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Prevalence of Amblyopia and Refractive Errors in an Unscreened Population of Children

Polling, Jan-Roelof*; Loudon, Sjoukje E.; Klaver, Caroline C. W.

doi: 10.1097/OPX.0b013e31826ae047
Clinical Report

Purpose: To describe the frequency of refractive errors and amblyopia in unscreened children aged 2 months to 12 years from a rural town in Poland.

Methods: Five hundred ninety-one children were identified by medical records and examined in a standardized manner. Visual acuity was measured using LogMAR charts; refractive error was determined using retinoscopy or autorefraction after cycloplegia. Myopia was defined as spherical equivalent (SE) ≤−0.50 D, emmetropia as SE between −0.5 D and +0.5 D, mild hyperopia as SE between +0.5 D and +2.0 D, and high hyperopia as SE ≥+2.0 D. Amblyopia was classified as best-corrected visual acuity ≥0.3 (≤20/40) LogMAR, in combination with a 2 LogMAR line difference between the two eyes and the presence of an amblyogenic factor.

Results: Refractive errors ranged from 84.2% in children aged up to 2 years to 75.5% in those aged 10 to 12 years. Refractive error showed a myopic shift with age; myopia prevalence increased from 2.2% in those aged 6 to 7 years to 6.3% in those aged 10 to 12 years. Of the examined children, 77 (16.3%) had refractive errors, with visual loss; of these, 60 (78%) did not use corrections. The prevalence of amblyopia was 3.1%, and refractive error attributed to the amblyopia in 9 of 13 (69%) children.

Conclusions: Refractive errors are common in Caucasian children and often remain undiagnosed. The prevalence of amblyopia was three times higher in this unscreened population compared with screened populations. Greater awareness of these common treatable visual conditions in children is warranted.



Departments of Ophthalmology (JRP, SEL, CCWK) and Epidemiology (CCWK), Erasmus Medical Center, Rotterdam, the Netherlands, and Department of Optometry and Orthoptics, University of Applied Sciences, Utrecht, the Netherlands (JRP).

Refractive errors and amblyopia are the most common causes of visual loss in children.1–6 Frequencies, however, show large differences around the world.7 Methods of detection vary widely, ethnicities differ, some countries have a screening program, and most countries use different methods for measurement of visual acuity.7–12

A number of studies report on prevalence of refractive error and myopia in children, and they generally find differences in prevalence of myopia according to age and ethnicity.12–16 Two studies in the United Kingdom including Caucasian children aged 6 to 7 years and 12 to 13 years, respectively, found a myopia prevalence of 2.8 to 5.7% in the youngest and 17.7 to 18.6% in the older age-group.4,13,17 South Asian children had significantly higher prevalence: 10.8% in those aged 6 to 7 years and 36.8% in those aged 12 to 13 years. Ojaimi et al. also studied schoolchildren aged 5 to 8 years in Australia and found an overall myopia prevalence of 1.4%. They found a significant difference between white European children (0.79%) and those belonging to other ethnicities (2.73%, p < 0.001).17 Ip et al. studied the same Sydney Myopia Study children aged 11 to 14 years and found an overall myopia prevalence of 11.9%. Large differences in prevalence were found between European Caucasian (4.6%) and East Asian (39.5%) children.18 Other Asian studies found high myopia prevalence varying between 15 and 25% at the age of 10 years.14

There are some studies that investigated refractive error in Polish children. Czepita et al. studied myopia in rural children aged 10 to 14 years from the southeast part of Poland and found a myopia prevalence of 6.3% at the age of 10 years increasing to a prevalence of 9.7% at the age of 12 years.6 In another Polish study in semirural population of children aged 6 to 18 years, the prevalence of myopia was slightly higher: 11.3% in those aged 10 years to 14.4% in those aged 12 years.6,19 However, these may be overestimates, as neither study used full cycloplegia to estimate refractive error.

