Optic chiasm

Last revised by Assoc Prof Frank Gaillard on 25 May 2022

The optic chiasm or chiasma is the midline structure where the nasal (medial) fibers of the optic nerves decussate to continue posteriorly as the optic tracts. It lies in the chiasmatic cistern and along with the pituitary stalk, is completely encircled by the circle of Willis.

The circle of Willis encircles the pituitary stalk and optic chiasma.

The anterior-to-posterior location of the chiasm relative to the pituitary fossa is variable 4:

  • prefixed: tuberculum sellae (15%)
  • normal: diaphragma sellae (70%)
  • postfixed: dorsum sellae (15%)

Historically, it has been believed that the crossing fibers made an anterior bend into the prechiasmatic optic nerve; this is known as Wilbrand knee. More recent work, however, has suggested this is artifactual. 

Small branches of the anterior cerebral artery and the superior hypophyseal artery supply the chiasm and intracranial portion of the optic nerves whereas the optic tracts are supplied by small branches of the anterior choroidal and posterior communicating arteries.

Lesions compressing the chiasm classically produce the visual field defect of bitemporal hemianopia, where there is loss of the temporal fields.

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Cases and figures

  • Figure 1: optic nerve and chiasm (Gray's illustration)
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  • Figure 2: optic nerve and chiasma anatomy
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  • Figure 3: optic chiasm
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  • Figure 4: midsagittal section
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