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Methods
Study Population
This study was based on data obtained from the 2008 Korean National Health and Nutrition Examination Survey (KNHANES), a cross-sectional and nationally representative survey conducted by the Korean Ministry of Health and Welfare. The target population of the survey comprised noninstitutionalized civilians who have resided in Korea for more than 1 year. The sampling units were households selected through a stratified, multistage, probability sampling design based on geographic area, sex, and age group, using household registries. In this study, 12,528 participants from 4,000 households were included, and the response rate was 82.8%. Weights indicating the probability of being sampled were assigned to each participant, thus producing results that represent the entire Korean population. Participants completed the four parts of the questionnaire: health interview survey, health behavior survey, health examination survey, and nutrition survey. The health interview survey and the health examination survey were completed by 9,308 (74.3%) of the 12,528 participants. We excluded male participants, individuals younger than 19 years, those without laboratory data (such as lipid and fasting plasma glucose levels), and those who had not fasted for at least 12 hours before blood sampling. Participants with a history of glaucoma were also excluded. After these exclusions, 4,524 women were included in our final analysis. This study was approved by the institutional review board of Yonsei University College of Medicine (Seoul, Republic of Korea).
Data Collection
At the time of the 2008 KNHANES, citizens were informed that they had been randomly selected as a household to voluntarily participate in the nationally representative survey conducted by the Korean Ministry of Health and Welfare, and that they had the right to refuse to participate in accordance with the National Health Enhancement Act supported by the National Statistics Law of Korea. The participants gave a written informed consent form to participate in the study. The Korea Centers for Disease Control and Prevention also obtained a written informed consent form from the participants for the use of their blood samples for further analysis. The health examinations, which were performed in 2008, included medical history, physical examination, a questionnaire about health-related behaviors, and anthropometric and biochemical measurements. Physical examinations were performed by trained medical staff based on standardized procedures. Participants were asked about lifestyle behaviors, including cigarette smoking, alcohol consumption, and dietary habits.
Smoking status was categorized into current smoker (a person who smokes cigarettes daily), ex-smoker (a person who smoked in the past but does not smoke cigarettes currently), and never smoker (a person who has never smoked a cigarette). Based on how often participants consumed any type of alcohol, alcohol consumption was assessed into three categories: one drink or less per week, two or three drinks per week, and four drinks or more per week. Physical activity was divided into three groups according to frequency of exercise: one time or less per week, two or three times per week, and four times or more per week. KNHANES adopted the International Physical Activity Questionnaire for the frequency of physical activity. If they were being treated for any disease, they were asked for data on the diagnosis and for a list of medications being taken. Menopause was defined as the participant's self-reported menopause status or having had hysterectomy. Completed questionnaires were reviewed by trained staff and entered into a database. Body weight and height were measured to the nearest 0.1 kg and 0.1 cm, respectively, with participants wearing light indoor clothing without shoes. Body mass index was calculated as the ratio of weight (kg) to height squared (m). Blood pressure was measured on the right arm using a standard mercury sphygmomanometer (Baumanometer, Copiague, NY). Systolic and diastolic blood pressure readings were recorded twice at 5-minute intervals and averaged for analysis. After a 12-hour overnight fast, blood samples were obtained from the participants through an antecubital vein. Fasting plasma glucose, triglyceride, and high-density lipoprotein cholesterol levels were measured using an ADVIA1650 autoanalyzer (Siemens Medical Solutions Diagnostics, Erlangen, Germany). Ophthalmologic examinations were performed by study ophthalmologists, using a slit lamp (Haag-Streit model BQ-900; Haag-Streit AG, Koeniz, Switzerland), to determine the presence of diseases in the anterior segment of the eye and to measure intraocular pressure. Intraocular pressure in both eyes was measured with a Goldmann applanation tonometer, and the mean of the values was used as outcome.
Definition of Metabolic Syndrome
The National Cholesterol Education Program Adult Treatment Panel III’s modified definition of metabolic syndrome was used. Metabolic syndrome was defined by the presence of three or more of the following risk factors: (1) abdominal obesity (waist circumference ≥80 cm); (2) elevated blood pressure (systolic/diastolic blood pressure ≥130/85 mm Hg and/or taking antihypertensive medication); (3) impaired fasting glucose (≥100 mg/dL), diagnosis of diabetes mellitus, and/or taking insulin or diabetic pill; (4) raised plasma triglycerides (≥150 mg/dL); and (5) low high-density lipoprotein cholesterol (<50 mg/dL).
Statistical Analysis
Data from the 2008 National Census, carried out by the Korea National Statistical Office, were used to define the standard population. Statistical estimates were weighted to represent the total population in Korea. Demographic and biochemical characteristics of the study population, according to menopause status, were compared using independent two-sample t test for continuous variables and χ test for categorical variables. Mean intraocular pressure according to the number of metabolic syndrome characteristics was calculated using one-way analysis of variance. Mean age-adjusted intraocular pressure according to metabolic syndrome components was calculated using analysis of covariance. Multiple linear regression analysis was used to assess the relationship between metabolic syndrome and intraocular pressure after adjusting for confounding variables according to menopause status. All analyses were conducted using SAS statistical software, version 9.1 (SAS Institute Inc., Cary, NC). All statistical tests were two-sided, and statistical significance was determined at P < 0.05.
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