Assessment of the optic disc (optic nerve head) in glaucoma Flashcards
what is glaucoma described as
a group of diseases characterised by retinal ganglion cell dysfunction and death
the is the detection of glaucoma and it’s progression based on
identification of abnormalities or changes in the optic nerve head (ONH) or the retinal nerve fibre layer (RNFL), either functional or structural
so the eye does not have glaucoma unless the ONH is damaged = optic neuropathy but does go hand in hand with other features such as raised IOP, narrow angles etc
raised IOP and narrow angles = a suspect status but unless the eye is damaged at the level of the ONH, then is actually doesn’t have glaucoma
name 4 points about the relationship between glaucoma and visual fields results and what do all these points mean
- Common statement….“50% nerve fibre loss before VF (Goldman bowl perimetry) defect…” - certain amount of damage to the ONH has to occur before the defect occurs
- RG cells mapped with threshold VF data
- A 5-dB VF loss ~25% RGC loss - a 25% ON fibre loss has to occur before a repeated VF loss is detected on modern images
- Abnormal points at p less than 0.5% have a mean
RGC loss of ~29%
= Extent of nerve fibre loss overstated, therefore the ONH is something we should observe first to prevent a VF defect from forming
list 5 features of a ‘normal’ optic disc
- Neural rim - yellowish/pink
- Physiological cup - avascular and pale
- White physiological scleral rim (Elschning) - scleral rim = pale colour halo around edge of disc (is where the neural tissue actually begins, so most accurately estimate disc margin if not will over estimate the CD ratio)
- Yellow-white ‘sieve-like’ appearance of lamina cribrosa
- Rim width configuration - ISNT (but not just the only guide for glaucoma)
why is the ISNT rule rim width configuration not the only guide for detecting glaucoma
because it doesn’t apply with a px who has, a tilted disc or myopic px’s etc so is just a guide for detecting, but the rule can be broken with glaucoma
when considering the c/d ratio, what else do you have to consider and why
you have to consider the height of the optic disc as well because
- 2 million nerve fibres passing through a large scleral opening gives a large cup
- 2 million nerve fibres passing through a small scleral opening gives a small cup and the nerve fibres are just more stuffed together
so if the scleral opening is large then will have a large cup = doesn’t mean px has glaucoma its just a physiological difference
list the 6 optic disc changes in glaucoma
- Enlargement of the optic cup (goes hand in hand with reduction in NRR tissue)
- Loss of disc rim (neural tissue)
- Vascular changes
- Increased pallor (of NRR, not the cup which is avascular)
- Peri-papillary atrophy (changes to edge of disc)
- RNFL changes
list 6 signs that can be seen in the enlargement of the optic cup in glaucoma
- Generalised: enlargement of cup is circumferential 360 deg
- Localised: if extremely localised = focal notch of NRR
- Vt-Hz disproportion: if cup is vertically elongated = suspicious as the disc is normally vertical and the cup is normally round or horizontal
- Inter-eye asymmetry: hallmark of glaucoma
- Lamina baring or
backward bowing: pore pattern is visible = deeper (but no point measuring dioptric value as deep cup can be normal) - ‘ISNT’ and ‘notch’
how do you assess the c/d ratio correctly
do not rely on the change in cup pallor as it does not correlate with the contour of the disc
the cup begins when theres a first discernible change in contour away from the surface of the retina and that may not match with the level of the pallor (thats why a stereoscopic view of the disc is important)
what do you need to know in order to evaluate the c/d ratio
what are 2 disadvantages of assessing c/d ratio to detect glaucoma
- Need to know optic disc
size - to interpret the amount of cupping - Poor sensitivity/specificity: won’t allow us to distinguish glaucoma alone adequately, and can better discriminate with combine with other features
- Inter/intra-observer
variability
how can the size of the optic disc be measured clinically and how can the size of the disc have an impact on your judgement of a glaucomatous disc
by using binocular indirect ophthalmoscopy - volk lens they will have correction factors: Superfield 1.5 90D 1.4 78D 1.1 66D 1.0
can measure height of the disc by changing the size of your slit lamp beam vertically with the upper and lower edge of the ONH ad read off the graticule when doing volk
which is the best way to do it unless your using OCT or other photographic method
multiply the size on the graticule of the SL with the correction value of the volk lens - for 66D no correction factor required
a c/d ratio of 0.5 in a small ONH is more suspicious of a c/d ratio of 0.7 in a large ONH
what is the size of a normal ONH
what size is considered a small ONH
what size is considered a large ONH
normal = 1.7-1.8mm high
small = 1.1-1.2mm height
large = 2.1mm or more in height
what is the difference between a physiological and a pathological neural rim
what is the most important this to look for when assessing neural rim
physiological: follows the ISNT rule (even if it has a large cup in a large disc)
pathological: does not follow the ISNT rule e.g. superior NRR rim is thinner than temporal
most important thing to look for is asymmetry between both eyes e.g. less cupping in one eye and a larger amount of cupping in the other eye = different c/d ratios
and asymmetry in cupping between the superior and inferior hemifields of one eye in VFs
how can you detect asymmetry of neural rim between both eyes and in one eye and how will this appear
in a VF test
will show a arcuate defect where the nrr tissue has been lost
asymmetry in the results between the lower and the upper hemifield
what are the 4 types of vascular changes that could be found in glaucoma
- Vessel configuration Nasalization Bayonetting Flyover/overpass Circumlinear baring
- Calibre of vessels: near optic disc is thinned (but is not a early sign)
- Collateral vessels: not leaky, but are shunt vessels
- Haemorrhage
describe the 4 different changes that can occur with vessel configuration in glaucoma
