Abstract
Background
Rocuronium bromide has been evaluated as a mydriatic agent in birds, but the species applied were limited and the dose and effect were variable.
Objective
This study aims to evaluate the efficacy of topical rocuronium bromide as mydriatics in 4 species according to horizontal palpebral fissure length: Feral pigeon (Columba livia), Common kestrel (Falco tinnunculus), Northern boobook (Ninox japonica), and Eurasian eagle owl (Bubo bubo).
Methods
A total of 32 birds (8 for each species) were included as pre-releasing examination. Rocuronium bromide was instilled in one randomly selected eye of each bird based on palpebral fissure length criteria (0.5 mg/50 µL for pigeons, 1 mg/100 µL for kestrels and boobook owls, and 2 mg/200 µL for eagle owls). The contralateral eye was used as control and treated with normal saline. After instillation of the drug, pupil diameter, pupillary light reflex, intraocular pressure, heart rate, and respiratory rate were evaluated at 10 min intervals up to 180 min and at 30 min intervals up to 360 min.
Results
Statistically significant mydriasis was obtained in all birds (p < 0.001). However, in boobook and eagle owls, marked mydriasis persisted until 360 min. Side effects including corneal erosion and lower eyelid paralysis were common, which was observed in 26/32 birds. Blepharospasm was also noted during this study. No systemic adverse signs were observed.
Conclusions
Rocuronium bromide could be a good mydriatics option for 4 species of birds, however, further studies are needed to find lowest effective dose to reduce drug-related side effects.
Keywords: Birds, mydriasis, mydriatics, neuromuscular nondepolarizing agents, adverse effects
INTRODUCTION
Mydriasis is essential to make an examination of the posterior segment of the eye. Since striated fibers are main components of avian iris muscles, parasympatholytic agents used as mydriatics in mammals do not effectively induce mydriasis in birds [1]. So far, efficacies of several agents including d-tubocurarine [2], phenylephrine and atropine combination [3], and non-depolarizing neuromuscular blocking agents (NMBAs) (e.g., vecuronium [3], alcuronium [4], pancuronium [5]) have been evaluated as mydriatics in various avian species with varied results.
There are a few reports on the efficacy of rocuronium bromide, one of NMBAs, as a mydriatic agent in parrots, pigeon, and raptors [6,7,8,9,10,11,12,13]. In most studies, rocuronium bromide showed sufficient mydriatic effects. However, each study was conducted with different doses of drugs in different species of birds. Duration of mydriasis and side effects were inconsistent.
The purpose of this study was to evaluate the efficacy of topical rocuronium bromide as a mydriatics in 4 species of birds with different sizes of eyes as standards for determining the dose of this drug for each instillation.
MATERIALS AND METHODS
Animals
The present study was approved by the Institutional Animal Care and Use Committees (IACUC) of the Jeonbuk National University, Republic of Korea (JBNU 2020-0138). All birds in this study were rescued to the wildlife center and they were examined as a pre-release evaluation. Eight juvenile and adult Feral pigeons (Columbia livia) weighing 215 g to 280 g, 8 adult Common kestrels (Falco tinnunculus) weighing 189 g to 210 g, 8 adult Northern boobooks (Ninox japonica) weighing 148 g to 190 g, and 8 juvenile and adult Eurasian eagle owls (Bubo bubo) weighing 1,200 g to 1,950 g were included. Sex information was not considered.
Study design
Gross examination for external abnormalities of eyes and ophthalmic examinations including intraocular pressure (IOP) measurement with a rebound tonometer (Tonovet®, iCare USA, USA), neurologic evaluation, slit lamp biomicroscopy (DC3, Topcon, Japan), and fluorescein staining (FS) were performed. Only birds with normal results on each examination were included in this study.
To determine the drug instillation dose for each eye by species, the dose was adjusted for eye size rather than body weight. The length of the horizontal palpebral fissure was measured with a caliper. Average lengths of horizontal palpebral fissures of Feral pigeons, Common kestrels, Northern boobooks, and Eurasian eagle owls were 8.08 mm, 11.1 mm, 18.2 mm and 26.7 mm, respectively. According to the average length, four species were divided into three groups of less than 10 mm (Feral pigeons), 10 to 20 mm (Common kestrels and Northern boobooks), and over 20 mm (Eurasian eagle owls). Although no statistics were performed to determine if there were significant differences in horizontal palpebral fissure length between individuals within the same species, no notable differences were identified at the time of measurement. Therefore, the lengths were measured for each individual and then statistically averaged by species to determine the drug dosage. In order of decreasing length, each group received 25 µL (0.25 mg), 50 µL (0.5 mg), and 100 µL (1 mg) of the drug in one single eye of each bird. In total, each species included eight treated eyes and eight contralateral eyes.
