Abstract
Objectives
The aim of this study was to assess the effect of topical tetracaine hydrochloride 0.5% on intraocular pressure (IOP) in ophthalmologically normal cats.
Methods
Twenty domestic shorthair cats (40 eyes) were used in this study. Each cat was randomly allocated to one of two groups (treatment or control). Baseline IOP (T0) was measured in each cat, and then one drop of tetracaine hydrochloride 0.5% or artificial tears was administered into a randomly chosen eye of each cat in the treatment and control groups, respectively. Repeat IOP measurements were performed at 2 mins (T2), 5 mins (T5), 15 mins (T15) and 30 mins (T30) with a rebound tonometer.
Results
Mean baseline IOP in all eyes was 20.6 ± 2.5 mmHg. After the unilateral administration of tetracaine, mean IOP decreased significantly in the treated eye at T2 (P = 0.01). Mean IOP returned to baseline values at T15. The mean IOPs in the treated eyes at T0, T2, T5, T15 and T30 were 20.6 ± 3.3 mmHg, 18.2 ± 2.5 mmHg, 18.2 ± 3.4 mmHg, 20.2 ± 3.2 mmHg and 19.8 ± 2.7 mmHg, respectively. A significant difference in IOP was found at all time points between the tetracaine and control groups (P <0.03).
Conclusions and relevance
The results of the present study showed a statistically significant reduction in mean IOP 2 mins after the administration of tetracaine hydrochloride 0.5% in the treated eyes of the cats.
Introduction
Measuring intraocular pressure (IOP) with a Schiotz or applanation tonometer, cytological sampling and ocular ultrasonography requires topical anaesthesia. Tetracaine and oxybuprocaine are commonly used as topical anaesthetics for these diagnostic purposes. 1 The onset of action of tetracaine is about 15 s and its effects persist for a maximum of 50 mins.1,2 Tetracaine has been reported in dogs and humans to have local adverse reactions, such as conjunctival hyperaemia, chemosis and ocular discomfort.1,2 In horses, the anaesthetic effect of tetracaine has been reported to be more intense than that of proparacaine. 3
The effect of tetracaine on IOP in humans, dogs and horses has been evaluated. In humans and dogs, IOP is decreased after the application of tetracaine,4–6 and in horses, an insignificant increase in IOP has been reported after the administration of tetracaine. 7
To our knowledge, no study has reported the effects of topical anaesthetic agents on IOP in cats. Thus, it was hypothesised that topical tetracaine may decrease IOP in cats.
Materials and methods
This study was approved by the Iranian Society for Prevention of Cruelty to Animals, using Iranian ethical codes for studying on animals. This work was conducted in accordance with the ARVO Statement for Use of Animals in Ophthalmic and Vision Research. Twenty client-owned intact adult domestic shorthair cats (10 males and 10 females) were used in this study. All animals were enrolled in this study after a complete physical examination was performed on them. All cats were presented to the clinic for routine general and ophthalmic examinations, and tonometry was performed as part of the complete ophthalmic examination (eg, checking for hypotony). A full ophthalmic examination was performed in all cats, which included: slit-lamp biomicroscopy for assessment of the cornea, anterior chamber, iris and lens; direct and indirect ophthalmoscopy for evaluation of the fundus; fluorescein staining for the presence of any ulcerative keratitis and tear film break-up time; and a Schirmer tear test for quantitative measurement of the tear film.
Each cat was randomly allocated into one of two groups (treatment or control). Before administration of tetracaine hydrochloride 0.5% or artificial tears, the baseline IOP (T0) of each cat was measured. In the treatment group, one drop of tetracaine hydrochloride 0.5% (Anestocaine 0.5%; Sinadarou Labs) was administered into one eye, selected at random, of each cat. In the control group, one drop of artificial tears was administered into one randomly selected eye. IOP measurements were performed at 2 mins (T2), 5 mins (T5), 15 mins (T15) and 30 mins (T30) after the administration of tetracaine or artificial tears with a rebound tonometer (TonoVet; Jorgensen Laboratories) with ‘d’ calibration.
There were no signs of ocular discomfort in any of the animals during the study for up to 24 h after the measurements were taken. A single investigator (SMR) performed all ophthalmic examinations and diagnostic testing. IOP measurements were performed between 10 am and 11 am, to minimise any potential disparities related to diurnal changes. Cats with both dark- and light-coloured irises participated in this study. Owing to the small sample size, statistical analysis to compare for differences in IOP in relation to iris colour was not performed.
Statistical analysis was performed in SPSS for Mac version 23 (IBM SPSS Statistics, SPSS). A one-sample Kolmogorov–Smirnov was used to test the normality of the data. A paired samples t test was also used to compare the IOP values obtained from the right and left eyes. Two-way repeated measures ANOVA with Bonferroni adjustment was employed to compare the means of the obtained IOP values. A P value <0.05 was considered to be statistically significant.
