Optometry & Vision Science:
Characteristics of Astigmatism in a Population of Schoolchildren, Dezful, Iran
Fotouhi, Akbar*; Hashemi, Hassan†; Yekta, Abbas Ali‡; Mohammad, Kazem§; Khoob, Mehdi Khabaz∥
Department of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran (AF, KM), Noor Ophthalmology Research Center, Noor Eye Hospital, Tehran, Iran (HH, MKK), Farabi Eye Hospital, Tehran University of Medical Sciences, Tehran, Iran (HH), and Department of Optometry, Mashhad University of Medical Sciences, Mashhad, Iran (AAY).
This work was supported, in part, by Noor Vision Correction Center, Tehran.
Received June 12, 2010; accepted April 5, 2011.
Akbar Fotouhi, Department of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, PO Box: 14155-6446, Tehran, Iran, e-mail: firstname.lastname@example.org
Purpose. To study and analyze astigmatism and its characteristics in a cross-sectional study of schoolchildren from Dezful, Iran.
Methods. In a cross-sectional study using random cluster sampling on 460 schools in Dezful (clusters), 39 clusters were selected. The study was conducted after coordinating with schools and obtaining written consent from students' parents. Cycloplegic refraction was done for primary and junior high school students and non-cycloplegic refraction was done for high school students. Astigmatism was defined as the cylinder power of 0.75 diopter (D) or more.
Results. Of 5726 selected students, 5544 (96.8%) participated in the study. The prevalence of astigmatism was 13.47% (95% confidence interval: 11.90 to 15.04) and was not significantly related to age and gender. Regarding axis, 45.76, 48.14, and 6.09% of astigmatic schoolchildren had with-the-rule (WTR), against-the-rule (ATR), and oblique astigmatism, respectively. An increase in age was accompanied by a decrease in the prevalence of WTR astigmatism and an increase in the prevalence of ATR astigmatism (p < 0.001). The association between astigmatism and myopia [odds ratio = 8.81] was stronger than its association with hyperopia (odds ratio = 3.81). Those with high values of spherical error had high values of cylindrical error, as well. Mean sphere in WTR, ATR, and oblique astigmatism was 1.93, 1.37, and 0.88 D, respectively (p < 0.001). The highest values of spherical refractive error were observed in WTR astigmatism group.
Conclusions. The prevalence of ATR astigmatism was high in this study. It appears that the decrease in the prevalence of WTR and the increase in the prevalence of ATR astigmatism as a result of aging happened earlier in our study compared with other studies. Astigmatism was found to have a strong correlation with myopia, although individuals with high hyperopia also had high astigmatism. Individuals with high ametropia mostly had WTR astigmatism although the percentage of ATR astigmatism was high in those with low ametropia.
Astigmatism is of one the most common eye conditions with a reported prevalence of 3.5% in children to 77% in individuals above 50 years.1,2 Different articles have studied the relationship between various demographic and ophthalmic factors and astigmatism.3–9 Age and gender are the most important demographic factors.6,7,10,11 However, available reports on the prevalence of astigmatism have shown conflicting results regarding these two factors.
Although, there is still no common agreement on the effect of age and gender on astigmatism, on the basis of most studies, it can be said that astigmatism axis is importantly affected by age in a way that the prevalence of with-the-rule (WTR) astigmatism decreases and the prevalence of against-the-rule (ATR) astigmatism increases with age.10,12,13 Some studies have shown the effect of lid tension on the cornea as a probable cause of changing astigmatism axis with age.14–16
Most studies have reported no difference in astigmatism prevalence between genders but a few have addressed the type of astigmatism in males and females.1,7,12,17–21 In addition to demographic factors, there are some studies that confirm the relationship between astigmatism and ocular components, particularly the spherical component of refraction.4 However, there are still some unknown aspects of the relationship between astigmatism and spherical errors. It has been shown that there is a direct correlation between myopia, and even the severity of myopia, and astigmatism.5,22,23 However, there is little known regarding the relationship between hyperopia and astigmatism.4
What is more, only a few articles have addressed the variations of astigmatism axis in different refractive errors.4,12 We previously studied the prevalence of refractive errors in Dezful school students and briefly reported on the status of astigmatism in school students.24 Because some aspects of astigmatism were not covered in the previous report, this report is presented with the aim of a more in-depth analysis of astigmatism in Dezful school students and its relationship with factors such as age, gender, and spherical refractive errors.
