Effects of medetomidine and xylazine on intraocular pressure and pupil size in healthy Beagle dogs.

Objective: To determine the effects of intramuscular (IM) administration of medetomidine and xylazine on intraocular pressure (IOP) and pupil size in normal dogs.
Study Design: Prospective, randomized, experimental, crossover trial.
Animals: Five healthy, purpose-bred Beagle dogs.
Methods: Each dog was administered 11 IM injections of, respectively: physiological saline; medetomidine at doses of 5, 10, 20, 40 and 80 μg kg(-1), and xylazine at doses of 0.5, 1.0, 2.0, 4.0 and 8.0 mg kg(-1). Injections were administered at least 1 week apart. IOP and pupil size were measured at baseline (before treatment) and at 0.25, 0.50, 0.75, 1, 2, 3, 4, 5, 6, 7, 8 and 24 hours post-injection.
Results: A significant decrease in IOP was observed at 6 hours after 80 μg kg(-1) medetomidine compared with values at 0.25 and 0.50 hours, although there were no significant changes in IOP from baseline. In dogs treated with 8.0 mg kg(-1) xylazine, significant reductions in IOP were observed at 4 and 5 hours compared with that at 0.25 hours after administration. In dogs treated with 5, 10, 20 and 40 μg kg(-1) medetomidine and 0.5, 1.0 and 2.0 mg kg(-1) xylazine, there were no significant changes in IOP. Pupil size did not change significantly after any of the medetomidine or xylazine treatments compared with the baseline value.
Conclusions and Clinical Relevance: Low or moderate doses of medetomidine or xylazine did not induce significant changes in IOP or pupil size. In contrast, high doses of medetomidine or xylazine induced significant changes up to 8 hours after treatment, but values remained within the normal canine physiological range. The results of this study suggest a lack of significant change in IOP and pupil size in healthy dogs administered low or moderate doses of xylazine or medetomidine.
(© 2015 Association of Veterinary Anaesthetists and the American College of Veterinary Anesthesia and Analgesia.)