Compensatory Ocular Growth Responses to Positive Lens Defocus in the Tree Shrew

Abstract

Abstract: : Purpose: Previous reports of ocular growth responses to positive lens defocus have provided variable outcomes in different animal models. Chicks show appropriate compensatory responses to large degrees of positive lens defocus whereas monkeys compensate appropriately to positive lenses when starting lens powers are small and increased in small steps (sequential positive lens strategy). The responses in tree shrew to positive lens defocus are equivocal. This study was carried out to determine if tree shrew eyes respond to positive lenses when a sequential positive lens strategy was employed as used by Smith and Hung (1999) in monkeys. Methods: Maternally reared tree shrews were fitted with head mounted goggles on the 14^th day after eye opening. The treated group had equal powered positive lenses fitted binocularly, starting with a power of +2D (n=4). The effective lens power was subsequently increased to +4D, +6.2D, +8.3D and +10D (one week each of +2D/+4D and two weeks each of +6.2D/+8.3D/+10D). Animals wearing zero powered lenses for an identical duration served as controls (n=4). Ocular growth responses were measured every week using retinoscopy and A–scan ultrasonography starting from baseline and continuing up to eight weeks of treatment. Results: Refractive status at baseline was not significantly different between the two groups [Treated/control +10.1±0.2D vs +10.0±0.2D; p=0.78; Mean±SE] nor was the vitreous chamber depth significantly different [Treated/control 2.77±0.03mm vs 2.82±0.02mm; p=0.16]. The treated group remained significantly more hyperopic than the control group after one week of treatment [Treated/control +10.7±0.3D vs +8.7±0.2D; p<0.01] and remained more hyperopic at all time points thereafter (p<0.01). These refractive differences were predominantly due to a shorter vitreous chamber depth in the positive lens treated animals compared to the controls. Conclusions: The data demonstrates that increasing positive lens powers induce hyperopia in tree shrews, relative to control animals, and this relative hyperopia is due to reduced vitreous chamber elongation. These findings indicate that tree shrews do respond to positive lens defocus.

Keywords: refractive error development • hyperopia • emmetropization