Refractive error correction among urban and rural school children using two self-adjustable spectacles

Introduction

Access to primary eye care services is essential to prevent avoidable visual impairment (VI) and blindness in children. Global estimates in 2010 indicated that 19 million children below the age of 15 years have VI, out of which 12 million are due to refractive error and more than two-thirds of them are found in low-income and middle-income countries.1 Compared with the other major causes of VI, refractive error sets in at a younger age, and constitutes the most prevalent cause of disabling visual disorder in children.1 Therefore, if not corrected, refractive error causes more blind-years than other causes in school-going children, particularly those from low-income and middle-income countries, who represent the greater portion of the vulnerable group with significant problems of undetected or uncorrected refractive error,2–5 leading to considerable impact on their participation and learning in class.3 According to the WHO, in low-income and middle-income countries, children with vision loss are 2–5 times less likely to be in formal education, but refractive error correction by spectacles can reduce the odds of failing a class by 44%.6

However, in many low-income and middle-income countries, various barriers prevent many children with refractive errors from accessing eye health services and achieving the classroom success. These barriers include the cost of spectacles,7 concerns about cosmetic appearance8 and inadequate understanding of the benefits of refractive correction among children, parents and teachers.9 10 In addition, although refractive error may be safely and effectively corrected with spectacles, there is a lack of well-trained optometrists and ophthalmologists in settings of limited resources.11 12 To help solve the problems associated with the traditional way of correcting refractive error, self-refracting spectacles (SRSs) or adjustable spectacles were developed to provide a simple, effective and affordable means of vision correction whereby it is possible for the wearer to adjust the refractive power of each lens independently to optimise his or her vision. This approach makes vision correction accessible to those in areas of the world where there is either a lack of professionally trained optometrists or where the cost of traditional spectacle lenses and professional consultation is expensive. Specifically, SRSs have been recommended for school children in rural communities who have no or little access to refractive services.13 14 Examples of such commercially available SRS are eye adjusters (Eyejusters 2015, Oxford, UK), Adspecs, (Centre for Vision in the Developing World, Oxford, UK), Superfocus device (Superfocus, Van Nuys, California, USA), Adlens (Eyejusters 2015, Oxford, UK) and FocusSpecs (Focus-on-Vision, Eindhoven, Netherlands).

Notwithstanding the intended use of SRS, to be able to use the SRS effectively, the child must understand and perform the self-adjustment according to a manufacturer instruction. However, commercially available SRSs have different optical designs (usually Alvarez and fluid-filled) and mechanisms of administering the self-refraction, which may affect the refractive and visual outcome. Though few studies15–18 have compared the accuracy of some SRS to the traditional way of refraction, to the best of our knowledge, no study has investigated the influence of refraction experience among school children of different settings on the refractive and visual accuracy of SRSs.

As known, the burden of VI caused by refractive error is not evenly distributed across and within countries, as over 70% of VI due to refractive error are borne by low-resourced countries and rural settings.19 20 A recent study found that myopia affects about 1 in 20 African school children and in Ghana, the reported myopia prevalence ranged from 1.7% to 22.6%.21 Another study found that more than 72% of VI in school children was caused by inadequately corrected refractive error.20 Urban and rural school children can be said to have different experiences in terms of access to refractive services, compliance and acceptance of recommended optical wear.18 In effect, urban school children may be more likely to be efficient in the use of the SRS compared with their rural counterparts. Given these differences, it is essential that the relative self-refraction experiences of urban and rural children are measured. This study, therefore, compared the accuracy and refractive outcome of two self-refraction spectacles among rural and urban school children to results obtained from three traditional refraction modalities: cycloplegic subjective refraction (CSR), non-cycloplegic refraction and autorefraction. The finding will be relevant in tailoring specific SRS instructions, design and mechanism of refraction to suit the population of interest and promote the use of adjustable spectacles to meet the visual needs of school-going children in low-income and middle-income countries who may have limited access to comprehensive refractive services.

