Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 117

Because of the significant refractive role of the lens, cataract surgery (or any surgical lens extraction) resulting in aphakia leaves the eye severely hypermetropic. The aphakic eye lacks the refractive contribution of the lens; therefore, light is not sufficiently 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.