Key Features

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  Retinal vascular anatomy.

  
  
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  Choroidal vascular anatomy.

  
  
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  Inner and outer blood–retinal barriers.

  
  
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  Choroidal and retinal blood flow measurements.

Introduction

Because many of the important diseases of the posterior segment are caused by changes in the vasculature of the retina and choroid, it is important to understand the circulatory systems involved to better recognize and treat disease states of the posterior segment. In this chapter, the anatomy and physiology of these circulatory systems are discussed.

Posterior Segment Vascular Anatomy

Retinal Vascular Anatomy

The retinal blood vessels provide nourishment for the inner two thirds of the retina. The central retinal artery, which is the first branch of the ophthalmic artery, is an end artery that has no significant anastomoses. In the area of the lamina cribrosa, its lumen measures about 170 µm in diameter. Typically, just before its exit from the optic nerve, the central retinal artery divides into the superior and inferior papillary arteries, which, in turn, divide into nasal and temporal quadratic branches ( Fig. 6.3.1A ). The anatomic division of the retinal arteries into superior and inferior halves is usually maintained throughout the retina because normal retinal vessels rarely cross the horizontal raphe (see Fig. 6.3.1A ). Cilioretinal arteries, derived from the posterior ciliary arteries, are variably present and emanate from the temporal rim of the optic nerve head toward the macula (see Fig. 6.3.1A ).

Angiogram From Scanning Laser Ophthalmoscopy.

(A) Normal fluorescein angiogram shows the normal filling of retinal arteries and veins; note the cilioretinal artery (green arrow). (B) Normal indocyanine green angiogram shows the normal filling of choroidal vessels. (C) Magnified area from image A shows the retinal capillaries (green arrows) close to the foveal avascular zone. (D) Indocyanine green angiogram shows a choroidal neovascularization (green arrow) secondary to age-related macular degeneration (AMD).

Arteries and veins remain in the nerve fiber layer. Throughout the retina, the capillaries are arranged in laminar meshworks. Depending on the thickness of the retina, the capillary meshwork can vary from three layers at the posterior pole to one layer in the periphery. The arterial intraretinal branches supply three layers of capillary networks:

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  The radial peripapillary capillaries.

  
  
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  Superficial capillaries in the ganglion cell and nerve fiber layers.

  
  
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  Deep, denser, capillaries in the inner nuclear layer.

Optical coherence tomography angiography (OCTA) reveals the microvasculature and the blood flow in the retinal capillaries in a noninvasive manner. Automated segmentation of the full-thickness retinal OCTA scans show in vivo the “superficial” and “deep” retinal vascular plexuses, and choriocapillaris. The superficial plexus shows a continuous and linear shape with a homogeneous wall. The vessels are evenly distributed and resemble a spider’s web. The deep network in healthy eyes has a regular distribution around the foveal avascular zone (FAZ), with more complex minute interconnections.

Like capillary networks elsewhere in the body, the retinal capillaries assume a meshwork configuration to ensure adequate perfusion to all inner retinal cells (see Fig. 6.3.1C ).

A capillary-free zone is present around each of the larger retinal arteries and veins, but it is more prominent around arteries, where it measures up to 100 µm in diameter. In the fovea and the far retinal periphery, retinal capillaries are absent. The FAZ is 400–500 µm in diameter in normal eyes.

The venous drainage of the retina generally follows the arterial supply. Retinal veins (mainly venules) are present in the inner retina, where they occasionally interdigitate with their associated arteries. When two vessels cross, the artery usually lies anterior to the vein, and the two vessels share a common adventitial coat. Many more arteriovenous crossings occur temporally than nasally because the nasal vessels assume a much straighter course. The crossings are important because they represent the most common site of branch retinal vein obstructions. The retinal veins drain into the central retinal vein, which also acts as the major efferent channel for the vessels of the optic nerve (see Fig. 6.3.1A ).

Retinal Vascular Anatomy

The retinal blood vessels provide nourishment for the inner two thirds of the retina. The central retinal artery, which is the first branch of the ophthalmic artery, is an end artery that has no significant anastomoses. In the area of the lamina cribrosa, its lumen measures about 170 µm in diameter. Typically, just before its exit from the optic nerve, the central retinal artery divides into the superior and inferior papillary arteries, which, in turn, divide into nasal and temporal quadratic branches ( Fig. 6.3.1A ). The anatomic division of the retinal arteries into superior and inferior halves is usually maintained throughout the retina because normal retinal vessels rarely cross the horizontal raphe (see Fig. 6.3.1A ). Cilioretinal arteries, derived from the posterior ciliary arteries, are variably present and emanate from the temporal rim of the optic nerve head toward the macula (see Fig. 6.3.1A ).

Angiogram From Scanning Laser Ophthalmoscopy.