Studies on the frequency of amblyopia have been carried out as well. Remarkable is the wide variation in criteria used for amblyopia.20 Consensus criteria defined by a joint classification are best-corrected visual acuity ≥0.3 (≤20/40) LogMAR in the affected eye, no underlying structural abnormality of the eye or visual pathway, a 2 LogMAR line difference between the two eyes, and the presence of an amblyogenic factor.21 A clinic-based study among Polish immigrants in the United States using different criteria found an amblyopia percentage as high as 9%.22 Studies using the consensus criteria generally found an amblyopia prevalence of ∼2.5 to 3% in populations without a vision screening program, whereas a prevalence of 0.8 to 1.1% was found in populations with these programs.2,23 Apart from criteria, methodology of screening also varies widely between countries.24–27 Some countries use visual acuity to screen for amblyopia, whereas others only screen for amblyogenic risk factors such as anisometropia.26,28–31 Most countries use their own visual acuity charts, which generally lack good internal and external reproducibility.2,20,27 All these factors are known to distort prevalence estimates of amblyopia.2 Vision screening alone detects amblyopia or refractive errors in need of correction but is not successful in detecting refractive errors per se.32

The aim of the current study was to determine the prevalence of refractive error and amblyopia in unscreened young Polish children of the same ethnicity. The examination included cycloplegic refraction in all children and visual acuity testing in those old enough to be screened using the internationally accepted LogMAR chart. We used consensus criteria to define amblyopia, and explored its prevalence and causes.10

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Study Population

The Mieroszów eye project is a cross-sectional population-based study including children aged 2 months to 12 years from Mieroszów, a village located in the southwest of Poland. The village is rural, has a low population density (7582 inhabitants on 76 sq km of land), and has a lack of full medical health service.33 Six hundred twenty-eight children were identified by medical records from the only general practitioner in the village. All children were of Caucasian origin. The research protocol adhered to the Declaration of Helsinki for research involving human subjects, and informed consent was obtained from all parents and guardians before the examination.

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Eye Examination

The eye examination took place at the Mieroszowski Centrum Kultury in the center of Mieroszów. A complete medical history was obtained, with assistance of Polish medical students. Three trained ophthalmic nurses, three orthoptists, and one optometrist performed complete ophthalmological examination. Monocular visual acuity measurement was preformed using LogMAR-based charts at 3 m distance. Visual acuity was tested in all cooperative children aged ≥2 years. The type of chart depended on the age of the child: Lea Hyvärinen symbols were used for those aged 2 to 3 year, HOTV charts were used for those aged 4 to 6 years, and ETDRS letter charts were used for those aged ≥7 years. A linear visual acuity was used, and acuity was scored using the ETDRS-Fast method.34 To pass a line on the chart, three of five symbols or letters needed to be answered correctly. Subjects who generally wore prescription glasses wore them during the test. Those who had ≥0.2 (≤20/32) LogMAR visual acuity were retested with trial glasses after refraction in a trial frame with their full spherical and cylindrical value. In children aged <2 years, visual acuity was scored based on the absence or presence of monocular fixation and pursuit movement. Stereovision was examined using the Lang II test (Lang-Stereotest, Forch, Switzerland) according to the instructions in the information manual accompanying the test. Strabismus was tested using the cover test for near and distance fixation according to standard clinical procedures. Ocular movement was tested using a penlight for near. Refraction was measured after 30 to 45 min of cycloplegia with 1 drop of 1% cyclopentolate instilled in each eye. In children aged 2 to 12 years, refractive error was measured using a Nikon Retinomax 2 autorefractor (Nikon, Japan); in younger or uncooperative children, this was determined by retinoscopy using a Heine retinoscope (Heine Optotechnik, Herrsching, Germany) and lenses according to standard protocols. Ophthalmoscopy was performed using a Keeler binocular indirect ophthalmoscope by the optometrist.

