Refractive and structural measures of lid-suture myopia in tree shrew.

In order to study more thoroughly the refractive and structural changes associated with lid-suture myopia, five tree shrews were raised for approximately 16 weeks with monocular visual experience produced by lid closure. Four animals raised with normal laboratory visual experience served as a control group. Compared to the paired open eye, lid-sutured eyes were myopic (-12.1 +/- 6.3 diopters by retinoscopy), corneas were flatter (0.26 +/- 0.18 mm radius increase by photokeratometry) and axial lengths were greater (0.32 +/- 0.17 mm longer by A-scan ultrasonography). The axial length increase was due to elongation of the vitreous chamber (0.38 +/- 0.19 mm longer by A-scan ultrasonography). The open eyes of experimental animals were not significantly different than the normal eyes of control animals. Two of these treatment effects, namely, refractive state changes and axial length increases, were demonstrated with independent techniques: streak retinoscopy was compared to coincidence optometry, and A-scan ultrasound was compared to axial measurements of photographs of frozen, sectioned eyes. The three main ocular effects of eyelid closure were stable over three measurement sessions completed within a 4 week period. Additional refractive and A-scan measurements taken 7.5 months later showed no significant changes. Optical modelling showed that the observed myopia of the lid-sutured eye is consistent with the observed elongation of the vitreous chamber coupled with the flattened cornea although other changes could not be ruled out. We conclude that an axial myopia is produced reliably in tree shrews by raising them with eyelid closure and that the measurement techniques used in this study have sufficient resolution to study the development of myopia in this species.