Optic Chiasm

The optic nerves join at the optic chiasm, which lies in the suprasellar region, superior to the diaphragma sella, inferior to the third ventricle and hypothalamus, and just anterior to the infundibular stalk. The chiasm tilts forward at an angle of 45 degrees and most commonly lies directly above the pituitary fossa, although occasionally it may be prefixed or postfixed in position relative to the sella. Within the chiasm, fibers from the nasal retina cross, and the most ventral axons from the inferior nasal retina bend temporarily through the contralateral optic nerve (Wilbrand's knee), whereas the fibers from temporal retina remain ipsilateral (Fig. 8-3 (Figure Not Available) ). The existence of Wilbrand's knee has come into question. M The ratio of crossed to uncrossed fibers is 53 to 47 percent, respectively. The fibers transmitting visual information from the superior retina remain superior in the chiasm, whereas those from the inferior retina remain situated inferiorly. The papillomacular bundles lie superiorly and posteriorly within the chiasm. [1 , [8

Most of the fibers traveling through the optic chiasm are destined for the optic tracts and lateral geniculate bodies. Retinal projections may also exist from the posterior chiasm to the hypothalamus, specifically the suprachiasmic nucleus or the supraoptic nucleus. y This pathway is thought to mediate the visual input responsible for diurnal variations of neuroendocrine systems. The chiasm's neuroanatomical locale in relation to surrounding vascular structures is important, given the potential aneurysms and subsequent visual field defects that they may produce. The carotid arteries ascend lateral to the optic chiasm, and the precommunicating segments of the anterior cerebral arteries lie superior to the chiasm. [6

Figure 8-3 (Figure Not Available) Optic chiasm: correlation of lesion site and field defe(From Liu GT: Disorders of the eyes and eyelids. In Samuels MA, Feske S [eds]: The office practice of neurology. New York, Churchill-Livingstone, 1996, p 46. Adapted from Hoyt WF, Luis O: The primate chiasm. Arch Ophthalmol 1963; 70:69-85. Copyright© 1963, American Medical Association.)

The chiasm derives its blood supply from an inferior and superior anastomotic group of vessels. y The inferior group is made up of the superior hypophyseal arteries, which derive their blood supply from the internal carotid, posterior communicating, and posterior cerebral arteries. The superior group of vessels consists of precommunicating branches of the anterior cerebral arteries. y

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