Abstract
Purpose:
Scotopic microperimetry measures retinal sensitivity under very low light and may be useful in conditions characterized by nyctalopia, such as retinitis pigmentosa and age-related macular degeneration. The Scotopic Macular Integrity Assessment device enables two-color perimetry to isolate rod and cone responses. This study assesses the reliability, test–retest repeatability, and sensitivity in healthy participants aiming to establish normative values.
Methods:
Scotopic microperimetry was performed using cyan and red stimuli on a 37-point radial grid after dark adaptation on control participants with no history of eye disease and visual acuity of 0.1 logarithm of the minimum angle of resolution or better. Fixation stability, fixation losses, and identification of the rod-free zone were used as reliability metrics. A subset underwent repeat testing within 4 weeks.
Results:
Thirty-nine participants (19 male and 20 female), median age 24 years (interquartile range, 9.5 years) and 23 years (interquartile range, 9 years) for the right and left eyes, respectively, completed testing. Overall 77 eyes underwent scotopic testing, with 82% meeting reliability criteria. Mean cyan and red sensitivities were 19.9 ± 1.1 dB and 20.9 ± 1.2 dB in right eyes, and 20.1 ± 1.4 dB and 21.3 ± 1.4 dB in left eyes, respectively. Volumetric cyan and red sensitivities were 2868 ± 157 dB.deg2 and 3077 ± 176 dB.deg2 in the right eyes, respectively, and 2892 ± 205 dB.deg2 and 3126 ± 207 dB.deg2 in the left eyes, respectively. Mean sensitivity coefficients of repeatability (CoR) were ± 1.4 dB (cyan) and ± 2.1 dB (red) while pointwise coefficients of repeatability were ± 7.2 dB (95% confidence interval, 6.5–7.6 dB) for cyan and ± 7.9 dB (95% confidence interval, 7.3–8.4 dB) for red, with no significant differences between eyes or genders. Fixation stability assessed using the 95% bivariate contour ellipse area for cyan was 2.9 ± 5.9 deg2 and 2.3 ± 2.2 deg2 for the right and left eyes, respectively, and for red were 0.7 ± 0.6 deg2 and 0.9 ± 0.8 deg2 for the right and left eyes, respectively. Again, there were no significant differences between cyan and red tests (Friedman test, bivariate contour ellipse area 63%, P = 0.455; bivariate contour ellipse area 95%, P = 0.432).
Conclusions:
Scotopic microperimetry using the Scotopic Macular Integrity Assessment device was feasible and well-tolerated. Repeatability metrics demonstrated limitations in fine spatial mapping of scotopic retinal sensitivity.
Translational Relevance:
This study highlights potential areas for future improvements in scotopic microperimetry before its use as an outcome measure in clinical trials for retinal disease.
This study demonstrates the application of volumetric analyses in scotopic central retinal sensitivity using the S-MAIA device with the extended dynamic range, providing a normative dataset for mean and volumetric sensitivity. The reliability of the dataset was ensured across multiple measures, including fixation losses, fixation stability, and the identification of a physiological rod-free zone. Assessment of scotopic function is more logistically complex to undertake and time consuming, in comparison with other forms of microperimetry, owing to the requirement for full dark adaptation before testing, followed by a challenging examination. Viewing threshold stimuli in complete darkness requires good concentration and lacks real-world generalizability.
Most participants were able to perform microperimetry reliably with 82% satisfying the inclusion criteria. Scotopic microperimetry testing is difficult and tiring for the participant. We believe the high rate of exclusions reflects the difficulty of the test. On the assumption that an examination is deemed reliable if it falls within the 95% confidence interval in a control group, our dataset indicates that in healthy participants, a BCEA95 of approximately 5 deg
2 or less may be considered a stable fixation. BCEA values were consistent with previous measures of fixation stability on radial testing grids under scotopic conditions
18 and mesopic conditions.
29,30 P1 and P2 act as complementary measures of fixation stability and only one test in this study was considered relatively unstable for only meeting one of these criteria. Fixation stability is an important consideration in deciding on the reliability of any given examination. Previous work has found that owing to the limitations of the 25-Hz refresh rate of the scanning laser ophthalmoscope used to perform retinal tracking, a good fixation performance is required to avoid undetected stimulus placement errors owing to saccadic eye movements overwhelming the tracking capabilities.
31 This is likely to be the source of high variability found at the borders of deep scotomas.
32
Owing to the physiological absence of rod photoreceptors in the central fovea, cyan sensitivity at this location should have a significantly reduced or absent threshold value in comparison with the paracentral test loci. Therefore, positive identification of the rod-free zone acts as an additional measure of test reliability (incorporating both fixation stability and false positive testing). This assumes accurate positioning of the test grid and correspondence between the anatomical and physiological fovea. There are limitations to this approach: the 37-point radial default grid used in this study includes only a single central test point. Improving the validity of this approach may involve adapting the testing algorithm to include multiple test loci in the central fovea within the rod-free zone. Alternatively, introducing formal false-positive catch trials, following strategies used in static perimetry,
33 may improve reliability assessment in microperimetry. Both options, however, would increase testing times significantly in an already long and difficult testing procedure.