Birds were manually restrained in dorsal recumbency. Topical proparacaine (Alcaine®, Alcon, Belgium) was administered to both eyes one time. After 3 minutes in the same position, rocuronium bromide (Esmeron® 10 mg/mL, MSD, USA) was instilled into one randomly selected eye. The contralateral eye was regarded as control and the same dose of normal saline was instilled. Birds were restrained in the same position for 3 more minutes. Afterwards, pupil diameter (PD), direct pupillary light reflex (PLR), IOP, heart rate, and respiratory rate were measured in a standing position prior to drug administration (T0) and every 10 min post administration up to 150 min. Thereafter, measurements were performed every 30 min up to 360 min. A slit lamp-mounted camera was used for taking images of the eye on each measurement. The intensity of the slit lamp light was set constant throughout the study. PD measurement was made horizontally across the center of the pupil using an electronic caliper embedded in the slit lamp capture program by one observer. PLR was scored according to the self-established scoring system (0: absent 0.5: markedly decreased 1: decreased 1.5: slightly decreased 2: normal) using a Finnoff transilluminator. FS was performed for both eyes on the last measurement of the study for evaluating corneal epithelial defect. Evaluations of PD, PLR and FS were conducted in a dark room throughout the study. During the period of experiment, birds were monitored for signs of adverse effects including blepharospasm, chemosis, conjunctival hyperemia, paralysis of the eyelid, neck, wing, or limb, and respiratory distress.
Statistical analysis
PD results are described as mean ± standard deviation. PD of each species were analyzed by comparing values of the treated eye over time as well as values between the control eye and the treated eye at each time points. Dunn’s multiple comparison after Kruskal-Wallis test was performed to compare the initial PD of the treated eye and PD values over time. Same method was applied to PLR results. Wilcoxon matched-pairs signed rank test was applied to assess PD results of the control eye and the treated eye at each time point. Multiple comparisons among all time points were run with Holm-Sidak’s correction method. IOP values were compared between every time point using multiple Mann-Whitney tests with Holm-Sidak’s correction. All p-values less than 0.05 were considered significant. SPSS V27 (IBM SPSS, USA) and GraphPad Prism V9 (GraphPad Software, USA) were used for all statistical analyses.
RESULTS
All species showed significant differences in mean PD between rocuronium-treated eyes and control eyes (p < 0.001). PD values, IOP, and PLR by species are shown in Fig. 1 and Table 1, 2, 3, 4. Although there were some differences in the maintenance period and the degree of mydriasis among individuals, dilatation of the pupil required for the posterior ocular examination was obtained sufficiently in all 32 individuals (Fig. 2). The dilatated status which can achieve fundus examination even if it is not statistically significant described as effective or marked. After 24 h, all subjects showed no difference in the PD between both eyes.
Fig. 1. Pupil diameters of rocuronium-treated eyes and control eyes in 4 species (Feral pigeon, Common kestrel, Northern boobook, and Eurasian eagle owl). Statistical difference was marked (*) compared to pupil diameter of T0.
Table 1. Data statistics of Feral pigeon (n = 8).
