ANATOMY OF VISUAL PATHWAY,FIELD DEFECTSANDITS LESIONS.

By-Dr. Ruchi Pherwani

Visual field

Each eye sees a part of the visual space that defines its visual field.

The visual fields of both eyes overlap extensively to create a binocular visual field. The total visual field is the sum of the right and left hemifields and consists of a binocular zone and two monocular zones.

Visual Pathway

Each eyeball act as camera ;it perceives the images & relay the sensations to the brain(occipital cortex) via the VISUAL PATHWAY.

VISUAL PATHWAY

Vision is produced by photoreceptors in the retina, a layer of cells behind the eye. The information leaves the eye by manner of the optic nerve, and there is partly crossing of axons at the optic chiasm.

Afterward the chiasm, the axons are termed as the optic tract. The optic tract wraps about the midbrain to acquire to the lateral geniculate nucleus (LGN), where all axons should synapse.

From there, the LGN axons fan out via the deep white matter of the brain as the optic radiations, that will ultimately travel to primary visual cortex, at behind the brain.

VISUAL PATHWAY comprises of:

Optic Nerve Optic ChiasmaOptic TractLateral Geniculate BodyOptic RadiationsVisual cortex

OPTIC NERVE2nd cranial nerve.47-50 mm in length.Starts from optic disc & extends upto optic chiasma

where the two nerves meet.Backward continuation of nerve fibre layer of retina

which consist of axons originating from ganglion cells.

Contains the afferent fibres of light reflex

Has 4 parts : 1)intraocular (1mm) 2)intraorbital (30mm) 3)intra canalicular (6-9mm) 4)intracranial (10mm)

Morphologicaly & Embryologicaly optic nerve is compared to sensory tract of Brain(white matter) : Optic nerve is an

outgrowth of brain. Not covered by

neurilemma so does not regenerate when cut.

Fibres of optic nerve are very thin(2-10 um in diameter)& are million in number.

surrounded by meninges unlike other peripheral nerves.

Both primary & secondary neurons are in retina.

1)Intra ocular Part :About 1mm in size, passes through

sclera , choroid & finally appears in eye as optic disc.

Divided in 4 portions from anterior to posterior :a)surface nerve fibre layer.

b) prelaminar region. c) lamina cribrosa. d) retrolaminar region.

2) Intraorbital part : Extends from back of eyeball to

optic foramina. This part slightly sinuous to give

play for the eye movements. Here optic nerve is surrounded

by all 3 layers of meninges & subarachnoid space.

The central retinal artery along with enters the subarachnoid space to enter the nerve on its inferomedial aspect.

Near optic foramina,optic nerve is closely surronded by annulus of zinn & the origin of four recti muscles.

Some fibres of superior & medial rectus are adherent to its sheath & account for painful occular movements in retrobulbar neuritis.

3) Intracanalicular Part

This part is closely related to ophthalmic artery.

it crosses the nerve from medial to lateral side in dural sheath.

Sphenoid & posterior ethmoidal sinuses lie medial to it & seperated by thin bony lamina, this relation accounts for retrobulbar neuritis following infection of sinuses.

4) Intracranial part

About 10mm

Lies above cavernous sinus & converges with its fellow to form chiasma.

Ensheathed in pia mater.

Internal carotid artery runs below then lateral to it.

OPTIC CHIASMA Flattened structure,12 mm

horizontally & 8mm anteroposteriorly.

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.

Anatomical variation in position of normal optic chiasma:

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.

Relations of chiasma :Anterior - anterior cerebral

arteries & its communicating arteries.

Posterior- tuber cinereum, infundibulum ,pitutary body ,posterior perforated substance.

Superior- third ventricle.Inferior- hypophysisLateral- extra cavernous

part of internal carotid artery& anterior perforated substance.

OPTIC TRACTS Cylindrical bundle of nerve

fibres.

Run outwards & backwards from posterolateral aspect of optic chiasma ,between tuber cinereum & anterior perforated substance to unite with cerebral peduncle.

Fibres from temporal half of retina of same eye & nasal half of opposite eye.

Posteriorly each ends in Lateral Geniculate Body.

LATERAL GENICULATE BODYOval structures situated at termination

of the optic tracts.Each consist of 6 layers of

neurons(grey matter) alternating with white matter (optic fibres)

Fibres of 2nd order neuron coming via optic tract relay here.

OPTIC RADIATIONS (Geniculo-Calcarine Pathway)From LGB to the occipital cortex.Pass forwards then laterally through the area

of wernicke as optic peduncles.Anterior to lateral ventricle ,traversing the

retrolenticular part of internal capsule,medial to auditory tract.

Its fibres then spread out fanwise to form medullary optic lamina.

Inferior fibres subserve upper visual fields & sweep anteroinferiorly in meyer’s loop & temporal lobe to visual cortex.

Superior fibres subserve inferior visual field proceed posteriorly through parietal lobe to visual cortex.

VISUAL CORTEXLocated on the medial aspect of

occipital lobe, in & near calcarine fissure. Visual cortex

Visuopsychic area

Peristriate area 18

Parastriate area 19

Visuosensory area

Striate area 17

Modified nomenclature recognizing five visual areas :

Visual area (V1) in area

17

Second visual area (V2)

occupying greater part of area 18.