This cross-sectional study was conducted on the schoolchildren of Dezful, a city in the south west of Iran, between 2004 and 2005 using random cluster sampling in a student population of 83,250 in 460 schools of the city. Of these schools as clusters, 39 schools were selected randomly with regard to the ratio between urban and rural areas, and all the students from each of the 39 schools invited to participate in the study.
Uncorrected visual acuity, best-corrected visual acuity (BCVA), and visual acuity with glasses (GVA) for those with glasses and objective refraction using a Topcon KR 8800 refractometer (Topcon Corporation, Tokyo, Japan) were determined on all students. The validity and reliability of the Topcon KR8000 in measuring astigmatism has already been reported.25,26
In this report, analyses were performed on all data obtained via auto-refraction. Cycloplegic refraction with the auto-refractometer was measured on primary and junior high school students after administering 3 droplets of cyclopentolate 1%; non-cycloplegic refraction with the auto-refractometer was performed on high school students. BCVA and subjective refraction were determined on all students whose uncorrected visual acuity was worse than 20/20. Those students whose BCVA was worse than 20/32 were referred to an ophthalmologist for further examination. Analysis was performed on data obtained from these students as well.
Considering the high correlation between the left and the right eyes regarding astigmatism (r = 0.758, p < 0.001), only the results of the right eye were analyzed. We defined astigmatism as a cylinder power of at least −0.75 diopter (D). On the basis of astigmatism axis, an axis of 180 ± 30 was classified as WTR, an axis of 90 ± 30 was considered ATR, and others were considered oblique astigmatism. A spherical equivalent of −0.5 D or worse was defined as myopia and a spherical equivalent of +2.0 or more as hyperopia. In Iran, children start primary school at the age of 7, junior high school at the age of 12, and high school at the age of 15, provided that they do not fail a grade.
The study protocol was approved by the Ethical Committee of Tehran University of Medical Sciences. The local school board and school principals also approved the study. A written informed consent letter from a parent or guardian, in addition to the assent of each student was obtained before examination.
The prevalence rates of astigmatism were calculated in the study population. The design effect of cluster sampling was considered in calculating the 95% confidence interval (CI). The 95% CIs were calculated by assuming a normal approximation. A binomial distribution was used for cases with few outcomes that did not follow a normal distribution. Simple and multiple logistic regression models were used to investigate the relationship between different variables and astigmatism. The χ2 test for trend was used to show the relationship between astigmatism axis and age.
Of 5726 selected students, 5544 participated in the study (96.8% response rate). Because 193 individuals did not have refraction data, analysis was performed on 5351 participants. Of the participants, 41.0% (2273), 25.3% (1400), and 33.7% (1871) were primary, junior high school, and high school students, respectively. Of them, 54.9% were females and 69.2% lived in urban areas. The mean age of the participants was 12.4 ± 3.1 years (6 to 20 years).
Randomly, 323 students had both cycloplegic and non- cycloplegic refractions. The mean and SD of the cylinder power of the students was −1.44 ± 1.0 and −1.42 ± 1.0 based on cycloplegic and non-cycloplegic refraction, respectively (p = 0.296). Also, these two methods were found to have a high correlation (r = 0.935).
J0 and J45 were used to compare astigmatism axis between the two methods. Mean ± SD of J0 was 0.001 ± 0.60 and −0.037 ± 0.59 for non-cycloplegic and cycloplegic refraction, respectively, and their difference was not statistically significant (p = 0.396). Similarly, mean ± SD of J45 was 0.063 ± 0.62 for non-cycloplegic and −0.055 ± 0.65 for cycloplegic refraction revealed no statistically significant difference (p = 0.837).