| Time (min) | Pupil diameter (mm) | IOP (mmHg) | PLR (score) | |||
|---|---|---|---|---|---|---|
| Treated | Control | Treated | Control | Treated | Control | |
| 0 | 3.38 ± 0.56 | 3.29 ± 0.40 | 10.00 | 10.25 | 2.00 | 2.00 |
| 10 | 4.67 ± 0.76 | 2.93 ± 0.61 | 10.50 | 9.75 | 0.50 | 2.00 |
| 20 | 5.23 ± 0.76 | 2.87 ± 0.32 | 10.50 | 10.00 | 0.25 | 2.00 |
| 30 | 4.93 ± 0.61 | 2.91 ± 0.43 | 10.25 | 10.75 | 0.13 | 2.00 |
| 40 | 4.86 ± 0.55 | 2.9 ± 0.21 | 10.50 | 10.75 | 0.25 | 2.00 |
| 50 | 4.96 ± 0.80 | 2.83 ± 0.18 | 10.00 | 9.75 | 0.13 | 2.00 |
| 60 | 4.85 ± 0.64 | 2.84 ± 0.23 | 9.75 | 10.00 | 0.13 | 2.00 |
| 70 | 4.75 ± 0.60 | 2.96 ± 0.57 | 10.00 | 10.25 | 0.13 | 2.00 |
| 80 | 4.86 ± 0.64 | 2.97 ± 0.31 | 9.75 | 10.25 | 0.13 | 2.00 |
| 90 | 4.86 ± 0.52 | 2.99 ± 0.35 | 10.25 | 10.25 | 0.13 | 2.00 |
| 100 | 4.59 ± 0.45 | 3.00 ± 0.31 | 9.25 | 9.75 | 0.50 | 2.00 |
| 110 | 4.59 ± 0.51 | 2.95 ± 0.47 | 10.50 | 9.50 | 0.88 | 2.00 |
| 120 | 4.41 ± 0.50 | 3.04 ± 0.18 | 9.75 | 10.50 | 1.00 | 2.00 |
| 130 | 4.27 ± 0.49 | 2.97 ± 0.35 | 10.25 | 10.25 | 1.00 | 2.00 |
| 140 | 4.08 ± 0.52 | 2.95 ± 0.34 | 10.00 | 10.25 | 1.00 | 2.00 |
| 150 | 4.19 ± 0.41 | 3.29 ± 0.44 | 10.75 | 10.25 | 1.13 | 2.00 |
| 180 | 3.80 ± 0.39 | 3.06 ± 0.27 | 10.25 | 9.50 | 1.50 | 2.00 |
| 210 | 3.55 ± 0.24 | 2.92 ± 0.22 | 10.25 | 10.25 | 1.75 | 2.00 |
| 240 | 3.48 ± 0.04 | 3.08 ± 0.21 | 10.25 | 10.50 | 1.75 | 2.00 |
| 270 | 3.29 ± 0.18 | 3.09 ± 0.44 | 10.50 | 9.75 | 1.88 | 2.00 |
| 300 | 3.29 ± 0.28 | 3.12 ± 0.24 | 10.25 | 10.25 | 2.00 | 2.00 |
| 330 | 3.34 ± 0.35 | 3.20 ± 0.31 | 10.50 | 10.25 | 2.00 | 2.00 |
| 360 | 3.23 ± 0.27 | 3.03 ± 0.27 | 10.50 | 10.00 | 2.00 | 2.00 |
Mean pupil diameters ± standard deviation, mean intraocular pressures, and mean pupillary light response were described.
IOP, intraocular pressure; PLR, pupillary light reflex.
Table 2. Data statistics of Common kestrels (n = 8).
| Time (min) | Pupil diameter (mm) | IOP (mmHg) | PLR (score) | |||
|---|---|---|---|---|---|---|
| Treated | Control | Treated | Control | Treated | Control | |
| 0 | 3.94 ± 0.63 | 4.02 ± 0.77 | 14.75 | 15.25 | 2.00 | 2.00 |
| 10 | 5.74 ± 0.57 | 4.11 ± 0.72 | 15.50 | 15.25 | 0.44 | 2.00 |
| 20 | 5.61 ± 0.48 | 4.07 ± 0.85 | 15.00 | 15.00 | 0.44 | 2.00 |
| 30 | 5.84 ± 0.44 | 4.10 ± 0.83 | 14.25 | 15.00 | 0.44 | 2.00 |
| 40 | 5.80 ± 0.62 | 4.11 ± 0.69 | 14.75 | 14.50 | 0.31 | 2.00 |
| 50 | 5.95 ± 0.45 | 3.93 ± 0.71 | 15.00 | 15.00 | 0.00 | 2.00 |
| 60 | 5.84 ± 0.70 | 3.99 ± 0.61 | 14.50 | 15.75 | 0.00 | 2.00 |
| 70 | 6.04 ± 0.57 | 3.98 ± 0.65 | 14.75 | 15.00 | 0.00 | 2.00 |
| 80 | 5.82 ± 0.36 | 3.90 ± 0.63 | 15.00 | 15.50 | 0.00 | 2.00 |
| 90 | 6.04 ± 0.65 | 4.14 ± 0.65 | 13.75 | 15.50 | 0.00 | 2.00 |
| 100 | 6.08 ± 0.38 | 3.93 ± 0.66 | 15.00 | 14.50 | 0.19 | 2.00 |
