FIGURE 13-11

Vascular supply of visual pathway from retina to occipital cortex. (From Harrington DO: The visual fields, ed 5, St Louis, 1981, Mosby.)

central retinal artery. The circle of Zinn, the anastomotic ring of branches of the short ciliary arteries, and peripapillary vessels supply the optic disc.6 Capillaries within the optic nerve are composed of nonfenestrated endothelium joined by zonula occludens, thus the vessels perfusing the nerve head are part of the blood-brain barrier.10,39 Pial vessels supply the optic nerve throughout its length; the intraorbital portion is supplied by branches from the ophthalmic artery, and the intracranial optic nerve is nourished by branches of the ophthalmic, anterior cerebral, anterior communicating, and internal carotid arteries.6,8

The blood supply to the optic chiasm is rich and anastomotic, with arterioles from the circle of Willis forming capillary beds at two levels.40,41 The superior network is supplied by the anterior cerebral and anterior communicating arteries, whereas the inferior network is supplied by the internal carotid, posterior cerebral, and posterior communicating arteries.6,8,12,13 The anterior choroidal artery, a branch of the internal carotid, is a primary supplier of the optic tract, although small branches from the middle cerebral artery also contribute.6,8,12,42,43 The blood supply to the LGN is derived from the anterior choroidal artery and the lateral choroidal and posterior choroidal branches of the posterior cerebral artery.8,42,44

The optic radiations can be divided into three sections: (1) the anterior radiations, which pass laterally over the inferior horn of the ventricle, are supplied by

the anterior choroidal artery and the middle cerebral artery; (2) the middle group of fibers passing lateral to the ventricle is supplied by the deep optic branch of the middle cerebral artery; and (3) branches of the posterior cerebral artery, including the calcarine branch, supply the posterior radiations as they spread out in the occipital lobe. Branches from the middle cerebral artery also contribute.6,8,12 The calcarine branch of the posterior cerebral artery is the major blood supply for the striate cortex, often supplemented by the posterior temporal or parietooccipital branch of the posterior cerebral artery or the occipital branch of the middle cerebral artery.6,8,12,45

F I B E R O R I E N T A T I O N A N D V I S U A L F I E L D S

With the eye looking straight ahead and fixating on an object, one is able to detect other objects around the point of regard, although the details may not be discernible. This entire visible area is termed the visual field. Information from the visual field is taken in by the retina and processed through the afferent visual sensory pathway. The location and orderly arrangement of the fibers throughout this pathway have been extensively studied. Damage in this afferent visual pathway will cause a defect in the visual field. Knowledge of the fiber patterns in the pathway can help to identify the location of the lesion on the basis of the resultant visual field defect.

RETINA

The axons of the retinal ganglion cells form characteristic patterns in the nerve fiber layer. The group of fibers that course from the macular area to the optic disc is called the papillomacular bundle. The superior and inferior temporal fibers, separated at the 180th meridian by the horizontal retinal raphe, must arch superiorly and inferiorly around the macular area, forming characteristic arcuate patterns in their course to the optic disc; the temporal retinal vessels usually do not cross the horizontal raphe either. The nasal fibers can travel directly to the optic disc and are described as radiating (Figure 13-12). Nasal and temporal fibers are separated by a theoretic vertical line passing through the center of the fovea. The long nerve fibers, from the peripheral retina, are more vitread in location than are the short peripapillary fibers, with extensive intermingling in the prelaminar optic nerve.46

OPTIC DISC

All of the axons in the nerve fiber layer come together at the optic disc, creating a specific pattern. The nasal fibers radiate directly to the nasal side of the disc, whereas the papillomacular bundle courses directly to the temporal

FIGURE 13-12

Nerve fiber pattern of retina in its relationship to retinal vascular tree. (From Harrington DO: The visual fields, ed 5, St Louis, 1981, Mosby.)

side of the disc.46-48 The fibers from the superior temporal retina arch around the papillomacular bundle to enter the superior pole of the disc; fibers from the inferior temporal retina curve below the papillomacular bundle to the inferior pole.48 The macular fibers take up approximately one third of the disc, although the macular area encompasses only one twentieth of the retinal area.1,8 The temporal fibers occupy approximately one third of the disc, as do the nasal fibers (Figure 13-13, A). The boundaries between each set of fibers are not always clear-cut in all parts of the pathway. The fibers from the peripheral retina are more superficial than those coming from the central retina.48

OPTIC NERVE

Near the lamina cribrosa, the fibers have the same orientation as they do in the disc, but within a short distance the macular fibers move to the center of the nerve.3,8 The rest of the fibers take up their logical positions: superior temporal fibers in the superior temporal optic nerve, inferior temporal fibers in the inferior temporal nerve, superior nasal fibers in the superior nasal nerve, and inferior nasal fibers in the inferior nasal optic nerve (Figure 13-13, B).

