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Comparison of Near Addition Value Prescription Methods for Myopic Children

Yu, Xinping*; Bao, Jinhua*; Drobe, Björn; Lv, Wenhui*; Wu, Ge*; Xu, Jinling*; Chen, Hao*

doi: 10.1097/OPX.0000000000000747
Original Articles

Purpose To compare personalized near addition values for myopic children that induce a reasonable phoria (Ph) (meeting Sheard's criterion) and induce a null lag of accommodation at near and to evaluate changes in these metrics after lens adaptation.

Methods Fifty-three myopic children participated in this study. Accommodative response, Ph, and fusional amplitudes (FAs) were measured at 33 cm through multiple addition lenses (0D, +1.00D, +1.50D, +2.00D, +2.50D, +3.00D). The adaptation effect on measured parameters was evaluated after 6 min of near work with each addition. The FA/Ph ratios were calculated for each addition and fitted using a rational function to obtain the optimal addition value satisfying Sheard's criterion (FA/Ph ≥ 2). Lag of accommodation change in association with addition value was assessed using linear regressions to obtain the addition values inducing a null lag.

Results Lag of accommodation (r = −0.987, p < 0.001) and Ph (r = −0.999, p < 0.001) decreased linearly with addition value. The 6-min adaptation induced a small but significant increase in accommodative lag and an exophoric shift (p < 0.05). Three FA/Ph patterns with respect to addition lenses were observed based on the near Ph: orthophoria/exophoria, low esophoria, and large esophoria. Addition values based on Sheard's criterion (mean ± SD, +2.16 ± 0.79D) were significantly lower than those based on null lag (+2.83 ± 0.44D, t = 19.86, p < 0.001). The FA/Ph ratio was less than 2.0 in 75.5% of subjects with additions determined through null accommodative lag. Additions determined through FA/Ph ≥ 2 induced a lag of accommodation of 0.38 ± 0.42D.

Conclusions Personalized near addition values using Sheard's criterion were lower than those obtained through null lag of accommodation. These values should allow a better binocular balance especially for exophoric children.

*MD

PhD

The Eye Hospital of Wenzhou Medical University (XY, JB, WL, GW, JX, HC); WMU-Essilor International Research Centre, Wenzhou, China (XY, JB, BD, WL, GW, JX, HC); and R&D Asia, Essilor International, Singapore (BD).

Hao Chen No. 270 Xueyuan Xi Rd Wenzhou 325027 China e-mail: chenhao823@mail.eye.ac.cn

Near addition lenses have been prescribed to children for several decades to slow the progression of myopia.1 Myopes, and especially progressing myopes, often have higher accommodative lags than emmetropes.2–4 This induces hyperopic retinal defocus and possible axial elongation of the eye, eventually leading to the development of myopia.5 Near additions are prescribed to myopic children to eliminate/decrease high accommodative lags.6

The Correction of Myopia Evaluation Trial demonstrated that the effects of near addition lenses on myopia progression in all children were inconclusive. Specifically, few benefited from progressive addition lenses (PALs) with a standard near addition value (+2.00D).7 The child's accommodative and phoria (Ph) status might play a significant role in the mechanism of myopic progression reduction with plus lenses (i.e., PALs or bifocal lenses). The greatest myopic progression reductions have been observed in children with high accommodative lags,8,9 esophoria,10,11 or both.12

One possible explanation for these reductions is that subjects with exophoria or orthophoria experienced increased near exophoria with near addition lenses, which would lead to a noneffective use of the lenses.13 It should be noted that most studies evaluating the effect of near addition lenses on myopia progression have not accounted for accommodative lag in association with binocular vision balance (i.e., Ph and fusion while wearing PALs or bifocal lenses).

