Open access Original research BMJ Open: first published as 10.1136/bmjopen-2019-032833 on 12 October 2020. Downloaded from Effect of birth order on stereoacuity in Chinese preschool children: a cross- sectional study Shu Han,1 Xiaohan Zhang,2 Rui Li,3 Haohai Tong,3 Xiaoyan Zhao,3 Yue Wang,3 Qingfeng Hao,3 Dan Huang,3 Hui Zhu,3 Xiaojun Zhang,1 Hu Liu 3 To cite: Han S, Zhang X, Li R, ABSTRACT Strengths and limitations of this study et al. Effect of birth order Objective This study aimed to investigate the relationship on stereoacuity in Chinese between birth order and stereoacuity among Chinese ► To our limited knowledge, no report has demon- preschool children: a cross- children aged 60–72 months. sectional study. BMJ Open strated the correlation between birth order and Design Cross- sectional. 2020;10:e032833. doi:10.1136/ stereoacuity. Participants 1342 children with complete data on the bmjopen-2019-032833 ► With the two- child policy in China, evaluating the ef- questionnaire, stereoacuity and refraction were included. fects of this baby boom on stereoacuity might bene- ► Prepublication history for Results The mean stereoacuity was 53.2±1.7, 56.9±1.9 fit both families and the society. this paper is available online. and 60.9±1.5 s of arc in the first- born group, second- born ► This is a population- based study comprising 1342 To view these files, please visit group and third- born group, respectively. Lower birth the journal online (http:// dx. doi. children aged 60–72 months. order was significantly correlated with better stereoacuity org/ 10. 1136/ bmjopen- 2019- ► This study provided data, such as mean stereoacu- (p=0.036). Third- borns (OR=3.02, p=0.027) were at higher 032833). ity, which could be valuable to primary eye care in risk of having subnormal stereoacuity compared with first- China. SH and XHZ contributed equally. borns in the multivariate analysis. ► Some children without examinations were excluded Conclusion Later- borns had poorer stereoacuity than which might bias the results, and some inaccurate Received 09 July 2019 first- borns. Revised 07 July 2020 stereoacuities might exist due to limited compliance. Accepted 04 August 2020 INTRODUCTION in first- grade children.7 8 However, to our Due to the Chinese two-child policy, we are limited knowledge, there is a serious lack of caught on the horns of a dilemma of an study on whether birth order is correlated http://bmjopen.bmj.com/ increasing demand for paediatric vision care with stereoacuity in Chinese preschool chil- and a decreasing professional care due to dren. In addition, it has been estimated that 1 second- child maternity leave. To the best 90 million Chinese couples could have a of our knowledge, studies on Chinese later- second child following the two- child policy borns are insufficient, and there is dire need in China.1 Thus, assessing the relationship to identify these Chinese later- borns. between birth order and stereoacuity has Birth order could predict intelligence, certain value in China nowadays. on September 27, 2021 by guest. Protected copyright. © Author(s) (or their personality and behaviours, according to This study is part of the Nanjing Eye Study employer(s)) 2020. Re- use extensive research conducted around the (NES) and was designed to investigate the 2–4 permitted under CC BY-NC. No world. In particular, several studies have association between birth order and stereo- commercial re- use. See rights reported that higher birth order was associ- and permissions. Published by acuity in Chinese preschool children and to BMJ. ated with higher risk of neurodevelopmental provide some insights into paediatric care, 5 6 1Ophthalmology, The Second disorders. As an important part of early and is timely with the increasing burden on Affiliated Hospital of Nanjing neurological development, a reduced level primary care in China. Medical University, Nanjing, of stereoacuity has a negative impact on the China ability of an individual to perform many 2 Ophthalmology, The Affiliated tasks, thus leading to bad interactions with MATERIALS AND METHODS Wuxi Children's Hospital of Nanjing Medical University, the world. This cross- sectional study is part of NES, a Wuxi, China As we all know, stereoacuity is the highest population- based cohort study designed to 3Ophthalmology, The First fusion sensory of binocular vision and has longitudinally assess the onset and progres- Affiliated Hospital of Nanjing impact on the performance of fine visual and sion of paediatric ocular diseases in Eastern Medical University, Nanjing, motor action. Children with poor stereoacuity China.9–12 As described previously,9 12 all China might have serious visual developmental children born between September 2011 and Correspondence to disorder, which could lead to reading difficul- August 2012 and resided in Yuhuatai District, Dr Hu Liu; liuhu@ njmu. edu. cn ties in kindergarten and to poor intelligence Nanjing, China were invited to participate Han S, et al. BMJ Open 