Anesthesia for ophthalmic surgery

CHAPTER

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Anesthesia for ophthalmic surgery Kirk N. Gelatt

Chapter contents Introduction

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Local or regional eyelid injections/nerve blocks

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Ophthalmic effects of general anesthetics

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Retrobulbar injections/nerve blocks in animals

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Preanesthetic medications

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Injectable general anesthetics

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Choice of general anesthetic for selected ophthalmic surgical procedures

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Inhalational general anesthetics

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Neuromuscular blocking agents

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ADAPTATIONS FOR LARGE ANIMALS AND SPECIAL SPECIES

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Ophthalmic drug and anesthetic drug interactions

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Horse

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Cattle

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Systemic diseases and general anesthesia

Introduction The advances in veterinary anesthesia have paralleled those in veterinary ophthalmic surgery, and have resulted in less anesthetic risk, and improved patient care and management. Studies in different animal species, including humans, suggest frequent similar ophthalmic responses to tranquilizers, narcotic analgesics, dissociative anesthetics, inhalational general anesthetics, and neuromuscular relaxants; however, species differences can occur. Combinations of critical peri-, intra-, and postoperative topical and systemic drugs are common in veterinary ophthalmic patients, and should be accommodated by the choice of general anesthetic agents and protocols. At the same time, concurrent general anesthesia and ophthalmic needs must avoid drug selections that are contraindicated or incompatible when used simultaneously. A significant number of ophthalmic patients are old and may have other diseases that may influence the choice of general anesthetics. In this chapter the impact on the eye and associated structures of drugs administered as part of general anesthesia will be summarized.

Ophthalmic effects of general anesthetics Intraocular pressure Intraocular pressure (IOP) results from a relative equilibrium between aqueous formation, aqueous humor outflow,

and the resistance of the fibrous tunics, e.g., the cornea and sclera, to pressure. The different drugs administered to tranquilize, sedate, and/or anesthetize animals may affect IOP directly by influencing the aqueous humor dynamics, or indirectly by causing hypercapnia, changes in extraocular muscle tone, hypoxemia, and hypothermia. Most general anesthetics lower IOP through actions on the central nervous, respiratory, and circulatory systems. The reduction in IOP is also related directly to the depth of general anesthesia. Most general anesthetics seem to lower IOP by an increase in the rate of aqueous humor outflow. Drugs that directly cause ocular hypotension can also produce ocular hypertension secondary to respiratory depression and acidosis that sometimes occurs with prolonged general anesthesia. As a general observation, drugs that produce an abrupt increase in arterial blood pressure will result in a moderate increase in IOP. This elevation in IOP is usually transient as the aqueous humor dynamics rapidly readjust and return to normal levels of IOP. The major percentage of the resistance of aqueous humor outflow is determined by the episcleral venous pressure. Drugs that produce marked increases in central venous pressure and episcleral venous pressure can also temporarily elevate IOP. The increased central venous pressure may also expand the anterior and posterior uveal vascular beds, indirectly increasing IOP. Expansion of the uveal vascular channels may produce pressure on the vitreous when the globe has been opened, and force the vitreous, its patellar fossa, and even the lens toward the anterior chamber.

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Anesthesia for ophthalmic surgery

Drug-induced changes in extraocular muscle tone can influence IOP. As most animal species have lower scleral rigidity than humans, relaxation of the extraocular and retrobulbar muscles markedly decreases the pressure of these orbital tissues upon the globe. The ketamineassociated elevation in IOP that occurs in humans is thought to be directly related to the increase in extraocular muscle tone. In lightly anesthetized dogs, intravenous administration of succinylcholine can result in a shortterm elevation in IOP. This 5–10 min elevation in IOP is thought to be related to the unusual sensitivity of the extraocular muscles to succinylcholine, and the initial muscle fasciculations that occur during the onset of the drug’s action. When the insertions of the extraocular muscles are severed in the cat, succinylcholine administration does not change IOP. Animals, in general, possess lower ocular rigidity than humans. As a result, when either the cornea or the sclera is incised and IOP released, the entire globe tends to collapse. The sclera in both the dog and cat possesses elastic fibers, in addition to the major complement of collagen, and as a result the sclera lacks rigidity. When the globe is collapsed, corneal and intraocular surgical procedures are more difficult to perform. The level of IOP in animals with low ocular rigidity also enhances the effects of retrobulbar muscle tone. Once the globe has collapsed, extraocular muscle tone may become of greater concern, tending to distort and displace forward the vitreous, lens, and anterior uvea.

Corneal drying and exposure Corneal abrasions, drying of the cornea, conjunctival irritation, and reduced tear formation have been associated with general anesthetics in humans and animals. Ketamine in cats has been associated with corneal drying, although the individual roles of reduced tear formation rates and loss of the protective blink reflex have not been differentiated. Hence, in cats undergoing ketamine anesthesia the corneas should be protected by copious amounts of ophthalmic petroleum-based bland ointment and/or the eyelids closed temporarily by tape. The same applies in dogs, and petroleum-based bland ointment should be applied to both eyes, depending on the type of ophthalmic surgery. The rate of aqueous tear formation in dogs, as determined by Schirmer’s tear test, after combinations of subcutaneous atropine, intravenous thiamylal sodium, and halothane or methoxyflurane was reduced by about 70% within 10 min and by 97% after 60 min. Another study has indicated that subcutaneous atropine reduces Schirmer’s tear test levels in normal dogs by about 55% at 60 min after drug administration. In dogs and cats with reduced levels of tear formation, the administration of parenteral and/or topical atropine can abruptly lower Schirmer’s tear test levels to zero, and initiate the clinical signs of keratoconjunctivitis sicca. Although the topical effects of general anesthetics have not been reported in large and special species animals, petroleum-based bland ointment is applied liberally to protect the corneoconjunctival surfaces during prolonged general anesthesia.

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Oculorespiratory cardiac reflex The oculocardiac or oculorespiratory cardiac reflex was first described by Aschner and Dagnini in 1908 in two simultaneous but independent reports. The afferent aspect of this reflex is carried in the long and short ciliary nerves, the ciliary ganglion, and the ophthalmic branch of the trigeminal nerve via the gasserian ganglion to the trigeminal sensory nucleus. Short internuncial fibers within the reticular formation connect the trigeminal sensory nucleus to the visceral motor nucleus of the vagus nerve and its descending nerve to complete the efferent limb to the heart. The afferent limb of the ophthalmic division of the trigeminal nerve does not appear to be unique. Intraorbital stimulation of the third, fourth, and sixth cranial nerves will also produce consistent respiratory prolongation, but more variable cardiac responses. There appears to be significant species differences for the oculorespiratory cardiac reflex (documented in dogs, cats, horses, and birds), and whether the cardiac, the respiratory, or a combination of both components occurs. In addition to the induced bradycardia, in some species such as the dog, the concurrent respiratory depression can be more profound. The reflex may be initiated by a number of ophthalmic manipulations, including ocular pressure massage for glaucoma, intraorbital injections of local anesthetics (which are also used to block this reflex), surgical traction of the extraocular muscles, and manipulations of the eyelid muscles. In dogs under general anesthesia, neuromuscular blocking agents, and controlled ventilation, only the cardiac portion of this reflex can be appreciated. There may be some individual animal variations relative to the oculorespiratory cardiac reflex in the dog and cat, with only some animals demonstrating this reflex consistently. The primary effect in the cat seems to be respiratory; in the dog, respiratory depression is the dominant response, but bradycardia can develop. To manage the oculorespiratory cardiac reflex, a number of strategies have been developed. To diminish or completely block the vagal effect on the heart, intravenous atropine is the standard treatment. Unfortunately, atropine administration yields inconsistent results. Consequently, the rationale to administer parenteral atropine preoperatively in dogs differs among veterinary anesthesiologists. One school recommends against the routine administration of parenteral atropine preoperatively. If bradycardia develops during the surgical procedure, surgery is temporarily halted and atropine is administered intravenously. Other veterinary anesthesiologists continue to recommend routine use of preoperative parenteral atropine to prevent the potential oculorespiratory cardiac reflex from developing. Unfortunately, intravenous atropine may not only increase the heart rate in the dog, but also increase the possibility of ventricular dysrhythmias. The intravenous dose of atropine to treat and/or prevent the oculorespiratory cardiac reflex in the dog seems critical. In children, although the prophylactic use of parenteral atropine seems to lower the incidence of the oculocardiac reflex, it has also been associated with severe and prolonged ventricular dysrhythmias. Low levels (0.015 mg/kg IV) of atropine in dogs with an experimentally induced oculorespiratory cardiac reflex may actually enhance respiratory depression. Higher doses of

