The effect of pupil size on visual performance with center-distance soft multifocal contact lenses

Abstract

Significance:

Multifocal contact lenses have been shown to slow myopia progression in children. Understanding their impact on visual performance is important. While these lenses have little effect on high-contrast visual acuity, they reduce low-contrast visual acuity and contrast sensitivity, with these reductions being greater as pupil size increases.

Purpose:

To determine the effect of pupil size on visual performance with center-distance multifocal contact lenses used for myopia control.

Methods:

Twenty-four myopic, nonpresbyopic adults were fitted with a single vision (Biofinity) and a center-distance multifocal (Biofinity D +2.50 add) contact lens, in random order. The pupil was dilated, and visual acuity (high- and low-contrast) and contrast sensitivity were measured through a 3mm and 6mm aperture.

Results:

Visual acuity differed by contrast level and depended on lens design and pupil size (p = 0.0040). With both pupil sizes, high-contrast acuity with the multifocal lens was only 2 letters worse than with the single vision lens (p = 0.0001). Lens design effects on low-contrast acuity were pupil size dependent (p = 0.0006). Compared with single vision lenses, low-contrast acuity with multifocal lenses was about 3 letters worse for a 3mm pupil (mean ± SEM = 0.07 ± 0.02 logMAR, p = 0.0010) and about 2 lines worse with a 6mm pupil (0.19 ± 0.02 logMAR; p < 0.001). While wearing the multifocal lens, low-contrast acuity decreased by nearly 2 lines when pupil size increased from 3mm to 6mm (0.17 ± 0.02 logMAR; p < 0.0001). The effect of lens design on the area under the log contrast sensitivity function (AULCSF) was pupil-size dependent (p = 0.024). When compared to a 3mm pupil, the greatest reduction in AULCSF at the larger 6mm pupil size was with the multifocal lens causing a 17% decrease compared with a 9% decrease for the single vision lens (p < 0.0001).

Conclusions:

Multifocal contact lenses perform similarly to single vision lenses under high contrast and smaller pupil conditions; however, visual performance reductions increase with lower contrast and a larger pupil size.

Myopia is associated with increased risk of ocular comorbidities such as glaucoma, retinal detachments, and myopic maculopathy.^1,2 By 2050, it is projected that 50% of the world’s population will be myopic, with over 1 billion people having high myopia (−5.00D or more myopic).³ Animal models have shown that plus lenses can slow eye growth.^4,5 This work has led to the investigation of optical designs that incorporate plus power into the lens to create myopic retinal defocus while simultaneously providing clear foveal vision to slow growth of the eye, potentially reducing the long-term risk of pathological conditions associated with myopia. Clinical trials have been conducted demonstrating that optical designs including multifocal soft contact lenses, orthokeratology, and novel spectacle designs can slow axial eye growth and myopia progression.^6–11

Multifocal soft contact lenses are increasingly being used for myopia control. Currently, MiSight (CooperVision, San Ramon, CA) is the only FDA-approved option for myopia control in the United States.¹⁰ Other contact lenses with center-distance designs that were originally approved for the correction of presbyopia are also being used off-label for myopia control. The Biofinity multifocal D (CooperVision, San Ramon, CA) has also been shown in a 3-year clinical trial to slow myopia progression.¹¹ The NaturalVue Multifocal (Visioneering Technologies, Inc., Alpharetta, GA) is also currently being studied in a randomized clinical trial.¹² The growing use of multifocal contact lenses makes it imperative to better understand not only their ability to slow axial elongation, but their impact on visual performance.

Previous studies have reported the effects of multifocal contact lenses on high- and low-contrast visual acuity and reading speed. Center-distance multifocal contact lenses generally perform similarly to single vision lenses with high-contrast targets with any differences being not clinically meaningful;^13–15 however, clinically meaningful reductions in low-contrast vision have been reported.^13,16 Center-distance multifocal contact lenses have also been reported to reduce contrast sensitivity compared with single vision contact lenses, demonstrating that contrast sensitivity provides additional information regarding the effect of multifocal optics on visual performance beyond visual acuity alone.^17,18 That said, previous studies have not specifically controlled pupil size when measuring these changes in visual performance.

