Organ
Optic chiasm
Partial crossing of the visual pathways
High-yield
Crossing nasal retinal fibers represent the temporal visual fields. Chiasmal compression can therefore reduce temporal vision in both eyes, although real visual-field defects are often incomplete or asymmetric.
Overview
The two optic nerves meet above the pituitary in the suprasellar cistern. Axons from each nasal hemiretina cross to the opposite side, while temporal retinal axons remain on the same side. The resulting optic tracts therefore each represent the opposite visual hemifield, using information from both eyes.
Reference facts
- Location
- Suprasellar cistern, above the pituitary fossa and beneath the hypothalamus, adjoining the anterior floor of the third ventricle.
- Parts
- Nasal retinal axons crossing the midline intermix with uncrossed temporal retinal axons; the posterior continuations are the optic tracts.
- Blood supply
- Small branches of nearby arteries, particularly the internal carotid, anterior cerebral and anterior communicating systems, with variable contributions.
- Relations
- Pituitary and diaphragma sellae below; hypothalamic and third-ventricular structures above; internal carotids laterally and anterior communicating complex nearby superiorly.
- Function
- Combines inputs so that each postchiasmal pathway represents the opposite half of visual space from both eyes.
Clinical correlation
An enlarging pituitary mass may compress the chiasm from below. Superior temporal field loss is a classic early pattern, but other patterns occur because the chiasm's position and the extent of compression vary. A lesion confined behind the chiasm instead tends to affect matching sides of both eyes' visual fields.
Frequently asked questions
Where is the optic chiasm located?
Suprasellar cistern, above the pituitary fossa and beneath the hypothalamus, adjoining the anterior floor of the third ventricle.
What does the optic chiasm do?
Combines inputs so that each postchiasmal pathway represents the opposite half of visual space from both eyes.
What are the parts of the optic chiasm?
Nasal retinal axons crossing the midline intermix with uncrossed temporal retinal axons; the posterior continuations are the optic tracts.
What is the blood supply of the optic chiasm?
Small branches of nearby arteries, particularly the internal carotid, anterior cerebral and anterior communicating systems, with variable contributions.