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Optometry & Vision Science:
doi: 10.1097/OPX.0000000000000221
Original Articles

Prevalence of High Astigmatism in Children Aged 3 to 6 Years in Guangxi, China

Xiao, Xin*; Liu, Wei-Min; Ye, Ying-Jia; Huang, Jian-Zhong; Luo, Wu-Qiang; Liu, Hong-Ting; Lin, Quan; Zhao, Wu-Xiao; Lin, En-Wei

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Author Information

*MPH

MSc

MBBS

Visual Science and Optometry Center of Guangxi, People’s Hospital of Guangxi Zhuang Autonomous Region, Nanning, Guangxi, China (XX, W-ML, J-ZH, WQ-L, H-TL, QL, W-XZ, E-WL); and Wuhan Aier Ophthalmological Hospital, Wuhan, Hubei, China (Y-JY).

Wei-Min Liu Visual Science and Optometry Center of Guangxi People’s Hospital of Guangxi, Taoyuan Rd Nanning, Guangxi 530021 China e-mail: lwm3s@yahoo.com.cn

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Abstract

Purpose: To investigate the prevalence and type of high astigmatism among children aged 3 to 6 years in Guangxi, a relatively undeveloped province in western China, and to examine the correlation between astigmatism and visual acuity.

Methods: Children aged 3 to 6 years in Nanning, the capital of Guangxi Province, participated in a population-based survey using a cluster random sampling technique. Eye examinations included autorefraction, visual acuity measurements, and assessments of the external eye, anterior segment, media, and fundus. Data for the right eyes were analyzed.

Results: Among the 2304 children examined, the overall prevalence of high astigmatism (≥1.25 diopters by noncycloplegic SureSight autorefraction) was 12.7% (95% confidence interval, 11.3 to 14.0%). The age-specific prevalences of high astigmatism in 3-, 4-, 5-, and 6-year-old children were 13.8, 13.2, 12.9, and 8.1%, respectively. The prevalence of high astigmatism did not vary with age or gender (p > 0.05). The majority of cases of high astigmatism were with-the-rule astigmatism (82.9%), followed by against-the-rule (12.6%) and oblique (4.5%) astigmatism. A linear correlation was found between astigmatism magnitude and visual acuity (logMAR acuity = 0.068 + 0.055 × astigmatism) in all participants. Multiple linear regression analysis further showed that the correlation of astigmatism with visual acuity was magnitude dependent (β = 0.240). When with-the-rule astigmatism was used as a reference group, against-the-rule astigmatism (β = 0.137) and oblique astigmatism (β = 0.154) were closely correlated with visual acuity.

Conclusions: High astigmatism was moderately prevalent among children aged 3 to 6 years in Guangxi Province. With-the-rule astigmatism was the dominant form of astigmatism. Magnitude- and orientation-dependent correlations of astigmatism with visual acuity were confirmed.

Increasing evidence has revealed that astigmatism caused by unequal power in the two orthogonal planes (meridians) is a leading cause of reduced visual acuity among children younger than 7 years.1,2 Astigmatism in early childhood is associated with the development of amblyopia3–5 (especially for meridional amblyopia) and worsening myopia.6–8 Meridional amblyopia resulting from astigmatism can further reduce visual performance, such as contrast sensitivity, stereopsis, letter acuity, grating acuity, and Vernier acuity.9,10 To improve our understanding of the etiology and epidemiology of astigmatism, it is important to know the distribution of astigmatism among children.

