THE OPTIC NERVE

Sanket Parajuli

Embryology of optic nerve

Optic nerve

1) Optic nerve fibers…….from nerve fiber layer of retina….pass through choroidal fissure to optic stalk

2) Glial system of nerves….from neuroectodermal cells forming the outer wall of optic stalk

3) Glial septa……composed of astroglia that forms from cells of inner wall of optic stalk

4) Sheath of ON: from mesenchyme surrounding optic stalk

5) Myelination : begins from chaisma at about 7th month… goes distally and reaches lamina cribosa just before birth

Applied anatomy

• Myelinated nerve fibers occur in 1% of the population. It usually observed as an asymptomatic, isolated finding following routine examination

• It occurs more commonly in association with neurofibromatosis, Down syndrome and cranial dysostosis

BERGMEISTER'S papillae• Bergmeister's papilla is formed by focal

proliferation of glial cells at the optic nerve head by 9 weeks of gestation

• The base of the cells are spread along the internal limiting membrane and the apex extends into the vitreous

• Subsequent focal necrosis and tissue remodeling are responsible for its atrophy.

• However, Bergmeister's papilla can remain, together with remnants of the hyaloid artery, on the optic disc with no apparent harmful visual effects.

Tilted disc:• Common anomaly usually B/L describes oblique entry of ON

into globe• Associated with myopia and astigmatism

Coloboma of the optic nerve

• Defect of the inferior aspect of the optic nerve.

• Incomplete closure of the embryonic fissure

• A varying amount of glial tissue typically fills the defect, manifests as a white mass.

ON head drusen::

Deposits usually calcified within the substance of ON

head present upto 2% of population

Optic nerve:

• 2nd CN• Starts from optic disc to optic chaisma• Backward continuation of nerve fiber layer of retina which

consists of axons of ganglion cells• Length: 35-45 mm • About 90% of nerve fibers in ON are small caliber fibers and

are thinly myelinated

• Supporting cells of ON:

a) Oligodendroglia: produce and maintain myelin sheaths

b) Microglia: immunocompetent phagocytic cells

c) Astroglia: structural support to nerve and coveys nutrients from vessels to nerve

Topographical division:

• Intraocular portion: optic disc/optic nerve head ==1mm

• Intraorbital portion : located within the muscle cone == 25 mm

• Intracanalicular portion: located within optic canal==5mm

• Intracranial portion: extends upto optic chaisma==10mm

Optic nerve head

• Site where axons of millions of retinal ganglion cells converge and leaves eye by penetrating sclera to from the optic nerve (ON)

• Represents the part of ON lying within eye

Special feature:• Among 10 layers of retina only two layers:– Nerve fiber layer– Internal limiting membrane

• Disc contains no photoreceptor cells----physiologic blind spot

• Site of entry of central retinal artery and exit site of central retinal veins

• Histological zones of ONH:– Surface nerve fibre layer:

composed of nerve fibres of retina and astrocytes

– Prelaminar zone: anterior part terminating at the vitreous

– Laminar zone: formed by lamina cribrosa

– Retrolaminar zone: continuous with retrobulbar ON

1. Surface nerve fiber layer:• Composed of axonal bundles i.e nerve

fibers of retina (94%) and astrocytes (6%)

Optic disc is covered by • A thin layer of astrocytes• The internal limiting membrane of

Elschnig Is continuous with internal limiting layer

of retina When central portion is thickened its K/a

central meniscus of Kuhnt Intermediate layer of kuhnt: collar of

astrocytes that separates ON from terminating outer retinal layer

2. prelaminar region:The predominant structures at this level are neurons and significantly increased quantity of astroglial tissue

The border of jacoby is a cuff of astrocytes that separates nerve from the choroid

3. Lamina cribosa region:• Composed of lamina cribrosa• Sieve-like fenestrated sheet of scleral

connective tissue lined by glial tissue• Bridges posterior scleral foramina/scleral canal

4. Retrolaminar region:• Decreased no. of astrocytes• Addition of myelin sheath supplied

by oligodendrocytes• ON diameter doubles in size

(from1.5 to 3.0 mm )

Intraorbital part(25mm)

• Extension: Back of eyeball to optic foramen

• Within orbit, posteriorly near the optic foramina: surrounded by rectus sheath (sheath of SR and MR muscle)

• Covered by three meningeal sheaths: – Dura– Arachnoid – Pia mater

• Intraorbital portion(25mm) length>distance between eyeball and optic canal(18mm)…..thus runs a tortuous course(forward bulging of eyeball in proptosis)

