A naturally occurring model of axial myopia in the Labrador retriever

Purpose. Numerous animal species undergo increased axial elongation and therefore develop myopia in response to deprivation of form vision and other manipulations of the visual environment. To date, this experimental myopia has served as the animal model of human refractive error. We wanted to know if the myopia occasionally seen in Labrador retrievers might serve as a spontaneously occurring model of axial myopia. Methods. Refractive error and ocular component dimensions were measured in 56 Labrador retrievers using cycloplegic retinoscopy, A-scan ultrasonography, and a video-based kerato-phakometer (measuring corneal and lenticular radii of curvature in the horizontal meridian). All dogs were brought by their owners for a Canine Eye Registry Foundation examination, and were either household pets or working field trial retrievers. Results. Of the 56 dogs, 6 (10.7%) were myopic by -0.50 D or more in the horizontal meridian. Myopic dogs had a mean (+/-SD) refractive error of-1.5 +/- 1.6D, while non-myopic dogs had an average (+/-SD) of +0.9 +/- 0.6D of hyperopia). Refractive error was significantly correlated with vitreous chamber depth for the sample as a whole (Spearman r =-0.37, p = 0.005), and was significantly longer in myopic compared to non-myopic dogs (10.9 +/- 0.37mm vs. 10.0 +/- 0.40mm; p<0.0001, Kruskal-Wallis). No other ocular components differed between myopic and non-myopic dogs. Conclusions. Axial myopia occurs in a small proportion of the breeding stock of Labrador retrievers. If this trait is heritable, establishing colonies may be beneficial. For example, the efficacy of growth-modulating agents may be evaluated in this naturally occurring, mammalian model of myopia compared to an avian model using form deprivation.