Abstract
Purpose: :
For reasons that are not clear, the prevalence of myopia is typically higher a) in individuals living in urban rather than rural environments, and b) in first-born (FB) rather than non-first born (NFB) children, although the latter association has only been examined in UK-based cohorts (Peckham et al. Br Med J 1997; 1:542-5; Rudnicka et al. Am J Clin Nutr 2008;87:1392-9; McMahon et al. 2009;ARVO abstract E3954). Reasoning that the myopia-birth order association might be a general, global, phenomenon, we explored the extent to which differences in the Total Fertility Rate (TFR) might contribute to rural versus urban differences in myopia prevalence.
Methods: :
Data for birth order in urban and rural communities from different countries were obtained from the MEASURE Demographic and Health Survey (DHS) project website (http://www.measuredhs.com/). We carried out simulations assuming an overall myopia prevalence of either 20 or 40%, equal numbers of subjects living in urban and rural locations, and birth orders corresponding to those for China in 1980. In Model A, the relative risk (RR) of myopia in FB vs. NFB was varied over the range 1.1 - 1.4. In Model B, the RR of myopia was varied over the range 1.1 - 1.4, such that subjects with a birth order of n were at an increased risk compared to those with a birth order of n+1.
Results: :
Across global populations, TFR was generally lower in urban compared to rural locations, which led to the proportion of FB subjects being higher in urban than in rural environments. The most extreme difference was seen in China, where (in 1980) the proportion of FB children was 73% and 36% in urban and rural areas, respectively. In simulations, the difference in myopia prevalence between urban and rural locations increased linearly with the RR of birth order for Model A, but non-linearly (reaching a plateau) for Model B. For a population with an overall myopia prevalence of 40% and an RR = 1.4, the simulations predicted a 7-18% greater prevalence in urban vs. rural subjects, depending on the model.
Conclusions: :
Differences in TFR may explain, in part, the higher rate of myopia seen in urban vs. rural communities. China’s one-child policy would exacerbate such an effect.
Keywords: myopia • emmetropization • clinical (human) or epidemiologic studies: natural history