Effect of Individualized Ocular Refraction Customization Spectacle Lens Wear on Visual Performance in Myopic Chinese Children

Abstract

Purpose: Individualized ocular refraction customization (IORC) lenses can be individually adjusted depending on the initial relative peripheral refraction to determine the myopic defocus (MD). We aimed to compare visual performance of children wearing IORC lenses with different amounts of MD to determine whether higher MD resulted in greater visual compromise.

Methods: This study included 184 myopic children aged eight to 12 years, and 172 completed the trial. The participants were randomly assigned to wear IORC lenses with low (IORC-L, 2.50 D), medium (IORC-M, 3.50 D), or high (IORC-H, 4.50 D) MD or single-vision spectacle lenses (SVL). Distance and near best-corrected visual acuity (BCVA), contrast sensitivity function (CSF) and questionnaires were evaluated at baseline and after six and 12 months.

Results: CSF over all frequencies and distance and near BCVA were not affected by lens design (all P > 0.05). The SVL group outperformed the three IORC lens groups in terms of ghosting images at baseline, and IORC-H and IORC-M groups outperformed IORC-L group (all P < 0.001); however, no differences were observed at the six- or 12-month visit. There were no significant differences among the four groups for any other subjective variables at any of the follow-up visits regarding vision clarity, vision stability, eyestrain, dizziness, headache, or overall vision satisfaction (all P > 0.05).

Conclusions: The IORC lenses with an actual MD of 4.50 D provided acceptable objective and subjective visual performance and were well tolerated by children.

Translational Relevance: IORC lenses with an actual MD of 4.50 D provided acceptable visual performance.

Introduction

In recent decades, myopia has emerged as a global public health concern.^1–3 Multiple optical interventions have been used to slow myopia progression in children, including specialized spectacle lenses, orthokeratology lenses, and multifocal soft contact lenses. Because of their noninvasive features, lack of ocular infection concern and lower cost than contact lenses, spectacle lenses are the most common choice for myopia control.^4,5 Recent studies have suggested that specialized spectacle lenses designed to impose myopic defocus (MD) on the peripheral retina, such as defocus incorporated multiple-segment (DIMS) spectacle lenses^6,7 and highly aspherical lenslets (HAL),^8,9 could result in significant myopia control effects. However, the effects of myopia control vary among myopic children because of differences in their baseline relative peripheral refraction (RPR) profiles.¹⁰ Therefore mapping the RPR profile to customize the optimal amount of MD could be vital for optimizing myopia control effects. 

Achieving an appropriate balance between slowing the progression of myopia and optimal visual performance is crucial in myopic children. Several clinical trials have shown that these specialized spectacle lenses have no significant impact on central visual acuity,¹¹ binocular visual function,¹² visual field sensitivity,¹³ peripheral visual functions,¹⁴ or peripheral motion detection.¹⁴ However, the long-term impacts of various aspects of subjective visual performance, which are much more realistic indicators in the real visual world for evaluating multifocal lens performance, have yet to be investigated.^15,16 Moreover, peripheral MD is considered one of the most accepted mechanisms underlying the myopia control effect of specialized spectacle lenses.¹⁰ Although recent studies have suggested that a greater amount of myopic defocus has a stronger effect on myopia control,⁸ it may also provide greater visual compromise, which renders myopic defocus less tolerable in children.¹⁵ However, the maximum amount of myopic defocus a child can tolerate when wearing specialized myopia control spectacle lenses has not been determined. This topic is of interest for practitioners who seek to fit myopia control spectacle lenses with the optimal amount of MD for maximum acceptability for children whose visual systems are still developing and for manufacturers optimizing myopia control lens design with the optimal dosage. 

To date, peripheral refraction can be measured rapidly and accurately by multispectral refraction topography (MRT) in clinical settings, and the RPR can be calculated easily as the central refraction subtracted from the peripheral refraction.^17–19 The RPR profile across the retina varies among myopic children.²⁰ When wearing specialized spectacle lenses with a constant MD, children might receive a different summation of MD because of their initial RPR.^10,21 For instance, for children with hyperopic RPR at the mid-periphery retina when provided with 3.50 D of MD from a specialized spectacle lens, the MD power would counterbalance the initial hyperopic RPR; therefore less than 3.50 D of MD would be received by the retina. That is, when children wear conventional specialized spectacle lenses with constant MD, the actual amount of MD perceived by the eye remains unknown. Thus further investigations of customized myopic defocus methods for each particular eye to determine the optimal dosage of a myopia control spectacle lens while not impacting visual performance and achieving good tolerance in myopic children are needed. 

