Acta Ophthalmologica 2014

Prevalence and risk factors of visual impairment and blindness in Korea: the Fourth Korea National Health and Nutrition Examination Survey in 2008–2010 Tyler H. T. Rim,1,* Jae S. Nam,2,* Moonjung Choi,1 Sung C. Lee1 and Christopher S. Lee1 1

Department of Ophthalmology, Institue of Vision Research, Yonsei University College of Medicine, Seoul, Korea Department of Medicine, Yonsei University College of Medicine, Seoul, Korea


ABSTRACT. Purpose: To describe the age, gender specific prevalence and risk factors of visual impairment and blindness in Korea. Methods: From 2008 to 2010, a total 14 924 randomly selected national representative participants of the Korea National Health and Nutrition Examination Survey underwent additional ophthalmologic examinations by the Korean Ophthalmologic Society. Best Corrected Distance Visual Acuity was measured using an international standard vision chart based on Snellen scale (Jin’s vision chart). Independent risk factors for visual impairment were investigated using multivariate logistic regression analysis. Results: The overall prevalence of visual impairment (≤20/40) of adults 40 years and older was 4.1% (95% CI, 3.6–4.6) based on the better seeing eye. The overall prevalence of blindness (≤20/200) for adults 40 years and older was 0.2% (95% CI, 0.1–0.3). Risk indicators of visual impairment were increasing age, low education status, living in rural area, being unemployed, being without spouse and the absence of private health insurance. The visually impaired were more likely to have eye diseases compared with the normal subjects, and they were less likely to utilize eye care. Conclusion: The prevalence of visual impairment was demonstrated to be higher while that of blindness was similar to previous population studies in Asia or U.S. Sociodemographic disparities are present in the prevalence of visual impairment and more targeted efforts are needed to promote vision screening in high risk groups. Key words: blindness – Korea National Health and Nutrition Examination Survey – risk factors – visual impairment

Acta Ophthalmol. 2014: 92: e317–e325 ª 2014 Acta Ophthalmologica Scandinavica Foundation. Published by John Wiley & Sons Ltd

doi: 10.1111/aos.12355

*These authors contributed equally to this work.

Introduction Visual impairment and blindness are important global health issues that

exert significant influence not only on the individual’s quality of life, but also on the society due to its economic and social impact. Recent estimates by the World Health Organization (WHO)

suggest that 285 million people are visually impaired, 121 million of which are due to uncorrected refractive errors and 39 million are blind worldwide. The WHO states that there is a paucity of population-based national data on the prevalence and causes of blindness and visual impairment in developing and undeveloped countries. Such data are essential for planning services for the realization of the goals of VISION 2020: the Global Right to Sight initiative (World Health Organization. Visual impairment and blindness. Available at ness/Vision2020_report.pdf Accessed 13 November 2011). Numerous studies have been carried out to study the prevalence and socioeconomic risk factors for blindness and visual impairment across the continents, some of them also investigating the racial and ethnic differences (Munoz & West 2002; Varma et al. 2004; Maberley et al. 2006; Limburg et al. 2008; Yamada et al. 2010). In Asia, several studies have been conducted in Singapore (Saw et al. 2004; Ong et al. 2012), China (Xu et al. 2006a,b) and Japan (Iwano et al. 2004; Nakamura et al. 2010); however, there are limited national epidemiological data for the causes and prevalence of visual impairment and blindness in Asian countries. Previous research in 1971 and 1972 in Korea on 94 799 eye patients reported prevalence of 4.8% for single eye blindness and 1.9% for blindness in both eyes (BS & BS 1974). Unfortunately, as this research was reported about 40 years


Acta Ophthalmologica 2014

ago, researchers have yet to conduct a population-based study to accurately define the present state of ocular health in Korea. We investigated the independent risk factors based on a national health survey in Korea: The Fourth Korea National Health and Nutrition Examination Survey 2007–2010 (KNHANES IV and V-1), a national representative survey conducted by the Ministry of Health and Welfare that provides data on vision status and sociodemographic factors of about 15 000 adults aged over 30 years. Over the past several decades, South Korea, as well as other several Asian countries, has experienced rapid socioeconomic growth. Understanding the current status of vision-related socioeconomic disparities is important for the design, implementation and evaluation of programmes intended to reduce these disparities. Therefore, this study aims to report the prevalence of visual impairment and blindness and its association with sociodemographic factors in Korea and to provide useful data for targeted prevention.

