Anatomy Of Optic Nerve And ItsBlood SupplyVijay Joshi
Embryology Of Optic Nerve• The optic nerve head is formed late in the embryonic periodas the optic stalk encloses the hyaloid artery (the eighthweek, 20-mm stage).• Physiologic cup of the optic disc develops at the 15th week ofgestation.• The optic nerve develops from the embryonic optic stalk,which appears at the fourth week and connects the opticvesicle to the forebrain.• As the stalk lengthens, it becomes thinner and the lumen isprogressively occupied by the axons growing from theganglion cells of the retina (the seventh week, 15-mm stage).• At the eighth week, axons fully occupy the stalk and reachthe brain, and a rudimentary optic chiasm is established.• The axons of the optic nerve are surrounded by myelinsheaths.• Myelinization (produced by oligodendrocytes) beginscentrally, progresses in a centrifugal direction, myelinizationis usually complete shortly after birth.
• Optic Nerve Sheaths• The sheaths of the optic nerve begin to appearat the end of the seventh week.• Thin, elongated mesenchymal cells surroundthe optic nerve (10-mm stage)• The pia mater can be identified by the ninth totenth week of gestation (45- to 50-mm stage),followed by the dura mater at the fifth monthof gestation and the arachnoid sheath by thesixth and seventh months of gestation.• Both the pia mater and the arachnoid arederived from the neural crest.
• Glial Element• At the ninth week (45-mm stage), the glial cells inthe optic nerve are oriented in rows between thefascicles of axons.• A peripheral layer of glial cells forms a gliallimitans made up of immature astrocytes with glialfilaments under the thin meningeal sheath.• The glial limitans is separated from the pia materby a complete basement membrane.• Astrocytes line the connective tissue septa andcapillaries.• The glial cells in the optic nerve and retina maydifferentiate into astrocytes and oligodendrocyte
• Vasculature• The development of capillaries in the optic nerveand the CNS is similar.• At the 11th week (65- to 73-mm stage), vesselsand connective tissue from the pia mater begin toenter the proximal optic nerve and slowly enlargethe connective tissue septa during the next fewmonths.• The capillaries within the optic nerve areseparated from the axons by a relatively completeastrocyte sheet and perivascular space.• In the 18th week (160-mm stage), vascularizationof the optic nerve is completed, and there may beanastomoses anteriorly by this stage with thearterial circle of Zinn-Haller.
1. Outer layer of opticstalk2. Lumen of optic stalk3. Inner layer of opticstalk4. Nerve fibers5. Hyaloid artery6. Mesenchyme7. Closed choroid fissureHuman astrocytes filling spaces bet the myelinated fibres of ON
Glimpse Of Embryology Of Optic NvWeek of Gestation Length (mm) Developing Events4 2.5–6 mm Short optic stalk5 5–9 mm Development of hyaloidvasculature6 8–14 mm Embryonic cleft closes7 13–18 mm Growth of axonsFormation of optic nerve8 18–31 mm Stalk fully occupied byaxonsAxons of optic nerves reachthe brainRudimentary optic chiasmestablishedOptic nerve vascularizationstarting to formOptic nerve head startingto form
Embryology Of Optic NerveWeek of Gestation Length (mm) Developing Events11 65–73 mm Vascular-connective septainvade the nerve12 80 mm Pia mater, arachnoid, anddura mater distinguishableGlial filaments appear14 105 mm Subarachnoid spaceappears15 117–123 mm Physiologic cup starts toform18 160 mm Vascularization of the opticnerve completed23 220 mm Myelinization starts
Optic Nerve• Second cranial nerve• Starts from optic disc, extends upto optic chiasma• Backwards continuation of nerve fibre layer ofretina( axons of ganglion cells)• Also contains afferent fibres of light reflex andsome centrifugal fibres• Comparable to sensory tract(white mater) of brainbecozi. Optic nerve is an outgrowth of brainii. not covered by neurilemma( doesn’t regenerate)iii. Surrounded by meningesiv. Fibres are 2-10um in dia as compared to 20 umin sensory nerves.v. Both primary and sensory neurons are in retina.