Results
The IOP values obtained in both groups were distributed normally based on a one-sample Kolmogorov–Smirnov test (P >0.4). Repeated measures ANOVA found a significant difference in feline IOP between baseline and T2 (P = 0.01). In the treatment group, the difference in IOP between baseline and T5 or T15 was not significant (P = 1.00). Mean IOP returned to the baseline values at T15. There were no significant differences in IOP between baseline and different time points in the control group (P >0.25). Only a small alteration (up to 1 mmHg) was noted in this group.
Inter-group comparison revealed a significant difference in IOP at all the time points between the tetracaine and the control groups (P <0.03). There were no significant differences between male and female cats at any time point (P >0.45). Figure 1 shows the mean IOP values in treated and untreated eyes in the treatment group. Mean IOPs are summarised in Table 1.
Box and whisker plot of intraocular pressure (IOP) values after the administration of tetracaine hydrochloride 0.5% in the treated and untreated eyes of cats in the treatment group. The IOPs are shown at baseline (0) and 2 mins (2), 5 mins (5), 15 mins (15) and 30 mins (30) after treatment. Treated = treated eyes; untreated = untreated eyes
Intraocular pressure (IOP) in eyes treated with either tetracaine hydrochloride 0.5% (treatment eyes) or artificial tears (placebo eyes), and control eyes of 20 domestic shorthair cats
Data are mean ± SD
T0 = baseline; T2 = 2 mins post-treatment; T5 = 5 mins post-treatment; T15 = 15 mins post-treatment; T30 = 30 mins post-treatment.
Discussion
The results of the present study showed changes in IOP in the studied animals. The maximum decrease in mean IOP in treated eyes was 2.4 mmHg at 2 mins after drug administration. Although there were statistically significant changes in IOP in the treated cats, the small variation seemed clinically insignificant bearing in mind the wide range of normal IOP values.
A study by Sarchahi and Eskandari compared the effects of four different anaesthetic agents on IOP in dogs by means of the rebound tonometer. 6 In dogs, tetracaine decreased the IOP after 15 mins, while proparacaine was reported to reduce IOP after 10 mins. Topical bupivacaine and lidocaine had no significant effect on the IOP in dogs. 6 Parchen et al reported a significant decrease in IOP in dogs 30 mins and 50 mins after tetracaine and proparacaine administration, respectively. 8 In a study by Boillot et al, 5 IOP decreased 1 min after tetracaine administration in the treated eye in dogs. 5 All these studies in dogs showed significant alterations in IOP at different time points. Age, breed, time of measurements, variable corneal thickness and inter-observer difference were the possible reasons for the difference between these studies.
In rabbits, IOP decreased immediately after the administration of tetracaine; this decrease was reported to persist for 20 mins. 9 However, in horses, a non-significant increase in IOP was reported 2 mins after the application of topical tetracaine. 7 The results of our study were in agreement with these previous studies in dogs and rabbits, but with some disparity. In the present study, IOP decreased after 2 mins in cats in both the treated and non-treated eyes, but the decrease was only significant in the treated eyes.
Iris pigmentation and melanin have been reported to have a significant effect on IOP in the presence of tetracaine in the canine eye. 5 In the present study, IOP decreased in cats with both dark- and light-coloured irises. There is no accurate mechanism to explain this finding. It is believed that tetracaine is able to make the tear film unstable, which can then result in changes in corneal thickness. 10 However, a study has reported an increase or decrease in corneal thickness of up to 30 μm after topical anaesthetic administration in humans. 11 As has been published previously, an increase in corneal thickness would increase the IOP in dogs. 12
Although the results of this study confirmed our hypothesis, we were unable to find an exact cause for this finding in cats. There were some limitations to the present study. The sample size was small and only normal participants were included. Further study in normal and glaucomatous cats for a longer period of time is required to monitor IOP and corneal thickness after the administration of tetracaine.
Conclusions
The results of this study revealed a significant decrease in mean IOP 2 mins after the administration of tetracaine hydrochloride 0.5% to the treated eyes of cats.
Footnotes
Conflict of interest
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.
Ethical approval
This work involved the use of non-experimental animals (owned or unowned) and procedures that differed from established internationally recognised high standards (‘best practice’) of veterinary clinical care for the individual patient. The study therefore had ethical approval from an established committee as stated in the manuscript.
Informed consent
Informed consent (either verbal or written) was obtained from the owner or legal custodian of all animals described in this work (either experimental or non-experimental animals) for the procedure(s) undertaken (either prospective or retrospective studies). For any animals or humans individually identifiable within this publication, informed consent (either verbal or written) for their use in the publication was obtained from the people involved.
