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Logo of cjvetresCVMACanadian Journal of Veterinary ResearchSee also Canadian Journal of Comparative MedicineJournal Web siteHow to Submit
 
Can J Vet Res. 2016 October; 80(4): 329–334.
PMCID: PMC5052886

Language: English | French

Degree of corneal anesthesia after topical application of 0.4% oxybuprocaine ophthalmic solution in normal equids

Abstract

Oxybuprocaine hydrochloride ophthalmic solution has been widely used off-label in horses and donkeys, despite lack of data demonstrating efficacy and safety in these species. The objective of this study was to assess anesthetic efficacy of 0.4% oxybuprocaine hydrochloride ophthalmic solution in horses (n = 5) and donkeys (n = 24) and compare the effects with 0.5% proparacaine hydrochloride ophthalmic solution. The baseline corneal touch threshold (CTT) was measured with a Cochet-Bonnet esthesiometer. Donkeys (n = 12) and horses (n = 5) in group A received sterile ophthalmic solutions 0.4% oxybuprocaine with fluorescein (also termed benoxinate with fluorescein, abbreviated as ben + flu) instilled in one eye and 0.9% sterile sodium chloride solution (NaCl) with fluorescein (Na + flu) in the contralateral eye. Donkeys (n = 12) and horses (n = 5) in group B received sterile ophthalmic solutions (ben + flu) in one eye and 0.5% proparacaine with fluorescein (prop + flu) in the contralateral eye. The CTT was measured at 1 and 5 min post-application and at 5-minute intervals until 75 min after treatment. The CTT changes over time differed significantly between oxybuprocaine-treated and control eyes (P < 0.001). The CTT continued to decrease throughout the duration of the study when compared with baseline values. No statistically significant difference in onset, depth, or duration of corneal anesthesia was found between oxybuprocaine and proparacaine treated eyes during the time of the study. Interestingly, horses were shown to have a significantly more sensitive cornea than donkeys (P = 0.002). Oxybuprocaine and proparacaine reduced corneal sensitivity in donkeys and horses. No local irritation was observed with 0.4% oxybuprocaine.

Résumé

La solution ophtalmique d’hydrochlorure d’oxybuprocaïne a été utilisée extensivement en dérogation chez les chevaux et les ânes, malgré le manque de données démontrant son efficacité et son innocuité chez ces espèces. L’objectif de la présente étude était d’évaluer l’efficacité anesthétique d’une solution ophtalmique d’hydrochlorure d’oxybuprocaïne 0,4 % chez des chevaux (n = 5) et des ânes (n = 24) et comparer les effets avec une solution ophtalmique d’hydochlorure de proparacaïne 0,5 %. La valeur de base du seuil de contact cornéen (SCT) a été mesurée à l’aide d’un esthésiomètre Cochet-Bonnet. Les ânes (n = 12) et chevaux (n = 5) du groupe A ont reçu une solution ophtalmique stérile d’oxybuprocaïne 0,4 % avec de la fluorescéine (également appelée benoxinate avec fluorescéine, abrévié ben + flu) dans un oeil et une solution stérile de chlorure de sodium 0,9 % (NaCl) avec de la fluorescéine (Na + flu) dans l’oeil contra-latéral. Les ânes (n = 12) et chevaux (n = 5) du groupe B ont reçu les solutions ophtalmiques stériles de (ben + flu) dans un oeil et de la propacaïne 0,5 % avec de la fluorescéine (prop + flu) dans l’oeil contra-latéral. Le SCT a été mesuré à 1 et 5 min post-application et à des intervalles de 5 min jusqu’à 75 min après le traitement. Les changements dans le temps du SCT différaient de manière significative entre les yeux traités à l’oxybuprocaïne et les témoins (P < 0,001). Le SCT continua de diminuer tout au long de la durée de l’étude lorsque comparé aux valeurs de base. Aucune différence significative dans le début, la profondeur, ou la durée de l’anesthésie cornéenne ne fut trouvée entre les yeux traités à l’oxybuprocaïne et la proparacaïne durant la durée de l’étude. De manière intéressante, les chevaux avaient une cornée significativement plus sensible que les ânes (P = 0,002). L’oxybuprocaïne et la proparacaïne ont réduit la sensibilité cornéenne chez les ânes et les chevaux. Aucune irritation locale ne fut observée avec l’oxybuprocaïne 0,4 %.