Higher-order aberrations are known to have a greater effect on image quality as pupil size increases.^19,20 Understanding the effect of pupil size on the reductions in visual performance reported in previous studies caused by multifocal contact lenses would further inform practitioners fitting these lenses. The purpose of this study was to determine the effect of pupil size on visual performance (visual acuity and contrast sensitivity) when wearing a center-distance multifocal soft contact lens that has been shown to slow the progression of myopia.

METHODS

Overview

The protocol was approved by the Institutional Review Board at the University of Houston, and the study followed the tenets of the Declaration of Helsinki. Prior to any study procedures, all participants provided written informed consent. Twenty-four myopic participants were recruited for this one-visit study. Recruitment took place via email within the College of Optometry and word of mouth. Eligible participants were 18–39 years old with best corrected visual acuity of 20/25 or better in the right eye. Their spherical refractive error was between −1.00D and −8.00D with astigmatism less than −1.00D at the corneal plane. Participants were excluded if they had any ocular or systemic conditions affecting vision, refraction, or the ability to wear a soft contact lens. Other exclusion criteria include history of ocular trauma or surgery that could result in abnormal vision, current rigid gas permeable contact lens wearers, and those who were pregnant and/or lactating.

Refractive error was determined using a most plus, least minus, standardized subjective refraction to achieve best-corrected distance visual acuity. This refraction was vertexed to the corneal plane, and the spherical equivalent used to determine the initial contact lens power. The right eye was fitted with Biofinity sphere and Biofinity multifocal D +2.50 add (CooperVision, Inc., San Ramon, CA) lenses in random order with the participant masked to the lens. The initial lens power was placed on eye and allowed to settle for at least 5 minutes. A most plus to best-corrected spherical over-refraction was used to optimize distance acuity, and lens fit was assessed. The power of the contact lens was updated to incorporate any over-refraction that was found. Contact lens centration was assessed using a Haag-Streit reticle under 10x magnification by measuring the nasal and temporal distance between the limbus and the contact lens edge. Lens decentration for each lens type was calculated as the temporal overlap minus nasal overlap, where positive values indicate temporal decentration and negative values indicate nasal decentration.

Visual acuity and contrast sensitivity testing

After verifying successful fit of each contact lens, the lens was removed and the pupil was dilated using one drop of 1% tropicamide, followed by a second drop 5 minutes later. Thirty minutes after the initial drop of tropicamide, the first lens type that was fitted was placed on the eye. The left eye was occluded during all testing. Visual performance (right eye) was assessed through two different artificial pupil apertures (3mm and 6mm diameter), in random order, with the left eye patched. The artificial pupils were aligned with the geometric center of the participant’s dilated right pupil using a unit magnification telescope that has been previously described.²¹ Each artificial aperture was fixed to a rotary device in the optical setup that allowed for the aperture size to be changed. The head of each participant was positioned using an adjustable chin and head rest, and a pupil monitoring camera was used to ensure that the artificial pupil remained imaged at the plane of and centered within the participant’s dilated pupil throughout testing. After being aligned, the participant’s high-contrast logMAR visual acuity, low-contrast logMAR visual acuity (10% Michaelson), and contrast sensitivity were measured at 4 meters through each artificial pupil using the M&S Clinical Trial Suite (CTS) 1500 (M&S Technologies, Niles, IL). The CTS 1500 includes an automated ETDRS chart in which the participant must correctly identify at least 3 of the 5 letters on a given line to move to the next smaller line. The test terminates when three or more letters are missed on a given line. Each letter is worth 0.02 logMAR, and the system tests down to −0.3 logMAR. Measurements were made under photopic lighting conditions (~367 Lux).