It is reasonable to explore optimal near addition lenses considering the accommodative and Ph status. Jiang et al.14 evaluated the optimal power value of near addition lenses that would induce the least accommodative error and vergence responses in young adults. These authors found that the average values at a 33-cm viewing distance, associated with 0 retinal defocus, were +1.28D and +0.20D when associated with 3 prism diopters (pd) near exophoria. Similarly, Cheng et al.13 used near base-in (BI) prisms to reduce addition lens–induced exophoria. These authors found that a +2.25D near addition associated with a 3 BI pd on each eye could minimize the lag of accommodation and addition-induced exophoria. However, the subjects in the study performed by Jiang et al.14 were young adults (including emmetropes and hyperopes). Furthermore, the vergence balance was only defined as being within the “normal range” in both studies, without accounting for Ph relative to the patients' fusional status or evaluating adaptation to wearing addition lenses during near work.13,14

As near addition usage is believed to be linked to comfortable binocular vision,13 a potential way to determine the optimal addition value would be to use a binocular vision comfort criterion (e.g., Sheard's criterion: the fusional vergence in reserve should be at least twice the amount of heterophoria [i.e., mean fusional amplitude (FA)/Ph ≥ 2]). Meeting Sheard's criterion is suggested to be associated with binocular comfort,15,16 especially in subjects with exophoria.17 The highest addition value that satisfies this criterion should represent the best compromise between reducing the lag of accommodation while retaining binocular comfort.

The present study aimed to compare two personalized near addition methods for myopic children, including the following: (1) inducing a Ph at near that satisfies Sheard's criterion and (2) inducing a null lag of accommodation at near. Moreover, we analyzed the effects of near work adaptation on these near addition values. Finally, this study aimed to identify a personalized addition power to be tested in a clinical trial (Personalized Addition value Clinical Trial [PACT]), which evaluates the effect of personalized near addition on myopia progression, compared with a fixed near addition (+2.00D) and single-vision lenses.

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METHODS

Subjects

Fifty-three patients were recruited from the Department of Optometry of the Eye Hospital of Wenzhou Medical University between October 2013 and March 2014. The patients (21 females, 32 males) ranged in age from 8 to 11 years (mean, 9.4 ± 0.95 years), with myopia (spherical equivalent, with full manifest correction obtained using cycloplegic refraction) between −1.0 and −3.0D (−2.08 ± 0.62D) in both eyes and with myopia progression of at least 0.5D in the past year, and assessed either by previous records or by self-reporting (for those without any refraction history records, the parents or guardians were asked to assess the myopia progression in the last year). All subjects had a best corrected visual acuity of less than or equal to 0 LogMAR. Twenty subjects had esophoria (2.0 to 15.5 pd [5.6 ± 3.7 pd]), 12 had exophoria (−2 to −5.5 pd [−3.5 ± 1.2 pd]), and 21 had orthophoria (−1.5 to 1.5 pd [−0.66 ± 1.1 pd]) at near measured using the modified Thorington test (“+” indicates esophoria, “-” indicates exophoria). Informed consent was obtained from all subjects and their parents or guardians. The study was approved by the WMU Ethics and WEIRC Scientific Committees. All procedures and data collection were performed according to the Declaration of Helsinki standards. Thirty-two of the 53 subjects were later enrolled in the PACT.

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Measurements

All subjects underwent a complete eye examination (including case history, visual acuity, cover test subjective refraction, slitlamp examination, near Ph testing using the modified Thorington test, BI FAs (in case of near esophoria) or base-out FAs (in case of near exophoria). Accommodative response, Ph, and FAs through six near addition lenses (+3.00D, +2.50D, +2.00D, +1.50D, +1.00D, and 0D) were measured at 33 cm, which is a distance commonly used for near refraction in children12 with a phoropter (Ph and FA) and trial frame (accommodative response). Near addition lenses were ordered from highest to lowest to avoid any near adaptation effects of accommodation.14 A Rapid Serial Visual Presentation with a speed of 75 characters/min18 (i.e., different Chinese characters were presented consecutively) was the accommodative target. The character size was 4.57 mm in height (visual angle of 0.79 degrees). Each subject was asked to find specific characters (e.g., or ) and indicate when they were present with a mouse click. The target was aligned with each subject's right eye under binocular viewing conditions. Accommodative responses were measured with a Shin Nippon (Nvision-K5001, Ryusyo Industrial Co. Ltd., Kagawa, Japan) open-field autorefractor (10-static-measurement). The spherical equivalent power for each measurement was calculated and then averaged over the 10 readings. During the measurements, the subjects wore their distance refraction lenses mounted on a trial frame at a vertex distance of 12 mm. The data were recorded in Microsoft Excel, and the average was obtained using the =average() function. Because the Shin Nippon was set for spectacle correction, we converted all readings to the power at the subject's corneal plane.2 The lag of accommodation was then calculated at the subject's corneal plane, as described previously.2