Materials and Methods Design and study population

A detailed description of the sampling, enumeration, visual acuity (VA) and ocular examination procedures has already been published (Yoon et al. 2011). KNHANES IV (2007–2009) was conducted as a national health survey in Korea that used a stratified, multistage, clustered sampling method based on 2005 National Census data to randomly select a population-based sample of 24 871 individuals across 500 national districts to represent the civilian, noninstitutionalized, South Korean population, and sample design and size were estimated properly so that annual survey results could represent the whole population in Korea. Ophthalmologic interviews and examinations were conducted from July 2008. KNHANES V (2010–2012) involved a population-based random sampling of households across 576 national districts (192 each year), which were selected by a panel to represent the South Korean population using a stratified, multistage, clustered sampling method based on 2009 National Resident demographics. All members of each selected household were asked to participate in


the survey. KNHANES V-1 refers to the first year (2010) of KNHANES V. Surveys prior to KNHANES IV were able to be analyzed and could be considered a national representative sample after 3 years when the survey was completed, but rolling survey sampling methods were applied from KNHANES IV which allowed annual analysis of national representative sample data possible. All examination and health interviews by trained teams including Ophthalmology residents were conducted in mobile centres while nutrition survey was carried out in household. This survey is aimed to determine the prevalence of the vision status and common eye diseases in a population-based sample in Korea. The ophthalmologic survey was designed to continue for 5 years from July 2008 to 2013. This study includes all results of ophthalmologic survey in KNHANES IV and V-1 including data from a survey conducted from July 2008 to December 2010. During this period, a total of 22 477 participants were recruited and received an eye examination. Ages of the study participants ranged from 3 to 95 years of age and 10 178 were men and 12 299 were women. Of the total 22 477 participants, 14 924 over the age of 30 were selected for this study. The participation rates were 74.3% (9308 of the 12 528 subjects) in 2008, 79.2% (10 078 of the 12 722 subjects) in 2009, and 77.5% (8473 of the 10 938 subjects) in 2010. VA testing

Uncorrected VA and/or Best Corrected Distance Visual Acuity (BCVA) were measured at a distance of 4 m using an international standard vision chart based on Snellen scale (Jin’s vision chart, Seoul, Korea; Jin 1997). The participant’s VA was measured in each eye, right followed by left with his or her existing refractive correction if he or she had one. The participant was asked to read numbers from 0.2, proceeding to the next line if he or she read more than three letters among five letters correctly. The participant’s VA was defined as the line with the smallest numbers in which he or she read more than three characters accurately. Automated refraction was performed in all participants using the autorefractor-

keratometer (KR8800; Topcon, Tokyo, Japan), followed by retesting of VA after applying pinhole in patients whose VA was below 0.8 based on Snellen chart. Eye clinic use

Subjects aged over 19 years were asked the question ‘when was the last time you had an eye examination by an ophthalmologist?’ with possible responses of ‘≤1 month’, ‘>1 month and ≤1 year’, ‘>1 year and ≤3 years’, ‘> 3 years’, and ‘never’. Definitions based on better or worse seeing eye

As there is no worldwide consensus on the definition of visual impairment, we used three definitions of visual impairment and two definitions of blindness based on the VA to allow comparison with other population-based studies (Varma et al. 2004). Visual impairment definition 1: BCVA of 20/40 or worse (including 20/40). This definition of visual impairment has been used in the Beaver Dam Eye Study (Klein et al. 1991) and the Blue Mountain Eye Study (Attebo et al. 1996). Visual impairment definition 2 (U.S. definition): BCVA worse than 20/40 but better than 20/200 (not including 20/40 or >20/200). This definition has been used in the Baltimore Eye Survey (Tielsch et al. 1990), the Barbados Eye Study (Hyman et al. 2001), the Rotterdam Study (Klaver et al. 1998), Proyecto VER (Munoz et al. 2002), and the Salisbury Eye Evaluation (Munoz et al. 2000). Visual impairment definition 3 (WHO definition). BCVA worse than 20/63 but better than or equal to 20/400 (not including 20/63 but including 20/400). This definition has been used by the Baltimore Eye Survey (Tielsch et al. 1990), the Barbados Eye Study (Hyman et al. 2001), the Rotterdam Study (Klaver et al. 1998), and the World Health Organization (Available at: html). Blindness definition 1. BCVA of 20/200 or worse This definition has been used in the United States by different federal

