Marco Rispoli, MD and Bruno Lumbroso, MD
Retinal vein occlusion (RVO) is the second most common retinal vascular disorder, after diabetic retinopathy, and is considered an important cause of visual loss. It has an acute origin secondary to thrombotic events, external compression, or vessel wall pathology. Some authors have hypothesized that branch retinal vein occlusions (BRVOs) may be the result of multiple interconnected events: compression of veins on arterovenous crossings, degenerative changes within venous walls, and hypercoagulability.1–4
Occlusion of the major veins of the retinal circulation leads to increased intraluminal pressure, hemorrhages, and edema. Macular edema and nonperfused areas induce visual acuity decrease in RVOs.5
Traditional fluorescein angiography (FA) shows enlargement of intercapillary spaces in the retinal obstructed areas, but is unable to detect single layer alterations.6 Optical coherence tomography angiography (OCTA) is able to enhance separately both superficial and deep vascular plexa.7
Figure 23-1 shows the OCTA capability to enhance separately full thickness retinal vasculature (A) between the inner limiting membrane and the Bruch’s membrane, the superficial vascular plexus (B), and the deep vascular plexus (C). (D) shows the detail of (B) and (C) segmented areas on the cross-section structural scan.
In eyes affected by RVO, OCTA shows vascular network anomalies, such as nonperfused areas (capillary dropout), that correspond to the same areas shown by traditional FA. On OCTA these areas are very sharpened because there are no masking effects due to late dye leakage typical in FA.8–10 Vessel shape observed in RVO has an inhomogeneous aspect, with some vessels increasing in size and others thinning.11,12
RVO area in OCTA shows a grayish background color with irregular network and texture.13 In OCTA vascular details are observable better than in FA as arterovenous anastomosis and vascular loops.
The vascular texture can show variable aspects from fine to granular. Frequently the capillaries’ nonperfusion areas (capillary dropout) are trunks, with sudden interruptions, or arterovenous anastomosis. The anastomoses in these areas are established deep in the vascular capillary network layer inside the inner nuclear layer.
In FA the areas of retinal edema are not easily detectable because there is impregnation by the dye. In OCTA an enlargement and distortion of the capillary network interconnections can be observed in combination with a decrease in the sharpness of dilated capillaries. In cases of cystoid edema a real displacement of the capillaries is observed. This feature is more evident at the level of the deep vascular plexus. The cystoid edema in FA shows the vascular wall impregnated with coloring (impregnation/staining), while the OCTA reveals a very thin stream (which is the same lumen) surrounded by a dark area that may correspond to the thickened vascular wall. In conclusion, in cystoid edema there is a different visualization between FA and OCTA: OCTA allows the visualization of the blood system without a dye-masking effect (staining and leakage) as observed in FA. Figure 23-2 shows the different visualizations of macular edema by FA (A) and OCTA (B) and (C).
Figure 23-1. Normal eye vascular segmentation. (A) Full-thickness OCTA (white). All capillary layers are overlapped. (B) Superficial capillary plexus (purple). (C) Deep vascular plexus (green). (D) Cross-sectional OCT showing capillary plexa segmentation areas. The superficial capillary plexus is shown in purple; the deep capillary plexus is shown in green.
Figure 23-2. (A) Traditional FA shows a juxtafoveal edematous area (yellow circle) with some microaneurysms. There is evidence of cystic cavities pooled by dye and juxtafoveal staining that blocks visualization of the capillary network. (B) OCTA of the superficial capillary plexus (purple) delineates the vascular texture in the edematous area. Texture is enlarged and the vessel size is irregular. (C) OCTA of the deep capillary plexus (green) shows the vascular anomalies down the edematous area. Some capillaries appear congested with focal aneurysmatic dilations (red arrow).
Retinal hemorrhages in OCTA are detectable as masked areas but are less evident than in FA.
In the ischemic area the color of the basal texture can change from light gray to a grayish granulation.
In vascular occlusions it is possible to observe changes in the structure of the superficial complex especially in the condition of macular ischemia. In these cases the vessel silhouette shows focal deviations, the wall thickness is discontinuous with focal segmentation and lumen restriction.14 The course of the vessel shows sudden interruptions with terminal enlargement around the foveal avascular zone (FAZ) that appears enlarged compared to that of healthy individuals. Flows appear segmented along the vessel occlusion area.
The reason for evaluating the superficial and deep networks separately is the potentially differential involvement of the 2 layers in some pathologies.15,16 Traditional FA does not visualize these features satisfactorily. According to Ishibazawa et al16 and Kuehlewein et al,14 the nonperfused area is well defined by OCTA. They reported the ischemic macular area identification in diabetic macular retinopathy. We can recognize 4 different OCTA features in RVO:
- FAZ enlargement
- Capillary nonperfusion (CNP) occurrence
- Microvascular abnormality appearance
- Vascular congestion signs (VCs)
FAZ enlargement is present in both vascular networks, while the CNP seems to appear mainly in the superficial capillary network. In the microvascular abnormalities there are several vessel aspects such as multiple intraretinal loops in the ischemic area, mainly on the superficial vascular network.
Figure 23-3. Capillary alteration in a case of RVO studied by OCTA 6 × 6 mm square. The purple image on the left indicates the superficial vascular plexus while the green image on the right indicates the deep vascular plexus. The red arrow indicates a capillary pathological anastomosis. The blue arrow indicates the capillary displacement toward the septa between cystic cavities. The yellow arrow indicates vascular congestion of the venous bed in the occluded area.
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