OPTIC PATHWAY AND LESIONS

UMA CHIDIEBERE JOHN

• Beginning in the retina, the visual pathway continues• through the optic nerves, • optic chiasm,• and optic tracts to synapse in• the lateral geniculate nucleus (LGN).• From the LGN, it extends through• the temporal and parietal lobes• to terminate in the occipital lobes

• A healthy disc is normally pink to orange in color, with well delineated margins and a small cup to disc ratio (<0.3)

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Why the normal disc is pink?• Thickness and the

cytoarchitecture of fiber bundles passing between glial columns containing capillaries

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• The retina is a thin, multilayered tissue sheet containing three developmentally distinct, interconnected cell groups that form signal processing networks:

• • Class 1 :: sensory neuroepithelium (SNE) ::photoreceptors and BCs• • Class 2 :: multipolar neurons :: GCs, ACs, and axonal cells

(AxCs)• • Class 3 :: gliaform neurons :: horizontal cells (HCs)

Receptors:1. Rods

2. Cones

Neurons:1. First order: bipolar cells

2. Second order: ganglion cells

3. Third order: LGB

Visuo-striate area (17):Both walls of calcarine sulcus involving Cuneus and lingual gyrus

Components of the visual pathwayA. Optic nerveB. Optic chiasmaC. Optic tractsD. Lateral geniculate bodiesE. Optic radiationsF. Visual cortex

VISUAL PATHWAY

VisualPathway

Optic chisam• Floor of the third ventricle.• 5-10 mm above the diphragma sella and the hypophysis cerebri.• 12mm wide, 8mm A-P , 4 mm thick.• 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.

1. OPTIC NERVE (STUMP)

2. 0PTIC CHIASMA

3. OPTIC TRACTS

VisualPathway

Optic chiasma:1. Junction of the floor and

the anterior wall of the third ventricle

2. Rests on the diaphragma sellae

3. Wilbrand’s loop

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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.

VisualPathway

Optic tracts:1. Medial root

Gudden’s commisure2. Lateral root

a. LGBb. Superior colliculus &

pre-tectal nucleusc. Supra-chiasmatic

nucleus

VisualPathway

Lateral geniculate body:1. Part of meta-thalamus2. Connected to superior

colliculus via superior brachium

3. Cells arranged in six layers

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.

VisualPathway

Optic radiation:1. From LGB to striate area

of visual cortex (17)2. Passes through the retro-

Lentiform part internal capsule

3. Meyer’s loop

Primary Visual Cortex????• Area 17• located in the occipital

lobe in the calcarine fissure region extending to the pole

• large representation in visual cortex for the macula (region for highest visual acuity)

• receives the primary visual input

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primary VISUAL CORTEX• FROM THE LATERAL GENICULATE NUCLEUS, THE FIBRES PASS BY THE

OPTIC RADIATIONS TO THE PRIMARY VISUAL CORTEX.• IT IS LOCATED IN THE CALCARINE FISSURE AREA OF THE MEDIAL

OCCIPITAL LOBE. (BRODMANN’S AREA 17 OR V1)

Secondary Visual Areas• visual association cortex (area

18, 19)• responsible for analyzing the

visual information• area for 3 dimensional

position, gross form, and motion

• area for color analysis

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Retinal Projections to Subcortical Regions

• suprachiasmatic nucleus of the hypothalamus

• control of circadian rhythms??• pretectal nuclei

• pupillary light reflex• accommodation of the lens

• superior colliculus• rapid directional movement of both eyes

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The Autonomic Nerves to the Eyes

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The Autonomic Nerves to the Eyes

• The eye is innervated by both parasympathetic and sympathetic neurons.

• Parasympathetic fibers arise in the Edinger-Westphal nucleus, pass in the 3rd cranial nerve to the ciliary ganglion.

• Postganglionic fibers excite the ciliary muscle and sphincter of the iris.

• Sympathetic fibers originate in the intermediolateral horn cells of the superior cervical ganglion.

• Postganglionic fibers spread along the corotid artery and eventually innervate the radial fibers of the iris.

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Control of Pupillary Diameter• miosis: decreasing of pupillary

aperture due to stimulation of parasympathetic nerves that excite the pupillary sphincter muscle

• mydriasis: dilation of pupillary aperture due to stimulation of sympathetic nerves that excite the radial fibers of the iris

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Fig. 16.07

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Visual reflexes• Light reflex or pupillary

reflex:When light is shown to one eye, normally the pupils of both eyes constrict.

