Intraocular lens power calculation for the equine eye

BACKGROUND: Phacoemulsification and intraocular lens (IOL) implantation during cataract surgery in horses occur with increasing frequency. To reduce the postoperative refractive error it is necessary to determine the proper IOL power. In the present study retinoscopy, keratometry and ultrasonographic biometry were performed on 98 healthy equine eyes from 49 horses. The refractive state, corneal curvature (keratometry) and the axial location of all optical interfaces (biometry) were measured. The influences of breed, height at the withers, gender and age on values obtained and the comparison between the left and right eye were evaluated statistically. Corresponding IOL power were calculated by use of Binkhorst and Retzlaff theoretical formulas. RESULTS: Mean ± SD refractive state of the horses was + 0.32 ± 0.66 D. Averaged corneal curvature for Haflinger, Friesian, Pony, Shetland pony and Warmblood were 21.30 ± 0.56 D, 20.02 ± 0.60 D, 22.61 ± 1.76 D, 23.77 ± 0.94 D and 20.76 ± 0.88 D, respectively. The estimated postoperative anterior chamber depth (C) was calculated by the formula C = anterior chamber depth (ACD)/0.73. This formula was determined by a different research group. C and axial length of the globe averaged for Haflinger 9.30 ± 0.54 mm and 39.43 ± 1.26 mm, for Friesian 10.12 ± 0.33 mm and 42.23 ± 1.00 mm, for Pony 8.68 ± 0.78 mm and 38.85 ± 3.13 mm, for Shetland pony 8.71 ± 0.81 mm and 37.21 ± 1.50 mm and for Warmblood 9.39 ± 0.51 mm and 40.65 ± 1.30 mm. IOL power was calculated with the Binkhorst and Retzlaff theoretical formulas. Calculated IOL power for the several breeds ranged from 18.03 D to 19.55 D. The mean value across all horses was 18.73 D determined with Binkhorst formula and 18.54 D determined with Retzlaff formula. CONCLUSIONS: Mean result of this study is: an 18.5 D IOL seemed to be the most appropriate to achieve emmetropia after IOL implantation in horses. Cataract surgery without IOL implantation results in hyperopic and visual compromised horses. Retinoscopy, keratometry and ultrasonographic biometry should be performed on every affected horse and postoperative visual outcome should be determined.