Degree and duration of corneal anesthesia after topical application of 0.4% oxybuprocaine hydrochloride ophthalmic solution in ophthalmically normal dogs in: American Journal of Veterinary Research Volume 74 Issue 10 ()

Objective—To assess the anesthetic efficacy and local tolerance of topically applied 0.4% oxybuprocaine ophthalmic solution to in dogs and compare its effects with those of 1% tetracaine solution.

Procedures—Dogs were assigned to 2 groups, and baseline corneal touch threshold (CTT) was measured bilaterally with a Cochet-Bonnet aesthesiometer. Dogs of group 1 (n = 22) received a single drop of 0.4% oxybuprocaine ophthalmic solution in one eye and saline (0.9% NaCl) solution (control treatment) in the contralateral eye. Dogs of group 2 (n = 12) received a single drop of 0.4% oxybuprocaine ophthalmic solution in one eye and 1% tetracaine ophthalmic solution in the contralateral eye. The CTT of each eye was measured 1 and 5 minutes after topical application and then at 5-minute intervals until 75 minutes after topical application. anti depressant medicine

Results—CTT changes over time differed significantly between oxybuprocaine-treated and control eyes. After instillation of oxybuprocaine, maximal corneal anesthesia (CTT = 0) was achieved within 1 minute, and CTT was significantly decreased from 1 to 45 minutes, compared with the baseline value. No significant difference in onset, depth, and duration of corneal anesthesia was found between oxybuprocaine-treated and tetracaine-treated eyes. Conjunctival hyperemia and chemosis were detected more frequently in tetracaine-treated eyes than in oxybuprocaine-treated eyes.

Conclusions and Clinical Relevance—Topical application of oxybuprocaine and tetracaine similarly reduced corneal sensitivity in dogs, but oxybuprocaine was less irritating to the conjunctiva than was tetracaine.

Objective—To assess the anesthetic efficacy and local tolerance of topically applied 0.4% oxybuprocaine ophthalmic solution to in dogs and compare its effects with those of 1% tetracaine solution.

Topical ophthalmic anesthetics permit various diagnostic and therapeutic procedures to be performed in dogs by reducing the pain or discomfort of patients associated with these procedures.1,2 They render the cornea and conjunctiva insensitive to noxious stimuli by interfering with the generation and propagation of action potentials in the sensory nerves located in the corneal and conjunctival epithelium; these effects are mediated through actions on voltage-gated sodium channels of the axons, which result in a transient blockade of sodium influx.3–5

Application of anesthetic solutions to desensitize the ocular surface has been used since 1884, when it was determined that application of cocaine to the eyes could be used in place of general anesthesia for ophthalmic procedures.6,7 Currently, local anesthetics consist of an aromatic ring (lipophilic) and an amino group (hydrophilic) linked by ester or amide bonds. In ophthalmologic preparations, the esters are typically used as topical anesthetics and the amides as injectable local anesthetics, although amide-type anesthetic agents such as lidocaine and bupivacaine have been recommended for topical ocular application in humans8 and horses.9

Commonly used topical ophthalmic anesthetics include proparacaine, tetracaine, and oxybuprocaine. Proparacaine (proxymethacaine), a benzoic ester available as a 0.5% ophthalmic solution, is the topical anesthetic most commonly used in human and veterinary medicine in the United States.2,3 Tetracaine is another popular topical ocular anesthetic available in the form of 0.5% and 1.0% aqueous solutions or a 0.5% viscous formulation.9–13 Oxybuprocaine, an ester of para-aminobenzoic acid, was developed in the 1950s in Switzerland, where the hydrochloride salt was known as benoxinate, to serve as a new compound for rapid anesthesia of the ocular surface in humans.14,15 Anesthetic properties of oxybuprocaine after application to the human cornea were initially reported in 1955.16 Although the 0.4% solution was used for many years in the United States, this preparation has been discontinued, and the drug is currently available only in combination with a vital dye (eg, sodium fluorescein or disodium fluorexon) for use in applanation tonometry.17,18 Oxybuprocaine hydrochloride 0.4% ophthalmic solution is currently used routinely in Europe for diagnostic and surgical procedures involving the eyes of humans.19–23 It also is a topical anesthetic of choice for veterinary ophthalmology.24,25 To our knowledge, there have been no data published on the effects of oxybuprocaine ophthalmic solution on corneal sensitivity in dogs.

