Fig. 3.1
SS-OCTA orthogonal view of branch retina vein occlusion. The left eye of a 61-year-old Caucasian female using the VCSEL SS-OCTA. (a) Unflattened structural en face 3 × 3 mm SS-OCT with (b), the corresponding 3 × 3 mm X-Fast SS-OCT B-scan, and (c), the corresponding 3 × 3 mm Y-Fast OCT B-scan. (d) Unflattened structural en face 6 × 6 mm SS-OCT with (e) the corresponding 6 × 6 mm X-Fast SS-OCT B-scan and (f) corresponding 6 × 6 mm Y-Fast SS-OCT B-scan. The SS-OCTA shows microvascular abnormalities such as areas of capillary non-perfusion, capillary loops, and microaneurysms
Fig. 3.2
OCTA orthogonal view of macular edema due to central retina vein occlusion (CRVO). The left eye of a 72-year-old Caucasian male using the VCSEL SS-OCTA. (a), 3 × 3 mm SS-OCT angiogram and (b), the corresponding structural en face SS-OCT. (c), 3 × 3 mm X-Fast SS-OCT B-scan and (d), the corresponding 3 × 3 mm Y-Fast SS-OCT B-scan. The areas of macular edema appear as dark, cystic areas with well-delineated borders
In retinal vein occlusions, the ultra-high speed SS-OCTA does not offer a clear advantage over SD-OCTA systems in visualization of the vascular abnormalities of the superficial and deep plexuses. However, long wavelength SS-OCTA is better able to visualize the choriocapillaris and choroidal vessels and ischemic changes that may occur as part of the global ischemia that occurs. Additionally, SS-OCTA can be used to visualize the progression or reperfusion of ischemia over time, as it is an imaging study that can be easily performed at multiple follow-up visits.
Alteration of the retinal contour may occur secondary to macular edema in patients with vein occlusions, which can cause segmentation error in automated segmentation algorithms on commercially available devices. Post-acquisition image processing uses Bruch’s membrane, the internal limiting membrane (ILM), or other distinct layer in the chorioretinal anatomy as a reference with respect to which en face image planes are segmented [7]. However, manual segmentation and flattening of these prototype images can help reduce segmentation error and better visualize the vasculature in one plane simultaneously without needing to scroll through a three-dimensional volume. Future commercially available SS-OCT systems should include automated flattening software for better visualization of SS-OCTA features.
3.1.2 Retinal Artery Occlusion
There are two main forms of retinal artery occlusion: branch retinal artery occlusion (BRAO) and central retinal artery occlusion (CRAO). BRAO occurs when the artery becomes blocked, usually due to an embolus. BRAO is usually the result of embolus that lodges at the bifurcation of a retinal arteriole. In the obstructed area, capillary dropout due to occlusion of blood flow is evident. Histopathologic studies have shown that in acute BRAO, there is an area of ischemia in the corresponding retinal quadrant, which is followed by inner retinal atrophy in long-standing cases. On SS-OCT, CRAO shows a distinct pattern of increased reflectivity and thickness of the inner retina in the acute phase and a corresponding decrease in reflectivity of the outer layer of the retina, retina pigment epithelium (RPE), and choriocapillaris. Similar to retinal vein occlusions, the ultra-high speed SS-OCTA may not offer a clear advantage over SD-OCTA systems in visualization of the vascular abnormalities of the superficial and deep plexuses, where the most obvious and dramatic changes occur. As well as the SD-OCTA, SS-OCTA of an acute BRAO show an ischemic area (see Fig. 3.3c) in the corresponding retinal quadrant of the branch marked by inner retinal edema at the initial stage followed by atrophy in long-standing cases (see Fig. 3.3).
Fig. 3.3
Multimodal image of branch retina artery occlusion (BRAO) with color fundus photo. The right eye of a 70-year-old Caucasian man using the VCSEL SS-OCTA. In the superficial plexus, it is possible to visualize the main superficial retina vessels in the arterial occluded area that lose some, but not all, collateral branches (asterisk) after the ischemic event. (a) Color fundus photo zoomed in to an approximately 3 × 3 mm area centered at the optic nerve showing RAO (asterisk). (b) Unflattened structural en face 3 × 3 mm SS-OCT. (c) 6 × 6 mm SS-OCTA of the superficial vascular plexus. The yellow asterisk corresponds to an occluded vessel that can be faintly seen on the structural en face SS-OCT, but not the SS-OCT angiogram. (d) Corresponding 6 × 6 mm X-Fast SS-OCT B-scan. The stars mark the corresponding shadowing artifact from the overlying vessels (e) unflattened structural en face 6 × 6 mm SS-OCT at the level of the RPE. (f) Corresponding 6 × 6 mm Y-Fast SS-OCT B-scan
3.2 Non-neovascular Age-Related Macular Degeneration
Dry, or non-neovascular, age-related macular degeneration (AMD) is a progressive chronic disease that is one of the leading causes of irreversible legal blindness in developed countries in adults older than 50 years of age. It is almost always bilateral and primarily affects the macula. There are both genetic and lifestyle risk factors that are related to the development and progression of dry AMD [8, 9].
