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Refractive Errors in School-age Children in Qazvin, Iran


1 Department of Public Health, Qazvin University of Medical Sciences, Qazvin, IR Iran
2 Department of Optometry, Shahid Beheshti University of Medical Sciences, Tehran, IR Iran
3 Department of Medical Physics, Medical Faculty, Tabriz University of Medical Sciences, Tabriz, IR Iran
*Corresponding author: Mohammad Khalaj, Department of Public Health, Qazvin University of Medical Sciences, Qazvin, IR Iran. Tel: +98-2833669585, Fax: +98-2833345862, E-mail: mohammadkhalaj82@yahoo.com.
Biotechnology and Health Sciences. 2014 August; 1(2): e22087 , DOI: 10.17795/bhs-22087
Article Type: Research Article; Received: Jun 10, 2014; Revised: Mar 21, 2014; Accepted: May 16, 2014; epub: Aug 28, 2014; ppub: Aug 2014

Abstract


Background: Refractive error remains one of the primary causes of visual impairment among school children all over the world, and its prevalence varies widely.

Objectives: The present study was aimed to determine the prevalence of refractive errors in school children aged 7 to 18 years in Qazvin, Iran.

Patients and Methods: In this cross-sectional study, 11821 students (aged 7 to 18 years) were recruited from different schools. Emmetropia was defined as refractive status between +0.25 and -0.25 D sphere. A -0.50 D or greater spherical considered as myopia, -6.00 D or more as high myopia, and +0.50 D or more as hyperopia, and a cylinder refraction greater than 0.75 D was defined as astigmatism. Visual acuity and refraction of all students were tested. Anterior and posterior segment examination and ocular motility evaluation were also performed to rule out the pathological causes of visual impairments.

Results: The study was performed on 5641 (47.72%) male and 6180 (52.28%) female students. The prevalence of myopia (from 32.96% at the age of 7 to 79.02% at the age of 18 years) significantly increased (P < 0.001), and hyperopia significantly decreased (from 47.07% in 7-year- old individuals to 8.32% in 18-year- old subjects) with age (P < 0.001). There were significant differences in refractive errors between males and females. Hyperopia and myopia was more common among female in comparison to males (P < 0.001). Astigmatism greater than 0.75 D in one or both eyes was found in 990 children (8.37%). Astigmatism increased from 6.04% in 7-year-old students to 9.86% in 15-year-old-students e and then no more difference was found in age group ranged from 15 to 18 years.

Conclusions: Based on our study, the prevalence of myopia is more than other types of refractive error, which is similar to that reported in previous studies on other school-age populations in some Asian countries. The high prevalence of refractive error among school-age children indicated that untreated refractive error is one of the most common public health problems.

Keywords: Refractive Error; Myopia, Hyperopia; School children; Age; Sex

1. Background


The prevalence of refractive error in different age groups has been reported by a number of previous studies. It has been believed that uncorrected refractive error is one of the main causes of treatable visual impairments, and is one of the most common eye disorders in children around world (1). Under corrected refractive errors among school-age children, particularly myopia, is a critical public health problem and is the second leading cause of treatable blindness (2, 3). As refractive errors are the major causes of mild to moderate visual impairment in patients, knowledge about the prevalence of refractive error would be of great help in planning public health strategies (4). Some studies indicated that in children the prevalence of hyperopia decreased and that of myopia significantly increased (P < 0.001) by getting older (5-10). In contrast with the increment of myopia with age, some studies reported that the reduction of myopia and increment of hyperopia with aging (11, 12). Among refractive errors, myopia is the most common type of refractive errors among 6 to 12-year-old children (13). Several epidemiologic studies on the prevalence of refractive errors have been conducted in Asia- pacific regions and many other countries.