The Cochet-Bonnet aesthesiometer induces a brief, localized noxious stimulus to the corneal surface. It has been used to quantify corneal sensitivity in humans26,27 and domestic animals.28–31 The concept for the device is based on the von Frey principle, which states that the force required to distort a long hair when applied axially against the cornea is constant and proportional to the diameter and length of the hair.32 The Cochet-Bonnet aesthesiometer currently in use has a 0.12-mm-diameter flexible nylon thread, the length of which can be changed from 0.5 to 6 cm to control the intensity of the pressure applied by the tip of the nylon thread to the cornea (ie, the longer the thread, the lower the intensity of the pressure).33 Mechanical stimuli that evoke corneal sensations correspond to pressures of 11 to 145 mg/0.0113 mm2 (the cross-sectional area of the 0.12-mm-diameter filament),34 and these corneal stimulations appear to be conducted by unmyelinated C and thin myelinated Aδ fibers in corneal neurons.27 The Cochet-Bonnet aesthesiometer has been the standard clinical method used to evaluate and monitor the depth and duration of corneal analgesia after topical ocular application of proparacaine, tetracaine, and nalbuphine in several species of animals.12,13,35–39

The study reported here was designed to evaluate the analgesic effect of 0.4% oxybuprocaine ophthalmic solution on corneal sensitivity in ophthalmically normal dogs and to compare the pharmacological and potential toxic effects of oxybuprocaine with those of 1% tetracaine solution. Our hypothesis was that topical ocular application of oxybuprocaine and tetracaine would result in similar corneal anesthetic effects in ophthalmically normal dogs.

Animals—Thirty-four healthy adult Beagles (25 females and 9 males) were used in the study. The dogs were university-owned research animals. They were 1 to 11 years old (mean, 4.4 years; median, 3.4 years). All dogs were housed in laboratory animal facilities for at least 3 months before the start of the study. Rooms were maintained at a mean ± SD ambient temperature and relative humidity of 19 ± 1°C and 50 ± 5%, respectively. Dogs were included in the study if they were considered ophthalmically normal on the basis of results of an ophthalmic examination that included slit-lamp biomicroscopy, indirect ophthalmoscopy, and rebound tonometry. Dogs with Schirmer tear test values < 10 mm/min were excluded from the study. All procedures were performed in compliance with institutional and national guidelines in accordance with the European Community Council directive 86/609/EEC.

Procedures—Dogs were allocated via a randomization procedure (with a table of random numbers) into 2 groups (22 dogs in group 1 and 12 dogs in group 2). In each group, CTT measurements were obtained by use of a Cochet-Bonnet aesthesiometera for both eyes of each dog immediately before topical administration of solutions (time 0; baseline). Solutions then were topically administered to the dogs, and CTT values were obtained for both eyes of each dog 1 and 5 minutes after topical administration and then at 5-minute intervals until 75 minutes after topical administration.

After baseline CTT measurements were obtained, dogs of group 1 received a single drop of 0.4% oxybuprocaine hydrochloride ophthalmic solutionb in one randomly selected eye and a single drop of sterile saline (0.9% NaCl) solution (control treatment) in the contralateral eye. Unpreserved oxybuprocaine was supplied in 5-mL multidose containers. After each dropper bottle was opened, it was stored at 6° to 8°C and used for only 2 or 3 days to reduce the risk of bacterial contamination of the solution, as has been recommended.40

After baseline CTT measurements were obtained, dogs of group 2 received a single drop of a 0.4% ophthalmic formulation of oxybuprocainec in one randomly selected eye and a single drop of 1% tetracaine ophthalmic solutiond in the contralateral eye. Both topical anesthetics were unpreserved solutions supplied in unit-dose applicators.