Unlike conventional spectacle lens designs,²² Thondar Technology Co., Ltd. recently introduced a novel lens design, the individualized ocular refraction customization (IORC) spectacle lens (Thondar, Shenzhen, China). The front surface of the IORC lens comprises a central optical zone for correcting distance refractive errors and an annular multiple focal zone with multiple lenslets having a relative plus power. The back surface of the IORC lens is customized with an individual's actual RPR across the retina as measured by MRT,^17,18 with the goal of correcting both central and peripheral refractions. Thus the IORC lens can be individually adjusted depending on the initial RPR across the retina to obtain the actual amount of MD provided by multiple lenslets. This spectacle lens is available not only with low or medium but also with very high MD (the range of MD: 2.50–4.50 D), which might allow investigation of the effect of MD with different levels of magnitude on the visual performance of participants (Fig. 1). 

Figure 1.

 

The design of the IORC spectacle lens. (a) The front surface design of the IORC lens. (b) The back surface design of the IORC lens. An individual's actual relative peripheral refraction across the retina was corrected by free-form surface turning at the back surface of the lens. (c) Illustration of an IORC lens design that simultaneously provides 2.50/3.50/4.50 D myopic defocus and peripheral refraction correction. The 2.50/3.50/4.50 D myopic defocus produces a second focal point. Ideally, the distance focus lies on the retina, and the near focus lies approximately 0.83, 1.16, and 1.50 mm, respectively, front of the retina. The calculations are based on the Gullstrand eye model.³⁵

Accordingly, this exploratory study was conducted to (1) compare 12-month patient-reported vision responses among myopic children wearing IORC lenses with low (2.50 D), medium (3.50 D), or high (4.50 D) MD and single-vision spectacle lenses (SVL) as evaluated through subjective questionnaires; (2) measure objective visual function after the discontinuation of spectacle lens use; and (3) test the hypothesis that myopic children wearing IORC lenses with 4.50 D of actual MD achieve acceptable objective visual function and subjective visual performance. 

Methods

Study Design

This exploratory study was a part of an IORC lens assessment study designed to assess the efficacy and subjective acceptance of IORC lenses versus SVL lenses in myopic children for a 12-month period. This prospective, randomized and double-masked clinical trial was conducted at West China Hospital of Sichuan University between September 2022 and November 2023. The primary outcomes were changes in spherical equivalent refraction (SER) and axial length from baseline. Here, we report on the secondary outcomes in terms of best-corrected visual acuity (BCVA), contrast sensitivity function (CSF), and self-reported questionnaires. The study protocol followed the tenets of the Declaration of Helsinki, was approved by the local ethics committee (No 2021-1191) and was registered on ClinicalTrials.gov (ChiCTR2200063036). Written informed consent was obtained from the participants and their guardians at the first visit after a detailed explanation of this study and the possible risks and benefits. 

Participants

Potential participants were recruited through one of two processes: (1) ophthalmologist referrals for those who only required myopia control treatment and did not have any eye disease during daily clinics and (2) optometrist referrals from optometry clinics for patients being seen for myopia treatment. Phone and visual screenings were performed to determine whether the participants met the study criteria. The inclusion criteria were as follows: (1) aged eight to 12 years, (2) had an SER ranging from −0.75 to −4.00 D, (3) had interocular anisometropia <2.00 D, and (4) had a BCVA of at least 1.0 (≤0.0 logarithm of the minimal angle of resolution (logMAR) in each eye). The exclusion criteria were as follows: (1) had strabismus and binocular vision abnormalities; (2) had ocular and systemic abnormalities; and (3) had prior experience with myopia control inventions, such as orthokeratology and pharmaceutical treatment (e.g., atropine). 

Four lens designs were used in this trial: traditional SVL (control group) and IORC lenses with a low (IORC-L, 2.50 D), medium (IORC-M, 3.50 D), or high (IORC-H, 4.50 D) MD. All participants were randomly assigned to one of four groups at a 1:1:1:1 ratio by an unmasked investigator (Q.C.) using computer-generated random numbers (Fig. 2). Participants and parents/legal guardians were masked by removing all spectacle lens labels before receiving the lenses. Neither the participants (together with their parents/guardians) nor the masked examiners were aware of the group allocations throughout the clinical trials. The masked examiners were responsible for the refraction test, SER, axial length, BCVA, CSF, and self-reported questionnaires. The unmasked examiners are in charge of dispensing and documenting adverse events. 

