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Clinical and Epidemiologic Research  |   August 2013
Prevalence and Risk Factors for Myopia in a Rural Korean Population
Author Affiliations & Notes
  • Jin Hae Lee
    Department of Ophthalmology and Visual Science, Seoul St. Mary's Hospital, College of Medicine, Catholic University of Korea, Seoul, Korea
  • Donghyun Jee
    Department of Ophthalmology and Visual Science, St. Vincent's Hospital, College of Medicine, Catholic University of Korea, Suwon, Korea
  • Jin-Woo Kwon
    Seoul Regional Military Manpower Administration, Seoul, Korea
  • Won Ki Lee
    Department of Ophthalmology and Visual Science, Seoul St. Mary's Hospital, College of Medicine, Catholic University of Korea, Seoul, Korea
  • Correspondence: Donghyun Jee, Department of Ophthalmology and Visual Science, St. Vincent's Hospital, College of Medicine, Catholic University of Korea, #93-6Ji-dong, Paldal-gu, Suwon 442-723, Korea; donghyunjee@catholic.ac.kr
Investigative Ophthalmology & Visual Science August 2013, Vol.54, 5466-5471. doi:10.1167/iovs.13-12478
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      Jin Hae Lee, Donghyun Jee, Jin-Woo Kwon, Won Ki Lee; Prevalence and Risk Factors for Myopia in a Rural Korean Population. Invest. Ophthalmol. Vis. Sci. 2013;54(8):5466-5471. doi: 10.1167/iovs.13-12478.

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Abstract

Purpose.: To assess the prevalence and risk factors of myopia in 19-year-old males in Jeju, a rural area of Korea.

Methods.: A total of 2805 subjects were included. Refractive examination was performed with cycloplegia to test for myopia (<−0.5 diopters [D]) and high myopia (<−6.0 D). Possible associated factors were evaluated including height, weight, educational level, and color vision deficiency.

Results.: The prevalence of myopia and high myopia were 83.3% and 6.8%, respectively, in 19-year-old males in Jeju. University students in their fourth to sixth years showed a higher risk for myopia (odds ratio [OR] 2.04; 95% confidence interval [CI], 1.52–2.71, P < 0.001) than those with lower academic achievement.

Conclusions.: The prevalence of myopia was relatively high (83.3%) in a rural area of Korea, while the rate of high myopia was relatively low (6.8%) compared with that (20.6%) in an urban area of Korea.