Materials and methods

Self-refracting spectacles

Two Alvarez SRS, with significant differences in instructional protocol and optical designs, were used. They both give variable spherical power and are fabricated for relatively low-cost markets. The first of these, the FocusSpec (Focus-on Vision, www.focus-on-vision.org/) is an antireflection coated, polycarbonate lenses (refractive index 1.59) and is available with a power range of −1.00 to −5.00 D for myopes and +0.50 to +4.50 D for hyperopes (see figure 1). The second design, the Adlens Adjustables (Adlens, https://adlens.com/uk/) also uses polycarbonate lenses and has a power range −6.00 to +3.00 D. Both designs have nominally zero astigmatic power at all spherical power settings. The assumption is that the wearer will, by observing a task at the appropriate distance, be able to adjust the spherical power of the lenses to obtain optimal correcting power and vision. Both designs have individual adjustments for the right and left eye lenses, so that the needs of anisometropes are catered for, although at the expense of an increased time requirement for optimising the powers. Both self-refraction spectacles are commercially available and remain stable on the same dioptre after setting. Figure 1 shows a picture of the two SRS, FocusSpec (A) and Adlens (B)

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Figure 1

Picture depicting the two SRS, FocusSpec (A) and Adlens (B). SRS, self-refracting spectacles.

Results

Distribution of VA obtained without correction and with the different methods of refraction correction

The median and mean uncorrected VA in the better-seeing eye for urban schools were 0.3 (6/12) and 0.27 (roughly equivalent to 6/12) while that for rural schools were 0.2 (6/9.5) and 0.26 (roughly equivalent to 6/12), respectively. For the worse-seeing eye, the median and mean uncorrected VA for the urban schools were 0.6 (6/24) and 0.57 (roughly equivalent to 6/24), while that for rural schools were 0.5 (6/19) and 0.53 (roughly equivalent to 6/19), respectively. The median and mean VA which accounted for similar VA in both eyes among urban schools were 0.45 (between 6/19 and 6/15) and 0.514 (roughly equivalent to 6/19) while those among rural schools were 0.3 (6/12) and 0.35 (between 6/15 and 6/12), respectively. Inspection of the distribution of VA between the right eyes and left eyes showed a similar distribution, so analysis of the results was done using the right eye only. Details of visual acuities obtained for rural and urban children without correction and with the different methods of refraction are presented in figure 2. Figure 2 also shows the proportion of children who achieved VA of ≥6/7.5 with self-refractions using FocusSpecs, Adlens, autorefraction and with CSR (as the standard) among those in urban schools were 92.6%, 92.4%, 60% and 92.6%, respectively, while those in rural schools were 81.6%, 86.2%, 54.0% and 95.4%. The median VA for those with similar VA across all methods of refraction in both eyes was 6/6 (0.0). A Wilcoxon signed-rank test conducted to determine if there was a statistically significant difference in the visual acuities obtained between those in urban and rural schools against the different methods of refraction showed that VA distributions differed from each other among rural and urban children (p<0.001) and among the different methods in rural children (p<0.001). Improvement over uncorrected VA in the right eye was greater in urban children than rural children for both self-refraction methods.

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Figure 2

Distribution of visual acuity (logMAR) in the right eye without correction and achieved with the different methods of refraction (top panel) and proportion who achieved visual acuity of ≥6/7.5 with the different methods of refraction (bottom panel) among the rural and urban children. logMAR, logarithm of the minimum angle of resolution.

Comparison of self-refraction to clinical standard

The mean SE refractive powers found in all the subjects for FocusSpec, Adlens, autorefraction and CSR were −0.75±0.57 D, −0.74±0.54 D, −0.49±0.01 D and −0.51±0.33 D, respectively.

The mean difference between the SE refractive error for FSSR and ASR was not significant (Wilcoxon signed-rank test; Z=−1.078, p=0.281), with a mean difference of −0.02±0.61 D, and a 95% limit of agreement between −0.10 and +0.10. The mean difference in SE refractive error between FSSR and autorefraction was significant (Z=−5.999, p<0.0001), with a mean difference of −0.26±0.044 D, and a 95% limit of agreement between −0.38 and −0.14. Similarly, the mean difference between ASR and autorefraction was also significant (Z=−4.785, p<0.001), with a mean difference of –0.24±0.04 D, and 95% limit of agreement between −0.35 and −0.14. The mean difference between FSSR and CSR was significant (Z=−5.071, p<0.001), with a mean difference of – 0.24±0.05 D, and 95% limit of agreement between −0.39 and −0.10. The mean difference between ASR and CSR was also significant (Z=−4.212, p<0.0001), with a mean difference of – 0.22±0.049 D, 95% limit of agreement between −0.35 and −0.10.