Parts of optic nerve• Is about 47-50 mm in length• Parts1.intraocular-1mm.2.intraorbital-30mm.3.intracanalicular-6-9mm.4.intracranial-10mm
• Intraocular part:• Passes through sclera, choroid, & appears in eye as opticdisc.• 1.5 mm in dia• Expands to 3 mm behind sclera due to +nce of myelinsheath• Divided into 4 portions( from ant to post)i. SNFL- has axonal bundles ( 94% nv fibres of retina + 5 %astrocytes)• Optic disc is covered by ILM of Elschnig• thin layer of astrocytes• Seprates it from vitreous• Continuous with ILM of retina• When central portion of this membrane gets thickenedk/as central meniscus of kuhnt• Near the optic nerve all layers of retina( except NFL) areseparated from it by partial rim of glial ts k/asintermediate ts of KUHNT
• Prelaminar region:• Predominant stuctures at this level are neurons andsignificantly inc astroglial tissue• Border tissue of JCOBY ( a cuff of astrocytes)separates the nerve from the choroid• Lamina cribrosa:• The bundles of ON fibres leave the eye through LC (fibrillar sieve composed of fenestrated sheets ofscleral connective tissue lined by glial tissue)• Rim of collagenous connective tissue with someadmixture of glial cells which intervenes b/w thechoroid and sclera & ON fibres is k/as border ts ofElsching
•Retrolaminar region:Decrease in astrocytes& acquisition ofmyelin, supplied byoligodendrocytes.•Myelin sheathdoubles dia of ON(from 1.5 to 3.0mm) asit passes through thesclera.•The post extent ofthe retrolaminarregion is not clearlydefined.
Ophthalmoscopic Features Of ONHead• Part of nerve head visible with ophthalmoscopeis termed the optic disc.• Its intra papillary parts- optic cup & neuroretinalrim, separated, by scleral ring of Elsching, from azone of parapapillary atrophy variably present ontemporal side• Optic disc colour: pink due to rich capillarysupply to its rim.• Nerve fibre loss in chronic glaucoma leads to incexposure of the lamina as axons are lost so thatits pores become more visible as the cupenlarges, so there is inc white reflex at disc.
• Disc shape:• usually oval in shape, its vertical diameter being onavg 9% longer than its horizontal.• Disc size:• Normal disc area ranges widely, from 0.86 mm sqto 5.54mm sq.• Macrodiscs area > 4.09sq mm, microdisc < 1.29 sqmm.• Applied: primary macrodiscs may be ass. with condsuch as pits of the ON `morning glory syndrome`• Secondary macrodiscs ass with high myopia andbuphthalmos (upto 20mmΛ2 )• NAION is commoner in smaller ON heads due toproblems of vascular perfusion and of limited space• Same true for optic nerve head drusen, due toblockage of orthograde axoplasmic flow.
• Pseudo papilloedema is also encountered withsmaller optic nerve head.• Susceptibility of the sup & inf disc regions todamage may be ass with the higher pore- to-discarea in these regions.• Optic cup:• Funnel shaped depression, varies in form & size,usually off-centre towards the temporal side.• Cup correlates with disc, thus large in large discs,small In small discs. May be absent in small disc.• Three dimensional mearsurements of cup shapecan be done using confocal microscopy orstereoscopic techniques
• Neuroretinal rim:• Tissue outside the cup, & contains the retinal nerveaxons as they enter the nerve head.• It is broadest in lower segment of the disc, thenabove, then nasally & then temporally.• There is greater axonal mass & vascularity in theinferotemporal region.• APPLIED: In pri open angle glaucoma prog loss ofretinal ganglion cells occurs, leading toenlargement of cup, particularly at upper & lowerpoles of disc, leading to vertically oval cup.• Occurrence of flame shaped hagge on rim, usuallyat inf or sup temporal margin, is early sign ofglaucoma.