Introduction
Myopia is a very common worldwide condition leading to visual disturbance. The high costs associated with the correction of myopia have resulted in public health and economic problems. 1,2 In particular, high myopia (<−6.0 diopters [D]) may have a devastating visual prognosis due to complications such as macular degeneration, glaucoma, and retinal detachment, 3,4 whereas low-to-moderate-grade myopia (−0.5 D to −6.0 D) can be corrected with glasses and contact lenses. Thus, the prevalence of high myopia is regarded as an important variable, and many studies have examined the pathophysiology of high myopia. 
The prevalence of myopia is increasing at an epidemic rate, particularly in East Asia, and differences exist between urban or rural areas. 59 However, there have been few studies of myopia in the Korean population. We previously reported an extremely high prevalence of myopia (96.5%), and high myopia (20.6%) among 19-year-old males in Seoul, Korea. 7 However, Seoul is a highly urbanized metropolitan city, and there may be a lower prevalence of myopia in rural areas. 10 The prevalence of myopia in rural area in Korea has not yet been evaluated. 
An association of myopia with height has been suggested due to significant positive correlations between eye growth and overall physical development. 11 The fact that cessation in the growth of eye axial length and height may occur at similar ages suggests that these two traits may share similar developmental pathways. 12 However, results of studies of this association have been inconsistent, 1316 and this association was not shown in our previous study with respect to 19-year-old males in Seoul. 7  
Color vision may influence the development of myopia through chromatic aberrations. In the human eye, long-wavelength (700 nm) and short-wavelength (450 nm) perception are separated by approximately 1.7 to 2.0 D, with very small individual variations. 17 Given that multichromatic images simultaneously stimulate the retina with different amounts of defocus, chromatic aberrations affect the emmetropization process. That is, changes in axial length compensate for defocused images. The contribution of the chromatic component of light to refractive development has been investigated in a number of animal models including chicks, 18 fish, 19 and guinea pigs. 20 Guinea pigs raised under long wavelength develop a significant myopia, and this myopiagenic effect was reversed by allowing the animals to recover under mixed-wavelength light conditions. 20 Thus, color vision deficiency that is caused by the absence or alteration of specific cone cells may affect the development of myopia. This idea is supported by the observations of Qian et al. 21 of a lower prevalence of myopia in Chinese children with color vision defects. 
To address these issues, we examined the prevalence of myopia in a 19-year-old male population in a rural area of Korea. We also investigated the associations of myopia with body stature, education levels, and color vision deficiency. 
Subjects and Methods
Study Population
The study design followed the tenets of the Declaration of Helsinki for biomedical research and was approved by the Institutional Review Board of the Catholic University of Korea in Seoul, Korea. 
A population-based cross-sectional study was performed in consecutive male conscripts who were aged 19 years and normally resident in Jeju during the study period (2011). Jeju is the largest island in the republic of Korea (South Korea) and is located in the south part of Korea. Jeju has a lower personal income (US $19,829, 2010) than that of Seoul (US $24,988, 2010), and has a higher proportion of farming and fishing industry (17.7%, 2010) than that of Seoul (0.2%, 2010). Because all 19-year-old males in South Korea have a legal obligation to undergo a physical examination for conscription, all 19-year-old males residing in Jeju Island were eligible for inclusion, yielding a total sample of 2805 individuals. Males exempted from military service were included in this study because they did undergo physical examinations in the process of determining exemption from military service. Exclusion criteria in this study included persons who had surgery for retinal detachment—such as vitrectomy, encircling, or buckling—because refractive errors may change after these surgeries. For subjects who had received refractive surgery, we obtained data concerning the preoperative refractive error from the hospitals in which they received surgery and included them using their hospital data. 
Data Collection
Cycloplegia was performed in all participants. Three drops of cyclopentolate 1% were administered at 5-minute intervals to both eyes. Autorefraction was measured by a professional optometrist with the Jeju Regional Military Manpower Administration using an autorefractor (AR-500; Canon, Inc., Tokyo, Japan), while one of the authors (JWK) supervised the procedures. Three readings were taken for each eye, and the average value was recorded. The results for each eye were converted into their spherical equivalents (sphere + 1/2 cylinder). Myopia was defined as <−0.50 D. Mild myopia was defined as <−0.50 and >−3.0 D; moderate myopia was defined as <−3.0 D and >−6.0 D; and severe myopia was defined as <−6.0 D. The examination procedure for refractive errors was the same as that in the previous report for Seoul. 7  
For anthropometric measurements, height was measured using a wall-mounted measuring scale, and weight was measured in kilograms using calibrated electronic scales. Each individual was instructed to remove any footwear and heavy clothing before height and weight measurements, which were then used to determine each participant's body mass index (BMI) using the universally recognized formula: weight (kg)/height (m) 2 . The variables of body stature were divided into four quartiles. 
Educational level was obtained from administrative recruitment documents and was classified on a scale of 1 to 3, as follows: 1 = high school education or less; 2 = 2- to 3-year college education; and 3 = 4- or 6-year university education. 
Color vision deficiency was evaluated using the Ishihara color test, which consists of many colored plates on which numbers or letters are printed in dots of primary colors surrounded by dots of other colors. These numbers or letters are clearly visible to those with normal color vision, but invisible or difficult to see to those with a color vision deficiency. 
Statistical Analyses
Analyses were performed using the Statistical Package for the Social Sciences (SPSS v. 14.0; SPSS, Inc., Chicago, IL). Logistic regression analysis was used to assess the association of myopia with body stature (based on height, weight, and BMI), education level, and color vision. 
Results
A total of 2805 19-year-old males residing in Jeju were enrolled in our study. In the educational level classification, 4- to 6-year university students made up the largest proportion (50.5%; Table 1). Mean height, weight, and BMI are shown in Table 1
Table 1
 
