Nolte J. The Human Brain: An Introduction to Its Functional Anatomy. 5th ed. St Louis: Mosby; 2002. Rootman J, Stewart B, Goldberg RA. Orbital Surgery: A Conceptual Approach. Philadelphia: Lippincott; 1995.
Afferent Visual Pathways
It is important to recognize that any disturbance in afferent function may result in the same symptoms of vision loss as observed with pathology affecting the retina, optic nerve, and visual pathways (Fig 1-17).
Figure 1-17 Basal view of the brain showing the anterior and posterior visual pathways. (Illustration b y Dave Peace.)
Retina
The afferent visual pathway begins within the retina. Details of retinal anatomy can be found in BCSC Section 2, Fundamentals and Principles of Ophthalmology, and Section 12, Retina and Vitreous.
The following discussion focuses on key points relevant to neuro-ophthalmology. The absence of retinal receptors over the optic disc creates a physiologic scotoma (the blind spot), located
approximately 17° from the fovea and measuring approximately 5° × 7°. The fovea (approximately 1.5 mm, or 1 disc diameter) is located approximately 4 mm (or 2.5 disc diameters) from and 0.8 mm lower than the optic disc.
The retinal pigment epithelium (RPE) is in direct contact with the retinal photoreceptor cells. Between the outer and inner retinal layers, the retinal signal starting in the rods and cones is processed primarily through the bipolar cells that connect the photoreceptors to the retinal ganglion cells (RGCs). A newly described subset of RGCs containing melanopsin—known as intrinsically photosensitive retinal ganglion cells (ipRGCs)—serve primarily nonvisual light-dependent functions such as the pupillary light reflex.
Horizontal, amacrine, and interplexiform cells (which communicate horizontally between neighboring cells) permit signal processing within the retinal layers. The glial support cells—Müller cells and astrocytes—also affect image processing and probably play a metabolic role as well.
There is a variable ratio of photoreceptor cells to ganglion cells in different regions of the retina. The ratio is highest in the periphery (at more than 1,000:1) and lowest at the fovea (where a ganglion cell may receive a signal from a single cone). Because of the increased density of ganglion cells centrally (69% within the central 30°), the bipolar cells are oriented radially within the macula. This radial arrangement of the axons of the bipolar cells (the Henle layer) is responsible for fluid accumulation in a star-shaped pattern. Another key anatomical feature of the retina is the location of the optic disc and the beginning of the optic nerve nasal to the fovea. Thus, although ganglion cell fibers coming from the nasal retina can travel uninterrupted directly to the disc, those coming from the temporal retina must avoid the macula by anatomically separating to enter the disc at either the superior or the inferior pole (Fig 1-18). This unique anatomy means that some of the nasal fibers (nasal within the macula) enter the disc on its temporal side (papillomacular bundle). Focal loss of the nerve fiber layer may appear as grooves or slits or as reflections paralleling the retinal arterioles where the internal limiting membrane drapes over the vessels, whereas diffuse nerve fiber layer loss is often more difficult to detect and brings the retinal vessels into sharp relief.
Figure 1-18 A, Pattern of the nerve fiber layer of axons from ganglion cells to the optic disc. Superior, inferior, and nasal fibers take a fairly straight course. Temporal axons originate above and below horizontal raphe (HR) and take an arching course to the disc. Axons arising from ganglion cells in the nasal macula project directly to the disc as the papillomacular bundle (PM). B, Lesions involving the decussating nasal retinal fibers, represented by the dashed red line, can result in bow-tie atrophy. C, Schematic depiction of damage to nasal and macular fibers of the retina and patterns of nasal and temporal optic nerve atrophy (represented by red outlined triangles) corresponding to damage to crossing nasal fibers. Therefore, band, or bow-tie, atrophy occurs with loss of nasal macular and peripheral fibers in the contralateral eye of a patient with a pregeniculate homonymous hemianopia or a bitemporal hemianopia. D, Clinical photograph of a right optic
nerve demonstrating bow-tie atrophy. (Part A reprinted from Kline LB, Foroozan R, eds. Optic Nerve Disorders. 2nd ed. Ophthalmology Monograph 10. New York: Oxford University Press, in cooperation with the American Academy of Ophthalmology; 2007:5; part B illustration b y Christine Gralapp; part C courtesy of Neil Miller, MD; part D courtesy of Lanning Kline, MD.)
