This document discusses the anatomy and variations of the optic chiasm. It describes the chiasm as a flattened structure located above the pituitary gland where the optic nerves from each eye partially cross. The temporal fibers remain uncrossed while the nasal fibers cross over. Variations in the position of the chiasm can impact which structures are involved in pituitary tumors. Lesions in the center of the chiasm cause bitemporal hemianopia while lateral lesions cause binasal hemianopia. The blood supply and relations to surrounding structures are also outlined.

1. CHIASMA
Dr Md Ferdous Islam
Department of Ophthalmology
CMH,DHAKA

2. Visual Pathway
Optic Nerve
Optic Chiasma
Optic Tract
Lateral Geniculate Body
Optic Radiations
Visual cortex

3. Chiasma
•Flattened structure,12 mm horizontally & 8mm
anteroposteriorly, 4 mm thick
•Ensheathed by pia & surrounded by CSF
• Lies over diaphragma sellae so visual field defects
seen in patient with pituitary tumor having
suprasellar extension
•Posteriorly chiasma continous with the optic tracts
& form the anterior wall of 3rd ventricle
•Nerve fibres arising from nasal half of two retina
decussate at the chiasma

4. • Floor of the third ventricle
• 5-10 mm above the diphragma sella and the
hypophysis cerebri
• Important relations: 3rd ventricle, hypothalmus,
pituitary stalk, sella, dorsum sellam anterior and
posterior clinoid processes, cavernous sinus
• Nasal fibers cross ; temporal fibers do not (53:47)
• Wilband’s knee

6. Anatomical Variation
a) central : lies directly over sella,
expanding pituitary tumor involves
chiasma first
b) pre-fixed : lies more anteriorly
over tuberculum sellae,pituitary
tumor involves optic tract first.
c) post-fixed : lies more posterior
over dorsum sellae,pituitary tumor
damage optic nerve first

8. Relations Of Chiasma
• Anterior - anterior cerebral arteries & its
communicating arteries
• Posterior- tuber cinereum, infundibulum ,pitutary
body ,posterior perforated substance
• Superior- third ventricle
• Inferior- hypophysis
• Lateral- extra cavernous part of internal carotid
artery& anterior perforated substance

9. Arrangement Of Nerve Fibers
• Temporal fibers from retina
remains uncrossed and runs
backward in lateral part of
optic chaisma
• Nasal peripheral fibers
o ¾ of fibers
o Cross over to enter medial
part of opposite optic tract in
fol
 lower nasal fibers in optic
tract traverse chiasma low and
anteriorly
 Upper nasal fibers in optic
tract trasverse chiasma high
and posteriorly

10. Arrangement Of Nerve Fibers
Macular fibers
• Some fibers crossed n
runs backward in opposite
optic tract
• Some fibers uncrossed n
runs on same side in optic
tract

11. Blood Supply Of Optic Chiasma
Arterial
Venous
• B/o anterior cerebral & anterior
communicating arterySuperior aspect
• B/o internal carotid artery ,posterior
communicating artery ,anterior superior
hypophyseal artery
Inferior aspect
• Superior chiasmal vein drains into
anterior cerebral vein
Superior
aspect
• Pre-infundibular vein draining into
basilar veinInferior aspect

12. Central Lesions Of Chiasma (Sagittal)
 Causes
 suprasellar aneurysm
 tumors of pituitary gland
 craniopharyngioma
 suprasellar meningioma & glioma of 3rd ventricle
 third ventricular dilatation due to obstructive
hydrocephalus
 chronic chiasmal arachnoiditis
 Characterised by
 Bitemporal hemianopia
 Bitemporal hemianopic
paralysis of pupillary reflex (usually lead to partial
descending optic atrophy)

13. Lateral Chiasmal Lesions
Causes
• Distension of 3rd ventricle causing pressure on
each side of optic chiasma
• Atheroma of carotids & posterior communicating
artery
Characterised by
• Binasal hemianopia
• Binasal hemianopic
parallysis of pupillary reflex (usually lead to partial
descending optic atrophy)