Safety and efficacy of bilateral topical application of rocuronium bromide for mydriasis in European kestrels (Falco tinnunculus).
Key words: mydriasis, rocuronium bromide, neuromuscular blocking agent, avian, raptor, kestrel, Falco tinnunculus
In birds, the achievement of mydriasis, necessary for a complete ophthalmoscopic examination, is more difficult than in mammals. The musculature of the avian iris is mainly composed of striated muscle fibers, (1,2) and parasympatholytic agents used in mammals to induce mydriasis are ineffective in birds. (1-3) Therefore, general anesthesia (4,5) or local neuromuscular blocking agents (NMBA) have been used. (2,6-10) Local NMBAs, such as d-tubocurarine and its derivatives, can be applied topically (8-12) or injected into the anterior chamber of the eye through the limbus. (2,7) In the latter case, clinicians must take care to prevent intraocular damage. (7) If used at an excessive dose, NMBAs used in birds as mydriatic agents can have systemic adverse effects, represented by eyelid, neck, and hindlimb muscle paralysis, and possible death. (8-10) Muscle paralysis usually starts from the eyelids, proceeds to the hindlimb muscles, and is followed by paralysis of the neck and finally of the respiratory muscles.
In 1 study in kestrels (Falco tinnunculus), the mydriatic effect of topical administration of 3 different NMBAs (alcuronium chloride, pancuronium bromide, and vecuronium bromide) was evaluated. (8) Analysis of the results suggested that only vecuronium bromide, administered at a dose of 2 drops, 3 times, at 15-minute intervals, provides an effective mydriasis of the pupil without adverse effects. (8) However, in this study, the NMBAs were applied to 1 eye only; therefore, the safety of bilateral topical administration of these drugs was not evaluated. (3)
Recently, good mydriasis without systemic adverse effects was obtained in nocturnal and diurnal birds of prey by using topically applied rocuronium bromide, a neuromuscular blocking agent and derivative of vecuronium bromide. (11,12) In common buzzards (Buteo buteo) and little owls (Athene noctua), rocuronium was topically applied concurrently in both eyes without the occurrence of any adverse effects. (12)
The objectives of this study were to evaluate the efficacy of topically applied rocuronium bromide as a mydriatic agent in kestrels, to verify the possibility of inducing bilateral concurrent mydriasis in these birds, to assess possible local or general adverse effects due to the use of the drug, and to compare the results from this study with those obtained from a study on the use of 3 different NMBAs in the same species of bird of prey.
Materials and Methods
Kestrels used in this study were brought to the Department of Veterinary Clinics of Pisa University (Italy) to undergo a complete ophthalmologic examination before release into the wild. Any bird that presented with an ophthalmic abnormality was excluded from the study to minimize the possible influence of an ophthalmic disease on mydriasis. Approval of this study was obtained from the local ethics committee on animal experimentation (D.Lgs.vo 116/92). The eyes of all the birds were examined, in a well-lit room, for gross abnormalities. Schirmer tear test readings were obtained for both eyes (OU) by using commercially available test strips (Dina strip Schirmer-Plus, GECIS sarl, Neung sur Beuvron, France) and palpebral, corneal, and direct pupillary light reflexes were evaluated OU. The intraocular pressure was assessed by means of applanation tonometry (Tonopen-XL, Mentor, Norwell, MA, USA) after a topical administration of 0.4% oxibuprocaine chlorhydrate (Benoxinato chlorhydrate INTES, ALFA INTES Industria Terapeutica Splendore S.r.1., Naples, Italy). A slit-lamp (Kowa SL-14, Kowa Company, Tokyo, Japan) was used to examine the adnexae and anterior segment OU. To allow a complete evaluation of the fundus, mydriasis was induced by using a single topical application of 0.12 mg of rocuronium bromide (Esmeron, Organon Italia S.p.a., Rome, Italy) concurrently in both eyes (total dose, 0.24 mg/bird) by means of a mechanical pipette (Pipetman 0-999 mcL, Gilson International-France SAS, Villiers-le-Bel, France). The drug, which is available as a 10 mg/mL aqueous solution, was applied without dilution. During the administration of the drug, the third eyelid of each eye was retained with a lid retractor for a 1-minute period to prevent its excursion, which could remove part of the mydriatic agent and, therefore, reduce its efficacy. The dosage of rocuronium for kestrels was established on the basis of previously published studies by maintaining a similar or lower total dose (mg/kg) compared with that used in other investigated species. (11,12)
Pupillary light reflex and pupillary diameter were recorded before the application of rocuronium (Tbase), after 10 minutes and then every 20 minutes after administration for 270 minutes (T270). The direct pupillary light response was assessed in a darkened room by using a 15-W halogen lamp (SL-14; Kowa) as a light source (intensity selected, 1/4; distance from the eye, approximately 30 cm). Because adult kestrels have a dark brown iris, and it is difficult to distinguish the pupil from the iris in ambient light conditions, the pupillary light reflex was evaluated based on a 3-point scale with a score of 2 (normal), 1 (decreased), and 0 (absent) light reflex. The pupillary diameter was measured by using a pupillary gauge calibrated to the nearest 0.5 mm only during the assessment of pupillary light response.
