Retinoscopy is an objective technique used to measure the refractive state of an eye. To better understand the optical factors that affect visual acuity, the refractive states of many species have been determined.1–21 The technique has been used in ophthalmogically normal, pathologically affected, and surgically manipulated eyes of dogs1,22–26 and cats3,20,27,28 to determine species reference limits as well as factors that affect refractive error. In 1 study,29 investigators indicated that the mean resting refractive state of 240 dogs was within 0.25 D of emmetropia, and breeds predisposed to development of myopia were also found. A more recent study1 of 1,440 dogs found the mean ± SD refractive state of all eyes examined was −0.05 ± 1.36 D; breeds found to be myopic (< −0.5 D) included the Rottweiler, Collie, Miniature Schnauzer, and Toy Poodle. Furthermore, the degree of myopia increased with increasing age among all dog breeds evaluated.1 An increase or decrease in axial length can result in incongruity between axial and focal lengths of the refractive elements of an eye, which results in ametropia.30 Axial myopia, which is attributable to an increase in the vitreous chamber depth, has been identified in Labrador Retrievers.30,31 The mean ± SD refractive state of horses in a recent study32 was −0.06 ± 0.68 D, and vitreous body length and age were negatively correlated with refraction values. In another study33 in which investigators evaluated the refractive state in pseudophakic eyes of horses, the mean preoperative refractive state of enucleated eyes was between −0.46 and 0.08 D. The development of the refractive state with maturation of an animal has also been evaluated in a number of species.11,34–40 Eyes of ostriches are characterized by myopia at the time a chick hatches and become emmetropic as a chick ages.11 This is true of American kestrels as well, and both American kestrels and ostriches are the exception in that most of the species that have been evaluated, including nonhuman primates and humans, appear hyperopic at birth and develop emmetropia as animals mature.11,34–37,39 To our knowledge, the refractive states of eyes and the association between ametropia and age and breed in cats has not been reported. The purpose of the study reported here was to assess the refractive state of a mixed population of ophthalmologically normal domestic cats and to identify natural and biometric factors that affect the refractive state.
Eye Care for Animals, Pasadena, Calif.
Kowa SL-14 portable slit lamp, Kowa Optimed, Torrence, Calif.
Keeler wireless, Keeler Ltd, Windsor, Berkshire, England.
Heine BETA TR 3.5 V, Heine Optotechnik, Herrsching, Germany.
TonoVet, Tiolat TV01, Helsinki, Finland.
Cyclopentolate HCl 1%, Alcon Laboratories Inc, Fort Worth, Tex.
L2 retinoscopy racks, Luneau Ophthalmologie, Chartres Cedex, France.
Proparacaine 0.5%, Alcon Pharmaceuticals Ltd, Fort Worth, Tex.
Ultraview 2.0 imaging console, E-Technologies Inc, Bettendorf, Iowa.
Grafco ultrasound transmission gel, Graham-Field Health Products, Atlanta, Ga.
SPPS, version 18, SPSS Corp, Chicago, Ill.
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McMullen RJ, Davidson MG, Campbell NB, et al. Evaluation of 30- and 25-diopter intraocular lens implants in equine eyes after surgical extraction of the lens. Am J Vet Res 2010; 71:809–816.
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