Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 116

The “purpose” of refraction and the accommodative processes described in the previous sections is to focus an image on the outer segments of the photoreceptors. An emmetropic eye is one in which parallel light rays (from a distant object) are focused on the outer segments when the eye is disaccommodated. A nonemmetropic, or ametropic, eye is one in which the focused image (from a distant object) falls anterior to the retina (i.e., nearsighted or myopic eye) or posterior to it (i.e., farsighted, hyperopic or hypermetropic eye) (see Figure 2.12).

Retinoscopy is the most commonly used clinical method to determine the refractive state of the eye in young children and animals. It is based on two assumptions: first, that light emerging from the eye (i.e., emergent rays) follows the same optical path as light entering the eye; and second, that the fundus reflex originates at the level of the outer segments. If those two assumptions hold, then emergent rays exit an emmetropic eye as parallel rays, a hypermetropic eye as diverging rays, and a myopic eye as converging rays. Therefore, the location of the focal point formed by the emergent rays can be used to determine the refractive state of the eye. Table 2.15 lists refractive errors in select species. Most of these values have been determined using streak retinoscopy, though autorefractors have also been used in veterinary medicine. A large survey found that, on average, dogs are indeed emmetropic, with a mean refractive error of −0.05 D. Nine dog breeds, including English Springer Spaniel, German Shepherd, Golden Retriever, Siberian Husky, Shetland Sheepdog, Labrador Retriever, Border Collie, Samoyed, and “other” terriers were found to be emmetropic (defined as having a mean refractive error <0.5 D in either direction). The same study also found that 8% of all dogs were hypermetropic, with a refractive error of up to +3.25 D. Three breeds (Australian Shepherd, Alaskan Malamute, and Bouvier des Flandres) were found to have a mean refractive error that was hypermetropic.

Figure 2.12 (a) In emmetropia, parallel light rays are focused on the retina. (b) In a farsighted (hypermetropic or hyperopic) eye, light rays are focused behind the retina. (c) In a nearsighted, or myopic, eye, the light is focused in front of the retina.

A study in cats reported that kittens (≤4 months) are myopic, with a mean error of −2.45 D, while adult cats are close to emmetropia, with a mean error of −0.39 D, thus demonstrating a significant effect of age. It is interesting to note that myopia decreases with age in cats, but in horses and in some dog breeds, notably the English Springer Spaniel and Beagle, it increases with age.

Several large studies have shown horses to be overall emmetropic. However, only 48–68% of horses are emmetropic in both eyes, with hyperopia and myopia reported in equal proportions in the ametropic horses, with errors of up to ±3 D. Age and breed may affect the refractive error in horses.

A large range of retinoscopy values is reported in species with small eyes. For example, values range from +20 to −13 D in the rat, and from −0.7 to +13.7 D in C57BL/6J mice.