(Traduit par Docteur Serge Messier)

Introduction

Horses and donkeys commonly suffer from ocular injuries, a consequence of the anatomical size and positioning of the eyes accompanied by their inborn “fight or flight” response, which creates a need for further investigation of topical ophthalmic anesthetics in order to select the most effective medication for each case. The cornea is the most sensitive tissue in the body (1). Its sensitivity has been documented by use of an esthesiometer (2,3). The Cochet-Bonnet corneal esthesiometer (CBA) has become the most commonly used esthesiometer in clinical and research settings (1,4). The filament of the CBA becomes increasingly more rigid as it is shortened and exerts an increased pressure on the corneal surface. The corneal touch threshold (CTT) is the minimal amount of corneal stimulation resulting in a consistent blink reflex and is measured by the use of a corneal esthesiometer (5,6). Corneal sensitivity has been measured using an esthesiometer in horses (79) as well as many other species (9); however, no studies measuring corneal sensitivity were found for donkeys. In previous studies mean corneal sensitivity measured by CTT using a CBA in healthy adult horses has been reported to range from 2.12 + 0.62 cm to 5.01 + 0.61 cm (68,1012). The central portion of the equine cornea is the most sensitive as determined by previous studies (7,8).

Commonly used topical ophthalmic anesthetics include: 0.5% proparacaine, 0.5% tetracaine, and 0.4% oxybuprocaine (benoxinate); proparacaine and tetracaine solutions being the most widely used in veterinary ophthalmology (4). Ocular sensitivity manifested by symptoms of acute conjunctival hyperemia, chemosis, and nictitans protrusion has been reported in dogs following topical application of tetracaine (4,13). In dogs, 0.4% oxybuprocaine hydrochloride (benoxinate) is appropriately suited as a topical ocular anesthetic agent that can be used in many clinical settings with few risks of conjunctival changes compared to tetracaine solution (14,15). Its anesthetic effects are comparable in depth and duration to that of proparacaine solution (16,17). In the United States, proparacaine solution is a common topical anesthetic solution used in veterinary practice (4,18). In the dog, studies have shown that significant anesthetic effects on the cornea can be measured for 45 min after the instillation of proparacaine (15) as well as oxybuprocaine solutions (14). The use of proracaine has been evaluated in many veterinary species and has also been proven to be a safe and effective corneal anesthetic agent in cats (16), dogs (9), and horses (6,11).

The rationale for this study was to determine the degree of corneal anesthesia provided by 0.4% oxybuprocaine with fluorescein (ben + flu) ophthalmic solution compared to 0.5% proparacaine with fluorescein (prop + flu) solution in donkeys and horses in addition to evaluating ben + flu for adverse effects in horses and donkeys. This study also allowed for comparison of response of ben + flu solution between donkeys and horses. The hypothesis was that a single topical application of ben + flu or prop + flu solution would provide similar anesthetic depth, but ben + flu would elicit longer anesthetic duration compared to 0.5% proparacaine solution in clinically normal donkeys and horses. It was anticipated that both donkeys and horses would respond similarly to the topical application of ben + flu solution.

Materials and methods

Animals

Twenty-four healthy adult castrated male Abyssinian donkeys ranging from 4 to 12 y of age (mean: 6.8 y, SD +\− 2.47 y) and 5 healthy adult horses (3 geldings and 2 mares) ranging from 10 to 32 y in age (mean:L 21.6, SD +\− 8.1 y) were used in the study. Horses were mixed breed Quarter Horse and Thoroughbred. Donkeys were randomly divided into 2 groups of 12 for the study. Each horse was used in both group A and group B with a 2-week washout period between treatments. All animals were university-owned and were housed on-pasture throughout the duration of the study. Animals were brought into the same treatment room for the study days to ensure constant ambient temperature, relative humidity, and elimination of environmental artifacts. All animals were considered healthy based on physical examination findings. Only ophthalmologically normal donkeys and horses, demonstrating a Schirmer Tear Test (Schirmer tear test strips; Schering-Plough Animal Health, Union, New Jersey, USA) value > 10 mm/min, absence of corneal fluorescein stain (Bio-Glo fluorescein sodium ophthalmic strips; HUB Pharmaceuticals, Rancho Cucamonga, California, USA) retention, and absence of active corneal or adnexal disease were included in the study. Animals were monitored for adverse effects including blepharospasm, epiphora, and chemosis throughout the study and the investigators, upon completion of the procedure, performed fluorescein staining of the cornea. Blepharospasm, epiphora, and conjunctival hyperemia were graded on a scale from 0 to 3 for each of the criteria evaluated. Grade 0 was considered normal, grade 1 mildly affected, grade 2 moderately affected, and grade 3 severely affected.

