Comparison of refractive state and circumferential morphology of retina, choroid, and sclera in chick models of experimentally induced ametropia

Purpose: Simultaneous comparisons of the circumferential morphological tiss ue profiles and final refractions from form-deprivation myopia (FDM), defoc us-induced myopia (DIM), and defocus-induced hyperopia (DIH) models of amet ropia have been made to test the hypothesis that changes in the thickness p rofiles of the three coats of the eye, and particularly that of the choroid , can be predicted from the degree of induced refractive error. Methods: Ha tchling chickens (n = 23) were raised for 2 weeks wearing either a monocula r translucent diffuser (FDM, n = 8), monocular -10.00 D lens goggle (DIM, n = 7), monocular +10.00 D lens goggle (DIH, n = 7), or nothing (Norm, 1). A ll animals were refracted using retinoscopy and were then sacrificed, and w hole eyes were processed for scanning electron microscopy. Retinal, choroid al, and cartilaginous sclera (CS) thickness measurements were made from pho tographic collages of the entire circumference of the globe. Of the 23 chic kens, complete morphological profile data were available for both eyes of 1 0 animals (nine treated and one normal). The contralateral fellow eyes (FEy es) of all nine experimental chickens were used as experimental controls as paired comparisons for statistical analyses. Results: Morphological profil es of control and experimental eyes revealed significant systematic regiona l variations in tissue thickness. This variation was related to nasal or te mporal eccentricity with the nasal side generally thinner than the temporal . Retinal, choroidal, and CS tissue from FDM and DIM eyes showed very simil ar anatomical responses despite significantly different degrees of refracti ve change. DIH eyes showed significant increases in choroidal thickness but none in retinal or CS thickness. Analysis of fellow control eyes indicated that in both myopia models (FDM and DIM), significant changes in all tissu es of the untreated fellow eyes occur whereas only the choroid of the fello w eye was affected in the hyperopic (DIH) model. Conclusions: The morpholog ical similarity observed in the circumferential profiles of the retina, cho roid, and cartilaginous sclera of the FDM and DIM eyes despite similar to 2 0 D difference in final refraction suggests that choroidal thickness is not a good predictor of final refractive error across models. Similarly, the f inal refractive difference of similar to 20 D between the DIM and the DIH e yes did not receive a major contribution from the final difference in choro idal thickness (with its implied effect on vitreous chamber length).