Myopia has already become the more prevalent refractive error worldwide, and its incidence is increasing particularly in young adults and school-aged children.1,2 For decades, researchers had typically focused on the genetic basis of the myopia. While, genetic factor alone cannot explain this dramatic increase. Increasing evidence suggests a role of environmental factors in etiology of the myopia or even myopia progression, such as outdoor activities, physical activities,3 socioeconomic factors and near work activities such as studying, reading, watching television, and using computer.4

There are some available options for myopia correction including spectacle, contact lens and refractive surgery. Spectacles are the most acceptable method in comparison with contact lens and refractive surgery, which is less invasive approach for correction.5 It has been approved that while myopia occurs early in life, it has a tendency toward progression due to related possible factors.6 Different approaches have been used in order to slow or reduce the progression of myopia, such as cycloplegic drugs (Atropine),7–9 specially designed contact lenses such as orthokeratology contact lenses,8–10 multifocal contact lenses,11,12 specially designed spectacles such as bifocal or multifocals13,14 and under-correction of the refractive error.6,9

While there is no evidence for FDA (United State Food and Drug Association) approved pharmaceutical agents which slow the myopia progression, there are some studies concern the possible effect of some drugs on controlling myopia progression such as atropine.15,16 Contributed studies show that atropine is an effective method in slowing the myopia progression.17–19 However, in a study which considered the different concentrations of atropine, side effects were cautioned with 1% concentrate of atropine and also with long term usage.19 Chia et al. also compared three different concentrations of atropine and reported that 0.01% concentration has less side effects compared with 0.50 and 0.1%.20 Although it has been shown that, lower dosages of atropine were found to have less rebound myopic progression.21

Special design of contact lenses such as orthokeratology, rigid contact lenses, and soft multifocal contact lenses,11,12 are alternative methods to control the progression of myopia.22 Orthokeratology contact lenses control myopia through reshaping and flattening the cornea during sleep, these lenses provide the clear vision in all day. It has been proved that, providing the successful fit, it will slower the progression of the myopia, particularly in low-to-moderate myopic patients.23–26 It has been shown that multifocal soft contact lenses reduce myopia progression through providing less peripheral refractive error.27 Similar to contact lenses, bifocal or mulifocal spectacles also control myopia progression through reducing accommodative effort at near vision.13

Under-correction is one of the interventions to reduce or slow myopia progression. One hypothesis about under-correction is that it may decrease the accommodative effort and also the lag of accommodation and subsequently slow or reduce the myopia progression.6,28,29 Some studies have already compared under-correction versus full correction in myopic population.6,30–33 Whereas, there is contradiction in study's findings with regards to the under-correction or full correction of myopia, it seems valuble to have a definite conclusion and provide clinically an appropriate and effective method of correction. Therefore, we aimed to conduct a meta-analysis to investigate whether the under-correction is the method of choice for myopic patients or the full correction is the better way.

Current meta-analysis was performed based on PRISMA statement, the Preferred Reporting Items for Systematic Reviews and Meta-Analysis. The PRISMA Statement consists of a 27-item checklist and a four-phase flow diagram. The aim of the PRISMA Statement is to help authors improve the reporting of systematic reviews and meta-analyses. PRISMA is used as a basis for reporting systematic reviews, particularly evaluations of interventions.34

Study selection

Studies concerning the difference between under-correction and full-correction of myopia were considered for inclusion. The exclusion criteria were shown in Flow chart. (Fig. 1) Two researchers (NY and SH) independently reviewed the titles and abstracts of the retrieved articles, applying mentioned inclusion and exclusion criteria and also reviewed the full-text version of the remaining articles to verify their suitability for inclusion.

Figure 1.

Flow chart of included studies.

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PRISMA flow chart (PRISMA 2009 flow diagram) of the study has been presented in Fig. 1. Overall six studies were included in this meta-analysis. Two studies with Randomized Controlled Trial (RCT) design, two studies with Cross-sectional and two Cohort studies. (Table 1). Among included studies, 2 studies, used cycloplegic auto refraction (group1) and 4 studies, used non-cycloplegic refraction with retinoscopy and auto refractometer, which were followed by refined subjective refraction based on maximum plus sphere as the end point for best corrected distance visual acuity (group2).

The pooled difference in mean of myopia progression using cycloplegic refraction was −0.179 D [lower and higher limits: −0.383, 0.025], which was higher in the full- correction group as compared to the under-correction group (p=0.085). In group 2, the pooled difference in myopia progression was 0.128 D [lower and higher limits: −0.057, 0.312] higher in under-correction group compared with full-correction group (p=0.175). Fig. 2 shows the forest plot of the myopia progression. The Cochrane Q value was 6.99 (p=0.008) and I2=85.69 for cycloplegic studies and Q value was 1.48 (p=0.686) and I2=0.000 for non-cycloplegic studies. Fig. 3 shows the funnel plot of the included studies. Estimated Eggers regression intercept was 3.70, p=0.33, which indicates a symmetrical funnel plot.

Figure 2.

Forest-plot of subgroup analysis based on refraction. Left column shows the identity of each included study and name of the first author. Statistics for each study part presents difference in means of myopia progression, confidence interval and p-value. In left hand column, boxes show the effect estimates from the single studies, while the diamond shows the pooled result. MPMVA=Maximum Plus to Maximum Visual Acuity.

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Figure 3.

Funnel plot of meta-analysis. Each dot represents a single study. The y-axis is shows the standard error of the effect estimate. The x-axis shows “Difference in means”.