Drusen, pigmentary changes, and photoreceptor and retina pigment epithelium loss can be observed clinically on fundoscopic examination. Photoreceptor changes can be caused by RPE dysfunction or can be secondary to choriocapillaris loss or both. The RPE provides nutrients to the overlying photoreceptors. RPE changes are the hallmark of late-stage dry AMD, and these areas of atrophy are commonly known as geographic atrophy (GA).
The SS-OCT prototype used in this chapter utilizes a longer wavelength, 1050 nm which has increased penetration through the RPE compared to shorter wavelength, ~840 nm, commercial SD-OCT systems. Depending on the imaging regime, SD-OCT may also suffer from sensitivity roll-off when imaging beneath the RPE. One of the most important advantages of SS-OCT in dry AMD is the enhanced visualization of the choriocapillaris and the ability to visualize of the presence and pattern of ischemia in this layer.
3.2.1 SS-OCTA in Early- and Intermediate-Stage Dry AMD
SS-OCTA enables precise correlation of structural and microvascular changes in the layers of the retina and choroid, which has improved our understanding of this condition and its pathogenesis. Despite limited clinical symptoms in early AMD, it is possible to visualize chorioretinal changes in early-stage AMD on SS-OCT. Drusen presents as hyper-reflective material between Bruch’s membrane and the RPE. SS-OCTA, with its enhanced visualization of the choroid and choriocapillaris, can help to better visualize the relationship between the RPE, Bruch’s membrane, and the choriocapillaris (CC) in the pathogenesis of AMD [10].
By simultaneous visualization of structural and microvascular information on SS-OCTA, it is possible to visualize drusen and observe vascular changes in the choriocapillaris both underneath and surrounding drusen. It has been noted that early dry AMD is associated with focal areas of choriocapillaris loss and a general reduction in choriocapillaris density when compared to age-matched normal controls. It is also possible to visualize the large choroidal vessels that lie below these areas of choriocapillaris loss. These SS-OCTA findings are supported by histopathologic data which have noted that drusen form over areas devoid of capillary lumens and extend into the intercapillary pillars, and that increased drusen density is associated with a reduction in the vascular density of the choriocapillaris [11–13].
3.2.2 SS-OCTA in Advanced Dry AMD
Geographic atrophy (GA) occurs in late-stage AMD and SS-OCTA in this area shows loss of choriocapillaris underlying large areas of atrophy. Figure 3.4 shows that areas of choriocapillaris loss can be correlated to areas of RPE atrophy.
Fig. 3.4
Non-neovascular AMD. SS-OCT angiogram fields of view with color fundus photo. The left eye with geographic atrophy (GA) of a 62-year-old Caucasian man using the VCSEL SS-OCTA. (a) Color fundus photo showing the GA. (b) Fundus autofluorescence showing the GA. (c) Structural en face 9 × 9 mm showing the GA. (d) The 3 × 3 mm SS-OCTA of the choriocapillaris demonstrates flow impairment in a similar area as the area of RPE atrophy and larger choroidal vessels have been push inward into the area of choriocapillaris atrophy. (e, f) The 6 × 6 mm SS-OCTA of the choriocapillaris with a red dotted line around an approximately 3 × 3 mm area centered at the macula. (f) Structural en face SS-OCT-B 3 × 3 mm showing the GA
SS-OCTA has been used to show that alterations in the choriocapillaris within the borders of GA tend to be primarily atrophic, while changes in the choriocapillaris beyond the GA borders appear to be mostly areas of impaired flow. When the area of choriocapillaris under the GA is compared to the surrounding normal areas, it is noted that there is considerable loss of flow in the choriocapillaris.
3.3 Central Serous Choroidal Retinopathy
Central serous chorioretinopathy (CSCR) usually affects young or middle-aged healthy individuals and can result in both acute, usually reversible, visual loss or in some cases a chronic decrease in visual acuity. Acute CSCR is caused by the rapid accumulation of subretinal fluid (SRF) in the macular area secondary to a focal leak in the RPE. The chronic form is believed to be due to more diffuse RPE changes located in the macula and is characterized by diffuse RPE leakage on the fluorescein angiography. It may or may not be associated with the presence of SRF. One or more serous retinal detachments of the neurosensory retina can be associated with the concurrent presence of one or more retinal pigment epithelium detachments (RPED) that can be located foveally, subfoveally, and perifoveally. Photoreceptor damage and permanent vision loss can result from persistent SRF [14].
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