Optic Nerve
The optic nerve begins anatomically at the optic disc but physiologically and functionally within the ganglion cell layer that covers the entire retina. The first portion of the optic nerve, representing the confluence of approximately 1.0–1.2 million ganglion cell axons, traverses the sclera through the
lamina cribrosa, which contains approximately 200–300 channels. The combination of small channels and a unique blood supply (largely from branches of the posterior ciliary arteries) probably plays a role in several optic neuropathies. The axons of the optic nerve depend on metabolic production within the ganglion cell bodies in the retina. Axonal transport—both anterograde and retrograde—of molecules, subcellular organelles, and metabolic products occurs along the length of the optic nerve and is an energy-dependent system requiring high concentrations of oxygen. The anterograde axonal transport system can be subdivided into slow, intermediate, and fast speeds. The axonal transport system is sensitive to ischemic, inflammatory, and compressive processes. Interruption of axonal transport, from whatever cause, can produce disc edema.
Just posterior to the sclera, the optic nerve acquires a dural sheath that is contiguous with the periorbita of the optic canal and an arachnoid membrane that supports and protects the axons and is contiguous with the arachnoid of the subdural intracranial space through the optic canal. This arrangement permits free circulation of CSF around the optic nerve up to the optic disc. Just posterior to the lamina cribrosa, the optic nerve also acquires a myelin coating, which increases its diameter to approximately 3 mm (6 mm in diameter, including the optic nerve sheath) from the 1.5 mm of the optic disc. The myelin investment is part of the membrane of oligodendrocytes that join the nerve posterior to the sclera.
The intraorbital optic nerve extends approximately 30 mm to the optic canal. The extra length of the intraorbital optic nerve allows unimpeded globe rotation as well as axial shifts within the orbit. The CRA and CRV travel within the anterior 10–12 mm of the optic nerve. The CRA supplies only a minor portion of the optic nerve circulation; most of the blood supply comes from pial branches of the surrounding meninges, which are in turn supplied by small branches of the OphA (see Fig 1-10). Topographic (retinotopic) representation is maintained throughout the optic nerve. Peripheral retinal receptors are found more peripherally, and the papillomacular bundle travels temporally and increasingly centrally within the nerve.
As the optic nerve enters the optic canal, the dural sheath fuses with the periorbita. It is also surrounded by the annulus of Zinn, which serves as the origin of the 4 rectus muscles and the superior oblique muscle. Within the canal, the optic nerve is accompanied by the OphA inferiorly and separated from the superior orbital fissure by the optic strut (the lateral aspect of the lesser wing of the sphenoid), which terminates superiorly as the anterior clinoid. Medially, the optic nerve is separated from the sphenoid sinus by bone that may be thin or dehiscent. The optic canal normally measures approximately 8–10 mm long and 5–7 mm wide but may be elongated and narrowed by processes that cause bone thickening (eg, fibrous dysplasia, intraosseous meningioma). The canal runs superiorly and medially. Within the canal, the optic nerve is relatively anchored and can easily be injured by shearing forces transmitted from blunt facial trauma (see Chapter 4).
At its intracranial passage, the optic nerve passes under a fold of dura (the falciform ligament) that may impinge on the nerve, especially if it is elevated by lesions arising from the bone of the sphenoid (tuberculum) or the sella. Once it becomes intracranial, the optic nerve no longer has a sheath. The anterior loop of the carotid artery usually lies just below and temporal to the nerve, and the proximal anterior cerebral artery passes over the nerve. The gyrus rectus, the most inferior portion of the frontal lobe, lies above and parallel to the optic nerves. The 8–12 mm intracranial portion of the optic nerve terminates in the optic chiasm.
Optic Chiasm
The optic chiasm measures approximately 12 mm wide, 8 mm long in the anteroposterior direction, and 4 mm thick (Fig 1-19). It is inclined at almost 45° and is supplied by small branches off the proximal anterior cerebral and anterior communicating arteries. The chiasm is located just anterior to the hypothalamus and the anterior third ventricle (forming part of its anterior wall and causing an invagination) and approximately 10 mm above the sella. The exact location of the chiasm with respect to the sella is variable. Most of the time it is directly superior, but in approximately 17% of individuals it is anterior (prefixed), and in approximately 4% it is posterior (postfixed) (Fig 1-20).
Figure 1-19 Anatomical dissection of the optic chiasm and surrounding structures. A, Sagittal view. B, Superior view.
(Courtesy of Alb ert L. Rhoton, Jr, MD.)
Figure 1-20 Position of the optic chiasm in relationship to the tuberculum sella. (Illustration b y Dave Peace.)
Within the chiasm, the fibers coming from the nasal retina (approximately 53% of total fibers) cross to the opposite side to join the corresponding contralateral fibers. The inferior fibers (subserving the superior visual field) are first to cross. Evidence suggests that the anterior loop of fibers into the contralateral optic nerve (Wilbrand knee) is an artifact; however, the finding of a superior temporal visual field defect contralateral to a central scotoma is helpful clinically in localizing pathology to the junction of the optic nerve and chiasm. The macular fibers tend to cross posteriorly within the chiasm; this arrangement underlies the bitemporal scotomatous field defects observed with posterior chiasmatic compression.