Week 9 - Disc assessment and SIGN Guideline Flashcards
Disk assessment using, and benefits:
• Disc Assessment using slit lamp biomicropscopy i.e. Volk lens or equivalent
•Why use slit lamp biomicroscopy?
• Better field of view
• Improved illumination
• Stereoscopic view of the fundus
• Easier to navigate the fundus
• Image size less affected by refractive error
• Better view few partially opaque media e.g. cataract
• AND can measure disc size!
Preparation for Volk Disc assessment
• Dilate the pupil (if necessary).
• Darken the room illumination so that the intensity of the slit lamp beam can be minimised
•The practitioner should be comfortable. If available, slit lamp elbow rests can be used.
• Position the patient comfortably at the slit lamp microscope.
• Instruct the patient to fixate a target straight ahead, e.g. the practitioners ear, stare wide and to blink normally.
Slit lamp settings.
• Start with 15x magnification. This can be increased later as
Disc assessment:
• Need to assess disc size
• Pay attention to where the disc rim is narrowest
• Scrutinise where the neuroretinal rim ends around the whole disc
• Base your judgement of where edge of the cup is on bending of the vessels - NOT ON PALLOR
• Look for glaucomatous disc features (Dr Sidiki’s Lecture - Clinical
Ophthalmology)
• Familiarise yourself with the DDLS scale and SIGN guidelines
- Narrowest Rim to disc ratio
• Need to consider which part of the neuroretinal rim is narrowest
• Do not refer based purely on violation of the ISNT rule
• Always refer optic disc retinal nerve fibre layer haemorrhages irrespective of other signs of glaucoma
• Refer based on Narrowest Rim to Disc Ratio - based on Disc Damage
Likelihood Scale (DDLS)
What about the ISNT rule
•Whilst the INST rule is followed by most healthy discs… Most discs which violate the ISNT rule are NOT glaucomatous
• One study showed that the sensitivity of the ISNT rule for detecting glaucoma is good (95%)
• However, the Specificity is poor 12%
• As a result the SIGN guidelines state referral for assessment of glaucoma should NOT be made solely on violation of the ISNT rule
When should a patient be referred irrespective of IOP?
• Patients with an optic disc haemorrohage should be referred irrespective of other signs of glaucoma
• Patients with cup to disc asymmetry - (difference in C:D of 0.2 or greater consider referral?)
What is the DDLS?
• Recently the Disc Damage Likelihood Scale(DDLS) has come to prominence as referral based on disc signs for glaucoma is Scotland should be made with reference to the DDLS.
• The grading on the DDLS has been shown to have
- good repeatability
- a strong correlation with glaucomatous visual field damage
- better specificity than cup to disc for detecting glaucomatous disc damage
3 steps to disc assessment under sign guidelines:
- Measure Disc Size
- Assess width of Neuroretinal Rim
- Grade using DDLS Scale
Disc size measurement:
• Difficult to do accurately using a photo
• Cannot be done using ophthalmoscopy
• Use a volk lens in combination with a slit lamp which can record the height of the slit
• Beware of peripapillary atrophy and myopic crescents when estimating disc size
• Reduce the width of the beam to 1-2mm
• Reduce the height of the beam to match the size of the disc and read of the value from the slit lamp height graticule
•Apply lens correction factor to the value obtained from the slit lamp height graticule
Considerations when choosing your lens:
• What part of the fundus are you most interested in in THIS patient?
• Macula or Disc - higher magnification and lower field of view -
- 60D
- 66D
- 78D
• Peripheral funds - lower magnification and larger field of view -
- 90D
- Superfield
Correction factor for disc size:
Corrective Factor for disc Volk Lens size
• 60: ×0.88
• Super 66: X1
• 78: × 1.08
• 90: × 1.30
• Superfield NC: x1.30
Size of discs and size of discs using 66, 78 and 90D lenses:
• Small Disc Size - less than 1.5mm
• Medium Disc Size - 1.5-2.0mm
• Large Disc Size - greater than 2.0mm
• When using a 66D lens the graticule reading will be 1.5-2.0 for a medium disc
• When using a 78D lens the graticule reading will be 1.25-1.75 for a medium disc
• When using a 90D lens the graticule reading will be 1.1-1.5 for a medium disc
narrowest rim to disc ratio:
• Identify the narrowest rim ratio (C:D 0.2)
• OR estimate the degrees of absent rim if rim is completely lost (Rim absent less than 45°)
• small discs (<1.5 mm in diameter) where the narrowest rim/disc ratio is <0.3
• medium discs (1.5-2.0 mm in diameter) where the narrowest rim/ disc ratio is <0.2
• large discs (>2.0 mm in diameter) where the narrowest rim/disc ratio is <0.1
• On DDLS always refer if grade 4 or above
What is a rule of thumb we can use to simplify DDLS?
• Small disc (+1)
• Medium disc (+0)
• Large disc (-1)
Limitations of DDLS:
• Small. coloboma, tilted, myopic
• Helps monitor focal loss, However;
- Poorer at monitoring diffuse NR loss
- Doesn’t work well with unusual discs, Neither does C:D
- Monitors the thinnest area of NRR but if focal loss is occurring in another location the DDLS grade may not change
Disc Drawing:
• The NRR thickness and the narrowest point of NNR thickness
• Blood vessel pathway as they emerge from the cup
• Any unusual/glaucomato us feature