Contrast sensitivity was measured at 1.5, 3, 6, 12, and 18 cycles per degree using a sinusoidal bull’s eye target illuminated to 85 cd/m² with the M&S Clinical Trial Suite’s Automated Contrast Sensitivity Function System.²² Participants used a tablet that they held while looking through the telescope at a screen to indicate if they saw a sinusoidal bullseye or a blank target – a two-alternative-forced-choice test. Participants completed a trial run first to ensure they understood the instructions and how to indicate whether they saw the target or not. Blank targets are shown approximately 25% of the time in a random pattern. For contrast sensitivity at each cycle per degree tested, the algorithm starts with step sizes of 0.3 log units. Once an incorrect answer is reported, the algorithm brackets first by 0.2 log unit steps, then 0.1 log unit steps to determine the final contrast sensitivity. Measurements are complete once the best contrast is seen correctly two times. Both high- and low-contrast visual acuity and contrast sensitivity testing were completed with each pupil size and each lens type.

The area under the log contrast sensitivity function is a useful metric that provides a single number to characterize the performance of the eye over the measured spatial frequencies.²³ We considered a change in AULCSF of more than 10% to be clinically meaningful. The area under the log contrast sensitivity function across the measured spatial frequencies was calculated using the “trapz” function in MATLAB (MathWorks; Natick, MA). Contrast sensitivity at each spatial frequency was connected with straight lines, which resulted in trapezoidal shapes organized from lowest to highest spatial frequency. The areas of the trapezoids were calculated and summed, giving the area under the log contrast sensitivity function.

Statistical analysis

A sample size of 23 participants was calculated to give the study 90% power at an alpha level of 0.05 to detect a 0.1 logMAR (one line) difference in visual acuity assuming a standard deviation (repeatability) in visual acuity of 0.14 logMAR.²⁴ We recruited 24 participants. Statistical analyses were conducted using IBM SPSS Statistics, version 28 (IBM, Armonk, NY). Data were analyzed using repeated-measures analyses of variance (RM-ANOVA), with Benjamini-Hochberg adjusted post-hoc t-tests, when appropriate. For visual acuity measures, the RM-ANOVA model included three factors: lens type (single vision or multifocal lens), pupil size (3mm or 6mm), and contrast level (high or low contrast). For the contrast sensitivity data, the RM-ANOVA model included three factors: lens type, pupil size, and spatial frequency (1.5, 3, 6, 12, and 18 cycles per degree). Area under the log-Contrast Sensitivity function was analyzed using a RM-ANOVA model that included factors for lens type and pupil size. Each model included all possible interactions between the factors included in the model. Greenhouse-Geiser adjusted p values were used to account for departures from the sphericity assumption. For this analysis, we considered a difference of acuity of a half line (3 letters) or more to be clinically meaningful.

RESULTS

The mean (± standard deviation) age and spherical-equivalent refractive error of the 24 participants were 23.9 ± 2.3 years old (range: 19 to 30 years) and −4.01 ± 1.81D (range: −1.12 to −8.25D), respectively. Eighteen (75%) of the participants were female. The mean decentration was significantly different than zero for both the Biofinity single vision lens (0.52 ± 0.38mm temporal; p < 0.0001) and the Biofinity multifocal lens (0.53 ± 0.31mm temporal; p < 0.0001). There was no difference in decentration between the two lens types (p = 0.90).

Visual Acuity

Visual acuity differed by contrast level (high vs low) and depended on lens design (single vision vs multifocal) and 3mm versus 6mm pupil size (contrast × lens × pupil interaction term; p = 0.0040; Figure 1). There was a significant difference in high-contrast visual acuity between the single vision and multifocal lens, but this difference did not vary with pupil size (lens by pupil interaction term; p = 0.33). Regardless of pupil size, while high-contrast visual acuity with the multifocal contact lens was about two letters worse than with the single vision contact lens (lens type main effect; p = 0.0001), this difference was not clinically meaningful. Regardless of lens type (sphere or multifocal), there was a statistically significant 1.5 letter decrease in high-contrast visual acuity with the 6mm pupil versus the 3mm pupil (pupil size main effect; p = 0.0078), though this difference was also not clinically meaningful.