| 110 | 6.12 ± 0.31 | 4.00 ± 0.42 | 13.50 | 15.25 | 0.19 | 2.00 |
| 120 | 5.96 ± 0.43 | 3.92 ± 0.56 | 15.00 | 15.25 | 0.19 | 2.00 |
| 130 | 6.02 ± 0.35 | 3.90 ± 0.55 | 14.00 | 15.00 | 0.19 | 2.00 |
| 140 | 6.15 ± 0.47 | 4.11 ± 0.74 | 14.25 | 14.75 | 0.19 | 2.00 |
| 150 | 6.06 ± 0.48 | 4.25 ± 0.59 | 14.25 | 14.75 | 0.31 | 2.00 |
| 180 | 6.17 ± 0.37 | 3.79 ± 0.64 | 14.50 | 15.00 | 0.31 | 2.00 |
| 210 | 5.87 ± 0.31 | 3.97 ± 0.58 | 15.00 | 15.25 | 0.56 | 2.00 |
| 240 | 5.73 ± 0.19 | 4.01 ± 0.54 | 13.75 | 15.25 | 0.63 | 2.00 |
| 270 | 5.16 ± 0.14 | 3.97 ± 0.63 | 15.50 | 15.25 | 0.81 | 2.00 |
| 300 | 4.89 ± 0.26 | 3.96 ± 0.63 | 15.00 | 15.25 | 1.31 | 2.00 |
| 330 | 4.73 ± 0.23 | 3.96 ± 0.60 | 14.75 | 14.75 | 1.63 | 2.00 |
| 360 | 4.58 ± 0.50 | 4.08 ± 0.77 | 15.00 | 14.25 | 2.00 | 2.00 |
Mean pupil diameters ± standard deviation, mean intraocular pressures, and mean pupillary light response were described.
IOP, intraocular pressure; PLR, pupillary light reflex.
Table 3. Data statistics of Northern boobook (n = 8).
| Time (min) | Pupil diameter (mm) | IOP (mmHg) | PLR (score) | |||
|---|---|---|---|---|---|---|
| Treated | Control | Treated | Control | Treated | Control | |
| 0 | 6.06 ± 0.90 | 6.07 ± 1.13 | 10.00 | 10.75 | 2.00 | 2.00 |
| 10 | 7.17 ± 0.54 | 6.28 ± 0.57 | 9.63 | 10.75 | 1.75 | 2.00 |
| 20 | 8.09 ± 0.94 | 6.27 ± 0.60 | 9.25 | 9.50 | 1.31 | 2.00 |
| 30 | 9.31 ± 1.12 | 6.31 ± 0.87 | 9.00 | 8.63 | 0.81 | 2.00 |
| 40 | 10.35 ± 0.77 | 5.94 ± 0.71 | 8.50 | 9.50 | 0.56 | 2.00 |
| 50 | 10.70 ± 0.48 | 5.99 ± 0.26 | 8.50 | 9.13 | 0.25 | 2.00 |
| 60 | 10.62 ± 0.56 | 5.46 ± 0.62 | 8.00 | 9.50 | 0.25 | 2.00 |
| 70 | 10.50 ± 0.63 | 5.62 ± 0.68 | 8.38 | 8.38 | 0.25 | 2.00 |
| 80 | 10.70 ± 0.51 | 6.06 ± 1.03 | 8.88 | 7.88 | 0.13 | 2.00 |
| 90 | 10.72 ± 0.61 | 5.62 ± 0.74 | 8.50 | 8.38 | 0.13 | 2.00 |
| 100 | 10.81 ± 0.85 | 6.14 ± 0.69 | 8.88 | 8.75 | 0.13 | 2.00 |
| 110 | 10.61 ± 0.81 | 5.62 ± 0.49 | 8.38 | 9.75 | 0.19 | 2.00 |
| 120 | 10.57 ± 0.78 | 5.73 ± 0.62 | 8.13 | 7.88 | 0.19 | 2.00 |
| 130 | 10.52 ± 0.82 | 5.84 ± 0.74 | 8.25 | 8.38 | 0.13 | 2.00 |
| 140 | 10.61 ± 1.00 | 5.81 ± 0.70 | 8.25 | 8.88 | 0.13 | 2.00 |
| 150 | 10.85 ± 0.85 | 5.54 ± 0.63 | 8.50 | 8.50 | 0.13 | 2.00 |
| 180 | 11.27 ± 0.55 | 5.60 ± 0.90 | 7.75 | 7.88 | 0.13 | 2.00 |
| 210 | 11.28 ± 0.52 | 5.68 ± 0.75 | 8.25 | 9.13 | 0.25 | 2.00 |
| 240 | 11.38 ± 0.44 | 5.80 ± 0.77 | 8.25 | 9.63 | 0.50 | 2.00 |
| 270 | 10.93 ± 0.86 | 5.78 ± 0.89 | 7.88 | 9.00 | 0.63 | 2.00 |
| 300 | 10.42 ± 1.15 | 5.49 ± 1.14 | 8.38 | 9.63 | 0.75 | 2.00 |
| 330 | 10.31 ± 1.11 | 5.36 ± 0.48 | 8.25 | 9.13 | 0.88 | 2.00 |
| 360 | 10.06 ± 1.30 | 5.62 ± 0.66 | 9.38 | 8.75 | 0.94 | 2.00 |
Mean pupil diameters ± standard deviation, mean intraocular pressures, and mean pupillary light response were described.
