Anatomy of optic nerve and its clinical Significance

• Presented By• Pabita Dhungel• B. Optometry• 2nd year• Institute of Medicine

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References

• Walsh and Hoyt’s clinical Ophthalmology• Wolf’s anatomy• Duane’s Ophthalmology-2005• AAO – Fundamentals & Principles of

Ophthalmology-sec 2• Embryology – Duke Elder• Anataomy and physiology of Eye – A.K

Khurana 2nd edition

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Presentation layout

• Embryology of optic nerve • Introduction•Blood supply•Few optic nerve diseases•Summary

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Formation of optic vesicle and optic stalk

• Neural plate destined to form prosencephalon depresses to form optic sulcus•Neural plate converts into prosencephalic vesicle•Optic sulcus deepens and prosencephalon bulge out to form optic vesicle•Proximal part of optic vesicle becomes constricted and enlongated to form optic stalk

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Embryology of optic nerve

Optic nerve headThe optic nerve head is formed late in the embryonic period as the optic nerve stalk encloses the hyaloid artery(8th week,20-mm stage)From the hyaloid artery ,the vascular bud develops(13th week,96-mm) within Bergmeister papilla and through it into the nerve fibre layer of the retina

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Contd…

• Glial cells form the sheaths of the vessels• Eventually hyaloid artery disapears before

birth, Bergmeister’s papilla becomes atrophic, and the physiological cup of the optic disc develops at 15th week of gestation

• Optic nerve• Develops from embryonic optic stalk at 4th

week and connects the optic vesicle to forebrain

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Contd…Week of Gestation

Length (mm)

Developing events

4 2.5-6mm

Short optic stalk

5 5-9mm Development of hyaloid vasculature

6 8-14mm Embryonic cleft closes

7 13-18mm

Growth of the axon, formation of optic nerve

8 18-31mm

Stalk fully occupied by axon, axons of optic nerve reaches the retina, rudimentary optic chiasm established, optic nerve vascularization starting to form, optic nerve head starting to form

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Contd…Week of gestation

Length (mm)

Developing events

11 65-73mm

Vascular-connective septa invade the nerve

12 80mm Pia mater, arachnoid, and dura mater distinguishable, glial filaments appear

14 105mm Subarachnoid spaces appears

15 117-123mm

Physiological cup starts to form

18 160mm Vascularization of the optic nerve completed

23 220mm Myelinization starts

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Embryology contd…

• Glial elements are formed from 9th week (45mm stage) to 18th week

• Vasculature develops at 11th week(65-73mm) in the same way as CNS

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Anatomy of optic nerve

• Optic nerve- more than 1 million axons.• Starts from optic disc upto optic chiasma.• Backward continuation of the nerve fiber

layer of retina.• Consisting of axons originating from ganglion

cells.• Contains the afferent fibers of light reflex.• Elongated tract of white matter • Not covered by neurilemma.

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Contd…

• Optic nerve divided in topographic areas.Intraocular portion of optic nerve- optic disc or nerve head,pre laminar and post

laminar.Intraorbital portion.Intracanalicular portion.Intracranial portion.

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Intraocular optic nerve

• 1 mm in length.• 1.5 mm diameter.• Which expands approximately 3-4 mm behind

the sclera.• Optic nerve head divided in 4 parts(ant to

post)

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Intraorbital optic nerve

Relation of ophthalmic arteryInitially infero-lateral-mediallyAt the optic foramen-inferior and lateral

Lateral to optic nerve (in posterior orbit)-Inferior division of 3rd nerve-Nasociliary artery-Sixth Nerve-Ciliary ganglion

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Intraorbital optic nerve

At the orbital apex – optic nerve surrounded by annulus of Zinn.

Blood supply - Ophthalmic artery with meningeal branches

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Intracanalicular optic nerve

• 9 mm• Tightly fixed within the canal (compressive

optic neuropathy)• Blunt trauma • Optic nerve edema• Blood supply - pial branches from ophthalmic

artery.

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Intracranial optic nerve

• Length-10mm• Dm-4.5mmExtends post & medially ascending at an angle

of 45º to join the chiasmaBlood supply- pial vessels arising from ICA

branches from ant cerebral and anterior communicating artery

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Optic nerve head

4 layers of Optic Nerve Head:• Surface nerve fiber layer.• Prelaminar layer consisting of retinal nerve

fibres angled posteriorly from the plane of retina visible only within the central cup.