Figure 1.

High- and low-contrast logMAR visual acuity with Biofinity sphere and Biofinity multifocal D (+2.50 add) under 3mm and 6mm pupil conditions. Error bars represent the standard error of the mean.

As expected, the effect of multifocal optics on low-contrast vision was more pronounced. When measuring low-contrast visual acuity, the effect of lens type on acuity was pupil size-dependent (lens × pupil; p = 0.0006). With a 3mm pupil, low-contrast visual acuity with the multifocal lens was about 3 to 4 letters worse than with the single vision lens (mean ± SEM = 0.07 ± 0.02 logMAR; p = 0.0010). With a 6mm pupil, the multifocal contact lens resulted in an almost 2-line reduction in low-contrast visual acuity compared with the single vision lens (0.19 ± 0.02 logMAR; p < 0.0001). When looking at the effect of pupil size, low-contrast acuity with the single vision contact lens was, on average, 3 letters worse with the larger 6mm pupil than with the 3mm pupil (mean ± SEM logMAR; 0.06 ± 0.02, p < 0.018). However, when wearing the multifocal contact lens, the effect of the larger 6mm pupil was more pronounced and resulted in an almost 2-line reduction in low-contrast acuity compared with the 3mm pupil (0.17 ± 0.02 logMAR; p < 0.0001).

Contrast Sensitivity

There were overall differences in contrast sensitivity caused by the lens designs (sphere versus multifocal) that were pupil size dependent (lens × pupil interaction term, p = 0.039), and these differences did not depend on the spatial frequency tested (lens × pupil × spatial frequency interaction term, p = 0.32; Figure 2). There was no difference in contrast sensitivity between the 3mm and 6mm pupil with the single vision contact lens (p = 0.77). However, with the multifocal lens, contrast sensitivity was (mean ± SEM) worse with the 6mm than the 3mm pupil by 0.10 ± 0.04 (p = 0.041). For the 3mm and 6mm pupil, the multifocal lens caused a mean reduction in contrast sensitivity across all tested spatial frequencies of 0.13 ± 0.03 (p = 0.013) and 0.24 ± 0.04 (p < 0.0001), respectively.

Figure 2.

Visual performance measured with contrast sensitivity across 5 spatial frequencies with Biofinity sphere and multifocal (+2.50 add) lenses under 3mm and 6mm pupil sizes. Error bars represent standard error of the mean.

The effect of the multifocal optics on the area under the log contrast sensitivity function also depended on pupil size (lens × pupil interaction term; p = 0.024; Figure 3). There was no difference in area under the log contrast sensitivity function between pupil sizes with the single vision contact lens (p = 0.68); however, while wearing the multifocal lens, the area under the log contrast sensitivity function was reduced by 8% with the 6mm pupil compared with the 3mm pupil (p = 0.014). With the 3mm pupil, the multifocal contact lens caused an average reduction of 9% in the area under the log contrast sensitivity function when compared with the single vision lens (p = 0.0031). This reduction was greater for a 6mm pupil, where the multifocal optics caused an average decrease in the area under the log contrast sensitivity function of 17% compared with the single vision lens (p < 0.0001).

Figure 3.

Box plots of the area under the log contrast sensitivity function with Biofinity sphere and multifocal (+2.50 add) under 3mm and 6mm pupil sizes. Lower and upper box limits indicate the 25th and 75th percentiles, and the center line within each box is the median. Whiskers extend 1.5 times the interquartile range.

