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Refraction in Australian Children
Author Response: Refraction in Australian Children
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Barbara M Junghans, Academic University of New South Wales, Australia, Sheila Crewther
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b.junghans{at}unsw.edu.au Barbara M Junghans, et al.
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We are delighted that Prof. Mitchell and his group1 have detected similar or even lower proportions of children with myopia in the early school years than found by us in the same city, as described in 20032 and more recently in 2005.3 We also note that we were able to report a similarly low proportion of children with myopia despite Prof. Mitchell's previously forcefully expressed view that cycloplegic refractions are more meaningful and a better representation of the prevalence of refractive error (see prepublication history3). The difference between the studies due to the effects of cycloplegia can be determined by comparing the difference in mean spherical refractive error for a given age group in each study. Thus, approximately 0.60 dioptres more hyperopia was found in the children under cycloplegia. Hence, those children who presented with exactly a -0.50 D refractive error in our non-cycloplegic studies (refer to our tables2,3) would most likely have appeared 0.60 more hyperopic, that is emmetropic, had a cycloplegic been used. By subtracting those children from the "equal to or more than 0.50 D myopic" group, only 2.3% of the 954 6-7-year-olds seen between 1990-2004 would be estimated to have myopia of any degree (including 0.25D) had a cycloplegic been used. Secondly, we were surprised to note that Prof. Mitchell's group have made little of the fact that relatively low levels of myopia have also been reported by several other large studies in Australia over the last 30 years,4-6 reinforcing our view3 that environmental influences are likely to be an extremely important factor driving the apparent epidemic of myopia worldwide. Although the early studies assessed children generally from the socio-economic extremes, and hence precluded judgements regarding the prevalence of refractive error in young Australian school children, our more recent investigations in Sydney were designed to include a much more diverse unselected non-clinical group of children7 – a total of 4258. The combination of such large numbers and a participation rate of well over 90% and the fact that this population includes some children examined recently and some examined 10 years ago gives further credence to our contention that there is no epidemic of myopia in Australia. We are also curious to know whether the 21% of the target population missed in the current study involved a considerable number of children already under eye care, as this may significantly influence interpretation of the data from the viewpoint of population statistics. Barbara M. Junghans1 1School of Optometry and Vision Science, University of New South Wales, Sydney, Australia References 1. Ojaimi E, Rose KA, Morgan IG, et al. Distribution of ocular biometric parameters and refraction in a
population-based study of Australian children. Invest Ophthalmol Vis Sci. 2005;46:2748-2754. |
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Paul Mitchell
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paul_mitchell{at}wmi.usyd.edu.au Paul Mitchell
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In our opinion, accurate estimates of the prevalence of myopia in young people critically depend upon cycloplegic refraction. The approach used by Dr. Junghans and Dr. Crewther in their letter to correct their non-cycloplegic measures to the cycloplegic gold standard is flawed, because they have assumed that the difference between cycloplegic and non-cycloplegic refractions is uniform across the entire range of spherical equivalent measures. We have both cycloplegic and non-cycloplegic refractions in over 200 children from our 6-7-year-old sample. The differences are relatively small for myopic children, but are major and quite variable for emmetropic and hyperopic children. These observations are completely consistent with those from the Shunyi RESC study.1 There is thus no simple algorithm to convert non-cycloplegic to cycloplegic measures. This appears to be also true in our 12-year-old sample, currently being reported. These data suggest to us that cycloplegic refraction must now be considered obligatory in studies of school-aged children for definitive data. Whether the prevalence of myopia is increasing in Australian school children cannot be established from the existing data, because of the absence of a solid baseline from which to make comparisons. This is being provided by our studies across different age groups, and planned follow-up of these cohorts. There are some indications of a possible increase, and a trend is seen even in the data of Dr. Junghans and Dr. Crewther.2 This is acknowledged rather obscurely in their paper as "a trend towards an increasing shift away from the hyperopic refraction in the later cohort." This may indicate that the increasing prevalence and earlier onset seen in urban East Asia may be starting to happen in Australia as well, although it is clear that changes of the magnitude seen in East Asia have not, as yet, taken place. Our data suggest that the prevalence of myopia in Australia is lower than in other parts of the world that are comparable in terms of urbanization and educational performance, even after matching for ethnicity. For example, the prevalence of myopia in 6-7-year-old children of East Asian origin in Sydney is 3.4%, which dramatically contrasts with the 25% prevalence of myopia in similarly-aged Chinese children in Singapore.4 To tease apart the complicated interaction of location, ethnicity, education and lifestyle requires precise measures of the factors involved, necessitating a sophisticated sampling framework. Even allowing just for the contribution of a changing ethnic balance to changes in prevalence requires a precise identification of ethnicity in the sample, rather than reliance on general statistical data and ethnicity inferred from family names. We have collected risk factor data in a detailed questionnaire which hopefully will provide some clues as to why the prevalence of myopia is different between ethnic groups in Australia and why, in age- and ethnicity-matched children, it is much lower in Sydney than in the urbanized environments of East Asia. Ethical considerations preclude us from collecting specific information about the children who declined to participate in the Sydney Myopia Study. There was, however, no indication of preferential non-participation by those with glasses, based on careful scrutiny of children in their classrooms. In addition, a significant proportion of the participants did have some form of eye care, as we have documented.5 While selective non-participation as mentioned by Dr. Junghans and Dr. Crewther needs to be considered, it is also important to include visually impaired students in special schools, units and classes in developing population statistics. Because of the precise documentation of our samples, we will be able to weight our data to give a comprehensive picture of the eye health of young Australians in urban environments. Kathryn A. Rose1 1School of Applied Vision Sciences, Faculty of Health Sciences, University of Sydney, Australia References 1. Zhao J, Pan X, Sui R, Munoz SR, Sperduto RD, Ellwein LB. Refractive Error Study in Children: results from Shunyi District, China. Am J Ophthalmol. 2000;129:427-435. |
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