• Before passing into optic canal it traverses into through annular tendon where attachments of MR and SR are adherent to its dural sheath (Pain in adduction or upgaze in retrobulbar neuritis---as meninges are supplied by sensory fibers)

• Central retinal artery and vein crosses subarachnoid space and enter the nerve on its inferomedial aspect about 10 mm front the eyeball

• Anteriorly : orbital fat separates ON from the EOMs

• Long and short ciliary nerves and arteries surround ON before they enter eyeball

• Between the ON and LR muscle :– Ciliary ganglion– oculomotor nerve – nasociliary nerve – sympathetic nerve– abducent nerve

• Structures crossing ON superiorly (lateral to medial):– Ophthalmic artery– Superior ophthalmic vein – nasociliary nerve

Intracanalicular part (5mm)• Closely related to ophthalmic artery (OA)

• OA crosses nerve inferiorly from medial to lateral side in dural sheath

• Leaves the sheath at orbital end of canal

• Medially: sphenoid and posterior ethmoidal sinus

Intracranial part • Length : ~ 1 cm(10mm)• Lies above cavernous sinus

• Meets with ON from opposite side

Optic chaisma (over the diaphragm sellae)

• Covered by pia mater

Relation: • ICA runs below it and then

laterally• Ophthalmic artery arises

from ICA below ON (at about its midpoint)

Meningeal sheath of ON• Intracranial part covered by pia only• Intracanalicular and intraorbital covered by all 3 layers• Anteriorly all 3 layers fuse and become continuous with

sclera• Posteriorly at the apex of orbit dura splits into 2 layers • Superficial layer becomes continuous with periosteum• Deep layer forms dural sheath of ON

BLOOD SUPPLY::

1. Intraocular part ( optic nerve Head)• Surface nerve fiber layer: central

retinal artery• Prelaminar layer : SPCA,

cilioretinal art , choroidal arteries• Laminar layer : SPCA, branches

from circle of zinn• Retrolaminar layer: SPCA ,Plial

plexus Central retinal artery, Recurrent choroidal arteries

2. Intraorbital1. Periaxial system of vessels:• Derived from branches of ICA :– Ophthalmic artery– Long posterior ciliary

arteries– Short posterior ciliary

arteries– Lacrimal artery– Central artery of retina

(before it enters ON and circle of zinn)

2. Axial system of vessels:• Derived from:– Intraneural branches of CRA– Central collateral arteries (CRA

before it pierces ON)– Central artery of the ON

3) Intracanalicular part: plial plexus branches from oph art

4) Intracranial part: • Periaxial system of vessels• Four sources of pial plexus:

Arterial branch from Supply to

ICA Inferior aspect of ON (Lower retinal fibers)

Anterior cerebral artery Inferior aspect of ON(Upper retinal fibers)

Branches from Ophthalmic artery

Twigs from Anterior communicating artery

Venous drainage:

• ONH-----central retinal vein

• Intraorbital and intracanalicular- peripheral plial plexus and by central retinal vein

• Intracranial part: plial system of veins which ends in anterior cerebral vein and basal vein

Ophthalmoscopic features ONH• That part of the nerve head visible with the

ophthalmoscope is termed the optic disc

• Its intrapapillary parts are the optic cup and the neuroretinal rim, which are separated, by the scleral ring of Elschnig, from a zone of para papillary atrophy variably present on the temporal side

• The scleral ring represents the border tissue of the nerve

Black arrow—scleral ring

Optic disc colour

• Pink….due to the rich capillary supply to its rim, which can be demonstrated by fluorescein angiography. This is most noticeable inferotemporally where the rim is widest.

Disc shape • The optic disc may be round, but is usually oval

in shape, its vertical diameter being on average 9% longer than its horizontal. The cup is 8% wider in the horizontal, so that the rim is wider above and below

Disc size

• Normal disc area = 2.69 ± 0.7 mm2

• Ophthalmoscopically, the disc corresponds in size to the internal opening of the scleral canal

• Primary macrodiscs, whose areas lie greater than two standard deviations above the mean (>4.09 mm2), and microdiscs hose areas are less than two standard deviations below (<1.29 mm2)

• Primary macrodiscs =='Morning Glory' syndrome

• Secondary macrodiscs are associated with the acquired globe enlargement which accompanies high myopia and buphthalmos and may reach upto to 20 mm2 in area.