Preanesthetic medications

atropine (0.023–0.04 mg/kg) may eliminate the bradycardia but prolong the apnea. Of these two complications, clinical management of apnea with controlled ventilation is the most feasible solution. An alternative to atropine in the dog is glycopyrrolate (0.01 mg/kg IV, usually given in two divided doses; often the second dose is not necessary) which appears as effective in preventing the oculorespiratory cardiac reflex but produces tachycardia. Under most circumstances, if the oculorespiratory cardiac reflex develops during ophthalmic surgery, surgery is suspended for several minutes and the depth of general anesthesia is increased. Less aggressive surgery is then slowly resumed while the respiratory and cardiac rates are carefully monitored. Fortunately, the onset of the oculorespiratory cardiac reflex is usually in the early aspects of surgery, and in intraocular surgical procedures before critical manipulations have begun.

Eye position During the induction of general anesthesia most injectable and inhalational anesthetics produce a downward and inward rotation of the eye that limits access to the cornea, anterior chamber, and anterior segment. As the globe is rotated ventromedially, the nictitating membrane simultaneously protracts to nearly cover the cornea. Some degree of enophthalmia also develops, decreasing further the exposure of the cornea and globe for surgery. In the large and giant breeds of dogs, access to the eye is already limited, and these drug effects can severely compromise surgical exposure of the eye. This poor positioning of the globe can handicap the surgeon by impairing observation of the entire cornea and anterior segment, increasing the difficulties of surgical manipulations, and unnecessarily prolonging the surgery. Several strategies have been developed to correct the rotation of the globe and exposure difficulties associated with general anesthetics. Unfortunately, most of these remedies to improve exposure may also result in some additional operative risks. Sutures may be placed in the anterior sclera or the rectus muscle insertions, and anchored to the eyelid specula or drapes. Scleral clips may be used similarly. Retrobulbar injections with saline positioned directly into the extraocular muscle cone to push the eye forward, or external to the extraocular muscle cone to turn the globe, may also produce noticeable inward compression of the posterior segment and additional pressure on the vitreous body. The animal cornea and sclera unfortunately lack rigidity, unlike humans and primates in general, and with traction or compression these tunics may become distorted. For certain types of conjunctival and corneal surgery, the distortion of the globe associated with these procedures may be inconsequential. However, when intraocular surgical procedures are planned, any preventable pressure on the globe, in whole or in part, should be avoided. Administration of the different neuromuscular blocking agents has replaced the need for extrabulbar injections to manipulate the position of the globe for surgery.

Pupil size Pupil size has been used historically to monitor the depth of general anesthesia. Without local control by topical mydriatics or miotics, pupil size may vary from marked dilatation to pinpoint in the lighter levels of general anesthesia, to

progressive mydriasis with deep general anesthesia. For conjunctival and corneal surgical procedures, pupil size is often adjusted preoperatively depending upon the concurrent ophthalmic disease. Often the pupil is dilated. Druginduced iridocycloplegia helps reduce the pain associated with preoperative anterior uveitis, and pupil dilatation reduces the likelihood of posterior synechiae formation. In the event of corneal and intraocular surgery, control of pupil size may become critical. Maximum mydriasis is essential for cataract extraction; preoperative pupillary dilatation is usually achieved with 0.3% scopolamine combined with 10% phenylephrine, 1% atropine, or a combination of 1% atropine and 10% phenylephrine. Topical non-steroidal anti-inflammatory agents, such as 0.03% sodium flurbiprofen, can also assist in the maintenance of pupillary dilatation. Prostaglandins appear to be released when the anterior chamber is entered surgically and initiate strong miotic activity. Endocapsular phacoemulsification of canine cataracts requires maximal mydriasis. Without the combination of topical mydriatics, topical and parenteral corticosteroids, and non-steroidal anti-inflammatory agents, the microsurgical refinements and higher success rates for canine cataract surgery would not have been possible.

Extraocular muscular tone The extraocular muscles are well developed in the dog and, in addition to the four rectus and two oblique extraocular muscles, include the retractor oculi muscle that inserts onto the sclera under the rectus muscle insertions and behind the globe’s equator. This bulk of extraocular muscles may produce pressure and indent the posterior segment of the globe, even with optimal general anesthesia. The extraocular muscle pressure, combined with the low scleral rigidity of the dog, seems to be more important once the anterior chamber has been entered, as during cataract and lens removal. If general anesthetics also increase central venous pressure, additional orbital pressure on the globe may develop from the extensive venous plexuses within the orbit. In the cat, the effects of the extraocular muscles during general anesthesia seem less important than in the dog. This may be caused by the poorly developed cat extraocular muscles and the limited orbital space. As a result, increased pressure on the posterior segment is less important and does not appear to be a problem clinically. Several strategies have been developed to address the potential extraocular muscle pressure and its adverse effects when the anterior chamber has been entered surgically. Neuromuscular blocking agents have now become routine for canine and equine intraocular surgery; in addition to greatly reducing extraocular muscle tone, these agents result in optimal eye position for microsurgery.

Preanesthetic medications Preanesthetic medications are designed to facilitate a smooth induction of general anesthesia, and help prevent possible drug-related complications. The controversial routine use of parenteral atropine as an anticholinergic agent has already been discussed. Parenteral glycopyrrolate (0.01 mg/kg IM) is preferred because of fewer cardiac effects.