Procedures

All animals in the study were randomly allocated to treatment order. For each animal the baseline CTT was determined in both eyes by use of a Cochet-Bonnet esthesiometer (Cochet-Bonnet esthesiometer; Luneau Ophtalmologie, Chartres, France) immediately prior to topical administration of solutions. All measurements were obtained with minimal restraint and without the use of sedation.

One investigator applied CBA to the cornea while another investigator visualized the application of the esthesiometer while wearing head loupes with a 4× magnification.

All solutions were drawn up in a sterile fashion using a new sterile 25-gauge needle and 1-mL syringe. A new sterile fluorescein strip was used to create the saline with fluorescein and the proparacaine with fluorescein. In order to administer the treatments, 0.2 mL of the selected drug solution was drawn into a 1-mL syringe via a 25-gauge needle. The needle was broken off at the hub and the drug solution was gently sprayed onto the corneal surface (6,11). The same bottle of each drug solution was used throughout the study period and stored according to the manufacturer’s guidelines. In group A, CTT results were compared after topical administration of 0.9% sterile sodium chloride solution (NaCl) with fluorescein (Na + flu) solution in 1 eye and topical administration of ben + flu (Fluorescein sodium and benoxinate hydrochloride ophthalmic solution, USP 0.25%/0.4%; Bausch & Lomb, Tampa, Florida, USA) in the contralateral eye. Corneal touch threshold measurements were obtained immediately after baseline for Group A, 0.2 mL of ben + flu was administered in 1 eye and 0.2 mL of Na + flu was administered in the contralateral eye. Investigators were masked as to which solution was being placed in which eye. In group B, CTT results were compared after a topical administration of ben + flu was instilled in one eye and a topical administration of prop + flu solution was instilled in the contralateral eye. For all animals, CTT was measured at time 0 (baseline), at 1 min, and 5 min, continuing every 5 min until 75 min after medications had been administered. Following data collection, the cornea was re-stained with fluorescein dye to ensure no corneal damage was sustained. The protocol used for this study was approved by the Institutional Animal Care and Use Committee of the Ross University School of Veterinary Medicine.

Measurement of corneal sensitivity

The CBA was used to measure the sensitivity of the central portion of the cornea. It has a 0.12 mm diameter flexible monofilament nylon thread which has a range of 5 to 60 mm in length to control the intensity of corneal pressure applied by the monofilament. The length of the nylon filament corresponds to pressures of 11 to 200 mg/0.0113 mm2, respectively. The CBA is calibrated by the manufacturer and the length of exposed filament determines the pressure exerted on the cornea. The CTT is the mean filament length in mm at which a consistent blink response (3 out of 5 stimulations) is elicited. The CTT readings in mm can be converted to applied force measurements using mg per S where S is 0.0113 mm2 of sectional area of the filament. The conversion of measurements made with the esthesiometer (Luneau SAS, Chartres Cedex, France) is shown in Table I. To determine corneal sensitivity, the filament of the esthesiometer was advanced toward the globe and applied perpendicular to the central portion of the cornea. The filament was pressed to the cornea until slight deflection of the filament was evident. The filament was initially applied at 60 mm (maximum length) and decreased in 5 mm increments until a consistent blink was elicited. Investigators wore 4× binocular loupes to improve accuracy of detection of filament bending. The CTT, or corneal sensitivity, was recorded as the length of the esthesiometer filament that induced a blink reflex for at least 3 of 5 stimulations for a specified filament length (14). Two investigators observed each treatment and concurred with all results.

Table I
Esthesiometer conversion

Treatments

Using sterile technique, fluorescein stain was added to the 0.5% proparacaine solution (Proparacaine hydrochloride ophthalmic solution; Akorn, Lake Forest, Illinois, USA) and to the 0.9% NaCl by a licensed veterinary technician to create a color consistent with that of the ben + flu solution. This was done to mask investigators as to which solution was placed in the eye. After obtaining the baseline CTT for each eye, ben + flu solution, prop + flu solution, or Na + flu was applied to the eye. For Group A, ben + flu was instilled in 1 eye and Na + flu was instilled in the contralateral eye. For Group B, ben + flu was instilled in one eye and prop + flu solution was instilled in the contralateral eye. For each eye in all animals, the CTT value was determined at 1 min, at 5 min, and at 5-minute intervals until 75 min following administration of solution (Figures 1 to to44).

Figure 1
Donkeys: 0.4% oxybuprocaine versus 0.5% proparacaine.
Figure 4
Horses: 0.9% sodium chloride (NaCl) versus 0.4% oxybuprocaine.

Evaluation of potential adverse effects

Throughout the duration of the study, animals were monitored for evidence of blepharospasm, conjunctival hyperemia, and epiphora. Immediately after CTT measurements were obtained, all corneas were stained with fluorescein stain and evaluated with an ophthalmoscope (Ophthalmoscope; Welch Allyn, Skaneateles Falls, New York, USA) to identify any injury to the cornea during the procedure.