Previous epidemiological investigations have documented the prevalence of astigmatism among children of different ethnicities. A high prevalence of astigmatism was observed in children of East Asian11,12 and Native American13 descent, and the distinguishing eyelid morphologies14,15 and orbital structures16 exhibited by these races have been postulated as a risk factor for astigmatism.11,17 In China, the prevalence of astigmatism in children is relatively high8,18–22 and ranges from 6.819 to 21.1%8 between different geographical regions. However, most of the previous studies concerning the prevalence of astigmatism among Chinese children were conducted in Hong Kong,8,20 Taiwan,18 and the eastern or coastal parts of China.19,21 Little data are available on the prevalence of astigmatism in children from western China.23

Western China, which is a relatively undeveloped area of China, includes 12 provinces and accounts for one-fourth of the total Chinese population.24 Guangxi Province, a transportation hub of the ASEAN (Association of Southeast Asian Nations) in western China,25 is a typical undeveloped area in western China. The nominal gross domestic product per capita in Guangxi Province in 2011 was ranked 27th among the 31 provinces in China.24 To date, there have been no reports regarding the prevalence of astigmatism in children from Guangxi Province or its capital, Nanning City; therefore, an assessment of the prevalence of astigmatism among children in these areas is needed.

We conducted a cross-sectional study to examine the prevalence and type of high astigmatism in children aged 3 to 6 years in Nanning City, Guangxi Province. In addition, we explored the correlation between the magnitude and axis of astigmatism and visual acuity using simple and multiple linear regressions.

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METHODS

Subjects

This study followed the tenets of the Declaration of Helsinki and was approved by the institutional review board of the People’s Hospital of Guangxi Zhuang Autonomous Region. Written informed consent was obtained from the parents of all participants, and verbal assent was obtained from the children before the examination. From February to August 2011, we surveyed children aged 3 to 6 years, who were enrolled in kindergarten, in Nanning City using a random sampling method. The sample size was estimated using the formula: n = Z2 ρ (1ρ)/B2 (where ρ = 0.6, B = 0.03, and Z = 1.96). With a random cluster sampling and an assumed no-response rate of 20%,26 a minimum sample size of 1846 was calculated.

The sampling frame consisted of 397 kindergartens in Nanning City. Among 2426 randomly selected children from eight kindergartens, 2304 (participation rate, 95.0%) children completed eye examinations. Approximately 5.0% (122 of 2426) children were excluded for various reasons: 55 refused to participate, 28 were absent when the survey was conducted, 20 could not finish the examination because of lack of cooperation, 15 had other eye discomforts, and 4 submitted incomplete background information.

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Procedures

The ocular examination consisted of noncycloplegic autorefraction, monocular visual acuity measurements, and assessments of the external eye, anterior segment, media, and fundus. Noncycloplegic autorefraction was performed using the Welch Allyn SureSight autorefractor (Welch Allyn, Skaneateles Falls, NY), which has been confirmed to have satisfactory accuracy and repeatability for measurement of astigmatism without cycloplegia.27–29 Measurements were performed using the “child” mode at a working distance of 35 cm. The sphere, cylinder, and axis readings for measurements with a confidence score of 6 or higher were recorded. This instrument can measure astigmatism values ≤3.00 diopters (D) and displays a value of 9.99 for astigmatism out of range (>3.00 D). Visual acuity was tested using an E-letter standard logarithmic visual acuity chart (Shanghai Bolan Optical-Electric, Co., Ltd., Shanghai, China). Visual acuity was expressed in decimal notation and analyzed using a logarithm of the minimum angle of resolution (logMAR) equivalents. Corneal light reflex with the cover/uncover test at 0.5 m was used to exclude children with strabismus. The ocular anterior segment and media were inspected with a slit lamp microscope, and fundus examinations were performed using a direct ophthalmoscope (Beta 200; Heine Optotechnik GmbH & Co., Herrsching, Germany).

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Quality Assurance

Field evaluations using the Welch Allyn SureSight autorefractor for preschoolers revealed that this instrument had a tendency toward an overestimation of astigmatism without cycloplegia.30,31 To estimate the overestimation of astigmatism measured by the SureSight autorefractor, a comparison was performed between the SureSight without cycloplegia and gold standard cycloplegic retinoscopy. A total of 213 children underwent SureSight refraction without cycloplegia and cycloplegic retinoscopy with 1% atropine sulfate eye drops. The results revealed that the mean amount of overestimation was 0.25 D. Therefore, a higher than normal astigmatism criterion (see Definitions section) was recommended in this study to avoid overestimation, in accordance with previous studies.30 SureSight autorefractor measurement found that only one eye with astigmatism was beyond the measurement range (9.99). We also retested this eye with value of 9.99 using the cycloplegic retinoscopy mentioned above. The result revealed that the astigmatism value for this eye was 3.25 D.