IOP, intraocular pressure; PLR, pupillary light reflex.
Table 4. Data statistics of Eurasian eagle owl (n = 8).
| Time (min) | Pupil diameter (mm) | IOP (mmHg) | PLR (score) | |||
|---|---|---|---|---|---|---|
| Treated | Control | Treated | Control | Treated | Control | |
| 0 | 11.55 ± 0.60 | 10.67 ± 0.60 | 14.00 | 13.00 | 2.00 | 2.00 |
| 10 | 10.87 ± 0.84 | 10.59 ± 0.94 | 13.50 | 13.75 | 2.00 | 2.00 |
| 20 | 11.40 ± 1.67 | 9.60 ± 0.67 | 14.00 | 13.00 | 2.00 | 2.00 |
| 30 | 12.72 ± 1.37 | 10.25 ± 0.36 | 13.25 | 12.50 | 2.00 | 2.00 |
| 40 | 13.43 ± 1.62 | 10.06 ± 1.23 | 12.75 | 13.25 | 1.81 | 2.00 |
| 50 | 14.91 ± 1.67 | 10.84 ± 0.96 | 13.50 | 13.75 | 1.81 | 2.00 |
| 60 | 15.56 ± 1.82 | 10.92 ± 1.31 | 12.75 | 14.50 | 1.69 | 2.00 |
| 70 | 15.45 ± 1.38 | 10.43 ± 1.77 | 12.00 | 13.25 | 1.44 | 2.00 |
| 80 | 15.68 ± 1.66 | 11.05 ± 0.69 | 12.25 | 13.75 | 1.19 | 2.00 |
| 90 | 16.18 ± 0.86 | 10.96 ± 0.50 | 11.75 | 13.50 | 1.19 | 2.00 |
| 100 | 16.38 ± 0.78 | 10.70 ± 0.58 | 12.00 | 12.75 | 1.00 | 2.00 |
| 110 | 16.70 ± 1.23 | 10.99 ± 0.59 | 12.25 | 13.00 | 1.00 | 2.00 |
| 120 | 16.07 ± 0.47 | 10.54 ± 0.93 | 12.25 | 12.25 | 1.00 | 2.00 |
| 130 | 16.61 ± 0.96 | 11.43 ± 0.73 | 12.25 | 13.25 | 0.81 | 2.00 |
| 140 | 16.74 ± 1.15 | 11.10 ± 0.74 | 12.75 | 13.00 | 0.69 | 2.00 |
| 150 | 16.67 ± 1.68 | 10.99 ± 0.57 | 12.75 | 13.00 | 0.69 | 2.00 |
| 180 | 16.45 ± 2.16 | 10.41 ± 0.34 | 12.50 | 12.50 | 0.69 | 2.00 |
| 210 | 16.59 ± 1.55 | 11.30 ± 0.82 | 12.50 | 13.50 | 0.69 | 2.00 |
| 240 | 16.41 ± 0.56 | 11.24 ± 1.13 | 12.25 | 13.75 | 0.81 | 2.00 |
| 270 | 15.40 ± 1.46 | 10.27 ± 1.45 | 12.50 | 12.75 | 1.00 | 2.00 |
| 300 | 14.63 ± 1.77 | 9.96 ± 1.15 | 12.25 | 13.00 | 1.56 | 2.00 |
| 330 | 15.32 ± 0.82 | 10.51 ± 1.13 | 12.00 | 12.50 | 1.56 | 2.00 |
| 360 | 15.07 ± 1.35 | 10.47 ± 0.72 | 11.75 | 12.50 | 1.56 | 2.00 |
Mean pupil diameters ± standard deviation, mean intraocular pressures, and mean pupillary light response were described.