Statistical analysis

Repeated measures analysis of variance (ANOVA) models were run to account for the clustering of CTT measurements of one eye over time for each of the treatments. Each ANOVA model (for each treatment comparison) had CTT as the outcome and time, treatment, and animal identification as factors. For each treatment in a treatment comparison group (in both horses and donkeys) a univariate ANOVA model was run to compare mean CTT measured at time 0 (baseline) and time 1 (1 min after treatment). A P < 0.05 was considered significant.

Statistical software (Stata statistical software, Version 13; Stata Corp, College Station, Texas, USA) was used to perform the ANOVA analysis.

Results

There was no evidence of conjunctival hyperemia, blepharospasm, or epiphora noted at any time throughout the study (Grade 0 out of 3). All eyes were evaluated for corneal injury immediately after data collection using fluorescein staining and ophthalmoscope evaluation.

Minimal adverse ocular effects in this experimental protocol were noted in a single donkey. This animal developed a mild superficial corneal abrasion associated with fractious behavior resulting in the handle of the esthesiometer instrument abrading the cornea. The corneal abrasion was treated topically with an ophthalmic triple antibiotic ointment (Neomycin and Polymyxin B sulfates and bacitracin zinc ophthalmic ointment; Akorn, Lake Forest, Illinois, USA) every 6 h after the procedure. No stain uptake was noted by 36 h post-abrasion, at which time treatment was discontinued. The CTT data from this animal were eliminated from statistical analysis due to the development of the corneal abnormality.

Oxybuprocaine (benoxinate) compared to saline

Oxybuprocaine (benoxinate) solution significantly decreased CTT in both donkeys (P < 0.001) and horses (P < 0.001) versus the controls (Na + flu). Means +/− SD, with associated P-values, are included in Table I. Significant differences (P < 0.01) were determined by ANOVA for mean CTT values between time 0 and 1 min, as well as all other time points post-administration of ben + flu, while no significant differences were noted between any of the post-administration times. The ANOVA models in both species showed no significant difference in mean CTT between baseline and any time points, nor within post-administration times for the control groups (P = 0.184 donkeys and P = 0.854 horses). In this study, several donkeys and horses were noted to reach an individual CTT of 0 mm (no blink response at a filament length of 5.5 mm) for several min with ben + flu.

Oxybuprocaine (benoxinate) compared to Proparacaine

Compared with baseline values, CTT was significantly decreased (P < 0.01) by 1 min after treatment in the ben + flu and prop + flu treated eyes for both donkeys and horses. Donkeys showed no significant difference (P = 0.60) between ben + flu treated eyes and prop + flu treated eyes at any time point. There was a statistically significant difference (P = 0.001) between the 2 anesthetic solutions in horses. There were no significant differences in the CTT between donkeys and horses relating to ben + flu (P = 0.0983) or prop + flu (P = 0.2435). This study was unable to assess duration of medications as no animal had returned to baseline values by the end of the evaluation period.

Donkeys compared to horses

Horses were shown to have a significantly more sensitive cornea (P = 0.002) than donkeys at baseline (Mean CTT = 5.4 +/− 0.68 cm and 4.22 +/− 1.09 cm respectively). The CTT was not significantly different between donkeys and horses receiving ben + flu solution (P = 0.0983) or prop + flu solution (P = 0.2435).

Figures plotting the means over time for each group are included, and were generated using a commercial spreadsheet program (Excel; Microsoft Office, https://products.office.com/en-us/excel).

Discussion

This study assessed and compared efficacy of corneal anesthesia after application of ben + flu and prop + flu ophthalmic solutions to normal eyes of horses and donkeys. To the author’s knowledge, this is the first study to evaluate the efficacy of topical ophthalmic administration of 0.4% oxybuprocaine (benoxinate) hydrochloride and compare efficacy between ben + flu and prop + flu solutions in the eyes of healthy donkeys and horses. In this study ben + flu solution significantly decreased CTT in donkeys (P < 0.001) and in horses (P = 0.0001) versus the controls. In donkeys, no significant difference was noted between ben + flu treated eyes and prop + flu treated eyes (P = 0.60); however in horses, a statistically significant difference was noted (P = 0.001). In a previous study, the duration of corneal anesthesia in horses using proparacaine solution was 25 min, with maximal corneal desensitization achieved by 5 min (11). In the current study, several donkeys and horses were noted to reach an individual CTT of 0 mm by 1 min post-administration, remaining for several min with both ben + flu and prop + flu solutions. The reason for this finding is unknown but it is possible that behavioral attributes might partially account for the difference. Although unknown, it is possible that the horses and donkeys in this study might be considered more tolerant as they are handled and examined daily by faculty and students. The reason for the apparent difference in corneal sensitivity between the donkey and horse is unknown. It is possible that a species difference in sensitivity exists. The authors were unable to identify a study examining the differences in sensitivity between these 2 species; therefore, this presents an opportunity for further investigation.