Figure 3 shows the distribution of SE refractive power indicating the range of myopia and hyperopia measured and figure 4 shows Bland-Altman plots comparing the SE refractive error of both self-refractions against CSR (as the gold standard) in right eye for children in urban and rural schools. Negative values indicate myopia and positive values, hyperopia. Overall, the mean±SD SE errors in urban schools for FSSR, ASR and CSR were −1.05±0.61 D, –0.97±0.58 D and −0.78±0.53 D while those in rural schools were −0.47±0.51 D, –0.55±0.43 D and −0.27±0.11 D, respectively. The difference between FSSR and ASR in urban schools was not statistically significant (one sample t-test, t=0.850, p=0.3966 and a 95% limit of agreement between −0.1059 and 0.2659), with a mean difference of −0.08±0.094 D. The difference between FSSR and ASR in rural schools was also not statistically significant (t=−1.119, p=0.265 and a 95% limit of agreement between −0.2212 and 0.0612), with a mean difference of −0.08±0.07 D. The difference between FSSR and CSR in urban schools was statistically significant (t=2.988; p=0.0033; 95% limit of agreement between −0.0916 and 0.4484), with a mean difference of −0.27±0.09 D while the difference between FSSR and CSR in rural schools was also statistically significant (t=3.596; p=0.0004; 95% limit of agreement between 0.0902 and 0.3098), with a mean difference of −0.20±0.06 D. The difference between ASR and CSR in urban schools was statistically significant (t=2.163; p=0.0320; 95% limit of agreement between 0.0165 and 0.3635), with a mean difference of −0.19±0.09 D while the difference between ASR and CSR in rural schools was also statistically significant (t=5.918; p<0.0001; 95% limit of agreement between 0.1866 and 0.3734), with a mean difference of −0.26±0.047 D.

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Figure 3

Distribution of spherical equivalent refractive among urban (top panel) and rural (bottom panel) children measured by FocusSpecs self-refraction, Adlens self-refraction, autorefraction and cycloplegic subjective refraction.

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Figure 4

Shows Bland-Altman plots comparing the spherical equivalent refractive error of both self-refractions methods against cycloplegic subjective refractive spherical equivalent refractive error among urban (left panels) and rural (right panels) school children. Difference between the measurements is plotted on the vertical axis, and their mean is plotted on the horizontal axis. The middle horizontal line represents the mean difference and the two horizontal lines, one above and the other below, are the 95% limits of agreement between measurements. ASR, Adlens self-refraction; CSR, cycloplegic subjective refraction.

Discussion

A number of studies13 17 23 have indicated that a range of good refractive results and VA were achievable with self-refraction in children although variations in the results reported. The discrepancies in the accuracy of self-refraction reported in their results may be attributed to several factors including the background and refraction experience of the subjects. The current investigation was, therefore, carried out to assess the accuracy of two commonly available self-refraction spectacles in urban and rural school children in the Central Region of Ghana.

The distribution of refractive error among the study population showed that myopia was more common in urban children (54.4%), while hyperopia was more common in rural children (58.0%). The differences in the prevalence of refractive error could be attributed to modern lifestyle activities, especially near work with technological devices (eg, computers, laptops, phones and video games) which may cause myopia. On the other hand, less access to activities with demanding near visual activity and nutritional changes in rural children may predispose them to hyperopia.24 25

The results from this study indicate that while the differences in refractive power achieved with the two Alvarez spectacles were not different from each other they were appreciable when compared with the traditional clinical gold standard of refraction in children (ie, CSR). The refractive results from both self-refractions also differed from refraction obtained with autorefraction. The refractive results from both self-refractions also differed from refraction obtained with autorefraction. The results are consistent to a previous study conducted on Chinese children16, which found differences in the methods and reported that SE differed by 1.0 D in either direction from CSR, which was more frequent for self-refraction (11.2%) than non-cycloplegic autorefraction (6.0%). In a study conducted in Ghana among myopes,17 it found that the mean SE refractive error measured by CSR and self-refraction were significantly different by −0.44 D. The study found that more children undercorrected or overcorrected their myopia greater than 0.50 D (15.3%) and greater than or equal to −1.00 D (8.4%) with self-refraction using FocusSpec.