Nasalisation: major trunk vessels have shunted in the nasal region (normally they should have curved around more) seen in advanced stage
Bayonetting: can be normal in deep/steep cupping, or seen in glaucoma where can see a sharp edge to the cup and then see a BV deflecting underneath the RIM, so it emerges at the base of the cup in a different position
Flyover/overpass: vessel crossing the central part of the disc and its lost contact support with the disc tissue underneath
Circumlinear baring: vessel follows a course/curved pattern with an area of pallor above that BV which tends to imply that cupping has increased but that BV has been beared/exposed as theres cupping above an below it
what type of sign in glaucoma is disc haemorrhaging and where is it mainly located
what does it indicate
which type of glaucoma can it be commonly seen in
what is the difference in outcome compared to someone who has glaucoma and no disc haemorrhaging
what is the ddx of disc haemorrhaging in glaucoma
an active sign of progression seen more commonly infer-temporally (which correlates with supers-nasal vf defect)
it indicates the glaucoma is not controlled and needs to be closely monitored
commonly seen in normal tension glaucoma
haemorrhage in glaucoma will show larger progression of VF loss compared to px with glaucoma and no disc haemorrhage
ddx - disc haemorrhaging can also happen in PVD
what is peri-papillary atrophy
what type of feature is it
which type of peri-papillary atrophy is uncommon in normals and hence seen in glaucoma
what outcome is it associated with
thinning of the retina and choroid around the edge of the disc
is a second order feature = can be acquired due to glaucoma
a-zone and b-zone
b-zone uncommon in normals, so rare in patients who don’t have a pathology and hence may be seen in glaucoma
it is visible sclera b zone = closer to the disc and a bad form of NRR loss (if inferiorly will form a superior arcuate VF loss)
NRR and VF loss
what are all the 8 questions you will want in your optic disc checklist
- Is the vertical C/D ratio more than 0.5?
- Is the C/D ratio consistent with disc size?
- Is the cup more vertically oval than the disc?
- Does rim configuration differ from ‘ISNT’?
- Are there any notches or pallor regions in the nrr?
- Are there any disc haemorrhages?
- Is the inter-eye C/D ratio asymmetry more than 0.2?
- Has there been a more than 0.15 change in C/D ratio?
when can a c/d ratio asymmetry of more than 0.2 be regarded as normal
when the size and the height of the disc between the 2 eyes is different. so account for that
how should you record your assessment of the optic disc
if theres a fundus machine then take a picture
if there isn’t then draw it
write the c/d ratio
describe the appearance of the ONH and any pathology seen
record which volk lens was used and the correction factor when measuring the height of the disc
what is the name of a scale that can be used to assess a optic disc in glaucoma
what does it incorporate
what is a advantage to this scale
disc damage likelihood scale
Incorporates into a grading scale:
- The effect of disc size
- Focal rim width
- Advantage: Highly reproducible - correlates with VF loss
what is the method of using the disc damage likelihood scale
- Measure disc size/height and correct for Volk lens used
- discs categorised as 1 of these 3 categories:
S less than 1.5mm
M 1.5 – 2.0mm
L more than 2.0mm - Measure width of thinnest part of the rim - Rim:Disc ratio
- If no rim present at thinnest part value = 0, if rim as thick as possible (no cup) value = 0.5
- Extent of rim absence is measured in degrees
- It estimates the likelihood of a px having glaucoma e.g. over a 5 year period
which 2 places can optic disc observer variability occur
what are the 2 sources of measurement error and what is the difference in variability between both
Valid and reliable measures are needed, occurs in:
- General glaucoma clinic monitoring
- Co-management/shared care schemes
2 sources of measurement error:
- Intra-observer variability (within) i.e. yourself = has a tighter level of agreement than inter
- Inter-observer variability (between) i.e. 2 colleagues = better level of agreement
what is the criteria for change in c/d ratio which brings suspicion of glaucoma
and what system is there a benefit of using
- C/D ratio has to change by equal/more than 0.15 (within/intra)
- C/D ratio has to change by equal/more than 0.25 (between/inter)
- Benefits of using a 20 point scale*
(i. e 0.0-1.0 in 0.05 steps)
how can you do a RNFL assessment and what signs show a defect in the RNFL
what is the appearance of a pseudo defect
can only detect with monochromatic light i.e. red free (green filter on SL or OCT)
a dark wedge shaped defect which can reach the optic disc = thinning of RNFL
pseudo defect = less than a vessels width
list 3 optic nerve head imaging systems that can be used
- Heidelberg Retina Tomograph (HRT II)
- GDx VCC
- Optical Coherence Tomography (OCT)
how is sensitivity and specificity related to glaucoma
sensitivity = about detecting diseased individuals
specificity = about normals
so if trying to discriminate between glaucoma and normals = need good sensitivity and specificity
can be:
- Technique employed
- Competence/experience
list 6 congenital disc anomalies that can mimic the appearance of glaucoma and hence be a ddx
- Coloboma
- Optic disc pit
- Tilted disc
- Drusen
- Myelinated fibres
- ‘Myopic’ disc
list 5 other conditions apart from glaucoma that have nerve fibre bundle defects, paracentral, arcuate and nasal step
Can also arise due to:
- Optic nerve head drusen
- Congenital optic nerve head pits
- Coloboma of optic nerve
- Anterior ischaemic optic neuropathy
- Tilted disc
what is the NICE key implementation point (what 5 tests you should offer) for a px who has diagnosis
At diagnosis offer all people who have COAG,
who are suspected of having COAG or who have OHT all of the following tests:
- IOP measurement using GAT
- central corneal thickness (CCT) measurement
- peripheral anterior chamber configuration and depth assessments
using gonioscopy - visual field measurement using standard automated perimetry (central thresholding test)
- optic nerve assessment, with dilatation, using stereoscopic slit
lamp biomicroscopy with fundus examination