OPTIC CHIASM

In the optic chiasm the nasal fibers cross (decussate); the ratio of crossed to uncrossed fibers in the chiasm is approximately 53 to 47.49 The crossing pattern

FIGURE 13-13

A, Surface of optic disc showing orientation of nerve fibers as they enter disc. B, Coronal section showing orientation of nerve fibers in optic nerve proximal to chiasm. (Right disc and nerve viewed from front.) ST, Superior temporal; SN, superior nasal; IT, inferior temporal; IN, inferior nasal; M, macular.

depends on processes that occur during embryologic development, with certain molecular guides directing the path taken by nerve fibers. The inferior nasal retinal fibers are inferior in the anterior chiasm; they cross to the other side, and many loop into the terminal part of the opposite optic nerve before turning to run back through the chiasm into the contralateral optic tract1 (Figure 13-14). These anterior loops (anterior knees of Wilbrand) bring fibers from the opposite eye into the posterior optic nerve.50 Some investigators believe that these “knees” are artifacts and suggest that the fibers shift into such a location after loss from fiber degeneration.51

The superior nasal fibers enter the superior chiasm, where they cross and then leave the chiasm in the contralateral optic tract; some of these fibers loop posteriorly into the optic tract on the same side before crossing. The fibers were historically called the posterior knees of Wilbrand1,8 (see Figure 13-14). The fibers from the temporal retina course directly back through the chiasm into the optic tract. The nasal macular fibers also cross and are spread throughout most of the chiasm.51

A small number of fibers have been identified that exit the posterior of the chiasm and enter the suprachiasmatic nucleus in the hypothalamus and have a role in synchronization of circadian rhythm.52-54

OPTIC TRACT

As the fibers leave the chiasm in the optic tract, the crossed and uncrossed fibers intermingle. The superior fibers (the fibers from both the ipsilateral superior temporal retina and the contralateral superior nasal retina) move to the medial side of the tract. The fibers from the inferior retina (ipsilateral inferior temporal retinal fibers and contralateral inferior nasal retinal fibers) occupy the lateral area of the tract.1,3 Figure 13-15 shows the regrouping that occurs as the fibers pass through the chiasm and into the optic tract. The macular fibers, crossed

and uncrossed, are located between these two groups1 (Figure 13-16).

LATERAL GENICULATE NUCLEUS

Fibers from the superior retinal quadrants terminate in the medial aspect of the LGN, whereas fibers from the inferior retinal quadrants terminate in the lateral aspect.8 A dorsal wedge, composing two thirds to three fourths of the LGN, represents the macula.3,55,56 Based on animal study mapping, each of the magnocellular and parvocellular layers receives input from just one eye: Layers 1, 4, and 6 receive fibers from the contralateral nasal retina, whereas layers 2, 3, and 5 receive ipsilateral temporal retinal fibers49 (Figure 13-17). Most of the structure, including the wedge representing the macula, contains all layers, although in the far medial and lateral aspects, some of the layers merge.6,55

The anatomic structure of the human LGN is similar to that of the monkey, so detailed maps of the monkey LGN have been applied to the human structure.57 Each layer of the LGN contains a retinotopic map or representation of the contralateral hemifield of vision. A retinotopic map is a “point-to-point localization” of the retina.14 These maps are stacked on one another, such that if a line (called a line of projection) were passed through all six layers, perpendicular to the surface, the intercepted cells all would be carrying information about the same point in the visual field. This alignment is so precise that there is a gap in each contralateral layer along the line of projection that corresponds to the location of the optic disc.15 Thus the fibers that carry information from the same site in the visual field of each eye terminate in adjacent layers of the LGN, right next to one another8 (see Figure 13-17). The fibers course through the posterior limb of the internal capsule as they leave the LGN to form the optic radiations.