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. Once crossed, the inferonasal fibers may even briefly loop back into the contralateral optic nerve sheath, before returning to the chiasm. This bend into the contralateral optic nerve is termed Wilbrand’s knee. However, it has been suggested that Wilbrand’s knee is actually an artifact of the methodology for tract tracing in humans.1
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 tuburculum sellae and behind by the dorsum sellae. Behind the<unk> ies the floor of the third ventricle.
Chiasmal Syndrome
Diencephalic syndrome
Cerebellar stroke syndrome
Arnold–Chiari malformation
Basal ganglia disease
Spinal cord tumor
Organic brain syndrome
Brain stem tumor