Role of the cornea in emmetropia and myopia.

The purpose of this paper is to review the available evidence concerning the role of the cornea in achieving and maintaining emmetropia, as well as its role in the development and progression of myopia. The role of the cornea in emmetropization is most evident at the two extremes of life. During infancy, when the axial length increases rapidly, both the cornea and the lens become flatter, moving the refractive state of the eye toward emmetropia; during the later years of life, when the axial length remains constant or even shortens, the cornea appears to steepen somewhat. When myopia develops, in some cases the cornea may undergo a "paradoxical" steepening, which accelerates, rather than retards, the development of myopia. The effect of the cornea on the development and progression of myopia is evident when the ratio of axial length to corneal radius--the AL/CR ratio--is considered. Recent studies suggest that a high AL/CR ratio, which has a value of approximately 3.0 in the emmetropic eye, is a risk factor for youth-onset myopia. Several studies have shown that myopic eyes have greater mean corneal powers than emmetropic eyes; this may be mostly due to greater corneal powers being a risk factor for youth-onset myopia.