• Cup/disc ratio:• Is ratio of cup & disc width, measured in same meridian,usually vertical or horizontal.• Its median value 0.3, doesn’t differ by more than 0.2 in99% subjects.• APPLIED: asymmetry of greater than 0.2 is of diagnosticimp in glaucoma.• Parapapillary chorioretinal atrophy:• Cresentric region of chorioretinal atrophy, foundtemporally in normal disc.• May be exaggerated in chronic glaucoma or in highmyopia.• Two zones of PPCRA are described :• Zone alpha/ choroidal cresent: is more peripheral zone &is an irregular hypo- or hyper pigmented regionassociated histologically with irregularities of the RPE ¶papillary choroid.
• Peripherally, it is adjacent to the retina ¢rally to zone beta if present, or directly toscleral ring if nt.• It corresponds RPE, that failed to extend to thedisc margin.• Zone beta or scleral cresent: is related to disccentrally or zone alpha peripherally.• It consists of marked atrophy of pigment epi &choriocapillaries, with good visibility of largerchoroidal vessels• APPLIED: The zones are larger in total area &individually in the presence of chronicglaucoma.
Optic Nerve HeadNormal ON head ON head in advanced glaucoma
• Retinal vessels: emerge on medial side of cup,slightly decentered superonasally.• The temporal arteries, taking an arcuate course asthey leave the disc• The nasal take more direct, though curved course• The course of veins & arteries is similar but notidentical & this avoids excessive shadowing ofrods & cones• Venous pulsations are observed at disc in 15-90%of normal subjects due to pulsatile collapse of theveins as ocular pressure rises with arterial inflowinto uvea.• Visible arterial pulsations are rare, usuallypathological. For eg. High ocular pressure or aorticin competence.
• Intraorbital part :• Extends from back of the eyeball to the optic foramina.• Sinuous course to give play for the eye movements• Covered by dura, arachnoid and pia.• The pial sheath contains capillaries and sends septa todivide nerve into fasciculi.• The SAS containing csf ends blindly at the sclera butcontinues intracranially. CRA, accompaning vein enterthis space inferomedially about 10 mm from the eyeball.• Post near optic foramina, the ON is closely surroundedby annulus of zinn & origin of four rectus muscles• Applied: Some fibres of SR & MR are adherent to itssheath account for the painful ocular movements seen inretrobulbar neuritis
• Relations of intraorbital part of ON :• The long & short ciliary nerves & arteriessurround the ON before these enter the eyeball• B/w ON & LR muscle are situated the ciliaryganglion, divisions of the oculomotor nerve, thenasociliary nerve, the sympathetic & theabducent nerve• The ophthalmic artery, sup ophthalmic vein &the nasociliary nerve cross the ON sup from thelat to med side.
• Intracanalicular :• Closely related to ophthalmic artery whichcrosses the nerve inferiorly from medial tolateral side in the dural sheath• Leaves the sheath at the orbital end of thecanal• Sphenoid and post ethmoidal sinuses liemedial to it and are separated by a thin bonylamina.• Applied: This relation accounts forretrobulbar neuritis following infection of thesinuses.
• Intracranial:• Lies above the cavernous sinus & convergeswith its fellow (over the diaphragma sellae) toform the chiasm.• Ensheathed in pia mater, but receivesarachnoid & dural sheaths at the point of itsentry into the optic canal.• ICA runs, at first below & then lateral to it.• The anterior perforated substance, themedial root of the olfactory tract & the antcerebral artery lie above this part of the ON.
