INTRODUCTION
The analytical study of retinal anatomy in relation to fluorescence angiography requires the retina to be subdivided into the posterior pole that lies inside the vascular arcades, and the peripheral retina that is the area around the posterior pole and the optic disk. The peripheral retina is further divided into mid-periphery, far-periphery, and extreme periphery (ora serrata): It is also subdivided in nasal periphery and upper and lower temporal periphery.
POSTERIOR POLE
The longer horizontal axis of the oval-shaped posterior pole measures 8–10 mm (30–35°). Fluorescein angiography with a 45° angle allows for a single frame picture to be taken of the posterior pole, the optic disk, and part of the vascular arcades. Modern devices allow wide-angle angiography.
MACULAR AREA
The macula, with a thickness of 100 microns, is located at the center of the posterior pole where it forms a slight depression centered on the fovea and the foveola. It has a 1,200-micron diameter. Within the macular area fluorescein angiography, we observe the avascular area that has a diameter of 450–500 microns and is sharply highlighted by the dye in the capillary phase. This dark avascular area is delimited by a continuous ring of fine anastomoses of the perifoveal vascular network, consisting of a single layer of capillaries.
The fovea itself, which is anatomically an area with a larger concentration of cones, is contained within a concentric circle having a diameter of 350 microns, hence it lies inside the fluorescein angiographic avascular area, while the foveola that corresponds to the macular center, measures about 100 microns. In the macula, there is a strong density of xanthophyllic pigment and at this level the cells of the pigment epithelium are higher and contain a bigger density of pigment granules. Apart from the absence of rods, the other specific feature of the macula is the decreased thickness of the retina and the thickening of the inner nuclear layers at the macula margins.
Around the macula area, the retina is 350 microns thick. Let us recall the nine layers of the retina: from the inner layer toward the outer layer, we find the internal limiting membrane, the nerve fibers layer, ganglion cells layer, inner plexiform layer, inner nuclear layer (bipolar cell bodies), outer plexiform layer, outer nuclear layer (cones and rods cellular bodies), outer limiting membrane, cones and rods layer. Then there is the external retinal layer consisting of a single layer of pigmented epithelial cells in close contact with each other through their “tight junctions”. These pigment epithelium cells are linked to each other and to Bruch's membrane or to the lamina vitrea by a cementing substance. The lamina vitrea is part of the choroid that is divided into choriocapillaris and choroid proper.
RETINAL ARTERIES AND VEINS
From the optic disk, take origin of the retinal arteries that form an arcade encircling the macula. Nerve fibers and central veins enter the optic disk. Arteries supply the inner layers of the neurosensory retina. A ciliary artery fills before the other retinal arteries. As the choroidal lobules it receives blood directly from the short posterior ciliary arteries (Figure 1).
The sensory retina thickness is supplied by two separate vascular systems. The inner side is supplied by retinal vessels; the most important branches are located in the nerve fiber level and form a loose network whereas the capillaries are observed in the inner half of the retina up to the inner nuclear layer and form a closer-knit network. The outer third of the retina instead is supplied by the choroid vascular system.
RETINAL PERIPHERY
Peripheral retina is generally subdivided into three zones:
- The mid-periphery, outside the vascular arcades
- The far-periphery, between equator and ora serrata, where the vortex veins are located
- The ora serrata or extreme periphery
In the periphery, the retina is less thick and has simpler structure. Neurons are bigger and scarcer.
Figure 1: Retina periphery. Peripheral retina is subdivided into three zones. (1) The mid-periphery, outside the vascular arcades; (2) the far-periphery, between equator and ora serrata, where the vortex veins are located; and (3) the ora serrata or extreme periphery.
OPTIC DISK
The optic disk has a diameter of 1,500 microns whereas the retinal veins at the border of the disk have a maximum diameter of about 120 microns. At the mid-periphery, their diameter is about 60 microns. The retinal arteries have a slightly smaller diameter (respectively 80 and 50 microns). The capillaries in immediate contact with the retinal vessels are very scarce and virtually absent (periarterial avascular area). At the arteriovenous crossings, arteries and veins are united by a common adventitia. The size of retinal capillaries ranges between 5 and 10 microns.
THE CHOROID-VORTEX VEINS
The choroid is part of the vascular tunica of the eye and consists of dense vascular tissue where the larger vessels are positioned externally, close to the sclera (Haller layer), the smaller vessels are in the inner part (Sattler layer), and the choriocapillaris is in contact with the retina.
The choroid is supplied by the posterior ciliary arteries (about 15–20 branches). The vascular supply to the choroid is schematized in Figure 2 that indicates the districts supplied by each branch of the short and long posterior ciliary arteries. The districts are clearly separated from each other and form distinct wedges or triangles (Figure 2).
This figure helps to understand the fluorescein angiographic aspect of certain occlusive lesions of the choroid (triangular syndromes). The most important vessels are impermeable to fluorescein and are in contact with the sclera. They are visible on the fluorescein angiography in the presence of sclerosis of the choriocapillaris and of the retinal pigment epithelium (window lesions). The arteries subdivide rapidly forming lobules, small, irregular, distinct independent, and not interlinked units. Each unit or lobule is functionally independent from the others. The lobule measures about 250 × 300 microns. All the lobules together form the choriocapillaris whose capillaries are larger in size than other capillaries (20 microns) and they have fenestrated walls that allow the fluorescein to rapidly diffuse. The choriocapillaris provides nutrition and oxygen to the outer third of the retina (pigment epithelium, cones and rods).
Figure 2: Choroid vascular supply. Choroid is supplied by the posterior ciliary arteries (from Hayreh, modified) – vascular districts form distinct wedges or triangle.
The veins of the choroid, originating from each lobule, flow into the 4 or 6 vortex veins located in the four quadrants of the bulb at the equator.