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Flow Characteristics in Retinal Vasculitis Using Optical Coherence Tomography Angiography

Phoebe Lin, MD, PhD; Miao Zhang, PhD; and Liang Liu, MD

Retinal vasculitis is a sight-threatening type of intraocular inflammation affecting the retinal vessels. It may occur as an isolated idiopathic condition, as a manifestation of infectious or neoplastic disorders, or in association with systemic inflammatory disease or other ocular inflammatory disease entities. On a cellular level, retinal vascular inflammation is associated with the slowing of intravascular leukocyte travel as they adhere to the endothelial cells, followed by extravasation of leukocytes into the extravascular space.¹ Retinal vasculitis can lead to complications including cystoid macular edema (CME), retinal ischemia, and neovascularization. Clinical exam typically reveals perivascular sheathing, intraretinal hemorrhage, cotton-wool spots, or vascular occlusion.^2,3 Fluorescein angiography (FA) is currently one of the main tools for evaluating the activity of retinal vasculitis by demonstrating leakage of the dye due to breakdown of the blood-retinal barrier at the retinal vasculature, including staining of or leakage from the vessel wall, vascular occlusion, areas of capillary dropout, and neovascularization in severe cases. FA, which involves intravenous injection of fluorescein dye, can have side effects. For instance, between 1% and 10% of patients will develop nausea and/or vomiting, but more serious adverse effects can occur, including urticaria, syncope, and very rarely, death.⁴

Optical coherence tomography angiography (OCTA) is a noninvasive imaging technique that uses the contrast between static and nonstatic tissue to acquire 3-dimensional volumetric angiographic information.^5,6 In retinal vasculitis, OCTA has some advantages over FA. First, after segmentation, en face images can be generated from different slabs. The vascular plexus of the inner retina, outer retina, and choriocapillaris can be visualized individually and flow characteristics extrapolated from these different vascular beds, thus potentially allowing one to better evaluate retinal vasculitis independently of choroiditis with the same imaging technique. While OCTA cannot provide vascular leakage information as FA does, one disadvantage of FA is that it provides two-dimensional image sets from which it can sometimes be difficult to distinguish from which vascular bed there is leakage or staining. Second, OCTA provides increased resolution of retinal vessel detail over FA.⁷ The macular vessel pattern and foveal avascular zone (FAZ) vessels can be captured clearly even in the presence of CME. In FA, capillary details and nonperfused areas can sometimes be obscured by adjacent leakage. Third, and perhaps most important, OCTA can quantitate certain aspects of the retinal microcirculation such as vessel density, flow indices, and areas of nonperfusion, in addition to the structural elements of OCT that provide highly reproducible measurements of retinal thickness important in the evaluation of CME. The additional blood flow metrics from OCTA complement the structural measurements and can be useful in assessing progression of disease or potentially response to treatment. Finally, OCTA requires less capture time than FA, and does not confer the risks and side effects associated with fluorescein injection.

Case 1 is a 42-year-old White woman with a history of systemic lupus erythematosus-associated retinal vasculitis and choroiditis who was unable to tolerate multiple types of immunosuppression, including oral corticosteroids, because of a history of severe steroid-induced multifocal central serous chorioretinopathy. She presented with photopsias and blurry vision despite low-dose azathioprine usage. She was 20/40 in the right eye and 20/30 in the left eye. The FA images (Figures 27-1A and 27-1B) showed diffuse retinal vascular leakage and macular leakage. There was also diffuse deep peripapillary leakage through the retinal pigment epithelium (RPE), possibly from a choroidal source. In the 6 × 6 mm macular region, the FA fails to demonstrate vascular detail whereas in the same region, the en face OCTA generated from the retinal vessels within the area bordered by the internal limiting membrane (ILM) to the outer border of the inner plexiform layer (IPL) (Figure 27-1C) demonstrated an attenuated vascular network in the vasculitis eye compared with the normal eye (Figure 27-1G). The fundus image in this location is shown in Figure 27-1D. To quantify the macular microcirculation, we use a fixed-threshold method to calculate the vessel density. The vessel density, defined as the percentage area occupied by the large vessels and microvasculature, was calculated from the 6 × 6 mm region, excluding the FAZ. The vessel density in this vasculitis eye was 80%, which is lower than the normal age-similar control (93%). The structural cross-sectional image (Figure 27-1E) demonstrates the presence of CME in the middle layers of the retina. When segmenting and quantifying just from the inner layer, the vessel density remains lower than normal, excluding any potential signal perturbations caused by presence of intraretinal fluid. The structural image with flow signal (Figure 27-1F) shows blood flow in individual layers of the retina.⁷ Flow index, which is derived from the split-spectrum amplitude-decorrelation angiography value in capillaries, as well as the area of the large vessels on the en face angiogram,⁵ was also reduced in this patient compared to a normal individual (0.056 vs 0.072, respectively). After enhanced treatment of her retinal vasculitis with cyclophosphamide, this patient had slightly improved leakage on FA (Figures 27-2A, 27-2B, 27-2D, and 27-2E), which was difficult to quantitate. The 3 × 3 mm OCTA (Figures 27-2C and 27-2F) demonstrated the vessel pattern around the fovea more clearly than FA. Also, the response to treatment could be quantified as macular vessel density, which increased from 76% to 83%. This improvement was greater than what we expect from variability in scanning and quantification in retinal vasculitis patients. When we investigated 7 eyes of 5 patients with retinal vasculitis, we found that the average macular vessel density of retinal vasculitis patients was significantly lower than age-matched normal individuals (P = 0.006).⁸

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