The ophthalmic questionnaires included history of ocular examinations; recent ocular examinations; family history of eye diseases; history of cold extremities and migraines; UV exposure time; ophthalmic surgery history; history of diagnosis by ophthalmologist including DED, cataract, ARMD, and glaucoma; symptoms of DED; current treatment for glaucoma; causes of visual impairment in cases of Snellen VA less than 0.32; near work duration; and parental history of myopia. The time points and age groups subjected to the questionnaire survey varied by year and are described in Table 1. Briefly, in KNHANES IV-V (2008-2012), ophthalmologic investigators queried participants about their history regarding ocular examination, cold extremities, migraines, and UV exposure time. The possible responses for UV exposure time included: "<5 hours" or "≥5 hours" in the KNHANES IV (2008-2009) and "<2 hours," "two-five hours," or "≥5 hours" in the KNHANES V (2010-2012). History of recent ocular examination and family history of eye disease were evaluated in KNHANES IV to VI (2008-2015), and history of ophthalmic surgery was investigated during KNHANES V-VI (2010-2015). In the KNHANES V (2010-2012), subjects were asked about a history of diagnosis of DED, cataract, ARMD, and glaucoma by ophthalmologists. To make data collection more accurate, subjects were also asked about symptoms they had experienced related to DED such as dryness, foreign body sensation, or burning and about the current status of medical treatment for glaucoma in the KNHANES V (2010-2012). In the cases of Snellen VA less than 0.32, interviews were conducted about the causes of visual impairment in the last year of KNHANES V (2012). Questions about near-work duration and parental history of myopia were added to the questionnaire in the KNHANES VI (2013-2015).

A list of all examination procedures according to age group and test period is shown in Table 2. Uncorrected VA and/or best-corrected distance VA were measured at a distance of 4 meters using an international standard vision chart based on the Snellen scale (Jin's vision chart, Seoul, Korea) [52]. Participant VA was measured in each eye, right side followed by left side. Each participant was asked to read numbers in the 0.2 line of the VA chart and to proceed to the next line if he or she correctly read at least three of the five letters. Participant VA was defined as the line with the smallest numbers in which the participant accurately read more than three characters. For those participants who presented with VA score lower than 0.8, corrected VA was measured using autorefraction. Automated refraction was performed using an autorefractive keratometer (KR8800; Topcon, Tokyo, Japan), followed by VA retesting using a pinhole in patients with Snellen VA lower than 0.8. Spherical equivalent refractive error was calculated as sphere +1 / 2 cylinder. These examinations were carried out throughout the KNHANES IV-VI (2008-2015).

Strabismus testing conducted from 2008 to 2011 included the cover-uncover test, prism and alternative cover test, and/or Krimsky test. Strabismus was defined as a manifest or latent ocular deviation at distance or near fixation with or without spectacle correction, esodeviation of 10 or more prism diopters, exodeviation of 15 or more prism diopters, or any vertical deviation.

Blepharoptosis was determined by measuring the marginal reflex distance 1 in the KNHANES IV and V (2010-2015). Participants were positioned at physician eye level and instructed to look straight ahead and relax while focusing on a distant target. After shining a penlight into the participant's eye, the distance from the corneal light reflex to the upper eyelid margin was measured in millimeters. Differential diagnosis of blepharoptosis was made with particular attention to pseudoptosis associated with eyebrow ptosis and dermatochalasis. Blepharoptosis was defined as a marginal reflex distance 1 of 2 mm or less.

Investigators also conducted structured slit-lamp examinations (Haag-Streit model BQ-900; Haag-Streit AG, Koeniz, Switzerland) to test for diseases in the anterior segment of the eye, such as pterygium (2008-2011) and cataract (2008-2012), and to measure the IOP (2008-2012) and anterior chamber depth using the Van Herick method (2008-2012) [53]. Standardized Lens Opacities Classification System (LOCS) III photographic images were used to assess cataracts. Cataract was defined as nuclear (LOCS III score ≥4 for nuclear opalescence or nuclear color), cortical (LOCS III score ≥2 for cortical cataracts), posterior subcapsular (LOCS III score ≥2 for posterior subcapsular), or mixed (more than one type per eye) based on comparison with these standard photographs [54]. Pseudophakic and aphakic eyes were also documented. Pterygium was defined as a radially-oriented fibrovascular lesion crossing the nasal or temporal limbus. Grading was based on the visibility of the underlying episcleral blood vessels [55]. IOP was measured once in each eye from the right side to left side by a trained ophthalmologist with a Goldmann applanation tonometer (Haag-Streit AG).