Figure 2.

 

Consolidated standards of reporting trials flow diagram of the study. IORC-L, individualized ocular refraction customization spectacle lens with low myopic defocus; IORC-M, individualized ocular refraction customization spectacle lens with medium myopic defocus; IORC-H, individualized ocular refraction customization spectacle lens with high myopic defocus; SVL, single-vision spectacle lens.

Sample Size Calculation

We calculated the sample size according to statistical analysis and recommendations from recent publications. To achieve 80% power in detecting a 0.50 D minimum difference (0.70 D of SD) in myopia progression between the two groups with an α level of 0.05 (two-tailed),^8,23 the minimum number of participants needed in each group was 37. Assuming a dropout rate of approximately 10%, we determined that 41 participants were required for each group. 

Measurements and Procedures

Refractive Test and Spectacle Prescriptions

The refraction test was performed according to a standard cycloplegia protocol. The suggested dosage for cycloplegia refraction is one drop of 1% tropicamide 4 times at 5-minute intervals. We obtained cycloplegic refraction data 30 to 45 minutes after the first drop of tropicamide was instilled, which ensured maximal cycloplegia effect. The refractive examination was performed using static retinoscopy and subjective refraction to determine the final distance prescription (Nidek RT-600; Gamagori, Aichi, Japan). At the initial spectacle dispensing and at each follow-up visit, participants and their guardians received face-to-face instruction about the purpose, use, and care of the lenses. All participants wore the lenses full time for at least 12 hours per day throughout the trial. The spectacle lenses were replaced with an updated prescription when the change in SER was greater than 0.25 D during any follow-up visit. Before ordering a new prescription, a trial frame was used to confirm the comfort of each participant. 

CSF and Distance and Near BCVA

We sought to determine whether long-term wearing of IORC lenses, rather than current wearing, affected visual function. Thus, CSF and distance and near BCVA were evaluated with the participant's best-corrected refraction mounted in a trial frame at baseline (after wearing lenses for 30 minutes) and after six and 12 months of lens wearing. 

CSF was performed binocularly using the CSV-1000 test (Vector Vision, Greenville, OH, USA) at spatial frequencies of 3, 6, 12, and 18 cycles per degree (cpd) under mesopic (3 cd/m²) conditions, with contrast levels reduced in steps corresponding to 0.15 logCS at a distance of 2.5 m. Before the test, two practice trials were implemented to eliminate the effect of familiarity. 

Distance BCVA was measured using a logarithmic high contrast (95%) visual acuity chart (NIDEK; Gamagori, Aichi, Japan) at 5 m. Near BCVA was measured using a high contrast reading chart (Wenbang Technology, Taizhou, China) at 40 cm. Both distance and near BCVA were measured monocularly and recorded in logMAR units. The children were asked to identify the characters in the smallest row that they could read. The test was stopped when three or more of the five letters per row were read incorrectly. 

Self-Reported Questionnaires

Subjective visual performance with the test lenses was evaluated through two subjective questionnaires (Supplementary Table S1), which were designed based on similar layouts used in previous studies related to wearing comfort and frequency of visual symptoms with specialized spectacle lenses.^6,24 Questionnaire 1 consisted of nine items addressing visual symptoms, such as lens adaptation, overall vision satisfaction, acceptance of the lens, vision clarity, and vision stability at distance, intermediate, and near tasks. Lens adaptation was defined as wearing the study lenses with no discomfort, complaints, or decreases in BCVA. Questionnaire 2 consisted of four items addressing visual symptoms, such as ghosting images, eyestrain, dizziness and headache. Cronbach's alpha coefficients of the reliability analysis of questionnaire 1 and questionnaire 2 were 0.85 and 0.88, respectively, and the Kaiser‒Meyer‒Olkin values of the validity analysis of questionnaire 1 and questionnaire 2 were 0.82 and 0.91, respectively, indicating high internal consistency and reliability of the questionnaires. 

The children were requested to answer the questionnaires alone, without the participation of their parents or guardians. The children could seek help from the examiner if they did not understand the meaning of the questions. The participants were instructed to rate the presence of any symptoms on a five-point Likert scale. For example, on the item “Are you satisfied with the visual clarity of your spectacle lenses at distance?,” participants selected their answer from five possible options (from very dissatisfied as 1 to very satisfied as 5). The participants were asked to complete the questionnaires at baseline and at the six- and 12-month visits. The scores for each item of the questionnaires were used for further analyses. 

