Intraocular Pressure Measurement by Applanation Tonometry: Baseline Assessment in Exotic Carnivores and Non-Human Primates
American Association of Zoo Veterinarians Conference 2012

Jessica Lovstad1,2; Kathryn Gamble1, DVM, MS, DACZM; Gillian McLellan2, BVMS, PhD, DVOphthal, DECVO, DACVO, MRCVS

1Lincoln Park Zoo, Chicago, IL, USA; 2School of Veterinary Medicine, University of Wisconsin, Madison, WI, USA


Intraocular pressure (IOP) is measurement of fluid pressure within the anterior chamber of the eye. Glaucoma or uveitis may present with abnormal IOP. Applanation tonometry measures IOP by the force required to flatten the cornea.4,6,7,10

In domestic animals, IOP can be measured under manual restraint with topical anesthesia.1,2 Although anesthesia can affect IOP,2 general anesthesia seldom can be avoided in exotic species.3,5,8,9 Normal IOP reference ranges have been established for domestic species, but little information is available for exotics.

In this retrospective study of one zoological collection, IOP measurements (n=100) were collected over a five-year period at a single institution in 22 mammalian species opportunistically during annual examinations. In 73% of the individuals, measurements were obtained more than twice at repeated physical examinations, and in 25% of the individuals, measurements were made from juvenile to adulthood. Anesthetic protocols were maintained consistently within each species. Typically, during measurement animals were positioned laterally. Measurements (in mm Hg) were obtained in triplicate using a Tonopen® XL (Medtronic, Jacksonville, FL, USA) and measurements in the dependent eye were recorded.

IOP of the right and left eye was compared by paired t-test by species. No difference was identified at p<0.05 significance. Normal IOP (18 mm Hg) within mammalian species studied was generally consistent. Therefore, differences greater than two standard deviations from this baseline may indicate underlying ocular pathology. Caution should be exercised in interpretation of IOP between eyes of laterally recumbent animals as IOP may be elevated artifactually in the dependent eye.1

Literature Cited

1.  Broadwater JJ, Schorling JJ, Herring IP, et al. Effect of body position on intraocular pressure in dogs without glaucoma. Am J Vet Res. 2008;69(4):527–530.

2.  Brunson DB. Anesthesia in ophthalmic surgery. Vet Clin North Am Small Anim Pract. 1980;10(2):481–495.

3.  Burke JA, Potter DE. The ocular effects of xylazine in rabbits, cats, and monkeys. J Ocul Pharmacol. 1986;2(1):9–21.

4.  Gelatt KN, Peifer RL, Gum GG, et al. Evaluation of applanation tonometers for the dog eye. Invest Ophthalmol Visual Sci. 1977;16(10):963–968.

5.  Liang D, Alvarado TP, Oral D, et al. Ophthalmic examination of the captive western lowland gorilla (Gorilla gorilla gorilla). J Zoo Wildl Med. 2005;36(3):430–433.

6.  Moses RA, Marg E, Oechsli R. Evaluation of the basic validity and clinical usefulness of the Mackay-Marg tonometer. Invest Ophthalmol Visual Sci. 1962;1(1):78–85.

7.  Nagata N, Yuki M, Hasegawa T. In vitro and in vivo comparison of applanation tonometry and rebound tonometry in dogs. J Vet Med Sci. 2011;73(12):1585–1589.

8.  Ofri R, Horowitz I, Jacobson S, et al. The effects of anesthesia and gender on intraocular pressure in lions (Panthera leo). J Zoo Wildl Med. 1998;29(3):307–310.

9.  Ofri R, Steinmetz A, Thielebein J, et al. Factors affecting intraocular pressure in lions. Vet J. 2008;177(1):124–129.

10.  Rusanen E, Florin M, Hassig M, et al. Evaluation of a rebound tonometer (Tonovet) in clinically normal cat eyes. Vet Ophthalmol. 2010;13(1):31–36.


Speaker Information
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Jessica Lovstad
Lincoln Park Zoo
Chicago, IL, USA

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