Near Ph was measured at 33 cm using the modified Thorington test (3-measurement average) and either BI (near esophoria with near addition lenses) or base-out FAs (near exophoria with near addition lenses) were measured with a phoropter. Before the measurements, all subjects participated in a training session with the modified Thorington test. We showed the children cards with illustrations to allow them to better understand the test. A tangent scale was used to measure Ph, which consisted of a small central aperture for the light source and a horizontal row of numbers that were spaced at 3.3 mm (1 pd apart at a distance of 33 cm). A red Maddox rod was placed in the phoropter in front of the subject's right eye, and Ph was measured using a “flashing technique.”19 The subjects verbally reported which letter was closest to the red line. Near Ph was defined as the average of three measurements. A previous study showed that, in myopic children, most of the binocular adaptation occurs within the first 3 minutes of near work.20 To study adaptation to near additions, each subject watched 6 min of video with each addition on an iPad at 33 cm, and the accommodative response, near Ph, and FAs were measured again. After resting for 1 min, the subject was examined with new addition lenses.

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Definition of Addition Lens for Sheard's Criterion (Faph2add)

The highest addition capable of satisfying Sheard's criterion that induces the smallest accommodative error. Addition values may be too high for traditional multifocal lens prescriptions for some highly esophoric subjects. In such cases, the maximum value was limited to +3.50D.

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Definition of Addition Lens for Null Accommodative Lag (Nulllagadd)

The lowest addition value needed to induce a null, or the lowest accommodative error.

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Statistical Analysis

Linear regressions were used to analyze the influence of addition values on Ph and accommodative lag. Repeated-measures analyses of variance were performed to test differences in accommodative lag, Ph, and the FA/Ph ratio before and after 6 min of near work. Optimal addition lenses were determined by fitting the lag of accommodation through intercept of linear regression (with the x axis of y = 0) and through nonlinear regression using an FA/Ph = │a/(b*Addition + c)│ rational function, where a represents a form factor and -c/b the addition value for which the Ph is null. An independent t-test was performed to identify differences in addition lens values between these methods, and a paired t-test was performed to identify differences in addition lens values before and after adaptation. All statistical analyses were performed with SPSS version 17.0 (SPSS, Chicago, IL).

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RESULTS

Accommodative Lag with Near Addition Lenses

The lag of accommodation decreased linearly with increasing addition value (Fig. 1), from 1.33 ± 0.31D without addition to 0.50 ± 0.25D with +2.00D addition and −0.12 ± 0.29D with +3.00D addition (y = −0.493x + 1.441, r = −0.987, p < 0.001). The lag increased significantly after 6 min of adaptation (F = 51.968, p < 0.001), although the difference was minimal (mean ± SD, 0.02 ± 0.007D).

FIGURE 1

FIGURE 1

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Near Ph with Near Addition Lenses

Phoria at near also decreased linearly toward greater exophoria with increasing addition lens power (Fig. 2), from 1.11 ± 4.36 pd esophoria without addition to −6.16 ± 4.18 pd exophoria with +2.00D addition and −9.46 ± 4.17 pd with +3.00D addition (y = −3.506x + 0.935, r = −0.999, p < 0.001). The Ph difference at near before and after 6 min of adaptation was significant (F = 65.906, p < 0.001). This value became more exophoric after adaptation, but the difference was not clinically relevant (mean ± SD, 0.18 ± 0.12 pd).