Statistical Analysis

All statistical analyses were performed using SPSS 27.0 (SPSS Inc., Chicago, IL, USA). All the data from participants who completed the 12-month follow-up visit were analyzed. The data are presented as the mean (±SD) for continuous variables. Only right-eye data were used for the analysis. To compare the baseline characteristics among the four groups, one-way analysis of variance (ANOVA) was used for continuous variables, and the χ² test was used for categorical variables. Levene's test for homogeneity of variance and the Shapiro‒Wilk test for normality were both performed. The changes in the distance and near BCVA, CSF, and subjective variables at the three visits (at baseline, six months, and 12 months) among the four groups were compared using repeated-measures analysis of covariance (RM-ANCOVA), adjusted for baseline SER. For significant outcomes, post hoc comparisons were conducted using Bonferroni correction. The χ² test was used to compare lens adaptations and the acceptance rate among the four groups. If >20% of the cells in the analysis had an expected count <5, Fisher's exact test was applied instead. P < 0.05 was considered to indicate statistical significance for all abovementioned analyses. 

Results

Participant Characteristics

This study initially included 184 participants, but two dropped out at baseline, and 10 were lost to follow-up because of changes in spectacle or orthokeratology lens use (n = 3), refusal to return for follow-up (n = 2), frequent damage to the spectacle frames (n = 1), unwillingness to wear spectacle lenses (n = 3), or adaptation problems (n = 1). Consequently, 172 participants were included in the final analysis. Of these, 42 participants were in the IORC-L group, 44 participants were in the IORC-M group, 43 participants were in the IORC-H group, and 43 participants were in the SVL group (Fig. 2). There were statistically significant differences among the four groups in terms of baseline SER (P = 0.041), whereas no significant differences were found in any other baseline demographics among the four groups, including age, sex, corneal power and axial length (P = 0.556; P = 0.097; P = 0.328; P = 0.069; respectively) (Table 1). 

Table 1.

 

Baseline Demographics of Participants Who Completed the 12-Month Follow-Up in Each Treatment Group

 

Comparison of Distance and Near BCVA and CSF Among the Four Groups

All three IORC lens groups yielded excellent distance and near BCVA values. There were no significant differences in distance or near BCVA among the three follow-up visits among the four groups (both P > 0.99). Furthermore, the interaction effects between lens types and visits were not significant (both P > 0.99) (Table 2). 

Table 2.

 

Comparsion of Distance and Near BCVA at Baseline and Six-Month Intervals Over 12-Month Spectacle Lens Wear Among the Four Groups

 

The patterns of CSF in this study followed the typical physiological shape of CSF; they peaked at 6 cpd and started to decrease as the spatial frequency increased. As expected, there were no significant differences in CSF at any frequency (3, 6, 12, or 18 cpd) among the three follow-up visits among the four groups (all P > 0.99). After 12 months of lens wear, the four groups exhibited significant improvements in CSF at 3, 6, and 12 cpd (P = 0.005; P < 0.001; P = 0.015, respectively), and there were no significant differences in any CSF measurements among the four groups (all P > 0.99) (Fig. 3). 

Figure 3.

 

Comparsion of contrast sensitivity function after 30-minute (at baseline, a), and after six (b) and 12 months (c) of lens wearing among four groups. Values are expressed as mean logCS ± standard error.

Comparison of Subjective Visual Performance Among the Four Groups

The SVL group experienced significantly less ghosting images than the three IORC lens groups at baseline. Among the three IORC lens groups, the IORC-H and IORC-M groups experienced significantly less ghosting images than the IORC-L group (overall P < 0.001; P = 0.011 between IORC-H and SVL; P = 0.008 between IORC-M and SVL; P < 0.001 between IORC-L and SVL; P>0.99 between IORC-H and IORC-M; P < 0.001 between IORC-H and IORC-L; P < 0.001 between IORC-M and IORC-L), but no differences were observed at the six-month and 12-month visits (P = 0.734; P = 0.968, respectively). There were no significant differences among the four groups in terms of any other subjective variables at baseline or at the two follow-up visits regarding vision clarity, vision stability, eyestrain, dizziness, headache, or overall vision satisfaction (all P > 0.05). The four groups exhibited significant improvements in all of the subjective ratings except for eyestrain after 12 months of lens wear, and at the 12-month visit, there were no significant differences among the four groups. There were also no interactions between lens types and visits (all P > 0.05) (Table 3). 