IOP, intraocular pressure; PLR, pupillary light reflex.
Fig. 2. Maximal dilatated pupil in treated eyes of (A) Feral pigeon, (B) Common kestrel, (C) Northern boobook, and (D) Eurasian eagle owl.
In Feral pigeons, mean PD of control eye was 3.01 ± 0.13 mm. Mydriasis was observed from T10 and significant difference was shown from T20 to T90 (p < 0.05) compared with the T0 value. After T90, PD of the treated eye decreased. From T270 onward, no prominent difference between both eyes was observed. The largest mean PD was 5.23 mm at T20. PLR significantly decreased from T10 and started to increase from T100 (p < 0.05, Fig. 3). Mean IOP values of control eyes and treated eyes were 10.13 ± 0.63 mmHg and 10.20 ± 0.65 mmHg, respectively. There was no significant difference in IOP between both eyes or with time (Fig. 4).
Fig. 3. Pupillary light reflex results in 4 species (Feral pigeon, Common kestrel, Northern boobook, and Eurasian eagle owl). Time points with statistically significant differences confirmed are marked by (*).
Fig. 4. IOP among 4 species (Feral pigeon, Common kestrel, Northern boobook, and Eurasian eagle owl). It showed no significant difference between treated eyes and control eyes, as well as no significant increase or decrease when time elapsed.
IOP, intraocular pressure.
Mean PD of control eyes of Common kestrels was 4.01 ± 0.10 mm. Marked dilatation of pupil was observed from T10, and maximal mydriasis was maintained from T30 to T210. Maximal mean PD was 6.17 mm, which was obtained at T180. The PD of the treated eye was statistically different from the initial value from T10 to T240 except for T20 (p < 0.05). PLR markedly reduced from T10 to T180 (p < 0.05), showing a score of less than 0.5. Mean IOP values of control eyes and treated eyes were 15.05 ± 1.45 mmHg and 14.64 ± 1.38 mmHg, respectively. No significant difference was evidenced in IOP between both eyes and with time.
As for Northern boobooks, mean PD of the control eye was 5.81 ± 0.28 mm and maximal mean PD was 11.38 mm at T240. Dilatation of pupil was observed since T10 visually. Significant difference was detected from T40 to T330 (p < 0.05). Although the PD of the treated eye gradually reduced from T270, the marked dilatation continued until the end of the experiment in all boobook owls. Two individuals examined up to T450 showed noticeable mydriasis until that point. Significantly decreased PLR was maintained from T40 to T240 (p < 0.05). Four birds showed no PLR until T360. Initial IOP of the treated eye was 10.38 mmHg. Mean IOP values of control eyes and treated eyes were 9.03 ± 1.87 mmHg and 8.57 ± 2.14 mmHg, respectively. There was no statistical difference of mean IOP of both eyes over time. However, 3 birds showed noticeable decrease or fluctuation of IOPs in both eyes.
In Eurasian eagle owls, mean PD of control eye was 10.66 ± 0.46 mm. Dilatation of pupil was observed since T20 in all eagle owls. Statistical difference was evidenced from T60 to T240 (p < 0.05) except for T70. Maximal mydriasis was observed from T110 to T240, which was 16.53 mm in average. Even without statistical significance, sufficient mydriasis for posterior ocular examination was maintained until T360 in 7 birds. Mean PLR score was the lowest from T140 to T210, and only one individual showed fully absent PLR from T100 to T270. Significant differences were detected from T100 to T270 (p < 0.05). Mean IOP values of control eyes and treated eyes were 13.13 ± 2.33 mmHg and 12.60 ± 2.02 mmHg, respectively.
