The optic chiasm is the X-shaped junction where the two optic nerves meet above the pituitary gland. Here the fibres from the nasal half of each retina cross to the other side, while fibres from the temporal halves stay on the same side. As a result, each side of the brain receives information from the opposite half of the visual field.
| Location | Floor of the forebrain at the junction of the anterior wall and floor of the third ventricle, above the diaphragma sellae and pituitary gland, in the chiasmatic cistern |
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| Connections |
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| Blood Supply | Superior hypophyseal arteries; Anterior cerebral and anterior communicating artery branches; Internal carotid artery branches |
Sorts visual fibres so that each optic tract carries information from the opposite half of the visual field from both eyes.
A pituitary tumour pushing up from below compresses the crossing fibres and causes bitemporal hemianopia, loss of the outer half of vision in both eyes, often starting in the upper fields. Craniopharyngioma, pressing from above, tends to affect the lower fields first. Visual field testing is a key part of assessing any sellar mass.
Loss of both outer (temporal) visual fields from compression of crossing fibres, classically by a pituitary macroadenoma.
Sudden bleeding or infarction of a pituitary tumour that rapidly compresses the chiasm, causing severe headache, vision loss and hormone failure.
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