DISCUSSION

We determined the effect of pupil size on high- and low-contrast logMAR visual acuity and contrast sensitivity with the Biofinity sphere and Biofinity multifocal D with +2.50 add. In previous work, visual performance with these lenses was determined; however, measurements were made using the participant’s habitual pupil size.^{13, 18} In the present study, we used standardized pupil sizes of 3mm and 6mm to determine the specific effect of pupil size on visual performance with multifocal optics.

Because of the growing popularity of center-distance multifocal contact lens designs in myopia control, it is important to understand the effect of such lens designs on visual performance.^11–13 Taking pupil size into consideration when evaluating visual performance is crucial to better understand how these lenses affect vision. Consistent with our study results, previous studies have shown that various multifocal contact lens designs have no clinically meaningful effect on high-contrast visual acuity under photopic conditions (i.e., no reduction to up to a 2 letter reduction).^13,25–30 That said, previous studies have found that various multifocal contact lens designs consistently reduce low-contrast visual acuity by roughly one line.^25,27,31 Gregory et al investigated the effect of different multifocal lens designs on visual performance under different lighting conditions.¹³ Photopic high-contrast visual acuity was not meaningfully affected by multifocal designs; however, low-contrast acuity decreased by one line under photopic conditions with multifocal contact lenses, similar to our study, and by two lines under mesopic conditions. Natural pupil sizes were measured under photopic and mesopic conditions in that study, and as expected, participants’ pupils were smaller under photopic than mesopic lighting. That said, they were unable to isolate the effect of the lower mesopic lighting on acuity when wearing multifocal lenses from the effect of the larger pupil size.

In the present study, we quantify the reduction in low-contrast visual acuity under photopic conditions that is specific to the increase in pupil size. When wearing the multifocal contact lens, a 6mm versus 3mm pupil diameter resulted in a nearly two-line reduction in low-contrast acuity versus the 3-letter reduction with increased pupil size when wearing a single vision lens. By comparison, the effect of pupil size on high-contrast acuity was negligible (a few letters at most), emphasizing that the standard high-contrast acuity measures typically done in a clinical exam setting do not provide a comprehensive picture of the effect of multifocal optics on the patient’s visual experience. Our results support those of previous studies in finding that low-contrast acuity serves as a more sensitive measure of visual performance and demonstrates that providers should expect that the effect of multifocal optics on visual performance will be greater for patients with habitually larger pupils and in everyday situations where the patient’s pupil is larger, such as dim lighting.

While visual acuity is one measure of visual performance, measuring contrast sensitivity at multiple spatial frequencies provides a more comprehensive assessment of visual function beyond the visual acuity cutoff spatial frequency. Nti et al investigated how single vision and various multifocal contact lens designs influence contrast sensitivity under photopic and mesopic conditions with habitual pupil sizes and reported that multifocal contact lenses reduced contrast sensitivity with mesopic conditions exacerbating the reductions in contrast sensitivity when wearing multifocal lenses.¹⁸ That said, the study was not designed to determine the effect of pupil size on contrast sensitivity and failed to find an effect of habitual photopic or mesopic pupil size on contrast sensitivity. The results from our study expand upon this finding by demonstrating that, as one might expect, reductions in contrast sensitivity caused by multifocal optics depend on pupil size with greater reductions with a larger pupil. With the single vision lens, both the area under the log contrast sensitivity function and contrast sensitivity at each spatial frequency measured was no different for a 3mm versus 6mm pupil. However, the multifocal design caused reductions in contrast sensitivity across all spatial frequencies for both a 3mm and 6mm pupil, with the effect being greater for the 6mm pupil.

The area under the log contrast sensitivity function is a single metric used to characterize contrast sensitivity across the spatial frequencies measured. Compared to the single vision lens, the area under the log contrast sensitivity function with the multifocal lens was reduced by 9% with a 3mm pupil and by 17% with a 6mm pupil. The reduction in the area under the log contrast sensitivity function with multifocal optics for a 3mm pupil found in the present study is comparable to the roughly 10% reduction reported previously with habitual pupils under photopic testing conditions with multifocal lenses.¹⁸ These changes describe reductions in visual performance that are not traditionally assessed with standard high contrast visual acuity. In isolation, the area of under the curve could mask differences in contrast sensitivity at a specific spatial frequency; however, in our study, changes in contrast sensitivity due to pupil size and lens optics were consistent across all special frequencies measured. Overall, these findings confirm that, on average, patients wearing multifocal contact lenses should expect some reduction in visual performance in situations where their pupil is larger.