Arrangements of nerve fibres in opticnerve• In the optic nerve:1. In the optic nerve head:• Arrangement of nerve fibres in the ONhead is exactly same as in retina, i.e., no overlapb/w lower & upper halves of fibres.• Temporal macular fibres remain on same side, &nasal one cross• Upper temporal retinal fibres are separated from lower by macularfibres2. In the distal region:• NF are distributed exactly as in retina i.e upper temporal & lowertemporal fibres are situated on the temporal half of the optic nerveand are separated from each other by a wedge shaped areaoccupied by the papillomacular bundle.• The upper nasal and lower nasal fibres are situated on the nasalside3.In the proximal region(near the chiasma):• Here the macular fibres are centrally placed.
Arrangement Of Fibres In Optic NerveArrangement of fibres in ON head & distalregionArrangement of fibres in proximal region of ON
Intraocular Part:1.Surface nerve fibre layer- supplied byi. Peripapillary arterioles of CRA originii. Epipapillary arterioles of CRA Originiii. Rich anastomosis with prelaminar regioniv. Occasional anas with chorocapillariesv. Precapillary branches from cilioretinal arterieswhen present2.Prelaminar &laminar region derive from shortposterior ciliary arteries.Arterial circle of zinnBlood supply of optic nerve
• 3.Retrolaminar region: receives its supplymainly from arteries & arterioles of pialsheath of neighbouring leptomeninges.• Applied-optic disc edema occurs prelaminaraxons swell from orthogonal axoplasmic flowat level of lamina cribrosa.• Insufficient bld flow thru posterior ciliaryarteries due to thrombosis, hypotension,vascular occlusion cause ON head infarction.
• Veonus drainage of optic neve head:• In each zone venules drain into CRV, or whenpresent into a duplicated vein( prob anembryological perisitence of hyaloid veins,hayreh 1963)• Occasionally septal veins in retrolaminar regiondrain into pial veins.• Some small venules from prelaminar region orfrom SNFL (optiociliary veins) drain intochoroid.• APPLIED: Optiociliary veins may enlarge inoptic nerve sheath meningiomas.
• Intraorbital part:• Supplied by 2 systems of vessles: a periaxial andan axial.• Preaxial consists of 6 branches of ICA namely:ophthalmic, LPCA, SPCA, lacrimal artery & centalartery of retina before it enters ON & circle ofzinn• The axial system consists of :• i) intraneural branches of CRA,• ii) central collateral arteries which comes offfrom CRA before it pierces the nerve,• iii) Central artery of ON
• Intracanalicular Part:• Ophthalmic artery is sole supply to thisportion, except occ branch from CRA on its infaspect. ( hayreh 1963)• Branches from ophthalmic artery arises withinthe canal or in the orbit.• Pial network is poor in this region, becozarteries reach pia along conn tissue bandsbinding the nerve to surr dural sheath.• APPLIED: this supply is vulnerable to shearinginjury in skull fracture.
• Intracranial Part:• Prechiasmal Artery: branch of sup hypophysealbranch of ICA. ( hayreh 1963)• Runs back along the medial side of the ON, joinsits fellow of the opp side along the ant border ofthe chiasm & supplies both.• It is probably the largest supply to intracranialpart.• Ophthalmic artery gives no of small collateralarteries running backwards along inf surface ofnerve, winding round its margins in sup aspect.( hayreh 1963)• Add branches from ant cerebral art & ant com artalso supply the same.• APPLIED: Carotid artery aneurysms, displacementof carotid artery can compress ON.
Blood Supply Of The Various Parts Of The OpticNerve
• VENOUS DRAINAGE:• Chiefly by central retinal vein & to lesser extentvia pial venous system.• Both systems drain into the ophthalmic venoussystems in the orbit & less commonly directlyinto cavernous sinus.
• BLOOD BRAIN BARRIER AT THE OPTIC NERVE:• The capillaries of ON head, the retina, and theCNS, have non-fenestrated endothelial linings withtight junctions b/w adjacent endothelial cells.• These are responsible for blood tissue barrier tothe diffusion of small molecules across capillaries.• However it is incomplete as a result of continuityb/w the extracellular spaces of choroid and ONhead at level of choroid(in pre laminar region)• There is no barrier to diffusion across the highlyfenestrated capillaries of the choroid.
