Aspects of automated perimetry 1 and 2 Flashcards
name 2 disadvantages to the Kinetic visual fields (Goldmann)
- Difficult to measure
- Poor reproducibility
what are the 2 conventional examination strategies of visual field testing
- kinetic
- static
out of static vf, what are the 2 types of strategies available
- threshold HFA, full threshold or SITA
- supra threshold henson perimeters
what does the supra threshold programme in the hesnon not do
does not measure the depth of a vf loss
it is used for case finding for eye disease
list 6 unconventional examination strategies of visual fields i.e. newer types of perimetry
- Ring perimetry
- Frequency doubling
- Displacement perimetry
- Noise perimetry
- Resolution perimetry
- Blue on yellow perimetry
what is the HFA considered to be
where is it commonly found
where is it starting to be found more
- Full threshold perimetry: ‘gold-standard’ = gives best measurement
- Commonly found in secondary care setting (hospitals)
- Now commonly found in optometric practices as well
what is the background luminance used in the HFA
what is the stimulus presentation time
what is the standard size of the stimulus that is used in practice
- 31.5 asb background luminance
- Stimulus time 200ms
- Goldmann Size III
what does research suggest about the target size III used in the HFA
that we should move to a bigger stimuli as it gives more reliable results
what is point wise retinal sensitivity and what is it measured in
what is a normal value
what value is a absolute defect
- the measurement of each point of retinal sensitivity which varies across the vf and with age
- measured in decibels dB
- normal = 33-35dB (varies across the vf and with age)
- absolute defect = 0dB (very bight stimuli)
what is a newer generation of the HFA called
list 4 improvements to this machine
HFA II
- Good compact design
- User friendly
- Gaze tracker
- ‘Smart’ data acquisition (SITA)
what are the 2 degrees of area that the HFA can measure monocularly
how many test points does each setting have
which one is used more in hospitals and why
which setting is supposed to pick up a nasal step and hence which type of field and by how much % is detected with this setting
- Central 30-2
- Central 24-2
- Central 30-2 = 76 test points
- Central 24-2 = 54 test points
- More used in hospitals = Central 24-2
- because its quicker, can be done fast = only reason
- Central 24-2 is supposed to pick up nasal step
- Central 24-2 - 90% of initial glaucomatous field defects in this area
how much of a persons vf does the HFA C 24-2 only cover and hence how much is neglected
- only covers 20% of vf
- 80% is neglected
how much of the vf does the estermann binocular vf cover and what is it used for
- beyond 120 degrees of vf
- for DVLA standards, fitness to drive
how does the testing of a HFA full threshold start and what is it called
how is the threshold of a px established by the HFA
- testing starts at 4 primary points - so spends most of its time looking at 9 deg at each different 4 points
- called primary points
- established with a ‘staircase strategy’ using 2 reversals
list 5 advantages of automated perimetry
- Stimulus parameters standardised and can be varied
- Examination strategy is known and reproducible
- No observer bias
- Computer records
- Examination delegated to non-qualified staff
within the staircase procedure in the full threshold programme of the HFA, how is the threshold established
it is the difference between the second reversal and the last unseen point
what is the first ever vf a patient has done known as
their baseline vf e.g. can be done when they got glaucoma
what is a change in someone’s vf re done on the same day not linked to if they have glaucoma and why
what is any difference seen on the same day down to
- it is not linked to physiological problem of the patient’s glaucoma
- because it cannot get worse on the same day
- so any difference seen is purely down to the measurable variability as you can never get them to look identical even on the same day
what is a difficulty found in automated perimetry and list the 2 main things that cause this
and 5 other causes based on how you set up the machine
- Noise - differences that shouldn’t be there = a variability
- Artefacts; pupil size; media opacity (cataract)
- Response reliability and poor fixation
Other things that will affect the variability based on how you set up the machine:
- Lens Rim
- Refraction
- Pupil size
- Lid/brow
- Blinking
- some machines can measure the noise
what 2 things causes the response reliability and poor fixation as a contribution to noise difficult to control (variability) in your vf results and explain each what each one means
- Learning effects
Performance (sensitivity) ‘improves’ as patient does more tests. Research indicates that the first or, more commonly, the first two fields sessions should be regarded as training visits - Fatigue effects
Can cause a reduction in sensitivity and noise increases. Becomes more apparent in tests longer than 10 mins. Minimised by rest periods between tests and quicker tests (SITA)
what is the only way around the learning affect (which causes noise/variation in a vf result)
to give the px more time to practice after learning as this will cause the px to perform better
