Keratometry - A Scan and Prediction of IOL Lens Power in Felines.
Intraocular lens (IOL) implantation is the only effective treatment to restore the vision of a cataract eye. The incidences of cataract in domestic short haired cats are increasing day by day. Fourteen eyes from seven clinically normal cats were examined for intraocular distances (using A-scan ultrasound biometry) and corneal curvature (using keratometry) and by using these data appropriate posterior chamber IOL strength to achieve emmetropia after lens removal has been predicted. Three theoretical formulas were used to calculate the diopteric strength of IOL. The overall average predicted IOL power of all the eyes were 52.68 [+ or -] 2.23D. The calculated mean IOL power in cats was more than the power required in dogs.
Keywords: Cat; IOL; strength
The lens plays a major role in forming an image in the retina by refracting light. Any opacity of lens, cataract, impairs the eye's ability to absorb and transmit light to retina, interfering vision of the animal. Cats commonly do not suffer from cataract unlike dogs. Secondary cataract is more prevalent then primary and mostly due to trauma, anterior uveitis, lens luxation (Frankel, 2001) or nutritional deficiencies (Richter et al., 2002). The goal of treatment is to restore the cat's vision and effective treatment option is surgical removal of affected lens. Cats became extremely far sighted with little useful vision without a lens (aphakic vision). Therefore replacement of the cataract lens with an artificial one is more important to restore the vision. Prediction of the dioptric strength of intra ocular lens (IOL) can be achieved by using several theoretical formulas based on optical models of eye.
The purpose of this study was to measure the intraocular distances (using A-scan ultrasound biometry) and corneal curvature (using keratometry) of feline eye and to use these data to predict appropriate posterior chamber IOL strength to achieve emmetropia after lens removal.
Materials and Methods
Fourteen eyes (7 right and 7 left eyes) from seven (3 male and 4 female) clinically normal adult domestic short haired cats weighing between 1.9kg and 4.9kg were used for this study. The age of the cats ranges from 5.5 years to 13.6 years with an average 9.52 [+ or -] 2.59 years and were found normal in ocular and general physical examination. All 14 eyes were subjected to ultrasonic biometry and keratometry.
A-mode ultrasonic biometry using 10 MHz linear transducer in amplitude mode was used to measure different intraocular distances (anterior chamber depth, lens thickness and axial length). The cats were restrained manually and Paraprocaracain 0.5% was instilled topically before taking the measurements.
All the cats were sedated using intramuscular injection of Xylazine and Ketamine in combination and positioned properly. The keratometer was placed at appropriate distance from the eye to be measured, forming the image of 3 rings and was brought into focus on the surface of cornea. The reflected images were aligned by rotating the measuring drums on keratometer. The K1 (horizontal corneal curvature) and K2 (vertical corneal curvature) values for each eyes were recorded.
Results and Discussion Biometry
The average measurements of various intraocular distances (Axial length, Anterior chamber depth and Lens thickness) of right, left and both eyes were not significantly different from each other (Table 1). The overall means of different distances for all the 14 eyes were 18.50 [+ or -] 0.80 (Axial Length), 4.15 [+ or -] 0.50
(Anterior Chamber Depth) and 7.15 [+ or -] 0.25 (Lens Thickness). Gilger et al. (1998) used ultrasonic biometry for the measurement of different intraocular distances and reported average axial length (20.91 [+ or -] 0.53), anterior chamber depth (5.07 [+ or -] 0.361) and lens thickness (7.77 [+ or -] 0.23) which were larger than our findings.
The average diopteric values of [K.sub.1] and [K.sub.2] of right and left eye were not significantly different from each other (table 2). The overall means for [K.sub.1] and [K.sub.2] values for both eyes were 38.68 [+ or -] 1.43 and 38.11 [+ or -] 0.98 respectively which were very similar to the finding of Gilger et. al. (1998).
Intraocular Lens Power Calculation
The predictable IOL power for all the 14 eyes were found out from the biometric and keratometric values. Three theoretical formulas were used to calculate the diopteric strength of IOL (Haigis, Hoffer Q. and SRK II) (Table 3). There was no significant difference in the predictable IOL power of right eye and left eye which was similar to the findings of Gilger et al. (1998). The overall average predicted IOL power of all the eyes were 52.68 [+ or -] 2.23 (using Haigis formula) which was within the range given by Gilger et al. (1998). The calculated mean IOL power in cats was more than the power required in dogs. It may be due to the more posterior position of the lens in cats than in dogs.
Cats need higher diopteric strength IOL for restoration of emmetropia after removal of cataractous lens. With the increasing incidence of cat cataract it is necessary to make artificial IOL specific for the cats. Further study is required to clinically evaluate the predictability of IOL power in case of cats.
Frankel, D. J. (2001). Malnutrition-induced cataracts in an orphaned kitten. Can. Vet. J. 42: 653-54.
Gilger, B.C., Davidson M.G. and Howard P.B. (1998). Keratometry, ultrasonic biometry, and prediction of intraocular lens power in the feline eye. Am. J. Vet. Res., 59:131-34.
Gilger, B.C., Davidson M.G. and Colitz C.M. (1998). Experimental implantation of posterior chamber prototype intraocular lenses for the feline eye. Am. J. Vet. Res. 59: 1339-43.
Mirshahi, A., Shafigh, S. and Azizzadeh, M. (2012). B-mode ultrasound of the normal eye in persian cats. In: 16th IVRA meeting and EVDI Annual Meeting, 08-26.
Richter, M., Guscetti, F. and Spiess, B. (2002). Aldose reductase activity and glucose-related opacities in incubated lenses from dogs and cats. Am. J. Vet. Res. 63:1591-97.
Arup Das (1), C. Ramani and Md. Shafiuzama
Department of Veterinary Surgery and Radiology Madras Veterinary College
Tamil Nadu Veterinary and Animal Sciences University (TANUVAS)
Chennai--600007 (Tamil Nadu)
(1.) Corresponding author.
Table 1: Averages of different biometric measurements Axial length Anterior chamber Lens thickness (mm) depth (mm) (mm) Right eye 18.85 [+ or -] 1.09 4.18 [+ or -] 0.69 7.20 [+ or -] 0.35 Left eye 18.35 [+ or -] 0.23 4.11 [+ or -] 0.28 7.11 [+ or -] 0.10 Both eyes 18.50 [+ or -] 0.80 4.15 [+ or -] 0.50 7.15 [+ or -] 0.25 Table 2: Averages of different keratometric measurements Horizontal Vertical corneal corneal curvature [(K).sub.1] curvature [(K).sub.2] Right eye 38.71 [+ or -] 1.25 38.64 [+ or -] 0.69 Left eye 38.64 [+ or -] 1.70 37.57 [+ or -] 0.97 Both eyes 38.68 [+ or -] 1.43 38.11 [+ or -] 0.98 Table 3 : Calculated IOL power in Diopter (D) Haigis Hoffer Q. SRK II Right eye 52.53 [+ or -] 2.11 45.98 [+ or -] 2.51 39.31 [+ or -] 1.29 Left eye 52.80 [+ or -] 2.31 45.75 [+ or -] 2.39 39.29 [+ or -] 1.43 Both eyes 52.68 [+ or -] 2.23 45.87 [+ or -] 2.35 39.30 [+ or -] 1.69
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|Title Annotation:||Short Communication; intraocular lens|
|Author:||Das, Arup; Ramani, C.; Shafiuzama, Md.|
|Date:||Jul 1, 2016|
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