Optic chiasm
Chiasm & tract · Bitemporal hemianopia
The decussation in which nasal-retinal fibres of each eye cross to the opposite side while temporal fibres stay ipsilateral. It lies in the suprasellar cistern about 10 mm above the pituitary and diaphragma sellae, beneath the floor of the third ventricle, with the internal carotid arteries lateral and the anterior cerebral and anterior communicating arteries above.
Traced from the start
Detail
- Crossing
- Nasal fibres (about 53%) cross; temporal fibres (about 47%) do not
- Below
- Diaphragma sellae and pituitary gland
- Lateral
- Supraclinoid internal carotid arteries
- Above and behind
- Anterior communicating artery, hypothalamus and third ventricle
- Fibre order
- Inferior nasal fibres cross low and anterior, superior nasal fibres cross high and posterior
When it goes wrong
Pituitary macroadenoma extending up through the diaphragma sellae to compress the chiasm from below
Bitemporal hemianopia beginning in the superior temporal quadrants (inferior nasal fibres are hit first), with headache and features of hypopituitarism or hyperprolactinaemia
Craniopharyngioma or third-ventricle mass compressing the chiasm from above and behind
Bitemporal hemianopia beginning in the inferior temporal quadrants; in a child, short stature and diabetes insipidus
Lesion at the junction of one optic nerve with the chiasm
Junctional scotoma: central or complete loss in the ipsilateral eye plus a superior temporal defect in the other eye
Practise this structure
2 questions in the bank tagged Pituitary.
A 52-year-old woman has progressive headaches and difficulty seeing objects to either side. MRI shows a 2.6-cm sellar mass elevating and compressing the optic c…
Clinical rotations · middle twist
A 44-year-old man has an incidental 8-mm pituitary lesion discovered during MRI for headaches. He has normal visual fields, normal pituitary hormone testing, an…
Clinical rotations · middle twist