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Anesthesia for ophthalmic surgery

Sedatives and tranquilizers are often employed preoperatively, especially before intraocular surgery. Both sedatives and tranquilizers lower IOP, probably by increasing the outflow of aqueous humor. Among the phenothiazine tranquilizers, acepromazine maleate is the most frequently recommended. Acepromazine maleate (0.03–0.1 mg/kg IM) is frequently utilized, not only because of the resultant tranquilization, but also for its anti-arrhythmogenic effect associated with the stabilization of the myocardium against catecholamine stimulation and arrhythmogenic agents. The phenothiazine tranquilizers also possess an anti-emetic action perioperatively. Both postoperative vomiting and retching in humans can elevate IOP indirectly by abrupt venous pressure increases. The same effect probably occurs in dogs. Xylazine is not recommended perioperatively in ophthalmic patients because it can cause vomiting and severe bradycardia. Acepromazine may slightly prolong the recovery from general anesthesia, hypothermia, and arterial hypotension, but usually provides a smoother, less traumatic recovery. Most narcotics seem to slightly lower IOP in those animal species studied. The two major advantages of narcotics are that: 1) these drugs are potent analgesics; and 2) they can be chemically antagonized if drug reversal is necessary. Unfortunately, most of these agents except for meperidine are also potent vagotonic and respiratory depressants. Use of narcotic derivatives prior to and following ophthalmic surgery has become more frequent. Occasionally, if the postoperative recovery becomes traumatic, parenteral narcotics are quite effective, probably because of the analgesic effects.

Injectable general anesthetics Most barbiturates lower IOP in animals. This ocular hypotension seems to result from depression of the diencephalon, an increased facility of aqueous humor outflow, and relaxation of the extraocular muscles. Ultrashort-acting barbiturates, such as thiopental and thiamylal (8–12 mg/kg IV) are effective induction agents. The reduction in IOP after administration of these drugs seems to be related to relaxation of the extraocular muscles and an increase in aqueous humor outflow rather than from arterial blood pressure changes. As these agents are potent respiratory depressants, endotracheal intubation should follow immediately after barbiturate administration. Intubation should be standard protocol in all ophthalmic surgical patients. Intermittent and often copious lavage of the corneal and conjunctival surfaces during surgery may exit the nasolacrimal system and accumulate in the mouth and pharynx. Ketamine may be an exception to the rule for injectable anesthetics. Elevated IOP has been associated with ketamine, with increased tone of the extraocular muscles in humans. Ketamine, recommended for the cat, has been reported to either not change or increase IOP in the cat by 10%. Ketamine, used alone, is not recommended for the dog because of its tendency to produce seizures. Ketamine, a dissociative anesthetic, may be injected intramuscularly for the induction of general anesthesia in cats. Ketamine is often combined with diazepam in dogs to reduce the possibility of seizures and produce muscular relaxation. An anticholinergic, such as atropine, is also administered to

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minimize salivation. After general anesthesia is sufficiently deep to permit intubation, general inhalational anesthesia may be initiated for longer duration surgeries. There are other injectable agents that are now used as induction agents for small animals, and experiences with some of these agents, such as propofol, midazolam, and TelazolW, have been excellent. Propofol, as an induction agent, has a recommended intravenous dose in small animals of 6 mg/kg, and then to effect, and has largely replaced the barbiturates. After rapid, smooth and excitement-free onset of general anesthesia, the duration is also relatively short (range 2.5–9 min). Usually administered as a slow bolus injection (to avoid apnea) for the induction of inhalational anesthesia, propofol can be injected repeatedly; however, its short duration of effect requires several injections for relatively short periods of time. Recovery after propofol is usually very rapid, and excitement free. Propofol is thought to lower IOP. Propofol has become the preferred induction agent world-wide in small animals, and the sole general anesthetic for many short-term ophthalmic procedures. Propofol (2,6-diisopropylphenol) is only soluble in water, and is mixed immediately before use. It comes in sterile glass ampoules and without preservatives. It fits a twocompartment open model, with rapid distribution from the plasma into the tissues and rapid metabolic clearance from plasma. It is metabolized by conjugation primarily by the liver and kidney. It is administered as an intravenous bolus at doses that range from 2.5 mg/kg (sedated dog) to 8 mg/kg (unsedated dog) to allow tracheal intubation and the initiation of inhalational anesthesia. Propofol’s anesthesia is quite brief; in unsedated dogs recovery is only 15 min. Propofol can also be used for the maintenance of anesthesia administered by continuous infusion or intermittent bolus. Propofol has minimal analgesic effects, and drugs with analgesic effects, such as opiates, should be administered concurrently. Propofol lowers IOP in humans, and this effect has also been reported in dogs (a decline of 26%). Midazolam is a water-soluble benzodiazepine. It does not induce anesthesia when used alone; hence midazolam is often combined with ketamine, or one of the ultrashortacting thiobarbiturates (thiamylal or thiopental) to induce general anesthesia. TelazolW consists of equal parts of a dissociative agent, tiletamine, and a benzodiazepine, zolazepam. Once in solution, TelazolW has a limited shelf-life of 4 days at room temperature and 14 days at 4 C. The recommended dose for dogs is 6.6–13.2 mg/kg IM or SC, and for cats 9.7– 15.8 mg/kg IM or SC. After deep intramuscular injection of TelazolW, onset of anesthesia is within 2–5 min and the recovery to walking requires 3–5 h. Induction of anesthesia with TelazolW is usually smooth, but recovery can be traumatic.

Inhalational general anesthetics All of the available inhalational general anesthetics lower IOP. The extent of ocular hypotension is directly related to the depth of general anesthesia. The lowering of IOP after inhalational anesthetics seems to result from collective drug actions on the respiratory, circulatory, and central nervous systems. The changes in arterial pressure associated with inhalational anesthetics do not seem to lower the IOP per se, but central venous pressure can be important. Changes

Neuromuscular blocking agents

Neuromuscular blocking agents Neuromuscular blocking agents have recently been added to the general anesthetic protocol for ophthalmic patients to improve exposure of the cornea and eye during intraocular surgery. Administration of neuromuscular blocking agents in dogs produces relaxation of all of the extraocular muscles, and within 30–60 s causes the eye to return to its normal axis from the ventromedial deviation associated with most general anesthetic agents (Fig. 3.1). In cats administered succinylcholine, the globe may assume a superolateral divergent position. Neuromuscular blocking agents have the potential to influence IOP depending on their mechanism of action. In humans, d-tubocurarine lowers IOP by relaxation of the extraocular muscles; however, if the patient hypoventilates,

A

B Fig. 3.1 Position of the canine eye under general anesthesia (a) before and (b) after the administration of neuromuscular blocking agents. Note the improved exposure of the cornea and globe.

leading to hypercarbia and hypoxemia, IOP may increase. In contrast, succinylcholine administered intravenously can elevate IOP in several animal species including the dog and cat (Fig. 3.2). The transient increase in IOP occurs almost immediately after succinylcholine administration, and appears to be directly associated with initial contraction of the extraocular muscles during the depolarization process. In humans, reported sensations after administration of succinylcholine include a feeling of increased orbital pressure, vertigo, and diplopia. The effect on IOP by parenteral succinylcholine is influenced by the level of general anesthesia at the time succinylcholine is administered. In unanesthetized persons, the average elevation of IOP after succinylcholine administration was 15 mm Hg. In light levels of general anesthesia after succinylcholine administration, IOP is usually increased 2–4 min later; during deep general anesthesia IOP is not affected. The period for drugrelated elevation in IOP is usually 5–6 min. 50