(A) Normal fluorescein angiogram shows the normal filling of retinal arteries and veins; note the cilioretinal artery (green arrow). (B) Normal indocyanine green angiogram shows the normal filling of choroidal vessels. (C) Magnified area from image A shows the retinal capillaries (green arrows) close to the foveal avascular zone. (D) Indocyanine green angiogram shows a choroidal neovascularization (green arrow) secondary to age-related macular degeneration (AMD).

Arteries and veins remain in the nerve fiber layer. Throughout the retina, the capillaries are arranged in laminar meshworks. Depending on the thickness of the retina, the capillary meshwork can vary from three layers at the posterior pole to one layer in the periphery. The arterial intraretinal branches supply three layers of capillary networks:

• •
  

  The radial peripapillary capillaries.

  
  
• •
  

  Superficial capillaries in the ganglion cell and nerve fiber layers.

  
  
• •
  

  Deep, denser, capillaries in the inner nuclear layer.

Optical coherence tomography angiography (OCTA) reveals the microvasculature and the blood flow in the retinal capillaries in a noninvasive manner. Automated segmentation of the full-thickness retinal OCTA scans show in vivo the “superficial” and “deep” retinal vascular plexuses, and choriocapillaris. The superficial plexus shows a continuous and linear shape with a homogeneous wall. The vessels are evenly distributed and resemble a spider’s web. The deep network in healthy eyes has a regular distribution around the foveal avascular zone (FAZ), with more complex minute interconnections.

Like capillary networks elsewhere in the body, the retinal capillaries assume a meshwork configuration to ensure adequate perfusion to all inner retinal cells (see Fig. 6.3.1C ).

A capillary-free zone is present around each of the larger retinal arteries and veins, but it is more prominent around arteries, where it measures up to 100 µm in diameter. In the fovea and the far retinal periphery, retinal capillaries are absent. The FAZ is 400–500 µm in diameter in normal eyes.

The venous drainage of the retina generally follows the arterial supply. Retinal veins (mainly venules) are present in the inner retina, where they occasionally interdigitate with their associated arteries. When two vessels cross, the artery usually lies anterior to the vein, and the two vessels share a common adventitial coat. Many more arteriovenous crossings occur temporally than nasally because the nasal vessels assume a much straighter course. The crossings are important because they represent the most common site of branch retinal vein obstructions. The retinal veins drain into the central retinal vein, which also acts as the major efferent channel for the vessels of the optic nerve (see Fig. 6.3.1A ).

Choroidal Vascular Anatomy

The choroid is, by far, the most vascular portion of the eye and, by weight, one of the most vascular tissues in the body. The choroid is responsible for the vascular support of the outer retina (see Fig. 6.3.1B ). A structurally and functionally normal choroidal vasculature is essential for retinal function: Abnormal choroidal blood volume and/or compromised flow can result in photoreceptor and retinal pigment epithelium (RPE) dysfunction and death. Other likely functions include light absorption, thermoregulation via heat dissipation, and modulation of intraocular pressure (IOP) via vasomotor control of blood flow. The choroid also plays an important role in the drainage of the aqueous humor from the anterior chamber via the uveoscleral pathway.

The blood supply to the choroid is from branches of the anterior and posterior ciliary arteries, branches of the ophthalmic artery. The overall structure of the choroid is segmental; this segmental distribution of blood begins at the level of the posterior ciliary branches and is mirrored in the vortex drainage system. As a result of the segmental distribution, the large and medium-sized choroidal arteries act as end arteries.

Histologically, starting from the retinal (inner) side, the choroid is divided into five layers:

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  Bruch’s membrane.

  
  
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  Choriocapillaris (layer of capillaries).

  
  
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  Sattler’s layer (layer of medium diameter blood vessels).

  
  
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  Haller’s layer (layer of large diameter blood vessels).

  
  
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  Suprachoroidea (transitional zone between choroid and sclera).

The choriocapillaris is a highly anastomosed network of capillaries, forming a thin sheet apposed to Bruch’s membrane. The fibrous basement membrane of the capillary endothelial cells forms the outermost layer of Bruch’s membrane in humans. The choriocapillaris is about 10 µm thick at the fovea, where there is the greatest density of capillaries, thinning to about 7 µm in the periphery. The capillaries arise from the arterioles in Sattler’s layer, each of which gives rise to a hexagonal (or lobular) shaped domain of a single layer of capillaries, giving a patch-like structure to the choriocapillaris. The choriocapillaris has large diameter capillaries of 20–25 µm, which allow the passage of multiple red blood cells at any moment in time. Unlike the retinal capillaries, the choriocapillaris has fenestrations of 700–800 nm diameter, which allows more rapid transport of molecules (leakage).

Besides the choriocapillaris, the choroid presents two other vascular regions: the outer Haller’s layer of large blood vessels and the inner Sattler’s layer of medium and small arteries and arterioles that feed the capillary network, and veins. The stroma (extravascular tissue) contains collagen and elastic fibers, fibroblasts, nonvascular smooth muscle cells, and numerous very large melanocytes that are closely apposed to the blood vessels. As in other types of connective tissue, there are numerous mast cells, macrophages, and lymphocytes.