in which types of tests does the fatigue affect become more apparent and which 2 ways can it be minimised
- tests longer than 10 minutes
- rest periods in-between
- quicker tests such as SITA
what does SITA stand for
what is it regarded as
- Swedish Interactive Testing algorithm
- The new goldstandard for testing
list 5 points that summarises SITA as being now the standard method of testing
- Based on data from Normals and glaucoma – Uses mathematical models
- Uses previous responses
- Results from neighbouring test points used
- Present more stimuli near threshold
- Saves most time with new ways of measuring response reliability
what does the SITA algorithm not use
and what advantage does this have
it does not use individual testing locations like the full threshold
it uses a special revelation in the resting and it also tests the patient’s reliability quicker
list 2 advantages and 2 disadvantages to SITA testing
Advantages:
- Faster than traditional methods! Intelligent testing
- Less fatigue and preferred by patients
Disadvantages:
- ‘Black box’ method
- SITA Fast is even quicker, using larger step size in staircase not useful for follow-up as it gives unreliable data due to doing the test too quickly
why are reliability indices of a vf test important
because if the vf test values are unreliable, then theres no point looking at those results any further
what are the 3 reliability indices used in a HFA
- fixation losses
- false positives
- false negatives
what is short term fluctuation SF
how it is measured
give 2 disadvantages to this method
- Measure of variance/SD (to work out if someone was reliable or not)
- Uses 10 repeat values
- If same, then variance/SD is zero
- If large, differences then variance/SD is large
i. e. the points would be retested at 10 locations, not once but twice during the examination and if theres a difference between the first and second go = unreliable data
Disadvantages:
- Extra testing
SITA doesn’t use this approach and this reduces test time
- Ten points cannot be representative of whole vf
how is fixation loss FL measured
name one technique that measures this and how is it done
give 4 disadvantages to this method
- Blind Spot monitoring
- Heijl-Krakau technique: Establish BS at beginning of test. If BS presentation seen (i.e. px pushes button in area of BS) then assume loss of fixation
This is done a number of times (trials) with % seen an estimate of FL
Disadvantages
- Incorrect location of BS
- BS is large area as the size of the stimulus II which is only 0.43 deg is small compared to the optic disc so if px moves, it still cannot be seen
- Time consuming: it is not done enough as theres not enough time
- Sampling - Poor precision as to get very accurate sampling you will have to map the blind spot 50 times during the test which is too time consuming, but its only done 10 times which gives poor sampling
how is fixation losses measured in the newer HFA
what are the 2 disadvantages to this technique
SITA can ‘switch off’ Heijl-Krakau technique (saves test time)
- Uses Eye Movement recorder instead Optical tracking device Real time graph monitoring steady fixation and blinks: down = blinks spikes = fixation losses
Disadvantages:
- Problems with small pupils causes problems with the eye tracker
- Doesn’t monitor head movements: as we can move out heads and maintain fixation
what is the best reliability data and why
measuring false positives as its the most informative data
what is false positives
how does the HFA full threshold programme measure this
what is a disadvantage to the way this is done
how does the SITA programme measure this
what is a advantage to the way this is done
- ‘Trigger Happy’ Patients = keep pushing button when stimulus is not there
- No stimulus but patient presses button: a trick where the machine doesn’t show a stimuli for awhile
- Small samples: Imprecise, because the machine doesn’t do this enough
- SITA: FP rate estimated by analysis of response times, so uses the px’s own reaction times, if it varies a lot then the results are unreliable
- No extra testing
which type of reliability data is least reliable
false negatives
what is false negatives
how does the HFA full threshold programme measure this
what are 3 disadvantages to the way this is done
how does the SITA programme measure this
what is a advantage to the way this is done
- patients who have lost interest/falling asleep, so dont push the button when a stimulus is presented/seen
- Stimulus 9dB above recorded threshold i.e. a stimulus brighter than what a px has previously seen and patient doesn’t press button
- Most people don’t have any
- Dependent on extent of loss: related to how bad vf is e.g. if px has glaucoma, vf is going to be recored how bad the vf is, even is px is trying their best, so there is a relationship between the level of vf sensitivities and false negatives = not useful
- Small samples: imprecise as can only play this trick so many times during a test, usually only around 10x
- SITA: FN rate takes into account threshold sequence at every test location: does not use technique of very bright stimulus, but does present a bright stimulus where previously missed
- No extra test time
how can you improve someones response reliability and avoid high values in their reliability summary
- Re-instruct or redo!
