Refractive development III: Variations in emmetropia and ametropia.

PURPOSE: The purpose of this study was to investigate biometric variations in emmetropia and ametropia, focusing on the differences and correlations found in adult eyes. Bigaussian analyses were performed to characterise the biometric properties of the Regulated (or Emmetropised) and Dysregulated subpopulations identified in earlier works.

METHODS

This work analyses the biometric and refractive error data of 2000 adult participants previously recruited during the first phase of the Shahroud Eye Cohort Study (Iran). Measurements included cycloplegic subjective refractive error, corneal radii of curvature and intraocular distances. The study employed multivariate bigaussian fits and statistical analyses to explore the relationships between ocular biometry parameters.

RESULTS

Significant correlations were found between ocular dimensions, suggesting the existence of ocular shape factors. Meanwhile, there was a large overlap in biometric values across refractive groups, especially within ±3 D. The 979 emmetropic eyes, defined as having a refractive error exceeding ±0.5 D, had axial lengths ranging between 20.86 and 25.62 mm, with matching corneal and lens powers that together ensured emmetropia. The distribution of both the refractive errors and the ocular biometry parameters was leptokurtic and skewed, and could be described accurately by a bigaussian function, representing the Regulated and Dysregulated subpopulations. Regulated eyes had well-matched biometric parameters, while in Dysregulated eyes one or more deviations were seen.

CONCLUSION

The terms refractive and axial ametropia are inadequate given the many interactions between ocular dimensions during eye growth, while emmetropisation is largely determined by the ability of the crystalline lens to lose power as the eye grows. 'Regulated' and 'Dysregulated' eyes are suitable alternatives, distinguished based on their relative biometry within a population. While the biometry of each eye is unique, large overlaps exist in the values of different refractive groups. These observations underscore the importance of using comprehensive rather than partial biometric data when studying or managing refractive errors.