LITMIR.BIZ

Figure 2.11 The effect of vitreous elongation on ocular refraction. (a) A focused emmetropic eye. (b) The refractive power of the eye has not changed, and the light is focused on the same spot as in panel (a). However, due to vitreous elongation, the retina has moved posteriorly, and therefore the light is now focused in front of the retina. As a result, the eye is now nearsighted or myopic.

      Pupil

      The pupillary aperture is not considered to be a classic refractive structure as it has no refractive index, but it does make an important contribution to the resolving power of the eye. As the pupil dilates in dim light, the number of photons entering the eye increases, resulting in increased retinal illumination. However, there is “a price to be paid” for this increased illumination, as mydriasis also decreases the depth of focus of the eye. This means that as the pupil dilates, the range of distances at which objects remain in focus decreases.

      Abnormal Refractive States and Optical Errors

      Emmetropia and Ametropia

      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.