The prevalence of this disorder in Asian countries varies from 50% in China to 84% in Taiwan and Hong Kong (14-19). The comparison of the prevalence of refractive errors in Malay, Chinese, and Indian children in Malaysia and Singapore indicated that the prevalence of myopia was higher in children in Singapore from all the ethnic groups. Ethnic-specific hyperopia rates did not differ in Singapore and Malaysia (20). The highest prevalence of myopia was found among urban Chinese populations such as Hong Kong, Taiwan, Singapore and Southern China, and the lowest was reported in non-Chinese rural populations such as Nepali and Indians (21-26). In Taiwan, two studies involving school children aged 6 to 18 years old showed a prevalence of more than 80% by the age of 18 (22), another study in a Japanese student population showed that an overall prevalence of approximately 50% (27). The age-adjusted overall prevalence rates of myopia, hyperopia, astigmatism, and anisometropia (SE difference of +1.0 D) in Indonesia were 26.1%, 9.2%, 18.5%, and 15.1% (95% CI: 12.9-17.4), respectively (1). Anisometropia > 1.00 D was found in 4.6% of primary school children in South Limburg, the Netherlands. The high prevalence of myopia in Chinese and Japanese students was reported (27, 28). Furthermore, 9.8% of children aged 2-6 years, 6.4% of children aged 7-11 years, 3.7% of adolescents, and 2.9% of adults were hyperopic (P = 0.380). The prevalence of myopia in female students (23.6%) was significantly higher in comparison with male ones (14.6%, P = 0.018). The mean prevalence of myopia among 7-year-old individuals increased from 5.8% in 1983 to 21% in 2000 in Taiwan (5). The high prevalence of myopia in Chinese and Japanese population was reported (27, 28). Furthermore, the prevalence of myopia has also increased among young Asian adult populations, as reported in a longitudinal 13-year- study on students aged between 3 and 17 years by Matsumura et al. (29). The progression of myopia was also noted to be more prevalent in older children, and was much higher than those reported in Western countries. In Sumatra, Indonesia, the prevalence of moderate hyperopia was 13.2% among children aged 6 years, and this amount decreased to 5% in 12-year–old individuals; this was more frequent in children with Caucasian ethnicity (15.7% and 6.8%, respectively) than those from ethnic groups (30).

The prevalence of myopia in Europe seems to be slightly lower than in Asian countries (19). That was reported to be 36.8% among Greek students aged 15 to 18 years old (31). In Germany, similar to other countries, the prevalence of myopia increased with age, it was 0% in children aged 2-6 years, 5.5% in children aged 7-11 years, 21.0% in adolescents (aged 12-17 years) and 41.3% in adults aged 18-35 years (32). Among US population, aged 12 to 54 years this was estimated to be significantly higher in 1999-2004 than those reported in 1971-1972 (41.6% and 25.0%, respectively; P < 0.001) (33). Among the students in Northeast Brazil, hyperopia was the most common refractive errors with 71%, followed by astigmatism (34%) and myopia (13.3%) (34).

2. Objectives


The aim of our study was to determine the prevalence of refractive errors among school-age children in Qazvin province, Iran.

3. Patients and Methods


In this cross-sectional study, school children aged 7 to 18 years old recruited from 20 primary schools and 10 high schools using random cluster-sampling method, from September 2003 and May 2010. Students were invited by school authority to refer to Eye Clinic of Bouali Hospital in Qazvin city. There are two big educational areas in Qazvin province (area 1 and 2) with 2209 primary, secondary and high schools with 222713 students. We selected mentioned schools with no consideration on the students' social and economic position. Only schools with mentally disabled children were excluded from the selection. In this study, 1329 out of 13150 eligible students were excluded from the study because of various reasons such as refusing to participate in the eye examinations, pathological conditions and, inability to continue the examination (10). Therefore, 11821 students were entered to this study. Parents and children were asked to fill out a questionnaire about the history of their refractive errors and spectacle uses (24). Children who were unwilling to undergo the examination due to fear, sickness or those who were under medication for some other reasons; having ocular effect were excluded from this study (35). All subjects underwent a complete ophthalmic examination. Parents or legal guardians signed an informed consent, and expressed that they did not have no history of systematic cardiovascular or nervous diseases, such as congenital heart diseases, hypoxic-ischemic encephalopathy, and learning difficulties (10).

Distance visual aquity (VA) was assessed for each eye separately using a log minimum angle of resolution 9 (MAR) chart with and without any spectacle correction, if worn. In the cases of poor vision or failure in measuring the visual acuity with the normal chart, the following tests were used sequentially: counting fingers, hand movements, and light perception (36). Dry refraction and cycloplegic auto refraction were tested on all children less than 15 years of age. The cycloplegia induced by administering, one drop of 1% Cyclopentolate eye drop instilled twice at a 5-minute intervals, and at least 35 minutes after the last drop, the cycloplegic refraction was tested using streak retinoscopy, and one of two auto refractors (Canon, Nidek) (37), because this method of examination provides more reliable measurements in young children (35).

Then subjective refraction was performed to achieve best corrected visual acuities, along with refractive error under cycloplegia (38) for students when best corrected visual acuity was not achieved to 20/20 (39).

