Physiological strategies for emmetropia

Purpose: To identify relationships among age-independent ocular biometric variables which contribute to ocular refraction in adult human eyes, and to identify differences in those relationships between emmetropes and ametropes. Methods: Manifest refraction (DRS methodology), corneal refractive power (keratometry), and axial anterior segment (anterior cornea to posterior lens), vitreous cavity (posterior lens to anterior sclera) and total globe (anterior cornea to anterior sclera) length (A-scan ultrasonography) were determined in 185 unaccommodated right eyes of adult humans aged 18 to 70 years. There were 136 emmetropes with absolute refractive error ≤ 2.0 diopter, and 49 ametropes (47 myopes, 2 hyperopes) with absolute refractive error of 2.25 - 11.0 diopters. Results: Refraction decreased significantly with increasing globe and vitreous cavity length in emmetropes and ametropes. Anterior segment length was also significantly negatively correlated with refraction in emmetropes, but not in ametropes. Corneal refractive power was not correlated with refraction in either group, but decreased significantly with increasing globe length in both, more strongly in emmetropes. Globe and vitreous cavity length were significantly positively correlated in both groups, more strongly in ametropes. Anterior segment length increased significantly with increasing globe length in emmetropes but not in ametropes. Anterior segment length decreased significantly with increasing vitreous cavity length in ametropes but not in emmetropes. Conclusion: These findings indicate an 'inflatable anterior segment' as well as the classic 'inflatable globe' mechanism of emmetropization. This newly described anterior segment mechanism involving increased separation between the cornea and the lens with increasing globe size appears to be absent in adult human myopia.