To assess the reproducibility of the logMAR visual acuity value, 5% of the children in each age group (total, n = 115) were retested for visual acuity on the same day by another examiner, who was unaware of the first result. Although visual testing of younger children is more time and labor intensive than that in adults, the subsample reproducibility of the logMAR visual acuity value in 3-, 4-, 5-, and 6-year-old children was satisfactory (κ = 0.71, 0.70, 0.72, and 0.75, respectively).

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Definitions

Astigmatism was expressed in the minus cylinder format. The axes of astigmatism were divided into with-the-rule ([WTR] cylinder axis of 1 to 15 degrees or 165 to 180 degrees), against-the-rule ([ATR] cylinder axis of 75 to 105 degrees), and oblique ([OBL] cylinder axis of 16 to 74 degrees or 106 to 164 degrees)32 astigmatism. The high astigmatism was defined as a cylinder power ≥1.00 D for cycloplegic retinoscopy. Taking into consideration the overestimation of the SureSight refractor (0.25 D), a ≥1.25-D standard for SureSight refraction was used in practice. The 1.00-D standard for cycloplegic retinoscopy was used for comparison with other studies (Table 1). We also present less severe cases of astigmatism (<1.25 D for SureSight) in the Results section. We only analyzed data collected from the right eyes.

Table 1
Table 1
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Data Analysis

Data were analyzed using SPSS statistical software for Windows (version 13.0; SPSS Inc., Chicago, IL). One-way analysis of variance and the Student-Newman-Keuls multiple-comparison test were performed to compare the mean astigmatism magnitude across different age groups. The χ2 test was applied to verify gender and age variations in high astigmatism prevalence or axis. Furthermore, multivariate linear regression analysis was performed to confirm the association between astigmatism and visual acuity. Statistical significance was set at p < 0.05.

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RESULTS

Subject Characteristics

A total of 2304 children aged 3 to 6 years (mean, 4.35 years; SD, 0.89 years), consisting of 1249 boys and 1055 girls (male-to-female ratio, 1.18:1.00), underwent ocular examination. The spherical equivalent (SE) ranged from −3.00 to +5.25 D (mean, 1.19 D; SD, 0.67 D). The cylinder diopter range was 0.0 to 3.25 D (mean, 0.67 D; SD, 0.48 D).

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Prevalence and Axis of High Astigmatism

The overall prevalence of high astigmatism ≥1.25 D was 12.7% (95% confidence interval [95% CI], 11.3 to 14.0%). The prevalence of high astigmatism was not significantly correlated with age or gender (p > 0.05). Among the 292 cases with high astigmatism (≥1.25 D), most were WTR (82.9%), followed by ATR (12.6%) and OBL (4.5%) astigmatism. No statistically significant differences were found in the axis of high astigmatism by age or gender (p > 0.05) or the mean astigmatism values across the astigmatism axis (F = 0.426, p = 0.654). However, the mean visual acuity of WTR astigmatism was better than that of ATR and OBL astigmatism (0.11, 0.19, and 0.20 logMAR for WTR, ATR, and OBL astigmatism, respectively; F = 6.531, p = 0.002).

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Astigmatism Magnitude and Axis for All Participants

A polar plot33 was constructed to illustrate the distributions of astigmatism magnitude and axis for all participants. The data in Fig. 1 indicate that 94.5% of astigmatism cases were ≤1.50 D, and WTR was predominant among the three types of astigmatism orientation. A significant change in the mean astigmatism magnitude occurred across age (0.70, 0.69, 0.67, and 0.58 D for 3-, 4-, 5-, and 6-year-old children, respectively; F = 3.588, p = 0.013) but not gender (t = 0.296, p = 0.767). Post hoc comparisons using the Student-Newman-Keuls test indicated a significant difference between the 6-year-old group and the 3-, 4-, and 5-year-old groups. Pearson correlation analysis indicated that astigmatism magnitude weakly, but significantly, correlated with age (r = −0.059; p = 0.004).

