Optic Nerve Anatomy
The entire visual pathway is described and illustrated in BCSC Section 5, Neuro-Ophthalmology. For further discussion of retinal involvement in the visual process, see Section 12, Retina and Vitreous.
An understanding of the normal and pathologic appearance of the optic nerve allows the clinician to detect and monitor glaucoma. The optic nerve is composed of the axons of retinal ganglion cells (RGCs), glial tissue, extracellular matrix, and blood vessels. The human optic nerve consists of approximately 1.2–1.5 million RGC axons, although there is significant individual variability. The cell bodies of the RGCs lie in the ganglion cell layer of the retina, and their axons synapse in the lateral geniculate nucleus. The intraorbital portion of the optic nerve has 2 components: the anterior optic nerve and the posterior optic nerve. The anterior optic nerve extends from the retinal surface to the retrolaminar region, where the nerve exits the posterior aspect of the globe.
Figure 5-1 Anatomy of retinal nerve fiber distribution. Inset depicts cross-sectional view of axonal arrangement. Peripheral fibers run closer to the choroid and exit in the periphery of the optic nerve, whereas fibers originating closer to the nerve head are situated closer to the vitreous and occupy a more central portion of the nerve. In this schematic, the fovea is depicted as being located along the horizontal meridian through the center of the optic disc. In most eyes, however, the fovea is located inferior to the horizontal meridian.
(Reproduced with permission from Shields MB. Textbook of Glaucoma. 3rd ed. Williams & Wilkins; 1992.)
The average diameter of the optic nerve head (ONH)* is approximately 1.5–1.7 mm as measured with planimetry, but it varies widely among individuals and ethnic groups. Immediately upon exiting the globe, the optic nerve expands to approximately 3–4 mm. This increase in size is accounted for by axonal myelination, glial tissue, and the beginning of the leptomeninges (optic nerve sheath). The axons are separated into fascicles within the optic nerve, with the intervening spaces occupied by astrocytes.
Figure 5-1 shows the distribution of nerve fibers as they enter the ONH. The arcuate nerve fibers entering the superior and inferior poles of the optic disc are more susceptible to glaucomatous damage, possibly because of the larger size of the pores of the lamina cribrosa in these regions. This susceptibility explains the frequent occurrence of arcuate visual field defects in eyes with glaucoma. The arrangement of the axons in the ONH and their differential susceptibility to damage determine the patterns of visual field loss in glaucoma, which are described and illustrated in Chapter 6 in this volume.
*Note: The terms optic disc and optic nerve head are used interchangeably in the literature and in this book.
The anterior optic nerve can be divided into 4 layers:
The most anterior zone is the superficial nerve fiber layer region, which is continuous with the nerve fiber layer (NFL) of the retina. This region is composed primarily of the axons of the RGCs in transition from the superficial retina to the neuronal component of the optic nerve. The NFL can best be visualized with slit-lamp biomicroscopy or in fundus photographs with a red-free (green) filter. Immediately posterior to the NFL is the prelaminar region, which lies adjacent to the peripapillary choroid. More posteriorly, the laminar region is continuous with the sclera and is composed of the lamina cribrosa, a structure consisting of fenestrated connective tissue lamellae that allow the transit of neural fibers through the scleral coat. Finally, the retrolaminar region posterior to the lamina cribrosa is marked by the beginning of axonal myelination and is surrounded by the leptomeninges of the central nervous system.
The lamina cribrosa provides the main structural support for the optic nerve as it exits the eye. It consists of a reticulated network of connective tissue beams that are composed primarily of collagen (Fig 2-5). Other extracellular matrix components include elastin, laminin, and fibronectin. The laminar beams also contain the capillaries that nourish this critical region. Neural components of the optic nerve pass through these connective tissue beams. In addition, relatively large central fenestrations allow transit of the central retinal artery and central retinal vein. Scanning electron microscopy of the normal lamina cribrosa discloses a lower density of connective tissue and extracellular matrix material at its inferior and superior poles, where the laminar pores are larger in comparison to the nasal and temporal regions. This difference may explain the greater susceptibility to damage of the inferior and superior regions of the optic nerve. The pores of the lamina cribrosa may often be seen by ophthalmoscopy at the base of the optic disc cup. Between the optic nerve and the adjacent choroidal and scleral tissue lies a rim of connective tissue called the ring of Elschnig.