Table 3.

 

Comparsion of Subjective Questionnaires at Baseline and Six-Month Intervals Over 12-Month Spectacle Lens Wear Among the Four Groups

 

No significant difference was observed in the proportion of participants who adapted to the spectacle lenses among the four groups (χ² test, P = 0.998). All treatment groups were adapted to the study lenses within seven days, with no adverse events, complaints, or discomfort reported (Table 4). There was a significant difference among the four groups in terms of the proportions of participants who were willing to continue wearing the IORC-H, IORC-M, IORC-L and SVL lenses at the 12-month visit (97.7% [42/43], 90.9% [40/44], 88.1% [37/42], and 76.7% [33/43], respectively; χ² test, P = 0.024), and post hoc multiple comparison testing revealed that the percentage of SVL wear acceptance was significantly lower than that for the IORC-H lens; moreover, no significant differences were observed between the other lens groups (Fig. 4). 

Table 4.

 

Frequency Distribution of Four Groups Who Adapted to the Spectacle Lenses With no Reported Adverse Events, Complaints, or Discomfort

 

Figure 4.

 

Proportions of participants who are willingness to continue to wear the test lens at the 12-month visit. Asterisks signify significant difference between groups.

Discussion

This study presented 12-month patient-reported visual performance and objective visual functions using IORC lenses with low, medium, and high MD and compared them with those of myopic children using the SVL. Overall, IORC lenses had no clinically meaningful influence on objective visual functions after 12 months of lens wear. The IORC lens design also did not affect most of the subjective ratings at baseline or at two follow-up visits, and children adapted well to all three IORC lens designs. This study also showed that a substantially highest proportion of participants who were willing to continue wearing IORC-H lenses after 12 months of lens wear. 

Objective Visual Function

Our results showed that there were no significant differences in distance or near BCVA changes after wearing the lenses for 12 months between the three IORC lens designs and the SVL. The results reported by Lam et al.¹² showed that there were no significant differences between the DIMS lens and SVL groups when comparing changes in distance and near BCVA after two years of lens wear, which is fairly consistent with the findings of this study, as both studies recruited participants of the same ethnicity, wore similar specialized spectacle lens designs and performed similar study protocols, confirming that IORC lenses with low, medium and high MD do not compromise distance or near BCVA in children during long-term follow-up with respect to classical SVL. 

CSF is much more informative than high-contrast BCVA for evaluating visual performance in patients with multifocal lens wear.^25,26 Therefore, an additional CSF assessment was performed in this study. As expected, the CSF test results showed a trend similar to that of the BCVA test; specifically, the three IORC lens designs did not significantly affect the CSF in children, and the differences in the CSF over time did not depend on the treatment group. However, no other study in the literature has analyzed the effects of long-term specialized spectacle lens wear on CSF; thus, direct comparisons with other reports are not possible. In this study, the four groups exhibited significant improvements in CSF at 3, 6, and 12 cpd after 12 months of lens wear, possibly because the children became more experienced and familiar with the measurements at the 12-month visits.¹² However, no significant improvements were observed in the CSF at 18 cpd in any group of children. Each participant underwent CSF testing from low (3 cpd) to high (18 cpd) spatial frequencies. It is possible that some children may have become bored or tired when they were repeatedly tested with similar procedures. This factor might limit possible improvements in CSF parameters at the very high spatial frequency of 18 cpd, which was tested in the final session of each measurement. Furthermore, differences in the difficulties presented by CSF tests with different spatial frequencies might also have influenced the amount of CSF improvement at 18 cpd.²⁷ Overall, IORC lenses with low, medium and high MD had no long-term impact on objective visual functions regarding CSF or BCVA relative to SVL wear. 

