2.1 Sample Size Calculation
A pre-survey was conducted in a randomly selected high school (323 subjects were involved). The prevalence rate of DED was 73.07%. The formula n = Z2p(1-p)/δ2 was used in the study (here, α=0.05, Z=1.96, p=73.07%, δ=0.02). Additionally, a nonresponse rate of 5% was considered. Therefore, a minimum of 1990 subjects was required in this study.
2.2 Sampling and Survey
A cross-sectional survey was conducted among public high schools in Hongkou District in Shanghai from November to December 2020. Letters were sent to the principals of all 11 public high schools (according to the public high school list published by Hongkou District Education Bureau; http://www.shhk.gov.cn/hkjy/) located in Hongkou district in Shanghai. The letter explained the purpose of the study. Finally, nine public high schools consented to the study. All students in the nine schools were recruited. A total of 4825 subjects were included in our survey. Written informed consent was obtained from all subjects and parents of subjects of age group less than 16. The study was approved by the Ethics Committee of Shanghai General Hospital and adhered to the tenets of the Declaration of Helsinki (NO. 2020KY026).
2.3 OSDI Questionnaire
The ocular surface disease index questionnaire (OSDI questionnaire) is one of the most widely used questionnaires to evaluate DED.[8, 9] It is viewed as one of the gold standards regarding DED according to the Dry Eye Work Shop (DEWS) consensus. In this study, a simplified Chinese version of the OSDI questionnaire, whose reliability and validity was previously verified, was utilized. It was divided into three dimensions: ocular symptoms, vision-related function, and environmental trigger scores. The ocular symptoms score consisted of three items, namely “eyes that are sensitive to light,” “eyes that feel gritty,” and “painful or sore eyes.” The vision-related function score consisted of five items: “blurred vision,” “poor vision,” “reading,” “driving at night,” “working with a computer or bank machine,” and “watching TV.” The environmental triggers score consisted of three items: “windy conditions,” “places or areas with low humidity,” and “areas that are air conditioned.” The severity was graded on a scale of 0 to 5 according to the frequency regarding each item, which represented “none of the time,” “some of the time,” “half of the time,” “most of the time,” and “all the time.” The OSDI score was based on the formula: [(sum of scores for all questions answered) *100] ⁄ [(total number of answered questions) *4]. Using the formula, the total OSDI score could be calculated.
2.4 Definition of DED
According to the total OSDI score, DED was considered as “symptomatic DED” and was defined as a subject whose OSDI score was greater than or equal to 13[12, 13]. Additionally, subjects were classified into 4 categories by OSDI score: normal (scores 0–12), mild DED (13–22), moderate DED (23–32), and severe DED (33–100).
2.5 Definition of risk factors
In total, five questions were related to potential risk factors, namely “psychological stress,” “VDT use,” “wearing contact lenses,” “sleep quality,” and “gender.” To measure and evaluate psychological stress, the Chinese version of the perceived stress scale (PSS) was utilized. The PSS questionnaire is one of the most widely used psychological instruments for measuring the perception of stress. Developed by Cohen in 1983, it has shown sufficient reliability and validity. The Chinese version of the PSS questionnaire was also found to be reliable.[15, 16] Originally, it consisted of 14 items that were designed to determine how unpredictable, uncontrollable, and overloaded respondents find their lives. To facilitate the implementation, the four items version of PSS was adapted for this study. This version consisted of four PSS items, two of which were negative (“how often have you felt that you were unable to control the important things in your life” and “how often have you felt difficulties were piling up so high that you could not overcome them”), while the other two items were positive (“how often have you felt confident about your ability to handle your personal problems” and “how often have you felt that things were going your way”). The subjects were required to answer each question using a five-point Likert scale score ranging from 0 (never) to 4 (very often) and report the event frequency correlated with the PSS items in the last month. PSS scores were obtained by reversing responses (0 = 4, 1 = 3, 2 = 2, 3 = 1, 4 = 0) to the two positively stated items and summing all scale items. The total scores ranged from 0 to 16, and subjects with higher scores had higher psychological stress levels. The video display terminal (VDT) use question was defined as “the average hours spent on VDT devices, such as iPad, iPhone, or computers, per day for the last week.” The wearing contact lenses question was defined as wearing a contact lens at least once a week for the last three months. Poor sleep quality was defined as difficulty in falling asleep.
2.6 Research process
Before the formal survey began, the project leader trained project members and standardized the survey process. On the day of the survey, the subject had to fill out a questionnaire that consisted of four parts (Supplemental Figure 1):
(1) General information: name, gender, age, school, class, and student identification number
(2) OSDI questionnaire
(3) DED risk factors (contact lens wear, sleep quality and VDT use) and medical history regarding eye diseases (self-reported ocular inflammation, such as conjunctivitis and keratitis, or previous ocular surgery history within the last 6 months)
(4) PSS questionnaire
Questionnaires with omissions were considered invalid. Two project members transferred the data to a database independently. Subjects who had a history of any ocular inflammation or ocular surgery within the last six months were excluded from the survey.
2.7 Statistical Analyses
The normality of the data distribution was tested using the Kolmogorov–Smirnov test. Normal distribution parameters were expressed as the mean (standard deviation, SD). Non-normal distribution parameters were expressed as the median (interquartile range, IQR). The prevalence of DED was calculated as a ratio. In univariate analysis, normally distributed continuous variables were compared by a T-test, while non-normally distributed continuous variables were compared by the Mann-Whitney U test. Chi-square tests were used for comparisons between categorical variables. All risk factors were then introduced into a binary logistic regression model. Relative risks were estimated as odds ratios (OR) with a 95% confidence interval (CI). P<0.05 was regarded as a significant difference. Data analyses were carried out using SPSS version 21.0 (SPSS, Chicago, IL, USA).