2020;10:e032833. doi:10.1136/bmjopen-2019-032833 1 Open access BMJ Open: first published as 10.1136/bmjopen-2019-032833 on 12 October 2020. Downloaded from in NES to undergo comprehensive eye examinations. In to point out the shapes on the left page of the booklet 2017, efforts to disseminate the benefits of our study to or to name each shape seen in test 1. If the child could the legal guardians of these preschool children led to respond correctly to the 200 s of arc test object (top of test a participation rate of 83.5% (1920 of 2300). The data 1), the child was encouraged to proceed to the 100 s of analysed for this study were obtained in 2017, when these arc test object (bottom of test 1). If the child passed test children were 60–72 months old. 1, testing proceeded to test 2 at 60 and 40 s of arc level, as Written informed consent was obtained from the described in test 1. If correct responses were not obtained parents or legal guardians of all participants. Oral at the 200 s of arc level, the child proceeded to test 3 at assent was obtained from all children right before the 800 and 400 s of arc level. If the child was unable to consis- examination. tently identify most of the two-dimensional shapes on the left- hand pages, the subject was scored with ‘unable’ Ocular examinations stereoacuity. Otherwise the smallest disparity at which the A comprehensive eye examination of participants was child was able to identify two of the three test shapes was performed by a team composed of six trained ophthal- recorded as the final stereoacuity. mologists and four optometrists. Children roster and basic information including name, gender and birth date Questionnaire were all obtained from each kindergarten’s principal A comprehensive questionnaire composed of six parts and the information was then verified. Measurements was distributed to the legal guardian of each participant. included anthropometric parameters, distance visual Detailed data concerning basic information of the chil- acuity (VA), anterior segment and fundus examination, dren and their parents, history of pregnancy, birth and refraction before and after cycloplegia, Randot Preschool feeding, daily activities, sleep quality of children, home Stereoacuity Test, ocular alignment and motility, ocular environment, and guardian’s concerns over the eyes of biometric parameters, intraocular pressure, accommo- each child were all collected. Maternal age, gestational dative response, and optical coherence tomography. age at birth (weeks), birth weight, Apgar score at 5 min, Children with suspected or confirmed eye problems delivery mode, neonatal oxygen supplementation, birth were referred to senior ophthalmologists and underwent order, smoking exposure, breast feeding and so on were further examinations. collected in the history of pregnancy, birth and feeding Distance VA was measured with a retroilluminated sections of the questionnaire. (ESV1200 Illuminated Cabinet, Good-Lite, Perth Amboy, New Jersey, USA) linear HOTV logMAR (logarithm of Definitions the minimum angle of resolution) chart (600 017, Good- Spherical equivalent (SE) was calculated as spherical Lite) at a distance of 3 m, right eye followed by left eye, dioptre (D) plus half of the dioptre of cylindrical power. as described before.10 12 Additionally, children who wore Myopia was defined as SE ≤−0.50 D in either eye, hyper- http://bmjopen.bmj.com/ spectacles were measured both with and without specta- opia defined as SE ≥+2.00 D in either eye, astigmatism cles. Best corrected VA was recorded with full correction defined as cylindrical power ≥1.00 D in either eye, and under cycloplegia. anisometropia was defined as SE difference ≥1.00 D Refractive status of each participant was measured between the two eyes. Strabismus was defined if any tropia before cycloplegia using table- mounted autorefrac- was present at a distance or near, with or without spec- tion (Canon R- F10, Tokyo, Japan) and photorefraction tacles. Amblyopia was defined similar to the Multiethnic (PlusoptiX, Nuremberg, Germany). Cycloplegic refrac- Pediatric Eye Disease Study.13 Subnormal stereoacuity tion was performed after cycloplegia using table- mounted was defined as stereoacuity worse than 40 s of arc (stereo- on September 27, 2021 by guest. Protected copyright. autorefraction and retinoscopy. One drop of topical 1.0% acuity value >40 s of arc).14 cyclopentolate eye- drop (Cyclogyl, Alcon Pharmaceuti- cals, Belgium) was administered to each eye twice at 5 min Statistical analyses intervals. Fifteen minutes later, a third drop was adminis- Stereoacuity scores in seconds of arc were first trans- tered if the pupil size was <6 mm or if the pupillary light formed into a logarithmic value before statistical model- reflex was still present. ling, and the results in log units were transformed back Before cycloplegia, stereopsis thresholds were to the original value after statistical analysis for easy clin- measured with the Randot Preschool Stereoacuity Test ical interpretation of results.