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Optical Coherence Tomography Angiography of Macular Telangiectasia Type 2

Ching J. Chen, MD; Brian Tieu, MD, PhD; and Matthew Olson, CRA

Macular telangiectasia (Mac Tel), also known as idiopathic juxtafoveal retinal telangiectasia or parafoveal telangiectasia, is a relatively rare and poorly understood disease. There are nonproliferative changes characterized by loss of retinal transparency, intraretinal crystalline deposits, pigment clumping, and capillary telangiectasia in the parafoveal area. Moreover, there may be loss of inner and outer retinal tissue that can eventually lead to cystic cavities or even macular hole formation. Furthermore, in the late stage of the disease, the proliferative finding of a subretinal neovascular membrane (SRNV) may develop and cause significant vision loss.

The prevalence is 0.1% according to the Beaver Dam Eye Study.¹ According to the Mac Tel Project the mean age of diagnosis is 61 ± 9 years. There is a slight female gender predilection, and a high association with hypertension and diabetes.² The disease is underdiagnosed because patients have very minimal visual complaints initially,^3–6 and the early vascular changes occur in the deep retinal capillary plexus, which may not be detectable on standard retinal examination.^3–7 Type 2 idiopathic macular telangiectasia (Mac Tel 2) is thought to be a neurodegenerative disease. One theory of pathogenesis is that Müller cell dysfunction may cause a secondary vascular insult, eventually leading to irreversible retinal tissue damage.^3,8–14

The term idiopathic juxtafoveal retinal telangiectasia was first coined by Gass⁷ in 1968. Gass and Oyakawa¹⁵ later provided the first classification system in 1982, based on the progression of disease as observed by funduscopy and fluorescein angiography (FA). This classification system was modified in 1993 by Gass and Blodi,¹⁶ who divided the disease into 3 groups. In an attempt to simplify this classification, Yannuzzi et al¹⁷ proposed the division of idiopathic Mac Tel into 2 types based on clinical observation, FA, indocyanine green angiography, and spectral domain optical coherence tomography (SD-OCT) findings. The Gass types 1A and 1B were merged into a group as aneurysmal telangiectasia, now known as type 1 idiopathic macular telangiectasia (Mac Tel 1). Gass type 2A was renamed idiopathic perifoveal telangiectasia, now known as Mac Tel 2. Gass types 2B, 3A, and 3B were deleted because of their extreme rarity. The Yannuzzi classification system was endorsed by Chew et al¹⁸ and subsequently adopted by the majority of the ophthalmic community. Based on the likely sequence of angiogenesis, Yannuzzi et al¹⁹ further described the clinical staging of Mac Tel 2.

Diagnosis of Mac Tel is usually based on FA^{4–7,15–19} and SD-OCT.^20–25 Many other ancillary tests and imaging techniques, however, are now available for the study of Mac Tel 2. Results of multifocal electroretinography,²⁶ microperimetry,²⁷ autofluorescence, macular pigment optical density measurement,^28–30 and adaptive optics imaging^31,32 have provided much additional valuable information. More recently, optical coherence tomography angiography (OCTA) has emerged as a very useful tool to evaluate retinal structural and vascular changes.

OCTA is a novel technology with the capability to image retinal blood flow without the need for contrast media injection.33–37 It is now possible to generate high-quality OCTA images with a relatively short scan time on a commercial SD-OCT system (Avanti RTVue-XR, Optovue Inc) using an efficient, split-spectrum amplitude-decorrelation angiography (SSADA) algorithm (AngioVue software, Optovue Inc). The OCTA scan generates a 3-dimensional image set that is segmented into pre-set tissue slabs of the superficial retinal capillary plexus, deep retinal capillary plexus, outer retina, and the choroid. The default setting described in this chapter refers to 4 pre-set tissue slabs. The upper border of the “superficial retinal slab” is 3 μm below the internal limiting membrane, and the lower border is 15 μm below the inner plexiform layer (IPL). The total thickness of the segmentation can be adjusted by altering the upper or lower borders. The default upper border of the “deep retinal slab” is 15 μm below the IPL, and the lower border is 70 μm below the IPL with the segmentation depth set at 55 μm. The “outer retinal slab” includes the retinal pigment epithelium (RPE), with a default upper border 70 μm below the IPL, and lower border 30 μm below the RPE reference plane (best fit continuous surface under the RPE, approximating the position of Bruch’s membrane). The default upper border of the “choroid capillary slab” is 30 μm below the RPE reference, and the lower border is 60 μm below the RPE reference. The total thickness is 30 μm. The boundaries of these predefined slabs can be adjusted manually by the user relative to the computer-segmented reference planes. The reference planes can also be manually adjusted to allow for a more precise determination of the location of pathology. These unique abilities make OCTA a powerful tool for the study of vascular pathologies involving the superficial, deep, outer retinal and choroidal layers such as Mac Tel 2.^38–41

