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