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The subgroup analysis of the results of the studies in group 1 and 2 demonstrated pooled progression rates of −0.26 diopter (95% CI −0.24 to −0.29) in group 1, in comparison with 0.15 diopter (95% CI 0.10 to 0.21) in group 2. In the first group of studies, myopia progression was significantly higher in full-corrected subjects in relation to under-corrected cases. (p<0.001) While, in the group 2, myopia progression decreased significantly with full-correction (p<0.001). Although, difference in means of progression was statistically significant in both types of refraction, but the difference was not clinically significant.

Today myopia is the main global cause of visual impairment, especially in Asia.39,40 Rapid increase of myopia prevalence has become a considerable global concern for both visual science researchers and practitioners 41 and assessment of approaches to slow down the myopia progression are the main topics of recent studies. Different approaches, such as single-focus spectacles, contact lenses and atropine eye drops, have been employed in an attempt to reduce myopia progression.42–46 Spectacles are still the most common and easiest option. The corrected amount of myopia (full-correction or under-correction) has been the topics of several studies and discussed in several literatures.6,30,31,33,37,38 Under-correction has been believed that can reduce the accommodative stimulus and demand at near,47 and subsequently reduce the blur drive for accommodation, which may be a myopigenic factor.48 Moreover, animal studies have also revealed that myopic defocus resulting from under correction or lack of correction could slow myopia.49–51 A controversial hypothesis regarding under-correction is that defocused retinal image might cause myopia progression due to form deprivation myopia.52 On the other hand, it has been hypothesized that full-correction of myopia optically position the far point of the eye close to the infinity and thus reducing any retinal defocus-induced, blur signals at distance.53 Several studies have compared the effect of under-correction and full-correction of myopia on its progression, but there are discrepancies in findings. Some studies showed that under- correction slows down the myopia progression,31,37 however, other studies reveled that full-correction could be effective in reducing myopia progression.6,30,33,38

This systematic review and meta-analysis showed that the rate of myopia progression significantly differs between under-correction and full-correction approaches. The difference was statistically significant but clinically did not reach the significance level. According to the findings in this meta-analysis, in non-cycloplegic sub-group (retinoscopy followed by maximum plus to the best visual acuity), under-correction caused significantly greater degree of myopia progression as compared to full-correction. The possible rational belief behind it, is that full-correction could prevent any induced defocus of retinal image53 and form deprivation,54 which is believed to be an effective factor in myopia progression or incidence. The present findings are in consistence with Adler,6 Chung,30 Chen38 and Vasuadevan33 studies, which reported that full-correction could significantly reduce the myopia progression compared with under-correction group. Besides, eyes with under-correction are continuously stimulated by blurred image which could causes greater rate of progression.30 Regarding studies with cycloplegic refraction, under-correction could significantly reduce myopia progression compared with full-correction. The possible reason is that under-correction reduce the accommodative stimulus and demand at near,48 and thus reduce the blur drive for accommodation, which is thought to be a myopigenic factor.

The reason that inference on the basis of between sub-groups differences is that there may be another factor, aside from types of refractions, which could be possible reason that could explain the difference between cycloplegic and non-cycloplegic sub-groups. Amount of under-correction may be more than what is really applied in cycloplegic group, as it has been reported myopia progression significantly decreased with increasing amount of under-correction.31 Besides, it has been approved as if under-correction continues, then myopia progression shows higher degree of reduction.55

Moreover, our findings show that, the results of two studies with cycloplegic refraction31,37 could not be comparable with other four non-cycloplegic studies.6,30,33,38 The reason may be that Li31 and Sun,37 in their studies, have applied different amount of under-correction, and just in one study31 the exact rate of progression was reported based on each amount of under-correction, which was a limitation in Sun study. Sun et al.37 have just reported the myopia progression for no correction group and compared it with full-correction. Another reason concerns the definition of under-correction could be the cause of error. In Li31 study, under-correction of myopia was defined as improved presenting visual acuity for at least 2 lines with subjective refraction. Using this criterion of under-correction of myopia may not have detected all under-corrected children because myopes may possess reduced blur sensitivity. Lastly, further randomized control trials investigations are needed using cycloplegic refraction, which in essence serve as the gold standard for precise refraction, to understand better the relation between amount of correction and myopia progression.

This study has also some limitations. Possible heterogeneity has already been reported in all meta-analysis, especially those which evaluate the pooled estimate. Our results showed the high heterogeneity which is likely to arise through differences in inclusion criteria, amount of under-correction and types of refraction. Also, the studies quality might affect the heterogeneity of the results. Therefore, any conclusion was limited by the characteristics of the studies. Otherwise, heterogeneity can be considered as an important drawback in meta-analysis to find the variables associated with variation across the primary studies can help future research on a topic. Publication bias is a major concern in all systematic reviews. The only method in order to avoid publication bias is searching a broad number of databases to locate all possible studies. We did our best to increase the sensitivity of our search strategy. However, publication bias is a major concern in all systematic reviews and cannot be avoided altogether. Citation bias is always a concern in systematic reviews. However, we only used citation analysis as an adjunct to the main search strategy.

The myopic eyes which are fully corrected with non-cycloplegic refraction with maximum plus sphere, are less prone to myopia progression, in comparison with those which were under corrected. Although, our results show statistically significant difference in myopia progression comparing under-correction and full-correction, in either cyclo or non-cyclo groups, this difference might not be clinically considerable. Further studies are warranted to provide reliable evidences in this regard, particularly with cycloplegic refraction.