• Laminar layer: Lamina cribrosa and nerve fibre bundles running through it.

• Retro laminar layer.

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Optic nerve head

• Lamina Cribrosa - Consists of series of plates of collagenous connective tissues.

• -Perforated by 200 - 400 openings.• -Superficial openings – appear as grey

dots deep within the optic cup.• -Large pores have thin connective

tissue supports and contain large nerve fibres.

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Contd…

• Optic Cup - 3-dimensional pale depression in the centre. Size of the cup related to dm of the disc.

• Neuroretinal rim- tissue between outer edge of the cup and outer margin of the disc.

• Orange or pink colour• Uniform width• Retinal blood vessels

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Optic chaisma

Nerve fibres:-Lower nasal fibres traverse the chiasma low and anteriorly (vulnerable-expanding intrasellar lesions)

Upper nasal fibres traverse high and posteriorly (craniopharyngioma)

Crossing fibres from infero-nasal quadrant loop ant into post part of contralateral optic nerve –Wilbrand’s Knee

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Optic tract

• Each optic tract contains ipsilateral temporal & contralateral nasal fibres from optic nerves.

• Nerve fibres rotates a 90º so that superior fiber – medially & inferior – laterally .

• Macular fibres - dorsolateral orientation. • Blood supply - anterior choroidal artery &

branches from posterior communicating artery.

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Contd…

• Visual field defects in optic tract lesion • incongruous homonymous hemianopia • Bilateral retinal layer atrophy / optic atrophy • Pupillary reflex – RAPD in contralateral eye

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Lateral geniculate bodyPart of Thalamus• Site of termination of afferent fibres• 6 major layers:-

Magnocellular (1 ,2 )Parvocellular (3,4,5,6 )

• Visual field defect in lesions of LGB• Incongrous homonymous hemianopia• No pupillary change

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Contd…

• Primary target of optic tract is dorsal lateral geniculate body(dLGB), a thalamic neuclei

• dLGB consists of 3 distinct regions• Dorsal four layers, consisting small neurons

called parvo, or P-cells, are parvocelluar layers• Larger neurons called magno or M- cells

comprises two ventral magnocellular layers• Intermediately lies small konio cells

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Contd…

• Parvo, magno and konio pathways are referred to as parallel pathways

• Parvo pathway encodes detail and color while magno pathway encodes fast movement

• The role of konio pathway is less well understood

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Optic radiation• 3 main groups:-

- Inferior portion (serving superior V.F.)

- Superior portion (serving inferior V.F.)

- Central portion (macular fibres)• As they exit from LGB the rotation of fibres

occurs in reverse 90º so superior fibres lie superiorly and inferior fibres lie inferiorly.

• Blood supply:- anterior choroidal artery & posterior cerebral artery

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Visual cortex

• Thinnest portion of cerebral cortex• Area - 20-45 sq cm• Occupies 3-5% of brain• Situated along calcarine fissure on medial

side• Macular fibres – tip of occipital lobe and tip of

lateral side• Ocular Dominance Column

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Visual cortex

• Blood supply: Middle cerebral artery Posterior cerebral artery

• Visual field defect - occlusion of the Posterior cerebral artery - congruous homogeneous hemianopia

• Head injury, gun shot injury, cerebral tumours.

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1 lesion of optic nerve

2..lesion through thru proximal part

3.Sagittal (central) lesion of chaisma

4.Lateral chaismal lesion

5. Lesions of optic tract

6. Lesions of LGB

7. Lesion of optic radiation in temporal lobe

8. Lesions of optic radiation in parietal lobe

9.Optic radiation

10.Visual cortex sparing macula

11.Visual cortex only macula 37

Blood supply

• Intraocular part- a)surface nerve fiber-capillaries from retinal

artery. b)prelaminar-short post ciliary artery. c)lamina cribrosa-short post ciliary artery and

circle of Zinn-Haller. d)retrolaminar-ciliary and retinal circulation.