Characteristics of Study Participants
Table 1
 
Characteristics of Study Participants
Variable N Proportion, % Mean ± SD
Age
 19 y 2805 100.0
Sex
 Male 2805 100.0
Educational level
 High school graduate or less 436 15.5
 Student in a 2- to 3-year  college 951 33.9
 Student in a 4- to 6-year  university 1418 50.5
Height, cm 173.82 ± 5.872
Weight, kg 69.19 ± 13.09
BMI, kg/m2 22.86 ± 3.97
The refractive error for each participant was expressed as the average of the right and left spherical equivalents, because the spherical equivalent values did not differ significantly (P > 0.05). Myopia was found in 83.3% of the sample (Table 2). The prevalence of mild, moderate, and high myopia was 48.0%, 28.5%, and 6.8%, respectively. The mean refractive error with standard deviation was −2.37 ± 2.14 D, and distribution of refraction was skewed toward myopia (Figure). Prevalence of myopia according to the body statue and educational level is shown in Tables 3 and 4, respectively. Protanopia and deuteranopia had a prevalence of 1.68% and 2.28%, respectively (Table 5). 
Figure
 
Distribution of spherical equivalent in 19-year-old Korean rural population (skewness, −0.621; kurtosis, 0.492).
Figure
 
Distribution of spherical equivalent in 19-year-old Korean rural population (skewness, −0.621; kurtosis, 0.492).
Table 2
 
Prevalence of Myopia in Rural Population of Korea
Table 2
 
Prevalence of Myopia in Rural Population of Korea
Classification Subjects, n Prevalence, % (95% CI)
Myopia, >−0.50 D 2336 83.3 (81.8–84.7)
Mild myopia, −0.5 to −2.99 D 1346 48.0 (46.1–49.9)
Moderate myopia, −3.0 to −5.99 D 800 28.5 (26.8–30.2)
High myopia, >−5.99 D 189 6.8 (5.8–7.7)
 Moderate high myopia,  −6.0 D to −7.99 D 159 5.7 (4.9–6.6)
 Severe high myopia,  −8.0 D to −9.99 D 28 1.0 (0.6–1.4)
 Very severe high myopia,  >−10.0 D 2 0.1 (0.0–0.2)
Table 3
 
Prevalence of Myopia According to Body Stature in 19-Year-Old Males in Korean Rural Area
Table 3
 
Prevalence of Myopia According to Body Stature in 19-Year-Old Males in Korean Rural Area
Myopia Patients, n Prevalence of Myopia, % (95% CI)
Weight
 First quartile: <60 510 81.9 (78.8–84.9)
 Second quartile: 60–67 630 84.3 (81.6–86.9)
 Third quartile: 67–75 535 81.7 (78.7–84.6)
 Fourth quartile: ≥75 660 84.7 (82.1–87.2)
Height
 First quartile: <170 502 81.1 (78.0–84.1)
 Second quartile: 170–174 615 84.4 (81.7–87.0)
 Third quartile: 174–178 627 83.8 (81.1–86.4)
 Fourth quartile: ≥178 591 83.5 (80.7–86.2)
BMI
 First quartile: <20.14 578 82.9 (80.1–85.6)
 Second quartile: 20.14–22.12 585 83.9 (81.1–86.6)
 Third quartile: 22.12–24.67 578 82.2 (79.3–85.0)
 Fourth quartile: ≥24.67 594 84.0 (81.2–86.7)
Table 4
 
Prevalence of Myopia According to Educational Level in 19-Year-Old Males in Korean Rural Area
Table 4
 
