Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

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СКАЧАТЬ refracted, resulting in image formation “behind” the retina. Since the early 1980s, veterinary ophthalmologists have sought to alleviate this problem by implanting IOLs in dogs' eyes following cataract extraction. The purpose of these implants is to compensate for loss of refraction by the lens, thereby returning the eye to an emmetropic state. Following the results of studies involving large numbers of dogs of various breeds, it has been determined that the canine IOL should have a power of 40.0–41.5 D. The 1.5 D range of recommended values probably results from breed differences. Use of 41 D IOLs in 60 dogs resulted in an average refractive error of ≤1.2 D. However, it is important to note that though 41 D IOLs are used to bring aphakic dogs to emmetropia, this does not mean that aphakic dogs suffer from hypermetropia of 41 D.

While canine IOLs are widely used by veterinary ophthalmologists, their development and use in other species is lagging behind. A study in horses concluded that an IOL of 25–30 D overcorrects the aphakic equine eye, even though preliminary calculations showed a theoretical power of up to 30 D. Subsequent studies, supported by a calculated IOL power of 15.4 D, have shown that a 14 D IOL brought 5/6 horse eyes to within 0.4 D of emmetropia.

Table 2.15 Refractive errors in selected animal species.^a

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. СКАЧАТЬ