VF Representation of Sensitivity/Stimulus Parameters Flashcards
What abnormalities does the total deviation map measure?
Diffuse and focal i.e. overall loss
What abnormalities does the pattern deviation map measure?
Focal loss
How is the pattern deviation map made from the total deviation map?
General height adjustment/elevator
Average reduction of the hill of vision (when compared to normal values is added to TD values (subtracted if vision better than average) to reveal focal loss
What is the p-value?
Probability analysis of deviation values
Indicates how likely there is to be an abnormality
If the p-value is darker, what does this mean?
It’s more likely to be abnormal
If the value of the absolute value is higher, what does this mean?
The patient has seen a dimmer stimulus i.e. they have a higher sensitivity at that location
What are some limitations of the grayscale map?
Not age-compensated
Not eccentricity compensated
Underestimates/masks early loss
Continuous nature of grayscale means that the difference between each point is cut out, making borders of loss less obvious
How does a Humphrey fields machine change the brightness of the stimulus?
Adds progressively darker ND filters in front of the light
What effect does a small pupil have on the results?
Diffuse ‘loss’ similar to cataracts
What can cause artefacts?
Upper lid
Glasses/trial lens edge
Incorrect positioning
What are the VF indices?
Mean Deviation (MD)
Pattern Standard Deviation (PSD)
Visual Field Index (VFI)
Short-term Fluctuation (SF)
Corrected Pattern Standard Deviation (CPSD)
Which of the VF indices are present on modern perimeters?
Mean Deviation (MD)
Pattern Standard Deviation (PSD)
Visual Field Index (VFI)
Which of the VF indices are present on older perimeters?
Mean Deviation (MD)
Pattern Standard Deviation (PSD)
Short-term Fluctuation (SF)
Corrected Pattern Standard Deviation (CPSD)
What is the equivalent of mean deviation on an Octopus machine? What’s the difference?
Mean Defect
Not weighted, and a reduced VF has positive values
What is the mean deviation?
What does it give an idea of?
A weighted mean of the TD values (weighted centrally - i.e. central values are given more importance)
More negative MD = more abnormal
Value given is how far away from a normal hill of vision the result is
Gives an idea of overall loss
What is a normal MD?
~-1
What is Pattern Standard Deviation (PSD)?
The standard deviation of the MD
Gives more idea of the shape of the hill of vision
What does a large difference between TD values indicate (analysed by PSD)?
Presence of focal loss
What does a more positive PSD score indicate?
More severe focal loss
If the PSD score is over +12dB, what does this indicate?
Diffuse loss
Large areas of focal loss becoming confluent
What is Short-term Fluctuation?
Weighted standard deviation of repeated measurements of sensitivity at the same specified locations - checks px consistency and variability
What could high SF indicate?
Abnormality (increased depth of defect, increased overall severity of loss)
Increased eccentricity
Px bad at doing test!
What is a normal value for SF?
Equal to or less than 1.5dB
On a HFA, how is SF tested?
10 locations tested twice
What is the equivalent of PSD on an Octopus machine? What is the difference?
Loss variance
Squared SD, so number is larger
What is Corrected PSD?
Why is it helpful?
PSD with the SF effects removed
Shows a more consistent shape of the hill of vision, makes focal loss more obvious and gives a good indication of variability
What is the Visual Field Index?
A centrally weighted percentage based on the probability map - tells you proportion of normal vision
Is the VFI clinically useful?
No - use as a tool to inform pxs, but be careful as 95% can seem good when it’s not
What does the Glaucoma Hemifield Test (GHT) indicate?
Whether one hemifield is likely to be glaucomatous when compared to the other hemifield (based on fact that one hemifield becomes glaucomatous before the other)
What values does the GHT use for comparison?
Pattern deviation probability values (split into boxes of different areas for comparison)
What causes the GHT to show “Within normal limits”?
No probability values are present in either hemifield
What causes the GHT to show “Outside normal limits”?
More probability values present in one hemifield than the other
What causes the GHT to show “Borderline”?
The software is unable to distinguish if it is glaucomatous, but it is not within normal limits
What causes the GHT to show a general reduction of sensitivity?
An equal increase in probability values in both hemifields
What causes the GHT to show an abnormally high sensitivity?
Px overclicking
What are the stimulus parameters?
Colour
Size
Location
Algorithm
What colour are the stimuli and background?
White
What conditions does the white background create? How bright is it?
Photopic conditions
31.5ASB
What is the standard stimulus size?
III
(2.26mm - subtends 0.43 degrees)
How many stimuli sizes are available?
6
For which stimulus sizes are normal values available?
III and V
By how much does the stimulus size increase between every size?
Doubles
Why would you increase the stimulus size?
Very poor response to standard size - needs bigger size to see stimulus
How much of the VF does a 30-2 program measure?
27 degrees
How much of the VF does a 24-2 program measure?
21 degrees
What is the difference between a 30-2 and 24-2?
Outer locations are removed in a 24-2 (except 2 nasal locations - 1 above and 1 below midline) due to increased variability at peripheral locations.
Nasal are kept to allow for nasal step defect to be detected
How much of the VF does a 10-2 program measure?
9 degrees
What is the difference between a 24-2 and a 24-2c?
24-2c has more central locations - allows for earlier detection of glaucoma in central retina
Why is it useful to perform a 10-2?
Smaller separation of points (2 degrees rather than 3)
Screens central retina
Both of these are useful for any central retina disease or to measure central vision when peripheral is gone (e.g. in end stage glaucoma)
What is the staircase method of Threshold estimation?
Determines threshold by starting above or below threshold, then descends or ascends in 4dB steps, then returns the opposite way in 2dB
What improves threshold accuracy?
Reduced step sizes
Increased reversal
Increased repetitions
What does increased threshold accuracy result in?
Reduces reliability
What is the threshold algorithm of FASTPAC (HFA)?
3dB steps, single crossing of the threshold
What is the threshold algorithm of Dynamic (Octopus)?
2-10dB steps dependent on px
What is the threshold algorithm of SITA Standard (HFA)?
4-2dB steps
What is the threshold algorithm of SITA Fast (HFA)?
4dB steps
What is the Error Related Factor (ERF) on SITA?
How similar px’s FOS curve is to normal and glaucomatous FOS curves
How does the SITA algorithm reduce time spent doing the test?
Tries to get as close as possible to threshold before presenting the stimulus
Adapts stimulus presentation speed to px reaction times
What increases the threshold variability?
Increased defect depth
Increased eccentricity
Increased overall loss
