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  • The NPVP was conducted from 2011 to 2012 and aimed to estimate the burden of common pediatric ocular disorders of preschool children aged 3–6 years in the Yuhuatai District, Nanjing, China. Nanjing City is the capital of Jiangsu Province, a traditional economic and cultural hub of eastern China and has a population of 8.1 million according to the China Sixth National Population Census (2010). Yuhuatai District is one of 11 municipal districts of Nanjing and has a relatively stable population structure (approximately 413 thousand residents) and a medium socioeconomic status in eastern China, which makes it representative of that area. The study adhered to the Declaration of Helsinki and was approved by the Institutional Review Board of Jiangsu Province Hospital. Written informed consent was obtained from parents or legal representatives of all participating children. Every 150 to 250 children who studied in the 43 kindergartens in Yuhuatai District were grouped according to geographic location, which were defined as clusters. In total, 48 clusters were established, and every cluster had 200 children on average. All of the clusters were numbered according to their locations and were randomly selected using a random numbers table. The eye examinations were performed by a team of two senior ophthalmologists, two junior ophthalmologists, two assistants and two optometrists. If glasses were worn, testing was performed with and without correction both. Ocular alignment was assessed using the Hirschberg light reflex test at a distance of 33 cm, the cover-uncover test and the alternate cover test with fixation targets at both 33 cm and 6 m [11]. Binocular and monocular ocular movements were examined at nine diagnostic positions of gaze with the head in a stationary position. If strabismus was suspected, a prism cover test was performed to detect the degree of eye misalignment. All children had distance VA measured, with or without spectacles, using the Early Treatment Diabetic Retinopathy Study (ETDRS) visual acuity chart (Precision, Vision, LaSalle, IL, USA) at a distance of 4 m. For children with distance VA < 6/12 or a two or more lines difference between eyes, subjective refraction was performed to obtain best corrected VA. All participants had the measurement of refractive error using an autorefractor (Suresight, Welch Allyn, USA) under non-cycloplegic conditions. The refraction status of children who were found abnormal in the examinations of ocular alignment, ocular movement and distance VA, was further evaluated under cycloplegic conditions if agreement was obtained from parents or legal representatives. One drop of topical 1.0% cyclopentolate (Cyclogyl, Alcon Pharmaceuticals) was administered to each eye twice at 5-minute intervals. Fifteen minutes later, a third drop of cyclopentolate was administered if the pupil size was < 6 mm or if the papillary light reflex was still present. Children with abnormalities found in the examinations of ocular alignment, ocular movement and distance VA needed to have further ocular examinations, including stereopsis screening using children random-dot stereograms (edited by Shaoming Yan, People’s Medical Publishing House, 2006, China), slit lamp examination, and fundus examination. Strabismus was determined if any tropia was present at distance or near, with or without spectacles and then classified according to the primary direction (esotropia, exotropia, vertical) of the tropia. Constant or intermittent heterotropia was defined as well. Vector analysis was used to determine the J0 (power in the vertical or horizontal meridian) and J45 (power in the oblique meridian) vector components of astigmatism [12]. Potential risk factors were spherical equivalent (SE) refractive error of less hyperopia eye, astigmatism of less astigmatic eye, SE anisometropia, J0 anisometropia (interocular difference in J0), and J45 anisometropia (interocular difference in J45). The dioptric criteria for levels of magnitude are provided in Table 1. The less hyperopic eye was chosen for analysis because that accommodative convergence (a potential contributor to convergent strabismus) is likely to be caused by accommodation in the less hyperopic eye if anisometropia is present. Table data removed from full text. Table identifier and caption: 10.1371/journal.pone.0120720.t001 Frequency Distributions of Refractive Error Risk Factors in 3 to 6 Years-Old Children with and without Strabismus in the Nanjing Pediatric Vision Project. D = diopters; J0 = power in the vertical or horizontal meridian; J45 = power in the oblique meridian; SE = spherical equivalent. *Percentage of participants with stated outcome status.§Chi square or Fisher exact test where applicable.†Level of refractive error defined by the less hyperopic eye for SE refractive error, and the less astigmatic eye for astigmatic refractive error. Binary logistic regression model was fitted to explore the associations of potential risk factors separately for concomitant esotropia and concomitant exotropia. Only age, gender and factors that were significant at the P<0.10 level were retained in further stepwise multiple logistic analyses. All analyses were performed using SPSS software (version 17.0, IBM, China) for Windows 7.0.
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