visual pathway & optic nerve tumours

THE VISUAL PATHWAY AND TUMOURS OF THE OPTIC NERVE

P.SAMIRROLL NO.130

TOPICS DISCUSSED :

• Anatomy of visual pathway• Lesions of visual pathway• Tumours of optic nerve

ANATOMY OF VISUAL PATHWAY

Components of the visual pathway

A. Optic nerveB. Optic chiasmaC. Optic tractsD. Lateral geniculate bodiesE. Optic radiationsF. Visual cortex

A. THE OPTIC NERVE

• The optic nerve is the second cranial nerve.

• It starts from the optic disc and ends at the optic chiasma where the two nerves meet.

Parts of the optic nerve:• The optic nerve is 47-50mm in length and is divided into 4 parts:

1. Intraocular part - Starts from optic disc, pierces choroid and sclera.Becomes continuous with intraorbital part at back of eyeball

2. Intraorbital part – Extends from back of eyeball to optic foramina. Closely surrounded by the annulus of Zinn and origin of the four rectii muscles posteriorly. Anteriorly it is separated from ocular muscles by orbital fat.

3. Intracanalicular part – Closely related to the ophthalmic artery which crosses obliquely over it to its medial side. Medial to this are the sphenoid and posterior ethmoidal sinuses separated by a thin bony lamina.

4. Intracranial part – Optic nerve lies above cavernous sinus and converges with the one from the other side to form optic chiasma, over the diaphragma sellae.

B. OPTIC CHIASMA• Flattened structure• Measures 12mm horizontally and 8mm

anteroposteriorly• Lies over tuberculum and diaphragma sellae• Fibres originating from nasal half of the retina

cross over (decussate) here.

C. OPTIC TRACTS

• Cylindrical bundles of nerve fibres running outwards and backwards from the posterolateral aspect of the optic chiasma

• Each tract consists of temporal fibres from retina of the same side and nasal fibres of the opposite side

• Posteriorly, each optic tract ends in the lateral geniculate body.

D. LATERAL GENICULATE BODIES

• Oval structures• Consist of 6 layers of neurons (grey matter)

alternating with white matter (formed by optic fibres)

E. OPTIC RADIATIONS

• These extend from lateral geniculate bodies to the visual cortex and consist of axons of the third order neurons of the visual pathway.

F. VISUAL CORTEX

• Located medially on the occipital lobe above and below the calcarine fissure.

• Subdivided into :I. Visuosensory area – Striate area 17, that

receives fibres of the optic radiationsII. Visuopsychic area – Peristriate area 18 and

parastriate area 19

BLOOD SUPPLY OF VISUAL PATHWAY

• Mainly supplied by the pial network of vessels.• Orbital part of optic nerve is also supplied by

an axial system derived from the central artery of the retina.

• The pial plexus itself gets contribution from different arteries.

• Blood supply of the optic nerve head : Surface layer- supplied by capillaries derived from

retinal arterioles Prelaminar region – mainly supplied by centripetal

branches of prepapillary choroid with some contribution from vessels of lamina cribrosa

Lamina cribrosa – supplied by branches from posterior ciliary arteries and arterial circle of Zinn

Retrolaminar part – supplied by centrifugal branches ofrom pial plexus formed by branches from the choroidal arteries, circle of Zinn, central retinal artery and ophthalmic artery

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 sclerosis

II. Extrinsic causes – Compressive lesions. Eg. Pitutary adenoma, meningioma

III. 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

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.

TUMOURS OF THE OPTIC NERVE

I. Optic nerve glioma

II. Optic nerve sheath meningioma

III. Optic nerve melanocytoma

I. Optic nerve glioma

It is a slow growing tumour arising from the astrocytes. Ususally occurs in the first decade of life.

Clinical features :• Gradual visual loss associated with gradual,

painless, unilateral axial proptosis occuring in a child usually between 4-8 years of age

• Fundus examination may show optic atrophy or pappiloedema and venous engorgement.

Diagnosis :• X-ray showing uniform, regular, rounded

enlargement of optic foramen in 90% of the cases.• CT scan and Ultrasonography depicting a fusiform

growth in relation to the optic nerve.

Treatment : • Observation without treatment is

recommended for patients having stationary tumour with good vision and non disfiguring proptosis.

• Surgical excision of tumour mass by lateral orbitotomy in case of disfiguring proptosis.

• Radiotherapy in unoperable cases• Nowadays a technique called as gamma knife

surgery is being used for treatment. It is non invasive and apparently doesn’t cause harm to surrounding tissues.

OPTIC NERVE GLIOMA

II. Optic nerve sheath meningiomas

It is a rare benign tumour of meningothelial cells of the meninges that usually occurs in mid age. It has slight female preponderance.

Clinical features :• Early visual loss• Limitation of ocular movements• Optic disc edema or atrophy

• Slow progressive unilateral proptosis

Treatment :• Observation is recommended if visual acuity is

good• Surgical excision is recommended for severe

proptosis with blind eye or threat to chiasma• Prognosis for life is good

OPTIC NERVE SHEATH MENINGIOMA

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