Effect of succinylcholine on IOP in the dog

40 IOP (mmHg)

in blood gases and the methods of ventilation may also influence IOP. Some of these agents may also lower IOP by increasing the facility of aqueous humor outflow. Prolonged general anesthesia with changes in blood pH and other complications may actually increase IOP. For instance, experiments in dogs with increased inspired CO2 indicate that IOP may rise after initial depression. Hypoventilation that leads to hypercapnia and hypoxemia can eventually result in elevation of IOP. Although some species differences may be present, the reduction in IOP after inhalational general anesthetics may be substantial. Methoxyflurane in humans can lower IOP by 15–25%. Halothane-induced ocular hypotension in children may be of greater magnitude in glaucomatous eyes than in normal eyes, but with more variability. Methoxyflurane in dogs presented for cataract surgery lowers IOP an average of 11 mm Hg with a range of 5–20 mm Hg. Isoflurane is the most frequently used inhalational general anesthetic for veterinary ophthalmic surgery (dog, cat, and horse most frequently), and has largely replaced halothane during the last decade. Halothane produces rapid induction and recovery associated with dose-related depression of the central nervous system. When combined with preanesthetics, such as acepromazine, the need for halothane is reduced. Halothane produces more depression of cardiac function than methoxyflurane, and causes more cardiac dysrhythmias than isoflurane. Halothane causes higher cardiac sensitivity to catecholamines; lidocaine (2–4 mg/kg IV in dogs and 1–2 mg/kg IV in cats) can be used to treat these arrhythmias. If adrenaline (epinephrine) is planned for hemostasis during ophthalmic surgery, isoflurane should be selected over halothane. Halothane is a poor analgesic, and is usually combined with nitrous oxide. Isoflurane is now preferred to halothane. The drug is nonflammable at anesthetic concentrations, is highly stable, and is not broken down by sunlight. Isoflurane has a faster onset of action and recovery because of its low blood solubility. Like halothane, isoflurane depresses cardiovascular function in a dose-dependent manner, but is less arrhythmogenic than halothane. Isoflurane may be used in patients with hepatic disease because it is minimally metabolized by the liver. At this time, isoflurane is recommended as the general anesthetic of choice for most aged and debilitated small animal patients. Sevoflurane is also becoming popular in many veterinary practices world-wide.

Drug

30 20

Control

10 0

0:00 0:01 0:02 0:03 0:04 0:05 0:06 0:07 0:08 0:09 0:10 Minutes n = 5 eyes

Fig. 3.2 The effect of IV succinylcholine on intraocular pressure (IOP) (mm Hg) in dogs under light general anesthesia.

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Studies in normal cats suggest that succinylcholine not only elevates IOP an average of 10–12 mm Hg, but also causes a forward displacement of the lens and iris when the anterior chamber is open. In an extracapsular cataract extraction, any forward displacement of the iris, lens, and presumably the vitreous is a cause for concern. If this effect occurs during the latter part of extracapsular cataract extraction, the thin posterior lens capsule is a weak barrier to increased pressure generated with the vitreous body. While succinylcholine acts as a depolarizing neuromuscular agent, members of the non-depolarizing relaxants, including d-tubocurarine, gallamine triethiodide, and pancuronium bromide, have not been associated with a drug-related elevation in IOP. Newer neuromuscular blocking agents used clinically, such as pancuronium bromide, vecuronium, and atracurium besylate, have shorter dose-related effects (Table 3.1). As inactivation of these neuromuscular blocking agents depends on the patient’s plasma anticholinesterases, concurrent use of potent topical anticholinesterase miotics in glaucomatous patients, such as echothiophate iodide and demecarium bromide, is contraindicated. Drugs available for reversal of these neuromuscular relaxants include edrophonium, neostigmine, and pyridostigmine. The neuromuscular blocking agents include atracurium, pancuronium, alcurium, and vecuronium. Atracurium besylate has minimal cardiovascular effects at recommended doses, has a relatively short duration of action without apparent cumulative effects, and elimination is independent of liver and kidney function. An intravenous bolus of atracurium (0.25 mg/kg), injected over 1 min, usually provides paralysis of the dog for about 30 min. For an additional intravenous bolus of atracurium, the dosage is reduced to 0.15 mg/kg. Atracurium can be antagonized, if necessary, with intravenous edrophonium at a dose of 0.5 mg/kg. Bradycardia has been associated with edrophonium administration. Either atropine administered previously or the very slow injection of edrophonium minimizes this effect. Neostigmine (2.5 mg), combined with atropine (1.2 mg), can also be used intravenously to antagonize the effects of atracurium. The total dose of neostigmine should not exceed 0.1 mg/kg. Pancuronium bromide is another frequently used neuromuscular blocking agent for canine intraocular surgery. An intravenous injection of pancuronium (0.06 mg/kg) causes a maximum neuromuscular blockage of 3–5 min that produces skeletal muscle relaxation and apnea for about 40 min. However, these muscle relaxants are not anesthetics. For their proper use, the patient’s respiration is carefully controlled by mechanical ventilation, neuromuscular and cardiovascular functions are adequately monitored, and the Table 3.1 Doses and length of paralysis with non-depolarizing neuromuscular blocking agents in dogs

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Drug

IV dose (mg/kg)

Length of effect (min)

Atracurium

0.5

15–80

Alcurium

0.06–0.1

30–40

Pancuronium

0.06–0.1

20–40

Vecuronium

0.06–0.1

15–20

inhalational general anesthetic is administered at sufficient levels to maintain unconsciousness and analgesia. Without adequate ventilation, these agents may cause respiratory acidosis within as short a time as 5 min. Most general anesthetics cause a ventromedial rotation of the globe, and local attempts to improve exposure of the eye have significant limitations and complications. After the administration of neuromuscular relaxants, the globe position returns to normal, permitting optimum surgical exposure, and IOP appears low. With pancuronium (0.06 mg/kg IV), the ocular changes persist for about 20–30 min following drug administration. The relaxation of the extraocular muscles releases the normal pressure on the globe, and the forward displacement of the vitreous, lens, and anterior uvea is minimal. Use of these neuromuscular blocking agents may contribute directly to improved cataract surgery results in dogs. Once the anterior chamber is surgically entered, and the majority of the cataract removed via extracapsular extraction or phacoemulsification, the forward displacement or central protrusion of the posterior lens capsule within the pupil is reduced as much as possible (Fig. 3.3). With reduced vitreous pressure on the posterior lens capsule, posterior lens capsule tears during surgery are kept to a minimum. Neuromuscular blocking agents that provide about 20– 40 min of muscle relaxation are the most useful (see Table 3.1). These agents are usually administered just before the anterior chamber is entered, and should provide extraocular muscle relaxation for the duration that the anterior chamber is open, the lens is delivered, and most if not all of the time for the apposition of the corneal or limbal wound. If an additional dose is necessary, as with bilateral cataract surgeries, the second dose of neuromuscular blocking agent is usually reduced by one-half.

Ophthalmic drug and anesthetic drug interactions Sometimes the ophthalmic medications and drugs associated with general anesthesia have possible conflicts. For instance, the preoperative treatment of the eye may involve a topical cholinergic miotic, but the anesthetic protocol includes the administration of parenteral anticholinergic agents, such as atropine. Fortunately, the effects of topical ophthalmic drugs are usually predominant because of the systemic dilution that occurs with parenteral drugs. For instance, studies indicate that parenteral glycopyrrolate (0.01 mg/kg IM) in normal dogs does not have any effect on IOP and pupil size. Parenteral atropine and glycopyrrolate at the recommended clinical doses in dogs with glaucoma do not elevate IOP. Other ophthalmic drugs may impact the management of the patient about to be anesthetized. Systemic carbonic anhydrase inhibitors are administered to lower IOP, but can produce a temporary metabolic acidosis and considerable diuresis. Anticholinesterase miotics used for the treatment of glaucoma can reduce the levels of plasma and red blood cell cholinesterases, rendering the patient more sensitive to the neuromuscular blocking agents and causing a prolonged effect. Fortunately, use of miotics for the treatment of glaucoma has become infrequent, and generally replaced by the prostaglandins.