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Contd…

• Intraorbital part- a)periaxial system-internal carotid artery. ophthalmic artery long posterior ciliary artery short post ciliary artery lacrimal artery central artery of retina circle of Zinn

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Contd…

b)axial system-

intraneural branches of central retinal artery

central collateral arteries

central artery of optic nerve

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Contd…

• Intracanalicular part-

ophthalmic artery &

periaxial system

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Contd…

• Intracranial part-

periaxial & pial plexus(4 sources)

internal carotid artery

anterior cerebral artery

ophthalmic artery

anterior communicating

artery

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Venous drainage

• Primarily by central retinal vein.• Orbital part –pial plexus and central retinal

vein.• Intracranial part-anterior cerebral and basal

vein.

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•Clinical significance of optic nerve

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• Optic nerve is formed by axons of 1.2 million ganglion cells of retina

• Measures about 1.5mm in diameter and lies betn 10-15 degrees from fixation in the nasal retina

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Contd…

• 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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Contd…

• Pathologies of the optic nerve, even though not always detected on ophthalmoscopic exam, may compromise its function and cause the following sign;

• Reduced VA• Afferent pupillary defects• Visual field defects• Dischromatopsia(impaired CV)

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Contd…

• Diminished light brightness sensitivity• Diminished contrast sensitivity• Optic disc changes on fundoscopy can include

disc edema, hyperemia, paleness and atrophy• Many a times though the disc looks

completely normal yet it harbors a major underlying disc

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Congenital anomalies

1. Coloboma•Defect resulting from malclosure of the fetal cleft•Usually inferior and can involve the retina and choroid

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Optic disc showing coloboma (rt)

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Contd…

2. Hypoplasia •Smaller disc that carries fewer axon than normal and may be associated with poor VA, field defects, strabismus.•Intake of alcohol, steroids and insulin by mother during first trimester of pregnancy increases the risk

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Anomalies associated with axial length and refractive error

• Hyperopes eyes are smaller , disc usually lacking physiological cup, have crowded appearance mimicking papilledema

• Myopic discs are larger and surrounded by white crescent of bare sclera, on the temporal side ,the CDR is bigger mimicking glaucomatous cupping

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Myelinated nerve fibre

• Myelinating process which is completed by the 9th month of gestation sometimes extends onto the surface of the disc or surrounding retina in a radiating fashion causing feathery patches

• Condition is usually benign

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Drusens

• Yellow opalescent hyaline excrescences derived from calcified axonal debris present on the surface of the disc or burried in it

• Optic nerve head is full and small mimicking papilledema

• May be associated with RP • Autofluoresence before FFA may help in

diagnosis

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Optic nerve head swelling

• Common causes• Congenital: ex drusens• Systemic diseases: ex; anemia, hypoxemia,

uremia, HTN• Tumors: ex; hemangiomas, orbital glioma,

meningioma• Infiltrative diseases: ex; lymphoma

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Severe disc oedema in multiple sclerosis

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Contd…

• Vascular diseases: ex; AION, giant cell arteritis and other autoimmune vasculitides, DR

• Ocular diseases: ex; uveitis, hypotony, CRVO• Inflammatory diseases: optic neuritis, papillitis

and neuroretinitis• elevated ICP (intracranial pressure) : ex; mass

occupying lesions, pseudotumor cerebri...

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Common causes of optic nerve edema

Papilledema Optic neuritis AIONNon inflammatory edema secondary to increased ICP

Inflammatory swelling Vascular accident (occlusion of short posterior ciliary artery causing retrolaminar nerve infarction

Brain tumors , abscesses, hematomas,meningitis might be underlying etiology

Multiple Sclerosis is highly associated

Hypertension, giant cell arteritis, hypercoagulable state are possible factors

Bilateral , may be asymmetric

Unilateral Unilateral

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Contd…Papilledema Optic neuritis AIONHeadache, nausea, vomittingNo visual loss usually, only enlarged blind spot and possible hyperopia

Retrobulbar pain, especially on ocular movement, early central scotoma, decreased acuity, impaired color vision,presence o APD

Acute painless visual loss, usually hemialtitudinal defect involving the lower visual field

Variable degree of disc swelling, hemorrhages and cystoid infarcts

Fewer hemorrhages and cotton wool spots

Pale segmental swelling and splinter hemorrhages at its margins

Prognosis usually good if primary cause of increased ICP is treated

Vision usually returns to normal

Poorer prognosis with permanent . loss. Second eye is ultimately involved in one third of idiopathic cases.

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