Ocular components in three breeds of dogs with high prevalence of myopia

Experimental models of human myopia have been developed using animals of various species. However, most of these are an induced rather than a spontaneous, naturally occurring myopia. This study was conducted to evaluate whether the spontaneous myopia found in three canine breeds was axial in nature and therefore similar to humans.

Refractive error was measured by cycloplegic retinoscopy and ocular components by A-scan ultrasound (ocular axial dimensions) and videophakometry (corneal and lens radii and powers) in 83 dogs of three breeds [English Springer Spaniels (n = 33), Toy Poodles (n = 36), and Collies (n = 14)]. Dogs with refractive errors equal to or more myopic than -0.5 diopters spherical equivalent were considered myopic.

Myopia was most common in Toy Poodles (63.9%), followed by English Springer Spaniels (36.4%) and Collies (35.7%). Axial lengths and vitreous chamber depths were not different between myopic and non-myopic dogs (p = 0.84 and 0.63, respectively). The anterior crystalline lens radius was steeper and the lens power was greater in myopic compared with non-myopic dogs (p = 0.048 for each).

Spontaneous myopia was very common in all three breeds in this sample of dogs, with Toy Poodles being most affected. However, the cause of the myopia appeared to be refractive, that is from a steeper, more powerful crystalline lens, rather than from excess axial elongation. These breeds do not appear to be promising models for human axial myopia.