6: OCT of the optic nerve head

what is the optic nerve head ONH

the point of exit for retnal ganglion cell axons leaving the eye ( forming the optic nerve as they pass through the onh)

most anterior portion of optic nerve

ganglion cells spread out whole retina and project axons in bundles across retina and they all head for onh

entr and exit point for major retinal arteries and veins

ong anatomy

avrage of around 1.8mm diaeter

margin on outisde followed by NRR . margin represents a break in the retina, all layers stop and no photoreceptors = blind spot

NRR is the nerve fibre ganglion cells coming into the onh from the retina

cup is simply the gap in between the nerve fibre bundles as they splay out

larger deper cups- will see the lamina cribose ( the holes)

why do we examine the onh

shows sings of sight threatening eye diseases eg glaucoma, optic neuritis, anterior ischaemic optic neuropathy and optic atrophy

swelling of the onh can indicate life threatnening condition eg raised intracranial pressure, cerebrospinal inflammation, tumpurs and malignant hypertension

some things to look for/ record

margins- are they distinct or not

cup to disc ratio

NRR intact all the way around?

NRR perfusion- does it look pale

prescence of RNFL defects in surrounding area

haemorrhages

prescence of peripapillary atrophy PPA

how can we image the onh and surround?

funsus photography- images for colour and haemorrhages

confocal scanning laser opthalmoscopy HRT

scanning laser polarimetry GDx

optical coherence tomography OCT

confocal scanning laser opthalmoscopy HRT

older method to image and quantify onh and peripapillary RNFL

measured height of the retina at many points

mesured surface topography

would define where the disc was and a reference plane within it and anything below it would be cu anything above would be the rim

scanning laser polarimetry GDx

measured birefringence of RNFL- estimated thickness of nerve fibre layer

lower resolution

no longer commercially available

OCT scans of the ONH (1): optic disc cube

A scans taken, using a raster scan approach capturing the disc and peripapillary area

building up like a 3 cube

typically around 200 × 200, 6mm x 6mm on the retina

can be used to analyse disc topography and surrounding RNFL thickness

measurements of rim/disc area, C/D ratio, cup volume

horizontal and vertical scan through onh centre

(1) /; otpic disc cube; onh topography analysis

defines a point, anythign deeper is cup anything above is rim

for analysis the cup is anything below the reference line

reference line is set 150 micrometeres above the adjacent retinal pigment epithelium- this is completely arbitary, not a reason why it is placed at that point

RNFL thickness analysis of the optic disc cube

heat map of the rnfl thickness . hotter colours indicate thicker rnfl

deviation of the rnfl thickness form normative data: red/yellow = thinner than normal

rnfl thickness along 3.4mm diameter circle around disc and comparison to normative data

analyisi of rnfl thickness in different segments along that circle

oct scans of the onh (2): BMO scans

6 radial B scans through the onh

b scan along each of the white lines

identify bruchs membrane opening as edge of onh

used for identifying minimum rim width around disc

BMO scans for assessing nrr

bright line with red dot at end of it whih is the end of bruchs membrane

so everything inside that is part of the onh

has ideniftied minimum distance between red dot ( bruch membrane ending) and top of the nerve fibre layer

sometimes the subjective disc margin ( green dots) does not correspond to the actual opening in bruchs membrane (red dots); not in the same place as the red dots. mismatch

so what does the mismatch mean then for analysis?

can therefore sometimes identify areas of thinner rim than subjectively assessed

useful in glaucoma

OCT scans of the ONH (3): cpRNFL scans

circumpapillary retinal nerve fibre layer thickness

measured along 3.4-3.5 mm diameter circle centred onh

can be extracted from cube scan

or caputred directly

most used and most useful scna of the disc for glaucoma

why is cpRNFL scans useful

all retinal ganglion cells from retina send their axons to form bundles that course through the rnfl and exit the eye at the onh, therefore virtually all pass through this circle

thinning of the rnfl indicatses loss of retinal ganglion cells= glaucoma

white circles in pic slightly away from the disc to maximise thickness of rnfl= greatest measurement range and minimise effects of variation in disc size/shape

bottom image shows localised loss of nerve fibres ( dark areas)

thickness comparison of cpRNFL

thickness compared to normative databse along its length and in sectors

inner limiting membrane and rnfl are the two easiest layers to segment

enables accurate measurements of thickness

right picture shows healthy eye. thicker nerve fibre layer in inferior temporal and superior temporal area

left; inferior temporal shows it is thinner as it drops off in graph.