No systemic adverse signs were identified during this study, although various ocular side effects were observed (Fig. 5). The most common finding was corneal erosion in the treated eye which was found in 3/8 of Feral pigeons, 8/8 of Common kestrels, 8/8 of Northern boobooks, and 7/8 of Eurasian eagle owls. In addition, 2/8 of Feral pigeons, 4/8 of Common kestrels, 4/8 of Northern boobooks, and 4/8 of Eurasian eagle owls showed multifocal punctate staining (MFPS) in the control eye. Paralysis of the eyelid in the treated eye which was presented as elevation of the lower eyelid was observed in 6/8 of Feral pigeons, 5/8 of Common kestrels, 6/8 of Northern boobooks, and 4/8 of Eurasian eagle owls. Eyelid paralysis persisted for 120 min on average. One Common kestrel showed blepharospasm in the treated eye after administration of rocuronium bromide and lasted until the end of the experiment.
Fig. 5. Various side effects were identified. Paralysis of the lower eyelid was observed in (A) Feral pigeon, (B) Common kestrel, (D) Northern boobook, and (E) Eurasian eagle owl. (C) Common kestrel having corneal erosion on the lateral cornea at T360. Corneal erosions were identified in all 4 species (Feral pigeon, Common kestrel, Northern boobook, and Eurasian eagle owl). (F) One Northern boobook showing persisted mydriasis even at T360.
DISCUSSION
Parasympatholytic agents commonly used as mydriatics in dogs and cats are ineffective in birds due to characteristics of the avian iris muscle, which is composed mostly of striated muscles [1]. Therefore, a neuromuscular paralyzing agent can be used to induce mydriasis through iris muscle paralysis [14]. The efficacy of a single instillation of rocuronium bromide was demonstrated in 4 avian species as a mydriatics in this study. We designed different doses of drugs for each species considering difference in amount of iris muscle according to eye size. We hypothesized that eye size would be proportional to horizontal palpebral fissure length, so we determined the drug instillation dose based on horizontal palpebral fissure length. However, the time to reach mydriasis and duration varied among species even when the drug was instilled based on eye size criteria. Mydriasis was induced rapidly in pigeons and kestrels from T10. Maximum mydriasis was maintained for less than 2 h in pigeons and 4 h in kestrels. The onset of mydriasis was observed between T10 and T20 in the 2 species of owls as well. However, the PD gradually increased, and the maximum mydriasis took about an hour. Marked dilatation was maintained for more than 360 min in 2 owl species.
The eagle owl has a much larger eyelid fissure. Thus, 2 drops were instilled accordingly. However, it seemed that 2 drops did not appear to be doubling the effect. In the other 3 species, mydriasis occurred to the extent that the iris was almost invisible, whereas the eagle owl was the only one to have about 70% mydriasis, which was slightly more limited than that induced by general anesthesia. There is a report that single-drop and double-drop instillations do not cause a significant difference in mydriasis of Tawny owl [11]. In addition, the actual amount of 0.1 mL was a little too much to stay on the ocular surface making some of the drug lost.
Since 7 boobook owls maintained PD nearly twice that of the control group even at T360, it was considered necessary to reduce the dose for this species. In Little owl with body weight similar to nocturnal raptors, mydriasis was maintained above that required for posterior ocular examination even at half the concentration used in this study [10]. Although rarely reported in birds, it has been demonstrated that excessive light exposure can cause retinal damage in rats and monkeys [15,16]. Therefore, for the purpose of simply examining the posterior portion, the PD graph of pigeons is considered the most ideal. Another study using kestrels found that when instillation was applied to both eyes, the maximum mydriasis lasted about 2 h, although a dose equivalent to 25% of that used in the present study was administered to one eye [9]. Even though higher dose was used to assess the efficacy by species with similar sized eyes, it is estimated that lower dose than our study might be sufficient for simple ophthalmic examination in kestrels. Establishing the minimum dose is required for ocular examination for each species. In addition, to prevent excessive light exposure, it is recommended to place the bird in a dark environment for at least 24 h after inducing mydriasis.
Iris pigmentation, difference of metabolism, and characteristics of species have been suggested as reasons for different onsets or durations between species [3,10,12]. This was because birds with dark iris had a delayed time to reach peak PD or had mydriasis persisting longer [3,10]. However, the Common kestrel's eyeballs with darker iris had shorter effective duration in our study, whereas Northern boobooks having bright iris with the same dose took a longer time to achieve maximal pupil with mydriasis lasting much longer. Additionally, there were no differences of mydriatic effects between pigeons with different iris colors (3 types) in this study. In the author’s opinion, physiological differences by species might have influenced absorption of the drug. It might not be an accurate comparison because it does not accurately compare the exposed surface area of the eye or the actual size of eyeball. Nevertheless, this suggests that there are differences in sensitivity between species. Thus, individual criteria are needed for each species. Furthermore, examination of other species with similar size eyeballs or surface area is necessary to investigate the evidence.