Subjective Visual Performance

In this study, the results comparing vision clarity and vision stability among the three IORC lens groups and the SVL group exhibited similar trends. No significant differences were found between the three IORC lens groups and the SVL group in terms of vision clarity or visual stability at baseline or at the two follow-up visits. Recent studies that examined visual acuity when participants fixated through the periphery of the HAL showed that visual acuity was unaffected during this task.^11,14 Since the IORC lens used in this study is very similar to the lenses used in those studies except for the MD customization, neither the vision clarity nor the vision stability should be affected by the novel IORC lens when the subject fixates through the periphery of the lens with multiple lenslets. In addition, given the large central clear zone (9 mm in diameter) of the IORC lens for correcting distance refractive errors, it is reasonable to expect a low likelihood of detecting a significant difference between the IORC lens and SVL with respect to vision clarity and visual stability. When the participants first used the IORC lenses, we told them to fixate through the center of the lens. After wearing the lenses for half an hour, the children learned to use the lenses to find the clear central area; thus, the influence of the peripheral lenslets was reduced.^13,26 Overall, the clear visual acuity and good visual stability of the participants provided assurance of good compliance in this longitudinal study. 

Images produced by bifocal, multifocal, or specialized lenses can include both a focused image and a simultaneously present defocused or “ghost” image,^28,29 and participants with such lenses may experience poor subjective visual performance, such as these ghosting images. Contrary to expectations, the current study showed that the SVL group experienced significantly less ghosting images than the three IORC lens groups at baseline. Among the three IORC lens groups, the IORC-H and IORC-M groups experienced significantly less ghosting images than the IORC-L group. 

Several factors could have influenced the magnitude of the ghosting images, such as MD power and lens design (i.e., power profiles) differences.³⁰ The differences in the quality of the retinal image generated by the three IORC lens designs might explain why the IORC-L lens induced more ghosting images than the IORC-H and IORC-M lenses. With the IORC-L lens (2.50 D) used in the current study, the peripheral image of the lenslets was only slightly blurred, so light beams from the periphery might be interrupted with clear central images. However, when the IORC-H and IORC-M lenses (4.50 D and 3.50 D) were used, the heavily blurred peripheral image of the lenslet could be treated as irrelevant background and ignored, but the clear central image could capture more attention. This possible explanation is in accordance with the blur ignored hypothesis.^31,32 Thus this study potentially provides evidence that the lower the magnitude of MD is, the significantly more noticeable symptoms of ghost images children might experience when wearing IORC lenses. 

However, there were no significant differences in ghosting images between the three IORC lens groups and the SVL group at the six- and 12-month visits. It is possible that the wearers may, at least in part, have adapted to the optics of these lenses and, accordingly, experienced fewer ghosting images after longer periods of lens wear. Similar experiences have been noted with multizone contact lenses, with increased perception of ghosting images often reported upon lens dispensation. However, the participants experienced fewer ghosting images after several weeks or months of wear, even in the absence of improved visual acuity.²⁸ That is, although the IORC lens could introduce subjective ghosting images at the initial lens assessment, children might gradually adapt and become less sensitive to defocus signals after six and 12 months of lens wear, resulting in less perception of ghosting images. 

Although wearing of the IORC lenses might have led to increases in the perception of ghosting images, the incidence of eyestrain, dizziness, and headache were not different between the three IORC lens groups and the SVL group at baseline or at two follow-up visits. The overall vision satisfaction questionnaire item, which encompasses all aspects of the visual experience, was also not significantly different among the four groups. One recent study investigated the effects of DIMS lenses on eye strain via a demanding search task and found that they robustly decreased eye strain relative to SVL, achieving results consistent with those of this study.³³ In this study, all treatment groups adapted to the study lenses within a week, with no adverse events, discomfort or complaints reported. A recent study by Bao et al.⁸ showed that myopic children wearing HAL adapted to the lenses within a week, which is fairly consistent with the findings of this study, justifying the minimum seven-day wearing period to adapt to specialized spectacle lenses. 

This study revealed that a substantially greatest proportion of participants were willing to continue wearing IORC-H lenses after 12 months of lens wear. The participants wearing IORC-H lenses experienced acceptable overall visual performance, and no treatment-related adverse events, such as falling, were reported. Previous studies reported that multifocal contact lenses with 3.00 D or greater added power would result in more vision-related issues than single-vision lenses.¹⁵ Multifocal contact lenses that have distinct power junctions across the optic zone have concentric rings of distance and near power so that the power changes abruptly between the zones.¹⁶ The basic rationale of IORC lens design is to exert myopic defocus in the mid-peripheral retina while maintaining the central retina in focus, and the myopic defocus area is distributed in a uniform manner with a clear vision area with a 50:50 area ratio. Thus, because of the different lens designs between multifocal contact lenses and IORC lens designs, we assume that there might be less risk of visual performance compromise with these new spectacle lens designs than with traditional multifocal contact lenses. 