Prevalence of Myopia According to Educational Level in 19-Year-Old Males in Korean Rural Area
Education Level Myopia Patients, n Prevalence of Myopia, % (95% CI)
High school graduate or less 347 79.6 (75.8–83.3)
Student in a 2- to 3-year college 732 77.0 (74.3–79.6)
Student in a 4- to 6-year university 1256 88.7 (87.0–90.3)
Table 5
 
Prevalence of Myopia According to Color Vision Deficiency in 19-Year-Old Males in Korean Rural Area
Table 5
 
Prevalence of Myopia According to Color Vision Deficiency in 19-Year-Old Males in Korean Rural Area
Variable Myopia Patients, n Prevalence of Myopia, % (95% CI)
Normal, n = 2694, 96.04% 2243 83.4 (81.8–84.9)
Deuteranopia, n = 64, 2.28% 53 82.8 (73.5–92.0)
Protanopia, n = 47, 1.68% 37 78.7 (66.9–90.4)
In a univariable analysis, myopia was not associated with height, weight, or BMI (P > 0.05 for each variable). Four- to six-year university students (OR 2.01, P < 0.001) showed a significantly higher risk of myopia than those with a lower academic achievement of a high school diploma or less. Deuteranopia and protanopia had no significant associations with myopia (OR 0.96, P = 0.904 and OR = 0.73, P = 0.194, respectively, Table 6). Multivariable logistic regression analysis showed that myopia was associated with educational level (OR 2.04, P < 0.001, Table 6). 
Table 6
 
Risk Factors for Myopia by Univariable and Multivariable Logistic Regression Analysis
Table 6
 
