Survey and subjects
This study used data from the Korean National Health and Nutrition Examination Survey (KNHANES-V), which was performed in 2010–2012 by the Korean Centers for Disease Control and Prevention and the Korean Ministry of Health and Welfare, Sejong, Republic of Korea. A total of 25,534 individuals in KNHANES-V were identified as candidates for this study. Participants aged less than 19 years (n = 5,935) were excluded, as were adults with a previous history of eyelid or intraocular surgery, or with a medical condition that might affect the position of the eyelid or motility, including thyroid disease, systemic collagen disease, myopathy and cerebrovascular disease (n = 1,721). In total, 17,878 adults were included in the final analyses. All participants provided written consent forms prior to enrollment. The Institutional Review Board of the Korea Centers for Disease Control and Prevention (KCDC) reviewed and approved this nationally representative data study.
Sociodemographic and lifestyle variables
Educated interviewers from the KNHANES-V administered standardized health examinations and questionnaires. Current smokers were defined as participants who currently smoked and had smoked more than 100 cigarettes in their lifetime. Participants were also categorized based on the quantity of alcohol consumed per day for the month prior to the interview; participants were considered heavy drinkers if they had consumed more than 30 g per day. Participants who performed moderate exercise at least five times per week for 30 minutes or more per session, or who performed vigorous exercise at least three times per week for 20 minutes or more per session, were considered regular exercisers. The education level was classified as high if the respondent had completed a high school education. A monthly household income of less than 1,092.40 United Sates dollars (USD) corresponded to the lowest income quartile.
Anthropometric and biochemical measurements
Anthropometric measurements were performed by specially trained examiners. Body weight and height, measured with participants barefoot and wearing light clothing, were used to calculate the body mass index (BMI). Waist circumference (WC) was estimated to the nearest 0.1 cm in a horizontal plane at the midpoint from the iliac crest to the costal margin at the end of a normal expiration. Blood pressure was checked every 8 hours via the right arm while the participant was in a sitting position, after about 5 minutes of relaxation, by a mercury sphygmomanometer (Baumanometer; Baum, Copiague, NY, USA). The definitive blood pressure was acquired by mean values of the second and third estimates. A participant was classified as hypertensive if the systolic blood pressure was more than 140 mmHg or the diastolic blood pressure was more than 90 mmHg, or if the participant had been diagnosed with high blood pressure and was taking high blood pressure medication. The percent body fat (fat mass÷total mass × 100, %) and sum of the lean soft tissue mass of the arms and legs were obtained by dual-energy X-ray absorptiometry (DXA, QDR 4500 A; Hologic, Waltham, MA, USA) at mobile examination centers.
The eyelid positions of all participants were examined by specially educated opticians who were resident physicians. The quality control of the ophthalmic survey was performed by the Epidemiologic Survey Committee of the Korean Ophthalmologic Society. Marginal reflex distance 1 (MRD1) was defined as the distance from the upper eyelid margin to the corneal light reflex in the primary position [13,14]. A differential diagnosis of blepharoptosis was made with particular attention paid to pseudoptosis associated with eyebrow ptosis and dermatochalasis. MRD1 values were obtained and sorted into five subclassifications: (1) ≤ 4.0 mm, (2) 3.0–3.9 mm, (3) 2.0–2.9 mm, (4) 1.0–1.9 mm, and (5) < 1.0 mm. Before data analyses, we defined blepharoptosis as an MRD1 of less than 2 mm for either eye. Levator function test (LFT) was also evaluated by measuring the upper eyelid excursion from downgaze to upgaze, excluding any influence of frontalis muscle function, and sorted into four subclassifications: (1) ≤ 12 mm, (2) 8–11 mm, (3) 5–7 mm, and (4) < 4 mm.
Statistical analyses were conducted using the survey procedure of SAS Windows software (ver. 9.3; SAS Institute, Cary, NC, USA) to account for the complex sampling design from the KNHANES which can supply nationally representative blepharoptosis prevalence estimated values. A two-sided P value less than 0.05 was allowed for statistically important. Fundamental characteristics of participants with and without blepharoptosis were expressed as proportions (% standard error, SE) for categorical variables and as means ± SE for continuous variables. The chi-square test or independent two-sample t-test were performed to compare differences in participant characteristics depending on having blepharoptosis or not. The cutoff age in the prediction of blepharoptosis was defined as the point of largest statistical value by the Rao-Scott chi-square test.