Lesions of the Visual PathwayI. LESIONS OF THE OPTIC NERVE• Complete blindness on the affected side• Abolition of direct light reflex on ipsilateral side &consensual on contralateral side• Near (accomodation) reflex is present• Causes – optic atrophytraumatic avulsion of optic nvindirect optic neuropathyacute optic neuritisII. LESIONS THROUGH PROXIMAL PART OF OPTICNERVE• Ipsilateral blindness• Contralateral hemianopia• Abolition of direct light reflex on affected side &consensual on contralateral side• Near reflex is intact
Tilted Disc• The optic nerve exits the scleraat a 90º angle.• A tilted optic nerve occurs whenthe nerve exits the eye at anoblique angle.• Tilted disc is usually a bilateralcondition in which thesuperotemporal disc is raised,simulating disc swelling, whilethe inferonasal disc is flat ordepressed.• This results in an oval-shapeddisc with the long axis at anoblique angle.• The blood vessels also enter theglobe at an oblique angle.• There is thinning of the RPE andchoroid in the inferior nasalquadrant.
Optic Nerve Head Drusen• (ONHD) are globules ofmucoproteins andmucopolysaccharides thatprogressively calcify in theoptic disc.• They are thought to be theremnants of the axonaltransport system ofdegenerated retinal ganglioncells.• ODD have also been referredto as congenitally elevated oranomalous discs,pseudopapilledema,pseudoneuritis, buried discdrusen, and disc hyalinebodies.• They may be associated withvision loss of varying degreeoccasionally resulting inblindness.
Optic Pit• An optic pit appears asa round or oval, gray orwhite depression in theoptic disc.• They are mostcommonly foundtemporally, but can befound in any area of thedisc
Myelinated Nerve Fibers• Myelinated nerve fibers manifestas white, feathery patches thatfollow the NFL bundles and have astriated appearance .• The peripheral edges appearfanned out.• The myelination can simulate discedema due to elevation of theoptic nerve and obscuration of thedisc margins and retinalvasculature.• Normally, myelination does notextend past the lamina cribrosa;however, in 0.6% to 1.0% of thepopulation, myelinated NFLoccurs.• Myelination is bilateral in just 8%of cases and is continuous with theoptic nerve head in only 33% ofeyes.
Coloboma Disc• Colobomas result from anincomplete closure of theembryonic fissure.• They can be either unilateralor bilateral and are oftenfamilial.• A coloboma of the optic discappears as a white, bowlshaped excavation of theinferior optic nerve head.• The optic disc is typicallyenlarged. The inferiorneuroretinal rim is thin orabsent, and the superiorneuroretinal rim is relativelynormal.• The coloboma may involve thechoroid and retina.• Iris and ciliary colobomas mayalso be present.
Morning Glory Disc• Morning glory disc anomaly isevidenced by a congenitalfunnelshaped excavation ofthe posterior pole .• The disc appears enlargedand may be recessed orelevated centrally.• A white tuft of glial tissuecovers the central portion ofthe cup.• Blood vessels appear to beincreased in number andemanate from the edge ofthe disc.• After arising from the disc,the vessels turn sharply atthe edge of the cup and havean abnormally straightpattern in the peripapillaryregion.
Optic Nerve Hypoplasia• In this case, the opticnerve head appearsabnormally small due to alow number of axons.• The disc may appear grayor pale and is surroundedby a light-coloredperipapillary halo.• At the normal junctionbetween the sclera andlamina cribrosa, there isanother change inpigmentation, a “doublering sign,” associated withthe hypoplasia.
Aicardi Syndrome• Aicardi syndrome is arare inherited (genetic)disorder in which thestructure that connectsthe two sides of thebrain (corpus callosum)is partly or completelymissing• optic disc coloboma anddome shaped loci ofpale areas with sharpborders nasal to theoptic disc suggestive ofchorio retinal lacunae.
