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Retinal Arterial Occlusion

Xiaogang Wang, MD, PhD; Jing Dong, MS; and Yading Jia, BS

As the main branches of the ophthalmic artery, the central retinal artery, travelling with the optic nerve, provides nutrients to the inner retina and the surface of the optic nerve. Acute retinal artery occlusion (RAO) is a common cause of sudden, painless, and profound vision loss in one eye. The common causes for RAO include the following: atherosclerosis-related thrombosis, especially at the level of the lamina cribrosa; cardiac embolism; carotid embolism; giant cell arteritis; periarteritis, associated with polyarteritis nodosa; Wegener’s granulomatosis; systemic lupus erythematosus; dermatomositis; thrombophilic disorder; and retinal migraine.^1–3

Fluorescein angiography (FA) relays delayed retinal arterial filling, delayed arteriovenous transit time, and masked background fluorescence because of retinal swelling in the involved area in acute RAO cases. Traditional FA, however, provides incomplete morphologic evaluation of the intermediate and deep capillary plexuses, which is helpful to identify deep capillary ischemia.⁴

As a novel, noninvasive, imaging technology, optical coherence tomography angiography (OCTA) can clearly demonstrate the retinal and choroidal microvasculature without the administration of exogenous dyes (Figure 24-1).⁵ Moreover, OCTA can quantitatively evaluate the retinal microcirculation information of any interesting areas in a reproducible way.⁶ Shadow effects, segmentation error, and projection artifacts, however, make some evaluation of the deep capillary plexus difficult and even unreliable at the upper limits of the retinal ischemic region (Figure 24-2).

CENTRAL RETINAL ARTERY OCCLUSION

Central retinal artery occlusion (CRAO) is usually characterized by painless catastrophic vision loss with a relative afferent pupil defect without disc swelling. The pathogenesis of occlusion is embolism from the heart or the atherosclerotic carotid arteries.⁷ Twenty-five percent of CRAO eyes have patent cilioretinal arteries, sparing the fovea and potentially preserving central vision. Color fundus photography of an acute CRAO shows an opacified retina resulting from ischemia and an evident central cherry-red spot in the foveola as a result of the demonstration of the retinal pigment epithelium (RPE) and choroidal pigment. FA traditionally shows delayed central retinal arterial filling and delayed arteriovenous transit time with boxcarring of the blood column.⁸ However, standard FA fails to demonstrate adequate perfusion information of ischemic inner and deeper retinal layers as reported in a previous histopathologic study.⁹ With the advent of OCTA imaging technology, understanding the ischemic pathophysiology of CRAO in vivo was possible. During the acute phase, structural OCT demonstrated diffuse thickening and enhanced reflectivity for both the inner and middle retinal layers, which may represent intracellular edema, ischemic cellular damage, and by-product accumulation. For acute CRAO, the histopathologic study reveals the presence of intracellular edema but not extracellular edema, which is in agreement with the common lack of low reflectivity intraretinal fluid spaces in cross-sectional OCT images.9 An OCTA scan centered on the fovea showed the same regions of decreased vascular perfusion both in superficial and deep capillary plexuses.¹⁰ If the cilioretinal artery-sparing area exists, focal restoration of the deep capillary plexus perfusion may be observed. For an OCTA scan centered on the optic nerve head, the radial peripapillary capillaries (RPCs) system showed diffuse attenuation. The diffuse thickening and hyper-reflectivity of the inner and middle retinal layers may block optical reflections from the outer retinal layers and the RPE/choriocapillaries complex. These structural shadow effects will also be noted as nonperfusion or relatively low perfusion on the OCTA at the choriocapillary level. In the chronic phase, diffuse thinning and atrophy of the inner and middle retinal layers and incomplete deep capillary plexus was demonstrated using OCT and OCTA; however, FA in the chronic phase failed to show any information of retinal ischemia or nonperfusion.¹¹ Therefore, a patient with chronic CRAO usually demonstrates a characterless retinal manifestation, but the precise diagnosis could be established using an OCT and OCTA scan.

Branch retinal artery occlusion (BRAO) is usually characterized by sudden segmental visual field loss, and visual acuity is decreased if there is foveal involvement. The embolism, mainly coming from the carotid artery and heart, is by far the most common cause of nonarteritic BRAO. Superficial retinal whitening and segmental retinal infarction occurs in the region of the occluded arteriole. FA confirms a delay or a lack of perfusion and boxcarring of the blood column in the vessels of the involved branch retinal artery.12 If FA was performed soon after the vision loss, nonperfusion was observed in the region of the involved retinal arteriole, and retrograde circulation begins to fill from the adjacent normal retinal vessels later. Intracellular edema and ischemia of the inner and middle retinal layers was found in the histopathologic study of acute RAOs. The structural OCT findings of acute BRAO are identical with the above-mentioned histopathologic report. The ischemic retinal region affected by the occluded vessel demonstrates an increased thickness without cystic spaces of low reflectivity, which correlated well with the histopathologic findings that retinal artery occlusive edema is in the intracellular, instead of the extracellular, space. Moreover, the whitened retinal area corresponds to diffuse hyper-reflectivity of the inner retina, which may be a result of coagulative necrosis or ischemia of these layers. OCTA demonstrated focal restoration of deep capillary plexus perfusion in the ischemic regions where the superficial capillary plexus perfusion was abnormal. For eyes with BRAO, focal attenuation of the RPC system in the distribution of the occluded retinal artery branch was revealed on the OCTA. The inner retinal layer hyper-reflectivity can shadow the optical signal of the outer retinal layer and the RPE/choriocapillaris complex beneath. BRAO shares the same shadow effects of OCTA with CRAO.

PARACENTRAL ACUTE MIDDLE MACULOPATHY

Paracentral acute middle maculopathy (PAMM), a novel presentation of deep retinal capillary ischemia, is usually characterized by paracentral scotoma or a decrease in vision.¹³ Focal retinal capillary ischemia has been proposed as the main mechanism for the development of these lesions. PAMM has been reported in conjunction with other retinal disorders, including CRAO, BRAO, central retinal vein occlusion, diabetic retinopathy, Purtscher retinopathy, and sickle cell retinopathy.^11,14–16 PAMM, in association with retinal arterial occlusive disease, shares similar color fundus photography and FA features with RAO. Using OCT, the area affected by PAMM demonstrates plaque-like hyper-reflectivity of the middle retinal layer at the level of the inner nuclear layer. En face OCT showed confluent hyper-reflective regions of the middle retina corresponding to the involved area with 3 distinct patterns: arteriolar, fern-like, and globular.¹⁷ OCTA highlighted variable regions of capillary dropout located in the superficial and deep retinal capillary plexus in the affected area.

CASE 1: ACUTE CENTRAL RETINAL ARTERY OCCLUSION

Clinical Summary

A 65-year-old male complained of blurry vision of the left eye for one day. His medical history was significant for hypertension. Visual acuity was 20/40 in the left eye. A dilated fundus examination showed diffusely attenuated arteries and ischemic retinal whitening surrounding the fovea, except for the nonischemic region of the retina perfused by the cilioretinal artery (white arrow) deriving from the choroidal circulation but not the central retinal artery (Figure 24-3A). FA (Figure 24-3B) revealed severe nonperfusion of the retina, except for the cilioretinal artery-sparing area of more than 44 seconds after injection of the fluorescein dye.

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