A standard output of the device is the cyan minus red difference. The S-MAIA has calibrated the decibel values to account for greater retinal sensitivity toward cyan. Hence, loci with a negative cyan minus red value would theoretically indicate a cone dominant region, loci with a positive value would indicate a rod dominant region and those with zero value would indicate either healthy retina or equal rod/cone dysfunction. Whilst appealing, there is a great deal of potential for misinterpretation. For example, in the most extreme case of complete rod dysfunction in a given region, a cyan stimulus is unlikely to elicit no response at the brightest level (0.0 dB), but rather will elicit a response from any healthy cones that are present. This creates ambiguity in interpreting with confidence whether a region is truly rod or cone dominant. Additionally, when the S-MAIA reports mean difference, it does not account for the central rod-free zone, which skews toward a negative cyan–red difference. For this reason, we do not consider cyan minus red to be a viable metric and have not considered it in detail in this work.
Pfau et al
18 (2017) investigated scotopic microperimetry in a similar cohort of participants with no visual impairment, however, this study only had a dynamic range of 0.0 to 20.0 dB (stimulus luminance range, 0.0025–0.25 cd/m
2) and many participants experienced threshold ceiling effects. In this study, with the increased dynamic range (0.0–36.0 dB) we demonstrate the upper limit of effective dynamic range to be 32.0 dB for cyan and 34.0 dB for red stimuli. This finding suggests that the S-MAIA with extended dynamic range is sufficient in removing the ceiling effect relevant for healthy retinas.
In a recent study by the authors involving scotopic microperimetry in a cohort of choroideremia patients, repeatability for scotopic pointwise CoR was ±15.5 dB and ±12.4 dB, with a scotopic mean sensitivity CoR of ±3.3 dB and ±1.4 dB for cyan and red stimuli, respectively.
27 In a similar study using mesopic microperimetry with patients with
RPGR-related retinitis pigmentosa,
12 it was reported that CoR for pointwise sensitivity were ±9.5 dB and ±9.3 dB, in the right and left eyes, respectively and the mean sensitivity CoR for the right and left eyes was ±0.7 dB and ±1.3 dB. Comparatively, in this study of healthy eyes, a difference exceeding approximately 8.0 dB on a pointwise level is necessary to identify meaningful changes. Our findings demonstrate a high pointwise CoR, in contrast with previous reports of the CoR for pointwise sensitivity in healthy controls.
18,34 However, the CoR for mean sensitivity remains similar to previous studies.
35,36 This discrepancy may be in a significant part owing to the underestimation of CoR where data are not nested in a linear mixed effect model framework. These findings may also suggest that repeatability under scotopic conditions is less robust than that seen in mesopic microperimetry, demonstrated by a CoR of 0.7 dB for mean sensitivity reported by Higgins et al.
36 The relationship between stimulus location and threshold sensitivity/repeatability was not explored here. This has been previously explored by Welker et al.
35 and Pfau et al.,
37 who present normative data for scotopic microperimetry assessed using the MAIA device. They demonstrate there is little variation in sensitivity or repeatability at greater eccentricities under scotopic conditions.
This study has several limitations. First, there is a skew in the age of the study population toward patients aged 20 to 40 years. As a result, any trends in scotopic sensitivity or reliability in older participants may not have been captured adequately. Furthermore, there was no formal ocular assessment before recruitment, exclusion of participants with known eye disease, or a history of eye surgery was based on participants' reports. Therefore, participants may have been included who had unknown eye disease; however, because the cohort of participants was young, it is unlikely they would have had significant age-related ocular changes, such as cataracts or age-related macular degeneration. Pupil dilation was not performed. Previously, pupil dilation was shown to have no effect on mesopic microperimetry testing, provided patients had a minimum pupil diameter of 2.5 mm.
20 Because, in scotopic testing conditions are darker this factor encourages natural pupil dilation, another study showed no clinically significant differences in scotopic perimetry sensitivity, using a modified Octopus 900 perimeter (Haag-Streit, Köniz, Switzerland), with and without pupil dilation.
34 Therefore, pharmacological pupil dilation is unlikely to have a significant impact on the results. In addition, owing to the MAIA's autofocusing capabilities, spectacle correction is not required for testing, therefore refractive errors were not collected as part of this study and it was assumed participants fell within the MAIA's corrective range (−15.00 to +10.00 DS). The consistent use of the right eye for the repeatability testing was a further limitation. Randomizing the eye to be repeated would have been a more robust repeat testing regime. However, in a healthy control population such as this, we would not expect any significant changes to these results between eyes. Furthermore, the repeatability analyses are limited by a small number of repeat tests, repeatability using more tests would yield a greater degree of confidence in the resulting CoR values; however, it was not possible to ask patients to return on many occasions.
Funded by the National Institute for Health and Care Research (NIHR) under its Research for Patient Benefit (RfPB) Programme (Grant Reference Number NIHR202821). NIHR Oxford BRC grant. The views expressed are those of the author(s) and not necessarily those of the NIHR or the Department of Health and Social Care.
Disclosure: D.A.O. Adeyoju, None; A.S. Josan, None; L.J. Taylor, None; R.E. MacLaren, None