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EMBRYOLOGY, ANATOMY, AND PHYSIOLOGY OF THE AFFERENT VISUAL PATHWAY A 35 B Figure 1.37. Wilbrand s knee in human tissue.

1 EMBRYOLOGY, ANATOMY, AND PHYSIOLOGY OF THE AFFERENT VISUAL PATHWAY A 35 B Figure Wilbrand s knee in human tissue. A, Woelcke myelin stain of a horizontal section through the optic chiasm from a patient whose left eye was enucleated 5 months before death. There is reduced myelin staining in the left optic nerve (arrow), but there is no evidence of Wilbrand s knee. B, Woelcke myelin stain of a horizontal section through the optic chiasm from a patient whose right eye was enucleated 2 years before death. There is more pronounced atrophy of the right optic nerve, and a small Wilbrand s knee is evident (arrow). (Courtesy of Dr. Jonathan Horton.) nerve corresponds generally to their distribution in the retina. The outflow from the papillomacular bundle is positioned temporally within the anterior optic nerve (Fig. 1.35), then gradually moves centrally in the more posterior optic nerve ( ). Intracranially, the axons partially lose their retinotopy because of the decussation of some axons (306). The macular projection does not have a precise localization in the posterior nerve (307). Historically, a Wilbrand s knee was believed to exist at the junction of the optic nerve and chiasm, where a small number of inferonasal retinal axons were believed to cross into the opposite optic nerve and pass anteriorly for a few millimeters, then turn back posteriorly to enter the chiasm (308,309). The knee is now believed to be an artifact (310). A knee can be created by removal of one eye (310). Apparently, the gliotic reaction secondary to optic nerve atrophy over a period of years distorts the anterior chiasm, pulling some axons into an aberrant location (Figs 1.36 and 1.37). Thus, a knee is not present normally. A paradox therefore exists, given that visual field defects produced by compressive lesions at the anterior chiasm often include a contralateral, superotemporal field depression (i.e., junctional scotoma), which heretofore was explained by the presence of a knee. OPTIC CHIASM EMBRYOLOGY The optic chiasm is a commissure formed by converging optic nerves anteriorly and diverging optic tracts posteriorly (Fig. 1.38). During development, the chiasmal anlage separates from the floor of the third ventricle, maintaining contact only at the boundary between its posterior aspect and the anterior-inferior wall of the third ventricle (Fig. 1.39). The location of the chiasm is established by the first RGC fibers that arrive, which occurs between the fourth and sixth week of development. The next fundamental step is the proper routing of the incoming axons. Numerous factors contribute to the directional preferences of the arriving axons (311,312) (Fig. 1.40). Glia within the optic nerve and some highly conserved molecules are believed to be instrumental to the process (306, ). For instance, Zic2 is expressed early in retinal development in the cells that will project ipsilaterally, and thus Zic2 may endow these cells with response properties that influence their trajectory at the chiasm. The differential spatial and temporal expression of the Pax2 gene by the growing axons and the sonic Hh gene at the developing chiasm are also likely relevant to the formation of the decussation (319). Specialized glial cells in the developing diencephalon extend their radial processes, which physically interact with the incoming axons (306). These glial cells express Eph/ephrinA molecules, which influence the growth of axons within the chiasm (see also the section on the retina). Metalloproteases, which modulate the interaction of local environmental cues with growth cones extending from the distal axons, also seem to be relevant for proper wiring (320). Axons that remain uncrossed tend to reach the chiasm before those that will cross, which might expose the different populations to time-varying changes in the local environment (321,322). The decision point is apparently not binary for these incoming axons. Real-time video microscopy has revealed that axons move through the chiasm in a pulsed, saltatory fash-

36 CLINICAL NEURO-OPHTHALMOLOGY Figure 1.38. The optic chiasm viewed from below. Note relationships of the mammillary bodies, oculomotor nerves (III nerve), and cerebral peduncles to the chiasm.