Megalopapilla• An abnormally large opticdisc, usually occursbilaterally and is associatedwith a large cup-to-discratio .• These discs have a surfacearea greater than 2.5mm2.• The round or horizontallyelongated cup, as well asthe lack of rim notching,helps distinguishmegalopapilla from normaltension glaucoma.• The neuroretinal rim maybe pale due to axons beingspread over a larger surfacearea.
Peripapillary Staphyloma• In this condition, the areaaround the disc is deeplyexcavated, with atrophicchanges in the retinalpigment epithelium.• It is generally unilateral,and the disc may benormal or appear pale.• As opposed to those inmorning glory discanomaly, the bloodvessels have a normalpattern.• These eyes are normallyemmetropic or slightlymyopic, but they can alsobe highly myopic.
Optic Nerve Hypoplasia• Medical condition arisingfrom theunderdevelopment of theoptic nerve not fitting inother clinical cond.• This condition is the mostcommon congenital opticnerve anomaly.• The optic disc appearsabnormally small, becausenot all the optic nerve axonshave developed properly.• It is often associated withendocrinopathies (hormonedeficiencies),developmental delay, andbrain malformations
• Optic atrophy• Primary optic atrophy• Primary optic atrophy occurs without antecedent swelling ofthe optic nerve head.• It may be caused by lesions affecting the visual pathways fromthe retrolaminar portion of the optic nerve to the lateralgeniculate body.• Lesions anterior to the optic chiasm result in unilateral opticatrophy, whereas those involving the chiasm and optic tract willcause bilateral changes.• Causes• • Optic neuritis.• Compression by tumours and aneurysms.• Hereditary optic neuropathies.• Toxic and nutritional optic neuropathies.• Trauma.
• Secondary optic atrophy• Secondary optic atrophy is preceded by long-standing swelling of the optic nerve head.• Causes include chronic papilloedema, anteriorischaemic optic neuropathy and papillitisPrimary Optic Atrophy Secondary Optic Atrophy
Non-arteritic anterior ischaemic optic neuropathy• NAION is caused byocclusion of the shortposterior ciliary arteriesresulting in partial or totalinfarction of the opticnerve head.• Diffuse or sectoralhyperaemic disc swelling,often associated with afew peripapillary splinterhaemorrhages• The swelling graduallyresolves and pallorensues 3–6 weeks afteronset.
Arteritic anterior ischaemic optic neuropathy• Arteritic anterior ischaemicoptic neuropathy (AAION)is caused by giant cellarteritis.• Giant cell arteritis (GCA) isa granulomatousnecrotizing arteritis with apredilection for large andmedium-size arteries,particularly the superficialtemporal, ophthalmic,posterior ciliary andproximal vertebral• A strikingly pale (‘chalkywhite’) oedematous disc isparticularly suggestive ofGCA
Papilloedema• Papilloedema is swelling of the optic nerve head secondary toraised intracranial pressure.• TYPES:• Early:• Mild disc hyperaemia with preservation of the optic cup• Indistinct peripapillary retinal nerve striations and disc margins(initially nasal, later superior, inferior and temporal).• There is loss of previous spontaneous venous pulsation – thismay not be significant because it is also absent in about 20% ofnormal individuals• Established:• Severe disc hyperaemia, moderate disc elevation with indistinctmargins and absence of the physiological cup• Venous engorgement, peripapillary flame haemorrhages andfrequently cotton wool spots• The optic nerve head appears enlarged, as the swelling inc• Circumferential retinal folds (Paton lines) may developtemporally.
• Chronic: Severe disc elevation without cotton wool spotsand haemorrhages.• Optociliary shunts and drusen-like crystalline deposits(corpora amylacea) may be present on the disc surface.• Atrophic (secondary optic atrophy):• The optic discs are a dirty grey colour, slightly elevated,with few crossing blood vessels and indistinct margins.Early Established Chronic Atrophic