Purpose: :
For reasons that are not clear, the prevalence of myopia istypically higher a) in individuals living in urban rather thanrural environments, and b) in first-born (FB) rather than non-firstborn (NFB) children, although the latter association has onlybeen 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; McMahonet al. 2009;ARVO abstract E3954). Reasoning that the myopia-birthorder association might be a general, global, phenomenon, weexplored the extent to which differences in the Total FertilityRate (TFR) might contribute to rural versus urban differencesin myopia prevalence.

Methods: :
Data for birth order in urban and rural communities from differentcountries were obtained from the MEASURE Demographic and HealthSurvey (DHS) project website (http://www.measuredhs.com/). Wecarried out simulations assuming an overall myopia prevalenceof either 20 or 40%, equal numbers of subjects living in urbanand rural locations, and birth orders corresponding to thosefor China in 1980. In Model A, the relative risk (RR) of myopiain FB vs. NFB was varied over the range 1.1 - 1.4. In ModelB, the RR of myopia was varied over the range 1.1 - 1.4, suchthat subjects with a birth order of n were at an increased riskcompared to those with a birth order of n+1.

Results: :
Across global populations, TFR was generally lower in urbancompared to rural locations, which led to the proportion ofFB 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 ruralareas, respectively. In simulations, the difference in myopiaprevalence between urban and rural locations increased linearlywith the RR of birth order for Model A, but non-linearly (reachinga plateau) for Model B. For a population with an overall myopiaprevalence of 40% and an RR = 1.4, the simulations predicteda 7-18% greater prevalence in urban vs. rural subjects, dependingon the model.

Conclusions: :
Differences in TFR may explain, in part, the higher rate ofmyopia seen in urban vs. rural communities. China’s one-childpolicy would exacerbate such an effect.