The finding in this study is also consistent with previous studies which indicated that there was a higher propensity for some of the children to either undercorrect or overcorrect their refractive errors by self-refraction, as indicated by relatively high SD. For instance, about 4% and 3% (translating to about six individuals) of the children undercorrected and overcorrected their myopia with FocusSpec, respectively, while about 7% of the children undercorrected or overcorrected the refractive errors over the clinical subjective values using Adlens. Correspondingly, the visual outcomes determined by CSR were better than those by self-refraction, as the former gold standard method included correction for existing astigmatism.17

The results also showed that generally, there were no statistically significant differences in mean SE refractive value correction between the two self-refraction devices in urban and rural children, with negligible mean differences. However, when compared with the clinical standard, the mean differences between FSSR and CSR in urban (−0.27±0.09 D) and rural (−0.20±0.06 D) children were significant. Similarly, there were statistically significant differences in mean SE refractive values between ASR and CSR in urban (−0.19±0.09 D) and in rural (−0.26±0.047D) children. These results revealed that difference in refractive error correction using the two Alvarez self-refraction methods was not different in both urban and rural children. However, when compared with cycloplegia subjective refraction, the refraction differed for all methods in urban and rural children. This finding is similar to previous studies that also reported differences in refractive outcomes between self-refracting methods and clinical standard of CSR due to mechanical and optical limitations of adjustable glasses.18 23 25–27 The results, however, are contrary to suggestions that children from less socioeconomic background are less likely to accurately perform self-refraction compared with their counterparts from more privileged backgrounds.17 It is suggested that children from less resourced environments, who are likely not to habitually wear spectacles were at greater risk for less accurate results with adjustable glasses possibly because such children are more tolerant of imperfectly corrected VA and were thus less inclined to carefully adjust the SRSs until optimal VA had been achieved.17 18

In addition, in this study, the findings revealed that refractive correction using the two self-refraction spectacles were similar and produced similar SE refractive values within acceptable margins of optical error for Alvarez designs,28–30 irrespective of the background of the end user. This was despite the fact that the study found low spectacle wear among the children, with only about a quarter of the study population reporting wearing spectacle, and nearly two-thirds (72.5%) of those wearing spectacles were from urban areas compared with slightly over one-third (27.5%) of rural children. The finding does not support our presumption that refraction and spectacle wear experience could influence self-refraction regarding appreciation of power changes.

On the other hand, the finding observed could be associated with the relative ease of manipulating the two adjustable devices, even though the styles of manipulating the adjustable are different. In performing self-refraction, the instruction for FocusSpec requires scrolling the wheel to adjust the lens power, with about 2.5 turns of the adjustment screw producing a 4D power change. That translates to a setting accuracy of 0.25 D which corresponds to screw scrolling of about 60°. On the other hand, Adlens design requires dialling or twisting the wheel on the sides to adjust the lens power, with about five complete rotations producing a 9.0 D power change. That translates to a setting accuracy of 0.25 D corresponding to screw rotations of about 50°.29 These angular rotations are quite large so that, from the purely mechanical point of view, no great skill is required when adjusting the lens powers. This perhaps explains why there was no significant difference in results among the children in adjusting for the lenses, by dialling or twisting the wheel on the sides to the Adlens adjustable or scrolling the wheel on the sides of the FocusSpec adjustable to recognise changes in refractive power. The results also revealed that refraction among myopes from both urban and rural schools across all refraction methods were statistically not significant, but there were statistically significant differences in mean SE refractive value obtained in hyperopes from both urban (FocusSpec and CSR, −0.28 D; Adlens and CSR, −0.28 D) and rural schools (FocusSpec and CSR, −0.35 D; Adlens and CSR, −0.38 D). This finding perhaps indicated that the children easily appreciated differences in minus correction than spherical blur plus corrections. Indeed, inspection of that data showed that myopes from urban schools tended to over minus with self-refraction compared with rural children, with the over minus more with FocusSpec (more myopic) than with Adlens.

