This study sought to quantify the impact of pupil diameter on the determination of refractions by using two different metric-optimized techniques. Refractions at 4-mm and 6-mm pupil diameters differed more for eyes of subjects with DS than controls; however, the differences in refractions were still relatively small, with 85% of refractions for DS eyes having differences less than 0.50 D when utilizing VSX. These findings are in line with reports of test-retest for subjective refraction of typical subjects with myopia reported by Raasch et al.
23 That study reported median test-retest of 0.20 D with 95% limits of agreement of 0.62 D for myopic adults.
23 The median and 95% limits of agreement for the present study was 0.28 D (1.23) for subjects with DS and 0.16 (0.31) for controls when combining both metric techniques. Although the 95% limits of agreement are larger for the DS eyes, the difference in refractions is more comparable to controls from both the present study and the Raasch study than the individuals with keratoconus reported in the Raasch et al.
23 study (0.75 D [6.01]). These findings are also consistent with the hypothesis that elevated higher order aberrations may negatively impact repeatability of refraction determination either through the subjective process or when pupil diameter is altered in an objective method. As shown in
Table 1, the aberrations of the subjects with DS were slightly elevated compared to the controls but not approaching the levels previously reported for individuals with keratoconus. The pattern observed in the differences between refractions is of a similar magnitude as the pattern for differences in the magnitude of higher order aberrations for controls, patients with DS, and patients with keratoconus. An individual analysis of the two metric techniques found that the refractions differed more between pupil diameters for PFSt-optimized refractions than VSX-optimized refractions for both subjects with and without DS. Given that PFSt is a metric that specifically analyzes the WFE over the entire pupil diameter, giving equal weight to each tessellation analyzed, this outcome is not unexpected. VSX, by contrast, represents a more vision-related analysis of the wavefront without consideration of individual tessellations. Further inspection of the data also revealed that of the five individuals with DS having dioptric differences greater than 1 D between 4- and 6-mm derived refractions with PFSt, four of the individuals had spherical equivalent refractive error of −10.00 D or greater. In the control sample, there were no individuals with dioptric differences greater than 1 D, but also only one subject had a spherical equivalent refractive error of −10 D. The relationship between high myopia and the impact of pupil diameter on objective refractions optimizing PFSt may warrant further investigation. Despite these differences in identifying best refractions, metric was not a factor in the loss of acuity with increasing pupil diameter for a fixed refraction.
This study also sought to determine the acuity loss predicted to occur when a patient's natural pupil dilates from 4 mm to 6 mm while wearing a refraction determined from analysis of a 4-mm pupil diameter. Although the acuity change was statistically significantly greater for the eyes with DS, the overall decrease in acuity was only 4.5 letters (compared to a decrease of 3.5 letters for control eyes), and thus, the difference between groups is not clinically meaningful. The acuity drop predicted from the DS eyes was also the same as the test-retest of the Bailey Lovie-style acuity testing on control observers (4.5 letters) by using the same acuity system as in the present study and, thus, is not likely to be clinically meaningful even in isolation.
1
Although pupil diameter may impact the resultant refraction identified from an objective metric optimization process, the differences occurring for 4-mm versus 6-mm diameters were within the test-retest variability of standard clinical refraction and thus do not create any less certainty in the endpoint in its utilization. We chose to evaluate the impact of increasing pupil diameter on visual acuity by applying the refraction determined at 4 mm, as we felt this moderate pupil diameter would most likely represent the pupil diameter experienced in typical room illumination. In using a 4-mm pupil diameter for refraction determination, acuity was not predicted to decrease beyond the repeatability of visual acuity testing, as individuals experience pupil dilation up to 6-mm diameter in their daily activities. As is typical with all refractive corrections, patients are predicted to perform worse in dim illumination whereupon the pupil dilates to a large diameter, but the detrimental effects for eyes of patients with DS is not predicted to be clinically worse than those without DS.
One limitation of this study is that only 4-mm and 6-mm pupil diameters were considered, and thus, the findings are not indicative of the impact of pupil diameters outside this range. However, the range of pupil diameters tested in this study is a reasonable estimation of the dynamic pupil range experienced by nonpresbyopic adults in photopic conditions.
26 Smaller pupils of a 3-mm diameter could reasonably be expected but are likely to provide improvements in visual acuity
27,28 due to limiting the exposure to higher order aberrations, and pupil diameters greater than 6 mm are less likely to occur unless under dark viewing conditions for which the limited luminance would likely reduce acuity more substantially than any impact from exposure to higher order aberrations.
28 In moving forward with objective prescribing techniques, however, it may be best to customize the refraction determination to the individual subject's habitual pupil diameter in the examination room rather than applying a single common pupil diameter to all.