FIGURE 2

FIGURE 2

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Fa/Ph Ratio with Near Addition Lenses

Three patterns of FA over near Ph variation with addition lenses were observed. First, for subjects with orthophoria or exophoria at near, FA/Ph decreased with increasing addition lens power. Fig. 3 shows the FA/Ph variation with addition lenses for an 8-year-old boy with −4.5 pd exophoria at near. The FA/Ph ratio was 4.44 without addition, which was reduced to 1.73 with +2.00D addition lenses and 1.36 with +3.00D addition lenses. Second, for subjects with large esophoria at near, the FA/Ph ratio increased with the addition lens power. Fig. 4 represents a 9-year-old boy with 15 pd esophoria at near. The FA/Ph ratio was 0.97 without addition lenses, which improved to 1.76 with +2.00 addition lenses and to 2.73 with +3.00 addition lenses. Third, for subjects with low esophoria, the FA/Ph ratio increased and subsequently decreased, whereas the addition value increased. Fig. 5 shows the FA/Ph variation for a 10-year-old boy with 4 pd esophoria at near. The FA/Ph ratio was 3.5 without addition lenses, and it increased to 10 with +1.50D addition lenses. For higher addition values, the ratio decreased to 3.4 with a +3.00D addition. The FA/Ph ratio decreased significantly, by an average of 0.21, after 6 min of adaptation (F = 4.837, p < 0.032).

FIGURE 3

FIGURE 3

FIGURE 4

FIGURE 4

FIGURE 5

FIGURE 5

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Comparison of Addition Values Determined through a Null Accommodative Lag and Fa/Ph ≥ 2

Addition values determined through FA/Ph ≥ 2 (Fig. 6) were significantly lower (+2.16 ± 0.79D) than those determined through a null accommodative lag (+2.83 ± 0.44D) (Table 1). The average FA/Ph ratio was 2.26 ± 2.76 (range, 0.8 to 19.0) when addition values were determined through a null accommodative lag, with 75.5% of cases showing ratios less than 2.0. The 13 remaining cases with FA/Ph ≥ 2 were all esophoric or orthophoric at near. The addition value determined through FA/Ph ≥ 2 induced an average lag of accommodation of 0.38 ± 0.42D (−0.5 to 1.38D). Eighteen (34%) of these cases had accommodative lags over 0.50D, and 8 (15%) had a lead of accommodation (−0.03 to −0.50D). After 6 min of adaptation, addition values determined through FA/Ph decreased by 0.05 ± 0.11D (−0.52 to 0.12D) (t = 2.003, p = 0.050), whereas addition values determined through null lag increased by 0.018 ± 0.018D (−0.2 to 0.13D) (t = 3.16, p = 0.003).

TABLE 1

TABLE 1

FIGURE 6

FIGURE 6

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DISCUSSION

With the benefit of reducing accommodative error, near addition lenses induced an obvious exophoric shift in myopic children. This exophoric shift was linearly correlated with near addition power.13 This increase in exophoria would be expected to reduce the positive-lens treatment effect in exophoric children.21 Near addition values were determined by Sheard's criterion (i.e., the highest addition values capable of satisfying a binocular vision comfort criterion). While reducing the lag of accommodation, such personalized near addition values would be expected to be well tolerated by the wearer, especially by children and young adults with near exophoria or orthophoria because fusional reserves would be at least two times higher than the Ph to compensate.

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Sample Comparison with Previous Pal/Bifocal Myopia Control Studies

The accommodative lag, Ph status, and age of subjects in our study were consistent with those of previous myopia control investigations with PAL or bifocal lenses (Table 2). Previous studies have typically used +2.00D or +1.50D near addition lenses to evaluate the effects of PALs/bifocal on myopia progression.7–9,22,23 The accommodative lag with +2.00D near addition lenses in our study was 0.50 ± 0.25D (−0.18 to 1.01D). In addition, 51% of the subjects had less than 0.50D. The FA/Ph ratio was 4.31 ± 6.45 (1.09 to 34.0) with this near addition value, and 42% of the subjects had a ratio of less than 2.0. The accommodative lag was 0.78 ± 0.27D (0.15 to 1.48D) with +1.50D near addition lenses. A total of 87% of subjects demonstrated values larger than 0.50D, and 19% had an FA/Ph ratio less than 2.0.