Local or regional eyelid injections/nerve blocks

Fig. 3.3 Effects of extraocular muscle tone during general anesthesia on the posterior lens capsule (a) after extracapsular cataract removal. (b) Changes in the posterior lens capsule after the administration of neuromuscular blocking agents.

A

B

Hyperosmotic agents, such as mannitol and glycerol, are used in veterinary ophthalmology for short-term reduction of IOP, and to reduce the size of the vitreous body preoperatively. In patients with cardiac and pulmonary disease, acute increases in vascular volume associated with hyperosmotic agents may be highly significant. Topical sympathomimetic agents, such as 2% adrenaline (epinephrine) and 10% phenylepinephrine, are important to the veterinary ophthalmologist for their effect on IOP and as mydriatics. Adrenaline (epinephrine) may also be injected (1:10 000 to 1:100 000 concentrations) into the anterior chamber for mydriasis, and to control iridal hemorrhage. Use of halothane as the general anesthetic with these adrenergic agents is associated with occasional extrasystoles and arrhythmias, because the myocardium has been sensitized to these catecholamines. Selection of isoflurane as the general anesthetic for these patients is recommended.

Systemic diseases and general anesthesia Many ophthalmic surgical candidates may have certain systemic diseases that potentially can affect the choice of general anesthesia, the duration of general anesthesia, and the administration of other drugs. In cataract surgery in dogs, animals with diabetes mellitus are the second largest group of patients following those with inherited cataracts. In fact, cataract secondary to diabetes mellitus is the most frequent type of metabolic cataract in the dog and the second most frequent cataract surgery in America. Successful clinical management of the diabetic dog with cataract must not only accommodate the daily control of blood glucose levels, but also control the lens-induced uveitis for optimal success rates after cataract removal. One strategy in diabetic dogs is to substitute aspirin for systemic prednisolone. Topical antiprostaglandins can also reduce the dosage for systemic corticosteroids as well as antiprostaglandins, such as carprofen (RimadylW; Pfizer Animal Health, Exton, PA) and flunixin meglumine (BanamineW; Schering-Plough, Kenilworth, NJ). Administration of topical corticosteroids and even systemic prednisolone may be necessary in some diabetic dogs for treatment of lens-induced uveitis after cataract surgery. Some elevation of blood glucose levels may occur with both topical and systemic corticosteroids in postoperative diabetic dogs. Maintenance of preoperative levels of insulin

doses is usually best in these dogs, until the topical and/or systemic levels of corticosteroids can be reduced or eliminated. Oral glycerin to lower IOP and reduce the vitreous space is not recommended in diabetic dogs as the glycerin is converted to blood glucose. Intravenous mannitol does not elevate blood glucose, and is the recommended systemic osmotic agent for the dog and cat. Systemic hypertension occurs mainly in older dogs and cats, and its presence can complicate intraocular surgery as well as potentially contribute to postoperative intraocular hemorrhage and retinal detachments. The development of these sequelae following apparent successful cataract surgery with an intact corneal or corneoscleral incision should necessitate periodic monitoring of blood pressure. Clinical management of these complications must include successful treatment of the systemic hypertension. Dogs with advanced renal, cardiovascular, and hepatic diseases are not usually candidates for elective intraocular surgeries. Unless these diseases are successfully treated and the animals’ life span significantly increased, the risks and costs of general anesthesia and time for surgery generally negate elective intraocular surgeries in these patients.

Local or regional eyelid injections/ nerve blocks Eyelid injections are used more often in large animals than in small animals for eyelid akinesia, but not local anesthesia, and are targeted at the palpebral nerve and its branches as it extends forward to innervate the orbicularis oculi muscle, the powerful sphincter muscle that closes the upper and lower eyelids. In general, as these palpebral nerve blocks are placed closer to the palpebral fissure, the effects are more localized as the main nerve trunk branches into numerous smaller nerves. Often topical anesthetics are instilled along with eyelid nerve blocks to provide surface anesthesia and permit detailed ophthalmic examinations, subconjunctival injections, collection of samples from the cornea and conjunctiva for cytology or culture, applanation tonometry, and other minimally invasive procedures. The exception in the horse is the supraorbital nerve block, in which local anesthetic is injected at the supraorbital foramen, which provides both mid upper lid akinesia and local anesthesia. Local ophthalmic nerve blocks in large animals

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are used for ophthalmic examinations, especially in the horse, and as an adjunct for local or general anesthesia as part of eyelid, orbital or ocular surgery.

Eyelid injections/nerve blocks in the dog Eyelid injections are used in the dog both for eye examination in dogs with very painful ophthalmic disorders, as well as for therapy. Spastic entropion in the dog, a relatively rare condition, may also benefit immediately from the palpebral nerve block. For akinesia of the dog eyelid, local anesthetic (1–3 mL) may be injected subcutaneously along the upper zygomatic arch at its most lateral position or approximately 1–2 cm posterior to the lateral canthus (Fig. 3.4). An effective anesthetic block is demonstrated by drooping of the upper eyelid, an inability to close the palpebral fissure with a fixed and relaxed orbicularis oculi muscle, and an everted lower eyelid. The dog continues to have complete sensitization of the cornea, conjunctivae, and lid surfaces, as well as ocular mobility, and topical anesthetics are usually also instilled to permit minimally invasive diagnostic and therapeutic procedures.

Eyelid injections/nerve blocks in the cat Eyelid injections have not been reported in the cat, but the palpebral nerve pathway in this species is similar to that of other carnivores.

Eyelid injections/nerve blocks in the horse Because the horse’s orbicularis oculi muscle is very powerful, eyelid closure can occur during an eye examination as well as during drug administration in spite of manual efforts by the veterinarian or owner to maintain the palpebral fissure open. As a result, eyelid nerve blocks for akinesia, as well as combined akinesia–anesthesia nerve blocks, are available and used very frequently in the horse (Fig. 3.5).

Fig. 3.5 To perform the palpebral nerve block in the horse, local anesthetic is injected subcutaneously at the highest projection of the zygomatic arch (palpebral nerve only – A), in the groove immediately caudal of the zygomatic arch (B – contains the auriculopalpebral nerve, artery, and vein), or in the supraorbital fossa within the supraorbital process of the frontal bone (C). The first two nerve blocks provide only lid akinesia while the supraorbital nerve block provides both akinesia and analgesia of the mid upper eyelid.

Two different palpebral nerve blocks are frequently used in the horse to produce eyelid akinesia. The most popular is injection of about 1–3 mL of local anesthetic subcutaneously at the highest point of the zygomatic arch, midway in the arch. In the second method, 1–3 mL of local anesthetic is injected in the depression or groove of the ventral edge of the temporal portion of the zygomatic arch, just caudal to the posterior ramus of the mandible. Just before injection of local anesthetic, aspiration is used to check the needle position and avoid injection into the rostral auricular artery or vein. As this nerve block is close to the main trunk of the auriculopalpebral nerve, occasionally more extensive facial nerve block effects may occur, including muscle block effects down to the nostril, as well as a drooping and immobile ear. Topical anesthetic is also instilled for surface anesthesia of the cornea and conjunctiva. The combined akinesia and local analgesia (anesthesia) supraorbital nerve block in the horse is used for diseases localized to the mid upper eyelid. Between 2 and 4 mL of local anesthetic is injected about the supraorbital foramen within the supraorbital arch. Topical anesthetic is also instilled for surface anesthesia of the cornea and conjunctiva.