CTT measurement—The CTT measurements were performed with each dog minimally restrained in a sitting position on an examination table. A Cochet-Bonnet aesthesiometer with a 0.12-mm-diameter nylon filament was used to measure sensitivity of the central portion of the cornea, as described elsewhere.28–31 Briefly, the tip of the filament was applied perpendicularly to the cornea until a minimal bending of the filament was observed. Initial measurements were obtained with the nylon filament at the maximum length (6 cm); the filament length was then gradually shortened by 0.5-cm increments until a blink reflex was observed.30 When a blink reflex was noticed, the filament length was then increased by 0.5 cm and measurements repeated until the CTT was determined. The CTT value was the shortest filament length that induced a blink reflex for at least 3 of 5 stimulations.30 For the posttreatment period, the initial length for the filament was the shortest length that had not induced a blink response during the pretreatment measurement. To minimize variation for the technique, all measurements were obtained by 1 observer (JYD), who used binocular head loupes with 4.0× magnificatione to accurately determine the initial bending of the nylon filament when the tip was applied to the cornea.

Evaluation of potential adverse effects—For dogs of group 2, blepharospasm, bulbar conjunctival hyperemia, and bulbar conjunctival edema were recorded for each eye by 1 observer (AR), who was unaware of the treatment applied to each eye. These variables were subjectively evaluated on a scale of 0 to 3 (0 = none, 1 = mild, 2 = moderate, and 3 = severe), as described elsewhere.38 Other possible adverse effects were scored as present or absent. After CTT measurements were obtained, the corneas were stained with fluorescein and rose bengal and examined via slit-lamp biomicroscopy to determine whether changes to the corneal surface had been induced by the procedures or topical treatments.

Statistical analysis—Data were reported as mean ± SD. In both study groups, changes in CTT between treatments were compared by use of a 2-way analysis that included the main effects of treatment, time, and their interaction. Differences over time were investigated by use of linear contrasts with the Tukey multiple comparison test. For each eye treated with a topical anesthetic, duration of complete corneal anesthesia and interval until corneal sensitivity returned to baseline values were recorded. Duration of complete corneal anesthesia was defined as the time during which there was a lack of a blink response to the maximal corneal stimulation (ie, filament length of 0.5 cm, which was equivalent to a CTT value of 0). Interval until corneal sensitivity returned to baseline values was determined as described elsewhere.41 A 1-way ANOVA was used to compare duration of complete corneal anesthesia and interval until corneal sensitivity returned to baseline values between the oxybuprocaine- and tetracaine-treated eyes of dogs in group 2. The standard assumption of variance homogeneity was confirmed for each analysis. For all comparisons, values of P < 0.05 were considered significant.

Group 1—Mean ± SD pretreatment CTT values did not differ significantly between the control eyes (1.54 ± 0.41 cm) and oxybuprocaine-treated eyes (1.54 ± 0.40 cm). The mean CTT value remained fairly constant over the observation period in eyes treated with saline solution, and there was no significant change in CTT value after instillation of saline solution, compared with the baseline value (Figure 1). The 2-way analysis revealed that the CTT pattern during the observation period differed significantly (P < 0.001) between the oxybuprocaine-treated eyes and control eyes. Following instillation of oxybuprocaine, mean corneal sensitivity decreased significantly from 1 to 45 minutes, compared with the baseline value. Complete corneal anesthesia (corresponding to a CTT value of 0) was achieved by 1 minute after instillation and was maintained in all 22 oxybuprocaine-treated eyes for a minimum of 15 minutes. The duration of complete corneal anesthesia ranged from 15 to 50 minutes (mean ± SD, 31.6 ± 9.7 minutes) for the 22 oxybuprocaine-treated eyes. The interval for return of corneal sensitivity to baseline values after oxybuprocaine instillation ranged from 25 to 70 minutes (mean ± SD, 52.3 ± 11.4 minutes) among the 22 treated eyes (Table 1).

Mean ± SD CTT before (time 0) and after topical application of a single drop of 0.4% oxybuprocaine ophthalmic solution in one eye (black circles) and a single drop of sterile saline (0.9% NaCl) solution in the contralateral eye (white circles) of 22 dogs (A) and a single drop of 0.4% oxybuprocaine ophthalmic solution in one eye (black circles) and a single drop of 1% tetracaine ophthalmic solution in the contralateral eye (white squares) of 12 dogs (B). Values represent the filament length of a Cochet-Bonnet aesthesiometer. Complete corneal anesthesia was defined as lack of a blink response to maximal corneal stimulation (ie, filament length of 0.5 cm), which was equivalent to a CTT value of 0. *Within a treatment group, value differs significantly (P < 0.05) from the value before topical application (time 0; baseline).

