The Lhasa Childhood Eye Study (LCES) was a cross-sectional, school-based, longitudinal cohort study conducted in Lhasa, Tibet, China. The study was mainly designed to estimate the prevalence and associated risk factors of ocular diseases in school-age children during a 5-year follow-up (Chinese Clinical Trial Registry [ChiCTR], Identifier: ChiCTR1900026693). Ethics committee approval was obtained from the Institutional Review Board of Beijing Tongren Hospital, Capital Medical University (TRECKY2019-146) in accordance with the Declaration of Helsinki principles. The first visit of LCES was between September 2019 to October 2019. Written informed consent forms were obtained and signed by all parents or guardians before the examinations. The procedures were modified from the Anyang childhood eye Study (ACES) conducted in central China.12
There are in total 28 elementary schools in Lhasa. Officials from the health and education departments of Lhasa summoned the principals of the 28 schools before recruitment, to inform them the LCES would be conducted from 2019 to 2024. Twenty-seven out of the 28 elementary schools in Lhasa were available to join and were stratified into three levels based on the evaluation of local government. Voluntary Grade one students who had been living in Lhasa city for at least half a year and would continue to live there for at least 5 years until they enter different middle schools were included in LCES. Individuals suffering from mental illness or other medical conditions who were unable to cooperate with the baseline survey were excluded. According to geographical characteristics and required sample size, 1943 Grade one students of seven primary schools were randomly sampled by stratified cluster sampling. All the clusters were numbered according to their locations and were randomly selected using a random numbers table. Participants were followed for 5 years until they entered different middle schools and could withdraw from the study at anytime for any reason.
The questionnaires were mainly derived from the version of that used in the ACES12 and were modified to make sure the questionnaires were culturally appropriate and linguistically accurate. The questionnaire contained information about indoor and outdoor activity, history of the birth and ophthalmic treatment, and habits of reading, writing, living, eating and so on, as well as parents’ information such as refractive and socioeconomic status, education, pregnancy history, medical records and other aspects. For example, the questionnaire would ask 1)‘What is your child’s current preferred writing posture?’ and 2)‘As far as you remember, did your child prefer to tilt his head when he/she was writing?’ with the following options: Yes or NO or Not sure.
Blood pressure and heart rate were measured using a digital automatic blood pressure monitor (HK-808, HSH, Shenzhen, China). Oxygen saturation was measured using a digital fingertip pulse oximeter (YX301, YUWELL, Jiangsu, China). Height and weight (without shoes or heavy clothing) were determined using a weighing scale. Body Mass Index (BMI, BMI = weight/height2 kg/m2) was also calculated.
The comprehensive eye examination was performed by a team of two optometrists and four ophthalmologists from Beijing Tongren Hospital who were trained and certified using standardized study protocols.
Distant and near visual acuity: Uncorrected and presenting distant visual acuity (VA) were measured for the right eye and left eye using Lea Symbols ETDRS 3 meter set charts (250300, Goodlite, IL, USA) at a distance of 3 meters based on the instructions and standard procedure. The last line attempted, combined with the number of mistakes made on that and previous lines, was used to calculate a letter-by-letter logMAR VA score. Pinhole and best-corrected distant VA (BCVA) were obtained after subjective refraction test for students with uncorrected distance VA˂20/20 (˃LogMAR 0.0).
Refraction: Objective refraction was measured before and after cycloplegia using an autorefractor (KR-800, Topcon, Tokyo, Japan). The cycloplegic procedure for each student was to first administer one drop of topical anesthetic agent (Alcaine, Alcon), followed by two drops of 1% cyclopentolate (Alcon) and one drop of Mydrin P (Santen, Japan) given 5 minutes separately. Thirty minutes after the last drop was instilled, a third drop of cyclopentolate was administered if the pupillary light reflex was still present or the pupil size was less than 6.0 mm.
Ocular movements: Nine directions of gaze including primary, secondary and tertiary were examined and recorded by asking students to fix on a moving penlight without moving their heads.
Ocular dominance: Students were asked to hold a card with a central hole and fixate on a distant object while holding their head stationary. The examiner covered the students’ eyes one after the other and repeated it three times to determine the dominant eye.
Cover test: Unilateral and alternating cover tests were performed at far (6 m) and near (33 cm) to detect heterotropia and heterophoria by an experienced pediatric ophthalmologist under natural conditions (with and without spectacles), respectively, and the magnitude of the deviation was measured using loose prisms. The prism was placed with the base along the reversed direction of deviation and was adjusted until no movement could be detected.
Stereopsis: Stereo Fly Test (S0001, STEREO, USA) was used to quantitatively measure stereo acuity for students at 40 cm.
Diseases definition and classification
Strabismus was defined if any heterotropia was present at near or distance, with or without spectacles and then classified according to the primary direction of the tropia.13
Depending upon the clinic-etiological features, primary exotropia was classified into basic non-specific type (exotropia approximately equal for near and distance), convergence insufficiency type (exotropia greater for near than distance, ≥10 prism diopters [PD]), and divergence excess (exotropia greater for distance than near, ≥10 PD). Esotropia was classified into accommodative esotropia (due to overaction of convergence associated with accommodation reflex) and non-accommodative esotropia (all those primary esodeviations in which the amount of deviation is not affected by the state of accommodation and roughly equal in amount at distance and near fixation).14
Micro-strabismus was defined as a deviation of＜10 PD in the presence of demonstrable binocular vision on the Lang II test. In the absence of demonstrable binocular vision, a deviation of this magnitude was classified simply as strabismus.15
The prevalence of strabismus was calculated and analyzed as the percentile of the number of individuals with different types of strabismus to the total number of individuals evaluated. Continuous variables were given as the mean ± standard deviation (SD), and categorical variables were given as the absolute value and relative frequency. Prevalence of strabismus and its 95% confidence interval (CI) was estimated using a general linear model. Risk factors associated with high level strabismus were evaluated using a linear regression model. The potential risk factors including general characters like age, gender, ethnicity, BMI, heart rate, oxygen saturation, and habits of reading, living, eating, as well as parents’ information such as pregnancy history and so on. The analyses were implemented with SAS software version 9.4 (SAS institute Inc. Cary, NC). χ2 tests and t-tests were also used. A p-value of <0.05 was considered statistically significant.