Risk Factors for Myopia by Univariable and Multivariable Logistic Regression Analysis
Univariable Analysis Multivariable Analysis
OR (95% CI) P OR (95% CI) P
Education level <0.001
 High school graduate or less 1.0 1.00
 Student in a 2- to 3-year college 0.86 (0.65, 1.13) 0.277 0.87 (0.65, 1.15) 0.334
 Student in a 4- to 6-year university 2.01 (1.51, 2.67) <0.001 2.04 (1.52, 2.71) <0.001
Weight 0.342
 First quartile: <60 1.00 1.00
 Second quartile: 60–67 1.19 (0.89, 1.58) 0.223 1.15 (0.79, 1.69) 0.451
 Third quartile: 67–75 0.99 (0.74, 1.31) 0.933 1.06 (0.62, 1.80) 1.064
 Fourth quartile: ≥75 1.22 (0.92, 1.63) 0.152 1.48 (0.73, 3.00) 0.277
Height 0.344
 First quartile: <170 1.00 1.00
 Second quartile: 170–174 1.25 (0.94, 1.67) 0.114 1.19 (0.87, 1.61) 0.277
 Third quartile: 174–178 1.20 (0.91, 1.59) 0.186 1.13 (0.81, 1.58) 0.455
 Fourth quartile: ≥178 1.17 (0.88, 1.56) 0.258 1.04 (0.71, 1.52) 0.857
BMI 0.803
 First quartile: <20.14 1.00 1.00
 Second quartile: 20.14–22.12 1.07 (0.81, 1.42) 0.614 0.95 (0.66, 1.37) 0.820
 Third quartile: 22.12–24.67 0.95 (0.72, 1.25) 0.727 0.81 (0.49, 1.32) 0.401
 Fourth quartile: ≥24.67 1.08 (0.81, 1.43) 0.582 0.79 (0.41, 1.53) 0.502
Color vision deficiency 0.429
 Normal 1.0
 Deuteranopia 0.96 (0.49, 1.85) 0.904 0.87 (0.45, 1.71) 0.703
 Protanopia 0.73 (0.364, 1.49) 0.194 0.84 (0.41, 1.73) 0.846
Discussion
The prevalence of myopia in Jeju, a rural area of Korea, was found to be high (83.3%), similar to that found in an urban area (Seoul, 96.5%). 7 It is well known that the prevalence of myopia is generally lower in rural than in urban areas. 22,23 A recent study in China demonstrated that the rate of myopia in schools located in an urban center (78.4%) was nearly double that in rural countryside schools (36.8%). 10 A similar study in Poland also showed a large difference of myopia prevalence between urban (13.9%) and rural (7.5%) children. 24 Unlike previous studies, we did not find a large difference between rural and urban areas of Korea. This finding could be due to exceptionally high educational pressures in South Korea, which is supported by the fact that even in this rural area, over 50% of young males are enrolled in 4- to 6-year university programs, and also the evidence that South Korea is one of the top countries in international educational surveys such as Program for International Student Assessment carried out by the Organisation for Economic Co-operation and Development. 25  
The prevalence of myopia in the present study (83.3%) is higher than that reported for other adolescents from rural areas of Asia and Europe. Although direct comparison is difficult due to different age groups, it is higher than that in a rural area in southern China (53.9% in 17-year-olds) 5 ; rural Shunyi District in China (36.7% in male, 55.0% in 15-year-old females) 26 ; and a rural area in Greece (36.8% in 15–18-year-olds). 27 In addition, our finding is very much higher than that of a rural Mongolian area (6.7% in 17-year-olds). 28 When compared with that reported from other military conscripts, it is higher than that in Danish conscripts (12.8% in 18-year-olds) 29 ; in Finland recruits (22.2% in 19-year-olds) 30 ; and is slightly higher than Singapore conscripts (79.3% in 17-year-olds). 31 It is often suggested that the prevalence of myopia is highest in those of Chinese ethnicity, but the prevalence we report for 19-year-old male Koreans is, to the best of our knowledge, the highest population-based prevalence of myopia yet reported, which indicates that a high prevalence of myopia is not a uniquely Chinese characteristic. 32  
Another prominent finding is that the prevalence of high myopia is relatively low (6.8%) in Jeju compared with that in Seoul (20.6%), despite the similar overall prevalence of myopia. 7 This discrepancy suggests that environmental factors in rural areas more strongly affect the development of high myopia than low-to-moderate myopia in Korea. One possible reason for this large difference is the light exposure time of subjects and onset age of myopia. People in rural area have a greater chance of increased daytime outdoor light exposure, which is a significant preventive factor for myopia. 33 Subsequently, more sunlight exposure time in rural areas may delay the age of onset of myopia. Recent studies suggest that children who become myopic early (5–6 years) can progress easily to reach high myopia. 34,35 Thus, if children become myopic slightly later in Jeju than in Seoul due to more sunlight exposure during the day, they will have less time to be highly myopic, and thus there will be a greater difference in the prevalence of high myopia than in the overall prevalence of myopia. This implies that public health care for prevention of high myopia may be more important in children at early ages. 
Distribution of refractive error in the Korean rural population had a skew to myopia and a low kurtosis. It contrasts with that reported from Western countries, in which distributions of refractive error show little skew and much higher kurtosis. 36 In addition, our finding is consistent with the distribution of refractive error for Singapore conscripts, 31 where the myopia prevalence is also high. This implies that a significant portion of young adults in Korea have myopia, whereas those in Western countries have symmetrical distribution for both myopia and hyperopia. 
Association of height with myopia was not shown in the present study. This may be because refractive error, not axial length, was measured in the present study. Most previous studies evaluating association of height with myopia showed that height was associated with axial length, but not with refractive error. 13,15,16,37,38 This suggests that emmetropization adjusts the axial length to the flat corneas of bigger eyes to produce emmetropia. 
Education was confirmed as a risk factor for myopia in the present study. However, the effect is actually small in percentage terms. In the present study, subjects with a university education had two times higher risk than those with low grade education; whereas in a Singapore study, people with a university education had 5.4 times higher risk than those without education. 39 This finding suggests that even the people with low grade education may be substantially myopic in Korea. 
The association of color vision with myopia was not shown in the present study, although subjects with color vision deficiency have a tendency of low myopia prevalence. In a recent study, students with color vision deficiencies presented a lower prevalence of myopia than those with normal color vision. 21 Considering that this study used 309 subjects with color vision deficiency, while the present study used 101 subjects of color vision deficiency, the lack of association with color vision in the present study could be from lack of statistical power. 
A limitation of this study is that the participants were a homogeneous group of 19-year-old males. The subjects used in this study may not be representative of the population of rural areas in Korea, because the rate of myopia in young people tends to be higher than in older people, and females have a higher tendency to have myopia than males. Thus, further epidemiologic studies involving all age and sex groups is needed. However, this study of the prevalence of myopia in young people is important for public health, given that the prevalence of myopia is high in young adults. Another limitation is a lack of information on near work, outdoor activity, and the family history of myopia, which are known as important risk factors. 
In conclusion, to the best of our knowledge, this is the first study to provide population-based data on the prevalence of myopia in young adult males living in a rural area in Korea. The prevalence of myopia was high (83.3%) in Jeju, a rural area of Korea. However, the rate of high myopia was lower (6.8%) compared with that in Seoul. This finding suggests that environmental factors in a rural area may reduce the development of high myopia rather than low to moderate myopia through the delay of onset of myopia. Thus, public health interventions may be important in children at early ages for prevention of high myopia. 
Acknowledgments
Supported by the Catholic Medical Center Research Foundation fund for settlement of a newly appointed professor (5-2012-B0001-00240) in the program year of 2012. The authors alone are responsible for the content and writing of the paper. 
Donghyun Jee had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. 
Disclosure: J.H. Lee, None; D. Jee, None; J.-W. Kwon, None; W.K. Lee, None 
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Figure
 
