Automated perimetry 3 - visual field progression and newer techniques Flashcards
which 2 ways can disease progression in glaucoma be monitored
- clinical management
- clinical trials
what is measuring vf more important than when investigating glaucoma in a patient and why
- more important than assessing the onh
- because the vf also tells you what the patient can do in a functioning of everyday life, which is what matters to the px most
vf/perimetry is __________ in monitoring a px for glaucoma over time = __________ _________ __________
vf/perimetry is critical in monitoring a px for glaucoma over time = critical clinical measurement
list 3 key things you have to do to when measuring vf during perimetry that gives better results
- explain the test clearly to the px and show them the vf chart - makes a difference and you get a better response
- supervise the px
- the environment - must be quiet and dark
what 3 things can make is difficult to detect a true vf progression
- noise/variability
- no ‘gold standard’ method
- difficult to detect change:
Slow process
No natural history
list 6 factors that can cause noise/variability in someone vf progression
- Pupil size
- Refractive error, cataract
- Response reliability
- Poor fixation
- Learning effects
- Fatigue effects
how does visual ‘clinical’ inspection assess a visual field progression and what are the 3 disadvantages of this
Consider all of the output - a forced choice question answered by a ophthalmologist and they base their decision on previous vf’s they have seen and decide if it is good or bad from that
Disadvantages:
- Difficult to do - as assessment is subjective
- Subjective
- Not quantitative - as its based on the decision of the ophthalmologist
what was each series of the glaucoma specialist ophthalmologist’s response classified into with the visual ‘clinical’ inspection assess a visual field progression
and what was the outcome of this method
Classified into:
- Definitely Stable
- Probably Stable
- Probably Progressing
- Definitely Progressing
not all of the glaucoma specialist ophthalmologists agreed with each other about the same vf’s
what was the next best way of assessing the progression of a visual field in glaucoma from visual ‘clinical’ inspection
a quantitative method by letting the VF machine decide using:
Global indices
e.g. Mean Defect (MD)
Average of all deviations - a single number that tells us how bad the VF is
Rate of loss (dB/year), more -ve = worse
what is the 2 advantages of using global indices like MD to assess the progression of the VF in someone with glaucoma
and what is the 3 disadvantages to using this method
Advantage:
- can plot the MD over time and can see if the value changes and can do a statistical analysis and see he trend/rate of loss
- good specificity - if MD worse then you know the VF is getting worse
Disadvantages:
- Data reduction
- No spatial info. about loss - i.e. doesnt tell you where the VF loss is e.g. a central VF loss is much worse than periphery, but both can have same MD value
- Poor sensitivity - if px has a slight change in VF, the number might not change that much, so difficult to track small changes over time (only way to resolve is to split the VF into individual parts)
what is a better way of assessing VF progression than using the MD which is just 1 whole number to represent the VF progression
what uses this method
why is this method better than just using the MD
- Point by point methods
GPA - “Glaucoma progression analysis”
= Change relative to baseline and a database of ‘stable patients’ - used by Zeiss HFA
- it does a more sensitive analysis than MD as it uses a point by point method
why are the limits represented in the glaucoma progression analysis method of assessing VF progression not parallel when the VF damage gets worse
how is the progression plotted onto a graph
- because the more damage to the VF from e.g. glaucoma - the more variation there is and thats why the limits aren’t parallel
- every single point in the VF is assessed separately and for each point there is a graph that represents the change in VF progression over time for each individual point
- the Y axis is the retested threshold
- the X axis is the tested threshold
- the higher your plot on the Y axis = the better you did from previous/baseline and the lower is you did worse
- the left side of the X axis = you did well (normal vision) and the right side is impaired vision (so did badly)
- if your sensitivity was recorded the same at both visits, it would be recorded along the same line
- if it was worse on the second visit, it will be recorded below the line but still stay within a certain limit which means it hasn’t fallen enough away from the line for us to be certain it is a true change because there is so much variability therefore that point is classed as ‘not progressing’
- in these measurements we want to make sure theres a really big change before we can be confident theres a true change
- anything that falls outside the limits = most likely a definite change as its a big change
what does each of the 5 symbols in a glaucoma progression analysis GPA plot stand for
- . = no change from baseline e.g. if started at 28dB and still 28dB on subsequent visit, OR 26dB on subsequent visit as its not a big enough change as its still within the limits
- outlined triangle = significant change e.g. started with 24dB then dropped to 18dB = 6dB drop/significant change
- half black filled triangle = change will be confirmed at next visit
- full black solid triangle = change confirmed at next 2 visits (true change) i.e. 3 times that it dropped below the base line
if theres 3 of these full black filled triangles then = vision is worsening - x = too much variability or defect at baseline (no decision)
i. e. that point was too damaged in the start for the machine to determine any further change recurring e.g. may have been a 0dB point/blind spot
how many tests does a px have to have to establish their baseline and therefore how long can it take to detect a full black filled triangle in their results in a glaucoma progression analysis GPA
- a test has to have 2 visits to have a baseline established
- px has to have 5 tests to establish a solid black filled triangle - so if every done 6 months, it can take 2 years or so to see VF is declining overtime, but has to be this way because VF measurements are so noisy
what is the advantage and 3 disadvantages of the glaucoma progression analysis GPA
however why does the disadvantages still make this a well used method
where is this method used
- Very sensitive
- Dependent on Baseline e.g. px may have done badly on the test the first time
- Database?
- Does not use all the fields
- however we have to rely on the software to determine the results of the VF test because good clinical software will help with clinical decisions better than an expert assessing if by eye as they all have different decisions to each other
- Used in clinical trials