Mean and SD of cylinder power in students was −0.35 ± 0.53 D. The prevalence of astigmatism was 13.47% (95% CI: 11.90 to 15.04). There was no significant statistical difference between females and males (p = 0.459) and between age and cylinder power (p = 0.112). The prevalence of astigmatism according to age is shown in Table 1. Multiple logistic regression also revealed no significant difference between astigmatism prevalence and either age or gender.
Regarding axis, 6.39% (95% CI: 5.21 to 7.57), 6.28% (95% CI: 5.18 to 7.38), and 0.80% (95% CI: 0.49 to 1.11) of the students had WTR, ATR, and oblique astigmatism, respectively. In individuals with astigmatism, the prevalence of WTR, ATR, and oblique astigmatism was 45.76% (95% CI: 37.79 to 53.73), 48.14% (95% CI: 41.15 to 55.14), and 6.09% (95% CI: 2.84 to 9.35), respectively. WTR astigmatism was significantly higher in females (p = 0.019) and ATR astigmatism was significantly higher in males (p = 0.012), whereas the prevalence of oblique astigmatism did not show any significant difference between the two genders (p = 0.731).
In those with astigmatism, the distribution of astigmatism axis in different age groups is shown in Fig. 1. The prevalence of WTR astigmatism in 7- and 8-year-old participants was 70.79%, which decreased to 18.46% in those aged ≥16 years. The logistic regression showed a decrease in the prevalence of WTR astigmatism as age increased (p < 0.001). Aging was accompanied by an increase in the prevalence of ATR astigmatism from 23.13% in 7- and 8-year-old individuals to 64.1% in those aged 16 years and above (p < 0.001). Regarding the oblique astigmatism prevalence, no significant difference was observed between different age groups (p = 0.121). In a multiple logistic regression model, age and gender had a significant association with WTR and ATR astigmatism whereas oblique astigmatism was not significantly related to age adjusted for gender.
The Association between Spherical and Cylindrical Components
Although we found that astigmatism and its axis were not affected by cycloplegia, to evaluate the correlation between cylinder and sphere, analysis was performed on individuals with cycloplegic refraction because sphere is affected by cycloplegic refraction.
In all students, the prevalence of emmetropia, myopia, and hyperopia was 78.25, 9.34, and 12.41%, respectively based on spherical equivalent. These values, based on cycloplegic refraction in primary and junior high school students, were 79.38, 2.29, and 18.33%, respectively. In students who had received cycloplegic refraction, the prevalence of astigmatism in emmetropic, myopic, and hyperopic individuals was 9.31% (95% CI: 7.68 to 10.94), 47.50% (95% CI: 38.24 to 56.75), and 28.13% (95% CI: 22.39 to 33.86), respectively. Considering emmetropia as a reference group, the association between astigmatism and myopia (odds ratio = 8.81, 95% CI: 6.00 to 12.95) was stronger than its association with hyperopia (odds ratio = 3.81, 95% CI: 2.95 to 4.93).
Fig. 2 shows the average amount of astigmatism according to spherical values. As shown in the Figure, those individuals with myopia worse than −5.0 D and those with hyperopia more than +5.0 D had the highest values of cylinder. Analysis of variance showed a significant difference between different values of refractive errors and cylinder (p < 0.001).
Mean sphere in WTR, ATR, and oblique astigmatism was 1.93 ± 2.31, 1.37 ± 0.92, and 0.88 ± 4.45 D, respectively (p < 0.001). Distribution of axis type in astigmatic students by spherical values is shown in Fig. 3. As seen in this Figure, individuals with high ametropia mostly had WTR astigmatism whereas ATR astigmatism was more prevalent in those with low ametropia (p < 0.001).
A detailed report of myopia and hyperopia among schoolchildren in Dezful has already been published.24 This report aimed at a more detailed analysis of astigmatism among schoolchildren. Different definitions of astigmatism have been presented in various studies. However, in those on schoolchildren, it has been defined as cylinder power of 0.75 D or more, and we used the same definition for appropriate comparison.1,7,8,17–21,24,27–30 It should also be noted that there was no significant difference in cylinder power and astigmatism axis between cycloplegic and non- cycloplegic refraction; therefore, we reported the data of the two groups together.
