Chiasmal Syndrome - Anatomy

Anatomy

The optic chiasm is formed by the union of the two optic nerves. The nasal fibers of each optic nerve decussate (cross) across the chiasm to the opposite side while the temporal fibers course posteriorly to form the optic tract on the same side. This arrangement allows the left half of the visual field to end up on the right side of the brain and the right half of the visual field to locate to the left side.

The optic nerves consist of the axons from the retinal ganglion of each eye. At the chiasm, 53% of the axons from the nasal retina cross the midline to join the uncrossed temporal fibers. These nasal fibers carry information from the temporal visual field. Similarly, the temporal fibers transmit images from the nasal field. The two optic tracts, representing the right and left visual fields, emerge posteriorly from the posterior chiasm. Most of these fibers synapse in the lateral geniculate nucleus or the pretectal nucleus.

The crossing of the nasal half of macular fibers of central vision occurs posteriorly in the chiasm. The inferior and superior fibers remain inferior or superior, respectively. However, the inferonasal fibers pass more anteriorly in the chiasm while the superonasal fibers pass more posteriorly. Classical teaching was that, once crossed, the inferonasal fibers briefly loop back into the contralateral optic nerve sheath, before returning to the chiasm. This bend into the contralateral optic nerve had been called Wilbrand's knee. However, today there is significant evidence that Wilbrand’s knee is simply an artifact (error). Optic nerve axons from one eye can only be selectively studied in the human after enucleation of the contralateral eye and thus degeneration of the axons on one side. After several years, the occurring optic nerve atrophy results in artifactual looping of the axons into the atrophic nerve. This looping was initially described by Wilbrand, who studied subjects with severe optic nerve atrophy after enucleation, and who then appears to have exaggerated the importance of this looping in later drawings1. Clinically, no optic field deficits have been observed in a small series of optic nerve sections at the optic nerve-chiasm junction2.

Several important structures are located adjacent to the optic chiasm. The supraclinoid branches of the internal carotid artery flank the chiasm. The cavernous sinuses are lateral and inferior to the chiasm. The frontal lobe of the brain lies above. The pituitary gland sits below in the sella turcica. The sella turcica is bound in front by the tuberculum sellae and behind by the dorsum sellae. Behind the chiasm lies the floor of the third ventricle.

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