Effects of Form Deprivation on Peripheral Refractive Errors and Ocular Shape in Infant Rhesus Monkeys

Abstract

Purpose: : In humans, the pattern of peripheral refractions and the shape of the posterior globe vary with the refractive error at the fovea. The purpose of this study was to determine whether form deprivation, which typically produces central axial myopia, also alters the pattern of peripheral refractions and eye shape in infant macaque monkeys.

Methods: : Monocular form-deprivation was imposed in 10 rhesus monkeys at 3 weeks of age by securing a diffuser spectacle lens in front of one eye; the fellow eyes were allowed unrestricted vision. Each eye’s refractive status was measured longitudinally by retinoscopy along the pupillary axis and at 15 degree intervals along the horizontal meridian out to eccentricities of 45 degrees. Axial length was determined by A-scan ultrasonography. Control data were obtained from seven normal monkeys. Near the end of the diffuser-rearing period (about 150 days of age), the shape of the posterior globe was determined by MRI for representative animals. Specifically, the distance between the posterior lens surface and the retina was determined as a function of eccentricity along the horizontal meridian.

Results: : At about 150 days of age, both eyes of the normal monkeys were well-matched and exhibited low degrees of central hyperopia and small amounts of relative myopia in the periphery. The vitreous chamber depths were relatively constant across the central 45 degrees of retina. For the treated monkeys, the interocular differences in central refraction varied between +2.69 and -10.31 D (treated eye - fellow eye). All seven treated monkeys that developed at least 2.00 D of relative central axial myopia in their treated eyes also showed relative hyperopia in the periphery that increased in magnitude with eccentricity. The one treated animal that developed relative central hyperopia in its treated eye exhibited relative myopia in the periphery. The interocular differences in refractive errors were highly correlated with the interocular differences in vitreous chamber depth at all eccentricities (r² > 0.80, p < 0.05).

Conclusions: : Like myopic humans, monkeys with form deprivation myopia exhibit relative hyperopic refractions in the periphery and more prolate shaped eyes. Thus, in addition to producing central refractive errors, vision-induced alterations in central axial length can alter the shape of the posterior globe and the pattern of peripheral refractive errors in infant primates.

Keywords: emmetropization • refractive error development • myopia