CLINICAL OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHIC STAGING AND FINDINGS OF MACULAR TELANGIECTASIA 2

Mac Tel 2 can be divided into 2 major categories, nonproliferative and proliferative, with 5 stages.

Nonproliferative Stages

Depending on the layer of the retina involved in the vasculopathic process, nonproliferative Mac Tel 2 can be further divided into 3 stages.

Stage 1

Deep retinal capillary telangiectasia: The vascular changes involve only the deep retinal capillary plexus. The most common OCTA findings in this stage include the following:

• Normal superficial retinal capillary plexus.
  
• Telangiectasia in the deep retinal capillary plexus predominantly in the temporal parafoveal area.
  
• No abnormal blood flow is detected in the outer retinal slab.

Figure 18-1 demonstrates a case with typical clinical and OCTA findings of Stage 1 Mac Tel 2. Color fundus photo shows no obvious abnormal findings in the macula (Figure 18-1A). FA shows no abnormal fluorescein leakage in the early venous phase (Figure 18-1B), but low-grade hyper-fluorescence is noted in the temporal parafoveal area in the late phase (Figure 18-1C). SD-OCT shows a hypo-reflective cystic cavity in the inner retina (red arrow) and mild disruption of the ellipsoid zone (Figure 18-1D). The superficial retinal slab on OCTA shows no abnormal blood flow in the superficial retinal capillary plexus (Figure 18-1E), but telangiectasia (yellow ellipse) is noted in the temporal parafoveal capillary plexus of the deep retinal slab (Figure 18-1F).

OCTA is more sensitive than FA in detecting telangiectatic vascular changes of the deep retinal capillary plexus in Mac Tel 2. This is well demonstrated in Figure 18-2. Color fundus photo shows no definite abnormal findings in the macula (Figure 18-2A). FA in the venous phase (Figure 18-2B) and late phase (Figure 18-2C) show no fluorescein leakage or staining. SD-OCT (Figure 18-2D) shows a small hypo-reflective cystic cavity (red arrow). OCTA reveals no abnormal blood flow in the superficial retinal slab (Figure 18-2E). An early telangiectatic vascular change (yellow ellipse) is noted in the temporal parafoveal deep capillary plexus in the deep retinal slab (Figure 18-2F).

Stage 2

Inner retinal capillary telangiectasia: The vascular changes involve both the superficial and deep capillary plexuses. The most common OCTA findings include the following:

• Mild to moderate dilation of the superficial retinal capillary plexus.
  
• Moderate to significant telangiectatic change of the deep retinal capillary plexus.
  
• Visible communication of dilated vessels between the superficial and deep capillary plexuses.
  
• Loss of vascular density due to capillary dropout in both the superficial and deep capillary plexuses.
  
• One or more sets of 90-degree dilated feeder and draining vessels may be seen.
  
• The outer retinal slab shows no abnormal flow signal.

The case shown in Figure 18-3 demonstrates some typical Stage 2 Mac Tel 2 findings. Fundus photo shows small crystalline deposits (blue arrows) inferior and temporal to the fovea (Figure 18-3A). FA shows noticeable telangiectatic vessels (yellow ellipse) temporal to the fovea in the early venous phase (Figure 18-3B), and significant fluorescein leakage from this telangiectatic vascular net (green ellipse) in the late phase (Figure 18-3C). SD-OCT shows multiple small hyper-reflective spots (purple arrows), cystic cavities (white arrow), and disruption of the ellipsoid zone (Figure 18-3D). OCTA shows telangiectasia of the temporal parafoveal capillary plexus (yellow ellipses) in both the superficial retinal (Figure 18-3E) and deep retinal slabs (Figure 18-3F).

