Emmetropic eye growth in East Asians and non-East Asians

Abstract

Purpose: To compare axial length (AL) growth curves in East Asian (EA) and non-EA emmetropes.

Methods: A meta-regression of 28 studies with emmetrope-specific AL data (measured with optical biometry) was performed. Emmetropia was defined as spherical equivalent refraction (SER) between -0.50 and +1.25 D, determined under cycloplegia if the mean age was ≤20 years. The AL growth curve (mean AL vs. mean age) was first fitted to the full dataset using a weighted nonlinear mixed-effects model, before refitting the model with ethnicity as a two-level grouping variable (EA vs. non-EA). Ethnic differences in growth curve parameters were tested using the Wald test.

Results: A total of 3331 EA and 1071 non-EA emmetropes (mean age: 6.5-23.1 years) were included. There was no evidence of an ethnic difference in either final AL (difference: 0.15 mm, 95% CI: -0.04 to 0.35 mm, p = 0.15) or initial AL, as represented by the amount that the final AL needed to be offset to obtain the y-intercept (difference: -2.77 mm, 95% CI: -10.97 to 5.44, p = 0.51). Likewise, AL growth rate (curve steepness) did not differ between ethnic groups (difference: 0.09, 95% CI: -0.13 to 0.31, p = 0.43). Collectively, AL growth rate decreased from 0.24 mm/year at 6 years of age to around 0.05 mm/year at 11 years of age, after which it dipped below the repeatability of optical biometry (±0.04 mm) and practically plateaued around 16 years of age (final AL: 23.60 mm).

Conclusions: EA and non-EA emmetropes have comparable AL growth curves.

Keywords: axial elongation rate; axial length; emmetropia; ethnicity; eye growth; myopia.

FIGURE 1

PRISMA flow diagram. From the 129 studies that entered full‐text screening, 46 were removed due to the absence of emmetrope‐specific axial length (AL) data, 15 were removed due to the use of non‐cycloplegic refraction in children aged 20 years or below, 12 were removed due to the use of ultrasonography to measure AL, 9 were removed due to the use of the same sample as at least one other included study, 7 were removed because the definition of emmetropia was outside the range of −0.50 to +1.25 D (e.g., −1.25 to +1.25 D), 4 were removed because the characteristics of the study participants did not match the target population (e.g., younger than 6 years of age) and 3 were removed because there was insufficient information to assess their eligibility (e.g., whether cycloplegic refraction was performed). One retracted article was further removed.

FIGURE 2

Combined (left) and ethnic‐specific (right) axial length (AL) growth curves. Bubble size corresponds to study weight. Studies with large weight (>1.5) are annotated, where SH refers to Shanghai, HK refers to Hong Kong, BJ refers to Beijing, NI refers to Northern Ireland, SE Norway refers to South‐eastern Norway, SG refers to Singapore and Anyang refers to Anyang City in Mainland China.

FIGURE 3

Annual axial length (AL) growth rate as a function of age derived from the present work (blue), Wong et al. (maroon; rate of change in vitreous chamber depth used as surrogate for AL growth rate because the presented AL growth model unrealistically predicts shrinking AL when extrapolated beyond 12 years old) and Zadnik et al. (green). Extrapolated data are represented by dotted lines.