TABLE 2

TABLE 2

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Respect of Sheard's Criterion with Near Addition Values Used in Previous Myopia Control Studies

The Correction of Myopia Evaluation Trial showed that myopic children with high accommodative lag and near esophoria achieved better results with +2.00D addition PALs (0.64D with 3 years of follow-up)12 than children wearing single-vision lenses. However, subsequent studies have not been able to replicate these findings. These studies obtained statistically, but not clinically, significant results (0.18D24 after 1 year in children with high lags; 0.28D25 after 3 years in esophoric children with high lags). The accommodative lag in our study was 1.48 ± 0.23D (0.94 to 2.04D) for the 20 esophoric subjects; this lag decreased to 0.63 ± 0.19D (0.32 to 1.04D) with +2.00D near addition lenses, and 19 (95%) subjects had FA/Ph ratios greater than 2.0. The accommodative lag with +2.0D near addition lenses was 0.42 ± 0.25D (−0.18 to 0.88D), and the FA/Ph ratio was 1.09 to 2.53 in the 33 subjects with near orthophoria or exophoria. Of these cases, 21 (64%) had FA/Ph ratios less than 2.0 (Fig. 7). These results suggest that the previously observed superior treatment effects associated with +2.00D near addition lenses in subjects with near esophoria may be caused by the better binocular balance in esophores compared with exophores or orthophores. It is possible that this increased binocular balance may have led to more frequent use of the near-vision component of multifocal lenses. Therefore, less hyperopic defocus would be experienced during near tasks. Near addition lenses of more than +2.00D could be prescribed to subjects with near esophoria to obtain even lower accommodative lags while maintaining good binocular balance. Therefore, a better myopia control effect may be achieved.

FIGURE 7

FIGURE 7

Our results also suggest that near addition lenses with Ph change compensations through BI prisms can be used to achieve better treatment effects. This finding is supported by Cheng et al.,26,27 who found that subjects with 6 BI prismatic bifocal lenses had a lower myopia progression than those with single-vision lenses or bifocal lenses after 2 and 3 years. These findings suggest that good treatment results may be achieved with near additions determined through FA/Ph ≥ 2. These results may be especially beneficial for children with near exophoria or orthophoria, as they induce lower accommodative lags while maintaining binocular balance.

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Comparison with Previous Methods Used to Determine Personalized Near Addition Lenses

Two previous studies13,14 have aimed to personalize addition lens power drawing on accommodation and/or Ph. Jiang et al.14 determined optimal addition values that induce a null accommodative lag or 3 pd exophoria at near in 30 adults (17 myopes, 11 emmetropes, and 2 hyperopes). Cheng et al.13 found that an addition of +2.25D, combined with a 6 pd BI prism, produced an optimal accommodation and Ph compensation compromise. Table 3 compares the results of the present study with the results of the two studies mentioned above. In our study, 75.5% of subjects had FA/Ph ratios less than 2.0 with near addition lens values determined through null accommodative lag. This result indicates that most cases could not achieve satisfying binocular balance. A previous study suggested that failing to meet Sheard's criterion was correlated with exhibiting subjective asthenopic symptoms.16 Moreover, Sheard's ratio increased with improved subjective symptoms through vision therapy.28 Accommodative lag was 0.38 ± 0.42D with near addition lens values determined through FA/Ph ≥ 2, with 66% of subjects exhibiting values less than 0.50D. The average near addition lens value needed to obtain a null accommodative lag was much higher in our study than in the study performed by Jiang et al.14 This difference may be caused by the younger age of our subjects (children vs. young adults) and refractive error (myopia vs. myopia and emmetropia or hyperopia). Indeed, children generally have higher lags than adults,29 and myopes have higher lags than emmetropes.2,30 In our study, 13 subjects (25%) with +2.25D near addition were orthophoric, esophoric, or had less than 6 pd of exophoria (+8.5 to −5.5 pd). These subjects, especially those with esophoria or orthophoria at near, might not need any BI prism as described by Cheng et al.13 because their Ph status remains within a comfortable, or more comfortable (high esophores), range with this near addition. Therefore, Ph status should have been accounted for when determining the prism addition.

