Effects of chronic optical defocus on the kitten's refractive status

Lid closure initiated early in life consistently produces axial myopia in a variety of species. It is not known what aspects of the anomalous visual experience associated with lid closure disrupt the emmetropization process and cause abnormal axial elongation. This study was designed to determine if a degradation in the quality of the spatial characteristics of the retinal image alone is sufficient to produce an experimental myopia. Optical rearing procedures were employed to defocus retinal images in one eye of developing kittens from three to fourteen weeks of age. Retinoscopic and ultrasonic procedures were used to evaluate the kitten's refractive status and ocular axial length. The types of defocusing lenses, the nature of the pretreatment visual experience, and the magnitude of the lens power were used to evaluate variables that may have confounded previous investigations. The major finding of the present study is that early chronic optical defocus resulted in axial myopia in kittens, irrespective of the pretreatment visual experience. The frequency and magnitude of the induced myopia were dependent on the amount of optical defocus, but they were not affected by the sign of the defocusing lens or its type (goggles or soft contact lens). The results demonstrate that a clear retinal image is important for the normal regulation of ocular growth and the maintenance of an emmetropic refractive state.