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Clinical Outcomes and Statistical Analysis

Main outcomes of the study were refractive error and amblyopia. Spherical equivalent (SE) was calculated as the sum of the full spherical value and half of the cylindrical value. We used the mean SE of both eyes in the analysis. Myopia was defined as SE ≤−0.50 D, emmetropia as SE between −0.5 D and +0.5 D, mild hyperopia as SE between +0.5 D and +2.0 D, and high hyperopia as SE ≥+2.0 D.15,35,36 Analyses for amblyopia were performed in children who had reliable measurements of visual acuity (i.e., aged 3 years and older in this population). Amblyopia was defined as best-corrected visual acuity ≥0.3 (≤20/40) LogMAR in the affected eye, together with a 2 LogMAR line difference between the two eyes and the presence of an amblyogenic factor.21,27,37 Amblyopia was categorized in three groups: (1) refractive amblyopia due to anisometropia of at least a 1.0 D difference in SE refraction between the two eyes in the absence of strabismus, (2) strabismic amblyopia in the presence of a strabismus or a history of strabismus surgery without anisometropia or high refractive error, or (3) a combination of strabismus and anisometropia.

All statistical analyses were performed using the PASW Statistics 17. Sample means and medians and their mean differences are reported with their range. Frequency differences between continuous and categorical variables were analyzed using Mann-Whitney test and Kruskal-Wallis test, and differences between continuous variables were analyzed using Spearman ρ. Linear regression was used to explore correlations.

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Of the 628 eligible children, 591 (94.1%) consented to examination at the research center. The median age was 7 years (range, 2 months to 12 years), and the gender distribution was equal (51% boys). The number of children and the refractive error defined in categories is presented per age-group in Table 1. Visual acuity increased significantly (Spearman ρ = −0.316, p < 0.001) with age, with a mean of 0.3 at 3 years to −0.04 at 12 years of age. (Fig. 1) The range of the SE was −5 D to +7.75 D, with a median of +1 D. The mean SE for boys was +1.1 D (standard deviation, 1.1) and for girls was +1.2 D (standard deviation, 1.0; p = 0.08). SE showed a significant reduction with age (p < 0.001) from +2 D at 2 months of age to +0.75 D at 12 years of age, with the strongest decrease in hyperopia in the first year of life. The distribution of refractive error by category for all the children is presented in Fig. 2. Of all children, 16.3% (n = 77 of 584) had decreased visual acuity; refractive error was the only cause. Of these 77 children, 13 (17%) had myopia (SE ≤−0.5 D), 2 (4%) had combined astigmatism with a mean emmetropic SE, 20 (26%) had mild hyperopia, and 42 (54%) had high hyperopia. Astigmatism <−0.5 D or more was found in 58 children (9.8%); astigmatism <−1.25 D was found in 19 children (3.2%). Astigmatism showed no relation with age (p = 0.53). Refractive error had not been corrected in 60 (78%) of the 77 children with decreased visual acuity, and wearing glasses did not appear to relate to refractive error (p = 0.72 for difference in SE between those with and those without glasses).

LogMAR visual acuity could not be measured in 164 children (27%) because of young age or non-cooperation. However, all these children had stable fixation and smooth pursuit. Visual acuity could be measured in 95% of children aged >5 years. Of the 420 children with reliable measurements, 13 (3.1%) had amblyopia according to our definition. The average age of the children with amblyopia was 6.9 (range, 3 to 11) years; 11 children were older than 6 years. Amblyopia was caused by strabismus in three, by anisometropia in five, and by combined mechanisms of anisometropia and strabismus in four children. Average visual acuity in the amblyopic eye due to amblyopia due to strabismus and combined mechanism was 0.6 (20/80) LogMAR; average visual acuity in those with amblyopia due to anisometropia was 0.4 (20/50) LogMAR. No ocular abnormalities such as retinopathy of the prematurity, cataracts, or other pathology were found.

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This study in unscreened children living in a rural area of Poland shows that refractive errors are very common and shift toward myopia with age, amblyopia is higher in this unscreened populations than in screened ones, and that uncorrected anisometropia is a prominent cause of amblyopia. Emmetropia occurred only in 12% of children aged <2 years, and increased to 26% in children aged 10 to 12 years. Prevalence of significant hyperopia decreased from 28% in those aged <2 years to 7% in those aged 10 to 12 years. The first occurrence of myopia was at the age of 1 year, and its prevalence increased from the age of 5 years onward to 2.2% in those aged 6 to 7 years and 6.3% in those aged 10 to 12 years. Comparison with earlier studies that had been performed in 10-year-old Polish children from another rural area shows highly comparable data (6.3% in 10-year-old and 9.7% in 12-year-old children).6 The prevalence of myopia, however, was considerably lower than that found in all Asian studies of young children, even in rural areas.14,15,38,39 Of all children, 16.3% (n = 77) had decreased visual acuity due to refractive error, and only a small proportion of these had received correction. There was no difference in refractive error between those who wore glasses and those who did not. Economic reasons may have played a more important role herein than refractive errors per se.