Distribution of spherical equivalent in 19-year-old Korean rural population (skewness, −0.621; kurtosis, 0.492).
Figure
 
Distribution of spherical equivalent in 19-year-old Korean rural population (skewness, −0.621; kurtosis, 0.492).
Table 1
 
Characteristics of Study Participants
Table 1
 
Characteristics of Study Participants
Variable N Proportion, % Mean ± SD
Age
 19 y 2805 100.0
Sex
 Male 2805 100.0
Educational level
 High school graduate or less 436 15.5
 Student in a 2- to 3-year  college 951 33.9
 Student in a 4- to 6-year  university 1418 50.5
Height, cm 173.82 ± 5.872
Weight, kg 69.19 ± 13.09
BMI, kg/m2 22.86 ± 3.97
Table 2
 
Prevalence of Myopia in Rural Population of Korea
Table 2
 
Prevalence of Myopia in Rural Population of Korea
Classification Subjects, n Prevalence, % (95% CI)
Myopia, >−0.50 D 2336 83.3 (81.8–84.7)
Mild myopia, −0.5 to −2.99 D 1346 48.0 (46.1–49.9)
Moderate myopia, −3.0 to −5.99 D 800 28.5 (26.8–30.2)
High myopia, >−5.99 D 189 6.8 (5.8–7.7)
 Moderate high myopia,  −6.0 D to −7.99 D 159 5.7 (4.9–6.6)
 Severe high myopia,  −8.0 D to −9.99 D 28 1.0 (0.6–1.4)
 Very severe high myopia,  >−10.0 D 2 0.1 (0.0–0.2)
Table 3
 
Prevalence of Myopia According to Body Stature in 19-Year-Old Males in Korean Rural Area
Table 3
 