FL > 20%
FP & FN > 33%
= not reliable data so not useful - Patients should be monitored
Unreliable patients can be advised or restarted (ultimately saves time)
what is meant by quantification of a vf result
The reduction of the data presented in the visual field chart to either one or a series of numbers which represent certain characteristics of the visual field data
i.e. numerically summarises the vf
which piece of numerical data on a HFA is best for case detection/finding such as glaucoma
the pattern deviation plot
what plot do you need first to understand a pattern deviation plot and what does this plot consist of
- total deviation plot
- it is a comparison of whats been recorded with the expected value at every single location
what are probability maps all based on
what is meant if the value is 0
what is meant if the value is more negative
in a total deviation map
- All based on an age-matched normal reference field (NRF)
Statistical comparison to normal subjects at each point - 0 = normal
- -ve = more further away from a normal
what does the distribution curve of a total deviation show
what a visually healthy person will see at each value
if the px is at the tail of the distribution = really abnormal
how is a pattern deviation plot established
adjustments are made for the overall sensitivity from the total deviation plot
the vf instrument brings values back to the normal hill of vision by correcting the overall sensitivity loss and from there can detect early glaucomatous loss i.e. localised loss
what 3 things does the global indices consist of
- mean deviation
- pattern standard deviation
- short term fluctuation
what are the global indices values based on
on an age-matched normal reference field (NRF)
what does the mean deviation represent
- Average of differences between measured field and NRF
- Overall depression in sensitivity of field
- the more -ve the value = the worse the vf
- 0 = perfect
what does the pattern standard deviation represent
- Difference between measured and NRF once taken account of the average deviation (MD)
- ‘Lumpiness’ in the field – localised loss
- the more +ve the value = the worse and is a sign of localised vf loss
- 0 = perfect
what is the global indices: short term fluctuation
a reliability measure
list 4 good values of global indices in a vf result
- A quick way to ‘score’ the field
- Population studies
- Large amounts of data
e. g. Does sensitivity change with age?
e. g. does ‘drug x’ preserve sensitivity? - For monitoring visual field progression
what is the glaucoma hemifield test done for
how many areas does it cover
how does it establish it’s results
what is it sensitive to
what is it insensitive to
how does it present it’s result
what should be done if it comes back negative
- Detects glaucomatous defects
- 5 areas
- compare below / above
Vertical asymmetry
It makes the assumption that in a normal/healthy vf, there will be symmetry in the superior and inferior vf
so it looks for asymmetry by looking at the psd in 3 location in the inferior vf
if it is different in the different areas and it looks for abnormality
principle of test = uses asymmetry of vf - sensitive to local change
- insensitive to diffuse loss
- it has no value/number to it’s results, just says within normal limits or outside normal limits
- if outside normal limits - test should be repeated
what 2 questions do you want to ask yourself when your evaluating someones vf results
and what do you never want to refer on
- Is the field reliable?
- Does the field fit in with the rest of your examination?
- Never refer on a single test alone