Morning Glory Syndrome - central excavation surrounded by an elevated rim of pink neuroglial tissue with the vessels emerging radially from the disc as spokes in all directions.

• Microdisc: <2 SD below mean

• Small optic disc have smaller number of ON fibers and smaller scleral canal than larger disc

• Non arteritic ischemic optic neuropathy is common in small disc—d/t vascular perfusion problem and limited space

optic cup • funnel-shaped depression which varies in form

and size and is usually off-centre towards the temporal side.

• Cup area correlates with disc area and, hence, is large in large discs and small in small discs. It is common for the cup to be absent from a small disc.

• White appearance of the cup is due to scattering of light by the lamina cribrosa and the sparse vascularity of this region.

• There is little light-scattering by the nerve axons themselves, which are unmedullated and relatively transparent.

• Nerve fiber loss in chronic glaucoma leads to increasing exposure of the lamina as axons are lost so that its pores become more visible as the cup enlarges

• Normal cup—horizontally oval• Chronic glaucoma– vertically oval

Glaucomatous cup

The neuroretinal rim • The tissue outside the cup is termed

the neuroretinal rim • contains the retinal nerve axons as

they enter the nerve head. Rim area ranges from 0.8-4.66 mm2 (1.97 ± 0.5 mm2) and correlates with disc area

• It is broadest in the lower segment of the disc, then above, then nasally and then temporally.

NRR configuration correlates

• With diameter of retinal artery and vein which is larger below than above and temporal

• With the nerve fiber visibility more detectable in the inferior temporal arcade than superior

• There’s greater retinal axonal mass and vascularity in the inferotemporal sector

• In POAG and other chronic glaucoma a progressive loss of retinal ganglion cell occur

• This leads to characteristic pattern of axonal loss at NRR with enlargement of the cup—upper and lower poles of the disc

• Flame shaped hemorrhages on the rim (inferior and superior temporal margin) is seen in glaucomatous ON damage

CDR:

• Ratio of cup to disc measured in the same meridian

• Median value of 0.3 • Increases with chronic glaucoma• Doesn’t differ by 0.2% in 99% of people• Asymmetry of 0.2%--signifies enlargement—

important in glaucoma diagnosis• In small disc CDR 0.2-0.3 is significant while in

macrodisc CDR 0.8 may be normal

Parapapillary chorioretinal atrophy:• A crescentric region of chorioretinal atrophy is a common finding

at temporal margin of normal disc• It is exaggerated in chronic glaucoma or in high myopia

• 2 zones : zone alpha and zone beta

A. Zone alpha: • more peripheral • Irregular hypo or hyper pigmented

region associated with irregularities of RPE and parapapillary choroid

• It corresponds to choroidal crescent• It contributes to relative scotoma

B. Zone Beta: related to disc centrally and to the retina or zone alpha peripherally

It consists of larger choroidal vessels, a marked atrophy of RPE and choriocapillariesIt corresponds to scleral crescentIt contributes to absolute scotomaThese zones enlarge in glaucoma

Retinal vessels: • Emerges on medial side of cup • Slightly decentered superonasally• Temporally arteries take a arcuate course and nasally curved

course as they leave the disc• Artery is usually nasal to vein• Arteries are narrower and brighter red than the veins which

have purplish tint and are often more convulated

• Venous pulsation is seen in 15-90% of the normal individuals—is due to pulsatile collapse of vein as ocular pressure rises with inflow of blood into uvea

• Arterial pulsation is pathological—implies high IOP or aortic incompetence

• Venous pulsation disappears: a) High IOPb) Low BPc) papilledema

• Opthalmoscopically what is seen is the blood column and not the vessel wall

• AV ratio: ratio of width of arteriole to venule—2:3

AV changes:

a) Diabetics—veins are abnormally wideb) HTN—attenuation of arteriolesc) Papilledema---veins are abnormally wided) Optic atrophy– both or arteries alone are attenuated

Arteriolar light reflex:

• Ratio of width of light being reflected off surface of artery to overall width of artery

• As wall thickens—less light allowed to pass across and more reflected back—thus arteriolar light reflex exaggerated in arteriolosclerosis

PAPILLEDEMA

Edema of the optic disc secondary to an increase in intracranial pressure (ICP)

ICP pressure meningeal sheaths around ONH

Slows axoplasmic flow ganglion fibers

Fluid to accumulate within the fibers

Swell (disc with elevated ONH with blurring of the disc

margins ) Is or will become bilateralEdema of the optic from cause other than increases ICP=“Edema of the optic disc.”

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