Phacoemulsification is the treatment of choice for vision-impairing cataracts. After lens removal in dogs, cats, and humans, an IOL is routinely implanted to correct hyperopia (farsightedness).1 The IOL minimizes postoperative refractive error, thereby improving the quality of postoperative vision.2
Phacoemulsification is increasingly being used to treat horses with vision-impairing cataracts.3 However, IOLs are not routinely implanted in horses after lens removal.3 Although vision is improved, horses are markedly hyperopic after surgery.4,5,a Difficulties with contrast and vision at night have also been reported.4,6,7 To improve postoperative visual acuity, equine-specific IOLs have been developed. In 1 study,8 implantation of a +18 D IOL was recommended to restore emmetropia. In another study,9 results close to emmetropia were detected after implantation of +14 D IOLs.9
When a human patient undergoes cataract surgery, IOLs with a wide variety of strengths are available. The AGL, corneal curvature, and estimated postoperative ACD are used to calculate an ideal dioptric strength for each individual.10,11 This individualized approach is required because of the wide variation in globe size and preoperative refractive error. In contrast, when canine and feline patients undergo cataract surgery, an IOL with a standardized strength is used for each species.12 This approach is possible because most of the animals are emmetropic prior to surgery and globe sizes are extremely similar.1
Most horses are emmetropic5,a; however, horses of various breeds, heights, body types, and ages can have globe lengths that differ by ≥ 10 mm.3,7,8,13–17 Variations in corneal curvature also exist, and breed, body weight, height, age, and AGL all significantly influence the degree of corneal curvature.13 However, to the authors' knowledge, the impact of variation in AGL and corneal curvature on appropriate selection of an equine IOL has not been reported. Both factors contribute to the dioptric strength of the IOL required to restore emmetropia after cataract surgery. The variation among equine eyes may require a more individualized approach to selection of an IOL with the appropriate strength.
The purposes of the study reported here were to determine whether there was a significant difference in the calculated IOL strength among horses; to assess correlations between the calculated IOL strength and height, body weight, age, and corneal diameter of horses; and to determine whether there was a significant difference in corneal curvature measured with B-mode ultrasonography and a modified photokeratometer. The hypotheses of the study were that a significant difference would exist in the calculated IOL strengths among a population of adult horses; a correlation would exist between the calculated IOL strength and height, weight, age, and corneal diameter for a population of adult horses; and a significant difference would exist in corneal curvature when measured with B-mode ultrasonography and a modified photokeratometer.
Anterior chamber depth
Axial globe length
Crystalline lens thickness
Vitreous body length
Stuhr C, Abrams G, Bullimore M. The normal refractive state of the equine (abstr), in Proceedings. 30th Annu Meet Am Coll Vet Ophthalmol 1999;74.
Accutome UBM Plus, provided by Accutome Inc, Malvern, Pa.
Ramsey D, Mutti D, Zadnik C, et al. Refractive error in Rocky Mountain horses with cornea globosa and with normal corneas (abstr). Invest Ophthalmol Vis Sci 2000;41:S135.
Purdue Research Machinery Services, Purdue University, West Lafayette, Ind.
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