The fundus was examined with a binocular indirect ophthalmoscope (Omega 180, Heine, Berlin, Germany) with a 40- and a 78-dioptries lens. At the end of the clinical examination, a fluorescein stain was performed in both eyes to exclude corneal lesions.
Possible local and/or general adverse effects secondary to the topical use of rocuronium bromide were monitored. In particular, the occurrence of signs of ocular irritation (lacrimation, blepharospasm, conjunctival hyperemia, chemosis), eyelid, wing, hind limb, and neck muscle paralysis, and any kind of respiratory distress were evaluated. After the ophthalmologic procedures, the birds were kept in single cages and observed for a period of 24 hours to exclude subsequent adverse effects.
Data were analyzed by using GraphPad Prism4 (San Diego, CA, USA). Data were evaluated for normal distribution by using the Kolmogorov-Smirnov method, and the Wilcoxon signed rank test was used to define Gaussian distribution. Data were summarized as mean (SD). A 1-way analysis of variance Friedman test with a Dunns post hoc test was used to assess differences between Tbase and subsequent time intervals within the observation time frame. A Mann-Whitney test was used to assess differences in the pupillary light reflex score between the 2 eyes. A 1-way analysis of variance for repeated measures was used to assess differences in values recorded at Tbase and subsequent time intervals within the observation time frame for the pupil diameter. Post hoc evaluation was performed by using the Tukey test. An unpaired Student's t test was used to compare values of the pupillary diameters between the 2 eyes. Differences were considered statistically significant at P [less than or equal to] .05.
[FIGURE 1 OMITTED]
Of a total number of 29 kestrels examined at the clinic in the period of the study, 19 birds were excluded because of evidence of ophthalmic disease. Ten healthy adult kestrels (n = 20 eyes) of both sexes, weighing from 200 to 250 g, were enrolled in the study. After application of rocuronium bromide, maximal mydriasis was achieved at T90 min, with a mean (SD) pupillary diameter of 6.30 (0.42) mm in the right eye and 6.35 (0.41) mm in the left eye. For both eyes, pupillary diameter differed significantly (P < .05) between Tbase and all subsequent time periods, with the exception of T270 minutes (P > .05) (Fig 1). Pupillary light response score for both eyes differed between Tbase and T20 minutes until T110 minutes (P > .05) (Fig 2). At T20 minutes, pupillary light reflex was absent in all of the treated eyes. No significant differences were found in pupillary diameter and pupillary light response between the right eye and the left eye. In all 20 eyes examined, the level of mydriasis achieved allowed a complete ophthalmoscopic examination. In 6 of 10 kestrels, examination of the posterior segment of the eye was possible beginning at T30 minutes. A normal pupillary light response was reobtained at approximately T270 minutes, whereas the pupillary diameter was still slightly larger than the pre-administration values. Local or systemic adverse effects were not observed in any of the birds.