Additional considerations include repeated administration of topical anesthetic as well as volume instilled in each treatment. In a previous study in horses, tetracaine hydrochloride was found to significantly increase the depth and length of corneal anesthesia when administered twice (1 min between treatments) (12). A similar experiment in dogs proved that 1 drop of 0.5% proparacaine hydrochloride ophthalmic solution applied twice with 1 min between doses resulted in significantly increased depth and duration of corneal anesthesia compared with a single drop dosage (15). Although there was a significant reduction in corneal sensation with a single application of ben + flu solution, further studies to evaluate whether or not more complete corneal anesthesia can be achieved with additional applications of the solution (i.e., a second topical administration 1 min after initial treatment) are warranted.

The impact of volume instilled within the eye was not evaluated in this study. The volume instilled, which is approximately equal to 4 drops of solution, is greater than the volume used in studies evaluating the onset and duration of proparacaine solution in cats and dogs (11,15). However, this volume represents the amount typically instilled in clinical application in horses (11).

The Cochet-Bonnet esthesiometer is reliable for measuring corneal sensitivity in normal eyes of horses (1,4), and with exception of one donkey whose behavior created mild injury, resulted in no significant ophthalmic trauma in this study. The application of the Cochet-Bonnet esthesiometer is relatively simple and safe to perform. Limitations of the esthesiometer include subjective interpretation of the response, variations in technique as well as a lack of standardization of deflection pressure (1,6). Variations in humidity and ambient temperature can also alter results by affecting the rigidity of the esthesiometer’s filament (1). Corneal touch threshold measurement was agreed upon by 2 investigators as they concurrently observed the animals’ responses, requiring the animal to blink 3 times out of 5 consecutive applications of the Cochet-Bonnet esthesiometer. This procedure was used in attempt to limit the subjective evaluation of the positive response in the CTT measurements. Standardization of environmental factors was achieved by having all study subjects assessed in the same air-conditioned treatment room. One limitation of this study is that CTT measurements ended prior to all animals returning to baseline values. This precluded comparison of duration of effect between proparacaine and oxybuprocaine. Further studies conducted until all animals return to baseline CTT values are necessary for comparison of these medications as well as to definitively determine the duration of topical anesthesia provided by the ben + flu solution.

Though the use of the Cochet-Bonnet esthesiometer is a reliable technique for measuring corneal sensitivity in veterinary studies, alternatively, a non-contact corneal esthesiometer (NCCE) such as the modified Belmonte esthesiometer, could have been used to evaluate corneal sensitivity. These devices use controlled pulses of air of varying pressures to stimulate the cornea. The advantage of a NCCE over the CBA is that a large, continuous range of stimulus intensities can be produced (19,20). While air-puff esthesiometers are commonly used in research and clinical evaluations in human medicine, the filament-based esthesiometer is routinely used in veterinary research. As few to no studies could be found for such evaluation in equids, for comparison basis, the authors of this study chose to use the filament-based esthesiometer.

This study assessed animals with a normal, healthy cornea and did not evaluate animals with corneal disease such as ulcerations. It has been shown that repeated applications of topical anesthetic agents might cause injury, local irritation, and corneal epithelial toxicity (4,17). These were not observed at any point during this study. Corneal ulceration and perforation may occur with repetitive or prolonged use and is therefore discouraged (4). Future studies need to be conducted to determine the efficacy and duration of effect of oxybuprocaine hydrochloride solution in donkeys and horses with corneal disease. Use of the medications and the application of the Cochet-Bonnet esthesiometer resulted in no long-term damage in healthy horse and donkey eyes for the length of this study. The medications used are combination products normally used in ophthalmologic examinations. The techniques and medications described are relatively simple and safe to perform and administer.

In conclusion, the use of ben + flu solution is effective and would pose minimal risk to donkeys and horses with normal corneas.

Figure 2
Donkeys: 0.9% sodium chloride (NaCl) versus 0.4% oxybuprocaine.
Figure 3
Horses: 0.4% oxybuprocaine versus 0.5% proparacaine.

Acknowledgment

Funded with an intramural grant from Ross University School of Veterinary Medicine.

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