Blood Supply to the Optic Nerve
The vascular anatomy of the anterior optic nerve and peripapillary region has been studied extensively (Fig 3-5). The arterial supply of the anterior optic nerve is derived entirely from branches of the ophthalmic artery via 1–5 posterior ciliary arteries. Typically, between 2 and 4 posterior ciliary arteries course anteriorly before dividing into approximately 10–20 short posterior ciliary arteries prior to entering the posterior globe. Often, the posterior ciliary arteries separate into a medial and a lateral group before branching into the short posterior ciliary arteries. These arteries penetrate the perineural sclera of the posterior globe to supply the peripapillary choroid, as well as most of the anterior optic nerve. Some short posterior ciliary arteries course, without branching, through the sclera directly into the choroid; others divide within the sclera to provide branches to both the choroid and the optic nerve. Often a discontinuous arterial circle, the circle of Zinn-Haller, exists within the perineural sclera. The central retinal artery, also a posterior orbital branch of the ophthalmic artery, penetrates the optic nerve approximately 10–15 mm posterior to the globe. The central retinal artery has few, if any, intraneural branches; the exception is an occasional small branch within the retrolaminar region, which may anastomose with the pial system. The central retinal artery courses adjacent to the central retinal vein within the central portion of the optic nerve.
Figure 5-2 Imaging of the lamina cribrosa, which consists of a reticulated network of connective tissue beams. A, Anterior view of the human lamina cribrosa from a healthy donor, obtained from 3-dimensional episcopic autofluorescent reconstruction illustrating the reticular network of supportive connective tissue. B, A scanning electron micrograph of a nonglaucomatous human lamina cribrosa after trypsin digestion viewed en face. The arrow points to the central retinal vessels. S and T indicate the superior and temporal regions of the lamina cribrosa, respectively. The density of the connective tissue and the size of the laminar pores vary by region. The lamina cribrosa is thought to be more structurally vulnerable to damage in the inferior and superior poles of the optic nerve head (ONH) because of the larger pores in these locations.
(Part A courtesy of Crawford Downs, PhD, and Christopher Girkin, MD; part B courtesy of Harry A. Quigley, MD.)
The superficial NFL is supplied principally by recurrent retinal arterioles branching from the central retinal artery. These small vessels, referred to as epipapillary vessels, originate in the peripapillary NFL and run toward the center of the ONH. The capillary branches from these vessels are continuous with the retinal capillaries at the ONH margin, but they also have posterior anastomoses with the prelaminar capillaries of the optic nerve. The temporal NFL may have an arterial contribution from the cilioretinal artery, when it is present.
The prelaminar region is principally supplied by direct branches of the short posterior ciliary arteries and by branches of the circle of Zinn-Haller, when it is present. In eyes with a well-developed circle of Zinn-Haller, arterial branches emerge to supply both the prelaminar and laminar regions. Similar to the prelaminar region, the lamina cribrosa region receives its blood supply from branches of the short posterior ciliary arteries or from branches of the circle of Zinn-Haller. These precapillary branches perforate the outer aspects of the lamina cribrosa before branching into an intraseptal capillary network. Arterioles also branch from the short posterior ciliary arteries and the circle of Zinn-Haller and course posteriorly to supply the pial arteries. These pial arteries often contribute to the laminar region. As in the prelaminar region, the larger vessels of the peripapillary choroid may contribute occasional small arterioles to this region, although there is no connection between the peripapillary choriocapillaris and the capillaries of the optic nerve.
Figure 5-3 Anterior optic nerve vasculature. A, Arterial supply to the anterior optic nerve and peripapillary choroid. B, Venous drainage of the anterior optic nerve and peripapillary choroid. C = choroid; CRA = central retinal artery; CRV = central retinal vein; LC = lamina cribrosa; NFL = nerve fiber layer; ON = optic nerve; PCA = posterior ciliary artery; PL = prelamina; R = retina; RL = retrolamina; S = sclera.
(Reprinted with permission from Cioffi GA. In Ritch R, Shields MB, Krupin T, eds. The Glaucomas. 2nd ed. Mosby; 1996:178, 183, Figs 8-2, 8-12.)
The retrolaminar region is also supplied by branches from the short posterior ciliary arteries, as well as by the pial arterial branches coursing adjacent to the retrolaminar optic nerve region. The pial arteries originate both from the central retinal artery, before it pierces the retrobulbar optic nerve, and from branches of the short posterior ciliary arteries more anteriorly. The central retinal artery may supply several small intraneural branches in the retrolaminar region.
The rich capillary beds of each of the 4 regions within the anterior optic nerve are anatomically confluent. Venous drainage of the anterior optic nerve occurs almost exclusively via a single vein, the central retinal vein. In the NFL, blood drains directly into the retinal veins, which then join to form the central retinal vein. In the prelaminar, laminar, and retrolaminar regions, venous drainage also occurs via the central retinal vein or axial tributaries to the central retinal vein.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.