split-spectrum amplitude-decorrelation angiography (SSADA) algorithm in flow detection, improves the signal-to-noise ratio. Although OCTA is a rapid three-dimensional scan with many

advantages, it has also some limitations including the limited normative database, small field of view, more prone to image artifacts, obscuration by hemorrhage or fluid and the inability to show leakage (FA will remain the gold standard in this) [2]. However, FFA cannot separately visualize the major capillary networks; (superficial retinal, deep retinal, and choriocapillaris) or radial peripapillary network, with the possibility of systemic side effects and allergic reaction to the injected dye. OCTA vascular changes may be affected by the axial length and individuals' systemic vascular risks [3].

NB: A detailed description of OCTA principles was mentioned in the first chapter "OCT from the Past to the Future."

### **2. OCTA in age-related macular degeneration**

Age-related macular degeneration (AMD) is the commonest cause of irreversible visual loss in the elder age group (above 65 years old and more). AMD is classified into two clinically distinct types, that is dry (non-neovascular) AMD and wet (neovascular) AMD. In the former, the clinical hallmark is *drusen* which are yellowishwhite deposits within the RPE-Bruch's membrane complex believed to be secondary to metabolic RPE dysfunction as well as impaired conductance of Bruch's membrane. In the clinical course of the disease, dry AMD can either progress to advanced dry AMD with geographic macular atrophy or to wet AMD with the development of choroidal neovascularization which can be sub-RPE (type I), subretinal type II) or intraretinal (type III CNV; also called retinal angiomatous proliferation (RAP).

Different CNV-vessel patterns based on OCTA according to Sulzbacher et al. were assessed. The loose-net (LN) presented as large diameter vessels, well-defined and discernible with a low branching index showing no capillary sprouting. The densenet (DN) appeared as a hyperreflective vascular net with dense capillary branching. Lesions with a ratio of approximately 50% of areas with large vessel diameter/low branching index and approximately 50% of areas with dense capillary branching were identified as the mixed type. Unidentifiable CNV pattern term was used when no neovascular vessels were detectable (neither in the choriocapillaris CC nor in the outer retina). Darwish classified AMD (according to OCTA lesions) into 2 patterns; pattern I requiring treatment and pattern II not requiring treatment. Pattern I showed all or at least three of the following five features; a well-defined CNV (tortuous lacy-wheel shaped), branching pattern (numerous tiny capillaries), presence of anastomoses and loops, the morphology of the vessel terminals (presence of a peripheral arcade) and perilesional hypo-intense halo. Coscas et al. provided these criteria as a basis for analysis and evaluation of CNV activity and the degree of CNV proliferation, persistence and/or recurrence; conversely, they provide for the stabilization and healing of vessels that become mature or quiescent. While a CNV lesion was considered as pattern II if it showed less than three of the previously reported OCTA features [4–6].

In AMD, OCTA has the advantage of dual appraisal of structural RPE and photoreceptor changes as well as vascular changes of choriocapillaris either choriocapillaris loss in dry AMD or the advent of choroidal neovascular membrane which is the defining feature of wet AMD. The longitudinal correlation between outer retinal and RPE structural changes and choriocapillaris vascular alterations in the clinical context of AMD constitutes the basis for OCTA utility in AMD which serves not only diagnosis but also follow-up and appraisal of response to treatment, for example anti-VEGF (**Figures 1** and **2**). However, OCTA choriocapillaris slabs must be assessed cautiously because they are liable to projection and masking artifacts of overlying structures given

*Role of OCT Angiography OCTA in the Diagnosis of Macular Diseases DOI: http://dx.doi.org/10.5772/intechopen.111673*

#### **Figure 1.**

*A: Active type 2 CNVM lesion imaged by Optovue Angiovue OCTA.* **A:** *At the choriocapillaris level, there is a densely packed vascular net formed of loops, and peripheral anastomoses and surrounded by a hypointense halo. The lesion is seen invading the RPE. The B-scan showed accumulated subretinal fluid.* **B:** *After injection, the CNVM shows inactivity. Large mature vessels are seen in a "dead tree" appearance showing no peripheral anastomosis or loops. No fluid accumulation is visible on SD-OCT with macular thinning (atrophy).*

#### **Figure 2.**

*A: Active type 2 CNVM lesion imaged by Optovue Angiovue OCTA,* **A:** *With a mixed net configuration at choriocapillaris showing minimal activity and increased central thickness in B-scan.* **B:** *After repeated injections, the vascular tree in OCTA gets mature with decreased CMT in SD-OCT scan.*

their deep anatomical location. Therefore, OCTA choriocapillaris slabs must always be correlated with the structural en-face images. OCTA can also show the early subclinical CNV before the signs of activity in the conventional FFA or SD-OCT [7, 8].
