Choroidal Neovascularization: OCT Angiography Findings
Choroidal neovascularization (CNV) is part of the spectrum of exudative age-related macular degeneration (AMD) that consists of an abnormal growth of vessels from the choroidal vasculature to the neurosensory retina through the Bruch's membrane. CNV can also develop in a number of other conditions such as myopic degeneration, chronic central serous chorioretinopathy, macular telangiectasia type 2, various white dot syndromes and other uveitic processes, and some choroidal tumors. Leakage of retinal edema and hemorrhage from CNV threatens visual acuity.
Etiology of CNV is multifactorial. Alterations in Bruch's membrane, migration of macrophages and production of vascular endothelium growth factor (VEGF), play an important role in the development of this disease.
The incidence and progression of AMD are related to age and genetic factors. With aging, the lysosomal activity for the degradation of external segments of photoreceptors decreases. This leads to subsequent accumulation of lipofuscin, which affects the normal function of the RPE. Another important risk factor for the development of CNV is the presence of large, confluent soft drusen.
Oxidative stress may play an important role in AMD. Several modifiable risk factors have been identified, including quitting smoking, dietary intake of omega-3 fatty acids, consuming vegetables and fruit with antioxidants including lutein and zeaxanthin, exercise, wearing sunglasses, and maintaining a healthy weight.
Alterations in the normal transport of metabolites, ions and water through Bruch's membrane in AMD, alter the nutrition and stability of retinal pigment epithelium (RPE) from choriocapillaris and the transport of waste out from the neurosensory retina. Hypoxia leads to VEGF being released by the RPE, which initiates a cascade of angiogenic responses at the level of the choroidal endothelium. Bruch´s membrane damage is required to allow the passage of abnormal neovascular vessels from the choroidal vasculature through the breaks in Bruch’s membrane to the retina. This impairment is part of the pathological course of AMD.
Histologically, neovascular membranes are classified into:
- Type 1 ("occult"), when the neovascular membrane is located below the RPE. Type 1 CNV demonstrates occult leakage on fluorescein angiography. Polypoidal choroidal vasculoplathy (PCV) is a subtype of Type 1 CNV that is characterized by the presence of polyp-like aneurysmal dilations of the branching vascular network.
- Type 2 ("classic"), passes through the RPE and is located above the RPE in the subretinal space. This is related to the angiographic classification of a classic CNV.
- Type 3 is defined as Retinal Angiomatous Proliferation (RAP), which corresponds to neovascularization that develops within the neurosensory retina an progresses posteriorly into the subretinal space.
In the presence of CNV, the patient experiences an acute decrease in visual acuity, relative scotoma, and/or metamorphopsia. The retinal examination shows a grayish macular lesion associated with subretinal fluid, cystoid macular edema, exudation, and/or hemorrhages.
En face OCT angiography (OCTA) is a new technology that has a great ability to show the retinal and choroidal microcirculation in detail without contrast medium or without invasive means. Instead, it uses motion contrast by comparing the decorrelation signal between repeated B-scans obtained at a given retinal cross-section to detect blood flow. It utilizes the principle that theoretically only circulating RBCs within the retinal vasculature should be moving/changing in the retina. OCTA is available on both spectral domain and swept source OCT devices. OCTA allows for three-dimensional analysis of the retinal and choroidal vasculature, and can be segmented to view each of the vascular plexuses individually. Each en face OCT angiogram is cross-registered with the corresponding OCT B-scans, an OCT thickness map, and a structural en face OCT, which allows for concurrent visualization of structure and blood flow.
For the purpose of visualizing changes in eyes with CNV or suspected CNV, a segmentation of the outer retina (extending from the outer plexiform layer to Bruch's membrane) and a segmentation of the choriocapillaris (approximately 20um thick region just below the RPE) are most useful. CNV can be seen as a seafan or coraliform neovascular complex within the outer retina, which is ordinarily devoid of blood flow in normal eyes. The PCV subtype of CNV can also be seen as a branching neovascular network within the outer retina but with concurrent aneurysmal dilations. After repeat pharmacologic intervention the large branches of the CNV become pruned and the smaller capillaries and any polyps may no longer be visualized (whether due to slow or absent flow, or complete regression). The choriocapillaris layer may demonstrate decreased flow or flow voids adjacent to the CNV complex. Additionally, choriocapillaris hypoperfusion may be seen underlying any areas of RPE atrophy.
Type 1 CNV is observed in OCTA as a neovascular complex between the RPE and Bruch's membrane, originating in the choroid. The type 2 CNV is visualized as a neovascular network that grows from the choroid vasculature and traverses the RPE-Bruch's membrane complex into the subretinal space. Type 3 CNV is clinically seen as tiny intra- and subretinal hemorrhages that correlate on OCTA as an intraretinal anastomosis originating in the deep capillary plexus of the retina.
Taking into account the numerous recent studies on the treatment of CNV in AMD, it has been shown that antiangiogenic therapy shows the best result both histologically with the regression of the neovascular lesion and functionally with improvement of the visual acuity. Although the treatment is the same for all types of CNV, it is important to differentiate them, since they do not all respond identically and some of them have a higher rate of recurrence.
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