Evaluation of healthy equine eyes by use of retinoscopy, keratometry, and ultrasonographic biometry

Abstract

Objective - To assess natural variations in degree of refraction, corneal curvature, corneal astigmatism, corneal radius, and intraocular distance of healthy equine eyes. Animals - 159 horses with healthy eyes that were admitted to a veterinary teaching hospital for nonophthalmic surgeries. Procedures - Eyes of horses were examined with a retinoscope prior to anesthesia and with a keratograph and A- and B-scan ultrasonographic biometers during surgery. In addition, manual caliper measurements of horizontal and vertical corneal radii were obtained. Results - Mean ± SD degree of refraction in the horizontal meridian of eyes was -0.06 ± 0.68 diopters (D). Vitreous body length and horse age correlated negatively with refraction values. The horizontal corneal radius (15.96 ± 1.28 mm) was larger than the vertical corneal radius (15.02 ± 1.09 mm). Accordingly, the vertical corneal curvature (21.56 ± 1.68 D) was greater than the horizontal corneal curvature (22.89 ± 1.65 D). Axial globe length (40.52 ± 2.67 mm), anterior chamber depth (6.35 ± 0.59 mm), lens thickness (12.30 ± 0.83 mm), and vitreous body length (21.87 ± 1.85 mm) were positively correlated with body weight, height, and age. Results of keratograph and caliper measurements correlated well for horizontal corneal diameter but poorly for vertical corneal diameter. Results of A- and B-scan ultrasonography differed by ≤ 1 mm in 64% of measured eyes. Conclusions and Clinical Relevance - Results of keratometry and ultrasonographic biometry varied widely. Additional research is needed to validate the keratograph used in our study for measurements in equine eyes.