- Direct light reflex: The constriction of pupils

upon which light is shown is called direct light reflex.

- Indirect or consensual:The constriction of pupil on the other eye even though no light is shown

Pathways of direct and indirect light reflexes (v.imp.)

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Pupillary light reflexDirectConsensual

Accommodation

ACCCOMODATION REFLEX• When the eyes are focussed

from a distant to near object, three reactions take place

• 1. Constriction of pupils• 2. thickening of lens due to

contraction of ciliary muscles• 3. Convergence of both eye

ballsThese three reactions together

constitute Accommodation or near reflex

Accommodation reflex

LESIONS OF THE VISUAL PATHWAY

Optic nerve lesions (A,B)Causes : Optic atrophy, traumatic avulsion, acute optic neuritis etc.

1.Distal optic nerve lesion (A)• Complete blindness of affected side• Abolition of direct light reflex on affected side• Accommodation reflex intact

2. Proximal optic nerve lesion (B)• Blindness on affected side• Contralateral hemianopia• Abolition of direct light reflex on affected side• Accommodation reflex intact

Chiasmal lesions (C,D)1.Central chiasmal lesion (C)

• Bitemporal hemianopia• Bitemporal hemianopic paralysis of pupillary reflexes

2.Lateral chiasmal lesion (D)• Binasal hemianopia• Binasal hemianopic paralysis of pupillary reflexes

Causes : I. Intrinsic causes – Lesions which produce thickening of

chiasma. Eg. Gliomas, multiple sclerosisII. Extrinsic causes – Compressive lesions. Eg. Pitutary

adenoma, meningiomaIII. Other causes – Include metabolic, toxic and

inflammatory syndromes. Eg. Lymphoid hypophysitis, sarcoidosis

Optic tract lesions (E)Causes :

I. Intrinsic causes – Demyelinating diseases and infarction.II. Extrinsic causes – Compressive lesions. Eg. Pitutary adenomas, tumours of

optic thalamusIII. Other causes – syphilitic meningitis, tubercular meningitis

Optic tract lesions• Incongruous homonymous hemianopia• Contralateral hemianopic pupillary responses (Wernicke’s

reaction)• Optic disc changes – Descending type of partial optic

atrophy is produced characterized by temporal pallor on the side of the lesion and bow tie atrophy on the contralateral side.

• Visual acuity is intact

Pitutary adenoma• Visual fields ; bitemporal hemianopia,junctional

scotoma, bitemporal hemianopic scotoma• Colour vision; early red deficit• Visual acuity tends to reduce• Optic disc- bow tie atrophy rarely papilloedema• Extraocular movements: cranial nerve palsies,see

saw nystagmus,spasm nutans.

Lateral geniculate nucleus lesions(E)• Incongruous homonymous hemianopia• Pupillary reflexes are normal as the fibres go to pretectal nucleus and

not the LGN• Optic disc pallor may occur due to partial descending atrophy

Lesions of optic radiations (F,G)Common lesions include :

• Vascular occlusions• Tumours• Trauma• Temporal lobectomy for seizures

Lesions of optic radiations• Superior quadrantic hemianopia(F) – Pie in the sky lesions.

It is explained by the fact that inferior fibres of optic radiations contain fibres from ipsilateral lower temporal retina and contralateral lower nasal retina.(part of optic radiations in temporal lobe)

• Inferior quadrantic hemianopia(G) – Pie on the floor lesions. This is the same as above. Difference being the superior fibres are affected. (part of optic radiations in parietal lobe)

• Complete homonymous hemianopia(H) – produced when all fibres of optic radiations are involved sometimes sparing the macular fibres as they lie centrally.

• Pupillary reflexes are spared• Optic disc atrophy does not occur

Visual cortex lesions (I,J,K)• Congruous homonymous hemianopia – macular field of vision is

spared. It is a feature of occlusion of posterior cerebral artery.• Congruous homonymous macular defects – occurs in lesions at the tip

of occipital cortex following head injuries or gun shot injuries

• Bilateral homonymous macular defects – presenting like bilateral central scotoma occur in bilateral lesions of occipital cortex

• Pupillary light reflexes are normal• Optic atrophy doesn’t occur.

Other manifestations of occipital lobe lesions include :• Cortical blindness• Dyschromatopsia

• Visual hallucinations• Palinopsia – Persistent perception of visual image• Visual anesthesia – transposition of visual stimulus from

one hemifield to another• Polyopsia – multiple images of single object which do not

disappear on closing the eye.