Developmental visual system anomalies and the limits of emmetropization

Optical defocus can within certain limits predictably alter ocular growth a nd refractive development in infant monkeys. However defocus, particularly unilateral defocus associated with anisometropia, can also promote abnormal sensory and motor development. We investigated the relationship between th e effective operating range for emmetropization in infant monkeys and the r efractive errors that produced amblyopia. Specifically, we examined the ref ractive-error histories of monkeys that did not demonstrate compensating oc ular growth for imposed refractive errors and used operant psychophysical m ethods to measure contrast sensitivity functions for 17 infant monkeys that were reared with varying degrees of optically imposed anisometropia. Impos ed anisometropias that were within the operating range of the monkey's emme tropization process were eliminated by differential interocular growth and did not produce amblyopia. On the other hand imposed anisometropias that fa iled to initiate compensating growth consistently produced amblyopia; the d epth of the amblyopia varied directly with the magnitude of the imposed ani sometropia. These results indicate that amblyopia and anisometropia are fre quently associated because persistent anisometropia causes amblyopia. Howev er, the failure of emmetropization in infants with refractive conditions th at are known to promote sensory and motor anomalies indicates that factors other than optical defocus, presumably factors associated with the developm ent of amblyopia and/or strabismus, can also influence early refractive dev elopment and in some cases cause anisometropia. (C) 1999 The College of Opt ometrists. Published by Elsevier Science Ltd. All rights reserved.