Prevalence of Myopia According to Body Stature in 19-Year-Old Males in Korean Rural Area
Myopia Patients, n Prevalence of Myopia, % (95% CI)
Weight
 First quartile: <60 510 81.9 (78.8–84.9)
 Second quartile: 60–67 630 84.3 (81.6–86.9)
 Third quartile: 67–75 535 81.7 (78.7–84.6)
 Fourth quartile: ≥75 660 84.7 (82.1–87.2)
Height
 First quartile: <170 502 81.1 (78.0–84.1)
 Second quartile: 170–174 615 84.4 (81.7–87.0)
 Third quartile: 174–178 627 83.8 (81.1–86.4)
 Fourth quartile: ≥178 591 83.5 (80.7–86.2)
BMI
 First quartile: <20.14 578 82.9 (80.1–85.6)
 Second quartile: 20.14–22.12 585 83.9 (81.1–86.6)
 Third quartile: 22.12–24.67 578 82.2 (79.3–85.0)
 Fourth quartile: ≥24.67 594 84.0 (81.2–86.7)
Table 4
 
Prevalence of Myopia According to Educational Level in 19-Year-Old Males in Korean Rural Area
Table 4
 
Prevalence of Myopia According to Educational Level in 19-Year-Old Males in Korean Rural Area
Education Level Myopia Patients, n Prevalence of Myopia, % (95% CI)
High school graduate or less 347 79.6 (75.8–83.3)
Student in a 2- to 3-year college 732 77.0 (74.3–79.6)
Student in a 4- to 6-year university 1256 88.7 (87.0–90.3)
Table 5
 
Prevalence of Myopia According to Color Vision Deficiency in 19-Year-Old Males in Korean Rural Area
Table 5
 
Prevalence of Myopia According to Color Vision Deficiency in 19-Year-Old Males in Korean Rural Area
Variable Myopia Patients, n Prevalence of Myopia, % (95% CI)
Normal, n = 2694, 96.04% 2243 83.4 (81.8–84.9)
Deuteranopia, n = 64, 2.28% 53 82.8 (73.5–92.0)
Protanopia, n = 47, 1.68% 37 78.7 (66.9–90.4)
Table 6
 
Risk Factors for Myopia by Univariable and Multivariable Logistic Regression Analysis
Table 6
 
Risk Factors for Myopia by Univariable and Multivariable Logistic Regression Analysis
Univariable Analysis Multivariable Analysis
OR (95% CI) P OR (95% CI) P
Education level <0.001
 High school graduate or less 1.0 1.00
 Student in a 2- to 3-year college 0.86 (0.65, 1.13) 0.277 0.87 (0.65, 1.15) 0.334
 Student in a 4- to 6-year university 2.01 (1.51, 2.67) <0.001 2.04 (1.52, 2.71) <0.001
Weight 0.342
 First quartile: <60 1.00 1.00
 Second quartile: 60–67 1.19 (0.89, 1.58) 0.223 1.15 (0.79, 1.69) 0.451
 Third quartile: 67–75 0.99 (0.74, 1.31) 0.933 1.06 (0.62, 1.80) 1.064
 Fourth quartile: ≥75 1.22 (0.92, 1.63) 0.152 1.48 (0.73, 3.00) 0.277
Height 0.344
 First quartile: <170 1.00 1.00
 Second quartile: 170–174 1.25 (0.94, 1.67) 0.114 1.19 (0.87, 1.61) 0.277
 Third quartile: 174–178 1.20 (0.91, 1.59) 0.186 1.13 (0.81, 1.58) 0.455
 Fourth quartile: ≥178 1.17 (0.88, 1.56) 0.258 1.04 (0.71, 1.52) 0.857
BMI 0.803
 First quartile: <20.14 1.00 1.00
 Second quartile: 20.14–22.12 1.07 (0.81, 1.42) 0.614 0.95 (0.66, 1.37) 0.820
 Third quartile: 22.12–24.67 0.95 (0.72, 1.25) 0.727 0.81 (0.49, 1.32) 0.401
 Fourth quartile: ≥24.67 1.08 (0.81, 1.43) 0.582 0.79 (0.41, 1.53) 0.502
Color vision deficiency 0.429
 Normal 1.0
 Deuteranopia 0.96 (0.49, 1.85) 0.904 0.87 (0.45, 1.71) 0.703
 Protanopia 0.73 (0.364, 1.49) 0.194 0.84 (0.41, 1.73) 0.846
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