Eyelid injections/nerve blocks in the cow Eyelid injections in cattle are usually combined with retrobulbar nerve blocks which provide orbital and ophthalmic akinesia and analgesia (anesthesia) during orbital and eyelid surgery. Local anesthetic (2–4 mL) is injected just caudal of the lateral canthus to block the terminal branches of the palpebral nerve.

Retrobulbar injections/nerve blocks in animals

Fig. 3.4 For the palpebral nerve block in the dog, the local anesthetic injection is positioned either immediately above the most lateral projection of the zygomatic arch or about 1–2 cm from the lateral canthus.

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In non-primates the lateral and part of the caudal floor of the bony orbit is usually open and devoid of bony walls. These areas, consisting of fascial tissue (periorbita), muscles, and blood vessels, and covered with either skin or mucosa, represent potential entry sites for access to the orbital tissues and the injection of drugs. In non-human primate species,

Retrobulbar injections/nerve blocks in animals

orbital access is generally limited to the frontal approach around the intact globe.

Retrobulbar injections/nerve blocks in dogs Access to the cornea and anterior globe may present exposure problems in small animals, especially in certain breeds of dogs. Fortunately, the lateral and dorsolateral aspects of the dog orbit are incomplete, and accommodate retrobulbar injections. Injections of sterile 0.9% saline can enhance the presentation of the cornea and globe, but only with some risk. The injection is performed with the dog under general anesthesia with the objective of forcing the globe further rostrad in the orbit, or to turn the globe and improve exposure of a selected area of the cornea and/or anterior segment. Providing retrobulbar anesthesia is another consideration. The amount of sterile saline injected is ascertained as the injection is performed, and the response of the globe to the space-occupying solution is assessed. The hypodermic needle may be inserted caudal to the junction of the lateral orbital ligament and dorsal aspects of the zygomatic arch (Fig. 3.6). The needle is directed towards the retrobulbar space in a ventromedial direction toward the opposite mandibular joint. The solution may be injected in the lateral aspects of the extraocular muscle cone, or immediately caudal to the globe and within the retrobulbar muscle mass. Injections external to the retrobulbar muscle cone will rotate the globe laterally; injections immediately behind the globe will push the globe forward. The volume injected should be limited to produce the desired outcome, but not result in undue pressure and distortion of the globe. Another injection site is ventral to the anterior zygomatic arch and rostrad to the vertical portion of the ramus of the mandible, the Barth’s nerve block (Fig. 3.7). The hypodermic needle, after passing the ramus of the mandible, is directed

Fig. 3.7 Barth’s method for retrobulbar injection in the dog consists of placement of a 5–8 cm, 22 g hypodermic needle inserted beneath the zygomatic arch at the level of the lateral canthus. The needle must pass rostrad to the vertical portion of the ramus of the mandible and directed to the orbital fissure.

toward the orbital fissure. Injections external to the retrobulbar muscle cone in the orbital floor and the medial orbit wall are possible with this method, and can be used to shift the globe dorsally. Retrobulbar injections can also be performed with curved, 5 cm long hypodermic needles directed through the conjunctiva or the eyelids to deposit solution beside or caudal to the globe (Dietz’s method). The volume and position of the injection within the orbit will shift the eye accordingly.

Retrobulbar injections/nerve blocks in cats Retrobulbar injections in the cat are not recommended because of the limited retrobulbar space and difficulty in proper positioning of the injection.

Retrobulbar injections/nerve blocks in horses

Fig. 3.6 Retrobulbar injections can be positioned in the dog with an 8 cm, 22 g hypodermic needle inserted caudal to the lateral orbital ligament and directed toward the opposite mandibular joint. Because the dog lacks ocular rigidity, extraocular injections that are several milliliters can indent the globe. Injections within the extraocular muscle cone may have greater effects than those injected next to the orbital walls.

Retrobulbar local anesthetic injections have been described in the horse by Berge and Lichenstern. The posterior orbit and entry of the critical cranial nerves in the horse are about as deep as in cattle (10–12 cm), but the posterior orbit is more conical. With gas inhalation general anesthesia, and often neuromuscular blocking agents and forced ventilation, retrobulbar nerve blocks in the horse are unnecessary and redundant. In the Berge method, an 8–10 cm, 18 g needle is inserted caudal to the supraorbital process of the frontal bone near the supraorbital foramen. The long needle is directed ventromedial (about 40 from the vertical) and slightly caudal toward the area of the orbital fissure where 15–20 mL of local anesthetic is injected (Fig. 3.8). In a modification of the Berge technique, in the Lichenstern’s method, an 8–10 cm, 18 g needle is inserted 1.5 cm caudal to the middle of the supraorbital process. The needle is directed toward the opposite last upper premolar tooth. The taut extraocular muscles’ fascial cone may be felt as

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combined with neuromuscular blocking agents is highly recommended.

Retrobulbar injections/nerve blocks in cattle

Fig. 3.8 In Berge’s retrobulbar nerve block in the horse for injection within the orbital fissure, the needle is inserted behind the supraorbital foramen of the dorsal orbital rim, inclined 40 to the vertical, and directed medioventrally and somewhat caudally.

the needle penetrates it. Approximately 20 mL of local anesthetic is injected near the orbital fissure. As a third method, the lateral and medial canthal routes may be used to inject about 10–15 mL of local anesthetic at each site. Of the large animal species, intraocular surgery is performed most often in the horse. As this species has considerable high scleral elasticity (low scleral rigidity), sizeable volume retrobulbar injections can markedly indent the posterior segment (as viewed by ophthalmoscopy), and increase the likelihood of vitreous prolapse and posterior lens capsule rupture during cataract surgery. Therefore, when intraocular surgery is considered in this species, general gas anesthesia

A

Because of inherent problems associated with general anesthesia in cattle, as well as economics, regional nerve blocks are common in this species. In fact, most orbital, eyelid, conjunctival, and corneal surgery is performed with regional injectable anesthesia. Of the three different routes for orbital injections of regional anesthesia in the cow, i.e., Peterson’s, Schreiber’s, and Hare’s, Peterson’s is the most common in America, but somewhat more difficult. A relatively simple method in cattle, the four-point block, uses more local anesthetic than the Peterson method, and delivers retrobulbar anesthetic through the dorsal, medial, lateral, and ventral conjunctival fornices directly into the retrobulbar space. In Peterson’s regional nerve block, an 8–10 cm, 18–20 g slightly curved hypodermic needle is inserted at the posterior angle of the zygomatic arch and lateral orbital rim, and directed anterior of the coronoid process of the mandible and inferomedially to the pterygopalatine fossa near the foramen orbitorotundum (Fig. 3.9). After aspiration (avoiding the internal maxillary artery), 15–20 mL of local anesthetic is injected. The auriculopalpebral nerve is blocked by placing 3–5 mL of local anesthetic subcutaneously along the dorsal zygomatic arch. Successful nerve blocks result in mydriasis, lack of globe mobility, loss of corneal sensation, and loss of eyelid movement. The globe in some cows can be proptosed moderately, and maintained in position by the eyelids.

