Anisometropia and its ocular biometric correlates in young adults across the full range of refractive status: findings from the Anyang University Students Eye Study

To investigate the prevalence and characteristics of anisometropia in young adults across the full range of refractive status and to identify the ocular biometric factors associated with interocular refractive asymmetry. This cross-sectional study included participants from the Anyang University Students Eye Study, a university-based eye health study conducted in central China. Young adults aged 18–25 years with complete cycloplegic refraction and ocular biometric measurements were included, with no restriction on refractive status; only extreme axial length values (> 3 SD from the population mean) were excluded. Anisometropia was defined as the absolute interocular difference in spherical equivalent refraction (SER_diff, D). Associations between anisometropia and refractive status were evaluated using regression analyses stratified by refractive status categories defined according to the more myopic eye (primary analysis) or the less myopic eye (sensitivity analysis). A multivariable linear regression model including all candidate interocular biometric differences was used to identify ocular biometric parameters associated with anisometropia, with LASSO regression performed as a collinearity check. Among 6479 participants, the mean age was 20.28 ± 1.39 years, the median SER_diff was 0.38 D, and the prevalence of anisometropia was 47.2%, 20.5%, 10.7% and 6.0% using thresholds of ≥ 0.50 D, ≥ 1.00 D, ≥ 1.50 D and ≥ 2.00 D, respectively. Participants with anisometropia showed greater interocular differences in most ocular biometric parameters, with the largest differences observed for axial length and axial length-to-corneal radius ratio (AL/CR). The interocular correlation of SE varied across refractive status categories, being weakest in emmetropia (β = 0.214) and strongest in high myopia (β = 0.673), while the prevalence of anisometropia (≥ 1.00 D) was lowest in emmetropia (6.5%) and increased toward both hyperopia (9.7%) and high myopia (28.9%). Multivariable regression identified interocular differences in AL/CR and axial length as the major biometric factors associated with anisometropia (model R² = 0.888), with minor contributions from corneal curvature, lens power and cylinder. Anisometropia is common among young adults across the full range of refractive status, with the lowest prevalence near emmetropia and increasing prevalence toward both refractive extremes, a pattern consistent with coordinated binocular emmetropization constraining interocular differences. Axial length and AL/CR asymmetry are the predominant biometric correlates of greater anisometropia, highlighting the importance of considering binocular asymmetry in the evaluation of refractive development.