Investigative Ophthalmology & Vision Sciences MSc Course. Glaucoma Module. Visual Field Reliability Indices. David Henson 2014.

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1 Investigative Ophthalmology & Vision Sciences MSc Course Glaucoma Module Visual Field Reliability Indices David Henson 214 Variability 1

2 Variability Clinical problem How can we judge whether the change from one visit to the next is significant? We know that some patients are more variable than others A small change in a reliable patient may be significant while a similar change is a less reliable patient may not We need a measure of patient reliability (variability) that can be obtained within a single test 2

3 Reliability measures This presentation covers the existing measures of reliability that are incorporated in the current generation of computerised perimeters Reliability criteria ONLY FOR SINGLE STIMULUS TESTS WHERE PATIENT USES A RESPONSE BUTTON Standard Automated Perimetry (SAP) Single Stimulus supra-threshold perimetry Frequency Doubling Technology (FDT) Threshold and supra-threshold Motion Displacement Perimetry etc NOT FOR Multiple stimulus perimetry 3

4 Reliability criteria Loss of Fixation Direct observation Often with aid of video camera subjective Blind spot sampling Heijl/Krakau technique Used in HFA and most other modern perimeters Eye movement recorder 4

5 Blind spot sampling Method BS located at beginning of test Occasionally present stimuli at BS location Record the number of times the patient responds and the number of trials, e.g. 5/15 Frequent problem with misslocation of BS Insensitive to small fixation errors as BS measure 5x7 degrees Increases test time Small sample size- imprecise.5 deg stimulus projected on BS Eye movement recorder Attractive proposition Objective Continuous with feedback potential Ignore results when eye not fixating Are you recording eye position or eye fixation? Eye position: Is the eye correctly located Eye fixation Is the eye looking at the fixation light 5

6 Point of regard? Camera Monitor Point of regard? Camera Monitor Fixation OK, lateral movement of eye 6

7 Point of regard? Camera Monitor Loss of fixation Lateral movement of the eye makes little difference to position of stimulus on retina x 3 24 μ θ 1 tan 24=x/3 = If eye is displaced 1mm then stimulus position error is ~.3 at 24 eccentricity. tan μ=1/3 = 1.9 tan θ=123.6/3 = 22.4 θ+μ=24.3 7

8 Eye Movement Recorders Current generation are sensitive to both displacement of the eye and rotation If the eye moves to one side then it appears as if fixation is lost when this need not be the case Conclusions regarding fixation error Fixation is very important. If the eye is not looking in the right direction then the stimulus is falling on the wrong part of the retina. Frequent fixation losses increase variability We need to differentiate between Errors in eye position Errors in fixation Current eye movement recorders do not 8

9 False positive errors Patient presses response key when there was no stimulus Trigger-happy patients No relationship with extent of loss Small samples: imprecise In example to the right only 16 trials False Negative errors Patient does not press response key At already tested location At intensity which should easily be seen 9dB above threshold on HFA Small samples: imprecise In example to the right only 11 trials Measure of attention 9

10 Accepted standards Humphrey Visual Field Analyzer (HFA) recommendations Fixation losses <2% False positives <33% False negatives <33% <66% of patients meet these criteria (Birt 1997) Changing Fixation losses to <33% improved this to ~75%. Extent of loss, test time, and age were identified as factors influencing the reliability of the Humphrey visual fields. Birt CM, et al. Analysis of reliability indices from Humphrey visual field tests in an urban glaucoma population. Ophthalmology 1997;14(7): Are Reliability Indices Reliable? Test patients twice Difference (test-retest variability) is a measure of reliability See how good indices are at predicting testretest variability False positives False negatives Fixation losses 1

11 1 st result 2 nd result Difference - = Calculate the standard deviation of the differences. If all the differences are then the SD will be Full Threshold Fixation losses <= 5 (1,15] (2,25] (3,35] (4,45] > 5 SITA Standard Fixation losses <= 5 (1,15] (2,25] (3,35] (4,45] > 5 Distribution of reliability indices 74 eyes from 74 Pts Tested twice Two types of test Full threshold SITA Number of observations 7 6 False Positives False Positives <= 2 (6,8] (12,14] (18,2] (24,26] <= 2 (6,8] (12,14] (18,2] (24,26] 4 35 False Negatives False Negatives <= 2 (6,8] (12,14] (18,2] (24,26] <= 2 (6,8] (12,14] (18,2] (24,26] Test-retest variability Test-retest variability <=.1 (.2,.3] (.4,.5] (.6,.7] (.8,.9] <=.1 (.2,.3] (.4,.5] (.6,.7] (.8,.9] Frequency of index (%) 11

12 1 8 Full Threshold Fixation losses SITA Standard Fixation losses False Positiv es False Positiv es First Test Indices (% 5 False Negativ es False Negativ es Variability (test-retest) Results/Conclusions Very weak relationship between test-retest variability and current reliability indices Strongest (but still weak) is between false negatives in SITA strategy Current reliability indices are very unreliable 12

13 Summary reliability indices As there are no good (valid) measures of variability (reliability) Perimetrists should be encouraged to grade patient reliability Reliability V Good Good Average X Poor V Poor Response window Patients normally press the button within a given time interval When they press the button outside of this time window it is usually an error (false positive) response latency (ms) stimulus FP catch-trial RT window presentation 13

14 Response window SITA uses patient response times to estimate the false positive rate After each test it calculates the normal response window for the patient It then calculates the number of outliers and translates this to a false positive rate Reduces test time by a small amount Based on larger sample More repeatable 14

15 False positives and supra-threshold tests Need to continue using false positive trails Otherwise normal Pts would see all the stimuli and would get into habit of pressing the button every time- increasing FP rate. Can use response times to reduce effect of false positives at end of the test work out the patients response window delete responses obtained outside of this window and re-test them. False positives and supra-threshold tests response latency (ms) stimulus FP catch-trial RT window presentation Re-test locations where RT was outside normal response window Henson perimeters automatically do this at end of an extended supra-threshold test. Henson DB, Artes PH. Ophthalmic & Physiol Optics 22;22:

16 Conclusion Described the 3 reliability indices used in modern perimeters Shown that they are not very reliable, i.e. they give a poor estimate of test-retest variability That the perimetrist can often give a better estimate of patient reliability than the reliability indices References Birt CM, et al. Analysis of reliability indices from Humphrey visual field tests in an urban glaucoma population. Ophthalmology 1997;14: Henson DB, Artes PH. New Developments in supra-threshold perimetry. Ophthalmic & Physiol Optics 22;22: Bengtsson B. Reliability of computerized perimetric threshold tests as assessed by reliability indices and threshold reproducibility in patients with suspect and manifest glaucoma. Acta Ophthalmol. Scand. 2;78: