The prevalence and severity of myopia globally have risen drastically during the last decade1,2 After onset, myopia may progress rapidly in children,3 while in adults, myopia progression typically occurs at a slower rate.4 Myopia is of particular concern not only because of the associated pathological changes, such as retinal breaks,5,6 glaucoma,7 and an increased risk of requiring cataract surgery,8 but also the high costs of long-term myopia vision correction9 and quality of life issues associated with this.10
Many strategies, including single-vision spectacle lenses,11 bifocal spectacle lenses,12 progressive addition lenses,13 and anticholinergic agents,14 have been used in an attempt to reduce the progression of myopia, with varying degrees of success. Contact lens corrections have also been used in an attempt to reduce myopia progression, including gas-permeable,15 silicone hydrogel,16 orthokeratology,17 bifocal,18 and dual-focus (DF) lenses,19 as well as other novel contact lens designs.20
It is beyond the scope of this article to discuss the underlying mechanisms for the different myopia control treatments previously described. However, contact lenses may be preferred over spectacles to deliver myopia control treatment to children. Contact lenses provide a constant treatment regardless of gaze positioning,20 and it has been shown that children can not only successfully wear contact lenses21 but contact lens–wearing children demonstrate improvement on a range of quality of life measures over spectacles.22
Recent studies by Anstice and Phillips19 as well as Sankaridurg et al.20 demonstrated that two different soft contact lens designs may be successful in reducing myopia progression in children, with demonstrated reductions in axial lengths of 0.11 ± 0.09 mm (10 months) and 0.27 ± 0.05 mm (12 months), respectively, and re-fractive error reductions of −0.44 ± 0.33 diopters (D) (10 months) and −0.57 ± 0.12 D (12 months), respectively, compared with control subjects. Both lens designs incorporate a central zone to correct myopic refractive error, but one design uses a concentric zone approach to simultaneously deliver peripheral myopic defocus,19 whereas the second produces a peripheral hyperopia-reducing effect.20 In both studies, high-contrast visual acuities (VAs) with the contact lenses were shown to be similar to single-vision distance contact lenses and spectacles, respectively. One study reported similar contrast sensitivities between the study lens and a spherical contact lens control; however, neither study reported patient-reported visual performance.
One of the designs, the DF lens, is commercially available and being prescribed in some parts of the world. Dual-focus lenses contain a large central correction area surrounded by concentric zones of alternating distant and near powers. The near power is intended as a “treatment” zone to prevent myopic progression.19 Whereas the DF lenses differ from and cannot be classified as either a multifocal or bifocal lens in the traditional sense, the lenses do incorporate optical powers that produce two focal planes. However, little is understood about the overall visual performance of the lenses at different distances and under various viewing conditions. For example, the lenses may reduce the progression of myopia over the long-term (benefit) but may degrade visual performance to such a degree on a daily basis (cost). The cost-benefit of this approach needs to be fully evaluated. The purpose of this study was to evaluate the potential cost, if any, of these DF lenses on the visual performance of the wearer. Results will be compared with those of a commercially available multifocal (i.e., two-zone lens) center distance design soft contact lens worn bilaterally.
Subjects, Lenses, and Wear Schedule
This study was conducted at the Indiana University School of Optometry. Ethical approval was obtained from the Indiana University Institutional Review Board, and written consent was obtained from each subject after the study purpose, procedures, and risks were explained in accordance with the tenets of the Declaration of Helsinki.
The study was not designed to investigate the myopia control benefits, if any, of the lenses. It was specifically designed to evaluate the visual performance attained with the lenses during short-term wear. Therefore, the study was of short duration and did not include any of the measures that might typically be obtained in a study designed to evaluate the effectiveness of a lens in controlling myopia progression, such as axial length, near-point phoria, accommodative status, or peripheral refraction. In addition, the study was not conducted on young children but rather on a sample of young adults who represent a convenient subject population with high visual demands similar to those in children.
Twenty-four young myopic adults (13 men and 11 women) with an average age of 22.5 ± 1.6 years and mean distance spherical equivalent refractive error of −3.23 ± 1.60 D were enrolled and completed the study. Twelve of the subjects were habitual, soft, single-vision contact lens wearers, and 12 had never worn contact lenses previously. All subjects were between 18 and 25 years old, had VA of 20/25 or better in each eye, less than 1.00 D astigmatism in each eye, no strabismus, no habitually uncorrected anisometropia 2.00 D or greater, and a spherical distance contact lens power requirement between −1.00 and −6.00 D for both eyes. All subjects were free from ocular and systemic disease that could prevent safe contact lens wear, and all were willing to comply with the prescribed contact lens wear and study visit schedule.
At the initial visit, subjective refraction was conducted, baseline patient-reported responses were collected for participants’ habitual vision correction, and objective vision assessments were evaluated with the subject’s best-corrected refraction mounted in a trial frame. In previous studies involving multifocal contact lenses, VA measurements have been shown to lack the sensitivity to distinguish decrements in vision performance that are perceptible to the wearer.23 It was suggested that patient-reported responses are a more useful tool for evaluating multifocal lens performance.23 Therefore, the main outcomes of the current study are patient-reported measures of visual performance, although some VA measures are included for clinical comparison. The integrity of the anterior ocular surface was verified using a slit-lamp biomicroscope before the study lenses being fitted.
Subjects were then randomly fitted with one of the study lens pairs. The DF lenses used in this study were the MiSight (CooperVision, Hong Kong) and the multifocal lenses were Proclear (CooperVision, USA). Both lenses are composed of omafilcon A material and have a 8.7 mm base cure. The MiSight© lens has a water content of 60%, lens diameter of 14.2 mm, and a treatment zone power of +2.00 D. The Proclear is also a 2 zone lens with a similar water content and diameter of 62% and 14.4 mm, respectively. The add power of this lens is +2.00 D. Each of the study lens types were worn bilaterally. Fig. 1 depicts the nominally reported dimensions of the DF19 (left panel) and Proclear Multifocal +2.00 D Add D (MF)24 (middle panel) lenses. Although not used in the current study, as a comparative control for graphical purposes only, the dimensions of the ACUVUE Bifocal contact lens are also provided25 (right panel).
A monocular distance spherical overrefraction was performed to confirm the study lens power in each eye as the maximum plus lens that provided optimal VA. Once the final study lens power had been selected and after a 20-minute settling period had elapsed, an abbreviated vision assessment was conducted on the first pair of lenses. Subjects who demonstrated an acceptable lens fit and who were sufficiently satisfied with their vision to continue in the study were dispensed with the study lenses. Subjects were instructed to wear the lenses for a minimum of 8 h/d, 5 d/wk, and at least 2 hours before attending the follow-up visit after 1 week of daily wear. Subjects who had no previous contact lens wear experience were instructed lens insertion and removal techniques and other relevant safety information. Subjects were instructed to dispose of the study lenses on a daily basis. Therefore, no lens care products were issued. However, if required, the use of commercially available rewetting drops was allowed on the condition that they were not used within 4 hours of a scheduled follow-up visit, and any use was reported to the investigators.
At the 1-week follow-up for the first lens pair, a comprehensive assessment of objective and patient-reported vision measures was conducted. Subjects were then crossed over and fit with the alternate lens type. Given that both study lenses were manufactured and verified by the same manufacturer using the same technology and distance power labeling procedure, the same distance lens powers were prescribed for both study lens pairs to ensure that any performance differences between the lenses were caused by differences in lens design. After the settling period, the same procedures were repeated. After 1 week of wear a follow-up visit occurred with the same procedures completed. After this visit, anterior ocular health was checked, and subjects were exited from the study.
Objective Visual Assessment
LogMAR VA was assessed under high illumination (160 cd/m2) and high contrast (90%) (HIHC) at distance (4 m) and under low illumination (16 cd/m2) and low contrast (10%) (LILC) at distance (4 m), intermediate (1 m), and near (40 cm). As HIHC acuity has been found to be less sensitive to subtle visual and optical changes,26 such as those found with multizone lenses,19 our acuity analysis will focus on LILC measures.
Patient-Reported Measures of Lens Performance
Patient-reported lens performance was evaluated under several viewing contexts using a verbal numerical rating scale that ranged from 0 (representing the poorest performance) to 100 (representing the best possible performance). Only the two end points of the scale were semantically anchored. Subjects were instructed and free to use any integer value between 0 and 100 inclusively. In addition to rating overall lens performance, participants evaluated each lens specifically for daytime, nighttime, and ghosting performance under six viewing contexts: distance, intermediate (i.e., computer), near, texting, watching television, and sports/recreational activities.
Power and Sample Size Calculations
A statistical power analysis was performed to determine the number of subjects needed to detect a standardized effect size of 0.6. This value represents the expected magnitude of the difference between two correlated means (0.06) divided by an estimated SD of 0.1 units. Assuming a fixed type I error rate of 0.05 and two-sided probability, the analysis indicated that this effect could be detected with 80% or better power for both the objective and patient-reported measurements with a sample of N = 24 subjects.
To ensure that the investigator remained masked from the lens design, a clinician not involved in the collection of data inserted the study lenses and dispensed an adequate supply of coded daily disposable lenses to the subject. In addition, there were no distinctive features on the lenses or lens packaging that could potentially unmask the examiner.
Subjects were also masked as to the lens type worn because both study lens types dispensed were packaged in identical blister packs and were overlabeled. Each label contained information limited to the study lens code, subject number, and the eye to which the lens was assigned.
Planned Statistical Analyses
Each analysis began with an omnibus repeated-measures (RM) analysis of variance (ANOVA) to test all fixed main effects and interactions. The significance level of each univariate within-subject effect was based on an F test statistic calculated using unadjusted degrees of freedom when the assumption of sphericity was met and corrected degrees of freedom (i.e., Greenhouse-Geisser and Huynh-Feldt) when the assumption of sphericity was not met. For models with a statistically significant interaction, a simple effects analysis was performed using a one-way RM ANOVA to isolate the effects of one factor at a specific level of another factor. Correlated-samples t tests were then used to estimate pairwise mean differences among dual-focus, multifocal, and best-correction/habitual lens types.
The VA data (logMAR VA) measured under LILC were analyzed as a two-factor within-subject design. Thus, the initial omnibus RM ANOVA included a significance test for the main effect of lens type with three categorical levels (dual-focus, multifocal, and best-correction), the main effect of viewing distance with three ordered levels (distance, intermediate, and near), and the interaction effect between lens type and viewing condition. Note that the HIHC data were used as a reference condition only and were not included in the statistical models. In addition, patient-reported data were compared with that of the entering habitual correction, whereas the acuity data were compared with the best-corrected data obtained during the study.
The patient-reported ratings of overall lens performance were analyzed using a one-way RM ANOVA with three categorical levels of lens type (dual-focus, multifocal, and habitual) as the only within-subject factor. Ratings of visual ghosting were analyzed using a two-factor RM ANOVA with three levels of lens type and six categorical levels of viewing context (i.e., distance, intermediate, near, texting, watching television, and activity related) as the within-subject factors. The patient-reported ratings under daytime and nighttime illumination were analyzed as a three-factor RM ANOVA that included tests for seven possible effects: three main effects (lens type, viewing context, and daytime vs. nighttime), three two-way interactions (lens type × context, lens type × day/night, viewing context × day/night), and one three-way interaction (lens type × viewing context × day/night).
Although the F statistic and follow-up paired sample t test comparisons are generally robust to violations of normality,27 we also performed a series of permutation tests to check the validity of statistical inferences (i.e., normal distribution, heterogeneity of variance) for the factorial ANOVAs and pairwise t tests. In all F tests, sphericity was assumed as Mauchly criterion was found to be not statistically significant; therefore, univariate within-subject F tests with unadju-sted degrees of freedom as opposed to the more conservative tests based on corrected degrees of freedom (e.g., Greenhouse-Geisser, Huynh-Feldt, etc) are reported.
Correlation between VA and Patient-Reported Ratings
After testing for average performance differences separately for objective and subjective measures, we evaluated the LILC VA data and nighttime patient-reported ratings for evidence of an association across these two measurement domains. Although the primary analyses provide critical information regarding various model effects in terms of mean differences, they do not provide information regarding consistency within subjects across measures. To address this issue, we estimated rank-based correlations between LILC VA and the nighttime patient-reported at distance, intermediate, and near.
Descriptive Summary of the Raw Data Distributions
Table 1 presents descriptive summary statistics for both the objective and patient-reported data. Although the LILC logMAR VA are fit reasonably well by a normal model regardless of lens type, Shapiro-Wilks tests indicated that the patient-reported data are not. Specifically, the observed ratings for habitual lenses are negatively skewed and exhibit a high degree of kurtosis. This result is likely related to ratings obtained with habitual lenses that are consistently high across all viewing distances.
The mean LILC logMAR VA and 95% confidence interval (CI) estimates grouped by lens type for distance, intermediate, and near are presented in Fig. 2. The mean distance HIHC logMAR VA and 95% CI are plotted as a reference. There was a significant difference in LILC acuities between lens types (p < 0.0001) and viewing distance (p < 0.01) that are qualified by a significant lens × distance interaction (p < 0.01).
Simple-effects analyses were performed to test for the effect of (1) lens type at each viewing distance and (2) viewing distance with each lens type. These one-way RM ANOVA results indicated significant differences caused by lens type when viewed at distance (p < 0.001) and at intermediate range (p < 0.0001) but not when viewed at near (p = 0.285). There was a significant difference between viewing distances for the multifocal (p < 0.05), DF (p < 0.001), and best-correction lenses (p < 0.05). Acuity was poorest at distance regardless of lens type and, on average, was poorest for the DF lenses and best for best-correction lenses regardless of distance. The average difference in LILC logMAR VA between lenses decreases significantly with viewing distance with the largest difference between lenses at distance and the least difference at near. Correlated-samples t test results between lens types at each viewing distance found no significant differences among lenses when viewed under high illumination and high contrast. However, when measured under LILC, there were significant mean differences between each study lens and best-correction viewed at distance (p = 0.001 and p = 0.016, respectively) and intermediate (p < 0.0001 and p < 0.0001, respectively). There were no significant differences in LILC logMAR VA between study lenses detected at any distance.
Patient-Reported Ratings Data
Patient-reported measures of overall lens performance were significantly different between lens types (p < 0.01) (Table 2). Although average ratings for both DF and MF study lenses were significantly lower than those for habitual lenses, there were no significant differences between the DF and MF lenses (p = 0.448).
Results of the two-factor within-subject ANOVA of lens ghosting ratings under different viewing contexts demonstrated a significant difference between lens types (p < 0.0001) and viewing contexts (p < 0.01). Lens type results did not vary as a function of viewing context. The pairwise differences in Table 2 show that habitual lenses were consistently rated as having less ghosting compared with either study lens, with mean differences ranging between 14 and 27 rating scale units. Although there is some evidence of a small consistent bias favoring MF over DF lens (between 4 and 9 units), none of the mean differences reach a Bonferroni-corrected significance level of 0.017.
Fig. 3 shows the means and 95% CIs of the mean for the patient-reported vision ratings data conditioned by lens type, that is, habitual (HAB), DF, and MF as a function of viewing distance and lighting condition. The location of the sample median is also shown for each condition (character, e.g., d for day). Significant differences were found between lens types (p < 0.0001), day/night (p < 0.0001), and viewing context (p < 0.05), as well as an interaction between day/night and viewing context (p < 0.0001), indicating differing day/night responses varying with viewing context. The results comparing the vision quality ratings between lenses follow similar trends to the ghosting ratings. Specifically, both DF and MF study lenses received lower ratings, on average, than habitual lenses under all viewing contexts, with significant mean differences ranging between approximately 9 and 19 patient-reported rating scale units. This effect is depicted graphically in Fig. 4A. Mean differences between the MF and DF lenses were small (ranged between 1 and ∼4 units across viewing contexts) and not significantly different. Details of the pairwise comparisons are provided in Table 3.
To identify specific sources of variance underlying the interaction between day/night and viewing context, simple effects analyses were performed using one-way RM ANOVAs. These tests revealed no significant difference in viewing contexts under daytime conditions (p = 0.53), with average ratings ranging from 80.33 (distance) to 82.83 (near). However, there was a significant difference between contexts under nighttime conditions (p < 0.001), with average ratings ranging from 73.76 (distance) to 80.40 (near). Fig. 4B shows that, regardless of viewing context, the average patient-reported ratings for daytime are significantly higher than those for nighttime (81.67 va 78.64), especially at distance (80.33 vs. 73.76) and less so at near (81.57 vs. 80.25).
Follow-up Permutation Test Results
The follow-up permutation tests on the objective and patient-reported measures yielded results that were generally consistent with those based on the parametric omnibus analyses. Only the lens type by viewing distance interaction for LILC logMAR VA was not significant (p = 0.13) in the permutation analysis. The pattern of results of the pairwise comparisons using permutation tests was fully consistent with the matched-samples t test results as previously described. Specifically, for the objective data, there were no significant pairwise differences among lenses under HIHC.
Association between logMAR VA and Patient-Reported Ratings
Scatter plots of the LILC logMAR acuity data and nighttime patient-reported ratings for the three viewing distances are presented in Fig. 5. Note that, regardless of viewing distance, there is considerable overlap in the distributions with the bivariate means (open symbols) for both DF and MF falling within the 95% probability contour (i.e., data ellipse) of the habitual lenses. None of the estimated correlations differed statistically from zero.
