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 arterial branches from the proximal ACA and ACoA. 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 varies but is usually directly superior (Fig 1-20).
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 (those subserving the superior visual field) are the first to cross. The anterior loop of fibers into the contralateral optic nerve (Wilbrand knee) may be an artifact in monkeys, but not necessarily in humans. The finding of a superior temporal visual field defect contralateral to a central scotoma localizes the 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.
Figure 1-19 Anatomical dissection of the optic chiasm and surrounding structures. A, Sagittal view. B, Superior view.
(Courtesy of Albert L. Rhoton Jr, MD.)
Figure 1-20 Position of the optic chiasm in relationship to the tuberculum sella.
(Illustration by Rob Flewell, CMI.)
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.