One of the most common side effects observed in this study was corneal erosion in treated eyes. It might be caused by a low pH of the rocuronium [12]. It may be preferable to apply other methods of mydriasis, such as general anesthesia, for individuals with corneal abnormalities. To minimize the corneal toxicity, further studies with more diluted concentrations of the drug are needed to lower damage by the drug. Flushing out eye after absorption of the drug or applying lubricants could be other options. Interestingly, a few control eyes also showed positive results in FS test with MFPS pattern on the last measurement of the study. This could be induced by the topical anesthetic which could be toxic to the cornea and possibly lead to superficial punctate keratitis [17]. Nevertheless, without corneal anesthesia, the birds showed marked pain reactions and induced most of the drug loss from the ocular surface which lead to no efficacy of rocuronium in our pilot study, it was considered to be necessary. Corneal erosions of treated eyes and MFPSs of control eyes were both observed at the temporal part of the cornea. This might be because the avian nictitating membrane can move from dorsonasal to ventrotemporal part of the eye. Thus, the drugs might be concentrated in the temporal cornea [18]. After one to two weeks, all birds recovered from corneal erosions.
Blepharospasm was observed only in one Common kestrel, although it was predominantly observed in the pilot study. Since blepharospasm, head shaking, and rapid blinking from discomfort caused drug loss and insufficient mydriasis were observed during the study period in all 4 species, we decided to instill topical anesthetic prior to administration of rocuronium, and the blepharospasm was reduced. Since the rocuronium solution has a pH of 3.2–4 [7,12], it would cause ocular irritation and pain. In other studies of rocuronium in birds, however, the blepharospasm was rarely reported [6,7,8,9,10]. This might be because on pre-ophthalmic examination, those studies measured IOP using applanation tonometry, which required instillation of a local anesthetic before the measurement. Since blepharospasm was not mentioned in the report using a pH-adjusted dilution solution, dilution might be considered as a method [12]. One report described that mydriasis was obtained more effectively with proparacaine than the experiment in which rocuronium was applied without corneal anesthesia since proparacaine could enhance drug absorption [19]. This could be because rocuronium was quickly removed from the ocular surface due to pain.
Three Northern boobooks showed IOP decrease or fluctuation in both treated and control eyes during this study. Based on human reports of IOP lowering after systemic administration of rocuronium [20], IOP decrease in Northern boobooks could have been induced by systemic absorption of rocuronium. Another possible factor was, since birds were manually restrained, the stress might have affected the IOP. However, to the author’s knowledge, there is no report of normal IOP in this species. Considering that the other 3 species have variable ranges of normal IOP (Domestic pigeon reference range: 7.0–13.0 mmHg [18], Eurasian kestrel reference range: 4.0–15.0 mmHg [21], Eurasian eagle owl reference range: 7.0–14.0 mmHg [22]), changes of IOP in Northern boobooks in this study might be normal variations.
In conclusion, rocuronium bromide could be a good option of mydriatic for 4 different species of birds. Results of this study might have suggested appropriate effective doses of topical rocuronium for ocular examination in pigeons and kestrels. However, given the corneal damage, clinician should be careful to use it selectively depending on the corneal condition. In addition, further studies are warranted to minimize these side effects and determine effective minimum doses for boobook and eagle owls.
Footnotes
Funding: This research was supported by the National Institute of Wildlife Disease Control and Prevention as “Specialized Graduate School Support Project for Wildlife Disease Specialist”.
Conflict of Interest: The authors declare no conflicts of interest.
- Conceptualization: Rhim H, Jung S, Han JI.
- Data curation: Rhim H, Jung S.
- Formal analysis: Rhim H, Han JI.
- Funding acquisition: Han JI.
- Investigation: Rhim H, Jung S, Han JI.
- Methodology: Jung S.
- Project administration: Rhim H, Han JI.
- Resources: Kim N, Han JI.
- Software: Rhim H.
- Supervision: Kim N, Han JI.
- Validation: Rhim H, Jung S, Han JI.
- Visualization: Rhim H, Jung S.
- Writing - original draft: Rhim H, Jung S.
- Writing - review & editing: Rhim H, Jung S, Kim N, Han JI.
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