My study showed that the ophthalmic artery in man gives out one (in 3 %), two (in 48 %), three (in 39 %), four (in 8 %), or five (in 2 %) PCAs. When there are more than three PCAs, the additional arteries are usually small in size.

I have designated the PCAs according to their relation to the optic nerve near their site of entry into the eyeball (Fig. 4.2). They are called:

According to some authors, the PCAs arise from the first part of the ophthalmic artery [53, 56, 57, 60], or the second part superior to the nerve [54, 59, 62], or medial to the nerve [55, 59]; others said that they arose on either side of the optic nerve. von Sömmerring [50] mentioned that the lateral PCA arose earlier than the lacrimal, while the medial PCA arose above the optic nerve. Meyer [52] described the medial PCA and the central retinal artery arising together first, followed by the lateral PCA. In 11 of his 17 specimens, in which the ophthalmic artery crossed over the optic nerve, the medial PCA with the central retinal artery arose before the lateral PCA; in the remaining 6 the lateral PCA (alone or with other branches such as the central retinal artery or muscular arteries) arose before the medial PCA. In the 3 specimens in whom the ophthalmic artery crossed under the optic nerve, the lateral PCA always arose first, followed by the central retinal artery and then the medial PCA in common with the muscular arteries.

In my series [65], however, the medial and lateral PCAs originated from the first part in 25 specimens, and from the angle in 39, and 48 medial and 37 lateral PCAs arose distal to the angle (Fig. 4.3). Where the origin was distal to the angle, the PCAs arose not only from the second part but even from the bend and the initial portion of the third part, especially the medial PCA. In nine specimens, all the PCAs arose distal to the angle, and in all these the ophthalmic artery crossed over the optic nerve. In the remaining specimens, one or both of the medial and lateral PCAs arose before the angle. There was no relationship of any kind between the number of PCAs, which might be present in a specimen, and their site of origin. Those in which one of the PCAs was situated distal to the angle were more numerous than those in which they arose before the angle and there were no special accessory branches and anastomoses in such cases. All these points contradict the views expressed by Sudakevitch [5]. PCAs may arise in common with the central retinal artery (see Chap. 2) – this has clinical importance (see Chap. 13). The PCAs and their branches are usually markedly tortuous.

The various PCAs run forward, divide into a large number of small branches and pierce the sclera near the optic nerve (Fig. 4.2). At that point, they are of two types:

Experimentally in rhesus monkeys, I cut one or another or all the PCAs in 85 eyes and evaluated the area of their distribution in the choroid and ONH and anastomoses, by fluorescein fundus angiography (Figs. 4.4 and 4.5) [15, 23, 24, 37]. PCA occlusion caused by thrombosis is well documented in patients with giant cell arteritis, by various histopathological studies [33, 35, 70–75]. That provided a model of PCA occlusion in man, to evaluate the in vivo distribution and anastomoses of the PCA in the choroid and ONH by fluorescein fundus angiography [30, 31, 42, 76–78]. These experimental and clinical studies showed that each PCA has a segmental distribution in the choroid and ONH, and they do not anastomose either with the adjacent PCAs or with the anterior ciliary arteries [15, 23, 24, 30, 31, 37, 42, 67, 68, 76–79]. Similarly when I produced anterior ciliary artery occlusion by recession and resection of the various recti in rhesus monkeys as well as in patients with strabismus, the PCAs did not fill the area of the anterior uvea supplied by the occluded anterior ciliary artery [34, 45].

The PCAs not only supply the choroid but also play an important role in the blood supply of the ONH [19, 32, 35, 42, 67, 68, 77, 79]. These studies showed that the lateral and medial PCAs supply the corresponding parts of the choroid [15, 20, 22, 30–33, 35, 37, 39, 42, 46]. There is marked interindividual variation in the area supplied by the two PCAs in man [30–32, 35, 39, 42, 46]. The medial PCA may supply the entire nasal choroid up to the level of the fovea, including the ONH (Figs. 4.6 and 4.7), or its supply may stop short nasal to the nasal peripapillary choroid (Fig. 4.8), so that it may take no part in the blood supply of the ONH, or there may be any variation between these two extremes (Fig. 4.9). The lateral PCA supplies the area of the choroid not supplied by the medial PCA or vice versa (Figs. 4.4, 4.5, 4.6, 4.7, 4.8, and 4.9). When there are more than one medial or lateral PCAs, the area supplied by each of them may be one quadrant or only a sector (Figs. 4.10a, 4.11, 4.12, and 4.13). When the superior PCA is present, it would accordingly supply a superior sector (Fig. 4.14). Given the marked interindividual variation in number reported in the literature (Table 4.1) and in my study (see above), and distribution by the various PCAs, there can be an extremely variable pattern of distribution by them in the choroid and ONH. My clinical and experimental studies have shown that the various PCAs do not anastomose with one another, and each behaves like an end artery [15, 20, 22, 30–33, 35, 37, 39, 42, 46].