Also to rule out the pathological causes of visual impairment, anterior and posterior segment examinations, identification of amblyopia, and ocular motility evaluation, were measured. A cover test was performed to detect the strabismus (40). The data collectors were medical students and an experienced, qualified optometrist from the Department of Qazvin Medical Sciences (41). Parents were asked for permission to use their child’s hospital records if needed. Other data, including sociodemographic information (ethnicity, parental education, employment, and home ownership) and the time that the child devotes to near work and outdoor activities were also obtained in parent questionnaires. The 12-year-old and older children completed a separate questionnaire, in which similar questions about time spent in near-work and outdoor activity were included (25). Activities included outdoor leisure activities, watching television/videos, near-work activity such as reading for pleasure, using a computer, and playing console games were recorded.

Analysis was performed using the Statistical Package for Social Science (SPSS Inc. Chicago, USA). Spherical equivalent refraction (SER) was calculated as the numerical sum of the sphere and half of the cylinder. Emmetropia was defined as refractive status between +0.25 D and -0.25 D Sphere. Myopia was considered when the measured objective refraction was greater than or equal to -0.50 spherical diopters in one or both eyes, hyperopia as ≥ 0.50 D and astigmatism was considered to be significant if it was ≥ 0.75 D. The chosen cut off values for myopia (0.50 D or less) is similar in the literature (42-44). Anisometropia was defined as difference of SER of 1.00 D or more between the two eyes.

4. Results


From total of 13150 eligible school children, 11821 (89.9%) cases responded to the study. The age group ranged from 7 to 18 years, with the mean age of 12.5 ± 3.61 years including 6180 (52.28%) females and 5641 (47.72%) males. The prevalence of myopia differed significantly in all age groups (32.96% in children aged 7 years, 58.74% in children aged ≤ 10 years, 67.9% in children aged ≤ 14 years and 79.02% in children aged ≤ 18 years) (Pearson’s Chi-square, P ≤ 0.0001). Furthermore, the prevalence of hyperopia differed in all groups: 47.07% of children aged 7 years, 26.23% of children aged 10 years, 15.23% in children aged 14 years, and 8.23% in children aged 18 years were hyperopia (Table 1).

The prevalence of myopia in females (52.28%) was significantly higher than in males 47.72%, (P = 0.018). The reduced visual acuity was observed because of refractive errors. In a total of 11821 subjects, myopia was detected in 7826 cases (66.2%), hypermetropia in 2256 (19.08%), astigmatism in 990 (8.37%), and amblyopia in 749 cases (6.34%). Out of total myopia, high myopia (-6.00 D or more) was in 550 (4.65%), of all refractive errors, anisometropia was in 644 (5.45%), (Table 2)

The prevalence of emmetropia and hyperopia significantly decreased with age (P < 0.001), and the prevalence of myopia and high myopia significantly increased with age (P < 0.001). In 7- year-old group, hyperopia had the most prevalence (47.07%), followed by myopia (32.96%), and astigmatism (6.04%). This distribution was changed in the 18- year-old age group, myopia (79.02%), followed by hyperopia (8.32%), and astigmatism (7.98%). Among all the participants with astigmatism (cylindrical error greater than 0.75 DC), 290 subjects (29.29%) had with-the-rule (WTR), 530 (53.54%) against-the-rule (ATR) astigmatism, and 170 (17.17%) oblique astigmatism. The prevalence of WTR and ATR astigmatism significantly increased with age (P < 0.001). Astigmatism increased from 6.04% in children aged 7 years to 9.86% in those 15 years of age and then no more difference was found in age group ranged from 15 to 18 years. Refractive error was detected in 11821 students and myopia was the most common type of refractive detected in 7826 students, contributing more than 66.2% of the refractive errors. Hyperopia was detected in 2256 students (19.8%), followed by astigmatism (cylinder power of 0.75 D or worse) in 990 (8.37%) of students (Table 3). Table 3 shows the distribution of hyperopia and myopia between 0.50 D and 10.00 D or greater, and astigmatism between 0.75 D and 6.00 D or greater in students aged 7 to 18 years. In hyperopia, myopia and astigmatism of right and left eyes, distribution of ± 2.00 D or less was more prevalent in those categorized in Table 3.