To our knowledge, this is the first study in which customized MD was deployed for each particular eye to evaluate visual performance with and acceptance of specialized spectacle lenses. A recent study showed that specialized spectacle lenses are well tolerated and accepted by Chinese children; however, the conventional spectacle lenses used in that study were not individually adjusted for the amount of MD.³⁴ The RPR profile across the retina varies among myopic children,²⁰ and when children wear conventional specialized spectacle lenses with constant MD, the actual amount of MD perceived by the eye remains unknown. Thus, we were unable to determine the maximum amount of MD the children in this study could tolerate when wearing specialized myopia control spectacle lenses at the start of the study. Due to our concerns that high levels of MD may result in adaptation issues and safety concerns.¹⁴ the highest amount of MD in this study was designed to be 4.50 D. The results of this study indicated that IORC lenses with an actual MD of 4.50 D provided acceptable visual performance and overall vision satisfaction. Thus, the findings from this study might provide preliminary evidence for subsequent studies on lenses with customized MD, myopia control effects and acceptability for myopic children. Our current description of the effect of different amounts of MD on visual performance may only be part of the a picture, and further study is required to determine the optimal amount of MD for maximum myopia control and lens acceptability. 

Limitations

This study has several limitations. First, the dropout rate of this study was 6.5% (12/184). The main reason for three children who were unwilling to wear spectacle lenses was a lack of motivation to wear spectacle lenses because of aesthetic issues, two children refused to attend follow-up visits because they were too busy with the school curriculum, three children wanted to try another intervention because they did not achieve a satisfactory myopia control effect, and only one of them discontinued the trial because he reported a slight perception of blur in peripheral vision on the first day of using the study lens, and he did not want to continue wearing the lens. Although the rate of adaptation failure could be underestimated, we consider the rate of adaptation failure to have little impact on the overall results and conclusions. Second, it was not entirely possible to mask the patients between the use of one of the three IORC lens types and the SVL because the lenslets are visible under certain lighting conditions. However, complete masking was possible among the three IORC lens types. Third, this exploratory clinical study measured contrast sensitivity using the CSV-1000E test because the CSV-1000E test is economical and easy to perform. However, the CSV-1000E test provides low measurement precision because of its coarse spatial frequency and contrast sampling. Although it is not routinely measured in clinical settings, future studies could measure the full CSF using a quick CSF procedure to obtain a precise and efficient CSF assessment.³⁴ Fourth, the observation period of one year was too short to draw any conclusions about the long-term influences of IORC lens use on visual performance. Thus, it is necessary to design a longitudinal study to explore the long-term influences of IORC lens use on visual performance. Finally, we evaluated CSF and distance and near BCVA with the participant's best-corrected refraction mounted in a trial frame at baseline and after 6 and 12 months of lens wearing. However, testing visual function with actual IORC lenses is also needed. Whether current wearing IORC lenses with different amounts of MD affects visual performance on daily tasks in children's lives, such as reading and writing, needs further investigation. 

Conclusions

The IORC lenses with 2.50, 3.50, and 4.50 D of actual MD had no clinically meaningful influence on the objective visual function of myopic children after 12 months of lens wear. The three IORC lens designs also did not affect most of the subjective visual performance, and the IORC lens with an actual MD of 4.50 D received the highest proportion of acceptance by myopic Chinese children. This study provides potential preliminary evidence that IORC lenses with an actual MD of 4.50 D provide acceptable visual performance and overall vision satisfaction, which might provide further insight into subsequent studies on customized MD, myopia control effects and acceptability for myopic children. 

Acknowledgments

The authors thank Chengdu Huashi Micro-Professional Vision Technology Co., Ltd. employee Xin Ma for his help in providing research grants, and Shenzhen Thondar Technology Co., Ltd. employees Serena ZheChuang LI and Xue Chuan Dong for their help in developing, manufacturing and supplying the investigational products used in the study. 

Supported by Chengdu Huashi Micro-Professional Vision Technology Co., Ltd. and Shenzhen Thondar Technology Co., Ltd. The former corporation provided research grants, and the latter provided spectacle lenses and frames for the study. Both funders had no role in the study design, study implementation, data analysis or manuscript preparation. 

Disclosure: Y. Wu, None; J. Kou, None; S. Lei, None; L. Xiong, None; Q. Chen, None; M. Zhang, None; L. Liu, None