Complications of retrobulbar injections/ nerve blocks Retrobulbar injections require care, and can induce retrobulbar hemorrhage. Hence, after needle placement and before injection, aspiration is attempted to minimize injection into the ocular vasculature. The animal orbit contains large veins

B

Fig. 3.9 In the Peterson retrobulbar nerve block in cattle: (a) View from side of orbit: A slightly curved, 10 cm hypodermic needle is inserted in the caudal angle (arrow) of the supraorbital process and zygomatic arch, and manipulated in front of the coronoid process of the mandible. (b) Frontal view: The hypodermic needle is then directed medial and somewhat ventrally to enter the floor of the pterygopalatine fossa and the orbitorotundum foramen (arrow). After aspiration to make certain the maxillary artery has not been entered, approximately 15–20 mL of local anesthetic is injected. As the hypodermic needle is withdrawn, an additional 3–5 mL of local anesthetic is injected subcutaneously for akinesia of the eyelids.

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Horse

and venous plexuses, but hemorrhage sufficient to produce additional pressure of the globe, and even enter the subconjunctival spaces, fortunately occurs infrequently. If this occurs, surgery should be delayed until the hemorrhage has reabsorbed. Inadvertent puncture of the globe with the needle is rare, but a serious complication. The retrobulbar saline is usually reabsorbed within 30–60 min. With the use of intravenous neuromuscular blocking drugs, use of retrobulbar injections to manipulate the globe is less common and may be redundant. A few cases of cattle have been reported to suffer respiratory collapse and sudden death after the Peterson retrobulbar block, presumably from accidental anesthetic injection within the optic nerve meninges or the cerebrospinal space.

Choice of general anesthetic for selected ophthalmic surgical procedures The ophthalmic surgical procedure may influence the choice of induction agent and general anesthetic based on the expected duration of the surgery, and the level of pain and discomfort expected postoperatively.

Orbital surgery Orbital surgery is expected to result in some blood loss, and excessive globe traction may initiate the oculorespiratory cardiac reflex. In small animals, a combination of injectable and inhalational anesthetics is used to provide general anesthesia for about 60 min. The administration of acepromazine preoperatively will usually assist in promoting a smooth recovery.

Eyelid surgery Surgical procedures of the eyelids usually require induction with injectable anesthetics and continued general anesthesia with inhalational agents. Occasionally multiple administrations of only injectable anesthetics will suffice. In very young puppies with entropion, correction with the ‘tacking procedure’ often uses halothane anesthesia induced via mask.

Nasolacrimal flush and catheterization Only topical anesthesia (occasionally combined with acepromazine tranquilization, or in non-cooperative patients) is necessary for many of the manipulations required for the nasolacrimal system, including flushes, catheterization, and incisions to open the imperforate lacrimal punctum.

Conjunctival and nictitating membrane surgeries To perform nictitating membrane flaps and small eyelid or conjunctival tumor removals in small animals, propofol in dogs and cats, or ketamine in cats, is recommended. For conjunctival grafts, the time taken to perform these procedures is about 30–60 min. As a result, induction with a shortacting intravenous barbiturate, endotracheal intubation, and maintenance with inhalational anesthesia is recommended.

A smooth recovery is necessary, and usually an analgesic, such as butorphanol tartrate, is indicated.

Corneal and intraocular surgeries For corneal and intraocular surgical procedures, tranquilization with acepromazine, propofol for induction, and inhalation agents for maintenance of general anesthesia are recommended. The administration of neuromuscular blocking agents (such as pancuronium 0.06 mg/kg IV) is initiated once general anesthesia is stabilized and a few minutes before entry into the anterior chamber is achieved. Both a smooth onset and recovery from general anesthesia are anticipated. Some degree of analgesia is usually necessary during the immediate postoperative period, and often butorphanol tartrate or a similar drug is administered.

Recovery from general anesthesia During the recovery period after ophthalmic surgery, the patient should slowly and smoothly recover from general anesthesia. Excessive whining, yelping, barking, vomiting, and uncoordinated movement and thrashing are to be avoided. These effects may threaten the integrity of the surgical wounds and can result in trauma and swelling of the eyelids, subconjunctival hemorrhages, hyphema, and anterior uveitis. The optimum clinical management of these complications is to prevent their occurrence by the appropriate selection of perioperative drugs and general anesthetics. Tranquilizers, such as acepromazine, have a reasonably long duration of action, and this effect usually includes the critical postoperative period. The usual dosage for acepromazine is 0.03– 0.1 mg/kg administered intramuscularly or subcutaneously about 15 min before ophthalmic surgery. The lower dosage level is best for older canine patients; the higher dosage level is recommended for young, healthy, and excitable patients. Butorphanol tartrate, an opiate agonist/antagonist analgesic with strong antitussive activity, may also be administered postoperatively (0.5–1.0 mg/kg SC) to promote a smooth recovery from general anesthesia. Restraint devices, such as the Elizabethan collar, hobbles, and bandaging of the front paws, are additional measures that can protect the eye during the recovery phase from general anesthesia.

ADAPTATIONS FOR LARGE ANIMALS AND SPECIAL SPECIES

Horse The introduction of many drugs for sedation, analgesia, and restraint in large animals, especially the horse, has advanced significantly in the past 40 years. In the 1960s, sedation of horses for eye examination or minor invasive procedures involved only acepromazine, and was not very satisfactory. The introduction of xylazine in the 1970s, used singly or best combined with acepromazine, provided much improved sedation and adequate restraint for detailed ophthalmic examinations in horses with considerable

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eye-related pain, as well as more invasive procedures (corneal cytology, nasolacrimal system cannulation, etc.). Nowadays, the availability of detomidine combined with butorphanol (introduced in the 1990s) provides deep sedation and the best possible restraint. In ranking available drugs for sedation and restraint in the horse, we would rank (from lowest to highest sedation):

• • • •

acepromazine (0.02–0.05 mg/kg IV; avoid in stallions because of the potential for priapism) xylazine (usual dose: 0.2–0.4 mg/kg IV) xylazine (0.6 mg/kg IV) combined with acepromazine (0.02 mg/kg IV) detomidine (our dose is 0.005–0.012 mg for an ophthalmic examination rather than the recommended 0.01–0.02 mg/kg used for pain or placing a lavage system) combined with butorphanol (0.02–0.03 mg/kg IV).

Probably most, if not all, sedatives lower IOP in horses. Acepromazine and xylazine lower IOP in horses by about 10–20%. Both xylazine and detomidine are a2-agonist sedatives. IOP decline seems secondary to venous pressure, direct pressure on the globe, blood pressure, tone in the extraocular muscles, head position, and the rate, dose, and rapidity of drug administration. Decline in IOP secondary to lower systemic blood pressures is the likely explanation. Intravenous xylazine at dosages of 0.3 mg/kg (23%), 1.0 mg/kg (27%), and 1.1 mg/kg combined with ketamine (2.2 mg/kg), lower IOP. The majority of ophthalmic surgeries performed in horses use general anesthesia, and for corneal surgery often involving penetrating corneal wounds, as well as surgery of the iris, lens, and cataracts, and neuromuscular paralysis for optimal globe positioning and ocular hypotony. However, general anesthesia is not without risk in the horse! Smooth recovery does not always occur in a horse coming out of general anesthesia, and a smooth recovery is hopefully

characterized by a horse which stands on its first attempt, and does not repeatedly struggle to stand and fall! Unsatisfactory and prolonged recoveries risk injury to the eye and limbs and other serious mishaps. Mortality with general anesthesia in the horse has been reported as 1.9%, and excluding those horses with emergency abdominal procedures, the death rate is 0.9%. Although complications with general anesthesia in horses undergoing ophthalmic surgery have not been reported, fatalities after ear, nose, and throat (ENT) procedures have been reported as 0.88%. As the length of time for the ophthalmic surgical procedure and general anesthesia increases, the likelihood of complications also increases. Fortunately, most ophthalmic procedures in the horse take less than 1 h, or perhaps slightly more! Standing ophthalmic procedures, including standing enucleation, have been reported in horses. Most equine standing surgeries are relatively minor; however, some may become too difficult, requiring use of general anesthesia for successful completion.

