Purpose: Although many aspects of vision-dependent eye growth are qualitatively similar in many species, the failure to observe recovery from form-deprivation myopia (FDM) in higher primates represents a significant potential departure. The purpose of this investigation was to re-examine the ability of rhesus monkeys (Macaca mulatta) to recover from FDM.
Methods: Monocular form deprivation was produced either with diffuser spectacle lenses (n = 30) or by surgical eyelid closure (n = 14). The diffuser-rearing strategies were initiated at 24 +/- 3 days of age and continued for an average of 115 +/- 20 days. Surgical eyelid closure was initiated between 33 and 761 days of age and maintained for14 to 689 days. After the period of form deprivation, the animals were allowed unrestricted vision. The ability of the animals to recover from treatment-induced refractive errors was assessed periodically by retinoscopy, keratometry, and A-scan ultrasonography. Control data were obtained from 35 normal monkeys.
Results: At the onset of unrestricted vision, the deprived eyes of 18 of the diffuser-reared monkeys and 12 of the lid-sutured monkeys were at least 1.0 D less hyperopic or more myopic than their fellow eyes. The mean (diffuser = -4.06 D, lid-suture = -4.50 D) and range (diffuser = -1.0 to -10.19 D, lid-suture = -1.0 to -10.25 D) of myopic anisometropia were comparable in both treatment groups. All 18 of these diffuser-reared monkeys demonstrated recovery, with 12 animals exhibiting complete recovery. The rate of recovery, which was mediated primarily by alterations in vitreous chamber growth rate, declined with age. None of the lid-sutured monkeys exhibited clear evidence of recovery. Instead, 8 of the 12 lid-sutured monkeys exhibited progression of myopia.
Conclusions: Like many other species, young monkeys are capable of recovering from FDM. However, the potential for recovery appears to depend on when unrestricted vision is restored, the severity of the deprivation-induced axial elongation, and possibly the method used to produce FDM.