Fig. 8.1
Display OCT angiography of the iris. (a) Reveals the iris microvasculature in patient with active anterior uveitis. The anatomy of the iris on OCTA reveals densely packed radial small vessels toward the center of the pupil and irregular less densely packed vessels toward the iris root. A single branch of a long posterior ciliary artery (LPCA) is visible. Major arterial circles are observed around the iris root and the ciliary body from which small arteries branch out and extend toward the pupillary margin. In the pupillary margin, the lesser iris circles with corn-shaped arcades are clearly visible, bridging adjacent vascular networks. At the root of the iris, a hyperreflective band runs almost continuously, representing the limbal circulation. Anterior to that the ciliary circulation is obscured due to the fact that the vessels dive deep toward the ciliary bodies thus being below the focal plane in our imaging study. (b and c) Display OCTA of the iris of the same patient with active disease (b) and inactive disease (c). Three vessels are identified [1–3] and compared between images. Note the decrease in intensity of the vessels (c) with reduction of inflammation. Color heat maps of relative flow density reveal a reduction in flow in the iris with improvement in inflammation. Flow density measurements made by native software reveal reduction of flow in the central circle (termed parafoveal in table) between active (b) and inactive disease (c)
Split-Spectrum Amplitude OCT of the Superficial Capillary Plexus in Uveitis
Fluorescein angiography (FA) has long been the best method for assessing microvascular details but requires acquisition of early-phase images to allow visualization of the capillary bed before dye leakage conceals its visibility (Fig. 8.2). Several reasons may hinder the visualization of the capillary network on FA in uveitis: missing of the early frames, problem of focusing in case of large macular edema, media opacities (cataract), and dye leakage from the capillaries. OCT angiography provides fine microvascular detail, ideal for the evaluation of subtle retinal vascular changes, and eliminates the problem of dye leakage as no dye is utilized. Birdshot chorioretinopathy is an excellent example of how OCTA can objectively grade and linearly assess microvascular status better than FA. OCTA images of the superficial capillary plexus provide a more precise delineation of the foveal avascular zone and its enlargement in birdshot chorioretinopathy than does FA (Fig. 8.2). OCT angiograms of birdshot chorioretinopathy also demonstrate an increased intercapillary space throughout the posterior pole (Fig. 8.2a, b). It is intriguing to speculate whether these changes cause a relative ischemia that could lead to decrease of retinal thickness on SD-OCT (Fig. 8.2c, d). Significant changes in capillary density and morphology have been noted in eyes with various types of uveitides compared to healthy controls [5]. The superficial macular retina has a high oxygen demand, and the parafoveal superficial capillary, as a watershedlike region, is at higher risk of ischemic insult. It could also be assumed that, in the context of a global slowdown of retinal circulation, the oxygen uptake in the inner retina will provide desaturated blood to the superficial capillary plexus. A more likely explanation would be that slowdown of the blood flow was so important in the superficial capillary plexus that it was barely detectable using the split-spectrum amplitude-decorrelation angiography algorithm. It has indeed been suggested that split-spectrum amplitude-decorrelation angiography does not enable detecting retinal capillary flow outside the range of 0.3–2 mm/s.
Fig. 8.2
In birdshot chorioretinopathy , OCT angiography (b) can better visualize FAZ enlargement and superficial capillary ischemia than conventional FA (a). OCT thickness map of same patient reveals a thin retina (c). Flow density heat map (d) reveals reduced flow density throughout the macula
Split-Spectrum Amplitude OCT of the Outer Retina in Uveitis
Inflammatory infiltrates and inflammatory choroidal neovascular membranes are not mutually exclusive entities in uveitis, and it is possible that many lesions contain both [6, 7]. For example, in multifocal choroiditis (MFC), lesions are located on SD-OCT at the level of the RPE and inner choroid leading to disruptions of the choriocapillaris–Bruch’s membrane–RPE complex (Fig. 8.3). In some cases, there is infiltration of the retina, essentially a chorioretinal inflammatory spot. In other cases, choroidal vessels proliferate, and they penetrate from the choriocapillaris–Bruch’s membrane–RPE complex anteriorly and laterally. The inflammatory-related CNV beneath the neurosensory retina leads to accumulation of blood and proteinaceous exudate in the subretinal space. Conventional fluorescein angiography provides dynamic retinal blood flow images of the inflammatory CNV, but at the same time, it demonstrates intense hyperfluorescence within the inflammatory exudates. OCTA delivers static information by delineating vasculature features, including vessel size. In healthy eyes, the outer retina is avascular. An OCTA of this region in MFC patients can confirm the vascular nature of RPE elevations while not detecting blood flow inside the inflammatory lesions (Fig. 8.3). However, imaging suspicious elevations with OCTA requires an understanding of projection artifacts [8]. Even in the absence of new vessels, fragments of images of the overlying retinal vessels on RPE elevations may be seen. For accurate clinical assessment, OCTA projection artifacts can be readily identified by examining sequential en face images at different depths [8].