Citation: American Journal of Veterinary Research 74, 10; 10.2460/ajvr.74.10.1321

Mean ± SD values for CTT, duration of complete anesthetic effect, and interval until return to baseline corneal sensitivity after topical administration of oxybuprocaine and tetracaine to ophthalmically normal dogs.

Dogs of group 1 (n = 22) received a single drop of 0.4% oxybuprocaine ophthalmic solution in one eye and saline (0.9% NaCl) solution (control treatment) in the contralateral eye. Dogs of group 2 (n = 12) received a single drop of 0.4% oxybuprocaine ophthalmic solution in one eye and 1% tetracaine ophthalmic solution in the contralateral eye. The CTT of each eye was measured before topical application (time 0; baseline), 1 and 5 minutes after topical application, and then at 5-minute intervals until 75 minutes after topical application. Complete corneal anesthesia was defined as lack of a blink response to maximal corneal stimulation (ie, filament length of 0.5 cm), which was equivalent to a CTT value of 0.

Group 2—Analysis of mean ± SD pretreatment CTT values revealed no significant difference between the oxybuprocaine- and tetracaine-treated eyes (1.41 ± 0.14 cm and 1.45 ± 0.15 cm, respectively). After treatment, there was a significant decrease in mean CTT value by 1 minute, compared with the baseline value, for both the oxybuprocaine- and tetracaine-treated eyes. Overall, the decreases in CTT value in response to oxybuprocaine and tetracaine ophthalmic solutions were of the same magnitude and duration, and the 2-way analysis revealed that the changes in CTT values over time did not differ significantly (P = 0.066) between the 2 topical anesthetics (Figure 1). Complete corneal anesthesia was achieved by 1 minute after instillation of both treatments, and mean duration of complete anesthesia did not differ significantly (P = 0.52) between oxybuprocaine- and tetracaine-treated eyes (34.5 ± 11.7 minutes and 31.6 ± 10.0 minutes, respectively; Table 1). Similarly, the mean interval until the return to baseline corneal sensitivity was not significantly (P = 0.90) different between the oxybuprocaine- and tetracaine-treated eyes (55 ± 8.7 minutes and 54.5 ± 7.5 minutes, respectively).

No signs of ocular discomfort (eg, blepharospasm or rubbing of the eyes) were detected in any of the dogs after instillation of oxybuprocaine or tetracaine. Conjunctival changes were observed in 1 of 12 oxybuprocaine-treated eyes and 8 of 12 tetracaine-treated eyes. Conjunctival hyperemia and edema were mild (grade 1) after instillation of oxybuprocaine, whereas conjunctival changes were mild (grade 1) in 5 dogs and severe (grade 3) in 3 dogs after instillation of tetracaine. In all dogs, the conjunctival changes were transient and resolved within 10 to 30 minutes after anesthetic administration. Slit-lamp examination with fluorescein and rose bengal staining performed at the completion of CTT measurements did not reveal surface irregularities or defects of the corneas.

The purpose of the present study was to determine the analgesic properties of a topical application of 0.4% oxybuprocaine to the cornea of ophthalmically normal eyes of dogs; a Cochet-Bonnet aesthesiometer was used to quantify changes in corneal sensitivity. The 0.4% oxybuprocaine eye drops used in the study were preservative-free solutions supplied in a multidose bottle or unit-dose applicators, which are currently used in human and veterinary ophthalmology in France. The Cochet-Bonnet aesthesiometry procedure was similar to the standard procedure used to evaluate the pharmacological efficacy of various ocular topical anesthetics in other studies,12,13,35–39,41 and the investigators were careful to control environmental and technical factors that could affect reliability of the results. To minimize the effects of ambient temperature and humidity on the stiffness of the nylon thread of the Cochet-Bonnet aesthesiometer,42,43 all CTT measurements were performed on dogs housed in facilities where temperature and humidity were controlled and maintained at consistent values throughout the study period. Only 1 breed of mesaticephalic dog was used in the present study to reduce variation in CTT values possibly linked to differences in facial conformation among dogs.28 Because measurement of the stimulus intensity to the central portion of the cornea is achieved indirectly by assessing the extent of bending of the filament, corneal contact should be detected as the smallest visible bending of the nylon monofilament. Precise observation of corneal contact has been conducted in human ophthalmology by attaching the Cochet-Bonnet aesthesiometer to a slit-lamp biomicroscope41; in the present study, binocular magnifier loupes with 4× magnification were used to observe corneal contact.

