In this study, we found that the test–retest repeatability was worse at the border of deep scotomas (using the ONH in normal participants as a model for this) than at areas of normal retina, using the MAIA microperimeter with a larger stimulus dynamic range. These findings suggest that it would be inappropriate to use a single estimate of test–retest repeatability for both the border of a deep scotoma and the other retinal regions when attempting to determine whether a significant change in visual function has occurred with disease progression using microperimetry.
Using the MP-1 microperimeter for the same participants in this study, we were unable to compare test–retest repeatability at the border of deep scotomas with the adjacent area of normal retina because there was a significant decrease in microperimetric sensitivity between the two examinations. This decrease is most likely attributed to the limited dynamic range of this microperimeter, where the sensitivity measured in these normal areas often reached a ceiling effect, and could thus only either stay the same or decrease on the repeated examination. In other areas, where the sensitivity measured was near the ceiling of 20 dB, a negative bias would also likely be present because the ceiling effect limited the full potential of a positive change on retest. For example, a point measured at 18 dB can only exhibit a positive improvement of +2 dB but has the potential to exhibit a change of ≤−2 dB. It is also unlikely that this decrease was due to a fatigue effect, because the examinations were short, and a fatigue effect should have had an influence on all regions tested, rather than on just the regions where a ceiling effect was present. Such a decline in sensitivity between examinations was not reported in a previous study using the MP-1 microperimeter.
16 However, it is not known in that previous study whether the “follow-up” option was used, which would set the initial intensity of the stimulus at each point on retest based on the threshold obtained on the first test. When this option is not selected, the initial stimuli will be presented at the same intensity (set at a default of 16 dB for the MP-1) for both the first and the second examination, thus reducing the influence of the ceiling effect. Regardless, we found that the CoR of PWS at the border of the ONH was ±7.52 dB in this study (without excluding floor and ceiling effects), which is higher than those reported previously in the macular region (central 10° radius) of older participants with macular disease where the CoR of PWS was ±5.56 dB and ±4.94 dB, with and without excluding floor and ceiling effects, respectively.
17
Our findings of poorer test–retest repeatability using the MAIA microperimeter are not consistent with those of a recent study that found the test–retest repeatability of microperimetry similar at both the border of degenerative changes or ONH and other areas of retina in eyes with hereditary retinal degeneration.
8 Although there are several differences in the testing procedures used (including the chromaticity of the stimuli and background and stimulus pattern), a key difference was the use of a three-point spatial moving average to the microperimetric data, and the study reported a PWS CoR of ±4.21 dB that was applicable across all locations.
8 Applying the same type of spatial averaging to our data, we found a similar PWS CoR of ±4.65 dB that was similar across all locations. It is important to note that such averaging can obscure the changes that occur with disease progression, especially at the border of a deep scotoma where spatial averaging will include both a point inside the atrophic or degenerative change and a point outside sampling an area of healthier retina, both of which are less likely to progress as rapidly as the border of these changes.
The PWS CoR of ±3.81 dB in the macular region of the normal participants in this study, using the newer MAIA microperimeter, was also consistent with that of our previous study, where the PWS CoR was ±3.74 dB at the macular region for older normal participants.
18 The PWS CoR at the border of deep scotomas was ±12.99 dB in this study, which was also higher than those with AMD in our previous study, where the PWS CoR for points at the macular region was ±4.37 dB or less.
18 Although the PWS CoR was higher (indicating greater test–retest variability) using the MAIA unit than the MP-1 microperimeter in this study, it is likely that the smaller dynamic range of the MP-1 may have limited the full extent of the test–retest variability that could have occurred.
The increased limits of the test–retest repeatability observed at the border of a deep scotoma is most likely due to a combination of the limits of the fundus-tracking systems used in the currently commercially available microperimeters and stimulus parameters such as its size and presentation duration. With the imaging systems of the microperimeter tracking the fundus at a rate of 25 frames per second, small degrees of eye movements that occur during the presentation of the stimulus (200 ms) can result in a neighboring retinal location being sampled during the gap between each frame tracked. Therefore, the test–retest repeatability may differ depending on the fixation stability of the participant and requires further detailed examination. However, this finding still highlights the potential limitation of applying the same limits of test–retest repeatability at the border of a deep scotoma. These findings have important implications for clinical studies using microperimetry for monitoring longitudinal changes and response to treatment for slowly progressive conditions, although care must be taken when generalizing these findings to such conditions where the gradient of sensitivity may differ from those examined in this study.
In summary, this study showed that the test–retest repeatability at the border of deep scotomas was worse than that in other areas of normal retinas, highlighting the potential limitation of applying a single estimate of test–retest repeatability to determine whether a significant change has occurred in these regions. These findings are important to consider when measuring functional decline at the border of a deep scotoma.