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G. McMahon, K. Northstone, T. Zayats, J. A. Guggenheim, C. Williams; Birth Order Is Associated With Myopia in Two UK Cohorts. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3954.
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The Avon Longitudinal Study of Parents and Children (ALSPAC) is an ongoing birth cohort study and The Family Study of Myopia (FSM) is a high myopia genetics study. This investigation aimed to test for an association between birth order and myopia in these two different cohorts.
ALSPAC: Non cycloplegic autorefraction was available for 7,150 participants aged 11-yrs. FSM: Subjective refractive error measures were available for 184 individuals in 77 sibships. In both datasets, "likely myopia" was defined as an average spherical equivalent (avSE) for the two eyes of ≤ -1.50 D, and "likely emmetropia" between -1.50 and +2.00 D. The relationship between birth order and (1) avSE or (2) the odds of "likely myopia" were assessed.
ALSPAC: 51% of participants were female. The mean age was 11.7 years. The prevalence of "likely myopia" was 4.9%. Birth order was significantly associated with avSE (Kruskal-Wallis, P < 0.01). In the unadjusted logistic regression model, the likelihood (Odds Ratio, OR) of being myopic for first-born children versus not first-born was OR = 1.48 (95% CI 1.17-1.88, P < 0.01). After adjusting for sex, number of myopic parents, hours per day spent reading in school holidays, hours per day spent outdoor in summer holidays and paternal social class, the odds ratio for first born versus not first-born was: OR = 1.40 (95% CI 1.06-1.85, P = 0.02). FSM: 68% of participants were female. The mean age was 45 years. The mean avSE for first-born and not first-born subjects were not significantly different (-7.46 D and -7.21 D, respectively, P = 0.46). The odds ratio for first born versus not first-born was: OR = 4.63 (95% CI 1.00-21.30, P < 0.05). Neither age or sex were associated with refractive error.
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