The prevalence of amblyopia in these children was 3.1% (n = 13), almost three times higher than in screened populations.1,2,20,37 The most important single cause of amblyopia was anisometropia.

There are strengths and limitations to this study. Strengths are the large age range, with incorporation of very young children, the high participation rate, the comprehensive methods of visual acuity and refractive error measurements, and the identical ethnic background of all children. Among the limitations is the relatively low number of children in all age-groups.

Normal development of refraction in children varies by genetics, environment, and epoch.10,15,16,39,40 Our study confirms the emmetropization process in the first decade, which is known to be strongest in the first 2 years of life.41,42 A distinct finding of this study is that the decline continues gradually in the years thereafter, with a slight mean hyperopia refractive error at the age of 12 years. For the population at large, visual acuity could be reliably measured from the age of 5 years onward. The mean visual acuity in younger children was <0.1 (20/25) LogMAR, but worse vision at a single examination in this age-group does not necessarily indicate pathology. With our single test, visual acuity measurement was possible in 42% of the 3-year-old, 77% of the 4-year-old, and 95% of the 5-year-old children. More attempts for visual acuity testing would improve this fraction.

After uncorrected refractive error, amblyopia was the most important cause of decreased visual acuity in our study. The amblyopia prevalence of 3.1% was high when compared with that of screened populations.2,5 At present, there is no population-based screening program available in Poland. The degree of visual loss depended on the cause of amblyopia. Amblyopia with visual acuity >0.4 (<20/50) LogMAR only corresponded with anisometropia, whereas amblyopia with visual acuity >0.6 (<20/60) was only associated with strabismus.

What do our findings imply for screening programs in young children? Successful screening can reduce the prevalence of untreated amblyopia (LogMAR acuity >20/50).2 An important factor for success is screening for visual acuity, as screening for refractive error alone will not detect amblyopia caused by strabismus.2,27,30 A beneficial side effect of visual acuity screening is the detection of only the refractive errors that are in need for correction, and not those that do not interfere with visual function.10,32

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Refractive errors are common in very young children and show a myopic shift with age. The prevalence of amblyopia (3.1%) was relatively high in this unscreened Caucasian population. A national screening program including measurement of visual acuity may help reduce amblyopia prevalence. Improving awareness by education of parents, teachers, and health care providers may lead to reduction of uncorrected refractive errors.

Jan-Roelof Polling

Erasmus MC, University Medical Center Rotterdam

’s Gravendijkwal 230

3015 CE Rotterdam

The Netherlands


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We thank the Mieroszów Screening team and the Vision in Poland Foundation for recruitment of participants, logistics, and help in ophthalmologic examination: Ryszard Chmielowski, Piotr Polanski, Andrzej Laszkiewicz, Victoria Chmielowska, Esma Aygün, Heleen Schreuders, Pascal van Rossum, Resan Sa-Ardnuam, Talitha Sa-Ardnuam, Reinier van Petegem, Arnoud den Ambtman, Els Smith, Feike van der Zee, Sjoerd van Dijk, Joanna Luteńko, Dorota Nagórna, Michał Kwapisz, Krystyna Grzymisławska, Anna Nadkańska, Małgorzata Henig, Lucyna Polańska, Ewelina Gąsiorek, Małgorzata Szczepanik, and all local volunteers. This research was supported by Vision in Poland Foundation. The authors have no declared conflict of interest or financial disclosure.

Received January 23, 2012; accepted June 19, 2012.

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vision screening; amblyopia; refractive error; visual acuity in children

© 2012 American Academy of Optometry