Influence of Methadone on Intraocular Pressure, Pupil Size, and Aqueous Tear Production in Healthy Dogs

Intraocular pressure (IOP), pupil size (PS), and tear production are variables important in maintaining eye homeostasis. The purpose of this study was to determine the effects of methadone on IOP, PS, and tear production measured by Schirmer I tear test (STT-I) in healthy nonpainful dogs. A prospective, randomized, “double-blind” clinical study was performed. A total of 40 healthy conscious client-owned dogs were included in the study. Dogs were allocated randomly to 1 of 3 groups and given intravenous methadone 0.3 mg/kg (Met-IV, n = 15), intramuscular methadone 0.3 mg/kg (Met-IM, n = 15), or saline 0.3 mL/kg (SAL, n = 10). IOP, PS, STT-I, heart rate (HR), and mean arterial pressure (MAP) were measured prior to (baseline) and at 2, 5, 10, 20, and 30 minutes after drug administration. Data were analyzed using 1-way and 2-way repeated measures ANOVA or their nonparametric equivalents (P < .05). No significant differences in IOP and PS within or between the groups were detected. In the Met-IV group, the STT-I decreased significantly after 30 minutes (P = .025), however, the values remained within the physiological ranges. In Met-IV group, HR decreased significantly at 5, 10, 20, and 30 minutes, respectively. No other significant differences were observed. Methadone administered at a dose of 0.3 mg/kg intravenously or intramuscularly seems to cause within 30 minutes no clinically important effect on IOP, PS, and STT-I in healthy conscious nonpainful dogs without ocular abnormalities.