The blood supply of the optic nerve varies from one segment of the nerve to another. Although the blood supply can vary widely, a multitude of studies have revealed a basic pattern (Fig 3-12). See Table 3-1, which summarizes the blood supply of the optic nerve. The blood supply of the visual pathway is summarized in Table 3-2 and depicted in Figure 3-13. The following sections discuss the vascular supply of the intraocular and intraorbital segments in greater detail.
The ophthalmic artery lies inferior to the optic nerve. The CRA and, usually, 2 long posterior ciliary arteries branch off from the ophthalmic artery after it enters the muscle cone at the annulus of Zinn.
The lumen of the CRA is surrounded by nonfenestrated endothelial cells with typical zonulae occludens that are similar to those in retinal blood vessels. The CRA, however, differs from retinal arterioles in that it contains a fenestrated internal elastic lamina and an outer layer of smooth muscle cells surrounded by a thin basement membrane. The retinal arterioles have no internal elastic lamina, and they lose their smooth muscle cells shortly after entering the retina. The CRV consists of endothelial cells, a thin basal lamina, and a thick collagenous adventitia.
Figure 3-12 Schematic representation of the vascular supply to the optic nerve and ONH. Intraocular view (A), lateral view (B), and sagittal view (C) of the ONH. Short posterior ciliary arteries supply centripetal capillary beds of the anterior ONH. The central retinal artery (CRA) contribution is restricted to nerve fiber layer capillaries and capillaries of the anterior intraorbital optic nerve. Capillary beds at all levels drain into the central retinal vein (CRV). A = arachnoid; Ch = choroid; ColBr = collateral branches; D = dura; LC = lamina cribrosa; NFL = superficial nerve fiber layer of the ONH; ON = optic nerve; P = pia; PCilA = posterior ciliary artery; R = retina; RA = retinal arteriole; S = sclera; SAS = subarachnoid space.
(Part C reproduced with permission from Hayreh SS. The blood supply of the optic nerve head and the evaluation of it—myth and reality. Prog Retin Eye Res. 2001;20(5):563–593.)
The lamina cribrosa is supplied by branches of the arterial circle of Zinn-Haller (Fig 3-14). This circle arises from the para-optic branches of the short posterior ciliary arteries and is usually embedded in the sclera around the nerve head. It is often incomplete and may be divided into superior and inferior halves. Involvement of the inferior half is the likely cause of altitudinal (superior or inferior hemifield) visual field defects following an episode of nonarteritic anterior ischemic optic neuropathy.
Of note, the posterior ciliary arteries are terminal arteries, and the area where the respective capillary beds from each artery meet is termed the watershed zone. When perfusion pressure drops, the tissue lying within this area is the most vulnerable to ischemia. Consequences can be significant when the entire ONH or a part of it lies within the watershed zone.
Figure 3-13 Vascular supply of the optic nerve and visual pathway.
(Modified with permission from Forrester JV, Dick AD, McMenamin PG, Roberts F, Pearlman E. The Eye: Basic Sciences in Practice. 4th ed. Edinburgh: Elsevier; 2016:98.)
Tan NY, Koh V, Girard MJ, Cheng CY. Imaging of the lamina cribrosa and its role in glaucoma: a review. Clin Exp Ophthalmol. 2018;46(2):177–188.
The intraorbital region of the optic nerve is supplied proximally by the pial vascular network and by neighboring branches of the ophthalmic artery. Distally, it is supplied by intraneural branches of the CRA. Most anteriorly, it is supplied by short posterior ciliary arteries and infrequently by peripapillary choroidal arteries.
Table 3-2 Blood Supply of the Visual Pathway
Figure 3-14 Circle of Zinn-Haller. Electron microscopy of the retrolaminar vascular circle (left). Branches from the circle to the optic nerve (right).
(Reproduced with permission from Spalton D, Hitchings R, Hunter P. Atlas of Clinical Ophthalmology. 3rd ed. New York: Elsevier/Mosby; 2005:563.)
Figure 3-15 Oculomotor nucleus complex. Note that all extraocular muscles served by CN III are innervated by their respective ipsilateral nuclei except the superior rectus muscle. Parasympathetic fibers traveling to the pupillary sphincter muscle synapse in the ciliary ganglion in the orbit. m. = muscle.
(Illustration by Christine Gralapp.)
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.