Table 1.
Distribution of Refractive Error a
Table 2.
Distribution of Visual Impairment For Age Groups of 7 to 18 Aged and Gender a
Table 3.
Distribution of Types of Refractive Errors in School-Age Children in Qazvin, Iran a,b
Figure 1.
Distribution of Refractive Error Types

5. Discussion


To our best of knowledge, this is one of the largest cross-sectional studies examining the prevalence of refractive errors in primary and high school students in Iran. Students with a mean age of 12.5 ± 3.61 years were randomly cluster selected from all over Qazvin province from different academic and socioeconomic backgrounds. In our study, myopia was the most common type of refractive errors among children aged 7 to 18 years, comprised 66.2% of all the errors, followed by hypermetropia (19.8%), astigmatism (8.37%) and amblyopia in 749 subjects (6.34%). This observation was similar to the study on Swedish children age of 12-13 years in which the prevalence of myopia was 49%.These values are consistently lower than our findings (45). The distributions of refractive frequency in various age groups displayed a clear change, the prevalence of hyperopia significantly decreased and the prevalence of myopia increased (10). It has been reported a prevalence of myopia of 11.6% for children aged 5 to 17 years and 20% for children aged 12 years and increased to 33% for those aged ≥ 20 years (46-48), and in study on 2353 Austeralian students aged 11-15-years old, the prevalence of myopia was 4.6% and this amount was 6.1% among European Caucasian and Middle Eastern children respectively, whereas prevalence of myopia was 31.5% among South Asian children (49). Similar to our findings, the increasing prevalence of myopia with age was observed in the population-based studies in some countries such as Taiwan (5), Australia (6), Singapore (7), Malaysia (13), and Indonesia (50). The prevalence of myopia and high myopia in our study was 66.2% and 4.65%, respectively, which significantly increased with age (from 32.96% in 7-year-old children to 79.02 in 18-year-old ones). These findings are in contrast with other population-based studies from different parts of the world that reported a decreasing trend of myopia with age, in countries such as India, hypermetropia was the commonest refractive error among children, that is accounted for 23% of all errors followed by astigmatism with myopia (12), and in primary schools in Kampala, Astigmatism was the most frequent refractive error, accounting for 52% of all the errors, followed by hyperopia, and myopia, respectively (36). In Northeast Brazil hyperopia was the most common type of refractive errors with a prevalence of 71%, followed by astigmatism (34%) and myopia (13.3%) (30). These results are in contrast with previous publications, mainly from the orient, where myopia is always considered as the most common refractive error among students (30).

In Asia the prevalence of myopia has the highest rate in comparison to the other countries of the world. For example Hong Kong has one of the highest incidence of myopia in the world, and it is likely that both the rate and severity of myopia will increase over the time (21). Similar trends are observed among Iran's East Asian neighbors, which causes important medical, social, and public health concerns (21-26). Several factors could interfere with the prevalence of myopia. In addition to heredity that influences the prevalence of myopia, which is believed as the most important associated factor to juvenile myopia, several other environmental factors could also be responsible for the increasing myopia among school-age children (especially among those with family history of myopia) such as; sleeping on night lighting (51), computer vision syndrome (52), and UV exposure (53).

The prevalence rates of amblyopia in this study were slightly different from the results of epidemiological studies from other countries. In studies on Singaporean 30 to 72-month old children, 7-year- old children in the United Kingdom and Australian children aged 6 years; the prevalence of amblyopia was 1.19%, 3.6% and 0.7%, respectively (54-56). The reported prevalence of amblyopia in a questionnaire-based study on Japanese children aged between 1.5 and 12 years was between 0% to 0.2% and this amount in Korean children aged 3 to 5 years was 0.4% of the 43% who responded (57-59). Amblyopias was detected in Taiwan in approximately 5% of 625 preschool children and in 3% of 4368 Guangzhou children, aged 5 to 15 years (24, 60). It was concluded that the high prevalence of ocular blindness among primary and high school children was observed in school children and refractive errors are the most common ocular disorders. Finally, the aim of all programs for controlling the blindness should be to publicizethe public awareness about eye care and to teach the essentials of eye healthcare (61).

In this study the overall prevalence of refractive errors and progression of myopia among school-age children were 66.2% for myopia, 19.8% for hyperopia, and 8.37% for astigmatism. The prevalence of myopia in this study was slightly higher than previous studies in other countries. The large increase in prevalence of myopia reported in all studies may be affected by several factors including the environmental factors and also the genetic features.

The results of the present study represents that patients in adolescent age group are at high risk for visual impairment especially myopia. Therefore, visual screening at the pre-school and school ages should be performed periodically. This could be useful in detecting the correctable causes of decreased vision especially refractive errors and eventually leading to improvement level of students, education. In addition, children at these ages and their parents should be educated about the signs and symptoms of refractive errors, ocular hygiene and also the risk factors involved in the development of these errors and other ocular pathological problems. Uncorrected refractive errors are the potential causes of learning difficulties and may limit the choices children make in their daily activities. New policies are needed to be developed to address this public health problem (46). Suggesting that we should develop preventive strategies and also develop approaches towards early treatments of these sorts of visual disabilities.

