Mydriatics are routinely administered in the eyes of dogs to facilitate examination of the lens and posterior segment of an eye. They are also used to dilate the pupils in various situations, such as in preparation for electroretinography, cataract surgery, or any surgical procedure that requires mydriasis. Tropicamide is a synthetic antimuscarinic agent that inhibits the action of acetylcholine on the iris sphincter1 and is commercially available in 0.5% and 1% ophthalmic solutions. In dogs, tropicamide is the most frequently used topical drug for induction of mydriasis for diagnostic purposes (diagnostic mydriasis) because of its rapid onset of action (10 to 20 minutes), completeness of pupil dilation, and relatively short duration of action (6 to 8 hours).2,3 Drugs for diagnostic mydriasis should be administered in accordance with the principle of safe mydriasis outlined by Mapstone4 in 1970. However, tropicamide-induced mydriasis has been associated with an abnormally increased IOP in dogs,5 cats,6 and humans.7,8 Results of an ultrasound biomicroscopy study9 indicate that, in dogs, a slight narrowing of the ciliary cleft entry is associated with pupil dilation induced by topical instillation of tropicamide in the eyes. Therefore, an important concern for dogs receiving mydriatic agents is the potential risk for induction or exacerbation of an acute increase in IOP.
In most dogs, ophthalmic examinations can be performed without medication; however, use of sedatives and related agents may be necessary to perform examinations in uncooperative or reluctant patients. In fact, administration of a sedative is sometimes essential to achieve adequate immobilization for a thorough ophthalmic examination.8 Sedatives and related agents have the potential to decrease pain and stress for the patient and to enhance safety for both the patient and examiner. A number of chemical agents and a combination of techniques are available for sedation, and the agent or technique selected is generally dependent on the physiologic state of the patient and type of procedure that is being performed.10 Factors that affect eye position, pupil size, and IOP are important considerations during selection of sedation protocols for patients undergoing ophthalmic examinations.11
Butorphanol tartrate is a synthetic κ-opiod receptor agonist and μ-opiod receptor antagonist. In veterinary practice, butorphanol is generally considered to have low analgesic properties12 and is commonly used as a premedication in combination with other drugs.13 In dogs and cats, butorphanol is also recommended for achieving moderate short-term sedation with minimal adverse effects.14,15 Results of 1 study16 suggest that, in dogs, butorphanol can induce adequate sedation for ophthalmic examination but may interfere with some ophthalmic variables. Most notably, substantial pupillary constriction and an increase in IOP were observed in dogs for up to 45 minutes after IM administration of 0.2 mg of butorphanol/kg.16
The purpose of the study reported here was to evaluate changes in pupil size and IOP after topical application of a 0.5% tropicamide ophthalmic solution in the eyes of dogs sedated with butorphanol. We hypothesized that systemic administration of butorphanol might affect tropicamide-induced mydriasis and that butorphanol and tropicamide may act synergistically to increase the IOP of dogs.
SL-14, Kowa Co, Tokyo, Japan.
Tonovet, Icare, Tiolat, Helsinki, Finland.
Test de Schirmer, Virbac, Carros, France.
Beta 200, Heine Optotechnik, Herrsching, Germany.
Dolorex, MSD Animal Health, Boxmeer, Netherlands.
Mydriaticum 0.5%, Laboratoire Théa, Clermont-Ferrand, France.
Rosix, Dioptrix, Toulouse, France.
SPSS SYSTAT, version 12.0, IBM Corp, Chicago, Ill.
3. Gelatt KN. Eye examination and diagnostics. In: Gelatt KN, ed. Essentials of veterinary ophthalmology. 3rd ed. Philadelphia: John Wiley & Sons Ltd, 2014;101–144.
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Description of scoring system used to assess the extent of sedation for 12 healthy adult Beagles before and after IM administration of butorphanol (0.2 mg/kg) or an equal volume of sterile saline (0.9% NaCl) solution.
|0||No signs of sedation.|
|1||Mild sedation (subject remains standing or sitting but appears calm, is aware of its surroundings, and reacts to verbal stimulation).|
|2||Moderate sedation (subject appears sleepy and remains sitting or assumes sternal recumbency; subject does not react to verbal stimulation but can be aroused with physical stimulation).|
|3||Heavy sedation (subject is inactive and in lateral recumbency and is difficult to arouse with physical stimulation).|
Adapted from sedation scoring systems described previously.18–20