All axons that approach the chiasmal midline display especially long pauses in activity, with ameboid movements of their growth cones, which assume a more complex morphology than is typical of

2 36 CLINICAL NEURO-OPHTHALMOLOGY Figure The optic chiasm viewed from below. Note relationships of the mammillary bodies, oculomotor nerves (III nerve), and cerebral peduncles to the chiasm. (Redrawn from Pernkopf E. In: Ferner HA, ed. Atlas of Topographical and Applied Human Anatomy. Vol 1. Philadelphia, WB Saunders, 1963.) ion. All axons that approach the chiasmal midline display especially long pauses in activity, with ameboid movements of their growth cones, which assume a more complex morphology than is typical of advancing axons (323,324). Following this complex orchestration near the end of 3 months of gestation, an adult-like hemidecussation is established. The shape of the developing optic chiasm differs from that of the mature chiasm. The evolution in shape partly relates to the shift from a lateral to eventually a frontal position of the eyes in the head. In the second half of gestation the gross anatomic changes include (a) progressive narrowing of the angle between the two optic nerves to about 45 degrees, without a corresponding change in the angle formed by the optic tracts; (b) separation of the chiasm from the diaphragma sellae and pituitary gland with development of the chiasmatic cistern; (c) progressive elevation of the chiasm above the optic foramina; and (d) thinning of attachments of the chiasm to the floor of the third ventricle. Rarely, a chiasm does not form, and this can occur despite normal development of the fovea (325,326). Malformation of the fovea and chiasm occurs in patients with albinism. Deficient melanin production leads to an abnormally small uncrossed projection, perhaps because of the timing of RGC development (discussed previously) rather than to some influence at the chiasm per se (312). In particular, albino mice have fewer Zic2-positive retinal neurons than their pigmented counterparts, which (given the putative role of Zic2 described above) may explain the diminished ipsilateral projection (327). The disordered chiasmal projection in albinos can be recognized by MRI, which can reveal a smaller chiasmal width and a relatively wide angle between the optic nerves and the optic tracts (328). GROSS ANATOMY OF THE CHIASM AND PERICHIASMAL REGION The optic chiasm has a transverse diameter of mm, an anteroposterior width of 4 13 mm, and thickness of 3 5 mm (262,329). The optic chiasm is covered, except where attached to the brain, by arachnoid and pia mater, the latter

EMBRYOLOGY, ANATOMY, AND PHYSIOLOGY OF THE AFFERENT VISUAL PATHWAY 37 Figure 1.39.

Atlas of Topographical and Applied Human Anatomy. Vol 1. Philadelphia, WB Saunders, 1963.) being continuous with the pia of the optic nerves and part of the optic tracts.

3 EMBRYOLOGY, ANATOMY, AND PHYSIOLOGY OF THE AFFERENT VISUAL PATHWAY 37 Figure Midsagittal section through the cerebral hemispheres, showing the position of the optic chiasm relative to the third ventricle and basal cisterns. (Redrawn from Pernkopf E. In: Ferner HA, ed. Atlas of Topographical and Applied Human Anatomy. Vol 1. Philadelphia, WB Saunders, 1963.) being continuous with the pia of the optic nerves and part of the optic tracts. The optic chiasm is in direct contact with CSF anteriorly within the subarachnoid space, and posteriorly within the third ventricle (Fig and 1.41), features easily visualized with MRI (Fig. 1.42). Inferiorly, the optic chiasm lies over the body of the sphenoid bone, typically above the diaphragma sellae and (paradoxically) only rarely within the sulcus chiasmatis (330) (Fig. 1.25). The relative position of the chiasm over the sella turcica is variable. The chiasm is (a) above the tuberculum sellae (i.e., prefixed ) in 12%; (b) above the diaphragma sellae in 79%; and (c) above the dorsum sellae (i.e., postfixed ) in 4% of cases (264,330) (Fig. 1.41). In patients with brachycephaly, the chiasm typically is more rostral and dorsal than in dolichocephaly. This variability of position partially accounts for the variable patterns of visual field defects seen in patients with upwardly expanding pituitary adenomas. The intracranial optic nerves do not lie on a horizontal plane; rather, they rise upward from the optic canals at an angle of degrees (331). The chiasm lies above the diaphragma sellae, the dural covering of the sella turcica, by approximately 10 mm ( ) (Fig. 1.43). The sphenoid sinuses lie under the chiasm only when they extend far back into the body of the sphenoid bone. In one third of adults, an arachnoid membrane extends outward from the infundibulum to fuse with arachnoid around the carotid arteries and the inferior surface of the chiasm (336). The shape of the pituitary gland varies considerably among individuals. The width of the gland is typically greater than or equal to its depth or length. The lateral and superior margins of the gland are more variable in shape because they are not confined within bone. Rarely, the pituitary gland protrudes inferiorly into the sphenoid sinus (264). Superiorly, the lamina terminalis, which defines the anterior end of the diencephalon, extends upward from the chiasm to the anterior commissure. The A2 segments of the anterior cerebral arteries pass medially and upward above the optic chiasm. The anterior cerebral and anterior communicating arteries may be situated above the chiasm or the