The mean baseline CTT values during the pretreatment periods in the present study were not significantly different within group 1 and group 2 dogs, and they were extremely similar between dogs of groups 1 and 2. These values were consistent with corneal sensitivity of canine eyes established with the Cochet-Bonnet aesthesiometer28 and were in accordance with the pretreatment CTT values reported in other investigations on the effects of topical administration of proparacaine35 and tetracaine12 in healthy dogs.

Analysis of results of the present study revealed that when 1 drop of preservative-free 0.4% oxybuprocaine ophthalmic solution was topically applied to ophthalmically normal dogs, there was rapid and pronounced corneal anesthesia, as assessed with a Cochet-Bonnet aesthesiometer. Complete desensitization was evident at 1 minute after application, and duration of maximal corneal anesthesia ranged from 15 to 50 minutes (mean ± SD, 31.6 ± 9.7 minutes) for the 22 treated corneas of group 1 dogs. These findings suggested that diagnostic or minor surgical ophthalmic procedures can be performed with a maximum of comfort for canine patients for at least 15 minutes after 1 drop of 0.4% oxybuprocaine has been applied to an eye. By comparison, duration of complete corneal anesthesia assessed with the Cochet-Bonnet aesthesiometer was found to range from 5 to 20 minutes in human eyes treated with 1 drop of 0.4% oxybuprocaine ophthalmic solution.41 In group 1 dogs, the time until corneal sensitivity returned to baseline values after oxybuprocaine administration had wide intersubject variation, with a range of 25 to 70 minutes (mean, 52.3 ± 11.4 minutes). Such duration of action would be sufficient for most clinical applications in dogs. In eyes of humans topically treated with 0.4% oxybuprocaine, wide ranges in recovery of corneal sensitivity were observed as determined by use of wisps of cotton (10 to 50 minutes) or a Cochet-Bonnet aesthesiometer (30 to 60 minutes).15,44 Measurement of corneal sensitivity with a noncontact corneal aesthesiometer in humans has revealed that the pattern of sensitivity loss (and subsequent return) after application of 1 drop of 0.4% oxybuprocaine was a rapid decrease that reached a peak loss at 15 minutes, which was followed by a gradual return of corneal sensitivity over the next 45 minutes.33

Currently, the 1% tetracaine ophthalmic preparation administered to group 2 dogs is available for veterinary ophthalmology in Europe. It was supplied in preservative-free unit-dose applicators similar to those used for the oxybuprocaine solution. No significant differences in onset, depth, and duration of corneal desensitization were found between the tetracaine- and oxybuprocaine-treated eyes in group 2 dogs. Although results from group 1 and group 2 dogs were not compared, similar corneal anesthetic properties were evident between the 2 oxybuprocaine preparations used in the study groups (Table 1). Results of the present study indicated that in ophthalmically normal dogs, the depth and duration of corneal anesthesia after administration of 1 drop of 0.4% oxybuprocaine solution or 1% tetracaine solution are extremely similar to those reported after administration of 1 drop of 0.5% proparacaine solution.35 Similarly, duration of corneal anesthesia was almost the same after topical administration of tetracaine (9.4 minutes) or proparacaine (10.7 minutes) in humans.11 Anesthetic properties of the 1% tetracaine ophthalmic solution applied in the present investigation were consistent with those reported in a study12 conducted to evaluate the pharmacological effects of 1% tetracaine combined with 0.1% phenylephrine on corneal sensitivity in dogs by use of a Cochet-Bonnet aesthesiometer. However, in that study,12 complete anesthesia of the cornea was detected 10 to 15 minutes after administration, whereas it was detected by 1 minute after treatment in the study reported here. Similar delays of 5 and 10 minutes until maximum corneal anesthetic effect after administration of 1 drop of 1% and 0.5% tetracaine solutions have been reported in horses.13,39 These differences might be related to the rapidity with which a drug diffuses through the corneal epithelium, which depends on the ratio of the ionized (hydrosoluble) and nonionized (liposoluble) forms of the molecule and is directly influenced by the pH of the ophthalmic preparation. The oxybuprocaine and tetracaine ophthalmic solutions of the present study had pH values of 4.26 to 4.55 and 4.64 to 4.71, respectively, which were in agreement with data in other reports.11,14 Thus it seems unlikely that differences in pH can account for the differences in the onset of tetracaine effects observed between the present study and other studies.12,13,39 Similarly, the subtle difference between the pH of the oxybuprocaine and tetracaine solutions applied to eyes of group 2 dogs may not explain the higher incidence of conjunctival changes observed after tetracaine administration, which suggests that tetracaine is probably more irritating than oxybuprocaine, irrespective of pH. Although topical administration of 1% tetracaine solution reportedly is tolerated well by horses with healthy eyes,39 it induced chemosis in 36% of the eyes of dogs when administered in combination with 0.1% phenylephrine.12 Acute chemosis has also been observed in a few dogs after application of a 0.5% tetracaine ophthalmic solution.45