A digital nonmydriatic fundus camera (TRC-NW6S; Topcon) and Nikon D-80 digital camera (Nikon, Tokyo, Japan) were used to obtain digital fundus images throughout the KNHANES IV and V (2008-2012). Digital images were captured under physiological mydriasis in all participants 19 years of age or older. For each participant, one 45° nonmydriatic digital retinal image centered on the fovea was taken per eye (two images per person). Optic nerve configuration and any retinal pathologic findings were recorded. Patients were considered to have early ARMD if they demonstrated presence of soft indistinct drusen or reticular drusen or presence of hard or soft distinct drusen with pigmentary abnormalities (increased pigmentation or hypopigmentation of the retinal pigment epithelium) in the absence of signs of late ARMD. Late ARMD included the signs of wet ARMD or geographic atrophy. Wet ARMD was defined as retinal pigment epithelial detachment or serous detachment of the sensory retina, subretinal or sub-retinal pigment epithelium hemorrhages, and subretinal fibrous scars. Geographic atrophy was defined as a circular discrete area (175 microns in diameter or greater) of retinal depigmentation with visible choroidal vessels, in the absence of signs of wet ARMD [56]. In participants with a history of DM or random blood glucose level of 200 mg/dL or higher and/or suspicion of DR in nonmydriatic fundus photography findings, seven standard field photographs were obtained from each eye after pharmacological pupil dilatation, as per the Early Treatment for Diabetic Retinopathy Study protocol, throughout years 2008 to 2011 [57]. On the other hand, in the last year of the KNHANES V (2012), fundus photography was performed for every participant with DM without pharmacological pupil dilatation. DR was identified in the presence of any characteristic lesion determined by the Early Treatment for Diabetic Retinopathy Study severity scale: microaneurysms, hemorrhages, hard exudates, cotton wool spots, intraretinal microvascular abnormalities, venous beading, and retinal new vessels [58,59]. The prevalence of DR among individuals with DM was estimated. Each fundus image was graded twice. First, preliminary grading was conducted onsite by ophthalmologists or ophthalmologic residents trained by the Korean Ophthalmologic Society. Retinal specialists with expertise in DR grading and ARMD diagnosis then performed detailed final grading using protocols from the Early Treatment for Diabetic Retinopathy Study and International Age-related Maculopathy Epidemiological Study Group.

In the KNHANES IV-V (2008-2012), automated visual field testing (Humphrey Matrix frequency-doubling perimeter; Carl Zeiss Meditec Inc., Dublin, CA, USA) with the N-30-1 screening program was performed on participants with any of the following five conditions: (1) elevated IOP (≥22 mmHg), (2) horizontal or vertical cup-to-disc ratio ≥0.5, (3) presence of optic disc hemorrhage, (4) presence of retinal nerve fiber layer defect, or (5) violation of the inferior-superior-nasal-temporal rule. Frequency doubling technology was repeated once if the initial results were deemed unreliable. Patients were considered to have primary open angle glaucoma if they met any one of the category I or II diagnostic criteria previously described [60]. In 2012, however, frequency doubling technology perimetry was conducted for all participants aged 40 years or older, regardless of glaucoma suspicion.

The Hardy-Rand-Rittler pseudoisochromatic color vision test was performed throughout the KNHANES VI (2013-2015) [61]. There were six screening plates, four for protan-deutan deficiencies and two for tritan deficiencies. If all six boxes showed correct responses, the patient had normal color vision, and no more testing was conducted. If the fifth or sixth plates were not noted, the patient was considered to have defective blue-yellow vision, and the examiner proceeded to show plates 21 to 24. If any of the boxes corresponding to plates 7 to 10 were not noted, the patient had defective red-green vision, and the examiner proceeded to show plates 11 to 20. After testing was complete, the total numbers in each column were summed in the spaces under the responses for plates 20 and 24. Participants were diagnosed as protan or deutan if the total numbers of marks in the protan or deutan columns, respectively, was greater than that in the opposite column. In cases of blue-yellow deficiency, participants were diagnosed as tritan or tetartan if the number of errors in the tritan or tetartan columns, respectively, was greater than that in the other column. Participants were diagnosed as having unclassified red-green or blue-yellow defects if the number of marks were the same in the protan and deutan or tritan and tetartan columns or if errors were made only in the screening plates. Color vision deficiency was graded as mild, medium, or strong, depending on whether the participants saw the symbols on the more saturated plates. There were 10 grading plates for protan/deutan defects: patients who made one or more errors in the two plates with the most saturated colors were graded as severe, and those who made an error in the next three most saturated plates were graded as medium. Those who made errors only in the five least-saturated plates were graded as mild. Similar interpretation was performed in the case of blue-yellow deficiency (Fig. 2).