Acta Ophthalmologica 2014

agencies to define disability caused by blindness, as well as in most population-based studies of eye disease (Tielsch et al. 1990; Klein et al. 1991; Attebo et al. 1996; Klaver et al. 1998; Munoz et al. 2000, 2002; Hyman et al. 2001). Blindness definition 2 (WHO definition). BCVA of worse than 20/400 (not including 20/400) This definition of blindness has been used by the World Health Organization (Available at: inf-fs/en/fact145.html) and in many population-based studies (Tielsch et al. 1990; Attebo et al. 1996; Taylor et al. 1997; Klaver et al. 1998; Munoz et al. 2000; Hyman et al. 2001).

Independent variables

From the KNHANES IV and V-1 data set, we collected data of various sociodemographic factors, which were obtained through direct interviews using standardized questionnaires. Current age (30–39/40–49/50–59/60– 69/70 or older), sex (men/women), monthly household income (lowest quintile/2nd–4th quintile/highest quintile), and highest educational level reached (elementary school graduates or lower/middle school graduates/high school graduates/university graduates or higher). residential area (urban/ rural), occupation (Administrator, Management, Professional/Business and financial operations occupations/ Sales and related occupations/Farming, fishing, and forestry occupations/ Installation, maintenance, repair occupation, and technicians/Labourer/ Unemployed), having a spouse (living with spouse/living without a spouse), having National Health Insurance (NHI) or Medicaid (NHI/Medicaid) and having Private Health Insurance (PHI; yes/no). Household monthly income was divided into tertiles. Income per adult equivalent was calculated using the formula household income/square root of number of persons in the household (Deaton & Lubotsky 2003). Under the NHI system, almost the entire population have been eligible for medical security (medical insurance and medical aid) in South Korea since 1991. In Korea, supplementary PHI usually covers additional medical costs and special medical needs such as hospital-

ization, heart and brain diseases, and cancer. The term ‘spouse’ was applied to individuals who were legally married or cohabiting, the term ‘without spouse’ was applied to single, divorced, or separated individuals.

occupation, spouse, and insurance status. All statistical tests were two-sided at 95% CI and were performed using the STATA/SE 12.1 software (StataCorp).

Statistical analysis

Study population

Basic characteristics of the study population were reported by descriptive statistics. To calculate the weight of KNHANES IV in accordance with the guidelines of the 2005 Census of Korea, poststratification adjustment based on response rates and extraction rate to include the same distribution of 2005 Korean population in sex and age group of 5 years interval was performed. Finally, the sum of the weight of KNHANES IV is equal to the Korean population as of 2005. We calculated the weight of KNHANES V-1 in a similar manner based on the 2010 Korean population (in accordance with the 2010 Census of Korea) in regards to sex and age groups at 5-year intervals. Prevalence estimates for all outcomes were performed for the overall sample and then in age and gender stratified groups. Based on weight, the prevalence of visual impairment and blindness in Korea was calculated by age and gender. Data were analyzed using the survey procedure of STATA/SE version 12.1 software (StataCorp, College Station, TX, USA) to account for the sample design and sampling weight, which was adjusted for oversampling, nonresponse, and the Korean population. Multivariate logistic regression analysis was used to determine independent predictive factors for visual impairment. The adjusted odds ratios (aOR) and 95% confidence interval (CI) were calculated. To evaluate the eye clinic utilization in subjects with visual impairment, the proportion of eye clinic use in visual impairment group by each definition was evaluated using unadjusted mean and adjusted mean based on linear regression analysis as the last visit to the eye clinic being

Prevalence and risk factors of visual impairment and blindness in Korea: the Fourth Korea National Health and Nutrition Examination Survey in 2008-2010.

To describe the age, gender specific prevalence and risk factors of visual impairment and blindness in Korea...
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