Spherical equivalent versus AL/CR ratio ‐ a tool for classifying refractive error

Abstract

Abstract Purpose To demonstrate that refractive development in children can be analysed in terms of three phases of development, defined in plots of cycloplegic spherical equivalent refraction (SER) versus the ratio of the axial length to the corneal radius (AL/CR ratio), rather than as a unitary process of emmetropisation. Methods Schools were randomly selected, with stratification by SES. Partipation was sought from students in Year 3 (age 6‐7) and Year 7 (age 12‐13). Participation was high (>75%). Cycloplegic SER (autorefraction), AL and CR (IOL Master) and height were measured in over 4000 children, giving two representative cross‐sectional samples. Results Plots of SER versus AL/CR ratio were triphasic, consisting of a hyperopic wing (SER >2D, an internediate sone (SER between ~0‐0.5D and 2D) and a myopic wing (SER <0‐0.5D) The relationship between differences in SER and AL changed between the phases (>2D/mm in hyperopic and myopic wings, but <1D/mm in the the intermediate zone). Correlations between SER, AL and height decreased across the three phases. Conclusion We interpret these plots as defining a developmental trajectory for refractive development. In the first year of life, rapid adjustment of AL to CR produces leptokurtic distributions of SER and AL/CR, at mildly hyperopic mean SER. Children with SER >2D after the first few years of life may be unable to emmetropise. The impact of continuing increases in AL on SER is reduced by compensatory reductions in lens power, in an anti‐emmetropic phase. For SER of <~0‐+0.5D, lens power changes cease, and further AL increases lead to myopia. The utility of analysis in terms of these phases is illustrated by the systematic changes in relationships between SER, AL and height. This analysis may have wider application.