Results of the present study show that topically applied rocuronium bromide induced a consistent mydriasis in all of the treated eyes of the birds in this study. Concurrent bilateral instillation of the drug did not cause any noticeable adverse effects in the examined birds. For this reason, rocuronium bromide at the dose used in this study appears to be a safe and effective topical mydriatic agent in kestrels when administered bilaterally. In some species of raptors, and especially in nocturnal birds of prey, pupillary apertures are large enough in a darkened environment to allow a good examination of the fundus without the use of a mydriatic agent. This is not true in kestrels, in which the pupillary light reflex is very fast and the pupil diameter, secondary to the stimulation of the examination light, is too small to be able to observe the posterior segment of the eye. Thus, in these birds, achieving mydriasis is an essential step in the ophthalmic examination. In addition, achieving a concurrent bilateral mydriasis is also very important in avian ophthalmology because it allows examination of both eyes at the same time and reduces excessive handling of the birds and the related stress. All of the kestrels examined in this study showed maximal bilateral mydriasis at T90 minutes, but in 6 of them, consistent pupil dilation of both eyes was already present at T30 minutes, which allows for a complete fundic examination. Bilateral topical administration of an NMBA for mydriasis induction was previously described only in a study that involved common buzzards and little owls. (12) The NMBA used was also rocuronium, and analysis of the results suggested a good efficacy of the drug without adverse effects in all the birds examined. (12)
[FIGURE 2 OMITTED]
In a previous study, 3 different neuromuscular blocking agents were applied locally to induce mydriasis in kestrels. (8) The application was always unilateral. The NMBAs used were alcuronium chloride (5 mg/mL), pancuronium bromide (2 mg/mL), and vecuronium bromide (4 mg/mL), applied topically at different dosages. When considering that 1 mL = 20 drops, the researchers in this study instilled 1 drop (0.05 mL), 2 times, at 15-minute intervals (alcuronium chloride); 2 drops, 2 times, at 15-minute intervals (pancuronium bromide); and 2 drops, 3 times, at 15-minute intervals (vecuronium bromide). At these dosages, only topical application of vecuronium was considered efficacious and safe. Pancuronium did not induce a sufficient mydriasis, whereas use of alcuronium achieved complete mydriasis, but most of the birds examined developed adverse effects, such as inferior eyelid paralysis in 5 birds and full neck and hindlimb muscle paralysis in 1 bird. Instead, use of vecuronium achieved a good mydriasis, but only after 3 repeated administrations; thus, the cumulative dose (total dose, 1.2 mg/bird) of the drug was high, and this is probably the reason why 1 eye of each bird was tested.
In the present study, each bird received a single dose of 0.12 mg/eye (total dose, 0.24 mg/bird) of rocuronium bromide applied topically by using a mechanical pipette. Rocuronium bromide was able to induce a consistent pupil dilation in both eyes of each bird without adverse effects and thus seems to be a better mydriatic agent in kestrels compared with vecuronium. In fact, at the dose used, rocuronium produces a secure and reliable bilateral mydriasis with a single topical administration in each eye, greatly reducing the stress caused by handling and optimizing the ophthalmologic examination. From these results, we conclude that rocuronium bromide is an efficient and safe mydriatic agent in kestrels.
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(11.) Barsotti G, Briganti A, Spratte JR, et al. Mydriatic effect of topically applied rocuronium bromide in tawny owls (Strix aluco): comparison between two protocols. Vet Ophthalmol. 2010;13(suppl 1):9-13.
(12.) Barsotti G, Briganti A, Spratte JR, et al. Bilateral mydriasis in common buzzards (Buteo buteo) and little owls (Athene noctua) induced by concurrent topical administration of rocuronium bromide. Vet Ophthalmol. 2010;13(suppl 1):35-40.
Giovanni Barsotti, DVM, PhD, Angela Briganti, DVM, PhD, Johanna R. Spratte, DVM, Renato Ceccherelli, DVM, and Gloria Breghi, DVM
From the Department of Veterinary Clinics, Faculty of Veterinary Medicine, University of Pisa via Livornese lato monte, 56122, San Piero a Grado, Pisa, Italy (Barsotti, Briganti, Spratte, Breghi); and the WildLife Rescue Center of Sea and Water Birds, via delle sorgenti 430, 57121, Livorno, Italy (Ceccherelli).
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|Title Annotation:||Original Studies|
|Author:||Barsotti, Giovanni; Briganti, Angela; Spratte, Johanna R.; Ceccherelli, Renato; Breghi, Gloria|
|Publication:||Journal of Avian Medicine and Surgery|
|Date:||Mar 1, 2012|
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