Potential limitations of the present study were that the anesthetic effects of the drugs were evaluated after application of only 1 drop, that experiments were performed on ophthalmically normal eyes, and that ophthalmic solutions in multidose bottles and unit-dose applicators were used. In dogs and horses, application of 2 drops of proparacaine or tetracaine a few minutes apart significantly extended the duration of both the anesthetic effect and complete corneal anesthesia.35,39 It has been anecdotally reported that repeated administration of the 0.4% oxybuprocaine solution for 5 minutes increases the duration of corneal anesthesia to approximately 2 hours in dogs.10 To our knowledge, there are no data to support such anecdotal reports; thus, further studies are warranted. Experiments of the present study were performed on dogs with healthy corneas, which, on the basis of results of slit-lamp examination and fluorescein staining, did not have evidence of any changes after treatment with oxybuprocaine and tetracaine. However, these results cannot be definitively generalized to diseased corneas, as indicated in a recent study.9 An early study46 revealed that topical administration of oxybuprocaine to dogs with glaucoma and corneal edema may cause extensive sloughing of the corneal epithelium, which heals in 24 to 48 hours. Similarly, superficial punctate epithelial defects have been reported in glaucomatous human eyes after treatment with oxybuprocaine.15

Ophthalmic solutions in multidose and unit-dose containers were used in the present study, and it must be considered that differences in the drug volume instilled were a source of intersubject variation in the anesthetic effects observed. Both the type of container and design of the tip can affect repeatability of drop size, which is one of the determinants of the extent of ocular effects.47–49 Although we did not evaluate variation in the volume of the drops expelled by the containers in the present study, we can hypothesize that variation in the volume instilled had a possible effect on the degree and duration of corneal anesthesia among the dogs. Intraindividual differences probably also accounted for the intersubject variation in the results because of similar observations after topical application of oxybuprocaine in eyes of humans33,41 and tetracaine in eyes of dogs.12

To our knowledge, the study reported here is the first in which objective information has been obtained on the analgesic effects for topical administration of 0.4% oxybuprocaine hydrochloride ophthalmic solution to healthy eyes of dogs. Results indicated that onset, depth, and duration of corneal anesthesia provided by this anesthetic agent were not different from those observed after administration of 1% tetracaine solution to ophthalmically normal eyes of dogs. These results suggested that oxybuprocaine is appropriately suited as a topical ocular anesthetic agent for dogs that can be used in a broad spectrum of clinical settings and with fewer risks of conjunctival changes, compared with those for tetracaine.

Cochet-Bonnet aesthesiometer, Luneau Ophthalmologie, Chartres, France.

Cebesine 0.4% eye drops, Chauvin Bausch & Lomb, Montpellier, France.

Oxybuprocaine hydrochloride Théa 1.6 mg/0.4 mL eye drops in single-dose container, Laboratoires Théa, Clermont-Ferrand, France.

Tetracaine 1% single-dose eye drops, TVM, Lempdes, France.

Head loupes HRP 4X/340, Heine Optotechnik GmbH & Co, Herrsching, Germany.

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