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.
Davidson MG. Retinoscopy (oral presentation). 9th Biannual William Magrane Basic Sci Course Vet Comp Ophthalmol, Raleigh, NC, June 2010.
Kubai MA, Bently E, Miller PE, et al. Refractive states of eyes and association between ametropia and breed in dogs. Am J Vet Res 2008; 69:946–951.
Hung LF, Ramamirtham R, Huang J, et al. Peripheral refraction in normal infant rhesus monkeys. Invest Ophthalmol Vis Sci 2008; 49:3747–3757.
Gilger BC, Davidson MG, Colitz CM. Experimental implantation of posterior chamber prototype intraocular lenses for the feline eye. Am J Vet Res 1998; 59:1339–1343.
Marsh-Tootle WL, Norton TT. Refractive and structural measures of lid-suture myopia in tree shrew. Invest Ophthalmol Vis Sci 1989; 30:2245–2257.
Mutti DO, Zadnik K, Johnson CA, et al. Retinoscopic measurement of the refractive state of the rat. Vision Res 1992; 32:583–586.
Frederiksen R, Warrant EJ. Visual sensitivity in the crepuscular owl butterfly Caligo memnon and the diurnal blue morpho Morpho peleides: a clue to explain the evolution of nocturnal apposition eyes? J Exp Biol 2008; 211:844–851.
Murphy CJ, Kern TJ, Howland HC. Refractive state, corneal curvature, accommodative range and ocular anatomy of the Asian elephant (Elephas maximus). Vision Res 1992; 32:2013–2021.
Murphy CJ, Bellhorn RW, Williams T, et al. Refractive state, ocular anatomy, and accommodative range of the sea otter (Enhydra lutris). Vision Res 1990; 30:23–32.
Ofri R, Millodot S, Shimoni R, et al. Development of the refractive state in eyes of ostrich chicks (Struthio camelus). Am J Vet Res 2001; 62:812–815.
Gur M, Sivak JG. Refractive state of the eye of a small diurnal mammal: the ground squirrel. Am J Optom Physiol Opt 1979; 56:689–695.
Howland HC, Howland M, Giunta A, et al. Corneal curvatures and refractions of central American frogs. Vision Res 1997; 37:169–174.
Hueter RE, Murphy CJ, Howland M, et al. Refractive state and accommodation in the eyes of free-swimming versus restrained juvenile lemon sharks (Negaprion brevirostris). Vision Res 2001; 41:1885–1889.
Hughes A, Vaney DI. The refractive state of the rabbit eye: variation with eccentricity and correction for oblique astigmatism. Vision Res 1978; 18:1351–1355.
Murphy CJ, Howland HC, Kwiecinski GG, et al. Visual accommodation in the flying fox (Pteropus giganteus). Vision Res 1983; 23:617–620.
Yinon U, Koslowe KC, Rassin MI. The optical effects of eyelid closure on the eyes of kittens reared in light and dark. Curr Eye Res 1984; 3:431–439.
Murphy CJ, Howland HC. Owl eyes: accommodation, corneal curvature and refractive state. J Comp Physiol 1983; 151:277–284.
Black J, Browning SR, Collins AV, et al. A canine model of inherited myopia: familial aggregation of refractive error in Labrador Retrievers. Invest Ophthalmol Vis Sci 2008; 49:4784–4789.
Davidson MG, Murphy CJ, Nasisse MP, et al. Refractive state of aphakic and pseudophakic eyes of dogs. Am J Vet Res 1993; 54:174–177.
Gaiddon J, Bouhana N, Lallement P. Refraction by retinoscopy of normal, aphakic, and pseudophakic canine eyes: advantage of a 41-diopter intraocular lens? Vet Comp Ophthalmol 1996; 6:121–124.
Smith EL III, Maguire GW, Watson JT. Axial lengths and refractive errors in kittens reared with an optically induced anisometropia. Invest Ophthalmol Vis Sci 1980; 19:1250–1255.
Cremieux J, Orban GA, Duysens J, et al. Experimental myopia in cats reared in stroboscopic illumination. Vision Res 1989; 29:1033–1036.
Mutti DO, Zadnik K, Fusaro RE, et al. Optical and structural development of the crystalline lens in childhood. Invest Ophthalmol Vis Sci 1998; 39:120–133.
Mutti DO, Zadnik K, Murphy CJ. Naturally occurring vitreous chamber-based myopia in the Labrador Retriever. Invest Ophthalmol Vis Sci 1999; 40:1577–1584.
Grinninger P, Skalicky M, Nell B. Evaluation of healthy equine eyes by use of retinoscopy, keratometry, and ultrasonographic biometry. Am J Vet Res 2010; 71:677–681.
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.
Andison ME, Sivak JG, Bird DM. The refractive development of the eye of the American kestrel (Falco sparverius): a new avian model. J Comp Physiol A 1992; 170:565–574.
Graham B, Judge SJ. Normal development of refractive state and ocular component dimensions in the marmoset (Callithrix jacchus). Vision Res 1999; 39:177–187.
Norton TT, McBrien NA. Normal development of refractive state and ocular component dimensions in the tree shrew (Tupaiabelangeri). Vision Res 1992; 32:833–842.
Zhou X, Qu J, Xie R, et al. Normal development of refractive state and ocular dimensions in guinea pigs. Vision Res 2006; 46:2815–2823.
Zhou X, Shen M, Xie J, et al. The development of the refractive status and ocular growth in C57BL/6 mice. Invest Ophthalmol Vis Sci 2008; 49:5208–5214.
Ofri R, Millodot S, Tadmor Y, et al. The development of the refractive state in the newborn Thomson gazelle. J Comp Physiol A Neuroethol Sens Neural Behav Physiol 2004; 190:831–835.
Ribeiro AP, Silva ML, Rosa JP, et al. Ultrasonographic and echobiometric findings in the eyes of Saanen goats of different ages. Vet Ophthalmol 2009; 12:313–317.
Cottrill NB, Banks WJ, Pechman RD. Ultrasonographic and biometric evaluation of the eye and orbit of dogs. Am J Vet Res 1989; 50:898–903.
Gilger BC, Davidson MG, Howard PB. Keratometry, ultrasonic biometry, and prediction of intraocular lens power in the feline eye. Am J Vet Res 1998; 59:131–134.
Belkin M, Yinon U, Rose L, et al. Effect of visual environment on refractive error of cats. Doc Ophthalmol 1977; 42:433–437.
Yinon U, Rose L, Shapiro A. Myopia in the eye of developing chicks following monocular and binocular lid closure. Vision Res 1980; 20:137–141.
Bradley DV, Fernandes A, Lynn M, et al. Emmetropization in the rhesus monkey (Macaca mulatta): birth to young adulthood. Invest Ophthalmol Vis Sci 1999; 40:214–229.
Moodie KL, Hashizume N, Houston DL, et al. Postnatal development of corneal curvature and thickness in the cat. Vet Ophthalmol 2001; 4:267–272.
Corboy J. Spherical errors. In: Corboy J, ed. The retinoscopy book: an introductory manual for eye care professionals. 5th ed. Thorofare, NJ: SLACK Inc, 2003;49–59.