Although a comparison of the distribution of refractive error values obtained is an easier, simple and objective way to compare the refractive modalities, the range of visual acuities achieved is in many ways of greater clinical significance. Inspection of the VA outcomes after refraction showed that the proportion of urban children who achieved VA of ≥6/7.5 for all refraction methods were more than rural children, except CSR. In percentage terms, 11%, 6.2% and 6.0% more urban children obtained better VA with FocusSpec, Adlens and autorefraction, respectively, than in rural children. For urban children, when the self-refraction methods were compared against each other and to the clinical gold standard, there was no difference in the proportions who achieved the expected VA of ≥6/7.5 in urban schools. In contrast, for rural schools, the proportion of children failing to achieve VA of ≥6/7.5 with both self-refractions differed significantly from each other (<5%), and from the proportion failing to achieve the same VA with CSR for FocusSpecs (13.8%) and Adlens (9.2%). Self-refraction outcome was, however, better when autorefraction results are compared with results of CSR in urban (32.6%) and rural children (41.4%) or to FocusSpec (32.6%; 27.6%) and Adlens (32.4%; 32.2%) self-refractions in urban and rural children, respectively. The results further showed that more urban school children achieved VA of ≥6/7.5 with both types of self-refraction devices compared with rural children. Improvement in worse VA was better in urban school children across all methods. Indeed, this study is the first to report on the refractive and visual outcome among rural and urban children in the same study, other studies have reported in urban15 and rural16–18 30 31 children separately.

While greater proportions of children that achieved desired visual acuities have been reported elsewhere,15–17 the proportion that achieved VA of ≥6/7.5 in this study is consistent with findings in urban15 and rural children.16 In reported studies in China using Adspecs self-refraction spectacles (fluid-filled lenses of refractive index 1.579) found the proportion of urban children with VA of 6/7.5 in the better eye with habitual correction, self-refraction, non-cycloplegic autorefraction and CSR were 34.8%, 92.4%, 99.5% and 99.8%, respectively, while another study among rural children found the proportion with similar VA to be 5.2% for uncorrected vision, 30.2% for currently worn spectacles, 96.9% for self-refraction, 98.4% for automated refraction and 99.1% for subjective refraction.15 16 In Ghana, a previous study in Ghana in myopic school children indicated that the proportion of children correctable to greater than or equal to 6/7.5 in the better eye by CSR was 99.0%, followed by cycloplegic retinoscopy (94.1%) and self-refraction was 85.2%.18 It must be noted that while the proportion of rural and urban school pupils in China who achieved the desired VA with self-refraction and clinical standards were similar, the study in Ghana found substantial difference in VA outcomes between the refractive methods, perhaps due to refraction experience of the subjects used. While the findings on the value of SE is consistent with other studies, and relatively similar for both self-refraction devices used, the difference in the visual outcome between rural and urban may be due to differences in refraction experience. In China, both rural and urban children may have relatively better access to refractive services, and therefore, have comparable refraction experience.

The results and the implications of this study must be understood within the context of its limitations. As indicated, self-refraction spectacles are incapable of correcting high astigmatism, which resulted in the exclusion of children with high astigmatic error from the study. Again, the limits of spherical correction for FocusSpecs are +0.50 to +4.50 and −1.00 to −5.00 D, and+3.00 to −6.00D for Adlens meant that the devices could not be used to correct very high amount of refractive errors. This may have resulted in failure to achieve ≥6/7.5 as seen in some cases. A larger sample size could have resulted in greater percentages of agreements with more predictive power. In addition, a reliability study using intraobserver measurement (test, retest) and interobserver measurement was not performed due to the time and scheduling constraint as the study was performed during school hours. Despite the study’s limitations, the results here are able to provide previously unavailable comparative data on the potential use of different self-refraction spectacles to improve vision in young people in urban and rural schools who carry a considerably large burden of inadequately corrected refractive error. The findings also show that self-refraction produces better refractive and visual outcome than that of autorefraction, which seems to indicate that their results may be less prone to accommodative inaccuracy than non-cycloplegic autorefraction,15 another modality advocated for use in areas where access to eye care providers is limited.

Conclusion

The study revealed that the background and refraction experience of school children did not significantly affect self-refraction, however, the results differed when compared with the clinical standard in urban and rural schools. For the visual outcome, the results revealed that, there was no difference in the proportion of children who obtained the desired VA with both self-refraction and the clinical standard in urban children, but the proportion that obtained the desired VA significantly differed among rural schools. The results indicate that both self-refraction devices are capable of achieving acceptable refractive accuracy and VA outcomes in children from low-resourced environment, but an easier mechanism of self-adjusting the spectacles must be taken into account to match clinical standards and ensure high wearing compliance.