Optic chiasm

The optic chiasm is a flattened, transversely oriented X-shaped structure at the base of the diencephalon, formed by the junction and partial crossing of fibres of the right and left optic nerves (CN II). Each optic nerve enters its anterolateral aspect, while the optic tracts emerge posterolaterally.

Anatomical relations

It lies in the suprasellar region, immediately superior to the diaphragma sellae and pituitary gland, posterior to the tuberculum sellae, and anterior to the infundibulum (pituitary stalk). Laterally, it is closely related to the internal carotid arteries.

Nerve fibres of the visual pathway

Within the chiasm, fibres arising from the nasal half of each retina decussate, whereas fibres from the temporal half remain ipsilateral. Consequently, each optic tract contains fibres representing the contralateral visual hemifield.

Clinical correlate (bitemporal hemianopia)

Clinically, compression of the central optic chiasm, classically by an enlarging pituitary mass, preferentially affects the crossing nasal retinal fibres and may produce bitemporal hemianopia.