The prevalence of astigmatism was 13.47% in Dezful students. The findings of other studies are summarized in Table 2. The lowest prevalence has been reported in Nepal and the highest in southern China.1,27 The prevalence of astigmatism in this study was found to be moderate. In view of the diversity of the results from other studies, it appears that environmental and racial factors are among important causes of astigmatism. As seen in this Table, the prevalence of astigmatism is higher in East Asian countries. Some reports have also discussed the role of ethnicity. Read et al.,10 in a review article, showed that both astigmatism and its type were affected by ethnicity.
In this study, the total prevalence of astigmatism did not show a significant correlation with age. Other studies have reported controversial results.1,2,8,11,18,19,21,27,28,30–33
We observed no statistical difference in the prevalence of astigmatism between the two genders. Similarly, other studies of similar age groups performed by Tong et al.,11 Dandona et al.,17 Goh et al.,18 Naidoo et al.,20 and Kleinstein et al.32 did not notice any differences in astigmatism between the two genders. However, in some reports by Pokharel et al.,1 Murthy et al.,7 Maul et al.,19 Zhao et al.,21 and He et al.,27 a higher prevalence of astigmatism was reported in women.
As reported, 48.14% of the school students with astigmatism had ATR astigmatism. This rate reached 64% in subjects between 17 and 20 years. However, based on other studies on schoolchildren, we expected to see a higher prevalence of WTR astigmatism.11,13,34 Similar to our finding, Mandel et al.12 reported a higher prevalence for ATR astigmatism compared with WTR astigmatism and the age range in Mandel study was 16 to 22 years, making his findings more expected.
The prevalence of ATR astigmatism in our 17 years or more old participants was even higher than its reported prevalence in middle-aged or elderly individuals.35 Two points must be taken into account in this regard: first, the definition of astigmatism axis is different in different studies, and second, racial and genetic differences cannot be ignored. Consistent with our study, previous studies have also reported that ATR astigmatism increases and WTR astigmatism decreases with age.12,13,34 It appears that one possible explanation is the change that the curvature of the cornea goes through with age. Also, decreasing weight of the upper eyelid with age can be another possible reason for this relationship. However, it appears that this process (i.e., the increase in ATR astigmatism and decrease in WTR astigmatism with age) happened earlier in our study compared with other studies, resulting in a higher prevalence of ATR astigmatism in individuals between 6 and 20 years.
Most of the studies on the correlation between refractive errors and astigmatism have been performed on myopic subjects.4,5,12,22,23 As with previous studies, we observed the relationship between astigmatism and myopia.5,11,21,22 Gwiazda et al.5 showed that newborns with ATR astigmatism were more prone to myopia.
To date, the correlation between astigmatism and hyperopia has been addressed by few studies. Farbrother et al.4 has already reported this correlation. It was an interesting and unexpected finding in our study. However, further studies are warranted to confirm this.
Another interesting finding of our study was the relationship between the degree of the spherical refractive error and the type of astigmatism axis, which has only been addressed in a few studies.4,6,12 Some studies have showed that most myopic subjects have WTR astigmatism.4,12 However, we found that WTR astigmatism was seen in individuals with high hyperopia in addition to its relationship with high myopia. In other words, those with high ametropia mostly had WTR astigmatism. The same finding was previously reported by Farbrother et al.4 and Mandel et al.12
As we have also shown in Fig. 3, the percentage of WTR astigmatism was higher in individuals with high myopia than those with high hyperopia. This may be due to the fact that individuals with WTR astigmatism have a longer axial length.4,36
The prevalence of astigmatism was moderate in this population but the prevalence of ATR astigmatism was high. The decrease in WTR astigmatism and increase in ATR astigmatism with aging in this study seemed to occur earlier than other population studies. High values of astigmatism were observed in those with high values of spherical refractive errors. Astigmatism showed a stronger association with myopia than with hyperopia. Individuals with high ametropia mostly had WTR astigmatism whereas the percentage of ATR astigmatism was higher in those with low ametropia.
This study was a research project by the Institute of Public Health Researches, affiliated to Tehran University of Medical Sciences.
Department of Epidemiology and Biostatistics
School of Public Health
Tehran University of Medical Sciences
PO Box: 14155-6446
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