optic chiasm

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.

Gross anatomy

Relations

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 :

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

Blood supply

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.

Related pathology

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