Figure 1
Figure 1
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Correlation between Astigmatism and Visual Acuity

A scatter plot (Fig. 2) was used to represent the relationship between visual acuity and astigmatism magnitude for all participants. Simple linear regression analysis indicated a significant correlation between visual acuity and astigmatism magnitude (r = 0.244, p < 0.001; logMAR acuity = 0.068 + 0.055 × astigmatism). In a previous study,13 the correlation between astigmatism and visual acuity was found to be confounded by the effects of age, gender, and SE; therefore, a multiple linear regression analysis was performed to further investigate the correlation between the magnitude and axis of astigmatism and visual acuity while adjusting for age, gender, and SE. In this multivariate model, logMAR acuity was the dependent variable and SE (SEs were divided into four groups: ≤−0.50 D, −0.49 to 0.50 D, 0.51 to 2.00 D, and >2.00 D), age, gender, astigmatism magnitude, and astigmatism axis were the covariates. The results indicated that the correlation of astigmatism magnitude (standardized coefficient β = 0.240, p < 0.001) with visual acuity was statistically significant after adjusting for the effects of age, gender, and SE. In the astigmatism axis, when WTR astigmatism was used as a reference group, ATR astigmatism and OBL astigmatism were significantly correlated with visual acuity (β = 0.137 and 0.154, all with p < 0.01).

Figure 2
Figure 2
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DISCUSSION

This study is the first to provide cross-sectional data on the prevalence of high astigmatism among 3- to 6-year-old children from Guangxi. The overall prevalence of high astigmatism (≥1.25 D [based on noncycloplegic SureSight], equivalent to ≥1.00 D for cycloplegic retinoscopy) in children aged 3 to 6 years was 12.7% (95% CI, 11.3 to 14.0%), which was higher than that in Australia34 and Xiamen (Fujian Province, China)19 but slightly lower than that in Hong Kong,8 Taiwan,18 and in other regions of the world.13,32,35–37 The age-specific prevalence of high astigmatism (Table 1) was comparable to that in Chinese children living in Taiwan18 and Singapore22 but lower than that in children living in Hong Kong (29.0%).8 However, the sample size of the 6-year-old group was small (n = 42) in the Hong Kong study,8 which created a wider CI and weakened the reliability of the results. Moreover, our survey revealed that the children we sampled from an underdeveloped region in China had a similar prevalence of astigmatism as that found in Hong Kong,8 Taiwan,18 and the developed coastal regions of China.19,21 This similarity in prevalence could be caused by several factors, including similar genetics,17 environmental and lifestyle influences,38,39 and similarities in morphological structure of eyelids and orbital bones across populations.11,17

Previous reports on age-related changes in pediatric astigmatism are contradictory. Some studies reported that the prevalence of astigmatism decreased with age,8,17,20,32,40 whereas other studies indicated that it increases22 or is stable with age.8,18 Our results also seem to be conflicting. On the one hand, the prevalence of high astigmatism was stable with age but, on the other hand, the mean astigmatism magnitude of all participants decreased with age, and a significant negative correlation was observed between age and astigmatism magnitude in all participants. However, after a closer inspection of the data, we observed that the change in astigmatism magnitude from the age of 3 to 6 years was clinically insignificant (0.70 − 0.58 D = 0.12 D, <0.25 D), and a weak correlation between age and astigmatism magnitude was observed (r = −0.059); therefore, we inferred that astigmatism was stable with age, which is in agreement with the results reported in studies conducted in Hong Kong8 and Taiwan.18 No gender variation in astigmatism prevalence or magnitude was observed in this study (p > 0.05), which is consistent with the results of Huynh et al.,34 Lai et al.,18 and Tong et al.41