4 38 CLINICAL NEURO-OPHTHALMOLOGY Figure Trajectory of retinal ganglion cell axons during early and late phase of chiasm formation. Horizontal view of axon growth and cells of the ventral diencephalon during the early (E12 E13) and later (E15 E16) phases of axon growth. Specialized radial glia (small dots) form palisades on either side of the midline and express RC2 as well as Slit2 (rostrally), EphA and EphB receptors, and NrCAM. The early-born neurons (large dots) express CD44 and SSEA-1 as well as ephrinas, Slit 1, Robo1 and Robo2, and disulfated proteoglycans. Slit1 is expressed dorsal to and around the optic nerve as it enters the brain and more weakly by the CD44/SSEA neurons. Slit2 is strongly expressed in the preoptic region directly dorsal and anterior to the chiasm. A, At E15 E16, during the major phase of retinal axon divergence, growth cones have different forms depending on their locale and behavior. Crossing (thick line) and uncrossed (thin line) fibers have slender streamlined growth cones in the optic nerve and optic tract. Near the midline, all axons pause and have more spread forms. Uncrossed growth cones extend along the midline in highly complex shapes before turning back to the ipsilateral optic tract. B, In the early phase of retinal axon growth, the first-born uncrossed retinal axons from the dorsocentral retina (DC) enter the ipsilateral optic tract directly, quite far from the midline. In contrast, in the later period, uncrossed axons travel toward the midline and diverge from crossing axons within the radial glial palisade. Crossed axons at both ages traverse the midline close to the rostral tip of the early-born neurons. All retinal axons at both ages grow around the contours of the early-born neurons. C, Maneuvers of retina axons with respect to the resident cells of the optic chiasm. DC, dorsocentral; D, dorsal; V, ventral; N, nasal; T, temporal. (From Mason C, Erskine L. The development of retinal decussations. In: Chalupa LM, Werner JS, eds. The Visual Neurosciences. Cambridge, MA, MIT Press, ) optic nerves, or they may rest directly on these structures (Fig. 1.44). The junction of the anterior communicating artery with the A1 segments usually occurs above the chiasm rather than the optic nerves (263). Laterally, the ICAs emerge from the cavernous sinuses and approach the posterior optic nerves and sides of the chiasm. Occasionally, the ICAs contact and compress these structures. Posteriorly, the chiasm is bounded by the third ventricle, which explains the vulnerability of the chiasm to compression or infiltration by lesions within the ventricle or even an expanded ventricle. ORGANIZATION OF NERVE FIBERSWITHIN THE OPTIC CHIASM Several generalizations can be made about the topography of fibers passing through the chiasm, although much of our knowledge in this regard derives from nonhuman primates.

a postfixed chiasm above the dorsum sellae (right). The W-shaped clear zone behind the chiasm is the anterior aspect of the third ventricle. (Redrawn from Rhoton AL Jr, Harris FS, Renn WH.

St Louis, CV Mosby, 1977 75 105.) Figure 1.42. MRIs of the normal optic chiasm. A, Noncontrast T1-weighted image shows position of the optic chiasm in sagittal section.

5 Figure Three sagittal sections of the optic chiasm and sellar region showing the positions of a prefixed chiasm above the tuberculum sellae (left), a normal chiasm above the diaphragma sellae (center), and a postfixed chiasm above the dorsum sellae (right). The W-shaped clear zone behind the chiasm is the anterior aspect of the third ventricle. (Redrawn from Rhoton AL Jr, Harris FS, Renn WH. Microsurgical anatomy of the sellar region and cavernous sinus. In: Glaser JS, ed. NeuroOphthalmology Symposium of the University of Miami and the Bascom Palmer Eye Institute. St Louis, CV Mosby, ) Figure MRIs of the normal optic chiasm. A, Noncontrast T1-weighted image shows position of the optic chiasm in sagittal section. Note the angle made by the incline of the intracranial portion of the optic nerve as it approaches the chiasm (arrowhead). B, T1-weighted image after contrast administration shows the position of the body of the chiasm in coronal section. Figure Relationships of the optic nerves and optic chiasm to the sellar structures and 3rd ventricle (III); C, anterior clinoid; and D, dorsum sellae. (Redrawn from Glaser JS. Neuro-ophthalmology. Hagerstown, MD, Harper & Row, 1978.) 39

40 CLINICAL NEURO-OPHTHALMOLOGY Figure 1.44.

intracranial optic nerves and optic chiasm. Gyri recti and olfactory nerves are located superiorly. (From Perlmutter D, Rhoton AL Jr.

