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).
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.
Figure 1-17 Basal view of the brain showing the anterior and posterior visual pathways.
(Illustration by Dave Peace.)
The absence of retinal receptors over the ONH creates a physiologic scotoma (blind spot), located approximately 17° from the fovea and measuring approximately 5° by 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 ONH.
The RPE is in direct contact with the retinal photoreceptor cells. Between the outer and inner retinal layers, the retinal signal generated in rods and cones is processed primarily through the bipolar cells that synapse on retinal ganglion cells (RGCs). A subset of melanopsin-containing RGCs, 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—play an important metabolic role.
The ratio of photoreceptor cells to RGCs varies depending on the region of the retina. The ratio is highest in the periphery (at more than 1000:1) and lowest at the fovea (where an RGC may receive a signal from 1 single cone). Because of the increased central density of RGCs (69% within the central 30°), the foveal bipolar cells and RGCs are displaced radially from the fovea, with the bipolar cells receiving input via long cone axons that make up the Henle layer. The radial nature of the Henle layer is responsible for the accumulation of fluid in the macula in a star-shaped pattern.
Another notable anatomical feature of the retina is the location of the ONH 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 ONH, those coming from the temporal retina must avoid the macula by anatomically separating to enter the ONH 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 ONH on its temporal side (papillomacular bundle). Focal loss of the nerve fiber layer may appear as either grooves or slits or as reflections that parallel the retinal arterioles where the internal limiting membrane drapes over the vessels. Diffuse loss of the nerve fiber layer is often more difficult to detect and brings the retinal vessels into sharp relief.
Figure 1-18 Anatomy of retinal ganglion cell axons in the retina and optic nerve. A, Pattern of the nerve fiber layer of axons from ganglion cells to the ONH. Superior, inferior, and nasal fibers take a fairly straight course. Temporal axons originate above and below the horizontal raphe (HR) and take an arching course to the ONH. Axons arising from ganglion cells in the nasal macula project directly to the ONH as the papillomacular bundle (PM). B, Lesions involving the decussating nasal retinal fibers (represented by the dashed red line) can result in bowtie atrophy. C, Schematic depiction of damage to nasal and papillomacular 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 Monographs 10. New York: Oxford University Press, in cooperation with the American Academy of Ophthalmology; 2007:5; part B illustration by Christine Gralapp; part C courtesy of Neil Miller, MD; part D courtesy of Lanning Kline, MD.)
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.