Cattle Cattle are most often physically restrained using its stanchion (dairy cattle) or a squeeze chute and variable head restraint (beef cattle). Sedation is generally not employed because of the bovine’s very high sensitivity to drugs, such as xylazine, and must be carefully administered. The recommended dose (see Plumb’s Veterinary Drug Handbook) is 0.05–0.15 mg/kg IV or 0.10–0.33 mg/kg IM. Pretreatment with atropine can decrease the bradycardia and hypersalivation. Xylazine should be avoided in pregnant cattle (last trimester) and in animals that are dehydrated, have urinary tract obstructions, or are debilitated.

Further reading General Auer U, Mosing M, Moens YPS: The effect of low dose rocuronium on globe position, muscle relaxation, and ventilation in dogs: a clinical study, Vet Ophthalmol 10:295–298, 2007. Brunson DB: Anesthesia in ophthalmic surgery, Vet Clin North Am Small Anim Pract 10:481–495, 1980. Gelatt KN: Anesthetic agents. In Veterinary Ophthalmic Pharmacology and Therapeutics, ed 2, Bonner Springs, 1978, VM Publishing, pp 23–28. Hall LW, Clarke KW: Veterinary Anaesthesia, ed 9, London, 1991, Baillie`re Tindall, pp 105–133. Kern TJ: Anesthetic considerations of the ophthalmic patient. In Short CE, editor: Principles and Practice of Veterinary Anesthesia, Baltimore, 1987, Williams and Wilkins, pp 173–176. Langley MS, Heel RC: Propofol: a review of its pharmacodynamic and pharmacokinetic properties and use

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as an intravenous anaesthetic, Drugs 35:334–372, 1988. Plumb DC: Plumb’s Veterinary Drug Handbook, ed 5, Ames, 2005, Blackwell, pp 327–329 and 1156–1160. Rubin LF, Gelatt KN: Analgesia of the eye. In Soma LR, editor: Textbook of Veterinary Anesthesia, Baltimore, 1971, Williams and Wilkins, pp 489–499. Wilson RP: Complications associated with local and general anesthesia, Int Ophthalmol Clin 32:1–22, 1993.

Canine Bagley LH, Lavach JD: Comparison of postoperative phacoemulsification results in dogs with and without diabetes mellitus: 153 cases (1991–1992), J Am Vet Med Assoc 205:1165–1169, 1994. Batista CM, Laus JL, Nunes N, PattoDos Santos PS, Costa JL: Evaluation of intraocular pressure and partial CO2 pressures in dogs anesthetized with propofol, Vet Ophthalmol 3:17–19, 2000.

Clutton RE, Boyd C, Richards DLS, Schwink K: Significance of the oculocardiac reflex during ophthalmic surgery in the dog, J Small Anim Pract 29:573–579, 1988. Frischmeyer KJ, Miller PE, Bellay Y, Smedes SL, Brunson DB: Parenteral anticholinergics in dogs with normal and elevated intraocular pressure, Vet Surg 22:230–234, 1993. Gelatt KN, Gwin RM, Peiffer RL, Gum GG: Tonography in the normal and glaucomatous Beagle, Am J Vet Res 38:515–520, 1977. Hazra S, De D, Roy B, et al: Use of ketamine, xylazine and diazepam anesthesia with retrobulbar block for phacoemulsification in dogs, Vet Ophthalmol 11:255–260, 2008. Hofmeister EH, Williams CO, Braun C, Moore PA: Influence of lidocaine and diazepam on peri-induction intraocular pressure in dogs anesthetized with propofol– atracurium, Can J Vet Res 70:251–256, 2006. Hofmeister EH, Williams CO, Braun C, Moore PA: Propofol versus thiopental:

Further reading effects of peri-induction intraocular pressures in dogs, Vet Anaesth Analg 35:275–281, 2008. Joffe WS, Gay AJ: The oculorespiratory cardiac reflex in the dog, Invest Ophthalmol 5:550–554, 1966. Magrane WG: Methoxyflurane (metofane) anesthesia in intraocular surgery, Pract Vet 3:75–76, 1967. Seim HB, Creed JE, Smith KW: Restraint techniques for prevention of self–trauma. In Bojrab MJ, editor: Current Techniques in Small Animal Surgery, ed 3, Philadelphia, 1990, Lea and Febiger, pp 42–49. Sullivan TC, Hellyer PW, Lee DD, Davidson MG: Respiratory function and extraocular muscle paralysis following administration of pancuronium bromide in dogs, Vet Ophthalmol 1:125–128, 1998. Vestre WA, Brightman AH, Helper LC, Lowery JC: Decreased tear production associated with general anesthesia in the dog, J Am Vet Med Assoc 174:1006–1007, 1979.

Young SS, Barnett KC, Taylor PM: Anaesthetic regimes for cataract removal in the dog, J Small Anim Pract 32:236–240, 1991.

Feline Hahnenberger EW: Influence of various anesthetic drugs on the intraocular pressure of cats, Von Graefes Archiv fu¨r klinische und experimentelle Ophthalmologie 199:179–186, 1976. Mester U, Stein HJ, Pillat-Moog U: Experiences gained with a combination ketamine anaesthesia for eye surgery on cats, Von Graefes Archiv fu¨r klinische und experimentelle Ophthalmologie 201:289–294, 1977.

Horse Brooks DE: Ophthalmology for the Equine Practitioner, 2009, Teton New Media, Jackson, pp 17–29. Hendrix DVH: Eye examination techniques in horses, Clinical Techniques in Equine Practice 4:2–10, 2005.

Miller-Michau T: Equine ocular examination: basic and advanced diagnostic techniques. In Gilger BC, editor: Equine Ophthalmology, St Louis, 2005, Saunders, pp 1–62. Robertson SA: Standing sedation and pain management for ophthalmic patients, Vet Clin North Am Equine Pract 20:485–497, 2004.

Food and fiber-producing animals Donaldson LL, Holland M, Koch SA: Atracurium as an adjunct to halothane– oxygen anesthesia in a llama undergoing intraocular surgery: a case report, Vet Surg 21:76–79, 1992. Pearce SG, Kerr CL, Boure LP, Thompson K, Dobson H: Comparison of the retrobulbar and Peterson nerve block techniques via magnetic resonance imaging in bovine cadavers, J Am Vet Med Assoc 223:852–855, 2003. Skarda RT: Local and regional anesthesia in ruminants and swine, Vet Clin North Am Food Anim Pract 12:579–626, 1996.

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