6 40 CLINICAL NEURO-OPHTHALMOLOGY Figure Anterior views of the A1 and proximal A2 segments of the anterior cerebral arteries, anterior communicating arteries, and recurrent arteries, showing variations in their relationship to the intracranial optic nerves and optic chiasm. Gyri recti and olfactory nerves are located superiorly. (From Perlmutter D, Rhoton AL Jr. Microsurgical anterior cerebral anterior communicating recurrent artery complex. J Neurosurg 1976; ) First, the proportion of crossed fibers is always greater than the uncrossed population, typically with a ratio of roughly in humans (293). Second, retinal fibers projecting to the ipsilateral dlgn come only from the temporal retina, whereas fibers projecting to the contralateral dlgn come only from the nasal retina. The separation between crossed and uncrossed fibers begins just as the fibers reach the chiasm. Uncrossed fibers, both dorsal and ventral, maintain their relative position through the lateral chiasm and pass directly into the ipsilateral optic tract (310). Crossed fibers from the dorsal retina project more caudally in the chiasm than crossed fibers of the ventral retina. An exception to these principles occurs for fibers that project to the superior colliculi, which project from throughout the retinae (337). Third, macular projections are both crossed and uncrossed and constitute a large percentage of the chiasmal fibers. Macular fibers are more concentrated dorsally and centrally and are generally not present in the inferior rostral and caudal regions of the chiasm (68,307,329,338). Fourth, retinohypothalamic axons exit the chiasm posteriorly (without entering

BLOOD SUPPLY The distal optic nerves, optic chiasm, and proximal optic tracts pass below the anterior communicating artery and anterior cerebral arteries and above the posterior communicating

7 EMBRYOLOGY, ANATOMY, AND PHYSIOLOGY OF THE AFFERENT VISUAL PATHWAY 41 the tracts) to reach the hypothalamus. Finally, some retinal fibers, especially those of melanopsin-containing RGCs (discussed previously), bifurcate and project to two central sites after they emerge from the chiasm. BLOOD SUPPLY The distal optic nerves, optic chiasm, and proximal optic tracts pass below the anterior communicating artery and anterior cerebral arteries and above the posterior communicating arteries, PCAs, and basilar artery (339) (Figs and 1.44). The chiasm obtains blood from branches of all of these surrounding vessels. There is considerable interindividual variation in the blood supply of the chiasm (329). In general, the chiasmal blood supply comes from a dorsal and ventral group of feeder vessels. The dorsal supply derives from branches of the ICA, usually via superior hypophyseal arteries, the A1 and occasionally the A2 segments of the anterior cerebral artery, and the anterior communicating artery. Branches from more distal stretches of the anterior cerebral artery typically supply some of the dorsal chiasm (265,340). The ventral supply derives from branches of the posterior communicating artery, PCA, and basilar artery (339). The superior hypophyseal artery, a branch of the ICA, often provides blood to the ventral chiasm (284). There is often significant collateralization among these vessels (341), which explains the utter rarity of chiasmal infarction. The optic tract, which is fully formed by the 13th week of gestation, is the segment of the afferent visual pathway between the chiasm and dlgn. From their origin, the optic tracts diverge in front of the interpeduncular space and wind laterally above the uncus and then around the internal cap- OPTIC TRACT sule to reach the dlgn (Figs. 1.38, 1.45, and 1.46). Unlike the intracranial optic nerves, the optic tracts are firmly attached to the brain throughout their course by glial cells. Each tract contains crossed and uncrossed fibers, most of which synapse in the dlgn. A few tract fibers turn medially Figure The optic tracts and lateral geniculate body viewed in sagittal section. Note the relationship of the optic tract to the internal capsule and corticospinal tract. The pulvinar is just medial to the lateral geniculate body. (Redrawn from Pernkopf E. Atlas of Topographical and Applied Human Anatomy. Vol 1. Philadelphia, WB Saunders, 1963.)

T HE visual field changes that accompany

T HE visual field changes that accompany J. Neurosurg. / Volume 31 / September, 1969 The Arterial Supply of the Human Optic Chiasm RICHARD BERGLAND, M.D.,* AND BRONSON S. RAY, M.D. Department of Surgery (Neurosurgery), New York Hospital-Cornell