Little is known about how OK retards axial elongation in childhood myopia. It has been postulated that growth of the eye is modulated by a visual feedback response to the defocus produced by the eye's refractive state.
32,33 Recently, some experimental studies of primates have yielded data to support the concept that the peripheral retina is able to influence ocular growth and refractive development.
34 –37 Similarly in humans, it is known that the peripheral retina can influence refractive development (e.g., retinal diseases that affect mainly the peripheral retina)
38,39 and that peripheral laser photocoagulation for retinopathy of prematurity
40,41 is often associated with significant refractive errors. Mutti et al.
42,43 reported that myopic children have greater relative hyperopia in the periphery with respect to axial refraction, indicating a prolate ocular shape (an axial length exceeding the equatorial diameter), compared with relative peripheral myopia and an oblate shape (an equatorial diameter exceeding the axial length) for emmetropes and hyperopes, suggesting that hyperopic defocus in the periphery promotes axial myopia in humans. Treatment strategies and methods for correcting not only central refractive error but also peripheral defocus and image quality may be effective to control eye growth in myopic children.
34,44 From this viewpoint, OK seems to be a promising method, inducing a reduction in central epithelial thickness and an increase in the more peripheral annular zone to yield a flatter central area.
29,45 Although these corneal changes eliminate myopic refractive error in the central area, there is probably an increase in myopia in intermediate peripheral areas caused by the annular ridge of epithelial and stromal thickening.
46 Therefore, light passing through the flattened central area of the cornea forms an image on the fovea, while light passing through the midperipheral cornea, which became steeper than the original shape after OK, focuses at a point closer to the retina at the periphery. In other words, this type of corneal morphology can eliminate or decrease relative peripheral hyperopia that is often associated with myopic eyes.
47 At this stage, however, this hypothesis remains speculative and should be tested in a detailed evaluation of ocular shape and peripheral refractive status, before and after the start of OK, in addition to the parameters we examined in the present study. In addition, it may be useful to examine the relationship between peripheral aberrations and axial elongation, because substantial increases in higher-order aberrations after OK were observed also in the peripheral field of the eye.
47