Pre-treatment choroidal thickness is not predictive of susceptibility to form-deprivation myopia in chickens

Purpose: Chicks developing experimentally-induced myopia show profound thinning of the choroid. We observed a wide range of choroidal thicknesses in a sample of normal chicks prior to their use in a pedigree-based study of form-deprivation myopia. Hence, we tested whether pre-treatment choroidal thickness predicted susceptibility to myopia. Methods: Retinal, choroidal and scleral thickness were measured using A-scan ultrasonography in normal White Leghorn chicks (n = 891) aged 4 days old, and again (n = 498) after 4 days of monocular form-deprivation at age 8 days of age. Refractive error was assessed by retinoscopy. Relationships between pre-treatment choroidal thickness and other variables were investigated using general linear models and variance components analysis. Results: Untreated 4 day-old male chicks had choroids approximately 10% thinner than females (p < 0.001), but sex explained <2% of the overall variability in choroidal thickness. Axial eye length in these untreated chicks was not significantly associated with choroidal thickness (p = 0.25). Moreover, pre-treatment choroidal thickness was not predictive of susceptibility to form-deprivation myopia (p = 0.89). Heritability analysis suggested that at least 50% of the variation in pre-treatment choroidal thickness was determined by additive genetic effects (p < 0.001). Conclusions: Parental choroidal thickness is the major determinant of choroidal thickness in untreated 4-day old chicks. Despite choroidal thickness potentially being indicative of ongoing emmetropisation to innate refractive errors, in this study it was not predictive of subsequent susceptibility to form-deprivation myopia.