VF artefacts and Suprathreshold Flashcards
What can increase the variability of a VF test, both within and between tests?
Depth of defect (deeper = more variability)
Eccentricity (more peripheral = more variability)
Overall severity of loss
What ocular factors can affect the variability of a VF test?
Media opacities
Pupil size (smaller = more variability, more likely to cut off peripheral field)
What physical factors can result in poor perimetry?
Incorrect trial lens power
Incorrect trial lens position
Upper lid, head and body position of px
By how much does the wrong lens power affect the variability of a VF test?
1DS defocus = ~1dB reduction
What behavioural factors can result in poor perimetry?
Learning effect
Fatigue effect
False positive effect
Unreliable response
What is the learning effect?
Improvement of threshold as test goes on - within and between eyes - as the px learns how the test works
Which areas and types of defect exhibit the most improvement due to the learning effect and why?
Peripheral areas - bc central areas have higher sensitivity so less improvement needed. Peripheral takes time to adapt.
Moderate defects - areas with moderate thresholds (~14-20dB) have the most variability so have more chance to improve.
What is the fatigue effect?
Worsening of threshold as test goes on - with and between eyes - as the px tires of test and loses concentration
Which areas and types of defect exhibit the most reduction due to the fatigue effect and why?
Peripheral areas - these areas have a lower sensitivity so are the first to go when concentration is lost
Moderate defects - areas with moderate thresholds (~14-20dB) have the most variability so are more likely to worsen on lack of concentration
What type of artefact can occur when the fatigue effect takes place?
Clover leaf field
What is the false positive effect?
Px continuously pressing button - ‘trigger happy’ to ‘please’ technician no matter what they can/can’t see
Will result in hyper abnormal results and white areas on report
What classes as an unreliable response?
High rate of false positives, false negatives or fixation loss
What is suprathreshold static perimetry?
The stimulus is presented at a known higher level than the threshold (normally 8dB) so every spot expected to be seen. If not seen, spot repeated at a much higher level (40dB) but can only qualitatively measure, i.e. ‘seen’ or ‘not seen’.
What is the suprathreshold increment?
The difference between the expected threshold and the known luminance
OR
how much brighter the stimulus is than the age corrected threshold value
What does gradient adapted mean in suprathreshold perimetry?
The suprathreshold increment is the same across the whole field, i.e. the values are different due to differences in sensitivity but it’s the same difference between the expected threshold and known luminance.
