Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

Species Refractive value (D) References Cat by habitat Belkin et al. (1977) Street cat −0.8 Laboratory cats 1.4 Cat by age Konrade et al. (2012) Kitten (≤4 months) −2.45 Adult (>1 year) −0.39 Cat by coat length Konrade et al. (2012) DSH −1.02 DLH −0.13 Dog – mean value −0.05 to −0.39 Murphy et al. (1992b); Gaiddon et al. (1996); Kubai et al. (2008); Groth et al. (2012) Dog by habitat Gaiddon et al. (1996) Indoor dogs −0.64 Outdoor dogs 0.17 Dog by breed −1.87 to +0.98 For specific breeds, see Mutti et al. (1999), Black et al. (2008), Kubai et al. (2008), Williams et al. (2011), and Kubai et al. (2013) Horse −0.17 to +0.33 Harman et al. (1999); Rull‐Cotrina et al. (2013); Bracun et al. (2014) Horizontal meridian −0.06 to +0.41 Grinninger et al. (2010); McMullen et al. (2014) Vertical meridian 0.25–0.34 McMullen et al. (2014) Rabbit (New Zealand White) 1.7 Herse (2005) Chicken (Cornell‐K) 4.1, 3.7 (4 and 17 weeks old, respectively) Wahl et al. (2015) Guinea pig (pigmented) 0.7 Howlett & McFadden (2007) Rat (Norway brown) 4.7, 14.2 (infant and adult, respectively) Guggenheim et al. (2004) Mouse (CBL75/6) −1.5, 4.0 (10 and 102 days old, respectively) Zhou et al. (2008)

      ^a See reference list for additional refractive studies in wildlife and aquatic species.

      DSH, domestic shorthair; DLH, domestic longhair.

      Studies in the cat indicate that IOLs for this species should have a power of 52–53 D. The difference between the canine and feline IOL values stems from differences in the anterior chamber depth of the dog and cat.

      Astigmatism

Astigmatism is a state of unequal refraction of light along the different meridians of the eye. Normally, a given refractive structure of the eye (e.g., the cornea or lens) has a constant curvature radius in all its meridians (though the cornea may flatten toward the limbus). Astigmatism occurs when the horizontal and vertical meridians of the cornea or lens have different curvature radii. Because of these differing curvatures, light entering the eye through the vertical meridian may be refracted more (i.e., direct, or with‐the‐rule, astigmatism) or less (i.e., indirect, or against‐the‐rule, astigmatism) than light entering through the horizontal meridian. Astigmatism is diagnosed by refracting the eye in both the horizontal and vertical meridians. A difference of 0.5 D or more in the refractive power of the horizontal and vertical meridians in the same eye is defined as astigmatism.

      Static Accommodation

      Several avian and reptilian species possess lower‐field myopia. The eyes of these animals are emmetropic along the horizontal and in the upper visual field, but they become progressively myopic below the horizontal. In other words, different parts of the eye have a different refractive power because the shape of the eye is more like a flattened circle, so that the posterior focal length differs for different meridians. This adaptation can be regarded as a static accommodation mechanism. Hence, the animal shifts its gaze to see the object with the appropriate refractive power, and can match the average viewing distances of different areas of the visual field. This allows the animal to keep the ground in focus with relaxed accommodation while foraging for food and, at the same time, monitor the sky for predators while focused at infinity. The same principle is also found in eyes of pinnipeds, where regional changes in the refractive powers of different parts of the cornea allow these animals to maintain high‐resolution vision in both water and air.

      Spherical and Chromatic Aberrations

      Spherical Aberrations

      The eye is not a perfect optical system. Two of the most significant optical problems that affect the eye are spherical and chromatic aberrations. Positive spherical aberrations occur because in both the cornea and the lens, rays passing through the periphery are refracted more than rays passing through the center. Therefore, rays passing through the periphery are focused closer to the cornea (or lens) than rays passing through its center. Obviously, as the image is not uniformly focused on the retina, the aberration causes blurred vision. A comparative study found significant degrees of spherical aberrations in the lenses of dogs, cats, and rats, but minimal lenticular aberrations in cows, sheep, and pigs.

      Emmetropia and Accommodation Underwater

      In aquatic species, the cornea is in contact with water rather than air. Because of the very small (∼0.003) difference between the refractive indices of the cornea and water, the cornea of these species has virtually no refractive power. In fact, because the anterior corneal surface has lower curvature than the posterior surface, under water the cornea acts as a weak divergent lens. Fish are forced to compensate for the absence of corneal refraction by increasing the refractive power of other ocular structures, usually the lens. For this reason, as noted earlier, the lenses of fish eyes are very spherical. Their increased curvature results in significantly larger refractive power.

      The problem of refraction under water is further complicated in species that move in and out of water because it is physically impossible for an eye to be emmetropic both in air and under water. Eyes that are emmetropic in the air will be hypermetropic under water because the refractive power of the cornea is lost due to its submersion in water. Therefore,