what do the colours mean on comparisons to normative data

measurements are compared to those taken from a large number of healthy volunteers; called normative database

green= within normal limits= greater than 5th percentile of normative data

yellow= borderline, between the 1st abd 5th percentile

red= outside normal limits below 1st percentile of normative data

what does measurments falling out the normal limits mean

means that it is smaller than that measurement for 99% of healthy eyes in normative database

indication that disease may be present

doesnt mean that disease is present

similarily, a measurement in green means that at least 5% of the healthy yes in database had a smaller measurement, doesnt mean disease is not present

rnfl thickness of healthy eye

above at least 5% of healthy eyes in the database

rnfl thickness in an eye with glaucoma

nerve fibre layer thickness falls into the red, so its thinner than 99% of those with normal

would do VFT if red seen

key ONH/RNFL features of glaucoma

reduced rnfl thickness , often localised in early cases

increased c/d ratio ± cup volume

reduced NRR thickness and area

papilloedema

onh swelling

margins not distinct

on oct shows very thick as above normal limits

more papilloedema

margins slghtly blurred

onh profile raised and smooth on B scan

rnfl thickness above normal limits

papilloedema vs optic disc drusen

drusen is lumpy calcium/proteins deposits in onh

canmake onh margins appear indistiny and mimic papilloedema

hard to distinguish even with oct if rnfl thickness is increased

other athologies: optic neuritis

inflammation of the optic nerve

swelling and haemorrhage of the left onh

lernfl thickness above normal limits

re rnfl thickness within normal limits

optic neuritis: same ox just 17 days later

in left eye, swleling has reduced, some back into the green

right eye starting to swell up

bordered red was 17 days prior , bottom after

high myopia

often large and/or tilted discs

typically etxensive peripapillary atrophy

difficult to assess tilted discs for cupping/NRR health

may have rnfl and visual field defects that look like glaucoma

the px in picture shows high myopia, extensizzve PPA, thin rnfl and an enlarged blind spot

normative da tabases and high myopia

normative databases often exclude high myopes ( more than -5.00 or -600

due to larger eyes/ stretching of ocular structures , tissue profiles may differ from normative database in myopic px

rnfl

thinning of rnfl is seen in glaucoma

rnfls thickness measurements are accurate because the boundaries are easy to segment

rnfl follows a double hump profile in healthy eyes

different normative databases can be chosen to best match the px

swelling of onh results in thickening of rnfl measureable by oct

normative database statements

within normal limists DOES NOT mean that measurement is healthy

outside normal limits does not mean measurements is below average; means its below the 1st percentile

it is possible to have measurements that are above normal limits

what can go wrong

• using incorrect normative database or no normative database for px

no databases for under 18 px , and availability for different ethnicities variable

is the px well represented in database, eg excludes over -5.00 myopes

• if they have unsual discs

• scan capturue issues

• scan misalignment

• segmentation errors

• blood vessels affecting rnfl thickness

unusual disc

px with glaucoma

extensive ppa - can be a sign in glaucoma

ppa is also present in healthy eyes as retinal layes not in tact where the ppa is

oct struggles to measure the rnfl thickness inferiorly where scan circle crosses ppa

results in an artefactual inferior defect in both eyes, not possible to monitor change in that region

scan capture issues

misalignment of instrument results in dark/ noisy, griany and distorted image

software has attempted to analyse but still failed

scan capture issues: same eye, 30 secs apart

lower image slightly out of focus as blood vessels not as crisp as the top image

rnfl thickness measurements affected

effect uneven around profile

scan capture issues

same eye same session

incorrect device positioning

oct is out of range

results in sections cut off that cant be analysed

scan capture issues; same eye same session

effect of the scan circle misalignment

large effects on the rnfl thickness measurements

circle too far over to right, left

scan capture issue: improper correction for corneal curvature

C - curve = mean of vertical and horizontal corneal radius

device positions can circle according to assumptions about the eye size and optics

incorrect C curve means scan will be too small or too large , leading to misleading measurements

segmentation errors

retinal layers segmented automatically by device software

distance between ILM an RNFL/GCL boundary = RNFL thickness

sometimes automated segmentation chooses the wrong reflectance change eg a shadow

results in incorrect measurements but can be manually corrected by drawing on where you wnat it

effects of blood vessels

large prominent blood vessels cause a shadow in oct image

may cause segmentation error as shadows reduce the reflected light reaching deeper layers so deeper layers appear darker

software struggled todetect the correct layer boundary

also rnfl is thicker when blood vessels are there both due to the thickness of the vessel itself and vessels tending to be in areas of thick rnfl

isolated peaks in rnfls thickness can result

effect of blood vessels

severe rnfl loss due to advanced glaucoma

generalised thinning of rnfl

most or all retinal nerve fibres lost

some residual thickness remains due to remaining supportive tissue

isolated peaks remain- caused by blood vessels

monitoring change

major advantage of imaging in general, particularily quantitatve measurements in those from oct

image at baseline eg start of treatment or just when a new px is seen first

compare images at future visits to baseline

powerful method for early detection especially in glaucoma

monitoring change on oct

can compare the numbers on the oct printout to previous

scan must be correctly positioned ie in the same place as last time

most oct devices have a built in software for monitoring change overtime