Third visual area(V3) occupying narrow strip over anterior

part of area 18

Fourth visual area(V4) in

area 19

Fifth visual area (V5) at posterior end of superior temporal gyrus.

Blood supply of Visual Pathway

Arterial Circle of Willis

Carotid arterial system

Vertebral arterial system

Circle of Willis :

Blood supply of Optic Nerve:

A. Intraocular part

•Peripapillary choroidal vessels

Prelaminar

•Posterior choroidal vessels

Lamina cribrosa region

•Centrifugal branches from central retinal artery•Centripetal branches from pial vessels

Retrolaminar

B . Intraorbital part :

Periaxial system of vessels

•Derived from 6 branches of internal carotid artery: ophthalmic, long & short posterior ciliary artery.•Lacrimal artery.•Central artery of retina.

Axial system of vessels

•Intraneural b/o central retinal artery.•Central collateral b/o central retinal artery.•Central artery of optic nerve.

C .Intracanalicular part : periaxial system of vessels.

D . Intracranial part : Pial system of vessels

B/o internal carotid artery

B/o anterior cerebral artery

B/o ophthalmic artery

Twigs from anterior

communicating artery

Venous drainage :

Optic nerve head

•Central retinal vein

Orbital part

•Peripheral pial plexus•Central retinal vein

Intracranial part

•Pial plexus which ends in anterior cerebral & basal vein

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.

Blood supply of optic tract:Arterial: Pial plexus receiving

contribution from posterior communicating artery, anterior choroidal artery & middle cerebral artery.

Venous drainage: anterior cerebral vein & basal vein.

Blood supply of lateral geniculate body:

Posterior cerebral artery •Supply fibres coming from superior homonymous quadrant of retina.

Anterior choroidal artery •Supply fibres coming from inferior homonymous quadrant of retina.

Macular fibres over region of hilum

•Supplied by anastomosis from posterior cerebral & anterior choroidal artery.

Blood supply of Optic radiations

•Anterior choroidal artery

Anterior part

•Deep optic artery b/o middle cerebral artery.

Middle part

•Calcarine branches from posterior cerebral artery.

Posterior part

Blood supply of visual cortex

Visual cortex

Calcarine artery b/o Posterior

cerebral artery

Terminal b/o middle cerebral artery & anastomosis

between middle& posterior cerebral artery.

Venous drainage of visual cortex

Medial aspect

•Internal cerebral vein drains in great cerebral vein of galen & straight sinus

Superolateral aspect

•Inferior cerebral vein drains in cavernous sinus.

LESIONS OF VISUAL PATHWAY

1) LESIONS OF OPTIC NERVE : Causes: optic atrophy indirect optic neuropathyacute optic neuritis traumatic avulsion of optic nerve. Characterised by: complete blindness in

affected eye with loss of both direct on ipsilateral & concensual light reflex on contralateral side. Near reflex is preserved.

Eg. Right optic nerve involvement

2)Lesions through proximal part of optic nerve :

ipsilateral blindness. contralateral hemianopia abolition of direct light reflex on

affected side & concensual light reflex on contralateral side.

near reflex intact. Eg. Rt optic nerve Involvement in Proximal part

3)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)

4)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)

5)Lesions of optic tract :Causes: Syphilitic meningitis/ gumma. Tuberculosis Tumors of optic thalamus Aneurysm of superior cerebellar or posterior

cerebral arteries.Characterised by :• Incongruous homonymous hemianopia with C/L

hemianopic pupillary reaction( wernicke’s reaction)

• These lesions usually lead to partial descending optic atrophy & may be associated with C/L 3rd nerve paralysis & ipsilateral hemiplegia.

6)Lesions of lateral geniculate body :

leads to homonymous hemianopia with sparing of pupillary

reflexes & may end in partial optic atrophy.

7)Lesions of optic radiations :Causes: Vascular occlusion Primary & secondary tumors Trauma Characterised by :

TOTAL OPTIC RADIATION

INVOLVEMENT

COMPLETE HOMONYMOUS

HEMIANOPIA( sometimes sparing

macula)

LESIONS OF

PARIETAL LOBE

(involving superior fibres of

optic radiations)

INFERIOR QUADRANTI

C HEMIANOPIA( PIE ON

THE FLOOR)

LESIONS OF TEMPORAL

LOBE (involving

inferior fibres of optic

radiations)

SUPERIOR QUADRAN

TIC HEMIANOPIA( PIE ON THE ROOF)

Pupillary reactions are normal as fibres of light reflex leave the optic tracts to synapse in the superior colliculi.

Lesions of optic radiations do not produce optic atrophy as the 1st order neurons (optic nerve fibres) synapse in LGB.

8)Lesions of visual cortex : pupillary light reflex is normal & optic atrophy does not occur following visual cortex lesions.

Congruous homonymous hemianopia(sparing macula)

Occlusion of posterior cerebral artery supplyin anterior part of

occipiatl cortex

Thank you…