Acknowledgments

The authors would like to thank all the children and parents and medical students who participated and helped in this study.

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Table 1.

Distribution of Refractive Error a

Age, yMyopiaHyperopiaAstigAmblyopiaTotal
7180 (32.96)257 (47.07)33 (6.04)76 (13.92)546
8220 (40.5)240 (44.2)35 (6.45)48 (8.84)543
9380 (51.98)226 (30.92)58 (7.93)67 (9.17)731
10430 (58.74)192 (26.23)60 (8.2)50 (6.83)732
11640 (64.2)200 (20.01)84 (8.43)73 (7.32)997
12560 (62.57)210 (23.46)86 (9.61)39 (4.36)895
13584 (63.83)217 (23.72)69 (7.54)45 (4.92)915
14753 (67.9)169 (15.23)106 (9.58)81 (7.3)1109
15834 (71.53)152 (13.04)115 (9.86)65 (5.57)1166
161006 (75.19)142 (10.61)118 (8.82)72 (5.38)1338
171090 (78.14)130 (9.32)110 (7.89)65 (4.66)1395
181149 (79.02)121 (8.32)116 (7.98)68 (4.68)1454
Total7826 (66.2)2256 (19.8)990 (8.37)749 (6.34)11821
a Data are presented as No. (%).

Table 3.

Distribution of Types of Refractive Errors in School-Age Children in Qazvin, Iran a,b

Hyperopia (Spherical ≥ +0.5 D) Myopia (Spherical ≤ 0.5 D) Astigmatism (Cylindrical ≥ 0.75)
Diopter RE LE Diopter RE LE Diopter RE LE
≥ + 0.5 585 (25.93) 530 (23.49) ≤ 0.5 1580 (20.19) 1640 (20.96) 0.75-1 582 (58.79) 579 (58.48)
≥ + 1 1004 (44.5) 952 (42.2) ≤ 1 2130 (27.22) 2019 (25.8) ≥ 1 127 (12.83) 117 (11.82)
≥ + 2 280 (12.41) 320 (14.18) ≤ 2 1720 (21.98) 1800 (23) ≥ 2 83 (8.38) 95 (9.6)
≥ + 3 145 (6.41) 188 (8.33) ≤ 3 981 (1255) 965 (12.33) ≥ 3 74 (7.47) 70 (7.07)
≥ + 4 90 (3.99) 97 (4.3) ≤ 4 498 (6.36) 477 (6.09) ≥ 4 57 (5.75) 58 (5.86)
≥ + 5 30 (1.33) 57 (2.53) ≤ 5 366 (4.68) 375 (4.79) ≥ 5 54 (5.45) 56 (5.66)
≥ + 6 28 (1.24) 39 (1.73) ≤ 6 210 (2.68) 205 (2.62) ≥ 6 13 (1.31) 15 (1.52)
≥ + 7 14 (0.62) 5 (0.22) ≤ 7 125 (1.6) 130 (1.66) - - -
≥ + 8 20 (0.89) 34 (1.51) ≤ 8 85 (1.09) 90 (1.15) - - -
≥ + 9 12 (0.53) 15 (0.66) ≤ 9 58 (0.74) 65 (0.83) - - -
≥ + 10 48 (2.13) 23 (1.02) ≤ 10 72 (0.92) 60 (0.77) - - -
Total 2256 (100) 2256 (100) total 7826 (100) 7826 (100) total 990 (100) 990 (100)
a Abbreviations: LE, left eye; RE, right eye.
b Data are presented as No. (%).

Table 2.

Distribution of Visual Impairment For Age Groups of 7 to 18 Aged and Gender a

Refractive ErrorsMaleFemaleTotal
Myopia3733 (47.7)4093 (52.3)7826 (66.2)
Hyperopia1093 (48.45)1163 (51.55)2256 (19.08)
Astig480 (48.48)510 (51.52)990 (8.37)
Amblyopia335 (44.73)414 (55.27)749 (6.34)
Anisometropia355 (55)289 (45)644 (5.45)
a Data are presented as No. (%).

Figure 1.

Distribution of Refractive Error Types
Hypermetropia A, myopia; B, astigmatism; C, in school-age children of Qazvin Iran.