One or more set of dilated 90-degree feeder and draining vessels can be seen in this stage. In Figure 18-4, fundus photo shows temporal parafoveal telangiectasia in the superficial retinal capillary plexus. A dilated 90-degree draining vessel (blue arrow) is noted (Figure 18-4A). FA shows fluorescein leakage from the temporal parafoveal area in both the early venous phase (Figure 18-4B) and late phase (Figure 18-4C). SD-OCT shows cystic cavities (white arrows) in both the inner and outer retina. Disruption of the external limiting membrane and ellipsoid zone are noted (Figure 18-4D). OCTA shows a dilated feeder vessel (yellow arrows) and draining vessel (blue arrows) connected to telangiectatic vascular nets in both the superficial and deep retinal slabs (Figures 18-4E and 18-4F). There is decreased vascular density with slight widening of the intervascular spaces.

Stage 3

Vascular invasion involves the full thickness of the retina. The angiogenic process extends from the subinternal limiting membrane in the foveal area to the RPE. The most common OCTA findings of this stage include the following:

• The foveal avascular zone (FAZ) may become distorted and irregular.
  
• Subinternal limiting membrane vascular ingrowth into the FAZ may be detected.
  
• Dilated vessels in the superficial and deep capillary plexuses with greater capillary dropout and widening of the intervascular spaces are seen.
  
• There is an optical shadowing effect due to the presence of pigment clumps.
  
• A lamellar or full-thickness macular hole may be present.
  
• Dilated feeder and draining vessels are commonly seen.

The size of the pigment clumps can vary in this stage. In Figure 18-5, only a very small, subtle pigment clump is present. Fundus photo reveals supratemporal parafoveal telangiectasia (Figure 18-5A). FA in the early venous phase shows a small pigment clump (red arrow) with hypo-fluorescence (Figure 18-5B). Significant temporal parafoveal telangiectasia is noted with fluorescein leakage in the late phase (Figure 18-5C). SD-OCT shows a large cystic cavity (partial layer macular hole) and multiple small hyper-reflective spots (white arrows) in both the inner and outer retina with disruption of the ellipsoid zone (Figure 18-5D). OCTA shows vascular dropout with significant enlargement of the FAZ, and no flow in the temporal parafoveal area corresponding to the location of the lamellar macular hole in both the superficial and deep retinal slabs (Figures 18-5E and 18-5F). Dilated feeder (yellow arrows) and draining vessels (blue arrows) are best demonstrated in the superficial retinal slab (Figure 18-5E), but are also visible in the deep retinal slab (Figure 18-5F). Abnormal blood flow is detected at the RPE level in the outer retinal slab (yellow ellipse), confirming the vascular invasion to the RPE (Figure 18-5G).

A significant, more prominent pigment clumping is demonstrated in Figure 18-6. Vascular invasion into the irregular-shaped FAZ is also noted. Fundus photo demonstrates irregular, heavy pigment clumping (yellow arrow) and surrounding moderate pigment atrophy (white arrow). Stretching and an irregular course of the macular vessels are noted as well (Figure 18-6A). FA shows moderate-severe fluorescein leakage in both the late venous and late phase. Blockage of choroidal fluorescence by the pigment is noted (Figures 18-6B and 18-6C). SD-OCT shows pigment in the inner retina with hyper-reflection (white arrows). An optical shadowing effect is noted due to pigment blockage (yellow arrows). There is a mild epimacular membrane with retinal folds (Figure 18-6D). The superficial and deep retinal slabs on OCTA show distorted and dilated vessels with an irregular and smaller FAZ, presumably due to a subinternal limiting membrane vascular ingrowth into the FAZ (Figures 18-6E and 18-6F). Multiple, dilated, large vessels are noted in the macula, as well as multiple, focal areas of capillary dropout. The vascular invasion reaches the outer retina and RPE. Blood flow is detected by OCTA at the RPE level (yellow circle) in the outer retinal slab (Figure 18-6G).

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