Determination of the Appropriate Size Intraocular Lens for Cataract Surgery in the Grevy’s Zebra (Equus grevyi)
Our study sought to determine if intraocular lens recommendations for the domestic horse can be extrapolated to other equidae and to provide recommendations on intraocular lens selection likely to achieve emmetropia in Grevy’s zebra. Anterior chamber depth, lens thickness, axial length and corneal curvature of 12 eyes of 8 Grevy’s zebra aged 4–14 years were measured with ultrasound biometry. Theoretical replacement lens powers (D) were calculated using the Binkhorst and Retzlaff equations. Using 10 MHz B-scan ultrasonography, mean and standard deviation of axial length, anterior chamber depth; lens thickness and cornea to posterior capsule distance were 40.65±1.11 mm, 6.85±0.30 mm, 13.83±4.24 mm, and 19.47±0.52 mm respectively (n=12). Mean anterior chamber depth utilizing 20 MHz B-scan ultrasonography was 6.67±0.19 mm (n=12). Utilizing A-scan ultrasound, mean axial length, anterior chamber depth and lens thickness were 41.33±0.84 mm, 7.05±0.42 mm and 12.46±0.32 mm, respectively (n=4). Corneal curvature calculated from 20 MHz images was 29.87±01.19 D (n=9). The estimated IOL powers with the Binkhorst and Retzlaff formulas were 8.62±1.62 D and 9.19±1.58 D, respectively. The mean and standard deviation of the intraocular pressure using applanation tonometry was 21.77 mm Hg with a 95% confidence interval of 19.09–24.45 mm Hg. Use of the equine model for IOL calculation in Grevy’s zebras is not appropriate, but with ultrasound, IOL strength is readily calculated. Further study is warranted to determine appropriate artificial lens characteristics/specifications for equids other than horses.
Abbreviations: IOL—Intraocular lens, D—Diopter.