Subjects
This study was a school-based cross sectional, case-control study. We performed ophthalmologic examinations on 630 children from eight primary schools in Seoul, Korea (urban, male:female = 318:312) and in 286 children in Paju, Korea (rural, male:female = 168:118) from May through October 2015. Seoul is the capital city of South Korea and a large urban center with a population of approximately 10 million. Paju is a city with a population near 400,000. We performed ocular exams on children in younger (1st to 3rd grade, aged 7 to 9) and older (4th to 6th grade, aged 10 to 12) grades from elementary schools in these two regions.
This study was performed according to the Declaration of Helsinki on Biomedical Research Involving Human Subjects. The Institutional Review Board of Chung Ang University of Medicine Review Board approved the clinical study. Both parents and children themselves provided written informed consent after being given a detailed explanation of the study. We confirmed that parents had seen the manuscript and patient data and agreed to its publication in a journal.
The relative humidity in this region of Korea during the time of the study (May to October) ranged from 57.8 to 58.2 % in urban areas (Seoul) and 58.1 to 58.4 % in rural areas (Paju). There was no significant difference in relative humidity between the two regions.
Exclusion criteria were: (1) children who underwent any type of eye surgery in the past 6 months, (2) children who had nocturnal lagophthalmos, (3) children who had eyelid problems (trichiasis, districhiasis, or epiblepharon), (4) children who had allergic conjunctivitis with the use of antihistamine drugs, (5) children who wear contact lenses, and (6) children who had congenital, endocrinal, or autoimmune disease.
Ocular examinations
A single examiner performed all ocular exams. These included visual acuity tests (best corrected VA with trial lens, LogMAR), autokeratomery, slit-lamp examinations of the cornea and conjunctiva, and evaluation of eyelid problems, allergic conjunctivitis, and exposure keratitis. Autokeratometry was measured using Topcon Auto Kerato-Refractometer KR-8800 by a single examiner; we reported the mean value of the three repeated measurements.
Tear break-up time (TBUT) was measured with a fluorescein strip (Haag-Streit Inter- national, Koniz, Switzerland) coated with one drop of balanced salt solution (BSS; Alcon Laboratories, Inc., Fort Worth, TX). After applying the strip to the inferior conjunctival fornix, the participant resumed normal blinking for several seconds. After the fluorescein solution spread across the corneal surface, the participant was asked to keep his or her eye open until the first defect of the tear film occurred. TBUT was defined as the interval between the last complete blink and the first appearance of a dry spot on the pre-corneal surface of the tear film [8]. The procedure was repeated three times for each eye tested, with results reported as the mean value of the three measurements.
A single examiner evaluated punctate epithelial erosions (PEE) in the cornea and conjunctiva using a slit lamp according to the Oxford Scheme Panel from 0 to 5 grades [9]. The study protocol is included (Additional file 1).
Questionnaires
A self-administered questionnaire was given to the children and their parents and both children and their parents completed each questionnaire. Questionnaires were designed to obtain information regarding risk factors for dry eye disease, including the mean daily duration of video display terminal (smartphone, computer, television) use, learning (reading and writing), outdoor activities, and past history of allergic disease and antihistamine drug use. Subjective ocular symptoms were measured with the modified ocular surface disease index (OSDI) score. This index was an objective parameter for DED diagnosis. Modified OSDI scores range from 0 to 100 points, and higher scores indicate greater discomfort due to dry eye disease. We added the OSDI index and ophthalmologic questionnaire (Additional file 1).
Diagnostic criteria of dry eye disease
Dry eye disease was diagnosed using a combination of questionnaire data and clinical ophthalmologic testing that elicited information on signs and symptoms based on the 2007 Dry Eye Workshop (DEWS) guidelines [10].
Objective signs included (1) TBUT less than 10 s or (2) positive corneal and conjunctival fluorescein staining. A modified OSDI score greater than 20 points was used for a positive measure of subjective symptoms. Children who had one or more objective signs and more than 20 points on the modified OSDI score were analyzed in the DED group.
Statistical analysis
We divided the school children into two groups and made the following comparisons: DED vs. control, urban vs. rural, younger graders vs. older graders. We used multivariate logistic regression analysis to assess DED risk and protective factors. Then, during 4 weeks of smartphone cessation in the DED group, we compared before and after parameters. Statistical analyses were performed using SPSS for Windows (version 21; SPSS, Inc., Chicago, IL). Continuous variables were compared between groups using the Student’s t-test. The chi-square test was used to compare non-continuous variables between the two groups. P values less than 0.05 were considered statistically significant.