Choroidal thickness predicts ocular growth in normal chicks but not in eyes with experimentally altered growth

Abstract

Background: In hatchling chicks, the thickness of the choroid is quite variable. It has been postulated that thickness per se or the changes occurring during early life might play a causal role in the regulation of ocular growth. We tested this notion by measuring ocular dimensions in several experimental conditions that alter ocular growth and in the fellow eyes.

Methods: Chicks aged 12 to 14 days wore monocular lenses or diffusers (+10 D, n = 23; -10 D, n = 16; diffusers, n = 16) for four to five days. Fellow untreated eyes served as controls. A separate group of completely untreated birds aged eight days were also tested (n = 12). We tested two drugs known to alter ocular growth. The dopaminergic agonist quinpirole was injected daily for five days into eyes wearing negative lenses (n = 47). The muscarinic agonist oxotremorine was injected one time into normal eyes (n = 27). All eyes were measured using high-frequency A-scan ultrasonography at the start and end of the experiment. Spearman's correlation coefficient was used in all analyses.

Results: Choroidal thickness predicted ocular growth rates in normal eyes: eyes with thinner choroids grew faster than those with thicker choroids (p = 0.0001). Furthermore, there was a negative correlation between initial thickness and the change in thickness (p = 0.0353). By contrast, eyes wearing lenses or diffusers did not show a correlation between initial thickness and growth rate. For lens-wearing eyes injected with quinpirole, which slowed growth, initial choroidal thickness predicted subsequent growth rate (p = 0.0126), similar to normal eyes. This was not so for oxotremorine, which stimulated growth.

Conclusions: The loss of the association between choroidal thickness and subsequent growth rate in eyes with experimentally altered growth implies an uncoupling of the choroidal response from the visual regulation of ocular growth. The negative correlation between initial thickness and ocular growth in eyes injected with quinpirole suggests potential therapeutic benefits to thicker choroids.

Keywords: chicks; choroid; defocus; emmetropisation; form-deprivation; myopia.