The effect of pupil size on visual performance in multifocal contact lenses is likely caused by the higher-order aberrations induced by the lenses.^32,33 When a smaller pupil is present, the eye becomes more diffraction limited, reducing the influence of the aberrations created by the multifocal optics. As pupil size increases, the influence of these higher-order aberrations on image quality increases, with visual performance as measured by low-contrast acuity and contrast sensitivity being more susceptible than with high-contrast acuity.^16,17,32

No study is without limitations. The design of the unit magnification telescope only allowed one eye to be measured with a set artificial pupil; therefore, it is possible that there may be some overestimation of reduced performance due to the lack of binocular summation. Our study also only assessed visual performance with one multifocal contact lens design. It is possible that results could vary if other lens designs were tested. That said, given that the low-contrast visual acuity reductions measured in this study with a multifocal contact lens are similar to those of previous studies using different lenses, it is reasonable to suggest that the results would not be dramatically different.

It is important to note that the power profile of the multifocal contact lens changes with lens power. Nti et al. measured power profiles of several multifocal lens designs at various amounts of minus power.³⁴ For the Biofinity multifocal D +2.50 add lens, the max amount of add occurred near the edge of the optic zone for lenses that correct less myopia (positive spherical aberration) versus closer to the center of the lens for powers that corrected more myopia (negative spherical aberration). Given that the power profile varies with lens power and that this power profile in combination with the unique aberrations of each participant’s eye yields the ultimate image quality and visual performance, it is important to note that we can only report the average visual performance for this specific cohort of participants. Nti et al. reported a consistent 8mm optic zone diameter for lenses from −1.00 D to −6.00 D; however, the optic zone diameter for more myopic powers were not reported. Of the participants in the current study, only two participants had a vertexed refractive error more myopic than −6.00D (−7.25D and −7.50D). While it is possible for there to be decreases in optic zone diameter at more myopic powers that could yield different proportions of light passing through the intended optical profile of the lens at larger pupil sizes once also factoring in any lens decentration, our results represent the average visual performance of our cohort when this commercially-available lens is fitted clinically.

Participants in this study were recruited from our College of Optometry and were primarily optometry students. An assumption may be made that students familiar with contact lens designs and methods for assessing visual performance may potentially introduce bias. That said, the contrast sensitivity task in this study is not one that participants had previous experience with, and Snellen rather than logMAR acuity is generally used clinically in the examination of patients. Additionally, we had to select two specific pupil sizes for the purpose of our testing. Realistically, patients’ pupil sizes fall within a wide range and the average changes found in the present study could vary from person to person. Lastly, all visual performance measurements in this study were made the same day that the lens was fitted. We do not know if the visual performance outcomes reported here would change if more time had been allowed to adapt to each contact lens.

With increasing use of multifocal contact lens designs to slow the progression of myopia, it is important for clinicians to understand their effect on visual performance. By utilizing artificial pupils, we were able to quantify reductions in visual performance specifically related to pupil size. We found clinically meaningful reductions in visual performance with multifocal contact lenses as measured by low-contrast logMAR visual acuity and contrast sensitivity. These reductions, in most cases, increased with a larger pupil size. Clinicians can use these results to definitively understand the role of pupil size on visual performance. Multifocal lens designs cause changes in visual performance that are not detected by standard high-contrast visual acuity, and these changes are greater as pupil size increases. It is important for the clinician to understand the effect of multifocal designs so they have a complete understanding of the interaction between the environment and pupil size on their patients’ visual experience throughout the day.