Despite the differences in design between the DF and MF lenses evaluated in this study, there were no discernible differences in any objective or patient-reported measures between the two lens types. Although no differences were noted in objective HIHC VA measurements between both the DF or MF lens type and best correction, the patient-reported responses indicated substantial differences between subjects’ habitual vision correction and both study lenses. Likewise, LILC visual performance with both the DF and MF lenses were significantly worse than that with best correction. These results are consistent with the observations of that HIHC VA measurements may be insensitive to detect performance differences that are caused by subtle optical design differences, which may otherwise be perceptible to MF lens wearers (i.e., based on patient-response measures).23
Naturally, the measurable decrements in LILC and patient-reported vision previously described raise questions regarding the cost-benefit of these types of lenses to control myopia progression in young wearers. The current study demonstrated that the DF lenses decreased average logMAR acuity by only approximately one line and approximately 10 rating units and that this decrement was similar to that experienced with currently available MF designs often commonly prescribed for presbyopic adults with high visual needs. Apart from the reported benefits of the DF lenses in terms of reducing the progression of myopia,19 however, it is unknown whether there are any adverse consequences of reducing the patient-reported clarity of children’s vision by this seemingly small amount. Although it seems unlikely in many instances, it is plausible that the slightly reduced vision caused by these lenses may affect the “real-world/activity” performance of some children highly active in activities that require high acuity (e.g., baseball to see the rotation of the ball to be hit). Sports vision performance specialists are constantly aiming to enhance sporting performance of many young athletes by enhancing acuity as much as possible by assuring optimal optical correction28 and even through the use of colored filters,29,30 visual training exercises,31,32 and so on. However, those interested in curbing the progression of myopia may have to knowingly not provide what may be the optimal optical correction. Previous studies have shown that it is possible for wearers to adapt to the optics provided by lenses.33 However, until more is known on the long-term effects of these lenses, practitioners may choose to weigh these potential costs and benefits with their patients on a case-by-case basis.
Previous reports have evaluated the effectiveness of other approaches aimed at slowing the progression of myopia. Many of these approaches, however, may also slightly (e.g., two to three letters) decrease VA or performance in some instances relative to best correction, including orthokeratology,34 bifocal/multifocal contact lenses,35,36 progressive addition spectacles,37 and gas-permeable lenses.38 Those attempting to use any of these approaches will have to weigh this potential decrease in visual performance provided by these various corrections against the relative reduction in myopia progression that may ultimately be attainable by each approach.
There are several important limitations to this study that should be realized. First, the lens evaluated in the current study has limited availability around the world. However, the design of the lens has some similar features to lenses currently available throughout most of the world that are sometimes used off label to control myopia progression.18 For example, the ACUVUE Bifocal contact lens, like the DF lens, is a distance center, concentric ring design. One main difference between these designs is that the central distance zone is larger in the DF lens. Fig. 1 highlights the design similarities and differences between the DF lens and one possible similar lens design, the ACUVUE Bifocal. Notice, however, that the central zone of the DF lens is considerably larger than that of the ACUVUE Bifocal lens. These design differences aside, the current report on the DF lens may provide some insight into what may be expected of other similar designs. For example, visual performance similar to that currently reported with the DF lenses has been reported with ACUVUE Bifocal lenses in presbyopic adults.39,40
Second, it may be argued that the MF lenses used in this study were not prescribed according to the manufacturer’s fitting guide (+2.00 Add D lenses worn bilaterally vs. a D lens in the dominant eye and an N lens in the nondominant eye), and therefore, the vision data may not be typical of MF lens performance. However, the N (near-powered optics in the center) lens would typically serve to boost only near vision performance in presbyopes and might actually reduce distance vision performance. Given that the study population was a group of young adults who were able to freely accommodate, it is possible that the vision performance of the MF lenses evaluated in this study was in fact enhanced compared with what might be expected clinically had the lenses been fitted according to the manufacturer’s instructions.
The current study only evaluated visual performance during 1 week of wear. It is possible, however, that wearers of these lenses may adapt to the optics of these lenses33 and accordingly experience improved visual performance after longer periods of wear. Similar experiences have been noted with multizone lens performance, with decreased visual performance often reported on lens dispensing, however, improved performance after several weeks/months of wear even in the absence of improved measured acuity.41 Nonetheless, if this is the case, the current report may underestimate the true long-term visual performance experienced by wearers of these lenses.
The current results demonstrate performance with the DF lens to be slightly worse at nighttime than during the daytime. Many contact lens–wearing children, however, may not be awake to perform many nighttime visual tasks. It is possible the current results on young adults with high nighttime visual demands may actually overestimate the true visual degradation experienced by most wearers of these lenses (e.g., children who do not drive and go to bed at 8:00 PM).
Nonetheless, the current results demonstrate that DF lenses designed to control myopia progression provide a level of vision similar to that attained with typical MF lenses. Practitioners who are comfortable in prescribing MF lenses for their patient’s visual needs should feel comfortable in considering myopia-control lenses for their young myopic patients after talking with them individually about their vision needs and goals.
Pete S. Kollbaum
800 East Atwater Ave
Bloomington, IN 47405
This study was funded by CooperVision Inc, USA.
Received August 8, 2012; accepted November 14, 2012.
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