Age, gender and regional/ethnic variations in emmetropic axial growth rate

To determine age-specific axial growth rate in emmetropic eyes and investigate the effect of sex and region/ethnicity using population-based data.

A retrospective analysis of five population-based studies conducted in the United Kingdom, Sweden and China. A total of 16,526 datapoints from 6753 participants, aged 6-16.9 years, with spherical equivalents (SE) from -0.49 to +1 D were analysed. Axial length was modelled using a Generalised Estimating Equation with region/ethnicity and sex included as fixed factors and age, SE and corneal radius of curvature as covariates. Model-based estimates of axial length were used to derive age-specific axial growth rates, maintaining SE at 0.00 D and constant corneal radius of curvature.

Within this emmetropic population, axial length was weakly correlated with SE (r = -0.24) but strongly associated with corneal curvature (r = 0.76). Gender, region/ethnicity, SE, corneal curvature and inverse function of age were associated with axial length of emmetropic eyes. Axial length was longer in males than females by 0.55 mm (95% CI: 0.53-0.56 mm) in East-Asian emmetropic eyes and by 0.50 mm (95% CI: 0.49-0.52 mm) in European eyes; however, axial growth rate was marginally greater in males by 7%. Axial length of East-Asian eyes was significantly greater than Europeans by 0.14 mm (95% CI: 0.12-0.16 mm) in males and 0.12 mm (95% CI: 0.11-0.14 mm) in females, but axial growth rate was not significantly different between regions/ethnicities (p = 0.06). Axial growth rate decreased non-linearly from 0.17 to 0.03 mm/year in males and 0.16 to 0.02 mm/year in females between 6 and 16 years.

Emmetropic axial growth rate between 6 and 16 years is non-uniform with greater growth rate at younger ages and in males. Growth rates estimated by maintaining constant SE and corneal curvature are lower or similar to previous estimates and may be used to set goals for myopia treatment.