Retinal venous occlusive disease (RVO) is characterized by vascular changes, which include dilation and engorgement of central or branch retinal veins, retinal hemorrhages, accumulation of intraretinal or subretinal fluid, and varying degrees of retinal ischemia. If the vascular changes affect the central macular area, RVO causes varying degrees of vision loss. Fundus fluorescein angiography has been the gold standard imaging technique to assess changes in retinal vasculature in RVO; however, it is an invasive technique and it lacks information about deeper vascular network. Optical coherence tomography (OCT) is a useful and widely popular imaging tool that provides high-resolution cross-sectional scans of the retina. OCT angiography (OCTA) has recently emerged as a novel noninvasive imaging modality that can generate a full map of the retinal vasculature that allows accurate evaluation of retinal vasculature in RVO. Recent studies have shown that OCTA is an excellent imaging modality to show vascular abnormalities associated with RVO including enlargement and disruption of the foveal avascular zone (FAZ), macular edema, microaneurysm formation, vascular looping, formation of venous–venous collaterals, capillary nonperfusion, and abnormal retinal neovascularization. Owing to its novelty, OCTA is not yet widely available in retina practices. Despite its limitations, such as small field of view, OCTA still offers numerous advantages over conventional fluorescein angiography. In addition, it is the first noninvasive imaging modality that can visualize the deep vascular network.
Keywords: optical coherence tomography, angiography, retinal vein occlusion, superficial vascular network, deep vascular network
Retinal venous occlusive disease (RVO) is a vascular disorder that features dilation and engorgement of central or branch retinal veins, retinal hemorrhages, accumulation of intraretinal or subretinal fluid, and varying degrees of retinal ischemia. RVO is a very common cause of retinovascular vision loss, ranking second only to diabetic retinopathy.1,2 Several population-based studies reported variable estimates of RVO prevalence that range from 0.3 to 2.3% in different ethnic groups.3,4,5,6
Significant vision loss in RVO typically occurs due to macular edema, macular ischemic changes, or as a sequela of complications related to retinal neovascularization. Fundus fluorescein angiography (FFA) is currently the gold standard imaging modality to assess retinal vasculature. FFA allows evaluation of vascular abnormalities in RVO such as retinal perfusion status, retinal neovascularization, delayed vascular filling, and intraretinal fluid leakage secondary to abnormal vascular permeability.7,8 In the less severe nonischemic form of RVO, FFA shows staining along retinal veins, microaneurysms, dilated optic nerve head capillaries, and, occasionally, minimal areas of capillary nonperfusion. In the more severe ischemic form of RVO, FFA often shows marked hypofluorescence indicative of either capillary nonperfusion or blockage from diffuse retinal hemorrhages.9 Intravenous fluorescein angiography, however, is invasive and involves intravenous injection of fluorescent chemical dye with the risk of associated adverse events such as nausea, vomiting, and, uncommonly, anaphylaxis. In addition, the detection and evaluation of deeper layers of retinal vasculature is not possible with FFA due to blockage by the fluorescence of the superficial vasculature.7
Optical coherence tomography (OCT) is a noninvasive imaging technique that provides high-resolution structural tomographic images of the multiple layers of the retina.10 Optical coherence tomography angiography (OCTA) is a recent advance that compares the decorrelation (change) in the intensity of the OCT signal and/or phase variance of reflected light waves between successive cross-sectional B-scans to detect erythrocyte motion to construct high-resolution three-dimensional en face angiograms of chorioretinal vasculature. Generated angiograms can also be correlated with corresponding high-resolution OCT B-scans for better and more comprehensive assessment.11 The authors have experience with the commercially available device Optovue XR Avanti with AngioVue software (Optovue, Fremont, CA). With this system, en face areas of 2 × 2, 3 × 3, 6 × 6, and 8 × 8 mm can be acquired. Since the same number of B-scans is acquired regardless of the scanned area, the larger the field of view, the lower the scan resolution. The 3 × 3 mm scans provide higher resolution compared to FFA without the risk of systemic complications.
9.2 Evaluation of the Fovea Avascular Zone
Physiologically, the fovea avascular zone (FAZ) is a capillary-free circular zone, approximately 450 to 600 µm in diameter at the center of the fovea surrounded by a ring of retinal capillaries.12 In RVO, different morphological alterations take place in the FAZ especially in ischemic disease, which is characterized by disruption and enlargement of the FAZ ring. Enlargement of the FAZ indicates foveomacular ischemia.13 Vessels close to the FAZ ring may also show vascular attenuation, tortuosity, looping, or microaneurysm formation ( ▶ Fig. 9.1a–e). The high resolution of OCTA scanning makes it more likely to detect FAZ changes and allows it to more clearly delineate microvascular malformations, which could be obscured by dye leakage on FFA.14
Fig. 9.1 Details of the foveal avascular zone (FAZ) in a 55-year-old female patient with a history of superotemporal branch retinal vein occlusion in her right eye for 18 months. (a) Color fundus photography. (b) Venous phase fluorescein angiography. (c) Magnified fluorescein angiography image of the yellow square in (b); optical coherence tomography angiography at the level of the (d) superficial and (e) deep vascular networks showing enlargement of the FAZ (blue dashed line) and disruption of its normal contour. (d) and (e) also show nonperfused areas (red asterisks), vascular loops/tortuosity, and microaneurysms (yellow circles).
OCTA is the first imaging modality that enables noninvasive evaluation of the deep vascular plexus (DVP). There is a growing body of scientific evidence to support the belief that FAZ changes in RVO, especially disruption and enlargement of the FAZ ring, are often more pronounced at the level of the DVP.15 This finding is obscured in FFA and can only be appreciated by OCTA. A recent clinical study showed statistically significant enlargement of FAZ maximum diameter measured by OCTA in eyes with RVO compared to normal healthy eyes (p < 0.008). Moreover, the same study showed strong correlation between best corrected visual acuity (BCVA) and FAZ maximum area measured by OCTA at the level of the DVP, hence highlighting the increasing importance of OCTA in evaluating the DVP ( ▶ Fig. 9.1c–e).16
9.3 Macular Edema
Macular edema is a common feature in RVO that may be present in both ischemic and non-ischemic forms of the disease. It represents the most common cause of vision loss in both branch and central RVO. The majority of patients with CRVO develop macular edema, whereas the incidence of macular edema is 5 to 15% in patients with BRVO.17,18
Conventional FFA demonstrates macular edema in the form of cystoid spaces or dye leakage in the mid to late phases of the angiogram. OCT, however, is more useful and more commonly used in quantifying and monitoring macular edema in patients with RVO, and is critical to the decision-making process in whether to treat or observe.19 OCTA has the advantage of cross-registration of the en face scans of superficial vascular plexus (SVP) and DVP with the cross-sectional high-resolution OCT B-scans. Cystoid spaces of macular edema can be seen with OCT-A at the level of superficial or deep vascular networks as defined rounded or oblong areas with smooth borders that lack OCT signals and correspond well to the intraretinal cysts on OCT B-scan ( ▶ Fig. 9.2a–c).
Fig. 9.2 Macular edema in a case of central retinal vein occlusion. (a) Cross-sectional optical coherence tomography (OCT) showing hyporeflective areas that represent intraretinal cysts. (b) OCT en face and (c) OCT angiography at the level of the deep vascular network showing lack of OCT signal in well-defined rounded or oblong areas with smooth borders corresponding to the intraretinal cysts (blue dashed circle).