Emmetropisation and the aetiology of refractive errors

Abstract

The distribution of human refractive errors displays features that are not commonly seen in other biological variables. Compared with the more typical Gaussian distribution, adult refraction within a population typically has a negative skew and increased kurtosis (ie is leptokurtotic). This distribution arises from two apparently conflicting tendencies, first, the existence of a mechanism to control eye growth during infancy so as to bring refraction towards emmetropia/low hyperopia (ie emmetropisation) and second, the tendency of many human populations to develop myopia during later childhood and into adulthood. The distribution of refraction therefore changes significantly with age. Analysis of the processes involved in shaping refractive development allows for the creation of a life course model of refractive development. Monte Carlo simulations based on such a model can recreate the variation of refractive distributions seen from birth to adulthood and the impact of increasing myopia prevalence on refractive error distributions in Asia.

Figure 1

A representation of the refractive data from Sorsby et al and a prediction for the distribution of human refraction based on an uncorrelated combination of ocular components as measured by Steiger.

Figure 2

Four distributions of refraction from two different studies ((a) Mutti et al and (b) Ingram and Barr) from 3 months of age to 3.5 years. (a) 3–9 months. (b) 1–3 years.

Figure 3

Histograms of the distribution of refraction in 11–13 years in Australia, Northern Ireland, and Japan.

Figure 4

Mean and standard deviation of refraction from the ages of 6–14 in the CLEERE (Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error) study (Zadnik et al).

Figure 5

Prevalence of myopia in Japanese school children from ages 3–17 over a 13-year period (1984–1996 inclusive, Matsumura and Hirai).

Figure 6

Distribution of refraction in adult population in Japan with a myopia prevalence showing a highly skewed distribution that has lost the classical feature of leptokurtosis seen, for example, in Figure 1.

Figure 7

An annotation of equation 1 describing the biological mechanisms associated with each component of the model (a) and an annotation of equation (1) describing the biological relevance of each component parameter (b).

Figure 8

Results of Monte Carlo simulations of human refractive development from 3 months of age to 6 years.

Figure 9

Results of Monte Carlo simulations of human refractive development from 8 years of age to 24 years. The left-hand panels model a population with a low tendency to develop myopia and the right-hand panels a population with a high tendency to develop myopia.