Experimental occlusion of one or another SPCA in 87 eyes of rhesus monkeys [28] resulted in a segmental choroidal filling defect in the area of distribution of the occluded artery (Figs. 4.15, 4.16, and 4.17). There are no anastomoses between the adjacent short PCAs [28, 32]. As described above, the SPCAs are of two types (Fig. 4.2) [17, 67]:

In addition to that, the short PCAs give off branches during their intraorbital course to the pial plexus on the optic nerve, at their site of penetration into the scleral and episcleral arterial plexus and during their course in the sclera to the arterial circle of Haller and Zinn (Fig. 4.21).

There was a stereotyped description of the long posterior ciliary arteries (LPCAs) in the textbooks, virtually unchanged since the early accounts [82, 83]. According to these earlier accounts, there were two such arteries, one medial and the other lateral (Fig. 4.2), arising from the ophthalmic artery either independently or by a common trunk, and sometimes one or the other arose from the ophthalmic artery in common with the central retinal artery or SPCAs [82, 84, 85]. From this description, it seems they were confused with the Main PCAs. The arteries pierced the sclera nasal and temporal to the optic nerve (the former 3.6 and the latter 3.9 mm from the optic nerve [85, 86]) and ran horizontally at first in a scleral canal (3–7 mm long [82] and 3–5 mm long [86]), and then in the suprachoroidal space, as far forward as the posterior part of the ciliary body, where each artery divided into two main divisions – superior and inferior. The superior and inferior divisions of the two LPCAs united to form the major arterial circle of the iris in the anterior part of the ciliary body behind the root of the iris. Of the anterior ciliary arteries (usually two from each rectus muscle except that there is only one from the lateral rectus), some joined the greater arterial circle of the iris and others the LPCAs or their branches [83]. The LPCA, according to all these descriptions, gave no branches throughout its entire course until it divided into its two main divisions in the ciliary muscle. It was held that recurrent choroidal branches, supplying the anterior part of the choroid up to the equator and anastomosing with the SPCAs at the equator, arose from the greater arterial circle of the iris, anterior ciliary arteries, and terminal part of the LPCAs [8, 9]. No anastomoses between the SPCAs and the recurrent choroidal arteries at the equator were found in rabbits [5, 82]. Thus, according to all the available descriptions, the LPCAs supplied the choroid only in front of the equator via the recurrent choroidal arteries, with no supply to the choroid behind the equator.

There are many studies on the effects of experimental occlusion of the LPCAs on the intraocular pressure and the anterior segment of the eye [87–93]. Most of these studies have been carried out in rabbits, and the production of ocular hypotony, phthisis bulbi, iris neovascularization, cataract, and anterior segment necrosis has been recorded. The occurrence of anterior segment ischemia following diathermy, scleral buckling, and some other procedures for retinal detachment has been attributed to interference with the LPCAs. Unsuccessful attempts were made to find out if occlusion of the LPCA alone or with anterior ciliary arteries would help to control the raised intraocular pressure (lOP) in glaucoma patients [90, 94–101]. Conflicting claims of temporary falls in lOP, of variable amounts and for varying periods, have been made.

My study, in 22 rhesus monkeys after occlusion of the temporal LPCA and evaluation by fluorescein fundus angiography, revealed that the LPCA supplies the choroid forward from the point where it enters the eyeball, and occlusion of the LPCA produces no significant effect on the IOP or the eye.

This part of the choroid is supplied by branches from the short PCAs. As in the rest of the choroid, postmortem cast studies have consistently shown free anastomoses in the peripapillary choroidal arteries so as to form a continuous vascular network [12]. Shimizu and Ujiie [12] in their choroidal casts found that branches of the short PCAs run centripetally while undergoing a series of bifurcations; an incomplete arterial ring was found along the margins of the optic nerve head, formed by the numerous arteries entering the arterial ring, and the latter was regarded by the authors as an interarterial anastomosis. They concluded that the peripapillary choroidal arterioles therefore cannot be regarded as end arteries.

The peripapillary choroid is a very important part of the choroidal vasculature, since it is the main source of blood supply to the prelaminar and retrolaminar parts of the optic nerve (Fig. 4.1) [12, 19, 20, 32, 42, 46, 105]. There are some serious misunderstandings prevailing on the subject. It is not uncommon to find authors stating that if the peripapillary choroid is so very important in the blood supply of the anterior part of the optic nerve, then why does the optic disc remain normal in eyes with peripapillary choroidal atrophy? The branches supplying the optic nerve arise essentially from the main arteries in the peripapillary choroid and not from the small branches, and the choriocapillaris stops abruptly at the disc margin (Figs. 4.26 and 4.27). In almost all peripapillary choroidal atrophy which is ophthalmoscopically visible, the loss is limited to the choriocapillaris, fine vessels, and overlying retinal pigment epithelium, with the large vessels still intact (Fig. 4.28).