CHIASMA

Dr Md Ferdous Islam Department of Ophthalmology

CMH,DHAKA

Visual Pathway

Optic Nerve Optic ChiasmaOptic TractLateral Geniculate BodyOptic RadiationsVisual cortex

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

• 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

Anatomical Variation

a) central : lies directly over sella, expanding pituitary tumor involves chiasma firstb) 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

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

Arrangement Of Nerve Fibers

• Temporal fibers from retina remains uncrossed and runs backward in lateral part of optic chaisma

• Nasal peripheral fiberso ¾ of fiberso 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

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

Blood Supply Of Optic Chiasma Arterial

Venous

Superior aspect • B/o anterior cerebral & anterior communicating artery

Inferior aspect

• B/o internal carotid artery ,posterior communicating artery ,anterior superior hypophyseal artery

Superior aspect

•Superior chiasmal vein drains into anterior cerebral vein

Inferior aspect

•Pre-infundibular vein draining into basilar vein

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)

Lateral Chiasmal Lesions

Causes• Distension of 3rd ventricle causing pressure on

each side of optic chiasma• Atheroma of carotids & posterior communicating

arteryCharacterised by • Binasal hemianopia• Binasal hemianopic parallysis of pupillary reflex (usually lead to partial

descending optic atrophy)