Axial length, corneal radius, and age of myopia onset

This study was designed to establish variations in the biometry of the ocular globe according to refractive state, and to determine the extent to which these characteristics contribute to the appearance and degree of myopia.

Subjects recruited for the study were 583 university students of mean age 20.32 +/- 2.82 years. Objective refraction without cycloplegia and the corneal radius (CR) of curvature were determined using an autokeratorefractometer. The axial length (AL) of the eye was measured by ultrasound biometry. The population was then characterized according to these three variables to establish relationships among them. Data were stratified by both refractive state and age of onset of myopia.

The prevalence of myopia (more minus than 0.50 D) was 57.78%; half the refractive defects being less minus than -3.00 D. The AL of the eye was found to be more related to the refractive error than the CR, especially in subjects with moderate myopia. In emmetropes or subjects with low myopia, the CR was directly correlated with AL, although this correlation was somewhat diminished in moderate myopias. Nonetheless, in the latter and in juvenile-onset myopia, a trend towards a shorter radius of curvature was observed.

The AL of the eye is the main morphological variable related to myopia. The function of the cornea seems to compensate the possible myopizing effects of slight increases in AL. When increases in AL are excessive, this effect of the cornea tends to disappear.