TABLE 3

TABLE 3

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Adaptation to Near Work with Addition Lenses

Sreenivasan et al.20 found that most adaptation to +2.00D lenses occurred within 3 min of near work for myopic children. In our study, addition lens values determined through both methods were statistically significant after 6 min of adaptation. However, this change was clinically insignificant. The FAPh2 Add decreased by 0.05 ± 0.11D (−0.52 to 0.12D), whereas NullLag Add increased by 0.018 ± 0.018D (−0.2 to 0.13D). A previous study in emmetropic adults showed that +2.00D addition lenses increased near exophoria.20 However, the same authors31 found that the lens-induced exophoric shift decreased after a 20-min near task adaptation to +2.00D addition lenses in myopes. These results suggested that near addition lens values determined through FA/Ph would be slightly higher after adaptation for orthophores and exophores but lower for esophores, as FA remained constant across time in most of our subjects. The effect of near work adaptation should be evaluated more specifically, that is, with near addition lenses of the same value or with optimal near addition lenses.

Another study31 showed that using +2.00D near addition lenses resulted in a small overaccommodation (−0.24 to −0.27D) at the beginning of near work in myopic children. This effect decreased during 20 min of sustained viewing through near addition lenses. Berntsen et al.32 studied the effects of a bifocal add on accommodative lag in myopic children with high accommodative lag for 6 months and showed that children mainly responded to bifocals by decreasing accommodation through studying. These two previous studies suggested that near addition values determined through null accommodative lag should be larger to account for the increase in accommodative lag after adaptation.

Myopes often show reduced completeness of vergence adaptation to +2.0D near addition lenses, with the degree of completeness being independent of AC/A.31 In our subjects, the response AC/A ratios were calculated from the +1.0D near addition, and we found no significant difference in the AC/A ratio between the three near Ph groups (3.39 ± 0.58 in subjects with orthophoria, 3.59 ± 0.35 in subjects with exophoria, and 3.34 ± 0.80 in subjects with esophoria; F = 0.57, p = 0.57, analysis of variance). Moreover, we found that AC/A did not remain constant when the near addition value changed. These results suggest that using only the AC/A ratio for determining the near addition value might not be adequately precise.

For practical reasons, Ph and fusion amplitudes in this study were measured in the primary position. However, if these measurements had been carried out in a gaze-down position (e.g., a typical reading posture), a more esophoric or less exophoric Ph would likely have been detected.33 To avoid accommodative adaptation, we tested the near addition lenses in order from highest to lowest as described in a previous study.14 However, this procedure might have induced adaptation effects in the vergence system; for example, children with exophoria need to exert greater fusional convergence effort to maintain single vision with the increased exophoria because of the high addition.34 The greater fusional convergence induced by high addition lenses would most likely have affected the fusion and Ph measurement results for the subsequent lenses. Moreover, in our study, we measured accommodation first, followed by Ph, and finally FAs. Subjects would most likely have already experienced lens-induced Ph adaptation during accommodation testing, which may have affected the Ph adaptation measurements during the near work in our study. Finally, a recent study found no further association between the severity of the clinical signs (e.g., Ph and Sheard's criterion) and the level of symptoms among symptomatic children with convergence insufficiency.35 Subjective evaluations of a participant's comfort would certainly provide additional insight that may be useful for prescribing personalized additions.

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CONCLUSIONS

Personalized near addition values obtained using an FA/Ph ratio ≥ 2 were lower than those obtained through null lag of the accommodation criterion. These values should allow a better binocular balance, especially for children with near exophoria and orthophoria. Near addition lens values determined through this FA/Ph ratio could be combined with accommodative lag measurements to avoid any lead of accommodation. The efficacy of personalized near addition lenses for the control of myopia is currently being tested in an ongoing clinical trial (PACT).

Hao Chen

No. 270 Xueyuan Xi Rd

Wenzhou 325027

China

e-mail: chenhao823@mail.eye.ac.cn

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ACKNOWLEDGMENTS

This study was supported by the International S&T Cooperation Program of China (Grant No. 2014DFA30940) and the Chinese Ministry of Health Research Projects (Grant No. 201302015), with partial funding provided by Essilor International S.A.

Part of this work was presented as a poster during the 2014 American Academy of Optometry Meeting in Denver, Colorado.

Received January 14, 2015; accepted August 27, 2015.

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Keywords:

accommodative lag; phoria; addition lens; myopic children

© 2016 American Academy of Optometry