Previous studies have shown that the dominant axis of astigmatism in children varied by ethnicity. In general, the WTR astigmatism was predominant in Asian8,18,20,22,41 and North American children,13,35,42,43 whereas ATR astigmatism was predominant in Spanish children,36,44 and OBL astigmatism was dominant in children from northern Ireland.37 Our study determined that WTR astigmatism was considerably more common than ATR or OBL astigmatism both in high astigmatism cases, as well as among all participants. This finding is in agreement with the aforementioned conclusions. With-the-rule astigmatism acounted for 82.9% of cases with high astigmatism in our study, which was comparable to that in Hong Kong (>80%)20 but lower than that in Singapore (≥95%)22 and higher than that in Canada (45%).43

The correlation of astigmatism and visual acuity has been reported previously,3,13,45 and the evidence seems to support the idea that the correlation of astigmatism with visual acuity is orientation3,44,45 and magnitude dependent3,13 (the higher the astigmatism magnitude, the poorer the visual acuity). In this study, significant linear regression between visual acuity and astigmatism magnitude (logMAR acuity = 0.068 + 0.055 × astigmatism) was found. Moreover, this linear correlation was further confirmed by multiple linear regression analysis (β = 0.240) after adjusting for the confounding effects of age, gender, and SE.

This study also found that ATR and OBL astigmatism from the cases with high astigmatism had poorer average visual acuity than WTR astigmatism, which suggests that the effect of astigmatism on visual acuity is not only dependent on the astigmatism magnitude but also on the astigmatism axis. Multivariate analysis for all participants also showed that, when WTR astigmatism was used as the reference group, ATR astigmatism (β = 0.137) and OBL astigmatism (β = 0.154) were significantly associated with visual acuity, thus suggesting that the effects of ATR and OBL astigmatism o visual acuity were 1.137 and 1.154 times that of WTR astigmatism, after adjusting for the confounding effects of astigmatism magnitude, age, gender, and SE.46,47 In short, the children with ATR and OBL astigmatism seemed to have poorer visual acuity compared with those children with WTR astigmatism. Based on previous work47 and our data, the worst visual acuity would likely be observed in children with OBL astigmatism. The pronounced blurring effect of OBL astigmatism may be caused by the effects of horizontal and nonhorizontal disparities and interocular differential image blurring.48 These phenomena have clinical implications. Thus, to acquire better therapeutic outcomes, it is important to correct astigmatism in preschoolers, particularly in those with OBL astigmatism.

Although the present results are valuable because they provide cross-sectional data on the prevalence of astigmatism in children from an undeveloped area of China, this study has three limitations. First, a hand-held, friendly, and easy-to-use autorefractor was used in this study to measure astigmatism without cycloplegia. This refraction method has a tendency to overestimate the astigmatism magnitude and to introduce measurement bias.30,31 Second, the age range of our subjects was very small (3- to 6-year-olds); therefore, the age-related changes in astigmatism prevalence should be interpreted with caution. Third, the magnitude- and orientation-dependent correlation between astigmatism and visual acuity was observed in the cross-sectional data. Therefore, further longitudinal studies with more convincing refraction methods are warranted.

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CONCLUSIONS

In summary, our study indicates that high astigmatism is moderately prevalent in children aged 3 to 6 years residing in undeveloped areas in China. With-the-rule astigmatism was the predominant form, and cross-sectional and magnitude- and orientation-dependent correlations of astigmatism with visual acuity were observed in this population. Because of the moderate prevalence and correlation of astigmatism with visual acuity, a kindergarten-based vision screening program for identification and correction of astigmatism is essential in undeveloped areas of China.

Wei-Min Liu

Visual Science and Optometry Center of Guangxi

People’s Hospital of Guangxi, Taoyuan Rd

Nanning, Guangxi 530021

China

e-mail: lwm3s@yahoo.com.cn

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ACKNOWLEDGMENTS

This research was supported by the Guangxi Health Department self-funded research project (Z2013345) from the Health Department of Guangxi.

Received January 14, 2013, accepted January 8, 2014.

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Keywords:

astigmatism; visual acuity; preschool children; Guangxi; western China

© 2014 American Academy of Optometry

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